microearthquakes and the nature of the creeping to locked transition of the san andreas fault zone near san juan bautista california abstract eighteen digital event recorders were deployed during may june 1981 along the creeping to locked transition of the san andreas fault zone near san juan bautista california as a supplement to the u s geological survey9s central california seismic network analysis of 18 well recorded microearthquakes 0 7 m l 2 8 located along the transition confirms the complexity of the crust and fault zone structure of the transition region seismic wave site amplification is 2 to 10 times greater at sites between the san andreas and sargent fault traces consistent with other evidence for lower velocities in the upper 3 km of crust there routine mislocation of epicenters 2 to 4 km northeast of the sargent fault are consistent with greater p wave velocity northeast of the sargent fault microearthquake source parameters are consistent with a more segmented and splayed fault geometry toward the northwest locked end of the transition p wave nodal planes for 10 microearthquakes located to the northwest 9 on the sargent fault and 1 near the san andreas are oriented more westerly than nodal planes commonly obtained for the frequent moderate size earthquakes on the creeping section of the san andreas fault to the southeast static stress drop dynamic stress drop and apparent stress estimates all increase with seismic moment with the apparent stress and dynamic stress drops equal to about 3 and 20 per cent respectively of the static stress drop average fracture energies calculated assuming complete stress drop generally increase with source size seismic moment rupture area seismic slip etc from 7 to 3000 j m 2 the two events with anomalously low fracture energies occurred on the creeping section of the san andreas fault

earthquake scaling laws for creeping and non creeping faults the dependence of the summed moment release on the fault dimension differs between non creeping faults and creeping faults or volcanic regions this can be attributed to different earthquake scaling laws the number n of events with fault length l scales for non creeping faults as n l l 2 and for creeping faults as n l l 3 this difference offers a means for mapping the distribution of creep in the seismogenic crust

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

moment magnitude relations based on strong motion records in greece in this work the variation of the local magnitude mlsm derived from strong motion records at short distances is examined in terms of moment magnitude m w strong motion data from greek earthquakes are used to determine the strong motion local magnitude mlsm by performing an integration of the equation of mo tion of the wood anderson wa seismograph subjected to an input acceleration the most reliable strong motion data are utilized for earthquakes with seismic mo ments log m 0 22 0 dynecm and calculated local magnitudes mls m 3 7 the correlation between the seismic moments log m0 and the calculated local magni tudes mlsm using strong motion records is given by log m 0 1 5 mcsu 16 07 which is very similar to that proposed by hanks and kanamori 1979 moreover it is shown that mls m is equal to moment magnitude mw for a large mls m range 3 9 to 6 6 comparison of the strong motion local magnitude and the me magnitude estimated in greece ml n and surrounding area shows a systematic bias of 0 4 to 0 5 similar to the difference that has been found between m w and mlgr for the same area the contribution of the local site effects in the calculation of the local magni tude mlsm is also considered by taking into account two indices of soil classification namely rock and alluvium or the shear wave velocity vs30 of the first 30 m based on nehrp 1994 and ubc 1997 an increase of mls m by 0 16 is observed for alluvium sites alternative relations showing the mls m variation with v 3 are also presented finally examination of the wa amplitude attenuation log a0 with distance shows that the jennings and kanamori 1983 relation for a 100 km is appropriate for greece the same results confirm earlier suggestions that the 0 4 to 0 5 bias between mlgr and mw also mlsa4 should be attributed to a low static magnification 800 of the athens wa instrument on which all other m l relations in greece have been calibrated

stress transfer during three sequential moderate earthquakes along the central calaveras fault california seismic and geodetic information on three sequential moderate earthquakes the 1979 m5 9 coyote lake the 1984 m6 2 morgan hill and the 1988 m5 1 alum rock along the central calaveras fault is used to examine how tractions on fault segments are transferred from one fault segment to the other geodetic data obtained before and after each earthquake are inverted for slip at depth the perturbed stress field induced by each earthquake is calculated using 3 d elastic dislocation theory although only long wavelength features of the slip distributions are well resolved the model suggests that the distribution of slip is nonuniform the largest slips tend to occur in regions having relatively few aftershocks or near the hypocenter according to the model the coyote lake earthquake induced 0 1 1 bar 0 01 0 1 mpa shear stress across the northern part of the morgan hill segment and only about 0 1 bar at the hypocenter of the morgan hill mainshock a shear stress of 0 5 bar was induced by the morgan hill earthquake at the hypocenter of the alum rock mainshock the average stress drops for the coyote lake morgan hill and alum rock earthquakes are estimated to be 30 25 and 15 bars respectively thus the earthquake induced shear stress of 0 1 0 5 bar at the hypocenter of the next earthquake is only a few percent of the total stress drop and cannot be solely responsible for the occurrence of the sequential earthquakes however the induced shear stress can trigger the adjacent earthquakes in the sense that adjacent earthquakes would have occurred at a later time if there was no such coseismic stress transfer it is inferred that fault geometry may influence the local stress field thereby initiation and termination of the rupture but not the regional stress accumulation

paleoseismology and seismic hazards evaluations in extensional volcanic terrains extensional structures in volcanic terrains are the surface expression of shallow dike intrusion and can be misinterpreted as structures associated with major tectonic faults dike induced structures can be distinguished from their tectonic counterparts by their association with cogenetic volcanic rocks and by several geometric relationships tensile fissures with little or no vertical displacement fissure swarms flexural monoclines and normal faults are commonly symmetrically distributed about a central eruptive fissure sometimes forming a graben above shallow dikes the structures typically occur within broad zones not narrow belts near a main fault zone reflecting their origin by repeated dike injection colluvial wedges containing records of single large earthquakes generally do not form instead fault scarps with several meter vertical displacements in volcanic bedrock may reflect the cumulative effects of many decimeter scale displacement events from several dike injection episodes the mechanism of dike intrusion and the nature of cointrusive seismicity have important implications for determination of the maximum magnitude and recurrence of earthquakes in extensional volcanic terrains observational seismicity from volcanic rift zones worldwide suggests the maximum magnitudes of dike induced earthquakes are 3 8 0 8 earthquakes are generally small to moderate because downdip extents of faults and fissures are controlled by the depth to the top of the associated dike usually 5 km permitting only small rupture areas also rupture and displacement on faults and fissures migrate incrementally at about the velocity of propagating dikes 0 5 m s as dike dilation stresses the zone above and ahead of the dike earthquake recurrence is tied to recurrence of volcanic cycles based on the geochronology of the associated volcanic materials based on these concepts innovative approaches have led to estimates of the maximum magnitude and recurrence for magma induced seismicity in eastern snake river plain esrp volcanic rift zones the most conservative approach uses surface length fault width downdip extent and rupture areas of normal faults and fissures produced by dike injection to estimate a maximum m5 5 earthquake for an episode of rift zone volcanism chronology of volcanic rocks suggests annual probabilities of 10 4 to 10 5 for volcanic rift zones near idaho national engineering laboratory facilities probabilistic assessments show that ground motion hazard due to volcanic rift zone seismicity is lower than the hazard from other earthquake sources this is because of the relatively low magnitudes and long recurrence intervals of cointrusive earthquakes in volcanic rift zones

silica diagenesis ii general mechanisms abstract amorphous silica phases opal a precipitate in nature due to the formation of dense colloids in supersaturated alkaline aqueous solutions with low relative concentrations of other ions opal a dissolves and yields solutions of still relatively high silica content in pore waters containing abundant cations open framework polymers form which flocculate to yield opal ct opal ct becomes increasingly ordered primarily due to preferential growth of cristobalite relative to tridymite and crystal size increase opal ct dissolves to yield pore waters of low silica concentration which allows slow growth of quartz crystals from monomeric solution the quartz crystals then slowly increase in size and crystallinity carbonates appear to enhance opal ct formation possibly due to the activity of positively charged hydroxyl complexes hence polymerization in relatively pure systems is involved in opal a formation and polymerization in impure systems is involved in opal ct formation and slow growth from monomeric low silica concentration solutions is involved in precipitation of quartz in sedimentary realms

