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Tectonophysics 400 (

Seismic activity along the Central America volcanic arc:

Is it related to subduction of the Cocos plate?

Marco Guzman-Spezialea,T, Carlos Valdes-Gonzalezb,Enrique Molinac, Juan Martın Gomeza

aCentro de Geociencias, UNAM, Campus Juriquilla, 76230 Queretaro, MexicobInstituto de Geofısica, UNAM, Cd. Universitaria, 04510 Mexico D.F., Mexico

cINSIVUMEH, Guatemala

Received 16 October 2003; accepted 1 March 2005

Available online 8 April 2005

Abstract

We determine seismic strain rate of tectonic earthquakes along the Central America Volcanic Arc. We then compare this

result to those obtained from earthquakes related to the convergence of the Cocos and Caribbean plates and to earthquakes in

the back-arc region of northern Central America.

The seismic strain-rate tensor for shallow-focus earthquakes along the Central America volcanic arc since 1700, has a

compressive eigenvector with a magnitude of 0.7�10�8 year�1, and oriented in a 3578 azimuth. The extensive eigenvector is

oriented in a 868 azimuth, with a magnitude of 0.82�10�8 year�1. When only Centroid Moment-tensor solutions (CMT) are

considered, the respective eigenvectors are 1.2�10�8 year�1 and 1.0�10�8 year�1.

The compressive eigenvector from the seismic strain-rate tensor for earthquakes along the Cocos-Caribbean convergent

margin is 2.0�10�8 year�1, plunging at 258, and oriented in a 298 azimuth. Its magnitude and direction are similar to those of

the compressive eigenvector for earthquakes along the volcanic arc. The extensive eigenvector along the convergent margin, on

the other hand, has a large vertical component. The compressive and extensive eigevenvectors are 4.9�10�8 year�1 and

4.6�10�8 year�1, using only CMTs as the database.

Earthquakes along the grabens of northern Central America yield a seismic strain-rate tensor whose extensive eigenvector

has a magnitude of 2.4�10�8 year�1, oriented in a 1098 azimuth. Magnitude and direction are similar to those of the extensive

eigenvector for earthquakes along the volcanic arc. The compressive eigenvector along the grabens is practically vertical.

Similarities in magnitudes and directions for compressive and extensive eigenvectors suggest to us that the strain field along

the Central America volcanic arc is the result of compression along the convergent Cocos-Caribbean margin, and extension in the

back-arc region, along the grabens of northern Central America. This field is resolved as strike-slip faulting along the arc.

D 2005 Elsevier B.V. All rights reserved.

Keywords: Central America volcanic arc; Cocos plate; Subduction; Compression; Extension; Seismic strain rate

T Corresponding author.

0040-1951/$ - s

doi:10.1016/j.tec

E-mail addr

2005) 241–254

ee front matter D 2005 Elsevier B.V. All rights reserved.

to.2005.03.006

ess: marco@geociencias.unam.mx (M. Guzman-Speziale).

M. Guzman-Speziale et al. / Tectonophysics 400 (2005) 241–254242

1. Introduction

Medium-sized, shallow earthquakes occur fre-

quently along the Central America Volcanic Arc.

These earthquakes, which have proved highly

destructive for some of the main cities in Central

America, are of tectonic origin and display strike-

slip faulting with one of the nodal planes aligned

parallel to the volcanic arc (Harlow and White,

1985; White, 1991; White and Harlow, 1993).

Several authors (e.g., Fitch, 1972; Harlow and

White, 1985; Guzman-Speziale, 1995a; DeMets,

2001) have argued that oblique plate convergence

is the driving mechanism responsible for these

shallow-focus earthquakes along the volcanic arc.

Recently, however, evidence has been presented

which suggests that this is not the mechanism that

produces these earthquakes (Guzman-Speziale and

Gomez, 2002).

-95

-95

-90

-90

10

15

Motagua F

Polochic Fault

Middle America Trench

NORTH AMERICA PLATE

COCOS PLATECARIBBEAN PLATE

° °

°°

°

°

Fig. 1. Tectonic framework of northern Central America. White, thin ar

Caribbean relative convergence, with length proportional to magnitude. G

Arrows in inset show direction of relative plate motion with respect to Nort

the help of GMT software (Wessel and Smith, 1991).

In this paper we calculate seismic strain rates

along the Central America Volcanic Arc, and also

along the convergent margin of the Cocos and

Caribbean plates, as well as the back-arc region of

northern Central America, to determine whether a

relationship exists between seismic activity along the

volcanic arc and along the plate interface and the

back-arc region.

2. Tectonic setting

Central America is located in the northwestern

corner of the Caribbean Plate, which is overriding the

subducted Cocos Plate along the Middle America

Trench (Fig. 1). Convergence of these two plates takes

place at a rate of 7–8 cm year�1 and an azimuth of

about 20–228, (e.g., DeMets et al., 1990; DeMets,

2001). The subducted slab dips at a fairly steep and

-85

-85

10

15

Grabensault

Volcanic Arc

°

°

°

°

rows along the Middle America Trench show direction of Cocos-

rey, thick arrows are oriented in a direction normal to the trench.

h America, with length proportional to speed. All figures drawn with

M. Guzman-Speziale et al. / Tectonophysics 400 (2005) 241–254 243

constant angle of about 458 (Bevis and Isacks, 1984;

Burbach et al., 1984).

