geochemical and sr nd isotopic constraints on the genesis of the cenozoic linzizong volcanic successions southern tibet abstract the linzizong volcanic successions that crop out in the lhasa terrane southern tibet have been conventionally regarded as the products of northward subduction of the neotethyan oceanic slab beneath south asia this study reports geochemical data of 100 volcanic rocks from the lhasa terrane to better constrain the temporal spatial distribution and petrogenesis of the linzizong volcanism the linzizong volcanic successions consist dominantly of calc alkaline rocks that erupted from ca 69 to 43 ma and show typical arc lava geochemical features marked with lile enrichment and hfse depletion in the spidergram their sr and nd isotope ratios e nd t 3 3 to 2 4 i sr 0 7048 0 7072 are generally similar to those of the associated gangdese i type granitoids the linzizong volcanism is characterized by a flare up period ca 50 ma that shows significant geochemical variations manifested by the coexistence of five types of volcanic rocks 1 the main suite of calc alkaline rocks sio 2 45 80 wt la 12 45 ppm e nd t 3 8 to 4 9 i sr 0 7037 0 7105 that could be interpreted by partial melting of the mantle wedge followed by assimilation and fractional crystallization afc processes with 10 crustal contamination and or magma mixing 2 the low k low ree suite sio 2 48 61 wt k 2 o 0 5 1 1 wt la 7 10 ppm that has the highest nd isotope ratios e nd t 5 9 to 3 5 suggesting a juvenile mantle origin possibly related to decompression melting of the asthenosphere 3 the shoshonitic suite sio 2 53 71 wt k 2 o 3 8 6 7 wt e nd t 2 8 to 6 1 from small degree melting of the metasomatized lithospheric mantle 4 the high ree suite sio 2 61 75 wt la 36 87 ppm e nd t 2 6 to 3 2 originating from remelting of a newly underplated basaltic lower crust and 5 the evolved suite of rhyolitic flows and ignimbrites sio 2 65 wt that have the lowest e nd t values of 14 to 18 representing remelting products of the basement or continental crust of the lhasa terrane such geochemical heterogeneities are attributed to breakoff of the subducted neotethyan slab under southern tibet that occurred in the early stage of the india asia collision

source parameters of some large earthquakes in northern aegean determined by body waveform inversion average source parameters for three large north aegean events are obtained from body wave inversion for the moment tensor the parameters for the events are as follows

moment magnitude relations based on strong motion records in greece in this work the variation of the local magnitude mlsm derived from strong motion records at short distances is examined in terms of moment magnitude m w strong motion data from greek earthquakes are used to determine the strong motion local magnitude mlsm by performing an integration of the equation of mo tion of the wood anderson wa seismograph subjected to an input acceleration the most reliable strong motion data are utilized for earthquakes with seismic mo ments log m 0 22 0 dynecm and calculated local magnitudes mls m 3 7 the correlation between the seismic moments log m0 and the calculated local magni tudes mlsm using strong motion records is given by log m 0 1 5 mcsu 16 07 which is very similar to that proposed by hanks and kanamori 1979 moreover it is shown that mls m is equal to moment magnitude mw for a large mls m range 3 9 to 6 6 comparison of the strong motion local magnitude and the me magnitude estimated in greece ml n and surrounding area shows a systematic bias of 0 4 to 0 5 similar to the difference that has been found between m w and mlgr for the same area the contribution of the local site effects in the calculation of the local magni tude mlsm is also considered by taking into account two indices of soil classification namely rock and alluvium or the shear wave velocity vs30 of the first 30 m based on nehrp 1994 and ubc 1997 an increase of mls m by 0 16 is observed for alluvium sites alternative relations showing the mls m variation with v 3 are also presented finally examination of the wa amplitude attenuation log a0 with distance shows that the jennings and kanamori 1983 relation for a 100 km is appropriate for greece the same results confirm earlier suggestions that the 0 4 to 0 5 bias between mlgr and mw also mlsa4 should be attributed to a low static magnification 800 of the athens wa instrument on which all other m l relations in greece have been calibrated

site amplifications for generic rock sites seismic shear wave velocity as a function of depth for generic rock sites has been estimated from borehole data and studies of crustal velocities and these velocities have been used to compute frequency dependent amplifications for zero attenuation for use in simulations of strong ground motion we define a genetic rock site as one whose velocity at shallow depths equals the average of those from the rock sites sampled by the borehole data most of the boreholes are in populated areas for that reason the rock sites sampled are of particular engineering significance we consider two genetic rock sites rock corresponding to the bulk of the borehole data and very hard rock such as is found in glaciated regions in large areas of eastern north america or in portions of western north america the amplifications on rock sites can be in excess of 3 5 at high frequencies in contrast to the amplifications of less than 1 2 on very hard rock sites the consideration of unattenuated amplification alone is computationally convenient but what matters for ground motion estimation is the combined effect of amplification and attenuation for reasonable values of the attenuation parameter k 0 the combined effect of attenuation and amplification for rock sites peaks between about 2 and 5 hz with a maximum level of less than 1 8 the combined effect is about a factor of 1 5 at 1 hz and is less than unity for frequencies in the range of 10 to 20 hz depending on t using these amplifications we find provisional values of about ao 70 bars and k 0 0 035 sec for rock sites in western north america by fitting our empirically determined response spectra for an m 6 5 event to simulated values the borehole data yield shear velocities 033o of 618 and 306 rrdsec for rock and soil sites respectively when averaged over the upper 30 m from this we recommend that v30 equals 620 and 310 m sec for applications requiring the average velocity for rock and soil sites in western north america by combining the amplifications for rock sites and the site factors obtained from our analysis of strong motion data we derive amplifications for sites with 17 3o 520 m sec nehrp class c corresponding to a mix of rock and soil sites and v3o 310 and 255 m sec average soil and nehrp class d sites respectively for the average soil site the combined effect of amplification and attenuation exceeds a factor of 2 0 for frequencies between 0 4 and about 4 hz with a peak site effect of 2 4 the peak of the nehrp class d site effect is 2 7

deterministic ground motion scenarios for engineering applications the case of thessaloniki greece abstract in this article we present a deterministic study to estimate seismic ground motions expected in urban areas located near active faults the purpose was to generate bedrock synthetic time series to be used as seismic input into site effects evaluation analysis and loss estimates for the urban area and infrastructures of thessaloniki northern greece two simulation techniques a full wave method to generate low frequency seismograms 1 hz were used to compute time series associated with four different reference earthquakes having magnitudes from 5 9 to 6 5 and located within 30 km of thessaloniki the propagation medium and different source parameters were tested through the modeling of the 1978 thessaloniki earthquake m 6 5 moreover two different nucleation points were considered for each fault in order to introduce additional variability in the ground motion estimates between the two cases the quasi unilateral rupture propagation toward the city produces both higher median peak ground acceleration pga and peak ground velocity pgv values and higher variability than bilateral ones conversely the low frequency ground motion that is peak ground displacement pgd is slightly influenced by the position of the nucleation point and its variability is related to the final slip distribution on the faults of the reference earthquakes and to the location of the sites with respect to the nodal planes of the radiation pattern to validate our deterministic shaking scenarios we verified that the synthetic peak ground motions pga and pgv and spectral ordinates are within one standard deviation of several ground motion prediction equations valid for the region at specific sites we combined the low and high frequency synthetics to obtain broadband time series that cover the entire frequency band of engineering interest 0 25 hz the use of synthetic seismograms instead of empirical equations in the hazard estimates provides a complete evaluation of the expected ground motions both in frequency and time domains including predictions at short distances from the fault 0 10 km and at periods larger than 2 3 sec

source spectral scaling and stress release estimates using strong motion records in greece a data set of 50 horizontal strong motion accelerograms from 18 main events from moderate and large earthquakes 5 2 mw 6 9 and distances 9 km r 116 km occurred at three different tectonic settings normal strike slip and thrust faulting were used to estimate stress parameters three different definitions of stress release were adopted the classical brune the dynamic rms root mean square and the apparent stress corner frequencies f0 based on the andrews 1986 methodology were computed and using these calculations seismic moments were estimated from independent sources teleseismic data the calculated corner fre quencies seem to be consistent based on seismic moments with previous estimates for the same area based on those source parameters a theoretical x square spectrum was derived from describing source radiation in terms of earthquake size frequency dependent site amplifications were included in the simulations separating the re cordings in three different categories b c and d corresponding to nehrp na tional earthquake hazards reduction program site classification the theoretical and observational spectra are in good agreement in a frequency range from 0 05 to 25 hz except for the cases where steep topography affects the recorded data a poor fit is also observed in cases where the distance of the recorded data is greater than 100 km and the attenuation model seems to be inadequate to describe the theoretical model the calculated parameters present a mean value of 55 16 bars for brune stress 72 26 bars for dynamic rms stress and 17 10 bars for apparent stress for all determined stresses the thrust faulting earthquakes exhibited higher values than the calculated normal and strike slip fault seismic events

along arc and back arc attenuation site response and source spectrum for the intermediate depth 8 january 2006 m 6 7 kythera greece earthquake an m 6 7 intermediate depth 66 km in slab earthquake occurring near the island of kythera in greece on 8 january 2006 was well recorded on networks of stations equipped with acceleration sensors and with broadband velocity sensors all data were recorded digitally using recording instruments with resolutions ranging from almost 11 to 24 bits we use data from these networks to study the distance dependence of the horizontal component fourier acceleration spectra fas and horizontal component pseudoabsolute response spectral acceleration psa for pur poses of simulating motions in the future we parameterize the distance decay using several forms of the geometrical spreading function for each of which we derive q as a function of frequency by extrapolating the distance decay back to 1 km we obtain a reference spectrum that can be used in future simulations this spectrum requires a more complicated spectral shape than the classic single corner frequency model in particular there appears to be an enhancement of motion around 0 2 0 3 hz that may be due to the radiation of a 3 5 sec pulse from the source we infer a 0 value of about 0 055 sec for rock stations and a stress parameter in the range of 400 600 bars we also find distinctive differences in the site response of stations on soft soil and soil both the fas and the 5 damped psa amplifications have similar peak amplitudes about 2 and 4 for soil and soft soil sites respectively relative to the rock sites at similar frequencies between about 0 4 and 2 0 hz with the soft soil amplifications peaking at somewhat lower frequencies than the soil amplifications one of the most distinctive features of the data is the clear difference in the motions for along arc and back arc stations with the former being significantly higher than the latter over a broad range of frequencies at distances beyond about 250 km the motions from the kythera earthquake are roughly comparable to those from intermediate depth earthquakes elsewhere but they appear to be significantly higher than those from re cordings of shallow earthquakes in greece of comparable magnitude and hypocentral distance

the effect of crustal structure on strong ground motion attenuation relations in eastern north america strong ground motion attenuation relations are usually described by smoothly decreasing functions of distance however consideration of wave propagation in the crust suggests that attenuation relations should be more complex such complexity may be present in strong ground motion data for eastern north american earthquakes which show amplitudes in the distance range of 60 to 150 km that lie above the trends at smaller and greater distances using a wavenumber integration method to compute green s functions and close in recordings of several earthquakes as empirical source functions we have generated synthetic seismograms that are in good agreement with regional and strong motion recordings of eastern north american earthquakes from these synthetic seismograms we have shown that the observed interval of relatively high amplitudes may be attributable to postcritically reflected s waves from the moho the presence and location of the interval of relatively high amplitudes is highly dependent on the crustal velocity structure and may therefore be expected to show regional variation however for any realistic structure model there will be a transition in the attenuation relation from an interval at shorter distances less than about 100 km that is dominated by direct waves to an interval at greater distances that is dominated by postcritically reflected waves the synthetic seismograms have response spectral velocities that match those of the recorded data and their m blg values are in good agreement with observed values

the 1997 1998 umbria marche sequence central italy source path and site effects estimated from strong motion data recorded in the epicentral area 1 we apply a nonparametric spectral inversion scheme to calculate source spectra s wave attenuation and site transfer functions from strong motion records in the epicentral region of the 1997 1998 umbria marche seismic sequence central italy we use records from moderate size earthquakes 4 6 ml 5 9 to parameterize the spectral amplitude decay in the distance range from 5 to 40 km we find that the average quality factor q can be approximated by q f 49 f0 9 between 0 5 and 8 hz and the geometrical spreading by r 0 9 at high frequencies f 8 hz the dependence of q on frequency weakens and it takes an approximate constant value of 318 we fit the source spectra to the square model and calculate an average stress drop of 2 1 mpa the average value is consistent with the previous estimates from the weak events 1 4 ml 4 5 of the umbria marche seismic sequence the most remarkable site effects are found in correspondence of large sedimentary basins filled by alternation of sandy clayey deposits the estimated spectral parameters are used to simulate acceleration spectra recorded during several earthquakes of the umbria marche sequence both point source and finite fault effects are considered furthermore attenuation relationships for peak ground velocity and ground acceleration are estimated using synthetic data and compared to existing relationships