short period strain 0 1 105 s near source strain field for an earthquake m l 3 2 near san juan bautista california measurements of dilational earth strain in the frequency band 25 10 5 hz have been made on a deep borehole strainmeter installed near the san andreas fault these data are used to determine seismic radiation fields during nuclear explosions teleseisms local earthquakes and ground noise during seismically quiet times strains of less than 10 10 on these instruments can be clearly resolved at short periods 10 s and are recorded with wide dynamic range digital recorders this permits measurement of the static and dynamic strain variations in the near field of local earthquakes noise spectra for earth strain referenced to 1 strain 2 hz show that strain resolution decreases at about 10 db per decade of frequency from 150 db at 10 4 hz to 223 db at 10 hz exact expressions are derived to relate the volumetric strain and displacement field for a homogeneous p wave in a general viscoelastic solid as observed on colocated dilatometers and seismometers a rare near field recording of strain and seismic velocity was obtained on may 26 1984 from an earthquake ml 3 2 at a hypocentral distance of 3 2 km near the san andreas fault at san juan bautista california while the data indicate no precursory strain release at the 5 10 11 strain level a coseismic strain release of 1 86 nanostrain was observed this change in strain is consistent with that calculated from a simple dislocation model of the event ground displacement spectra determined from the downhole strain data and instrument corrected surface seismic data suggest that source parameters estimated from surface recordings may be contaminated by amplification effects in near surface low velocity materials

seismic slip segmentation and the loma prieta earthquake we have plotted the cumulative seismic slip projected onto a vertical plane for earthquakes occurring during the last 20 years along 210 km of the san andreas fault that includes the section that moved in the loma prieta earthquake these plots illustrate the differences in depth and character of the seismicity between the locked and creeping portions of the fault or fault zone and define the segment upon which the loma prieta earthquake occurred working by analogy from the relation between pre main shock microseismicity and presumed main shock slip regions at parkfield and loma prieta we identify a segment on the san francisco peninsula where we believe the m 7 1838 earthquake occurred and which we believe may have accumulated sufficient strain that rupture should be expected in the coming decades

new empirical relationships among magnitude rupture length rupture width rupture area and surface displacement source parameters for historical earthquakes worldwide are compiled to develop a series of empirical relationships among moment magnitude m surface rupture length subsurface rupture length downdip rupture width rupture area and maximum and average displacement per event the resulting data base is a significant update of previous compilations and includes the additional source parameters of seismic moment moment magnitude subsurface rupture length downdip rupture width and average surface displacement each source parameter is classified as reliable or unreliable based on our evaluation of the accuracy of individual values only the reliable source parameters are used in the final analyses in comparing source parameters we note the following trends 1 generally the length of rupture at the surface is equal to 75 of the subsurface rupture length however the ratio of surface rupture length to subsurface rupture length increases with magnitude 2 the average surface displacement per event is about one half the maximum surface displacement per event and 3 the average subsurface displacement on the fault plane is less than the maximum surface displacement but more than the average surface displacement thus for most earthquakes in this data base slip on the fault plane at seismogenic depths is manifested by similar displacements at the surface log linear regressions between earthquake magnitude and surface rupture length subsurface rupture length and rupture area are especially well correlated showing standard deviations of 0 25 to 0 35 magnitude units most relationships are not statistically different at a 95 significance level as a function of the style of faulting thus we consider the regressions for all slip types to be appropriate for most applications regressions between magnitude and displacement magnitude and rupture width and between displacement and rupture length are less well correlated and have larger standard deviation than regressions between magnitude and length or area the large number of data points in most of these regressions and their statistical stability suggest that they are unlikely to change significantly in response to additional data separating the data according to extensional and compressional tectonic environments neither provides statistically different results nor improves the statistical significance of the regressions regressions for cases in which earthquake magnitude is either the independent or the dependent parameter can be used to estimate maximum earthquake magnitudes both for surface faults and for subsurface seismic sources such as blind faults and to estimate the expected surface displacement along a fault for a given size earthquake

seismic moments local magnitudes and coda duration magnitudes for earthquakes in central california onscale seismograms recorded at stations in the u s geological survey s usgs central california seismic network calnet have been used to estimate the seismic moment m and local magnitude ml for earthquakes of 1 ml 4 located on the san juan bautista and parkfield sections of the san andreas fault the coyote lake section of the calaveras fault the sargent fault and near livermore these data together with m and ml estimates for 4 ml 6 earthquakes in these areas cannot be fit with a single linear log m versus ml relation rather the data are consistent with log m 1 5 ml 16 for 3 ml 6 with log m 1 2 ml 17 for 1 1 2 ml 3 1 2 and with a slope of to 1 fro 1 2 ml 1 1 2 whereas usgs coda duration magnitude md is consistent with ml for 1 1 2 ml 3 md is larger than ml at ml 1 1 2 and smaller than ml at ml 3 log m can be estimated to a precision of 0 2 for 1 md 3 1 2 earthquakes in central california by applying log m 1 2 md 17 to the md that have been routinely published by the usgs

a note on seismic activity near the eastern termination of the izmit rupture in the hours preceding the duzce earthquake we use continuous recordings at four stations installed near the eastern termination of the izmit rupture to analyze the seismic activity there during the 5 hr leading to the duzce earthquake the records clearly show the presence of three groups of events one of the groups originates from the middle of the izmit rupture and consists of aftershocks of the m 5 7 spanca lake earthquake that had occurred a day earlier and was itself to date the second largest aftershock in the izmit sequence a second group of events is spread over the 30 km long eastern karadere segment of the izmit rupture and a comparison with the number of aftershocks expected based on decay laws for similar sequences in other regions suggests that seismic activity was high on the eastern segment in the hours leading to the duzce earthquake the third group of events occurs beyond the termination of the izmit rupture and clusters around the hypocenter of the duzce earthquake it shows that in the little over 5 hr leading to the duzce earthquake this destructive event was preceded by six precursory shocks ranging in magnitude from 0 9 to 2 6 manuscript received 30 august 2000

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

earthquake magnitude relationships for the saint peter and saint paul archipelago equatorial atlantic we have investigated several relationships between ml m neic and mw for the earthquakes locally recorded in the saint peter and saint paul archipelago spspa equatorial atlantic because we only have one station in the area we could not derive attenuation relations for events recorded at different distances at different stations our approach was then to compare our ml estimates with magnitudes reported by neic this approach produced acceptable results particularly for epicentral distance smaller than 100 km for distances greater that 100 km there is a systematic increase in the residuals probable due to the lack of station correction and our inability to accurately estimate q we also investigate the mw m neic relationship we find that mw estimates using s wave produce smaller residuals when compared with both m neic finally we also investigate the ml mw relationship and observe that given the data set we have the 1 1 holds we believe that the use of the present methodologies provide consistent magnitude estimates between all the magnitudes investigated that could be used to better assess seismic hazard in the region

hrtem of microcrystalline opal in chert and porcelanite from the monterey formation california microcrystalline opal was investigated using low dose transmission electron microscopy tem methods to identify microstructural characteristics and possible phase transformation mechanisms that accommodate silica diagenesis high resolution tem hrtem revealed that microcrystalline opal in opal ct chert 90 wt silica and opal ct porcelanite 50 90 wt silica from the miocene monterey formation of california displays various amounts of structural disorder and coherent and incoherent lamellar intergrowths species of microfibrous opal identified by hrtem in early formed opal ct chert include length slow opal c and unidimensionally disordered length slow opal ct open quotes lussatite close quotes these fibers often display a microstructure characterized by an aperiodic distribution of highly strained domains that separate ordered domains located at discrete positions along the direction of the fiber axes microfibrous opal occurs as several types of fiber aggregation forms tem revealed that the siliceous matrix in later formed opal ct porcelanite consists of equidimensional nanometer size opal ct crystallites and lussatite fibers pseudo orthorhombic tridymite po 2 was identified by hrtem in one sample of opal ct porcelanite burial diagenesis of chert and porcelanite results in the precipitation of opal c and the epitaxial growth of opal c domains on opal ct substrates diagenetic maturation of lussatite was identified by tem in banded opal ct quartz chert to occur as a result of solid state ordering the primary diagenetic silica phase transformationsmore between noncrystalline opal microcrystalline opal and quartz occur predominantly by a series of dissolution precipitation reactions however tem showed that in banded opal ct quartz chert the epitaxial growth of quartz on microfibrous opal enhances the rate of silica diagenesis less

silica diagenesis i solubility controls abstract the diagenetic sequence of silica is examined in terms of several thermodynamic and kinetic concepts the relationship between solubility and surface area or particle size is sufficient to explain simple opal a opal ct quartz transformations suggesting that the role of temperature and other physical parameters may not be as critical as previously thought complexation and adsorption in an impure system when added to the model allow for possibilities of opal a transformation directly to quartz of clay and zeolite formation from an opaline silica source and of variations in silica transformation rates due to presence of impurities in the system

