source spectra and spectral attenuation of seismic waves from mexican earthquakes and evidence of amplification in the hill zone of mexico city it is well known that seismic waves are enormously amplified at lake bed sites with respect to hill zone sites in mexico city it has been suggested that even hill zone sites suffer amplification in this paper we use eight recent well recorded earthquakes to determine spectral attenuation of seismic waves and to quantify the suggested amplification in the hill zone results show that indeed spectral amplitudes at these sites are roughly 10 times greater than expected for sites outside mexico city at similar hypocentral distances this implies that lake bed sites are amplified by a factor of 100 to 500 at some frequencies between 0 2 to 0 7 hz the source spectra retrieved from the regression analysis agree well with those obtained from epicentral recordings alone for two of the largest earthquakes ms 8 1 and 7 6 whose seismic moments are well determined and whose source spectra can be patched in a very broad band of frequencies 0 015 to 20 hz we observe clear deviation from an 2 model the spectra at intermediate frequencies from about 0 05 to 2 hz show a sag whether the observed spectra deviate from an 2 model for events with ms 6 depends on the reliability of the reported long period seismic moments if they are considered reliable then these events also deviate from the model if however the reported seismic moments of smaller events are ignored then the observed spectra can be fitted well with the 2 model

tectonic setting and source parameters of the september 19 1985 michoacan mexico earthquake analysis of body waves and long period surface waves from the september 1985 earthquake in coastal michoacan mexico shows that the event was an interplate subduction event with a low dip angle fault plane 9 striking parallel to the mid america trench 288 and a small component of left lateral motion 72 with a point source depth of 17 km and a seismic moment in excess of 1 10 28 dyn cm the earthquake was a multiple event with a second source of identical moment fault geometry and depth occurring approximately 26 s after the first directivity in the body wave time function indicates that the second event occurred roughly 100 km to the southeast of the first this suggests that the earthquake first broke the northern portion of the michoacan gap propagated with low moment release through the rupture zone of the 1981 playa azul earthquake and then broke the remaining asperity in the southern section of the gap the seismic moment determined from rayleigh and love waves is between 1 0 1 7 10 28 dyn cm m w 7 9 8 1 the largest moment determined to date for a mexico subduction earthquake comparison of seismograms at pasadena with records of other large mexico events shows that the michoacan earthquake is basically the same size as the 1932 jalisco mexico earthquake and clearly larger than other significant events in mexico since 1932 the seismic moment and the time since the last large earth quake in michoacan in 1911 fit an empirical relation between moment and recurrence time found for the guerrero oaxaca region of the subduction zone the large aftershock on september 21 m s 7 5 has the same geometry as the mainshock a somewhat larger source depth 22 km a simple time function and a seismic moment between 2 9 4 7 10 27 dyn cm m w 7 6 7 7

volcano tectonic evolution of the santa maria island azores implications for paleostress evolution at the western eurasia nubia plate boundary abstract the growth and decay of oceanic volcanoes developed close to plate boundaries are intrinsically related to a competition between construction and destruction processes partly controlled by tectonic strain and stresses from morphologic stratigraphic tectonic and new high precision k ar data we present a comprehensive picture of the volcano tectonic evolution of santa maria and discuss its significance regarding the stress evolution and regional deformation in the azores our new data show that 1 the western flat portion of the island is mostly composed of west dipping volcanic rocks here dated between 5 70 0 08 and 5 33 0 08 ma which we consider the remnants of an older shield volcano 2 more than half of this early volcanic complex has been removed by an east directed large scale sector collapse 3 a second volcano here coined the younger shield volcano grew rapidly on the collapse scar between at least 4 32 0 06 and 3 94 0 06 ma 4 more than half of this new volcano was removed by a second large scale sector collapse most probably around 3 6 ma based on the ages of parasitic scoria cones sitting unconformably on the younger shield volcano 5 the latest parasitic volcanic activity is here dated at 2 84 0 04 ma extending significantly the known eruptive history of santa maria morpho structural data shape of the island faults dikes and distribution of volcanic cones show a significant control of construction and destruction along the n045 and n150 directions the age of the lavas intruded by dikes suggests that the n045 and the n150 trends are ca 5 3 ma old and younger than ca 4 3 ma respectively based on the new data we conclude that a change in the regional stress field occurred between 5 3 and 4 3 ma most likely associated with a major reconfiguration of the eurasia nubia plate boundary in the azores

gravity anomalies and crustal signature of volcano tectonic structures of pico island azores pico island is the youngest 300 ka island of the azores archipelago dominated on its western sector by the stratovolcano of pico mountain which rises about 3 5 km above the surrounding ocean floor the other two major morpho tectonic units of the island include the shield volcano of topo and the achada plateau a 29 km long volcanic ridge with a wnw ese trend a gravimetric study on pico island coupled with geological and volcanological data was aimed towards a better understanding of its deep volcano tectonic structures from a total of 407 stations a regional model of density contrasts was developed in order to reproduce the crustal structure of the island in addition a local high resolution model of the sub surface structure of the westernmost part of the island is presented both models were obtained by means of an inversion global adjustment on fixed density contrasts in order to determine the three dimensional geometry of the sources of the anomalous gravity field the most prominent anomalous gravity structure found in pico island is a body with positive density contrast located on the se slopes of topo volcano this anomaly extends to depth of approximately 8 km mean depth of 4 5 km and has a volume of 310 km 3 this anomaly maybe best interpreted by the presence of a major intrusive system beneath topo volcano and can be also correlated to gravitational collapses at the shield volcano a second body of positive density contrast which appears to extend to 4 5 km depth is located near the town of madalena this anomaly may be correlated to a high density anomaly recently detected in the ne sector of faial island smaller sized high density bodies at shallow depths of around 1 3 km are located beneath pico mountain volcano and all together define a semi circular arc with an average diameter of 8 km we propose that this arcuate structure might outline a ring fracture associated with the collapse of the central sector of pico mountain volcano early in its growth low density bodies are located on the nw flanks of pico mountain along a linear ene trending structure these bodies extend to depths of 4 km and have a total volume of 69 km 3 they are interpreted as being associated with an old extensional fracture zone previous studies suggested the existence of a shallow and large magma reservoir with volumes up to 1000 km 3 beneath pico mountain volcano in this study we have not found any evidence of the existence of such huge magma body on the contrary we propose that those discrete low density bodies correspond to complex intersections of different sets of tectonic lineaments along which magma tends to intrude as shallow 2 5 km and small 10 20 km 3 magma pockets c 2006 elsevier b v all rights reserved

correlation between the fault rupture pattern and associated microseismic activity of the petatlan earthquake summary the fault rupture pattern of the 1979 march 14 petatlan mexico earthquake m 7 6 was analysed in previous studies using local and teleseismic data here we demonstrate that the local data are consistent with the complex source of this event inferred from recent p wave inversion studies the result is particulary important since this correlation is obtained from the analysis of the pre and post seismic activity as recorded by a local array although local seismic records are unable to establish the rupture complexity of a mainshock the foreshock and aftershock activity of the petatlan earthquake provided some of the source characteristics of this event

dynamic rupture of asperities and stress change during a sequence of large interplate earthquakes in the mexican subduction zone we investigate the spatial and temporal variations of shear stress due to the successive failures over an extensive segment of the mexican subduction zone during a sequence of large interplate earthquakes that occurred over a period of 13 yr for this purpose we develop 3d dynamic rupture models incorporating a shal lowly dipping fault located above the subducting plate the spatial distribution of dynamic stress drop over the fault has been estimated for each of the events through an inversion procedure using some of the previously derived kinematic fault param eters as observational constraints the results revealed quite heterogeneous stress changes during these earthquakes coming from medium to high dynamic stress drop due to the rupture of a few patch like asperities and from stress increase in between and around them two weak asperities located southeast of the michoacan segment were ruptured first by the 1979 petatlan event the 1981 playa azul event ruptured two asperities in the central zone with a stress drop higher than 80 bars the largest 1985 michoacan earthquake resulted from the rupture of two large size strong asperities located at both sides of the 1981 fault zone with high stress drop of 80 to 100 bars and from another two asperities at depth two days after this largest event two asperities were broken during the zihuatanejo aftershock in the southeastern adjacent zone many after shocks of these large events tend to be distributed in the zones of stress increase outside the asperities while only small numbers of aftershocks have been observed within these asperity zones it appears that several major asperities that existed in this extensive segment have been ruptured successively so as to fill unbroken gaps on the plate interface thus the stress change left over from the previous earthquake has dominant effects on the next event in this subduction zone

potbellies wasp waists and superparamagnetism in magnetic hysteresis because the response of a magnetic substance to an applied field depends strongly on the physical properties of the material much can be learned by monitoring that response through what is known as a magnetic hysteresis loop the measurements are rapid and quickly becoming part of the standard set of tools supporting paleomagnetic research yet the interpretation of hysteresis loops is not simple it has become apparent that although classic single domain pseudo single domain and multidomain loops described in textbooks occur in natural samples loops are frequently distorted having constricted middles wasp waisted loops or spreading middles and slouching shoulders potbellies such complicated loops are often interpreted in oversimplified ways leading to erroneous conclusions the physics of the problem have been understood for nearly half a century yet numerical simulations appropriate to geological materials are almost unavailable in this paper we discuss results of numerical simulations using the simplest of systems the single domain superparamagnetic sd sp system examination of the synthetic hysteresis loops leads to the following observations 1 wasp waisting and potbellies can easily be generated from populations of sd and sp grains 2 wasp waisting requires an sp contribution that saturates quickly resulting in a steep initial slope and potbellies require low initial slopes the sp contribution approaching saturation at higher fields the approach to saturation is dependent on volume hence the cube of grain diameter therefore there is a very strong dependence of hysteresis loop shape on the assumed threshold size 3 we were unable to generate potbellies using an sp sd threshold size as large as 30 nm and wasp waists cannot be generated using a threshold size as small as 8 nm the occurrence of both potbellies and wasp waists in natural samples is consistent with a room temperature threshold size of some 15 nm 5 4 simulations using a threshold size of 15 20 nm with populations dominated by sp grain sizes that is with a small number of sd grains produce synthetic hysteresis loops consistent with measured hysteresis loops and transmission electron microscopic observations from submarine basaltic glass 5 simulations and measurements using two populations with distinct coercivity spectra can also generate wasp waisted loops a relatively straightforward analysis of the resulting loops can distinguish the latter case from wasp waisting resulting from sp sd behavior

seismic moment catalog of large shallow earthquakes 1900 to 1989 we compile a worldwide catalog of shallow depth 70 km and large ms 7 earthquakes recorded between 1900 and 1989 the catalog is shown to be complete and uniform at the 20 sec surface wave magnitude ms 7 0 we base our catalog on those of abe 1981 1984 and abe and noguchi 1983a b for events with ms 7 0 those catalogs however are not homogeneous in seismicity rates for the entire 90 year period we assume that global rates of seismicity are constant on a time scale of decades and most inhomogeneities arise from changes in instrumentation and or reporting we correct the magnitudes to produce a homogeneous catalog the catalog is accompanied by a reference list for all the events with seismic moment determined at periods longer than 20 sec using these seismic moments for great and giant earthquakes and a moment magnitude relationship for smaller events we produce a seismic moment catalog for large earthquakes from 1900 to 1989 the catalog is used to study the distribution of moment released worldwide although we assumed a constant rate of seismicity on a global basis the rate of moment release has not been constant for the 90 year period because the latter is dominated by the few largest earthquakes we find that the seismic moment released at subduction zones during this century constitutes 90 of all the moment released by large shallow earthquakes on a global basis the seismic moment released in the largest event that occurred during this century the 1960 southern chile earthquake represents about 30 to 45 of the total moment released from 1900 through 1989 a frequency size distribution of earthquakes with seismic moment yields an average slope b value that changes from 1 04 for magnitudes between 7 0 and 7 5 to b 1 51 for magnitudes between 7 6 and 8 0 this change in the b value is attributed to different scaling relationships between bounded large and unbounded small earthquakes thus the earthquake process does have a characteristic length scale that is set by the downdip width over which rupture in earthquakes can occur that width is typically greater for thrust events at subduction zones than for earthquakes along transform faults and other tectonic environments

