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

fault plane orientations of small earthquakes of the 1997 umbria marche italy seismic sequence from p wave polarities and rise times summary a two step technique has been developed with the aim of retrieving the fault plane orientation of a small magnitude earthquake the technique uses a set of non linear equations which relate the rise times of first p waves to source parameters source dimension l dip and strike of the fault and intrinsic qp at the first step of the technique the inversion of p polarities provides two fault plane orientations for each focal mechanism solution at the second step the inversion of rise times is carried out to retrieve l and qp by fixing and to the values of the fault plane orientation inferred at the first step a robust analysis based on the random deviates technique allows us to evaluate if one fault plane exists which systematically better fits data and can be considered the true fault plane a parameter is introduced to quantify the level of resolution of the true fault plane a probabilistic approach based on the assumption that errors on data are gaussian distributed allows us to a posteriori validate or reject the true fault plane the technique has been applied to 47 small earthquakes 1 3 ml 3 9 occurred below the town of sellano during the 1997 umbria marche central italy seismic crisis the strike of the inferred fault planes is generally along the apennine direction fault plane solutions can be mainly subdivided into two groups a first group of sw dipping faults and a second group of ne dipping faults these results agree with the present day knowledge of the fault systems in the area an average qp equal to 354 63 has been estimated in agreement with previous attenuation studies

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

the relation between site amplification factor and surficial geology in central california abstract site amplification factors at frequencies of 1 5 3 6 and 12 hz were determined for 132 stations of the usgs seismic network in central california from coda waves of 185 local earthquakes in this area using a recursive stochastic inversion method we found that the site amplification at a station is systematically related to the geology underlying that station the site amplification is high for young quaternary sediments and decreases with increasing geologic age at all frequencies between 1 5 and 12 hz the rate of decrease varies with frequency where site amplification at low frequencies shows a faster rate of decrease with age than at higher frequencies to quantify the relation between site amplification factor and site geology the surface geology of station sites was classified into five groups namely 1 quaternary sediments 2 pliocene sediments 3 miocene through cretaceous sediments 4 franciscan formation and mesozoic granitic rocks and 5 pre cretaceous metamorphic rocks the station site amplification factors for each group were logarithmically averaged and the mean value was assigned to the median geologic age of that group a smooth power law relation was found between the mean site amplification and the median geologic age this relation provides a simple way of estimating site effect at a specific site with known surficial geology our site amplification factors were compared with magnitude station residuals determined by eaton 1990 for the same network a remarkable linear correlation was found between the logarithmic coda amplification factor and the magnitude station residual value confirming that the coda method provides an effective means of estimating site amplification factor comparison of our site amplification factor with strong motion results obtained from the loma prieta earthquake boore et al 1989 maley et al 1989 shakal et al 1989 chin and aki 1991 suggests that weak and strong motion site amplification factors correlate well in the region outside the epicentral source region beyond epicentral distances of about 50 km within the epicentral source region however the weak motion amplification factors estimated from coda waves do not agree with the observed for strong ground motion suggesting a nonlinear behavior at sediment sites these results together with studies on the correlation between weak motion site amplification and earthquake intensity borcherdt 1970 borcherdt and gibbs 1976 chavez garcia et al 1990 king et al 1990 demonstrate the importance of in situ determination of weak motion site amplification for seismic zonation

local observations of the onset of a large earthquake 28 june 1992 landers california the landers earthquake mw 7 3 of 28 june 1992 had a very emergent onset the first large amplitude arrivals are delayed by about 3 sec with respect to the origin time and are preceded by smaller scale slip other large earthquakes have been observed to have similar emergent onsets but the landers event is one of the first to be well recorded on nearby stations we used these recordings to investigate the spatial relationship between the hypocenter and the onset of the large energy release and to determine the slip function of the 3 sec nucleation process relative location of the onset of the large energy release with respect to the initial hypocenter indicates its source was between 1 and 4 km north of the hypocenter and delayed by approximately 2 5 sec three station array analysis of the p wave shows that the large amplitude onset arrives with a faster apparent velocity compared to the first arrivals indicating that the large amplitude source was several kilometers deeper than the initial onset an ml 2 8 foreshock located close to the hypocenter was used as an empirical green s function to correct for path and site effects from the first 3 sec of the mainshock seismogram the resultant deconvolution produced a slip function that showed two subevents preceding the main energy release an mw 4 4 followed by an mw 5 6 these subevents do not appear anomalous in comparison to simple moderate sized earthquakes suggesting that they were normal events which just triggered or grew into a much larger earthquake if small and moderate sized earthquakes commonly detonate much larger events this implies that the dynamic stresses during earthquake rupture are at least as important as long term static stresses in causing earthquakes and the prospects of reliable earthquake prediction from premonitory phenomena are not improved

