Comprehensive Summaries of Uppsala Dissertations from the Faculty of Science and Technology 834

Earthquake Sources and Hazard in Northern Central America

BY José Diego Cáceres Calix

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

I Cáceres D. and Kulhánek O. (2000). Seismic Hazard of Honduras. Natural Hazards 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 in northern Central America. Tectonophysics (Submitted)

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

V Cáceres Diego. Coulomb stress changes and the aftershock sequence of the July 11, 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 not included in this Thesis:

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

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

1. Introduction...... 1 2. Tectonic Settings ...... 3 3. Seismicity...... 5 3.1. Seismic data ...... 5 3.2. Seismicity distribution...... 7 3.3. Summary of seismic information...... 8 4. Probabilistic Seismic Hazard ...... 11 4.1. Introduction...... 11 4.2. Seismogenic zones...... 11 4.3. Attenuation relationships and uncertainties...... 12 4.4. Maps of hazard levels...... 13 5. Fault Geometry ...... 14 5.1. Introduction...... 14 5.2. Depth of faulting...... 14 5.3. Seismic moment release ...... 16 6. Plate Motion and Seismic Deformation Rates...... 17 6.1 Introduction...... 17 6.2. Rate of deformation from earthquakes...... 17 7. Earthquake Triggering...... 19 7.1. Introduction...... 19 7.2. Interaction between earthquakes...... 19 8. Summary...... 21 9. Summary of papers...... 23 10. Acknowledgements ...... 26 11. References...... 27 1. Introduction

Northern Central America, our area of study, is located in the northwestern corner of the Caribbean plate and represents a tectonically complex area. It is embedded between the Caribbean and North American plates and is bounded by the Cocos plate to the southwest. A cursory inspection of earthquake activity maps reveals high seismicity in the area. According to historical records, large earthquakes, reaching magnitudes up to M=8 have occurred in this region. In the last 25 years, 3 earthquakes with magnitude larger than 7 have struck northern Central America, causing great loss in terms of lives and economy.

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

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

Fault segments with a deficit in seismic moment release, determined through a detailed analysis of the seismic coupling coefficient along plate interfaces, help us to understand active tectonic processes. The seismic coupling coefficient is the ratio of the seismic moment release rate to the expected seismic moment estimated from plate tectonic convergence rates. The coefficient indicates the proportion of slip represented by earthquakes and hence the coupling along a given section.

1 The rationale of this thesis is firstly to analyse seismological, geological and tectonic information available for northern Central America in order to evaluate the seismic hazard for the region from a statistical point of view and secondly to improve the model of seismogenic sources complemented with physical properties e.g. depth of faulting, seismic coupling and the effects of previous 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 is presented. Paper I (Section 4) describes the estimation of seismic hazard levels assuming a Poisson distribution of seismicity. Paper II (Section 5) presents a more detailed description of the tectonic setting of the North America-Caribbean plate boundary and the techniques used to analyse relevant earthquakes, followed by a description of the seismic slip modelling and estimation of the seismic coupling coefficient. Paper III (Sections 3 and 6) uses newly available data to study seismicity in the region and analyses global 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 the interaction between earthquakes and concludes with an analysis of the relevance of results to the tectonics of the region. Finally, Paper V (Section 7) presents the relationship between a large earthquake and subsequent aftershocks by means of the Coulomb failure stress changes.

2 2. Tectonic Settings

North Central America is a tectonically complex area. It is located between the Caribbean, North American and Cocos plates. The latter is subducting under the Caribbean plate along the Central American and Mexican Pacific coasts (Figure 1). The relative motion and interaction among the three tectonic plates constantly accumulates stress along their boundaries and several faults in between them. Northern Central America consists 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.

