stress and earthquakes in southern california 1850 2004 1 we compute the stress tensor in the upper crust of southern california as a function of time and compare observed seismicity with the estimated stress at the time of each earthquake several recent developments make it possible to do this much more realistically than before 1 a wealth of new geodetic and geologic data for southern california and 2 a catalog of moment tensors for all earthquakes with magnitudes larger than 6 since 1850 and larger than 5 since 1910 we model crustal deformation using both updated geodetic data and geologically determined fault slip rates we subdivide the crust into elastic blocks delineated by faults which move freely at a constant rate below a locking depth with a rate determined by the relative block motion we compute normal and shear stresses on nodal planes for each earthquake in the catalog we consider stress increments from previous earthquakes seismic stress and aseismic tectonic stress both separately and in combination the locations and mechanisms of earthquakes are best correlated with the aseismic shear stress including the cumulative coseismic effects from past earthquakes does not significantly improve the correlation correlations between normal stress and earthquakes are always very sensitive to the start date of the catalog whether we exclude earthquakes very close to others and whether we evaluate stress at the hypocenter or throughout the rupture surface of an earthquake although the correlation of tectonic stress with earthquake triggering is robust other results are unstable apparently because the catalog has so few earthquakes

irregular recurrence of paleoearthquakes along the central garlock fault near el paso peaks california 1 new investigations at the el paso peaks paleoseismic site have refined and extended the record of paleoearthquakes for the central garlock fault event evidence in the form of buried fissures scarps folds and upward terminations has allowed us to identify six well resolved earthquakes designated as events w u r q k and f within the last 7000 years the exposures that we excavated did not reveal additional evidence for three poorly resolved events reported by mcgill and rockwell 1998 evidence for event y identified in the previous study was reexamined and found not to be a paleoearthquake radiocarbon dates of detrital charcoal combined with a method of interpolating the event ages with revised sedimentation rates provide our preferred constraints on the timing of faulting events the most recent surface rupturing earthquake event w occurred between a d 1450 and 1640 the penultimate event u occurred between a d 675 and 950 event r which was unrecognized in the previous study occurred between a d 250 and 475 event q occurred relatively shortly before event r between a d 25 and 275 event k occurred between 3340 and 2930 b c the oldest identified paleoearthquake event f occurred between 5300 and 4670 b c the event ages indicate that earthquake recurrence is highly irregular at the el paso peaks site with individual preferred intervals ranging from as little as 215 years to as many as 3300 years the irregular recurrence of surface rupturing earthquakes along the central garlock fault may be related to temporal and spatial clustering of earthquakes proposed for faults in the eastern california shear zone

rank ordering statistics of extreme events application to the distribution of large earthquakes rank ordering statistics provide a perspective on the rare largest elements of a population whereas the statistics of cumulative distributions are dominated by the more numerous small events the exponent of a power law distribution can be determined with good accuracy by rank ordering statistics from the observation of only a few tens of the largest events using analytical results and synthetic tests we quantify the systematic and the random errors we also study the case of a distribution defined by two branches each having a power law distribution one defined for the largest events and the other for smaller events with application to the worldwide harvard and southern california earthquake catalogs in the case of the harvard moment catalog we make more precise earlier claims of the existence of a transition of the earthquake magnitude distribution between small and large earthquakes the b values are b2 2 3 0 3 for large shallow earthquakes and b1 1 00 0 02 for smaller shallow earthquakes however the crossover magnitude between the two distributions is ill defined the data available at present do not provide a strong constraint on the crossover which has a 50 probability of being between magnitudes 7 1 and 7 6 for shallow earthquakes this interval may be too conservatively estimated thus any influence of a universal geometry of rupture on the distribution of earthquakes worldwide is ill defined at best we caution that there is no direct evidence to confirm the hypothesis that the large moment branch is indeed a power law in fact a gamma distribution fits the entire suite of earthquake moments from the smallest to the largest satisfactorily there is no evidence that the earthquakes of the southern california catalog have a distribution with two branches or that a rolloff in the distribution is needed for this catalog b 1 00 0 02 up to the largest magnitude observed mw 7 5 hence we conclude that the thickness of the seismogenic layer has no observable influence whatsoever on the frequency distribution in this region