comparison of four moderate size earthquakes in southern california using seismology and insar source parameters determined from interferometric synthetic aperture radar insar measurements and from seismic data are compared from four moderate size less than m 6 earthquakes in southern california the goal is to verify approximate detection capabilities of insar assess differences in the results and test how the two results can be reconciled first we calculated the expected surface deformation from all earthquakes greater than magnitude 4 in areas with available insar data 347 events a search for deformation from the events in the interferograms yielded four possible events with magnitudes less than 6 the search for deformation was based on a visual inspection as well as cross correlation in two dimensions between the measured signal and the expected signal a grid search algorithm was then used to estimate focal mechanism and depth from the insar data the results were compared with locations and focal mechanisms from published catalogs an independent relocation using seismic data was also performed the seismic locations fell within the area of the expected rupture zone for the three events that show clear surface deformation therefore the technique shows the capability to resolve locations with high accuracy and is applicable worldwide the depths determined by insar agree with well constrained seismic locations determined in a 3d velocity model depth control for well imaged shallow events using insar data is good and better than the seismic constraints in some cases a major difficulty for insar analysis is the poor temporal coverage of insar data which may make it impossible to distinguish deformation due to different earthquakes at the same location

san andreas fault zone california m 5 5 earthquake history the san andreas fault zone has been a very significant source of major california earthquakes from 1812 to 1906 it generated four major earthquakes of m 7 or larger in two pairs on two major portions of the fault a pair of major earthquakes occurred on the central to southern region where the 1857 faulting overlapped the 1812 earthquake faulting a pair of major earthquakes occurred on the northern region where the 1906 faulting overlapped the 1838 earthquake faulting also earthquakes of m 7 occurred in the san francisco bay area on the hayward fault in 1868 and the santa cruz mountains near loma prieta in 1989 and on the imperial fault near the border with mexico in 1940 the 1838 earthquake s damage effects throughout the bay area from san francisco to santa clara valley and monterey were unequalled by any historical earthquake other than the 1906 event this and numerous strong possible aftershocks during the following 3 years in the san juan bautista vicinity suggest 1838 faulting from san francisco to san juan bautista cycles of seismicity and quiescence were associated with the bay area earthquakes of 1868 1906 and 1989 the 1868 earthquake on the hayward fault was preceded by 12 earthquakes of m 5 5 from 1855 to 1866 within 60 km of the hayward fault and was followed by 13 quiet years the 1906 san andreas fault event was preceded from 1881 to 1903 by 18 earthquakes of m 5 5 and was followed by quiescence with only three earthquakes of m 5 5 until 1954 the bay area has been seismically quiet at the m 5 5 level since the 1989 loma prieta earthquake and its 1990 aftershocks which contrasts with the 10 years before 1989 when five m 5 5 6 2 events occurred the loma prieta earthquake is of similar magnitude to the 1868 hayward event and could be followed by a similarly short quiet period the 1857 earthquake had immediate foreshocks in the lonoak bitterwater region 50 km northwest of parkfield in the northern end zone of the 1857 rupture extending southeast from bitterwater 70 km to parkfield the rate of seismic moment release has decreased with time since 1857 this may reflect the decay with time of the stress loading due to the 9 m 1857 fault displacements 80 km southeast of parkfield and explain why the predicted earthquake which was based on the assumption of regular recurrence of parkfield earthquakes has not yet occurred the extent of the 1812 earthquake fault rupture is not well defined jacoby et al 1988 estimated that it extended 170 km from cajon pass to tejon pass based on this estimate we present the hypothesis that the rupture occurred in two segments in december 1812 the eastern segment generated the 8 december earthquake that damaged san juan capistrano san gabriel san fernando and san buenaventura thirteen days later the western segment ruptured generating the earthquake that damaged san fernando and san buenaventura again as well as santa barbara santa ynez and purisima concepcion

the livermore valley california sequence of january 1980 abstract at 19 h 00 m 09 46 s utc on 24 january 1980 a strong earthquake m l 5 5 that caused a surprising amount of damage occurred north of livermore valley about 12 km to the southeast of mt diablo and was associated with surface rupture along the greenville fault there was a foreshock m l 2 7 a minute and a half earlier and a sequence of 59 events m l 2 5 in the ensuing 6 days on 27 january at 02 h 33 m 35 96 s a larger magnitude earthquake occurred in the sequence m l 5 6 this second principal shock was located 14 km to the south of the first principal earthquake toward the southern end of the greenville fault preliminary estimates of the seismic moments of the two principal shocks are 5 3 10 24 and 1 3 10 24 dyne cm respectively in addition to the lower seismic moment the m l 5 6 shock on 27 january exhibits a clearly focused radiation pattern with large amplitudes toward the northeast field investigations after the first principal shock indicated surfaced rupture along the greenville fault zone for at least 6 km with both right lateral strike slip and some dip slip motion with the northeast side up variable offsets on surface cracks suggested displacements of a few centimeters with evidence of increases in some places after the second 27 january earthquake there were eight earthquakes with m l 4 0 in the sequence up to 5 february 1980 no foreshocks near the greenville fault m l 1 5 were observed by the university of california seismographic stations in the prior 3 months rapid deployment of field seismographs by a number of seismological organizations permitted precise locations and fault plane solutions some results on seismicity are as follows 1 the rupture propagated over 15 km to the southeast along the marsh creek greenville faults on 24 january and stopped in the vicinity of highway 580 this southern progression may have had some causal connection with the relatively high intensities reported near the southwest end of the greenville fault 2 the two principal shocks of the sequence have slight but significant differences in the fault plane solutions both are predominantly right lateral strike slip but the strike of the northern one is n13 w whereas the strike of the southern one is n39 w this change in strike is not evident in the mapped strikes of the marsh creek and the greenville faults 3 in contrast to the second principal earthquake the first principal shock was followed by two others m l 4 0 in rapid succession one 53 sec and the other 97 sec after this repetition gave a relatively long duration to the shaking on 24 january and was commented on in felt reports it may explain the greater intensity reported in many localities on 24 january compared to 27 january 4 the b value 0 64 0 13 for the sequence is somewhat lower than the b 0 70 0 17 for the recent coyote lake earthquake sequence on the calaveras fault on 5 august 1979 there are fewer earthquakes than normal in the range 3 0 m l

a comparison of coda and s wave spectral ratios as estimates of site response in the southern san francisco bay area abstract we calculate spectral ratios for s waves and codas to evaluate the amplification of 14 sites in the southern san francisco bay area relative to a nearby bedrock site in the coyote hills our data are seismograms written by loma prieta aftershocks the epicentral distances and azimuths to the stations are effectively the same all the sites in the study are amplified with respect to the reference site at frequencies from 0 5 to 7 hz the shapes of the s wave and coda spectral ratios are similar but the coda ratios are greater than the s wave ratios by as much as a factor of 4 the difference is larger for sites on alluvial and bay mud deposits particularly at frequencies around 1 hz suggesting the presence of waves trapped in the alluvial basin in general the length of the analysis window affects the s wave spectral ratios for alluvial sites longer windows give ratios similar to coda ratios apparently because these windows include more of the phases that contribute to the coda we classify the sites according to their geological characteristics and surficial shear wave velocities for the s wave ratios the differences between the classes showed no systematic trend the softest and hardest soil classes we consider have practically identical s wave amplifications the average coda ratios for the site classes clearly increase as the soil classes include slower and softer materials after correction for differences in reference sites the coda amplifications are very similar to the relative amplifications for these site classes estimated by borcherdt and glassmoyer 1992 from the strong motion recordings of the 1989 loma prieta earthquake

seasonal and long term groundwater unloading in the central valley modifies crustal stress changes in terrestrial water content cause elastic deformation of the earth s crust this deformation is thought to play a role in modulating crustal stress and seismicity in regions where large water storage fluctuations occur groundwater is an important component of total water storage change in california helping to drive annual water storage fluctuations and loss during periods of drought here we use direct estimates of groundwater volume loss during the 2007 2010 drought in california s central valley obtained from high resolution interferometric synthetic aperture radar based vertical land motion data to investigate the effect of groundwater volume change on the evolution of the stress field we show that gps derived elastic load models may not capture the contribution of groundwater to terrestrial water loading resulting in an underestimation of nontectonic crustal stress change we find that groundwater unloading during the drought causes coulomb stress change of up to 5 5 kpa and seasonal fluctuations of up to 2 6 kpa at seismogenic depth we find that faults near the valley show the largest stress change and the san andreas fault experiences only 40 pa of coulomb stress change over the course of a year from groundwater storage change annual coulomb stress change peaks dominantly in the fall when the groundwater level is low however some faults experience peak stress in the spring when groundwater levels are higher additionally we find that periods of increased stress correlate with higher than average seismic moment release but are not correlated with an increase in the number of earthquakes this indicates groundwater loading likely contributes to nontectonic loading of faults especially near the valley edge but is not a dominant factor in modulation of seismicity in california because the amplitude of stress change declines rapidly with distance from the valley by carefully quantifying and spatially locating groundwater fluctuations we will improve our understanding of what drives nontectonic stress and forces that modulate seismicity in california