3 The internal deformation of these blocks is controlled by the well-defined, seismically active, North America-Caribbean and Cocos-Caribbean plate boundaries. Gordon (1990) suggests that much of the Chortís block is part of a diffuse North America-Caribbean plate boundary zone, although, much of the deformation is associated with movements along the plate boundaries. The subduction of the Cocos plate beneath the Caribbean plate produces the Middle America trench. DeMets (2001) proposes that the right-lateral strike- slip motion along the Central America volcanic depression is due to the obliqueness of the subduction along the Middle America trench axis. The volcanic depression takes the name of Nicaragua depression on the south of the region (Figure 1). The Motagua and the Chixoy-Polochic fault systems, along with the Swan transform fault, make up the major boundary between the North American and Caribbean plates. This is characterised by a sinistral strike-slip motion and extends from western Guatemala to the Cayman spreading centre, including the Cayman trough, a pull-apart basin within the Caribbean Sea. The possible existence of a triple junction connecting the Motagua fault system with the Middle America trench is still a matter of debate. Minor strike-slip faults together with a series of basins may serve as an extended plate boundary zone, which may take up part of the stress produced 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 of the plate boundaries. As suggested by Gordon (1990), the Honduras depression may serve as an extended plate boundary where the North America-Caribbean plate boundary exerts major influence. The depression is composed of several grabens that run from the Caribbean coasts to the Pacific in Honduras (Guzmán-Speziale, 2001). Sub-parallel to the Swan transform fault there are a number of faults know collectively as the Honduras borderland faults (Rogers et al., 2002). These faults are active, as it can be seen from seismicity maps (Figure 2) and are evident already from the topography map (Figure 1). Finally, the extensive Guayape fault system crosses over Central America with a proposed right lateral motion (Gordon and Muehlberger, 1994).

4 3. Seismicity

3.1. Seismic data Northern 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). Many historical earthquakes in this region reach magnitudes around 8, like those of 1902 and 1942 in the Middle America trench, the 1816 event on the Polochic fault in Guatemala, and the shock of 1856 on the Swan transform fault. More recently, 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 central Nicaragua, and the earthquake of January 13, 2001, Mw= 7.6, off the coast of El Salvador, have caused great loss of life and had great negative economic impact on Central America. Earthquakes also take place in the interplate provinces, 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 resulted in destruction of parts of the city of Managua, Nicaragua, 1972.

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

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

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

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

3.2. Seismicity distribution The distribution of seismicity is not spatially uniform. High levels of seismicity along the plate boundaries contrast with the low and scattered levels in intraplate provinces. Nevertheless, perusal of the distribution of event epicentres reveals a good correlation with the tectonic structures already visible from the topographic and bathymetrical map (Figure 1).

Epicentres can be classified spatially into interplate, i.e. epicentres associated with the Middle America trench and the transcurent boundary between the North America and Caribbean plates, and intraplate earthquakes

7 Figure 4. Hypocentre cross-sections throughout northern Central America

located e.g. in the Honduras Depression, Guayape and along the volcanic chain (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. High concentrations are observed at about 33 km depth (Figure 5), which is, however an artefact originating from location techniques applied. There is a noticeable increase in the dipping angle of the subduction zone towards the south. 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 steeper angle in the area off coast of Nicaragua.

3.3. Summary of seismic information Figures 5, 6 and 7 present respectively the number of earthquakes as a function of focal depth, magnitude vs. time of occurrence and number of events vs. magnitude, for the compiled earthquake hypocenter catalogue for

8 the region outlined in Figure 2. Figure 6 represents the evolution of the seismicity through time while Figure 7 shows the number of earthquakes with respect to magnitude. From Figure 6 and Figure 7, we see that the catalogue can be considered as complete for magnitudes around 4.0 and larger from 1964 to 2002. Within the study area, the frequency of events larger 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) we converted MS magnitudes into seismic moment. We see that the number of earthquakes with magnitude less than MS=5.5 is about 3100 (89% of the total number), summing up a moment release of 3.75x1019 Nm, while the number of earthquakes with MS ≥ 5.5 totals 379 events corresponding to 8.09x1021 Nm of moment release. This implies that 11% of the events released 99.5% of the total seismic moment in the area during the observed 102 years.

