Pent-up stress putsPent-up stress puts‘the squeeze’ on Los Angeles‘the squeeze’ on Los Angeles

Ken Hudnut

U. S. Geological Survey

Pasadena Office

USGS Public LectureCaltech

May 1, 2001

A very brief historyA very brief historyof geodesyof geodesy

Geodesy literally means measurement of the Earth (we do geodesy to study strain build-up between earthquakes)

Eratosthenes (a.k.a. ‘Beta’) measured circumference of the Earth very accurately

– Knew that sun hit bottom of well at Syene

– At same time (high noon), he measured sun’s angle at Alexandria

– 500 miles away (stadia)

so C = 25,000 miles

More recently…More recently…

Conventionalgeodesy: triangulation trilateration

Spacegeodesy:

VLBI, SLRand GPS

Elastic rebound theoryElastic rebound theoryHarry Reid – looking atgeodetic data from the1906 San Franciscoearthquake – surmisedthat strain is accumulatedand then released in anearthquake… over andover again… with similarstrain accumulation andrelease – could it predictfault behavior?

fault

fenc

e

Earthquake recurrenceEarthquake recurrence Reid’s elastic rebound

theory was too good to be true - too simple

Earthquakes do not behave this way, even in a simple system

– Brick & bungee– B & K (1967)

Time predictable, slip predictable, or non-predictable, i.e., chaotic?

We are up against friction – notoriously unpredictable

slip

time

stra

in

even

t 1

even

t 2

Slightly more complex fault model – slider blocksSimple model of an earthquake fault, the Burridge-Knopoff slider block model (1967). A scaling distribution (fractal statistics) is seen for the slider block simulations, similar to form of Gutenberg-Richter distribution of naturally occuring relationship between number of earthquakes versus magnitude

Log10 {Area}L

og

10 {

Fre

qu

ency

}

Earthquake Earthquake TerminologyTerminology

Strike-slip FaultsStrike-slip FaultsRupturesurface

Hypocenter

Hypo-center

Epicenter

Faultplane

Fault

Plate tectonic motionsPlate tectonic motions As the Farallon

plate subducted, the San Andreas fault was born

In the past 5 million years, this motion has been steady at about 5 cm/yr (that’s 5 meters per century or 50 km per million years!)

Movie by T. Atwater, UCSB

Faults & Faults & EarthquakesEarthquakes

San Andreas fault zone– North American and Pacific

plate relative motions of 56 mm/yr in a right-lateral sense

Eastern Californiashear zone

– Accomodation of right-lateral motion inboard of Sierra Nevada block

– Estimated rates of some 8-12 mm/yr (geological & space geodetic)

– Easier to go through than the Big Bend?

Big bend of San Andreas– Compression across

Los Angeles basin Ventura basin

All of Earth’s tectonic plates are constantly moving, with respect to boththe geocenter, and all of the other plates. Nothing is staying fixed, as thepieces of crust move across the surface of our sphere. Space geodesyallows us to measure absolute plate motions. We can readily estimateand then remove poles and rotation rates from our data, reducing thevelocity field so that it is referred, say, to the ‘stable’ North American plate…

… remember thatnot too long ago,baseline lengthswere re-observedto obtain precisestrain data. Spacegeodesy enabledprecise referenceframe realization,helping us to puttogether the bigpicture, allowingrigorous tests ofplate tectonics andearthquake disloc-ation theory …

Southern California Integrated GPS NetworkSouthern California Integrated GPS Network

Studying earthquakes using GPS technology

State-of-the-art network for research and earthquake response

– Software and hardware development

– New networking technology– Automated processing

systems developed Data also used widely for

surveying, GIS mapping, engineering

A few acronymsA few acronyms

Global Positioning System (GPS)

Southern California Integrated GPS Network (SCIGN)

Plate Boundary Observatory (PBO)

Southern California Earthquake Center (SCEC)

GPS – Global Positioning SystemGPS – Global Positioning System

GPS is a U.S.-built constellation of navigation satellites

Normally it is used for ‘coarse’ positioning

– Handheld GPS units (~$100-$500) C/A code only 6 meter precision (with SA turned off)

We ‘earthquake people’ do precise GPS– Top-notch GPS receivers ($11,000)

P-code and phase on both L1 & L2 Differential phase - several millimeter

precision (1000x better!) Wide range of uses for GPS data

provided by SCIGN

GPS signal GPS signal correlationcorrelation

GPS phase GPS phase differencingdifferencing

‘‘normal’ vs. ‘precise’ GPSnormal’ vs. ‘precise’ GPS

5/2/2000ended SA

normal GPSimproved byabout 10x

Continuous GPS & Continuous GPS & strainmetersstrainmeters

Best tools ever devised for highly accurate, automated, constant monitoring of crustal strain for

