source rupture plane determination from directivity doppler effect for small earthquakes recorded by local networks abstract fault plane solutions from first motion polarity of p waves give two nodal planes among which it is not possible to distinguish the fault plane from the auxiliary one we found that for earthquakes recorded by a dense local network it is possible to identify the rupture direction from the doppler effect produced by directivity this principle was successfully applied using three different simple techniques direct inspection and time and frequency domains analyses to one m l 3 6 and six smaller 1 2 m l 2 0 well recorded earthquakes from northern baja california mexico the results indicate activity in faults conjugate to those currently mapped in this region

seismic site effects and the spatial interpolation of earthquake seismograms results using aftershocks of the 1986 north palm springs california earthquake abstract we address the following two questions can a microearthquake9s ground motions be modeled by incident p and s waves that excite a site transfer function that is a smooth function of incidence angle given recorded ground motions from a set of earthquakes having known locations and mechanisms can we derive such a site transfer function and use it to obtain the ground motions that would result from an earthquake source occurring somewhere in the same volume but having a location and mechanism that are different from the recorded events although many factors will cause two distinct microearthquake sources to have different seismograms at a common station in this paper we concentrate only upon the differences caused by source mechanisms p and s wave travel time variations and by variations in the site transfer function we specifically exclude the effects of waves scattered from heterogeneities in the geologic structure away from the seismic site we express the site transfer function as a sum of several terms having simple dependences upon incidence angle and azimuth each term is an independent function of time given a set of seismograms observed at the site we solve a linear system of equations for the time dependences of each term these time series may be used to calculate the seismograms that would have resulted from an earthquake having arbitrary mechanism and location this step is an interpolation we have applied this technique to seismograms after aftershocks of the 1986 north palm springs earthquake our interpolation technique works fairly well within the volume occupied by the recorded events but the method is not very successful at providing accurate seismograms for sources located outside the aftershock volume the primary causes of the inaccuracy are the inadequacy of our chosen angular functions to model the site response fully and the likely scattering of seismic waves by geological heterogeneities in this case the banning and mission creek faults near the seismic stations our methods could be used to determine the effects of single scattering from lateral heterogeneities in geologic structure

source parameters and fault plane determinations of the 28 december 1999 northeastern cairo earthquakes abstract the velocity records of the 28 december 1999 northeastern cairo egypt earthquake sequence have been analyzed for relocation focal mechanism source parameters and rupture directivity a cross correlation analysis was performed to recognize the waveform similarity and to obtain relative location on the basis of small time delays between first arrivals the three events exhibit good correlation and similarity in focal mechanisms actually the fault plane solutions display normal faulting with strike slip component dynamic source parameters were estimated for the three events using p wave spectrum for 21 selected seismograms at the seven nearest stations the corner frequency fo low frequency amplitude o and the average attenuation path were simulated by nonlinear least square best fitting the mean of the obtained values shows that the mainshock occurred on a fault length of 822 m with relative displacement of 51 cm seismic moment of 5 59 1015 n m and stress drop of 5 9 mpa the azimuthal variation of the obtained low frequency spectrum amplitude was used to estimate the rupture velocity and direction the mean rupture velocity estimated to be eight tenths of the local shear wave velocity as well as the azimuthal variation provides an indication for the rupture directivities of the three events that are of 175 5 estimated rupture directions were incorporated with p wave focal mechanisms to identify the actual fault planes of these events therefore the preferred fault planes are those that have the strike of 137 148 and 132 dip of 59 45 and 51 and rake of 39 33 and 45 respectively in this case the slip component in strike direction indicates a left lateral motion along the nw fault the obtained results imply the reactivation of the oligocene deep seated faults in the gulf of suez red sea trend nw se

rupture characterization of a low magnitude earthquake of central apennines italy abstract this paper describes a detailed study of the rupture mechanism of a m l 3 aftershock of the 1984 abruzzo central italy earthquake m s 5 8 to study the rupture extension shape and directivity the time durations of p and s far field pulses were analyzed in different directions corresponding to six recording stations the path corrected far field displacement p and s pulses at six seismic stations were determined using the empirical green function method the fault plane solution has been estimated by joint inversion of p polarities and s polarizations measured on three component records finally we used a method based on the plot of isochrons associated with the observed pulse widths and projected on each nodal plane the comparison of isochron plots for the two nodal planes suggests a rupture fault plane striking along the apenninic direction a weak source directivity up and west ward on the fault plane was observed a maximum ruptured surface of 0 4 km 2 was estimated assuming a sub shear rupture process the estimate of the seismic moment from spectral measurements gives the static stress drop ranging from 1 to 66 bar for the rupture velocity ranging between 0 2 and where is the shear wave velocity at the source

an evaluation of seismic decoupling and underground nuclear test monitoring using high frequency seismic data an effective solution to the problem of the detection and identification of low yield coupled and fully decoupled underground nuclear explosions appears available via use of high frequency seismic data ranging up to 30 or 40 hz in order to evaluate detection identification capabilities when using such data it is necessary to estimate 1 spectral characteristics and relative amplitudes of both p and s waves from explosions and earthquakes over the frequency band from 5 to 40 hz 2 signal transmission characteristics over this band through pertinent types of earth structure and 3 recording system and ground noise characteristics over this frequency band in this study each of these topics is considered in turn as they relate to detection and discrimination of the signals from low yield coupled and decoupled explosions in the regional and teleseismic distance ranges estimates of the capabilities of specific hypothetical networks to detect and identify insofar as signal to noise ratio is an important factor in identification explosions within the ussr are then considered these estimates of signal detection capability provide the central focus for the study as they serve to translate diverse and rather complex sets of observational data and theory into concrete predictions of monitoring capability following the assessment of detection capabilities the problem of identification of small events is considered with particular emphasis on discrimination at regional distances where the network is calculated to provide signals of high signal to noise ratio the principal results and conclusions of this study are as follows 1 seismic system noise can be suppressed to levels well below ground noise at quiet sites up to frequencies at least as high as 30 40 hz when using presently available hardware 2 average amplitudes of high frequency noise in a variety of geological environments are very low and change little with time or season 3 transmission of high frequency p and s wave signals in the regional distance range in stable continental areas and shields is nearly as efficient as at 1 hz with effective q factors in shield areas being about 9000 and 4000 for high frequency pn and sn respectively while the effective q for pn waves in tectonic areas is about 1000 4 a properly designed and deployed network of 25 simple three component nonarray stations internal to the ussr and 15 similar stations surrounding the ussr is predicted to be capable of multistation detection at high signal to noise ratio of fully decoupled 1 kt explosions located at all potential decoupling sites within the ussr 5 by inference from the quantitative agreement of empirical observations and theoretical predictions when using lower frequency data over a great range of explosion yields we conclude that procedures based on the use of both detectable p and s waves will serve to identify explosion generated seismic signals at least as small as those expected from a fully decoupled 1 kt explosion

