estimation of coda wave attenuation for nw himalayan region using local earthquakes abstract the attenuation of seismic wave energy in nw himalayas has been estimated using local earthquakes most of the analyzed events are from the vicinity of the main boundary thrust mbt and the main central thrust mct which are well defined tectonic discontinuities in the himalayas the time domain coda decay method of a single back scattering model is employed to calculate frequency dependent values of coda q qc a total of 36 local earthquakes of magnitude range 2 1 4 8 have been used for q c estimation at central frequencies 1 5 3 0 6 0 9 0 12 0 and 18 0 hz through eight lapse time windows from 25 to 60 s starting at double the time of the primary s wave from the origin time the estimated average frequency dependence quality factor gives the relation q c 158 f 1 05 while the average q c values vary from 210 at 1 5 hz to 2861 at 18 hz central frequencies the observed coda quality factor is strongly dependent on frequency which indicates that the region is seismic and tectonically active with high heterogeneities the variation of the quality factor q c has been estimated at different lapse times to observe its effect with depth the estimated average frequency dependent relations of q c vary from 85 f 1 16 to 216 f 0 91 at 25 to 60 s lapse window length respectively for 25 s lapse time window the average q c value of the region varies from 131 36 at 1 5 hz to 2298 397 at 18 hz while for 60 s lapse time window its variation is from 285 95 at 1 5 hz to 2868 336 at 18 hz of central frequency the variation of q c with frequency and lapse time shows that the upper crustal layers are seismically more active compared to the lower lithosphere the decreasing value of the frequency parameter with increasing lapse time shows that the lithosphere acquires homogeneity with depth

three dimensional velocity imaging of the kachchh seismic zone gujarat india abstract to understand the causative mechanism of the continued occurrence of earthquakes in kachchh gujarat for the last six years we estimated high resolution three dimensional vp vs and vp vs structures in the aftershock zones of the 2001 mw7 7 bhuj and 2006 mw5 6 gedi earthquakes we used 13 862 p and 13 736 s wave high quality arrival times collected from the seismograms of 2303 aftershocks recorded at 5 18 three component seismograph stations during 2001 06 seismic images revealed a marked spatial variation in the velocities from 20 to 14 in vp from 12 to 13 in vs and from 12 to 12 increase in vp vs in the 0 34 km depth range beneath the bhuj aftershock zone relatively more increase in vp than vs resulting in an increase in vp vs in the crust beneath the seismically active causative fault north wagad fault nwf zone of 2001 bhuj mainshock suggests a rigid mafic crust beneath the region they also delineate an increase of 8 in vp and 14 in vs and a decrease of 4 in vp vs in the almost vertical rupture zone of the 2006 gedi earthquake extending up to 12 km depth this high velocity body associated with the gedi mainshock is inferred to be a gabbroic intrusive the banni region and the wagad uplift are found to be associated with high velocity intrusive bodies inferred to be mafic extending from 5 to 35 km depth which might have intruded during the rifting in early jurassic 160 ma aftershock activity is mainly confined to the zones characterized by high vp high vs and low vp vs ratio which might be representing the strong competent and brittle parts of the fault zone intrusive bodies that could accumulate large strain energy for generating aftershocks for more than six years it is inferred that the crustal stress concentrations associated with the intrusive bodies are contributing significant perturbation to the crustal stress regime to generate the intraplate earthquakes in the kachchh rift zone a few patches of slow vp and vs and high vp vs between 10 to 30 km depth have also been detected on the causative 45 south dipping north wagad fault nwf for the 2001 mainshock which may be attributed to the fluid filled fractured rock matrix interestingly the 2001 bhuj mainshock hypocenter is found to be associated with such a low velocity patch thus it is inferred that the occurrence of 2001 bhuj mainshock might have facilitated by the presence of fluids at hypocentral depth

paleomagnetism of late cretaceous and eocene limestones and chalks from haiti tectonic interpretations we have conducted a paleomagnetic study of limestones and chalks from haiti which suggests that the island of hispaniola is a composite of at least two tectonically independent blocks sampling sites are distributed among three widely separated localities namely les cayes nine sites 18 3 n latitude 73 6 w longitude and beloc five sites 18 4 n latitude 72 6 w longitude which are located on the southern peninsula of haiti and a northern locality ennery six sites 19 5 n latitude 72 5 w longitude stable magnetization components of dual polarity and moderately high blocking temperature were revealed through progressive thermal demagnetization of samples from all of the beloc and ennery sites while seven of the nine les cayes sites possessed unstable magnetizations mean paleolatitudes calculated from beloc and ennery data show 8 of latitudinal separation that is of at least eocene age pole positions beloc 43 2 n latitude 185 8 e longitude 95 25 1 and ennery 66 1 n latitude 31 3 e longitude 95 19 7 show both colatitudinal and angular discordance with the north american apparent polar wander path based on these disparities we demonstrate that the implied tectonic displacements tend to support some aspects of previously published caribbean plate tectonic models

ground motion relations for eastern north america predictive relations are developed for ground motions from eastern north american earthquakes of 4 0 m 7 25 at distances of 10 r 500 km the predicted parameters are response spectra at frequencies of 0 5 to 20 hz and peak ground acceleration and velocity the predictions are derived from an empirically based stochastic ground motion model the relations differ from previous work in the improved empirical definition of input parameters and em pirical validation of results the relations are in demonstrable agreement with ground motions from earthquakes of m 4 to 5 there are insufficient data to adequately judge the relations at larger magnitudes although they are consistent with data from the saguenay m 5 8 and nahanni m 6 8 earthquakes the underlying model parameters are constrained by empirical data for events as large as m 6 8

attenuation of coda waves in new england abstract seismic wave attenuation was measured in new england from the time domain decay of coda wave amplitudes on narrow bandpass filtered seismograms of local earthquakes the frequency band of interest was 0 75 to 10 hz q c f was found to increase with frequency across this band but there was also a difference between the q c f values for short t t 100 sec lapse times of coda wave propagation for the short lapse time data corresponding to wave paths primarily in the upper crust q c f increases from 400 at 3 hz to 1300 at 10 hz for the large lapse time data corresponding to wave paths in the lower crust and upper mantle q c f was found to vary from 660 at 1 hz to 1500 at 10 hz if we interpret this dataset in terms of a model incorporating both scattering and anelastic attenuation we find that the mean free path of seismic waves over this frequency band is 80 km for short lapse times and 400 km for long lapse times if we assume that scattering is entirely responsible for the observed attenuation we find that the minimum mean free path for short lapse times is about 75 km over all frequencies whereas for long lapse times the minimum mean free path decreases from 400 km at 0 75 hz to 90 km at 10 hz

estimation of ground motion for bhuj 26 january 2001 mw 7 6 and for future earthquakes in india only five moderate and large earthquakes mw 5 7 in india three in the indian shield region and two in the himalayan arc region have given rise to multiple strong ground motion recordings near source data are available for only two of these events the bhuj earthquake mw 7 6 which occurred in the shield region gave rise to useful recordings at distances exceeding 550 km because of the scarcity of the data we use the stochastic method to estimate ground motions we assume that 1 s waves dominate at r 100 km and lg waves at r 100 km 2 q 508f 0 48 is valid for the indian shield as well as the himalayan arc region 3 the effective duration is given by fc 1 0 05r where fc is the corner frequency and r is the hypocentral distance in kilometer and 4 the acceleration spectra are sharply cut off beyond 35 hz we use two finite source stochastic models one is an approximate model that reduces to the x 2 source model at distances greater that about twice the source dimension this model has the advantage that the ground motion is controlled by the familiar stress parameter dr in the other finite source model which is more reliable for near source ground motion estimation the high frequency radiation is controlled by the strength factor sfact a quantity that is phys ically related to the maximum slip rate on the fault we estimate dr needed to fit the observed amax and vmax data of each earthquake which are mostly in the far field the corresponding sfact is obtained by requiring that the predicted curves from the two models match each other in the far field up to a distance of about 500 km the results show 1 the dr that explains amax data for shield events may be a function of depth increasing from 50 bars at 10 km to 400 bars at 36 km the corresponding sfact values range from 1 0 2 0 the dr values for the two himalayan arc events are 75 and 150 bars sfact 1 0 and 1 4 2 the dr required to explain vmax data is roughly half the corresponding value for amax while the same sfact explains both sets of data 3 the available far field amax and vmax data for the bhuj mainshock are well explained by dr 200 and 100 bars respec tively or equivalently by sfact 1 4 the predicted amax and vmax in the epi central region of this earthquake are 0 80 to 0 95 g and 40 to 55 cm sec respectively

