evidence for non constant energy moment scaling from coda derived source spectra 1 we present two lines of evidence showing the ratio of seismically radiated energy er to seismic moment mo increases with increasing moment for crustal events spanning 5 orders in seismic moment this ratio often referred to as the scaled energy er m0 has been elusive and the subject of recent debate within the seismological community because significant frequency dependent corrections must be made to account for source and path heterogeneity it is not uncommon that energy estimates of the same event by different researchers have significantly different values to minimize some of the potential problems we used the regional coda methodology outlined by mayeda et al 2003 on 4 large earthquake sequences mw 7 4 and 7 2 izmit duzce mw 7 2 gulf of aqaba mw 7 3 landers and mw 7 2 hector mine this methodology has been shown to provide the lowest variance estimate of the source spectrum when compared against traditional direct wave estimates in our first approach we made energy estimates from coda derived spectra and observed that increases with increasing moment for 3 7 mw 7 4 in our second approach we used the simple idea outlined by prieto et al 2004 to directly test for self similarity under self similarity earthquake source spectra scaled by 3 should have the same shape to test this one needs very stable spectra either through spectral stacking of direct wave spectra or by the use of stable coda derived spectra for all sequences we found that the mainshocks and larger aftershocks had significantly different spectral shapes than the smaller events the scaled 3 spectra of the larger events represented an upper bound having larger amplitudes near the corner frequency in fact an 3 5 scaling moved the larger events closer to the mean of the spectral population though the spectral shape differences persisted these results strongly suggest that earthquakes are not scaling self similarly and that complexities in the rupture process such as variable slip velocity exist between small and large events

gulf stream warm surges thermal anomalies resulting from cold core ring interactions observations from thermal satellite imagery reveal a thermal anomaly pattern in the sea surface temperature sst field of the gulf stream downstream of cape hatteras which is a direct result of gulf stream eddy interactions upstream of cape hatteras the general sequence of events observed in the images is as follows 1 detrainment of gulf stream surface water occurs as a result of an interaction with a cold core ring 2 downstream of the interaction the gulf stream s sst decreases several degrees over a few days owing to the loss of the upstream surface flow 3 a visible jet of warm surface water is advected down the major axis of the gulf stream after the stream ring interaction subsides and the detrainment diminishes the jet maintains a 1 3 c temperature difference with the surrounding gulf stream water as it propagates downstream this paired cooling warming process is labeled a warm surge event because the sharp leading edge of the warm jet i e the surge is easily detectable in the satellite images while the anomalous surface cooling preceding the passage of the warm jet is difficult to observe vertical profiles of temperature downstream velocity and cross stream velocity all indicate that the upper 100 150 m of the gulf stream are significantly altered during an event satellite and in situ data also indicate that just downstream of the interaction there is enhanced entrainment of sargasso sea surface waters into the gulf stream to replenish the detrained gulf stream water

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

effect of distance on local magnitudes found from strong motion records values of local magnitude m l are calculated from 56 strong motion accelerograms recorded in the imperial valley earthquake of 15 october 1979 according to procedures developed earlier kanamori and jennings 1978 these data plus similar data from the san fernando earthquake of 9 february 1971 and additional less numerous data from several other california earthquakes are used to investigate the use of different measures of distance in near field determinations of m l this investigation has relevance for similar uses of distances in determining seismic design criteria in addition the consistency of the values of m l found from the strong motion data is examined from the viewpoint of assessing the need for any correction in the standard attenuation curve log 10 a 0 it was found that the most consistent values of m l result when distance is measured to the closest point on the surface trace of the fault if a site lies within a circle with diameter equal to the extent of faulting and centered on the center of faulting center of energy release outside this circle the distance measured to the center of the circle is recommended a consistent trend in the values of m l found from strong motion records is seen in the data the values start at zero distance at essentially the far field value and then decrease to 1 4 unit at about 20 km then they rise to 1 4 unit at 50 to 60 km a smooth revision to the standard attenuation curve is presented which removes this systematic trend

apparent stress scaling and statistical trends abstract statistical relations between different earthquake parameters such as m0 seismic moment es seismic energy a apparent stress a rupture area g average slip acceleration are investigated for this purpose a kinematic earthquake model representing averaged earthquake rupture process is formulated the model implies a scaling relationship for a as a function of three other parameters related to kinematic m0 geometric a and material g source characteristics which according to the model can change independently this scaling relation is used to explain statistical trends that characterize different earthquake data sets including micro small moderate and large events plotted in the log a log m0 space and to determine the area in this space where typical earthquakes occur the scaling relationship is interpreted in terms of the apparent stress minimum i e the most uniform among the possible earthquake rupture patterns it is concluded that although the apparent stress increases on an average with increasing seismic moment small and large earthquakes are essentially similar

earthquake monitoring in southern california for seventy seven years 1932 2008 the southern california seismic network scsn has produced the scsn earthquake catalog from 1932 to the present a period of more than 77 yrs this catalog consists of phase picks hypocenters and magnitudes we present the history of the scsn and the evolution of the catalog to facilitate user understanding of its limitations and strengths hypocenters and magnitudes have improved in quality with time as the number of stations has increased gradually from 7 to 400 and the data acquisition and measuring procedures have become more sophisticated the magnitude of completeness m c of the network has improved from m c 3 25 in the early years to m c 1 8 at present or better in the most densely instrumented areas mainshock aftershock and swarm sequences and scattered individual background earthquakes characterize the seismicity of more than 470 000 events the earthquake frequency size distribution has an average b value of 1 0 with m 6 0 events occurring approximately every 3 yrs the three largest earthquakes recorded were 1952 m w 7 5 kern county 1992 m w 7 3 landers and 1999 m w 7 1 hector mine sequences and the three most damaging earthquakes were the 1933 m w 6 4 long beach 1971 m w 6 7 san fernando and 1994 m w 6 7 northridge earthquakes all of these events ruptured slow slipping faults located away from the main plate boundary fault the san andreas fault their aftershock sequences constitute about a third of the events in the catalog the fast slipping southern san andreas fault is relatively quiet at the microseismic level and has not had an m 6 earthquake since 1932 in contrast the slower san jacinto fault has the highest level of seismicity including several m 6 events thus the spatial and temporal seismicity patterns exhibit a complex relationship with the plate tectonic crustal deformation