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

spatial clustering and stress drops of foreshocks of the february 1990 tennyson and weber new zealand earthquakes the ability to distinguish foreshocks from background seismicity is very important in short term earthquake prediction to that end we have looked at spatial clustering using waveform cross correlation and stress drops of foreshocks of two new zealand earthquake sequences that occurred in 1990 the tennyson sequence located in a continental margin type strike slip environment consisted of a group of foreshocks an ml 5 8 mainshock and many aftershocks a cross correlation analysis showed five spatially close clusters of activity prior to the mainshock two were event pairs located within the final aftershock zone two were clusters of four events each located outside the aftershock zone and the fifth was a cluster of eight immediate foreshocks loated within the aftershock zone an analysis of two nearby control regions showed that pairs of identical events were not uncommon but larger clusters were stress drops of three events in the 12 days before the mainshock obtained by deconvolving small events as empirical green s functions were lower than for earlier preshocks and aftershocks source time functions derived from the green s function deconvolution indicated that a unilateral rupture model was more appropriate than a circular source model cross correlation values from the ml 5 9 weber sequence also showed spatial clustering but this was well removed from the mainshock in space and time a control area also showed similar clustering suggesting that it is a normal feature of the seismicity at a convergent margin the weber foreshocks only four in all were not highly correlated for both sequences foreshocks did not correlate with the aftershocks indicating that they occurred in a region of complete coseismic stress relief a stress drop of 1650 bars was obtained for a 44 km deep event that occurred within the upper part of the subducting pacific plate nearby but not related to the tennyson sequence

rupture properties of clustered microearthquakes near intersecting intraplate faults of new madrid seismic zone implications for fault weakening seismic waveforms from the portable array for numerical data acquisition network were used to identify sets of 21 similar microearthquakes near ridgely tennessee where two major segments of the new madrid seismic zone nmsz intersect the events are relocated with a precision of about 30 m and are found to have occurred along four subfaults that are aligned to n50 e subparallel to the southern arkansas segment of the nmsz the separations among the subfaults are a few hundred meters while the span of hypocenters within each subfault is typically a few tens of meters these subfaults are inferred from focal mechanism solutions to be oriented at high angles to n50 e trend striking roughly north northwest and dipping moderately with reverse slip this indicates that the subfaults belong to the central segment of the nmsz and are en echelon the similar events also cluster in time the typical time span of events over one subfaults is about 3 weeks while the intercluster times are a few months or more an empirical green s function analysis of the largest event placed a lower bound of its rupture dimension at 121 32 m suggesting that this rupture overlapped with the smaller ruptures along the same subfault on the basis of variations in fault zone properties previously inferred for the ridgely junction the rupture properties of the similar events and the result of a stress calculation we infer that the similar events may be triggered by temporal weakening of the subfaults possibly associated with a temporal elevation of pore fluid pressure the nearly constant depths 5 7 km and the reverse dip of the ruptures suggest that the en echelon subfaults may be intersected by subhorizontal veins which could have been developed by episodic influx of fluid at superhydrostatic pressures

preliminary results from vertical seismic profiling of oroville microearthquake s waves seismograms recorded along a 0 5 km deep borehole in oroville ca show that ground level s wave velocity spectra at this location are a product of site effects in the 6 25 to 50 hz range studied both the cleveland hill fault through which the borehole was drilled and the rock above it contribute to a substantial loss of s wave energy in first order approximation the average apparent s wave quality factor qsa along the well is 9 across the fault zone the qsa drops to a low of 3 downhole the seismograms can readily be separated into longitudinal and transverse components particle motion diagrams show that the s arrival is made up of two differently polarized waves this splitting is possibly an effect of wave propagation in the highly cracked and probably anisotropic rock of the region

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

the effects of attenuation and site response on the spectra of microearthquakes in the northeastern caribbean abstract the spectra of microearthquakes recorded by a seismic network in the northeastern caribbean were analyzed to determine the effects of attenuation and site response on these spectra the ratios of the spectral amplitudes at 5 and 20 hz were measured for several earthquakes at different distances from a fixed receiver to estimate q for p and s waves in the crust and upper mantle the average q p determined from three stations was 380 for epicentral distances between 40 and 200 km the average q s determined from two stations was estimated at 400 spectra of earthquakes closer than 40 km to one station indicate that a high q path q 800 exists for these short distances in the island arc platform the near equivalence of q p and q s observed in this study of the upper lithosphere contrasts with estimates of anderson et al 1965 that q p 9 4 q s in the asthenosphere this finding that q p is approximately equal to q s suggests that the attenuation mechanism in the upper lithosphere differs from that in the asthenosphere the spectra of nearby microearthquakes unaffected by crustal attenuation display corner frequencies which are uniform despite large differences in the seismic moments of the events the corner frequencies of these spectra appear to be characteristic of the receiver site and insensitive to the location and moment of the earthquakes at one station the s wave corner frequency of a nearby blast was identical to the corner frequency of a magnitude 4 8 earthquake derived from the record of a strong motion accelerograph at the same site

source processes of three aftershocks of the 1983 goodnow new york earthquake high resolution images of small symmetric ruptures broadband large dynamic range geos data from four aftershocks m l 2 to 3 of the 1983 goodnow new york earthquake recorded at hard rock sites 2 to 7 km away from the epicentral area are used to study rupture processes of three larger events with an m l 1 6 event as the green s function event we analyze the spectra and spectral ratios of ground velocity at frequencies up to 100 hz and conclude that 1 there are resolvable p wave f max 51 and 57 hz at two sites 2 there is abundant information on sources of larger events up to frequencies of 50 to 60 hz and 3 there is an unstable nonlinear instrument resonance at about 90 hz we analyze the artifacts of low pass filters in the deconvolved rupture process including the limited time resolution and biases in the rise time measurement some extensions of the empirical green s function egf method are proposed to reduce these artifacts and to precisely estimate the relative locations of events that are close both in time and in space applying the egf method to three aftershocks we find that these events have ruptures that are simple crack like characterized by small fault radii 70 to 120 m and static stress drops that vary depending on the size of the events between about 5 to 16 bars we also find that two of the events with origin times differing by 0 60 sec are separated by 190 110 meters assuming a causal relationship between the two would result in a slow propagation velocity 0 05 0 03 time of the local shear wave velocity we therefore interpret the corresponding ruptures as being distinct the area between the rupturing patches having a characteristic length much smaller than those of the rupturing patches comparison of the results of this study with those obtained for the goodnow main shock and microearthquakes in california and hawaii suggests that the stress drops of the goodnow aftershocks decrease considerably by up to a factor of 25 or more from those estimated for the main shock even after the estimated large uncertainty in the latter estimates is considered this decrease is similar to those reported for the nahanni earthquakes m b 5 0 to 6 5 in northwest territories and for the north palm springs earthquake m l 5 9 sequence in california the stress drops of the simple crack like microearthquakes are significantly variable by factors between 3 and 27 within the single source areas the median of stress drops of the intraplate goodnow aftershocks is lower by factors of 2 to 4 than the medians calculated for the other interplate microearthquake sequences

finite difference simulations of seismic scattering implications for the propagation of short period seismic waves in the crust and models of crustal heterogeneity synthetic seismograms produced by the finite difference method are used to study the scattering of elastic and acoustic waves in two dimensional media with random spatial variations in seismic velocity the results of this study provide important insights about the propagation of short period 5 the self similar medium is characterized by a scattering q that is constant with frequency whereas theory predicts that the apparent q in an exponential medium is proportional to frequency these alternative models of crustal heterogeneity can thus be tested by improved measurements of the frequency dependence of crustal q at frequencies greater than about 1 hz assuming that scattering is responsible for most of the attenuation at these frequencies measurements of the time decay of the synthetic coda waves clearly show that the single scattering model of coda decay is not appropriate in the presence of moderate amounts of scattering attenuation scattering q 200 in these cases q values derived from the coda decay rate using the single scattering theory do not correspond to the transmission q of the medium the cross correlation of synthetic waveforms observed for an array of receivers along the free surface is observed to be dependent on the correlation distance of the medium the self similar random medium proposed here for the crust produces waveform variations at high frequencies 15 30 hz similar to those reported for actual small scale seismic arrays with apertures of hundreds of meters