The North America Plate bounds the Caribbean

Plate to the north along a left-lateral transform

boundary which in Central America is marked princi-

pally by the Motagua-Polochic Fault System (e.g.,

Molnar and Sykes, 1969; Malfait and Dinkelman,

1972). Relative motion between the North America and

Caribbean plates is about 2 cm year�1 (e.g., Sykes et

al., 1982; Dixon et al., 1998; DeMets, 2001).

The volcanic front consists of 75 basaltic to dacitic

volcanoes with documented Holocene activity, 31 of

which have been active in historic times (Simkin et

al., 1981; Carr and Stoiber, 1990). They lie along a

line which closely parallels the Middle America

Trench, and some 150 km from it (Fig. 1). The

volcanic arc extends from the Motagua-Polochic

system to central Costa Rica, onshore of where the

Middle America Trench looses its surface expression.

Volcanoes are closely-spaced, 12–30 km apart, with

elevations ranging from 100 m to more than 4000 m

(Carr, 1984). In general, the volcanic front is 10–15

km wide (e.g., Carr and Stoiber, 1990). Only a few

Holocene volcanoes do not lie along the volcanic

front, the most notorious being the 10 or so which are

located behind the arc, in an extensional environment

(Burkart and Self, 1985) and whose volcanic products

are petrologically and geochemically distinct from the

basaltic cones along the arc (Walker, 1981).

Just south of the Motagua-Polochic system and

east of the volcanic arc lie a system of grabens

which are oriented N–S (Fig. 1) (e.g., Dengo, 1968;

Dengo and Bohnenberg, 1969; Weyl, 1980; Mann et

al., 1990; Gordon and Muehlberger, 1994). These

grabens are seismically active and recently Guzman-

Speziale (2001a) has calculated a rate of opening of

8 mm year�1. Further to the southeast, Donnely et al.

(1990), summarizing the results of earlier workers

(e.g., McBirney and Williams, 1965), identify Neo-

gene alkaline basalt centers located along north–

south alignments in eastern Nicaragua and eastern

Costa Rica, in the back-arc region, which they

suggest are associated with E–W extension. Mann

and Burke (1984) proposed that the N–S trending

Wagwater and Montpelier-Newmarket rifts in

Jamaica, as well as the Southern Nicaragua Rise

Graben, are part of this extensive regime along the

northern Caribbean Plate.

3. Seismic activity in Central America

Seismicity is dominated by shallow, thrust-faulting

earthquakes related to subduction of the Cocos Plate

beneath the Caribbean Plate (e.g., Molnar and Sykes,

1969; Dean and Drake, 1978; Burbach et al., 1984;

Dewey and Suarez, 1991; Pacheco and Sykes, 1992;

Ambraseys and Adams, 1996). These earthquakes

have magnitudes sometimes reaching 8.0. There is

also a well-defined Wadati-Benioff zone dipping at an

angle of about 458 and reaching depths to 250 km

(e.g., Burbach et al., 1984; Dewey and Suarez, 1991).

The boundary between the North America and

Caribbean plates is also seismically active. Several

large earthquakes have taken place along the Motagua

and the Polochic faults. White (1984) has catalogued

25 destructive historical earthquakes along the plate

boundary since 1530.

There is upper-crustal seismicity associated to the

N–S-trending grabens, including an M =6.0 after-

shock of the 1976 Motagua fault earthquake along the

Guatemala City Graben (Langer and Bollinger, 1979;

White and Harlow, 1979). These grabens have

experienced large historical earthquakes, with magni-

tudes sometimes reaching 7.0 or more (e.g., White,

1991).

Shallow-focus earthquakes with 5.7VMsV6.9occur along the volcanic arc at an average of one

every 2.5 years (Fig. 2). This activity is well

documented, at least since the 16th century (e.g., Carr

and Stoiber, 1977; White and Harlow, 1993; Peraldo

and Montero, 1999). The earthquakes have been

highly destructive, affecting most of the large cities

in Central America; the city of San Salvador, for

example, has been severely damaged in at least 12

occasions since 1594 (Harlow et al., 1993; Peraldo

and Montero, 1999). White and Harlow (1993)

compiled a catalog of destructive upper-crustal earth-

quakes in Central America since 1900, the vast

majority of which occurred at shallow depth and

within 20 km of the volcanic arc. Peraldo and

Montero (1999), on the other hand, collected histor-

ical documents on Central American earthquakes

from the 16th to the 19th century. Additionally, they

constructed, when possible, isoseismal maps, deter-

mined a range of probable magnitude (Ms), and made

a tectonic interpretation. Again, most of the events are

related to the activity along the volcanic arc.

-95˚ -90˚ -85˚

10˚ 10˚

15˚ 15˚

-95˚ -90˚ -85˚

10˚ 10˚

15˚ 15˚

Fig. 2. CMTs (Harvard University, 2004) used in this study. Also shown are the areas of the volumes considered: top, volcanic arc; bottom,

Cocos-Caribbean convergence zone and grabens of northern Central America.

