observing earthquakes triggered in the near field by dynamic deformations we examine the hypothesis that dynamic deformations associated with seismic waves trigger earthquakes in many tectonic environments our analysis focuses on seismicity at close range within the aftershock zone complementing published studies of long range triggering our results suggest that dynamic triggering is not confined to remote distances or to geothermal and volcanic regions long unilaterally propagating ruptures may focus radiated dynamic deformations in the propagation direction therefore we expect seismicity triggered dynamically by a directive rupture to occur asymmetrically with a majority of triggered earthquakes in the direction of rupture propagation bilaterally propagating ruptures also may be directive and we propose simple criteria for assessing their directivity we compare the inferred rupture direction and observed seismicity rate change following 15 earthquakes m 5 7 to m 8 1 that occurred in california and idaho in the united states the gulf of aqaba syria guatemala china new guinea turkey japan mexico and antarctica nine of these mainshocks had clearly directive unilateral ruptures of these nine seven apparently induced an asymmetric increase in seismicity rate that correlates with the rupture direction the two exceptions include an earthquake preceded by a comparable magnitude event on a conjugate fault and another for which data limitations prohibited conclusive results similar but weaker correlations were found for the bilaterally rupturing earthquakes we studied although the static stress change also may trigger seismicity it and the seismicity it triggers are expected to be similarly asymmetric only if the final slip is skewed toward the rupture terminus for several of the directive earthquakes we suggest that the seismicity rate change correlates better with the dynamic stress field than the static stress change manuscript received 1 march 2002

a study of possible ground motion amplification at the coyote lake dam california the abutment site at the coyote lake dam recorded an unusually large peak acceleration of 1 29g during the 1984 morgan hill earthquake following this earthquake another strong motion station was installed about 700 m downstream from the abutment station we study all events seven recorded on these stations using ratios of peak accelerations spectral ratios and particle motion polarization using holograms to investigate the relative ground motion at the two sites we find that in all but one case the motion at the abutment site is larger than the downstream site over a broad frequency band the polarizations are similar for the two sites for a given event but can vary from one event to another this suggests that the dam itself is not strongly influencing the records although we can be sure that the relative motion is usually larger at the abutment site we cannot conclude that there is anom alous site amplification at the abutment site the downstream site could have lower than usual near surface amplifications on the other hand the geology near the abut ment site is extremely complex and includes fault slivers with rapid lateral changes in materials and presumably seismic velocities for this reason alone the abutment site should not be considered a normal free field site

simulation of accelerograms peak ground accelerations and mmi for the tecoman earthquake of 21 january 2003 abstract the tecoman mexico earthquake 21 january 2003 20 06 shows directivity in the area in which it spread the colima graben the presence of soft soils and the location of 8 out of 10 major urban zones in the area are important factors concerning the acceleration peak ground acceleration pga and modified mercalli intensity mmi caused by this earthquake data are not available due to the lack of instrumentation in the area in this paper we simulate acceleration time histories whose main contribution is to reflect a model that considers the source the path and the site effects in order to reach this objective we used the source model generated by ramirez gaytan and aguirre 2008 for the tecoman earthquake the pga and mmi were estimated from the waveforms in 25 points within the state of colima and eight more outside thus the maximum accelerations obtained are compared with four different ground motion prediction equations gmpes our results show an expected and reasonable behavior in agreement with the considerations of each one of the statistical methods compared in the end we generated an isoseismal map for the region this map is overlaid by the isoseismal map for this earthquake generated by other authors who used traditional methods to determine mmi first we conclude that the empirical green s function method can be successfully applied to simulate the acceleration waveforms in sites without instrumentation during the mainshock second the simulated records can be successfully used in seismic risk studies

the pliocene initiation and early pleistocene volcanic disruption of the palaeo gediz fluvial system western turkey in this paper we report our latest observations concerning a pliocene and early pleistocene record from western turkey the sedimentary sequence described comprises the fluvial deposits of an early pleistocene palaeo gediz river system and its tributaries prior to the onset of volcanism around kula and the subsequent lacustrine volcaniclastic and fluvial deposits associated with the first phase of volcanism similar to 1 2 ma in this area early development of an east west drainage system in this area resulted from tectonic adjustments to north south extension and the formation of east west oriented grabens headward erosion of drainage entering the main alasehir graben led to the progressive capture of pre existing drainage systems as eastward headward erosion upstream tapped drainage networks previously formed in internally draining nne ssw oriented basins within one of these the selendi basin part of this evolutionary sequence is preserved as a buried river terrace sequence eleven terraces are preserved beneath alluvial fan sediments that are in turn capped by basaltic lava flows using the available geochronology these terraces are considered to represent sedimentation incision cycles which span the period similar to 1 67 1 2 ma although progressive valley incision is a fluvial system response to regional uplift the frequency of terrace formation within this time period suggests that the terrace formation resulted from sediment water supply changes a consequence of obliquity driven climate changes the production of sub parallel terraces suggests that during this period the river was able to attain a quasi equilibrium longitudinal profile adjusted to the regional uplift rate thus the incision rate of 0 16 mm a 1 during this period is believed to closely mirror the regional uplift rate after the onset of volcanism at similar to 1 2 ma there is a destruction of the dynamic link between fluvial system behaviour and climate change the repeated damming of the trunk river and its tributaries led to the construction of complex stratigraphic relationships during the first phase of volcanism the palaeo gediz river was dammed on numerous occasions leading to the formation of a series of lakes upstream of the dams in the palaeo gediz valley variations in lake level forced localised base level changes that resulted in complex fluvial system response and considerable periods of disequilibrium in profile adjustment furthermore response to these base level changes most likely disrupted the timing of the incisional adjustment to the on going regional uplift thus making the use of this part of the archive for inferring regional uplift rates untenable c 2007 elsevier ltd all rights reserved

seismicity and structure in central mexico evidence for a possible slab tear in the south cocos plate the morphology of the transition from flat to normal subduction in eastern central mexico is explored using intraslab earthquakes recorded by temporary and permanent regional seismic arrays observations of a sharp transition in slab dip near the abrupt end of the trans mexican volcanic belt tmvb suggest a possible slab tear located within the subducted south cocos plate the eastern lateral extent of a thin ultra slow velocity layer usl imaged atop the cocos slab in recent studies along the meso america subduction experiment array is examined here using additional data we find an end to this usl which is coincident with the western boundary of a zone of decreased seismicity and the end of the tmvb near the sharp transition in slab dip waveform modeling of the 2 d structure in this region using a finite difference algorithm provides constraints on the velocity and geometry of the slab s seismic structure and confirms the location of the usl analysis of intraslab seismicity patterns reveals clustering sudden increase in depth variable focal mechanism orientations and faulting types and alignment of source mechanisms along the sharp transition in slab dip the seismicity and structural evidence suggests a possible tear in the south cocos slab this potential tear together with the tear along the orozco fracture zone to the northwest indicates a slab rollback mechanism in which separate slab segments move independently allowing for mantle flow between the segments

the relationship between quaternary volcanism in central mexico and the seismicity and structure of subducted ocean lithosphere late quaternary volcanism in central mexico is related to the subduction of young ocean lithosphere at the middle america trench along arc variations in seismicity volcano structure and composition of volcanic products bear a remarkable correlation with the age and structural framework of the downgoing slab morphological and petrographic characteristics of major volcanoes within the trans mexican volcanic belt tmvb serve to distinguish two calc alkaline subprovinces 1 a western arc averaging 60 km in width associated with aseismic subduction of the rivera plate the main cones of this region are dominated by two pyroxene andesites comprise volumes 70 km 3 and stand less than 3 000 m above sea level 2 a broad central and eastern arc related to subduction of a gently inclined segment of the cocos plate bounded by the rivera transform and the tehuantepec ridge major volcanic edifices possess summit elevations in the range 4 000 to 6 000 m have appropriately larger volumes typically 200 km 3 and are constructed with a high proportion of amphibole bearing lavas the boundary between these subprovinces is marked by a north south oriented structural depression the colima graben and it coincides with a 100 km offset in the volcanic front extensional tectonism in the colima graben accompanied by mixed calc alkaline and alkaline volcanism of potassic affinity is likely related to a hinge type transform fault which marks the cocos rivera plate juncture in the downgoing slab a third segment of ocean floor is presently interacting with continental lithosphere south of the gulf of tehuantepec where quaternary volcanism is weakly developed within a tectonically complex region that marks the diffuse cocos noam caribbean triple junction the northern limit of this triple junction is defined by the seismically active isthmus fault which may be related to alkaline volcanic activity at san andres tuxtla a tectonic reconstruction based on the evolution of oceanic crust reveals that the distribution of intermediate depth earthquakes along the arc is directly dependent upon the age of the subducted slab ocean lithosphere younger than approximately 20 m y is subducted aseismically at convergence rates approaching 9 cm yr the length of the inclined seismic zone indicates that the time constant for thermal relaxation in the slab is approximately 4 m y the tmvb overlies the aseismic extension of this young ocean lithosphere several aspects of this study have a bearing on the segmented nature of converging margins in general 1 the tectonic evolution of the ocean floor may determine the nature of segmentation at the site of subduction 2 the complete record of volcanism in the tmvb over the past million years can be related to the present plate configuration 3 alkaline and calc alkaline volcanism have developed contemporaneously at a converging plate margin 4 lineaments in volcanic arcs may reflect the structural complexity of the crust rather than segment boundaries in the subducted slab