the dynamics of thrust and normal faults over multiple earthquake cycles effects of dipping fault geometry we perform dynamic simulations of thrust and normal faults over multiple earthquake cycles our goal is to explore effects of asymmetric fault geometry on the long term seismicity and dynamics of dipping faults a dynamic finite element method is used to model the interseismic and coseismic processes with a dynamic relaxation technique for the former the faults are loaded by stable sliding along the downward continuation of the faults the asymmetric fault geometry of dipping faults with respect to the free surface cause changes in normal stress during the interseismic and coseismic periods these changes are of opposite sign in the two periods resulting in a stabilization of the normal and shear stresses over many earthquake cycles both faults develop relatively stable event patterns in which a large event that ruptures the entire fault is preceded by a number of small events with various rupture lengths a strong asymmetry in fault and ground motion exists between thrust and normal faults and between the hanging wall and the footwall of both faults on faults dipping less than 70 in both normal and thrust faults the horizontal component of ground motion dominates on the footwall while the vertical component dominates on the hanging wall the above results may have implications in seismic hazard analysis and building design in regions where dip slip faults predominate

numerical models of crustal scale convection and partial melting beneath the altiplano puna plateau we employ 2 d thermo mechanical modelling to study possible mechanisms for generating large scale crustal magmas in the altiplano puna region of the central andes the peak of ignimbrite activity in the late miocene and pliocene is associated in space and time with tectonic shortening and plateau uplift a seismic low velocity zone and other geophysical observations indicate that partial melting in the mid crust is still present under the ignimbrite province today we show that neither radiogenic heat production in a thickening crust nor shear heating due to tectonic shortening nor heat brought by intrusions of arc magmas into the mid crust can heat the mid crust to the degree and within the time frame suggested by the geologic petrologic and geophysical observations a viable mechanism to achieve high temperatures within the time constraints is convective heat and mass transfer by partially molten lower crust which itself was heated by enhanced mantle heat flow possibly associated with delamination of the mantle lithosphere during tectonic shortening and intensified magmatic arc activity the thermo mechanical models explain the mid crustal low velocity zone and the high and strongly variable surface heat flow observed in the altiplano convection by bulk flow of the crust appears when the middle and lower crust are mechanically weak quartz dominated rheology the basal heat flow from the mantle is high s 60 mw m 2 and tectonic shortening is active we argue that these conditions are satisfied in the central andes orogen 6 2002 elsevier science b v all rights reserved

a process based model of pre eruption seismicity patterns and its use for eruption forecasting at dormant stratovolcanoes abstract using long term data sets the eruptive size timing and style can typically be successfully forecast for dormant volcanoes using current monitoring data and knowledge of precursory eruptive patterns the usaid usgs volcano disaster assistance program vdap utilizes seismic data over multiple eruption cycles and across volcano types to successfully forecast eruption size style and onset times as well as changes within ongoing eruptions from millions of seismic event observations we show that seismic data a proxy for rate dependent strain changes in the magmatic system combined with a process based conceptual geologic model enables eruption forecasting especially for stratovolcanoes dormant 20 years we show that this model often works for some open systems ones that sufficiently reseal to plug the conduit and also where a deep intrusion occurs after phreatic or phreatomagmatic explosions have begun this model is divided into four seismicity and geologic stages we describe each of the four stages of seismicity in detail and how they relate to the ongoing geologic process of that stage stage 1 deep intrusion and seismicity occurring at 10 to 40 km depth below the volcanic edifice stage 2 distal vt seismicity induced at distances from 2 to 30 km laterally from the summit by magma intruding into and inflating the upper crustal magma storage region stage 3 vent clearing seismicity associated with the initial vent creation and clearing phase of eruptive activity stage 4 repetitive event seismicity associated with the final ascent of magma from 2 km to the surface we present 36 examples of this seismic progression from 26 volcanoes we believe that this progression generally correlates with the magma flux and the eventual explosivity and is invaluable for forecasting eruptions

dependence of coda attenuation on frequency and lapse time in central greece the dependence of coda attenuationq c on frequency and lapse time was studied data from small local earthquakes recorded at three stations vmr vsi and vfi of the volnet network operating in central greece were used q c was estimated by applying the single scattering model to bandpass filtered seismograms over a frequency range of 1 to 12 hz analysis was performed every 10 s until the end on overlapping time windows q c is found to depend on frequencyf in hz according to a power law q c q0f n observedq0 ranges from 30 to 100 and the powern ranges from 0 90 to 0 70 q0 increases andn decreases with lapse time increasing a strong dependence ofq c on lapse time was also found in the frequency range of 1 to 8 hz and at a short lapse time q c values were found to be similar for all three stations on the other hand at the longest analyzed time window 50 s the estimatedq c values show a discrepancy which is more obvious at a higher frequency band the scattering coefficient around the central station vsi is found to range from 0 029 to 0 0041 km 1 q c from the single scattering model andq s from the amplitude ratio of directs to coda waves for the vsi station are similar we believe dependence of coda attenuationq c on frequency and lapse time is caused by a combination of geotectonic features and depth variation asq s

attenuation of coda waves in northern greece the single scattering model has been applied for the estimation of codaq values for local earthquakes that occurred in northern greece during the period 1983 1989 and recorded by the telemetered network of the geophysical laboratory of the university of thessaloniki codaq estimations were made for four frequency bands centered at 1 5 hz 3 0 hz 6 0 hz and 12 0 hz and for the lapse time windows 10 20 sec 15 30 sec 20 45 sec 30 60 sec and 50 100 sec the codaq values obtained show a clear frequency dependence of the formq c q 0 f n whileq 0 andn depend on the lapse time window q 0 was found equal to 33 andn equal to 1 01 for the time window of 10 to 20 sec while for the other windowsq 0 increased from 60 to 129 withn being stable close to 0 75 this lapse time dependence is interpreted as due to a depth dependent attenuation the high attenuation and the strong frequency dependence found are characteristic of an area with high seismicity in agreement with studies in other seismic regions

eocene neotethyan slab breakoff in southern tibet inferred from the linzizong volcanic record abstract formation of the linzizong volcanic successions in the lhasa terrane southern tibet has long been related to northward subduction of the neotethyan oceanic slab under asia here we report new 40ar 39ar age results for the volcanic rocks recovered from a large area 29 n to 32 n and 85 e to 93 e that together with literature data delineate two discrete stages of volcanism these are a widespread cretaceous stage and an intense but spatially confined paleogene stage the latter occurring only in the southern part of the lhasa terrane resulted in the linzizong volcanic successions our data furthermore suggest southward migration and intensification of the volcanism in the lhasa terrane with magmatic flare ups at ca 50 ma while the volcanic successions consist dominantly of calc alkaline rocks typical of arc lava geochemistry those formed during the flare up period show significant compositional variations from low k tholeiitic through calc alkaline to shoshonitic magma suites these observations enable us to interpret the volcanic southward migration and following flare ups as the consequences of rollback and breakoff of the subducted neotethyan slab that occurred ahead and in the early stage respectively of the india asia collision our interpretation that involves a major eocene tectonomagmatic activity and concomitant topographic uplift in southern tibet is consistent with himalayan metamorphic constraints regional sedimentary records and seismic tomography

the nature and timing of crustal thickening in southern tibet geochemical and zircon hf isotopic constraints from postcollisional adakites abstract rising as the roof of the world the tibetan plateau is now underlain with the thickest continental crust on earth how and when was this crust formed which would have exerted pivotal controls to the formation of the plateau has long been an issue of hot debates this paper reports zircon u pb ages and hf isotope data for postcollisional 30 9 ma adakites in the southern lhasa terrane southern tibet a comparative analysis of whole rock rare earth element geochemistry and zircon hf isotopes between the adakites and associated gangdese igneous rocks suggests that the tibetan crust underwent a major phase of tectonic thickening between ca 45 and 30 ma in the region the lower part of the thickened crust consisted prevailingly of mafic lithologies which we argue to have resulted from intense basaltic underplating and subsequent remelting that took place during the late cretaceous and eocene time related to the neotethyan subduction processes including breakoff of the subducted slab at ca 50 ma in the early stage of the india asia collision these processes were responsible for not only the juvenile crust formation but also for the creation of a thermally softened lithosphere in the southern lhasa terrane the indentation of india consequently caused distributed lithospheric thickening with formation of an orogenic root beneath southern tibet root foundering during the oligocene gave rise to the adakitic magmatism regional topographic uplift and onset of northward underthrusting of the indian plate that has since played a key role in forming the entire tibetan plateau

seismic estimates of stress release five seismic estimates of stress release are compared for eight multiply recorded aftershocks of the 1975 oroville california earthquake the dynamic stress drop the arms stress drop and the apparent stress provide stable estimates which are strongly correlated across the data set the uncertainties of these estimates the standard error of the mean divided by the mean are approximately 15 estimates of the brune stress drop are uncorrelated with estimates of the average static stress drop calculated from the seismic moments and the rupture geometries of the aftershocks determined from the signal durations of the recorded s waves the brune stress drops are most strongly correlated with the arms stress drops implying that the brune stress drop provides an estimate of the average dynamic stress drop as brune originally proposed

spectral scaling of the 1985 to 1988 nahanni northwest territories earthquakes abstract the sequence of earthquakes that began in october 1985 near the nahanni river northwest territories canada provides an exceptional opportunity to study the variation with magnitude of the source spectra from intraplate earthquakes we study the spectral scaling for the seven largest events using p waves recorded at stations of the western canada telemetred network wctn the eastern canada telemetred network ectn and the global digital seismograph network gdsn these earthquakes range in size from m 4 8 to m 6 8 we compare predictions from various source scaling models with observed spectral ratios and source acceleration spectra for the suite of events the spectral ratios show a strong azimuthal dependence which is probably due to differences in the focal mechanisms or rupture characteristics of the events taking the average of the ratios from individual gdsn stations as the best representation of the spectral ratios an squared model with a constant stress parameter or one that increases slightly with moment magnitude gives a simultaneous fit to both the spectral ratios and to the absolute spectra the high frequency spectral levels by themselves imply a somewhat stronger dependence of the stress parameter on moment than found from fitting the overall spectra the stress parameters for the scaling laws are less than 35 bars for earthquakes as large as m 7 recently proposed scaling laws in which the corner frequency is proportional to the inverse quarter root of seismic moment for an squared model are not consistent with the data and neither are cubed models the spectral scaling for the sequence of nahanni earthquakes may not be representative of intraplate magnitude scaling in general since there is some evidence that aftershocks do not exhibit the same scaling as main shocks the high frequency spectral levels of the aftershocks of both the nahanni and the 9 january 1989 miramichi new brunswick earthquakes imply stress parameters that increase with magnitude by contrast the spectral levels for a series of main shocks in eastern north america show less magnitude dependence of the stress and generally higher stress levels when compared to aftershocks of the same magnitude

variations of apparent stresses and stress drops prior to the earthquake of 6 may 1984 mb 5 8 in the adak seismic zone abstract on 6 may 1984 a m b 5 8 earthquake occurred within a subregion of the adak seismic zone that had shown clustering of m b 4 5 earthquakes during the last 3 yr this region has been examined for premonitory variations in stress drop and apparent stresses calculated from the s wave spectra of 69 microearthquakes m d 13 nm decrease with decreasing moment

s wave attenuation in the crust in northern greece abstract the attenuation of shear waves in the crust is estimated for frequencies between 1 5 and 12 0 hz by applying a single station method based on the rate of decay of the s wave to coda wave amplitude ratio with distance the data used come from local earthquakes that occurred in the thessaloniki area northern greece during the period 1983 through 1989 and were recorded by the telemetered network of the geophysical laboratory of the university of thessaloniki the q s values obtained are 115 244 477 and 755 for 1 5 3 0 6 0 and 12 0 hz respectively these values are very close to the coda q values estimated for the same area using the s to s single scattering model for lapse times between 30 and 100 sec but they are higher than the coda q values for lapse times between 10 and 30 sec the estimated q s is found to be strongly frequency dependent proportional to f 0 91 which is very close to the frequency dependence of the coda q

a synthesis of the recent activity of galeras volcano colombia seven years of continuous surveillance 1989 1995 abstract the current period of re activation since 1988 at galeras volcano colombia has been characterized mainly by the following events 1 a semi continuous series of vulcanian eruptions during 5 9 may 1989 2 emplacement of an andesitic lava dome at the bottom of the main crater in october november 1991 3 six vulcanian eruptions during 1992 1993 the first of which destroyed most of the dome on 16 july 1992 and 4 three volcano tectonic seismic crises in april 1993 november december 1993 and march 1995 during much of this seven year period several small ash and gas emissions also have taken place the 4 9 may 1989 eruptions originated from the secondary crater el pinta and deposited ash lapilli and blocks in the crater area the 1992 1993 eruptions originated from the main crater and were associated with obstruction of the conduit by magma from dome emplacement in late 1991 causing overpressurization of the system for the 1992 1993 eruptions pre eruptive seismicity deformation and so2 flux all exhibited very low levels the eruptions were characterized by their sudden initiation low intensity vei 1 small eruption columns and small volumes of erupted material the source of the volcano tectonic seismic crises is located approximately 3 km north and northeast of the crater some of these events were felt in pasto and other towns located around the volcano on one occasion causing loss of life injuries and damage to buildings