lithospheric stress states predicted from long term tectonic models influence of rheology and possible application to taiwan abstract the long term stress state of the lithosphere results from a combination of boundary forces surface loads and tractions from mantle flow here we have employed a numerical technique which allows for the self consistent treatment of the interaction between the lithosphere and mantle and takes into account the deformation of the free surface we have used this to study lithospheric dynamics in order to gain insights on the present day conditions of stress and deformation of the lithosphere our numerical approach takes the observed topography and far field plate velocities as input and computes the resultant deformation as a function of the rheological stratification our model application is inspired by taiwan tectonics stress data and earthquake focal mechanisms indicate that the upper crust in taiwan may be predominantly under compression the recent pingtung earthquake in 12 26 2006 however suggests that the sub moho mantle lithosphere under southern taiwan may be under extension if this earthquake indeed represents the local state of stress this raises the question as to why extension and compression can occur simultaneously in this region for addressing this issue we have performed a number of numerical simulations in which we studied the effects of rheological stratification boundary conditions and geometry on the stress state and lithospheric dynamics we have employed both layered viscosity models and models in which we use laboratory based non newtonian and temperature dependent creep laws results show that the rheological stratification plays an important role in lithospheric deformation the relatively hot lithospheric thermal state in taiwan makes it likely that part of the lower crust is relatively weak which results in decoupling from surface near deformation with deformation at depth bending of a subducting slab underneath taiwan causes an extensional stress state in the sub moho mantle lithosphere whereas the collision of the philippine sea plate with the eurasian margin is responsible for a compressional state of stress in the crust it is demonstrated that models with a weak lower crust are also in reasonable agreement with available gps data as well as with constraints on uplift rates from long term geodynamic modelling studies

crustal seismic velocity structure near faial and pico islands azores from local earthquake tomography the azores archipelago portugal is located on an oceanic plateau in a geodynamic environment prone to intense seismic and volcanic activity in order to investigate the crustal structure in this region we have conducted a local earthquake tomography study in the area of the islands of faial pico and s jorge using data recorded in july 1998 the july 9th 1998 earthquake near faial island triggered an aftershock sequence of thousands of events that lasted for several months and were recorded by a total of 14 stations located on the three islands surrounding the epicentral area in the upper crustal layers consistency is seen between the tomographic results and the islands surface volcanic units beneath the faial central volcano a low vp b6 0 km s anomaly roughly located at 3 7 km depth suggests a connection to the plumbing system possibly the presence of a magma chamber in ne faial a high vp n6 3 km s body was found located at mid lower crust most likely an intrusion of gabbroic composition that is bordered by the registered seismic activity its shape suggesting a tectonic controlled mechanism the relocated hypocenters together with the overall analysis of the tomographic model suggest a tectonic segmentation of faial island the crustal thickness under the islands volcanic buildings of the faial pico area was estimated at around 14 km 2007 elsevier b v all rights reserved

tectonic framework of the azores triple junction the lack of accurate and detailed magnetic information has in the past limited the development of well constrained models for the plate tectonic evolution of the azores triple junction an aeromagnetic survey made possible by the existing airport facilities has long been desired as it can provide high quality magnetic data whose homogeneity and coherency is far better than those provided by classic marine surveys the results presented in this paper concern only a part of the aeromagnetic survey conducted by the portuguese instituto nacional de meteorologia e geofisica and already allow an improved definition of the basic tectonic boundaries at the central part of the azores plateau

ground motion and tectonics in the terceira island tectonomagmatic interactions in an oceanic rift terceira rift azores triple junction abstract the interpretation of high resolution topography bathymetry gps and insar data and detailed structural geology indicate that 1 terceira developed at the intersection of two major volcano tectonic lineaments wnw ese local tr s direction and nnw sse submarine chain of volcanoes here firstly recognised and coined terceira seamount chain 2 terceira is affected by four main fault systems the ca n165 normal faults dipping to east and west mostly across the middle of the island the n140 normal faults mostly making up the lajes graben the n110 faults with oblique striations normal dextral making up the main volcanic lineament and the more subtle n70 the transform direction related to the nubia eurasia plate boundary seismicity gps data and faults displacing the topography indicate that all systems are active 3 the whole island is subsiding at a rate of ca 5 mm yr as attested by both gps and insar data which is exceptionally high for the azores islands common explanations like thermal contraction or bending of the lithosphere or magmatic processes or collapse of the island under its own weight likely cannot justify the observed subsidence rate the estimated average of tr s subsidence rate is also not enough therefore we conclude that the measured 5 mm yr can be a peak 4 the fault geometry and kinematics are consistent with the current direction of maximum extension in the azores ca n65 and the rotation of nubia relative to eurasia 5 given that the ne shoulder of the lajes graben is moving upwards at 5 mm yr and sits directly on the tr s ne shoulder we conclude that the tr s shoulder is moving up most likely as a result of the elastic rebound associated with rifting the elastic rebound in both ne and sw tr s shoulders is most likely responsible for the observed ridge morphology all along the tr

seismic strain release along the middle america trench mexico we measure the seismic moment of five great ms 7 8 earthquakes from 1928 to 1942 along the mexican portion of the middle american trench directly from recorded seismograms and obtain considerably lower moment values than had previously been estimated from ms such direct moment determinations are inherently more reliable as the average seismic slip rate along this portion of the arc appears to be essentially constant these measurements of the past seismic moment are used to estimate that the current end of 1980 moment deficiency is 46 1027 dyne cm or an average slip of 97 cm along the arc although this is much lower than past estimates at the current rate of increase 1 6 1027 dyne cm yr 3 4 cm yr of seismic displacement the moment deficiency by 1985 will be the equivalent of five magnitude 8 earthquakes and by 1990 six such events the slip along this portion of the middle america trench arc appears to fit shimazaki and nakata s slip predictable model in which the size but not the time of occurrence of future large events may be estimated

anisotropy of magnetic susceptibility of heated rocks abstract heating produces changes which does not always correspond to simple enhancement of the magnetic fabric two methods are proposed to determine the anisotropy of magnetic susceptibility of the ferrimagnetic minerals formed or that have disappeared by chemical change during successive heating the first diagonalizes the tensor from the difference between each tensor term before and after heating the second employs linear regression for each tensor term made with the values obtained throughout a thermal treatment when the same magnetic fabric is obtained from several thermal steps it cannot be related to randomly oriented ferrimagnetic minerals instead the newly formed fabric must be related to characteristics of the pre existing rock by comparing this ferrimagnetic minerals fabric with the initial whole rock fabric we can distinguish cases where heating simply enhances pre existing fabric from those where thermal treatment induces a different fabric relative to the pre heating fabric this different fabric may simply be an inverse fabric or one whose principal susceptibility axes are oriented in a different direction related to petrostructural elements other than those defining the initial fabric

the acapulco ometepec mexico earthquakes of 1907 1982 evidence for a variable recurrence history abstract large and great earthquakes occurring along the acapulco ometepec segment of the mexican subduction zone during this century are reexamined to better constrain locations of prior earthquake rupture zones and the history of earthquake recurrence the 7 june 1982 ometepec earthquakes are used as master events in a joint epicentral determination relocation of the 1937 m s 7 7 1950 m s 7 3 1957 m s 7 7 and 1962 m s 7 2 6 9 earthquakes and their principal aftershocks rupture zones for these older events are consistent with patterns of rupture seen for more recent events i e epicenters near the coast and a narrow zone of aftershock activity between the trench axis and the coastline comparison of the 1937 1950 and 1957 ruptures with that of the great m s 8 0 1907 earthquake suggests that the later three events may represent a delayed multiple rerupture of the 1907 zone the time interval between the 1937 and 1950 shocks is short compared to similar sized events in oaxaca similar epicentral locations overlapping aftershock zones and a large difference in magnitude for the 1937 and 1950 events suggest that both events are part of the same recurrence episode and hence do not constitute a recurrence interval based on this model repeat times for large m s 7 5 events range from 30 yr for ometepec to 50 yr for acapulco san marcos

reconstructing the architectural evolution of volcanic islands from combined k ar morphologic tectonic and magnetic data the faial island example azores the morpho structural evolution of oceanic islands results from competition between volcano growth and partial destruction by mass wasting processes we present here a multi disciplinary study of the successive stages of development of faial azores during the last 1 myr using high resolution digital elevation model dem and new k ar tectonic and magnetic data we reconstruct the rapidly evolving topography at successive stages in response to complex interactions between volcanic construction and mass wasting including the development of a graben we show that 1 sub aerial evolution of the island first involved the rapid growth of a large elongated volcano at ca 0 85 ma followed by its partial destruction over half a million years 2 beginning about 360 ka a new small edifice grew on the ne of the island and was subsequently cut by normal faults responsible for initiation of the graben 3 after an apparent pause of ca 250 kyr the large central volcano cv developed on the western side of the island at ca 120 ka accumulating a thick pile of lava flows in less than 20 kyr which were partly channelized within the graben 4 the period between 120 ka and 40 ka is marked by widespread deformation at the island scale including westward propagation of faulting and associated erosion of the graben walls which produced sedimentary deposits subsequent growth of the cv at 40 ka was then constrained within the graben with lava flowing onto the sediments up to the eastern shore 5 the island evolution during the holocene involves basaltic volcanic activity along the main southern faults and pyroclastic eruptions associated with the formation of a caldera volcano tectonic depression we conclude that the whole evolution of faial island has been characterized by successive short volcanic pulses probably controlled by brief episodes of regional deformation each pulse has been separated by considerable periods of volcanic inactivity during which the faial graben gradually developed we propose that the volume loss associated with sudden magma extraction from a shallow reservoir in different episodes triggered incremental downward graben movement as observed historically when immediate vertical collapse of up to 2 m was observed along the western segments of the graben at the end of the capelinhos eruptive crises 1957 58

statistics of small earthquakes and frequency of occurrence of large earthquakes along the mexican subduction zone abstract the validity of the gutenberg richter g r frequency magnitude relation over a wide range of magnitudes is usually accepted and the statistics of small earthquakes is often extrapolated to predict the frequency of occurrence of large earthquakes we take noaa and pde data from 1963 to 1981 related to the mexican subduction zone h 65 km and convert m b to m s in the range of 3 3 m b 5 7 for this conversion we use regression of m s on m b for mexican and central american events as well as an m s m b regression based on worldwide data wyss and habermann 1982 we also take into account the standard error in m s this may be important since the number of events decreases as m b increases the m s data of 18 5 yr is normalized to 75 5 yr and is compared to the statistics of observed large earthquakes m s 6 4 for the period 1906 to 1981 it is found that the g r relation in the range 4 5 m s 6 0 when extrapolated grossly underestimates the observed frequency of large earthquakes m s 6 5 for the oaxaca and jalisco regions there is strong evidence that the plate interface in the oaxaca region consists of a few 3 or 4 large fault zones each of which cyclically ruptures over its entire dimension without giving rise to smaller events in the manner implied by the g r relation the result is of profound importance in seismic risk analysis and in the understanding of fault mechanics this phenomenon has been reported for only a few regions e g turkey bath 1981 japan wesnousky et al 1983 it may have escaped detection in other regions due to a variety of causes the most important being the long recurrence periods of large earthquakes

the november 29 1978 oaxaca mexico earthquake a large simple event the recent oaxaca mexico earthquake november 29 1978 mw 7 6 ms 7 8 seismic moment m0 3 2 1027 dyne cm is of special interest because of its location within a predetermined seismic gap the event excited long period 100 200 s multiple rayleigh and love waves which were well recorded by the wwssn these data along with p wave first motion data and p waveforms were used to constrain the source mechanism the results indicate an oblique thrust mechanism consistent with subduction of the cocos plate to the northeast beneath mexico dip 14 strike n90 w rake 54 hence this event is indeed of the type anticipated by ohtake et al 1977 a local network of stations installed in a joint university of mexico california institute of technology program began operation 20 days prior to the mainshock gonzalez 1979 mcnally et al 1979 mcnally 1981 ponce et al 1980 the 43 foreshocks of magnitude m 2 8 were recorded by the network in a period of 20 days prior to the mainshock these events show an interesting spatial and temporal pattern which culminates in the last 1 8 days of the sequence with an apparent migration of activity toward the epicenter of the impending earthquake this pattern can be interpreted as a buildup of stress or migration of stress toward a fault zone asperity with supplemental stations the network continued operation until december 12 1978 singh et al 1980 and provided good hypocentral control for the more than 169 aftershocks of magnitude m 2 8 the area of the aftershock zone determined from these events is 5525 km2 85 km by 65 km in spite of the large size of the event p waves for the oaxaca event indicate an extremely simple source at the period range of the wwssn long period seismographs this simplicity suggests that the p waves were generated by a limited portion of the rupture surface perhaps by the breaking of a fault zone asperity this result may be further supported by the discrepancy between the larger surface wave moment compared with that determined from body waves such simplicity also appears to be the case for the 1965 and 1968 mexican earthquakes on the adjacent eastern and western ends of the oaxaca aftershock zone respectively chael et al 1980 this type of body wave simplicity for a large subduction zone earthquake is an important characteristic of the mode of strain release along some subducting plate boundaries

