Comprehensive Summaries of Uppsala Dissertationsfrom the Faculty of Science and Technology 834

ACTA UNIVERSITATIS UPSALIENSISUPPSALA 2003

Earthquake Sources and Hazard in Northern Central America

BY

José Diego Cáceres Calix

PAPERS INCLUDED IN THE THESIS

This thesis is based on the following papers, which are referred to in the text by theirRoman numerals:

I Cáceres D. and Kulhánek O. (2000). Seismic Hazard of Honduras. NaturalHazards 22(1): 49-69

II Cáceres D. and Arvidsson R. Seismic Properties of the Swan transform fault,Caribbean Sea. Journal of Seismology (Submitted)

III Cáceres D., Monterroso D. and Tavakoli B. Seismic Active deformation innorthern Central America. Tectonophysics (Submitted)

IV Cáceres D. and Arvidsson R. Static stress transfer along the western margin of theNorth America-Caribbean plate boundary. Geophysical Research Letters(Submitted)

V Cáceres Diego. Coulomb stress changes and the aftershock sequence of the July11, 1999, earthquake in the Gulf of Honduras, Caribbean Sea. (Manuscript)

Reprint I was made with kind permission from Kluwer Academic Publishers.

Additional papers written during my stay at the Department of Earth Sciences but notincluded in this Thesis:

Cáceres, D. and Arvidsson, R., 2000. Seismic hazard in northern Central America.Extended abstract, 12th World Conference on Earthquake Engineering, New ZealandSociety for Earthquake Engineering, Upper Hutt, New Zealand, 2000, Paper No. 2855

Tavakoli, B. and Cáceres, D., 2002. Subduction and crustal fault models tocharacterize seismogenic zones for seismic hazard in northern Central America.Submitted to Tectonophysics.

Contents

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

2. Tectonic Settings .............................................................................................3

3. Seismicity.........................................................................................................53.1. Seismic data .............................................................................................53.2. Seismicity distribution.............................................................................73.3. Summary of seismic information ............................................................8

4. Probabilistic Seismic Hazard ........................................................................114.1. Introduction............................................................................................114.2. Seismogenic zones.................................................................................114.3. Attenuation relationships and uncertainties..........................................124.4. Maps of hazard levels ............................................................................13

5. Fault Geometry ..............................................................................................145.1. Introduction............................................................................................145.2. Depth of faulting....................................................................................145.3. Seismic moment release ........................................................................16

6. Plate Motion and Seismic Deformation Rates..............................................176.1 Introduction.............................................................................................176.2. Rate of deformation from earthquakes..................................................17

7. Earthquake Triggering...................................................................................197.1. Introduction............................................................................................197.2. Interaction between earthquakes ...........................................................19

8. Summary ........................................................................................................21

9. Summary of papers........................................................................................23

10. Acknowledgements .....................................................................................26

11. References....................................................................................................27

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1. Introduction

Northern Central America, our area of study, is located in thenorthwestern corner of the Caribbean plate and represents a tectonicallycomplex area. It is embedded between the Caribbean and North Americanplates and is bounded by the Cocos plate to the southwest. A cursoryinspection of earthquake activity maps reveals high seismicity in the area.According to historical records, large earthquakes, reaching magnitudes upto M=8 have occurred in this region. In the last 25 years, 3 earthquakes withmagnitude larger than 7 have struck northern Central America, causing greatloss in terms of lives and economy.

Studies of earthquake activity provide insight into active tectonicprocesses in a region and are used to estimate hazard levels to prepare for thepossible effects of future events on the society and infrastructure. A key tothe estimation of seismic hazard lies in the identification of tectonicstructures and seismogenic sources which may put a region into peril.

The estimation of fault areas is an important factor in seismic hazardcalculations. Definition of the depth to which earthquakes rupture Earth’scrust using only catalogues of hypocentres is uncertain. Determination of thefocal depth of an earthquake is often fixed to a pre-determined value in orderto ensure convergence in the inverse problem. In some cases this problemmanifests itself in an artificial concentration of earthquakes at 33 kilometresdepth in earthquake catalogues. Also, unrealistic depth estimates e.g. “air-quakes” where the estimated source position is above Earth’s surface havebeen reported.

Fault segments with a deficit in seismic moment release, determinedthrough a detailed analysis of the seismic coupling coefficient along plateinterfaces, help us to understand active tectonic processes. The seismiccoupling coefficient is the ratio of the seismic moment release rate to theexpected seismic moment estimated from plate tectonic convergence rates.The coefficient indicates the proportion of slip represented by earthquakesand hence the coupling along a given section.

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The rationale of this thesis is firstly to analyse seismological, geologicaland tectonic information available for northern Central America in order toevaluate the seismic hazard for the region from a statistical point of view andsecondly to improve the model of seismogenic sources complemented withphysical properties e.g. depth of faulting, seismic coupling and the effects ofprevious earthquakes on subsequent seismic activity.