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

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

sedimentation rates and patterns in beaver ponds in a mountain environment abstract sediment depth was measured at several sites within each of eight beaver ponds in glacier national park montana and sediment samples wen collected from five of these ponds accumulation rates of sediments far exceeded published rates from boreal forest landscapes in eastem and central north america pond area strongly predicts volume of sedimentation textural differences illustrated spatial variations associated with position in a pond and along a pond sequence organic matter content was significantly higher in older ponds and has ramifications for the development of the benthos and the long term storage of matter in ponds the role of beavers as biogeomorphic agents is profound but requires further elucidation to distinguish between fluvial sediment deposition in ponds and sediment deposition associated with beaver excavational activity

a common origin for aftershocks foreshocks and multiplets we demonstrate that the statistics of earthquake data in the global cen troid moment tensor cmt and national earthquake information center neic catalogs and local california council of the national seismic system cnss catalog are consistent with the idea that a single physical triggering mechanism is responsible for the occurrence of aftershocks foreshocks and multiplets specifically we test the hypothesis that tectonic earthquakes usually show clustering only as a result of an initial earthquake triggering subsequent ones and that the magnitude of each trig gered earthquake is entirely independent of the magnitude of the triggering earth quake therefore a certain percentage of the time as determined by the gutenberg richter magnitude frequency relationship an earthquake should by chance be larger than or comparable in size to the earthquake that triggered it this hypothesis predicts that the number of times foreshocks or multiplets are observed should be a fixed fraction of the number of aftershock observations we find that this is indeed the case in the global cmt and neic catalogs the average ratios between foreshock aftershock and multiplet rates are consistent with what would be predicted by the gutenberg richter relationship with b 1 we give special attention to the solomon islands where it has been claimed that unique fault structures lead to unusually high numbers of multiplets we use monte carlo trials to demonstrate that the solomon islands multiplets may be explained simply by a high regional aftershock rate and earthquake density we also verify our foreshock results from the more complete recordings of small earthquakes available in the california catalog and find that foreshock rates for a wide range of foreshock and mainshock magnitudes can be projected from aftershock rates using the gutenberg richter relationship with b 1 and the relationship that the number of earthquakes triggered varies with triggering earthquake magnitude m as c10m where c is a productivity constant and is equal to 1 finally we test an alternative model that proposes that foreshocks do not trigger their mainshocks but are instead triggered by the mainshock nucleation phase in this model the nucleation phase varies with mainshock magnitude so we would expect mainshock magnitude to be correlated with the magnitude number or spatial extent of the foreshocks we find no evidence for any of these correlations

source properties of two microearthquakes at kilauea volcano hawaii abstract the source time functions of two microearthquakes with magnitudes 2 1 and 2 3 at kilauea volcano hawaii are determined by using the deconvolution technique p wave seismograms of a smaller event with magnitude 1 6 in a nearby location are treated as empirical green9s functions and deconvolved from the waveforms of the larger earthquakes by spectral division from the time domain analysis of the source time functions some of the source properties such as the complexity and directivity and some of the source parameters such as pulse width rise time and source dimension are determined with a good azimuthal distribution of stations the direction and magnitude of rupture velocity are estimated for one of the events the rupture direction is about n85 w and rupture velocity is 0 75 v s assuming v p 1 73 v s the result for direction agrees with one of the nodal planes of the focal mechanism our results indicate that there exist obvious variations in rupture durations for nearby small events with similar seismic moments suggesting significant differences in the static stress drops the stress drops of two events 1 5 and 16 5 bar estimated from the rise times differ by an order of magnitude implying heterogeneity of the stress distribution over this small area