M. Guzman-Speziale et al. / Tectonophysics 400 (2005) 241–254244

M. Guzman-Speziale et al. / Tectonophysics 400 (2005) 241–254 245

These events are apparently of tectonic origin,

because they bear no direct temporal relationship with

volcanic eruptions (White and Harlow, 1993) and

because the largest earthquakes due to motion of

magma are no larger than 5.5 (Okada, 1983). Available

fault-plane solutions (e.g., Montero and Dewey, 1982;

White and Harlow, 1993) (Fig. 2) show a strike-slip

faulting mechanism, either right-lateral along a NW–

SE plane or left-lateral along a NE–SW plane. The

former would mean along-the-arc faulting while the

latter would indicate that the fault plane is oriented

perpendicular to the volcanic chain.

In most cases, there is no direct evidence that either

of the planes is the fault plane because there is no

surface faulting that could be associated with the

earthquake. Indirect evidence suggests that some of

the events have a right-lateral, along-arc rupture

whereas for others faulting is left-lateral, perpendicular

to the arc.

For the following events there is evidence that

suggests right-lateral, along-the-arc, faulting: Earth-

quakes in Costa Rica in 1839 and 1841 show

isoseismals for intensities VII and VIII noticeably

elongated in a NW–SE direction (Peraldo and

Montero, 1999); for the El Salvador event of 1854

there are reports that damage occurred along a

narrow zone stretching from SE to NW (Peraldo

and Montero, 1999), additionally, isoseismals (Har-

low et al., 1993) are also significantly elongated in

this direction. The two Costa Rica earthquakes of

1910 show isoseismals in a NW–SE direction

(Montero and Dewey, 1982; White and Harlow,

1993). Three events in El Salvador from 1917 to

1919, very close in time and space, progressed from

W to E, suggesting faulting in this direction (White et

al., 1987; Harlow et al., 1993; White and Harlow,

1993). The foreshock–aftershock sequence of the

1965 El Salvador earthquake is oriented in a NW–

SE direction (Lomnitz and Schultz, 1966; White et al.,

1987). The distribution of aftershocks suggests a

NW–SE faulting plane for the February 2001, El

Salvador earthquake (Centro de Investigaciones Geo-

tecnicas, 2001).

According to available evidences, left-lateral fault-

ing, perpendicular to the arc is most probable for the

next events: Cracks in a N–S direction were reported

for the 1857 El Salvador earthquake (Peraldo and

Montero, 1999); surface faulting for the 1931 and

1972 Nicaragua earthquakes took place in a N–S

direction (e.g., White and Harlow, 1993); alignment of

aftershocks suggests a NNE–SSW faulting plane for

the 1982 Gulf of Fonseca earthquake; from aftershock

distribution, the 1986 El Salvador earthquake was

caused by a N25E-trending fault (e.g., White et al.,

1987; Harlow et al., 1993) but a foreshock swarm just

east of the epicentral area had an E–W distribution

(White et al., 1987).

4. Method and data

The method of Kostrov (1974) is now the standard

tool to determine seismic strain rate within a volume in

the Earth. It has been used to calculate deformation in

several areas, for example: The Mediterranean and

Middle East (Jackson and McKenzie, 1988), con-

tinental regions (Ekstrom and England, 1989), central

Greece (Papapzachos and Kiratzi, 1992), the Aegean

(Papazachos et al., 1992), the Anadaman Sea (Guzman

Speziale and Ni, 1993), the North and East Anatolian

faults (Kiratzi, 1993), Japan (Kiratzi and Papazachos,

1996) and the grabens of Central America (Guzman-

Speziale, 2001a), among other areas.

The average seismic strain rate from N earthquake

moment tensors Mij within a volume V and a time

period s is given by (Kostrov, 1974):

eeij ¼1

2lVs

XNn¼1

Mnij ð1Þ

l is the modulus of rigidity (3�1010 N/m2).

The sum of seismic moment tensors in the right-

hand side of Eq. (1) may be expressed as (e.g.,

Papapzachos and Kiratzi, 1992):

XNn¼1

Mnij ¼

XNn¼1

Mn0

!Fij ð2Þ

where the average shape tensor F is given by

(Papapzachos and Kiratzi, 1992; Kiratzi and Papaza-

chos, 1996):

Fij ¼1

N

XNn¼1

Mnij

Mn0

ð3Þ

In this manner, a tensor is obtained which

represents each of the seismic moment tensors

M. Guzman-Speziale et al. / Tectonophysics 400 (2005) 241–254246

involved, with equal weight. Historic earthquakes

(for which the seismic moment tensor is not known)

may be included in the calculation if Eq. (2), instead

of Eq. (1), is used The average shape tensor is first

calculated with available moment tensors and then

scalar moments are added. Knowing the surface-wave

magnitude Ms, a scalar seismic moment Mo may be

obtained by (Ekstrom and Dziewonski, 1988):

logMo ¼ 12:24þMs Msb5:3 ð4aÞ

logMo ¼ 23:20�ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi92:45� 11:40Ms

p

5:3VMsV6:8 ð4bÞ

logMo ¼ 1:5Msþ 9:14 MsN6:8 ð4cÞ

We obtained the average seismic strain rate tensor

for shallow crustal earthquakes along the Central

America volcanic arc and interplate earthquakes along

the Cocos-Caribbean interface. Based on the work of

Guzman-Speziale (2001a), we also updated the

average seismic strain rate along the grabens of

Central America.