The thesis is organised as follows: In Papers I and III (Sections 2 and 3)the tectonic framework and seismicity of northern Central America ispresented. Paper I (Section 4) describes the estimation of seismic hazardlevels assuming a Poisson distribution of seismicity. Paper II (Section 5)presents a more detailed description of the tectonic setting of the NorthAmerica-Caribbean plate boundary and the techniques used to analyserelevant earthquakes, followed by a description of the seismic slip modellingand estimation of the seismic coupling coefficient. Paper III (Sections 3 and6) uses newly available data to study seismicity in the region and analysesglobal plate motion and estimated convergence rates from earthquakes.Paper IV (Section 7) describes major earthquakes along the North America-Caribbean plate boundary, continues with a more detailed discussion of theinteraction between earthquakes and concludes with an analysis of therelevance of results to the tectonics of the region. Finally, Paper V (Section7) presents the relationship between a large earthquake and subsequentaftershocks by means of the Coulomb failure stress changes.

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2. Tectonic Settings

North Central America is a tectonically complex area. It is locatedbetween the Caribbean, North American and Cocos plates. The latter issubducting under the Caribbean plate along the Central American andMexican Pacific coasts (Figure 1). The relative motion and interactionamong the three tectonic plates constantly accumulates stress along theirboundaries and several faults in between them. Northern Central Americaconsists of the Maya block to the north and the Chortís block to the south(Figure 1).

Figure 1. Tectonic settings of northern Central America. HD= Honduras depression,HB= Honduras borderland faults, ND= Nicaragua depression.

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The internal deformation of these blocks is controlled by the well-defined,seismically active, North America-Caribbean and Cocos-Caribbean plateboundaries. Gordon (1990) suggests that much of the Chortís block is part ofa diffuse North America-Caribbean plate boundary zone, although, much ofthe deformation is associated with movements along the plate boundaries.The subduction of the Cocos plate beneath the Caribbean plate produces theMiddle America trench. DeMets (2001) proposes that the right-lateral strike-slip motion along the Central America volcanic depression is due to theobliqueness of the subduction along the Middle America trench axis. Thevolcanic depression takes the name of Nicaragua depression on the south ofthe region (Figure 1). The Motagua and the Chixoy-Polochic fault systems,along with the Swan transform fault, make up the major boundary betweenthe North American and Caribbean plates. This is characterised by a sinistralstrike-slip motion and extends from western Guatemala to the Caymanspreading centre, including the Cayman trough, a pull-apart basin within theCaribbean Sea. The possible existence of a triple junction connecting theMotagua fault system with the Middle America trench is still a matter ofdebate. Minor strike-slip faults together with a series of basins may serve asan extended plate boundary zone, which may take up part of the stressproduced along the major plate boundaries.

Several tectonic structures belong to the intraplate provinces in the region.The deformation processes on these structures are closely related to that ofthe plate boundaries. As suggested by Gordon (1990), the Hondurasdepression may serve as an extended plate boundary where the NorthAmerica-Caribbean plate boundary exerts major influence. The depression iscomposed of several grabens that run from the Caribbean coasts to thePacific in Honduras (Guzmán-Speziale, 2001). Sub-parallel to the Swantransform fault there are a number of faults know collectively as theHonduras borderland faults (Rogers et al., 2002). These faults are active, asit can be seen from seismicity maps (Figure 2) and are evident already fromthe topography map (Figure 1). Finally, the extensive Guayape fault systemcrosses over Central America with a proposed right lateral motion (Gordonand Muehlberger, 1994).

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3. Seismicity

3.1. Seismic dataNorthern Central America is one of the most seismically active regions in

the world. Most of the earthquake activity concentrates along the Caribbean-Cocos and Caribbean-North America plate boundaries (Figure 2). Manyhistorical earthquakes in this region reach magnitudes around 8, like those of1902 and 1942 in the Middle America trench, the 1816 event on the Polochicfault in Guatemala, and the shock of 1856 on the Swan transform fault. Morerecently, the February 2, 1976 earthquake on the Motagua fault, Mw= 7.6,the September 2, 1992, Mw= 7.5, event off the Pacific coast of centralNicaragua, and the earthquake of January 13, 2001, Mw= 7.6, off the coast ofEl Salvador, have caused great loss of life and had great negative economicimpact on Central America. Earthquakes also take place in the interplateprovinces, e.g. the shock of December 1915, MS=6.4, in western Honduras,and the shock located in the volcanic arc, magnitude MS= 6.5, which resultedin destruction of parts of the city of Managua, Nicaragua, 1972.