resolving fault plane ambiguity for small earthquakes summary we have developed an automated procedure to resolve fault plane ambiguity for small to medium sized earthquakes 2 5 ml 5 using synthetic green s tensors computed in a 3 d earth structure model and applied this procedure to 35 earthquakes in the los angeles area for 69 per cent of the events we resolved fault plane ambiguity of our cmt solutions at 70 per cent or higher probability for some earthquakes the fault planes selected by our automated procedure were confirmed by the distributions of relocated aftershock hypocentres in regions where there are no precisely relocated aftershocks or for earthquakes with few aftershocks we expect our method to provide the most convenient means for resolving fault plane ambiguity our procedure does not rely on detecting directivity effects therefore it is applicable to any types of earthquakes

earthquake source parameters and fault kinematics in the eastern california shear zone based on waveform data from a profile of aftershocks following the north south trace of the 28 june 1992 landers rupture across the mojave desert we construct a new velocity model for the mojave region that features a thin slow crust using this model we obtain source parameters including depth and duration for each of the aftershocks in the profile and in addition any significant m 3 7 joshua tree landers aftershock between april 1992 and october 1994 for which coherent terrascope data were available in all we determine source parameters and stress drops for 45 significant m w 4 earthquakes associated with the joshua tree and landers sequences using a waveform grid search algorithm stress drops for these earthquakes appear to vary systematically with location with respect to previous seismic activity proximity to previous rupture i e with respect to the landers rupture and with tectonic province in general for areas north of the pinto mountain fault stress drops of aftershocks located off the faults involved with the landers rupture are higher than those located on the fault with the exception of aftershocks on the newly recognized kickapoo landers fault stress drops are moderate south of the pinto mountain fault where there is a history of seismic swarms but no single throughgoing fault in contrast to aftershocks in the eastern transverse ranges and related to the 1992 big bear california sequence landers events show no clear relationship between stress drop and depth instead higher stress drop aftershocks appear to correlate with activity on nascent faults or those that experienced relatively small slip during mainshock rupture

the superstition hills california earthquakes of 24 november 1987 the 24 november 1987 superstition hills earthquakes occurred on the conjugate northwest striking right lateral superstition hills fault and a previously unknown northeast striking left lateral structure defined by a lineation of hypocenters extending from the superstition hills fault to the brawley seismic zone master event locations of the earthquakes using caltech usgs seismic network data reveal the following the first main shock 33 4 9 n 115 47 7 h 10 6 km m s 6 2 0154 gmt on the northeast striking structure and its foreshocks co locate events on the northeast structure cluster in space and time and some aftershocks occur in the brawley seismic zone the second main shock 33 0 9 n 115 50 9 w h 1 9 km m s 6 6 1315 gmt on the superstition hills fault was 12 hr after the first main shock and initiated at shallow depth where the two trends join aftershocks of the second main shock lie in a northwest trend between the superstition hills and superstition mountain faults and have a sharply defined western edge the extent of the northwest trend aftershocks is not coincident with surface rupture on the superstition hills fault in general the earthquakes in the northeast trend are deep and the earthquakes in the northwest trend are shallow we compare the earthquake distribution to the distribution of crystalline basement rocks defined from a refraction study of fuis et al 1982 the earthquake locations and extent of aftershock activity appear to be controlled by the presence of crystalline basement rocks

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

is the cauca patia and romeral fault system left or rightlateral extrusion driven collision is controlled by strike slip faults of opposite senses whereas subduction related oblique convergence is accommodated by margin parallel faults of identical senses northern andes geodynamics has been interpreted either as resulting from the obliquity of convergence between nazca and south american plates or from the collision of the carnegie ridge with ecuadorian margin the sense of the cauca patia and romeral fault system cprfs has been interpreted in two opposite manners right or left lateral agreeing with oblique or collisional patterns respectively we analyze arguments which favor south to 4 n a right lateral motion along the cprfs that is consistent with obliquity driven geodynamics however north of 5 n the cprfs is actually left lateral these opposite motions between the northern and southern part of the cprfs have to be accommodated around 5 n by n s stretching and or shortening existing data support n s shortening around 5 n this buttressing could be due to the collision between the panama and andean blocks