a geodetic study of the 2003 january 22 tecoman colima mexico earthquake we use coseismic displacements and aftershock information from global positioning system gps measurements at 27 sites in western mexico and a 12 station local seismic network to determine the characteristics of the 2003 january 22 m w 7 2 subduction thrust earthquake near tecoman colima mexico estimates of the earthquake moment slip direction and best fitting slip distribution are derived by optimizing the fit to the gps displacements for a 3 d finite element mesh that simulates the study area the calculated moment release is 9 1 10 19 n m m w 7 2 with maximum slip of 2 m at a depth of 24 km and a maximum rupture depth of 35 40 km the inversion indicates that coseismic rupture extended downdip from depths of 9 to 40 km along a 80 km along strike region that is bounded by the edges of the manzanillo trough the optimal solution is robust with respect to plausible changes in the subduction interface geometry and differing subsets of the data a comparison of the cumulative post seismic slip that can be inferred separately from earthquake aftershocks and gps measurements within a year of the earthquake indicates that 95 per cent or more of the post seismic deformation was aseismic near term post seismic measurements indicate that slip propagated downdip to areas of the subduction interface beneath the coastline within days following the earthquake as also occurred after the nearby m w 8 0 colima jalisco subduction earthquake in 1995 the similar behaviours and locations of the 1995 2003 earthquake sequence to two earthquakes in june of 1932 suggests that thrust earthquakes along the subduction interface northwest of the manzanillo trough may trigger earthquakes in the vicinity of the manzanillo trough however our modelling of coulomb stress changes caused by the 1995 earthquake indicate that it induced only modest unclamping of the subduction interface in the vicinity of the tecoman rupture in addition gps measurements indicate that elastic shortening characterized areas onshore from the tecoman rupture from mid 1997 up until the time of the rupture consistent with progressively stronger clamping of the subduction interface during this period this precludes any obvious triggering relationship with the 1995 earthquake the apparent coincidence of the edge of both the 1932 and 1995 2003 rupture sequences with the edge of the manzanillo trough may indicate that the trough is a mechanical barrier to along strike rupture propagation this implies a limit to the area of potential slip and hence rupture magnitude during future large earthquakes in this region

modeling of source rupture characteristics for the saguenay earthquake of november 1988 the magnitude mb 6 0 saguenay earthquake of 25 november 1988 in quebec canada was one of the largest to have occurred in eastern north america during the past half century recorded high frequency ground motions exceeded anticipated values for an event of its size by a factor of 10 on both the regional network and strong motion instruments two proposed explanations for the discrepancy are 1 that the source was a rare high stress drop event and 2 that it was an asymmetrical fractional stress drop rupture involving only normal effective stresses in this article detailed fault slip models are derived to fit characteristics of strong motion displacement velocity and acceleration data the results establish that the effective rupture stress was normal less than 100 bars that the fault rupture was highly asymmetrical with respect to the point of rupture initiation and that the average slip time for points within the rupture area approx 0 2 sec was considerably less than that associated with the standard brune 1970 source spectral model the rupture area developed in a number of episodes each widening or lengthening the previously ruptured area which may explain the short average slip time the results indicate that the widely used assumption in hazard analyses that earthquake spectra are adequately represented by the standard brune 1970 complete stress drop model may be seriously unreliable for prediction of strong ground motion in eastern north america

p wave velocity tomography from local earthquakes in western mexico in western mexico the subduction of the rivera and cocos plates beneath the north america plate has deformed and fragmented the overriding plate forming several structural rifts and crustal blocks to obtain a reliable subsurface image of the continental crust and uppermost mantle in this complex area we used p wave arrivals of local earthquakes along with the fast marching method tomography technique we followed an inversion scheme consisting of 1 the use of a high quality earthquake catalog and corrected phase picks 2 the selection of earthquakes using a maximum location error threshold 3 the estimation of an improved 1 d reference velocity model and 4 the use of checkerboard testing to determine the optimum configuration of the velocity nodes and inversion parameters surprisingly the tomography results show a very simple vp distribution that can be described as being controlled by geologic structures formed during two stages of the separation of the rivera and cocos plates the earlier period represents the initial stages of the separation of the rivera and cocos plates beneath western mexico the later period represents the more advanced stage of rifting where the rivera and cocos plates had separated sufficiently to allow melt to accumulate below the colima volcanic complex during the earlier period 14 or 10 1 6 ma ne sw oriented structures lineaments such as the southern colima rift were formed as the two plates separated during the second period 1 6 ma to the present the deformation is attributed to magma generated within and above the tear zone between the rivera and cocos plates rising beneath the region of the colima volcanic complex the rising magma fractured the overlying crust forming a classic triple rift junction geometry this triple rift system is confined to the mid to lower crust perhaps indicating that this rifting process is still in an early stage this fracturing along with fluid circulation and associated heat advection within the fractures can easily explain the observed distribution of vp as well as many of the results of previous seismological studies also surprisingly we find no evidence at deep crustal depths to support either a trenchward migration of the volcanic arc or toroidal asthenospheric flow through the slab tears bounding the jalisco block to the nw and se

subsidence and strike slip tectonism of the upper continental slope off manzanillo mexico the direction of convergence between the rivera and north american plates becomes progressively more oblique in a counter clockwise sense as measured relative to the trench normal direction northwestward along the jalisco subduction zone by analogy to other subduction zones the forces resulting from this distribution of convergence directions are expected to produce a nw moving fore arc sliver and a nw se stretching of the fore arc area also a series of roughly arc parallel strike slip faults may form in the fore arc area both onshore and offshore as is observed in the aleutian arc in the jalisco subduction zone the jalisco block has been proposed to represent such a fore arc sliver however this proposal has encountered one major problem namely right lateral strike slip faulting within the fore arc sliver and between the fore arc sliver and the north american plate should be observed however evidence for the expected right lateral strike slip faulting is sparse some evidence for right lateral strike slip faulting along the jalisco block north american plate boundary the tepic zacoalco rift system has been reported although some disagreement exists right lateral strike slip faulting has also been reported within the interior of the jalisco block and in the southern colima rift which forms the se boundary of the jalisco block threefold multi channel seismic reflection data were collected in the offshore area of the jalisco subduction zone off manzanillo in april 2002 during the famex campaign of the n o l atalante these data provide additional evidence for recent strike slip motion within the fore arc region of the jalisco subduction zone this faulting offsets right laterally a prominent horst block within the southern colima rift from which we conclude that the sense of motion along the faulting is dextral these data also provide additional evidence for recent subsidence within the area offshore of manzanillo as has been proposed

macroseismic study of the mw 7 5 21 january 2003 colima mexico across trench earthquake a field investigation of residential house damage in 83 localities of the occidental part of mexico allowed construction of a macroseismic map of the mo ment magnitude 7 5 2003 colima earthquake the 12 grade modified mercalli mm scale and the 7 grade damage scale of whitman 1988 were used for estimating the earthquake intensities the macroseismic effects of this earthquake were unexpected for the occidental part of mexico the earthquake occurred within the across trench structure of the el gordo graben and was characterized by the down dip across trench directivity of its rupture as reconstructed from seismic waveform modeling it was a reason for the severe damage to buildings in the localities situated at distances where usual along trench rupturing subduction earthquakes of the same magnitude do not produce any significant damage the macroseismic map of the 2003 earth quake shows that the narrow 30 km wide mm vii zone was elongated up to 60 km in the northeast direction toward the continental part of colima state as a result the intensity in colima city was 1 5 grades larger than would be expected for this mag nitude according to the proposed attenuation of intensity with distance relationship for earthquakes from the mexican subduction zone

the october 9 1995 colima jalisco mexico earthquake mw8 an aftershock study and a comparison of this earthquake with those of 1932 data from portable seismographs and a permanent local network called resco are used to locate the aftershocks of the october 9 1995 colima jalisco earthquake mw 8 0 the maximum dimension of the aftershock area which is rectangular in shape is 170 km 70 km our study shows that the mainshock nucleated 24 km south of manzanillo near the foreshock of october 6 1995 mw 5 8 and propagated 130 km to the nw and 40 km to se the aftershock area lies offshore and is oriented parallel to the coast the observed subsidence of the coast is a consequence of this offshore rupture area the aftershocks reach unusually close to the trench within 20 km this may be due to lack of sediments with high pore pressure at shallow depth there are some similarities between this earthquake and the two great earthquakes of 1932 3 june ms 8 1 18 june ms 7 8 which occurred in this region in both cases the aftershocks were located offshore and the coastline subsided the sum of seismic moments and the rupture lengths of the 1932 events 1 8 1021 n m and 280 km respectively however were greater than the 1995 earthquake also a comparison of seismograms of 1932 and 1995 earthquakes show great differences it seems that the 1995 event is not a repeat of either june 3 or june 18 1932 earthquakes

present day kinematics of the rivera plate and implications for tectonics in southwestern mexico we model the present day motion of the rivera plate relative to the pacific north america and cocos plates and combine the results with the nuvel i model for cocos north america motion to examine present day deformation in southwestern mexico the pacific rivera data which include 25 three m y average spreading rates determined from original surface ship magnetic data and 22 azimuthal data along the rivera transform are systematically misfit by the nuvel i pacific north america and pacifie cocos euler vectors indicating that the rivera plate is kinematically distinct from north america and cocos an f test shows that the improvement in fit to the pacific rivera data from a model with a distinct rivera plate exceeds that expected solely from adding three model parameters the rivera north america euler vector derived from closure of the rivera pacific north america plate circuit predicts slower and more trenchnormal convergence along the acapulco trench than prior models in addition the model predicts convergence normal to the eastern tamayo fracture zone at a rate 60 slower than the right lateral strike slip predicted by prior models an observed systematic misfit of the pacific rivera euler vector to azimuths from the western rivera transform fault may have several causes there may be a time averaging problem between the 3 0 m y average rates and the shorter time average transform fault azimuths and earthquake slip vectors alternatively rivera transform trends may be biased by deformation within lithosphere adjacent to the transform valley or future detailed mapping of the transform may reveal the misfit to be an artifact of the presently available bathymetry along the entire pacific rivera rise spreading rates averaged over the past 0 7 m y are systematically faster than 3 0 m y average rates with the difference increasing southward along the rise the post 3 ma change in the rate gradient has caused a southward migration of the pacific rivera euler pole since 3 ma possibly in response to the 3 ma completion of a spreading reorganization that doubled the length of the rivera transform fault and halved the area of the rivera plate we hypothesize that deformation in southwestern mexico including opening along the colima rift is related to oblique subduction of the cocos plate along the northern middle america trench three unes of evidence support this hypothesis first the sense of oblique subduction predicted by nuvel i along the middle america trench is consistent with a model in which part of western mexico located southeast of the colima rift moves to the southeast relative to north america second field geologic data and landsat imagery suggest that in the past few million years the chapala oaxaco fault zone has accommodated several kilometers of sinistral motion and sinistral transtension has occurred along the trans mexican volcanic belt either or both of these fault zones could accommodate any southeastward coastal block motion that might result from oblique subduction third the observation that earthquake slip vectors from the northern middle america trench trend systematically counterclockwise from the predicted cocos north america convergence direction is also consistent with southeastward motion the discrepancy between the observed and predicted slip directions along the middle america trench suggests sinistral slip is 0 10 mm yr 1 if oblique subduction is driving southeastward transport of a coastal sliver the colima rift is a passive pull apart zone at the northwestern end of this sliver rather than an incipient spreading ridge that will replace the pacific rivera rise