paleogene crustal anatexis and metamorphism in lhasa terrane eastern himalayan syntaxis evidence from u pb zircon ages and hf isotopic compositions of the nyingchi complex in the eastern himalayan syntaxis the southern lhasa terrane is dominated by middle to high grade metamorphic rocks nyingchi complex which are intruded by felsic melts u pb zircon dating and zircon hf isotopic composition of these metamorphic and magmatic rocks provide important constraints on the tectono thermal evolution of the lhasa terrane during convergent process between indian and asian continents u pb zircon data for an orthogneiss intruding the nyingchi complex yield a protolith magma crystallization age of 83 4 1 2 ma with metamorphic ages of 65 46 ma this orthogneiss is characterized by positive ehf t values of 8 3 and young hf model ages of 0 6 ga indicating a derivation primarily from a depleted mantle or juvenile crustal source zircons from a quartz diorite yield a magma crystallization age of 63 1 0 6 ma with ehf t values of 8 2 to 2 7 suggesting that this magma was sourced from partial melting of older crustal materials zircon cores from a foliated biotite granite show ages ranging from 347 to 2690 ma with age peaks at 347 403 ma 461 648 ma and 1013 1183 ma their zircon ehf t values range from 30 6 to 6 9 both the u pb ages and hf isotopic composition of the zircon cores are similar to those of detrital zircons from the nyingchi complex paragneiss implying that the granite was derived from anatexis of the nyingchi complex metasediments the zircon rims from the granite indicate crustal anatexis at 64 4 0 7 ma and subsequent metamorphism at 55 1 1 3 and 41 4 2 3 ma our results suggest that the late cretaceous magmatism in the southern lhasa terrane resulted from neo tethys oceanic slab subduction and we infer that paleocene crustal anatexis and metamorphism were related to the thermal perturbation caused by rollback of the northward subducted neo tethyan oceanic slab

a study on kappa value in taiwan using borehole and surface seismic array since the inception of the borehole seismic array deployed by the central weather bureau cwb in taiwan in recent years a large quantity of strong motion records have been accumulated from frequently occurring earthquakes around taiwan that provide the opportunity to investigate the spectral decay parameter kappa and to understand further the site effects of seismic stations in this study we used 133 earthquakes recorded at 30 borehole seismic arrays each array includes two force balance accelerometers one at the surface and the other inside the borehole based on the regression analysis between value and hypocentral distance for each surface borehole station pairs the resulting derived from surface stations are higher than results from borehole stations the estimated values are found to range from 0 032 to 0 097 s at surface and 0 012 to 0 078 s in borehole in comparison with a study of site corrections for earthquake magnitude determination these higher values associated with negative station corrections can correspond to the effect of soil conditions to evaluate station corrections of borehole seismic array we collected records from the cosite stations of taiwan strong motion instrumentation program tsmip and central weather bureau station network cwbsn and then derived a linear relationship between the values and station corrections finally both resulting station corrections and values reveal high coherence in local site conditions

emplacement and petrological evolution of the andesitic dome of galeras volcano 1990 1992 abstract since the reactivation of galeras volcano in 1988 the emplacement of a lava dome in the main crater of the volcano has been its most significant magmatic activity changes leading to the emplacement of the dome were present as early as june july 1990 when the trend of degassing for the major species mainly so 2 and co 2 in the summit fumaroles changed drastically and small vulcanian explosions become more frequent moreover after august 1990 an inflationary trend as measured by an electronic tiltmeter was observed to increase over background levels and the temperature of a newly formed fumarole besolima increased from 130 to 738 c within a month the dome was seen for the first time in early october 1991 at the base of the main crater the dome grew for about a month to a total volume of 4 10 5 m 3 during this time the growth rate was calculated to be 12 10 3 m 3 day or 0 14 m 3 s an explosive eruption on 16 july 1992 destroyed most of the dome the chemical composition of the dome ranges from 59 4 to 61 1 wt sio 2 anhydrous the presence of 1 large plagioclase phenocrysts with strongly inversely zoned rims and 2 small euhedral plagioclase crystals with normal zoning and ca rich cores indicates that a magma mixing event occurred early in the history of dome emplacement large amounts of crystal fractionation then took place about 70 fractionation can explain the mineral suite and the major and trace element compositions of the glasses the uppermost part of the dome degassing freely at the surface for ten months was able to isolate the magmatic system producing a highly evolved melt in the dome interior the july 1992 eruption tapped not only the uppermost part of the dome but also a deeper h 2 o saturated magma analyses from these samples show that most of the h 2 o was degassed while cl and b contents were higher than in the uppermost part of the dome however the f content is lower in this deeper part of the conduit while it increases in the matrix glass of the degassed upper part of the dome

the lower crust of the gangdese magmatic arc southern tibet implication for the growth of continental crust abstract magmatic arcs are thought to be the primary sites of modern day continental crustal growth and arc crustal sections provide an exceptional opportunity to directly observe the geological processes that occur there yet few deeply exposed arc sections are available for direct study the gangdese magmatic arc southern tibet formed during the mesozoic subduction of neo tethyan oceanic lithosphere and cenozoic collision between the indian and asian continents and represent juvenile continental crust however the petrological components and compositions of the lower crust of the gangdese arc remain unknown based on detailed geological mapping we conducted a systemic geochemical geochronological and zircon hf isotopic study of well exposed high grade metamorphic and migmatitic rocks from the lower crust of the eastern gangdese arc the results obtained show that late cretaceous garnet amphibolites dioritic and granitic gneisses and paleocene eocene garnet amphibolites and granitic gneisses are the main components of the gangdese lower arc crust these meta intrusive rocks witnessed a long period of magmatic and metamorphic and anatectic processes from the middle jurassic to the late eocene and have chemical compositions that range from ultramafic to felsic with an average sio2 content of 57 61 wt and mg value of 0 49 these new data indicate firstly that the gangdese lower arc crust has an overall intermediate composition and typical feature of juvenile crusts and therefore supports the recent proposition that continental lower crusts are relatively felsic in composition instead of mafic we consider that the downward transport of felsic intrusives and associated sedimentary rocks into the deep crustal levels and subsequent partial melting resulted in componential and compositional changes of the gangdese arc lower crust over time this is a potential key mechanism in transforming primary lower arc crust to mature continental lower crust for the magmatic arcs with a complete growth history

the gangdese magmatic constraints on a latest cretaceous lithospheric delamination of the lhasa terrane southern tibet abstract this paper presents a systematic study of late cretaceous to early tertiary magmatism in the eastern gangdese batholith lhasa nyingchi segment the southern lhasa terrane southern tibet based on zircon u pb age rock association and geochemistry the magmatism can be divided into three stages 95 86 ma stage 1 85 73 ma stage 2 and 68 60 ma stage 3 with a short magmatic quiescence or gap of 73 69 ma the stage 1 is composed mainly of intermediate mafic rocks including norite gabbro diorite charnockite and subordinate granodiorite whereas the stage 2 consists largely of felsic rocks including tonalite granodiorite monzogranite and granite in contrast the stage 3 comprises a wide range of rock types from gabbro to granite all these samples show i type geochemical characteristics with high zircon hf isotope ratios e hf t 7 1 13 0 medium k calc alkaline and metaluminous to weakly peraluminous features stage 1 granitic rocks show low sr y 31 43 and la yb n 6 3 8 3 ratios in contrast to stage 2 that show gradually increasing sr y 47 450 and la yb n 6 38 ratios their magma temperatures constrained by titanium in zircon thermometer decrease from 740 719 c stage 1 to 685 628 c stage 2 these are consistent with an interpretation of progressive magma source deepening coupled with crustal thickening stage 3 granitoids in turn show reduced sr y 3 40 and la yb n 3 5 17 6 ratios and elevated magma temperatures 755 676 c suggesting a high temperature low pressure melting condition and thus a shallow magma source which may imply the presence of thinner crust the contrasting temperature pressure conditions between stage 2 and stage 3 magmatism suggest an event of crustal thinning which we interpret through a synthesis of other lines of geologic evidence to have resulted from a lithospheric delamination of the lhasa terrane in the latest cretaceous in conclusion the lhasa terrane underwent continuous crustal thickening in the late cretaceous and lithospheric delamination in the latest cretaceous this implies that the overthickened crust of southern tibet is generated in the cenozoic

scaling of ground motion parameters state of stress and focal depth the ground motion parametersrv and ra where r is hypocentral distance v is peak velocity is density and a is peak acceleration are found observationally to be strong functions of stress state and linearly related to focal depth z if the stress state and focal depth are properly taken into account then the ground motion parameters depend on the seismic moment m0 as expected from basic scaling principles that is rv m01 3 and ra is independent of earthquake size regression lines fitted to observations covering broad ranges in seismic moment and focal depth indicate that for extensional and compressional tectonic regimes corresponding to normal and thrust or reverse faulting earthquakes respectively peak acceleration is given by ra 1 08 mpa 3 06 mpa km z normal and ra 5 65 mpa 8 76 mpa km z thrust and for peak velocity rv m01 3 10 4 m2 s n m 1 3 3 00 0 69 km 1 z normal and rv m01 3 10 4 m2 s n m 1 3 4 63 1 82 km 1 z thrust for strike slip earthquakes the data currently are insufficient to define regression fits but such lines would definitely lie between those for the normal and thrust stress regimes these equations are appropriate for ground motion at small hypocentral distance as recorded in a whole space it follows that for comparable hypocentral depth peak acceleration in a compressional tectonic regime for example much of the eastern united states is a factor of about 3 greater than in an extensional regime such as nevada and for peak velocity a similar comparison yields a factor of 2 difference the similarity in behavior of the peak acceleration parameter to crustal strength estimated from byerlee s law of friction with respect to depth and stress state indicates that crustal strength is probably the factor governing the seismic source processes that give rise to the high frequency peak ground motion analysis of the peak acceleration data in the context of crustal strength suggests bounds on this ground motion parameter as recorded at a typical surface site that depend on stress state specifically a normal 0 5 g and a thrust 1 9 g thus the state of stress as well as the focal depth clearly is an important factor to be taken into account in the prediction of seismic ground motion

determination of δσ and κ0 from response spectra of large earthquakes in greece we fit an 09 2 model to response spectra from eight recent greek earth quakes ranging in size from m 5 8 to m 6 9 the diminution parameter k o was determined for each site with a value near 0 06 for a typical soil site the stress parameter aa showed little variation from earthquake to earthquake and had a mean value of 56 bars over all earthquakes predictions of peak velocity peak acceleration rupture duration and fault length using the derived stress parameters are consistent with observations frequency dependent site amplifications were included in all es timates the combined effect of amplification and attenuation had a maximum value close to a factor of 2 5 for a typical soil site relative to the motions at the surface of a perfectly elastic uniform half space composed of materials near the source the results form the foundation for predictions of strong motions in greece for distances and magnitudes other than those for which data are available

twenty years 1990 2010 of geodetic monitoring of galeras volcano colombia from continuous tilt measurements abstract galeras an andesitic stratovolcano part of the galeras volcanic complex is one of the most active volcanoes in colombia historic activity is centered on a small volume cone inside the youngest amphitheater which breaches the west flank of the volcano at least 30 confirmed eruption periods have been recorded in the past 480 years with episodes of unrest ranging from weak fumarolic activity and ash emissions to larger explosive events the most recent eruption periods recorded instrumentally since 1988 have been characterized by minor explosive eruptions and the emplacement of three crater domes and small pyroclastic flow deposits in this paper we discuss the evolution of volcanic activity using a 20 year long record of tilt measurements in particular we focus on three episodes of unrest occurred in 1991 2006 and 2008 when the deformation was clearly associated with shallow magma intrusions and the emplacement and destruction of crater domes the depth of the intrusions varied from a few hundred meters august 2005 to two kilometers january 2009 while the volume change ranged from 10 4 m 3 may october 2009 to 10 6 m 3 january 2009 a comparison with seismic data indicates that the deformation sources were located within the cloud of hypocenters of the volcano tectonic events the lack of a clear correlation between the volume change and depth of the sources and the total so 2 flux could indicate that the unrest at galeras was related to a larger intrusive event with only a small part of the magma erupted in the form of tephra and lava domes

ground motion waveforms and source spectral scaling from close distance accelerograms in a compressional regime area friuli northeastern italy abstract an aftershock sequence was recorded by an sma 1 instrument installed at gemona del friuli on a massive limestone site during the period from may to october 1976 the sequence consisted of small to moderately large 10 21 m 0 10 25 dyne cm dominant thrust earthquakes that occurred within 15 km of the station the instrument was triggered by p waves for 24 events the resulting records are particularly suitable to the investigation of source properties for the friuli earthquakes acceleration velocity and displacement waveforms have been analyzed in order to investigate source properties durations t and areas 0 of the shear wave pulses have been measured from the displacement time histories and compared with the values of corner frequency f c and seismic moment m 0 estimated in the frequency domain from the acceleration spectra of the same events the comparison between these two set of data reveals that attenuation correction is important also for correct estimations of source parameters from ground motions at close focal distance and moderate magnitudes after correcting for attenuation stress drops are approximately constant 300 bars on the average in the investigated seismic moment range for this sequence the behavior of peak ground motions versus earthquakes size is consistent with the trend obtained from stochastic simulations assuming an omega squared spectral model with a constant stress drop of 300 bars the good results obtained by applying to friuli earthquakes an omega squared model with a constant stress drop of 300 bars are further confirmed by the analysis of spectral ratios for different size earthquakes of the sequence this analysis demonstrates that the finding of this spectral scaling model is not biased to first order by whole path or near site propagation properties whose effect can be significant when data from a single station are investigated

