no

on

logy,

al Au

interactions take place along the Swan Transform fault, are curvilinear, requiring rotation about a relatively

near by pole (Gordon and Muehlberger, 1994).

Works on this left-lateral strike slip plate boundary

Tectonophysics 404 (2005)the Motagua and ChixoyPolochic fault conforming the1. Introduction

Northern Central America is situated at the north-

western corner of the Caribbean plate. Most of its de-

formation processes are due to the interaction between

the North American Cocos and Caribbean plates. These

plate boundary between North American and Caribbean

plates (Fig. 1). The Swan transform fault connects the

southern end of the Cayman spreading axis with the

Motagua fault (Rosencrantz and Mann, 1991).

In Central America, the plate boundary continues as

the Motagua and ChixoyPolochic faults. These faultsReceived 25 November 2002; accepted 17 May 2005

Available online 15 June 2005

Abstract

Evaluation of the seismic moment tensor for earthquakes on plate boundary is a standard procedure to determine the relative

velocity of plates, which controls the seismic deformation rate predicted from the slip on a single fault. The moment tensor is

also decomposed into an isotropic and a deviatoric part to discover the relationship between the average strain rate and the

relative velocity between two plates. We utilize this procedure to estimate the rates of deformation in northern Central America

where plate boundaries are seismically well defined. Four different tectonic environments are considered for modelling of the

plate motions. The deformation rates obtained here compare well with those predicted from the plate motions models and are in

good agreement with actual observations. Deformation rates on faults are increasingly being used to estimate earthquake

recurrence from information on fault slip rate and more on how we can incorporate our current understanding into seismic

hazard analyses.

D 2005 Elsevier B.V. All rights reserved.

Keywords: Central America; Crustal deformation; Average strain rate; Seismic moment tensor summation; Seismic strain rate tensorCrustal deformation in

Diego Caceresa,b,*, David M

aDepartment of Earth Sciences, SeismobDepartamento de Fsica, Universidad Nacion0040-1951/$ - see front matter D 2005 Elsevier B.V. All rights reserved.

doi:10.1016/j.tecto.2005.05.008

* Correspondi

logy, Uppsala University, Uppsala, Sweden.

E-mail address: diego.caceres@chello.se (D. Caceres).rthern Central America

terrosoa, Behrooz Tavakolia

Uppsala University, Uppsala, Sweden

tonoma de Honduras, Tegucigalpa, Honduras

119131

www.elsevier.com/locate/tectokes (1969), Plafkerinclude those by Molnar and Syng author. Department of Earth Sciences, Seismo-(1976), Guzman-Speziale and Meneses-Rocha (2000)

among others. The rate of slip along this boundary

Fig. 1. Tectonic settings of northern Central America. FS is fault system, HD is Honduras depression, ND is Nicaragua depression. Values and arrows indicate relative plate velocities

(mm/year) with respect to North American plate from Nuvel-1a in DeMets et al. (1990). Values for the Middle America Trench are from McNally and Minster (1981). Values for the

North AmericanCaribbean plate boundary from DeMets et al. (2000).

D.Caceres

etal./Tecto

nophysics

404(2005)119131

120

Fig. 2. Seismotectonic map of northern Central America. Seismicity data (solid circles) are for Mz4.5, size of circles is proportional to earthquake magnitude. Selected fault planesolutions, according to a minimum magnitude, are from the CMT catalog. For zones Sz1, Sz2 and Sz3, minimum magnitude isM =6.5. For the rest of the zones, minimum magnitude

is M =5.5. Size of the beach-balls is proportional to magnitude. The polygons represent the seismogenic sources in which the area has been divided.

D.Caceres

etal./Tecto

nophysics

404(2005)119131

121

associated with these faults (Figs. 1 and 2).