banderas rift zone a plausible nw limit of the jalisco block 1 echo soundings recently made in bahia de banderas show that this region is a graben with steeply dipping walls and several basins it is the offshore continuation of the valle de banderas graben and of a branching rift rio ameca rift originating in the tepic zacoalco rift zone the general trend of the three structures is ene with some ne trending offsets and they have a total length of 150 km this banderas rift zone is proposed as the nw limit of the jalisco block the existence of this limit suggests that there is another platelet or block between the jalisco block and a portion of the rivera plate probably bounded by the tres marias escarpment the jalisco block and the north america plate

overview of recent coastal tectonic deformation in the mexican subduction zone holocene and pleistocene tectonic deformation of the coast in the mexico subudction margin is recorded by geomorphic and stratigraphic markers we document the spatial and temporal variability of active deformation on the coastal mexican subduction margin pleistocene uplift rates are estimated using wave cut platforms at ca 0 7 0 9 m ka on the jalisco block coast rivera north america tectonic plate boundary we examine reported measurements from marine notches and shoreline angle elevations in conjunction with their radiocarbon ages that indicate surface uplift rates increasing during the holocene up to ca 3 0 5 m ka in contrast steady rates of uplift ca 0 5 1 0 m ka in the pleistocene and holocene characterize the michoacan coastal sector south of el gordo graben and north of the orozco fracture zone ofz incorporated within the cocos north america plate boundary significantly higher rates of surface uplift ca 7 m ka across the ofz subduction may reflect the roughness of subducting plate absence of preserved marine terraces on the coastal sector across el gordo graben likely reflects slow uplift or coastal subsidence stratigraphic markers and their radiocarbon ages show late holocene ca last 6 ka bp coastal subsidence on the guerrero gap sector in agreement with a landscape barren of marine terraces and with archeological evidence of coastal subsidence temporal and spatial variability in recent deformation rates on the mexican pacific coast may be due to differences in tectonic regimes and to localized processes related to subduction such as crustal faults subduction erosion and underplating of subducted materials under the southern mexico continental margin

the great jalisco mexico earthquakes of 1932 subduction of the rivera plate we have analyzed seismograms of the 3 june 1932 ms 8 2 and the 18 june 1932 ms 7 8 jalisco earthquakes and their aftershocks at manzanillo mnz guadalajara gum and tacubaya tac the aftershocks locations the first motions at mnz and the isoseismic maps of the two main shocks strongly suggest that a the 3 june 1932 earthquake initiated nw of but close to mnz and propagated nw for an estimated length of rupture of 220 km b the 18 june 1932 earthquake nucleated sw of mnz offshore and perhaps ruptured a length of about 60 km and c the width of rupture was approximately 80 km there is no evidence of events occurring se of mnz even up to 1 1 2 yr after the first main shock a gap of about 60 km remains between the aftershock areas of the 1932 jalisco and the 1973 colima earthquakes whose seismic potential is unknown although the boundary between the rivera and cocos plates is uncertain there is little doubt that the 1932 earthquakes broke the shallow part of the rivera subduction zone our results may help in quantifying seismic potential of tectonically similar areas such as the juan de fuca subduction zone in the nw united states

seismic structure beneath the rivera subduction zone from finite frequency seismic tomography 1 the subduction zone of western mexico is a unique region on earth where microplate capture and overriding plate disruption are occurring today the young small rivera plate and the adjacent cocos plate are subducting beneath the jalisco block of mexico here we present a p wave tomographic model of the upper mantle to 400 km depth beneath the jalisco block and surrounding regions using teleseismic p waves recorded by the mapping the rivera subduction zone mars and colima volcano deep seismic experiment codex seismic arrays the inversion used 12 188 p wave residuals and finite frequency theory to backproject the 3 d traveltime sensitivity kernels through the model below a depth of 150 km the tomography model shows a clear gap between the rivera and cocos slabs that increases in size with depth the gap between the plates lies beneath the northern part of the colima graben and may be responsible for the location of colima volcano the images indicate that the deep rivera plate is subducting more steeply than does the adjacent cocos plate and also has a more northerly trajection at a depth of about 100 km both the rivera and cocos slabs have increased dips such that the slabs are deeper than 200 km beneath the trans mexican volcanic belt tmvb it is also found that the rivera plate is at roughly 140 km depth beneath the young central jalisco volcanic lineament our images suggest that the rivera plate and westernmost cocos plate have recently rolled back toward the trench this scenario may explain the unusual magmatic activity seen in the tmvb

structural architecture of the colima volcanic complex 1 the colima volcanic complex cvc is currently the most active mexican volcano and is located in the western sector of the trans mexican volcanic belt inside the active colima rift a regional n s striking extensional structure the colima rift is filled by a 1 km thick sequence of quaternary lacustrine sediments alluvium and colluvium mostly underling the about 3000 m thick volcanic pile of the cvc in this work we present the results of a detailed morphostructural and field study of quaternary faults and fractures in the cvc and the surrounding area including the regional structures of the colima rift we also present a geometrical modeling of the faults inside the volcano and a numerical model of the gravity induced stress and strain fields of the cvc the study attempts to characterize the geometry kinematics and dynamics of the deformation features of the cvc and relate it with the volcano structure the geology of the substratum and the geodynamic setting of the region our model considers that the observed deformation of the cvc and the surroundings results from the interplay between the active n s trending regional extensional tectonics and the southward spreading of the volcano over its basement forming an e w oriented volcanotectonic graben the interaction between regional tectonics and previously unrecognized volcanic spreading can control magma migration and flank instability in an area where eruptions and sector failures represent a potential high risk for more than 500 000 people

fracture zone subduction and reactivation across the puysegur ridge trench system southern new zealand a two dimentional kinematic model for lithospheric thrusting that considers the flexural interaction between the underriding and overriding plates was used to assess the mechanical implications of subducting trench parallel fracture zones on the topography and free air gravity anomalies of the puysegur ridge trench system fracture zones in the underriding plate are simulated by vertical discontinuities across which bending and shearing stresses cannot be supported the implication of reactivating fracture zones on the outer trench slope is to suppress the flexural bulge creating topographic relief between the fracture zone and the trench and reducing the trench gravity anomaly in the studied region the trench subparallel l atalante fracture zone accounts for the lack of a flexural bulge strong outer trench slope and gravity gradients the southward decrease of the trench fracture zone distance results in a decrease of the trench gravity anomaly the effect of subducting and vertically reactivating fracture zones within the underriding plate is to segment the load of the overriding plate creating a trough located above the underthrust fracture zone the vertical offset and gravity modifications are functions of the fracture zone position relative to the trench this model explains the 3500 m deep troughs and the 1700 m subsidence observed at the puysegur ridge we speculate that reactivating underthrust fracture zones can facilitate strike slip faults development within the upper plate in case of oblique subduction and are conducive to the formation and dispersion of terranes at convergent margins this process can account for basal tectonic erosion

the 11 december 1995 earthquake mw 6 4 implications for the present day relative motion on the rivera cocos plate boundary the 11 december 1995 earthquake is the largest and best constrained instrumentally recorded event which has occurred on the rivera cocos plate boundary the reported focal mechanism for this event indicates almost pure strike slip faulting with nodal planes oriented north south and east west a visual inspection shows that the seismograms recorded world wide strongly suggest a directivity effect indicative of a rupture propagating eastward from the epicenter this observation is confirmed by a directivity analysis which shows a sharp reduction in the difference between observed and synthetic seismograms when the rupture direction is at an azimuth of approximately 90 these results indicate that the east west trending nodal plane of the earthquake of 11 december 1995 is the actual fault plane considering that this is the largest instrumentally recorded earthquake in the region it strongly suggests that the relative motion of the rivera plate with respect to the cocos plate takes place along east west oriented faults and that the sense of motion is right lateral strike slip

active rifting in southwestern mexico manifestations of an incipient eastward spreading ridge jump a major ongoing episode of continental rifting has formed a system of three intersecting grabens in the western mexican volcanic belt these structures may mark the initial stage of an eastward spreading ridge jump beneath continental crust eastward ridge segment jumps have been propagating northward along this part of the east pacific rise for the past 12 m y but this is the first plate boundary reorganization event to involve continental crust since the separation of baja california in the miocene since pliocene time small volumes of unusual alkaline magmas have erupted in the graben areas in close association with the more abundant subduction related calc alkaline magmas erupted from andesitic stratovolcanoes the alkaline magmas are varieties found in zones of active rifting elsewhere in the world

rupture length of the october 9 1995 colima jalisco earthquake mw8 estimated from tsunami data we analyze tsunami data of the great 1995 colima jalisco mexico earthquake recorded in cabo san lucas baja california sur to estimate its rupture length l to model the tsunami arrival time we assume a rectangular source area oriented parallel to the trench whose se limit is fixed at the point of rupture initiation the nw limit of the source area i e l is varied between 120 and 200 km the comparison between synthetic and observed data strongly suggests that l of the earthquake was 160 20 km this length agrees with those reported in various other studies of the earthquake it however disagrees with a previous study based on the same tsunami data which suggested that the rupture may have extended 250 km nw from the epicenter tanioka and ruff 1996 the cause of this discrepancy is most likely an error in the timing of the records used by tanioka and ruff 1996 we conclude that the earthquake only partially ruptured the rivera north america plate interface a 120 km long segment in the nw extreme of this interface which apparently ruptured in 1932 remains presently unbroken

interrelation between fault zone structures and earthquake processes in order to develop capabilities for predicting earthquake processes on the basis of known fault zone structures and stress conditions we need to find relations between seismogenic structures and processes in the present paper we search for the scale dependence in various earthquake phenomena with the hope to find some structures in the earth that may control the earthquake processes among these phenomena we shall focus on 1 geologic structures which play some role in nucleation and stopping of earthquake fault rupture 2 depth ranges of the brittle seismogenic zone 3 asperities and barriers distributed over a fault plane 4 source controlled f max effect 5 nonfractal behavior of creep events and 6 temporal correlation between coda q 1 and seismicity of earthquakes with magnitude characteristic to a given area our review of various scale dependent phenomena leads us to propose a working hypothesis that the temporal change in coda q 1 may reflect the activity of creep fractures near the brittle ductile transition zone