9 Figure 6. Magnitude vs. time

Figure 7. Number of events vs. magnitude

10 4. Probabilistic Seismic Hazard

4.1. Introduction To 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. If we 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 once we know the past seismicity of an area, through statistical laws we can foresee how the seismicity will behave in the future. As soon as we establish the rules of the seismicity, we can quantify the hazard levels as probabilities of exceeding specified ground motion levels for specific periods of time. It is presupposed that the knowledge of the seismic history is adequate. The theoretical 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 will become the seismogenic source model. As a second step, it is necessary to estimate how seismic energy dissipates with respect to earthquake magnitudes and hypocentral distances for earthquake-site pairs of interest. The final step is the calculation of the levels of hazard. Results can be quantified in several units (acceleration, velocity, displacement); we have chosen the peak ground acceleration (PGA), which traditionally has been used.

4.2. Seismogenic zones To demarcate a seismogenic source zone, it is assumed that all earthquakes have equal and independent probability of occurrence within the zone. The set of zones, at the same time, are considered independent of each other. Furthermore, it is assumed that all seismicity within a zone has the

11 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 one of the primary reasons for differences in estimated hazard levels in different studies. In short, a source model is a collection of seismic sources with corresponding geographical location and activity rates (e.g., in terms of frequencies and magnitude levels). Together with the source zone definitions come seismic source parameters (b-value, maximum magnitude, threshold magnitude and activity rate), which are obtained through the statistical treatment of seismic catalogues. For each source zone, we compile a sub- catalogue of earthquakes that occurred within the zone. Then, we estimate for each source zone the probability of occurrence of future earthquakes with a given magnitude and within a given time period

4.3. Attenuation relationships and uncertainties In PSHA the rate at which seismic energy decays, as a function of distance from the source and source magnitude is specific for each region is and known as an attenuation relationship. In the relationship other parameters can be included to bring the relationship closer to reality (e.g. site specifics and tectonic style of the sources). However, a large collection of data is required and for some areas this is not feasible. Therefore, the analyst must select a relationship concordant with the region of study. This selection is a key for any seismic hazard study.

To model the uncertainties of parameters used in the PSHA, we make use of a logic tree. With this approach, the problem is partitioned into less complex elements so that each of them can be studied in detail, but at the same time the view of the whole problem is not lost. Logic trees provide a convenient, flexible, and powerful means of incorporating the uncertainty in modelling future seismicity in intraplate regions, explicitly in the estimate of the future hazard (Coppersmith and Youngs, 1986). Our logic tree includes different zonations, attenuations relationships and minimum and maximum magnitudes. At the end is the judgment of the analyst regarding which of the outcomes of the logic tree is the most appropriate to the studied region.

12 4.4. Maps of hazard levels For the area of Honduras (Paper I), and after considering all parameters involved in the PSHA, we performed hazard computations on a grid with 0.1° spacing. Figure 8 presents spatial trends of expected PGA values obtained from the analysis, reflecting the influence of the seismogenic source zones on the outcome. The highest expected acceleration for the 100- year return period is observed in and near areas of seismic sources with a high rate of seismic activity, especially along the subduction zone.

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

13 5. Fault Geometry

5.1. Introduction

In the estimation of seismic hazard, the contribution of all seismic sources is taken into account. A key parameter in the PSHA is the maximum magnitude a fault can generate. The maximum magnitude is controlled by the geometry (length, width, dipping angle of the plane) of the fault. An appropriate description of the location of the seismogenic source, style of faulting (e.g. strike-slip, normal or thrust) and its geometry is, therefore, fundamental. The depth of faulting is one of the parameters that involves large uncertainties, while the style of faulting, related to the radiation pattern, affects the ground motion expected at different distances and azimuths from a fault. Our goal is to estimate the geometry of the faults in the region, the predominant faulting process and to identify characteristic fault segments through the seismic moment release. In the present thesis, we studied in detail the Swan transform fault (Paper II).