foundering of lower island arc crust as an explanation for the origin of the continental moho a long standing theory for the genesis of continental crust is that it is formed in subduction zones however the observed seismic properties of lower crust and upper mantle in oceanic island arcs differ significantly from those in the continental crust accordingly significant modifications of lower arc crust must occur if continental crust is indeed formed from island arcs here we investigate how the seismic characteristics of arc crust are transformed into those of the continental crust by calculating the density and seismic structure of two exposed sections of island arc kohistan and talkeetna the kohistan crustal section is negatively buoyant with respect to the underlying depleted upper mantle at depths exceeding 40 kilometres and is characterized by a steady increase in seismic velocity similar to that observed in active arcs in contrast the lower talkeetna crust is density sorted preserving only relicts about ten to a hundred metres thick of rock with density exceeding that of the underlying mantle specifically the foundering of the lower talkeetna crust resulted in the replacement of dense mafic and ultramafic cumulates by residual upper mantle producing a sharp seismic discontinuity at depths of around 38 to 42 kilometres characteristic of the continental mohorovicic discontinuity the moho dynamic calculations indicate that foundering is an episodic process that occurs in most arcs with a periodicity of half a million to five million years moreover because foundering will continue after arc magmatism ceases this process ultimately results in the formation of the continental moho

dynamic stress changes during earthquake rupture we assess two competing dynamic interpretations that have been pro posed for the short slip durations characteristic of kinematic earthquake models de rived by inversion of earthquake waveform and geodetic data the first interpretation would require a fault constitutive relationship in which rapid dynamic restrengthen ing of the fault surface occurs after passage of the rupture front a hypothesized mechanical behavior that has been referred to as self healing the second inter pretation would require sufficient spatial heterogeneity of stress drop to permit rapid equilibration of elastic stresses with the residual dynamic friction level a condition we refer to as geometrical constraint these interpretations imply contrasting pre dictions for the time dependence of the fault plane shear stresses we compare these predictions with dynamic shear stress changes for the 1992 landers m 7 3 1994 northridge m 6 7 and 1995 kobe m 6 9 earthquakes stress changes are com puted from kinematic slip models of these earthquakes using a finite difference method for each event static stress drop is highly variable spatially with high stress drop patches embedded in a background of low and largely negative stress drop the time histories of stress change show predominantly monotonic stress change after passage of the rupture front settling to a residual level without significant evidence for dynamic restrengthening the stress change at the rupture front is usu ally gradual rather than abrupt probably reflecting the limited resolution inherent in the underlying kinematic inversions on the basis of this analysis as well as recent similar results obtained independently for the kobe and morgan hill earthquakes we conclude that at the present time the self healing hypothesis is unnecessary to explain earthquake kinematics

regional variation of the short period 1 to 10 second source spectrum we determined m b the body wave magnitude calculated from the maximum amplitude of short period p waves of 38 large earthquakes in various tectonic provinces the data are divided into three groups group 1 subduction zone thrust events group 2 nonsubduction zone dip slip and oblique slip events and group 3 strike slip events groups 2 and 3 include intraplate earthquakes comparison of m b values for these three groups of events suggests that the source spectral amplitudes of intraplate events at a period of about 1 4 sec is 2 to 5 times larger than those of subduction zone events with the same m w we also determined the source spectra of 28 large earthquakes m w 6 5 to 7 7 directly from global digital seismographic network gdsn data over a period range from 1 to 10 sec at periods from 1 to 2 sec the source spectral amplitudes of intraplate earthquakes are 2 to 4 times larger than those of subduction zone events with the same m w the difference decreases as the period increases to 10 sec

timing of india asia collision geological biostratigraphic and palaeomagnetic constraints 1 a range of ages have been proposed for the timing of india asia collision the range to some extent reflects different definitions of collision and methods used to date it in this paper we discuss three approaches that have been used to constrain the time of collision the time of cessation of marine facies the time of the first arrival of asian detritus on the indian plate and the determination of the relative positions of india and asia through time in the qumiba sedimentary section located south of the yarlung tsangpo suture in tibet a previous work has dated marine facies at middle to late eocene by far the youngest marine sediments recorded in the region by contrast our biostratigraphic data indicate the youngest marine facies preserved at this locality are 50 6 52 8 ma in broad agreement with the timing of cessation of marine facies elsewhere throughout the region double dating of detrital zircons from this formation by u pb and fission track methods indicates an asian contribution to the rocks thus documenting the time of arrival of asian material onto the indian plate at this time and hence constraining the time of india asia collision our reconstruction of the positions of india and asia by using a compilation of published palaeomagnetic data indicates initial contact between the continents in the early eocene we conclude the paper with a discussion on the viability of a recent assertion that collision between india and asia could not have occurred prior to 35 ma

early eocene crustal thickening in southern tibet new age and geochemical constraints from the gangdese batholith abstract the lhasa terrane southern tibet is where the thickest part of the earth s crust occurs as a result of the india asia collision this paper reports zircon u pb age and geochemical data of samples collected from the gangdese batholith southern lhasa terrane a subset of which has already been dated at 65 41 ma using zircon u pb geochronology ji w q wu f y chung s l li j x liu c z 2009a zircon u pb chronology and hf isotopic constraints on the petrogenesis of gangdese batholiths southern tibet chemical geology 262 229 245 to constrain the timing and mechanism s responsible for crustal thickening beneath southern tibet our new data indicate that the gangdese magmatism lasted continuously from ca 65 to 34 ma during which geochemical evidence for crustal thickening is observed crustal thickening is best illustrated by the progressive increase in la yb and sr y of the early to latest eocene magmatic rocks associated with the decrease in hf isotopic ratios of paleocene to eocene magmatic zircons we also identify in this study the oldest ca 51 ma post collisional adakitic granitoids in the gangdese batholith interpreted as products from partial melting of underplated basaltic lower crust therefore a thickened crust likely existed by that time at least locally in the southern lhasa terrane tibet

scaling differences between large interplate and intraplate earthquakes a study of large intraplate earthquakes with well determined source parameters shows that these earthquakes obey a scaling law similar to large interplate earthquakes in which m 0 l 2 or u l where l is rupture length and u is slip in contrast to interplate earthquakes for which 1 10 5 for for the intraplate events 6 10 5 which implies that these earthquakes have stress drops about 6 times higher than interplate events this result is independent of focal mechanism type this implies that intraplate faults have a higher frictional strength than do plate boundaries and hence that faults are velocity or slip weakening in their behavior this factor may be important in producing the concentrated deformation that creates and maintains plate boundaries

estimation of attenuation structure and local earthquake magnitude based on acceleration records in greece hatzidimitriou p papazachos c kiratzi a and theodulidis n 1993 estimation of attenuation structure and local earthquake magnitude based on acceleration records in greece in s j duda and t b yanoskaya editors estimation of earthquake size tectonophysics 217 spec sect 243 253 strong motion accelerograms are used for the estimation of attenuation coefficient in the upper crust in greece on the assumption that direct s waves are responsible for the strong near source motion this coefficient was then used to derive a calibration formula for the estimation of local earthquake magnitude for epicentral distances a 130 km and 4 0 i m i 6 1 q values are calculated by the spectral decay of the log spectrum of the accelerograms the resulting values exhibit the high attenuation of seismic waves in greece and indicate the existence of a highly attenuative surface sedimentary layer which strongly controls the total wave attenuation

origin and evolution of multi stage felsic melts in eastern gangdese belt constraints from u pb zircon dating and hf isotopic composition this integrated study of whole rock geochemistry zircon u pb dating and hf isotope composition for seven felsic rocks from the nyingchi complex in eastern himalayan syntaxis has revealed a complex magmatic history for the eastern gangdese belt this involves multiple melt sources and mechanisms that uniquely identify the tectonic evolution of this part of the himalayan orogen our u pb zircon dating reveals five stages of magmatic or anatectic events 165 81 61 50 and 25 ma the jurassic granitic gneiss 165 ma exhibits ehf t values of 1 4 to 3 5 the late cretaceous granite 81 ma shows variable ehf t values from 0 9 to 6 2 indicating a binary mixing between juvenile and old crustal materials the paleocene granodioritic gneiss 61 ma has ehf t values of 5 4 to 8 0 suggesting that it originated from partial melting of a juvenile crustal material the eocene anatexis is recorded in the leucosome which has hf isotopic composition similar to that of the jurassic granite indicating that the leucosome could be derived from partial melting of the jurassic granite the late oligocene biotite granite 25 ma shows adakitic geochemical characteristics with sr y 49 3 56 6 the presence of a large number of inherited zircons and negative ehf t values suggest that it sourced from anatexis of crustal materials in contrast to the gangdese batholiths that are mainly derived from juvenile crustal source in central tibet the old crustal materials play an important role for the magma generation of the felsic rocks suggesting the existence of a crustal basement in the eastern gangdese belt these correspond to specific magmatic evolution stages during the convergence between india and asia the middle jurassic granitic gneiss resulted from the northward subduction of the neo tethyan oceanic slab the late cretaceous magmatism is probably related to the ocean ridge subduction the paleocene eocene magmatism metamorphism and anatexis are interpreted to result from roll back and break off of the subducted neo tethyan slab that occurred in the early stage of the india asian collision respectively the late oligocene adakitic rocks resulted from the break off of the subducted indian continental crust starting at 25 ma

earthquake mechanisms of the adriatic sea and western greece implications for the oceanic subduction continental collision transition summary we present 21 focal solutions magnitude 5 5 reliably computed by body wave modelling for the western hellenic arc from yugoslavia to the southern peloponnese mechanisms located within the aegean show normal faulting the t axis trending n s in the centre and parallel to the active boundary in the external part mechanisms associated with the keffalinia fault are consistent with dextral strike slip motion reverse mechanisms located along the active boundary are remarkably consistent and do not depend on the nature of the active boundary continental collision or oceanic subduction the consistency in azimuth of the slip vectors and of the gps velocity relative to africa all along the active boundary suggests that the deformation is related to the same motion the discrepancy between seismic energy release and the amount of shortening confirms that the continental collision is achieved by seismic slip on faults but the oceanic subduction is partially aseismic the northward decrease in velocity between continental collision and oceanic subduction suggests the continental collision to be a recent evolution of the active subduction

adakites from continental collision zones melting of thickened lower crust beneath southern tibet adakites are geochemically distinct intermediate to felsic lavas found exclusively in subduction zones here we report the first example of such magmas from southern tibet in an active continental collision environment the tibetan adakites were emplaced from ca 26 to 10 ma and their overall geochemical characteristics suggest an origin by melting of eclogites and or garnet amphibolites in the lower part 50 km of thickened tibetan crust this lower crustal melting required a significantly elevated geotherm which we attribute to removal of the tectonically thickened lithospheric mantle in late oligocene time the identification of collision type adakites from southern tibet lends new constraints to not only the himalayan tibetan orogenesis how and when the indian lithosphere started underthrusting asia can be depicted but also the growth of the early continental crust on earth that consists dominantly of the tonalite trondhjemite granodiorite suites marked by adakitic geochemical affinities

empirical peak ground motion predictive relations for shallow earthquakes in greece in the present article new predictive relations are proposed for the peak values of the horizontal components of ground acceleration velocity and displace ment using 619 strong motion recordings from shallow earthquakes in the broader aegean area which are processed using the same procedure in order to obtain a homogeneous strong motion database the data set is derived from 225 earthquakes mainly of normal and strike slip focal mechanisms with magnitudes 4 5 m 7 0 and epicentral distances in the range 1 km r 160 km that have been relocated using an appropriate technique about 1000 values of peak ground acceleration pga velocity pgv and displacement pgd from horizontal components were used to derive the empirical predictive relations proposed in this study a term ac counting for the effect of faulting mechanisms in the predictive relations is intro duced and the ubc 1997 site classification is adopted for the quantification of the site effects the new relations are compared to previous ones proposed for greece or other regions with comparable seismotectonic environments the regression anal ysis showed a noticeable up to 30 variance reduction of the proposed relations for predicting pga pgv and pgd values compared to previous ones for the aegean area suggesting a significant improvement of predictive relations due to the use of a homogeneous strong motion database and improved earthquake parameter infor mation