In order to study the seismicity in northern Central America, we compileda list of earthquake hypocenter parameters covering the period 1900-2002.This set can be subdivided into three periods. 1900 to 1963, which is prior tothe establishment of the WWSSN (Worldwide Standardized SeismographNetwork), is compiled from the lists presented by Ambrasseys and Adams(2001), Rojas et al (1993) and White and Harlow (1993) for CentralAmerica. Data for the period 1963-1999 was selected from the extendedcatalogue of Engdahl et al. (1998). From the National EarthquakeInformation Centre (NEIC) and the International Seismological Centre, weextended the catalogue to the period 1999-2002. Our hypocentre catalogueincludes 3480 earthquakes in total.

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Figure 2. Seismicity for the period 1900-2002, magnitudes larger than 5

Also used is the catalogue of earthquake centroid-moment tensorsolutions (H-CMT, Dziewonski et al., 1981). For northern Central Americait starts in 1976 and is currently available up to 2002 (Figure 3). It consistsof hypocentre parameters, fault plane solutions, moment magnitudes andscalar seismic moments for moderate (Mw=5) to large earthquakes. The H-CMT catalogue covers approximately the last 30 years of seismicity whilethe hypocenter catalogue alone contains information about events that haveoccurred since 1900. Event parameters may be less precise as we go back intime from 1964, due to quality of the seismic instrumentation. Nevertheless,the hypocentre catalogue represents the best available information for aperiod that covers about 3/4 of the time span of the available data.

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Figure 3. Selected H-CMT fault plane solutions in Northern Central America

3.2. Seismicity distributionThe distribution of seismicity is not spatially uniform. High levels of

seismicity along the plate boundaries contrast with the low and scatteredlevels in intraplate provinces. Nevertheless, perusal of the distribution ofevent epicentres reveals a good correlation with the tectonic structuresalready visible from the topographic and bathymetrical map (Figure 1).

Epicentres can be classified spatially into interplate, i.e. epicentresassociated with the Middle America trench and the transcurent boundarybetween the North America and Caribbean plates, and intraplate earthquakes

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Figure 4. Hypocentre cross-sections throughout northern Central America

located e.g. in the Honduras Depression, Guayape and along the volcanicchain (Figure 2). A number of profiles in the region under review (Figure 4)show that events range in depths from 5 km to nearly 300 km. Highconcentrations are observed at about 33 km depth (Figure 5), which is,however an artefact originating from location techniques applied. There is anoticeable increase in the dipping angle of the subduction zone towards thesouth. The change is smooth, from Guatemala-El Salvador (Profiles A-A’and B-B’, Figure 4) via an intermediate angle at profile D-D’ to a steeperangle in the area off coast of Nicaragua.

3.3. Summary of seismic informationFigures 5, 6 and 7 present respectively the number of earthquakes as a

function of focal depth, magnitude vs. time of occurrence and number ofevents vs. magnitude, for the compiled earthquake hypocenter catalogue for

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the region outlined in Figure 2. Figure 6 represents the evolution of theseismicity through time while Figure 7 shows the number of earthquakeswith respect to magnitude. From Figure 6 and Figure 7, we see that thecatalogue can be considered as complete for magnitudes around 4.0 andlarger from 1964 to 2002. Within the study area, the frequency of eventslarger than Ms=7.0 is about one each 5 years.

Figure 5. Number of earthquakes vs. focal depth

Using the equations derived by Ekström and Dziewonsky (1988) weconverted MS magnitudes into seismic moment. We see that the number ofearthquakes with magnitude less than MS=5.5 is about 3100 (89% of thetotal number), summing up a moment release of 3.75x1019 Nm, while thenumber of earthquakes with MS ≥ 5.5 totals 379 events corresponding to8.09x1021 Nm of moment release. This implies that 11% of the eventsreleased 99.5% of the total seismic moment in the area during the observed102 years.

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Figure 6. Magnitude vs. time

Figure 7. Number of events vs. magnitude

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4. Probabilistic Seismic Hazard

4.1. IntroductionTo investigate the seismic hazard for any given point in an area, we need

to estimate the ground motions likely to be caused by a nearby earthquake. Ifwe make use of statistics, we obtain a probabilistic seismic hazard analysis(PSHA) for the site (or a number of sites) of interest. It is assumed that oncewe know the past seismicity of an area, through statistical laws we canforesee how the seismicity will behave in the future. As soon as we establishthe rules of the seismicity, we can quantify the hazard levels as probabilitiesof exceeding specified ground motion levels for specific periods of time. It ispresupposed that the knowledge of the seismic history is adequate. Thetheoretical basis and applicability to “real-world” scenarios is due to Cornell(1968) and many workers have introduced various modifications since then.The PSHA requires a demarcation of sources of seismic activity. This willbecome the seismogenic source model. As a second step, it is necessary toestimate how seismic energy dissipates with respect to earthquakemagnitudes and hypocentral distances for earthquake-site pairs of interest.The final step is the calculation of the levels of hazard. Results can bequantified in several units (acceleration, velocity, displacement); we havechosen the peak ground acceleration (PGA), which traditionally has beenused.