morpho structural evolution of a volcanic island developed inside an active oceanic rift s miguel island terceira rift azores the evolution of volcanic islands is generally marked by fast construction phases alternating with destruction by a variety of mass wasting processes more specifically volcanic islands located in areas of intense regional deformation can be particularly prone to gravitational destabilisation the island of s miguel azores has developed during the last 1 myr inside the active terceira rift a major tectonic structure materializing the present boundary between the eurasian and nubian lithospheric plates in this work we depict the evolution of the island based on high resolution dem data stratigraphic and structural analyses high precision k ar dating on separated mineral phases and offshore data bathymetry and seismic profiles the new results indicate that 1 the oldest volcanic complex nordeste composing the easternmost part of the island was dominantly active between ca 850 and 750 ka and was subsequently affected by a major south directed flank collapse 2 between at least 500 ka and 250 ka the landslide depression was massively filled by a thick lava succession erupted from volcanic cones and domes distributed along the main e w collapse scar 3 since 250 kyr the western part of this succession furnas area was affected by multiple vertical collapses associated plinian eruptions produced large pyroclastic deposits here dated at ca 60 ka and less than 25 ka 4 during the same period the eastern part of the landslide scar was enlarged by retrogressive erosion producing the large povoacao valley which was gradually filled by sediments and young volcanic products 5 the fogo volcano in the middle of s miguel is here dated between ca 270 and 17 ka and was affected by at least one southwards flank collapse 6 the sete cidades volcano in the western end of the island is here dated between ca 91 and 13 ka and experienced mutliple caldera collapses a landslide to the north is also suspected from the presence of a subtle morphologic scar covered by recent lava flows erupted from alignments of basaltic strombolian cones the predominance of the n150 and n75 trends in the island suggest that the tectonics of the terceira rift controlled the location and the distribution of the volcanism and to some extent the various destruction events

source characteristics of the 1985 michoacan mexico earthquake at periods of 1 to 30 seconds source characteristics of the sept 19 1985 michoacan mexico earthquake and its aftershock on sept 21 were inferred from broadband and short period teleseismic gdsn records we fourier transformed the p waves corrected for instrument response attenuation geometrical spreading and radiation pattern including the depth phases and then averaged to obtain the teleseismic source spectrum from 1 to 30 s the michoacan source spectrum is enriched at 30 s and depleted at 1 to 10 s relative to an average source spectrum of large interplate subduction events source spectra for the sept 21 aftershock 1981 playa azul 1979 petatlan and 1978 oaxaca events follow a trend similar to that of the 1985 michoacan event this spectral trend may characterize the mexican subduction zone a station by station least squares inversion of the michoacan earthquake records for the source time function yielded three source pulses which we interpreted as events on the fault plane the first two are similar in moment and the third contains only 20 of the moment of the first directivity is evident in the timing at each station we measured the time differences between the pulses and performed a least squares nonlinear estimation of the strike distance and time separation between the events to locate them relative to one another the second event occurred 26 s after the first and 82 km southeast of it indicating southeastward rupture along the trench the third event occurred 21 s after the second and about 40 km seaward of it the two large events are also seen in the near field strong motions the mainshock records spectrum and time functions contain less high frequency radiation than those of the 1985 valparaiso chile earthquake apparently the michoacan earthquake ruptured two relatively smooth strong patches which generated large 30 s waves but small 1 to 10 s waves such behavior contrasts with the valparaiso event which had a more complex rupture process and generated more 1 to 5 s energy this difference is consistent with the higher near field accelerations recorded for the valparaiso event

p and s wave displacements from kinematic dislocation models abstract numerical dislocation models based on haskells 1969 formulation are used to estimate the amount of normal motion necessary to produce the high p s spectral ratios observed from strong motion records of the guerrero mexico subduction zone castro et al 1991 while this depends on the nature of the assumed dislocations a normal motion with amplitude of less than 10 of the amplitude of shear slip is sufficient to produce p s values comparable with the observations we model a planar fault with random patches distributed on the fault plane and a nonplanar fault in which a dilatational jog connects en echelon fault segments a structural system proposed by sibson 1985 1989 which introduces normal motions on the fault that depend mainly on the geometry of the fault for the planar fault model the magnitude of the normal motion is prescribed both models introduce complexity in both body wave displacement time histories although for the p waves this complexity is accentuated at higher frequencies f 1 hz for the nonplanar model a jog with an angle of 10 introduces a normal component of 18 of the slip on the fault

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

the role of lithospheric processes on the development of linear volcanic ridges in the azores abstract linear volcanic ridges lvrs are widespread along the azores plateau and are often used as a tectonic marker of the surface stress field nevertheless the mechanisms that drive the emplacement and development of these structures are not well established and they have been attributed to the plateau diffuse deformation off rift extension or the result of the interaction between a hotspot and the brittle lithosphere this study hypothesizes that linear volcanic ridges are the result of magma emplacement into pre existing damaged lithosphere using a 3d finite element representation of the brittle lithosphere and underlying ductile mantle and assuming that the deformation is driven by plate boundary forces applied at the edges as describe by global plate kinematic models the brittle layer is described by an elastoplastic rheology with progressive damage where fractures are assumed to be analogous to localized shear bands the ductile mantle underneath is modeled as a viscoelastic layer that exerts a shear drag at the base of the brittle layer the modeling shows that lithospheric processes alone can justify the spatial distribution of linear volcanic ridges and even the development of the faial ridge the factors controlling the fracturing pattern are the plate geometry and velocity boundary conditions the shearing introduced at the east azores fracture zone gloria fault limit and the interaction between the viscous mantle and the spatially varying brittle plate thickness along the terceira rift the predicted fractures match the orientation of the lvrs in the second n135 n140 and third n150 to n s sectors and provide an explanation for the arcuate shape of the rift itself the brittle plate thickness variations are crucial for the development of the more recent lvrs which are predicted to occur along the faial ridge in the best fit model the top mantle viscosity is 1 10 22 pa s at 5 15 km depth and the present day fracture network takes 3 ma to develop

variation of magnetic properties in sedimentary rocks hosting the foum zguid dyke southern morocco combined effects of re crystallization and fe metasomatism the effects of dyke intrusion on the magnetic properties of host sedimentary rocks are still poorly understood therefore we have evaluated bulk magnetic parameters of standard palaeomagnetic samples collected along several sections across the sediments hosting the foum zguid dyke in southern morocco the study has been completed with the evaluation of the magnetic fabric after laboratory application of sequential heating experiments the present study shows that 1 close to foum zguid dykes the variations of the bulk magnetic parameters and of the magnetic fabric is strongly related with re crystallization and fe metasomatism intensity 2 the thermal experiments on ams of samples collected farther from the dyke and thus less affected by heating during dyke emplacement indicate that 300 400 c is the minimum experimental temperature necessary to trigger appreciable transformations of the pre existing magnetic fabrics for temperatures higher than ca 580 c the magnetic fabric transformations are fully realized with complete transposition of the initial fabric to a fabric similar to that of samples collected close to the dyke therefore measured variations of the magnetic fabric can be used to evaluate re crystallization temperatures experienced by the host sedimentary rock during dyke emplacement the distinct magnetic behaviour observed along the cross sections strongly suggests that samples collected farther from the dyke margins did not experience thermal episodes with temperatures higher than 300 c after dyke emplacement 3 ams data shows a gradual variation of the magnetic fabric with distance from the dyke margin from sub horizontal k3 away from the dyke to vertical k3 close to the dyke experimental heating shows that heat alone can be responsible for this strong variation therefore such orientation changes should not be unequivocally interpreted as the result of a stress field resulting from the emplacement of the dyke for instance 4 magnetic studies prove to be a very sensitive tool to assess rock magnetic transformations thermally and chemically induced by dyke intrusion in hosting sediments

post emplacement tilting of lava flows inferred from magnetic fabric study the example of oligocene lavas in the jeanne d arc peninsula kerguelen islands abstract statistical analysis of the magnetic fabric of samples from several successive lava flows emplaced under similar conditions can allow determination of the mean flow direction when magnetic fabric data from individual flows do not lead to reliable results a difference between the obtained flow direction and the present dip direction indicates that the flows were tilted after emplacement for 2 successive series of flows on the jeanne d arc peninsula presently nnw dipping this method shows lava emplacement along a ssw nne direction this indicates a gentle tilting acquired during a period of weak deformation in the whole archipelago additionally the magnetic fabric data allow the reconstruction of the different conditions of emplacement of these two series of lava flows and of formation of the local thick conglomerate interbedded between these series

ground motions from subduction zone earthquakes a total of 258 horizontal components of accelerogram data recorded at soil sites during thrust normal and strike slip earthquakes occurring in seven subduction zones around the pacific ocean were analyzed the results of statistical analyses of 5 per cent damped pseudo velocities psv computed from these accelerograms at 10 periods t between 0 1 and 4 sec indicated no significant differences in the average psv levels of the thrust normal and strike slip data from the northern honshu zone each of these groups of northern honshu data which together comprised one half of the total data base were fit equally well by the same attenuation model the analyses also revealed that at intermediate periods 0 8 t 3 sec the psv observed at stiff soil sites within the northern honshu nankai kuril mexico and alaska zones were on the average significantly greater than the psv observed at similar sites in the peru northern chile and new britain bougainville zones this observation indicates that differences in the source and or travel path characteristics between the two groups accounted for the differences in psv independent evidence supporting source differences is the correlation noted between psv in this period band and the characteristic source complexities for these zones which hartzell and heaton inferred from the teleseismic data of the larger magnitude earthquakes certain anomalous tectonic characteristics of the new britain bougainville zone were noted that may have contributed in some systematic manner to the relatively unusual spectral characteristics of the ground motions recorded on stiff soil sites within this zone some of the differences in the psv among the zones at intermediate and long periods were probably the result of differences in local geologic characteristics geology greatly influenced the mexican psv data from soft soil sites to a lesser extent geology probably affected the alaskan data which were recorded mostly on deep stiff soils and the new britain bougainville data recorded at yonki which is underlain by softer soil deposits

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

attenuation of modified mercalli intensity with distance in mexico abstract two relations are proposed to predict the attenuation of modified mercalli intensity i with distance d for mexican earthquakes i e ln i b 0 b 1 ln d d 9 b 2 d d 9 b 3 ln m s ln i b 0 b 1 d d 9 b 2 ln d d 9 b 3 ln m s m s is the earthquake surface wave magnitude d is a distance related to the maximum i mapped for an earthquake i or to m s the coefficients b i i 0 1 2 3 were obtained by fitting in a least square sense the information contained in the intensity maps of 32 events to the relations those events were classified in three groups according to their epicentral location focal mechanism and depth i e events related to the subduction zone intermediate depth earthquakes in south central mexico and to shallow crustal events along the trans mexican volcanic belt the i predicted by the proposed relations compare well with the i observed for historical earthquakes not included in the fitting results obtained from a parametrical study showed that the attenuation of i with d is different for each of the three types of earthquakes for distances of less than about 200 km the earthquakes associated with the subduction zone have a larger attenuation than the ones originating in the south central region of mexico for greater distances d 200 km the opposite behavior is observed the events located in the trans mexican volcanic belt have a larger attenuation with distance than that of events in the other two regions from these results it seems advisable in mexico to use several attenuation relations to estimate the seismic hazard at a site depending on the particular tectonic setting and the path of the events under consideration

large scale catastrophic flank collapses in a steep volcanic ridge the pico faial ridge azores triple junction abstract large scale flank collapses are common in the geological evolution of volcanic ocean islands in the atlantic to date catastrophic lateral collapses in the azores islands have been difficult to identify leading to suggestions that a lack of events may relate to the relatively small size of the islands here we show evidence for two major collapses on the northern flank of pico island pico faial volcanic ridge central azores suggesting that this island had a collapse incidence similar to that of other atlantic volcanic islands the study is based on the analysis of 1 offshore and onshore high resolution digital elevation models 2 field data focused on the n flank and 3 new k ar ages on selected lava flow samples pico sub aerial northern flank is marked by two conspicuous arcuate shaped depressions concave towards the sea here interpreted as landslide scars a main debris field is observed offshore the largest depression this deposit has 20 km of maximum length covers ca 150 km2 is composed of meter to hectometer blocks and has an exposed volume here estimated between 4 and 10 km3 though the actual volume probably exceeds 10 km3 debris flow towards the ese was apparently determined by the slope of the narrow wnw ese s jorge channel young lava flows cascade over the interpreted scars thus concealing the older volcanic sequence s affected by the landslide s new k ar ages measured on these lava flows provide a minimum age of ca 70 ka for the large scale collapse s in pico s northern flank