seismic imaging of the aftershock zone of the 2001 mw 7 7 bhuj earthquake india 1 detailed seismic tomography reveals that the 2001 mw 7 7 bhuj earthquake was associated with a 10 14 increase in vp and vs and a 10 decrease in vp vs in the 10 to 35 km depth range covering a 2750 km2 area beneath the aftershock zone this anomaly could be attributed to the existence of a lithological heterogeneity or a pluton of mafic composition that might have intruded during the rifting in early jurassic 160 ma the banni basin and the wagad uplift are found to be associated with high velocity intrusive bodies extending from 5 to 35 km depth a few patches of low vp and vs and high vp vs between 10 to 30 km depth have also been detected on the causative 45 south dipping north wagad fault nwf for the 2001 mainshock which may be attributed to a fluid filled fractured rock matrix

intrinsic and scattering attenuation in western india from aftershocks of the 26 january 2001 kachchh earthquake abstract 176 vertical component short period observations from aftershocks of the mw 7 7 26 january 2001 kachchh earthquake are used to estimate seismic wave attenuation in western india using uniform and two layer models the magnitudes mw of the earthquakes are less than 4 5 with depths less than 46 km and hypocentral distances up to 110 km the studied frequencies are between 1 and 30 hz two seismic wave attenuation factors intrinsic absorption qi 1 and scattering attenuation qs 1 are estimated using the multiple lapse time window method which compares time integrated seismic wave energies with synthetic coda wave envelopes for a multiple isotropic scattering model we first assume spatial uniformity of qi 1 qs 1 and s wave velocity a second approach extends the multiple scattering hypothesis to media consisting of several layers characterized by vertically varying scattering coefficient g intrinsic absorption strength h density of the media and shear wave velocity structure the predicted coda envelopes are computed using monte carlo simulation results show that under the assumption of spatial uniformity scattering attenuation is greater than intrinsic absorption only for the lowest frequency band 1 to 2 hz whereas intrinsic absorption is predominant in the attenuation process at higher frequencies 2 to 30 hz the values of q obtained range from qt 118 qi 246 and qs 227 at 1 5 hz to qt 4000 qi 4600 and qs 33 300 at 28 hz center frequencies being qt 1 a measure of total attenuation results also show that qi 1 qs 1 and qt 1 decrease proportional to f two rates of decay are clearly observed for the low 1 to 6 hz and high 6 to 30 hz frequency ranges values of are estimated as 2 07 0 05 and 0 44 0 09 for total attenuation 1 52 0 21 and 0 48 0 09 for intrinsic absorption and 3 63 0 07 and 0 06 0 08 for scattering attenuation for the low and high frequency ranges respectively despite the lower resolution in deriving the attenuation parameters for a two layered crust we find that scattering attenuation is comparable to or smaller than the intrinsic absorption in the crust whereas intrinsic absorption dominates in the mantle also for a crustal layer of thickness 42 km intrinsic absorption and scattering estimates in the crust are lower and greater than those of the mantle respectively

attenuation of coda waves in the saurashtra region gujarat india the attenuation characteristics based on coda waves of two areas jamnagar and junagarh of saurashtra gujarat india have been investigated in the present study the frequency dependent relationships have been developed for both the areas using single back scattering model the broadband waveforms of the vertical components of 33 earthquakes mw 1 5 3 5 recorded at six stations of the jamnagar area and broadband waveforms of 68 earthquakes mw 1 6 5 recorded at five stations of the junagarh area have been used for the analysis the estimated relations for the junagarh area are qc 158 5 f 0 99 0 04 lapse time 20 s qc 170 4 4 f 0 97 0 02 lapse time 30 s and qc 229 6 6 f 0 94 0 03 lapse time 40 s and for the jamnagar area are qc 178 3 f 0 95 0 05 lapse time 20 s qc 224 6 f 0 98 0 06 lapse time 30 s and qc 282 7 f 0 91 0 03 lapse time 40 s these are the first estimates for the areas under consideration the junagarh area appears to be more attenuative as compared to the jamnagar area the increase in qc values with lapse time found here for both the areas show the depth dependence of qc as longer lapse time windows will sample larger area the rate of decay of attenuation q 1 with frequency for the relations obtained here is found to be comparable with those of other regions of the world though the absolute values differ a comparison of the coda q estimated for the saurashtra region with those of the nearby kachchh region shows that the saurashtra region is less heterogeneous the obtained relations are expected to be useful for the estimation of source parameters of the earthquakes in the saurashtra region of gujarat where no such relations were available earlier these relations are also important for the simulation of earthquake strong ground motions in the region

opening of the grenada back arc basin and evolution of the caribbean plate during the mesozoic and early paleogene abstract geological and geophysical data indicate that the grenada basin was presumably created during the paleocene by sundering of a proto eastern caribbean arc into a remnant arc to the west aves swell and an active arc to the east lesser antilles ridge grenada basin spreading is thought to have been penecontemporaneous with the creation of the yucatan basin located at the opposite side of the caribbean sea i suggest that a continuous mesozoic caribbean arc m c a including the greater antilles the aves lesser antilles system and the aruba blanquilla chain netherland venezuelan antilles was initiated in the pacific probably about 130 120 ma ago its arrival in front of and its subsequent motion inside the central atlantic tethyan seaway caused the opening of both yucatan and grenada basins which occurred at the two initial points of contact with the north and south american cratons in contrast to the style of many other island arcs this back arc spreading event occurred only once in the long history of the m c a the lesser antilles appear to be the oldest currently active intra oceanic island arc

aftershock activity and frequency dependent low coda qc in the epicentral region of the 1999 chamoli earthquake of mw 6 4 on 28 march 1999 19 05 10 09 ut a significant earthquake of m w 6 4 occurred in the garhwal himalaya 30 555 n 79 424 e one hundred and ten well recorded aftershocks show a wnw ese trending northeasterly dipping seismic zone extending from a depth of 2 to 20 km as the main shock hypocenter occurred at the northern end of this seismic zone and aftershocks extended updip it is inferred that the main shock rupture nucleated on the detachment plane at a depth of 15 km and then propagated updip along a ne dipping thrust plane further the epicentral distribution of aftershocks defines a marked concentration near a zone where main central thrust mct takes a significant turn towards the north which might be acting as an asperity in response to the nne compression due to the underthrusting of himalayan orogenic process prevalent in the entire region presence of high seismicity including five earthquakes of magnitude exceeding 6 and twelve earthquakes of magnitude exceeding 5 in the 20th century is presumed to have caused a higher level of shallow crustal heterogeneity in the garhwal himalaya a site lying in the central gap zone of the himalayan frontal arc attenuation property of the medium around the epicentral area of the 1999 chamoli earthquake covering a circular area of 61 500 km2 with a radius of 140 km is studied by estimating the coda q c from 48 local earthquakes of magnitudes varying from 2 5 4 8 these earthquakes were recorded at nine 24 bit reftek digital stations two of which were equipped with three component cmg40t broadband seismometers and others with three component l4 3d short period seismometers the estimated q o values at different stations suggest on average a low value of the order of 30 0 8 indicating an attenuating crust beneath the entire region the frequency dependent relation indicates a relatively low q c at lower frequencies 1 3 hz that can be attributed to the loss of energy due to scattering on heterogeneities and or the presence of faults and cracks the large q c at higher frequencies may be related to the propagation of backscattered body waves through deeper parts of the lithosphere where less heterogeneities are expected an important observation is that the region north of mct more rigid highly metamorphosed crystalline rocks is less attenuative in comparison to the region south of mct less rigid slightly metamorphosed rocks sedimentary wedge the acceleration decays to 50 at 20 km distance and to 7 at 100 km hence even 1g acceleration at the source may not cause significant damage beyond 100 km in this region