empirical green s function analysis of loma prieta aftershocks abstract following the 18 october 1989 loma prieta earthquake an array of 22 three component iris passcal instruments was deployed in the santa cruz mountains we use an empirical green9s function method to study source properties of aftershocks as well as attenuation in the epicentral area using pairs of closely spaced events we show that the largest aftershocks are well modelled by an 2 source spectrum and we obtain estimates of the p wave corner frequency for a total of 15 events brune stress drops are observed to have no systematic variation over the range of moments spanned by the data set down to 10 19 dyne cm there is a suggestion that aftershocks that occurred outside the mainshock rupture have higher stress drops than those that occurred within the mainshock rupture a m l 3 4 foreshock in the subsequence associated with the 18 april 1990 m l 5 4 watsonville earthquake yields a stress drop estimate of 820 bars this value is well outside the stress drops obtained for all other events 6 to 266 bars site specific attenuation parameters suggest large lateral and vertical heterogeneity in p wave q structure station holy in a valley near the surface trace of the san andreas fault is characterized by low q values while stations to the north of the aftershock zone are characterized by the highest q values

source parameters of the 15 november 1995 border town nevada earthquake sequence abstract the november 1995 border town nevada earthquake sequence occurred near the california nevada border approximately 20 km northwest of reno nevada the largest earthquake of the sequence m w 4 5 was widely felt throughout the reno sparks truckee region this event occurred on a west dipping high angle fault at a depth of 14 km and shows dip slip motion on a preferred fault plane orientation of strike n10 e dip 70 w and rake 75 we have relocated 27 aftershocks and one foreshock of the event using records from the local network and two portable digital instruments the largest aftershocks also align along a n10 e trend and define the preferred fault plane all of the aftershocks occur within a small volume with a 2 by 2 km horizontal extent and between depths of 10 and 14 km simultaneous determination of m 0 f c and is made by fitting spectra using various starting models i e initial s and based on an f 2 spectral shape m 0 ranges from 10 18 to 10 23 dyne cm for 14 events and hypocentral distances were less than 15 km we fit s wave spectra with different starting models to test the stability of s the results show that s and converged to different values depending on the starting model and the magnitude because of an attenuation source dimension trade off for example a model starting with a stress drop of 10 bars produced an average s of 27 bars and an apparent scaling breakdown below m w 3 starting the search at 50 bars produced an average s of 69 bars and no magnitude dependence we cannot resolve any breakdown in self similarity because we find plausible spectral models for all magnitudes where s is nearly constant at 60 bars seismograms of the four largest aftershocks m w 3 0 to 3 4 were deconvolved using smaller aftershocks as empirical green9s functions event radii estimated from the pulse widths yield stress drop averaging 34 bars within the range of the frequency domain results

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

fault plane determinations for three small earthquakes along the san jacinto fault california search for cross faults using rupture directivity and aftershock locations three earthquakes ml 4 2 4 3 were studied to determine which of the two nodal planes of the focal mechanism was the rupture plane all three events occurred near the san jacinto fault zone during december 1989 and february 1990 and had a large component of strike slip motion with nodal planes oriented in the northeast and northwest directions smaller earthquakes were used as empirical green functions to deconvolve the path and site complications from the short period p waveforms the resultant far field source time functions were inverted for slip on a finite fault and tested to see which nodal plane provided a better fit to the data the results of the waveform inversion showed that the earthquake near the anza gap december 2 1989 and the earthquake near san bernardino december 28 1989 ruptured northwest trending planes the earthquake near buck ridge february 18 1990 appeared to rupture a northeast trending plane but the waveform results for this event were not considered as reliable available aftershock data for the three events were also examined using master event relocations the aftershock sequence following the buck ridge event showed a clear lineation trending toward the northeast the results of this study indicate that the anza and san bernardino earthquakes ruptured areas on or parallel to the san jacinto fault while the buck ridge event ruptured a northeast trending cross fault in addition to determining the fault plane the waveform analysis resolved details of the slip distribution for these relatively small earthquakes

scaling of source parameters for vrancea romania intermediate depth earthquakes the stress drop scaling of the vrancea romania intermediate depth earthquakes with seismic moments from 10 to 20 dyne cm is investigated the empirical green s function technique is used in order to retrieve the seismic source parameters for the smaller earthquakes 1021 1026 dyne cm available source parameters for the major vrancea earthquakes that occurred in 1940 1977 1986 and 1990 mw 6 9 as determined from teleseismic and local data were included a circular source model approximates well the deconvolved source time functions and source spectra for the analyzed earthquakes in case of the large earthquakes an equivalent circular area is considered corresponding to the zone of aftershocks the static stress drop is determined from seismic moment and source radius for brune s model a dependence of the stress drop on the seismic moment as m140 is outlined over the entire magnitude range

rupture characteristics of the three m 4 7 1992 1994 parkfield earthquakes slip on the san andreas fault was determined for three m 4 7 earthquakes using a tomographic inverse system beroza and spudich 1988 to invert seismic source time functions stfs from s waves stfs were obtained by deconvolving mainshock accelerograms by those from collocated smaller earthquakes accelerograms were from the u s geological survey parkfield small aperture array upsar and from a distributed array of digital accelerometer stations at parkfield eight or nine stfs are used in each of the three inversions stfs are typically symmetrical pulses with a duration of about 0 3 0 5 s in the inversion mainshock rise time was set to 0 05 s and we allowed the rupture time to vary slightly from a constant rupture velocity of approximately 0 85 rupture for all three events which are located in or close to the middle mountain preparation zone or box mmb quickly reaches a local maximum in slip and then propagates outward to peaks ridges or plateaus in the slip distribution slip for the october 20 1992 event located just inside the southern edge of the mmb propagates from an initial spike north and updip along a curving ridge for about 2 km the initial spike continued to grow in the november 14 1993 event located north of the october 20 1992 event just beneath the hypocenter of the 1966 parkfield earthquake which shows little directivity although there is a smaller patch of slip updip and to the south in contrast rupture for the december 20 1994 event located just south of the october 20 1992 event propagated north and slightly updip creating a rough plateau in slip a few kilometers wide on a side directivity for this event also is to the north directivity for all three events points in the approximate direction of the 1966 hypocenter small pulses which comprise a coda are found on the stfs for several seconds after the initial impulsive event several tests based on the assumption that the average of all stfs from upsar for each event is an estimate of the true slip at the source suggest that the codas in the stfs are s waves from a long duration source rather than uncorrected site response an initiation phase is found on the array average for the november 14 1993 and december 20 1994 events these precursory phases are the result of a spike in slip at the hypocenter a value of 2 4 4 mm is obtained for d c the slip weakening distance by interpreting the initial spike as a critical patch the few aftershocks for the october 20 1992 event are distributed to the north and updip of the mainshock but the november 14 1993 event had a strong burst of aftershock activity that propagated to the north of its hypocenter at roughly the same depth aftershocks of the december 20 1994 event are mostly updip the november 14 1993 event had the simplest slip distribution appeared to be the most impulsive and had the most active aftershock sequence and the greatest depth if the eventual parkfield earthquake initiates near the 1966 hypocenter then the directivity of the three events studied here will have pointed to it however it is certainly possible that both the initiation of characteristic parkfield shocks and the directivity of smaller events are controlled by fault properties on a larger scale such as by fault bends or jogs