Northern Central America is one of the most seis-

ophyranges between 11 and 25 mm/year (DeMets et al.,

2000). Historical earthquakes have ruptured segments

of the plate boundary, e.g., the earthquake of 1816 on

the Polochic fault (M 7.5 to 7.75) (White, 1985) and the

1856 event on the Swan transform fault (M 7 to 8).

In recent times, the Motagua earthquake of 1976 (M

7.5) produced about 2 m of slip (Kanamori and

Stewart, 1978).

Along the Pacific coasts of Central America, the

Cocos plate is subducted beneath the Caribbean plate

along the Middle America trench in an oblique, north-

east direction. The slip rate increases, from north to

south, from 60 to 85 mm/year (DeMets, 2001) along

the trench segment of the study area. Numerous earth-

quakes have been recorded along this boundary with

magnitudes reaching up to 8 (White and Harlow,

1993). An interesting feature of this subduction zone

is the change in the dipping angle (Bilek and Lay,

1999). Several authors, e.g., White (1991), Burkart

and Self (1985), McNally and Minster (1981) and

Bilek and Lay (1999) have discussed different aspects

of this subduction zone.

Parallel to the Middle America trench extends the

Central America volcanic front, along the axis of the

volcanic chain related to the subduction process.

Moderate-size earthquakes occur there showing dex-

tral strike slip motion, as reported, e.g., by Harlow

and White (1985) and Fitch (1972). DeMets (2001)

suggests that strike slip is a consequence of the

oblique nature of the subduction process along the

Middle America trench. Bounded by the North Amer-

icanCaribbean and the CocosCaribbean plate

boundaries, there is a broad zone of EW extension

grabens, the Honduras depression (Guzman-Speziale,

2001). During the period from 1964 to 2001, seis-

micity in the grabens has been scarce, with magni-

tudes reaching up to M 6.7 (ISC, 2001). To the east of

the Honduras depression, a conspicuous feature, the

Guayape fault system, runs throughout Honduras in

NESW direction. Finch and Ritchie (1991) and

Gordon and Muehlberger (1994) suggested a dextro-

giral sense of motion for this fault system.

According to R. Rogers (personal communication),

from 2-D offshore seismic data (LandSat imagery and

geomorphology), there is no evidence that the Guayape

system is accommodating motion along its structure in

D. Caceres et al. / Tecton122recent time. To the north of the Guayape fault and south

of the Swan transform fault, in the so-called Hondurasmically active zones in the world. Most of the seismic

activity is concentrated along the plate boundaries,

and is sparsely distributed on the intraplate provinces.

Recently, several sites along the region have been

equipped with GPS receivers, which would, eventu-

ally allow retrieving the rate of motion along faults

and plate boundaries. However, using seismic data, it

is possible to obtain, in some cases, a slip on a fault

due to large earthquakes, which may be close to the

full rate of plate motion that includes motion not

registered with the aid of seismometers. As pointed

out by Kostrov (1974) and Jackson and McKenzie

(1988) among others, the seismic moment tensor, as an

expression of the size of earthquakes, can be used to

determine the crustal deformation of a seismic volume.

The objective of the present study is to determine the

deformation rates from the seismicity pattern in North

Central America. First, we utilized geological and seis-

mic evidences, a segmentation model, a large number

of fault plane solutions and historical and instrumental

earthquake data to characterize a seismic model for

northern Central America. Then we applied a proce-

dure (estimation of the scalar seismic moment rate) for

the analysis of the strain rate tensor and the motion of

crustal blocks in the region. A detailed analysis of

uncertainties involved in the process is also performed

for reliable estimation of the crustal deformation. We

estimated the rates of crustal deformation for the west-

ern boundaries of the Caribbean plate, the Honduras

depression grabens and the Honduras borderlands

faults. We also compare the resulting deformation

rates derived from seismic data with the horizontal

component of the stress regime in order to obtain

complementary insights into the deformation.

