Volcano-Hazard Zonation for San Vicente Volcano, El Salvador · around San Vicente volcano, as well as the proxim-ity of major transportation routes, increase the risk that even small

Volcano-Hazard Zonation for San Vicente Volcano, El SalvadorVolcano-Hazard Zonation for San Vicente Volcano, El SalvadorVolcano-Hazard Zonation for San Vicente Volcano, El SalvadorVolcano-Hazard Zonation for San Vicente Volcano, El SalvadorVolcano-Hazard Zonation for San Vicente Volcano, El Salvador 11111

Open-File Report 01–367Open-File Report 01–367Open-File Report 01–367Open-File Report 01–367Open-File Report 01–367

U.S. Department of the InteriorU.S. Department of the InteriorU.S. Department of the InteriorU.S. Department of the InteriorU.S. Department of the InteriorU.S. Geological SurveyU.S. Geological SurveyU.S. Geological SurveyU.S. Geological SurveyU.S. Geological Survey

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22222 Volcano-Hazard Zonation for San Vicente Volcano, El SalvadorVolcano-Hazard Zonation for San Vicente Volcano, El SalvadorVolcano-Hazard Zonation for San Vicente Volcano, El SalvadorVolcano-Hazard Zonation for San Vicente Volcano, El SalvadorVolcano-Hazard Zonation for San Vicente Volcano, El Salvador

Cover photographCover photographCover photographCover photographCover photograph

San Vicente volcano viewed from the southeast. Note the dual cones of the volcano. The LaCarbonera complex is located to the west of the volcano and the youngest cone is on the right,indicating that the focus of volcanic activity has migrated roughly eastward with time. (Photographby J.J. Major, U.S. Geological Survey).


Volcano-Hazard Zonation forVolcano-Hazard Zonation forVolcano-Hazard Zonation forVolcano-Hazard Zonation forVolcano-Hazard Zonation forSan Vicente Volcano, El SalvadorSan Vicente Volcano, El SalvadorSan Vicente Volcano, El SalvadorSan Vicente Volcano, El SalvadorSan Vicente Volcano, El SalvadorByByByByBy J.J. Major, S.P. Schilling, C.R Pullinger, C.D. Escobar, and M.M. Howell J.J. Major, S.P. Schilling, C.R Pullinger, C.D. Escobar, and M.M. Howell J.J. Major, S.P. Schilling, C.R Pullinger, C.D. Escobar, and M.M. Howell J.J. Major, S.P. Schilling, C.R Pullinger, C.D. Escobar, and M.M. Howell J.J. Major, S.P. Schilling, C.R Pullinger, C.D. Escobar, and M.M. Howell

U.S. GEOLOGICAL SURVEYU.S. GEOLOGICAL SURVEYU.S. GEOLOGICAL SURVEYU.S. GEOLOGICAL SURVEYU.S. GEOLOGICAL SURVEYOpen-File Report 01-367Open-File Report 01-367Open-File Report 01-367Open-File Report 01-367Open-File Report 01-367

Vancouver, Washington U.S.A.2001


U.S. DEPARTMENT OF THE INTERIORGale Norton, Secretary

U.S. GEOLOGICAL SURVEYCharles G. Groat, Director

This report is preliminary and has not been reviewed for conformity with U.S. Geological Survey editorial standards. Any use oftrade, product, or firm names is for descriptive purposes only and does not imply endorsement by the U.S. Government.

For additional information write to: Copies of this report can be purchased from:

.S.UegrahC-ni-tsitneicS Geological SurveysecivreS noitamrofnIyevruS lacigoloeG .S.U

68252 xoB .O.P01 .gdlB ,truoC lanidraC ES 003152208 OC ,revneD38689 AW ,revuocnaV

0124-202 )303(0098-399 )063(FAX: (360) 993-8980


CONTENTSCONTENTSCONTENTSCONTENTSCONTENTS

Introduction....................................................................................................................................... 1

Volcanic Phenomena......................................................................................................................... 2

Pyroclastic flow and pyroclastic surge........................................................................................... 2

Lava flows and lava domes........................................................................................................... 4

Tephra......................................................................................................................................... 4

Volcanic gases............................................................................................................................. 5

Debris avalanche, landslide, and lahar........................................................................................... 6

Past Events at San Vicente Volcano................................................................................................... 7

Future Activity at San Vicente Volcano.............................................................................................. 8

Events at Other Volcanoes Can Affect the San Vicente Region........................................................... 9

Volcano-Hazard-Zonation Map......................................................................................................... 10

Proximal volcanic hazard zone...................................................................................................... 10

Lahar hazard zones....................................................................................................................... 10

Hazard Forecasts and Warnings......................................................................................................... 12

Protecting Communities and Citizens from Volcano-Related Hazards.................................................. 12

References........................................................................................................................................

13

Additional Suggested Reading............................................................................................................ 13

End Notes ......................................................................................................................................... 14

PLATEPLATEPLATEPLATEPLATE [In pocket]

1. Volcano-hazard zonation for San Vicente Volcano, El Salvador.

FIGURESFIGURESFIGURESFIGURESFIGURES

1. Location of major cities and significant Quaternary volcanoes in El Salvador.................................. 2

2. Simplified sketch showing hazardous events associated with a volcano like San Vicente................. 3



INTRODUCTIONINTRODUCTIONINTRODUCTIONINTRODUCTIONINTRODUCTIONSan Vicente volcano, also known as

Chichontepec, is one of many volcanoes along thevolcanic arc in El Salvador (figure 1). This com-posite volcano, located about 50 kilometers east ofthe capital city San Salvador, has a volume of about130 cubic kilometers, rises to an altitude of about2180 meters, and towers above major communitiessuch as San Vicente, Tepetitan, Guadalupe,Zacatecoluca, and Tecoluca. In addition to thelarger communities that surround the volcano,several smaller communities and coffee plantationsare located on or around the flanks of the volcano,and major transportation routes are located near thelowermost southern and eastern flanks of thevolcano. The population density and proximityaround San Vicente volcano, as well as the proxim-ity of major transportation routes, increase the riskthat even small landslides or eruptions, likely tooccur again, can have serious societal conse-quences.

The eruptive history of San Vicente volcano isnot well known, and there is no definitive record ofhistorical eruptive activity [1] (numerals in bracketsrefer to end notes in the report). The last signifi-cant eruption occurred more than 1700 years ago,and perhaps long before permanent human habita-tion of the area. Nevertheless, this volcano has avery long history of repeated, and sometimesviolent, eruptions, and at least once a large sectionof the volcano collapsed in a massive landslide.The oldest rocks associated with a volcanic center

at San Vicente are more than 2 million years old.The volcano is composed of remnants of multipleeruptive centers that have migrated roughly east-ward with time. Future eruptions of this volcanowill pose substantial risk to surrounding communi-ties.

Volcanic eruptions are not the only events thatpresent a risk to local communities. Anotherconcern is a landslide and associated debris flow (awatery flow of mud, rock, and debris--also knownas a lahar) that could occur during periods of novolcanic activity. An event of this type occurred in1998 at Casita volcano in Nicaragua when ex-tremely heavy rainfall from Hurricane Mitchtriggered a landslide that moved down slope andtransformed into a rapidly moving debris flow thatdestroyed two villages and killed more than 2000people. Historical landslides up to a few hundredthousand cubic meters in volume have been trig-gered at San Vicente volcano by torrentialrainstorms and earthquakes, and some have trans-formed into debris flows that have inundatedvillages down stream [1]. For example, a debrisflow in 1934 on the north side of San Vicentedestroyed the village of Tepetitan. Destructiverainfall- and earthquake-triggered landslides anddebris flows on or near San Salvador volcano, westof San Vicente, in September 1982 and January2001 demonstrate that such mass movements in ElSalvador have also been lethal.