use of seismoscope records to determine ml and peak velocities more information about ground motion can be extracted from seismoscope records than a single point on a response spectrum to demonstrate this the relation between seismoscope response and wood anderson instrument output and peak horizontal ground velocity has been studied by simulating the various responses for a range of distances and magnitudes the simulations show that the relation used by jennings and kanamori 1979 to convert from peak seismoscope readings to the peak response of a wood anderson instrument has a distance and magnitude dependent systematic error the error is negligible however for modern seismoscopes at distances of a few tens of kilometers at several hundred kilometers the relation underestimates the wood anderson response by as much as a factor of two the spread in jennings and kanamori s estimate of ml for the 1906 san francisco earthquake recorded on seismoscopes having relatively low natural frequencies 0 26 and 0 5 hz is reduced by the results in this paper the upper value from a seismoscope in carson city nevada at 290 km from the fault going from ml 7 2 to ml 7 0 and the lower value from yountville california r 60 km going from about 6 3 to 6 4 about 0 3 units of the remaining spread may be due to local geologic site conditions if the 0 3 units is distributed equally between the yountville and carson city recordings the estimates of ml for the san francisco earthquake then become 6 5 and 6 8 somewhat lower than jennings and kanamori s final estimates of 6 3 4 to 7 although the error in using the relation of jennings and kanamori to estimate wood anderson response was at most a factor of 1 6 for the 1906 earthquake the error can be substantially larger for smaller earthquakes recorded on similar low frequency seismoscopes the relation between wood anderson and seismoscope response used by jennings and kanamori can be combined with an empirical relation between peak horizontal velocity and wood anderson response to predict peak velocity from seismocope recordings the simulations show that this relation v max 8 1 awa where v max is the peak horizontal velocity in centimeters second and awa is one half the range of the wood anderson motion in meters forms a lower bound for estimates of peak velocity from seismoscope recordings the relation is good for stations within about 100 km of earthquakes with moment magnitudes of about 4 5 to 6 5 and it underestimates peak velocity by factors up to 2 or 3 for larger earthquakes at distances within 100 km an application of the simulation method to the 1976 guatemala earthquake moment magnitude 7 6 results in 37 cm sec as a lower bound to v max with 66 cm sec as a more likely value from the seismocope recording in guatemala city approximately 25 km from the motagua fault

anisotropic radiation modelling of macroseismic intensities for estimation of the attenuation structure of the upper crust in greece a method is suggested for the analysis of macroseismic intensity data in order to accurately determine an average attenuation structure of the upper part of the crust in an area the method is based on a model which assumes that the observed intensities depend on source properties radiation pattern size focal depth geometrical spreading and anelastic attenuation the method is applied to 13 008 intensity values observed in corresponding sites of greece and grouped in 4228 groups according to their spatial clustering in order to diminish observational errors and site effects an average intensity attenuation coefficient c 0 0039 0 0016 corresponding to a quality factor q 350 140 is determined for the upper 20 km of the crust in this area this value is relatively low in good agreement with the relatively high heat flow and high seismic activity of this area a byproduct of the present study is the determination for each earthquake of a macroseismic focal depth and of a macroseismic size which is strongly correlatted with both the earthquake s magnitude and its seismic moment determined by independent methods

on two characteristic frequencies of acceleration spectra patch corner frequency and fmax abstract we examine two issues related to the radiation of tectonic earthquake sources i the complexity of the shape of observed source spectra and ii the effect of recording site characteristics on the cut off frequency f max of acceleration spectra beyond which spectral amplitudes diminish sharply we compute the far field spectra of the 1979 imperial valley and the 1984 morgan hill earthquakes from the slip functions that were inferred from detailed inversion studies of strong motion and teleseismic data of these two california strike slip events these spectra along with data from the 1983 central japan sea earthquake sequence main shock and aftershocks demonstrate the existence of the so called patch corner frequency which is related to the scale length of heterogeneities of the fault plane such a frequency is a distinct feature of the source spectrum of the specific barrier model of papageorgiou and aki by analyzing accelerograms of the san fernando earthquake of 1971 we found that f max determined from records at soil stations is nearly the same as that determined from records at hard rock stations

stochastic finite fault modeling of ground motions from the 1994 northridge california earthquake i validation on rock sites the stochastic method of simulating ground motions from finite faults is validated against strong motion data from the m 6 7 1994 northridge california earthquake the finite fault plane is subdivided into elements each element is as signed a stochastic o92 spectrum and the delayed contributions from all subfaults are summed in the time domain simulated horizontal acceleration time histories and fourier spectra at 28 rock sites are compared with observations we first perform simulations using the slip distribution on the causative fault derived from strong motion teleseismic gps and leveling data wald et al 1996 we then test the performance of the method using quasi random distributions of slip and alternative hypocenter locations this is important because the rupture initiation point and slip distribution are in general not known for future earthquakes the model bias is calculated as the ratio of the simulated to the observed spectrum in the frequency band of 0 1 to 12 5 hz averaged over a suite of rock sites the mean bias is within the 95 confidence limits of unity showing that the model provides an accurate prediction of the spectral content of ground motions on average the maximum excursion of the model bias from the unity value when averaged over all 28 rock stations is a factor of approximately 1 6 at most frequencies it is below a factor of 1 4 interestingly the spectral bias and the standard deviation of the stochastic simu lations do not depend on whether the fault slip distribution and hypocenter location are based on data or are randomly generated this suggests that these parameters do not affect the accuracy of predicting the average characteristics of ground motion or they may have their predominant effect in the frequency range below about 0 1 hz below the range of this study the implication is that deterministic slip models are not necessary to produce reasonably accurate simulations of the spectral content of strong ground motions this is fortunate because such models are not available for forecasting motions from future earthquakes however the directivity effects controlled by the hypocenter location are important in determining peak ground ac celeration at individual sites although the method is unbiased when averaged over all rock sites the simulations at individual sites can have significant errors generally a factor of 2 to 3 which are also frequency dependent factors such as local geology site topography or basin propagation effects can profoundly affect the recordings at individual stations to generate more accurate site specific predictions empirical responses at each site could be established

evidence for the variation of stress drop between normal and thrust faulting earthquakes in italy the horizontal components of 45 strong motion accelerograms recorded in friuli italy from may to september 1976 have been analyzed in order to estimate the stress drop of eight thrust faulting earthquakes two different stress parameters the brune stress drop and the apparent stress a have been considered the friuli main shock has a strong stress drop 800 bars while the aftershocks have stress drops which range from 250 to 400 bars five accelerograms written by the montenegro yugoslavia earthquake of april 1979 have also been analyzed this thrust faulting earthquake had a stress drop of 900 bars the stress drops of these thrust faulting earthquakes are compared to the stress drops determined by rovelli et al 1988a for nine normal faulting earthquakes which occurred in the apennines region from 1979 to 1984 the radiated energies and apparent stresses have also been calculated for the set of apennine earthquakes the thrust faulting earthquakes in the alpine dynaride region exhibit higher values for both the brune and the apparent stress than the normal faulting earthquakes in the apennines region excluding the friuli and montenegro main shocks the brune stress drops of the thrust faults are approximately 3 times those of the normal faults while the apparent stresses are twice as large this difference is statistically significant at a confidence level of 95 the same difference also emerges from the scaling of the peak ground motions recorded in these two regions suggesting an enhanced severity of the seismic input in the alpine dynaride environment compared with the apennines

two stages of granulite facies metamorphism in the eastern himalayan syntaxis south tibet petrology zircon geochronology and implications for the subduction of neo tethys and the indian continent beneath asia the eastern himalayan syntaxis in southeastern tibet consists of the lhasa terrane high himalayan rocks and indus tsangpo suture zone the lhasa terrane constitutes the hangingwall of a subduction zone whereas the high himalayan rocks represent the subducted indian continent our petrological and geochronological data reveal that the lhasa terrane has undergone two stages of medium p metamorphism an early granulite facies event at c 90 ma and a late amphibolite facies event at 36 33 ma however the high himalayan rocks experienced only a single high p granulite facies metamorphic event at 37 32 ma it is inferred that the late cretaceous c 90 ma medium p metamorphism of the southern lhasa terrane resulted from a northward subduction of the neo tethyan ocean and that the oligocene 37 32 ma high p 1 8 1 4 gpa rocks of the high himalayan and coeval medium p 0 8 1 1 gpa rocks of the lhasa terrane represent paired metamorphic belts that resulted from the northward subduction of the indian continent beneath asia our results provide robust constraints on the mesozoic and cenozoic tectonic evolution of south tibet

large stress release during normal faulting earthquakes in western turkey supported by broadband ground motion simulations this article investigates the stress drop variability of shallow normal faulting earthquakes in western anatolia through strong motion simulations for this purpose source characteristics of three moderate to large magnitude events are constrained by the empirical green s function simulation in a broadband frequency range recordings of ten strong motion stations in 78 km epicentral distance range are utilized for the simulation estimated strong motion generation areas smgas are 22 km2 66 km2 and 110 km2 where rise times are 0 6 s 0 7 s and 0 6 s respectively for the 2011 simav mw 5 8 2017 lesvos mw 6 3 and 2017 bodrum kos mw 6 6 earthquakes those values are found to be consistent with global source scaling relationships one particular observation is that stress drop ratios between the mainshock and aftershock for all events are relatively large compared with those previously calculated for strike slip events in turkey stress drops of smgas for the simav and bodrum earthquakes are in the range of 25 mpa and this value drops to 19 mpa for the lesvos earthquake to further investigate the stress drop variation of earthquakes in western anatolia an earthquake source database fc mo offered by yamanaka et al iag iaspei 2017 s07 1 03 2017 is utilized brune journal of geophysical research 76 5002 1971 stress drop values of 360 small to moderate earthquakes mw 3 0 6 0 are calculated with the given corner frequency and seismic moment information assuming a constant shear wave velocity results indicate that the majority of the earthquakes have a stress drop value 5 mpa this value changes to between 5 and 57 mpa for the remaining earthquakes these high stress drop values support the former findings stating that normal faulting earthquakes may release higher stress than strike slip earthquakes this indicates that the regional stress regime in western turkey may cause relatively larger stress release during the normal faulting mainshocks

1989 1995 earthquake sequences in the galeras volcano region sw colombia and possible volcano earthquake interactions abstract the galeras volcanic complex located in the narino department sw colombia includes the most historically active volcano in the country galeras a 4276 m high stratovolcano located 9 km west of the city of san juan de pasto 400 000 inhabitants the area is also affected by the continental faulting represented by the buesaco aranda and pasto faults belonging to one of the most seismically active structures in colombia the romeral fault system several moderate to strong shallow earthquakes affected the city of san juan de pasto and its neighbouring region since the xvii century the coexistence of an active volcanic complex and an active fault system complicates the study and interpretation of the different processes taking place in the region as well as the identification of any connection or interaction among them the reactivation of the volcano in 1989 was characterized by three main magmatic events 1 a series of vulcanian eruptions during 4 9 may 1989 with semi continuous ash emissions from a secondary crater 2 the emplacement of an andesitic lava dome at the bottom of the main crater from september 1990 until december 1991 and 3 six vulcanian eruptions from the main crater during 1992 1993 with destruction of most of the dome during the first one on 16 july 1992 during the same period four earthquake sequences were located in a limited area n and ne of galeras volcano on august september 1989 aug1989 april june 1993 apr1993 november december 1993 nov1993 and march august 1995 mar1995 the last one included a ml4 7 main shock on 4 march 1995 producing moderate to high damage in the epicentral region msk maximum intensity viii and in the city of san juan de pasto vi vii the last damaging earthquake in the region was a msk intensity viii ix in 1947 a detailed analysis of the spatio temporal characteristics of the four earthquake sequences allowed identifying their different origin and suggesting some interrelationship between the reactivated eruptive process and the contemporaneous seismic activity the aug1989 sequence presents a typical volcanic swarm like pattern most probably related with the process of magma intrusion from depth at the beginning of the volcano s reactivation the apr1993 the nov1993 and the mar1995 sequences show a clear tectonic origin with events occurring on rupture planes almost vertical that can be associated to the active faults in the area the seismogenic process of these three sequences could have been activated or accelerated by the main eruptions during 1992 1993 these results suggest that constrains provided by improved relocations and the detailed analysis of the space time characteristics of earthquake sequences in galeras volcanic environment allow to establish the different generation mechanisms involved and to suggest feasible explanations on the possible interrelationships of the magmatic volcanic processes and the seismicity observed

origin of late oligocene adakitic intrusives in the southeastern lhasa terrane evidence from in situ zircon u pb dating hf o isotopes and whole rock geochemistry abstract we describe the whole rock geochemistry biotite ar ar and zircon u pb geochronology and in situ hf o isotopic compositions of the linzhi two mica granitoid in the southeastern lhasa terrane the granitoids are calc alkaline to high k calc alkaline in composition and they have geochemical affinities with adakites the whole rock geochemistry with high sio 2 and al 2 o 3 contents low mgo cr and ni contents low mg values negative e nd t 6 2 to 2 9 and low 87 sr 86 sr i 0 7059 0 7074 together with the in situ zircon hf o isotopic data e hf t 2 7 to 2 7 18 o 7 2 8 8 indicates that the granitoid was derived mainly from the partial melting of a rejuvenated lower crust underneath the lhasa terrane this lower crust was dominated by ancient crustal material with the minor involvement of juvenile mantle inputs 40 ar 39 ar ages for biotites and the presence of magmatic epidote suggest the granitoid was intruded at mid crustal depths indicating that the eastern lhasa terrane experienced a pulse of rapid uplift in the interval from 26 to 22 ma the geochemical features of the linzhi adakitic rocks are significantly different from those of eocene adakitic rocks from the north himalayan belt and this suggests that we can rule out the possibility of crustal thickening in the eastern part of lhasa terrane caused by impingement of the indian lower crust post collisional 26 10 ma adakitic rocks occur within an e w trending magmatic belt that runs parallel to the yarlung tsangpo suture in the lhasa terrane these rocks have distinctive variations in geochemistry along the strike of the lhasa terrane indicating that the lower crust is isotopically heterogeneous from west to east along the terrane these geochemical variations probably result from variable degrees of input along strike of juvenile mantle derived melts into an old lower crust the middle part of the lhasa terrane is marked by a juvenile lower crust that contains large amounts of mantle derived melt underplated during neo tethyan oceanic crust subduction in contrast the lower crust below the western and eastern parts of the terrane is relatively ancient therefore crustal thickening caused by underplating of mantle derived magmas could play an important role in the middle segment of the terrane whereas crustal compression and shortening as a result of the collision of the indian and asian plates could be the major cause for crustal thickening in the western and eastern segments