offshore evidence for a huge landslide of the northern flank of tahiti nui french polynesia a bathymetric analysis of the flanks of tahiti nui french polynesia reveals the existence of a major catastrophic north directed landslide during its eruptive history the related debris avalanche da deposits extend north 70 km from the island shore and have a total volume of about 800 km3 two contrasted facies of the debris are recognized 1 relatively small sized material including a few blocks with well developed flow structures elongate downslope directed ridges and transversal pressure ridges and 2 a scattering of large kilometer sized individual blocks that form in part a prominent bathymetric hummock in the proximal central part of the landslide deposits bathymetric analyses suggest that the tahitian failure evolved through two main successive shots the da debris were rapidly overlain by subsequent volcanic material producing a smooth surface down to 1500 m below sea level the on land and offshore structures indicate a listric fault geometry for the landslide scar the volume removed by the landslide is thus estimated between 400 km3 and 450 km3 discrepancy in the volume of the scar and the debris derived from it is due in part to expansion of the debris to fragmentation of the avalanche and to incorporation of sediments comparison with a recently recognized southern tahitian landslide clouard et al 2001 and with landslides recognized around the hawaiian islands and the canary islands shows that the huge northern tahitian landslide described here has a reduced mobility in its central part perhaps the older tetiaroa eruptive complex to the north acted as a buttress to inhibit movement of the da products the northern and southern tahiti nui landslides moved in opposite directions away from the main central e w directed rift zone that served as the main magma feeding structure then the huge northern landslide strongly controlled the later volcano structural evolution by capturing subsequent volcanic activity to the north despite the northwestward displacement of the pacific plate over the society hot spot

flexural uplift of rift flanks due to mechanical unloading of the lithosphere during extension we suggest that the uplift of rift flanks results from mechanical unloading of the lithosphere during extension and consequent isostatic rebound this mechanism is presented as an alternative to explanations for rift flank uplift involving thermal or dynamic processes and magmatic thickening of the crust our hypothesis is based on two critical concepts first the lithosphere retains finite mechanical strength or flexural rigidity during extension second isostatic rebound uplift of the lithosphere follows when the kinematics of extension produces a surface topographic depression that is deeper than the level to which the surface of the extended lithosphere would subside assuming local isostatic compensation we develop and analyze two kinematic models for instantaneous extension of the lithosphere to show that flexural rebound is a viable explanation for the uplift of rift flanks we first investigate the isostatic consequences of finite simple slip on an initially planar dipping normal fault cutting the entire lithosphere when the lithosphere retains flexural rigidity during extension the topography resulting from this model resembles a half graben and the footwall rift flank is flexurally uplifted this simple normal faulting model explains free air gravity anomalies and topography observed at rift flanks in oceanic lithosphere such as broken ridge in the eastern indian ocean the caroline ridges sorol trough in the western equatorial pacific and the coriolis trough behind the new hebrides island arc we then investigate a general kinematic model for lithospheric extension where simple slip on a surface of arbitrary shape is accompanied by pure shear extension in the upper and lower plates when the simple slip component is not zero or the distribution of pure shear in the upper and lower plates is not identical the surface of slip can be regarded as a detachment by simplification our general model accounts for pure shear extension of the lithosphere that is uniform with depth in this case detachments have no meaning in the geologic sense however the kinematics of depth independent pure shear may nevertheless be described in terms of a surface which we term a kinematic reference surface at some depth in the lithosphere we speculate that the depth of this surface may be rheologically controlled the magnitude of rift flank uplift by flexure depends critically on the depth of this reference surface in contrast if local isostasy is assumed when the lithosphere undergoes a given amount of depth independent pure shear the resulting topography will be the same regardless of how the kinematics of that extension are formulated the basin and rift flank topography and free air gravity anomaly over young continental rifts such as the rhine graben can be satisfied using our general extensional model with a small amount 5 km of extension along a listric shaped detachment soling into the crust mantle boundary because the flexural rebound mechanism explains the observed topography and gravity anomaly over both oceanic and continental extensional domains we suggest that rheological differences between the two lithospheric types may not be important in their overall response to extension

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

surface displacement field at terceira island deduced from repeated gps measurements abstract we present the results of six gps global positioning system campaigns held in terceira island azores between 1999 and 2006 for a group of 17 stations to estimate the surface displacement field induced by the eurasian nubian interplate motion and their relationship with the rifting along terceira rift and volcanic processes at the island scale we show that the whole island mainly behaves like a rigid body which absolute motion is closer to eurasia the magnitude of the computed intra island horizontal deformation field difference between the individual site velocities and the mean rigid body motion is still below the observation accuracy and so cannot be directly related with local processes we also show that the island is globally subsiding with an average rate close to 1 mm yr as it moves to the ne away from the terceira rift mostly attached to the eurasian plate the high subsidence rate can be the result of the volcanic processes that shaped the serreta ridge nw of terceira

evolution of the accretion processes along the mid atlantic ridge north of the azores since 5 5 ma an insight into the interactions between the ridge and the plume 1 high resolution bathymetry and gravity data north of the azores plateau show that this part of the north mid atlantic ridge is presently undergoing a phase of weak crustal production and magmatism most of the ridge segments are small and short lived suggesting a disrupted and highly variable accretion regime since anomaly 3a the influence of the nearby plume appears to be relatively minor and corresponds more to a weak thermal signal than to any major input of plume material and increased crustal production at the axis a period of increased magmatism was identified at the southern limit of the study area near 40 n around anomaly 5 this magmatic pulse caused the emplacement of a topographic high probably underlain by a thickened crust this pulse probably marks the northernmost and last significant arrival of material from the azores plume to the mar axis

seismic potential of acapulco san marcos region along the mexican subduction zone recently seismic quiescence based on p d e and i s c data has been reported in the acapulco san marcos region 16 17 2 n 98 5 99 7 w mcnally 1981 in order to assess the seismic potential of this region we have estimated seismic moments and slips associated with 1957 ms 7 5 and 1907 ms 8 0 events which occurred in this area historical data suggest that earthquakes occurred in this region in 1845 ms 7 9 and probably also in 1820 ms 7 6 we find preference for the time predictable model in the data set which if valid forecasts the next large earthquake between now and 1993 with 1985 as the most likely year although this forecast can not be taken as definitive since it is based on several assumptions each of which are reasonable but uncertain the implications of reported quiescence and the results of this paper suggest a careful monitoring of the area

paleomagnetic constraints on fault motion in the hilina fault system south flank of kilauea volcano hawaii movement of the south flank of kilauea volcano in hawaii has been associated with catastrophic landslide events the surface expression of this former movement is the hilina fault system with fault scarps as high as 500 m paleomagnetic directions for lava flows exposed in the hilina fault scarps at puu kapukapu and keana bihopa on the hilina pali are used to determine the average rate of movement along faults slip surfaces separating the two sections this paper reports results from two independent paleomagnetic studies within the hilina pali area twenty one paleomagnetic sites 143 cores were sampled by the michigan technological university group from lava flows between the mo o ash and middle pohakaa ash at keana bihopa in the footwall block of the 500 m high hilina pali fault scarp thirty paleomagnetic sites 152 cores were collected by the california institute of technology group from lava flows between the mo o ash and middle pohakaa ash in the 300 m high puu kapukapu section the hanging wall block a comparison of site mean directions show that lava flows in the lower part of the puu kapukapu section have been tilted more than lava flows in the upper part with respect to the keana bihopa section the systematic steepening of remanent directions downsection at puu kapukapu indicates that slippage of this block occurs along listric normal faults the average amount of backward tilt of the puu kapukapu block based on a comparison of mean directions from the two sections is 7 8 7 7 using slope stability methods the average rate of movement of the puu kapukapu block since deposition of the middle pohakaa ash is 1 7 2 4 cm yr and the average displacement subsidence is 680 740 m assuming that the average displacement resulted from a series of earthquakes producing subsidence equal to that observed in the 1975 kalapana earthquake 3 m of subsidence along a 40 km segment of coastline on the south flank of kilauea volcano one kalapana size earthquake occurring every 200 years would account for this displacement lastly overall mean directions for the two sections indicate that puu kapukapu has rotated counterclockwise with respect to the hilina pali by 14 8 8 5 about a nearby vertical axis this also suggests that slippage between the two blocks occurs along listric normal faults

the western guerrero mexico seismogenic zone from the microseismicity associated to the 1979 petatlan and 1985 zihuatanejo earthquakes abstract the western guerrero mexico seismogenic zone was completely ruptured by the 1979 m s 7 6 petatlan and 1985 m s 7 5 zihuatanejo earthquakes hypocenters of the petatlan aftershocks define an approximately 10 km thick wadati benioff zone of high seismic activity and a thinner seaward region that is primarily an extension of the deeper part of the 10 km thick zone the aftershocks of the zihuatanejo earthquake occurred in the seaward portion of the same epicentral region but the hypocenters were shallower the spatial distribution of the closely timed microseismicity following the two earthquakes outlines a seismogenic zone which begins at about 40 km from the trench axis of the western guerrero subduction region and extends approximately 90 km these results indicate that the maximum possible size of thrust earthquakes in the guerrero seismic gap is of m w 8 4

stress accumulation and release since 1882 in ometepec guerrero mexico implications for failure mechanisms and risk assessments of a seismic gap we found evidence that four episodes of large shallow h 60 km the time intervals between extensional and subsequent compressional events range from 1 6 to 9 9 years with an average of 6 3 3 7 years this apparent pattern which initiates at depth was further documented for the final 2 3 of the last seismic cycle from the spatiotemporal distribution of seismicity during this final period we identified three general stages of stress release stage i from 1964 to january 1980 was characterized by scattered seismicity stage ii from february 1980 to june 6 1982 showed a significant increase in downdip seismic activity while the shallow zone became quiescent there was also a period of apparent seismic quiescence from mid 1975 to early 1978 most seismic energy in stage ii was released on october 24 1980 by a downdip normal fault earthquake at intermediate depth ms 7 0 h 65 km stage iii began on june 7 1982 with the occurrence of two large interplate thrust fault earthquakes at shallow depths ms 6 9 h 18 km and ms 7 0 h 19 and 23 km the documentation of stage iii through june 1986 shows subsidence of previous downdip activity and an aftershock zone which is spatially coincident with the shallow quiet zone identified in stage ii the spatiotemporal characteristics of the 1982 main shock after shock sequence suggests that rupture propagated to the northwest toward the site of the 1957 ms 7 5 acapulco san marcos earthquake this propagation is of interest because the latter site is presently considered to possess high seismic potential we suggest that downdip normal fault seismic activity at intermediate depths reflects a symptomatic state of stress associated with some basic mechanism of plate interaction prior to interplate slippage in a large earthquake this mechanism may have an important bearing on the generation and possible intermediate term forecasts of large shallow interplate earthquakes along the subduction zone in mexico

some remarks on the behavior of the magnetotelluric phase the magnetotelluric phase in the medium of incidence over a good conductor varies from its value at the conductor surface usually close to 45 sup 0 to the characteristic standing wave phaseshift of 90 sup 0 over a distance of a few conductor skin depths much shorter than the vacuum wavelength at the frequencies considered over the top of a graben or at the surface of a two layer structure the basement can be looked upon as the matrix if the graben or the overburden is a better conductor it will carry an extra current density in phase with the electric field thus giving rise to an additional magnetic field also in phase with the electric field this drives the argument of the surface impedance toward smaller values for a more resistive graben or overburden the current is depressed corresponding to a reduction of the surface magnetic field by an amount in phase with the electric field this effect drives the phase of the impedance to values above the characteristic 45 sup 0 of a uniform conductor