the subducted rivera cocos plate boundary where is it what is it and what is its relationship to the colima rift gravity data the geometry of the wadati benioff zone beneath western mexico and the seafloor morphology of the rivera cocos plate boundary west of the middle america trench suggest that the subducted part of this boundary lies directly beneath and is oriented parallel to the southern colima rift thus the southern colima rift likely formed in response to divergence between the subducting rivera and cocos plates due to direct coupling between these two plates and the overriding north american plate in contrast the subducted plate boundary lies east of and oblique to the northern colima and central colima grabens east of the central colima graben a low density zone overlies the boundary and underlies surface exposures of cretaceous granitoids and associated thermal springs and shallow focus earthquakes characteristics that are explained by thermal convection induced in the upper mantle by divergence between the subducted rivera and cocos plates these characteristics along with the adjacent locations of the low density upper mantle and the central colima graben are consistent with crustal extension produced by the uniform sense normal simple shear mechanism

seismicity and tectonics of the rivera plate and implications for the 1932 jalisco mexico earthquake seismicity on and near the boundaries of the rivera plate is examined in order to determine the subduction regime of the rivera plate and its influence on the earthquake behavior of the northernmost section of the mid america trench in particular we address the question of a rivera cocos boundary there have been several large historic earthquakes in the coastal areas of the mexican states colima and jalisco but the last large event was in june 1932 the 1932 jalisco earthquake ms 8 1 the present quiescence has lasted longer than the average recurrence interval for the mexican subduction zone and the jalisco area is now categorized as a seismic gap however if subduction in this area is dominated by the motion of the rivera plate which is subducting at half the rate of the cocos plate the deficiency of earthquake activity may not be unusual in an attempt to better delineate the rivera cocos boundary intermediate sized earthquakes along and near the rivera fracture zone and the mid america trench were relocated with the joint epicenter determination method we found that in general the catalog locations are fairly accurate with the average change in epicenter upon relocation being 12 km seismicity along the rivera fracture zone follows the physiographic trend of the fracture zone but east of its intersection with the east pacific rise seismicity is more diffuse two intermediate sized earthquakes in this diffuse area have focal mechanisms suggesting that they represent rivera cocos boundary motion reexamination of arrival time data for the 1932 jalisco earthquake yields an epicenter at 19 57 n 104 42 w close to the boundary zone inferred from these two earthquakes we conclude that the possibility that the 1932 jalisco earthquake broke the northernmost section of the cocos north american plate interface as opposed to the rivera north american plate interface cannot be ruled out

relative motion between the rivera and north american plates determined from the slip directions of earthquakes so far the direction and rate of relative motion between the rivera and the north american plates riv nam has been determined by the combination of two euler poles rivera riv with respect to pacific pac and pac with respect to north america here we estimate the relative motion of this plate pair riv nam assuming that the horizontal projection of the direction of slip of the earthquakes occurring on the riv nam boundaries reflect their relative plate motion a catalog of earthquakes for which focal mechanisms are reported since 1976 is used in the analysis earthquakes were considered in the three segments of the riv nam plate boundary the subduction zone of the rivera plate beneath the jalisco block the tres marias escarpment and the events associated with the tamayo fracture zone the best fitting euler pole is determined using a grid search of 64 potential poles the slip direction predicted for each grid point is compared to the slip direction of the focal mechanisms of the earthquakes on the plate boundary the best fitting euler pole determined in a root mean square sense rms is located at 21 8 n 107 6 w a rate of rotation of 5 3 year is estimated assuming the seismic earthquake cycle of the 1932 and 1995 great earthquakes represents a lower bound of the rate of plate motion in the subduction zone the best fitting euler pole shows that the subduction of the rivera plate takes place in a direction perpendicular to the trench with a relative velocity of 4 3 cm year offshore manzanillo the rate of relative motion riv nam decreases from se to nw north of approximately 21 n the subduction of the rivera plate becomes oblique to the trench and the relative velocity between the two plates decreases to an average of 1 9 cm year this slow rate of convergence may explain the rapid decrease of seismicity in the trench and the apparent absence of large earthquakes in this region in the tres marias escarpment our best fitting pole suggests that subduction stops giving way to high angle reverse faulting perpendicular to the tres marias escarpment in agreement with the reverse faulting earthquakes occurring here to the north of 22 5 n the slip predicted by the best fitting pole suggests right lateral faulting in a direction parallel to the tamayo fracture zone at a very low velocity 0 5 1 0 cm year the best fitting euler pole determined here lies very close to the riv nam plate boundary in the vicinity of the tamayo fracture zone this location of our best fitting euler pole explains the low relative plate velocity the relatively low level of seismic activity and the presence of a broad zone of deformation that accommodates the riv nam motion

the colima mexico earthquake mw 5 3 of 7 march 2000 seismic activity along the southern colima rift the 7 march 2000 earthquake m w 5 3 located within the southern colima rift in southwestern mexico is the largest crustal earthquake ever recorded within the state of colima mexico the mainshock was located at 8 km depth and the aftershocks are distributed at depths between 5 and 10 km aftershock distribution follows a normal decay as measured by the modified omori s law with a p value of 0 96 both aftershock locations and focal mechanisms determined for the mainshock and some of the aftershocks delineate a fault dipping toward the northwest and striking west southwest this fault segment is parallel to the southern end of the tamazula fault which ends at manzanillo bay regional waveform modeling for the mainshock situates the earthquake on a normal fault with a significant left lateral strike slip component on a plane striking 260 and dipping toward the northwest at 46 we use a small aftershock with similar focal parameters as the mainshock to deconvolve p and s waves recorded at the permanent station coig from the deconvolution we obtained a source duration of 0 75 sec from the p waves and 0 9 sec from the s waves for the main event and a stress drop of 320 bars the aftershock area is much larger than the area measured from the apparent source time function from the aftershock area and the seismic moment obtained by harvard university centroid moment tensor catalog we compute a static stress drop of 20 bars fault plane solutions from the mainshock and some of the aftershocks indicate that the northern and larger segment of the tamazula fault is perpendicular to the extensional axis directed east southeast this study confirms the change in the direction of the extensional axis of the southern colima rift with respect to the northern colima rift extensional axis oriented almost east west here we show that the tamazula fault zone at least its eastern segment is active and a re evaluation of the seismic hazard posed by this fault to the state of colima and the cities of colima and manzanillo is urgent manuscript received 13 september 2002

differences in attenuation of ground motion perpendicular to the mexican subduction zone between colima and guerrero an explanation based on numerical modeling abstract a recent article tejeda jacome and chavez garcia 2007 showed that significant differences in seismic ground motion attenuation perpendicular to the pacific coast exist between guerrero and colima in western mexico for hypocentral distances larger than 100 km larger ground motions are predicted using a model derived from local data in the northern section of the subduction zone colima relative to ground motion models derived using data from the southern section guerrero in this article we consider two possible explanations for the differences the first possible explanation is differences in the geometry of the subducting slab between colima and guerrero the second is the varying shallow structure of the overriding plate due to the presence of the trans mexican volcanic belt tmvb oblique to the middle american trench we use 2d numerical modeling for p sv waves to investigate which of the two factors can explain the differences in the observed attenuation our results show that it is the presence of the tmvb closer to the coast in colima that better explains the differences

rupture history of the 1989 loma prieta california earthquake strong motion records of the 1989 loma prieta earthquake are inverted to determine a model of the rupture history uncorrected horizontal and vertical accelerograms are integrated to particle velocity time histories for 38 stations within an epicentral range of 75 km the time histories are bandpassed filtered with corners at 0 05 and 1 0 hz these bandpassed time histories are inverted using a nonlinear method to solve for the distribution of slip amplitudes and rupture times at specified locations on the fault plane the fault plane is specified a priori 38 km long and 17 km wide extending from 3 to 19 km depth at a constant dip of 70 starting models have rupture times based on constant rupture velocities of 2 5 2 8 and 3 0 km sec and uniform slip with rise times of 0 5 1 0 1 5 and 3 0 sec the waveform inversion results show the strike slip displacement is concentrated at the southern end of the rupture rake 156 and the dip slip displacement is concentrated at the northern end of the rupture rake 115 the average total slip is partitioned almost equally between strike slip and dip slip rake 137 the hypocentral area has an unusually small amount of slip with almost no slip in a region just to the north and up dip from the hypocenter the rupture front is complex propagating up dip to the south faster than it propagates to the north the region of maximum strike slip to the southeast radiates simultaneously with the region of maximum dip slip to the northwest the average rupture velocity is 3 0 km sec approximately 0 83 times the local shear wave speed the calculated seismic moment is 3 5 0 5 1026 dyne cm

strong motion seismology although this is the first review on this topic to appear in a quadrennial report the roots of strong motion seismology extend back to at least 1932 when far sighted engineers in the seismological field survey of the u s coast and geodetic survey installed rugged fieldworthy instruments designed to make on scale recordings of large earthquakes carder 1964 these instruments are called accelerographs for their output closely mimics ground acceleration hudson 1979 the original instruments and their offspring have provided a wealth of information about ground motions of direct use to engineers due in large part to the continued devotion of the seismological field survey now part of the branch of engineering seismology and geology at the u s geological survey the number of recordings has increased substantially particularly in the last two decades and multiple recordings of a few california earthquakes have provided the data necessary to begin to unravel the complexities of the ground motions and to predict these motions on an empirical basis although seismologists used accelerograph records in studies of earthquake energy and ground motion attenuation as long ago as 1942 gutenberg and richter 1942 1956 widespread seismological use of the records began with aki s 1968 analysis of the 1966 parkfield earthquake