estimation of near source ground motions from a teleseismically derived rupture model of the 1989 loma prieta california earthquake abstract a rupture model for the 1989 loma prieta earthquake obtained from inversion of teleseismic body waves is used to calculate the mainshock near source strong ground motions the effectiveness of both synthetic and empirical aftershock green9s functions is evaluated in terms of velocity response spectra over the period range from 0 1 to 10 sec a strictly deterministic approach is used in the simulation of near source ground motion with both the synthetic and aftershock green9s functions a deterministic method has advantages in that it is simple to apply and there is no stochastic element to introduce variability in the solution response spectral values are very sensitive to the choice of the mainshock source time function using a 1 0 sec triangular time function obtained from the teleseismic study gives good estimates of response spectra from 1 to 2 sec period for aftershock green9s functions and from 1 to 10 sec period for synthetic green9s functions the limited long period response of the aftershock summations is due to the restricted bandwidth of the recording sensor using a 0 5 sec kostrov time function extends the bandwidth of validity of the aftershock summations to 0 1 to 2 sec period and the synthetic green9s function summations to 0 3 to 10 sec period the limited short period bandwidth of the synthetic green9s function summations is due to the lower frequency content 5 hz and absence of accurate high frequency propagation effects in the synthetic seismograms although the teleseismic study could not distinguish between a 1 0 sec triangular time function and a 0 5 sec kostrov time function we feel justified in using the kostrov function because it lies within our broader knowledge of earthquake source time functions

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

physical model of aftershock temporal behaviour abstract the aim of the model is to describe the temporal behaviour of cumulative seismic moment released in aftershock sequences it therefore constitutes an alternative approach with respect to the omori and utsu laws which model the number of events against time static fatigue is assumed to be the principal explanation of the aftershock temporal behaviour as a result aftershocks can originate at a stress intensity factor lower than the fracture toughness under the condition that the mainshock causes a redistribution of stress the initial stress condition of the aftershock sequence at mainshock origin time t0 can be considered as the superposition of the stress before the mainshock and of the stress step d caused by the dynamic rupture of the mainshock the model can be written in the form s t d rt ln t where s t is the cumulative stress drop at time t after the mainshock t the temperature r the universal gas constant and a constant dependent on material type the model is derived from the empirical static fatigue law t s exp u rt widely applied in fracture mechanics is the stress s a constant and the other parameters as above the fit of the model to five aftershock sequences obtained by linear regression analysis is quite good with the r2 greater than 0 85 for all the sequences but the imperial valley sequence where r2 0 7 the results seem to favour the hypothesis that the decay of the rate of seismic moment release depends on temperature in agreement with other evidences based on the evaluation of the decay rate of omori law but in contrast to some static fatigue results obtained in laboratory tests on granite rocks the scarcity of field data on underground temperature does not allow any definitive conclusion about this subject

prediction of strong ground motion for the 1989 loma prieta earthquake using empirical green s functions abstract this article examines how the empirical green9s function seismogram synthesis method might perform as a tool for earthquake hazard analysis by predicting strong ground motion from the 18 october 1989 loma prieta earthquake a suite of accelerograms are synthesized for five sites 40 to 80 km from the nearest fault rupture from 25 different source models that might have been considered prior to the occurrence of the loma prieta earthquake three component full wavetrain ground accelerations are synthesized over the frequency range of 0 5 to 25 0 hz with a sparse distribution of source events for empirical green9s functions and fairly simple source models the suite of source models includes a range of rupture histories and geometries asperity models and slip models that reflect the uncertainties both in predicting the actual rupture history and in knowledge of modeling rupture processes the suite of synthesized accelerograms includes a good match to observed time series amplitudes frequency content energy distribution and some waveforms and demonstrates that the character of actual time series can be captured with a range of simple source models the average levels of peak acceleration pseudo velocity response and fourier amplitude spectra provide a good prediction to the observed values at four of the five sites the log normal standard deviations provide bounds on the predictions that are specific to the source and recording site

a study of ground motion attenuation in the southern great basin nevada california using several techniques for estimates of qs log a 0 and coda q as a first step in the assessment of the earthquake hazard in the southern great basin of nevada california this study evaluates the attenuation of peak vertical ground motions using a number of different regression models applied to unfiltered and band pass filtered ground motion data these data are concentrated in the distance range 10 250 km the regression models include parameters to account for geometric spreading anelastic attenuation with a power law frequency dependence source size and station site effects we find that the data are most consistent with an essentially frequency independent q and a geometric spreading coefficient less than 1 0 regressions are also performed on vertical component peak amplitudes reexpressed as pseudo wood anderson peak amplitude estimates pwa permitting comparison with earlier work that used wood anderson wa data from california both of these results show that q values in this region are high relative to california having values in the range 700 900 over the frequency band 1 10 hz comparison of ml magnitudes from stations brk and pas for earthquakes in the southern great basin shows that these two stations report magnitudes with differences that are distance dependent this bias suggests that the richter log a0 curve appropriate to california is too steep for earthquakes occurring in southern nevada a result implicitly supporting our finding that q values are higher than those in california the pwa attenuation functions derived from our data also indicate that local magnitudes reported by california observatories for earthquakes in this region may be overestimated by as much as 0 8 magnitude units in some cases both of these results will have an effect on the assessment of the earthquake hazard in this region the robustness of our regression technique to extract the correct geometric spreading coefficient n and anelastic attenuation q is tested by applying the technique to simulated data computed with given n and q values using a stochastic modeling technique we generate suites of seismograms for the distance range 10 200 km and for both wa and short period vertical component seismometers regressions on the peak amplitudes from these records show that our regression model extracts values of n and q approximately equal to the input values for either low q california attenuation or high q southern nevada attenuation regressions on stochastically modeled wa and pwa amplitudes also provides a method of evaluating differences in magnitudes from wa and pwa amplitudes due to recording instrument response characteristics alone these results indicate a difference between mlwa and mlpwa equal to 0 15 magnitude units which we term the residual instrument correction in contrast to the peak amplitude results coda q determinations using the single scatterer theory indicate that qc values are dependent on source type and are proportional to p where p 0 8 to 1 0 this result suggests that a difference exists between attenuation mechanisms for direct waves and backscattered waves in this region

size of earthquakes in southern mexico from indirect methods l g and duration magnitude formulas as well as seismic moment relations are derived for southern mexico the derivation uses a method based on the coda q value for this region obtained in another study canas et al 1988 that needs only one seismographic station

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

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

mining induced seismicity in faulted geologic structures an analysis of seismicity induced slip potential relationships between the locations of mining induced seismic events local fault structure and mine geometry were examined in a deep hard rock mine in northern idaho stopes experiencing rock bursts and other large seismic events were found to fall into two structural regimes the silver vein and the n48 w trend a steeply dipping plane of seismic activity that is subparallel to major local steeply dipping faults which bound blocky structures the n48 w trend also intersects a shaft that was seriously damaged when fault gouge was expelled into the opening during a 3 month period of high seismic energy release models of stress interaction are used to support the hypothesis that mining induced deformation was mobilized along a 1 5 km length of the n48 w trend specifically numerical models are used to simulate rupture of seismic events and estimate induced changes in the quasi static stress field a coulomb failure criterion is used with these results to estimate the spatial variation in potential for slip on planes parallel to local faulting increases in the potential for slip on fault planes subparallel to the n48 w trend are consistent with activation of deformation along its 1 5 km length for events with constant seismic moment stress drop is shown to be far more important than source dimension in elevating slip potential along the observed plane of seismic activity