variations of seismic velocities in the kachchh rift zone gujarat india during 2001 2013 abstract we herein study variations of seismic velocities in the main rupture zone mrz of the mw 7 7 2001 bhuj earthquake for the time periods 2001 05 2006 08 2009 10 and 2011 13 by constructing dvp dvs and d vp vs tomograms using high quality arrival times of 28 902 p and 28 696 s waves from 4644 precise jhd joint hypocentral determination relocations of local events differential tomograms for 2001 05 reveal a marked decrease in seismic velocities low dvp low dvs and high d vp vs in the mrz at 5 35 km depths during 2001 10 which is attributed to an increase in crack fracture density higher pore fluid pressure resulted from the intense fracturing that occurred during the mainshock and post seismic periods while we observe a slight recovery or increase in seismic velocities 2011 13 this could be related to the healing process lower pore fluid pressure due to sealing of cracks of the causative fault zone of the 2001 bhuj mainshock the temporal reduction in seismic velocities is observed to be higher at deeper levels more fluid enrichment under near lithostatic pressure than that at shallower levels fluid source for low velocity zone lvz at 0 10 km depths with high d vp vs could be attributed to the presence of meteoric water or soft alluvium sediments with higher water content while fluid source for lvz at 10 35 km depths could be due to the presence of brine fluids released from the metamorphic dewatering and volatile co2 emanating from the crystallization of carbonatite melts in the asthenosphere in fractures and pores we also imaged two prominent lvzs associated with the katrol hill fault zone and island belt fault zone extending from shallow upper crust to sub crustal depth which might be facilitating the deeper circulation of metamorphic fluids volatile co2 thereby the generation of lower crustal earthquakes occurring in the kachchh rift zone

estimates of q in central asia as a function of frequency and depth using the coda of locally recorded earthquakes digital recordings of microearthquake codas from shallow and intermediate depth earthquakes in the hindu kush region of afghanistan were used to determine the attenuation factors of the s wave coda qc and primary s waves q an anomalously rapid decay of the coda shortly after the s wave arrival observed also in a study of coda in central asia by rautian and khalturin 1978 seems to be due primarily to depth dependent variations in qc in particular we deduce the average qc in the crust and uppermost mantle 100 km depth is approximately four times lower than the deeper mantle 400 km depth over a wide frequency range 0 4 to 24 hz further while qc generally increases with frequency at any depth the degree of frequency dependence of qc depends on depth except at the highest frequency studied here 48 hz the magnitude of qc at a particular frequency increases with depth while its frequency dependence decreases for similar depths determinations of q and qc agree suggesting a common wave composition and attenuation mechanism for s waves and codas comparison of these determinations of qc in afghanistan with those in other parts of the world shows that the degree of frequency dependence of qc correlates with the expected regional heterogeneity such a correlation supports the prejudice that qc is primarily influenced by scattering and suggests that tectonic processes such as folding and faulting are instrumental in creating scattering environments

a frequency dependent relation of coda q c for koyna warna region india attenuation properties of the lithosphere around the koyna warna seismic zone is studied by estimating the coda q c from 30 local earthquakes of magnitude varying from 1 5 to 3 8 an average lapse time of 65 sec used in the single scattering model sampled a circular area with an average radius of 114 km the estimated q c values show a frequency dependent relation q c 169 f0 77 and range from 169 at i hz to 1565 at 18 hz a comparison of worldwide q studies reveals that for a large frequency range the q for active regions is low as compared to that for stable regions however south carolina and norway are exceptions in that their q is low in the low frequency range while new england and north iberia are exceptions as they have a q value similar to that for active regions like spain turkey italy and garhwal himalaya stih in the higher frequency range in contrast to this the q for the koyna warna area which belongs to a stable region is low in the entire frequency range as compared to the stable regions and similar to the active stih regions

lg coda q variation across eurasia and its relation to crustal evolution an extensive set of broadband and short period seismograms from regional seismic events has been used to derive a tomographic image of broad scale variations of lg coda q for most of eurasia the image indicates that lg coda q at a frequency of 1 hz varies between about 200 and 1000 lowest q values 800 are found in four regions the east european shield the siberian shield the indian shield and the southwestern and oldest portion of the altaid belt the pattern of lg coda q variation across eurasia suggests that crustal q in any region is directly proportional to the length of time that has elapsed since the most recent episode of large scale compressional tectonic activity or upper mantle heating that occurred there we interpret the low q as resulting largely from hydrothermal fluids generated by tectonic activity or heating that now reside in cracks and permeable portions of the eurasian crust crustal q increases with time after that activity as fluids are lost to the surface or absorbed by retrograde metamorphism

attenuation of high frequency p and s waves in the gujarat region india the local earthquake waveforms recorded on broadband seismograph network of institute of seismological research in gujarat india have been analyzed to understand the attenuation of high frequency 2 25 hz p and s waves in the region the frequency dependent relationships for quality factors for p qp and s qs waves have been obtained using the spectral ratio method for three regions namely kachchh saurashtra and mainland gujarat the earthquakes recorded at nine stations of kachchh five stations of saurashtra and one station in mainland gujarat have been used for this analysis the estimated relations for average qp and qs are qp 105 2 f0 82 0 01 qs 74 2 f1 06 0 01 for kachchh region qp 148 2 f0 92 0 01 qs 149 14 f1 43 0 05 for saurashtra region and qp 163 7 f0 77 0 03 qs 118 34 f0 65 0 14 for mainland gujarat region the low q 0 5 as obtained from present analysis indicate the predominant seismic activities in the region the lowest q values obtained for the kachchh region implies that the area is relatively more attenuative and heterogeneous than other two regions a comparison between qs estimated in this study and coda q qc previously reported by others for kachchh region shows that qc qs for the frequency range of interest showing the enrichment of coda waves and the importance of scattering attenuation to the attenuation of s waves in the kachchh region infested with faults and fractures the qs qp ratio is found to be less than 1 for kachchh and mainland gujarat regions and close to unity for saurashtra region this reflects the difference in the geological composition of rocks in the regions the frequency dependent relations developed in this study could be used for the estimation of earthquake source parameters as well as for simulating the strong earthquake ground motions in the region

estimation of site response in the kachchh seismic zone gujarat india abstract the horizontal components of s wave data from 38 aftershocks m w 3 0 5 6 of the 2001 bhuj earthquake m w 7 7 recorded at 18 strong motion sites in the kachchh seismic zone gujarat india were analyzed to estimate the site response in the frequency range 0 2 10 0 hz using the reference site generalized inversion technique the site response values thus obtained at each site were averaged logarithmically over two frequency bands 0 2 1 8 and 3 0 7 0 hz respectively to study the spatial variation of site response in the kachchh seismic zone in the 0 2 1 8 hz frequency band the high site response values 1 0 1 7 were observed to be associated with the regions south of the kachchh mainland fault that suffered extensive ground failure during the 2001 m w 7 7 bhuj earthquake in the 3 7 hz frequency band the average site response estimates in the area of jurassic and quaternary formations vary from 0 22 to 1 53 whereas those of tertiary formation lie between 0 60 and 1 30 the maximum site response values at both 0 2 1 8 hz 1 7 and 3 7 hz 1 53 are estimated at the sikara site which lies in a zone of geological contact between jurrassic and tertiary formations the deamplification observed at the ams station in the 0 2 10 hz frequency band could be attributed to a very localized geological variation of the soil type or geometry of the basin near the site it is apparent that in both 0 2 1 8 and 3 7 hz frequency bands the presence of geological contacts and sediment thicknesses controls the site responses in the kachchh seismic zone it is also inferred that larger site response 1 2 values in the 0 2 1 0 hz frequency band could be indicating the probable presence of soil class c and d 180 v s 360 m sec in the kachchh basin according to 1997 national earthquake hazards reduction program nehrp provisions