palaeomagnetism of late miocene to quaternary volcanics from the eastern segment of the trans mexican volcanic belt a systematic palaeomagnetic study in the eastern part of the trans mexican volcanic belt includes 39 miocene pliocene and quaternary volcanic rocks in the southeastern mexico basin sierra nevada and sierra de ri o fri o the altiplano area and the palma sola massif a total of 430 samples have been selectively demagnetized using mostly alternating field demagnetizing methods supplemented by thermal analyses most characteristic remanences are carried by low ti titanomagnetites with occasional titanohematites or slightly maghemitized low ti titanomagnetites of similar direction seven sites were discarded because they presented intermediate directions hydrothermal alteration or were remagnetized by lightning strikes the mean directions of 32 sites together with 24 sites from sierra de las cruces in the western mexico basin indicate rocks older than 2 ma are rotated some 10 counterclockwise with respect to quaternary rocks whereas there is no rotational difference between miocene and pliocene rocks statistical analyses between different regrouped populations confirm that the rotational pattern is due to the age of the volcanics rocks but not to their spatial distribution the quaternary mean direction from the three mexico basin ranges is consistent with the geographical reference pole in contrast the pliocene mean direction from volcanic rocks of the altiplano area and the sierra de las cruces is slightly rotated some 10 westwards with respect to the reference direction from north america no significant rotations have been observed in the eastern tmvb from the western mexico basin to the border of the altiplano between late miocene and late pliocene times it suggests that a very small counterclockwise vertical axis rotation may have been taken place in this segment of the tmvb between late pliocene and quaternary times comparisons of these results with a summary of the available palaeomagnetic data in the area indicate that the previously reported quaternary rotations are of questionable reliability and that the large counterclockwise rotations reported in cretaceous to miocene rocks probably took place before the late miocene these new palaeomagnetic data support the idea that the eastern tmvb since the late miocene has been a zone of extension with a little left lateral shear component

aftershocks of the coyote lake california earthquake of august 6 1979 a detailed study aftershock hypocenters and focal mechanism solutions for the coyote lake california earthquake reveal a geometrically complex fault structure consisting of multiple slip surfaces the faulting surface principally consists of two right stepping en echelon northwest trending partially overlapping nearly vertical sheets and is similar in geometry to a slip surface inferred for the 1966 parkfield california earthquake the overlap occurs near a prominent bend in the surface trace of the calaveras fault at san felipe lake slip during the main rupture as inferred from the distribution of early aftershocks appears to have been confined to a 14 km portion of the northeastern sheet between 4 and 10 km depth focal mechanisms and the hypocentral distribution of aftershocks suggest that the main rupture surface itself is geometrically complex with left stepping imbricate structure seismic shear displacement on the southwestern slip surface commenced some 5 hours after the mainshock aftershocks in this zone define a single vertical plane 8 km long between 3 and 7 km depth within the overlap zone between the two main slip surfaces the average strike of aftershock nodal planes is significantly rotated clockwise relative to the strike of the fault zone in close agreement with the stress perturbations predicted by crack interaction models aftershock activity in the overlap zone is not associated with a simple dislocation surface space and time clustering within the entire aftershock set suggest an alternation of seismic displacement between the component parts of the fault zone this alternation is consistent with local stress perturbations predicted by crack interaction models we conclude that the fault structure is geometrically complex and that the displacements that occur on its component surfaces during the aftershock process dynamically interact by generating perturbations in the local stress field which in turn control the displacements table 5 is available with entire article on microfiche order from american geophysical union 2000 florida avenue n w washington d c 20009 document j82 006 1 00 payment must accompany order

a review of the discrete wavenumber method we present a review of the discrete wavenumber dwn method the method introduced by bouchon and art 1977 allows the simple and accurate calculation of the complete green s functions for many problems in elastodynamics

earthquakes quaternary faults and seismic hazard in california data describing the locations slip rates and lengths of quaternary faults are the primary basis in this work for constructing maps that characterize seismic hazard in california the expected seismic moment m0e and the strength of ground shaking resulting from the entire rupture of each mapped fault or fault segment are estimated using empirical relations between seismic moment m0 rupture length source to site distance and strong ground motions assuming a fault model whereby the repeat time t of earthquakes on each fault equals m0e m0g where the moment rate m0g is proportional to fault slip rate it is observed that the moment frequency distribution of earthquakes predicted from the geologic data agrees well with the distribution determined from a 150 year historical record the agreement is consistent with the argument that the geologic record of quaternary fault offsets contains information sufficient to predict the average spatial and size distribution of earthquakes through time in california the estimates of t for each fault are the foundation for constructing maps that depict the average return period of 0 1g peak horizontal ground accelerations and the horizontal components of peak acceleration peak velocity and the pseudovelocity response at 1 period and 5 damping expected to occur at the level of 0 1 probability during a 50 year period of time a map is also formulated to show the probability that 0 1g horizontal ground accelerations will occur during the next 50 years the maps serve to illustrate the potential value of quaternary fault studies for assessing seismic hazard interpretation of available slip rates indicates that the largest and most frequent occurrence of potentially destructive strong ground motions are associated principally with the san andreas san jacinto calaveras hayward and ventura basin fault zones other regions of similarly high hazard may yet remain unrecognized this inadequacy results primarily from an incomplete data set numerous faults for example are mapped along the coastal region of northern california and within the modoc plateau but relatively few studies relating to fault slip rate are reported a similar problem exists for other stretches of coastal california where marine reflection studies provide evidence of active faulting offshore yet yield little or no information of fault slip rate geological and geophysical field studies can work to remove these deficiencies a concerted effort to locate and define rates of activity on all faults in california is the most promising means to further quantify present levels of seismic hazard in california

ambient seismic noise tomography of the colima volcano complex the colima volcanic complex cvc located in the western sector of the trans mexican volcanic belt contains the most active mexican volcano volcan colima the cvc is located within the colima rift a regional north south striking extensional structure we used ambient seismic noise recorded by stations deployed in western mexico during the mapping the rivera subduction zone mars and the colima volcano deep seismic experiment codex we computed the cross correlations of the vertical component of continuous records of ambient noise data to extract empirical greens functions these functions provide detailed images of rayleigh wave group velocity for different periods using the arrival travel time of these waves for a given period estimates can be obtained of the lateral variations in velocity for a given period using 2d tomography the study aims to better understand the geometry and the seismic surface wave velocity structure of the cvc and relate it to the volcanoes structure and the geologic setting of the region source of low velocity anomaly over cvc is distributed fairly continuously with depth in the subsurface which indicates magma rising along fractures the progressive increasing toward the south in the size of low velocity anomalies indicates migration towards the south of the melting that correlates with the trend of the stratovolcanoes that form the cvc the zone of magma generation presently fully developed under volcan de fuego might be starting to shift towards south to the area nw of armeria where a new void in the tear zone may be starting to form

implications of the october 1995 colima jalisco mexico earthquakes on the rivera north america euler vector rivera north america euler vectors fall into three groups based on the predicted motion of the rivera plate relative to the north america plate along the middle america trench north of 18 5 n namely those that predict 1 a slow more easterly directed motion 2 a slow more northerly directed motion and 3 a faster more northerly directed motion the focal mechanism solution rupture area and seismic moment of the great m w 8 0 colima jalisco mexico earthquake of october 9 1995 and its foreshocks and aftershocks provide additional constraints on the rivera north america euler vector the slip direction n30 e of the main event clearly favors those euler poles which predict a more northerly direction of motion this direction lies within 5 of those predicted by these poles whereas it differs by between 9 and 18 from those poles which predict a more easterly motion the amount of slip which occurred during the main event suggests a minimum convergence rate of between 4 3 and 6 5 cm yr this favors euler vectors which predict higher convergence rates however a firm conclusion cannot be made due to the uncertainty in the recurrence time estimation a new rivera north america euler pole calculated incorporating the information provided by the 1995 events lies at 21 87 n 109 58 w it predicts a n30 3 e directed motion of the rivera plate relative to north america at the location of the main event a difference of only 0 3 from the slip direction of the main event

evidence for and implications of self healing pulses of slip in earthquake rupture dislocation time histories of models derived from waveforms of seven earthquakes are discussed in each model dislocation rise times the duration of slip for a given point on the fault are found to be short compared to the overall duration of the earthquake 10 however in many crack like numerical models of dynamic rupture the slip duration at a given point is comparable to the overall duration of the rupture i e slip at a given point continues until information is received that the rupture has stopped propagating alternative explanations for the discrepancy between the short slip durations used to model waveforms and the long slip durations inferred from dynamic crack models are 1 the dislocation models are unable to resolve the relatively slow parts of earthquake slip and have seriously underestimated the dislocations for these earthquakes 2 earthquakes are composed of a sequence of small dimension short duration events that are separated by locked regions barriers 3 rupture occurs in a narrow self healing pulse of slip that travels along the fault surface evidence is discussed that suggests that slip durations are indeed short and that the self healing slip pulse model is the most appropriate explanation a qualitative model is presented that produces self healing slip pulses the key feature of the model is the assumption that friction on the fault surface is inversely related to the local slip velocity the model has the following features high static strength of materials kilobar range low static stress drops in the range of tens of bars and relatively low frictional stress during slip less than several hundreds of bars it is suggested that the reason that the average dislocation scales with fault length is because large amplitude slip pulses are difficult to stop and hence tend to propagate large distances this model may explain why seismicity and ambient stress are low along fault segments that have experienced large earthquakes it also qualitatively explains why the recurrence time for large earthquakes may be irregular

the rupture mechanism of the coyote lake earthquake of 6 august 1979 inferred from near field data the coyote lake earthquake ml 5 7 occurred within the dense network of seismic instruments of central california and was recorded by numerous accelerograph stations two of these stations located within the fault zone itself provide particularly remarkable records of the rupture process we use these data obtained in the immediate vicinity of the fault and the broadband seismograms recorded at berkeley about 100 km away from the epicenter to infer the velocity of propagation of the rupture the extent of the fractured area the rupture front geometry and other characteristics of the fracture mechanism in order to extract this information from the data we model the earthquake as a propagating dislocation starting at the hypocenter and spreading radially we compute the resulting ground motion at the receiver sites using the discrete wavenumber representation of green s functions bouchon 1981 the results yield a rupture velocity of 2 6 km sec a fault length of about 14 km and a fault slip of 15 to 20 cm they also show that the fault extends to shallower depth than indicated by the aftershocks the most important result is the finding that slip at any given location on the fault takes place in a very short time span and that simple uniform dislocation models give a good description of the rupture process