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

determination of fault planes in a complex aftershock sequence using two dimensional slip inversion summary the 1994 arthur s pass earthquake mw 6 7 south island new zealand had a complex aftershock sequence including events aligned with major mapped faults to determine whether the major ne sw trending strike slip faults in the region were activated during this aftershock sequence we investigate the largest well recorded aftershocks the arthur s pass earthquake itself was a reverse faulting event but the majority of the aftershocks were strike slip we use the empirical green s function method to obtain source time functions for four aftershocks ml 4 1 5 1 we then invert for slip on each nodal plane and compare the variance reduction to determine which is the fault plane the two largest earthquakes ml 5 1 and ml 4 2 located close to the mapped trace of the bruce fault both occurred on fault planes striking nnw sse perpendicular to the strike of the bruce and other regional strike slip faults the third earthquake studied ml 4 1 located on a lineation of aftershocks parallel to the regional mapped trend had a preferred fault plane with a ne sw strike the fourth aftershock ml 4 1 was located close to the main shock fault plane and had an oblique reverse mechanism this earthquake exhibited northward directivity but the fault plane could not be identified the earthquake stress drops ranged from 1 to 10 mpa

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

correlation of p wave amplitudes and travel time residuals for teleseisms recorded on the southern california seismic network p waves from 13 teleseismic events well distributed in azimuth and recorded on a large short period network in southern california show a positive correlation between travel time residuals and relative amplitudes slower arrivals are consistently larger than faster arrivals although there is a large amount of scatter the data show an amplitude increase of about a factor of 2 for a 1 s increase in travel time the correlation can be seen in various subsets of the data individual earthquakes single stations recording 10 or more earthquakes and data averaged over all recorded events at each station because the variations in amplitude and travel time can be associated with surface geology much of the data recorded at non bedrock sites can be explained in terms of a near surface layer of varying velocity the travel time residual would then be directly proportional to the velocity in the layer and the amplitude proportional to the square root of the velocity contrast at the base of the layer however some stations show azimuthally varying travel time residuals that are attributed to upper mantle velocity structures we use a two dimensional finite difference calculation to show that velocity anomalies in the upper mantle can also contribute to the observed correlation of travel time and amplitude

stress drops and radiated seismic energies of microearthquakes in a south african gold mine 1 we estimate stress drops and radiated seismic energies of 20 microearthquakes 0 0 mw 1 3 in a south african gold mine to investigate their rupture characteristics and scaling relationships to large earthquakes we analyze seismograms of borehole accelerometers recorded with high sampling rate 15 khz within 200 m of the hypocenters at the depth of 2650 m the waveform data have very high signal to noise ratio and no significant later phases are observed at all stations corner frequencies and quality factors of the anelastic attenuation q are estimated from spectra of velocity seismograms by assuming the omega squared model of boatwright 1978 we also investigate moment tensors for double couple solutions and volumetric components from the waveform inversion static stress drops of the 20 earthquakes calculated from the model of madariaga 1976 are from 3 2 to 88 mpa and scaled energies er mo the ratio of the radiated energy er to the seismic moment mo are from 4 2 10 6 to 1 1 10 4 we find that both the static stress drops and the scaled energies of the analyzed earthquakes are comparable to those values of larger earthquakes our results indicate that the dynamic rupture processes of these microearthquakes are similar to those of larger earthquakes

evidence for and implications of self healing pulses of slip in earthquake rupture dislocation time histories of models derived from waveforms of seven earthquakes are discussed in each model dislocation rise times the duration of slip for a given point on the fault are found to be short compared to the overall duration of the earthquake 10 however in many crack like numerical models of dynamic rupture the slip duration at a given point is comparable to the overall duration of the rupture i e slip at a given point continues until information is received that the rupture has stopped propagating alternative explanations for the discrepancy between the short slip durations used to model waveforms and the long slip durations inferred from dynamic crack models are 1 the dislocation models are unable to resolve the relatively slow parts of earthquake slip and have seriously underestimated the dislocations for these earthquakes 2 earthquakes are composed of a sequence of small dimension short duration events that are separated by locked regions barriers 3 rupture occurs in a narrow self healing pulse of slip that travels along the fault surface evidence is discussed that suggests that slip durations are indeed short and that the self healing slip pulse model is the most appropriate explanation a qualitative model is presented that produces self healing slip pulses the key feature of the model is the assumption that friction on the fault surface is inversely related to the local slip velocity the model has the following features high static strength of materials kilobar range low static stress drops in the range of tens of bars and relatively low frictional stress during slip less than several hundreds of bars it is suggested that the reason that the average dislocation scales with fault length is because large amplitude slip pulses are difficult to stop and hence tend to propagate large distances this model may explain why seismicity and ambient stress are low along fault segments that have experienced large earthquakes it also qualitatively explains why the recurrence time for large earthquakes may be irregular