study of the source process of the 1992 colombia ms 7 3 earthquake with the empirical green s function method the source time function stf of the october 18 1992 colombia ms 7 3 earthquake has been determined using long period teleseismic surface and body waveform data and the empirical green s function egf method love and sh waveforms of a ms 6 7 foreshock were treated as the egfs and deconvolved from the mainshock waveforms to obtain the relative stf this indicated that the ms 7 3 mainshock was a double event followed 100 seconds later by a large aftershock not previously reported the source durations for the mainshock and the aftershock are about 45 and 25 seconds respectively based on the source radiation directivity the rupture directions for the mainshock and aftershock are about s50 w the total length of the fault for the mainshock and aftershock is about 140 km which agrees with the fault dimension estimated using the aftershock distribution the seismic moment releases for the three asperities on the rupture surface are 1 0 1019 6 9 1019 and 2 3 1019 nm with respect to time

source parameters of the 23 april 1992 joshua tree california earthquake its largest foreshock and aftershocks abstract seismic moments corner frequencies and stress drops have been calculated for the 23 april 1992 joshua tree california earthquake its largest foreshock and 12 aftershocks using recordings from terrascope stations source parameters are determined from nonlinear least squares best fits to ratios of p wave fourier amplitude spectra assuming an omega squared source model the spectral ratios are the mainshock spectra divided by the foreshock and aftershock spectra seismic moments range from 2 10 21 to 2 10 25 dyne cm corner frequencies range from 0 51 to 4 8 hz aftershock stress drops range from 4 2 to 41 bars with a log average stress drop of 15 bars the foreshock stress drop is relatively large 31 bars but is within the aftershock stress drop range the mainshock stress drop of 203 bars is significantly larger than any of the aftershock or foreshock stress drops

an egf technique to infer the rupture velocity history of a small magnitude earthquake 1 an empirical green s function egf technique has been developed to detect the rupture velocity history of a small earthquake the assumed source model is a circular crack that is characterized by a single and unipolar moment rate function mrf the deconvolution is treated as an inverse problem in the time domain which involves an assumed form of the moment rate function mrf the source parameters of the mrf are determined by adopting a global nonlinear inversion scheme a thorough synthetic study on both synthetic and real seismograms allowed us to evaluate the degree of reliability of the retrieved model parameters the technique was applied to four small events that occurred in the umbria marche region italy in 1997 to test the hypothesis of a single rupture process the inversion results were compared with those arising from another egf technique which assumes a multiple rupture process for each event the best fit model was selected using the corrected akaike information criterion for all the considered events the most interesting result is that the selected best fit model favors the hypothesis of a single faulting process with a clear variability of the rupture velocity during the process for the studied events the maximum rupture speed can even approach the p wave velocity at the source as theoretically foreseen in studies of the physics of the rupture and recently observed for high magnitude earthquakes

source scaling of earthquakes in the shumagin region alaska time domain inversions of regional waveforms the scaling of pulse duration to seismic moment is estimated for earthquakes along an interplate thrust zone from digital waveforms recorded by short period and broad band instruments of the east aleutian shumagin seismic network we measure pulse duration using an empirical green s function technique based on damped time domain deconvolution from several thousand events 22 earthquakes with magnitudes 3 0 7 0 and depths 23 56 km are found to give reliable estimates of pulse duration durations are also determined directly from one parameter nonlinear inversions for a variety of simple functional forms of source time functions symmetric source pulses boxcar or triangle shapes fit waveforms better than an asymmetric model t exp 2t d for most 62 per cent of the waveform pairs while the asymmetric model fits best for only 8 per cent of the data pulse duration increases with the size of events from 0 1 to 10s over the seismic moment m0 range of 1014 to 3 1019 nm when normalized by the cube root of seismic moment pulse durations show 8 x variation comparable static stress drop estimates range from 0 2 to 135 mpa contrary to predictions of some laboratory and theoretical studies earthquakes at the deepest part of the thrust zone do not show significantly higher stress drops than do shallower events rupture properties however show a strong dependence on earthquake size the three largest events m0 5 1018nm have the three longest normalized durations on average 3 8 times longer than those for smaller events the durations require smaller events to have 10 100 x larger static stress drops or 4 x faster rupture velocities or some combination of the two possibly the largest events rupture both strong and weak patches while smaller events just rupture strong patches on the fault surface the characteristic dimension that separates large from small events 3 15 km is comparable to characteristic wavelengths of pacific basin bathymetry and may reflect the influence of the subducted sea floor upon fault zone heterogeneity

rupture process of the ms 6 6 superstition hills california earthquake determined from strong motion recordings application of tomographic source inversion abstract we analyze strong motion recordings of the m s 6 6 superstition hills earthquake to determine the timing location spatial extent and rupture velocity of the subevents that produced the bulk of the high frequency 0 5 to 4 hz seismic energy radiated by this shock the earthquake can be characterized by three principal subevents the largest ones occurring about 3 and 10 sec after initiation of rupture timing relationships between pulses on the seismograms indicate that the three subevents are located within 8 km of each other along the northern portion of the superstition hills fault the two largest subevents display different directivity effects we apply a tomographic source inversion to the integrated accelerograms to determine the slip acceleration on the fault as a function of time and distance based on a one dimensional fault model the azimuthal distribution of amplitudes for the second subevent can be largely explained by a rupture that propagated about 2 km to the southeast along the superstition hills fault at a velocity about equal to the p wave velocity an alternative model with rupture propagating to the northeast along a conjugate fault plane can also account for the observed directivity of this subevent but it is not supported by the aftershock distribution the third subevent ruptured to the southeast along an 8 km long portion of the superstition hills fault at about the shear wave velocity this rupture propagation caused the relatively large accelerations and velocities observed in strong motion records for stations southeast of the hypocenter the long time intervals between the subevents and their relative proximity to each other indicate a very slow component to the rupture development the southern half of the superstition hills fault did not generate significant high frequency strong ground motion although it showed substantial co seismic surface displacement the subevents are situated along the same northern portion of the fault where most of the aftershocks are located the locations of the subevents appear to be controlled by bends in the fault mapped at the surface and by changes in basement structure at depth

source pulse enhancement by deconvolution of an empirical green s function observations of the earthquake source time function are enhanced if path recording site and instrument complexities can be removed from seismograms assuming that a small earthquake has a simple source its seismogram can be treated as an empirical green s function and deconvolved from the seismogram of a larger and or more complex earthquake by spectral division when the deconvolution is well posed the quotient spectrum represents the apparent source time function of the larger event this study shows that with high quality locally recorded earthquake data it is feasible to fourier transform the quotient and obtain a useful result in the time domain in practice the deconvolution can be stabilized by one of several simple techniques in this paper the method is implemented and tested on high quality digital recordings of aftershocks of the jan 9 1982 miramichi new brunswick earthquake in particular seismograms from a jan 17 aftershock 017 13 33 gmt local mag 3 5 exhibit path or site effects which complicate the determination of source parameters after deconvolution the apparent far field source of this event is a simple pulse in displacement with duration 0 07 second for both p and s