4.2. Seismogenic zonesTo demarcate a seismogenic source zone, it is assumed that all

earthquakes have equal and independent probability of occurrence within thezone. The set of zones, at the same time, are considered independent of eachother. Furthermore, it is assumed that all seismicity within a zone has the

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same tectonic origin. Depending on the scale, one can define zoning for, e.g.large regions, or at a small scale, e.g. for cities (micro zoning).Unfortunately, there are no general rules how to delineate source zones.Personal judgment sways the final selection of the source model and is oneof the primary reasons for differences in estimated hazard levels in differentstudies. In short, a source model is a collection of seismic sources withcorresponding geographical location and activity rates (e.g., in terms offrequencies and magnitude levels). Together with the source zone definitionscome seismic source parameters (b-value, maximum magnitude, thresholdmagnitude and activity rate), which are obtained through the statisticaltreatment of seismic catalogues. For each source zone, we compile a sub-catalogue of earthquakes that occurred within the zone. Then, we estimatefor each source zone the probability of occurrence of future earthquakes witha given magnitude and within a given time period

4.3. Attenuation relationships and uncertaintiesIn PSHA the rate at which seismic energy decays, as a function of

distance from the source and source magnitude is specific for each region isand known as an attenuation relationship. In the relationship otherparameters can be included to bring the relationship closer to reality (e.g. sitespecifics and tectonic style of the sources). However, a large collection ofdata is required and for some areas this is not feasible. Therefore, the analystmust select a relationship concordant with the region of study. This selectionis a key for any seismic hazard study.

To model the uncertainties of parameters used in the PSHA, we make useof a logic tree. With this approach, the problem is partitioned into lesscomplex elements so that each of them can be studied in detail, but at thesame time the view of the whole problem is not lost. Logic trees provide aconvenient, flexible, and powerful means of incorporating the uncertainty inmodelling future seismicity in intraplate regions, explicitly in the estimate ofthe future hazard (Coppersmith and Youngs, 1986). Our logic tree includesdifferent zonations, attenuations relationships and minimum and maximummagnitudes. At the end is the judgment of the analyst regarding which of theoutcomes of the logic tree is the most appropriate to the studied region.

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4.4. Maps of hazard levelsFor the area of Honduras (Paper I), and after considering all parameters

involved in the PSHA, we performed hazard computations on a grid with0.1° spacing. Figure 8 presents spatial trends of expected PGA valuesobtained from the analysis, reflecting the influence of the seismogenicsource zones on the outcome. The highest expected acceleration for the 100-year return period is observed in and near areas of seismic sources with ahigh rate of seismic activity, especially along the subduction zone.

Figure 8. Seismic hazard map of Honduras for a return period of 100 years (0.4probability of exceedance in 50 years)

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5. Fault Geometry

5.1. Introduction

In the estimation of seismic hazard, the contribution of all seismic sourcesis taken into account. A key parameter in the PSHA is the maximummagnitude a fault can generate. The maximum magnitude is controlled bythe geometry (length, width, dipping angle of the plane) of the fault. Anappropriate description of the location of the seismogenic source, style offaulting (e.g. strike-slip, normal or thrust) and its geometry is, therefore,fundamental. The depth of faulting is one of the parameters that involveslarge uncertainties, while the style of faulting, related to the radiationpattern, affects the ground motion expected at different distances andazimuths from a fault. Our goal is to estimate the geometry of the faults inthe region, the predominant faulting process and to identify characteristicfault segments through the seismic moment release. In the present thesis, westudied in detail the Swan transform fault (Paper II).

5.2. Depth of faultingFrom different profiles of seismicity (hypocenter catalogue) throughout

the study area (Figure 4), we can distinguish a characteristic feature, namelythe clustering of events at about 33-km depth. These are depth-constrainedhypocentres and can be seen more clearly in the histogram of event focaldepths (Figure 5). With regards to the H-CMT catalogue, the clustering isaround the 10-15 kilometres depth range. This is ascribed to the conventionthat focal depth in the H-CMT method is constrained to depths of 10 km orgreater (Dziewonski et al., 1983). Hence, the clustering in both catalogues isan artefact of the procedures used to locate earthquakes.

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We should also be aware of the fact that the depth of faulting is related tothe rheology of the source, i.e. to temperature. In some cases, we have focaldepth determinations larger than 30 km on crustal faults. Heat flow datafrom several faults show that the occurrence of earthquakes is controlled bythe 400°C-600°C isotherms, constraining the depth of faulting to be, e.g. lessthan 20-30 km in non-subduction faults. Therefore, a more accurateestimation of hypocentral depth is required. One possibility is to useteleseismic body-waveform modelling since waveforms are more sensitiveto focal depth than travel time data. With this tool, we determined that thedepth of faulting on the Swan transform fault is limited to 20 km (Paper II).In Figure 9 we have related the hypocentral depth of earthquakes,determined with the body waveform inversion technique, with a simplecooling plate model for the Swan transform fault. We can see thathypocenters are bounded by the 400 isotherm.