low temperature magnetic behavior of titanomagnetites titanium substitution has a significant effect on the temperature dependence 5 300 k of low temperature saturation remanence and of initial susceptibility in synthetic and natural titanomagnetite fe3 xtixo4 0 x 0 4 and electronic and lattice relaxation phenomena over the entire titanomagnetite compositional range these features produce diagnostic magnetic behavior useful for titanomagnetite identification in natural samples moreover the apparent similarities between the thermal decay of low temperature remanence of titanomagnetite with pyrrhotite pure magnetite and superparamagnetic phases can lead to complications in interpreting low temperature remanence data when titanomagnetite is present

construction and destruction of a volcanic island developed inside an oceanic rift graciosa island terceira rift azores abstract there is a great lack of knowledge regarding the evolution of islands inside active oceanic rifts in particular the meaning of the different evolutionary steps therefore we conducted an investigation in graciosa island which lies at the northwestern end of the terceira rift in the azores triple junction with the objective of constraining the evolution of the island in terms of volcanic growth and mass wasting in particular the meaning and age of the destruction events from digital elevation model dem analysis stratigraphic and tectonic observations k ar dating on key samples and available bathymetry and gravity data we propose that graciosa comprises five main volcanic complexes separated by major unconformities related to large scale mass wasting 1 the older volcanic edifice serra das fontes complex grew until ca 700 ka and was affected by a major flank collapse towards the southwest which removed the whole sw flank the summit and a part of the ne flank 2 the baia do filipe complex developed between at least 472 ka and 433 ka in two different ways in the sw presently offshore as a main volcano and in the ne unconformably over the sub aerial remnants of the serra das fontes complex as secondary volcanic edifices 3 the baia do filipe complex was affected by a major flank collapse towards the sw again removing most of the edifice 4 the remnants of the baia do filipe complex were covered in unconformity by the serra dormida complex between ca 330 and 300 ka which in turn was unconformably covered by the younger basaltic cover complex between ca 300 ka and 214 ka these two units were affected by a third major sector collapse that removed the whole western flank the summit and part of the eastern flank of the serra dormida and basaltic cover complexes 5 despite the relatively young age of graciosa the collapse scars are not well preserved and not active anymore 6 a central type volcano has been growing since at least 60 ka at the southeastern end of the island inside the scar left by a fourth sector collapse towards the se which affected most previous complexes contemporaneously parasitic strombolian cones formed all over the island despite the location of graciosa inside the active terceira rift the new data indicate that the evolution of the island has been driven by a competition between volcano growth and repeated destruction by catastrophic sector collapses rather than by slow incremental faulting associated with the tectonics of the rift

distributed deformation close to the azores triple point abstract terceira rift and the northern and southern branches of the mid atlantic ridge mar form a triple junction close to 39 n known as the azores triple junction new swath bathymetric data are used to investigate the surface expression of faulting close to the triple junction by the systematic mapping of mar generated abyssal hills it is shown that close to the geometrical intersection between the three spreading axes there exists no single transform fault connecting terceira rift to the mar but a distributed tectonic deformation area characterized by mesoscale brittle deformation close to the surface covering approximately 90 km by 100 km and almost no volcanism which links terceira rift to the mar accommodating the relative displacement of the three plates close to the geometrically triple point magnetic chrons are used to compute the spatial variation of spreading velocity at the mid atlantic ridge and confirm the above interpretation they show a progressive increase of spreading velocity along a single mar segment between pure nubian at 38 30 n and pure eurasian at 39 25 n without the development of a transform fault that would integrate the eurasia nubia plate boundary the comparison of similar triple junctions where a slower axis joins two faster ridges shows that the slower arm does not reach the triple point and that there is always a finite triple junction area highly tectonized in which size is dependent on the angle between the two faster arms and consequently on the relative spreading velocity of the slower arm

source time functions of large mexican subduction earthquakes morphology of the benioff zone age of the plate and their tectonic implications we study source parameters of large shallow mexican subduction zone earthquakes 95 w to 106 w which occurred between 1928 and 1986 by modeling the p waves recorded on galitzin wilip seismograph in debilt dbn holland for post 1962 events the source parameters retrieved from dbn seismograms alone agree well with those obtained from long period world wide standardized seismograph network records giving us confidence in our results for pre 1962 events all earthquakes are shallow h 10 to 20 km with few exceptions the sources in oaxaca 95 w to 99 w are very simple to the northwest of 99 w they are simple as well as complex the ratio of surface wave to body wave seismic moment mos mop which is a measure of long to short period radiation is smaller in oaxaca 1 5 0 5 than in the regions northwest of 99 w 3 1 1 3 these results suggest a change in the plate interface characteristics near 99 w the sharp change in the rupture mode and the intersection of the o gorman fracture zone ofz with the trench occur near 99 w two strike slip events offshore close to ofz suggest a segmentation of the subducting plate near 99 w the age of the plate near the trench in oaxaca is not well known it is possible that it does not increase continuously from northwest to southeast in the region but jumps across 99 w if so then the older age of the subducted plate southeast of 99 w may be the cause of the distinct rupture mode of the oaxaca earthquakes the length of the benioff zone which is greatest below oaxaca 400 km and decreases toward the northwest can be explained by the correlation between the length of the subducted slab and the product of the lithosphere age and convergence rate the relative complexity of sources the weaker background seismicity and the lesser number of aftershocks northwest of oaxaca may be explained by a stronger interface coupling resulting from subduction of younger oceanic slabs 5 to 13 m y old in this region this however explains neither larger mos mop values northwest of oaxaca nor the low stress drop estimates obtained from the analysis of near field strong motion data for the michoacan earthquake of 1985 both of which indicate weaker coupling of the interface thus the issue of whether subduction of very young plates 10 m y old results in strong or weak coupling remains unsolved from the presently available mexican data

seismic gaps and recurrence periods of large earthquakes along the mexican subduction zone a reexamination abstract seismic gaps and recurrence periods of large shallow interplate earthquakes along the mexican subduction zone are reexamined after combining information from a catalog of nineteenth century9s earthquakes some relocated epicenters of the early part of this century source parameters of recent large earthquakes and redetermined magnitudes of great shallow earthquakes of this century tehuantepec and michoacan gaps have not experienced a large shock in this century and perhaps none in the past century they are either aseismic or have anomalously large repeat times guerrero jalisco and ometepec regions presently appear to have a high seismic potential observed average repeat times of large earthquakes m s 7 4 in six regions east central and west oaxaca san marcos petatlan and colima are between 32 to 56 yr data of this century indicate that the strain is released mostly in large events m s 7 4 a simple dislocation model with parameters obtained from the studies of recent earthquakes explains the observed recurrence periods quite well the b value for this zone is not meaningful an observation which is of significance for seismic risk estimation most of seismic moment or equivalently seismic energy release since 1800 appears to occur for 15 yr followed by relative quiescence in the next 15 yr

source characteristics of earthquakes in the michoacan seismic gap in mexico we investigated the source characteristics of large earthquakes which occurred in the michoacan mexico seismic gap during the period from 1981 to 1986 in relation to historical seismicity in the region the rupture pattern of the michoacan gap during this period can be characterized by a sequential failure of five distinct asperities before 1981 the michoacan gap had not experienced a large earthquake since 1911 when an m s 7 8 earthquake occurred the recent sequence started in october 1981 with the playa azul earthquake which broke the central part of the gap body wave modeling indicates that the playa azul earthquake is 27 km deep with a seismic moment of 7 2 10 27 dyne cm it is slightly deeper than the recent michoacan earthquakes and its stress drop is higher suggesting a higher stress level at depths in the michoacan gap the seismic moment of the 19 september 1985 m w 8 1 earthquake was released in two distinct events with the rupture starting in the northern portion of the seismic gap and propagating to the southeast with low moment release through the area already broken by the 1981 playa azul earthquake the rupture propagated further southeast with an m w 7 5 event on 21 september 1985 another aftershock occurred on 30 april 1986 to the northwest of the 19 september main shock body wave modeling indicates that this event has a simple source 10 sec long at 21 km depth and fault parameters consistent with subduction of the cocos plate 280 12 and 70 and m 0 2 0 to 3 1 10 26 dyne cm m w 6 8 to 6 9 although this distribution of asperities is considered characteristic of the michoacan gap whether the temporal sequence exhibited by the 1981 to 1986 sequence is also characteristic of this gap or not is unclear it is probable that depending on the state of stress in each asperity the entire gap may fail in either a single large event with a complex time history or a sequence of moderate to large events spread over a few years the seismic moment and the time since the last earthquake in michoacan in 1911 fit an empirical relation between moment and recurrence time found for the guerrero oaxaca region of the mexico subduction zone

conditional probabilities for the recurrence of large and great interplate earthquakes along the mexican subduction zone abstract time dependent conditional probabilities for the recurrence of large and great interplate earthquakes along the mexican subduction zone are presented for time intervals of 5 10 and 20 yr duration i e 1986 1991 1986 1996 and 1986 2006 at present the central oaxaca 97 3 to 97 7 w ometepec san marcos 98 2 to 99 5 w and central guerrero 100 to 101 w segments stand out as having the highest probability for the recurrence of large and great earthquakes in the near future segmentation of the margin is delineated by the rupture zones of the most recent earthquakes occurring in each area for segments of the mexican margin with one or more known recurrence intervals probability estimates are based on the observed average recurrence time for each segment and the lognormal probability distribution function use of a generic distribution function nishenko and buland 1987 allows a more stable estimate of average recurrence times than are available from a few observations a long term prediction time window is also defined based on the 90 per cent confidence interval for our estimates of the recurrence time use of a predetermined confidence interval conveys valuable additional information as to the precision and information content of the forecast for those segments of the margin with only one prior event and hence no historically observed recurrence times repeat times are estimated by extrapolating the observed recurrence time behavior in oaxaca subject to the assumptions that recurrence time scales only as a function of the ratio of seismic displacement and convergence rate and that all events are characterized as simple sources i e involve the rupture of a single asperity

study of the source processes of the 1965 1968 and 1978 oaxaca earthquakes using short period records the source processes of the 1965 1968 and 1978 oaxaca earthquakes are studied using teleseismic short period records which contain more detail of the high frequency component of the source rupture process than long period seismograms the locations and the fault mechanism solutions of these earthquakes are similar and the recording stations are chosen common to the three earthquakes so that the relative comparison of the source processes should be meaningful the source process of each earthquake that was obtained is as follows the 1965 event was a break of a single high stress zone the 1968 and 1978 events were complex breaks of several high stress zones the complexity of the source process has a good correlation with the foreshock activity mb 4 0 in the upcoming rupture zone that is the 1965 event had no foreshocks the 1968 event had several foreshocks and the 1978 event had one foreshock for the 1965 event the moment value 7 1025 dyn cm which is estimated for the single source is much smaller than that obtained from a surface wave analysis that is 2 2 1027 dyn cm by chael and stewart

mauna loa lava accumulation rates at the hilo drill site formation of lava deltas during a period of declining overall volcanic growth accumulation rates for lava flows erupted from mauna loa as sampled in the uppermost 280 m of the hilo drill hole vary widely for short time intervals several thousand years but overall are broadly similar to those documented elsewhere on this volcano since 100 ka thickness variations and accumulation rates for mauna loa lavas at the hilo drill site have been strongly affected by local paleotopography including funneling and ponding between mauna kea and kilauea in addition gentle submerged slopes of mauna kea in hilo bay have permitted large shoreline displacements by mauna loa flows ages of eruptive intervals have been determined from published isotopic data and from eustatic sea level curves modified to include the isostatic subsidence of the island of hawaii at 2 2 2 6 mm yr prior to 10 ka rates of mauna loa lava accumulation at the drill site varied from 0 6 to 4 3 mm yr for dateable intervals with an overall rate of 1 8 mm yr major eruptive pulses at about 1 3 and 10 ka each probably representing a single long lived eruption based on lack of weathering between flow units increase the overall accumulation rate to 2 4 mm yr the higher rate since 10 ka reflects construction of thick near shoreline lava deltas as postglacial sea levels rose rapidly large lava deltas form only along coastal segments where initially subaerial slopes have been submerged by the combined effects of eustatic sea level rise isostatic subsidence or spreading of volcano flanks overall accumulation of 239 m of lava at the drill site since 100 120 ka closely balances submergence of the hilo area suggesting that processes of coastal lava deposition have been modulated by rise in sea level the hilo accumulation rate is slightly higher than average rates of 1 2 mm yr determined elsewhere along the mauna loa coast based on rates of shoreline coverage and dated sea cliff and fault scarp exposures low rates of coastal lava accumulation since 100 ka near or below the rate of island wide isostatic subsidence indicate that mauna loa is no longer growing vigorously or even maintaining its size above sea level