a new observation of dynamically triggered regional seismicity earthquakes in greece following the august 1999 izmit turkey earthquake the m w 7 4 izmit turkey earthquake triggered widespread regional seismicity in greece over a study region extending from 400 km to nearly 1000 km away from the epicenter small events began immediately after the passage of the mainshock surface waves suggesting that the transient stresses of the seismic waves were the trigger the increase in cataloged earthquakes in ordinary continental crust is a new observation and is statistically significant at the 95 level unlike the previous example of distant triggering during the landers earthquake the activated seismicity occurred entirely in non volcanic areas the greek sites were triggered by waves with amplitudes at least a factor of 3 lower than the observed triggering threshold for imperial valley we speculate that dynamic triggering on a regional scale results in countrywide episodes of increased seismicity or superswarms in regions with low triggering thresholds such as greece

rupture process for micro earthquakes inferred from borehole seismic recordings we investigate the spatial extent of rupture and variability in fault slip for micro earthquakes by inverting seismic moment rate functions derived from empirical green s function deconvolution by using waveforms from an array of borehole seismometers we determine the spatial distributions of fault slip for m 3 earthquakes that occurred along the hayward fault in central california and identify a variety of slip behaviors including subevents directivity and high stress drop the 2013 m w 3 2 orinda earthquake exhibits a complex rupture process involving two subevents with northwest and up dip directivity the two subevents release 43 and 18 of the total seismic moment 6 7 1013 n m and their inferred peak stress drops are 18 and 8 mpa the 2011 m w 4 0 berkeley and 2012 m w 4 0 el cerrito earthquakes are marked by high stress drop the inferred peak and mean stress drops are about 100 130 and 40 mpa respectively which suggests that there are locally high levels of fault strength on the hayward fault our finite source modeling suggests that the radiation efficiency determined for these two earthquakes is very low 0 1 and implies that most energy is dissipated during the earthquake rupture process

seismic wave attenuation from borehole and surface records in the top 2 5km beneath the city of basel switzerland summary we investigate attenuation q 1 of sediments of 2 5 3 5 km thickness underneath the city of basel switzerland we use recordings of 195 induced events that were obtained during and after the stimulation of a reservoir for a deep heat mining project in 2006 and 2007 the data set is ideally suited to estimate q as all events are confined to a small source volume and were recorded by a dense surface network as well as six borehole sensors at various depths the deepest borehole sensor is positioned at a depth of 2 7 km inside the crystalline basement at a mean hypocentral distance of 1 8 km this allows us to measure q for frequencies between 10 and 130 hz we apply two different methods to estimate q first we use a standard spectral ratio technique to obtain q and as a second measure we estimate q in the time domain by convolving signals recorded by the deepest sensor with a q operator and then comparing the convolved signals to recordings at the shallower stations both methods deliver comparable values for q we also observe similar attenuation for p and s waves qp qs as expected q increases with depth but with values around 30 50 it is low even for the consolidated permian and mesozoic sediments between 500 and 2700 m

absolute stress fields in the source region of the 1992 landers earthquake earthquake focal mechanisms are often inverted to obtain the deviatoric stress field because shear stress is equal to the frictional strength of the fault at the time of an earthquake six components of the absolute stress tensor at the hypocenter can be obtained from a focal mechanism by combining deviatoric stress fields with the coulomb failure criterion for a data set of focal mechanisms determined for southern california earthquakes including the 1992 landers earthquake sequence we calculated the absolute stress tensors at their hypocenters using a standard intrinsic friction coefficient under three pore pressure conditions parameterized by the reference pore pressure at the optimally oriented faults to the stress field three absolute stress fields were obtained for southern california immediately before the landers main shock by applying each data set of the stress tensors to an inversion scheme based on bayesian statistical inference and akaike s bayesian information criterion the coseismic stress field was calculated to obtain the absolute stress fields immediately after the main shock the variations of the coseismic stress rotation were related to the reference pore pressure comparing this relation with that obtained through stress inversion we determined the absolute stress field and the most plausible reference pore pressure to be hydrostatic on average the maximum shear stresses immediately before the main shock were 44 15 and 79 24 mpa at depths of 5 and 10 km respectively earthquakes on the off plate boundary faults in southern california occur on faults that are loaded by anderson byerlee stress conditions

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

the 1988 and 1990 upland earthquakes left lateral faulting adjacent to the central transverse ranges two earthquakes m l 4 6 and m l 5 2 occurred at almost the same location in upland southern california in june 1988 and february 1990 and had similar strike slip focal mechanisms with left lateral motion on a northeast striking plane the focal mechanisms and aftershock locations showed that the causative fault was the san jose fault an 18 km long concealed fault that splays west southwest from the frontal fault of the central transverse ranges left lateral strike slip faults adjacent to the frontal faults may play an important role in the deformation of the transverse ranges and the los angeles basin as suggested by these upland earthquakes the left lateral strike slip 1988 m l 4 9 pasadena earthquake on the raymond fault 30 km to the west of upland and scattered background seismicity along other active left lateral faults these faults may transfer slip away from part of the frontal fault toward the south alternatively these faults could represent secondary faulting related to the termination of the northwest striking right lateral strike slip faults to the south of the range front the 1988 and 1990 upland earthquakes ruptured abutting or possibly overlapping segments of the san jose fault the edges of the overlapping aftershock zones which are sharply defined together with background seismicity outline a 14 km long aseismic segment of the san jose fault the 1988 mainshock originated at 9 5 km depth and caused aftershocks between 5 and 12 km in contrast the 1990 mainshock focus occurred at the top of its aftershock zone at 5 km and caused aftershocks down to 13 km depth these deep aftershocks tapered off within 2 weeks the rate of occurrence of aftershocks in magnitude time space was the same for both sequences the state of stress reflected in the focal mechanisms of the aftershocks is identical to that determined from background activity and did not change with time during the aftershock sequence the constant stress state suggests that the 1988 and 1990 events did not completely release all the stored slip on that segment of the fault the presence of 14 km of unbroken fault the abrupt temporal termination of deep aftershocks and the constant stress state all suggest that a future moderate sized earthquake m l 6 0 6 5 on the san jose fault is possible with a rupture length of at least 14 km and possibly 18 km