2. Method, data and seismogenic zoning

2.1. Methodborderlands, there is a series of left-lateral oblique

faults, e.g., the Nueva Esperanza, Pueblo Viejo fault,

Aguan fault, La Ceiba faults and some unnamed off-

shore faults. Moderate and scarce seismicity can been

sics 404 (2005) 119131The method for data analysis we follow in this

work is outlined in Papazachos and Kiratzi (1992)

ophyand based on works from Kostrov (1974), Molnar

(1979) and Jackson and McKenzie (1988). The aver-

age strain rate tensor, eij for a seismic zone withknown dimensions is related to the sum of the seismic

moment tensor as:

ePij

XN

n1Mnij

2lVs M

0FP

ij

2lVi; j 1; 2; 3 1

where l is the rigidity in crustal rocks;P

Mij is the

sum of moment tensors of earthquakes in the volume

V subjected to deformation during a given period of

time s. On the right hand side of Eq. (1), FPij representsthe geometrical features of the tensor, also called the

focal mechanism tensor, which is assumed to be con-

stant over the time s. Mo represents the rate of the sizeof earthquakes; it is the scalar annual seismic moment

rate, calculated following Molnar (1979):

M o A

1 B M1B

0;max : 2

Here, M0,max is the scalar moment of the max-

imum magnitude event observed in the volume. A=

10[a(bd/c)] and B =b/c. Constants a and b are calcu-lated from the GutenbergRichter magnitudefrequen-

cy relation for each volume. Coefficients c and d are the

constants of the empirical momentmagnitude relation

calculated for the whole area of study. The velocity

tensor U can be obtained from

Uii M0Fii

2llk lji 1; 2; 3k p i; ip j; jp k

U12 M0F12

ll1l3

Ui3 M0Fi3

ll1l2i 1; 2: 3

Above, length= l1, width= l2 and thickness= l3 of

the seismic volume form the local coordinate system

for all calculations. Fij (in the northeastdown coor-

dinate system) has to be rotated since the formulations

of ei and Uij given above are valid in the local coordi-nate system.

2.2. Data

We compiled an epicenter catalogue in order to

D. Caceres et al. / Tectoncalculate the constants a, b, c and d of the annual

seismic moment release (Eq. (2)). The compilationwas made from catalogues of Engdahl et al. (1998)

and the ISC (2001) for the period 1964 to 2001. Data

for the period between 1700 and 1964 are from

Ambraseys and Adams (2001), Rojas et al. (1993),

White and Harlow (1993) and Osiecki (1981). All

magnitudes are surfacewave magnitude (Ms) and the

epicenters are shown in Fig. 2. When a large time span

is considered, heterogeneity usually is present in the

data to be used to calculate the constants b and a in the

GutenbergRichter relation. To abridge the problem,

we followed the bmean valueQ method described inPapazachos (1990). The whole time interval is divided

into subintervals within which the earthquake data are

complete above a certain minimum magnitude. For

each interval, the frequency of shocks (counts) is

obtained. By means of least-squares inversion of the

counts, we then obtained the constants b and a.

The value for c was held fixed, equal to 1.5, follow-

ing Kanamori and Anderson (1975). Its standard error

was assumed to be rc=0.25, which is half the value ofthe last significant digit. To obtain the constant, d, we

performed a regression, in the least-squares sense, for

earthquakes havingMs andM0 reported simultaneous-

ly. The constant, d =9.39, was obtained with standard

error rd=0.04 and rcd=0.002. By converting thecorresponding maximum Ms magnitude observed on

each volume (listed in Table 1), with the aid of the

MsMo relationship, we obtained Mo,max (the maxi-

mum seismic moment) for each of the zones consid-

ered in the study. With all the parameters at hand, we

can calculate the seismic moment rate from (2).