For each of the elements involved (i.e., the

volcanic arc, the convergent margin, and the back-

arc grabens), the volume V must be determined. In

the case of the volcanic arc, only the volume where

there is seismic activity is considered (from latitude

�908 to �83.58, approximately). The interplate

region is that in front of the active part of the

volcanic arc (Fig. 2).

We consider the interplate region along the Cocos-

Caribbean interface to be 50 km deep and 1.48 (about155 km) wide (e.g., Pacheco et al., 1993). Because of

the change in azimuth and curvature of the Middle

America Trench, three separate segments are consid-

ered (Fig. 2). From elementary Calculus (e.g., Munem

and Foulis, 1978), the volume of these circular

Table 1

Parameters for the volume segments along the Cocos-Caribbean margin a

Segment Center of curvature Distance (degrees)a

Lat. Lon. Min. Max

1 32.59 �80.74 20.61 22.0

2 7.37 �90.63 4.84 6.2

3 13.58 �81.69 4.52 5.9

a Distances for convergent margin.b Distances for volcanic arc.

segments on a sphere may be obtained in spherical

coordinates by:

V ¼Z h2

h1

Z /2

/1

Z q2

q1

q2sin/dqd/dh ð5Þ

where the radius q goes from 6321 km (Earth radius

minus 50 km) to 6371 km, / is the angular distance,

and h is the azimuth from the center of curvature of

the segment. Parameters for each of the segments are

given in Table 1. Centers of curvature for each of the

segments are based on those given in Guzman-

Speziale (1995b). The total volume of the convergent

zone is 6.93 � 1015 m3.

We calculate the volume in the volcanic arc in a

similar manner. We take three segments along the arc

using the same centers of curvature, with a width of

0.68, which covers the deformation (seismically

active) region. White (1991) and White and Harlow

(1993) argue that the vast majority of earthquakes

along the volcanic arc take place in depths between 3

and 15 km, so we use 15 km as the seismogenic depth.

The total volume thus calculated is 8.94�1014 m3.

The volume for the grabens is taken directly from

Guzman-Speziale (2001a): 7.5�1014 m3.

Centroid moment-tensor solutions (CMTs)

reported by Harvard University (e.g., Dziewonski

and Woodhouse, 1983; Harvard University, 2004) are

used to calculate the average shape tensor associated

to the interface between the Cocos and Caribbean

plates, as well as the Central America volcanic arc,

and the grabens of Central America. For the con-

vergent margin, tensors with scalar seismic moment of

at least 2.5 � 1017 N m and T axis plunging 458 ormore (thrust-faulting mechanism, according to Froh-

lich and Apperson, 1992) are chosen, whereas B axis

must plunge at least 458 (strike-slip faulting earth-

quakes) for earthquake moment tensors along the

nd the Central America volcanic arc

Distance (degrees)b Azimuth

. Min. Max. Min. Max.

1 20.21 20.81 202.25 214.25

4 6.04 6.64 18.25 54.00

2 4.12 4.72 187.50 235.50

Table 2

Significant earthquakes along the Central America volcanic arc 1700–1978

N Date Latitude Longitude Msa Msb Mo (N m)c Mo (N m)d Ref.