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 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

detection of high frequency tensile vibrations of a fault during shear rupturing observations from the 2008 west bohemia swarm summary the analysis of 12 earthquakes of magnitudes between 1 7 and 3 7 that occurred in 2008 in west bohemia czech republic reveals that shear rupturing along a fault is accompanied by weak tensile vibrations of the fault the vibrations are of a narrow frequency band with frequencies distinctly higher than those of shear rupturing the tensile vibrations decay slowly in time and often resemble resonant waves they complicate the radiated p waveforms and the p wave radiation pattern the tensile vibrations can be observed in waveforms in directions near the nodal lines where the dominance of shear faulting radiation is lost the tensile vibrations are probably generated by opening of the fault during shear rupturing or by creating a wing tensile crack at the tip of the fault when shear fracturing stops

the 1992 landers earthquake sequence seismological observations the m w 6 1 7 3 6 2 1992 landers earthquakes began on april 23 with the m w6 1 1992 joshua tree preshock and form the most substantial earthquake sequence to occur in california in the last 40 years this sequence ruptured almost 100 km of both surficial and concealed faults and caused aftershocks over an area 100 km wide by 180 km long the faulting was predominantly strike slip and three main events in the sequence had unilateral rupture to the north away from the san andreas fault the m w6 1 joshua tree preshock at 33 n58 and 116 w19 on 0451 ut april 23 was preceded by a tightly clustered foreshock sequence m 4 6 beginning 2 hours before the mainshock and followed by a large aftershock sequence with more than 6000 aftershocks the aftershocks extended along a northerly trend from about 10 km north of the san andreas fault northwest of indio to the east striking pinto mountain fault the m w7 3 landers mainshock occurred at 34 n13 and 116 w26 at 1158 ut june 28 1992 and was preceded for 12 hours by 25 small m 3 earthquakes at the mainshock epicenter the distribution of more than 20 000 aftershocks analyzed in this study and short period focal mechanisms illuminate a complex sequence of faulting the aftershocks extend 60 km to the north of the mainshock epicenter along a system of at least five different surficial faults and 40 km to the south crossing the pinto mountain fault through the joshua tree aftershock zone towards the san andreas fault near indio the rupture initiated in the depth range of 3 6 km similar to previous m 5 earthquakes in the region although the maximum depth of aftershocks is about 15 km the mainshock focal mechanism showed right lateral strike slip faulting with a strike of n10 w on an almost vertical fault the rupture formed an arclike zone well defined by both surficial faulting and aftershocks with more westerly faulting to the north this change in strike is accomplished by jumping across dilational jogs connecting surficial faults with strikes rotated progressively to the west a 20 km long linear cluster of aftershocks occurred 10 20 km north of barstow or 30 40 km north of the end of the mainshock rupture the most prominent off fault aftershock cluster occurred 30 km to the west of the landers mainshock the largest aftershock was within this cluster the m w6 2 big bear aftershock occurring at 34 n10 and 116 w49 at 1505 ut june 28 it exhibited left lateral strike slip faulting on a northeast striking and steeply dipping plane the big bear aftershocks form a linear trend extending 20 km to the northeast with a scattered distribution to the north the landers mainshock occurred near the southernmost extent of the eastern california shear zone an 80 km wide more than 400 km long zone of deformation this zone extends into the death valley region and accommodates about 10 to 20 of the plate motion between the pacific and north american plates the joshua tree preshock its aftershocks and landers aftershocks form a previously missing link that connects the eastern california shear zone to the southern san andreas fault