source properties of earthquakes near the salton sea triggered by the 16 october 1999 m 7 1 hector mine california earthquake we analyze the source properties of a sequence of triggered earthquakes that occurred near the salton sea in southern california in the immediate aftermath of the m 7 1 hector mine earthquake of 16 october 1999 the sequence produced a number of early events that were not initially located by the regional network including two moderate earthquakes the first within 30 sec of the p wave arrival and a second approximately 10 minutes after the mainshock we use available amplitude and waveform data from these events to estimate magnitudes to be approximately 4 7 and 4 4 respectively and to obtain crude estimates of their locations the sequence of small events following the initial m 4 7 earthquake is clustered and suggestive of a local aftershock sequence using both broadband trinet data and analog data from the southern california seismic network scsn we also investigate the spectral characteristics of the m 4 4 event and other triggered earthquakes using empirical green s function egf analysis we find that the source spectra of the events are consistent with expectations for tectonic brittle shear failure earthquakes and infer stress drop values of 0 1 to 6 mpa for six m 2 1 to m 4 4 events the estimated stress drop values are within the range observed for tectonic earthquakes elsewhere they are relatively low compared to typically observed stress drop values which is consistent with expectations for faulting in an extensional high heat flow regime the results therefore suggest that at least in this case triggered earthquakes are associated with a brittle shear failure mechanism this further suggests that triggered earthquakes may tend to occur in geothermal volcanic regions because shear failure occurs at and can be triggered by relatively low stresses in extensional regimes

source parameters of the december 2011 january 2012 earthquake sequence in southern carpathians romania abstract the seismicity at the contact between the getic depression and the south carpathians is part of the overall seismicity characterizing the contact of the moesian platform and the south carpathians orogen the december 2011 january 2012 earthquake sequence that occurred close to tg jiu city provides the best data set for the seismic activity in the region the seismic source parameters are estimated for 15 events of the sequence using the empirical green s function and spectral ratio techniques we selected 3 main events and 12 associated collocated small events 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 drop values in the range of 6 112 mpa relative small radius of the source and high stress drops suggest an intraplate type behaviour the scaling relationships investigated comply well with similar relationships in other regions in the world and in other seismogenic areas in the south carpathians region

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

asperities barriers characteristic earthquakes and strong motion prediction recent results from paleoseismology suggest that earthquakes recurrent on a given fault may often have the same characteristic length and amount of slip stable asperities and barriers which survive many earthquakes can explain these results the activity of volcano usu japan since 1977 which produced a dacite dome with a total upheaval of 180 m offered a great opportunity to study the recurrence behavior of a fault seismologists at hokkaido university revealed a family of repeated earthquakes with nearly the same amount of slip generated from the same fault plane on the crater wall along which the dome moved upward this family can be explained by the existence of a stable asperity on the crater wall and is called the asperity type family they found however another type of family of earthquakes which share the same fault plane but show a great range of variation in the amount of slip the behavior of the latter family can be explained by the presence of weak barriers over a fault plane defined by strong stable barriers and this family is referred to as the barrier type family this type of earthquake model has been used successfully in the interpretation of observed strong motion acceleration spectra from california and the results reveal the stability of local stress drop among different earthquakes the agreement found between local stress drops estimated from strong motion data and those inferred from geological observation supports the possibility for predicting earthquake strong motion directly from geological observation of the causative fault

waveform analysis of two preshock main shock aftershock sequences in utah we use waveform cross correlation to study clustering of small earthquakes that occurred from 1981 to 1983 within 20 km of 1 an ml 4 3 main shock near salt lake city utah on 8 october 1983 and 2 an ml 4 0 main shock near richfield utah on 24 may 1982 no clusters of earthquakes were observed prior to the salt lake city event during the time period studied however a cluster of four nearly identical preshocks within 80 m of each other possibly representing the failure of a critical asperity occurred 5 km northwest of the richfield event during a 4 hour period 10 months before the main shock we found no similar clusters among the other preshocks within the richfield study area all of which were located more than 10 km from the main shock the aftershocks in both study regions during the first 17 days after the respective main shocks occupied areas smaller than the rupture areas estimated for the main shocks from initial pulse width measurements both aftershock sequences showed a spatial migration of the initial events that could only be discerned from the waveform data one possible explanation for this migration could be propagating stress changes caused by the occurrence of the aftershocks themselves or by postseismic creep

source time functions of nuclear explosions and earthquakes in central asia determined using empirical green s functions relative source time functions rstf have been estimated for four underground nuclear explosions and seven earthquakes in central asia using broadband p waveforms of nearby smaller events as empirical green s functions egf rstfs of the four explosions m sub b 5 3 to 6 5 are each dominated by a simple pulse with a source duration of 0 4 to 0 8 s rstfs for two of the explosions show a significant secondary pulse with a pulse width similar to that of the first pulse the authors conclude that the secondary phases are most likely associated with the spall slapdown phenomenon seismic moment releases by the spall phases are less than one third of those by the first explosion pulses elastic radii of the explosions are estimated to be about 0 25 to 0 5 km and stress drops of the explosion range from 13 to 52 mpa in contrast rstfs of earthquakes studied m sub b 5 5 to 6 6 typically comprise multiple source pulses with a total source duration from a few to several tens of seconds indication that the complex source process involves a fault dimension of several tens of kilometers stress drops of the earthquakes are much smaller than those of the nuclearmore explosions ranging from 0 5 to 3 5 mpa the authors study demonstrates the power of the egf method for retrieving rstfs and reveals that differences in rstfs and source parameters can be used to distinguish large nuclear explosions from moderate to large earthquakes m sub b 5 5 80 refs 11 figs 6 tabs less

rupture nucleations in the 1995 hyogo ken nanbu earthquake and its large aftershocks summary local seismograms of the hyogo ken nanbu earthquake m 7 2 and its largest aftershocks show that a slow rising initial phase precedes the main p phase high dynamic range short period seismograms of the foreshocks mainshock and aftershocks were recorded by the japan meteorological agency jma network and the net hyogo using these seismograms we investigate the initial rupture process of the mainshock and large aftershocks using an empirical green s function method the deconvolved moment rate function of the mainshock shows that it begins with a small pre event mw 4 0 then the moment releases gradually and after about 0 6 s it increases rapidly the first 0 7 s of the deconvolved moment rate functions are inverted to obtain the spatio temporal images of the initial rupture process the results show that at the hypocentral area less than 1 km from the hypocentre slip velocity rises slowly with a low peak in the surrounding area 1 km east and downward from the hypocentre slip velocity rises rapidly with a high peak it is also found that during the initial rupture process the slip velocity pulse grows continuously these observations are in accord with a theoretical model of the transition process from nucleation to high speed dynamic rupture in which slip velocity rises slowly at the nucleation zone we also analyse the initiation and the main dynamic rupture of large aftershocks spatio temporal images of the largest aftershocks m 5 1 and m 4 8 show that there is a clear initial rupture process with a low moment release and after about 0 2 s slip velocity increases rapidly we compare the initial rupture process of the mainshock with those of the large aftershocks m 4 0 5 1 the duration 0 6 s of the initial phase of the mainshock is longer than those 0 02 0 3 s of the large aftershocks furthermore the slip velocity of the initial rupture process of the mainshock is several times larger than those of the two largest aftershocks these results suggest that the mainshock starts in the region where the stress drop and fracture energy is higher