Standard deviations and covariance for all the para-

meters involved were calculated as well. The advan-

tage of this method lies in the possibility to use

historical as well as instrumental seismicity in the

calculation of the seismic moment rate for a given

zone. To calculate the Fij tensor in (3), we compiled a

catalogue of focal mechanisms from Harvard CMT

catalogue available on the Internet, Bradley and Drake

(1978), Molnar and Sykes (1969) and White and

Harlow (1993). For each zone, a simple averaging

procedure was applied to obtain a brepresentativeQfocal mechanism for each volume. Errors involved

in the method described above are mainly controlled

by the errors in the moment rate (Kiratzi and Papa-

zachos, 1996). It is assumed that a, b, c, d and MS,max

sics 404 (2005) 119131 123(maximum surface-waves magnitude) have normally

distributed random errors.

Table 1

Parameters for each zone used in the calculations in the present study

Source l1 (km) l2 (km) l3 (km) T0 (year) Tf (year) Mmax

Tz1 537 71 40 1855.6 2002.2 7.5

Tz2 245 78 40 1856.5 1999.7 7.5

Tz3 314 80 35 1816.5 2001.7 7.6

Sz1 222 88 60 1853.0 2002.3 7.9

Sz2 200 100 60 1776.0 2002.0 8.1

Sz3 438 90 60 1900.0 2001.8 7.9

Vz1 366 51 40 1906.4 2002.0 6.9

Vz2 371 74 50 1867.0 2002.2 7.4

Hdz 313 226 50 1733.0 2001.5 7.6

Hbz 126 95 40 1918.5 2000.5

T0 and Tf are the beginning and ending of the catalog time span respec

frequency constants. Azimuth is from north. Mmax is the maximum magn

D. Caceres et al. / Tectonophy124Errors for moment rate can be obtained by introduc-

ing Gaussian deviations in Eq. (2), as outlined in

Papazachos and Kiratzi (1992). Errors for a and b

are listed in Table 2. The standard error of Ms,max is

assumed to be 0.35 (Ekstrom and Dziewonski, 1988).

Errors of c and d are given above. The errors in the

F tensor do not need to be considered since the ten-

sor was calculated as a simple average (Kiratzi and

Papazachos, 1996). The stress data are from the

World Stress Map project (Mueller et al., 2000)

and display the orientations of the maximum hori-

zontal stress SH (Fig. 3).

2.3. Seismogenic zoning

Plate boundaries are seismically well defined in theCentral America region (Gordon and Muehlberger,

Table 2

Components of the strain rate tensor (factors of 107/year) for thezones in the present study

Source e11 e12 e13 e22 e23 e33

Tz1 0.4774 0.0781 0.0131 0.4044 0.1784 0.0731Tz2 0.9653 0.0216 0.1730 0.9449 0.2274 0.0204Tz3 0.5781 0.5917 0.2569 0.6124 0.0489 0.0343Vz1 0.2724 0.2249 0.1446 0.2951 0.0389 0.0227Vz2 0.4382 0.3546 0.1350 0.4505 0.2022 0.0122Sz1 0.1112 0.0879 0.2286 0.0632 0.1456 0.1744Sz2 0.1991 0.1559 0.1857 0.1027 0.1451 0.3018Sz3 0.0706 0.0306 0.1873 0.0127 0.0757 0.0832Hdz 0.1628 0.0340 0.0286 0.0033 0.0323 0.1661Hbz 0.0688 0.0282 0.1442 0.0043 0.0171 0.0646All values of strain are factors of 107/year.1994). For simplicity in the calculations, we have

drawn zones as rectangular as possible. The length,

width and azimuth of each zone were estimated as

indicated in Papazachos and Kiratzi (1992). Using the

catalogue of epicenters, we obtained a least-squares

best-fit line for each zone, the we projected the most

distant epicenters of the zone to the line to estimate

the length l1 of the zone. The width, l2, of each zone is

obtained taking 4r, where r is the standard deviationfrom the least-squares line obtained for each zone.

The azimuth of each zone is taken as the azimuth of

the line with the north.