source parameters and three dimensional attenuation structure from the inversion of microearthquake pulse width data method and synthetic tests we propose a new method to determine source parameters and attenuation structure of a three dimensional medium based on first p rise time and total pulse width measurements from microearthquake data the effects of fault finiteness on seismic radiation are taken into account by assuming the rupture model for a circular crack of sato and hirasawa 1973 ray theory synthetic seismograms in a constant q anelastic medium are computed to derive a set of nonlinear equations which relate the source and attenuation parameters fault radius orientation of the fault plane and quality factor to the pulse width data half and total duration of the p waveforms the numerically built relationships are used to compute the direct problem in the framework of a nonlinear inversion scheme based on the modified downhill simplex method the validity and robustness of the inversion method are tested by synthetic simulations by assuming the sources and receivers configuration of the seismic passive experiment conducted in the campi flegrei caldera southern italy during the last microearthquake crisis 1982 1984 different heterogeneous q models have been considered in order to assess the uncertainty and resolution of source and attenuation parameters for the given acquisition layout the results of this simulation study indicate that first pulse width data from a local network permit retrieval with sufficient accuracy of the heterogeneous q structure and fault radii a rather dense azimuthal coverage of the sources is instead needed to recover the angles in particular the fault strike which define the fault orientation

finite moment tensor of the 3 september 2002 yorba linda earthquake we have developed procedures for inverting broadband waveforms for the finite moment tensors fmts of regional earthquakes the fmt is defined in terms of second order polynomial moments of the source space time function it removes the fault plane ambiguity of the centroid moment tensor cmt and yields several additional parameters of seismological interest the characteristic length lc width wc and duration tc of the faulting as well as the directivity vector v d of the fault slip following mcguire et al 2001 we represent the observed waveform relative to the synthetic in terms of two frequency dependent differential times a phase delay p and an amplitude reduction time q which we measure using the generalized seismological data functional gsdf method gee and jordan 1992 we numerically calculate the fmt partial derivatives which allows us to use synthetics computed by using any forward modeling tools we have tested our methodology on southern california seismic network scsn recordings of the 03 september 2002 yorba linda earthquake m w 4 3 using 1d synthetic green s functions we determined the cmt and resolved fault plane ambiguity to resolve the details of source finiteness we employed a joint inversion technique that recovers the cmt parameters of the aftershocks and the cmt and fmt parameters of the mainshock the joint system of equations relating the data to the source parameters of the mainshock aftershock cluster is denuisanced for path anomalies in both observables this projection operation effectively corrects the mainshock data for path related anomalies in a way similar to but more flexible than empirical green s function egf techniques our results indicate that the yorba linda rupture occurred as left lateral slip on a fault patch conjugate to the nearby right lateral whittier fault we obtained source dimensions of lc 0 7 0 1 km wc 0 4 0 1 km and tc 0 2 0 05 sec implying a stress drop of about 3 2 mpa and we found a directivity of 0 8 0 2 oriented up and to the northeast the inferred fault plane is consistent with the mainshock aftershock distribution relocated by hauksson et al 2002

foreshock sequence of the 1992 landers california earthquake and its implications for earthquake nucleation the june 28 1992 landers california earthquake mw 7 3 was preceded for about 7 hours by a foreshock sequence consisting of at least 28 events in this study we examine the geometry and temporal development of the foreshocks using high precision locations based on cross correlation of waveforms recorded at nearby stations by aligning waveforms rather than trying to obtain travel time picks for each event independently we are able to improve the timing accuracy greatly and to make very accurate travel time picks even for emergent arrivals we perform a joint relocation using the improved travel times and reduce the relative location errors to less than 100 m horizontally and less than 200 m vertically with the improved locations the geometry of the foreshock sequence becomes clear the landers foreshocks occurred at a right step of about 500 m in the mainshock fault plane the nucleation zone as defined by the foreshock sequence is southeast trending to the south and nearly north trending to the north of the right step this geometry is confirmed by the focal mechanisms of the foreshock sequence which are right lateral and follow the trend as determined by the foreshock locations on the two straight segments of the fault and are rotated clockwise for foreshocks that occur within the step the extent of the foreshock sequence is approximately 1 km both vertically and horizontally modeling of the coulomb stress changes due to all previous foreshocks indicates that the foreshocks probably did not trigger each other this result is particularly clear for the mw 4 4 immediate foreshock since stress transfer in the sequence appears not to have played a significant role in its development we infer an underlying aseismic nucleation process probably aseismic creep other studies have shown that earthquake nucleation may be controlled by fault zone irregularities this appears to be true in the case of the landers earthquake although the size of the irregularity is so small that it is not detectable by standard location techniques