Figure 9. Hypocenters of earthquakes and isotherms along the Swan transform fault

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5.3. Seismic moment releaseThe general pattern of seismicity in the studied area (Figure 2) indicates

parts where deformation is taking place, it reveals that crustal blocks are inmotion. Unfortunately, a seismicity map does not provide more informationthan the epicentre locations and magnitudes of earthquakes that took place inthe area. It is possible to study how a region is being seismically deformedby means of images of the scalar seismic moment release (Figure 10). Thescalar seismic moment represents the size of an earthquake. At the same timeit is proportional to the deformation of the area where the event takes place.In Paper II, we examined the Swan transform fault for a 92-year period andestimated that the fault has a deficit in seismic moment release of the orderof 40 percent when a maximum earthquakes is assumed as Mw=7.7. On theother hand, for a 146-year period the fault presents no deficiency.

Figure 10. Seismic moment release for the period 1900-2002

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6. Plate Motion and Seismic DeformationRates

6.1 IntroductionThe activity rate of earthquakes on a fault is proportional to the slip rate

on a fault, i.e. to the speed of one block moving with respect to the other.The slip rates can be used to constrain the activity rate, and are therefore ofimportance in hazard investigations. Slip rates can be obtained from platemotion models, however these values are estimated for plate boundaries ingeneral. From Figure 1 and Figure 2, we can find tectonic structures forwhich seismic activity is low or non-existing. The Honduras depression,Honduras borderland faults and the Guayape fault system are examples ofthis situation. Therefore, estimation of slip rates for such structures willimprove the estimation of the PSHA for this region.

6.2. Rate of deformation from earthquakesThe overall velocities of deformation across a zone of distributed

deformation are related to the seismic moment that occurs within it (Jacksonand McKenzie, 1988). The resulting velocity across the zone can becompared (in the case of simple deformation) with the estimation ofvelocities from plate motions. The basis for the method is due to Kostrov(1974) and was modified by Papazachos and Kiratzi (1992). It relates thetotal seismic strain rate tensor to the annual seismic moment release ratecontained in the crustal volume of interest. The region of interest is dividedin tectonic volumes with uniform tectonic characteristics (i.e. similar focalmechanisms of earthquakes) to compute the velocity across the zone. Toestimate the seismic moment rate it is required to define a maximummagnitude and therefore a maximum moment released within the volume.Through applying the Gutenberg-Richter relationship to each volume, wecan obtain the tensor of seismic moment release rate and then we can deduce

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the velocity for each volume within the area of interest. In Paper IV, velocityfor the zones depicted in Figure 11 were estimated. The velocities across thedifferent zones along the Middle America trench, the volcanic chain and theNorth America-Caribbean plate boundaries show a good correlation withvelocities from global plate motion models (DeMets et al., 2000 andDeMets, 2001). In Paper IV we also estimated the velocity of deformationfor the Honduras depression (5mm/yr), that correlates well with previousstudies (Guzmán-Speziale, 2001). As a complement, we present velocitiesfor the Honduras borderland faults (2mm/yr) that correspond to observedgeological offsets (Rogers, personal communication 2002) and the velocityof the Guayape fault system (2mm/yr) from which there are no publishedvalues. Velocities for all the volumes contained in northern Central Americaare presented in Figure 11.

Figure 11. Deformation velocities (in mm/yr) for different volumes in northernCentral America (values in circles). Polygons represent crustal volumes. Blackarrows denote compression, gray arrows extension.

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7. Earthquake Triggering

7.1. IntroductionIn the Poisson model of seismicity for estimating PSHA, it is assumed

that there is a seismic cycle in which the level of stress rises to a thresholdvalue prior to the largest earthquake within a zone. As stated earlier, theseismogenic sources are supposed to be independent of each other. However,earthquakes like those of 1989 in Loma Prieta, 1992 in Landers, 1995 inKobe and 2001 in Istmitz present all a common characteristic, namelycorrelation of changes in the Coulomb stress and levels of seismicity. Inareas where the Coulomb stress change (dCFS) is positive, the seismicityrate increases and where the changes are negative, the seismicity ratedecreases. This suggests that the interaction between faults should not beneglected.