This report describes the kinds of hazardousevents that occur at volcanoes in general and the

Volcano-Hazard Zonation forVolcano-Hazard Zonation forVolcano-Hazard Zonation forVolcano-Hazard Zonation forVolcano-Hazard Zonation forSan Vicente Volcano, El SalvadorSan Vicente Volcano, El SalvadorSan Vicente Volcano, El SalvadorSan Vicente Volcano, El SalvadorSan Vicente Volcano, El SalvadorByByByByBy J.J. Major, S.P. Schilling, C.R Pullinger J.J. Major, S.P. Schilling, C.R Pullinger J.J. Major, S.P. Schilling, C.R Pullinger J.J. Major, S.P. Schilling, C.R Pullinger J.J. Major, S.P. Schilling, C.R Pullinger11111, C.D. Escobar, C.D. Escobar, C.D. Escobar, C.D. Escobar, C.D. Escobar11111, , , , , M.M. HowellM.M. HowellM.M. HowellM.M. HowellM.M. Howell

11111 Servicio Nacionale de Estudios Terretoriales, Ave. Roosevelt y 55 Ave. Norte, Torre El Salvador (IPSFA)


kinds of hazardous geologic events that haveoccurred at San Vicente volcano in the past. Theaccompanying volcano-hazards-zonation mapshows areas that are likely to be at risk whenhazardous events occur again.

VOLCANIC PHENOMENAVOLCANIC PHENOMENAVOLCANIC PHENOMENAVOLCANIC PHENOMENAVOLCANIC PHENOMENAVolcanoes pose a variety of geologic hazards--

both during eruptions and in the absence of eruptiveactivity (figure 2). Although it has been severalthousand years since the last significant eruption ofSan Vicente volcano, many of the hazardous eventsdepicted in figure 2 have occurred at San Vicente inthe past and will likely occur again. Most of theseevents are caused by the eruption of molten rock, ormagma, but some, like debris avalanches (land-slides) and lahars, can occur without eruptiveactivity. The nature and scale of eruptive activitydepends in part on the size and type of volcano, thecomposition of the magma, and on interactionsbetween magma and water.

Pyroclastic flow and pyroclastic surgePyroclastic flow and pyroclastic surgePyroclastic flow and pyroclastic surgePyroclastic flow and pyroclastic surgePyroclastic flow and pyroclastic surge

Explosive eruptions can produce mixtures of hotgases and volcanic rock particles that are denserthan air. Such dense mixtures behave like a fluid,stay close to the ground, and flow downslope awayfrom the volcano. If the mixture is made up mostlyof rock particles, then it has a high density and itspath will be confined to topographically low areas,much as topography will control the flow of water.This type of dense flow is called a pyroclasticflow. However, if the mixture is made up mostly ofgas with a small proportion of rock fragments, thenit will have a lower density and its path will be lesscontrolled by topography. This type of gas-richmixture is called a pyroclastic surge. Pyroclasticflows and surges also produce ash clouds that canrise thousands of meters into the air, drift down-wind, and transport ash for tens of kilometers ormore from a volcano.

Pyroclastic flows and surges often occurtogether, and both are exceedingly hazardous.

Figure 1. Location of major cities and significant Quaternary volcanoes in El Salvador. Circles indicate majorcities, triangles indicate major volcanoes. Lake Coatepeque and Lake Ilopango are large silicic calderas.

Lake Ilopango

Lake Coatepeque

Santa Ana

San Vicente

San Miguel

San Salvador

V. Santa Ana

V. Izalco V. San Salvador

V. San Vicente

V. San Miguel

V. Cochague

EL

SALVADOR

GUATAMALA

HONDURAS

Pacific Ocean40 km0

14oN.

90oW. 88

oW.


Figure 2. Simplified sketch showing hazardous events associated with a volcano like San Vicente. Events suchas lahars and landslides (debris avalanches) can occur even when the volcano is not erupting. Inset box showsclassification of magma types on the basis of silica content. Illustration by Bobbie Meyers, modified from USGSFact Sheet 002-97

Magma

Pyroclastic Flow

Pyroclastic Flow

Eruption Column

Lahar (Debris Flow)

Lava Flow

Prevailing Wind

MagmaReservoir

Dome Collapse

Fumaroles

Eruption Cloud

Ash (Tephra) FallAcid Rain

Bombs

Lava Dome

Conduit

Crack

GroundWater

Landslide(DebrisAvalanche)

Magma Silica (SiO2)Types Content

Rhyolite >68%Dacite 63-68%Andesite 53-63%Basalt <53%


They can move at such rapid speeds, 50 to 150kilometers per hour, that escape from their paths isdifficult or impossible. Temperatures in pyroclasticflows and surges commonly are several hundreddegrees Celsius or more. Owing to their highdensity, high velocity, and high temperature, pyro-clastic flows can destroy all structures and kill allliving things in their paths by impact, burial, orincineration. Although pyroclastic surges are moredilute and less dense than pyroclastic flows, surgescan affect larger areas and still be very destructiveand lethal. People and animals caught in pyroclas-tic surges can be killed directly by trauma, severeburns, or suffocation.

Pyroclastic flows and surges have occurredthroughout the eruptive history of San Vicente. Thelargest and most extensive pyroclastic flows wereassociated with explosive eruptions that occurredbefore construction of the modern edifice, perhapsmany tens to hundreds of thousands of years ago.Smaller pyroclastic flows and surges have occurredin association with lava-flow and lava-dome erup-tions. Some pyroclastic-flow deposits are found asfar as 15 kilometers from the present summit of thevolcano, but their relation to the eruptive history ofthe volcano is unclear.

Lava flows and lava domesLava flows and lava domesLava flows and lava domesLava flows and lava domesLava flows and lava domes

Lava is liquid magma that has reached Earth’ssurface nonexplosively. Depending on its viscosityand rate of discharge, lava will form a bulbous lavadome over a vent or a lava flow that can travelseveral kilometers or more down slope from a vent.Lava flows commonly move down slope as streamsof molten rock a few to tens of meters thick. Therate at which lavas flow depends chiefly upon theirchemical composition. The more silica in a lava,the more viscous it is and the more sluggishly itmoves (see inset figure 2). Basaltic lavas (<53%silica) of the kind observed in Hawaii can moverapidly, at tens of meters per minute, whereasandesitic lavas (silica contents ranging from about54% to 61%) of the kind commonly erupted by thecomposite volcanoes in El Salvador are moresluggish and move at most at a few tens of metersper hour. Although lava flows can be extremelydestructive, they typically are not life threatening.People and animals can usually walk out of the path

of an advancing flow. However, fronts on sluggishlava flows moving across steep slopes can some-times collapse and generate blocks of hot debristhat cascade down slope, break apart, and formhazardous, fast-moving pyroclastic flows or surges.

Lava domes can pose a greater hazard thanlava flows. Lava domes form from lava that ismore viscous than that in lava flows, and as a resultthey can grow on steep slopes or construct steep-sided landforms. As lava domes grow, they canbecome unstable and collapse repeatedly, alsogenerating blocks of hot debris that cascadedownslope, break apart, and form hazardous, fast-moving pyroclastic flows or surges.

Lava flows extend down all flanks of SanVicente, but have not traveled more than about 7kilometers from the summit. Future lava flows willlikely be confined to similar distances and will posea significant hazard to developed structures andagricultural crops, but because they move so slowlythey pose little threat to human life, unless a flowfront collapses and generates a pyroclastic flow orsurge. The location of the next lava flow at SanVicente volcano will be determined by the locationand geometry of the eruption vent at that time.