the ml scale in southern california measurements 9 941 of peak amplitudes on wood anderson instruments or simulated wood anderson instruments in the southern california seismographic network for 972 earthquakes primarily located in southern california were studied with the aim of determining a new distance correction curve for use in determining the local magnitude m l events in the mammoth lakes area were found to give an unusual attenuation pattern and were excluded from the analysis as were readings from any one earthquake at distances beyond the first occurrence of amplitudes less than 0 3 mm the remaining 7 355 amplitudes from 814 earthquakes yielded the following equation for m l distance correction log a 0 log a 0 1 110log r 100 0 00189 r 100 3 0 where r is hypocentral distance in kilometers a new set of station corrections was also determined from the analysis the standard deviation of the m l residuals obtained by using this curve and the station corrections was 0 21 the data used to derive the equation came from earthquakes with hypocentral distances ranging from about 10 to 700 km and focal depths down to 20 km with most depths less than 10 km the log a 0 values from this equation are similar to the standard values listed in richter 1958 for 50 200 km the effect at close distances is consistent with that found in several other studies and is simply due to a difference in the observed 1 r geometrical spreading for body waves and the 1 r 2 spreading assumed by gutenberg and richter in the construction of the log a 0 table m l s computed from our curve and those reported in the caltech catalog show a systematic dependence on magnitude small earthquakes have larger magnitudes than in the catalog and large earthquakes have smaller magnitudes by as much as 0 6 units to a large extent these systematic differences are due to the nonuniform distribution of data in magnitude distance space small earthquakes are preferentially recorded at close distances relative to large earthquakes for large earthquakes however the difference in the two magnitudes is not solely due to the new correction for attenuation magnitudes computed using richter s log a 0 curve are also low relative to the catalog values the differences in that case may be due to subjective judgment on the part of those determining the catalog magnitudes the use of data other than the caltech wood anderson seismographs the use of different station corrections or the use of teleseismic magnitude determinations whatever their cause the departures at large magnitude may explain a 1 0 0 7 proportionality found by luco 1982 between m l s determined from real wood anderson records and those from records synthesized from strong motion instruments if it were not for the biases in reported magnitudes luco s finding would imply a magnitude dependent shape in the attenuation curves we studied residuals in three magnitude classes 2 0 m l 3 5 3 5 m l 5 5 and 5 5 m l 7 0 and found no support for such a magnitude dependence based on our results we propose that local magnitude scales be defined such that m l 3 correspond to 10 mm of motion on a wood anderson instrument at 17 km hypocentral distance rather than 1 mm of motion at 100 km this is consistent with the original definition of magnitude in southern california and will allow more meaningful comparison of earthquakes in regions having very different attenuation of waves within the first 100 km

local magnitude determinations for intermountain seismic belt earthquakes from broadband digital data the university of utah seismograph stations uuss earthquake catalogs for the utah and yellowstone national park regions contain two types of size measurements local magnitude m l and coda magnitude m c which is calibrated against m l from 1962 through 1993 uuss calculated m l values for southern and central intermountain seismic belt earthquakes using maximum peak to peak p p amplitudes on paper records from one to five wood anderson w a seismographs in utah for m l determinations of earthquakes since 1994 uuss has utilized synthetic w a seismograms from u s national seismic network and uuss broadband digital telemetry stations in the region which numbered 23 by the end of our study period on 30 june 2002 this change has greatly increased the percentage of earthquakes for which m l can be determined it is now possible to determine m l for all m 3 earthquakes in the utah and yellowstone regions and earthquakes as small as m to maintain continuity in the magnitudes in the uuss earthquake catalogs we determined empirical m l station corrections that minimize differences between m l s calculated from paper and synthetic w a records application of these station corrections in combination with distance corrections from richter 1958 which have been in use at uuss since 1962 produces m l values that do not show any significant distance dependence m l determinations for the utah and yellowstone regions for 1981 2002 using our station corrections and richter s distance corrections have provided a reliable data set for recalibrating the m c scales for these regions our revised m l values are consistent with available moment magnitude determinations for intermountain seismic belt earthquakes to facilitate automatic m l measurements we analyzed the distribution of the times of maximum p p amplitudes in synthetic w a records a 30 sec time window for maximum amplitudes beginning 5 sec before the predicted sg time encompasses 95 of the maximum p p amplitudes in our judgment this time window represents a good compromise between maximizing the chances of capturing the maximum amplitude and minimizing the risk of including other seismic events

broadband modeling of local earthquakes three component broadband waveforms of two small earthquakes near upland california recorded on the pasadena broadband high dynamic range instrument were modeled to obtain useful green s functions for this path and to examine the sensitivity of the synthetic seismograms to perturbations of the crustal model we assumed that the source of each event was both simple and known as determined from the caltech usgs array first motions a trapezoidal time function was chosen to fit the width of the direct s wave generalized rays reflectivity and finite difference techniques were used to compute the synthetic seismograms we found that a simple layer over a half space model is an adequate approximation of the upper crust along this profile in particular the waveforms are controlled by a relatively slow 4 km thick surficial layer 4 5km s 1 2 6 km s 1 over a faster layer 5 9 km s 1 3 5 km s 1 the relative amplitudes of direct and multiple s indicate that the main shock occurred at a depth of 6 km while the aftershock occurred at a depth of 8 to 9 km sensitivity analyses indicate that for distances less than 50 km and for periods longer than 1 sec the synthetic seismograms are not very sensitive to perturbations of the deep crustal structure analysis of upper crustal model perturbations revealed that the surficial layer is between 3 to 5 km thick in addition the contact between this layer and the underlying material can be smoothed with a 2 km wide velocity gradient without adversely affecting the fit to the data two dimensional finite difference calculations show that a ridge structure beneath the recorder acts as a lowpass filter the lower frequency phases are largely unaffected other two dimensional models with ridges between the source and receiver clearly did not fit the data synthetic seismograms computed for the best fitting model were used to estimate a long period moment of 6 2 1022 dyne cm ml 4 6 and 1 1022 dyne cm ml 3 7 with identical triangular source time durations of 0 3 sec assuming the same fault dimension of 0 4 km from standard scaling laws stress drop estimates of 410 and 70 bars are obtained for the two events respectively generally we found that it is possible to reproduce local waveforms at frequencies up to 1 hz without a complete knowledge of fine structural detail resulting green s functions can be useful in studying historic events and in simulations of large events from a given source region