7.2. Interaction between earthquakesThe stress interaction method, as described in Stein et al. [1992] and King

et al. [1994] calculates static stress changes, by assuming that faults areplanar dislocation surfaces immersed in an elastic half space. These changesare then rotated to a system that has orthogonal axes along the slip directionof the fault and its normal (Hodgkinson et al, 1996). In Figure 12, dCFS arepresented for the period 1900 to 1977 using earthquakes with magnitudelarger than 6 along the plate boundaries of northern Central America (PaperIV). If a fault is within a zone of positive changes in dCFS, it is assumed thatthis fault has been brought closer to failure. Likewise, decrease in dCFSindicates that the fault failure may have been delayed. For the sequence ofaftershocks of the February 1976, Mw=7.6, earthquake, 88% took place onregions of positive increase of dCFS. As follows from Figure 12, the eventof August 1980, Mw=6.5, is located on a zone of positive dCFS, and theApril 1982, Mw=6.3, shock is located on a low positive dCFS zone.

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Figure 12. A: Earthquakes (filled circles) larger than Mw=6 for the period 1900-1977. B: Calculated Coulomb stress changes (dCFS) on optimally oriented strikeslip faults. The pre-existing dCFS were modified by the February 1976, Motagua,earthquake (star). Later earthquakes are filled hexagons. Filled circles represent theaftershock sequence that accompanied the 1976 shock. Contour values are given infractions of 105Pa.

Based on the available data, we believe that the evolution of the staticstress field studied here yields a good correlation of dCFS with theaftershocks and background seismic activity (Paper IV). We applied theconcept of changes of Coulomb stresses to a particular event, the July 11,1999, Mw=6.7, earthquake in the Gulf of Honduras. We correlated thechanges of dCFS caused by this earthquake with aftershocks that followed. It77% of the aftershocks fall on areas of positive increase in dCFS of at least0.1 bar (Paper V). The application of results from calculations of dCFS onseismic hazard is still in debate, especially with regards to the knowledge ofhow close major faults are to failure, limiting its use in predicting the timingof large earthquakes (Stein et al., 1994). Nevertheless, it is known that theoccurrence of a large earthquake changes the condition of failure on otherfaults in its vicinity, altering the probabilities of occurrence of future events.This result may be used to show where possible earthquakes may take place.

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8. Summary

The focus of this study was firstly to evaluate the Probabilistic SeismicHazard (PSH) for Honduras and secondly to refine fault geometries andassociated parameters for seismogenic sources in northern Central Americacausing deformation, that could lead to an improved estimation of PSH forthe region.

As in most cases, the deformation is not localised to plate boundaries butto large belts of deformation within tectonic plates. Particularities includelocking at plate boundary interfaces resulting in large earthquakes (MiddleAmerica trench, Motagua fault), aseismic slip estimated through momentdeficits (Swan transform fault) and thickening and rotation in some regions(Honduras depression) within the plates. Some of these characteristics arewell reflected on a PSH map, like the high expected acceleration valuesalong the Middle America trench and Motagua-Polochic fault systems.

Part of this study was dedicated to estimating the kinematics of faults.The seismic coupling fraction, the portion of the total plate motion that couldlead to earthquakes was computed for the Swan transform fault. We usedrecent and historical earthquakes through direct summation of scalar seismicmoments to estimate the coupling fraction leading to a conclusion that 40%of the motion is through stable sliding.

The deformation pattern obtained through seismic moment tensors innorth Central America presents a close resemblance to that obtained throughglobal plate motion estimates with respect to orientation and amount. Thegenerally low discrepancies between seismic and plate motion data maysuggests that the aseismic deformation along plate boundaries is not largebut at the same time large earthquakes are required. With this approachdeformation rates for faults for which previous estimations were non existing(Guayape and Honduras borderland faults) can be estimated.

These results have implications for the calculation of seismic hazard. Apriori assumptions about the geometry of the faults, for both, the estimationof the seismic coupling coefficient (using scalar seismic moments) or the

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deformation rates (using seismic moment tensors) involve uncertainties thataffect the results. Therefore, a better constraint on the geometry waspursued. The coverage of the seismic cycle is a limitation on the hypocentralcatalogue because of the lack of data from the pre-instrumental period. Atthe same time, earthquakes are not isolated from each other, and theinteraction between areas of increased or reduced stress may modify theestimations of seismic hazard.

We believe that, from the results obtained here, PSHA will be betterestimated taking into account that the definition of seismic sources andrelated parameters are better constrained. There are many areas of potentialfuture work to further investigate the deformation processes in northernCentral America. New seismic stations and Geographical Positioning System(GPS) sites would result in new models for crustal deformation and betterconstrains on the motion of the Caribbean plate in general.

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9. Summary of papers

I Cáceres D. and Kulhánek O. (2000). Seismic Hazard of Honduras.Natural Hazards 22(1): 49-69

In this paper we have described the procedures used, input data applied andresults achieved in our efforts to develop seismic hazard maps of Honduras. Theprobabilistic methodology of Cornell is employed. Numerical calculations werecarried out by making use of the computer code SEISRISK III. To examine theimpact of uncertainties in seismic and structural characteristics, the logic treeformalism has been used. We compiled a de-clustered earthquake catalogue for theregion comprising 1919 earthquakes occurring during the period from 1963 to 1997.Unified moment magnitudes were introduced. Definition of a seismotectonic modelof the whole region under review, based on geologic, tectonic and seismicinformation, led to the definition of seven seismogenetic zones for which seismiccharacteristics were determined. Four different attenuation models were considered.Results are expressed in a series of maps of expected PGA for 60% and 90%probabilities of non-exceedence in a 50-year interval which corresponds to returnperiods of 100 and 475 years, respectively. The highest PGA values of about 0.4g(90% probability of non-exceedence) are expected along the borders with Guatemalaand El Salvador.