rock albedo and monitoring of thermal conditions in respect of weathering some expected and some unexpected results broadly speaking there is at least within geomorphic circles a general acceptance that rocks with low albedos will warm both faster and to higher temperatures than rocks with high albedos reflectivity influencing radiative warming upon this foundation are built notions of weathering in respect of the resulting thermal differences both at the grain scale and at the scale of rock masses here a series of paving bricks painted in 20 per cent reflectivity intervals from black through to white were used to monitor albedo influenced temperatures at a site in northern canada in an attempt to test this premise temperatures were collected for five months for the rock surface and the base of the rock the blocks being set within a mass of local sediment resulting thermal data did indeed show that the dark bricks were warmer than the white but only when their temperatures were equal to or cooler than the air temperature as brick temperature exceeded that of the air so the dark and light bricks moved to parity indeed the white bricks frequently became warmer than the dark it is argued that this negating of the albedo influence on heating is a result of the necessity of the bricks both white and black to convect heat away to the surrounding cooler air the darker brick being hotter initially convects faster than the white as a product of the temperature difference between the two media thus where the bricks become significantly hotter than the air they lose energy to that air and so their respective temperatures become closer the albedo influence being superceded by the requirement to equilibrate with the surrounding air it is argued that this finding will have importance to our understanding of weathering in general and to our perceptions of weathering differences between different lithologies copyright 2005 john wiley sons ltd

seismic gaps and earthquakes 1 mccann et al 1979 published a widely cited seismic gap model ascribing earthquake potential categories to 125 zones surrounding the pacific rim nishenko 1991 published an updated and revised version including probability estimates of characteristic earthquakes with specified magnitudes within each zone these forecasts are now more than 20 and 10 years old respectively and sufficient data now exist to test them rather conclusively for the mccann et al forecast we count the number of qualifying earthquakes in the several categories of zones we assume a hypothetical probability consistent with the gap model e g red zones have twice the probability of green zones and test against the null hypothesis that all zones have equal probability the gap hypothesis can be rejected at a high confidence level contrary to the forecast of mccann et al the data suggest that the real seismic potential is lower in the gaps than in other segments and plate boundary zones are not made safer by recent earthquakes for the 1991 nishenko hypothesis we test the number of filled zones the likelihood scores of the observed and simulated catalogs and the likelihood ratio of the gap hypothesis to a poissonian null hypothesis for earthquakes equal to or larger than the characteristic magnitude the new seismic gap hypothesis failed at the 95 confidence level in both the number and ratio tests if we lower the magnitude threshold by 0 5 for qualifying earthquakes the new gap hypothesis passes the number test but fails in both the likelihood and likelihood ratio tests at the 95 confidence level

global seismicity of 2001 centroid moment tensor solutions for 961 earthquakes abstract centroid moment tensor cmt solutions are presented for 961 earthquakes occurring during 2001 the solutions are obtained using corrections for aspherical earth structure represented by the whole mantle shear velocity model sh8 u4l8 of dziewonski and woodward a m dziewonski r l woodward acoustic imaging at the planetary scale in h emert h p harjes eds acoustical imaging vol 19 plenum press new york 1992 pp 785 797 the model of anelastic attenuation of durek and ekstrom bull seism soc am 86 1996 144 is used to predict the decay of the waveforms