Prominent lava domes are not recognized onSan Vicente volcano. However, blocky pyroclastic-flow deposits, similar to those associated with thecollapse of lava domes at other volcanoes like SanVicente, are found interlayered with lava flowsaround the volcano, and they indicate that lavadomes may have formed and collapsed duringeruptions of the volcano. Pyroclastic flows relatedto lava-dome or lava-flow collapses have traveledat least 5 kilometers from the summit of the vol-cano; pyroclastic surges related to lava-flow ordome collapses probably traveled even farther.Pyroclastic flows formed by lava-flow or domecollapses at San Vicente volcano likely producedash clouds that drifted many kilometers away fromthe volcano.

TTTTTephraephraephraephraephra

As magma nears the surface of a volcano,gases dissolved in the magma are released. If thegas is released rapidly, then the magma can bebroken explosively into small fragments and be


dispersed into the atmosphere. Fragments fromsuch eruptions, which range in size from micro-scopic ash to meter-sized blocks, are collectivelycalled tephra. Tephras form deposits that blanketbroad areas downwind from a volcano. A deposit’sthickness and particle size generally decrease awayfrom the vent, but a deposit can cover large areastens to hundreds of kilometers from the source.The largest tephra fragments, called ballisticprojectiles, fall to the ground within a few kilome-ters of the vent.

Tephra falls seldom threaten life directly, exceptwithin a few kilometers of a vent. Large ballisticfragments are capable of causing death or injury byimpact. Large projectiles may also still be hotwhen they land and can start fires if they fall ontocombustible material. Most injuries and fatalitiesfrom tephra falls occur when the tephra accumula-tions are thick, or are saturated by rainfall, and thusare heavy enough to collapse building roofs. Finetephra suspended in the air can irritate eyes andrespiratory systems and exacerbate pulmonaryproblems, especially in the elderly and infants.

Indirect effects of tephra falls can be perhapsmore disruptive than the direct effects of tephrafalls. Even thin accumulations of tephra fall cansignificantly disrupt social and economic activitiesover broad regions. Tephra plumes can create tensof minutes or more of darkness, even on sunnydays, and tephra falls can reduce visibility andnavigability on highways. Tephra ingested byvehicle engines can clog filters and increase wear.Deposits of tephra can short-circuit or breakelectric transformers and power lines, especially ifthe tephra is wet, sticky, and heavy. Tephra cancontaminate surface-water drinking supplies, plugstorm- and sanitary-sewer systems, and clogirrigation canals. Even thin tephra accumulationsmay ruin sensitive crops. A serious potentialdanger of tephra stems from the threat of evensmall, dilute tephra clouds to jet aircraft that fly intothem. Ingestion of even small amounts of tephrainto jet engines can cause them to malfunction andlose power.

Lessons learned from the 1980 eruption ofMount St. Helens in the United States can helpgovernments, businesses, and citizens to preparefor future tephra falls. Communities downwind of

Mount St. Helens experienced significant disrup-tions in transportation, business activity, andservices from fallout of as little as 5 millimeters oftephra. The greater the amount of tephra fall, thelonger it took for a community to recover. Asperceived by residents, tephra falls of less than 5millimeters were a major inconvenience, whereasfalls of more than 150 mm constituted a disaster.Nonetheless, all of the downwind communitiesaffected by Mount St. Helens resumed normalactivity within about two weeks of the event.

The San Vicente volcano complex producedvoluminous, extensive tephra fall deposits once in itseruptive history. Those tephra fall deposits areassociated with a series of major explosive erup-tions that occurred less than 1 million years ago, butbefore construction of the modern edifice. Sincethen San Vicente volcano has not erupted extensivetephras. Instead, modest amounts of tephra havebeen produced during lava-flow and lava-domeeruptions. Most tephra fallout around the volcanohas come from clouds of sand- and silt-sizedparticles that rose from moving pyroclastic flowsproduced perhaps by lava-dome collapse. Suchclouds of tephra probably rose several thousands ofmeters into the air and drifted downwind. Depend-ing upon wind directions at the time of an eruption,future lava-flow and lava-dome eruptions at SanVicente could produce tephra clouds that affectcommunities such as San Vicente, Tepetitan,Tecoluca, and Zacatecoluca, among others. How-ever, tephra fallout from these types of eruptionsposes little threat to life or structures in nearbycommunities.

VVVVVolcanic gasesolcanic gasesolcanic gasesolcanic gasesolcanic gases

All magmas contain gases that are releasedboth during and between eruptions. Volcanic gasesconsist mainly of steam but also include carbondioxide and compounds of sulfur and chlorine, andminor amounts of several other gases.

Generally, volcanic gases are diluted rapidlydownwind from the vent, but within a few kilome-ters of a vent they can endanger life and health ifconcentrations are high and exposure is prolonged.Eyes and lungs of people and animals can beinjured by acids, ammonia, and other compounds.


People and animals can suffocate in denser-than-airgases like carbon dioxide, which pond and accumu-late in closed depressions.

The greatest hazards arising directly from gasesemitted at San Vicente volcano are likely to beconfined to the summit crater, and thus of concernto those who work or recreate within the crater.Outside the summit crater, direct hazards fromvolcanic gases are likely to be minor.

A wider spread, but indirect, hazard arisingfrom volcanic gases emitted by San Vicente volcanoinvolves formation of acid rain. Compounds ofsulfur are one of the main gases emitted by volca-noes, and excessive acidification of rainfall canoccur when sulfur compounds combine with watervapor and droplets and form sulfuric acid that isdeposited during storms. If such acid is sufficientlyconcentrated it can damage crops, reduce landproductivity, and pollute surface water. In ElSalvador, acid rain resulting from emission ofvolcanic gases has damaged coffee crops locally,particularly around Santa Ana volcano [2].

Debris avalanche, landslide, and laharDebris avalanche, landslide, and laharDebris avalanche, landslide, and laharDebris avalanche, landslide, and laharDebris avalanche, landslide, and laharThe slopes of a volcano may become unstable

and fail catastrophically, generating a rapidlymoving landslide called a debris avalanche.Slope instability at volcanoes can be caused bymany factors. Magma rising upward through avolcano can push aside older volcanic rock anddeform and steepen the flanks of a volcano, orwarm acidic ground water can circulate throughcracks and porous zones inside a volcano, alterstrong rock to weak slippery clay, and graduallyweaken the volcano so that it is susceptible todebris avalanches. A volcano’s slopes can also failwithout direct involvement of magma. Unexpectedearthquakes, torrential rains, or steam explosionscan trigger slope failures, but these failures arecommonly smaller in volume than those triggered bymagmatic intrusion. A debris avalanche can attainspeeds in excess of 150 kilometers per hour;generally, the larger the avalanche, the faster andfarther it can travel. Small-volume debris ava-lanches typically travel only a few kilometers fromtheir source, but large-volume debris avalanchescan travel tens of kilometers from a volcano.Debris avalanches destroy everything in their paths

and can leave deposits of 10 meters to more than100 meters thick on valley floors.

Deposits of at least one large debris avalanchehave been recognized southeast of San Vicente, inthe vicinity of Tecoluca, suggesting that a largesegment of the volcano collapsed at least once in amassive prehistoric landslide. The full extent of thisavalanche and its associated lahar is not preciselyknown, but it appears to have reached the RíoLempa, 25 kilometers from the volcano.

