source parameters and scaling relations for small earthquakes in the matsushiro region southwest honshu japan abstract using near field broadband data from a suite of crustal earthquakes which occurred in a small homogeneous source region we examine the scaling relationships among source parameters for earthquakes in the critical 1 5 to 4 5 magnitude range we take special precautions to insure that our conclusions are not biased by local structure seismic moment fault radius and stress drop relations were determined for 32 small shallow strike slip earthquakes 1 6 m l 4 3 in the matsushiro region of southwest honshu japan spectral and cepstral analyses of short period arrivals exhibit structure interpreted as resulting from reverberations within thin low velocity surface layers below the recording site furthermore calculated crustal transfer functions for sh predict a peak over the range of 2 to 4 hz this type of site response can bias estimates of spectral parameters particularly corner or characteristic frequencies for simple sources such a bias can be reduced by filtering in the cepstral domain the effect of cepstral filtering on corner frequency determinations is generally less than 20 per cent and tends to decrease them particularly for larger events where the structurally induced spectral enhancement is at frequencies above the source related corner frequencies estimates of the seismic moment brune stress drop and fault radius for each event were calculated from sh displacement amplitude spectra the seismic moments range from 10 18 to 10 22 dyne cm the stress drops range from 0 008 to 16 bars the relationship between log stress drop and log moment is approximately linear with a slope of 1 up to a moment of 2 10 21 dyne cm but for higher moments the slope is not significantly different from zero fault radii range from 0 3 to 1 2 km there is a physical scatter in radius for any given moment up to a factor of 1 5 for moments less than 2 10 21 dyne cm there is no significant trend in the radius as a function of moment but for higher moments the radius increases with moment the main conclusion is that the stress drop is the dominant scaling factor for moments less than 10 21 dyne cm for higher moment the source dimension becomes the controlling factor

a comparison between the tectonic stress measured in situ and stress parameters from induced seismicity at monticello reservoir south carolina during may and early june 1979 five three component digital seismographs were deployed near the monticello reservoir south carolina to determine the stress drops of reservoir induced earthquakes and compare them with earlier hydrofracture stress measurements of zoback and hickman this issue records for 327 events were obtained 10 of these events each recorded on four or more stations were analyzed to determine stress drops depths for the 10 events ranged from 0 5 to 1 4 km fault plane orientations were calculated from an inversion of the body wave displacements individual radiation pattern corrections were applied in calculating the moments at each station final estimates of seismic moment were obtained by averaging logarithmically the estimates from each component for each event an arithmetic moment was obtained from the individual estimates of the source radius and stress drop was then calculated once from the final estimates of seismic moment and source radius for each event moments of the 10 digitally recorded earthquakes ranged from 2 5 1017 to 2 7 1018 dyne cm for the p wave data and from 4 6 1017 to 4 6 1018 dyne cm for the s wave data the stress drops s wave data only generally ranged from about 0 2 to 4 bars four earthquakes were recorded by a strong motion instrument at the dam site magnitudes m for these events ranged from 2 8 to 3 0 moments taken as averages of the values from each component ranged from 1 7 to 3 4 1020 dyne cm and the stress drops from 13 to 92 bars large differences between the corner frequencies of the two components factors of 2 5 and 4 for two of the events correspond to a large relative uncertainty for these stress drop values nevertheless tens of bars of stress are apparently released for the largest events in the monticello sequence but only a few bars are released for the smaller magnitude events in comparison the in situ stress data measured in two holes give an estimate of shear stress of about 45 bars at 0 2 km depth and 30 to 70 bars at 1 km depth in the northern hole and from 15 20 bars near the surface to almost 70 bars at depth in the southern hole the variation in shear stress inferred from in situ measurements at the deeper levels indicates an inconsistency between the thrust faulting mechanisms of the earthquakes and the state of stress which is more consistent with thrust faulting at 0 2 km depth and strike slip or normal faulting below thus the estimates of shear stress from in situ measurements are of the same order as the stress drops of the m 2 8 3 0 events but about 1 order of magnitude larger than the stress drops obtained for the smaller events

high frequency 10 hz content of the initial fifty seconds of waveforms from the 2011 off the pacific coast of tohoku earthquake in our recent article regarding high frequency content of the initial fifty seconds of waveforms from the 2011 off the pacific coast of tohoku earthquake iwakiri and hoshiba 2012 figure 2a b y axis labels on upper and lower parts shown as cm s s2 in the print journal and in the initial online edition should have been cm s s for the upper part and cm s s s for the lower part figure 4 y axis labels on the right and left parts for each station

occurrence of low stress drop earthquakes in the garhwal himalaya region abstract the digital seismic data recorded by a telemetered seismic array has been used to compute seismic moments and stress drops of local earthquakes in the garhwal himalaya region based on brune s model of the earthquake source the seismic moments for 18 shallow focus earthquakes range from 7 10 18 dyn cm to 6 23 10 21 dyn cm these earthquakes are found to have very low stress drops less than 1 bar for seven events and between 1 bar and 10 bars for another seven events the maximum stress drop is found to be 38 bars for an earthquake of magnitude 4 2 with focal depth 15 km the shallow focus events having low stress drops in this region indicate that the crust has low strength to withstand the accumulated strain energy

source characteristics of the nw himalaya and its adjoining region geodynamical implications abstract in this study we propose a novel interpretation of source parameters estimated using brune model for the nw himalaya and its adjoining region analysis of 157 events 2 2 mw 4 9 resulted in the scaling relationship between seismic moment m0 and corner frequency fc for the region as m0 8 1022 fc 2 89 which is in good unison to the garhwal and the kumaon himalaya with analogous seismogenic potential variability of kappa 0 01 s 0 08 s with varying number of events 2 5 m 5 0 at different depths is indicative of nature and extent of structural heterogeneity beneath the source zone which controls the propensity of background seismicity that provides enough testimony to complex geodynamical set up of the nw himalaya and its adjoining region source radii of source zone varying from 148 m to 975 m with static stress drops calculated for a given constant stress of 0 1 bar 1 bar 10 bar and 100 bar correspond to an average stress drop varied from 0 4 bar to 99 9 bar seismic moments of events vary from 2 64 1019 dyne cm to 2 32 1023 dyne cm with increase of the source radii we infer that variability in stress drop for seismic moment mo is attributed to different seismic sources of varying strengths associated with different structural heterogeneities stress drop of the events has good correspondence with source depth having the linear relationship of shear stress regime in the brittle crust with stress drop and source depth suggesting the presence of intricate subsurface seismogenic sources beneath the nw himalaya and its adjoining region

the effect of crustal scattering on observed high frequency earthquake seismograms summary a comparison of high frequency 1 100 hz local out to 40 km earthquake seismograms at stations of the southern black forest digital seismic network is made in an attempt to isolate the effects of near receiver structure and of crustal scattering on the waveforms of the direct body waves and on the coda waves angle of incidence and azimuth of approach analyses from lower crustal earthquakes show that near surface weathering layers with low body wave velocities and with laterally varying thicknesses exist under all of the stations even those at hard rock sites of the black forest the similarity of direct p wave pulse shapes at each site for events of different sizes the time domain form of the so called fmax phenomenon is attributed to the response from this weathering zone the amplitude of the p wave coda of an event is found to be independent of the earthquake radiation pattern and to depend only on the absolute size of the event and on epicentral distance to the receiver furthermore the high frequency p wave coda is found to be generated at least in part from random crustal scattering including that within 100 m of the earthquake focus direct s waveforms are shown to be more complicated than their p wave counterparts probably due to the same near receiver and crustal scattering phenomena however in the case of s waves scattered energy can arrive before as well as at and after the direct s arrival making the identification of the direct s difficult especially for nodal arrivals

estimation and applicability of attenuation characteristics for source parameters and scaling relations in the garhwal kumaun himalaya region india abstract source parameters of the small to moderate earthquakes are significant for understanding the dynamic rupture process the scaling relations of the earthquakes and for assessment of seismic hazard potential of a region in this study the source parameters were determined for 58 small to moderate size earthquakes 3 0 m w 5 0 occurred during 2007 2015 in the garhwal kumaun region the estimated shear wave quality factor q f values for each station at different frequencies have been applied to eliminate any bias in the determination of source parameters the q f values have been estimated by using coda wave normalization method in the frequency range 1 5 16 hz a frequency dependent s wave quality factor relation is obtained as q f 152 9 7 f 0 82 0 005 by fitting a power law frequency dependence model for the estimated values over the whole study region the spectral low frequency spectral level and corner frequency and source static stress drop seismic moment apparent stress and radiated energy parameters are obtained assuming 2 source model the displacement spectra are corrected for estimated frequency dependent attenuation site effect using spectral decay parameter kappa the frequency resolution limit was resolved by quantifying the bias in corner frequencies stress drop and radiated energy estimates due to finite bandwidth effect the data of the region shows shallow focused earthquakes with low stress drop the estimation of zuniga parameter e suggests the partial stress drop mechanism in the region the observed low stress drop and apparent stress can be explained by partial stress drop and low effective stress model presence of subsurface fluid at seismogenic depth certainly manipulates the dynamics of the region however the limited event selection may strongly bias the scaling relation even after taking as much as possible precaution in considering effects of finite bandwidth attenuation and site corrections although the scaling can be improved further with the integration of large dataset of microearthquakes and use of a stable and robust approach