the energy release in earthquakes energy calculations are generally made through an empirical application of the familiar gutenberg richter energy magnitude relationships the precise physical significance of these relationships is somewhat uncertain we make use here of the recent improvements in knowledge about the earthquake source to place energy measurements on a sounder physical basis for a simple trapezoidal far field displacement source time function with a ratio x of rise time to total duration t 0 the seismic energy e is proportional to 1 x 1 x 2 m 0 2 t 0 3 where m 0 is seismic moment as long as x is greater than 0 1 or so the effect of rise time is not important the dynamic energies thus calculated for shallow events are in reasonable agreement with the estimate e 5 10 5 m 0 based on elastostatic considerations deep events despite their possibly different seismological character yield dynamic energies which are compatible with a static prediction similar to that for shallow events studies of strong motion velocity traces obtained near the sources of the 1971 san fernando 1966 parkfield and 1979 imperial valley earthquakes suggest that even in the distance range of 1 to 5 km most of the radiated energy is below 1 to 2 hz in frequency far field energy determinations using long period wwssn instruments are probably not in gross error despite their band limited nature the strong motion record for the intermediate depth bucharest earthquake of 1977 also suggests little teleseismic energy outside the pass band of a long period wwssn instrument

a hybrid method for calculating the radiated wave energy of deep earthquakes abstract the energy radiated as waves is a parameter vital to the understanding of earthquake rupture physics calculations of this energy however are difficult and wrought with uncertainty we developed a new hybrid method for the calculation of radiated wave energy and other key seismic parameters the method calculates these parameters in an internally consistent and objective manner allowing for robust comparison between parameters and across earthquakes in addition the method does not require deconvolution or filtering of data thus results contain all recorded high frequencies we use the method to calculate source time functions seismic moment duration and radiated wave energy for 19 recent deep depth 292 km earthquakes ranging in size from m w 6 5 to 8 2 we compute effective seismic efficiency for the three largest events the principle result is that the effective seismic efficiencies are less than 0 3 if earthquake rupture adhered to the orowan model then the average frictional stress during rupture is equal to the final shear stress on the fault plane and the seismic efficiency is 1 but since the measured seismic efficiency is less than 1 the rupture process of these large deep earthquakes partitions more energy into friction and the average frictional stress during rupture is higher than the final fault plane stress

teleseismic estimates of the energy radiated by shallow earthquakes the energy flux contained in the p wave groups p pp sp or the s wave groups s ps ss radiated by a shallow earthquake is modeled assuming that the energy flux in the direct and depth phases adds incoherently by defining generalized radiation patterns which incorporate this neutral interference the wave groups are analyzed as though they were comprised of a single phase measurements of the energy flux in the wave groups are corrected in the frequency domain for both the body wave attenuation and the frequency band of the recording the corrected measurements are then used to estimate the seismic energy radiated by the earthquake this analysis is applied to digital recordings of the teleseismic wave groups radiated by the may 2 1983 coalinga california earthquake and the october 28 1983 borah peak idaho earthquake for the coalinga earthquake an estimate of es 1 6 0 4 1021 dyn cm was determined from six p wave groups while the sh wave group recorded at station col returned an estimate of es 1 2 1021 dyn cm for the borah peak earthquake an estimate ofes 3 2 0 5 1021 dyn cm was determined from seven p wave groups the distribution of the isoseismals for the borah peak earthquake indicate that the energy radiated by this event was focussed to the northwest in the direction of rupture propagation correcting the teleseismic estimate for this focussing gives es 3 6 0 5 1021 dyn cm combining these estimates of the radiated energy with broad band estimates of the seismic moment yields estimates of the apparent stress of 17 and 7 bars for the coalinga and the borah peak earthquakes respectively

initial investigation of the landers california earthquake of 28 june 1992 using terrascope the 1992 landers earthquake m s 7 5 m w 7 3 was recorded at six terrascope stations in southern california peak accelerations ranged from 0 16 g at svd 63 km to 0 0092 g at isa 245 km decreasing with distance away from the fault zone the peak velocity showed a different pattern reflecting the rupture directivity from south to north the largest peak velocity 19 cm sec was observed at gsc 125 km moment tensor inversion of long period surface waves yielded a mechanism with m 0 1 1 10 27 dyne cm m w 7 3 dip 74 rake 176 and strike 340 inversion of teleseismic p and s waves revealed two distinct sub events of 6 and 8 sec duration and about 10 sec apart the source parameters for the first and second events are m 0 1 9 10 26 dyne cm dip 83 rake 179 strike 359 and m 0 6 1 10 26 dyne cm dip 87 rake 178 strike 333 respectively the radiated wave energy e s was estimated as 4 3 10 23 ergs the ratio e s m 0 3 9 10 4 corresponds to a stress drop of 280 bars and suggests that the landers earthquake belongs to the group of high stress drop earthquakes and occurred on a fault with a long recurrence time the rupture directivity can be seen clearly in the records from pfo 68 km located to the south and gsc located to the north of the epicenter the maximum displacement at pfo is only 13 of that at gsc despite the shorter epicentral distance to pfo than to gsc the slip distribution determined with the empirical green s function method indicates that the landers earthquake consists of two distinct sub events about 30 km apart with the second sub event to the north being about twice as large as the first one this slip distribution is consistent with the teleseismic data and the surface offsets mapped in the field