We divide the belt of the left-lateral North Amer-

icanCaribbean plate boundary into the Swan trans-

form fault (Tz1), Eastern MotaguaPolochic fault

system (Tz2) and Western MotaguaPolochic fault

a b Moment rate Azimuth (8)

4.50F0.13 0.60F0.02 4.77E18F3.13E18 693.38F0.17 0.45F0.03 4.61E18F2.25E18 603.59F0.09 0.47F0.02 4.64E18F2.26E18 1155.55F0.11 0.70F0.02 2.32E19F1.83E19 1185.22F0.20 0.64F0.03 3.03E19F2.33E19 1105.97F0.33 0.76F0.05 3.36E19F3.18E19 1263.82F0.62 0.51F0.11 1.82E18F1.83E18 1106.21F0.36 0.83F0.06 5.80E18F5.35E18 1304.72F0.60 0.62F0.10 3.74E18F3.86E18 70

4.68E17 50

tively. Constants a and b are the GutembergRitcher magnitude

itude, moment rates are in Nm.

sics 404 (2005) 119131system (Tz3) zones. Because of the concave geometry

of the plate boundary, we have divided it into zones

Tz2 and Tz3. Since we are interested only in the

active part of each belt, we have not included the

boundary segment in between Tz1 and Tz2 because

of lack of seismicity.

In the belt along the Middle America trench, the

rate of convergence between the Cocos and Carib-

bean plates increases from north to south according

to DeMets (2001). Because of the obliquity of the

subduction and the trend of seismicity, we have

divided this belt into Guatemala subduction zone

(Sz1), El Salvador subduction zone (Sz2) and Nicar-

agua subduction zone (Sz3). Earthquakes along the

belt of the volcanic chain are a result of the parti-

tioning of the oblique convergence of the Cocos

Caribbean plates along the Middle America Trench

Fig. 3. Maximum horizontal stress (SH) trends in northern Central America.

D.Caceres

etal./Tecto

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404(2005)119131

125

these faults may produce infrequent earthquakes of

M=7 to M =7.5.3

ponentsofthevelocity

tensorU

(inm/year)

ceU11

U12

U13

U22

U23

U33

k1

Az(8)

Pl(8)

k2

Az(8)

Pl(8)

k3

Az(8)

Pl(8)

0.0015

0.0104

0.0014

0.0212

0.0004

0.0003

0.0257

66.25

0.35

0.0036

203.00

23.58

0.0003

204.00

66.42

0.0001

0.0133

0.0009

0.0162

0.0021

0.0003

0.0238

60.82

3.38

0.0078

29.98

13.26

0.0003

15.17

76.30

0.0065

0.0149

0.0004

0.0068

0.0018

0.0001

0.0216

45.43

4.20

0.0084

45.00

6.29

0.0001

11.00

82.43

0.0013

0.0055

0.0001

0.0071

0.0012

0.0001

0.0105

59.29

5.90

0.0021

32.20

14.00

0.0001

8.55

74.68

0.0004

0.0087

0.0007

0.0134

0.0023

0.0001

0.0178

64.54

5.84

0.0052

152.67

17.69

0.0004

7.85

71.31

0.0226

0.0166

0.0283

0.0076

0.0160

0.0105

0.0506

31.78

28.02

0.0278

24.64

61.79

0.0030

120.19

2.97

0.0338

0.0334

0.0240

0.0163

0.0150

0.0181

0.0687

37.01

17.98

0.0276

192.14

70.31

0.0091

124.48

7.75

0.0167

0.0159

0.0206

0.0154

0.0128

0.0050

0.0436

40.62

26.24

0.0177

13.60

61.04

0.0012

124.93

11.38

0.0003

0.0004

0.0004

0.0055

0.0002

0.0008

0.0055

94.36

1.69

0.0011

1.38

60.72

0.0001

5.31

29.22

0.0002

0.0004

0.0008

0.0010

0.0008

0.0003

0.0017

115.05

29.14

0.0011

163.83

49.76

0.00002

40.2

25.11

azim

uth

andPlisplunge.