velocity field in asia inferred from quaternary fault slip rates and global positioning system observations we perform a joint inversion of quaternary strain rates and 238 global positioning system gps velocities in asia for a self consistent velocity field the reference frames for all geodetic velocity observations are determined in our inversion procedure india in moves relative to eurasia eu about a pole of rotation at 29 78 n 7 51 e 0 353 myr 1 which yields a velocity along the himalaya within india that is 73 76 of the magnitude of the in eu nuvel 1a velocity and a vector azimuth that is 8 10 clockwise of nuvel 1a in eu vector azimuth relative to eurasia south china moves at 9 11 mm yr in the direction 110 120 with a pole position 64 84 n 156 74 e 0 12 myr 1 amurian block motion has a pole position in a similar location but at a slower rate 64 61 n 158 23 e 0 077 myr 1 and most of the amurian eurasia motion is accommodated by extension across lake baikal tarim basin moves relative to eurasia about a pole of rotation at 39 24 n 98 2 e 0 539 myr 1 and 16 18 mm yr of shortening is accommodated across the west central tien shan there is distributed e w extension throughout both southern and north central tibet within southern tibet between the longitudes of 77 e to 92 e the deformation field accommodates 16 19 mm yr of e w extension we compare predicted seismic moment rates with those observed in this century in asia total observed seismic moment rates within the entire area of central and east asia 2 2 107 km2 in this century are 2 26 0 7 1020 n m yr 1 as compared with a predicted total rate of 2 03 0 066 1020 n m yr 1 comparisons between observed and predicted moment rates within 42 subregions reflect the generally unstable process of inferring long term seismic moment rates from a catalog of limited duration 94 years an observation period of 10 000 years would be required to reduce uncertainties in observed seismic moment rate to the same size as the uncertainties in model tectonic moment rates inferred from the joint inversion of gps and quaternary rates of strain we show that in general a better correlation with model tectonic moment rate is inferred from the seismicity catalog by considering the numbers of earthquakes above a cutoff magnitude mb 5 0 for the period january 1 1965 to january 1 1999

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

a spatial random field model to characterize complexity in earthquake slip 1 finite fault source inversions reveal the spatial complexity of earthquake slip over the fault plane we develop a stochastic characterization of earthquake slip complexity based on published finite source rupture models in which we model the distribution of slip as a spatial random field the model most consistent with the data follows a von karman autocorrelation function acf for which the correlation lengths a increase with source dimension for earthquakes with large fault aspect ratios we observe substantial differences of the correlation length in the along strike a x and downdip a z directions increasing correlation length with increasing magnitude can be understood using concepts of dynamic rupture propagation the power spectrum of the slip distribution can also be well described with a power law decay i e a fractal distribution in which the fractal dimension d remains scale invariant with a median value d 2 29 0 23 while the comer wave number k c which is inversely proportional to source size decreases with earthquake magnitude accounting for larger slip patches for large magnitude events our stochastic slip model can be used to generate realizations of scenario earthquakes for near source ground motion simulations

fractal dimension and b value on creeping and locked patches of the san andreas fault near parkfield california we tested the hypotheses 1 that the fractal dimension d of hypocenters are different in a locked and a creeping segment of the san andreas fault and 2 that the relationship d 2b holds approximately where b is the slope of the frequency magnitude relationship the test area was the 30 to 50 km fault segment north of park eld for which two earthquake catalogs exist the borehole high resolution seismic network data and the u s geological survey data which have a minimum magnitude of completeness of mc 0 4 and mc 1 0 1 2 respectively the relative location errors in the two catalogs are estimated as 0 25 km and less than 1 km respectively the periods of high quality data available extend from 1987 to 1998 5 and 1981 to 2000 2 respectively furnishing 2609 and 3775 events for analysis in the two catalogs in the locked part 0 5 b 0 7 and 0 96 d 1 14 whereas in the creeping segment 1 1 b 1 6 and 1 45 d 1 72 however the spatial distribution of the hypocenters in the creeping segment is not well approximated by a fractal distribution we conclude 1 that the frequency magnitude distribution as described by b as well as the fractal dimension d are different in the locked and creeping segments near park eld 2 that the spatial distribution in the creeping segment is not well approximated by a fractal distribution and 3 that the relationship d 2b holds in the locked segment where both parameters can be measured accurately thus we propose that the heterogeneity of seismogenic volumes lead to differences in d and b and that these differences where established by high quality data may furnish clues concerning properties of fault zones