1 1701.00.00 11.95 �86.05 5.4 5.4 0.4387e+18 0.4387e+18 2

2 1712.12.14 13.58 �88.83 5.4 6.2 0.4387e+18 0.3421e+19 2, 3

3 1739.00.00 11.80 �86.20 5.4 5.4 0.4387e+18 0.4387e+18 2

4 1748.03.03 13.60 �89.10 6.4 6.4 0.6099e+19 0.6099e+19 3

5 1765.04.14 13.70 �89.00 6.0 6.1 0.1977e+19 0.2592e+19 1, 2, 3

6 1772.02.15 10.00 �84.13 5.7 5.7 0.9095e+18 0.9095e+18 1, 2

7 1783.11.29 13.60 �88.80 5.4 6.0 0.4387e+18 0.1977e+19 2, 3

8 1798.02.02 13.65 �89.25 5.7 6.2 0.9095e+18 0.3421e+19 2, 3

9 1821.04.10 9.83 �84.08 5.0 5.9 0.1738e+18 0.1517e+19 4

10 1835.06.10 9.92 �84.17 5.4 5.4 0.4387e+18 0.4387e+18 2

11 1838.12.00 13.50 �88.40 5.4 6.0 0.4387e+18 0.1977e+19 2

12 1839.03.22 13.82 �89.25 6.0 6.2 0.1977e+19 0.3421e+19 1, 2, 3

13 1839.10.01 13.66 �89.22 5.4 5.9 0.4387e+18 0.1517e+19 1, 2, 3

14 1841.09.02 10.00 �83.92 6.5 6.5 0.8247e+19 0.8247e+19 1, 2

15 1842.03.21 9.97 �84.12 5.4 5.4 0.4387e+18 0.4387e+18 2

16 1851.03.18 10.13 �84.19 6.0 6.9 0.1977e+19 0.3090e+20 1, 4

17 1853.08.24 10.42 �84.90 5.4 6.0 0.4387e+18 0.1977e+19 2

18 1854.04.16 13.68 �89.18 6.0 6.6 0.1977e+19 0.1126e+20 1, 2, 3

19 1854.06.11 13.65 �88.83 5.4 6.2 0.4387e+18 0.3421e+19 1, 2, 3

20 1857.11.06 13.63 �89.00 6.0 6.4 0.1977e+19 0.6099e+19 1, 2, 3

21 1860.06.21 13.62 �88.91 6.0 6.1 0.1977e+19 0.2592e+19 1, 2, 3

22 1860.12.03 13.78 �89.33 6.0 6.0 0.1977e+19 0.1977e+19 1, 2

23 1867.03.21 13.76 �89.50 5.4 5.8 0.4387e+18 0.1171e+19 2, 3

24 1872.12.30 13.62 �88.66 5.4 5.8 0.4387e+18 0.1171e+19 1, 2, 3

25 1873.03.04 13.71 �89.20 6.4 6.4 0.6099e+19 0.6099e+19 1, 2, 3

26 1873.03.19 13.71 �89.20 6.2 6.5 0.3421e+19 0.8247e+19 1, 2

27 1878.10.03 13.28 �88.25 6.0 6.0 0.1977e+19 0.1977e+19 1, 2

28 1888.12.30 10.13 �84.20 6.0 6.0 0.1977e+19 0.1977e+19 2

29 1896.04.20 9.88 �83.92 5.0 5.9 0.1738e+18 0.1517e+19 4

30 1899.03.25 13.60 �88.80 6.1 6.1 0.2592e+19 0.2592e+19 3

31 1910.04.03 9.85 �83.92 5.8 5.8 0.1171e+19 0.1171e+19 5

32 1910.05.04 9.85 �84.33 6.4 6.4 0.6099e+19 0.6099e+19 5

33 1911.08.28 10.25 �84.32 6.0 6.0 0.1977e+19 0.1977e+19 5

34 1912.06.06 10.23 �84.28 6.5 6.5 0.8247e+19 0.8247e+19 5

35 1912.07.19 13.87 �89.57 5.9 5.9 0.1517e+19 0.1517e+19 5

36 1917.06.08 13.70 �89.50 6.5 6.5 0.8247e+19 0.8247e+19 5

37 1917.06.08 13.75 �89.27 6.4 6.4 0.6099e+19 0.6099e+19 5

38 1919.04.28 13.66 �89.17 6.0 6.0 0.1977e+19 0.1977e+19 5

39 1931.03.31 12.15 �86.17 6.0 6.0 0.1977e+19 0.1977e+19 5

40 1936.12.20 13.72 �88.93 6.1 6.1 0.2592e+19 0.2592e+19 5

41 1937.12.25 13.93 �89.78 5.8 5.8 0.1171e+19 0.1171e+19 5

42 1938.04.25 12.45 �86.85 5.9 5.9 0.1517e+19 0.1517e+19 5

43 1938.05.06 12.53 �86.87 6.1 6.1 0.2592e+19 0.2592e+19 5

44 1951.05.06 13.52 �88.40 6.0 6.0 0.1977e+19 0.1977e+19 5

45 1951.05.06 13.52 �88.40 6.2 6.2 0.3421e+19 0.3421e+19 5

46 1951.05.07 13.48 �88.45 5.8 6.0 0.1171e+19 0.1977e+19 5

47 1951.08.02 13.00 �87.50 5.8 5.9 0.1171e+19 0.1517e+19 5

48 1951.08.03 13.00 �87.50 5.9 6.0 0.1517e+19 0.1977e+19 5

49 1952.12.30 10.05 �83.92 5.9 5.9 0.1517e+19 0.1517e+19 5

50 1955.04.04 12.75 �87.17 6.2 6.2 0.3421e+19 0.3421e+19 5

51 1955.04.30 12.38 �86.52 6.0 6.0 0.1977e+19 0.1977e+19 5

52 1955.09.01 10.25 �84.25 5.8 6.0 0.1171e+19 0.1977e+19 5

(continued on next page)

M. Guzman-Speziale et al. / Tectonophysics 400 (2005) 241–254 247

N Date Latitude Longitude Msa Msb Mo (N m)c Mo (N m)d Ref.

53 1965.05.03 13.72 �89.12 6.0 6.0 0.1977e+19 0.1977e+19 5

54 1972.12.23 12.15 �86.27 6.2 6.2 0.3421e+19 0.3421e+19 5

55 1973.04.14 10.47 �84.97 6.5 6.5 0.8247e+19 0.8247e+19 5

A 1.27853e+20 1.984193e+20

References: 1. Carr and Stoiber (1977); 2. Peraldo and Montero (1999); 3. Harlow et al. (1993); 4. Montero-Pohly (1989); 5. White and Harlow

(1993) (and references therein).

We exclude events west of �908.a Minimum value reported.b Maximum value reported.c Calculated from minimum magnitude.d Calculated from maximum magnitude.