Lahars, also called mudflows and debris flows,are flowing masses of mud, rock, and water thatlook much like flowing concrete. They are pro-duced when water mobilizes large volumes of loosemud, rock, and volcanic debris. Commonly, land-slides and debris avalanches will transform intolahars as they move down valley. Lahars, likefloods, inundate floodplains and submerge struc-tures in low-lying areas. They can travel many tensof kilometers at speeds of tens of kilometers perhour. Lahars can destroy or damage everything intheir paths through burial or impact. They followriver valleys and leave deposits of muddy sand andgravel that can range from a few to tens of metersthick. They are particularly hazardous becausethey travel farther from a volcano than any otherhazardous phenomenon except tephra, and theyaffect stream valleys where human settlement isusually greatest. In some instances, landslides andlahars can clog a channel or block a tributarychannel and impound a lake behind the blockage.Commonly, the impounded water will spill over theblockage, quickly cut a channel, catastrophicallydrain the lake, and generate a flood that movesdown the valley endangering people and property.Breaching of the blockage may occur within hoursto months after impoundment.

Like floods, lahars range greatly in size. Thesmallest lahars recur most frequently (perhapsevery few years), whereas the largest recur on theorder of centuries to millennia. The size of lahars iscontrolled by both the amount of water and theamount of loose sediment or volcanic debris avail-able. Large debris avalanches or eruptions candump tens to hundreds of millions of cubic metersof sediment into channels and produce large lahars.Small debris avalanches or eruptions producesmaller lahars. Deposits of ancient lahars are


found in several channels around San Vicentevolcano, and historical landslides and lahars of morethan 100,000 cubic meters in volume have failedfrom all sides of the volcano’s steep upper slopesand have traveled more than 6 kilometers from theirsources [1].

Landslides and lahars can cause problems longafter the event that formed them ends. Oncelandslides and lahars fill stream channels withsediment, the streams begin to erode new paths,and the new stream channels can be highly unstableand shift rapidly as sediment is eroded and movedfarther down valley. Rapid stream shifting cancause rapid and dramatic bank erosion. Also,because stream channels are clogged with sedi-ment, they have less ability to convey water. As aresult, relatively small floods, which may havepreviously passed unnoticed, can pose potentiallysignificant threats to people living in low-lyingareas. In general, people living in low-lying areasalong river valleys are most susceptible to thesesecondary affects from landslides and lahars, butareas on higher ground adjacent to river channelsapparently safe from flooding may be threatened bybank erosion. Examples from many volcanoesaround the world show that the effects of sedimentdeposition by landslides and lahars in streamchannels can persist for years to decades [3].

PAST EVENTS APAST EVENTS APAST EVENTS APAST EVENTS APAST EVENTS AT SAN VICENTET SAN VICENTET SAN VICENTET SAN VICENTET SAN VICENTEVOLCANOVOLCANOVOLCANOVOLCANOVOLCANO

Details of the eruptive history of San Vicentevolcano are poorly known. The last major eruptionoccurred more than 1700 years ago, and probablyoccurred long before permanent human habitationof the area (~2000 B.C.). Nevertheless, we doknow that a volcanic complex at San Vicente has ahistory that extends more than 2 million years, andthat it has exhibited highly explosive eruptions aswell as emplacement of lava flows and lava domes.

Previous studies recognize the existence of atleast a three-stage evolution of San Vicente volcano[1]. The oldest rocks at San Vicente are between 1million and 2 million years old and are associatedwith a series of pronounced hills that lie immedi-ately west-northwest of the volcano. These hillsdelineate the edge of an annular feature thought to

be the remnants of an older volcanic center, knownas La Carbonera. Lava flows extruded overhundreds of thousands of years built the LaCarbonera complex.

Sometime after about 1 million years ago,relatively quiescent emplacement of basaltic andandesitic lavas at the ancestral La Carboneravolcanic center was interrupted by a phase of majorexplosive activity. The explosive eruptive phaseproduced pyroclastic flows, pyroclastic surges andthick tephra fall. Deposits of this explosive phaseof activity are rich in dacite pumice, a light frothyfragment of exploded magma having a silicacontent ranging from 63% to 68%, which indicatesthat gas-rich magma intruded the volcano anderupted violently. The earliest deposits from thisphase of activity are separated from later depositsby a thick, well developed paleosol, a buried soilhorizon, indicating that the explosive phase ex-tended over many thousands of years. Thecompositions, textures, and distributions of somenonpumiceous tephra and surge deposits within thesequence of deposits related to this explosive phaseof activity indicate that some explosions werephreatomagmatic and involved interactions ofmagma and water. The timing of this explosivephase of activity is unknown. Development of apair of thick, well developed paleosols within thissequence of deposits as well as the construction ofthe modern San Vicente volcano after this explosiveeruptive phase ended suggest that this period ofexplosive activity occurred many tens to hundredsof thousands of years ago.

The modern San Vicente volcano was built afterthe phase of explosive activity. Its edifice consistsof two prominent cones that are composed largelyof andesite lava flows. On the basis of theirmorphology, the easternmost cone appears to be theyoungest, suggesting that the focus of volcanism atthis center has migrated east-northeastward withtime. Local deposits of nonpumiceous pyroclasticflows interlayered with the lava flows suggest thatconstruction of the modern edifice included growthand collapse of small-volume lava domes.

Mass movements of sediment from the volcanoby landslides, lahars, and pyroclastic flows, andreworking of that sediment by streamflow, haveformed an apron of debris that has accumulated at


the base of the volcano. Of particular significanceis an extensive lahar-like deposit southeast of thevolcano that contains many small hills, composed ofvolcanic rock and debris, known as hummocks.This deposit, found near and southeast of Tecoluca,extends at least as far as the Río Lempa, 25kilometers from the volcano, and represents adebris avalanche and associated lahar that resultedfrom collapse of a massive segment of the volcano.

Most of the deposits in the apron of debrisaccumulated at the base of the volcano are prob-ably many thousands of years old. They commonlyare weathered and capped by a well developed soilhorizon, and are overlain by the Tierra BlancaJoven (TBJ) deposit, the youngest tephra depositfrom an eruption of Ilopango caldera. The TBJdeposit is more than 1700 years old [4].

Although there has been no historical eruptiveactivity at San Vicente volcano, lethal, and poten-tially lethal, volcano-related events have occurredseveral times. Known earthquake- and rainfall-triggered landslides and lahars occurred in 1774,1934, 1996, and 2001 [1]. Others may haveoccurred in historical time, but are not recorded.The 1774 lahar occurred on the northeast flank ofthe volcano and affected the village of San Vicente.The 1934 lahar occurred on the north flank of thevolcano and destroyed the village of Tepetitan,more than 6 kilometers from the summit of thevolcano. Landslides and lahars on the south flankof the volcano in 1996 damaged the major roadwaybetween Tecoluca and Zacatecoluca. Landslidestriggered by an earthquake in February 2001occurred on the north and northwest flanks of thevolcano, but they did not transform into lahars thatflowed down valley. However, these landslidesdumped more than 200,000 cubic meters of sedi-ment into channels that drain the volcano, andpotential remobilization of that sediment poses anincreased risk from destructive floods and lahars tothe downstream communities of Guadalupe andTepetitan. In September 2001, a rainfall-triggeredlahar from the northwest flank of the volcano,which possibly formed in material loosened by theFebruary 2001 earthquake, damaged the town ofGuadalupe.