deterministic earthquake scenario for the basel area simulating strong motions and site effects for basel switzerland 1 the basel earthquake of 18 october 1356 is considered one of the most serious earthquakes in europe in recent centuries i0 ix m 6 5 6 9 in this paper we present ground motion simulations for earthquake scenarios for the city of basel and its vicinity the numerical modeling combines the finite extent pseudodynamic and kinematic source models with complex local structure in a two step hybrid three dimensional 3 d finite difference fd method the synthetic seismograms are accurate in the frequency band 0 2 2 hz the 3 d fd is a linear explicit displacement formulation using an irregular rectangular grid including topography the finite extent rupture model is adjacent to the free surface because the fault has been recognized through trenching on the reinach fault we test two source models reminiscent of past earthquakes the 1999 athens and the 1989 loma prieta earthquake to represent mw 5 9 and mw 6 5 events that occur approximately to the south of basel to compare the effect of the same wave field arriving at the site from other directions we considered the same sources placed east and west of the city the local structural model is determined from the area s recently established p and s wave velocity structure and includes topography the selected earthquake scenarios show strong ground motion amplification with respect to a bedrock site which is in contrast to previous 2 d simulations for the same area in particular we found that the edge effects from the 3 d structural model depend strongly on the position of the earthquake source within the modeling domain

a model of vulcanian eruptions at galeras volcano colombia abstract a series of six vulcanian eruptions at galeras volcano colombia occurred during 1992 1993 these eruptions followed the emplacement of a lava dome in late 1991 which was accompanied by cooling crystallizing and partial solidification of the column of magma plugging the conduit five of the eruptions were preceded by episodes of monochromatic long period seismicity suggesting cyclic pressurization of the magma the quiescent intervals between eruptions generally decreased with each successive eruption we propose a model of gas release from magma into confined stationary pore spaces by 5 10 crystallization of anhydrous minerals causing repeated overpressurization of the magmatic system using initial lithostatic pressures of 20 100 mpa 800 3900 m depth gas buildup by crystallization indicates overpressures of 3 17 mpa which is the approximate fracture criterion for a magma chamber our galeras data suggest two models of pressurization 1 gas exsolution may have occurred at similar depths but with progressively smaller overpressures being developed before each successive eruption according to this model the comparatively long quiescent periods between the first eruptions resulted in relatively large amounts of gas exsolution from the magma and accumulation in pore spaces whereas the shorter quiescent periods for the later eruptions exsolved less gas the decrease in overpressure with time may have resulted from progressive fracturing of partly solidified magma during the eruptions which progressively decreased the tensile strength and increased the permeability of the magma 2 since overpressures are smaller at shallower depths for a given amount of crystallization the first eruptions of 1992 1993 were caused by gas exsolution at shallow depths while the last eruptions having shorter quiescent intervals resulted from a greater amount of gas exsolution at greater depths in this case the rate of gas exsolution varied as a function of depth the two scenarios for pressurization may be combined into a more general model involving edifice weakening and the development of a shallow hydrothermal system from continued degassing of the magma plug as weakening of the edifice formed an extensive system of fractures the shallow hydrothermal system deepened due to meteoric water input and magmatic degassing with time a greater number of fractures were filled and partly sealed by hydrothermal minerals at progressively deeper levels as a result the source of the overpressure also deepened with time

global patterns of radiated seismic energy and apparent stress radiated energies from shallow earthquakes with magnitudes 5 8 that occurred between 1986 and 1991 are used to examine global patterns of energy release and apparent stress in contrast to traditional methods which have relied upon empirical formulas these energies are computed through direct spectral analysis of broadband seismic waveforms velocity squared spectra of body waves are integrated after they have been corrected for effects arising from depth phases frequency dependent attenuation and focal mechanism the least squares regression fit of energy es to surface wave magnitude ms for a global set of 397 earthquakes yields log es 4 4 1 5ms which implies that the gutenberg richter relationship overestimates the energies of earthquakes the least squares fit between es and seismic moment m0 is given by the relationship es 1 6 10 5 m0 which yields 0 47 mpa as the average global value of apparent stress however the regression lines of both es ms and es m0 yield poor empirical predictors for the actual energy radiated by any given earthquake the scatter of data is more than an order of magnitude about each of the regression lines on the other hand global variations between es and mo while large are not random when subsets of es m0 are plotted as a function of seismic region and faulting type the scatter in data is substantially reduced the es m0 fits for many seismic regions and tectonic environments are very distinctive and a characteristic apparent stress c can be derived the lowest apparent stresses 3 0 mpa are associated with strike slip earthquakes that occur at oceanic ridge ridge transforms and in intraplate environments seaward of island arcs intermediate values of apparent stress 1 5 a 3 0 mpa are associated with strike slip earthquakes at incipient or transitional plate boundaries in general the dominant mode of failure for a tectonic environment is associated with the faulting type that has the lowest apparent stress an energy magnitude me can complement moment magnitude mw in describing the size of an earthquake me being derived from velocity power spectra is a measure of seismic potential for damage mw being derived from the low frequency asymptote of displacement spectra is more physically related to the final static displacement of an earthquake when earthquake size is ranked by moment a list of the largest events is dominated by earthquakes with thrust mechanisms when earthquake size is ranked by energy the list of the largest events is dominated by strike slip earthquakes

on relating apparent stress to the stress causing earthquake fault slip apparent stress a is defined as where is the average shear stress loading the fault plane to cause slip and is the seismic efficiency defined as ea w where ea is the energy radiated seismically and w is the total energy released by the earthquake the results of a recent study in which apparent stresses of mining induced earthquakes were compared to those measured for laboratory stick slip friction events led to the hypothesis that this hypothesis is tested here against a substantially augmented data set of earthquakes for which can be estimated mostly from in situ stress measurements for comparison with a the expanded data set which includes earthquakes artificially triggered at a depth of 9 km in the german kontinentales tiefbohrprogramm der bundesrepublik deutschland ktb borehole and natural tectonic earthquakes covers a broad range of hypocentral depths rock types pore pressures and tectonic settings nonetheless over 14 orders of magnitude in seismic moment apparent stresses exhibit distinct upper bounds defined by a maximum seismic efficiency of 0 06 consistent with the hypothesis proposed before this behavior of a and can be expressed in terms of two parameters measured for stick slip friction events in the laboratory the ratio of the static to the dynamic coefficient of friction and the fault slip overshoot typical values for these two parameters yield seismic efficiencies of 0 06 in contrast to efficiencies for laboratory events for which is always near 0 06 those for earthquakes tend to be less than this bounding value because ea for earthquakes is usually underestimated due to factors such as band limited recording thus upper bounds on appear to be controlled by just a few fundamental aspects of frictional stick slip behavior that are common to shallow earthquakes everywhere estimates of from measurements of a for suites of earthquakes using are found to be comparable in magnitude to estimates of shear stress on the basis of extrapolating in situ stress data to seismogenic depths

modeling finite fault radiation from the ωn spectrum the high frequency seismic field near the epicenter of a large earth quake is modeled by subdividing the fault plane into subelements and summing their contributions at the observation point each element is treated as a point source with an o92 spectral shape where co is the angular frequency ground motion contributions from the subsources are calculated using a stochastic model attenuation is based on simple geometric spreading in a whole space coupled with regional anelastic atten uation q operator the form of the co n spectrum with natural n follows from point shear dislocation theory with an appropriately chosen slip time function the seismic moment and comer frequency are the two parameters defining the point source spectrum and must be linked to the subfanlt size to make the method complete two coefficients aa and k provide this link assigning a moment to a subfault of specified size introduces the stress parameter ao the relationship between comer frequency dislocation growth rate and fault size is established through the coefficient k which is inherently nonunique these two parameters control the number of subsources and the ampli tudes of high frequency radiation respectively derivation of the model from shear dislocation theory reveals the unavoidable uncertainty in assigning ogn spectrum to faults with finite size this uncertainty can only be reduced through empirical vali dation the method is verified by simulating data recorded on rock sites near epicenters of the m8 0 1985 michoacan mexico the m8 0 1985 valparaiso chile and the m5 8 1988 saguenay qurbec earthquakes each of these events is among the largest for which strong motion records are available in their respective tectonic environ ments the simulations for the first two earthquakes are compared to the more de tailed modeling of somerville et al 1991 which employs an empirical source function and represents the effects of crustal structure using the theoretical impulse response both methods predict the observations accurately on average the precision of the methods is also approximately equal the predicted acceleration amplitudes in our model are generally within 15 of observations an unexpected result of this study is that a single value of a parameter k provides a good fit to the data at high frequencies for all three earthquakes despite their different tectonic environments this suggests a simplicity in the modeling of source processes that was unanticipated

heterogeneous distribution of dynamic stress drop and relative fault strength recovered from the results of waveform inversion the 1984 morgan hill california earthquake the dynamic rupture process of the 1984 morgan hill california earthquake m 6 2 has been investigated on the basis of a three dimensional 3d dynamic shear crack model from previous waveform inversion results for this purpose a locking fracture criterion is introduced for rupture propagation from which a lower bound of the peak shear stress just before rupturing and hence of the relative fault strength has been estimated at each fault segment the distribution of static and dynamic stress drops has also been independently evaluated from that of fault slips by linear and nonlinear inversion procedures respectively the results show that large slips located from the hypocenter to about 10 km and 14 to 17 km along the strike at depths between 8 and 12 km result from local dynamic stress drops exceeding 40 and 140 bars respectively negative stress drops down to 15 bars are required to explain very low slips over a shallow fault section if a non negative stress drop condition is imposed somewhat larger slips are obtained there than those from the waveform inversion the above results suggest that there could be a zone of velocity strengthening frictional behavior in the shallow crust which may have arrested slip motion during rupture propagation the strength excess is found to be generally small but somewhat larger at a small zone that has delayed rupture propagation the dynamic rupture initiated from a small nucleus zone with a low stress drop propagated southeastward breaking the deeper fault section with high stress drop and then broke a relatively high strength zone after a short time of arrest with highest stress drop

quasi dynamic models of simple earthquakes application to an aftershock of the 1975 oroville california earthquake a class of kinematic rupture models for simple earthquakes is proposed these models incorporate results from both theoretical and numerical solutions to the mixed boundary value problem of a three dimensional frictional rupture the analytic form for the slip velocity is divided into two phases the rupture growth during which the slip distribution is self similar kostrov 1964 and the healing during which the slip velocity decreases monotonically to zero the arrival of a p wave stopping phase propagating into the interior of the rupture area from the fault perimeter initiates the healing the duration of healing may be varied to produce a gradual or abrupt stopping of the rupture which permits modeling a wide range of high frequency spectral behavior these models are used to generate synthetics which are fit to the velocity and the velocity squared pulse shapes obtained from strong motion accelerograms of the 0103 aftershock of the 1975 oroville california earthquake the aftershock ml 4 6 has a moment of 1 8 1023 dyne cm and a total radiated energy of 2 7 1019 dyne cm the fitted source model propagates updip with a rupture velocity of 0 85 and a dynamic stress drop of 210 bars the arrival times of five stopping phases correlated across the suite of accelerograms are used to determine the relative locations of the hypocenter and five stopping events the description of rupture determined from these stopping events both corroborates the source model obtained from the waveform modeling and establishes an important counterpoint to the model by providing a dynamic interpretation of the unmodeled complexity of the waveforms

large scale inflation of tungurahua volcano ecuador revealed by persistent scatterers sar interferometry the tungurahua volcano in ecuador has been experiencing a substantial activity period since 1999 with several eruptions including those of 2006 and 2008 we use a persistent scatterers approach to analyze a time series of envisat synthetic aperture radar sar data over the period 2003 2009 to investigate surface deformation in the region of the volcano we measure a continuous large scale uplift with a maximum line of sight displacement rate of about 8 mm yr which is the first evidence of a sustained inflation in the andes for an active volcano encompassing several eruptions we model this signal as magma emplacement in a permanent storage zone at 11 5 km below sea level with a net inflow rate of 7 million m3 yr the paroxysmal eruptions in 2006 and 2008 did not seem to disrupt this long term signal however we observe significant deformation during the 2006 eruption consistent with an additional intrusion of 4 5 million m3 of magma