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

scaling law and its applications to earthquake statistical relations abstract the spectra of the far field body wave displacement for earthquakes of various magnitudes have been obtained using a two dimensional faulting model on a rectangular fault there are three corner frequencies f c 1 f c 2 and f c 3 in the displacement spectra f c 1 and f c 2 relate to faulting times for the long and wide directions on the rectangular fault respectively and f c 3 corresponds to the rise time of the source time function according to these three corner frequency values four spectral regions are defined u u 0 f 0 f f c 1 in the first region u 1 f f c 1 f f c 2 in the second region u 1 f 2 f c 2 f f c 3 in the third region and u 1 f 3 f f c 3 in the fourth region this results in different source parameter relations for different magnitude ranges i e different seismic moment ranges because the attenuation rates depend on frequency it is supposed that earthquake faulting is satisfied by three conditions of geometrical similarity stress environment and dynamic similarity therefore seismic moment faulting time along the length and width of the fault and rise time can all be scaled with fault length the constants in the scaling relations can be determined from seismic moment data for about 800 earthquakes that occurred during 1981 1983 and are given by dziewonski and woodhouse 1983 and m s and m b data from the isc bulletin thus the source parameter relations between m s and m b m s and m l m b and m l log m o and log s fault area log s and m s log l fault length and m s and log d average dislocation and m s can be deduced from the scaling law rather than from pure statistical relations these relations are statistically consistent with the observed data the average rupture velocity of the fault can also be estimated from the scaling law it is 2 65 km s the total time of rupture is 0 35 s km l km which is close to that estimated by kanamori 1981 and the dislocation slip rate is 287 1143 cm s

shear wave anisotropy and stress direction in and near long valley caldera california 1979 1988 we observed shear wave splitting birefringence for two temporary surface deployments of three component digital seismographs that were in place before and after m 6 earthquakes near the long valley caldera california in one of these deployments the data sample precedes the large events of the may 1980 mammoth lakes earthquake sequence by 6 months and cover the two most active months of the may 1980 aftershock sequence one of the stations wit from this deployment was reoccupied with identical instrumentation in 1988 another deployment preceded the 1986 chalfant valley mainshock by 2 days and recorded events for 6 days the polarization of the faster shear wave changes from n30 w at mammoth to due north at chalfant valley and in both cases is parallel to the strike of nearby surface faults and to the mean direction of p axes determined from focal mechanism groupings observations from nearby stations yield fast directions nearly at 23 from each other and time separations of fast and slow shear waves show considerable station to station variation showing no correlation with earthquake station distance or earthquake depth these observations suggest that the observed anisotropy results primarily from near station presumably shallow effects rather than from widespread aligned microcracks the records at long valley station wit show a slight variation of average fast shear wave polarization from 1979 to 1980 but with the limited amount of data available this difference is not statistically significant time separation measurements of up to 0 48 s at station wit yield estimates of 15 30 velocity anisotropy among the highest reported for local earthquakes secular variation in the time separation of fast and slow shear waves between events recorded at station wit in 1979 and 1980 is swamped by the scatter in the data if it is present the five events recorded in 1988 showed similar s wave polarizations and time separations to those in 1980 but there are not enough events for a statistical comparison unlocated events at long valley station mrs show a slight increase in time separation after the may 1980 m 6 0 earthquakes but the increase is not statistically significant

estimation of source parameters of local earthquakes originated in shillong mikir plateau and its adjoining region of northeastern india abstract we estimate the source parameters seismic moment source radius stress drop and source displacement and scaling laws for local earthquakes that occurred in the shillong mikir plateau assam valley and arunachal himalaya in northeast india during 2001 2008 the source parameters were determined using the spectral analysis of p waves from the vertical component seismograms after correction for attenuation seismic moments are observed within the range from 9 51 10 12 to stress drop ranges from 4 10 5 to 9 10 7 pa for the brune model and 7 10 5 to 1 10 8 pa for the madariaga model seismic events in this study are prominent with an average stress drop of 0 1 10 mpa the effect of site geology may be a contributing factor for such a variation in stress drop source dimensions are however found to be smaller within the major part of the plateau it is suggested that local earthquakes in the region are associated with a brittle shear failure mechanism on fault segments and or the presence of weakened zones and earthquakes are triggered by low deviatoric stress empirical relations between m w m l and m 0 m l are developed leading to the future prediction of calibration coefficients for the local earthquakes in the shillong mikir plateau and its adjoining region online material tables of source parameters

statistical relations between source parameters for low and intermediate magnitudes friuli 1976 a data set of three component short period digital seismograms recorded in friuli after the strong earthquake of 6th may 1976 allowed the local magnitude ml and the seismic moment m0 to be estimated in the range 0 ml 2 the data set including the same parameters for the higher magnitude friuli events ml 5 shows two different slopes for the relation log m0 cml d for the two different ranges of ml one finds c 1 0 for 0 ml 2 and c 1 5 for5 ml 6 2 respectively this implies that apparent stress release increases at low magnitudes while it appears to be comparatively independent of the magnitude and to have an average value of about 100 bar for higher magnitude earthquakes conversely the fault dimensions do not appear to be magnitude dependent for ml 2 for higher magnitude events the linear fault dimensions range from about 1 km at ml 5 to about 12 km for the strong earthquake of 6th may 1976 ml 6 2

source parameters for small events associated with the 1986 north palm springs california earthquake determined using empirical green functions using small events as empirical green functions which include path site and instrument effects on the p waveforms source parameters were estimated for 25 ml 3 4 to 4 4 events associated with the 1986 north palm springs earthquake the static stress drops ranged from 3 to 80 bars for moments of 0 7 to 11 1021 dyne cm there was a spatial pattern to the stress drops of the aftershocks which showed increasing values along the fault plane toward the northwest compared to relatively low values near the hypocenter of the mainshock the highest values were outside the main area of slip and are believed to reflect a loaded area of the fault that still has an higher level of stress which was not released during the main shock the relatively high stress drop of a preshock in 1983 located close to the hypocenter of the main shock is also interpreted as evidence that stress drops of magnitude 3 and 4 events can be used as indicators of the stress level in a particular area

dependency of near field ground motions on the structural maturity of the ruptured faults little work has been undertaken to examine the role of specific long term fault properties on earthquake ground motions here we empirically examine the influence of the structural maturity of faults on the strong ground motions generated by the rupture of these faults and we compare the influence of fault maturity to that of other source properties slip mode and blind versus surface rupturing we analyze the near field ground motions recorded at rock sites for 28 large mw 5 6 7 8 crustal earthquakes of various slip modes the structural maturity of the faults broken by those earthquakes is classified into three classes mature intermediate and immature based on the combined knowledge of the age slip rate cumulative slip and length of the faults we compare the recorded ground motions to the empirical prediction equation of boore et al 1997 at all frequencies earthquakes on immature faults produce ground motions 1 5 times larger than those generated by earthquakes on mature faults the fault maturity appears to be associated with larger differences in ground motion amplitude than the style of faulting factor of 1 35 between reverse and strike slip earthquakes and the surface rupture occurrence factor of 1 2 between blind and surface rupturing earthquakes however the slip mode and the fault maturity are dependent parameters and we suggest that the effect of slip mode may only be apparent actually resulting from the maturity control we conclude that the structural maturity of faults is an important parameter that should be considered in seismic hazard assessment

the nature of volcanic earthquake swarm preceding the 2000 flank eruption at usu volcano hokkaido japan summary the 2000 eruption of usu volcano hokkaido occurred on the northwestern flank of volcano on march 31 at 13 07 local time after about 4 days of intensive seismic activity we study the seismic process prior to the eruption using the routine earthquake catalogue of the japan meteorological agency jma and the digital seismograms of the short period station nbbt jma earthquakes began beneath the summit and southern slope of the volcano then seismic foci clustered along two linear zones intersecting to the south of the initial epicentral zone the majority of earthquakes occurred at depth within the range between 4 and 7 km the magnitude range of located earthquakes varied between 0 8 and 4 4 the earthquake epicentres did not reach the sites of the forthcoming eruption they stopped at a distance of about 1 3 km the eruption began during the ceasing of seismic activity a study of the nature of the swarm was performed for a sample of 303 earthquakes with a magnitude interval from 2 5 to 3 4 that were representative of the swarm visual analysis allowed separating the earthquakes into two groups 286 volcano tectonic vt earthquakes and 17 long period lp earthquakes the spectral analysis of s wave so f274 vt earthquakes in the frame of brune s model showed that the vt earthquakes may be separated into two groups as characterized by high stress drops hsd and low stress drops lsd the swarm began with hsd earthquakes the lsd and lp earthquakes were recorded about 30 40 hr later simultaneously with the beginning of deformation of volcanic edifice and accompanied the hsd earthquakes up to the beginning of eruption we attribute the hsd earthquakes to the probable product of magmatic thermal fracturing of host rocks the relatively long lsd ruptures may be attributed to the slips along the pre existing fractures during magma movement

source parameters and scaling relations for reservoir induced seismicity in the longtan reservoir area we investigate the relationship between the impoundment and seismicity in the longtan reservoir southwestern china and find evidence that the seismicity was reservoir induced after the reservoir impoundment a pronounced increase in seismicity was observed in five clusters mainly concentrated in the areas where few earthquakes had occurred before the first filling the observed induced seismicity shows a strong correlation with the filling cycles the activity levels in the five clusters are different due to differences in the structures and permeabilities of the faults source parameters for 1 616 earthquakes with m l 0 1 4 2 recorded by 24 fixed and temporary stations deployed around the reservoir were calculated after applying corrections for geometrical spreading frequency dependent q and site effects the static stress drop and apparent stress in this area both appear to increase with increasing seismic moment over the entire magnitude range our results show that reservoir induced earthquakes have ten times lower average stress drop than natural tectonic earthquakes these results may indicate that the reservoir induced seismicity can occur with a lower tectonic stress due to the high pore pressures of the underground medium and that the effect of the water decreases the coefficient of friction

high stress drop events in the victoria baja california earthquake swarm of 1978 march summary the victoria earthquake swarm of 1978 march with magnitudes up to ml 4 8 occurred near the northern end of the cerro prieto fault in northern baja california mexico accurate epicentre locations of a number of earthquakes in the swarm reveal that the activity concentrated in a zone of about 6 km radius projected on the horizontal with earthquakes occuring mostly at depths of around 12 km a composite fault plane solution prepared with data from the larger earthquakes of the swarm indicates right lateral strike slip motion along a vertical plane extending parallel to the cerro prieto fault seismic moment source radius and stress drop are calculated from strong motion records and digital seismograph records obtained at short epicentral distances in most cases less than 10 km from calculated displacement spectra and brune s model stress drops between 1 bar and 1 kbar were estimated for earthquakes with seismic moments in the range 1 4 1019 2 9 1023 dyne cm eighteen events of 44 studied here had stress drops higher than 90 bar a result in striking contrast to lower stress drop results from previous distant station studies of earthquakes in the region results of comparing near source and large distance spectral shapes for two earthquakes in the victoria swarm region indicate that scattering and attenuation are a likely cause of the severe reduction of high frequency energy clearly shown by the larger distance spectra and thus of the resulting lower stress drop determinations at large distances if these events are characteristic of other large events in the region then the low stress drop determinations of thatcher and thatcher hanks for this region may be in doubt very high peak horizontal ground accelerations up to 0 6 g were measured from the near source records which were characterized by short duration bursts of high frequency energy it is suggested here that these high peak accelerations resulted from a combined effect of high stress drop and sediment amplification using a crustal model derived from mcmechan mooney a sedimentary amplification factor of 3 4 is estimated and used as a correction in calculation of the earthquake source parameters four other high stress drop up to 2 5 kbar earthquakes in this area were also analysed the high stress drop values obtained from seismic spectra are corroborated by using the rms root mean square acceleration formulation introduced by hanks