low coda qc in the epicentral region of the 2001 bhuj earthquake of mw 7 7 on 26 january 2001 03 46 55 ut a devastating intraplate earthquake of m w 7 7 occurred in a region about 5 km nw of bhachau gujarat 23 42 n 70 23 e the epicentral distribution of aftershocks defines a marked concentration along an e w trending and southerly dipping 45 zone covering an area of 60 40 km2 the presence of high seismicity including two earthquakes of magnitudes exceeding 7 7 in the 200 years is presumed to have caused a higher level of shallow crustal heterogeneity in the kutch area a site lying in the seismic zone v zone of the highest seismicity for potentially m8 earthquakes on the seismic zoning map of india attenuation property of the medium around the epicentral area of the bhuj earthquake covering a circular area of 61 500 km2 with a radius of 140 km is studied by estimating the coda q c from 200 local earthquakes of magnitudes varying from 3 0 4 6 the estimated q 0 values at locations in the aftershock zone high seismicity are found to be low in comparison to areas at a distance from it this can be attributed to the fact that seismic waves are highly scattered for paths through the seismically active and fractured zone but they are well behaved outside the aftershock zone distribution of q 0 values suggests that the local variation in q 0 values is probably controlled by local geology the estimated q 0 values at different stations suggest a low value of q 102 0 80 f 0 98 0 02 indicating an attenuative crust beneath the entire region the frequency dependent relation indicates a relatively low q c at lower frequencies 1 3 hz that can be attributed to the loss of energy due to scattering attenuation associated with heterogeneities and or intrinsic attenuation due to fluid movement in the fault zone and fluid filled cracks the large q c at higher frequencies may be related to the propagation of backscattered body waves through deeper parts of the lithosphere where less heterogeneity is expected based on the attenuation curve estimated for q 0 102 the ground acceleration at 240 km distance is 13 of 1 g i e 0 13 g agreeing well with the ground acceleration recorded by an accelerograph at ahmedabad 0 11 g hence it is inferred that the q 0 value obtained from this study seems to be apt for prediction of ground motion for the region

results from local earthquake velocity tomography implications toward the source process involved in generating the 2001 bhuj earthquake in the lower crust beneath kachchh india to comprehend the source processes of the 26 january 2001 bhuj earthquake sequence of m w 7 7 and its influence on the seismic hazard of the kachchh we estimate various seismological parameters using reliable and accurate aftershock data the estimated parameters led to several important findings including the delineation of an east west trending south dipping 45 fault north wagad fault nwf which touches the surface about 25 km north of kachchh mainland fault kmf the aftershock zone is confined to a 60 km long and 40 km wide region lying between the kmf to the south and nwf to the north extending from 10 to 45 km depth focal mechanism solutions of the mainshock and 25 significant aftershocks of m w 3 0 obtained from waveform inversion of broadband data and local earthquake moment tensor inversion suggest that the region between the kmf and nwf is mainly characterized by reverse faults with east west trend and southerly dip matching with the geological faults in the region the tomographic inversion technique is used to invert 5516 p wave travel times and 4061 s p travel time differences from 600 aftershocks recorded at 8 18 stations tomographic results suggest a regional high velocity body characterized by high v p 7 0 8 5 km sec high v s 4 0 4 8 km sec and low 0 24 0 26 with a head extending 60 km in north south and 40 km in east west at 10 40 km depths this high velocity anomaly is inferred to be a mafic pluton rift pillow which might have intruded during the rifting time 135 ma this crustal mafic pluton must be contributing significantly in accumulating large crustal stresses resulting in the generation of large earthquakes in this intraplate area another important result of our study is the detection of a low velocity zone low v p 6 5 7 0 km sec low v s 3 6 4 0 km sec large 0 26 0 265 within the mafic body at the hypocentral depth of the mainshock 18 25 km which is inferred to be a fluid filled trapped aqueous fluid resulting from metamorphism fractured rock mass the analysis of depth distribution of b values suggests a high b value zone between 15 and 25 km depths which further supports this contention hence the presence of fluids at the hypocenter might have facilitated the occurrence of the 2001 bhuj earthquake within the inferred mafic body in the lower crust

tectonic evolution and mantle structure of the caribbean we investigate whether predictions of mantle structure from tectonic reconstructions are in agreement with a detailed tomographic image of seismic p wave velocity structure under the caribbean region in the upper mantle positive seismic anomalies are imaged under the lesser antilles and puerto rico these anomalies are interpreted as remnants of atlantic lithosphere subduction and confirm tectonic reconstructions that suggest at least 1100 km of convergence at the lesser antilles island arc during the past 45 myr the imaged lesser antilles slab consists of a northern and southern anomaly separated by a low velocity anomaly across most of the upper mantle which we interpret as the subducted north america south america plate boundary the southern edge of the imaged lesser antilles slab agrees with vertical tearing of south america lithosphere the northern lesser antilles slab is continuous with the puerto rico slab along the northeastern plate boundary this results in an amphitheater shaped slab and it is interpreted as westward subducting north america lithosphere that remained attached to the surface along the northeastern boundary of the caribbean plate at the muertos trough however material is imaged until a depth of only 100 km suggesting a small amount of subduction the location and length of the imaged south caribbean slab agrees with proposed subduction of caribbean lithosphere under the northern south america plate an anomaly related to proposed oligocene subduction at the nicaragua rise is absent in the tomographic model beneath panama a subduction window exists across the upper mantle which is related to the cessation of subduction of the nazca plate under panama since 9 5 ma and possibly the preceding subduction of the extinct cocos nazca spreading center in the lower mantle two large anomaly patterns are imaged the westernmost anomaly agrees with the subduction of farallon lithosphere the second lower mantle anomaly is found east of the farallon anomaly and is interpreted as a remnant of the late mesozoic subduction of north and south america oceanic lithosphere at the greater antilles aves ridge and leeward antilles the imaged mantle structure does not allow us to discriminate between an intra americas origin and a pacific origin of the caribbean plate key points tectonic reconstructions for the caribbean validated by seismic tomography cenozoic lesser antilles and mesozoic greater antilles subduction confirmed tomography gives insight into geodynamics of lesser antilles

identification and tectonic implications of a tear in the south american plate at the southern end of the lesser antilles 1 in the southeast corner of the caribbean westward subduction of atlantic oceanic south america beneath the lesser antilles transitions to east west transform motion between continental south america and the caribbean plate this geometry requires negatively buoyant subducting oceanic south american lithosphere to progressively detach from positively buoyant continental south american lithosphere the most widely accepted model is slab break off with oblique arc continent collision and northwest dipping continental subduction precipitating narrow rifting in the subducting slab in contrast the subduction transform edge propagator step model conceptualizes progressive detachment along a vertical dip slip tear through the lithosphere with stress focused at the edge of the propagating transform boundary we present four types of seismic data to resolve the ongoing lithospheric detachment local seismicity receiver functions wide angle seismic velocity inversion and a regional balanced cross section constrained by petroleum industry data these four data sets image a near vertical tear extending through the entire lithosphere revealing a key mechanism for the structural evolution of venezuela

stress drop variations and source scaling relations of moderate earthquakes of the indian tectonic plate we investigate the source characteristics of 53 earthquakes in and around the indian subcontinent through a grid search technique applied to the p and s wave displacement spectra of regional waveforms registered at 42 broadband seismic stations in addition we estimate the p and s wave attenuation factors representative of the various tectonic regions that constitute the indian shield we discuss the sensitivity of the displacement spectrum to various parameters and demonstrate that for a brune type source model the seismic moment is the best estimated parameter compared to others such as the corner frequency our estimates of moment magnitudes are within the range of 1014 1020 n m with the crack radii being less than 12 km the nonuniform distribution of the static stress drop along the himalaya burmese arc and the western indian regions indicates that the central himalayas are prone to frequent failures owing to the lesser stress drop in contrast with the northwestern himalaya and burmese arc regions our results suggest that median stress drops are independent of the earthquake mechanism online material list of events and associated estimated parameters