rupture characteristics and tomographic source imaging of ml 3 earthquakes near anza southern california the rupture characteristics of several ml 3 earthquakes near anza california are determined from time domain analysis of path corrected displacement pulses these pulses are corrected for propagation effects apparent attenuation scattering site resonance using the waveforms of adjacent small events ml 2 1 as empirical green s functions the earthquakes studied occurred during two swarm sequences near cahuilla in an area characterized by swarm activity at shallow depth 5 km the data consist of seismograms recorded by the anza seismic network a high dynamic range local array with digital telemetry the waveforms of adjacent small events ml 2 1 are deconvolved from those of the ml 3 events to yield the source time functions of the larger earthquakes the ml 3 earthquakes display a variety of rupture modes similar to those reported for large shocks but over much shorter time scales and distances events with single and multiple ruptures are observed relative locations of the subevents are obtained from azimuthal differences in the time separation of pulses in the deconvolved displacement waveforms some events as small as ml 2 4 display unilateral rupture propagation as manifested by azimuthal variations in the pulse widths of the deconvolved displacement pulses a tomographic back projection technique is applied to the deconvolved displacement pulses to image the slip velocity on the fault as a function of time after nucleation and distance along the fault using a one dimensional fault model the tomographic inversion for one of the events ml 2 4 reveals a unilateral rupture with rupture velocity about equal to the shear wave velocity vs and fault length of 300 m a similar inversion for an ml 3 7 event resolves two subevents separated by about 200 m and yields an average rupture velocity of 0 8vs these rupture velocities are similar to those reported for large earthquakes indicating that rupture velocity is independent of seismic moment down to 3 1 1019 dyn cm the smallest event studied there are clear variations in pulse width for neighboring events with similar seismic moments implying significant differences in static stress drops factor of five variations in stress drop 11 61 bars are calculated for events within 200 m of each other thus the stress and or strength along this fault zone varies considerably over distances of hundreds of meters

source parameters of the 23 april 1992 m 6 1 joshua tree california earthquake and its aftershocks empirical green s function analysis of geos and terrascope data source parameters of the m 6 1 23 april 1992 joshua tree mainshock and 86 m 1 8 to 4 9 aftershocks are determined using an empirical green s function methodology for the aftershocks deconvolved p and s wave spectra are calculated for 126 pairs of closely spaced events recorded on portable geos stations s wave spectra from the two horizontal components are averaged the deconvolved spectra are fit by a ratio of omega square source models yielding an optimal least squares corner frequency for both the large and the small event in each pair we find no resolved difference between the inferred p and s wave corner frequencies using the standard brune model for stress drop we also find no resolved nonconstant scaling of stress drop with moment although we also conclude that detailed scaling systematics would be difficult to resolve in particular a weak increase of stress drop with moment over a limited moment magnitude cannot be ruled out for magnitudes smaller than m 3 to 3 5 the inferred stress drop values will be limited by the maximum observable corner frequency value of 60 hz for the mainshock source time functions are obtained from mainshock recordings at three terrascope stations pfo pas and gsc using an m 4 3 foreshock as an empirical green s function the results indicate a fairly simple single pulse source time function with clear south to north directivity and an inferred rupture radius of 5 to 6 km the deconvolved source time functions are inverted to obtain a finite rupture model that gives a robust estimate of rupture dimension early aftershocks are found to lie along the perimeters of regions with high mainshock slip the inferred mainshock stress drop value 56 bars is within the range determined for the aftershocks our derived mainshock source spectra do not show resolvable deviation from the omega square model

source inversion of the 1988 upland california earthquake determination of a fault plane for a small event abstract we examined short period p waves to investigate if waveform data could be used to determine which of two nodal planes was the actual fault plane for a small m l 4 6 earthquake near upland california we removed path and site complications by choosing a small aftershock m l 2 7 as an empirical green function the main shock p waves were deconvolved by using the empirical green function to produce simple far field displacement pulses we used a least squares method to invert these pulses for the slip distribution on a finite fault both nodal planes strike 125 dip 85 and strike 221 dip 40 of the first motion focal mechanism were tested at various rupture velocities the southwest trending fault plane consistently gave better fitting solutions than the southeast trending plane we determined a moment of 4 2 10 22 dyne cm the rupture velocity and thus the source area could not be well resolved but if we assume a reasonable rupture velocity of 0 87 times the shear wave velocity we obtain a source area of 0 97 km 2 and a stress drop of 38 bars choice of a southwest trending fault plane is consistent with the trend of the nearby portion of the transverse ranges frontal fault zone and indicates left lateral motion this method provides a way to determine the fault plane for small earthquakes that have no surface rupture and no obvious trend in aftershock locations

the anza array a high dynamic range broadband digitally radiotelemetered seismic array a small aperture seismic array has been deployed along a 30 km stretch of the san jacinto fault in the vicinity of the town of anza california the array was installed to study specifically the scaling laws of body wave spectra the character of high frequency ground motion the physical interpretation of seismic stress drops and the interaction of earthquakes this region was chosen for these studies due to its high rate of seismic activity in the 2 m 4 5 range the likelihood of a m 6 event in the next decade and the existence of the southern california batholith on either side of the fault reducing problems associated with attenuation and large scale anisotropy these studies employ an instrument package with a frequency response of 100 hz a dynamic range of 138 db a sampling rate of 250 times per second per component and a 16 bit a d converter the array consists of 10 three component stations telemetered via digital vhf radio to a nearby mountain peak and thence via a microwave link to la jolla california a minicomputer system monitors the array s performance detects events and records data upon demand initial results demonstrate the feasibility of digital transmission with an inherent increase in data quality over analog systems

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

source processes of 15 september 1998 m 5 0 sendai northeastern japan earthquake and its m 3 8 foreshock by waveform inversion a moderate earthquake m 5 0 occurred on 15 september 1998 in sendai city northeastern japan causing slight damage in a portion of the city it was located at about 12 km depth on the nagamachi rifu fault an active reverse fault passing through the center of the city we investigate rupture processes of the earthquake and its largest foreshock m 3 8 by an empirical green9s function method we use waveform data recorded by nearby dense strong motion arrays and by short period and broadband stations waveform inversion results for the mainshock show that the spatial extent of its rupture area is about 3 3 km 2 which is roughly consistent with that of the aftershock area most of the moment release was contained in two asperities aftershocks precisely relocated by a homogeneous station method show that their activity is high in areas with a relatively small amount of moment release of the mainshock rupture of the largest foreshock m 3 8 that occurred 6 min before the mainshock started approximately at the mainshock hypocenter its rupture area with a radius of about 300 m does not strongly overlap with the asperities of the mainshock

a seismotectonic analysis of the anza seismic gap san jacinto fault zone southern california small earthquake epicenters near the anza seismic gap define a 20 km quiescent segment of fault bounded to the northwest and southeast by areas of relatively high seismicity recent moderate earthquakes on and near the san jacinto fault in the gap and their relatively depressed aftershock activity indicate that the fault is seismogenic and highly stressed but locked by some mechanism the locked nature of the fault may be due to relatively high compressive stress normal to the fault resulting from the convergent geometries of the local active discontinuous faults and the oblique orientation of the regional maximum compressive stress strain is not being relieved by aseismic fault creep a swarm of small earthquakes in the crustal block 13 km southwest of the anza gap beneath the cahuilla valley recently released stress in an area which was previously highly active before the 1918 m 6 8 and 1937 m l 6 0 earthquakes the occurrence of these periods of increased seismicity near cahuilla in the years immediately before the nearby closer than 35 km large earthquakes and the recent swarm suggest that the ground beneath cahuilla may be acting as a stress meter signaling the presence of high stresses before large local earthquakes the length of the quiescent fault segment suggests potential for about an m 6 5 earthquake if the entire segment ruptures at once