– long baselines– absolute ref. frame– displacement field– very high precision

SCIGN & other PBO elements require sub-millimeter velocities on the plate boundary scale in order to answer the scientific questions

regional active faultsregional active faultsYou neverknow whatyou’re goingto get…

Fort Tejon1857

Long Beach1933

Arvin-Tehachapi1952

San Fernando1971

ECSZ sequence1992-1999

Northridge1994

CaliforniaCaliforniarelative plate relative plate motions formotions forthe past 20 the past 20

million yearsmillion years

Movie by T. Atwater, UCSB

Using GPS to study earthquakesUsing GPS to study earthquakes

Measure station positions every day to within a few millimeters

Track stations through time (time series) Velocities derived from stations’ time series Strain derived from the velocity field Strain is proportional to seismic hazard and to

seismic risk

Geodesy for risk estimationGeodesy for risk estimation

Getting from GPS data to Getting from GPS data to strain, and from strain to riskstrain, and from strain to risk

Daily positions showStation trajectory vs. time(a time series of position)

In North-South, East-Westand Up-Down components,we fit a line to the data

This gives a vector for eachstation’s velocity

time 1

time 2

t1t2

North by 40 mm in 2 yrs.

West by 70 mm in 2 yrs.

From station velocities to strainFrom station velocities to strain

Velocities of all stations then are used to calculate strain (between each set of three sites)

First case – all of the velocities are the same, so there is no strain occurring in the triangle between these three sites for this time interval

From station velocities to strainFrom station velocities to strain

Velocities of all stations then are used to calculate strain (between each set of three sites)

Second case – not all of the velocities are the same, so there is strain occurring in the triangle between these three sites for this time interval

t1

t2

SCEC Crustal Motion MapSCEC Crustal Motion Map

Combined EDM,

VLBI, survey-

mode and

continuous GPS

rigorously Released as a

SCEC product Set the bar very

high for the

SCIGN project

Types of Types of StrainStrain

Shear strain– San Andreas fault has right-

lateral slip (that results from shear strain between North American and Pacific Plates)

Plane strain– Compression across the

system of thrust faults beneath Los Angeles

Dilatational strain

Rotational strain

t1 t2

From station velocities From station velocities to strainto strain

Velocities of all GPS stations are then used to calculate strain (between each set of three sites)

Maps can then be made to show each component of strain, (e.g., the shear strain both parallel to and perpendicular to the San Andreas fault)

Strain values can then be contoured, or used in hazard calculations, to make maps that are more generally used, e.g., by engineers

Probabilistic Seismic Hazard AnalysisProbabilistic Seismic Hazard Analysis

SCECled usinggeodeticdata asinput totheirseismichazardAnalysis(BSSA, 1995)

How to improve hazard informationHow to improve hazard informationand reduce earthquake risk?and reduce earthquake risk?

[SoCal is home to 15 million people, and poses half of [SoCal is home to 15 million people, and poses half of the national earthquake risk (FEMA #366)]the national earthquake risk (FEMA #366)]

Better geodetic and other data as input to geodynamic earthquake source models

– SCIGN: state-of-the-art continuous GPS net– LARSE: deep crustal imaging of faults– TriNet: recording earthquake shaking

Better methods of simulating shaking and estimating damage likely to be caused in scenario events

– USGS & CDMG – official maps to have future revisions– SCEC ‘IT’, RELM – developing & testing new methods– HAZUS – convert hazard to risk – how to reduce it?

Operational Groups:

Major Funding:

Total $18 M

SOPAC

SCIGN is an integral part

of SCEC

The major objectives of theThe major objectives of theSCIGN array are:SCIGN array are:

* To provide regional coverage for estimating earthquake potential throughout Southern California

To identify active blind thrust faults and test models of compressional tectonics in the Los Angeles region

To measure local variations in strain rate that might reveal the mechanical properties of earthquake faults

In the event of an earthquake, to measure permanent crustal deformation not detectable by seismographs, as well as the response of major faults to the regional change in strain

Proper tools for the job…

- air rotary rock drill & auger rig- 185 installations by contractor

Had to build deeply anchored tripod

‘monuments’

MonumentationMonumentation

SCIGN – a SCIGN – a greatgreat GPS network: GPS network: carefully planned - well reasonedcarefully planned - well reasoned