apparent stresses stress drops and amplitude ratios of earthquakes preceding and following the 1975 hawaii ms 7 2 main shock abstract s wave spectra for approximately 300 earthquake signals 2 8 m l 3 9 were obtained from hand digitized wood anderson records from hilo hawaii all sources were located within part of the rupture volume of the 1975 hawaii m s 7 2 earthquake the epicentral distance to hilo was about 45 3 km the backazimuth was 180 10 south and the sources were located at depths of 9 1 4 km the apparent stress and brune stress drop based on 3 sec windows of the s waves were examined for possible temporal and spatial dependence most of the data showed a remarkable constancy of the average apparent stress and stress drop per event as a function of time no measurable change in average signal content took place at the time of the main shock a statistically significant difference in the average stress parameters per event was found between neighboring subvolumes radius 5 km of the 1975 aftershock zone a volume previously proposed as asperity based on seismicity pattern interpretation contained earthquakes with higher apparent stresses of 9 0 8 8 bars compared to the neighboring volume with 4 2 6 1 bars seventy eight spectra obtained from a second station showed the same difference between the two volumes as that found at station hilo suggesting that the signal differences should be interpreted in terms of source differences in addition amplitude ratios of sv p waves measured at hilo showed that a systematic difference of 10 in fault plane orientation existed during years before the 1975 main shock this agrees with the rotation of the fault plane during the main rupture reported elsewhere we hypothesize that heterogeneities along the fault plane with dimensions of about 5 km cause changes in fault plane dip differences in average stress release per earthquake strong contrasts in seismicity patterns and differences in aseismic strain release by fault creep these same heterogeneities in the fault plane apparently also control the location of major earthquakes and the complexity of their ruptures

gravity changes and internal processes some results obtained from observations at three volcanoes temporal gravity changes provide information about mass and or density variations within and below the volcano edifice three active volcanoes have been under investigation each of them related to a plate boundary mayon luzon philippines merapi java indonesia and galeras colombia the observed gravity changes are smaller than previously expected but significant for the three volcanoes under investigation and within the observation period mainly the increase of gravity is observed ranging from 1 000 nm 2 to 1 600 nms 2 unexpectedly the gravity increase is confined to a rather small area with radii of 5 to 8 km around the summit at mayon and merapi the parallel gps measurements yield no significant elevation changes this is crucial for the interpretation as the internal pressure variations do not lead to significant deformation at the surface thus the classical mogi model for a shallow extending magma reservoir cannot apply to confine the possible models the attraction due to changes of groundwater level or soil moisture is estimated along the slope of merapi exemplarily by 2 d modelling mass redistribution or density changes were evaluated within the vent as well as deeper fluid processes to explain the gravity variations the results are compared to the model incorporating the additional effect of elastic deformation

the 25 november 1988 saguenay quebec earthquake source parameters and the attenuation of strong ground motion the saguenay earthquake of 25 november 1988 occurred close to the southern margin of the saguenay graben in southern quebec it was caused by almost purely dip slip faulting centered at a depth of 26 km with a p axis oriented northeast southwest this faulting mechanism is similar to those of the larger historical earthquakes in eastern north america but the focal depth is substantially greater than all but one of these events the seismic moment estimated from regional pni waves and teleseismic long period body waves is 5 10 24 dyne cm corresponding to a moment magnitude of 5 8 the source duration of the earthquake is estimated to be 1 8 sec corresponding to a stress drop of 160 bars which is not significantly higher than the average stress drop of 120 bars estimated from previous large earthquakes in eastern north america in order to simultaneously match the recorded ground motion amplitudes of strong motion acceleration strong motion velocity and teleseismic short period and long period body waves it is necessary to use a source function having a complex shape that implies the presence of asperities and larger local stress drops the large set of strong motion recordings of the saguenay earthquake has been used to validate a procedure for estimating strong ground motion attenuation based on a simple wave propagation model the most important feature of the recorded strong motions is that their peak amplitudes do not decay significantly with distance inside 120 km but then decay abruptly beyond 120 km profiles of recorded accelerograms with absolute times indicate that at distances beyond 64 km the peak ground motions are due to strong postcritical reflections from velocity gradients in the lower crust the principal shear wave arrivals and the variation of their peak amplitudes with distance were reproduced in synthetic seismograms generated using a regional crustal structure model the critical distances for the postcritical reflections were short because of the deep focal depth of the event causing the elevation of ground motion amplitudes out to 120 km similar studies of earthquakes in other regions of eastern north america indicate that the strength of the postcritical reflections and the distance ranges over which they are dominant are controlled by the focal depth and crustal structure regional variations in crustal structure thus give rise to predictable regional variations in strong ground motion attenuation

roughness at the base of the seismogenic zone contributing factors the cutout depth of microseismic activity in continental fault zones appears to correspond to the onset of greenschist metamorphic conditions at about 300 c it can generally be modeled as the transition from frictional to quasi plastic behavior in quartzofeldspathic crust shear resistance increases with depth through the frictional regime to peak at the transition beneath which it falls off exponentially with increasing temperature larger earthquake ruptures ml 5 5 nucleate around this transition depth where the highest concentrations of strain energy may accumulate varying depth and amplitude of the peak shear resistance along strike induce fluctuations in strain energy concentration at the base of the seismogenic zone factors affecting the depth of the transition include crustal composition geometry and mode of faulting fluid pressure levels in the frictional regime and water content in the quasi plastic regime quasi plastic strain rate and geothermal gradient evaluation of their relative importance is complicated because several are interdependent however compositional change may cause abrupt irregularities in seismogenic depth and peak shear resistance while regional variations in heat flow look to be particularly effective in creating long wavelength heterogeneities in strain energy concentration affecting faulting style

long period events the most characteristic seismicity accompanying the emplacement and extrusion of a lava dome in galeras volcano colombia in 1991 abstract since its reactivation in 1988 the principal eruptions of galeras volcano occurred on may 4 9 1989 july 16 1992 and january 14 march 23 april 3 april 14 and june 7 1993 the initial eruption was a phreatic event which clearly marked a new period of activity a lava dome was extruded within the main crater in october 1991 and subsequently destroyed in an explosive eruption on july 16 1992 the eruptions that followed were all vulcanian type explosions the seismicity accompanying the emplacement extrusion and destruction of the lava dome was dominated by a mix of long period lp events and tremor displaying a variety of waveforms repetitive lp events with dominant periods in the range 0 2 1 s were observed in october and november 1991 and visually correlated with short energetic pulses of gas venting through a crack bisecting the dome surface each lp event was characterized by a weak precursory signal with dominant periods in the range 0 05 0 1 s lasting roughly 7 s using the fluid driven crack model of chouet 1988 1992 we infer that two distinct cracks may have acted as sources for the lp and precursor signals spectral analyses of the data yield the following parameters for the lp source crack length 240 360 m crack width 130 150 m crack aperture 0 5 3 4 mm crack stiffness 100 500 sound speed of fluid 880 m s and excess pressure 0 01 0 19 mpa similar analyses yield the parameters of the precursor source crack length 20 30 m crack width 15 25 m crack aperture 2 3 8 7 mm crack stiffness 5 15 sound speed of fluid 140 m s and excess pressure 0 06 0 15 mpa combined with geologic and thermodynamic constraints obtained from field observations these seismic parameters suggest a gas release mechanism in which the episodic collapse of a foam layer trapped at the top of the magma column subjacent to the dome releases a slug of pressurized gas which escapes to the surface while dilating a preexisting system of cracks in the dome structure accordingly the fracture observed on the crystallized dome body is the surface extension of the lp source crack where lp activity is induced by the rapid emission and expansion of gas flowing through this conduit the width and aperture of the crack estimated in the model are in good agreement with the length and aperture of the fracture estimated from visual observations the source parameters of the precursor signal are suggestive of a nozzle like conduit connecting the lp source crack to the underlying magma reservoir excitation of this conduit segment is attributed to the rapid emission and acceleration of the frothy fluid resulting from the collapse of the foam layer at the top of the reservoir the calculated periodicity of foam collapse events is in agreement with the observed average rate of thirteen lp events per hour

zircon u pb chronology of the nyingtri group southern lhasa terrane tibetan plateau implications for grenvillian and pan african provenance and mesozoic cenozoic metamorphism abstract the nyingtri group of the lhasa terrane in southern tibet consists dominantly of metasedimentary rocks and orthogneiss these rocks have a similar mineral paragenesis of plagioclase k feldspar biotite quartz sillimanite garnet staurolite muscovite amphibole indicating amphibolite facies metamorphic conditions inherited detrital zircons from the metasedimentary rocks show magmatic features and yield widely variable 206pb 238u ages ranging from 3300 to 50 ma the data define two prominent age populations 1200 1000 and 600 500 ma indicating that the source of the nyingtri group preserves the records of both grenville and pan african magmatic thermal events inherited magmatic zircon cores from the orthogneiss yield a crystallization age of 496 ma limiting the depositional age of the metasedimentary sequence to cambrian or older overgrowth rims on the detrital zircons from one metasedimentary rock yield a metamorphic age of 32 ma on the basis of these results together with the

the andean type gangdese mountains paleoelevation record from the paleocene eocene linzhou basin abstract paleoelevation reconstruction using oxygen isotopes is making a significant contribution to understanding the cenozoic uplift of the himalayas and the tibetan plateau this paper presents new oxygen and carbon isotopic compositions from well dated tertiary paleosols lacustrine calcareous carbonates and marls from the nianbo 60 54 ma and upper pana formations 51 48 ma of the linzizong group in the linzhou penbo basin the sediments of the nianbo formation which are 180 m thick were deposited in alluvial fans braided rivers fan deltas and on nearshore to offshore lacustrine settings whereas those of the upper pana formation are 100 m thick and are comprised predominantly of proximal alluvial fan and braided river deposits correlations between the lithofacies and stable isotopic compositions suggest that the basin was mainly a hydrologically open environment it is confirmed that the 18 o c and 13 c c values from nianbo and pana formations have not yet been reset by late stage diagenesis based on petrographic examination oxygen isotope of the fossil ostracodes and tectonic deformation of strata the paleoelevations are reconstructed using the corrected most negative paleosurface water 18 o psw values these imply that the linzhou area had attained an elevation of 4500 400 m during the period of the indo asian collision i e achieved a near present elevation and may form an andean type mountain range stretching the gangdese arc before collision the gangdese mountains probably maintained high elevations since at least the paleocene and could play a crucial role in the climate change in the interior of the tibetan plateau during the early cenozoic the paleogeomorphic scenario of the eocene tibet is proposed to exist at two high mountains in excess of 4500 m that sandwiched a low elevation basin

geometry of continental normal faults seismological constraints teleseismic body waves from large earthquakes are used to study the downdip geometry of continental normal faults in the aegean waveform modeling techniques together with rigorous statistical tests are applied to put firm bounds on the amount of downdip curvature of these faults and the role of coseismic slip on a basal detachment synthetic modeling shows that good azimuthal station coverage and inclusion of sh waves are necessary to resolve fault curvature the data indicate ruptures of the aegean events occurred on planar faults extending across the entire brittle portion of the crust no seismogenic low angle detachment faulting at the base of the upper crust was detected for these events decoupling of the brittle upper crust from the plastic lower crust probably occurs aseismically in a ductile fashion

the multiparameter station at galeras volcano colombia concept and realization abstract volcanoes are complex systems in which the interaction of many different physical and chemical factors and processes contribute to changes in activity in the past 40 years our ability to observe and quantify short term changes in a volcano s activity has improved due to the installation of seismometers and tiltmeters and the continuous records they provide however due to instrumental limitations the observations have mainly been used phenomenologically to draw inferences about possible changes on the basis of previous experience since 1995 the bundesanstalt fur geowissenschaften und rohstoffe bgr and the instituto de investigacion e informacion geocientifica minero ambiental y nuclear ingeominas have been working to develop and deploy a multiparameter mp station on galeras volcano colombia this station is designed to concurrently measure various geophysical and geochemical parameters it includes three broadband seismometers at the crater rim as well as a more remotely located broadband seismic reference at other locations in the crater or on the rim electromagnetic probes an infrasound sensor and a weather station are operating the data from these sensors are digitized at each site with 24 bit digitizers and transmitted by spread spectrum radio via repeater when necessary to the observatorio vulcanologico y sismologico ovp in the city of pasto there they are received and displayed on a networked personal computer and recorded continuously the data flow into the routine analysis procedures of the ovp and the continuous data are archived on cd in addition to the other sensors a system of specially developed sensors continuously monitors the chemistry and physics of the gases at fumaroles on the active cone the data from this system are also transmitted in realtime to ovp and recorded the continuous recordings of the mp station are supplemented by regular thermographic measurements of the surface temperature in the crater using an infrared camera joint analysis and interpretation of the data streams from the many sensors of the mp station will improve our understanding of the physical processes occurring in galeras volcano