source and qp parameters from pulse width inversion of microearthquake data in southeastern sicily italy 1 source and qp parameters were estimated from the inversion of first arrival p waveform durations of about 300 microearthquakes recorded at a digital seismic network operating in southeastern sicily the average risetime and pulse width at each station do not show large differences allowing us to exclude significant differential attenuation site effects a first qp estimate was obtained by applying the classical risetime method under the assumption of a point like source time function in order to investigate the effect of directivity due to the finiteness of seismic sources new nonlinear relationships based on a circular crack model rupturing at a constant velocity were numerically built these relationships were used to formulate a nonlinear inverse method for retrieving source radius dip and strike of the circular crack and qp parameters from the inversion of risetime and pulse width data the application of the method produced a better fit of the observed data and a qp value higher than that obtained by applying the risetime method the discrepancy between the different q estimates may be due to a trade off among source dimension and qp as we inferred from a test on a subset of low magnitude events ml 2 5 a good agreement with independent estimates of fault plane solutions as inferred from p polarities and s polarizations was found the estimated stress drops are generally very low 0 1 10 bars this suggests that the background seismic activity in southeastern sicily is related to fault segments and or weakened zones where great stress accumulations are hindered

empirical green s functions from small earthquakes a waveform study of locally recorded aftershocks of the 1971 san fernando earthquake seismograms from 52 aftershocks of the 1971 san fernando earthquake recorded at 25 stations distributed across the san fernando valley are examined to identify empirical green s functions and characterize the dependence of their waveforms on moment focal mechanism source and recording site spatial variations recording site geology and recorded frequency band recording distances ranged from 3 0 to 33 0 km hypocentral separations ranged from 0 22 to 28 4 km and recording site separations ranged from 0 185 to 24 2 km the recording site geologies are diorite gneiss marine and nonmarine sediments and alluvium of varying thicknesses waveforms of events with moment below about 1 5 1021 dyn cm are independent of the source time function and are termed empirical green s functions waveforms recorded at a particular station from events located within 1 0 to 3 0 km of each other depending upon site geology with very similar focal mechanism solutions are nearly identical for frequencies up to 10 hz there is no correlation to waveforms between recording sites at least 1 2 km apart and waveforms are clearly distinctive for two sites 0 185 km apart the geologic conditions of the recording site dominate the character of empirical green s functions even for source spatial separations of up to 20 0 km the empirical green s functions at a particular site are consistent in frequency content amplification and energy distribution therefore it is shown that empirical green s functions can be used to obtain site response functions the observations of empirical green s functions are used as a basis for developing the theory for using empirical green s functions in deconvolution for source pulses and synthesis of seismograms of larger earthquakes

near surface attenuation and site effects from comparison of surface and deep borehole recordings abstract near surface attenuation and site effects are investigated using the seismograms of 17 local earthquakes recorded at depths of 0 0 3 1 5 2 5 and 2 9 km in the cajon pass borehole southern california the borehole penetrates 500 m of miocene sandstone and then crystalline granitic basement rock previous estimates of site response have been limited to shallower holes where the surface reflection interferes with the upgoing direct wave and the deepest sensor is not below the severe near surface effects in bedrock spectral ratios of the direct p and s waves for each earthquake between the different recording levels are well modeled with frequency independent q and amplification at the borehole q p 27 8 and q s 21 7 in the upper 2 9 km increasing from q p 26 and q s 15 in the upper 300 m to q p 133 and q s 47 between 1 5 and 3 km one event was also recorded at a surface granite site less than 1 5 km from the wellhead comparison of the 2 9 km recording with that at this granite site gives q p 50 and q s 23 the similarity of these values with those of previous studies at a wide range of sites suggests that q is very low in the near surface independent of rock type near surface amplification appears considerably more site dependent at the wellhead the amplifications at 1 hz of the direct p and s waves are 12 7 and 13 7 respectively these values include the free surface effect and are in reasonable agreement with the impedance contrast from the borehole logs at the granite site amplification of both p and s waves is less than four direct wave amplification at the wellhead is therefore at least three times that of the granite site the spectra of the direct and coda waves of the one earthquake recorded at both the wellhead and granite sites are compared with the corresponding 2 9 km recording coda wave amplification is in good agreement with the direct wave at the rock site but at the borehole the coda wave amplification factor overestimates the direct wave amplification by a factor of 3

source parameters of some large earthquakes in the eastern mediterranean region based on an itertive maximum entropy technique teleseismicp waves of some large earthquakes that occurred in the eastern mediterranean region have been analysed by using an iterative maximum entropy technique in order to obtain the independent spectral parameters the long period spectral level 0 and the corner frequencyf0 of the far field displacement spectra

influence of source region properties on scaling relations for m 0 events excavation induced seismic events with moment magnitudesm 0 are examined in an attempt to determine the role geology excavation geometry and stress have on scaling relations correlations are established based on accurate measurements of excavation geometry and methodology stress regime rock mass structure local tectonics and seismic locations scaling relations incorporated seismic moments and source radii obtained by spectral analysis accounting for source propagation and site effects and using madariaga s dynamic circular fault model observations suggest that the interaction of stresses with pre existing fractures fracture complexity and depth of events are the main factors influencing source characteristics and scaling behaviour self similar relationships were found for events at similar depths or for weakly structured rock masses with reduced clamping stresses whereas a non similar behaviour was found for events with increasing depth or for heavily fractured zones under stress confinement additionally the scaling behaviour for combined data sets tended to mask the non similar trends overall depth and fracture complexity initially thought as second order effects appear to significantly influence source characteristics of seismic events withm 0 and consequently favour a non similar earthquake generation process

simultaneous determination of site responses and source parameters of small earthquakes along the atotsugawa fault zone central japan velocity seismograms recorded at nine local stations from 102 small earthquakes on the atotsugawa fault zone central japan with seismic moment from 3 10 17 to 3 10 21 dyne cm are analyzed to obtain the s wave spectra a genetic algorithm is applied to determine the earthquake source parameters and site responses simultaneously the stability of both site responses and source parameters are experimentally tested as well as the effects of different q models the resulting source parameters show that the stress drops appear to vary systematically with the location of earthquakes events located on the mozumi fault or the junction of the mozumi and the atotsugawa faults have higher stress drops than those on the creeping section and southernmost part of the atotsugawa fault the relationship between seismic moment and corner frequency constructed by combining our results and those of iio 1986 for other regions in japan shows that a single scaling relation does not apply in a moment range of 10 10 to 10 25 dyne cm a steplike change in scaling relationship occurs between 5 10 18 and 5 10 20 dyne cm corresponding to corner frequencies between 15 and 25 hz this observation suggests that earthquakes are most likely a multihierarchical system divided by characteristic sizes according to aki9s 1980 model a characteristic source dimension 100 200 meters may represent a critical size of the barriers distributed in the fault zone at which the barrier effects are balanced by the dynamic effect of the source process

slip budget and potential for a m7 earthquake in central california the slip rate budget of the san andreas fault saf in central california which is approximately 33 mm yr is accounted for by a change in the slip release mechanism along the fault in the nw section of the fault between bear valley and monarch peak creep apparently accounts for the slip budget with the seismicity contributing negligibly the section at parkfield marks the transition from a creeping to a locked fault trace since the m8 1857 earthquake five m6 earthquakes have occurred but have not completely accounted for the slip budget southeast of parkfield the saf has been locked since 1857 from cholame to bitterwater valley this section now lags the deep slip by the amount of slip released in 1857 consequently faulting in this section could occur at the time of the next parkfield earthquake if this earthquake releases the slip deficit accumulated in the transition zone and in the locked zone the earthquake will have a moment magnitude m7 2

strain accumulation near the epicenters of the 1978 bishop and 1980 mammoth lakes california earthquakes abstract the bishop california earthquake m l 5 8 of 4 october 1978 occurred virtually beneath one of the stations in a trilateration network used by the u s geological survey to monitor strain accumulation in northern owens valley subsequently late may 1980 three earthquakes m l 6 0 6 3 and 6 0 the mammoth lakes sequence occurred on the edge of the trilateration network 10 to 20 km from the bishop earthquake epicenter the deformation of the network observed in the epoch 1976 to mid 1979 was not significantly different from that observed in the epoch 1973 to 1976 thus the bishop earthquake itself did not produce a measurable deformation nor was anomalous deformation detected prior to the mammoth lakes earthquakes the deformation of the trilateration network between july 1979 and september 1980 indicates significant extension 50 mm in 25 km in the direction n65 e essentially perpendicular to the alignment of the epicenters of the mammoth lakes earthquakes this deformation suggests normal faulting whereas the preliminary nodal plane solutions seem to require left lateral slip secular strain accumulation was detected in northern owens valley latitude 37 15 to 37 45 in the 1972 to 1979 interval the principal strain rates with standard deviations were 0 08 0 02 strain a n83 e 15 and 0 02 0 02 strain a n07 w 15 extension reckoned positive the observed strain accumulation is consistent with normal faulting along the sierra escarpment in owens valley north of bishop this accumulation appears to contrast with southern owens valley where strain accumulation is primarily right lateral shear acorss the axis of the valley

