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Earthquake Sources and Hazard in Northern Central America 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 ACTA UNIVERSITATIS UPSALIENSIS UPPSALA 2003 ! " # $ % & $ $ '% %( % ) * &%( +, ( !( * %- . / 0 % + ( 12 0 /3 + 4( ( 5!6( 7 ( ( 8/9: 7";;6;<";6 : % + 0 $ ) % + : % ( % 0 % . & % : % + % $ $ ( % ) % $$ $ % $ % %&% % & & ( % $ % & % ) % $ . % &% &( 8 ) ) $ ! % %- . ) % & & % = . % + & % % % 0 $ % 0 ( . % ) & $ % & "7<6 $ & % $ %- . % &% ' ( ' % 0 1'/4 $ % ; ) $ ( % %&% $ % 0 ) $ % 0 > % 0 ( 9 $ % $ $ & & % '/ (( $ & % ) % & " $ % )% % + ( ? % % : % + $ % $ ( $$ % - ) . % $ ) $ & ) & % $ % $ $ & .( % ) % $ . ) & % $ % $ ( @ ) $ % + % ) %- . % & $ ) & > %- .( : % + ' / 0 9 ) $ & / $ * %- . && & ! " ! # $ %&! ! "'()*+& ! A & +, ! 8//: "" 6!B 8/9: 7";;6;<";6 # ### !!57 1% #CC (.(C D E # ### !!574 PAPERS INCLUDED IN THE THESIS 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.
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