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

rupture complexity of a moderate intraplate earthquake in the alps the 1996 m5 epagny annecy earthquake summary the magnitude 5 epagny annecy earthquake of 1996 july 15 is the largest seismic event to have occurred in the alps since the introduction of modern digital instrumentation this strike slip event was located on the vuache fault near the town of annecy in the northern french alps the aim of our work was to retrieve the main parameters of the rupture process of this earthquake from seismograms recorded at local and regional distances 20 300 km to eliminate path and site evects from the seismograms we compared the main shock recordings at each station with those of the largest aftershocks nearby we used a combination of techniques including pulse width measurements and cross correlation of velocity traces comparison of p wave displacement pulses and empirical green s function deconvolution to retrieve the apparent duration of the rupture process as seen at each station our results demonstrate that in the absence of on scale data p wave pulse width measurements on clipped signals can be misleading if the rupture process is complex in the case of the annecy earthquake comparisons of on scale p wave displacement seismograms and the empirical green s function deconvolutions show that the rupture process consisted of at least two subevents separated by 0 2 0 3 s and with a total duration of about 0 5 s the systematic azimuthal dependence of both the shape and duration of the apparent source time function is consistent with a nearly unilateral propagation of the main rupture phase in a southeast direction along the fault plane and parallel to the direction of slip an isochron analysis reveals that the first subevent occurred slightly to the northwest of the nucleation point but that the second subevent was located further to the southeast thus confirming the overall rupture directivity towards the southeast an interpretation of our results in light of the previously documented aftershock distribution and of observations of ground cracks in the epicentral area suggests that the main shock occurred on the vuache fault and that rupture in a northwest direction was inhibited by a right lateral stepover in the fault accordingly the vast majority of the subsequent aftershocks which include several magnitude 3 4 events occurred on a fault segment that is slightly ovset from the inferred surface trace of the vuache fault and that was activated by the main shock

can observations of earthquake scaling constrain slip weakening summary we use observations of earthquake source parameters over a wide magnitude range m w 0 7 to place constraints on constitutive fault weakening the data suggest a scale dependence of apparent stress and stress drop both may increase slightly with earthquake size we show that this scale dependence need not imply any difference in fault zone properties for different sized earthquakes we select 30 earthquakes well recorded at 2 5 km depth at cajon pass california we use individual and empirical green s function spectral analysis to improve the resolution of source parameters including static stress drop and total slip s we also measure radiated energy e s w ecompare the cajon pass results with those from larger california earthquakes including aftershocks of the 1994 northridge earthquake and confirm the results of abercrombie 1995 e s m 0 where rigidity and both e s m 0 and increase as m 0 and s increases uncertainties remain large due to model assumptions and variations between possible models and earthquake scale independence is possible within the resolution assuming that the average trends are real we define a quantity g 2 es m 0 s 2w hich is the total energy dissipation in friction and fracture minus 1 s w here 1 is the final static stress if 1 d the dynamic shear strength during the last increments of seismic slip then g g the fracture energy in a slip weakening interpretation of dissipation

triggering of the 1999 mw 7 1 hector mine earthquake by aftershocks of the 1992 mw 7 3 landers earthquake 1 there is strong observational evidence that the 1999 mw 7 1 hector mine earthquake in the mojave desert california was triggered by the nearby 1992 mw 7 3 landers earthquake many authors have proposed that the landers earthquake directly stressed the hector mine fault our model of the landers aftershock sequence however suggests there is an 85 chance that the hector mine hypocenter was actually triggered by a chain of smaller earthquakes that was initiated by the landers main shock we perform our model simulations using the monte carlo method based on the gutenberg richter relationship omori s law bath s law and assumptions that all earthquakes including aftershocks are capable of producing aftershocks and that aftershocks produce their own aftershocks at the same rate that other earthquakes do in general our simulations show that if it has been more than several days since an m 7 main shock most new aftershocks will be the result of secondary triggering these secondary aftershocks are not physically constrained to occur where the original main shock increased stress this may explain the significant fraction of aftershocks that have been found to occur in main shock stress shadows in static coulomb stress triggering studies