intraplate and interplate earthquakes in chilean subduction zone a theoretical and observational comparison abstract during the last decade efforts to improve our knowledge of great chilean earthquakes have shown that not all of the major destructive events have occurred in the contact between the nazca and south american plates interplate earthquakes waveform analysis of records from the ms 8 0 1950 antofagasta and the ms 7 8 1939 chillan earthquakes have shown that these large events are tensional rupturing along nearly vertical intermediate depth fault planes within the subducting slab intraplate events previous studies found that other earthquakes in chile like santiago 1945 ms 7 1 la ligua 1965 ms 7 1 tal tal 1965 ms 6 9 tocopilla 1970 ms 6 5 and tarapaca 2005 mw 7 8 were also of tensional type in the present work we analyze theoretical and observational evidence comparing interplate and intraplate earthquakes we found clear differences in the source characteristics between these two kinds of events with intermediate depth intraplate earthquakes presenting larger corner frequencies and greater seismic energy release than interplate events for a given seismic moment this is also reflected in the higher averaged apparent stress drop for intraplate earthquakes a 90 bar compared to interplate events a 30 bar from theoretical computations we found that the rupture velocity has a minor effect on the resulting displacements while directivity and changes in the static stress drop produced notable and similar results we believe that the differences found in the data might be due to changes in the static stress drop the effect of directivity or both regardless of the cause of the observed differences in the apparent stress drop these results should be taken into consideration into the assessment of the seismic hazard in chile

the digital array at anza california processing and initial interpretation of source parameters seismicity along the san jacinto fault suggests that a 40 km long section near the town of anza may constitute a gap in the occurrence of ml 6 7 earthquakes the potential for a gap filling shock and the high rate of seismicity at the southern end of the gap five events of 4 0 ml 5 5 since 1970 provided the impetus for deploying a digital seismic array to collect high quality ground motion recordings of all events 2 ml 4 5 on scale recording for shocks with magnitudes above ml 4 can be obtained from an existing array of eight strong motion accelerographs the anza site also had the advantage of being in the southern california batholith which appears to be relatively homogeneous compared to the franciscan gablian contrast of the central san andreas we expected that the granitic rocks of the batholith would yield relatively accurate earthquake locations and efficiently propagate high frequencies the field instrumentation is specifically designed for broadband recording up to 70 hz and high dynamic range 96 db in the on site digitizer alone since both are necessary for determining the rupture history of earthquakes both local vhf and microwave digital telemetry transmit the data from the anza region to san diego for computer data logging in the first 30 months of the array s operation approximately 292 events have been recorded located and processed for source parameters most events occur in one of five clusters or in a diffuse zone near the buck ridge fault two of these clusters are located at right stepping en echelon offsets coyote creek san jacinto and san jacinto hot springs two others are directly below and about 8 km west of anza respectively the fifth cluster is just to the northeast of the hot springs fault although event depths are generally between 11 and 14 km at the southern end of the hot springs fault depths extend to 18 km these are some of the deepest strike slip earthquakes on the san andreas system we calculate source parameters such as the scalar moment and stress drop for the analysis of high frequency waves scaling relations and earthquake interaction the largest event recorded thus far had a moment of 4 4 1021 dyn cm m 3 8 and a stress drop of 55 bars both arms and brune stress drops increase with moment source radius increases only slowly with moment the maximum values of both the brune and arms stress drops increase with depth down to 10 km remain approximately constant to 14 km and may decrease below 14 km the data suggest that stress drops of a group of earthquakes can be related to the strength of the upper crust calculated from frictional and quasi plastic flow laws although individual events may not be related stress drops on the san jacinto fault are high compared to those on the central creeping section of the san andreas where stress drops are about 10 bars or less this observation is consistent with the relative rupture area of events of equal ml 5 5 and may be related to the lithologie differences between the two fault segments and the amount of normal stress compared to shear stress

variability of seismic source spectra estimated stress drop and radiated energy derived from cohesive zone models of symmetrical and asymmetrical circular and elliptical ruptures large variability of earthquake stress drops and scaled energy has been commonly reported in the literature but it is difficult to assess how much of this variability is caused by underlying physical source processes rather than simply observational uncertainties here we examine a variety of dynamically realistic rupture scenarios for circular and elliptical faults and investigate to what extent the variability in seismically estimated stress drops and scaled energy comes from differences in source geometry rupture directivity and rupture speeds we numerically simulate earthquake source scenarios using a cohesive zone model with the small scale yielding limit where the solution approaches a singular crack model with spontaneous healing of slip compared to symmetrical circular source models asymmetrical models result in larger variability of estimated corner frequencies and scaled energy over the focal sphere the general behavior of the spherical averages of corner frequencies and scaled energy in the subshear regime extends to the supershear regime although shear mach waves generated by the propagation of supershear rupture lead to much higher corner frequency and scaled energy estimates locally our results suggest that at least a factor of 2 difference in the spherical average of corner frequencies is expected in observational studies simply from variability in source characteristics almost independent of the actual stress drops translating into a factor of 8 difference in estimated stress drops furthermore radiation efficiency estimates derived from observed seismic spectra should not be directly interpreted as describing rupture properties unless there are independent constraints on rupture speed and geometry

the interpretation of the b and the b0 values and its implications on the regional deformation of the crust summary after reviewing the model and the interpretation of the negative power laws which represent the statistical distributions of the linear dimension of faults l and of the stress drops p of the earthquakes it is seen that the coefficient of the law log m0 m increases with m for smaller m it is verified that the parameters l and p of california earthquakes are independent and also it is suggested that one may consider a fractal dimension of earthquakes associated with p it is shown how these statistical distributions may allow distribution of slips in the seismic regions to be inferred where all the slips occur approximately in the same horizontal direction from the density distribution of the length of the faults it is found that in california 50 per cent of the regional displacement is due to the faults with linear lengths larger than 63 per cent of the maximum linear length of the faults similar results are obtained using the seismic moment m0 or the magnitude m instead of l in these cases however the density distribution of p is also needed for the exact computation of the displacement assuming that in california the regional displacement has been 5 cm yr 1 as measured with ground observations and space techniques it is tentatively estimated that the displacement due to earthquakes with m0 larger than 1027 1 may be as large as 4 2 cm yr 1 and that a cumulative displacement of at least 170 cm due to earthquakes with m0 larger than 1027 1 may have not been released since the great 1906 san francisco earthquake it is finally discussed how the variation in the density distribution of stress drops may be a precursor to earthquakes

earthquake source parameters and the frequency dependence of attenuation at coalinga mammoth lakes and the santa cruz mountains california least squares best fits were calculated to the fourier amplitude spectra of p and s waves of locally recorded earthquakes attenuation amplification and earthquake source parameters were calculated from parameters of the fits approximately 8200 spectra were fit at 39 sites at coalinga mammoth lakes and the santa cruz mountains california typically the spectra were fit from 2 to 45 hz for p waves and 2 to 20 hz for s waves the source spectral falloff of p waves was limited to be no greater than f 2 5 f is frequency by low attenuation sites in the sierra nevada attenuation was measured through t the travel time divided by q attenuation was considered to be frequency dependent according to t to f which is equivalent to q qof negative values of were found to minimize the sum of squared residuals at most of the sites studied attenuation and amplification varied from site to site systematically with site geology source parameters were determined for individual earthquakes by averaging fit parameters from between four and 18 spectra depending on the earthquake seismic moments ranged from roughly 5 1012 to 1015 nm at coalinga and mammoth lakes moments ranged from 2 1011 to 3 1014 nm in the santa cruz mountains when the fits were determined using negative values of a the seismic moments trended approximately as fc 3 fc is the corner frequency for 0 5 stress drops varied from roughly 0 2 to 50 mpa at coalinga and mammoth lakes stress drops were lower in the santa cruz mountains ranging from approximately 0 05 to 8 mpa

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

source parameters and rupture velocity of small m 2 1 reservoir induced earthquakes summary we calculate stress drop and rupture speed for ml 2 1 shallow reservoir induced earthquakes and find them to be similar to those of large natural earthquakes previous studies have suggested that hydrofractures mining and reservoir induced earthquakes have lower average stress drop than natural tectonic earthquakes this difference might result from the different tectonic setting or the shallower hypocentral depths of induced earthquakes alternatively difficulties in correcting for attenuation and site effects in earlier studies may lead to underestimation of stress drop in addition most studies assume the rupture velocity of small reservoir induced earthquakes to be the same as for the large earthquakes we analyse a set of 101 ml 2 1 earthquakes induced by changing water level in the acu reservoir ne brazil the earthquakes are shallow depth 5 km and the region has negligible natural seismicity we use three different approaches to calculate the source parameters of the six largest 1 9 ml 2 1 earthquakes we model the individual spectra to find corner frequency frequency independent q and long period amplitude we use collocated small earthquakes as empirical green s functions to calculate the spectral ratios and determine the relative source time functions estimates of the source duration and corner frequency imply stress drops in the range of 26 179 mpa these are similar to or higher than tectonic earthquakes suggesting that the shallow hypocentral depth and the presence of water do not affect stress drop we observe clear directivity for one of the earthquakes and use the azimuthal variation in pulse width to estimate a rupture velocity of 0 6

near surface velocities and attenuation at two boreholes near anza california from logging data to investigate near surface site effects in granite rock we drilled 300 m deep boreholes at two sites which are collocated with stations from the digital array at anza california the first borehole was sited at station knw keenwild fire station which is located along a ridge line about 8 7 km east of the san jacinto fault zone station pfo pinon flat observatory chosen for the second site is another 6 km further to the east of station knw and is located on a gently sloping hillside we logged each borehole for p and s wave velocities as well as for crack density and orientation p waves were generated by striking a plate with a hammer at the surface a tool consisting of weighted anvils driven by compressed air against end plates along a 3 5 m beam was used to generate shear waves signals were recorded downhole with a three component sensor package at 2 5 m intervals from the surface to 50 m depth and at 5 m intervals from 50 m depth to the bottom of the hole velocities were determined by differencing the measured arrival times of first arrivals or peaks over each interval in depth travel times were computed for the first breaks at shallow depths however below about 100 m depth times were computed for the first peaks rather than for first breaks since the first arrival was no longer clearly distinguishable the knw site yielded a shear velocity of 1 9 km sec by only 30 m in depth and reached close to 2 6 km sec at the bottom of the hole p wave velocities at knw were also high at 5 4 km sec starting at 120 m depth the pfo site had similar but slightly higher shear wave velocities the bottom hole shear wave velocity reached 3 0 km sec and the p wave velocity was 5 4 km sec shear wave attenuation was computed using both the pulse rise time and spectral ratio methods at station knw attenuation was significant only in an interval between 17 5 and approximately 40 m in depth over the top 50 m attenuation corresponding to a q of about 8 was obtained a total t of 0 004 sec was measured for this interval pulse rise times also increased rapidly in this zone the spectral ratio data for station pfo yields two peaks in attenuation above 50 m similar to the attenuation found for station knw the peak in attenuation corresponds to a q of about 11 averaged over the top 50 m spectra of the seismic pulses produced by the hammer give good signal between 20 to 80 hz significant motion perpendicular to the polarizations of the first shear wave arrival was recorded within a few meters of the surface apparently the rock structure is sufficiently complicated that body waves are being converted sh to sv at oblique incidence very close to the surface the presence of these elliptical particle motions within a mere few m of the pure shear wave source suggests that the detection of polarizations perpendicular to the main shear arrival at a single location at the surface is not by itself a good method for detecting shearwave splitting within the upper few tens of kilometers of the earth s crust crack densities and orientations were determined from televiewer records these records showed cracks with a preferred direction at station knw and of a greater density than at station pfo at station pfo crack densities were smaller and more diffuse in orientation