recent large earthquakes along the middle american trench and their implications for the subduction process several large shallow earthquakes ms 7 0 have occurred along the middle american trench since the installation of the wwssn network included are the 1965 1968 and 1978 oaxaca events the 1970 chiapas event the 1973 colima event and the 1979 petatlan event these earthquakes have been studied in an attempt to identify similarities and differences between them that may lead to a better understanding of fracture and subduction processes the events have seismic moments ranging from 1 0 1027 dyne cm for the 1968 event to 3 2 1027 dyne cm for the 1978 event all events are of predominantly thrust type consistent with subduction to the northeast of the cocos plate body waves for the 1965 1968 1978 and 1979 events along the trench indicate rather simple faulting processes these events all had focal depths of 15 to 20 km and stress drops of the order of 10 bars the 1970 and 1973 events the easternmost and westernmost respectively of the events studied here are located close to triple junctions for the cocos north america caribbean plates 1970 and the cocos north america rivera plates 1973 these two events generated more complex body waves than the oaxaca and petatlan quakes this source complexity may have been related to the more complicated tectonics near the 1970 and 1973 epicenters this may offer a new perspective to viewing source complexity in relation to subduction zones

stratification of the earth beneath the azores from p and s receiver functions abstract seismic recordings of iris ida gsn station cmla and of several temporary stations in the azores archipelago are processed with p and s receiver function prf and srf techniques contrary to regional seismic tomography these methods provide estimates of the absolute velocities and of the vp vs ratio up to a depth of 300 km joint inversion of prfs and srfs for a few data sets consistently reveals a division of the subsurface medium into four zones with a distinctly different vp vs ratio the crust 20 km thick with a ratio of 1 9 in the lower crust the high vs mantle lid with a strongly reduced vp vs velocity ratio relative to the standard 1 8 the low velocity zone lvz with a velocity ratio of 2 0 and the underlying upper mantle layer with a standard velocity ratio our estimates of crustal thickness greatly exceed previous estimates 10 km the base of the high vs lid the gutenberg discontinuity is at a depth of 80 km the lvz with a reduction of s velocity of 15 relative to the standard iasp91 model is terminated at a depth of 200 km the average thickness of the mantle transition zone tz is evaluated from the time difference between the s410p and sks660p seismic phases that are robustly detected in the s and sks receiver functions this thickness is practically similar to the standard iasp91 value of 250 km and is characteristic of a large region of the north atlantic outside the azores plateau our data are indicative of a reduction of the s wave velocity of several percent relative to the standard velocity in a depth interval from 460 to 500 km this reduction is found in the nearest vicinities of the azores in the region sampled by the prfs but as evidenced by srfs it is missing at a distance of a few hundred kilometers from the islands we speculate that this anomaly may correspond to the source of a plume which generated the azores hotspot previously a low s velocity in this depth range was found with srf techniques beneath a few other hotspots

the azores triple junction evolution since 10 ma from an aeromagnetic survey of the mid atlantic ridge abstract in the past two decades several models have been presented to describe the evolution and the present structure of the azores triple junction these models were mainly based on morphological analysis of sea bottom topography sparse magnetic profiling sidescan sonar surveying over the plateaus and global plate kinematic considerations for the north atlantic in this paper we follow a different approach from a detailed aeromagnetic survey covering both sides of the mid atlantic ridge between 37 n and 40 30 n the magnetic anomalies up to anomaly 5 are accurately identified allowing careful modelling of the kinematics of this region for the past 10 ma and thus establishing a coherent framework for the design of geophysical models for the azores triple junction the analysis of magnetic anomalies and the use of fourier domain inversion techniques show that the ridge is made up of six segments each one varying in length from 50 to 60 km the more continuous section of the ridge can be defined from the first four northern segments although the north azores fracture zone right offsets the ridge at 39 30 n 29 40 w the fifth and sixth segments are respectively right offset by the acor fracture zone at 38 23 n 30 15 w and by the princess alice fracture zone at 38 00 n 30 50 w anomaly identifications using a two dimensional model and plate tectonics reconstruction techniques allowed the calculation of rotation pole parameters the results thus obtained reveal that at least between anomalies 5 and 3 10 3 85 ma the azores displayed an independent motion relative to the neighbouring plates and after anomaly 2a 2 45 ma the azores moved attached to the eurasian plate the triple junction azores north america africa or eurasia north america africa moved northward from 38 00 n 30 50 w to 38 20 n 30 15 w between 4 and 3a and probably to 38 50 n 30 00 w at anomaly 2a time a detailed reconstruction model is presented

catastrophic flank collapses and slumping in pico island during the last 130 kyr pico faial ridge azores triple junction abstract the pico island constitutes the easternmost sub aerial domain of a steep wnw ese volcanic ridge which has developed within the nubia eurasia diffuse plate boundary azores triple junction the island comprises three volcanic systems from older to younger the topo volcano the fissural system and the pico stratovolcano from a high resolution digital elevation model 10 m and new bathymetric stratigraphic structural and high precision k ar data we reconstruct the main successive stages of growth and partial destruction of the island over the last 200 kyr we especially concentrate on the central sector of the island which has recorded gradual movements through slumping and catastrophic flank collapses since ca 130 kyr the remmants of the topo volcano are partly exposed on pico s se flank and are here dated between 186 5 and 115 4 ka topo was significantly destroyed by n and s directed large scale flank collapses between ca 125 and 70 ka on pico s n flank collapse seems to have removed all the unstable material but in the s the collapse structure is composite including a major flank collapse and a remnant slump complex that is still active a first episode of deformation occurred between ca 125 and 115 ka along the master fault of the slump between ca 115 and 69 ka most of the unstable material was removed by a major flank collapse leaving behind a still considerable volume of unstable material that comprises the active slump this first collapse was catastrophic and generated a large debris deposit recognized on the high resolution bathymetry with a minimum run out of ca 17 km the scar was partially filled by volcanic products erupted from volcanic cones developed within the slump depression and possibly also from the early wnw ese fissural system subsequent deformation in the slump area affected in part the filling units leading to the individualization of secondary curved faults younger volcanic products have gradually masked the mass wasting scars unlike the well known hilina slump hawaii pico s slump evolution might be controlled by an active regional tectonics

top down inflation and deflation at the summit of kīlauea volcano hawai i observed with insar 1 we use interferometric synthetic aperture radar insar to study deformation of the summit caldera at k lauea volcano during 2000 2008 which spanned both an east rift zone eruptive event in 2007 and the start of the ongoing summit eruption in 2008 the data set consists of small baseline subset sbas time series generated from 270 acquisitions on three separate beam modes from the radarsat 1 satellite we identify 12 time periods with distinct patterns of displacement that we attribute until late 2003 to secular tectonic driven deformation and from 2004 to 2008 to four different sources in the summit area we model the shallow magmatic system as a spherical reservoir at 1 9 0 1 km depth below the surface to the northeast of halemaumau source 1 and three vertically stacked sills at greater depths in the southern caldera area source 2 at the southern edge of the caldera at 2 9 0 2 km depth source 3 to the south southeast of the caldera at 3 4 0 5 km depth and source 4 south of the caldera at 3 6 0 4 km depth the sequence for filling of and withdrawal from these reservoirs reveals a top down process with sequences of both inflation and deflation initiating in the shallowest source inflation of source 3 is coincident with elevated seismic activity in the upper east rift zone in february 2006 and may 2007 source 4 is elongated toward the southwest rift zone and also shows elevated seismicity that extends toward the southwest rift zone

volcano tectonic evolution of a linear volcanic ridge pico faial ridge azores triple junction assessed by paleomagnetic studies abstract the morphology of volcanic oceanic islands results from the interplay between constructive and destructive processes and tectonics in this study the analysis of the paleomagnetic directions obtained on well dated volcanic rocks is used as a tool to assess tilting related to tectonics and large scale volcano instability along the pico faial linear volcanic ridge azores triple junction central north atlantic for this purpose 530 specimens from 46 lava flows and one dyke from pico and faial islands were submitted to thermal and alternating magnetic fields demagnetizations detailed rock magnetic analyses including thermomagnetic analyses and classical high magnetic field experiments revealed titanomagnetites with different ti content as the primary magnetic carrier capable of recording stable remanent magnetizations in both islands the paleomagnetic analysis yields a characteristic remanent magnetization which presents island mean direction with normal and reversed polarities in agreement with the islands location and the age of the studied lava flows indicating a primary thermo remanent magnetization field observations and paleomagnetic data show that lava flows were emplaced on pre existing slopes and were later affected by significant tilting in faial island magmatic inflation and normal faults making up an island scale graben can be responsible for the tilting in pico island inflation related to magma intrusion during flow emplacement can be at the origin of the inferred tilting whereas gradual downward movement of the se flank by slumping processes appears mostly translational

source spectra and rms acceleration of mexican subduction zone earthquakes strong motion sm data of six mexican subduction zone earthquakes 6 4 ms 8 1 recorded near the epicentral zone are analyzed to estimate their far field source acceleration spectra at higher frequencies f 0 3 hz apart from the usual corrections such as geometrical spreading 1 r average radiation pattern 0 6 free surface amplification a factor of 2 and equal partitioning of the energy into two orthogonal horizontal components a factor of 1 sqrt 2 the observed spectra are corrected for a frequency dependentq q 100f a site dependent filter e kf and amplification ofs waves near the surface a factor of about 2 atf 2hz we taker as the average distance from the rupture area to the site if we model the high frequency plateau f 1 hz of the source spectra by a point source 2 model and interpret them in terms of brune s model we obtain between 50 and 100 bars for all earthquakes the low frequency broadband teleseismicp wave spectra corrected witht 1 0 s agrees within a factor of two with sm source spectra near 1 hz the 2 model is inadequate to explain the observed source spectra in a broad frequency range these resemble spectra given bygusev 1983 with some differences

global survey of aftershock area expansion patterns we developed an objective method to define the aftershock areas of large earthquakes as a function of time after the main shock the definition is based upon the amount of energy released by aftershocks the spatial distribution of the energy release is first determined and is contoured the 1 day aftershock area is defined by a contour line corresponding to the energy release level of 10 15 6 ergs 100 km 2 day the 10 day 100 day and 1 y aftershock areas are similarly defined by contour lines corresponding to 10 14 8 10 14 0 and 10 13 5 ergs 100 km 2 day respectively we also define the expansion ratios at time t by the ratio of the aftershock area at t to that at 1 day using this method we study the aftershock area expansion patterns of 44 large m s 7 5 and five moderate shallow earthquakes which occurred from 1963 to 1980 each aftershock sequence is examined at four different times i e 1 day 10 days 100 days and 1 y after the main event we define the aftershock area expansion ratios and e by s 100 s 1 and l 100 l 1 respectively here s t and l t are the area and the length of the aftershock area respectively at time t our study suggests that a distinct regional variation of aftershock area expansion patterns is present it is strongly correlated with the tectonic environment in general the subduction zones of the mariana type have large expansion ratios and those of the chilean type have small expansion ratios some earthquakes that occurred in the areas of complex bathymetry such as aseismic ridges tend to have large expansion ratios these results can be explained in terms of an asperity model of fault zones in which a fault plane is represented by a distribution of strong spots called the asperities and weak zones surrounding the asperities the rupture immediately after the main shock mostly involves asperities after the main rupture is completed the stress change caused by the main shock gradually propagates outward into the surrounding weak zones this stress propagation manifests itself as expansion of aftershock activity in this simple picture if the fault zone is represented by relatively large asperities separated by small weak zones chilean type then little expansion of aftershock activity would be expected on the other hand if relatively small asperities are sparsely distributed mariana type significant expansion occurs the actual distribution of asperities is likely to be more complex than the two cases described above however we would expect that the expansion ratio is in general proportional to the spatial ratio of the total asperity area to the fault area

circum pacific seismic potential 1989 1999 the seismic potential for 96 segments of simple plate boundaries around the circum pacific region is presented in terms of the conditional probability for the occurrence of either large or great interplate earthquakes during the next 5 10 and 20 years i e 1989 1994 1989 1999 and 1989 2009 this study represents the first probabilistic summary of seismic potential on this scale and involves the comparison of plate boundary segments that exhibit varying recurrence times magnitudes and tectonic regimes presenting these data in a probabilistic framework provides a basis for the uniform comparison of seismic hazard between these differing fault segments as well as accounting for individual variations in recurrence time along a specific fault segment and uncertainties in the determination of the average recurrence time