crust and subduction zone structure of southwestern mexico southwestern mexico is a region of complex active tectonics with subduction of the young rivera and cocos plates to the south and widespread magmatism and rifting in the continental interior here we use receiver function analysis on data recorded by a 50 station temporary deployment of seismometers known as the mars mapping the rivera subduction zone array to investigate crustal structure as well as the nature of the subduction interface near the coast the array was deployed in the mexican states of jalisco colima and michoacan crustal thickness varies from 20 km near the coast to 42 km in the continental interior the rivera plate has steeper dip than the cocos plate and is also deeper along the coast than previous estimates have shown inland there is not a correlation between the thickness of the crust and topography indicating that the high topography in northern jalisco and michoacan is likely supported by buoyant mantle high crustal vp vs ratios greater than 1 82 are found beneath the trenchward edge of magmatism including below the central jalisco volcanic lineament and the michoacan guanajuato volcanic field implying a new arc is forming closer to the trench than the trans mexican volcanic belt elsewhere in the region crustal vp vs ratios are normal the subducting rivera and cocos plates are marked by a dipping shear wave low velocity layer we estimate the thickness of the low velocity layer to be 3 to 4 km with an unusually high vp vs ratio of 2 0 to 2 1 and a drop in s velocity of 25 we postulate that the low velocity zone is the upper oceanic crust with high pore pressures the low velocity zone ends from 45 to 50 km depth and likely marks the basalt to eclogite transition

rupture process of an earthquake with kilometric size fault inferred from the modeling of near source records abstract we use the exceptional set of strong ground motion records obtained during one of the largest aftershocks m l 5 0 of the 1979 imperial valley earthquake to study the time history of rupture on a kilometric size fault we apply single crack composite cracks and dislocation earthquake models to simulate the source and calculate the ground velocity and acceleration at the recording sites the best fit to the data is obtained for a crack like source where rupture is initiated at one edge and expands circularly over the fault the stress drop inferred is 720 bars and the fractured area has a diameter of about 1 km the fault is vertical and strikes in the n320 e direction this simple model explains surprisingly well the amplitudes and waveforms of the ground velocities and accelerations recorded in the near source region during the event

seismic imaging of the 1999 izmit turkey rupture inferred from the near fault recordings we use near fault accelerograms to infer the space time history of rupture on the fault during the izmit earthquake the records show that the ground displacement and velocity near the fault were surprisingly simple rupture propagated toward the west at a velocity of about 3 km s and toward the east at a remarkably high average velocity of 4 7 km s over a distance of about 45 km before decelerating to about 3 1 km s on the eastern segment slip on the fault is particularly large down to a depth of 20 km on the central portion of the fault where it reaches about 7 m slip is large also below 10 km on the eastern fault segment and this may have contributed to the loading of shear stress on the duzce fault on the western fault segment large slip seems confined to shallow depths

new insights into the subducting oceanic crust in the middle american trench off western mexico 17 19 n abstract we present new multibeam bathymetric and multichannel seismic data on the middle american trench between 17 and 19 n these data show north of the present tip of the east pacific rise unsedimented seafloor with an abyssal hill fabric bare of sediments at the outer wall of the trench 18 30 n two multichannel seismic profiles orthogonal to the middle american trench display clear oceanic crust extending more than 20 km beneath the overriding plate these seismic images suggest that the subducting oceanic crust along the middle american trench is younger than previously proposed this result therefore implies a new location for the east pacific rise termination in relation to the trench the subduction of this young oceanic crust off western mexico provides new constraints on the deformation and magmatism of the western mexican mainland

crustal deformation in great california earthquake cycles periodic crustal deformation associated with repeated strike slip earthquakes is computed for the following model a depth l h extending downward from the earth s surface at a transform boundary between uniform elastic lithospheric plates of thickness h is locked between earthquakes it slips an amount consistent with remote plate velocity vpl after each lapse of earthquake cycle time tcy lower portions of the fault zone at the boundary slip continuously so as to maintain constant resistive shear stress the plates are coupled at their base to a maxwellian viscoelastic asthenosphere through which steady deep seated mantle motions compatible with plate velocity are transmitted to the surface plates the coupling is described approximately through a generalized elsasser model we argue that the model gives a more realistic physical description of tectonic loading including the time dependence of deep slip and crustal stress buildup throughout the earthquake cycle than do simpler kinematic models in which loading is represented as imposed uniform dislocation slip on the fault below the locked zone parameters of the model are chosen to fit seismic and geologic constraints and the apparent time dependence of surface strain rates along presently locked traces of the 1857 and 1906 san andreas ruptures we fix vpl 35 mm yr tcy 160 years and l 9 11 km based on earthquake nucleation depths the geodetic data are then found to be described reasonably within the context of a model that is locally uniform along strike and symmetric about a single san andreas fault strand by lithosphere thickness h 20 30 km and elsasser relaxation time tr 10 16 years we therefore suggest that the asthenosphere appropriate to describe crustal deformation on the earthquake cycle time scale lies in the lower crust and perhaps crust mantle transition zone and has an effective viscosity between about 2 1018 and 1019 pa s depending on the thickness assigned to the asthenospheric layer predictions based on the chosen set of parameters are also consistent with data on variations of contemporary surface strain and displacement rates as a function of distance from the 1857 and 1906 rupture traces although the fit is degraded by asymmetry relative to the fault and by slip on adjacent fault strands

multichannel seismic imaging of the rivera plate subduction at the seismogenic jalisco block area western mexican margin during the tsujal marine geophysical survey conducted in february and march 2014 spanish mexican and british scientists and technicians explored the western margin of mexico considered one of the most active seismic zones in america this work aims to characterize the internal structure of the subduction zone of the rivera plate beneath the north american plate in the offshore part of the jalisco block to link the geodynamic and the recent tectonic deformation occurring there with the possible generation of tsunamis and earthquakes for this purpose it has been carried out acquisition processing and geological interpretation of a multichannel seismic reflection profile running perpendicular to the margin crustal images show an oceanic domain dominated by subduction accretion along the lower slope of the margin with a subparallel sediment thickness of up to 1 6 s two way travel time approx 2 km in the middle american trench further from these data the region appears to be prone to giant earthquake production the top of the oceanic crust intraplate reflector is very well imaged it is almost continuous along the profile with a gentle dip 10 however it is disrupted by normal faulting resulting from the bending of the plate during subduction the continental crust presents a well developed accretionary prism consisting of highly deformed sediments with prominent slumping towards the trench that may be the result of past tsunamis also a bottom simulating reflector bsr is identified in the first half a second twtt of the section high amplitude reflections at around 7 8 s twtt clearly image a discontinuous moho defining a very gentle dipping subduction plane

a quaternary monogenetic volcanic field in the xalapa region eastern trans mexican volcanic belt geology distribution and morphology of the volcanic vents abstract the most abundant volcanic manifestations along the east west trending trans mexican volcanic belt tmvb are the scoria cones these have been grouped by other authors in extended monogenetic volcanic fields such as michoacan guanajuato chichinautzin apan and los tuxtlas here we present geological and morphological data of a relatively unknown group of monogenetic volcanoes located on the east flank of the cofre de perote volcano cp around the city of xalapa in the state of veracruz mexico within an area of about 2400 km2 the xalapa monogenetic volcanic field xmvf contains over 50 late quaternary volcanoes most of them are scoria cones but small shield volcanoes and tuff rings also occur the lava flows produced by these volcanoes are constrained by an abrupt topography and cover a great percentage of the surface on the eastern and northeastern flanks of cp between 3000 and 500 m a s l the representative rocks of the different volcanic centers include olivine basalt basaltic andesite with phenocrysts of plagioclase pyroxene and minor olivine and andesite with phenocrysts of plagioclase and pyroxene sio2 and al2o3 contents of the rocks vary between 45 and 62 wt and 15 to 18 wt respectively most of the basaltic rocks have mgo contents between 4 2 and 9 wt ni and cr concentrations between 23 and 180 and 10 to 380 ppm respectively with a typical calc alkaline behavior trace elements suggest two types of magmas the most abundant are characterized by an enrichment of lile and lree with negative anomalies of nb and ti which denote a calc alkaline affinity others are lile depleted and show high concentrations of mgo cr and ni which is typical of primary calc alkaline magmas the mean scoria cone morphological values are cone height hco 90 8 m cone diameter wco 686 38 m crater diameter wcr 208 49 m and 0 12 km3 for the cone volume we dated twelve different scoria cones using the 40ar 39ar method for the other cones we used morphologic indicators such as the ratio of the scoria cone height to basal diameter hco wco and maximum slope angle additionally vegetation type and cone and lava flow morphologies were also considered in order to determine rough relative ages in general the ages fall into three groups those older than 2 0 ma those between 0 25 0 40 ma and those less than approximately 0 1 ma

shape of the subducted rivera and cocos plates in southern mexico seismic and tectonic implications the geometry of the subducted rivera and cocos plates beneath the north american plate in southern mexico was determined based on the accurately located hypocenters of local and teleseismic earthquakes the hypocenters of the teleseisms were relocated and the focal depths of 21 events were constrained using a body wave inversion scheme the suduction in southern mexico may be approximated as a subhorizontal slab bounded at the edges by the steep subduction geometry of the cocos plate beneath the caribbean plate to the east and of the rivera plate beneath north america to the west the dip of the interplate contact geometry is constant to a depth of 30 km and lateral changes in the dip of the subducted plate are only observed once it is decoupled from the overriding plate on the basis of the seismicity the focal mechanisms and the geometry of the downgoing slab southern mexico may be segmented into four regions 1 the jalisco region to the west where the rivera plate subducts at a steep angle that resembles the geometry of the cocos plate beneath the caribbean plate in central america 2 the michoacan region where the dip angle of the cocos plate decreases gradually toward the southeast 3 the guerrero oaxaca region bounded approximately by the onshore projection of the orozco and o gorman fracture zones where the subducted slab is almost subhorizontal and underplates the upper continental plate for about 250 km and 4 the southern oaxaca and chiapas region in southeastern mexico where the dip of the subduction gradually increases to a steeper subduction in central america these drastic changes in dip do not appear to take place on tear faults suggesting that smooth contortions accommodate these changes in geometry the inferred 80 and 100 km depth contours of the subducted slab lie beneath the southern front of the trans mexican volcanic belt suggesting it is directly related to the subduction thus the observed nonparallelism with the middle american trench is apparently due to the changing geometry of the rivera and cocos plates beneath the north american plate in southern mexico and not to zones of weakness in the crust of the north american plate as some authors have suggested