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

source parameters of the 1980 mammoth lakes california earthquake sequence from the more than 1500 mammoth lakes earthquakes recorded on three component digital seismographs spudich et al 1981 150 were used in an analysis of the locations mechanism and source parameters a composite fault plane solution of nine earthquakes 3 9 m 5 1 defines a right lateral strike slip mechanism on a steeply dipping nearly east west plane striking s75 e or left lateral strike slip on a nearly north south plane striking n10 e vertical cross sections of well located aftershocks indicate possibly three east west planes that coincide with the locations of the four largest earthquakes with ml 6 0 using the spectral analysis of s waves brune 1970 source parameters for 67 earthquakes were determined forty eight had magnitudes greater than or equal to 3 0 seismic moments ranged from 9 20 1018 dyn cm to 2 33 1024 dyn cm earthquakes with seismic moment greater than about 1 0 1021 dyn cm had nearly constant stress drops 50 bars earthquakes with seismic moment less than about 1 0 1021 dyn cm had stress drops that apparently decrease as seismic moment decreases

local magnitude and source parameters for earthquakes in the peninsular ranges of baja california mexico digital three component seismograms provided by the northwestern mexico seismic network were used to estimate local magnitudes and source parameters for 81 earthquakes located in the peninsular ranges of baja california the magnitude values were estimated from peak amplitudes as measured on equivalent wood anderson seismograms in combination with the attenuation law proposed by richter 1958 for the southern california region the source parameters were determined from spectral analysis based on the seismic source model proposed by brune 1970 1971 a comparison of our local magnitudes mlr with magnitudes reported by the california institute of technology and the u s geological survey shows residuals with a standard deviation of 0 21 concerning the source parameters stress drop determinations yielded values between 1 and 100 bars while the calculated seismic movements range from 2 4 1019 to 7 6 1021 dyne cm least square fits of the moment magnitude data give log m 1 11 0 02 mlr 17 32 0 05 for our data for small earthquakes 2 0 mlr 4 0 and log m 1 63 ml 14 87 for earthquakes in the same area having magnitudes between 4 0 ml 6 8 these two linear relationships intersect at a magnitude of about 4 6

the magnitude frequency distribution of earthquakes recorded with deep seismometers at cajon pass southern california abstract the cumulative b value the slope of the gutenberg richter relationship between earthquake occurrence rate and magnitude is commonly found to be constant 1 network catalogues however reveal a decrease at small magnitudes b value is not just an artifact of catalogue incompleteness but that small earthquakes are really not as numerous as a constant b value extrapolated from larger events would predict in the cajon pass area southern california the b value of seismicity recorded by the local network scsn appears to decrease below about m l 1 6 in order to investigate whether this decrease is real or simply represents the network detection threshold we use seismicity recorded by the deep 1 5 and 2 5 km seismometers deployed in the cajon pass scientific drillhole between april 1992 and october 1994 the maximum amplitudes recorded downhole are compared to scsn magnitudes for events recorded by the network to determine the relationship between amplitude and m l as a function of hypocentral distance from the borehole magnitudes are then calculated for 1300 earthquakes which occurred within 40 km of the borehole magnitude frequency curves are calculated for those events within 18 km of the borehole and a constant b value is observed to m l 0 5

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

an inversion method to analyze rupture processes of small earthquakes using stopping phases 1 we propose a method to obtain source parameters most importantly the rupture velocity of small earthquakes on the basis of stopping phases it is assumed that a rupture begins at one focus of an ellipse and spreads radially at a constant rupture velocity on the fault surface eventually terminating on the elliptical boundary this model allows us to investigate a wide variety of rupture configurations ranging from a circular to a unilaterally propagating rupture in this model we find that two high frequency stopping phases hilbert transformations of each other are radiated and that the difference in arrival times between the two stopping phases is dependent on the average value of the rupture velocity the radius of the major semiaxes of the ellipse and its ellipticity these parameters can be estimated by a nonlinear least squares inversion method we perform simulation tests which allow us to detect a hilbert transform pair whose arrival times coincide with ones predicted by an assumed source model from noisy data these simulation tests show that our inversion method is useful since our method does not require an a priori assumption of the average rupture velocity the orientation of the fault plane and the circular crack model it provides us with a tool for estimating accurate source dimensions as well as rupture processes of small earthquakes

seismic source spectrum of microearthquakes the seismic source spectra of microearthquakes having seismic moments between 1014 and 1018 dyne cm were investigated by using local recordings from an excellent hard rock site the p wave velocity near the site was estimated as about 6 km sec even immediately below the ground surface the effect of anelastic attenuation was thought to be very small since predominant frequencies of greater than 100 hz were detected in seismograms recorded at focal distances greater 10 km many seismograms with s p times of less than 0 6 sec were observed the first cycle of the p wave velocity seismogram was used in this study the waveforms after the first cycle are likely formed near the site since their periods are exactly the same for earthquakes that have different source processes in the high frequency portion of the estimated displacement source spectra the slopes of the fall off have values much greater than 2 the source process of microearthquakes is assumed to be very slow and smooth

magnitude and seismic moment scales in western yunnan peoples republic of china abstract seismograms and accelerograms from 77 earthquakes in 1982 to 1984 near the northwest end of the red river fault in western yunnan province peoples republic of china have been used to calculate seismic moment m 0 and local magnitude m l using techniques established in california for 1 1 2 m l 3 1 2 log m 0 16 97 1 17 0 05 m l consistent with the log m 0 m l relation appropriate for central california a comparison of these m l values and the coda duration magnitudes m and s wave amplitude magnitudes m assigned by the seismological bureau of yunnan province suggests that m and m values are generally larger than the m l values for 1 7 m 5 3 m l 0 49 0 67 0 04 m and log m 0 17 30 0 86 0 04 m for 2 0 m 5 4 m l 0 24 0 76 0 05 m and log m 0 17 06 0 95 0 05 m

source parameters and rupture velocities of microearthquakes in western nagano japan determined using stopping phases we use an inversion method based on stopping phases imanishi and takeo 2002 to estimate the source dimension ellipticity and rupture velocity of microearthquakes and investigate the scaling relationships between source parameters we studied 25 earthquakes ranging in size from m 1 3 to m 2 7 that occurred between may and august 1999 at the western nagano prefecture japan which is characterized by a high rate of shallow earthquakes the data consist of seismograms recorded in an 800 m borehole and at 46 surface and 2 shallow borehole seismic stations whose spacing is a few kilometers these data were recorded with a sampling frequency of 10 khz in particular the 800 m borehole data provide a wide frequency bandwidth with greatly reduced ground noise and coda wave amplitudes compared with surface recordings high frequency stopping phases appear in the body waves in hilbert transform pairs and are readily detected on seismograms recorded in the 800 m borehole after correcting both borehole and surface data for attenuation we also measure the rise time which is defined as the interval from the arrival time of the direct wave to the timing of the maximum amplitude in the displacement pulse the differential time of the stopping phases and the rise times were used to obtain source parameters we found that several microearthquakes propagated unilaterally suggesting that all microearthquakes cannot be modeled as a simple circular crack model static stress drops range from approximately 0 1 to 2 mpa and do not vary with seismic moment it seems that the breakdown in stress drop scaling seen in previous studies using surface data is simply an artifact of attenuation in the crust the average value of rupture velocity does not depend on earthquake size and is similar to those reported for moderate and large earthquakes it is likely that earthquakes are self similar over a wide range of earthquake size and that the dynamics of small and large earthquakes are similar

stress orientation profile to 3 5 km depth near the san andreas fault at cajon pass california a detailed profile of the orientation of the apparent maximum horizontal compressive stress was constructed over the depth interval 1 75 3 46 km in the cajon pass drill hole located 4 2 km from the san andreas fault in southern california the profile is based on the analysis of stress induced well bore breakouts and consists of 32 616 orientation determinations at a basic sampling interval of 4 cm the azimuth of the apparent maximum horizontal compressive stress around the borehole is 057 s d 19 depth dependent variations in breakout azimuths from a few degrees to as much as 100 occur over depth intervals of several centimeters to hundreds of meters in the borehole with wavelengths showing a self similar distribution over a range of depth intervals the variations in breakout orientation at depth seem to reflect stress fluctuations associated with active faults penetrated by the drill hole assuming linear elastic behavior perturbations in stress magnitude and orientation were computed for predrilling slip on these faults fault stress drops limited to the ambient shear stress i e up to total stress drop along with occasional local extreme stress drops and geometrical complications can explain the variations in breakout orientations in the drill hole the penetrative stress inhomogeneity may suggest that the average orientation of borehole breakouts in cajon pass which indicates left lateral shear on planes parallel to the san andreas fault may not be representative of the state of stress near the fault at depth in this region rather the stress orientation profile indicates the superposition of numerous local stress perturbations on an average stress field which is characterized by an absence of appreciable right lateral shear on planes parallel to the san andreas combining these observations with the highly variable stress state in shallow boreholes in the western mojave desert it is suggested that the heterogeneous stress field in this region representing a spectrum of seismic events may extend over substantial distances across and along the san andreas fault zone if the average measured breakout orientation in cajon pass is representative of the state of stress near the san andreas in this region then substantial left lateral shear stress is resolved on planes parallel to the fault in contradiction to the sense of its long term motion we use an elastic dislocation model to compute the net stress change near the cajon pass drill site since just prior to the large 1812 earthquake taking into account coseismic slip in major earthquakes and aseismic accumulation of slip on the san andreas and san jacinto faults the model suggests that the net change in fault parallel shear stress during the current earthquake cycle in the cajon pass area is nearly negligible the difference between the measured and computed results requires that additional left lateral shear stress be superimposed on a generally fault normal maximum horizontal compressive stress the results suggest that during the great fort tejon earthquake of 1857 left lateral shear stress parallel to the san andreas fault in the region of cajon pass may have played a role in terminating the southeastward extent of the rupture propagation