extensional tectonics at the geysers geothermal area california comparisons between the yearly volume of steam withdrawn from the geysers california geothermal reservoir and the number of earthquakes indicate that most of the seismicity is induced to establish the inducing mechanism an estimate of the stress field orientation was calculated from inversion of 210 fault plane solutions below 1 km depth the geysers is undergoing uniaxial extension with the least principal stress 3 oriented near horizontal at approximately 105 azimuth the consistency between the local and regional stress fields indicates that the regional tectonic stress is much larger than the stresses induced locally through geothermal activities the stress data are equivocal as to whether earthquakes are induced due to conversion of aseismic to seismic slip or by increased shear stresses arising from reservoir contraction the observed tendency for downward growth of seismicity may arise from a preferential fracture orientation that is near vertical in conjunction with a stress state where the gradient of 1 exceeds that of 3 evidence of extension at the geysers and in the surrounding clear lake region indicates a direct relationship to the presence of the quaternary clear lake volcanics

the rupture mechanism of the coyote lake earthquake of 6 august 1979 inferred from near field data the coyote lake earthquake ml 5 7 occurred within the dense network of seismic instruments of central california and was recorded by numerous accelerograph stations two of these stations located within the fault zone itself provide particularly remarkable records of the rupture process we use these data obtained in the immediate vicinity of the fault and the broadband seismograms recorded at berkeley about 100 km away from the epicenter to infer the velocity of propagation of the rupture the extent of the fractured area the rupture front geometry and other characteristics of the fracture mechanism in order to extract this information from the data we model the earthquake as a propagating dislocation starting at the hypocenter and spreading radially we compute the resulting ground motion at the receiver sites using the discrete wavenumber representation of green s functions bouchon 1981 the results yield a rupture velocity of 2 6 km sec a fault length of about 14 km and a fault slip of 15 to 20 cm they also show that the fault extends to shallower depth than indicated by the aftershocks the most important result is the finding that slip at any given location on the fault takes place in a very short time span and that simple uniform dislocation models give a good description of the rupture process

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

source parameters 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

duration magnitude scale and site residuals for northern morocco the first empirical duration magnitude m d formula is developed and tested for the northern morocco seismic network nmsnet this relationship is obtained by relating the ign instituto geografico national madrid body waves mblg ign to the duration and the epicentral distance at 25 analogue stations of the nmsnet for 479 earthquakes with 2 5 mb 5 4 from march 1992 to february 2001 m d estimates are significantly more precise while introducing a correction term for each of these stations csta j the magnitude for the i th event m d i is the mean value of individual m d ij 0 14 1 63 log 10 tau ij 0 031 triangle ij csta j

a method for calibration of the local magnitude scale based on relative spectral amplitudes and application to the san juan bautista california area we develop and use a spectral empirical green s function approach to estimate the relative source amplitudes of earthquakes near san juan bautista california we isolate the source amplitudes from path effects by comparing the recorded spectra of pairs of events with similar location and focal mechanism without computing the path effect with this method we estimate the relative moments of 1600 m 1 5 4 local earthquakes and we use these moments to recalibrate the duration magnitude scale in this region the estimated moments of these small earthquakes increase with catalog magnitude m d roughly proportionally to 101 1 m d slightly more slowly than a moment magnitude scaling of 101 5 m w this more accurate magnitude scaling can be used in analyses of the local earthquakes such as comparisons between the seismic moments and geodetic observations electronic supplement 1 additional description and figures showing estimated source amplitudes obtained with different parameters and observed amplitudes of some earthquakes with large misfits 1 http www bssaonline org lookup suppl doi 10 1785 0120160141 dc1

source parameters and scaling relationships of earthquakes in israel abstract determination of earthquake dynamic source parameters such as the seismic moment stress drop and rupture area are often based purely on theoretical considerations in reality however some of the corrected coefficients utilized in the computations should preferably be defined empirically in the present study 39 earthquakes of ml between 3 0 and 6 7 are used to define the correction coefficients associated with distance geometric and unelastic attenuation and ratios between zero frequency spectral amplitides of s waves recorded by vertical and horizontal seismographs these are used in turn to determine the seismic moment m0 and the empirical correlation between log m0 and ml magnitudes for that region the validity of using constant stress drop assumption and correlation between moment and corner frequency is also discussed

the afar depression interpretation of the 1960 2000 earthquakes summary we studied the seismic activity of the afar depression ad and adjacent regions during the period 1960 2000 we define seven distinct seismogenic regions using geological tectonic and seismological data based on the frequency magnitude relationships we obtain b values of about 1 for the different regions the pattern of the distribution of the location of epicentres fits with the known active fault zone in the ad and the axial volcanic ridges the bab el mandab area and the danakil aysha a blocks are less active for 125 intermediate to strong earthquakes the seismic moment and source parameters were calculated the results of the fault plane solutions for the afar depression indicate mainly strike slip and normal sense of movement originating from fault planes striking nw se these results indicate a clockwise block rotation described previously as a bookshelf model in central ad there are a few rightlateral faults east of massawa with e w striking fault planes at the southern red sea north of the danakil block the mixed focal mechanisms with axial plane striking nw se comprise several reverse faulting strike slip motion and normal faulting right lateral movement was also calculated for a cluster of seismic events between the manda hararo and alyata volcanic ridges along nw se striking faults along the n s striking faults in the escarpment at the western afar margins there are two distinct clusters of epicentres the strong earthquakes at the southern cluster exhibit normal or strike slip motions the intermediate to small earthquakes in the northern cluster exhibit reverse and strike slip motions mainly normal faults were calculated along ne sw striking faults of the ethiopian east african rift estimates of the seismic efficiency suggest that the maximal values are about 50 per cent or less implying that most of the motion is taken aseismically

uranium bearing opals products of u mobilization diffusion and transformation processes understanding the retention mechanism of u by amorphous silica i e opal in the environment is of great importance to nuclear waste disposal because opals can retain u for millions of years uraniferous opals from spor mountain and the thomas range utah u s a are examined in terms of their mineralogical chemical and isotopic compositions uranium rich zones composed of most likely vorlanite cauo 4 occur in fibrous opal ct termed lussatite along the interface of the phase with microcrystalline quartz red to black colored precipitates of vorlanite also occur in the interstices between fibers and grains in lussatite and massy opals respectively the high abundance of vorlanite in certain growth zones can be explained by the diffusion of ca and u along boundaries of layers grains and fibers and by the release of ca and u through the transformation of opal a into opal ct and from opal ct into microcrystalline quartz similar o isotope compositions of opal ct and associated microcrystalline quartz indicate that crystallization processes and deposition of subsequent layers of opal occurred from fluids of similar origin and t differences in the isotope and chemical composition between uraniferous opals microcrystalline quartz the sio 2 polymorph moganite and pyrolusite indicate the past occurrence of various alteration processes involving fluids of different composition and t the results of this study indicate that assemblages of opal and microcrystalline quartz can retain u for millions of years as long as confined pore spaces between different generations of opal and quartz or between growth features of opal provide space for the nucleation and adsorption of u bearing phases and species

driving forces for the weathering and alteration of silica in the regolith implications for studies of prehistoric flint tools a range of geochemical and mineralogical factors trigger the weathering and influence the stability of various silica phases under near surface temperatures and pressures based on the current knowledge of the crystallochemical basis of silica behaviour in near surface environments the mineralogical and petrographical transformations of silica under weathering conditions in the regolith are reviewed using data from wide ranging and detailed investigations of siliceous materials some of the investigations are of siliceous materials that evolved in near surface environments over geological time and thus the weathering and alteration features are relatively clear and well expressed examples include weathering of tightly cemented sandstones the formation of silica duricrusts pedogenetic silcretes and the weathering of flints the various micromorphological and mineralogical forms of silica as determined by high resolution petrographic and electron optical techniques can be related to geochemical and hydrological conditions in particular microenvironments however new studies have demonstrated that silica alteration and transformation mechanisms can also occur within prehistoric and historic time intervals these are also described but are more difficult to observe principally because of the microcrystalline character of the alteration phases and the infra millimetric scale on which they develop of many potential applications of knowledge about the near surface alteration of silica in regolith materials and the environmental conditions under which it occurs our studies can be used as a basis determining the provenance and post discard history of prehistoric flint tools and artefacts in part to determine the source of the primary raw material we suggest that the mineralogical and petrographical characteristics of the weathering rind cortex of the flint specifically the recrystallization dissolution and deposition of the silica phases and the spatial arrangement of surfaces that have undergone successive transformations are related to specific and identifiable weathering environments