quartz ribbons in high grade granite gneiss modifications of dynamically formed quartz c axis preferred orientation by oriented grain growth quartz ribbons form a well defined ls fabric in granitic gneisses sheared and metamorphosed in the upper amphibolite facies boundaries of quartz grains are smooth and sub perpendicular to ribbon walls suggesting post deformational growth whereas c axes display asymmetric girdle patterns consistent with the sense of shear indicated by mesoscopic fabrics subdivision of the microfabrics according to the proportion of ribbon length occupied by individual grains indicates that the c axes of relatively short grains exhibit across girdle pattern similar to that of the whole sample longer grains reproduce elements of the pattern of the short grains suggesting that oriented grain growth occurred grain boundary mobility which clearly ended in a post deformational static regime probably began during dynamic recrystallization owing to space limitations imposed by the feldspathic matrix grain growth in the ribbons ceased before the initial dynamic fabric was erased in summary this study shows that the c axes pattern of longer grains within quartz ribbons reproduces part of the dynamically formed pattern of the shorter grains reflecting an oriented grain growth which concluded in a static episode although possibly initiated in the dynamic regime

earthquake source scaling relationships from 1 to 5 ml using seismograms recorded at 2 5 km depth the scaling relationships of earthquake sources less than about magnitude 3 have been the subject of considerable controversy over the last two decades studies of such events have shown a tendency for the constant stress drop self similarity of larger earthquakes to breakdown at small magnitudes and an apparent minimum source dimension of about 100 m has been observed other studies showed that this apparent breakdown in scaling could be an artifact of severe near surface attenuation limiting the spatial resolution of surface data in this study source parameters are determined for over 100 nearby tectonic earthquakes from recordings at a depth of 2 5 km in granite in the cajon pass scientific drill hole southern california comparison of the seismograms recorded at this depth with those at the wellhead clearly demonstrates the effect of the severe attenuation in the upper kilometers of the earth s crust source parameters are calculated by spectral modeling of three component p and s waves assuming four source models based on the brune 2 n 2 model in model l n 2 is fixed and q of p and s waves is determined to be 912 581 1433 and 1078 879 1323 respectively the numbers in parentheses are 1 standard deviation in model 2 qp qs 1000 is assumed and n is allowed to vary the 2 model is a good average for the data set but there is some real scatter supported by the data in model 3 qp qs 1000 is also assumed and 2 is constrained and in model 4 attenuation is ignored and n is allowed to vary source dimensions of less than 10m are observed for all four models 10 times smaller than the proposed minimum no breakdown in constant stress drop scaling is seen in the downhole data approximately ml 1 to 5 5 m0 109 1016 nm the ratio between radiated seismic energy estimated by integrating the velocity squared spectra with adequate signal bandwidth and seismic moment appears to decrease gradually with decreasing moment in the magnitude range 1 to 7 this is not incompatible with a constant stress drop but could result from errors in calculating energy the ratio of the s wave energy to that radiated by the p waves is about 14 after correction for attenuation this low value is consistent with the corner frequency shift of about 1 3 this corner frequency shift is observed for all four source models and therefore is interpreted as being source controlled

small earthquakes tectonic forces earthquake scaling and frequency of occurrence relations require that small earthquakes be just as important as larger ones in redistributing the forces that drive relative displacements across active faults of any dimension including plate boundaries

earthquake slip distribution a statistical model 1 the purpose of this paper is to interpret slip statistics in a framework of extended earthquake sources we first discuss deformation pattern of the earth s surface from earthquakes and suggest that continuum versus block motion controversy can be reconciled by a model of the fractal distribution of seismic sources we consider earthquake slip statistical distributions as they can be inferred from seismic moment frequency relations and geometrical scaling for earthquakes using various assumptions on temporal earthquake occurrence these distributions are synthesized to evaluate the accuracy of geologic fault slip determinations and to estimate uncertainties in long term earthquake patterns based on paleoseismic data because the seismic moment distribution is a power law pareto a major part of the total seismic moment is released by major earthquakes m 10 19 5 n m moment magnitude m 7 for these large earthquakes the rupture is confined to the upper brittle crust layer we review the various moment frequency and earthquake scaling relationships and apply them to infer the slip distribution at area and site specific regions simulating the seismic moment and strain accumulation process demonstrates that some synthetics can be interpreted as examples of a quasiperiodic sequence we demonstrate the application of the derived slip statistical relations by analyzing the slip distribution and history of the san andreas fault at wrightwood california