geochemical constraints on the formation of late cenozoic ferromanganese micronodules from the central arctic ocean in order to determine geochemical compositions of late cenozoic arctic seawater oxide fractions were chemically separated from 15 samples of hand picked ferromanganese micronodules 50 300 pm the success of the chemical separation is indicated by the fact that 97 of the sr in the oxide fraction is seawater derived rare earth element ree abundances of the arctic micronodule oxide fractions are much lower than those of bulk fe mn nodules from other ocean basins of the world e g 33 vs 145 ppm nd but the arctic oxides are enriched in ce relative to nd ce nd 2 2 f 0 5 and have convex upward shale normalized ree patterns ndn gdn 0 61 f 0 06 gdn ybn 1 5 0 2 ndn yb 0 9 f 0 2 typical of other hydrogenous and diagenetic marine fe mn oxides bulk sediment samples from the central arctic ocean have ree abundances and patterns that are characteristic of those of post archean shale non detrital fractions calcite oxide coatings of recent arctic foraminifera have ree abundances and patterns similar to those of recent foraminifera from the atlantic ocean electron microprobe analyses n 178 of transition elements in 29 arctic fe mn micronodules from five different stratigraphic intervals of late cenozoic sediment indicate that oxide accretion occurred as a result of hydrogenetic and diagenetic processes close to the sediment seawater interface transition element ratios suggest that no oxide accretion occurred during transitions from oxic to suboxic diagenetic conditions only k is correlated with si and al and ratios of these elements suggest that they are associated with illite or phillipsite ca and mg are correlated with mn which indicates variable substitution of these elements from seawater into the manganate phase the geochemical characteristics of arctic fe mn micronodules indicate that the rees of the oxide fractions were ultimately derived from seawater however because of minute contributions of sr from siliciclastic detritus during diagenesis or during the chemical leaching procedure sr isotope compositions of the oxide fractions cannot be used to trace temporal changes in the sr sr ratio of arctic seawater or to improve the chronostratigraphy 0 1997 elsevier science b v kevwords 87sr 86sr arctic ocean ferromanganese nodules marine sediments rare earth elements

the effects of site response on source parameters deduced for the 1980 long valley california earthquake sequence data taken from the university of nevada unr array of surface located wideband digital displacement seismographs placed over the 1980 long valley earthquake sequence show very strong site effects which control spectral parameters and hence inferred source parameters up to ml 5 0 observations from 4 000 seismograms show that differences in site effects can lead to inferred seismic moments at least a factor of two lower than previously reported observed s wave spectral corner frequencies show upper bound cutoffs which are widely disparate at different sites e g 15 hz at mg1 and 6 hz at wt1 thus fmax is quite variable by site this high frequency cutoff clearly a site effect at wt1 could be misinterpreted as an apparent departure from source similarity for seismic moments below 1021 dyn cm estimates of seismic moment and source duration from the unr rock sites may be more accurate than those previously published both because the unr instrumentation has greater resolution at longer periods 0 1 to 1 0 hz and because of less near site signal distortion at high frequencies

high frequency observations and source parameters of microearthquakes recorded at hard rock sites we have estimated the source parameters of 53 microearthquakes recorded in july 1983 which were aftershocks of the miramichi new brunswick earthquake that occurred on 9 january 1982 these events were recorded by local three component digital seismographs at 400 sps component from 2 hz velocity transducers sited directly on glacially scoured crystalline basement outcrop hypocentral distances are typically less than 5 km and the hypocenters and the seven digital seismograph stations established all lie essentially within the boundaries of a granitic pluton that encompasses the faults that ruptured during the main shock and major aftershocks the p wave velocity is typically 5 km sec at the surface and at least 6 km sec at depths greater than about 1 km the events have s wave corner frequencies in the band 10 to 40 hz and the calculated brune model seismic moments range from 1015 to 1018 dyne cm the corresponding stress drops are generally less than 1 0 bars but there is considerable evidence that the seismic source signals have been modified by propagation and or site effects the data indicate a there is a velocity discontinuity at 0 5 km depth b the top layer has strong scattering attenuating properties c some source receiver paths differentiate the propagated signal d there is a hard rock site p wave f max between 50 and 100 hz and e some hard rock sites are characterized by p wave resonance frequencies in the range 50 to 100 hz comparison of this dataset with the january 1982 new brunswick digital seismograms which were recorded at sites underlain by several meters of low velocity surface sediments suggests that some of the hard rock site phenomena listed above can be explained in terms of a layer over a half space model for microearthquakes this result implies that spectrally determined source dimension scales with site dimension thickness of the layer more generally it emphasizes that it is very difficult to accurately observe the details of seismogenic processes from the earth s surface

anomalous concentrations of seismically triggered rock falls in pacoima canyon are they caused by highly susceptible slopes or local amplification of seismic shaking anomalously high concentrations of rock falls were triggered in pacoima canyon los angeles california during the 1994 northridge earthquake similar concentrations were also documented from the 1971 san fernando earthquake using an engineering rock mass classification that evaluates the susceptibility of rock slopes to seismic failure based on the fracture properties of a rock mass in terms of a numerical q value that describes rock quality the rock slopes in pacoima canyon were compared with rock slopes in surrounding areas where topography and lithology are similar but rock fall concentrations from the earthquakes were much lower a statistical comparison of q values from five sites surrounding pacoima canyon indicates that seismic susceptibilities are similar to those within pacoima canyon differences in the characteristics of rock slopes between these sites are not sufficient to account for the relatively high concentrations of rock falls within pacoima canyon as compared to low concentrations elsewhere by eliminating susceptibility differences as a cause the most likely explanation for the differences in rock fall concentrations is anomalously high shaking levels in pacoima canyon possibly resulting from topographic amplification within the canyon manuscript received 21 may 2001

modelling of multiple events using empirical green s functions method application to swarm earthquakes and implications for their rupture propagation summary recent studies of source time functions stfs of small earthquakes have shown that some of the ml 3 events may display complicated waveforms indicating multiple rupturing episodes the stfs of such earthquakes consist of several pulses whose relative positions provide information on the mutual position of the subevents i have used the waveform modelling method to analyse multiple events in order to disclose the geometry of the rupture the p and s waveforms of multiple events mes are modelled as the sum of waveforms of single subevents with different hypocentre coordinates and scalar moments to construct the waveform of each single event composing the me the waveform of a co located small event is used as an empirical green s function egf assuming similar focal mechanisms of the subevents and of the egf the method seeks the coordinates and origin times of the subevents and their relative seismic moments the non linear problem is solved using the genetic algorithms method synthetic tests have shown that the method is capable of locating reliably up to three subevents with an accuracy better than 40 m the method was applied to the records of the 2000 earthquake swarm in nw bohemia vogtland in central europe by the egf deconvolution 54 mes were identified in the magnitude range from 1 2 to 3 3 and 18 of them were successfully modelled as double or triple events with separate rupture positions the separation of subsources reached 100 ms in time and 320 m in space the relative positions of the subevents with respect to the orientation of the fault indicate that most of them occurred on a common fault plane the space time separation of the subevents corresponds to a speed of 3 0 0 9 km s 1 a value typical for rupture propagation of large earthquakes the later subevents occur farther than the nominal rupture radius of the first subevent and their mutual distance scales with magnitude these observations suggest that the analysed mes share a common fault surface and that their subevents represent individual rupture episodes the angular distribution of the position vectors of later subevents indicates that many of them result from slip parallel rupture growth while some of the ruptures propagate upwards

source parameters of the ensenada bay earthquake swarm baja california mexico seismic data collected from the ensenada bay earthquake swarm of late 1981 were used to calculate the spectra of ground displacement data from the stations of ensenada enx and cerro bola cbx

assessment of ground motion in palermo italy during the 6 september 2002 mw 5 9 earthquake using source scaling law in calderoni et al 2005 a section of text that was present in the original manuscript was inadvertently deleted during composition the section entitled random summation of aftershocks rnds is printed correctly below rnds joyner and boore 1986 generates synthetic seismograms of the target earthquake at

seismic source parameters for microearthquakes of the granada basin southern spain abstract ninety five microearthquakes of the granada basin southern spain with duration magnitudes ranging from 1 3 to 3 5 moment magnitude from 0 9 to 2 5 have been spectrally analyzed with digital recordings from ten stations of the andalusian seismic network the coda q quality factor qc assumed to be frequency dependent was used instead of q for the path correction and the q adopted was equal to 9 4 qc the k parameter for site attenuation correction had values in the range of 0 02 0 05 fourier displacement spectra were calculated for p and s waves and analyzed with respect to the source model of brune 1970 1971 spectral parameters were automatically determined by the method of snoke 1987 the seismic moment ranged from 1017 to 1020 dyne cm with average m0 p to m0 s ratio of 1 02 and the source radii spanned from 0 14 to 0 40 km with average r p to r s ratio of 1 3 the seismic energy values ranged from 1 103 to 9 107 j with p to s energy ratio from 1 to 10 for about 85 of the events whereas the static stress drop varied from 0 02 to 2 6 bar the scaling relation between the seismic moment and the stress drop indicates a decrease in stress drop with decreasing seismic moment apparent stress is linearly correlated with stress drop and have on average similar values and this points out to a partial stress drop

a summary of attenuation measurements from borehole recordings of earthquakes the 10 hz transition problem earthquake seismograms recorded by instruments in deep boreholes have low levels of background noise and wide signal bandwidth they have been used to extend our knowledge of crustal attenuation both in the near surface and at seismogenic depths site effects are of major importance to seismic hazard estimation and the comparison of surface shallow and deep recordings allows direct determination of the attenuation in the near surface all studies to date have found that q is very low in the near surface 10 in the upper 100 m and increases rapidly with depth unlike site amplification attenuation at shallow depths exhibits little dependence on rock type these observations are consistent with the opening of fractures under decreasing lithostatic pressure being the principal cause of the severe near surface attenuation seismograms recorded in deep boreholes are relatively unaffected by near surface effects and thus can be used to measure crustal attenuation to higher frequencies 100 hz than surface recordings studies using both direct and coda waves recorded at over 2 km depth find q to be high 1000 at seismogenic depths in california increasing only weakly with frequency between 10 and 100 hz intrinsic attenuation appears to be the dominant mechanism these observations contrast with those of the rapidly increasing q with frequency determined from surface studies in the frequency range 1 to 10 hz further work is necessary to constrain the factors responsible for this apparent change in the frequency dependence of q but it is clearly unwise to extrapolate q estimates made below about 10 hz to higher frequencies