Positiveandnegativeplungeindicates

that

theeigenvectorpointsinto

oroutofthesolidearthrespectively.

k1,k2and

k3areeigenvalues

ofthe

itytensor.

ophysics 404 (2005) 119131Summarizing, we divided the deforming area into

10 seismic zones based on seismicity patterns, tecton-

ics and similarity in style of focal mechanisms. Seis-

micity and focal mechanism for all zones are shown in

Fig. 2 and the parameters for each zone are presented

in Table 1.

3. Results and discussion

The resulting strain rate tensor elements for seis-

mogenic zones in North Central America are listed in

Table 2. Components of the velocity tensor, eigenva-

lues and corresponding azimuth and plunge for each

seismogenic source are presented in Table 3. The

eigenvectors correspond to directions of principal

axes of the diagonalized velocity tensor and their

magnitudes are the maximum, intermediate and min-

imum deformation rates, k1, k2 and k3. The discus-sion of the results for each volume is given below.

Due to insufficiency in the seismic record to calculate

b and a values for the zone Hbz, we follow the

alternative described in Guzman-Speziale (2001).

The difference in this case is that the seismic moment

rate is calculated as a direct summation of individual

moments over the time span covered by the catalogue(DeMets, 2001). The system accounts for the Northern

Volcanic chain (Vz1) and the Southern Volcanic chain

(Vz2).

We have included a triangular wedge east of the

Motagua fault and a triangular block to the west of the

Guayape fault, both described by Gordon and Muehl-

berger (1994) in the zone of the Honduras depression

(Hdz). Hdz is not continuous, but a zone of several

fault-bounded extensional grabens. Seismicity is mod-

erate and recent activity includes the earthquake of

April 27, 1982 (Mw=5.4). It is suggested that the

grabens of the Honduras depression are forming due

to interaction with the slip on the North American

Caribbean plate boundary (Gordon, 1990). South of

the Swan transform fault and northeast of the Hon-

duras depression we find the Honduras borderlands

faults (Hbz). A few earthquakes occur on these strike

slip faults. White and Harlow (1993) speculate that

D. Caceres et al. / Tecton126of the zone. Resulting deformation rates are presented

in Fig. 4. Table

Com

Sour

Tz1

Tz2

Tz3

Vz1

Vz2

Sz1

Sz2

Sz3

Hdz

Hbz

Azis

veloc

Fig. 4. Topography map and distribution of deformation velocities for seismogenic zones in which northern Central America was divided. Values in circles are in mm/year. Gray

arrows indicate compression, white arrows extension. Focal spheres represent the average focal mechanism for each zone.

D.Caceres

etal./Tecto

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404(2005)119131

127

ophy3.1. Results of deformation rate for each seismogenic

zone

3.1.1. Plate boundary between North American and

Caribbean plates

This belt is made up by the Tz1, Tz2 and Tz3

zones. 16 focal mechanisms, with left-lateral strike

slip, were used in the calculations. The slip velocity

ranges between 23 and 25 mm/year with a trend of

E68W. The western MotaguaPolochic fault system

Tz3 shows 21mm/year in the E90W direction. Pre-

dicted values from DeMets et al. (2000) for this plate

boundary are about from 18.6 mm/year (Fig. 1).

3.1.2. Plate boundary between Cocos and Caribbean

plates

From the Middle America subduction zone, 48

focal mechanisms mostly showing thrust motion are

used for the calculations. The Sz1 zone shows com-

pression of 51 mm/year trending N31E; Sz2: 69 mm/

year along N37E and Sz3 zone: 44 mm/year trending

N40E. The values predicted by NUVEL (DeMets et

al., 1990) are 69, 75 and 78 mm/year for the Sz1, Sz2

and Sz3 zones respectively.