aftershock zone scaling we investigate the distribution of aftershock zones for large earthquakes scalar seismic moment m 10 19 5 n m moment magnitude m 7 mainshocks are selected from the harvard centroid moment tensor catalog and aftershocks are selected from the preliminary determination of epicenters neic catalog the aftershock epicenter maps are approximated by a two dimensional gaussian distribution the major ellipse axis is taken as a quantitative measure of the mainshock focal zone size the dependence of zone length l on earthquake size is studied for three representative focal mechanisms thrust normal and strike slip although the numbers of mainshocks available for analysis are limited maximum a few tens of events in each case all earthquakes show the same scaling m l 3 no observable scaling break or saturation occurs for the largest earthquakes m 10 21 n m m 8 therefore it seems that earthquake geometrical focal zone parameters are self similar

earthquakes quaternary faults and seismic hazard in california data describing the locations slip rates and lengths of quaternary faults are the primary basis in this work for constructing maps that characterize seismic hazard in california the expected seismic moment m0e and the strength of ground shaking resulting from the entire rupture of each mapped fault or fault segment are estimated using empirical relations between seismic moment m0 rupture length source to site distance and strong ground motions assuming a fault model whereby the repeat time t of earthquakes on each fault equals m0e m0g where the moment rate m0g is proportional to fault slip rate it is observed that the moment frequency distribution of earthquakes predicted from the geologic data agrees well with the distribution determined from a 150 year historical record the agreement is consistent with the argument that the geologic record of quaternary fault offsets contains information sufficient to predict the average spatial and size distribution of earthquakes through time in california the estimates of t for each fault are the foundation for constructing maps that depict the average return period of 0 1g peak horizontal ground accelerations and the horizontal components of peak acceleration peak velocity and the pseudovelocity response at 1 period and 5 damping expected to occur at the level of 0 1 probability during a 50 year period of time a map is also formulated to show the probability that 0 1g horizontal ground accelerations will occur during the next 50 years the maps serve to illustrate the potential value of quaternary fault studies for assessing seismic hazard interpretation of available slip rates indicates that the largest and most frequent occurrence of potentially destructive strong ground motions are associated principally with the san andreas san jacinto calaveras hayward and ventura basin fault zones other regions of similarly high hazard may yet remain unrecognized this inadequacy results primarily from an incomplete data set numerous faults for example are mapped along the coastal region of northern california and within the modoc plateau but relatively few studies relating to fault slip rate are reported a similar problem exists for other stretches of coastal california where marine reflection studies provide evidence of active faulting offshore yet yield little or no information of fault slip rate geological and geophysical field studies can work to remove these deficiencies a concerted effort to locate and define rates of activity on all faults in california is the most promising means to further quantify present levels of seismic hazard in california

evidence for different scaling of earthquake source parameters for large earthquakes depending on faulting mechanism scaling relationships between seismic moment rupture length and rupture width have been examined for this purpose the data from several previous studies have been merged into a database containing more than 550 events for large earthquakes a dependence of scaling on faulting mechanism has been found whereas small and large dip slip earthquakes scale in the same way the self similarity of earthquakes breaks down for large strike slip events furthermore no significant differences in scaling could be found between normal and reverse earthquakes and between earthquakes from different regions since the thickness of the seismogenic layer limits fault widths most strike slip earthquakes are limited to rupture widths of between 15 and 30 km while the rupture length is not limited the aspect ratio of dip slip earthquakes is similar for all earthquake sizes hence the limitation in rupture width seems to control the maximum possible rupture length for these events the different behaviour of strike slip and dip slip earthquakes can be explained by rupture dynamics and geological fault growth if faults are segmented with the thickness of the seismogenic layer controlling the length of each segment strike slip earthquakes might rupture connected segments more easily than dip slip events and thus could produce longer ruptures than dip slip events of the same width