silica phase transformations during diagenesis within petrified woods found in fluvial deposits from thailand myanmar abstract transitions in petrified wood samples from several thailand myanmar sites included opal a to opal ct opal a to metastable crystobalite and reordered opal ct quartz the moganite in the samples indicates a potential volcanic silica source for the wood petrifactions silica phase transitions evident under sem include opal a opal ct opal a cuboid quartz pseudocubic and opal ct quartz certain stages of silica transformation and transition processes seem to overlap such overlapping results when lepisphere blades transform to the high crystallinity of prismatic quartz through a solid state mechanism the results provide some insights into the petrified wood genesis in the region thermal inputs into the silicification process may have pre dated final burial of woods in flood deposits

analysis of earthquake body wave spectra for potency and magnitude values implications for magnitude scaling relations we develop a simple methodology for reliable automated estimation of the low frequency asymptote in seismic body wave spectra of small to moderate local earthquakes the procedure corrects individual p and s wave spectra for propagation and site effects and estimates the seismic potency from a stacked spectrum the method is applied to 11 000 earthquakes with local magnitudes 0 m l 4 that occurred in the southern california plate boundary region around the san jacinto fault zone during 2013 moment magnitude m w values derived from the spectra and the scaling relation of hanks kanamori follow a gutenberg richter distribution with a larger b value 1 22 from that associated with the m l values 0 93 for the same earthquakes the completeness magnitude for the m w values is 1 6 while for m l it is 1 0 the quantity m w m l linearly increases in the analysed magnitude range as m l decreases an average earthquake with m l 0 in the study area has an m w of about 0 9 the developed methodology and results have important implications for earthquake source studies and statistical seismology

seismic strain and seismogenic stress regimes in the crust of the southern tyrrhenian region abstract an investigation has been performed to identify and characterize the seismic deformation zones active over the last decades in the region of italy that has experienced the strongest seismicity during the last centuries the study is based on the estimate of hypocenter locations fault plane solutions seismogenic stress and seismic strain tensor orientations carried out using the entire dataset of the national and local seismic networks and the recently improved three dimensional 3d crustal velocity model of the study area a series of simulation tests have been performed to evaluate the significance of the earthquake space distribution obtained and whether it was influenced by network geometry problems related to the sea and the lack of ocean bottom seismometers in the sectors where hypocentral location was synthetically proved to be reliable space distributions of earthquakes located with epicenter and focal depth errors less than 3 and 4 km respectively have been compared with local geology in order to identify seismogenic faults the dataset of 32 fault plane solutions estimated with fault parameter errors less than 20 has been used to investigate space variations of seismogenic stress and seismic strain orientations over the study area stress was found to be uniform in the messina strait and southern calabria where inversion of the available set of 11 fault plane solutions showed clear evidence of an extensional regime the different orientations of the minimum compressive stress and strain found in this sector together with the information available on local geology and tectonics lead us to propose that the seismicity occurring over the last decades in the messina strait and southern calabria was not in general produced at the main faults but at minor faults activated by the main tectonic stress field acting in the area to the west in the sector including western etna the nebrodi chain and the western aeolian islands analysis of the available set of 16 fault plane solutions revealed a certain degree of stress heterogeneity with an apparent prevalence of north south compression this east to west change of stress strain regimes is evaluated in the light of current hypotheses regarding the geodynamics of the study region

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

regional magnitude scaling transportability and ms mb discrimination at small magnitudes data sets of mb pn and mb lg measurements are presented for three continental regions in order to investigate scaling relationships with moment magnitude w and event discrimination at small magnitudes compilations of published measurements are provided for eastern north american and central asian earthquakes and new measurements are reported for earthquakes located in western united states statistical tests on w mb relationships show that the mb lg scale of nurru 1973 is transportable between tectonic regions and a single unified w mb lg relationship satisfies observations for m 6 5 in all regions a unified relationship is also developed for nuclear explosions detonated at the nevada test site and test sites of the former soviet union regional mb for explosions scale at higher rates than for earthquakes and of significance is the finding that mb pn for explosions scales at a higher rate than mb lg a model is proposed where differences in scaling rates are related to effects of spectral overshoot and near field rg scattering on the generation of pn and lg waves by explosions for earthquakes mb pn and mb lg scale similarly showing rates near 1 0 or 2 3 log1omo seismic moment

microearthquake imaging of the parkfield asperity microearthquake data from a downhole seismometer network on the san andreas fault appear to outline two aseismic asperities that may correspond to the locations of the foreshocks and main shocks of the parkfield characteristic earthquakes the source parameters of the microearthquakes show that a few of the earthquakes have significantly higher stress drops than most furthermore the magnitude frequency statistics suggest that at local magnitude 0 6 the cumulative number of small events begins to fall off the usual gutenberg richter b 1 relation in which the number of events increases exponentially with decreasing magnitude the downhole seismometer data establish a baseline from which the evolution of the earthquake process at parkfield can be monitored and suggest that different mechanical conditions than those that lead to the typical gutenberg richter relation may be operating for the smallest of parkfield microearthquakes

mesozooplankton abundance and distribution in association with hydrography on hanna shoal ne chukchi sea during august 2012 and 2013 abstract hanna shoal in the northeastern chukchi sea is potentially vulnerable to ecosystem disruption under ongoing climate change however aspects of its ecology particularly of its zooplankton have been poorly understood mesozooplankton distribution taxonomic composition and abundance were described from across hanna shoal in august 2012 and 2013 as part of the multidisciplinary comida hanna shoal program zooplankton were collected using vertical tows of paired bongo nets equipped with 150 m and 500 m mesh nets samples from the 150 m mesh nets were enumerated to identify and count taxa copepod species and copepod life stages haplotypes of the mitochondrial cytochrome c oxidase i mtcoi gene were used to differentiate bering and arctic haplotype groups of the copepod calanus glacialis and to differentiate c glacialis and its close congener c marshallae the meroplankton particularly bivalve larvae and in 2012 echinoderm larvae were an important component of the zooplankton and were of greater abundance on the eastern portion of the shoal than elsewhere regions identified on the basis of different taxonomic compositions were associated with different water masses and current pathways the northeast corner of the shoal in particular was distinct from the remainder of the shoal and from barrow canyon with both different life stage compositions and unique arctic haplotypes of the mtcoi gene for c glacialis marshallae suggesting populations at those locations originated in the arctic ocean rather than the bering sea bering sea summer water and intrinsic plankton was observed in the southwest portion of the shoal comparisons with historic and recent studies done near hanna shoal demonstrated that similar plankton compositions were present across a broad region of the chukchi sea and that abundances of the copepod c glacialis appear to be increasing on the time scale of decades potentially through increased input from the northern bering sea because the chukchi sea is highly advective it is likely that zooplankton populations over hanna shoal are lost to the canada basin to the north and must be re established annually through input from the northern bering sea under this scenario enhancement of a resident hanna shoal zooplankton community that would retain a significant proportion of the primary production in the water column is unlikely

diagenetic trends in the siliceous facies of the monterey shale in the santa maria region california x ray diffraction and oxygen isotopic analyses of outcrop and subsurface samples of siliceous rocks were used to reconstruct thermal and diagenetic histories of the miocene monterey shale near santa maria california within many stratigraphic sections soft porous diatomaceous rocks change gradationally to underlying hard and brittle chert porcellanite and siliceous shale the accompanying silica mineral zones are in descending stratigraphic order 1 biogenic silica opal a 2 cristobalitic silica opal ct and 3 microcrystalline quartz boundaries between silica mineral zones and stratigraphic horizons are often discordant within the opal ct zone the d 101 spacing of opal ct decreases in a smooth non linear fashion from about 4 10 a to 4 04 a in the santa maria valley and bradley oil field areas the thicknesses of the opal ct zones are greater and the present thermal gradients less than in the adjacent orcutt oil field thin opal ct zones at shallow maximum burial depths apparently correlate with higher thermal gradients using present thermal gradients and reconstructed maximum burial depths from well data in the santa maria region the ranges in temperatures for the top and base of the opal ct zone are 38 54 c and 55 110 c respectively the temperature difference between these two boundaries ranges from 17 to 60 c in comparison temperature ranges for these two boundaries computed from oxygen isotopic compositions of opal ct and quartz extrapolated experimental quartz water fractionations and assuming o18 0 o for the isotopic composition of the equilibrating fluid are 18 56 c and 31 80 c for the top and base of the opal ct zone respectively the temperature difference between these boundaries is 11 36 c using this method thermal gradients and sedimentation rates strongly influence rates of silica transformations reconstructed thermal and diagenetic histories of siliceous rocks of the monterey shale at four well sites in the santa maria region demonstrate that most silica conversions probably occurred during the last 3 4 myr in response to accelerated rates of sedimentation and therefore burial heating during the pliocene