Table 2 (continued)

M. Guzman-Speziale et al. / Tectonophysics 400 (2005) 241–254248

volcanic arc, and a normal faulting mechanism

(plunge of P axis z458) is required for earthquakes

in the region of the grabens. In all, 77 CMTs met the

requirements for the convergent margin, 23 for the

volcanic arc, and 5 for the grabens. Using only CMTs

to determine the average shape tensor ensures a

uniform data set, although focal mechanisms in

Central America are reported by other workers (e.g.,

Molnar and Sykes, 1969; Dean and Drake, 1978;

Burbach et al., 1984; White and Harlow, 1993).

Table 3

Significant thrust-faulting earthquakes along the Cocos-Caribbean plate m

N Date Latitude Longitude Msa

1 17190305 13.00 �89.50 7.2

2 17520507 12.30 �87.50 6.7

3 17760530 13.18 �90.08 7.2

4 18150820 12.75 �89.00 7.2

5 18260403 10.00 �85.50 6.7

6 18310207 13.20 �89.70 7.0

7 18331002 10.00 �85.50 7.0

8 18591208 13.20 �90.00 7.0

9 18670630 13.20 �89.16 6.5

10 18690301 13.00 �90.00 7.0

11 18820303 9.20 �84.20 6.7

12 18851012 12.08 �87.03 6.7

13 19000621 10.00 �85.50 7.1

14 19160227 10.70 �85.98 7.3

15 19210328 12.50 �87.50 7.2

16 19260208 13.00 �89.00 7.0

17 19391221 10.00 �85.00 7.1

18 19561024 11.50 �86.50 7.2

References: 1. Peraldo and Montero (1999); 2. Pacheco and Sykes (1992a Minimum value reported.b Maximum value reported.c Calculated from minimum magnitude.d Calculated from maximum magnitude.

Large historic earthquakes for Central America are

reported in various sources. We consider historic

those earthquakes that occurred prior to 1978, when

systematic reporting of CMTs by Harvard University

began, and on or after 1700.

Several authors (e.g., Carr and Stoiber, 1977;

Montero-Pohly, 1989; Harlow et al., 1993; White

and Harlow, 1993; Peraldo and Montero, 1999) report

historic earthquakes along the Central America

volcanic arc. We use 55 of these events (Table 2).

argin, 1700–1977

Msb Mo (N m)c Mo (N m)d Ref.

7.4 0.8710e+20 0.1738e+21 1

6.7 0.1553e+20 0.1553e+20 1

7.5 0.8710e+20 0.2455e+21 1

7.2 0.8710e+20 0.8710e+20 1

6.7 0.1553e+20 0.1553e+20 1

7.1 0.4365e+20 0.6166e+20 1

7.2 0.4365e+20 0.8710e+20 1

8.0 0.4365e+20 0.1380e+22 1

7.1 0.8247e+19 0.6166e+20 1

7.0 0.4365e+20 0.4365e+20 1

7.2 0.1553e+20 0.8710e+20 1

7.7 0.1553e+20 0.4898e+21 1

7.1 0.6166e+20 0.6166e+20 2

7.3 0.1230e+21 0.1230e+21 2

7.2 0.8710e+20 0.8710e+20 2

7.0 0.4365e+20 0.4365e+20 2

7.1 0.6166e+20 0.6166e+20 2

7.2 0.8710e+20 0.8710e+20 2

A 9.70437e+20 3.2126e+21

).

Table 4

Significant earthquakes along the grabens of Central America 1570–1978

N Date Latitude Longitude Msa Msb Mo (N m)c Mo (N m)d Ref.