paleogeodetic records of seismic and aseismic subduction from central sumatran microatolls indonesia we utilize coral microatolls in western sumatra to document vertical deformation associated with subduction microatolls are very sensitive to fluctuations in sea level and thus act as natural tide gauges they record not only the magnitude of vertical deformation associated with earthquakes paleoseismic data but also continuously track the long term aseismic deformation that occurs during the intervals between earthquakes paleogeodetic data this paper focuses on the twentieth century paleogeodetic history of the equatorial region our coral paleogeodetic record of the 1935 event reveals a classical example of deformations produced by seismic rupture of a shallow subduction interface the site closest to the trench rose 90 cm whereas sites further east sank by as much as 35 cm our model reproduces these paleogeodetic data with a 2 3 m slip event on the interface 88 to 125 km from the trench axis our coral paleogeodetic data reveal slow submergence during the decades before and after the event in the areas of coseismic emergence likewise interseismic emergence occurred before and after the 1935 event in areas of coseismic submergence among the interesting phenomenon we have discovered in the coral record is evidence of a large aseismic slip or silent event in 1962 27 years after the 1935 event paleogeodetic deformation rates in the decades before after and between the 1935 and 1962 events have varied both temporally and spatially during the 25 years following the 1935 event submergence rates were dramatically greater than in prior decades during the past four decades however rates have been lower than in the preceding decades but are still higher than they were prior to 1935 these paleogeodetic records enable us to model the kinematics of the subduction interface throughout the twentieth century

long term earthquake clustering summary we analyse statistically the long term properties of several instrumental earthquake catalogues complete catalogues exhibit both short and long term clustering for earthquakes of all depth ranges after accounting for the effect of short term clustering we find that in residual declustered catalogues long term clustering not periodicity characterizes the occurrence of all earthquakes shallow intermediate and deep the degree of clustering in residual catalogues is the same for earthquakes in different depth ranges circumstantial evidence indicates that the long term v ariation of seismicity is governed by a power law temporal distribution as in short term clustering it is scale invariant the fractal dimension of an earthquake set on the time axis is of the order of 0 8 0 9 therefore mainshock occurrence is closer to a stationary poisson process than standard aftershock sequences of shallow earthquakes

further applications of the stochastic stress release model to historical earthquake data abstract the stochastic stress release model provides a stochastic version of the elastic rebound model which may be used to fit simulate and forecast occurrence of large earthquakes within a seismic region this paper reviews its applications to historical earthquakes from china iran and japan thereby extending and supporting earlier work on the chinese historical data the model generally provides an improvement in fit over poisson models with either constant rate or linear trend a crucial factor is the identification of suitable subregions which can act as largely independent seismic units in northern china divisions into two and four such units are considered based on recognized geophysical subdivisions in iran the best results are obtained from a subdivision into three such units the situation in japan seems more complex a preliminary model with two units gives a significant improvement of fit in one out of the two cases in all regions treating the subregions separately gives marked improvements over treating the whole region as a single unit however the quality of fit within subregions differs sharply from case to case pointing to substantial differences in the seismic regimes as reflected in the historical data

a more precise chronology of earthquakes produced by the san andreas fault in southern california improved methods of radiocarbon analysis have enabled us to date more precisely the earthquake ruptures of the san andreas fault that are recorded in the sediments at pallett creek previous dates of these events had 95 confidence errors of 50 100 calendar years new error limits are less than 23 calendar years for all but two of the dated events this greater precision is due to larger sample size longer counting time lower background noise levels more precise conversion of radiocarbon ages to calendric dates and better stratigraphic constraints and statistical techniques the new date ranges with one exception fall within the broader ranges estimated previously but our estimate of the average interval between the latest 10 episodes of faulting is now about 132 years variability about the mean interval is much greater than was suspected previously five of the nine intervals are shorter than a century three of the remaining four intervals are about two to three centuries long despite the wide range of these intervals a pattern in the occurrence of large earthquakes at pallett creek is apparent in the new data the past 10 earthquakes occur in four clusters each of which consists of two or three events earthquakes within the clusters are separated by periods of several decades but the clusters are separated by dormant periods of two to three centuries this pattern may reflect important mechanical aspects of the fault s behavior if this pattern continues into the future the current period of dormancy will probably be greater than two centuries this would mean that the section of the fault represented by the pallett creek site is currently in the middle of one of its longer periods of repose between clusters and sections of the fault farther to the southeast are much more likely to produce the next great earthquake in california the greater precision of dates now available for large earthquakes recorded at the pallett creek site enables speculative correlation of events between paleoseismic sites along the southern half of the san andreas fault a history of great earthquakes with overlapping rupture zones along the mojave section of the fault remains one of the more attractive possibilities