II Cáceres D. and Arvidsson R. Seismic Properties of the Swantransform fault, Caribbean Sea. Journal of Seismology(Submitted)

In this paper we have compiled a series of seismic properties of the Swantransform fault, Caribbean Sea. Estimations of the focal mechanisms and centroiddepths for 9 of the largest earthquakes of the transform fault, for the period 1980-1999, are presented along with their corresponding estimated rupture area, averageslip and stress drop. A strike-slip mechanism is predominant on the events studiedhere in good agreement with the expected mechanisms on transform faultearthquakes; centroid depths of all 9 events range from 7 to 15 km. We haveestimated the maximum depth of seismic faulting from 20 to 25 km (from inversionof body waveforms). The results fit well with a simple thermal model for transformfaults, corresponding to the 600° isotherm, as well as the slip distribution of the

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largest earthquake in the studied period, that limits the faulting to 25 km. We havealso calculated the crustal stress orientation from focal mechanism along thetransform, indicating that the direction of the maximum component of stress is aboutN30E. We complemented a catalogue of recent seismicity with historical seismicityfor the transform fault and estimated that the seismic coupling coefficient is of about10 % for a period of 92 years.

III Cáceres D., Monterroso D. and Tavakoli B. Seismic Activedeformation in northern Central America. Tectonophysics(Submitted)

We have made use of the seismic moment tensor for earthquakes on plateboundary as a standard procedure to determine the relative velocity of plates, whichcontrol the seismic deformation rate predicted from the slip on a single fault. Themoment tensor is also decomposed into an isotropic and a deviatoric part to discoverthe relationship between the average strain rate and the relative velocity betweentwo plates. We utilize this procedure to estimate the rates of deformation in northernCentral America where plate boundaries are seismically well defined. Four differenttectonic environments are considered for modelling of the plate motions. Theresulting deformation rates estimated compare well with those determined from thepredicted velocities from the global plate motion model and gravity data. The ratesobtained from the model application are in good agreement with actual observations.Deformation rates on faults are increasingly being used to estimate earthquakerecurrence from information on a fault slip rate and more on how we can incorporateour current understanding into seismic hazard analyses.

IV Cáceres D. and Arvidsson R. Static stress transfer along thewestern margin of the North America-Caribbean plate boundary.Geophysical Research Letters (Submitted)

We investigate the Coulomb stress interaction for earthquakes from the NorthAmerican-Caribbean (NOAM-CARIB) plate boundary for the period 1976-1999. Aseries of maps containing the variation in stress estimated by means of the Coulombfailure criterion (CFC) for each earthquake is presented. A correlation betweenzones of stress increase and epicenters of earthquakes of the sequence is visible in 3out of 6 of the shocks, in the sense that each event is related to its predecessor.Analyzing the final state of stress in the sequence is possible to identify faultsegments where the next rupture is most likely to take place.

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V Cáceres D. Coulomb stress changes and the aftershock sequenceof the July 11, 1999 earthquake in the Gulf of Honduras,Caribbean Sea. (Manuscript)

Coulomb failure stress changes (dCFS) caused by the July 11, 1999, Gulfof Honduras, Caribbean Sea, earthquake are calculated. The trend anddipping angle of the relocated aftershock sequence show a good agreementwith the fault plane solution available for the main event as well as a goodcorrelation with the trend of the major Polochic fault that crosses the area.The correlation of aftershock locations with Coulomb stress changes,resolved for optimally oriented fault planes caused by the main shock showsthat 77 % of the aftershocks fall into regions where the dCFS is positive.

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10. Acknowledgements

I would like to thank the following people for helping me throughout theresearch process in Uppsala, without whom I wouldn't have reached thisstage; to Ota Kulhánek, Federico Güendel, Ronald Arvidsson and RolandRoberts for their helpful and interesting comments and suggestions atvarious stages of development. During my studies, I received grants from:the projects Seismotectonic Regionalization of Central America (SERCA)and Natural Disaster Mitigation in Central America (NADIMCA) bothsupported by the Swedish International Development Agency (SIDA-SAREC). I also thank for the fellowship received from the Department ofEarth Sciences, Uppsala University and the leave of absence permit from theDepartment of Physics of the University of Honduras (UNAH), to continuemy doctoral studies. I thank my colleagues and friends from the Departmentfor their friendship, critique, enlightenment and inspiration. I also thank JensHavskov, Kuvvet Atakan and Robert Rogers for their valuable contributionin the development of this work. I appreciate the help from innumerablepeople I do not mention here for any reasons.