rupture directivity and slip distribution of the m 4 3 foreshock to the 1992 joshua tree earthquake southern california details of the m 4 3 foreshock to the joshua tree earthquake were studied using p waves recorded on the southern california seismic network and the anza network deconvolution using an m 2 4 event as an empirical green s function corrected for complicated path and site effects in the seismograms and produced simple far field displacement pulses that were inverted for a slip distribution both possible fault planes north south and east west for the focal mechanism were tested by a least squares inversion procedure with a range of rupture velocities the results showed that the foreshock ruptured the north south plane similar to the mainshock the foreshock initiated a few hundred meters south of the mainshock and ruptured to the north toward the mainshock hypocenter the mainshock m 6 1 initiated near the northern edge of the foreshock rupture 2 hr later the foreshock had a high stress drop 320 to 800 bars and broke a small portion of the fault adjacent to the mainshock but was not able to immediately initiate the mainshock rupture

geoid and gravity anomaly data of conjugate regions of bay of bengal and enderby basin new constraints on breakup and early spreading history between india and antarctica 1 timing of breakup of the indian continent from eastern gondwanaland and evolution of the lithosphere in the bay of bengal still remain as ambiguous issues geoid and free air gravity data of bay of bengal and enderby basin are integrated with shipborne geophysical data to investigate the early evolution of the eastern indian ocean geoid and gravity data of the bay of bengal reveal five n36 w fracture zones fzs and five isolated ne sw structural rises between the eastern continental margin of india ecmi and the 85 e ridge 86 e fz the fzs meet the 86 e fz at an angle of 39 the rises are associated with low gravity and geoid anomalies and are oriented nearly orthogonal to the fzs trend the geoid and gravity data of the western enderby basin reveal a major kerguelen fz and five n4 e fzs the fzs discretely converge to the kerguelen fz at an angle of 37 we interpret the fzs identified in bay of bengal and western enderby basin as conjugate fzs that trace the early cretaceous rifting of south ecmi from enderby land structural rises between the fzs of bay of bengal may either represent fossil ridge segments possibly have extinct during the early evolution of the bay of bengal lithosphere or may have formed later by the volcanic activity accreted the 85 e ridge two different gravity signatures short wavelength high amplitude negative gravity anomaly and relatively broader low amplitude negative gravity anomaly are observed on south and north segments of the ecmi respectively the location of continent ocean boundary cob is at relatively far distance 100 200 km from the coastline on north ecmi than that 50 100 km on the south segment on the basis of geoid gravity and seismic character and orientation of conjugate fzs in bay of bengal and western enderby basin we believe that transform motion occurred between south ecmi and enderby land at the time of breakup which might have facilitated the rifting process in the north between combined north ecmi elan bank and macrobertson land and in the south between southwest sri lanka and gunnerus ridge region of east antarctica approximately during the period between the anomalies m1 and m0 and soon after detachment of the elan bank from north ecmi the rifting process possibly had reorganized in order to establish the process along the entire eastern margins of india and sri lanka

theory of time dependent rupture in the earth at a velocity x which is a well defined function of k this propagation is stable and quasi static and is referred to as subcritical crack growth we shall use the terms stable quasistatic and subcritical to mean propagation at velocities much less than the sonic velocities of the medium this behavior results from stress induced corrosion at the crack tip the principal corrodent for the present application being h20 this behavior has been firmly established in the laboratory for mode i tensile cracks in a wide variety of materials ineluding silicates and silicate glasses see cholz 1968a 1972a martin 1972 wiederhorn and bolz 1970 atkinson 1979 lawn and wilshaw 1975 knott 1973 the form of the relationship between k and x does not vary significantly with the material only the parameters in the law vary as an example we show in figure 1 data on subcritical crack growth in quartz after atkinson 1979 the empirical relationship is found to take the form