body wave crustal attenuation characteristics in the garhwal himalaya india we estimate frequency dependent attenuation of p and s waves in garhwal himalaya using the extended coda normalization method for the central frequencies 1 5 2 3 4 6 8 10 12 and 16 hz with earthquake hypocentral distance ranging from 27 to 200 km forty well located local earthquake waveforms were used to study the seismic attenuation characteristics of the garhwal himalaya india as recorded by eight stations operated by wadia institute of himalayan geology dehradun india from 2007 to 2012 we find frequency dependent p and s wave quality factors as defined by the relations q p 56 8f 0 91 0 002 and q s 151 8f 0 84 0 002 by fitting a power law frequency dependence model for the estimated values over the whole region both the q p and q s values indicate strong attenuation in the crust of garhwal himalaya the ratio of q s q p 1 obtained for the entire analyzed frequency range suggests that the scattering loss is due to a random and high degree of heterogeneities in the earth medium playing an important role in seismic wave attenuation in the himalayan crust

estimation of earthquake source parameters in the kachchh seismic zone gujarat india from strong motion network data using a generalized inversion technique the generalized inversion of s wave spectral amplitude data from 18 strong motion sites in the kachchh seismic zone gujarat india has been carried out to estimate source parameters of 38 aftershocks m w 2 93 5 32 from the 2001 bhuj earthquake m w 7 7 the result shows that the seismic moment m and source radius r of aftershocks are between 3 1 1013 and graphic 1 and 226 889 m respectively while stress drops vary from 0 11 to 7 44 mpa the regression analysis between m and r shows a break in linear scaling at graphic 2 the estimated stress drop values scatter significantly with m graphic 3 for larger aftershocks graphic 4 but vary in proportion with m graphic 5 in case of smaller magnitude events this can be due to the complex rupture process associated with relatively larger aftershocks in comparison to circular rupture for smaller events the high stress drop values are observed for events located on the north wagad fault at 15 30 km depths and gedi fault at 0 5 km depths this could be attributed to the large stress develop at the hypocentral depth as a result of high pore fluid pressure beneath these two fault zones the regression between m and corner frequency f c shows m is proportional to graphic 6 which is consistent for earthquakes with short rupture lengths and high stress drop values high stress drop values commonly associated with the events from kachchh and adjacent narmada son lineament rift zones indicate strong potential for generating high peak ground accelerations and hence high seismic hazard 1 embed inline graphic 1 gif 2 embed inline graphic 2 gif 3 embed inline graphic 3 gif 4 embed inline graphic 4 gif 5 embed inline graphic 5 gif 6 embed inline graphic 6 gif

seismic wave attenuation in the greater cairo region egypt in the present study a digital waveform dataset of 216 local earthquakes recorded by the egyptian national seismic network ensn was used to estimate the attenuation of seismic wave energy in the greater cairo region the quality factor and the frequency dependence for coda waves and s waves were estimated and clarified the coda waves q c and s waves q d quality factor were estimated by applying the single scattering model and coda normalization method respectively to bandpass filtered seismograms of frequency bands centering at 1 5 3 6 12 18 and 24 hz lapse time dependence was also studied for the area with the coda waves analyzed through four lapse time windows 10 20 30 and 40 s the average quality factor as function of frequency is found to be q c 35 9f 0 9 0 02 and q d 10 2f 0 9 0 02 for coda and s waves respectively this behavior is usually correlated with the degree of tectonic complexity and the presence of heterogeneities at several scales the variation of q c with frequency and lapse time shows that the lithosphere becomes more homogeneous with depth in fact by using the coda normalization method we obtained low q d values as expected for a heterogeneous and active zone the intrinsic quality factor q i 1 was separated from the scattering quality factor q s 1 by applying the multiple lapse time domain window analysis mltwa method under the assumption of multiple isotropic scattering with uniform distribution of scatters the obtained results suggest that the contribution of the intrinsic attenuation q i 1 prevails on the scattering attenuation q s 1 at frequencies higher than 3 hz

variation of seismic coda wave attenuation in the garhwal region northwestern himalaya seismic coda wave attenuation q text c 1 characteristics in the garhwal region northwestern himalaya is studied using 113 short period vertical component seismic observations from local events with hypocentral distance less than 250 km and magnitude range between 1 0 to 4 0 they are located mainly in the vicinity of the main boundary thrust mbt and the main central thrust mct which are well defined tectonic discontinuities in the himalayas coda wave attenuation q text c 1 is estimated using the single isotropic scattering method at central frequencies 1 5 3 5 7 9 12 16 20 24 and 28 hz using several starting lapse times and coda window lengths for the analysis results show that the q text c 1 values are frequency dependent in the considered frequency range and they fit the frequency power law q text c 1 left f right q 0 1 f n the q0 qc at 1 hz estimates vary from about 50 for a 10 s lapse time and 10 s window length to about 350 for a 60 s lapse time and 60 s window length combination the exponent of the frequency dependence law n ranges from 1 2 to 0 7 however it is greater than 0 8 in general which correlates well with the values obtained in other seismically and tectonically active and highly heterogeneous regions the attenuation in the garhwal region is found to be lower than the qc 1 values obtained for other seismically active regions of the world however it is comparable to other regions of india the spatial variation of coda attenuation indicates that the level of heterogeneity decreases with increasing depth the variation of coda attenuation has been estimated for different lapse time and window length combinations to observe the effect with depth and it indicates that the upper lithosphere is more active seismically as compared to the lower lithosphere and the heterogeneity decreases with increasing depth

estimation of source parameters for the aftershocks of the 2001 mw 7 7 bhuj earthquake india the source parameters for 213 bhuj aftershocks of moment magnitude varying from 2 16 to 5 74 have been estimated using the spectral analysis of the sh waveform on the transverse component of the three componnet digital seismograms as well as accelerograms the estimated stress drop values for bhuj aftershocks show more scatter mo0 5 to 1 toward the larger seismic moment values log mo 1014 5 n m larger aftershocks whereas they show a more systematic nature mo3 for smaller seismic moment log mo 15 mpa lying in 22 26km depth range beneath the region we suggest that the concentration of large stress drop values at 10 36km depth may be related to the large stress strain associvated with a brittle competent intrusive body of mafic nature

shear wave attenuation characteristics over the central india tectonic zone and its surroundings abstract the study area lies between latitude 18 26 n and longitude 73 83 e and mainly covers the central india tectonic zone citz the frequency dependent shear wave quality factor qs has been estimated over the citz and its surroundings using double spectral ratio dsr method we have considered 25 local earthquakes with magnitude ml varies from 3 0 to 4 7 recorded at 11 stations running under national seismic network the fast fourier transformed fft spectra were computed from the recorded waveform having time window from onset of s phase to 1 0 s and for a frequency band of 0 1 10 hz three different shear wave velocities i e 3 87 3 39 and 3 96 km s were obtained over the study area based on a pair of earthquakes recorded at a pair of stations the low qs values of 51 96 at 1 hz i e qs 51f0 49 qs 90f0 488 and qs 96f0 53 were found in the area covering the son narmada tapti sonata lineament citz eastern part of the satpura fold belt vindhyan and gondwana basins godavari and mahanadi grabens and southern part of gangetic plain intermediate qs values of the order of 204 277 i e qs 204f0 56 and qs 277f0 55 were noted in the cartonic areas namely bundelkhand dharwar bhandara and bastar while the higher qs values of 391 628 at 1 hz i e qs 391f0 49 qs 409f0 48 qs 417f0 48 qs 500f0 66 qs 585f0 65 and qs 628f0 69 were found in the eastern part of the sonata citz and the northeastern part of the satpura fold belt the low qs values might be attributing to the more heterogeneous sonata rift system low qs values further may presumably be associated with lower level of seismicity and apparently account for higher tectonic stress accumulation over long duration the long term accumulated stress is generally released through occasional triggering of moderate magnitude earthquakes in the sonata zone surrounding the sonata region the higher qs values possibly accounts for a more homogeneous subsurface structure along the sonata zone