effects of healing on the seismogenic potential of carbonate fault rocks experiments on samples from the longmenshan fault sichuan china fault slip and healing history may crucially affect the fault seismogenic potential in the earthquake nucleation regime here we report direct shear friction tests on simulated gouges derived from a carbonate fault breccia and from a clay carbonate fault core gouge retrieved from a surface exposure of the longmenshan fault zone lfz which hosted the 2008 wenchuan earthquake the experiments were conducted under dry and hydrothermal conditions at temperatures up to 140 c at an effective normal stress of 50 mpa and involved sequential velocity stepping vs slide hold slide shs and velocity stepping stages dry tests performed on breccia derived samples showed no dependence of quasi steady state friction ss on shs or vs history and a log linear relation between transient peak healing pk and hold time or classical dieterich type healing behavior by contrast all experiments conducted under hydrothermal conditions were characterized by non dieterich healing behavior this included 1 an increase in ss upon resliding after a hold period and 2 an increase in friction rate parameter a b after shs testing comparison with previous results suggests that the healing behavior seen in our wet tests may be attributed to solution transfer processes occurring during hold periods our findings imply that the shallow portions of faults with carbonate clay rich cores e g the lfz can heal much faster than previously recognized while the upper limit of the seismogenic zone may migrate to deeper levels during interseismic periods these effects have important implications for understanding the seismic cycle in tectonically active carbonate terrains

seismicity focal mechanisms and tectonics in the last four years there has been a tremendous increase in the detail of tectonic processes reported in the typical seismotectonic study new data from many local and regional networks of seismographs established during the previous four year period plus the now routine application of improved hypocentral location procedures long period body wave modelling and moment tensor inversion using digitally recorded data have increased resolution of both the hypocentral location and the nature of the faulting process consequently research emphasis has been on local tectonics or those aspects of regional and global tectonics requiring high resolution i briefly describe below some of the highlights of these studies

scaling law between corner frequency and seismic moment of microearthquakes is the breakdown of the cube law a nature of earthquakes 1 the corner frequency fc and the seismic moment m0 of microearthquakes local magnitude from 1 3 to 1 3 are estimated from the records of sensors located at an 1800 m deep borehole at the nojima fault central japan the digital waveforms of each event are recorded with two different sampling rates 10 khz and 100 hz the source parameters estimated from high sampling data satisfy a relationship of m0 fc 3 corresponding to a constant stress drop law while those calculated from low sampling data follow a relationship of m0 fc 4 a lack of high frequency components in the 100 hz sampling data seem to be obvious indicating that the value of fc and m0 are incorrectly estimated and the relationship of m0 fc 4 is an artifact caused by the limited frequency band of these data the scaling relationship of m0 fc 3 is thus also valid for microearthquakes and the breakdown of the constant stress drop law reported in the literature is not well founded

source parameters for the roermond aftershocks of 1992 april 13 may 2 and site spectra for p and s waves at the belgian seismic network summary one and three component digital recordings from six stations of the belgian seismic network were used to invert simultaneously for source velocity spectra of 18 aftershocks of the ml5 8 roermond earthquake and for site amplification spectra at these stations the events were selected to have well constrained fault plane solutions and the stations to be within 140 km epicentral distance frequency dependent quality factors were computed from the coda waves of the same events the joint source site determination method we used first corrects the body wave spectra for radiation pattern geometrical spreading attenuation and free surface effect and then is formulated as a linear inverse problem repeated over frequencies in this application between 1 and 30 hz with one reference station for which the site amplification spectrum is given as constant in this application la chartreuse the unbiased source velocity spectra were fitted with a brune type spectral function to obtain a scale factor and a corner frequency from which among others the seismic moment source radius stress drop final disolcation and fracture energy were calculated in the 1 9 3 4 ml magnitude range the stress drop seismic energy and fault radius tend to increase with seismic moment and seismic efficiency with fault dimension brune s stress drop varies between 0 4 and 4 6 mpa frictional overshoot prevails over partial stress drop the average ratio of p and s wave corner frequencies is 1 6 the site amplification spectra vary between 1 and 10 strong site effects were observed at stations membach and vianden

simultaneous inversion for q and source parameters of microearthquakes accompanying hydraulic fracturing in granitic rock an inversion that fits spectra of earthquake waveforms and gives robust estimates of corner frequency and low frequency spectral amplitude has been used to determine source parameters of 223 microearthquakes induced by hydraulic fracturing in granodiorite assuming a 2 source model the inversion fits the p wave spectra of microearthquake waveforms to determine individual values of corner frequency and low frequency spectral amplitude for each event and one average frequency independent q for all source receiver paths we also implemented a constraint that stress drops of all microearthquakes be similar but not equal and found that this constraint did not significantly degrade the quality of the fits to the spectra the waveforms analyzed were recorded by a borehole seismometer the p wave q was found to be 1070 for q values as low as 600 and as high as 3000 the misfit between model and spectra increased by less than 5 per cent and the average corner frequency changed by less than 15 per cent from those obtained with a q of 1070 average stress drop was 3 7 bars seismic moments obtained from spectra ranged from 1013 to 1018 dyne cm the low stress drops are interpreted to result from underestimation of the actual stress drops because of a nonuniform distribution of stress drop and slip along the fault planes spatially varying stress drops and slips result from the strong rock heterogeneity due to the injection of fluid into the rock stress drops were found to be larger near the edges of the seismic zone in regions that had not been seismically active during previous injections the seismic moments determined from spectra were used to obtain a coda length to moment relation then moments were estimated for 1149 events from measurements of coda lengths from events whose moments could not be measured from spectra because of saturation or a low signal to noise ratio the constant of proportionality between cumulative number of events and seismic moment is higher than that found for tectonic regions the slope is so high that the seismic energy release is dominated by the large number of small events in the absence of information about the number of events smaller than we studied we cannot estimate the total seismic energy released by the hydraulic injection

source parameters of earthquakes recorded near the itoiz dam northern spain we calculate the source parameters and attenuation from earthquakes recorded near the itoiz dam from 2004 to 2009 with magnitudes ranging between 1 2 and 5 2 we use a genetic algorithm in order to fit the three component p wave spectra with the spectral level corner frequency and attenuation factor as searching parameters the obtained moments range from 1 72 1011 to 2 65 1015 nm the radii span from 0 09 to 1 00 km and the stress drops vary from 0 006 to 29 462 mpa the maximum value for the q attenuation factor is 794 and the minimum value is 53 we find a good agreement between empirical and theoretical relationships between moment and magnitude there seems to be a breakdown of self similarity but it could be due to the method used we group the data by means of a self organizing map and the clusters found are related by their magnitude and not by other considerations

seismic moment assessment of earthquakes in stable continental regions ii historical seismicity summary this part i study in conjunction with part 11 develops a method to determine within specified uncertainty bounds the seismic moment and thus moment magnitude of all earthquakes of stable continental regions scr for which instrumental or intensity data exist its basis is polynomial regression analysis using a database of scr earthquakes with direct seismic moment determination the independent variables include modern teleseismic magnitudes and regional magnitudes part i and isoseismal areas or number of recording stations part 11 part i11 is an application of the methodology of parts i and i1 to several major historical earthquakes all data used in the regressions are assigned individual uncertainties estimated from the literature or from experience formal confidence limits 68 per cent or 95 per cent on both the regression formulas and the predicted seismic moment values are then possible via error propagation analysis the most complete development is for the teleseismic magnitudes m and mb for both the final regression for log m is a quadratic formula that closely emulates the relationship between amplitude magnitudes and log m expected from dislocation theory and source scaling arguments regressions are also derived for the regional magnitudes mlg and m because there are many scr events mainly pre 1964 that have no teleseismic magnitudes prediction uncertainties from teleseismic magnitudes in moment magnitude units are in the ao 18 0 28 range and from regional magnitudes in the ko 23 0 38 range over a wide magnitude band finally the methodology developed here is generic even though the database is specific application to plate boundary oceanic intraplate or active continental intraplate regions should be straightforward

dynamic source parameters of the 1981 gulf of corinth central greece earthquake sequence based on fft and iterative maximum entropy techniques teleseismic p waves of the 1981 gulf of corinth central greece earthquake sequence have been analysed using an iterative maximum entropy algorithm to obtain the corner frequency and the long period constant level of the far field displacement spectra these values have been used to compute the seismic source parameters such as seismic moment source dimension fault length average displacement on the fault plane and stress drop by applying madariaga s dynamic model the proposed iterative maximum entropy spectrum estimation algorithm finds smooth and more accurate spectra than those computed by the conventional fft routine for the main shock ms 6 7 of february 24 and its principal aftershocks ms 6 4 of february 24 and march 4 1981 a fault length of 20 20 18 km a seismic moment of 1 5 1026 4 8 1025 4 7 1025 dyne cm a stress drop of 58 17 16 bars and an average displacement on the fault plane of 159 cm 50 cm and 49 cm respectively were found these values are in agreement with the source parameters which have been proposed by other researchers

static stress drop in the 1994 northridge california aftershock sequence we use time domain pulse widths to estimate static stress drops for 279 ml 2 5 to 4 0 aftershocks of the 17 january 1994 mw 6 7 northridge california earthquake the stress drops obtained range from 0 02 to 40 bars with a log average of 0 75 bar error bars computed for our estimates are typically a factor of 5 indicating that the three order of magnitude scatter in stress drops is not solely a result of measurement errors and that there is a significant amount of heterogeneity in the static stress drops of the aftershocks stress drops might be expected to increase with depth since a fault can maintain a higher shear load at higher confining pressures we observe an increase in log average stress drop at about 15 km depth which is statistically significant at the 80 confidence level the increase is due primarily to a lack of lower stress drop events below this depth and may be controlled by material properties since the northridge aftershocks are observed to intersect an anomalously high velocity body at around this depth hauksson and haase 1997 an apparent increase in stress drop with magnitude is also observed over the entire magnitude range of the study although whether this trend is real or an artifact of attenuation of high frequencies in the upper crust is unresolved