FUTURE ACTIVITY AFUTURE ACTIVITY AFUTURE ACTIVITY AFUTURE ACTIVITY AFUTURE ACTIVITY AT SAN VICENTET SAN VICENTET SAN VICENTET SAN VICENTET SAN VICENTEVOLCANOVOLCANOVOLCANOVOLCANOVOLCANO

Although San Vicente volcano has a lack ofhistorical eruptive activity and a poorly knowneruptive history, vigorous hot springs and geother-mal development located on the north side of thevolcano suggest that past eruptions are youthfulenough that the volcanic system maintains residualheat from magmatic sources. As a result, thevolcano should be considered active and likely toerupt again.

On the basis of past eruptive activity, futureeruptive activity at San Vicente volcano will mostlikely involve emplacement of lava flows andgrowth and collapse of small-volume lava domes.Collapse of lava domes will generate small pyro-clastic flows and surges that may travel severalkilometers beyond the base of the volcano. Tephrafall associated with such eruptive events may travel10 kilometers or more from the volcano. Thevolcano has, however, erupted violently in the pastand could do so again in the future. Explosiveeruptions are more dangerous than those thatemplace lava flows or lava domes. Explosiveeruptions can produce large pyroclastic flows andsurges that simultaneously affect multiple sectors ofthe volcano, as well as produce tephra falls andlahars that could affect areas more than 10 kilome-ters from the volcano. Landslides and lahars,triggered by any of several mechanisms, can occuron any flank of the volcano.

Although the volcano has erupted violently in itspast, it apparently has not done so since the presentcone was constructed. Because thousands of yearshave elapsed since its last significant eruption, it isdifficult to forecast the style of future eruptions. Ifprecursory volcanic activity, such as increasedseismicity within the cone, increased gas emission,and edifice deformation, is detected at San Vicentevolcano it would be prudent to anticipate explosiveactivity at the onset of an eruption.

The largest magnitude events that are possibleat San Vicente volcano have very low annualprobabilities, but if they occur they will have veryserious consequences. Although preparing for suchrare events is not warranted, understanding poten-tial extreme-case scenarios is nonetheless prudent.


As demonstrated by the catastrophic 1980 eruptionof Mount St. Helens in the United States, two typesof large-scale hazardous events can occur atvolcanoes like San Vicente-- a large-volume debrisavalanche and lahar, and a large directed blast (atype of highly mobile pyroclastic flow). Hazardzones for such catastrophic events are shown onthe accompanying hazard-zonation map (plate 1).

The primary effects of future eruptions orlandslides at San Vicente will likely be confined towithin about 10 kilometers of the summit of thevolcano. However, large lahars could travel morethan 10 kilometers from the summit, and tephra fallcould be carried more than 10 kilometers down-wind.

Upper-level wind patterns in Guatemala be-tween 3000 and 15,000 meters altitude are stronglyseasonal [5]. Similar wind patterns are likely in ElSalvador. From January to March, westerly windsdominate. April and May are transitional months inwhich westerly winds give way to more northerlyand easterly winds. June through October arecharacterized by easterly winds, and November andDecember are transitional months during whichwesterly winds gradually become dominant. Thestrong seasonality of these winds will influenceareas affected by tephra falls. Erupted tephras willlikely fall eastward of the volcano from Januarythrough March, potentially cover broad regions tothe east, south, and west in April and May, affectareas west of the volcano from June throughOctober, and possibly areas west, north, and east ofthe volcano in November and December. Surfacewinds may also affect tephra distributions, and theirpatterns are diurnal as well as seasonal [5]. There-fore, all sectors around San Vicente volcano can beaffected by tephra fall, but some areas are morelikely to be affected than others depending upon theseason in which an eruption occurs.

The primary effects of eruptions and landslidesare serious, but secondary effects can be equally assevere, can affect areas that are beyond the zoneof primary impact, and can linger for several years.Such secondary effects, which are chiefly associ-ated with sediment deposited in river channels bylandslides and lahars, involve reworking and redis-tribution of sediment, bank erosion, loss of channelcapacity, and enhanced hazards of floods in low-

lying areas. Secondary effects that occur in theaftermath of an eruption of San Vicente volcano ora large landslide off the volcano can affect areasmany tens of kilometers downstream from thevolcano.

EVENTS AEVENTS AEVENTS AEVENTS AEVENTS AT OTHER VOLCANOES CANT OTHER VOLCANOES CANT OTHER VOLCANOES CANT OTHER VOLCANOES CANT OTHER VOLCANOES CANAFFECT THE SAN VICENTE REGIONAFFECT THE SAN VICENTE REGIONAFFECT THE SAN VICENTE REGIONAFFECT THE SAN VICENTE REGIONAFFECT THE SAN VICENTE REGION

San Vicente volcano is not the only source ofvolcanic hazards in the region. The mostdevastating volcanic events that have affected theSan Vicente region are related to large explosiveeruptions from Ilopango caldera, which is located tothe west of San Vicente (figure 1). Four explosiveeruptions from this caldera within approximately thepast 40,000 to 50,000 years [4] left tephra-fall andpyroclastic-flow deposits that are as much asseveral meters thick in the San Vicente region.Deposits of the youngest explosive eruption fromIlopango, the regional Tierra Blanca Joven (TBJ)unit, are dated at 260 A.D. [4], and these depositsextend from several kilometers east of San Vicentevolcano to several kilometers west of San Salvadorvolcano (figure 1). Such large catastrophiceruptions of Ilopango occurred about once every10,000 to 15,000 years during the past 40,000 to50,000 years, so the annual probability of anothereruption of this magnitude at Ilopango is very low.Depending upon the season and prevailing winddirections, tephra from eruptions of other volcanoessuch as Santa Ana, San Salvador, and San Miguel(figure 1), for example, could affect the San Vicenteregion.

Cerro Ramirez is a small volcano that formed onthe lowermost northeast flank of San Vicentevolcano near the town of San Vicente. Very little isknown about this volcano, but future eruptiveactivity at this site will pose significant risk to localpopulations, particularly to the town of San Vicente.

North-northeast of San Vicente volcano lies theApastepeque volcanic field, a dense cluster ofabout 25 small volcanoes contained within an areaof about 65 square kilometers [1]. These volcanoesconsist of small domes, cinder cones, and explosioncraters. Eruptions of these volcanoes are similar tothose of the small monogenetic volcanoes thatsurround San Salvador volcano, and include explo-


sive ejection of tephra, lava flows, and possiblypyroclastic flows and pyroclastic surges. Futureeruptions of the Apastepeque volcanic field mayproduce tephra falls that accumulate thinly oncommunities mainly to the north of San Vicentevolcano.

VOLCANO-HAZARD-ZONAVOLCANO-HAZARD-ZONAVOLCANO-HAZARD-ZONAVOLCANO-HAZARD-ZONAVOLCANO-HAZARD-ZONATION MAPTION MAPTION MAPTION MAPTION MAPBecause the details of past eruptions at San

Vicente are poorly known, we rely on informationabout the effects and consequences of eruptionsfrom volcanoes around the world that are similar toSan Vicente volcano to gain a general idea ofpossible eruption scenarios and hazards. This is areasonable method because similar types of eventsoccur at many volcanoes, even though the exacttypes of events that occur, and their relative fre-quencies and sizes, vary among volcanic centers.

The accompanying volcano-hazard-zonationmap (plate 1) shows areas that could be affectedby future hazardous geologic events at San Vicentevolcano. Individual events typically affect only partof a hazard zone. The location and size of anaffected area will depend on the location of anerupting vent or landslide, the volume of materialinvolved, and the character of an eruption, espe-cially its explosivity.