1 1586.12.23 14.60 �90.75 5.4 6.0 0.4387e+18 0.1977e+19 1, 5

2 1607.10.09 14.50 �90.50 5.4 6.2 0.4387e+18 0.1977e+19 1, 5

3 1651.02.18 14.52 �90.68 5.4 5.4 0.4387e+18 0.4387e+18 1, 5

4 1689.02.12 14.55 �90.75 6.0 6.0 0.1977e+19 0.1977e+19 1, 5

5 1717.09.29 14.52 �90.80 6.5 6.5 0.8247e+19 0.8247e+19 5

6 1733.04.00 14.20 �88.40 4.9 5.4 0.1380e+18 0.4387e+18 3, 5

7 1733.05.00 14.42 �89.28 5.4 7.5 0.4387e+18 0.2455e+21 3, 5

8 1743.10.15 15.00 �89.50 6.7 6.7 0.1553e+20 0.1553e+20 1, 3, 5

9 1765.06.02 14.83 �89.50 6.0 7.6 0.1977e+19 0.3467e+21 1, 5

10 1773.07.29 14.50 �90.80 6.5 6.5 0.8247e+19 0.8247e+19 1, 5

11 1773.12.14 14.50 �90.80 5.7 5.7 0.9095e+18 0.9095e+18 1, 5

12 1774.10.14 14.50 �87.66 5.4 6.0 0.4387e+18 0.1977e+19 2, 3, 5

13 1809.06.20 14.40 �87.66 5.0 5.7 0.1783e+18 0.9095e+18 2, 3, 5

14 1820.10.09 16.00 �87.85 6.0 6.5 0.1977e+19 0.8247e+19 2, 5

15 1830.04.21 14.47 �90.60 6.3 6.3 0.4550e+19 0.4550e+19 1, 5

16 1851.11.14 14.50 �87.70 6.0 6.5 0.1977e+19 0.8247e+19 1, 2

17 1885.12.18 14.41 �90.62 6.3 6.4 0.4387e+18 0.1977e+19 1, 5

18 1854.04.16 13.68 �89.18 6.0 6.6 0.4550e+19 0.6099e+19 1, 3, 4

19 1917.12.26 14.53 �90.53 5.8 5.8 0.1171e+19 0.1171e+19 1, 4

20 1917.12.29 14.55 �90.53 5.7 5.7 0.9095e+18 0.9095e+18 1, 4

21 1918.01.04 14.58 �90.53 6.1 6.1 0.2592e+19 0.2592e+19 4

22 1918.01.25 14.50 �90.53 6.2 6.2 0.3421e+19 0.3421e+19 4

23 1934.02.03 14.85 �89.15 6.2 6.2 0.3421e+19 0.3421e+19 1, 4

A 6.59383e+19 6.770012e+20

Table taken from Guzman-Speziale (2001a,b).

References: 1. Carr and Stoiber (1977); 2. Osiecki (1981); 3. White (1991); 4. White and Harlow (1993); 5. Peraldo and Montero (1999).a Minimum value reported.b Maximum value reported.c Calculated from minimum magnitude.d Calculated from maximum magnitude.

M. Guzman-Speziale et al. / Tectonophysics 400 (2005) 241–254 249

Data for 18 historic earthquakes along the convergent

margin come from Pacheco and Sykes (1992) and

Peraldo and Montero (1999) (Table 3). The 19 historic

events for the grabens come from Guzman-Speziale

(2001a) (Table 4).

5. Results

5.1. Cocos-Caribbean convergent margin

The average shape tensor F from Harvard CMTs

is:

F ¼� 0:4954 � 0:2842 0:6027� 0:2842 � 0:0746 0:40810:6027 0:4081 0:5698

24

35 ð6Þ

We obtained a minimum and a maximum value

for the sum of scalar seismic moments, adding the

moments calculated from the smallest and largest

magnitudes reported, and moments from CMTs.

These are: 1.51173�1021 N m and 3.75389�1021

N m, with an average of 2.63281�1021 N m, which

include the values reported in Table 3 and from the

77 CMTs. In a coordinate system where x1 is north,

x2 is east, and x3 is down, considering the average

sum of scalar seismic moments, the corresponding

volume (see above), and 304 years of data, the

average seismic strain-rate tensor calculated using

Eqs. (1) (2) and (3) is:

eij ¼� 1:0320 � 0:5921 1:2554� 0:5921 � 0:1554 0:85011:2554 0:8501 1:1868

24

35

� 10�8 yr�1 ð7Þ

M. Guzman-Speziale et al. / Tectonophysics 400 (2005) 241–254250

This tensor has the following eigenvalues, arranged in

decreasing order:

� ¼1:90670:1285

� 2:0358

24

35� 10�8 yr�1 ð8aÞ

and associated eigenvectors, in columnar form:

U ¼0:3296 � 0:5155 0:79090:2774 0:8536 0:44090:9024 � 0:0740 � 0:4244

24

35 ð8bÞ

The eigenvector associated to the largest (exten-

sive) eigenvalue is mostly vertical, with a small

horizontal component oriented in a 408 azimuth.

The intermediate eigenvector is practically horizontal

and oriented in a S298E direction while the smallest

(compressive) eigenvector has a large horizontal

component in a N298E direction and a small vertical

component plunging 258.

5.2. Central America volcanic arc

For the volcanic arc we obtained an average shape

tensor:

F ¼� 0:5844 0:0885 � 0:12650:0885 0:7093 � 0:0463

� 0:1265 � 0:0463 � 0:1249

24

35 ð9Þ

The minimum, maximum, and average sums of scalar

moments are 1.50396�1020 N m, 2.20963�1020 N

m, 1.85679�1020 N m, respectively. Taking the

average and 304 years as the time, the average seismic

strain-rate yields:

eeij ¼

� 6:6555 1:0081 � 1:44011:0081 8:0780 � 0:5273

� 1:4401 � 0:5273 � 1:4226

24

35

� 10�9 yr�1 ð10Þwith eigenvalues and eigenvectors:

� ¼0:8188

� 0:1115� 0:7073

24

35� 10�8 yr�1 ð11aÞ

U ¼0:0740 � 0:2368 0:96870:9951 0:0815 � 0:0560

� 0:0656 0:9681 0:2417

24

35 ð11bÞ

In this case, the largest eigenvector (extensive) is

horizontal and oriented in an E–W direction, the

intermediate eigenvector is mostly vertical, and the

compressive (smallest) eigenvector is almost horizon-

tal (a small vertical component with a 148 plunge),

oriented in the N direction.