5.2. Depth of faulting From different profiles of seismicity (hypocenter catalogue) throughout the study area (Figure 4), we can distinguish a characteristic feature, namely the clustering of events at about 33-km depth. These are depth-constrained hypocentres and can be seen more clearly in the histogram of event focal depths (Figure 5). With regards to the H-CMT catalogue, the clustering is around the 10-15 kilometres depth range. This is ascribed to the convention that focal depth in the H-CMT method is constrained to depths of 10 km or greater (Dziewonski et al., 1983). Hence, the clustering in both catalogues is an artefact of the procedures used to locate earthquakes.

14 We should also be aware of the fact that the depth of faulting is related to the rheology of the source, i.e. to temperature. In some cases, we have focal depth determinations larger than 30 km on crustal faults. Heat flow data from several faults show that the occurrence of earthquakes is controlled by the 400°C-600°C isotherms, constraining the depth of faulting to be, e.g. less than 20-30 km in non-subduction faults. Therefore, a more accurate estimation of hypocentral depth is required. One possibility is to use teleseismic body-waveform modelling since waveforms are more sensitive to focal depth than travel time data. With this tool, we determined that the depth 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 simple cooling plate model for the Swan transform fault. We can see that hypocenters are bounded by the 400 isotherm.

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

15 5.3. Seismic moment release The general pattern of seismicity in the studied area (Figure 2) indicates parts where deformation is taking place, it reveals that crustal blocks are in motion. Unfortunately, a seismicity map does not provide more information than the epicentre locations and magnitudes of earthquakes that took place in the area. It is possible to study how a region is being seismically deformed by means of images of the scalar seismic moment release (Figure 10). The scalar seismic moment represents the size of an earthquake. At the same time it 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 and estimated that the fault has a deficit in seismic moment release of the order of 40 percent when a maximum earthquakes is assumed as Mw=7.7. On the other hand, for a 146-year period the fault presents no deficiency.

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

16 6. Plate Motion and Seismic Deformation Rates

6.1 Introduction The 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 of importance in hazard investigations. Slip rates can be obtained from plate motion models, however these values are estimated for plate boundaries in general. From Figure 1 and Figure 2, we can find tectonic structures for which seismic activity is low or non-existing. The Honduras depression, Honduras borderland faults and the Guayape fault system are examples of this situation. Therefore, estimation of slip rates for such structures will improve the estimation of the PSHA for this region.

6.2. Rate of deformation from earthquakes The overall velocities of deformation across a zone of distributed deformation are related to the seismic moment that occurs within it (Jackson and McKenzie, 1988). The resulting velocity across the zone can be compared (in the case of simple deformation) with the estimation of velocities from plate motions. The basis for the method is due to Kostrov (1974) and was modified by Papazachos and Kiratzi (1992). It relates the total seismic strain rate tensor to the annual seismic moment release rate contained in the crustal volume of interest. The region of interest is divided in tectonic volumes with uniform tectonic characteristics (i.e. similar focal mechanisms of earthquakes) to compute the velocity across the zone. To estimate the seismic moment rate it is required to define a maximum magnitude and therefore a maximum moment released within the volume. Through applying the Gutenberg-Richter relationship to each volume, we can obtain the tensor of seismic moment release rate and then we can deduce

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

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

18 7. Earthquake Triggering

7.1. Introduction In 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 threshold value prior to the largest earthquake within a zone. As stated earlier, the seismogenic sources are supposed to be independent of each other. However, earthquakes like those of 1989 in Loma Prieta, 1992 in Landers, 1995 in Kobe and 2001 in Istmitz present all a common characteristic, namely correlation of changes in the Coulomb stress and levels of seismicity. In areas where the Coulomb stress change (dCFS) is positive, the seismicity rate increases and where the changes are negative, the seismicity rate decreases. This suggests that the interaction between faults should not be neglected.