Potentially hazardous areas around San Vicentevolcano are divided into proximal-volcanic and laharhazard zones depending primarily on the type ofhazard. The lahar hazard zones are subdividedfurther on the basis of the relative degree of hazardfrom lahars of various volumes. Hazard-zoneboundaries are drawn on the basis of (1) themagnitude of past events at the volcano, as inferredfrom deposits; (2) mathematical models that usecalibrations from other volcanoes to predict theprobable extent of lahars; and (3) our experienceand judgement derived from observations andunderstanding of events at similar volcanoes.

Although we show sharp boundaries for hazardzones, the limit of the hazard does not end abruptlyat these boundaries. Rather, the hazard decreasesgradually as distance from the volcano increases,and for lahars decreases rapidly with increasingelevation above channel floors. Areas immediatelybeyond outer hazard zones should not be regarded

as hazard-free, because the limits of the hazard canbe located only approximately, especially in areas oflow relief. Many uncertainties about the source,size, and mobility of future events preclude locatingthe boundaries of zero-hazard zones precisely.

Users of the hazard map in this report should beaware that the map does not show all hazardousareas subject to landslides and lahars from SanVicente volcano. The volcano is extensivelyincised, and landslides could occur in any drainage.For this report we defined zones of inundation fromlahars of various volumes for prominent channelsdirected toward populous areas. Other channelsfor which we have not modeled lahar inundationshould not be considered as areas devoid of laharhazard. Landslides and lahars from other un-mapped channels could just as well threaten life andproperty.

Proximal volcanic hazard zoneProximal volcanic hazard zoneProximal volcanic hazard zoneProximal volcanic hazard zoneProximal volcanic hazard zone

The proximal volcanic hazard zone includesareas immediately surrounding San Vicente vol-cano, and extends about 8 to 10 kilometersoutward from the summit depending upon localtopography [6]. This zone delineates areas subjectto devastating volcanic phenomena includingpyroclastic flows and surges, debris avalanches,lava flows, and ballistics. Owing to the speed anddestructiveness of many of these phenomena,escape or survival is unlikely in the proximalvolcanic hazard zone. Therefore, evacuating thishazard zone during periods of volcano unrest isrealistically the only way to protect lives. Debrisavalanches and lahars will originate in the proximalarea, and deposits from slides and flows less thanabout 500,000 cubic meters in volume will likely berestricted to this zone. Larger debris avalanchesand lahars will travel away from the volcano andonto adjacent lowlands. The extent of inundationfrom lahars of various volumes is the basis fordefining the lahar hazard zones.

Lahar hazard zonesLahar hazard zonesLahar hazard zonesLahar hazard zonesLahar hazard zones

Lahar hazard zones lie primarily along channelsthat drain San Vicente volcano. Depending on thedistance from the volcano, these areas will beaffected a few minutes to about one hour after the


onset of a lahar. Beyond 10 kilometers from thevolcano’s summit escape may be possible if peopleare given sufficient warning. Within 10 kilometersof the volcano lahars may happen too quickly toprovide effective warning.

We used a mathematical technique calibratedwith data from other volcanoes [7] to estimatepotential areas of inundation by lahars of variousvolumes. For each channel analyzed, we definefour nested hazard zones that depict anticipatedinundation by hypothetical “design” lahars havingdifferent volumes. The largest design lahar se-lected for initiation within a single channel, 1 millioncubic meters, reflects our estimate of the largestprobable lahar that could be triggered by earth-quakes or torrential rains at San Vicente [7]. Wealso define a hazard zone that depicts anticipatedinundation by a lahar having a volume of 100 millioncubic meters. A lahar of this size reflects ourestimate of the largest probable debris avalanchethat might descend suddenly from San Vicentevolcano [7]. A debris avalanche of this size re-quires catastrophic failure of a very large part ofthe volcano, followed by complete transformation ofthe avalanche into a lahar. Such an event mightoccur in conjunction with volcanic activity, such asintrusion of magma into the edifice, that would bedetected by monitoring. However, the possibilitythat large flank failures could be triggered bymechanisms other than magma intrusion, such as byweakening of rock by hydrothermal alteration,strong earthquakes, or torrential rains cannot bedismissed. In general, landslides and lahars trig-gered by mechanisms other than volcanic activityare most likely to be smaller than 1 million cubicmeters in volume.

The intermediate (300,000 to 500,000 cubicmeters) and smallest (100,000 cubic meters) designlahars are more typical lahar volumes for a small tomoderate eruption or for a landslide that occurswithout warning. Lahars of these sizes haveoccurred historically at San Vicente volcano, andlahars of these sizes and smaller are the most likelysizes to occur again.

Large lahars are less likely to occur than smalllahars. Thus, the nested lahar-hazard inundationzones show that the likelihood of lahar inundationdecreases as distance from the volcano and eleva-

tion above the valley floors increases. The annualprobability of lahars of various sizes is difficult toestimate. Ages and extents of lahars havingvolumes of 1 million cubic meters or more from SanVicente are unknown, but lahars of this size prob-ably have an annual probability of less than 1 in50,000 to 1 in 10,000 on the basis of soil develop-ment on such deposits [8]. Smaller landslides andlahars triggered by earthquakes or torrential rainsare much more likely to occur, but would probablyinundate only parts of the design hazard zonesadjacent to stream channels. Lahars of about500,000 cubic meters or less may have an annualprobability of about 1 in 50 to perhaps as great as1 in 10 [8].

In general, lahar hazard zones are within about10 kilometers of the summit crater, and fall mostlywithin the proximal volcanic hazard zone. Thehazard zone for the largest volume design lahar of100 million cubic meters, representing catastrophic,massive flank failure, extends as much as 25kilometers from the summit crater and broadlycovers the landscape surrounding the volcano.Local topography plays a large role in controllingthe runout of smaller lahars. Although landslidesand lahars originate in and flow along steeplyincised drainages on the flanks of the volcano,these channels abruptly shallow and the topographyabruptly flattens near the base of the edifice. As aresult, lahars rapidly spill out of channels, spread,and stop. The most distant hazard zones areassociated with the deepest incised channels inwhich lahars remain confined, particularly along thenorthern half of the San Vicente region. Despitetheir relatively short runout distances, even thesmallest lahars can be devastating. All majorcommunities near the volcano are located within 10kilometers of the summit. Our results suggest thatthe communities located generally northwest tonortheast of San Vicente volcano are at greatestrisk from inundation by lahars. However, severalsmaller communities and coffee plantations arelocated on the lower flanks of the volcano, and thehazard zones of even the smallest lahars extendwell into areas that are now settled or used foragriculture.


HAZARD FORECASTS ANDHAZARD FORECASTS ANDHAZARD FORECASTS ANDHAZARD FORECASTS ANDHAZARD FORECASTS ANDWARNINGSWARNINGSWARNINGSWARNINGSWARNINGS

Scientists normally can recognize and monitorseveral indicators of impending volcanic eruptions.Magma rising into a volcano prior to an eruptioncauses changes that can usually be detected byvarious geophysical instruments and visual observa-tions. Swarms of small earthquakes are generatedas rock breaks to make room for rising magma or asheating of fluids increases underground pressures.Heat from the magma can increase the temperatureof ground water and raise temperatures of hotsprings and steaming from fumaroles; it can alsogenerate small steam explosions. The compositionand volume of gases emitted by fumaroles canchange as magma nears the surface, and injection ofmagma into a volcano can cause swelling or othertypes of surface deformation.

El Salvador has a national seismic network, so asignificant swarm of earthquakes at San Vicentevolcano would be noticed quickly. At other volca-noes similar to San Vicente, notable increases inseismicity have occurred days to months beforeeruptions. An increase in seismicity near thevolcano should prompt deployment of additionalseismometers to better locate earthquakes, andstimulate other monitoring efforts that examinesigns of volcanic unrest.