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

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

the caledonian nappes of eastern north greenland reconnaissance work in kronprins christian land1 7 indicated the presence of one main nappe of late proterozoic sediments2 3 thrust over lower palaeozoic platform carbonates similar to peary land7 8 our 2 month helicopter supported work in 1980 now indicates the presence of three thrust bounded nappes the finderup land nappes which occur in northern kronprins christian land contain a late precambrian early cambrian sequence in thrust contact above silurian carbonates the younger vandredalen nappe in thrust contact above the finderup land nappes and silurian carbonates occurs throughout kronprins christian land and contains a thick sequence of late proterozoic elastics and carbonates in southern kronprins christian land the saefaxi elv nappe only contains ordovician silurian carbonates and is in thrust contact below vandredalen nappe the nappes were involved in westward directed thrusting 150 km and are probably a result of continental collision when the northern part of lapetus ocean closed in the late silurian

source characterization and fault plane determination for mblg 1 2 to 4 4 earthquakes in the charlevoix seismic zone quebec canada two earthquake doublets and two multiplets recorded by the charlevoix telemetered network cltn in the charlevoix seismic zone csz of southern quebec canada have been analyzed using an empirical green s function egf method to derive the relative source time functions rstf s of seven master events with mblg 1 2 to 4 4 we identified the doublets and multiplets using a waveform cross correlation and relative event location technique to verify that each earthquake pair had similar focal mechanisms and hypocentral locations three component s waveforms recorded by the high dynamic range 126 db instrumentation of the cltn were used to extract the rstf s the rstf s reveal that six of the seven events are simple with single source pulses having durations of 0 05 to 0 2 sec another earthquake 920310 0545 m 3 3 appears to be a double event with two episodes of rupturing azimuthal variations of the rstf pulse amplitudes and widths provide strong evidence for the rupture directivities of five of the earthquakes m 1 2 to 4 4 the azimuthal variations in the rstf pulse amplitudes were used to estimate the rupture directions and rupture velocities lower bound estimates of the rupture velocity range from 0 5 to 0 7 vs estimates of the rupture direction were combined with p wave focal mechanisms for the four largest events m 3 3 to 4 4 to identify the fault plane for these earthquakes source parameters were measured for the rstf s of the master events including seismic moments of 3 5 1018 to 5 3 1021 dyne cm fault radii of 100 to 330 m and static stress drops of 2 to 90 bars the fault radii and stress drop estimates for m 3 events agree well with estimates obtained by other researchers for m 3 to 4 5 earthquakes in the csz we also observed apparent scaling between the stress drop and the earthquake size which has been reported in other studies of stress drop in northeastern north america

rupture directivity of fluid induced microseismic events observations from an enhanced geothermal system the rupture process of fluid induced microseismic events is still poorly understood mainly due to usually small magnitudes and sparse monitoring geometries the high quality recordings of the earthquake sequence 2006 2007 at the enhanced geothermal system at basel switzerland constitute a rare exception allowing a systematic directivity study of 195 events using the empirical green s function method we observe clear directivity signatures for about half the events which demonstrates that rupture directivity persists down to small magnitudes ml 1 the predominant rupture behavior is unilateral we further find evidence that directivity is magnitude dependent and varies systematically with distance from the injection source whereas pore pressure seems to play the dominant role close to the injection source and no preferred rupture direction is observable directivity aligns parallel to the event distribution with increasing distance 100 m and is preferably oriented away from the injection point the largest analyzed events ml 2 show a distinct behavior they rupture toward the injection source suggesting that they nucleate in the vicinity of the pressure front and propagate backward into the perturbed volume this finding is of particular relevance for seismic hazard assessment of georeservoirs since it implies that maximum event size is related to dimension of the fluid perturbed volume our study also resolves rupture complexities for a small group of events this shows that small fault heterogeneities exist down to a scale of a few tens of meters the observation of directivity and complexity in induced microseismic events suggest that future source studies account for these phenomena

properties of the seismic nucleation phase abstract near source observations show that earthquakes begin abruptly at the p wave arrival but that this beginning is weak with a low moment rate relative to the rest of the main shock we term this initial phase of low moment rate the seismic nucleation phase we have observed the seismic nucleation phase for a set of 48 earthquakes ranging in magnitude from 1 1 8 1 the size and duration of the seismic nucleation phase scale with the total seismic moment of the earthquake suggesting that the process responsible for the seismic nucleation phase carries information about the eventual size of the earthquake the seismic nucleation phase is characteristically followed by quadratic growth in the moment rate consistent with self similar rupture at constant stress drop in this paper we quantify the properties of the seismic nucleation phase and offer several possible explanations for it

comprehensive analysis of earthquake source spectra in southern california we compute and analyze p wave spectra from earthquakes in southern california between 1989 and 2001 using a method that isolates source receiver and path dependent terms we correct observed source spectra for attenuation using both fixed and spatially varying empirical green s function methods estimated brune type stress drops for over 60 000 m l 1 5 to 3 1 earthquakes range from 0 2 to 20 mpa with no dependence on moment or local b value median computed stress drop increases with depth in the upper crust from about 0 6 mpa at the surface to about 2 2 mpa at 8 km where it levels off and remains nearly constant in the midcrust down to about 20 km however the results at shallow depths could also be explained as reduced rupture velocities near the surface rather than a change in stress drop spatially coherent variations in median stress drop are observed with generally low values for the imperial valley and northridge aftershocks and higher values for the eastern transverse ranges and the north end of the san jacinto fault we find no correlation between observed stress drop and distance from the san andreas and other major faults significant along strike variations in stress drop exist for aftershocks of the 1992 landers earthquake which may correlate with differences in main shock slip

tomographic imaging of the earthquake rupture process to advance our understanding of the earthquake rupture process we must develop methods that will map observations into the space time variation of seismic moment release on the fault plane while the temporal history of overall moment release is relatively easy to obtain from p waves there has been no objective quantitative method available to directly determine the spatial variation in moment release the theoretical connection between spatially varying moment release and the resultant seismogram is a linear integral relationship specifically a radon transform this paper outlines a linear inverse method that provides a high resolution tomographic image of the space time varying moment density function application of the method to p waves from the large 1976 mindanao earthquake demonstrates the ability of this method to reliably extract coherent pulses of moment release in space and time