strain and stress fields in the southern apennines italy constrained by geodetic seismological and borehole data summary we present an improved evaluation of the current strain and stress fields in the southern apennines italy obtained through a careful analysis of geodetic seismological and borehole data in particular our analysis provides an updated comparison between the accrued strain recorded by geodetic data and the strain released by seismic activity in a region hit by destructive historical earthquakes to this end we have used nine years of gps observations 2001 2010 from a dense network of permanent stations a data set of 73 well constrained stress indicators borehole breakouts and focal mechanisms of moderate to large earthquakes and published estimations of the geological strain accommodated by active faults in the region although geodetic data are generally consistent with seismic and geological information previously unknown features of the current deformation in southern italy emerge from this analysis the newly obtained gps velocity field supports the well established notion of a dominant ne sw oriented extension concentrated in a 50 km wide belt along the topographic relief of the apennines as outlined by the distribution of seismogenic normal faults geodetic deformation is however non uniform along the belt with two patches of higher strain rate and shear stress accumulation in the north matese mountains and in the south irpinia area low geodetic strain rates are found in the bradano basin and apulia plateau to the east along the ionian sea margin of southern italy in southern apulia and eastern basilicata and calabria geodetic velocities indicate nw se extension that is consistent with active shallow crustal gravitational motion documented by geological studies in the west along the tyrrhenian margin of the campania region the tectonic geodetic field is disturbed by volcanic processes comparison between the magnitude of the geodetic and the seismic strain rates computed using a long historical seismicity catalogue allow detecting areas of high correlation particularly along the axis of the mountain chain indicating that most of the geodetic strain is released by earthquakes this relation does not hold for the instrumental seismic catalogue as a consequence of the limited time span covered by instrumental data in other areas e g murge plateau in central apulia where seismicity is very low or absent the yet appreciable geodetic deformation might be accommodated in aseismic mode overall the excellent match between the stress and the strain rate directions in much of the apennines indicates that both earthquakes and ground deformation patterns are driven by the same crustal forces

seismicity at the volcanoes of katmai national park alaska july 1995 december 1997 abstract upper crustal seismicity located within katmai national park occurs mainly within four distinct clusters originating near martin mageik mm subnet volcanoes trident volcano katmai caldera and snowy volcano analyses of earthquake frequency magnitude distributions reveal high b values beneath mm subnet 1 5 normal b values at trident volcano 1 0 and intermediate b values at katmai caldera 1 3 for all seismicity occurring between july 1995 and december 1997 detailed analyses of subsets of b values and hypocenter locations at mm subnet reveal a temporal increase in b value associated with an increase in the maximum depth of seismicity the changes occurred during a shallow earthquake swarm beneath mageik volcano between october 16 and 25 1996 and again in november december 1997 before the swarm the weighted least squares b value was 1 01 at mm subnet increased to 1 59 during the swarm and remained anomalously high until april 1997 the corresponding maximum depth of seismicity is generally less than 5 km for well located earthquakes occurring after september 18 1996 but shifted to 10 km during both the october 1996 swarm and the november december 1997 period the november december 1997 event is not associated with an increase in the rate of seismicity or the b value the october 1996 swarm had a cumulative moment release of 5 0 10 20 dyn cm and decayed from a peak rate of 110 events per day with a modified omori law p value of 1 06 0 11 modelling by the flow law with a 10 km depth limit for seismicity suggests that temperature gradients are on the order of 20 40 c km 1 in agreement with the p value estimate we infer that a simple pressurizing intrusion was not associated with the october swarm because higher stresses should increase seismicity and lower the b value the opposite of what we observed alternatively an actively degassing intrusion would reduce the effective stress and increase the b value at shallow depths while the increased stress would induce seismicity at depth surface temperature measurements taken at the mageik crater lake in july 1995 and again in july 1997 revealed an increase of about 10 c in the lake water temperature consistent with a degassing intrusive event

stress strain changes and triggered seismicity at the geysers california the principal results of this study of remotely triggered seismicity in the geysers geothermal field are the demonstration that triggering initiation of earthquake failure depends on a critical strain threshold and that the threshold level increases with decreasing frequency or equivalently depends on strain rate this threshold function derives from 1 analyses of dynamic strains associated with surface waves of the triggering earthquakes 2 statistically measured aftershock zone dimensions and 3 analytic functional representations of strains associated with power production and tides the threshold is also consistent with triggering by static strain changes and implies that both static and dynamic strains may cause aftershocks the observation that triggered seismicity probably occurs in addition to background activity also provides an important constraint on the triggering process assuming the physical processes underlying earthquake nucleation to be the same gomberg this issue discusses seismicity triggered by the mw 7 3 landers earthquake its constraints on the variability of triggering thresholds with site and the implications of time delays between triggering and triggered earthquakes our results enable us to reject the hypothesis that dynamic strains simply nudge prestressed faults over a coulomb failure threshold sooner than they would have otherwise we interpret the rate dependent triggering threshold as evidence of several competing processes with different time constants the faster one s facilitating failure and the other s inhibiting it such competition is a common feature of theories of slip instability all these results not surprisingly imply that to understand earthquake triggering one must consider not only simple failure criteria requiring exceedence of some constant threshold but also the requirements for generating instabilities

garner valley downhole array of accelerometers instrumentation and preliminary data analysis the garner valley downhole array gvda in southern california 33 41 60 n 116 40 20 w is a five element array of three component dual gain force balance accelerometers that are capable of measuring accelerations from 3 10 6 to 2 0 g over a frequency range from 0 0 to 100 hz the accelerometers are placed at depths of 0 6 15 22 and 220 m the lithostratigraphy is 19 m of soil overlying a 24 m layer of weathered granite below which is granite tonalite the array only 7 km from the san jacinto fault is located at the northern end of the anza seismic gap on the san jacinto fault where an m 6 5 or greater earthquake can be expected it is only 35 km from the indio segment of the san andreas fault which last ruptured with an m 8 earthquake around 1700 a d from its installation in july 1989 through july 1991 gvda has recorded 280 earthquakes with magnitudes ranging from 1 2 to 4 7 and epicentral distances ranging from from 0 to 110 km the maximum acceleration recorded was 89 cm sec 2 from an m l 4 2 earthquake 15 8 km hypocentral distance from gvda the average amplification of the apparent seismic moment between 220 m depth and the surface is about 13 for over five orders of magnitude in the size of the recorded earthquakes the spectral ratio 0 220 of horizontal acceleration amplitude spectrum shows a mean amplification of about 10 for the frequency range of 2 0 to 30 hz for 17 events resonance peaks exist at about 1 7 3 0 and 12 0 hz where the spectral ratio 0 220 is nearly 40 analysis of the acceleration spectra of two specific earthquakes that have nearly the same hypocenter but with different magnitudes m 4 2 and 2 5 shows that the weathered granite zone beneath the soil strongly attenuates the spectral amplitudes for frequencies greater than 40 hz the impedance of the soil relative to the weathered granite amplifies all frequencies by about a factor of 3 leading to spectral levels that are greater at the surface than at 22 m depth near the top of the weathered granite the acceleration spectra at each depth show resonant peaks at the same frequencies the spectra at each depth are well correlated for both earthquakes the acceleration spectrum at 220 m is nearly constant for frequencies less than 60 hz which implies that there is little or no attenuation

the dependence of peak horizontal acceleration on magnitude distance and site effects for small magnitude earthquakes in california and eastern north america abstract one hundred and ninety free field accelerograms recorded on deep soil 10 m deep were used to study the near source scaling characteristics of peak horizontal acceleration for 91 earthquakes 2 5 m l 5 0 located primarily in california of the six attenuation relationships developed in this study the one considered most reliable is given by the expression ln p h a 2 5 0 1 0 6 2 3 m l 1 0 ln r 7 2 8 where pha is the mean of the two horizontal components of peak acceleration in g m l is local magnitude r is epicentral distance in kilometers and is a random error term with mean of zero and standard deviation of 0 506 an analysis of residuals based on an additional 171 near source accelerograms from 75 earthquakes indicated that accelerograms recorded in building basements sited on deep soil have 30 per cent lower accelerations and that free field accelerograms recorded on shallow soil 10 m deep have 82 per cent higher accelerations than free field accelerograms recorded on deep soil accelerations recorded on rock were found to be similar in amplitude to those recorded on deep soil a separate analysis of site effects for the imperial valley and oroville regions of california indicated that peak accelerations from individual sites can depart significantly from predictions based on broad site classifications an analysis of residuals based on 27 selected strong motion recordings from 19 earthquakes in eastern north america indicated that near source accelerations associated with frequencies less than about 25 hz are consistent with predictions based on attenuation relationships derived for california once site effects and differences in regional magnitude scales are taken into account