complex faulting deduced from broadband modeling of the 28 february 1990 upland earthquake ml 5 2 the 1990 upland earthquake was one of the first sizable local events to be recorded broadband at pasadena where the green s functions appropriate for the path are known from a previous study the synthetics developed in modeling the 1988 upland sequence were available for use in rapid assessment of the activity first motion studies from the caltech usgs array data gave two solutions for the 1990 main shock based on the choice of regional velocity models although these focal mechanisms differ by less than 5 in strike and 20 rake it proved possible to further constrain the solution using these derived green s functions and a three component waveform inversion scheme we obtain from long period waves a fault plane solution of 216 77 5 0 m 0 2 5 10 24 dyne cm depth 6 km and a source duration of 1 2 sec for which the orientation and source depth are in good agreement with the first motion results of hauksson and jones 1991 comparisons of the broadband displacement records with the high pass wood anderson simulations suggests the 1990 earthquake was a complicated event with a strong asperity at depth double point source models indicate that about 30 per cent of the moment was released from a 9 km deep asperity following the initial source by 0 0 to 0 5 sec our best fitting distributed fault model indicates that the timing of our point source results is feasible assuming a reasonable rupture velocity the rupture initiated at a depth of about 6 km and propagated downward on a 3 5 by 3 5 km length by width fault both the inversion of long period waves and the distributed fault modeling indicate that the main shock did not rupture the entire depth extent of the fault defined by the aftershock zone a relatively small asperity about 1 0 km 2 with a greater than 1 kbar stress drop controls the short period wood anderson waveforms this asperity appears to be located in a region where seismicity shows a bend in the fault plane

estimating the diminution of shear wave amplitude with distance application to the los angeles california urban area the rate of decay with distance of shear wave amplitude computed from 20 sec s wave spectra is determined from terrascope records of small earthquakes in the greater los angeles area piecewise log linear interpolation functions and traditional diminution functions are used to fit spectral decay to a maximum distance of 150 km simultaneously isotropic source and receiver terms are determined separate branches of the spectral decay function are found for two categories of source depth greater than 10 km and less than 10 km in the hypocentral distance range of 20 to 150 km and in the frequency range of 0 5 to 8 0 hz an important result of the investigation is that the horizontal component decay rate associated with deeper crustal sources is generally greater than that associated with shallower sources and is greater than that which is estimated using more traditional models of spectral decay with distance the same behavior generally holds for vertical component spectra the variation in apparent attenuation rate with source depth should affect seismic hazard estimates associated with the rupture of blind thrust faults in the los angeles basin and vicinity the results of the inversions suggest that interpolation function representations of spectral decay are sensitive to perturbations of s wave amplitude due to crustal reflectors such as post critical s wave arrivals from mid crustal to deep crustal velocity interfaces

stress drop heterogeneity within tectonically complex regions a case study of san gorgonio pass southern california in general seismic slip along faults reduces the average shear stress within earthquake source regions but stress drops of specific earthquakes are observed to vary widely in size to advance our understanding of variations in stress drop we analysed source parameters of small magnitude events in the greater san gorgonio area southern california in san gorgonio the regional tectonics are controlled by a restraining bend of the san andreas fault system which results in distributed crustal deformation and heterogeneous slip along numerous strike slip and thrust faults stress drops were estimated by fitting a brune type spectral model to source spectra obtained by iteratively stacking the observed amplitude spectra the estimates have large scatter among individual events but the median of event populations shows systematic statistically significant variations we identified several crustal and faulting parameters that may contribute to local variations in stress drop including the style of faulting changes in average tectonic slip rates mineralogical composition of the host rocks as well as the hypocentral depths of seismic events we observed anomalously high stress drops 20 mpa in a small region between the traces of the san gorgonio and mission creek segments of the san andreas fault furthermore the estimated stress drops are higher below depths of 10 km and along the san gorgonio fault segment but are lower both to the north and south away from san gorgonio pass showing an approximate negative correlation with geologic slip rates documenting controlling parameters of stress drop heterogeneity is important to advance regional hazard assessment and our understanding of earthquake rupture processes

determination of earthquake energy release and ml using terrascope we estimated the energy radiated by earthquakes in southern california using on scale very broadband recordings from terrascope the method we used involves time integration of the squared ground motion velocity and empirical determination of the distance attenuation function and the station corrections the time integral is typically taken over a duration of 2 min after the p wave arrival the attenuation curve for the energy integral we obtained is given by q r cr n exp kr r 2 2 h ref 2 with c 0 49710 n 1 0322 k 0 0035 km 1 and h ref 8 km where is the epicentral distance a similar method was used to determine m l using terrascope data the station corrections for m l are determined such that the m l values determined from terrascope agree with those from the traditional optical wood anderson seismographs for 1 5 6 5 m l saturates the ratio e s m 0 m 0 seismic moment a measure of the average stress drop for six earthquakes the 1989 montebello earthquake m l 4 6 the 1989 pasadena earthquake m l 4 9 the 1990 upland earthquake m l 5 2 the 1991 sierra madre earthquake m l 5 8 the 1992 joshua tree earthquake m l 6 1 and the 1992 landers earthquake m w 7 3 are about 10 times larger than those of the others that include the aftershocks of the 1987 whittier narrows earthquake the sierra madre earthquake the joshua tree earthquake and the two earthquakes on the san jacinto fault the difference in the stress drop between the mainshock and their large aftershocks may be similar to that between earthquakes on a fault with long and short repeat times the aftershocks which occurred on the fault plane where the mainshock slippage occurred had a very short time to heal hence a low stress drop the repeat time of the major earthquakes on the frontal fault systems in the transverse ranges in southern california is believed to be very long a few thousand years hence the events in the transverse ranges may have higher stress drops than those of the events occurring on faults with shorter repeat times such as the san andreas fault and the san jacinto fault the observation that very high stress drop events occur in the transverse ranges and the los angeles basin has important implications for the regional seismic potential the occurrence of these high stress drop events near the bottom of the seismogenic zone strongly suggests that these fault systems are capable of supporting high stress that will eventually be released in major seismic events characterization of earthquakes in terms of the e s m 0 ratio using broadband data will help delineate the spatial distribution of seismogenic stresses in the los angeles basin and the transverse ranges

the 3 december 1988 pasadena california earthquake evidence for strike slip motion on the raymond fault the pasadena earthquake m l 4 9 occurred on 3 december 1988 at a depth of 16 km the hypocenters of the earthquake and its aftershocks define a east northeast striking steeply northwest dipping surface that projects up to the active surficial trace of the raymond fault one of the nodal planes of the focal mechanism of the earthquake parallels the raymond fault with left lateral strike slip movement on that plane and is consistent with geomorphic and paleoseismic evidence that the raymond fault is dominantly a left lateral strike slip fault the existence of a component of sinistral slip along the raymond fault had been suspected prior to the earthquake but the northward dip of the fault and the prominent scarp along the western portion of its trace had led most workers to conclude that slip along the fault was dominantly reverse