characteristics of deformation and past seismicity associated with the 1819 kutch earthquake northwestern india the 1819 earthquake in kutch northwestern india is one of the most significant events to have occurred in a plate interior setting despite being the second largest among the stable continental region scr earthquakes this event has not been analyzed within the context of present day understanding of earthquake seismology coseismic changes related to this earthquake include massive ground deformation in a wide low lying tidal flat area although detailed historic accounts of this earthquake exist many questions regarding the mode of deformation and the seismic history of the region remain unresolved we explored the region nearly 180 years after the earthquake and the information gathered adds to our understanding of this event and provides a fresh perspective on this unique intraplate seismogenic zone a 90 km long tract of elevated land with a peak height of 4 3 m is the most visible surface expression of this earthquake we surveyed and analyzed the morphological features of this scarp and also carried out exploratory trenching in this region the scarp morphology is suggestive of a growing fold related to a buried north dipping thrust rather than a discrete fault that could have resulted from a surface rupture the extensive liquefaction field associated with the earthquake offered an ideal setting to explore the paleoearthquake history age data of liquefaction features suggest that a previous event of comparable size must have occurred 800 1000 years ago seismic activity appears to be related to the reactivation of an ancient rift in a stress regime that is dominated by nearly north south compression

sensitivity analysis of seismic hazard for the northwestern portion of the state of gujarat india we test the sensitivity of seismic hazard to three fault source models for the northwestern portion of gujarat india the models incorporate different characteristic earthquake magnitudes on three faults with individual recurrence intervals of either 800 or 1600 years these recurrence intervals imply that large earthquakes occur on one of these faults every 266 533 years similar to the rate of historic large earthquakes in this region during the past two centuries and for earthquakes in intraplate environments like the new madrid region in the central united states if one assumes a recurrence interval of 800 years for large earthquakes on each of three local faults the peak ground accelerations pga horizontal and 1 hz spectral acceleration ground motions 5 damping are greater than 1 g over a broad region for a 2 probability of exceedance in 50 years hazard level these probabilistic pgas at this hazard level are similar to median deterministic ground motions the pgas for 10 in 50 years hazard level are considerably lower generally ranging between 0 2 g and 0 7 g across northwestern gujarat ground motions calculated from our models that consider fault interevent times of 800 years are considerably higher than other published models even though they imply similar recurrence intervals these higher ground motions are mainly caused by the application of intraplate attenuation relations which account for less severe attenuation of seismic waves when compared to the crustal interplate relations used in these previous studies for sites in bhuj and ahmedabad magnitude m 7 3 4 earthquakes contribute most to the pga and the 0 2 and 1 s spectral acceleration ground motion maps at the two considered hazard levels d 2004 elsevier b v all rights reserved

estimation of sedimentary thickness in kachchh basin gujarat using sp converted phase an inexpensive method using natural earthquake data is utilized for determining the sedimentary thickness in kachchh the institute of seismological research isr is operating a network of broadband seismographs and strong motion accelerographs in gujarat we used data from 13 broadband seismographs and two strong motion accelerographs in the study the stations are within 5 to 80 km from the epicenters in this study the s to p converted phase sp is used this phase is generated due to large impedance contrast between sediments and basement this phase is clear in the vertical component the difference in the travel times of s and sp phases and velocities of p and s waves is used for determining the sedimentary layer thickness the thickness of sediments beneath each of these 15 stations was determined covering an area of 23 500 sq km

attenuation characteristics of coda waves in mainland gujarat india abstract the attenuation characteristics based on coda waves of mainland gujarat india have been investigated in the present study the broadband waveforms of 53 local earthquakes mw 1 1 3 3 having focal depths in the 6 0 33 6 km range recorded at five stations of mainland gujarat region has been used for the analysis the frequency dependent relationships q q0fn for coda q qc and dependency of coda q on lapse time windows have been determined for the said region the average lapse time dependent coda q relations estimated for the region are qc 87 13 f 1 01 0 06 lapse time 30 s qc 112 20 f 0 94 0 08 lapse time 40 s and qc 120 22 f 0 76 0 07 lapse time 50 s the increase in qc values with lapse time shows the depth dependence of qc as longer lapse time windows will sample larger area the observed quality factor is strongly dependent on frequency and lapse time which indicates that the upper lithosphere is more heterogeneous and seismotectonically active while the lower lithosphere is homogeneous and relatively less active a comparison of the coda q estimated for mainland gujarat region with those of nearby kachchh and saurashtra regions shows that mainland gujarat region is more heterogeneous the rate of decay of attenuation q 1 with frequency for the relations obtained here is found to be comparable with those of other regions of the world though the absolute values differ the obtained relations are expected to be useful for the estimation of source parameters of the earthquakes in the mainland gujarat region where no such relations were available earlier these relations are also important for the simulation of earthquake strong ground motions in the region

sediment thicknesses and qs vs qp relations in the kachchh rift basin gujarat india using sp converted phases delineation of the top sedimentary structure and its qs vs qp relationship using the travel time difference of direct s and converted sp phase is key to understanding the seismic hazard of any sedimentary basin area we constructed filtered displacement waveforms from local etna episensor acceleration recordings as well as local velocity recordings of aftershocks of the 2001 bhuj earthquake recorded by the kachchh seismological network of the national geophysical research institute ngri hyderabad india during 2001 2004 stations are within 15 70km of epicenters and the resulting displacement waveforms are generally simple displaying prominent p sp and s wave pulses particle motion of p and s waves suggest near vertical raypaths consistent with preliminary depth estimates the direct s wave on the horizontal component is characterized by lower frequency content than the converted sp phase on the vertical component this difference in frequency content between s and sp phases can be explained in terms of different attenuation effects for p and s waves in the unconsolidated sediments the sp phase is generated by s to p phase conversion at the base of mesozoic sediments of the kachchh basin travel time inversion velest of 2565 p and 2380 s arrivals from 658 well located aftershocks recorded at 8 14 three component local seismic stations led to 1 d velocity models indicated very slow sediments in the upper 0 2 km depth range vp 2 92 km s and vs 0 90 km s and an increasing trend of velocities with depth at 2 40 km depth the estimated sediment thicknesses beneath 12 accelerograph and 6 seismograph sites from the estimated velocity model and the travel time difference between s and converted sp phases reaches a maximum of 1 534 0 117 km beneath bandri near the location of 2001 bhuj mainshock and attains a minimum sediment thickness of 0 858 0 104 km beneath ramvav and burudia the spectral ratios between sp and s from 159 three component accelerograms have been used to study seismic wave attenuation in the kachchh rift basin the estimated qs vs qp relations for 12 accelerograph sites vary from qs 0 184 qp at chobari to qs 0 505 qp at dudhai for stations chobari chopdwa jahawarnagar vondh and tapar the spectral ratio slopes and hence the calculated qs vs qp relations are effectively the same and the correlation coefficients are quite high 0 91 0 93 stations adhoi manfara new dudhai dudhai and sikara have similar qs vs qp relationships to each other and also have high correlation coefficients 0 78 0 87 the spectral ratios for stations anjar and ramvav are small and poorly constrained resulting in less reliable qs vs qp relations this could be due to noisy data fewer available waveforms or scattering due to velocity heterogeneities and or interface irregularities

reduced seismic velocities in the source zone of new madrid earthquakes a three dimensional inversion of p wave travel time residuals from local earthquakes reveals a remarkable pattern of low seismic velocities in crustal rocks immediately adjacent to the active portions of the new madrid fault system seismic velocities are lowest in regions of greatest concentration of earthquake activity near two intersections of linear trends in seismicity the maximum reduction in compressional wave velocity is at least 7 per cent in the upper 5 km of the crust and at least 4 per cent in the depth range of 5 to 14 km the reductions are consistent with a velocity decrease which would be expected if crustal rocks in the source zone contain fluid filled cracks in which pore pressure is a substantial fraction of external pressure the presence or absence of such fluids may explain why some portions of the faults in and surrounding the upper mississippi embayment are active while others are not