7.2. Interaction between earthquakes The stress interaction method, as described in Stein et al. [1992] and King et al. [1994] calculates static stress changes, by assuming that faults are planar dislocation surfaces immersed in an elastic half space. These changes are then rotated to a system that has orthogonal axes along the slip direction of the fault and its normal (Hodgkinson et al, 1996). In Figure 12, dCFS are presented for the period 1900 to 1977 using earthquakes with magnitude larger than 6 along the plate boundaries of northern Central America (Paper IV). If a fault is within a zone of positive changes in dCFS, it is assumed that this fault has been brought closer to failure. Likewise, decrease in dCFS indicates that the fault failure may have been delayed. For the sequence of aftershocks of the February 1976, Mw=7.6, earthquake, 88% took place on regions of positive increase of dCFS. As follows from Figure 12, the event of August 1980, Mw=6.5, is located on a zone of positive dCFS, and the April 1982, Mw=6.3, shock is located on a low positive dCFS zone.

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

Based on the available data, we believe that the evolution of the static stress field studied here yields a good correlation of dCFS with the aftershocks and background seismic activity (Paper IV). We applied the concept of changes of Coulomb stresses to a particular event, the July 11, 1999, Mw=6.7, earthquake in the Gulf of Honduras. We correlated the changes of dCFS caused by this earthquake with aftershocks that followed. It 77% of the aftershocks fall on areas of positive increase in dCFS of at least 0.1 bar (Paper V). The application of results from calculations of dCFS on seismic hazard is still in debate, especially with regards to the knowledge of how close major faults are to failure, limiting its use in predicting the timing of large earthquakes (Stein et al., 1994). Nevertheless, it is known that the occurrence of a large earthquake changes the condition of failure on other faults in its vicinity, altering the probabilities of occurrence of future events. This result may be used to show where possible earthquakes may take place.

20 8. Summary

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

As in most cases, the deformation is not localised to plate boundaries but to large belts of deformation within tectonic plates. Particularities include locking at plate boundary interfaces resulting in large earthquakes (Middle America trench, Motagua fault), aseismic slip estimated through moment deficits (Swan transform fault) and thickening and rotation in some regions (Honduras depression) within the plates. Some of these characteristics are well reflected on a PSH map, like the high expected acceleration values along 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 could lead to earthquakes was computed for the Swan transform fault. We used recent and historical earthquakes through direct summation of scalar seismic moments 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 in north Central America presents a close resemblance to that obtained through global plate motion estimates with respect to orientation and amount. The generally low discrepancies between seismic and plate motion data may suggests that the aseismic deformation along plate boundaries is not large but at the same time large earthquakes are required. With this approach deformation 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. A priori assumptions about the geometry of the faults, for both, the estimation of the seismic coupling coefficient (using scalar seismic moments) or the

21 deformation rates (using seismic moment tensors) involve uncertainties that affect the results. Therefore, a better constraint on the geometry was pursued. The coverage of the seismic cycle is a limitation on the hypocentral catalogue because of the lack of data from the pre-instrumental period. At the same time, earthquakes are not isolated from each other, and the interaction between areas of increased or reduced stress may modify the estimations of seismic hazard.

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

22 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 and results achieved in our efforts to develop seismic hazard maps of Honduras. The probabilistic methodology of Cornell is employed. Numerical calculations were carried out by making use of the computer code SEISRISK III. To examine the impact of uncertainties in seismic and structural characteristics, the logic tree formalism has been used. We compiled a de-clustered earthquake catalogue for the region comprising 1919 earthquakes occurring during the period from 1963 to 1997. Unified moment magnitudes were introduced. Definition of a seismotectonic model of the whole region under review, based on geologic, tectonic and seismic information, led to the definition of seven seismogenetic zones for which seismic characteristics 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 return periods 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 Guatemala and El Salvador.