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 non self similarity and transitional increment of scaled energy in the 2005 west off fukuoka seismic sequence 1 we investigated seismic scaling relations of the 2005 west off fukuoka seismic sequence in japan using a coda based methodology two independent methods such as coda source spectrum method and coda spectral ratio method were applied to seismic waveform data recorded at four broadband seismic stations located around the epicenter of the mainshock the corner frequencies determined from both methods are consistent with each other and the scaling relations between the corner frequencies and the corresponding seismic moments are clearly non self similar in both cases moreover the scaled energy obtained from the coda source spectrum method shows that there is a transitional change in the scaled energy with seismic moment the scaled energy is larger for events with mw 5 0 than for events with mw 4 0 for events larger than mw 5 0 the scaled energy is almost constant and it means that they obey self similar scaling for events smaller than about mw 4 0 the scaled energy is more or less constant but it is difficult to decide whether it follows self similar or non self similar scaling because of scatter and the limitation of the minimum moment magnitude mw 3 4 the most important observation is a linear increment of the scaled energy between about mw 4 0 and 5 0 these observations imply that the rupture dynamics for larger and smaller earthquakes are different and that the change in dynamics may occur continuously between about mw 4 0 and 5 0 in this seismic sequence

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

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 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

synthesis of wood anderson seismograms from broadband digital records abstract wood anderson torsion seismograms are synthesized by convolution with cosited broadband digital seismograph recordings and compared with the photographically recorded wood anderson seismograms at berkeley and mt hamilton the objective is to use routinely the synthesized seismograms to determine local magnitude comparison of local magnitude values calculated from both synthesized and photographic seismograms shows that the variation in local magnitude between the two types of seismograms is less than the variation among the photographic seismograms consequently broadband digital recordings can provide reliable local magnitude estimates without introducing significant bias it is shown that the static magnification of standard t s 0 8 sec wood anderson torsion seismographs determined from measurement of the free period and tilt sensitivity is 2080 60 and not 2800 as often reported this discrepancy is due to the erroneous assumption that the taught wire suspension in a wood anderson seismograph does not distort significantly thus leading to an incorrect estimate of the radius of gyration and consequently to a theoretical geometrical magnification of 2800

a laboratory acoustic emission experiment under in situ conditions in this paper we revisit acoustic emission ae data from an in situ rock fracture experiment conducted at the underground research laboratory url in manitoba canada the mine by experiment a large scale excavation response test was undertaken at a depth of 420 m and involved the mechanical excavation of a cylindrical tunnel during the experiment a small array of 16 panametrics v103 ae sensors enclosed a 0 7 m 0 7 m 1 1 m rectangular prism of lac du bonnet granite located in the tunnel wall the v103 sensors were later calibrated in the laboratory and a source parameter analysis was undertaken using a spectral fitting method corner frequency and moment magnitude were found to be inside the ranges 250 khz fc 490 khz and 7 5 mw 6 8 respectively static stress drops ranged from 0 3 to 4 mpa which is consistent with large seismicity recorded at the url

a comparison of broadband source spectra seismic energies and stress drops of the 1989 loma prieta and 1988 armenian earthquakes broadband source spectra of the 1989 loma prieta mw 6 9 and 1988 armenian mw 6 7 earthquakes are computed at periods from 1 to 50 sec using digitally recorded teleseismic p body waves the effects of attenuation geometrical spreading and radiation pattern are removed from the spectra of individual stations which are then averaged the source spectra of the loma prieta and armenian earthquakes are higher for their seismic moments than the spectra of 11 intraplate earthquakes studied by zhuo and kanamori 1987 which in turn are 2 to 4 times larger than average spectra of interplate subduction zone earthquakes the seismically radiated energy can be computed from the source spectrum using haskell s 1964 formulation assuming a point source with no directivity an orowan stress drop can be obtained from the seismic energy and moment the orowan stress drops for the loma prieta and armenian earthquakes are both about 20 bars significantly higher than orowan stress drops of recent large interplate earthquakes there is a positive correlation between the orowan stress drops and the estimated repeat times consistent with the notion that mechanical fault strength increases with increasing interseismic period

broadband source spectrum seismic energy and stress drop of the 1989 macquarie ridge earthquake i compute the broadband source spectrum at periods from 1 to 50 seconds using teleseismic p body waves of the may 23 1989 macquarie ridge earthquake mw 8 1 recorded by the gdsn geoscope and ida networks the average source spectrum is obtained by windowing tapering and fourier transforming p waves removing from the spectra the effects of attenuation geometrical spreading and radiation pattern and averaging logarithmically over the stations the source spectrum for the strike slip macquarie ridge earthquake is higher than an average source spectrum of seven recent large earthquakes scaled to be comparable to a mw 8 1 earthquake by a factor of 2 to 3 at periods of 1 to 20 seconds these other earthquakes were underthrusting events in subduction zones using haskell s formulation assuming a point source with no directivity i estimate the seismically radiated energy from the source spectrum by integrating the square of the source spectrum in velocity and scaling the result the seismic energy thus estimated for the macquarie ridge earthquake is 3 to 8 1023 ergs an orowan stress drop can be obtained from the seismic energy and moment the orowan stress drop for the macquarie ridge earthquake is about 20 to 50 bars much higher than similarly determined stress drops of other recent large earthquakes there is a correlation between the orowan stress drops and time since the last earthquake of comparable or larger magnitude for seven recent large earthquakes this correlation suggests that a healing process operates that may control the mechanical strength of the fault and is important on time scales of tens to hundreds of years