site response attenuation and source spectra of s waves along the guerrero mexico subduction zone acceleration spectra from nine earthquakes and up to 11 stations per earthquake are inverted to find the source spectra the site response and the quality factor q of s waves this method is applied to moderate earthquakes 4 0 0 8 hz does not follow a simple linear relation as observed in previous studies of attenuation in other regions when the source spectra are interpreted in terms of brune s model the stress drops obtained are less than 20 bars for the four larger events analyzed the site functions obtained indicate that although all the stations used are on rock the effect of the near surface geology caused important amplifications at some stations

gps and tectonic evidence for a diffuse plate boundary at the azores triple junction abstract we use gps bathymetric structural and seismic data to define the pattern of present deformation along the northern half of the azores plateau where the nubia eurasia plate boundary terminates at the axis of the mid atlantic ridge mar new and existing campaign gps velocities from the azores islands reveal extension oblique to a series of en echelon volcanic ridges occupied by terceira s jorge pico and faial islands in a frame of reference defined by 69 continuous gps stations on the eurasia plate terceira island moves 2 1 mm yr away from eurasia consistent with the island s location within the terceira rift and plate boundary structure the volcanic ridges south of the terceira rift move toward wsw at progressively faster rates reaching a maximum of 3 5 0 5 mm yr 2 for the pico faial volcanic ridge the hypothesis that the terceira rift accommodates all nubia eurasia plate motion is rejected at high confidence level based on the motions of sites on s jorge island just west of terceira rift all of the islands move relative to the nubia plate with pico island exhibiting the slowest motion only 1 0 5 mm yr 2 detailed bathymetry from the interior of the hypothesized azores microplate reveals faults that crosscut young mar seafloor fabric these observations and the gps evidence for distributed deformation described above argue against the existence of a rigid or semi rigid azores microplate and instead suggest that nubia eurasia plate motion is accommodated by extension across a 140 km wide zone east of the mar axis most likely bounded to the north by the northern shoulder of the terceira rift the mar spreading rate along the western end of the azores deformation zone 38 5 n 39 5 n is intermediate between the eurasia north america rate measured at 39 5 n and the nubia north america rate measured at 38 5 n consistent with the joint conclusions that the nubia eurasia boundary is broad where it intersects the mar and the azores triple junction is diffuse rather than discrete

the tsunami earthquake of 1932 june 22 in manzanillo mexico seismological study and tsunami simulations summary we conduct a detailed seismological study of the large colima mexico earthquake of 1932 june 3 and of its aftershocks of june 18 and 22 the latter event iii generated a tsunami more devastating than that of the main shock despite much smaller seismic magnitudes thus qualifying as a so called tsunami earthquake relocation based on published arrival times shows that event iii took place up dip of the main shock the analysis of the spectral amplitude of mantle surface waves yields low frequency moments of 24 5 2 and 4 times 1027 dyn cm respectively with event iii featuring a moment growing with period which expresses the source slowness characteristic of tsunami earthquakes this is confirmed by a deficient energy to moment ratio as derived from high frequency p waves recorded at pasadena near field hydrodynamic simulations show that the effects of the main shock s tsunami are well modelled by a standard seismic source whereas the stronger tsunami from event iii can be modelled by rupture along a splay fault in a mechanically deficient material all our results then fit the model for tsunami earthquake aftershocks proposed for the kuril islands by fukao in 1979

seismic strain release in the mexican subduction thrust abstract in this paper the seismic strain release in the mexican subduction zone is discussed the paper is based on a revised catalog of large earthquakes in mexico new in this catalog are estimates of the seismic moment for many of the larger pre 1962 earthquakes of this century based on instrumental recordings from uppsala sweden a regression between moment and magnitude is established and used to estimate moments for the remaining events of this and the previous century the envelopes of the cumulative seismic moment curves can thus be developed consistent with the nineteenth century seismicity these indicate that the guerrero region is currently the most likely within the mexican subduction zone to experience one great event or several large events with a moment deficit approaching 20 10 27 dyn cm this is comparable with the moment released between 1899 and 1912 in the region in guerrero and also perhaps in michoacan the seismic strain release is consistent with plate tectonic expectations for a seismogenic zone 50 km in width in oaxaca either the seismogenic zone is much narrower and there is considerable aseismic slip or events to rupture the downdip extremes of the subduction zone have not occurred so far this century this last alternative arises because in guerrero the 1907 and 1957 earthquakes have relatively strong 40 s surface waves recorded at uppsala compared with all the other events such events which we infer are deeper present new problems for seismic hazard estimation in mexico since they are not represented in the available strong motion records

thick dyke emplacement and internal flow a structural and magnetic fabric study of the deep seated dolerite dyke of foum zguid southern morocco 1 knowledge on forced magma injection and magma flow in dykes is crucial for the understanding of how magmas migrate through the crust to the earth s surface because many questions still persist we used the long thick and deep seated foum zguid dyke morocco to investigate dyke emplacement and internal flow by means of magnetic methods structural analysis petrography and scanning electron microscopy we also investigated how the host rocks accommodated the intrusion regarding internal flow 1 important variations of the rock magnetic properties and magnetic fabric occur with distance from dyke wall 2 anisotropy of anhysteretic remanent magnetization reveals that anisotropy of magnetic susceptibility ams results mainly from the superposition of subfabrics with distinct coercivities and that the imbrication between magnetic foliation and dyke plane is more reliable to deduce flow than the orientation of the ams maximum principal axis and 3 a dominant upward flow near the margins can be inferred the magnetic fabric closest to the dyke wall likely records magma flow best due to fast cooling whereas in the core the magnetic properties have been affected by high temperature exsolution and metasomatic effects due to slow cooling regarding dyke emplacement this study shows that the thick forceful intrusion induced deformation by homogeneous flattening and or folding of the host sedimentary strata dewatering related to heat as recorded by thick quartz veins bordering the dyke in some localities may have also helped accommodating dyke intrusion the spatial arrangement of quartz veins and their geometrical relationship with the dyke indicate a preintrusive to synintrusive sinistral component of strike slip

aftershock area expansion and mechanical heterogeneity of fault zone within subduction zones we investigate global variations in the pattern of aftershock area expansion associated with large subduction zone earthquakes the expansion of aftershock areas with time is relatively small in the subduction zones of alaska the aleutians mexico and parts of the kuriles and south america these subduction zones are interpreted to be characterized by moderate to strong interplate coupling in contrast the extent of aftershock expansion is greater for events within the subduction zones of northeast japan and western and south pacific where interplate coupling is inferred to be weak these results are interpreted in terms of an asperity model where a fault zone is represented by a distribution of strong spots asperities and weaker zones the immediate aftershock area is assumed to extend over an area occupied by asperities and the subsequent expansion to occur into the surrounding weaker zones if large asperities abut each other on a fault plane interplate coupling is strong and little expansion of aftershock area occurs smaller and more sparsely distributed asperities cause weak interplate coupling and significant expansion of aftershock area small and densely distributed asperities result in moderate interplate coupling and little aftershock expansion

p waveform of large shallow earthquakes along the mexican subduction zone abstract we combine qualitative analysis of wiechert seismograms with quantitative analysis of galizin wilip and press ewing seismograms from europe to draw conclusions regarding simplicity complexity of p waveforms of large shallow earthquakes since 1907 which occurred along the mexican subduction zone these seismograms are compared with lp wwssn seismograms of recent earthquakes it is found that the earthquakes in oaxaca are simple with two exceptions the earthquake of 15 january 1931 m s 8 0 was a complex intraplate event with normal or strike slip faulting some earthquakes are simple and others are complex in the subregions of guerrero and michoacan all three earthquakes in jalisco are complex the depths of the events are shallow 10 h 20 km with the majority being at 16 km the simplicity of events the evidence of a characteristic magnitude of m s 7 8 and the ratio of surface wave to p wave seismic moment of about 1 5 in oaxaca suggest rather homogeneous asperities of nearly equal dimension separated by barriers elsewhere the interface of the plates is characterized by smaller asperities of differing sizes separated by areas where aseismic deformation may occur the earthquakes of 22 february 1943 m s 7 7 and 14 march 1979 m s 7 6 which were supposed to have broken the same area show drastically different p waveforms it appears that the location of the 22 february 1943 earthquake is in error alternatively the same area may have been broken with entirely different rupture processes

playa azul michoacan mexico earthquake of 25 october 1981 ms 7 3 abstract the playa azul earthquake broke the central part of the michoacan gap along the mexican subduction zone the aftershock area estimated from field data for a 6 day period beginning 19 hr after the main shock is 40 km 20 km preliminary focal mechanism of the main shock is consistent with a shallow dipping thrust fault although there is some evidence of seismic quiescence for 5 1 2 yr followed by a foreshock about 3 months before the main shock and an above average microseismic activity in the western part of the epicentral region during the last 4 1 2 months or more the data is not sufficient quality to assert this conclusion the michoacan gap was previously assigned category 3 singh et al 1981 based on the classification of mccann et al 1979 the seismic potential of the unbroken segments of this gap 45 km to nw and 35 km to the se of the playa azul aftershock area remains uncertain corresponding to a rupture length of 40 km each segment could generate an earthquake of similar magnitude to the playa azul earthquake unfortunately the recurrence period of large earthquakes in the region is not known and great major earthquakes cannot be ruled out

the terceira rift as hyper slow hotspot dominated oblique spreading axis a comparison with other slow spreading plate boundaries abstract we suggest the 550 km long terceira rift tr azores plateau is the world s slowest spreading hyper slow 4 mm a plate separation 2 3 3 8 mm a perpendicular to oblique axial segments organized accreting plate boundary in its slightly sinuous ca 300 km radius of curvature axial trace its oblique spreading angles ca 40 65 and in frequency and first motions of earthquakes the tr resembles better known ultra or super slow spreading ridges e g gakkel and southwest indian ridges interpreted simply as volcanically unfilled rift valley segments the inter island basins e g the 3200 m deep hirondelle basin are slightly wider 30 60 km but not significantly deeper 1000 2200 m than the mid atlantic ridge mar median valley 20 28 mm a 10 n 53 n however along axis segmentation wavelengths ca 100 km are double those along the central mar but make tr comparable to the ultra slow 15 16 mm a southwest indian and gakkel 7 13 mm a ridges if this segmentation wavelength reflects rayleigh taylor instabilities the viscosity contrast between the overlying axial lithosphere and the partial melt zones is about an order of magnitude greater at ca 4 16 mm a than at 20 30 mm a the tr differs dramatically from ultra slow ridges only in the large amplitude of along strike topography 2000 4000 m 4200 m total variation owing perhaps to a copious melt flux from the azores hotspot combined with a spreading rate determined greater axial flexural strength and plate thickness and slower export of volcanics from the rift axis the probable tr youth ca 1 ma requiring less than 4 km new oceanic crust suggests lack of steady state spreading conditions which may explain the published gravity evidence against tr spreading absolute plate motions support the creation of the azores plateau by successive ne jumps of the rift axis to maintain its position over a fixed hotspot

regional variation in the number of aftershocks mb 5 of large subduction zone earthquakes mw 7 0 it is reasonable to expect that the number of aftershocks following a large earthquake should reflect the degree of heterogeneity on and near the fault plane to investigate this question we plot n the number of aftershocks with mb 5 0 occurring within the next 30 days after large subduction zone thrust faulting earthquakes mw 7 0 versus the magnitude mw of the main shock only earthquakes that occurred after the installation of the world wide standard seismographic network stations are considered because the detection threshold of aftershocks in many regions is greater than mb 5 0 n is obtained by fitting the data to gutenberg and richter s relation log n a b mb for the same mw n varies by a factor of about 40 in different regions n is less than expected in the eastern pacific including alaska and is greater than expected in the western pacific relative lack and excess of n correlates well with regions of supposedly strong and weak coupling respectively while small and large n also show some correlation with the expansion rate of aftershock area with time the variation in n is much larger than would be expected from the mere increase of aftershock areas in some cases it appears that the number of aftershocks n for earthquakes of similar mw may also reflect the complexity and enrichment of the p wave amplitude spectra from 1 to 10 sec for example the valparaiso chile earthquake of 1985 mw 8 0 generated n 46 earthquakes whereas for the michoacan mexico event of 1985 mw 8 1 n is only equal to 5 furthermore for the valparaiso earthquake p waveforms are much more complex and the spectra are more energetic at 1 to 10 sec as compared to the 1985 michoacan event