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

In this paper we have compiled a series of seismic properties of the Swan transform fault, Caribbean Sea. Estimations of the focal mechanisms and centroid depths for 9 of the largest earthquakes of the transform fault, for the period 1980- 1999, are presented along with their corresponding estimated rupture area, average slip and stress drop. A strike-slip mechanism is predominant on the events studied here in good agreement with the expected mechanisms on transform fault earthquakes; centroid depths of all 9 events range from 7 to 15 km. We have estimated the maximum depth of seismic faulting from 20 to 25 km (from inversion of body waveforms). The results fit well with a simple thermal model for transform faults, corresponding to the 600° isotherm, as well as the slip distribution of the 23 largest earthquake in the studied period, that limits the faulting to 25 km. We have also calculated the crustal stress orientation from focal mechanism along the transform, indicating that the direction of the maximum component of stress is about N30E. We complemented a catalogue of recent seismicity with historical seismicity for the transform fault and estimated that the seismic coupling coefficient is of about 10 % for a period of 92 years.

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

We have made use of the seismic moment tensor for earthquakes on plate boundary as a standard procedure to determine the relative velocity of plates, which control 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 resulting deformation rates estimated compare well with those determined from the predicted velocities from the global plate motion model and gravity data. The rates obtained from the model application are in good agreement with actual observations. Deformation rates on faults are increasingly being used to estimate earthquake recurrence from information on a fault slip rate and more on how we can incorporate our current understanding into seismic hazard analyses.

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

We investigate the Coulomb stress interaction for earthquakes from the North American-Caribbean (NOAM-CARIB) plate boundary for the period 1976-1999. A series of maps containing the variation in stress estimated by means of the Coulomb failure criterion (CFC) for each earthquake is presented. A correlation between zones of stress increase and epicenters of earthquakes of the sequence is visible in 3 out 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 fault segments where the next rupture is most likely to take place.

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

Coulomb failure stress changes (dCFS) caused by the July 11, 1999, Gulf of Honduras, Caribbean Sea, earthquake are calculated. The trend and dipping angle of the relocated aftershock sequence show a good agreement with the fault plane solution available for the main event as well as a good correlation 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 shows that 77 % of the aftershocks fall into regions where the dCFS is positive.

25 10. Acknowledgements

I would like to thank the following people for helping me throughout the research process in Uppsala, without whom I wouldn't have reached this stage; to Ota Kulhánek, Federico Güendel, Ronald Arvidsson and Roland Roberts for their helpful and interesting comments and suggestions at various 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) both supported by the Swedish International Development Agency (SIDA- SAREC). I also thank for the fellowship received from the Department of Earth Sciences, Uppsala University and the leave of absence permit from the Department of Physics of the University of Honduras (UNAH), to continue my doctoral studies. I thank my colleagues and friends from the Department for their friendship, critique, enlightenment and inspiration. I also thank Jens Havskov, Kuvvet Atakan and Robert Rogers for their valuable contribution in the development of this work. I appreciate the help from innumerable people 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 science friends are also acknowledged.

26 11. References

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27 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 plate motions and seismic moment tensors, and the rates of active deformation in the Mediterranean and Middle East. Geophys. J. Int., 93, 45-73. King, G.C.P., Stein, R.S. and Lin, J., 1994. Static stress triggering of earthquakes. Bull. Seismol. Soc. Am., 84, 3, 935-953. Kostrov, V.V., 1974. Seismic moment and energy of earthquakes and seismic flow of rock. Izv., Earth Physics, 1, 23-40. Papazachos, C.B. and Kiratzi, A.A., 1992. A formulation for reliable estimation 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. Epirogenic uplift above a detached slab in northern Central America. Geology, 30, 11, 1031-1034. Rojas, W., Bungum, H. and Lindholm, C., 1993. Historical and recent earthquakes 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 in the southern San Andreas fault system caused by the 1992 magnitude=7.4 Landers earthquake. Science, 258, 1328-1332. White, R.A. and Harlow, D.H., 1993. Destructive upper-crustal earthquakes in Central America since 1900. Bull. Seismol. Soc. Am., 83, 1115-1142.

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