3.1.3. Internal deformation in the Caribbean plate

3.1.3.1. The volcanic chain. Two separated zones:

Vz1 and Vz2, create this belt. 12 focal mechanisms

were used for calculations in this zone. Deformation

velocities range from 10 mm/year towards W104E, to

17 mm/year towards W109E. Values estimated by

DeMets (2001) for the Vz1 and Vz2 zone are of

about 14 mm/year.

3.1.3.2. The Honduras depression faults (Hdz). The

deformation in this zone is characterized by an exten-

sion of 5 mm/year trending N94E. 3 focal mechan-

isms were used in the calculations. It can be observed

that there is a vertical motion of 1 mm/year suggesting

perhaps a crustal thinning. The NUVEL (DeMets et

al., 1990) model predicts a value of 5 mm/year while

Guzman-Speziale (2001) estimated an average exten-

sion rate at 8 mm/year.

3.1.3.3. TheHonduras borderland faults (Hbz). Seis-

D. Caceres et al. / Tecton128micity here is scarce, nevertheless, we were able to find

a focal mechanism for this system showing left-lateralstrike slipmotion. The deformation of this zone is taken

up by 1.7 mm/year towards N70E. The NUVEL

(DeMets et al., 1990) model predicts a value of 4.5

mm/year.

3.1.3.4. The Guayape fault system. The relative

velocity in plate tectonics can be applied to deter-

mine the relative movement of plates given insuffi-

cient seismicity or no earthquakes for the long-term

estimations of motion. The Guayape fault system, a

major tectonic feature in northern Central America,

is such a low seismicity zone. The upper-crustal

deformation of the fault is mostly aseismic and

must be accommodated by creep. A velocity diagram

analysis gives velocities of about 24 mm/year along

the dextral Guayape fault, but there is no indepen-

dent estimate of the velocities on the plates to com-

pare this result.

3.2. Discussion

Analysis of different data sets can provide a

complementary view of the deformation in an area

(Petit, 1998). In the following discussion, we corre-

late maximum horizontal stress SH orientations, from

the World stress map project (WSMP), with the

resulting rates of active seismic deformation, de-

scribed above, to better understand the deformation

process in northern Central America. The results

from the WSMP are not a result from a truly inde-

pendent data set (other than earthquakes), the process

of the data, however is, and some insight may be

gain when compared with the results obtained here.

The maximum horizontal stress indicates compres-

sion with an average trend of N30E (Fig. 3) which is

in good agreement with the calculated P axis trend

of N25E in this study. Along the Middle America

trench, the maximum horizontal stress presents an

average trend of N25E in good agreement with the P

axis trend of N30E for the zones Sz1 and Sz2. Along

the inland segment of the North AmericaCaribbean

plate boundary, it is possible to distinguish two

sectors according to the distribution of seismicity

which is clustered from around 898W and disappearseast of 888W. From the seismicity and topographymaps (Figs. 2 and 4), it is possible to suggest that

sics 404 (2005) 119131this segment connects the concave segment of the

plate boundary with its convex segment. The inflex-

geomorphology support the idea that the Guayape

system is not accommodating motion along its struc-

ture in recent time (Rogers, personal communication).

The Honduras borderlands zone, Hbz, exhibits rela-

tively high topography (up to 2400 m), and the de-

formation is dominated by extension as well as by

strike slip tectonics with the tension axis oriented

about N55W (Table 4, Fig. 4).

If we summarize values of seismic deformation

rates obtained here, we see that they are, in general,

in good agreement with results from the NUVEL-1A

model by DeMets et al. (1990, 1994) and the new

estimations in DeMets et al. (2000), DeMets (2001)

and Guzman-Speziale (2001). Although we used a

limited number of focal mechanisms to calculate the

rate of deformation for the HD and Hbz, we do not

ophysics 404 (2005) 119131 129ion points show a change in seismicity and the

maximum horizontal stress (WSMP) indicates a

compression axis with trend N25E in good agree-

ment with our calculated N30E axis trend. The

western segment of the plate boundary is character-

ized by a maximum horizontal stress has a trend of

about N35E while the calculated P axis from this

study reveals a trend of about N40E.