the 1991 sierra madre earthquake sequence in southern california seismological and tectonic analysis the m l 5 8 sierra madre earthquake of 28 june 1991 occurred at a depth of 12 km under the san gabriel mountains of the central transverse ranges since at least 1932 this region had been quiescent for m 3 the mainshock focal mechanism derived from first motion polarities exhibited almost pure thrust faulting with a rake of 82 on a plane striking n62 e and dipping 50 to the north the event appears to have occurred on the clamshell sawpit fault a splay of the sierra madre fault zone the aftershock sequence following the mainshock occurred at a depth of 9 to 14 km and was deficient in small earthquakes having a b value of 0 6 twenty nine single event focal mechanisms were determined for aftershocks of m 1 5 the 4 km long segment of the clamshell sawpit fault that may have ruptured in the mainshock is outlined by several thrust focal mechanisms with an east northeast striking fault plane dipping to the north to the west several thrust aftershocks with east striking nodal planes suggest some complexity in the aftershock faulting such as a curved rupture surface in addition several strike slip and normal faulting events occurred along the edges of the mainshock fault plane indicating secondary tear faulting the tectonic stress field driving the coexisting left lateral strike slip and thrust faults in the northern los angeles basin is north south horizontal compression with vertical intermediate or minimum principal stress axis

a method for mapping apparent stress and energy radiation applied to the 1994 northridge earthquake fault zone using the northridge earthquake as an example we demonstrate a new technique able to resolve apparent stress within subfaults of a larger fault plane from the model of wald et al 1996 we estimated apparent stress for each subfault using where g is the modulus of rigidity is the shear wave speed and is the average slip rate the image of apparent stress mapped over the northridge fault plane supports the idea that the stresses causing fault slip are inhomogeneous but limited by the strength of the crust indeed over the depth range 5 to 17 km maximum values of apparent stress for a given depth interval agree with a max 0 06s z where s is the laboratory estimate of crustal strength as a function of depth z the seismic energy from each subfault was estimated from the product ada where a is subfault area and d its slip over the fault zone we found that the radiated energy is quite variable spatially with more than 50 of the total coming from just 15 of the subfaults

regional seismic discriminants using wave train energy ratios we have examined broadband regional waveforms of recent since 1988 nevada test site nts underground explosions and earthquakes throughout the southwestern united states and baja mexico recorded by terrascope and other iris stations in order to characterize seismic sources for the purposes of event identification as expected earthquakes tended to be richer in long period surface wave and short period shear wave energy relative to explosions of comparable p wave strength also explosions in general were found to be richer in 1 to 6 sec surface wave rg energy and other late arriving coda energy than were earthquakes most earthquakes show relatively little long period t 6 sec rg and surface wave coda energy which we attribute to their deeper source depths whereas known shallow earthquakes do exhibit these phases we have developed several seismic discriminants based on our observations the most promising discriminant is the ratio of short period f 1 0 hz vertical component pnl wave train energy esppz to long period 0 05 to 0 167 hz three component surface wave energy e lp 3 for this ratio explosions tend to have a higher value than do earthquakes this discriminant works on the same premise as the teleseismic mb ms ratio for which earthquakes are richer in long period surface wave energy relative to explosions with the same body wave magnitude the long period passband was chosen to limit the effect of longer period noise and to remove the effect of the coda surface waves another potential discriminant examined is the ratio of short period f 1 0 hz vertical component p wave to s wave energy esppz espsz we find that this criterion only yields marginal separation of the source populations but becomes more effective at higher frequency bands f 4 0 hz or when looking at single station observations it does however help to quantify significant short period waveform differences between the three test subsites with pahute mesa shots generating relatively little s wave energy compared to those of yucca flat for which the s wave or lg is often the largest phase while rainier s shots are intermediate in character with distinct but less prominent s waves this s wave generation is thought to be caused by near source scattering to converted phases and appears to be highly dependent on the near source geology these two discriminants are useful in that they are simple and fast to calculate using regional stations for sources 200 to 1300 km away the magnitude threshold for the esppz e lp 3 discriminant is roughly ml 4 0 the limiting factor being the signal level of the airy phase while that for the esppz espsz discriminant is roughly ml 3 0 for the same distance ranges

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

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

the richter scale its development and use for determining earthquake source parameters abstract the m l scale introduced by richter in 1935 is the antecedent of every magnitude scale in use today the scale is defined such that a magnitude 3 earthquake recorded on a wood anderson torsion seismometer at a distance of 100 km would write a record with a peak excursion of 1 mm to be useful some means are needed to correct recordings to the standard distance of 100 km richter provides a table of correction values which he terms log a o the latest of which is contained in his 1958 textbook a new analysis of over 9000 readings from almost 1000 earthquakes in the southern california region was recently completed to redetermine the log a o values although some systematic differences were found between this analysis and richter s values such that using richter s values would lead to underand overestimates of m l at distances less than 40 km and greater than 200 km respectively the accuracy of his values is remarkable in view of the small number of data used in their determination richter s corrections for the distance attenuation of the peak amplitudes on wood anderson seismographs apply only to the southern california region of course and should not be used in other areas without first checking to make sure that they are applicable often in the past this has not been done but recently a number of papers have been published determining the corrections for other areas if there are significant differences in the attenuation within 100 km between regions then the definition of the magnitude at 100 km could lead to difficulty in comparing the sizes of earthquakes in various parts of the world to alleviate this it is proposed that the scale be defined such that a magnitude 3 corresponds to 10 mm of motion at 17 km this is consistent both with richter s definition of m l at 100 km and with the newly determined distance corrections in the southern california region aside from the obvious and original use as a means of cataloguing earthquakes according to size m l has been used in predictions of ground shaking as a function of distance and magnitude it has also been used in estimating energy and seismic moment there is a good correlation of peak ground velocity and the peak motion on a wood anderson instrument at the same location as well as an observationally defined and theoretically predicted nonlinear relation between m l and seismic moment an important byproduct of the establishment of the m l scale is the continuous operation of the network of wood anderson seismographs on which the scale is based the records from these instruments can be used to make relative comparisons of amplitudes and waveforms of recent and historic earthquakes furthermore because of the moderate gain the instruments can write onscale records from great earthquakes at teleseismic distances and thus can provide important information about the energy radiated from such earthquakes at frequencies where many instruments have saturated