estimation of coda wave attenuation for the national capital region delhi india using local earthquakes attenuation of seismic waves is very essential for the study of earthquake source parameters and also for ground motion simulations and this is important for the seismic hazard estimation of a region the digital data acquired by 16 short period seismic stations of the delhi telemetric network for 55 earthquakes of magnitude 1 5 to 4 2 which occurred within an epicentral distance of 100 km in an area around delhi have been used to estimate the coda attenuation qc using the single backscattering model the seismograms have been analyzed at 10 central frequencies the frequency dependence average attenuation relationship qc 142f1 04 has been attained four lapse time windows from 20 to 50 seconds duration with a difference of 10 seconds have been analyzed to study the lapse time dependence of qc the qc values show that frequency dependence exponent n remains similar at all the lapse time window lengths while the change in q0 values is significant change in q0 with larger lapsetime reflects the rate of homogeneity at the depth the variation of qc indicates a definitive trend from west to east in accordance with the geology of the region

stability of coda q in the region of parkfield california view from the u s geological survey parkfield dense seismograph array many investigators have proposed that changes in the rate at which the coda decays may be an intermediate term precursor to moderate to large earthquakes parkfield california on the san andreas fault is a promising location for studying premonitory changes in coda q qc because a large earthquake is likely to occur there we have investigated qc using recordings from the u s geological survey parkfield dense seismograph array which is a digital array with 14 triaxial sensors and an aperture of about 1 km for each earthquake we can measure qc from up to 42 recordings their average is more stable than the measurement from a single station using clustered seismicity we have developed criteria for selecting events and reducing scatter in the measurement the qc value determined from a seismogram depends on the position and length of the analysis window thus qc should always be measured from the same length window starting at the same lapse time regardless of the source location in addition the band limited signal to noise ratio at the end of the analysis window is important qc determined in two frequency bands 4 8 hz and 8 16 hz from a tight cluster of 26 events which occurred between december 1989 and january 1994 has not changed despite m 4 7 and m 4 6 events in october 1992 and november 1993 qc measured from local events 60 km in three frequency bands shows larger scatter but has also not changed during this period for monitoring qc observations should include array averaged measurements from a single lapse time because qc measurements made using an analysis window that starts at a constant multiple of the s wave lapse time depend on epicentral distance a procedure combining the evaluation of the time and distance dependences of qc also gives stable observations

coda wave seismic structure beneath the indian ocean region and its implications to seismotectonics and structural heterogeneity abstract the indian ocean region is one of the challenging areas for seismological research because of its complex seismotectonic settings and intricate structural heterogeneity prevails beneath the indian ocean region in order to address the problem we attempted to determine the attenuation structure for the indian ocean region using coda wave generation by single backscattering model in this study the central frequency of interest has been considered between 1 5 and 12 0 hz with lapse time window ltw of 20 30 40 50 and 60 s measured from the origin time of the earthquake we obtained the attenuation relation for different seismotectonic zones associated with varying estimate of attenuation parameters spreading zone qc spz 171f1 10 subduction zone qc sz 220f0 961 triple point junction qc tpj 156f1 15 prominent faults of indian ocean region qc pfioz 197f0 99 and volcanic and hotspot qc vz hs 209f0 97 we estimated average frequency dependent attenuation parameters of q0 that varies as 156 32 q0 220 33 with corresponding variability in frequency parameter as 0 96 0 05 1 15 0 09 at 40 s ltw for the entire indian ocean region suggesting that the entire indian ocean is seismo tectonically highly active and is attributed to the nature and extent of attenuation due to varying amount of structural heterogeneity in different tectonic zones characterized by the occurrence of earthquakes of varying strengths in the seismically active tectonic zones of the region we found that the triple junction rtj is associated with low qc and high attenuation because of deposition of a variety of sediments at the rtj which is in unison to the geological mapping of the region we observed that our estimated attenuation model is comparable that of the aleutian islands the adak seismic zone at low frequency 5 hz and to the philippine sea subduction zone petukhin at higher frequency range 5 hz which is very much correlative to seismo tectonic settings we infer that our assimilated attenuation model has potential to provide information on intricate seismotectonics and seismogenesis for computing earthquake source parameters and earthquake hazards that may be useful for assimilating detailed seismic velocity structure beneath the indian ocean region

quantifying the net slab pull force as a driving mechanism for plate tectonics 1 it has remained unclear how much of the negative buoyancy force of the slab fb is used to pull the trailing plate at the surface into the mantle here i present three dimensional laboratory experiments to quantify the net slab pull force fnsp with respect to fb during subduction results show that fnsp increases with increasing slab length and dip up to 8 12 of fb making fnsp up to twice as large as the ridge push force the remainder of fb is primarily used to drive rollback induced mantle flow 70 to bend the subducting plate at the trench 15 30 and to overcome shear resistance between slab and mantle 0 8

energy flux model of seismic coda separation of scattering and intrinsic attenuation a new model of seismic coda is presented based on the balance between the energy scattered from the direct wave and the energy in the seismic coda this energy flux model results in a simple formula for the amplitude and time decay of the seismic coda that explicitly differentiates between the scattering and intrinsic anelastic attenuation of the medium this formula is valid for both weak and strong scattering and implicitly includes multiple scattering the model is tested using synthetic seismograms produced in finite difference simulations of wave propagation through media with random spatial variations in seismic velocity some of the simulations also included intrinsic dissipation the energy flux model explains the coda decay and amplitude observed in the synthetics for random media with a wide range of scattering q in contrast the single scattering model commonly used in the analysis of microearthquake coda does not account for the gradual coda decay observed in the simulations for media with moderate or strong scattering attenuation scattering q less than or equal to 150 the simulations demonstrate that large differences in scattering attenuation cause only small changes in the coda decay rate as predicted by the energy flux model the coda decay rate is sensitive however more to the intrinsic q of the medium the ratio of the coda amplitude to the energy in the direct arrival is a measure of the scattering attenuation thus analysis of the decay rate and amplitude of the coda can in principle produce separate estimates for the scattering and intrinsic q values of the crust we analyze the coda from two earthquakes near anza california intrinsic q values determined from these seismograms using the energy flux model are comparable to coda q values found from the single scattering theory the results demonstrate that coda q values are at best measures of the intrinsic attenuation of the lithosphere and are unrelated to the scatteing q less

attenuative dispersion of p waves in and near the new madrid seismic zone a procedure for measuring the attenuative dispersion of body wave phases has been developed and applied to panda recordings of direct p waves in the new madrid seismic zone we have determined relative group delays of spectral components of those waves for 152 paths and related that observed dispersion to seismic wave attenuation using an expression we have derived for a continuous relaxation model the group delays are sensitive to batchmode documentclass fleqn 10pt legalpaper article usepackage amssymb usepackage amsfonts usepackage amsmath pagestyle empty begin document q mathrm m 1 end document the value of q 1 over the flat portion of the attenuation spectrum and 1 2 the frequency of the half amplitude point at the high frequency end of the spectrum q m increases with epicentral distance from about 45 to 145 between 10 and 60 km suggesting an increase of crustal p wave q with depth in addition q m values for paths that are entirely or mostly confined to the active portion of the new madrid seismic zone are lower than for paths that traverse a significant portion of crust outside the active region the reduced q in the source zone of new madrid earthquakes can be explained by fluids in permeable rock a mechanism previously proposed by al shukri and mitchell 1988 to explain reduced velocities there a comparison of p wave q values reported in this study of attenuative dispersion with those previously deduced from the decay of spectral amplitudes shows that mean values obtained by the two methods are about the same but that q values from attenuative dispersion measurements exhibit less scatter