uniform sense normal simple shear of the continental lithosphere geophysical studies suggest that the thin crust characteristic of the basin and range province extends eastward beneath the west margin of the colorado plateau and rocky mountain regions in arizona and utah zones perhaps over 100 km wide may be defined bounded on the west by the east limit of upper crustal normal faults that account for more that 10 extension and on the east by the east limit of thinning beneath the colorado plateau a discrepancy exists within these zones between the negligible extension measurable in the upper crust and the substantial extension apparent from crustal thinning assuming the discrepant zone crust was as thick as or thicker than the colorado plateau rocky mountain crust prior to extensional tectonism if various theories appealing to crustal erosion are dismissed mass balance problems evident in the discrepant zones are most easily resolved by down to the east normal simple shear of the crust moving lower and middle crustal rocks that initially were within the zones up and to the west to where they now are locally exposed in the basin and range province west of the discrepant zones in both arizona and utah east directed extensional allochthons with large displacement are exposed these geophysical and geological observations complement one another if it is accepted that the entire crust in both arizona and utah failed during extension on gently east dipping east directed low angle normal faults and shear zones over a region several hundred kilometres wide large scale uniform sense normal simple shear of the crust suggests the entire lithosphere may do the same such a hypothesis predicts major lithospheric thinning without crustal thinning will occur in plateau areas in the direction of crustal shear in the case of the arizona utah and red sea extensional systems and possibly the death valley extensional terrain a broad topographic arch typically 1500 2000 m higher than the extended terrain is present suggesting lithospheric thinning in areas predicted by the hypothesis

gravimetric analysis of the rifts and volcanic fields of the jalisco block mexico abstract the gravity map of the complete jalisco block jb is obtained from satellite derived measurements at a resolution of 1 km separation including the offshore region up to the middle america trench mat tepic zacoalco tz and colima delimiting rifts of the jb are identified as gravimetric minima the nw limit of the block appears also as a rifted zone with a gravity minimum connecting bahia de banderas with the tz rift defining a new triple point the compostela triple point rift rift rift that together with the guadalajara triple point determine the north boundary of the jalisco block the largest negative gravity anomalies are observed associated with the central jalisco volcanic lineament cjvl and bahia de banderas they appear to be connected since they are only separated by a small positive anomaly east of bahia de banderas the cjvl is parallel to the middle america trench exhibiting a discontinuous alignment of quaternary volcanism from the colima volcanic complex to the northwestern rifted limit of the jalisco block ameca and santiago rivers flow parallel to each other in a northwesterly direction for 100 km then santiago river changes course to continue 64 km in a north northwesterly direction up to aguamilpa dam ameca river should follow the same direction instead it abruptly changes its course to a southwesterly direction to discharge in bahia de banderas the drastic course deviation is attributed to the intersection of ameca river causeway with the bahia de banderas compostela rift proposed in this study to be the northwestern limit of the jalisco block

a geodynamical perspective on the subduction of cocos and rivera plates beneath mexico and central america abstract the middle america subduction zone masz is one of the world most complex convergent margins as it involves the subduction of the rivera and cocos young oceanic plates beneath the north american and caribbean plates and is bounded by the gulf of california rift and the panama slab window characterized by contorted and unusual slab geometry irregularly distributed seismicity and volcanism exceptionally large slow slip events sse and non volcanic tremors nvt this subduction system represents a great natural laboratory for better understanding geodynamic processes at a fundamental level based on a solid observational foundation and incorporating the latest experimental results into a coherent geodynamical framework we shed light on the main processes controlling the subduction system evolution in this region the tectonics volcanism slab geometry and segmentation along the margin are reviewed from a geodynamical perspective we proposed and discussed a series of evolutionary scenarios for the mexican and central american subduction zones providing a coherent starting base for future geodynamical modeling studies tailored to this active margin we discuss comparatively the recently discovered sses and nvts along the masz and try to differentiate among the proposed mechanisms responsible for these observations finally we discuss the recent seismic anisotropy observations in a geodynamic context offering an integrated view of mantle flow pattern along the entire active margin although the masz as a whole may be considered a fairly complicated region with many unusual features and sometimes controversial interpretations its complexity and unusual characteristics can improve our knowledge about the linkage between deep and surface processes associated with subduction zone dynamics

the near source ground motion of the 6 august 1979 coyote lake california earthquake a finite fault striking n24 w and extending to a depth of 10 km is proposed to explain the strong ground motion data for the 6 august 1979 coyote lake california earthquake m l 5 9 our source model suggests that right lateral faulting initiated at a depth of 8 km and ruptured toward the south with a velocity of 2 8 km sec this unilateral rupture can explain the large displacement recorded south of the epicenter however the waveform coherency across an array south and southwest of the epicenter suggests that the rupture length is less than 6 km the maximum dislocation is about 120 cm in a small area near the hypocenter and the total moment is estimated to be 3 5 10 24 dyne cm an abrupt stopping phase which corresponds to a deceleration of right lateral motion can explain the high peak acceleration recorded at array station 6 the stress drop in the hypocentral area is about 140 bars the average stress drop over the entire rupture surface is 30 bars the preferred finite source model can predict the p n1 waveforms and the beginning features in the teleseismic seismograms no clear arrivals can be observed in the near source data for the possible second and third smaller events suggested by nabelek personal communication

motion of the rivera plate since 10 ma relative to the pacific and north american plates and the mantle abstract to better understand the influence of rivera plate kinematics on the geodynamic evolution of western mexico we use more than 1400 crossings of seafloor spreading magnetic lineations along the pacific rivera rise and northern mathematician ridge to solve for rotations of the rivera plate relative to the underlying mantle and the pacific and north american plates at 14 times since 9 9 ma our comparison of magnetic anomaly crossings from the undeformed pacific plate to their counterparts on the rivera plate indicates that significant areas of the rivera plate have deformed since 9 9 ma dextral shear along the southern edge of the plate from 3 3 2 2 ma during a regional plate boundary reorganization deformed the rivera plate farther into its interior than previously recognized in addition seafloor located north of two rupture zones within the rivera plate sutured to north america after 1 5 ma anomaly crossings from these two deformed regions thus cannot be used to reconstruct motion of the rivera plate finite rotations that best reconstruct pacific plate anomaly crossings onto their undeformed counterparts on the rivera plate yield stage spreading rates that decrease gradually by 10 between 10 and 3 6 ma decrease rapidly by 20 after 3 6 ma and recover after 1 ma the slowdown in pacific rivera seafloor spreading at 3 6 ma coincided with the onset of dextral shear across the then incipient southern boundary of the rivera plate with the pacific plate the available evidence indicates that the rivera plate has been an independent microplate since at least 10 ma contrary to published assertions that it fragmented from the cocos plate at 5 ma motion of the rivera plate relative to north america has changed significantly since 10 ma in concert with significant changes in pacific rivera motion a significant and robust feature of rivera north america motion not previously recognized is the cessation of margin normal convergence and thus subduction from 2 6 to 1 0 ma along the entire plate boundary followed by a resumption of trench normal subduction along the southern half of the rivera north america plate boundary after 1 0 ma motion of the rivera plate relative to the underlying mantle since 10 ma has oscillated between periods of landward motion and seaward motion the evidence suggests that the torque exerted by slab pull on this young and hot oceanic plate is either minimal or is effectively counterbalanced by forces that resist its motion

preseismic and coseismic deformation associated with the coyote lake california earthquake supplement is available with entire article on microfiche orderfrom american geophysical union 2000 florida avenue n w washington d c 20006 document j79 008 01 00 payment mustaccompany order the coyote lake earthquake ml 5 9 august 6 1979 epicenter about 100 km southeast of san francisco occurred on the calaveras fault within a geodetic network that had been surveyed annually since 1972 to monitor strain accumulation the rupture surface as defined by aftershocks is a vertical rectangle 20 km in length extending from a depth of 4 km to about 12 km the observed deformation of the geodetic network constrains the average slip to be about 0 33 0 05 m right lateral although the geodetic data furnished an exceptionally detailed picture of the preearthquake deformation no significant premonitory anomaly associated with the coyote lake earthquake can be identified

slip partitioning of the calaveras fault california and prospects for future earthquakes examination of main shock and microearthquake data from the calaveras fault during the last 20 years reveals that main shock hypocenters occur at depths of 8 9 km near the base of the zone of microearthquakes the spatial pattern of pre main shock microseismicity surrounding the coyote lake and morgan hill hypocenters is similar to the pattern of the post main shock microseismicity microseismicity extends between depths of 4 and 10 km and defines zones of concentrated microseismicity and aseismic zones estimates of the fault regions which slipped during the coyote lake and morgan hill earthquakes as derived from seismic radiation coincide with zones which are otherwise aseismic we propose that these persistent aseismic zones represent stuck patches which slip only during moderate earthquakes from the pattern of microearthquake locations we recognize six aseismic zones where we expect future main shocks will rupture the calaveras fault from an analysis of historic seismic data we establish the main shock rupture history for each aseismic zone and identify two zones that are the most likely sites for the next m 5 earthquakes the first zone is located near gilroy and was last ruptured by a m5 2 earthquake in 1949 the second zone is located south of calaveras reservoir and north of the 1988 m5 1 alum rock earthquake it has not slipped seismically since at least 1903 and the size of the aseismic region is sufficiently large to sustain a m5 5 earthquake

crustal architecture at the collision zone between rivera and north american plates at the jalisco block tsujal project processing and analysis of new multichannel seismic records coincident with wide angle seismic profiles acquired in the framework of the tsujal project allow us to investigate in detail the complex structure of the oceanic domain in the collision zone between rivera plate and block jalisco at its northern termination the subducting rivera plate which is overridden by the north american plate jalisco block is clearly identified up to 21 5 n just south of maria magdalena island as a two clear reflections that we interpret as the interplate and moho discontinuities north of the tres marias islands the seismic images display a different tectonic scenario with structures that are consistent with large faulting and rifted margin a two dimensional velocity approach for the crustal geometry is achieved using joint refraction reflection travel time tomography the uncertainty of the results is assessed by means of monte carlo analysis our results show an average oceanic crustal thickness of 6 7 km with a moderate increase towards the jalisco block an anomalous thick layers 3 0 km displaying a relatively low velocity 5 5 km s underneath maria magdalena rise and an estimated moho depth deeper than 15 km in the collision zone between rivera plate and jalisco block we have also determined an anomalous crust on the western flank of the tres marias islands which may be related to the initial phases of continental breakup of the baja california peninsula and mexico mainland high resolution bathymetry provides remarkable images of intensive slope instabilities marked by relatively large slides scars of more than 40 km2 extent and mass wasting deposits probably triggered by the intense seismicity in the area