progressive cenozoic cooling and the demise of antarctica s last refugium the antarctic peninsula is considered to be the last region of antarctica to have been fully glaciated as a result of cenozoic climatic cooling as such it was likely the last refugium for plants and animals that had inhabited the continent since it separated from the gondwana supercontinent drill cores and seismic data acquired during two cruises shaldril i and ii in the northernmost peninsula region yield a record that when combined with existing data indicates progressive cooling and associated changes in terrestrial vegetation over the course of the past 37 million years mountain glaciation began in the latest eocene approximately 37 34 ma contemporaneous with glaciation elsewhere on the continent and a reduction in atmospheric co2 concentrations this climate cooling was accompanied by a decrease in diversity of the angiosperm dominated vegetation that inhabited the northern peninsula during the eocene a mosaic of southern beech and conifer dominated woodlands and tundra continued to occupy the region during the oligocene approximately 34 23 ma by the middle miocene approximately 16 11 6 ma localized pockets of limited tundra still existed at least until 12 8 ma the transition from temperate alpine glaciation to a dynamic polythermal ice sheet took place during the middle miocene the northernmost peninsula was overridden by an ice sheet in the early pliocene approximately 5 3 3 6 ma the long cooling history of the peninsula is consistent with the extended timescales of tectonic evolution of the antarctic margin involving the opening of ocean passageways and associated establishment of circumpolar circulation

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

seismic energy release over a broad frequency band there remains much uncertainty on the absolute elastic wave energy released by fault rupture few direct estimates of the partition of seismic wave energy in ground shaking have been made in this work ground particle velocities from integrated accelerograms are used to compute the kinetic energy crossing unit area per unit time simplified theory for the near field strong motion case would appear to give a valid lower energy bound the wave attenuation does not present a major problem the partition of energy in predominantly p s and surface wave portions for given frequency windows is tabulated using strong motion accelerograms recorded at different azimuths to the fault sources of six california earthquakes 5 5 m l 7 2 checks against earlier magnitudem l and momentm 0 correlations indicate significantly higher overall wave energy outputs than expected but further calibration is needed the study demonstrates that stable estimates of frequency dependent seismic wave energies in the nearfield can be obtained from strong motion records hence energy flux may have a wider application to risk mapping than previously thought in particular a shift from peak acceleration scaling to kinetic energy inputs for engineering design appears to involve only routine processing

the oroville earthquakes a study of source characteristics and site effects half space modeling in the time domain of broadband seismic records from the oroville aftershock sequence demonstrates that these events have simple sources systematic scaling between the far field source duration and seismic moment for the aftershocks studied here and other california earthquakes confirms brune s 1970 source model and shows that the stress drops for california earthquakes range between 10 and 100 bars these events also show no trend in stress drop as a function of size for 3 0 or m sub l or 6 4 analysis of the static deformation associated with the main shock shows that the slip occurred on a fault 81 km sup 2 in area corresponding to the zone defined by the first 3 days of aftershocks teleseismic studies indicate that the seismic source area was 50 km sup 2 corresponding to a zone completely free of aftershocks in the middle of the aftershock zone both results imply a stress drop of 29 39 bars and seismic moment between 5 7 and 8 7 x 10 sup 24 dyn cm the evidence suggests that faulting occurred seismically on the central portion of the fault and propagated out slowly enlarging the fault plane complicated waveforms recorded for the same events at nearby sites on alluvium indicatemore the critical role that propagation through complex structure plays in determining the character of the observed seismic signal at close in stations modeling of these records show that dipping structure may account for the observed waveform complexities less

a local magnitude scale for the western great basin eastern sierra nevada from synthetic wood anderson seismograms abstract we have computed synthetic wood anderson seismograms for over 1100 arrivals at 10 three component broadband digital stations in the unr western great basin eastern sierra nevada network these represent all the available records from local earthquakes over magnitude 3 5 between 1990 and june of 1993 plus selected events of smaller magnitude there were 77 events ranging in magnitude from 2 2 to 5 9 including four events over magnitude 5 the distances considered ranged from 15 to 600 km with the best represented range being from 30 to 450 km we invert these measurements to determine distance and station corrections appropriate for a local magnitude scale constrained by richter9s original definition that an earthquake of m l 3 will cause a 1 mm zero to peak deflection of the wood anderson seismogram at 100 km from the epicenter the results between 30 and 450 km were essentially independent of choice of curve fitting parameters in the 30 to 500 km distance region the smooth distance correction curves were very similar to that determined by richter 1958 which is still used for southern california earthquakes we propose to use richter9s distance correction curve in reporting amplitude magnitudes from our digital network

partitioning of seismoacoustic energy and estimation of yield and height of burst depth of burial for near surface explosions explosions near the earth s surface excite both seismic ground motions and atmospheric overpressure the energy transferred to the ground and atmosphere from a near surface explosion depends on yield w as well as the height of burst depth of burial hob dob for above belowground emplacements we report analyses of seismic and overpressure motions from the humble redwood series of low yield near surface chemical explosions with the aim of developing quantitative models of energy partitioning and a methodology to estimate w and hob dob the effects of yield hob and range on amplitudes can be cast into separable functions of range and hob scaled by yield we find that displacement of the initial p wave and the integral of the positive overpressure impulse are diagnostic of w and hob with minimal scatter an empirical model describing the dependence of seismic and air blast measurements on w hob dob and range is determined and model parameters are found by regression we find seismic amplitudes for explosions of a given yield emplaced at or above the surface are reduced by a factor of 3 relative to fully contained explosions below ground air blast overpressure is reduced more dramatically with impulse reduced by a factor of 100 for deeply buried explosions relative to surface blasts our signal models are used to invert seismic and overpressure measurements for w and hob and we find good agreement w errors 30 hob within meters with ground truth values for four noncircular validation tests although there is a trade off between w and hob for a single seismic or overpressure measurement the use of both measurement types allows us to largely break this trade off and better constrain w and hob however both models lack resolution of hob for aboveground explosions

near field investigations of the landers earthquake sequence april to july 1992 the landers earthquake which had a moment magnitude m w of 7 3 was the largest earthquake to strike the contiguous united states in 40 years this earthquake resulted from the rupture of five major and many minor right lateral faults near the southern end of the eastern california shear zone just north of the san andreas fault its m w 6 1 preshock and m w 6 2 aftershock had their own aftershocks and foreshocks surficial geological observations are consistent with local and far field seismologic observations of the earthquake large surficial offsets as great as 6 meters and a relatively short rupture length 85 kilometers are consistent with seismological calculations of a high stress drop 200 bars which is in turn consistent with an apparently long recurrence interval for these faults