source parameters of microearthquakes at mount st helens usa summary we estimate the source parameters for a selection of microearthquakes that occurred at mount st helens in the period 1995 1998 excluding the activity of 2004 september this time period includes the most intense episode of earthquake activity since the last dome building eruption in 1986 october 200 seismograms were processed to obtain seismic moments source radii stress drops and average fault slip the source parameters were determined from the spectral analysis of p waves after correction for attenuation and site effects in particular p wave quality qp and site s factors have been previously calculated in the frequency ranges 2 7 hz and 18 30 hz because it was impossible to perform corrections for qp and s over the whole spectrum we applied a new approach based on the notion of holed spectrum to estimate spectral parameters the term holed spectrum indicates a spectrum lacking corrected spectral amplitude values at certain frequencies we carried out a statistical study to verify that dealing with the holed spectrum does not lead to significant differences in the estimates of spectral parameters we also investigated the dependence of spectral parameters low frequency level corner frequency and high frequency decay on the bandwidth of spectral hole and defined the threshold values for three different spectral models displacement holed spectra corrected by attenuation and site response are then used to determine spectral parameters in order to calculate seismic source parameters seismic moments range from 1017 to 1019 dyne cm source dimensions from 100 to 350 m and average fault slip from 0 003 to 0 1 cm self similarity seems to break down in that stress drops are very low 0 1 1 bars we postulate that seismicity is associated with a brittle shear failure mechanism occurring in a highly heterogeneous material under a relatively low stress regime

frequency magnitude distribution of earthquakes in vrancea relevance for a discrete model a high resolution analysis of a homogeneous and complete data set of intermediate depth microearthquakes h 60 km extending over an interval of 9 3 years emphasizes a significant deviation at low magnitudes ml 4 in a linear frequency magnitude distribution it appears as a distinct change in the slope of the cumulative distribution and as a seismicity deficit followed by a relative enhancement in the corresponding noncumulative curve this is in agreement with the presence of two characteristic mechanisms and allows the identification of both the magnitude threshold of asperity like earthquakes and the transition zone ml 3 3 3 9 from crack like earthquakes background seismicity to asperity like events these features are better pointed out on well individualized active zones the background seismicity shows a tremendeous decrease in its b slope from 1 03 to 0 57 during a 6 year interval before the occurrence of a major earthquake in 1986 mw 7 3 followed by a fast recovery to 1 03 in 2 years after this event such a behavior could be correlated to the continuous growth of the shear stress free surface on the fault as in a percolation process followed by its sudden diminution due to the locking of the fault the above results provide relevance for the discrete character of faulting and enable a coherent modeling of the earthquake generation process from microearthquake to major event scale lengths

fault creep and microseismicity on the hayward fault california implications for asperity size 1 the hayward fault is documented to undergo significant creep with some patches accommodating 50 or more of the long term fault displacement in spite of this the fault has also experienced moderate to large earthquakes by comparing the patterns of microseismicity observed on the fault with models of fault zone creep we can investigate the long term displacement deformation history of the fault in terms of the relative roles of aseismic creep fault slip accommodated through microseismicity and strain accumulation slip deficit we find that microseismicity on the hayward fault produces a negligible percentage of the seismic moment dissipated on the fault combining seismicity with our fault creep models allows us to calculate the size of asperities on the creeping fault for small asperities associated with repeating earthquakes on the hayward fault the rupture areas of these asperities range from 20 to 60 m2

microearthquake spectra from the anza california seismic network site response and source scaling abstract we analyzed spectra of local microearthquakes recorded by the anza california seismic network to isolate the effects of site response and to investigate the scaling of source parameters for small earthquakes spectra of microearthquakes m m o 19 dyne cm at anza have shapes characteristic of the receiver sites and are generally independent of the source region thus the site response is a major conditioner of the observed spectral shape to remove the effects of site response from the spectra of a m 3 event and isolate its source spectrum we divided by the spectra of an adjacent aftershock used as an empirical green9s function event the spectral ratios indicate that the apparent corner frequencies of small earthquakes m o 19 dyne cm observed at even the high f max stations on hard rock are much lower than the source corner frequencies the spectral ratios are consistent with stress drop remaining constant with decreasing seismic moment for events with moments as small as 10 18 dyne cm the spectral ratios display remarkable agreement between sites which showed vast differences in their original spectra indicating that the spectral division effectively removed the site response the source spectrum of the m 3 event has a high frequency spectral fall off of about 2 an apparent dependence of high frequency fall off with seismic moment in the original spectra can also be explained by the effects of site response the difference between the p and s wave corner frequencies and high frequency roll offs in the observed spectra for these events is the result of the site response and is not a source property the shapes of the spectra of microearthquakes at anza can largely be explained by attenuation at shallow depth with a frequency independent q for some sites near surface resonances are also apparent in the spectra of microearthquakes it is indicated by t values determined for each site that q p q s for the shallow low q layer further evidence of low near surface q is observed in the anomalous spectra of an unusually shallow earthquake source depth 1 km in the network the spectra from this shallow event are depleted in high frequency energy at most stations relative to those of deeper events this observation can be explained by a low q surficial zone for stations of the network situated on alluvium this low q layer has a maximum thickness of about 3 km and maximum p and s wave q values of 30 to 50

seismic induced pore water pressure records from the mammoth lakes california earthquake sequence of 25 to 27 may 1980 abstract eight records of seismic induced pore water pressure and corresponding acceleration records were gathered from aftershocks of the 25 to 27 may 1980 mammoth lakes california earthquake sequence the records were obtained from lakeshore sediments at convict lake that had undergone liquefaction and subsequent lateral spreading failure the pore pressure records showed several similarities to corresponding strong motion acceleration records the pore pressure traces show clear p and s wave phases and are in phase with corresponding p wave components of the acceleration records s wave components of pore pressure records appear most similar in waveform to one or the other of the corresponding horizontal acceleration components although the p wave portions of pore pressure are in phase with their acceleration counterparts the amplitudes are not related by a strictly proportional factor s wave portions of the porepressure records show a phase lag compared to the s wave of the respective acceleration trace in most cases the frequency of the pore pressure s wave components are also about 5 hz lower in frequency than the acceleration s waves

coseismic crustal deformation on a fault zone defined by microseismicity in the pavliani area central greece summary pavliani is an area of high microseismicity in northern central greece there is no evidence in the historical record of large damaging earthquakes in this area since at least 600 bc we examine contemporary crustal deformation in this area using microearthquake data recorded during 1983 84 by the volos seismic network principally and to a lesser extent by the patras seismic network the microseismicity 1 4 6 ml defines an approximately vertical fault zone trending wsw ene of 55 km length and 25 km depth the fault plane solution for the 4 6 m earthquake shows a strike consistent with the trend of the microseismicity zone and an extension direction consistent with many large earthquakes in central greece associated with the extensional greater aegean spectra of the microearthquakes are estimated using p waves obtained by selective windowing designed to exclude other p phases from the coda seismic moments in the range 1 95 x 10 n m are obtained accompanied by estimates of seismotectonic source parameters including source radii average stress drop and average coseismic slip of prime utility is an excellent correlation found between seismic moment and magnitude and with coseismic slip this allows heterogeneity in the distribution of individual microearthquake source parameters to be mapped onto the vertical fault zone plane and the history of coseismic deformation for the whole seismicity observed during 1983 84 to be examined using cumulative slip and moment release rate peaks of high moment release rate occur in the east and centre of the fault zone that in the east is dominated by a few large moment release events that in the centre by a large number of small events both moment release rate peaks despite their different component seismicity are associated with high cumulative coseismic slip only the eastern peak is also associated with high stress drop it is also shallower and therefore any surficial interaction is less likely to be attenuated the surface projection of this peak is within 3 km of a mapped normal fault coincident with an alpine thrust front delineated by a major scarp of mesozoic limestone surface evidence of recent movement on this normal fault is only slight friable fault gouge is seen at a few localities displacement could not be determined because the limestone surfaces are deeply weathered and lack kinematic indicators however considering the whole fault zone as a cohesive entity and using a brune estimate of slip at the current moment release rate throughout the present extensional tectonic regime suggests a total vertical displacement of order 175 m similar to the elevation of the scarp

simulation of ground accelerations for the may 1980 mammoth lakes california earthquakes abstract strong motion accelerograms from the may 1980 mammoth lakes earthquake sequence are used to support the technique of estimating near source ground motion by the summation of existing records an sma 1 record recovered 3 km from the epicenter of the first magnitude m l 6 earthquake in the mammoth lakes sequence 25 may 16 33 44 utc is simulated by the summation of records from later events the source time history on which this summation is based comes from a study of the teleseismic p waves the summation method has several distinct advantages it is easy to apply inexpensive costly wave propagation codes are not needed and source to receiver propagation effects are automatically included the method however requires a good data base from which to perform the summation

determination of near surface attenuation with κ parameter to obtain the seismic moment stress drop source dimension and seismic energy for microearthquakes in the granada basin southern spain abstract a set of 43 microearthquakes of the granada basin with magnitude duration m d from 1 4 to 3 5 have been spectrally analysed the digital data used in this study was recorded by two short period digital seismic network five permanent stations using andalusian seismic network red sismica de andalucia rsa and three stations of a portable digital seismic network red sismica portatil rsp the displacement spectra for p and s waves were analysed with brune s j geophys res 75 1970 4997 j geophys res 76 1971 5002 source model and the spectra parameters were determined by snoke s model to correct spectra from path attenuation coda q was used for the spectra correction from near surface attenuation parameter was calculated obtaining it from acceleration spectra the values of parameter range from 0 01 to 0 04 s for p waves and from 0 006 to 0 04 s to s waves once the spectra of these attenuation effects was corrected we obtained the seismic moments that range from 5 45 10 17 dyne cm to 1 53 10 20 dyne cm the source radii are between 0 13 and 0 39 km the seismic energy ranges from 7 17 10 3 j to 1 13 10 8 j the stress drop values were below 4 bars the scaling relations between seismic moment and stress drop indicated decreasing stress drop with decreasing seismic moment