Along the volcanic depression, the maximum hor-

izontal stress has a trend of about N30W in the Vz1

zone and about N30W for Vz2. Our resulting P axis

shows trends of N50W for the Vz1 zone and N20W

for the Vz2 zone. In the interior of northern Central

America, thicker crust under the western side of the

Honduras depression (898W to 878W) correlateswith topography reaching up to 2000 m (Fig. 1,

Rogers et al., 2002) and also to intermediate level of

diffuse seismicity (Fig. 2). At the same time, there is

also a suggested possible discontinuity to thinner crust

at the northeast side of the HD, which presents lower

levels of seismicity.

The HD zone is undergoing an extensional tectonic

regime with maximum horizontal stress SH showing

orientations ranging from N40E to N5E in good

agreement with the orientation of the N20E axis

obtained here. A crustal thinning at a rate of about

1mm/year may be interpreted from the results in

Table 3. Trends of deformation velocities (Fig. 4) sug-

gest that the northeastern side of HD is rotating counter

clockwise while its western side is subjected to com-

pression, proposed earlier by Gordon and Muehlberger

(1994).

From the seismic catalogue prepared for this study,

we find that the earthquake of 1976 February 2 in

Guatemala, one of the largest events (M=7.6) during

a 100-year period, was located on the Motagua fault.

Aftershocks showed migration from east to west along

the fault and then from north to south along north

striking grabens (Fig. 1). According to Gordon

(1990), this event and its pattern of aftershocks

showed that the north striking normal faults on the

western side of the HD zone are forming in response

to slip on the North AmericaCaribbean plate bound-

ary. A low level of seismicity along the Guayape fault

system (Fig. 2) suggest that the area east of HD

(878W828W) is not undergoing a significant defor-

D. Caceres et al. / Tectonmation at present, and may behave as a rigid block.

2-D seismic data from offshore, LandSat imagery andexpect that the error induced may be large. As pointed

out by Papazachos and Kiratzi (1992), the error in the

magnitude of the strain and velocity rates is deter-

mined by the error factor in the scalar annual moment

rate. The effect of using low numbers of focal

mechanisms is in the direction of the eigenvalues of

the velocity and strain rate tensors. Unfortunately, the

seismicity is low on the HD and Hbz zones, but at the

same time, the tectonics structures in the block show

similar deformation style and its significance would

lie in the amount of deformation. The deformation rate

of Sz3 was estimated here as 44 mm/year while

DeMets (2001) gives 78 mm/year. This discrepancy

can be attributed to the relatively low level of seis-

Table 4

Estimated fault plane solutions for the average focal mechanism for

each zone

Source P T Strike (8) Dip (8) Rake (8)

/ d / d

Tz1 25 23 117 3 69 76 19Tz2 210 13 118 10 254 73 2Tz3 224 15 131 9 267 73 4Vz1 139 12 233 19 275 68 175

Vz2 157 14 252 18 294 67 177

Sz1 207 28 32 61 290 17 82

Sz2 213 19 31 71 305 26 92

Sz3 211 34 32 56 300 11 88

Hdz 8 77 99 4 20 50 74Hbz 149 55 56 32 224 78 78

P is pressure axis, T is tension axis, / is azimuth and d is dip foreach axis.

4. Conclusions

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This work has been carried out at the Department

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Our thanks go to Ota Kulhanek for critically reading

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Crustal deformation in northern Central AmericaIntroductionMethod, data and seismogenic zoningMethodDataSeismogenic zoning

Results and discussionResults of deformation rate for each seismogenic zonePlate boundary between North American and Caribbean platesPlate boundary between Cocos and Caribbean platesInternal deformation in the Caribbean plateThe volcanic chainThe Honduras depression faults (Hdz)The Honduras borderland faults (Hbz)The Guayape fault system

Discussion

ConclusionsAcknowledgmentsReferences