5.3. Grabens of Central America

The shape tensor is:

F ¼0:0942 � 0:2727 � 0:0177

� 0:2727 0:8037 0:0620� 0:0177 0:0620 � 0:8979

24

35 ð12Þ

6.6545�1019 N m, 6.7761�1020 N m, and

3.7209�1020 N m, are the minimum, maximum,

and average sums of scalar moments. The average

strain-rate yields:

eeij ¼

0:2562 � 0:7417 � 0:0480� 0:7417 2:1860 0:1686� 0:0480 0:1686 � 2:4422

24

35

� 10�8 yr�1 ð13Þ

The eigenvalues and eigenvectors are:

� ¼2:44440:0041

� 2:4485

24

35� 10�8 yr�1 ð14aÞ

U ¼� 0:3215 0:9469 0:00810:9462 0:3216 � 0:03510:0358 0:0036 0:9994

24

35 ð14bÞ

The grabens of Central America show eigenvectors

of the seismic strain-rate tensor oriented in an azimuth

of 1098 (extensive or largest one), and an almost

vertical smallest (compressive) one.

6. Discussion

Several authors (Fitch, 1972; Harlow and White,

1985; Guzman-Speziale, 1995a; DeMets, 2001) have

suggested that earthquakes along the Central America

volcanic arc are due to oblique subduction of the

Cocos Plate. Recently, however, Guzman-Speziale

and Gomez (2002) pointed out that this model

M. Guzman-Speziale et al. / Tectonophysics 400 (2005) 241–254 251

presents several problems, such as very small along-

arc components of relative plate motion (Fig. 1),

earthquake faulting planes perpendicular to the

volcanic arc for some of the earthquakes, and

buttressing of the supposedly detached forearc at its

northwestern end.

If not oblique subduction, what is the mechanism

that triggers these tectonic earthquakes? Guzman-

Speziale (2001b) suggested that a combination of

compression along the subduction zone and extension

in the back-arc region might yield strike-slip faulting

along the volcanic arc. We retake this idea here.

Earthquakes along the volcanic arc yield an

average seismic strain-rate tensor for which the largest

(extensive, or least compressive) and smallest (com-

pressive) eigenvectors are horizontal and oriented E–

W and N–S, respectively. Counterparts for these

eigenvectors may be found along the convergent

margin and in the zone of grabens.

The compressive (smallest) eigenvector along the

convergent margin plunges 258 and is oriented in a

298 azimuth. Its magnitude is 2.0�10�8 year�1. Its

-95

-95

-90

-90

10

15

o o

o

o

o o

Fig. 3. Horizontal direction of extensive (white arrows) and compressive (d

horizontal component along a NS direction is

1.6�10�8 year�1. The same eigenvector for earth-

quakes along the volcanic arc is oriented N–S with a

magnitude of 0.7�10�8 year�1. The NS, horizontal

component of the compressive vector is only two

times in magntitude, compared to the compressive

eigenvector of earthquakes along the volcanic arc. In

the case of the grabens, the extensive eigenvector is

oriented in a 1098 azimuth, with a magnitude of

2.4�10�8 year�1, compared to the 0.8�10�8 year�1,

that is, only about three times and a very similar

orientation.

Compressive strain-rate along the convergent

margin and along the volcanic arc are similar, in

direction and in magnitude. So are extensive strain-

rates along the grabens and the volcanic arc (Fig. 3).

This suggests to us that compression along the

convergent margin and extension along the grabens

are transmitted to the volcanic arc. We propose that

this strain combination is resolved along the volcanic

arc because it is a zone where lithospheric strength is

decreased due to a higher thermal gradient and a small

-85

-85

10

15

o

o

o

o

ark arrows) eigenvectors along tectonic elements of Central America.

M. Guzman-Speziale et al. / Tectonophysics 400 (2005) 241–254252

thickness. Extension from the convergent margin or

compression from the back-arc region does not

contribute to the state of stress along the volcanic

arc because both components are vertical.

Evidently, seismic activity along the convergent

margin is larger than either along the volcanic arc or

the grabens, both in number of earthquakes and in

magnitudes. Yet, seismic strain-rate is similar because

in the convergent margin it is distributed along a much

larger volume.

7. Conclusion

Evidence presented elsewhere (Guzman-Speziale

and Gomez, 2002) suggests that oblique plate

convergence may not be the driving mechanism for

tectonic earthquakes along the Central America

volcanic arc. The model first suggested by Guzman-

Speziale (2001b), in which the earthquakes are due to

a combination of compression from the Cocos-

Caribbean convergent margin and back-arc extension,

is shown here to be well supported by calculations of

the strain-rate tensor in all three tectonic elements. In

other words, our results suggest that, indeed, seismic

activity along the Central America volcanic arc is

related to subduction of the Cocos plate along the

Cocos-Caribbean interface.

Acknowledgements

This work was possible thanks to grants GEOF

3.4.2.42 from Instituto Panamericano de Geografıa e

Historia (IPGH), to Guzman-Speziale and Valdes,

and 36449-T from Consejo Nacional de Ciencia y

Tecnologıa (Conacyt), Mexico, to Guzman-Speziale.

We are grateful to the two referees, Carlos Mendoza

and Marino Protti, for their comments, which greatly

improved the manuscript. Centro de Geociencias,

UNAM, contribution 905.

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