Periods of unrest at volcanoes produce times ofgreat uncertainty. During the past few decadessubstantial advances have been made in volcanomonitoring and eruption forecasting, but stillscientists can often make only very general state-ments about the probability, type, and scale of animpending eruption. Precursory activity can gothrough accelerating and decelerating phases, andsometimes will die out without an eruption. Gov-ernment officials and the public must realize thelimitations in forecasting eruptions and must beprepared to cope with such uncertainty.

Despite advances in volcano monitoring anderuption forecasting, it is still difficult, if not impos-sible, to predict the precise occurrence of landslidestriggered by earthquakes or torrential rains. There-fore, government officials and the public need toidentify the locations of lahar hazard zones and

realize that potentially lethal events in these hazardzones can occur with little or no warning.

Protecting Communities and CitizensProtecting Communities and CitizensProtecting Communities and CitizensProtecting Communities and CitizensProtecting Communities and Citizensfrom Vfrom Vfrom Vfrom Vfrom Volcano-Related Hazardsolcano-Related Hazardsolcano-Related Hazardsolcano-Related Hazardsolcano-Related Hazards

Communities, businesses, and citizens must planahead to mitigate the effects of future volcaniceruptions, landslides, and lahars from San Vicentevolcano. Long-term mitigation efforts must includeusing information about volcano hazards whenmaking decisions about land use and siting ofcritical facilities. Future development should avoidareas judged to have an unacceptably high risk orbe planned and designed to reduce the level of risk.

When volcanoes erupt or threaten to erupt, arapid, well-coordinated emergency response isneeded. Such a response will be most effective ifcitizens and public officials have a basic under-standing of volcano hazards and have planned theactions needed to protect communities.

Because an eruption can occur within days tomonths after the first precursory activity andbecause some hazardous events, such as landslidesand lahars, can occur without warning, suitableemergency plans should be made in advance.Although it has been more than 2000 years sinceSan Vicente volcano last erupted significantly and itis unknown when it may erupt again, public officialsneed to consider issues such as public education,land-use planning, communication and warningstrategies, and evacuations as part of a responseplan. Emergency plans already developed forfloods may apply to some extent, but may needmodifications for hazards from lahars. For habitatsin low-lying areas, a map showing the shortest routeto high ground will also be helpful for evacuations.

Knowledge and advance planning are the mostimportant items for dealing with volcano hazards.Especially important is a plan of action based on theknowledge of relatively safe areas around homes,schools, and workplaces. All of the volcanohazards described in this report are serious, andmany different hazardous phenomena may affectan area that extends as much as 10 kilometers fromthe summit of San Vicente volcano. Lahars posethe biggest threat to people living, working, or


recreating along channels that drain San Vicentevolcano. The best strategy for avoiding a lahar is tomove to the highest possible ground. A safe heightabove river channels depends on many factorsincluding the size of the lahar, distance from thevolcano, and shape of the valley. For areas beyondabout 8 kilometers from the summit of the volcano,all but that largest lahars will rise less than 20meters above river level. San Vicente volcano willerupt again, and the best way to cope with futureeruptions is through advance planning in order tomitigate their effects.

REFERENCESREFERENCESREFERENCESREFERENCESREFERENCESBaum, R.L., Crone, A.J., Escobar, D., Harp, E.L.,

Major, J.J., Martinez, M., Pullinger, C.R., andSmith, M.E., 2001, Assessment of landslidehazards resulting from the February 13, 2001,El Salvador earthquake: U.S. Geological SurveyOpen-File Report 01–119, 22 p.

Brauer, J., Smith, J., and Wiles, V., 1995, On yourown in El Salvador: On Your Own Publications,Portland, OR, 260 p.

Hart, W.J.E., and Steen-McIntyre, V., 1983, TierraBlanca Joven tephra from the A.D. 260 eruption ofIlopango caldera, in Sheets, P.D., ed., Archaeologyand Volcanism in Central America: University ofTexas Press, Austin, p. 14–34.

Hayashi, J.N., and Self, S., 1992, A comparison ofpyroclastic flow and debris avalanche mobility:Journal of Geophysical Research, v. 97,p. 9063--9071.

Iverson, R.M., Schilling, S.P., and Vallance, J.W.,1998, Objective delineation of lahar-hazard zonesdownstream from volcanoes: Geological Society ofAmerica Bulletin, v. 110, p. 972–984.

Major, J.J., Pierson, T.C., Dinehart, R.L., andCosta, J.E., 2000, Sediment yield following severevolcanic disturbance--a two decade perspectivefrom Mount St. Helens: Geology, v. 28,p. 819--822.

Malin, M.C., and Sheridan, M.F., 1982, Computer-assisted mapping of pyroclastic surges: Science,v. 217, p. 637–640.

Mercado, R., Rose, W.I., Najera, L., Matías, O., andGirón, J., 1988, Volcanic ashfall hazards and

upper wind patterns in Guatemala, preliminaryreport: Publication of Department of GeologicalEngineering and Sciences, Michigan Technologi-cal University: Houghton, MI, 34 p.

Portig, W.H., 1976, The climate of Central America,in Schwerdtfeger, W., ed., World Survey ofClimatology, Climates of Central and SouthAmerica, v. 12: Elsevier, New York, p. 405–478.

Romano, L., 1997, Catalogo de desastres, accidentesy ecologia (1915-1990): Centro de Proteccion deDesastres.

Rose, W.I., Conway, F.M., Pullinger, C.R., Deino, A.,and McIntosh, W.C., 1999, An improved ageframework for late Quaternary silicic eruptions innorthern Central America: Bulletin of Volcanology,v. 61, p. 106–120.

Rotolo, S.G., Aiuppa, A., Pullinger, C.R., Parello, F.,Tenorio-Mejica, J., 1998, An introduction to SanVicente (Chichontepec) volcano, El Salvador:Revista Geológica de América Central, v. 21,p. 25–36.

Rotolo, S.G., and Castorina, F., 1998, Transition frommildly-tholeiitic to calc-alkaline suite: the case ofChichontepec volcanic center, El Salvador, CentralAmerica: Journal of Volcanology and GeothermalResearch, v. 86, p. 117–136.

Rymer, M.J., and White, R.A., 1989, Hazards in ElSalvador from earthquake-induced landslides, inBrabb, E.E., and Harrod, B.L., eds., Landslides:Extent and Economic Significance. Balkema,Rotterdam, p. 105--109.

Williams, H., and Meyer-Abich, H., 1955, Volcanismin the southern part of El Salvador: University ofCalifornia Publications in Geological Sciences,v. 32, 64 p.

ADDITIONAL SUGGESTED READINGADDITIONAL SUGGESTED READINGADDITIONAL SUGGESTED READINGADDITIONAL SUGGESTED READINGADDITIONAL SUGGESTED READINGBlong, R.J., 1984, Volcanic hazards: Academic Press,

Orlando, 424 p.

Sigurdsson, H., Houghton, B., McNutt, S.R., Rymer,H., and Stix, J., eds., 2000, Encyclopedia ofVolcanoes: Academic Press, San Diego, CA.,1417 p.

Tilling, R.I., ed., 1989, Volcanic hazards: Short coursein geology, v. 1, American Geophysical Union,Washington, D.C., 123 p.