My thanks go to my family for immense moral support, keen insight,endless advice incredible patience and love. My non-interested in sciencefriends are also acknowledged.

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11. References

Ambraseys, N.N. and Adams, R.D., 2001. The Seismicity of CentralAmerica: A descriptive catalogue 1898-1995. Imperial College Press,London, 309pp.

Coppersmith, K. J. and Youngs, R. R., 1986. Capturing uncertainty inprobabilistic seismic hazard assessments within intraplate tectonicenvironments. Proceedings of the Third U.S. National Conference onEarthquake Engineering. August 24 - 28, 1986, Charleston, South Carolina

Cornell, C. A., 1968. Engineering seismic risk analysis. Bull. Seismol.Soc. Am., 58, 1583-1606.

DeMets, C., Jansma, P.E., Mattioli, G.S., Dixon, T.H., Farina, F.,Bilham, R., Calais, E., Mann, P., 2000. GPS geodetic constraints onCaribbean-North America plate motion. Geophys. Res. Lett., 27, 437-440.

DeMets, C., 2001. A new estimate for present-day Cocos-Caribbean platemotion: Implications for slip along the Central American volcanic arc.Geophys. Res. Lett., 28, 21, 4043-4046.

Dziewonski, A.M., Chou, T.A. and Woodhouse, J.H., 1981.Determination of earthquake source parameters from waveform data forstudies of global and regional seismicity. J. Geophys. Res., 86, 2825-2852.

Dziewonski, A.M., Friedman, A., Giardini, D. and Woodhouse, J.H.,1983. Global seismicity of 1982, Centroid moment tensor solutions for 308earthquakes. Phys. Earth Planet. Inter., 33, 76-90.

Ekström, G. and Dziewonski, A.M., 1988. Evidence of bias inestimation of earthquake size. Nature, 332, 319-323.

Engdahl, E. R., van der Hilst, R and Buland R., 1998. Globalteleseismic earthquake relocation with improved travel times, and proceduresfor depth determination. Bull. Seismol. Soc. Am., 88, 722-743.

Gordon, M.B., 1990. Neogene tectonics of the Chortís block: A widezone of deformation responding to interaction between the North Americaand Caribbean plates. In: Strike slip faulting and basin formation at theGuayape fault, Valle de Catacamas intersection, Honduras, Central America,Doctoral Thesis, Graduate School of the University of Texas, Austin.

Gordon, M. and Muehlberger, W., 1994. Rotation of the Chortís blockcauses dextral slip on the Guayape fault. Tectonics, 13, 858-872.

Guzmán-Speziale, M., 2001, Active seismic deformation in the grabensof northern Central America and its relationship to the relative motion of theNorth America-Caribbean plate boundary. Tectonophysics, 337, 39-51.

Hodgkinson, M.K., Stein, R.S. and King, G.C.P., 1996. The 1954Rainbow Mountain-Fairview Peak-Dixie valley earthquakes: A triggerednormal faulting sequence. J. Geophys. Res., 101, 25459-25471.

28

International Seismological Centre, 2001, On-line Bulletin,http://www.isc.ac.uk/Bull, Int. Seis. Cent., Thatcham, United Kingdom.

Jackson, J. and McKenzie, D., 1988. The relationship between platemotions and seismic moment tensors, and the rates of active deformation inthe Mediterranean and Middle East. Geophys. J. Int., 93, 45-73.

King, G.C.P., Stein, R.S. and Lin, J., 1994. Static stress triggering ofearthquakes. Bull. Seismol. Soc. Am., 84, 3, 935-953.

Kostrov, V.V., 1974. Seismic moment and energy of earthquakes andseismic flow of rock. Izv., Earth Physics, 1, 23-40.

Papazachos, C.B. and Kiratzi, A.A., 1992. A formulation for reliableestimation of active crustal deformation and its application to central Greece.Geophys. J. Int., 111, 424-432.

Rogers, R.D., Kárasson, H. and van der Hilst, R.D., 2002. Epirogenicuplift above a detached slab in northern Central America. Geology, 30, 11,1031-1034.

Rojas, W., Bungum, H. and Lindholm, C., 1993. Historical and recentearthquakes in Central America. Rev. Geol. Amér. Central, 16, 5-22.

Stein, R.S., King, G.C.P. and Lin, J., 1992. Change in failure stress inthe southern San Andreas fault system caused by the 1992 magnitude=7.4Landers earthquake. Science, 258, 1328-1332.

White, R.A. and Harlow, D.H., 1993. Destructive upper-crustalearthquakes in Central America since 1900. Bull. Seismol. Soc. Am., 83,1115-1142.