hard rock spectral models for intermediate depth vrancea romania earthquakes the frequency dependent amplification for rock sites as well as apparent source spectra were studied using an earthquake ground motion database collected in romania almost all events occurred in the vrancea focal zone southeast carpathians which is characterized by a high rate of occurrence of large earthquakes in a narrow focal volume the seismic activity ranges within an almost vertical stripe in depths between 60 and 170 km the database used includes several hundred records from more than 100 small magnitude m w 3 5 5 3 earthquakes that occurred in 1996 2004 and six acceleration records obtained during four large m w 7 2 6 9 6 3 and 6 0 earthquakes the characteristics of amplification were evaluated using the well known technique of horizontal to vertical h v fourier spectral ratio of the s wave phase lermo and chavez garcia 1993 the apparent source spectra were obtained from horizontal components using site h v propagation path geometric spreading and anelastic attenuation and near surface attenuation kappa factor corrections seismic radiation within the frequency range from 0 2 0 3 to 15 20 hz during the earthquakes of various magnitudes in the vrancea zone may be described by an omega square single corner frequency spectral model 2 brune spectrum the stress parameter which controls high frequency spectral amplitudes increases with magnitude from 20 30 bars for m w 3 5 up to 200 250 bars for m w 4 8 5 3 and up to 1000 bars for the case of large m w 6 0 6 5 events empirical amplification h v for rock sites showing stability for particular locations reveal high variability from station to station which together with variations of values of kappa reflects influence of local geologic and geomorphologic factors

deformation associated with the extrusion of a dome at galeras volcano colombia 1990 1991 abstract during 1989 1992 two electronic tilt meters named crater and peladitos and a short level line named las piedras were used for monitoring galeras volcano the two tilt meters detected considerable changes of uplift mainly from june to november 1991 the crater station showed cumulative values of more than 600 microradians while the peladitos station exhibited values of more than 100 microradians the short level line showed inflationary tilt of more than 20 microradians in 1989 this behavior was related to increases in the daily occurrence of long period seismic events during july november 1991 caused by the rise and extrusion of a lava dome which was subsequently destroyed in july 1992 at the end of 1990 the deformation source was located at a depth of 2 3 km by december 1991 the source had migrated to less than 500 m in depth the magma ascent rate was 1 4 m day during this period the dome emplaced in the crater had a volume between 300 000 to 400 000 m 3 using a hydrodynamic model and the tilt changes registered by the peladitos station the volume of the magma body is estimated at approximately 2 8 10 6 m 3

earthquake source spectra in eastern north america source spectra for earthquakes of 3 m 7 in eastern north america ena are determined from regional seismograph recordings strong motion records teleseismic data and modified mercalli intensity data the inclusion of mmi data is made possible by the finding that the felt area of an earthquake is an accurate measure of its high frequency spectral level at the source current ground motion relations for ena are based on a brune source model with a stress parameter of 100 bars this model is a poor fit to ena source spectra for events of m 4 at high frequencies 5 to 10 hz observed spectral amplitudes are more nearly matched by a brune stress parameter of 200 bars intermediate frequency amplitudes near 1 hz are matched by a stress parameter of approximately 50 bars an empirical two corner source model is proposed to describe ena source spectra for events of 4 m 7

zircon u pb geochronology and hf isotopic constraints on petrogenesis of the gangdese batholith southern tibet abstract during the mesozoic cenozoic northward neotethyan subduction and subsequent india asia collision gave rise to the extensive transhimalayan magmatism that stretches from burma and western yunnan through southern tibet to the ladakh and kohistan complexes to understand the age distribution and petrogenesis of the gangdese batholith the largest intrusive exposure along the transhimalayan magmatic belt fifty granitic samples were selected for in situ zircon u pb and hf isotopic analyses the u pb data suggest four discrete stages of magmatic activity i e 205 152 109 80 65 41 and 33 13 ma respectively with the 65 41 ma stage being the most prominent the hf isotopic data indicate that the gangdese batholith is overwhelmed by positive e hf t values which are comparable to those of the kohistan ladakh batholiths in the west but differ markedly from those of the chayu burma batholiths in the east most of the gangdese granites show similar and young hf model ages 1000 300 ma indicating their derivation from juvenile crust however those formed in the 65 41 ma stage exhibit more heterogeneous hf isotopic ratios with negative e hf t values being observed in some granites younger than 50 ma suggesting the involvement of old indian continental crust in their petrogenesis this age may thus mark the onset of the india asia collision the newly established zircon u pb age and hf isotope database of the gangdese batholith can be used as a powerful tracer or fingerprint when studying the source to sink relation of the sediments eroded from the southern tibetan plateau

crustal deformation in the mid may 1991 crisis preceding the extrusion of a dacite lava dome at unzen volcano japan abstract tilt observations at fg1 station located about 680 m west of the central crater of unzen volcano southwestern japan revealed details about the process of magma ascent during the mid may 1991 crisis just before the appearance of a lava dome crustal deformations measured during this period can be interpreted by a combination of upward growth of a buried magma column and lateral intrusion of a dike modeled by a vertical line source and a planer tensile dislocation in an elastic half space marked change in tilt was observed at first in the ew direction at fg1 during may 11 14 suggesting the ascent of a magma column with a diameter of about 40 m from about 300 m to 160 m in depth beneath the central crater after the interruption of the rise of the magma column magma began to intrude laterally as a dike on may 15 toward approximately the n80 w direction this resulted in a large tilt change essentially in the ns component at fg1 the horizontal extent of the dike might have exceeded 400 m in a few days with a thickness of about 6 m on may 17 the magma column started to rise again probably because the impediment to magma ascent was fractured by the growth of the dike the top of the magma column finally reached to the crater bottom probably on may 19 and a new lava dome was actually observed on may 20 after which the remarkable upward tilt to the south at fg1 due to the dike intrusion decreased rapidly the deformation after may 20 can be interpreted by a gradual increase in the thickness of the dike up to 13 m by the end of may 1991 although large errors are inevitable in estimated values because of a lack of sufficient data the present results confirm the importance of making continuous observations of crustal deformation near the summit areas of active volcanoes in order to clarify the underground processes of magma movement leading to eruptions

tectonic and magmatic development of the salinian coast ridge belt california 1 we present new field structural petrographic and geochronologic data on a rare midcrustal 25 km exposure of a cordilleran arc the coast ridge belt located in the santa lucia mountains of central california the study area is composed primarily of a deformed suite of upper amphibolite to granulite facies rocks the sur series which is dominated by metaigneous tonalites diorites and gabbros with subordinate metasedimentary quartzite and marble inherited zircons in magmatic rocks suggest that the provenance of framework rocks is drawn heavily from miogeoclinal formations and that sedimentation occurred in the late paleozoic or later minor magmatism in the coast ridge belt began in the early or middle cretaceous but magmatic activity was most intense during a short period time from 93 to 81 ma based on u pb zircon ages of a felsic gneiss and two less deformed diorites the time period 93 81 ma also brackets a period of extensive thickening and high temperature ductile deformation while a thrusting cause for ductile deformation cannot be ruled out we favor the hypothesis that the exposed rocks correspond to a zone of return flow of supracrustal rocks locally displaced by granitoid plutons in the shallower crust magmatism ended throughout salinia between81and76ma coincidentwiththeattainmentof peak pressure and temperature conditions of 0 75 gpa and 800 c exhumation followed immediately bringing the coast ridge belt to the surface within 8 my at a rate of at least 2 3 mm yr exhumation was coincident with an episode of extensional collapse that has been documented elsewhere in the southern california arc during the early laramide orogeny and that may be related to underthrusting of the forearc at that time index terms 8102 tectonophysics continental contractional orogenic belts 8035 structural geology pluton emplacement 3640 mineralogy and petrology igneous petrology 8030 structural geology microstructures 8124 tectonophysics earth s interior composition and state 1212

comparison of source scaling relations of eastern and western north american earthquakes source scaling relations have been obtained for earthquakes in eastern north america and other continental interiors and compared with a relation obtained for earthquakes in western north america the scaling relation for eastern north american earthquakes was constructed from measurements of seismic moment and source duration obtained by the waveform modeling of seismic body waves the events used include nine events of m blg magnitude 4 7 to 5 8 that occurred after 1960 and four earlier events with magnitudes between 5 5 and 6 6 the scaling relation for events in other continental interiors was used for comparative purposes and to provide constraints for large magnitudes detailed analysis of the uncertainties in the scaling relations has allowed the resolution of two important issues concerning the source scaling of earthquakes in eastern north america first the source characteristics of earthquakes in eastern north america and other continental interiors are consistent with constant stress drop scaling and are inconsistent with nonconstant scaling models such as that of nuttli 1983 second the stress drops of earthquakes in eastern north america and other continental interiors are not significantly different from those of earthquakes in western north america and have median values of approximately 100 bars the source parameters of earthquakes in eastern north america are consistent with a single constant stress drop scaling relation whereas the source parameters of earthquakes in western north america are much more variable and show significant departures from an average scaling relation in which stress drop decreases slightly with seismic moment

age of initiation of collision between india and asia a review of stratigraphic data abstract the collision of india with asia is perhaps the most profound tectonic event to have occurred in past 100 ma it is responsible for the uplift of the himalayas and tibetan plateau and has been argued to have been responsible for geological geochemical and climatological consequences of global extent yet the age of initiation of this collision remains poorly constrained the literature is replete with estimates that range from the late cretaceous 65 ma to latest eocene

the seismic radiation from composite models of faulting abstract the failure of an asperity i e the dynamic rupture of a small fault area with finite stress drop surrounded by a broken or weak fault area which has no stress drop but which slips after the asperity fails is proposed as a model for the rupture process of a subevent in a composite earthquake the rupture area of the composite earthquake surrounding the subevent is modeled by the weak fault area surrounding the asperity in the subevent model the resulting seismic moment of the subevent is proportional to the stress drop and the rupture area of the subevent as well as the radius of the composite earthaquake by setting the stress drops of the asperity models equal to the dynamic stress drops of the subevents the composite earthquake can be modeled as the sum of a set of subevents which cover the rupture area of the composite earthquake the scaling of the high and low frequency radiation from composite earthquakes composed of asperities is commensurate with generally observed spectral scaling laws in contrast to composite earthquakes composed of cracks or smaller earthquakes a simple filtering strategy is proposed for filtering the waveforms radiated by cracks to approximate the waveforms radiated by asperities the p and s waves radiated by an m l 5 2 earthquake which occurred on 9 may 1983 at coalinga california are simulated using the p and s waves radiated by an m l 3 6 aftershock the aftershock waveforms are first filtered to approximate the radiation from asperities with the appropriate rupture areas and then the waveforms of 12 asperity subevents are summed together to simulate the waveforms and spectra of the composite earthquake

a source study of the thessaloniki northern greece 1978 earthquake sequence summary the thessaloniki northern greece earthquake sequence appears to have occurred along faults forming a graben structure this graben situated in the border region between the serbomacedonian massif and the vardar zone is bounded to the south west by clearly exposed north west striking north east dipping normal faults relative hypocentre determinations fault plane solutions surface faulting and the aftershock distribution suggest that some of these faults have been reactivated during the 1978 earthquakes the source parameters of the mainshock mb 6 1 ms 6 4 were determined by computing body wave synthetic seismograms in the time domain and comparing them with the observed this modelling constrained the orientation of faulting determined by the p wave fault plane solution it also constrained the source depth to 6 mp 2 km similar depths were calculated by a relative relocation method for the other three large events of the sequence the dislocation time function required for the far field had a total duration of 9 mp 1 5 s the body wave moment was 5 2 mp 1 8 times 1025 dyne cm for a fault length of 35 km and a fault width of 17 km both estimated from the aftershock distribution the static stress drop was found to be 4 bar an accelerograph record of the mainshock shows two distinct events 3 4 s apart these two events were unresolved by the long period data in the far field but the unusually long duration of the mainshock time function suggests an overall slow energy release which probably occurred as a sequence of events close in space and time seismic energy released in this fashion can account for waveform complexities observed at some stations

the corner frequencies and stress drops of intraplate earthquakes in the northeastern united states this article presents the estimation of stress drops for small to middle sized intraplate earthquakes in the northeastern united states the vertical component sg and lg waves of 49 earthquakes were analyzed and their seismic corner frequencies and seismic moments were determined for these events both short period and broadband records were obtained from stations in the region there are eight events each of which has an aftershock good enough to be treated as its empirical green s function and their corner frequencies were estimated from empirical green s function methods for the other events the corner frequencies were directly estimated by the spectral fitting of the vertical component of the sg or lg wave displacement spectrum with the square source spectral model using the available broadband and high frequency short period data and a frequency dependent q correction the static stress drops were then calculated from the corner frequency and seismic moment from our study the source corner frequencies estimated by fitting the lg displacement spectrum with the assumed square source model are more consistent with the corner frequencies measured from empirical green s function deconvolution method than those estimated from the intersection of horizontal low frequency spectral asymptote and a line indicating the 2 decay above the corner frequency the source corner frequencies we estimated proved to be most appropriate for the small to middle sized earthquakes the static stress drops calculated from these corner frequency estimates tend to be independent of seismic moment for events above a certain size for earthquakes with size less than about 2 1020 dyne cm the stress drop tends to decrease with decreasing moment suggesting a breakdown in self similarity below a threshold magnitude a characteristic rupture size of about 100 m is implied for these smaller earthquakes