END NOTESEND NOTESEND NOTESEND NOTESEND NOTES[1] The geologic data upon which this report is

based come largely from Rotolo et al. (1998);Rotolo and Castorina (1998); Williams andMeyer-Abich (1955); communications withpersonnel at Centro de InvestigacionesGeotécnicas, San Salvador; and our ownreconnaissance investigations. There is norecord of historical eruptive activity of SanVicente volcano, although Williams and Meyer-Abich (1955) make a vague reference to anunconfirmed eruption in 1643. Information onhistorical landslides and lahars comes mainlyfrom interviews of elderly residents ofTepetitan, communications with personnel atCentro de Investigaciones Geotécnicas andGeothermal Salvadorena, Brauer et al. (1995),Romano (1997), and Baum et al. (2001).

[2] Diario del Hoy reported on gas emissions, acidrain, and crop damage at coffee plantationsaround Santa Ana volcano in a story publishedon January 19, 2001.

[3] Analyses of limited data from volcanoes aroundthe world indicate that sediment yields fromriver channels filled with volcanic debris by aneruption can remain higher than typical back-ground levels for years to decades after aneruption. In some cases sediment yields canremain 10 to 100 times greater than typicalbackground levels for more than two decades(Major et al., 2000). River channels heavilyclogged with sediment typically are unstable.Heavy sediment deposition causes a river towander across the valley floor, which cantrigger significant bank erosion that further addsto a river’s sediment load.

[4] Ages of eruptions from large silicic calderas inCentral America are given in Rose et al. (1999).Detailed discussion of the Tierra Blanca Joven(TBJ) tephra from Ilopango caldera is given inHart and Steen-McIntyre (1983).

[5] Upper-level wind patterns in Guatemala aregiven in Mercado et al. (1988). Diurnal andseasonal surface-wind patterns in San Salvadorare given in Portig (1976).

[6] The maximum extent of the proximal volcanichazard zone is estimated from the formulaH/L = 0.2, where H is the elevation difference

between the summit of San Vicente volcano andthe hazard boundary line, and L is the horizontaldistance from the center of the summit to thehazard boundary line (see, for example, Malinand Sheridan, 1982; Hayashi and Self, 1992;and Iverson et al., 1998). The value 0.2 wasselected because it is consistent with the H/Lratio of proximal hazardous phenomena at manyother volcanoes.

[7] Lahar hazard zones were constructed bymodeling lahar volumes of 100,000; 300,000;500,000; 1 million; and 100 million cubicmeters. Using mathematical and digital carto-graphic techniques (Iverson et al., 1998), thesevolumes were used to compute the estimatedextent of inundation down stream from a sourcearea. Although there have been at least fourlarge landslides and lahars at San Vicentevolcano within the past 225 years, the volumesof these events are rather ill-constrained.Landslides at San Vicente volcano triggered by alarge (Mw 6.6) earthquake in February 2001 hadestimated volumes as great as 250,000 cubicmeters (Baum et al., 2001). Regional earth-quake- and rainfall-triggered landslides have hadvolumes of more than 10 million cubic meters,but most landslides triggered by these mecha-nisms have had volumes of a few hundred to afew tens of thousands of cubic meters (Rymerand White, 1989; Baum et al., 2001; E.L. Harpand A.J. Crone, U.S. Geological Survey,personal communication). At Casita volcano inNicaragua, extremely heavy rainfall fromHurricane Mitch triggered a landslide of about1.5 million cubic meters in volume, but as itmoved down slope it transformed into a laharthat scoured its channel and its volume enlargedto more than 3 million cubic meters (K.M.Scott, U.S Geological Survey, personal commu-nication). On the basis of these data, we selecta landslide and associated lahar of 1 millioncubic meters to be a probable maximum sizelikely to be triggered in any single channel atSan Vicente volcano by earthquakes or torrentialrainfalls.

A volume of 100 million cubic meters is consid-ered to be the largest likely debris avalanchefrom San Vicente volcano, on the basis of thefollowing analogy to the 1980 debris avalancheof Mount St. Helens. The 1980 Mount


St. Helens avalanche removed about 2300million cubic meters from the north flank of thevolcano, which had an average slope of about30 degrees. This avalanche removed about25% of the cone’s total volume above thealtitude at which the failure plane intersected thelower north flank. Similar to Mount St. Helens,San Vicente volcano is a massive, steep sidededifice, but one that is extensively incised.Slopes above 1600 meters altitude equal orexceed 30 degrees, and the volume of SanVicente volcano above this altitude is about 1billion cubic meters. However, that is thevolume of the combined “dual” edifice. Eachseparate cone of San Vicente volcano has avolume of about 500 million cubic meters. Ifwe assume that magma would most likelyintrude one cone rather than both simulta-neously, and apply the 25% value from MountSt. Helens to San Vicente, then the maximumprobable volume of a large debris avalanche andassociated lahar is slightly more than 100 millioncubic meters. For modeling purposes we haverounded this volume downward to 100 millioncubic meters. San Vicente has collapsed in amassive landslide at least once in its history;however, the volume of that collapse andassociated lahar is unknown.

[8] The annual probability of a lahar having avolume that equals or exceeds 1 million cubicmeters is probably less than 1 in 10,000. Lahardeposits around San Vicente volcano havingsuch approximate volumes are capped by welldeveloped soil horizons that are grossly similarto or older than those that cap some of theQuaternary tephra deposits from eruptions ofIlopango caldera (the TB2, TB3, and TB4units), and the deposits from those calderaeruptions have been estimated to range fromabout 15,000 to 50,000 years old (Rose et al.,

1999; J.W. Vallance, U.S. Geological Survey,personal communication). We estimate possibleannual probabilities of landslides and laharshaving volumes of 500,000 cubic meters or lessas follows. Historical earthquake-inducedlandslides have occurred throughout El Salvadorat least a dozen times from 1857 to 2001(Rymer and White, 1989; Baum et al., 2001).Volumes of these landslides have ranged from afew hundred to more than 10 million cubicmeters, but most have had volumes of less thana few to a few tens of thousands of cubicmeters. Thus, earthquake-induced landslides ofsmall to moderate volume occur in El Salvadorabout once every 12 years. At San Vicentevolcano, rainfall- and earthquake-triggeredlandslides and lahars occurred at least fourtimes in the past 225 years, and at least threetimes in the past 65 years, which suggest annualprobabilities of occurrence of about 1 in 60 to 1in 20. These historical lahars reached thecommunities of San Vicente, Guadalupe, andTepetitan as well as the highway betweenTecoluca and Zacatecoluca (see plate 1). Thevolumes of these lahars are not known, but theirextents suggest volumes on the order of300,000 to 500,000 cubic meters. Although theestimated probabilities are highly generalized, weconclude that the annual probability of land-slides and lahars ≤500,000 cubic meters in sizeat San Vicente volcano is about 1 in 50 toperhaps as great as 1 in 10.

[9] The potential extent of a regionally devastatingblast pyroclastic current is estimated from theformula H/L = 0.09, similar to that of the 1980directed blast at Mount St. Helens. Althoughsuch an event would impact a very large area,events of this type have a very low frequency ofoccurrence.

2222222222 Volcano-Hazard Zonation for San Vicente Volcano, El SalvadorVolcano-Hazard Zonation for San Vicente Volcano, El SalvadorVolcano-Hazard Zonation for San Vicente Volcano, El SalvadorVolcano-Hazard Zonation for San Vicente Volcano, El SalvadorVolcano-Hazard Zonation for San Vicente Volcano, El SalvadorPrinted on recycled paper

Major, and others—

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R—U.S. Geological Survey —

Open-File Report 01-367

Documents

Volcano-Hazard Zonation for San Vicente Volcano, El Salvador · around San Vicente volcano, as well as the proxim-ity of major transportation routes, increase the risk that even small