the steeply subducting edge of the cocos ridge evidence from receiver functions beneath the northern talamanca range south central costa rica the deep structure of the south central costa rican subduction zone has not been studied in great detail so far because large parts of the area are virtually inaccessible we present a receiver function study along a transect of broadband seismometers through the northern flank of the cordillera de talamanca south costa rica below moho depths the receiver functions image a dipping positive conversion signal this is interpreted as the subducting cocos plate slab compatible with the conversions in the individual receiver functions in finite difference modeling a dipping signal such as the one imaged can only be reproduced by a steeply 80 dipping structure present at least until a depth of about 70 100 km below this depth the length of the slab cannot be determined because of possible scattering effects the proposed position of the slab agrees with previous results from local seismicity local earthquake tomography and active seismic studies while extending the slab location to greater depths and steeper dip angle along the trench no marked change is observed in the receiver functions suggesting that the steeply dipping slab continues until the northern flank of the cordillera de talamanca in the transition region between the incoming seamount segment and cocos ridge considering the time predicted for the establishment of shallow angle underthrusting after the onset of ridge collision the southern costa rican subduction zone may at present be undergoing a reconfiguration of subduction style where the transition to shallow underthrusting may be underway but still incomplete

three dimensional attenuation structure in and around the new madrid seismic zone abstract digital seismograms from the central mississippi valley seismic network have been used to study the three dimensional attenuation structure of the new madrid seismic zone and its surroundings attenuation time t values were calculated for each of 252 seismograms using a spectral decay technique assuming that their values are independent of frequency the observed t values were inverted using a stochastic least squares inversion scheme to determine a three dimensional image of the attenuation structure of the upper crust homogeneous and heterogeneous starting models of the upper crust produce similar images of that structure the resulting models include a substantial increase of attenuation within and close to the zones that contain the most active faults of the new madrid seismic zone this region was previously found al shukri and mitchell 1988 to be characterized by lower velocities than surrounding regions the reduced velocities and increased attenuation values in the active portion of the new madrid seismic zone are explained as being caused by fluid filled cracks in the upper crust the presence or absence of fluid filled cracks may govern whether or not faults in this region are active or inactive

the deadliest intraplate earthquake the recent earthquake in the bhuj region took a toll of at least 30 000 lives in their perspective gupta et al 1 discuss what may be the cause of the large but infrequent earthquakes in this region which is not near any tectonic plate boundary the earthquake hazard in the region was well recognized before the disaster however and adherence to the building codes for high risk areas may have prevented much human tragedy 1 http www sciencemag org cgi content full 291 5511 2101

source parameters of the anjar earthquake of july 21 1956 india and its seismotectonic implications for the kutch rift basin abstract the kutch rift basin is a major seismic region in india characterized by a low earthquake occurrence rate but a very high maximum intensity the largest event in the region is the great kutch earthquake of 1819 estimated m w 7 8 however lacking instrumental data its exact source parameters are not well known the anjar or rann of kutch earthquake of july 21 1956 m w 6 0 took place very close to the 1819 kutch event and caused considerable damage and casualties our modeled waveforms of teleseismic p and sh waves and p wave first motions determine a reverse faulting mechanism strike 235 dip 46 5 and rake 85 as well as source parameters for the anjar earthquake a previous strike slip mechanism for this event has been eliminated on the basis of waveform constraints the two nodal planes of our solution both strike northeast sub parallel to the trend of the kutch rift very close to the trend of the isoseismals the seismic moment is 1 19 10 25 dyne cm the focal depth and the stress drop are 15 km and 162 bar respectively the anjar reverse fault may represent a reactivated normal fault undergoing tectonic inversion in the kutch rift zone under present day compressional stress two larger earthquakes adjacent to anjar but outside of the kutch rift are also found to be mainly reverse faulting with a minor strike slip component judging from available information the great kutch event of 1819 is most likely a reverse faulting event

a spectral analysis of the 21 may 1997 jabalpur india earthquake mw 5 8 and estimation of ground motion from future earthquakes in the indian shield region the jabalpur earthquake 23 08 n 80 06 e h 36 km is the first event in the indian peninsular shield region to be well recorded by a newly installed 10 station broadband seismographic network using these data we estimate q of lg wave in the indian shield region as q 508 0 48 1 20 hz the corrected source spectrum with m 5 4 1024 dyne cm reported in the harvard cmt catalog and an 2 source model requires a stress parameter p of 420 bar to explain high frequency spectral level the computed seismic energy from the records is 7 4 1020 erg which yields an apparent stress and brune stress drop of 62 and 270 bars respectively the analysis of the jabalpur earthquake provides some elements for the estimation of ground motions during future earthquakes in the indian shield region based on the results of the jabalpur earthquake and on studies of tectonically similar region of eastern north america we assume that 1 the sources follow an 2 model 2 s waves dominate at r 100 km and lg waves dominate at r 100 km 3 q f 508 0 48 4 the ground motion is a bandlimited finite duration gaussian white noise and 5 the effective duration of the ground motion equals 1 c 0 05 r where fc is the corner frequency we apply random vibration theory rvt to compute various measures of ground motion such as a max and v max at near source distances the source finiteness is approximately taken into account the attenuation curves for 5 5 mw 7 and for p of 100 400 bar are presented as expected the predicted values with p 420 bar agree reasonably well with the limited jabalpur data an a max of 150 gal is predicted in the epicentral region of this earthquake the predicted curves imply p 100 bar for the latur earthquake of 1993 mw 6 1 h 2 5 km to explain a max 1 g and the reported isoseismal intensities in the epicentral region for koyna earthquake of 1967 mw 6 3 the inferred a max and v max from isoseismal intensities and the recorded strong motions at the koyna dam site are in agreement with the prediction curves for p 100 bar the rvt predictions seem reasonable but need validation from more strong motion data which is presently lacking

ground motion attenuation relation from strong motion records of the 2001 mw 7 7 bhuj earthquake sequence 2001 2006 gujarat india predictive relations are developed for peak ground acceleration pga from the engineering seismoscope srr records of the 2001 mw 7 7 bhuj earthquake and 239 strong motion records of 32 significant aftershocks of 3 1 mw 5 6 at epicentral distances of 1 r 288 km we have taken advantage of the recent increase in strong motion data at close distances to derive new attenuation relation for peak horizontal acceleration in the kachchh seismic zone gujarat this new analysis uses the joyner boore s method for a magnitude independent shape based on geometrical spreading and anelastic attenuation for the attenuation curve the resulting attenuation equation is eqalign ln hbox y 7 9527 1 4043 hbox m rm w ln left hbox r rm jb 2 19 82 2 right 1 2 0 0682 hbox s hfill cr hbox for 3 1 hbox m rm w le 7 7 quad quad hbox std hbox dev left sigma right pm 0 8243 hfill cr where y is peak horizontal acceleration in g mw is moment magnitude rjb is the closest distance to the surface projection of the fault rupture in kilometers and s is a variable taking the values of 0 and 1 according to the local site geology s is 0 for a rock site and s is 1 for a soil site the relation differs from previous work in the improved reliability of input parameters and large numbers of strong motion pga data recorded at short distances 0 50 km from the source the relation is in demonstrable agreement with the recorded strong ground motion data from earthquakes of mw 3 5 4 1 4 5 5 6 and 7 7 there are insufficient data from the kachchh region to adequately judge the relation for the magnitude range 5 7 mw 7 7 but our ground motion prediction model shows a reasonable correlation with the pga data of the 29 march 1999 chamoli main shock mw 6 5 validating our ground motion attenuation model for an mw6 5 event however our ground motion prediction shows no correlation with the pga data of the 10 december 1967 koyna main shock mw 6 3 our ground motion predictions show more scatter in estimated residual for the distance range 0 30 km which could be due to the amplification noise at near stations situated in the kachchh sedimentary basin we also noticed smaller residuals for the distance range 30 300 km which could be due to less amplification noise at sites distant from the kachchh basin however the observed less residuals for the longer distance range 100 300 km are less reliable due to the lack of available pga values in the same distance range