using an enhanced dataset for reassessing the source region of the 2003 armeria mexico earthquake we present a fresh look at the source region of the 22 january 2003 m w 7 4 armeria earthquake which occurred off the pacific coast of the state of colima mexico near the town of armeria the effects of this earthquake in the neighboring states of colima and jalisco were different and stronger than those of previous recent major earthquakes in the region this earthquake and its aftershocks were recorded by two local telemetered seismograph networks resco and resjal from 22 january to 24 january 2003 no important seismicity was located on the plates interface or within the rivera plate and most epicenters were located west of the armeria river which is the western border of the colima graben and is located outside of the colima gap region from 24 january to 31 january the seismicity recorded by both networks showed a migration in depth with an almost vertical offshore distribution between 4 and 24 km in depth for this period a seven station portable digital seismograph network equipped with three component seismometers was deployed in the epicentral area to study the aftershock sequence in detail with this denser network more than 200 m l 2 0 aftershocks were recorded the aftershock foci were deeper than those recorded during the early period and most of them locate on a hypothetical 12 dipping interface between the rivera and north american plates composite focal mechanism solutions for the aftershocks located during both periods indicate a reverse fault character that changes with time analysis of the new dataset still indicates that the earthquake was a shallow intraplate event

anatomy of the colima volcano magmatic system mexico abstract colima volcano is one of the most active volcanoes in continental north america it is located within the colima graben on the western part of the colima rift zone although extensively studied the internal structure and deep magmatic system remains unknown this research gives new clues to understand how and where magmas are produced and stored at depth using ambient seismic noise we jointly invert for rayleigh and love wave dispersion curves for both phase and group velocity which is applied for the first time in a volcanic environment we invert for both the shear wave velocity and radial anisotropy the 3d high resolution shear wave velocity model shows a deep large and well delineated elliptic shape magmatic reservoir below the colima volcano complex at a depth of about 15 km on the other hand the radial anisotropy model shows a significant negative feature i e v s v v s h revealed from 35 km depth until the top of the magma reservoir at about 12 km depth the latter suggests the presence of numerous vertical fractures where fluids rooting from a well known mantle window can easily migrate upward and then accumulate in the magma reservoir furthermore the convergence of both a low velocity zone and a negative anisotropy suggests that the magma is mainly stored in conduits or inter fingered dykes as opposed to horizontally stratified magma reservoir

a pliocene ignimbrite flare up along the tepic zacoalco rift evidence for the initial stages of rifting between the jalisco block mexico and north america the tepic zacoalco rift a nw trending corridor 50 250 km is one arm of a triple rift system in western mexico together with the colima rift and the middle america trench it bounds the jalisco block a portion of western mexico that may be moving independently of north america the predominant basement rock types in the tepiczacoalco rift are rhyolitic ash fl ow tuffs and lavas which were previously assumed to be oligocene miocene in age related to the sierra madre occidental volcanic province or older new 40 ar 39 ar dates on 41 volcanic samples reveal a previously unrecognized voluminous fl are up of rhyolitic ignimbrites between 5 and 3 ma throughout the entire corridor of the tepic zacoalco rift they are often associated with pliocene high ti basalts the eruption rate during this pliocene time period was an order of magnitude higher hundreds of m m y than that documented in the tepic zacoalco rift over the last 1 m y the pliocene ash fl ow tuffs have been faulted along nw trending lineaments producing vertical offsets up to at least 500 m the voluminous ignimbrite fl are up in the tepiczacoalco rift at 5 3 ma may refl ect the initial stages of rifting of the jalisco block away from north america analogous to what occurred in the proto gulf region at 12 6 ma prior to the transfer of baja california from north america to the pacifi c plate additionally new 40 ar 39 ar dates show that the sierra madre occidental volcanic province extends across the entire width of the tepic zacoalco rift and terminates abruptly at the northern boundary of the jalisco block near the rio ameca in contrast paleoceneeocene basement from the jalisco block extends northward into the tepic zacoalco rift where it is locally overlain by sierra madre occidental rhyolites

relative motions of the pacific rivera north american and cocos plates since 0 78 ma we use magnetic anomaly and fracture zone crossings from 3 n 24 n along the east pacific rise to define the boundaries and relative velocities of the rigid pacific rivera north american and cocos plates since 0 78 ma and to test several hypotheses regarding the nature of deformation between these plates crossings of the 0 78 ma isochron from the pacific rivera rise north of 22 0 n are significantly better fit by the 0 0 78 ma pacific north america rotation than by a pacific rivera rotation thereby supporting a recently proposed model for accretion of the northern part of the rivera plate to north america a significant misfit of the pacific cocos rotation to crossings of anomaly ln from the east pacific rise north of 16 4 n suggests that seafloor north of 16 n moves relative to the rigid pacific or cocos plates we also find that nine crossings of the eastern rivera fracture zone and eight crossings of anomaly ln flanking the manzanillo spreading segment are fit well by a pacific rivera rotation the rivera cocos angular velocity derived from the best fitting pacific cocos and pacific rivera angular velocities predicts 19 3 mm yr 1 95 confidence limit of n s convergence near the center of the rivera cocos boundary nearby left lateral strike slip earthquakes with north trending fault planes suggest that convergence is accommodated via sinistral shear along a north trending zone the unambiguous kinematic and seismologic evidence for significant rivera cocos motion indicates that the absence of a well defined rivera cocos plate boundary cannot be interpreted as evidence that rivera cocos motion is slow or zero as is postulated in several previous studies the rivera cocos angular velocity predicts oblique rivera cocos convergence at the el gordo graben and beneath the manzanillo trough and southern colima graben any extension across these features since 0 78 ma thus cannot have resulted from divergence between the subducting rivera and cococs plates

source rupture process of the tecoman colima mexico earthquake of 22 january 2003 determined by joint inversion of teleseismic body wave and near source data the spatial and temporal slip distribution of the tecoman colima mexico earthquake is estimated from near source strong motion and teleseismic body wave data to perform a stable inversion we applied smoothing constraints and determined their optimal relative weights on the observed data using an optimized akaike s bayesian information criterion abic the source parameters are as follows strike dip slip 300 20 93 seismic moment m 2 3 1020 n m source duration 30 sec along strike distance 35 km along dip distance 70 km we found that the rupture process can be divided into three stages the rupture nucleated near the hypocenter stage i then it broke the first asperity centering about 15 km southwest from the epicenter stage ii and the rupture propagated to the northeast and the second asperity was broken stage iii we also estimated the shear stress change due to the rupture process of the mainshock on and around the major fault zone it appears that one cluster of aftershocks for the first 5 days which took place in and adjacent to the zones of stress increased due to the fault rupture during the mainshock but overall correlation between the aftershock location and the stress pattern is not clear manuscript received 14 may 2003

tectonic tremor and slow slip along the northwestern section of the mexico subduction zone abstract the southwestern coast of mexico is marked by active subduction of the rivera and cocos plates producing megathrust earthquakes that tend to recur every 50 100 yr herein we use seismic and gps data from this region to investigate the potential relationship between earthquakes tectonic non volcanic tremor and transient slip along the westernmost 200 km of the mexico subduction zone visual examination of seismograms and spectrograms throughout the 18 month long mars seismic experiment reveals clear evidence for frequent small episodes of tremor along the rivera and cocos subduction zones beneath the states of jalisco colima and michoacan using a semi automated process that identifies prominent energy bursts in envelope waveforms of this new data analyst refined relative arrival times are inverted for source locations using a 1 d velocity model the resulting northwest southeast trending linear band of tremor is located downdip from the rupture zones of the 1995 mw 8 0 colima jalisco and 2003 mw 7 2 tecoman subduction thrust earthquakes and just below the regions of afterslip triggered by these earthquakes despite the close proximity between tremor and megathrust events there is no evidence that the time since the last great earthquake influences the spatial or temporal pattern of tremor a well defined gap in the tremor beneath the western colima graben appears to mark a separation along the subducted rivera cocos plate boundary from the position time series of 19 continuous gps sites in western mexico we present the first evidence that slow slip events occur on the rivera plate subduction interface unlike the widely recorded large amplitude slow slip events on the nearly horizontal cocos plate subduction interface below southern mexico slow slip events below western mexico have small amplitudes and are recorded at relatively few mostly coastal stations the smaller slow slip beneath western mexico may be due to the steeper dip causing a narrower zone where the conditions for slow slip are met the prominent afterslip following the m 8 1995 colima jalisco and m 7 2 2003 tacoma megathrust earthquakes may also shrink the area available for episodic slow slip

slip kinematics and dynamics during and after the 1995 october 9 mw 8 0 colima jalisco earthquake mexico from gps geodetic constraints we use horizontal and vertical crustal displacements derived from gps measurements at 26 sites in western mexico to study the coseismic and post seismic kinematics and dynamics of the 1995 october 9 m w 8 0 colima jalisco earthquake along the middle america trench the measurements bracket the entire landward edge of the approximately 150 km long rupture zone and span a 4 yr period for most sites we solve for the temporal evolution of slip along the subduction interface by inverting gps displacements for the coseismic and four post seismic intervals march 1995 march 1999 subject to the assumption that the crust responds elastically to slip along a shallow dipping curved subduction interface coseismic rupture of up to 5 m was largely focused above depths of 20 km and was limited to a 120 140 km long segment of the subduction zone within one week of the earthquake post seismic slip migrated downdip to depths of 16 35 km where it has since decayed logarithmically we also find evidence for shallow aseismic slip during 1996 or early 1997 northwest of the coseismic rupture zone and increasingly widespread relocking of shallow regions of the subduction interface after early 1997 the relative lack of afterslip in shallow regions of the subduction interface suggests that the interface lies in the unstable frictional regime and hence is strongly coupled between earthquakes by 1999 the cumulative slip moment associated with post seismic slip equaled 70 per cent of the coseismic moment with nearly all of this slip occurring downdip from the coseismic rupture zone the migration of slip after the earthquake to a deeper and presumably velocity strengthening area of the subduction interface and the logarithmic decay of afterslip conform to the qualitative and quantitative predictions of a model in which the fault kinematics are prescribed by rate and state variable frictional laws however misfits to the geodetic displacements exceed the average displacement uncertainties for all epochs implying one or more of the following 1 the elastic response is heterogeneous due to slip along unmodelled upper crustal faults or variations in the elastic properties of the crust 2 other post seismic mechanisms such as viscoelastic or poroelastic effects contribute to or possibly dominate the post seismic response 3 we have underestimated the uncertainties in the gps displacements