Monuments– Each of 5 legs is drilled to 10

meters Lowermost 6 meters is anchored Upper 4 meters is isolated from soil

– Stainless for longevity Innovative geodetic tools

– SCIGN radomes & adaptors– Data acquisition software

Redundant precise processing– GIPSY and GAMIT– Rigorous comparisons ongoing

Accuracies are the highest ever achieved, exceeding even highly optimistic expectations for SCIGN

movie by John Galetzka, USGS

SCIGN project installation statusSCIGN project installation status

Los Angeles deformationLos Angeles deformation

Two main models advanced for the observed 5 mm/yr contractional strain across Los Angeles:

1. Strain on NW-SE and SW-NE strike-slip faults (e.g., Walls et al.)

2. Strain on thrust fault system [including blind thrust faults] (e.g., Argus et al.)

Deep crustal studies - LARSEDeep crustal studies - LARSE

Fuis et al. (SCEC & USGS) set off many explosions to reflect energy from deep faults

Tomographic imaging (like a CAT scan)

Shows deep geometry of thrust faults beneath Los Angeles

Images the faults, but doesn’t show if they are active or not

Geodesy can now be used to estimate rate of slip on these faults

Deformation across LADeformation across LA

SCEC and SCIGN GPS velocities - reduced so that San Gabriel Mtns. are held fixed (Argus et al., JPL)

Contraction may be in a narrow belt, concent-rated across the San Gabriel Valley

Simple elastic models don’t fit the GPS data?Simple elastic models don’t fit the GPS data?

So far, Argus et al. are finding that it’s going to take some fancy modeling to explain these GPS data

Bawden et al. find land subsidence effects on data, and after correcting for that, they can fit the data

Meanwhile, SCIGN is providing better data to help resolve these differences

and in the future…and in the future…

Soon we will build the SCIGN project’s 250th station

Early data have already been used for– Controversy over tectonic style in Los Angeles– SCEC v2 crustal motion map and Phase II hazards– Observation of static displacement field propagation and pushing the

real-time GPS envelope (broad-band seismology using GPS?)

Data so far are exceeding expectations, so we now expect to surpass original observational objectives (except where signals are masked by land subsidence)

Leading the world in continuous GPS network technology and software development

SCIGN project progress…SCIGN project progress…

GPS & telemetry/networkingGPS & telemetry/networking

Market for GPS boards is driven by Moore’s law (like PC’s) toward faster/better/cheaper, miniaturization, etc.

Spread spectrum radio and satellite telemetry leading to high bandwidth IP field networking (e.g., TDMA)

Allows higher sampling rates and more affordable real-time telemetry

Assess damage to infrastructureAssess damage to infrastructure

Were tilts or strains large enough to damage systems? (from regional measurements)

Did damage occur to critical structures or systems? (from site-specific monitoring)

Structure monitoringStructure monitoring Pacoima dam

GPS monitoring since Sept. 1995 with LA County

GPS data can indicate damage to engineered structures such as overpasses and tall buildings

New methods: high-resolution topographic New methods: high-resolution topographic mapping and digital photographymapping and digital photography

Laser scanning using an airborne platform requires high sampling-rate GPS data during flight to control aircraft position and attitude

SCIGN stations were operated at 1 and 2 sample per second rates via the radio network

The Plate The Plate Boundary Boundary

ObservatoryObservatory

SCIGN as a prototype Plans for 875 additional

continuous GPS stations, for earthquake research, throughout the Western U. S. A. (as well as in Canada and Mexico)

The San Andreas The San Andreas fault zone ‘focus fault zone ‘focus

array’ of PBOarray’ of PBO

Geodetic networks for earthquake research, especially for observing transitional behavior between creeping and locked fault surfaces, along-strike and at depth, and aseismic fault loading processes (that cannot be observed with seismological instruments)

ConclusionsConclusions SCIGN is the world-leading, state-of-the-art, continuously-

operating GPS network We have innovated & devised the best-ever geodetic methods

for earthquake research Having led in technology development, we now fully expect to

lead in research results (especially for the Los Angeles urban region) as the network is now nearly fully operational

– Original research on strain and fault motions– New and improved hazard and risk analyses

Data from SCIGN are freely available to all, and are used extensively in surveying, engineering, and for GIS mapping (spatial referencing users may become main sponsors of network operations, removing some burden from agencies like NASA, USGS & NSF)

For more information:For more information:

http://pasadena.wr.usgs.gov/scign/

http://www.scign.org/

Kenneth W. Hudnut, Ph.D.GeophysicistUSGS Pasadenahudnut@usgs.gov

Arthur C. Clarke's 2nd Law:

"The only way of discovering the limits of the possible

is to venture a little way past them into the impossible."