use of the mantle magnitude m m for the reassessment of the moment of historical earthquakes the mantle magnitudemm is used on a dataset of more than 180 wavetrains from 44 large shallow historical earthquakes to reassess their moments which in many cases had been previously estimated only on the basis of the earthquake s rupture area we provide 27 new or revised values ofmo based on the spectral amplitudes of surface waves recorded at a number of stations principally uppsala and pasadena among them and most significantly we document a large low frequency component to the source of the 1923 kanto earthquake the low frequency seismic moment is 2 9 1028 dyn cm in accord with geodetic observations on the other hand we revise downwards the seismic moment of the 1906 ecuador event which did not exceed 6 1028 dyn cm

the 1998 faial earthquake azores evidence for a transform fault associated with the nubia eurasia plate boundary abstract with very few exceptions m 4 tectonic earthquakes in the azores show normal fault solution and occur away from the islands exceptionally the 1998 shock was pure strike slip and occurred within the northern edge of the pico faial ridge fault plane solutions show two possible planes of rupture striking ene wsw dextral and nnw sse sinistral the former has not been recognised in the azores but is parallel to the transform direction related to the relative motion between the eurasia and nubia plates therefore the main question we address in the present study is do transform faults related to the eurasia nubia plate boundary exist in the azores knowing that the main source of strain is related to plate kinematics we conclude that the sinistral strike slip nnw sse fault plane solution is not consistent with either the fault dip ca 65 which is typical of a normal fault or the ca ene wsw direction of maximum extension both are consistent with a normal fault as observed in most major earthquakes on faults striking around nnw sse in the azores in contrast the dextral strike slip ene wsw fault plane solution is consistent with the transform direction related to the anticlockwise rotation of nubia relative to eurasia altogether tectonic data measured ground motion observed destruction and modelling are consistent with a dextral strike slip source fault striking ene wsw furthermore the bulk clockwise rotation measured by gps is typical of bookshelf block rotations observed at the termination of such master strike slip faults therefore we suggest that the 1998 earthquake can be related to the wsw termination of a transform ene wsw fault plane solution associated with the nubia eurasia diffuse plate boundary

a catalog of shallow earthquakes of mexico from 1900 to 1981 a catalog of shallow earthquakes depth 65 km of mexico 15 to 20 n 94 5 to 105 5 w covering the period 1900 to 1981 is presented the catalog includes only those events whose magnitudes m ms or magnitude from pasadena are 5 9 it is most likely complete for shocks with ms 6 5 from july 1906 to december 1981 for many shocks ms has been determined from uppsala bulletins and or uppsala and gottingen wiechert seismograms the statistics of incremental frequency n ms shows a peak at ms 7 7 reinforcing a previous observation by singh et al 1981 in the range 6 4 ms 7 5 the observed n ms deviates significantly from an exponential distribution the catalog provides a basis for further statistical analysis of earthquakes in mexico

precursory earthquakes of the 1943 eruption of paricutin volcano michoacan mexico abstract paricutin volcano is a monogenetic volcano whose birth and growth were observed by modern volcanological techniques at the time of its birth in 1943 the seismic activity in central mexico was mainly recorded by the wiechert seismographs at the tacubaya seismic station in mexico city about 320 km east of the volcano area in this paper we aim to find any characteristics of precursory earthquakes of the monogenetic eruption though there are limits in the available information such as imprecise location of hypocenters and lack of earthquake data with magnitudes under 3 0 the available data show that the first precursory earthquake occurred on january 7 1943 with a magnitude of 4 4 subsequently 21 earthquakes ranging from 3 2 to 4 5 in magnitude occurred before the outbreak of the eruption on february 20 the s p durations of the precursory earthquakes do not show any systematic changes within the observational errors the hypocenters were rather shallow and did not migrate the precursory earthquakes had a characteristic tectonic signature which was retained through the whole period of activity however the spectra of the p waves of the paricutin earthquakes show minor differences from those of tectonic earthquakes this fact helped in the identification of paricutin earthquakes except for the first shock the maximum earthquake magnitudes show an increasing tendency with time towards the outbreak the total seismic energy released by the precursory earthquakes amounted to 2 10 19 ergs considering that statistically there is a threshold of cumulative seismic energy release 10 17 18 ergs by precursory earthquakes in polygenetic volcanoes erupting after long quiescence the above cumulative energy is exceptionally large this suggests that a monogenetic volcano may need much more energy to clear the way of magma passage to the earth surface than a polygenetic one the magma ascent before the outbreak of paricutin volcano is interpretable by a model of magma filled crack formation proposed by weertman based on seismic data and other field observations

the 1980 1997 and 1998 azores earthquakes and some seismo tectonic implications abstract we have studied the focal mechanisms of the 1980 1997 and 1998 earthquakes in the azores region from body wave inversion of digital gdsn global digital seismograph network and broadband data for the 1980 and 1998 shocks we have obtained strike slip faulting with the rupture process made up of two sub events in both shocks with total scalar seismic moments of 1 9 1019 nm mw 6 8 and 1 4 1018 nm mw 6 0 respectively for the 1997 shock we have obtained a normal faulting mechanism with the rupture process made up of three sub events with a total scalar seismic moment of 7 7 1017 nm mw 5 9 a common characteristic of these three earthquakes was the shallow focal depth less than 10 km in agreement with the oceanic type crust from the directivity function of rayleigh lr waves we have identified the nw se plane as the rupture plane for the 1980 and 1998 earthquakes with the rupture propagating to the se slow rupture velocity about of 1 5 km s has been estimated from directivity function for the 1980 and 1998 earthquakes from spectral analysis and body wave inversion fault dimensions stress drop and average slip have been estimated focal mechanisms of the three earthquakes we have studied together with focal mechanisms obtained by other authors have been used in order to obtain a seismotectonic model for the azores region we have found different types of behaviour present along the region it can be divided into two zones zone i from 30 w to 27 w zone ii from 27 w to 23 w with a change in the seismicity and stress direction from zone i in zone i the total seismic moment tensor obtained corresponded to left lateral strike slip faulting with horizontal pressure and tension axes in the e w and n s directions respectively in zone ii the total seismic moment tensor corresponded to normal faulting with a horizontal tension axis trending ne sw normal to the terceira ridge the stress pattern for the whole region corresponds to horizontal extension with an average seismic slip rate of 4 4 mm yr

reevaluation of magnetic chrons in the north atlantic between 35 n and 47 n implications for the formation of the azores triple junction and associated plateau 1 in this paper we present a new magnetic compilation for an area of the north atlantic located between 35 n and 47 n and up to anomaly 33r we also present a strategy to pick magnetic isochrones and compute finite rotation poles this technique is based on a continuous reduction to the pole technique and some basic assumptions regarding the direction of the remanent magnetization vector a cost function that measures the misfit between interpreted and rotated isochrones and the systematic exploitation of the parameter space is used to compute the best set of finite eulerian rotations for the chrons 5 6 6c 11 12 13 18 20 21 22 23 24 25 26 28 29 30 32 33 and 33r this set of chrons and poles is used to discuss the evolution of the north atlantic close to iberia and in particular the onset and early development of the azores triple junction area we show that the relative motion between the eurasian and the african plates can be coherently described in terms of rigid plate kinematics respecting both the anomalies shapes and the precise location of the main structural elements of the area the pico fracture zone the east azores fracture zone and the gloria fault we distinguish from the magnetic point of view two different areas of the azores plateau the south azores domain where almost undisturbed nnw magnetic lineations can be found and the azores domain close to the topographic highs and with no systematic magnetic stripping with the exception of a few recent lineations probably matuyama and brunhes we present an approximate reconstruction of the plate configuration after chron 18 to conclude that the attachment of iberia to eurasia was younger than previously thought lower miocene triggering the formation of the azores domain in which stretching took place essentially in the last 20 ma at an average rate of 3 8 mm a and progressively attaching the south azores domain to the african plate by a northward progression of the triple junction

regional variations of seismic attenuation of lg waves in southern mexico attenuation of lg waves is estimated using 61 events located in the subduction zone of the middle american trench and recorded by the seismic network of laguna verde veracruz mexico we estimate the anelastic attenuation coefficient of lg waves or equivalently qlg by calculating the rate of acceleration spectra decay with distance we consider paths from two regions guerrero veracruz ne sw direction and oaxaca veracruz north south direction assuming a frequency dependence of of the form 0 we find that 0 175 0 05 for both regions and that 0 was lower for paths from guerrero to veracruz 0 0 0071 0 002 than for paths from oaxaca to veracruz 0 0 0161 0 003 in the frequency range from 2 to 7 hz in terms of the quality factor qlg values of qlg 134 30 0 83 0 2 and qlg 59 10 0 81 0 2 were found for the guerrero veracruz and the oaxaca veracruz paths respectively this difference in attenuation may be due to the state of stresses that prevails in both regions including the density and fluid content of fractures which are attenuation mechanisms also suggested for other regions mitchell 1995

vertical axis rotation of the upper portions of the north east rift of tenerife island inferred from paleomagnetic data abstract paleomagnetic sampling sites were established in 82 dykes along an 8 km long section of the north east rift zone nerz of tenerife canary islands spain of the 70 interpretable sites 16 are of normal polarity and 54 of reversed polarity four normal polarity sites and fifteen reverse polarity sites were excluded from the grand mean calculation for statistical reasons after inverting the reverse polarity sites through the origin the in situ grand mean yields a declination d 023 8 an inclination i 42 3 95 3 2 39 0 n 51 that is discordant to the expected late miocene to pleistocene field direction d 357 6 i 38 8 95 4 7 this discordance can be explained as either a 26 clockwise vertical axis rotation or a 28 wnw side down tilt about an average 009 horizontal tilt axis the sampled section is composed of numerous semi vertical dykes cutting mainly lava flow units that are sub horizontal and cross cut by steeply dipping faults 70 90 field evidence is therefore more compatible with a vertical axis rotation rather than a horizontal axis tilt of the drilled units we argue that this clockwise vertical axis rotation is likely related to strike slip movements that occurred along the edges of the collapse scars and accommodate the emplacement and growth of the underlying intrusive core and associated dykes six new 40ar 39ar age determinations constrain the main interval of dyke emplacement within the nerz between 0 99 ma and 0 56 ma the intrusive activity in the sampled section of the rift appears to have been almost continuous with several intrusion pulses that are probably related to flank destabilisation event s during the mid pleistocene our study thus demonstrates a long lived multi faceted history that shaped the nerz

an earthquake doublet in ometepec guerrero mexico on june 7 1982 an earthquake doublet occurred in a gap near ometepec guerrero mexico which had been given a high seismic potential the two earthquakes first event m s 6 9 m b 6 0 16 3 n 98 4 w d 25 km second event m s 7 0 m b 6 3 16 4 n 98 5 w d 8 km of the doublet occurred within five hours of each other we determine the source parameters of these events by inverting surface wave data at a period of 256 s the results are for the first event strike 116 dip 77 slip 88 and seismic moment of 2 8 10 26 dyne cm and for the second event strike 116 dip 78 slip 89 and seismic moment of 2 8 10 26 dyne cm modeling of long period p waves suggests that the first event has a depth of 20 km and is represented by a single trapezoidal source time function with an effective duration of 6 s the second event is best modeled by two sources at depths of 15 and 10 km the combined effective source duration time for the two sources is about 10 s the ratio of the seismic moment obtained from body waves to that from surface waves is 0 5 for the first event and 1 for the second adding the seismic moment of the two events and considering the first week aftershock area 3200 km 2 the stress drop is 4 bars these results suggest that the first event that involved a deeper asperity caused an incremental stress change large enough to trigger the second event if the two events of the doublet broke distinct areas of the subduction zone the coseismic slip is 0 58 m and accounts for about 25 of the total plate motion between the cocos plate and the north america plate accumulated since the last large earthquake in the region other doublets similar to the ometepec doublet have occurred along the middle america trench during the past 70 years a regional distribution of comparable size asperities may be responsible for this relatively frequent occurrence of doublets and for the simplicity of earthquakes in the region the high convergence rate which produces rapid strain accumulation and short recurrence intervals for large earthquakes and the smooth sea floor subducted along the middle america trench may contribute to the homogeneous distribution of comparable size asperities we found a relation log t 1 3 log m 0 t is the average recurrence time and m 0 is the average seismic moment for large earthquakes along the subduction zone in the guerrero oaxaca region where the convergence rate and the properties of the subducted plate are considered relatively uniform a simple asperity model predicts this relation