slip partitioning of the calaveras fault california and prospects for future earthquakes examination of main shock and microearthquake data from the calaveras fault during the last 20 years reveals that main shock hypocenters occur at depths of 8 9 km near the base of the zone of microearthquakes the spatial pattern of pre main shock microseismicity surrounding the coyote lake and morgan hill hypocenters is similar to the pattern of the post main shock microseismicity microseismicity extends between depths of 4 and 10 km and defines zones of concentrated microseismicity and aseismic zones estimates of the fault regions which slipped during the coyote lake and morgan hill earthquakes as derived from seismic radiation coincide with zones which are otherwise aseismic we propose that these persistent aseismic zones represent stuck patches which slip only during moderate earthquakes from the pattern of microearthquake locations we recognize six aseismic zones where we expect future main shocks will rupture the calaveras fault from an analysis of historic seismic data we establish the main shock rupture history for each aseismic zone and identify two zones that are the most likely sites for the next m 5 earthquakes the first zone is located near gilroy and was last ruptured by a m5 2 earthquake in 1949 the second zone is located south of calaveras reservoir and north of the 1988 m5 1 alum rock earthquake it has not slipped seismically since at least 1903 and the size of the aseismic region is sufficiently large to sustain a m5 5 earthquake

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

inferring the coseismic and postseismic stress changes caused by the 2004 mw 6 parkfield earthquake from variations of recurrence times of microearthquakes 1 kinematic models of coseismic stress inverted from ground motion data do not usually find good correlation between the location of aftershocks and high stress patches in particular numerous earthquakes are recorded in areas of the fault where the stress decreases however most of coseismic slip distributions have limited spatial resolution typically not better than 1 km here we investigate the stress changes produced by the 2004 mw 6 parkfield earthquake on and near its rupture zone at the scale of magnitude 2 earthquake asperities approximately tens of meters after relocating earthquakes in this zone between 1984 and 2007 we form repeating highly similar earthquake sequences and study how the quasiperiodicity of occurrence at each sequence observed during the 20 years preceding the 2004 main shock is perturbed by this event we apply a simple model of the seismic cycle to infer the coseismic and postseismic stresses experienced by the repeatedly failing asperities despite being spatially sparse these stress distributions have resolutions only limited by the typical scale of an asperity we propose that the high spatial variability of the seismicity patterns following the mw 6 earthquake results from an heterogeneous coseismic stress field the emergence of the omori utsu law observed at large scale greater than kilometers at parkfield is simply the outcome of averaging such quasi deterministic patterns over many sequences the fact that the coseismic stress can significantly change over distances of the order of 100 m adds credence to the hypothesis that earthquake rupture is intrinsically very heterogeneous

source duration scales with magnitude differently for earthquakes on the san andreas fault and on secondary faults in parkfield california we used a comparison of source time function pulse widths to show that a group of earthquakes on the san andreas fault near parkfield have a constant duration over a magnitude range of 1 4 3 7 earthquakes on secondary faults have an increase in duration with magnitude which is the expected relationship for the usual observation of constant stress drop the constant duration suggests that fault area is the same regardless of magnitude and that variations in stress drop are due entirely to variations in slip calculated stress drop values on secondary faults range from 0 31 to 14 mpa and stress drop values on the san andreas fault range from 0 18 to 63 mpa the observation of constant duration on the san andreas fault is consistent with a model of a locked asperity in a creeping fault the differences in durations between the events on the san andreas fault and on secondary faults suggest that earthquakes on the san andreas fault are inherently different we speculate that faults with more cumulative displacement have earthquakes that may rupture differently furthermore the differences in source properties between the two populations might be explained by differences in fault surface roughness online material station subsets used to evaluate cross fault seismic velocity contrasts

seismic activity along fault branches of the dead sea jordan transform system the carmel tirtza fault system abstract about 550 earthquakes 1 0 m l 5 3 along the carmel tirtza fault a branching fault of the jordandashdead sea transform system recorded by the israel seismograph network isn between 1984 and 1994 were analyzed the seismicity pattern reveals significant activity often confined to known surface traces of active faults or within local grabens earthquake clusters are also observed the seismicity pattern within the mediterranean seems more diffuse possibly due to observational inaccuracies and insufficient knowledge of how the carmel tirtza fault system extends towards the northwest the seismicity along the carmel tirtza fault system since the beginning of this century is characterized by a b value of about 0 9 for 15 events with 3 1 m l 5 3 we found seismic moment estimates m 0 of 0 7 10 14 m 0 10 17 nm and brune stress drop estimates between 0 9 and 16 4 mpa these characteristics are comparable to those for events occurring on the main dead sea jordan transform fault ten individual earthquake focal mechanisms and three joint focal mechanisms for earthquake clusters have been obtained towards the northeastern part of the carmel tirtza fault system a left lateral motion seems to dominate while the seismicity pattern and the focal mechanism solutions near the branching of the carmel tirtza fault and the dead sea jordan rift suggest the present day existence of a complicated tectonic regime stress inversion assuming one stress tensor for the whole region indicates nne ssw extension and wnw ese compression

source character of microseismicity in the san francisco bay block california and implications for seismic hazard abstract we examine relocated seismicity within a 30 km wide crustal block containing san francisco bay and bounded by two major right lateral strike slip fault systems the hayward and san andreas faults to determine seismicity distribution source character and possible relationship to proposed faults well located low level seismicity m d 3 0 has occurred persistently within this block throughout the recording interval 1969 to 1995 with the highest levels of activity occurring along or directly adjacent to within 5 km the bounding faults and falling off toward the long axis of the bay the total seismic moment release within the interior of the bay block since 1969 is equivalent to one m l 3 8 earthquake one to two orders of magnitude lower than activity along and within 5 km of the bounding faults focal depths of reliably located events within the bay block are generally less than 13 km with most seismicity in the depth range of 7 to 12 km similar to focal depths along both the adjacent portions of the san andreas and hayward faults focal mechanisms for m d 2 to 3 events within the bay block mimic focal mechanisms along the adjacent san andreas fault zone and in the east bay suggesting that bay block is responding to a similar regional stress field two potential seismic source zones have been suggested within the bay block our hypocentral depths and focal mechanisms suggest that a proposed subhorizontal detachment fault 15 to 18 km beneath the bay is not seismically active several large scale linear nw trending aeromagnetic anomalies within the bay block were previously suggested to represent large through going subvertical fault zones the two largest earthquakes both m d 3 0 in the bay block since 1969 occur near two of these large scale linear aeromagnetic anomalies both have subvertical nodal planes with right lateral slip subparallel to the magnetic anomalies suggesting that structures related to the anomalies may be capable of brittle failure geodetic focal mechanism and seismicity data all suggest the bay block is responding elastically to the same regional stresses affecting the bounding faults however continuous holocene reflectors across the proposed fault zones suggest that if the magnetic anomalies represent basement fault zones then these faults must have recurrence times one to several orders of magnitude longer than on the bounding faults

insights on earthquake triggering processes from early aftershocks of repeating microearthquakes characterizing the evolution of seismicity rate of early aftershocks can yield important information about earthquake nucleation and triggering however this task is challenging because early aftershock seismic signals are obscured by those of the mainshock previous studies of early aftershocks employed high pass filtering and template matching but had limited performance and completeness at very short times here we take advantage of repeating events previously identified on the san andreas fault at parkfield and apply empirical green s function deconvolution techniques both landweber and sparse deconvolution methods reveal the occurrence of aftershocks as early as few tenths of a second after the mainshock these events occur close to their mainshock within one to two rupture lengths away the aftershock rate derived from this enhanced catalog is consistent with omori s law with no flattening of the aftershock rate down to the shortest resolvable timescale 0 3 s the early aftershock rate decay determined here matches seamlessly the decay at later times derived from the original earthquake catalog yielding a continuous aftershock decay over timescales spanning nearly 8 orders of magnitude aftershocks of repeating microearthquakes may hence be governed by the same mechanisms from the earliest time resolved here up to the end of the aftershock sequence our results suggest that these early aftershocks are triggered by relatively large stress perturbations possibly induced by aseismic afterslip with very short characteristic time consistent with previous observations on bimaterial faults the relative location of early aftershocks shows asymmetry along strike persistent over long periods

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