the persistence of directivity in small earthquakes abstract we derive a simple inversion of peak ground acceleration pga or peak ground velocity pgv for rupture direction and rupture velocity and then test this inversion on the peak motions obtained from seven 3 5 m 4 1 earthquakes that occurred in two clusters in november 2002 and february 2003 near san ramon california these clusters were located on two orthogonal strike slip faults so that the events share the same approximate focal mechanism but not the same fault plane three earthquakes exhibit strong directivity but the other four earthquakes exhibit relatively weak directivity we use the residual pgas and pgvs from the other six events to determine station corrections for each earthquake the inferred rupture directions unambiguously identify the fault plane for the three earthquakes with strong directivity and for three of the four earthquakes with weak directivity the events with strong directivity have fast rupture velocities 0 63 v 0 87 the events with weak directivity either rupture more slowly 0 17 v 0 35 or bilaterally the simple unilateral inversion cannot distinguish between slow and bilateral ruptures adding a bilateral rupture component degrades the fit of the rupture directions to the fault planes by comparing pgas from the events with strong and weak directivity we show how an up dip rupture in small events can distort the attenuation of peak ground motion with distance when we compare the rupture directions of the earthquakes to the location of aftershocks in the two clusters we find than almost all the aftershocks of the three earthquakes with strong directivity occur within 70 of the direction of rupture

influence of convective adjustment on the stability of the thermohaline circulation abstract three different convective adjustment schemes are employed in the gfdl gcm to investigate if the spontaneous collapse of the thermohaline circulation under mixed boundary conditions as observed by f bryan depends on the parameterization of convective overturning it is found that both a procedure guaranteeing complete static stability and cox s implicit vertical diffusion scheme avoid the spontaneous collapse both schemes are also insensitive to the choice of time step whereas the standard gfdl convection algorithm in conjunction with mixed boundary conditions produces results that differ qualitatively from each other when different time steps are used

rupture characteristics of clustered microearthquakes and variations in fault properties in the new madrid seismic zone seismic waveforms from the portable array for numerical data acquisition panda network were used to relocate 45 events occurring in four spatial clusters in the northern new madrid seismic zone nmsz source time functions of the larger ml 2 5 events were obtained and yield estimates of rupture radii between 130 and 175 m and static stress drops at or below the megapascal level there is a tendency for closely located events within 200 m from one another to occur within short time intervals of minutes to days available focal mechanism solutions reveal that some of the clustered events have diverse slip directions indicating that the ruptures occur on adjacent fault patches rather than being repetitive over the same patch on the basis of this and a previous study of events farther south we infer that the properties of the faulting system in the nmsz are laterally heterogeneous and are subject to effects of some temporally and spatially localized processes that drive the faults toward seismic failure comparison of this observation with that in the san andreas fault near parkfield reveals that event clustering occurs in both fault systems on similarly short spatial and temporal scales the inferred localized processes may occur in both fault systems despite their different tectonic environment far field loading rate and geometric style of deformation on the other hand in the nmsz we have not observed any quasiperiodic and repetitive ruptures that have been observed near parkfield this is consistent with a slower or absent aseismic creeping in the nmsz

source process and tectonic implication of the january 20 2007 odaesan earthquake south korea abstract the source process for the 20th of january 2007 mw 4 5 odaesan earthquake in south korea is investigated in the low and high frequency bands using velocity and acceleration waveform data recorded by the korea meteorological administration seismographic network at distances less than 70 km from the epicenter synthetic green functions are adopted for the low frequency band of 0 1 0 3 hz by using the wave number integration technique and the one dimensional velocity model beneath the epicentral area an iterative technique was performed by a grid search across the strike dip rake and focal depth of rupture nucleation parameters to find the best fit double couple mechanism to resolve the nodal plane ambiguity the spatiotemporal slip distribution on the fault surface was recovered using a non negative least square algorithm for each set of the grid searched parameters the focal depth of 10 km was determined through the grid search for depths in the range of 6 14 km the best fit double couple mechanism obtained from the finite source model indicates a vertical strike slip faulting mechanism the nw faulting plane gives comparatively smaller root mean squares rms error than its auxiliary plane slip pattern event provides simple source process due to the effect of low frequency that acted as a point source model three empirical green functions are adopted to investigate the source process in the high frequency band a set of slip models was recovered on both nodal planes of the focal mechanism with various rupture velocities in the range of 2 0 4 0 km s although there is a small difference between the rms errors produced by the two orthogonal nodal planes the sw dipping plane gives a smaller rms error than its auxiliary plane the slip distribution is relatively assessable by the oblique pattern recovered around the hypocenter in the high frequency analysis indicating a complex rupture scenario for such moderate sized earthquake similar to those reported for large earthquakes

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

strong motion seismology strong motion seismology is concerned with earthquake ground motion in the amplitude range that poses the threat of human injury or property damage because the occurrence of strong motion cannot be predicted in advance and because the time between significant events at any one place may range from tens to thousands of years strong motion data is recorded on triggered instruments that typically must be maintained in readiness for years between periods of significant recording as a consequence the strong motion data set is sparse because of the engineering need for estimates of future ground motion the scope of the field of strong motion seismology encompasses seismic source theory and any other aspect of seismology that can provide insights helpful in making the best possible estimates given the limited data this relationship with the rest of seismology is a reciprocal one in that strong motion data gives perhaps the best information available on earthquake source processes

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

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

determination of rupture duration and stress drop for earthquakes in southern california a simple technique is developed for determining the rupture duration and stress drop of earthquakes between magnitudes 3 5 and 4 0 using the time between the p wave onset and the first zero crossing 1 2 on seismograms from local seismic networks this method is applied to 10 main shocks in southern california to investigate regional variations in stress drop the initial pulse widths of 65 foreshocks or aftershocks of these events were measured values of 1 2 for small earthquakes below about magnitude 2 2 are generally observed to remain constant with decreasing magnitude in four sequences studied the relative pulse width of a particular main shock m 3 5 at a given station is found to be correlated with the relative pulse width of its aftershocks recorded at that station these observations are interpreted to signify that the waveforms of these small events m 2 2 are essentially the impulse response of the path between the source and receiver values of 1 2 determined from small foreshocks and aftershocks are therefore subtracted in effect deconvolved from those of each main shock to obtain an estimate of the rupture duration of the main shock which is corrected for path effects significant variations in rupture duration and stress drop are observed for the main shocks studied aftershock locations and azimuthal variations in 1 2 both indicate that the rupture zone of one earthquake expanded unilaterally a factor of 10 variation in stress drop is calculated for two adjacent events of similar seismic moments occurring 1 hr apart on the san jacinto fault system the first event in this pair had the highest stress drop of the events studied 860 bars and was followed within 8 months by a magnitude 5 5 earthquake 2 km away

high frequency spectral scaling of a main shock aftershock sequence near the norwegian coast a high frequency seismic element was recently added to the noress regional array in norway this system can monitor seismic signals at frequencies up to 50 hz in february 1986 the high frequency seismic element recorded an mblg 4 7 main shock and several aftershocks which occurred 420 km northwest of noress off norway s western coast these events produced high frequency signals which were well above the background noise at the station p wave spectra of these events scale in a manner consistent with the square constant stress drop source model the data do not require any change in this scaling to magnitudes mblg below 2 in contrast to previous reports that constant stress drop scaling breaks down at smaller magnitudes

single station decomposition of seismograms for subevent time histories we have adapted an iterative lseast squares inversion with positivity constraints to the problem of deconvolving the recordings of a small earthquake from the recordings of a large composite earthquake the inversion uses an f test to minimize the number of non zero amplitudes or subevents in the solution set by requiring each added subevent to reduce the variance significantly we apply this inversion to the analysis of strong motion recordings of a set of moderate sized 4 0m l 6 4 aftershocks of the 1976 friuli earthquake