aftershock zones of large shallow earthquakes fault dimensions aftershock area expansion and scaling relations summary we determine the aftershock areas from relocated hypocentres for 64 dip slip and eight strike slip earthquakes in the period 1977 1996 together with those for three recent earthquakes the 1998 antarctic plate earthquake the 1999 izmit turkey earthquake and the 2000 wharton basin earthquake we also include the data for 27 strike slip earthquakes from pegler das 1996 we find that the location of the hypocentre is essentially random along strike for both strike slip and dip slip earthquakes subduction zone earthquakes appear to initiate more frequently towards the down dip edge of the fault whereas the non subduction zone dip slip earthquakes do not have any preferred depth of initiation the aftershock zones of subduction zone earthquakes often expand substantially along strike and up dip but far less in the down dip direction whereas those for non subduction zone earthquakes do not expand significantly in either the up or the down dip direction subduction zone thrust earthquakes have larger and more numerous aftershocks than earthquakes in all other tectonic settings for strikeslip earthquakes we find that slip increases at least linearly with length for dip slip earthquakes we find that the ratio of length to width increases systematically with length for lengths 40 km indicating that there is some restriction on fault width slip is found to be proportional to length over the moment range 10 17 nm m0 3r10 21 nm

changes in frequency size relationship from small to large earthquakes the constant b value observed in frequency magnitude distributions of earthquakes has been taken as an indication of self similarity at all magnitudes hence earthquake properties should scale uniformly in the same way from small to large earthquakes it has often been observed however that the seismic moment released in small earthquakes scales differently with rupture length than it does for large events1 2 where the crossover between small and large events is defined at a rupture dimension equal to the down dip width of the seismogenic zone a possible explanation for this contradiction is that frequency size distributions are biased by small earthquakes small events dominate most global earthquake catalogues because of the short time period covered another source of bias in size distributions is the saturation of earthquake magnitudes for large events here we correct biases in calculations of b values and present evidence for a change in b value in frequency size distributions we find that a break in self similarity from small to large earthquakes occurs at a point where the dimension of the event equals the down dip width of the seismogenic layer

southern california deformation southern california is one of the most seismically active places in the united states and has therefore been of long standing interest to geoscientists in their perspective jackson et al discuss recent work to collect geodetic data on the motion and earthquake potential of the faults in southern california

on moment length scaling of large strike slip earthquakes and the strength of faults 1 several large strike slip earthquakes have occurred in various tectonic settings in the past 5 years adding well documented data to the global collection of moment and length estimates for such earthquakes based on this augmented dataset we reexamine the controversial issue of scaling of seismic moment with length of rupture we find that the global dataset of large strike slip earthquakes follows a bi modal distribution most oceanic and or intraplate strike slip earthquakes have stress drops that are 5 times larger than interplate continental ones when distinguishing these two classes the scaling is compatible with that predicted by simple dislocation theory

fault behavior and characteristic earthquakes examples from the wasatch and san andreas fault zones paleoseismological data for the wasatch and san andreas fault zones have led to the formulation of the characteristic earthquake model which postulates that individual faults and fault segments tend to generate essentially same size or characteristic earthquakes having a relatively narrow range of magnitudes near the maximum analysis of scarp derived colluvium in trench exposures across the wasatch fault provides estimates of the timing and displacement associated with individual surface faulting earthquakes at all of the sites studied the displacement per event has been consistently large measured values range from 1 6 to 2 6 m and the average is about 2 m on the basis of variability in the timing of individual events as well as changes in scarp morphology and fault geometry six major segments are recognized along the wasatch fault on the basis of the most likely number of surface faulting events 18 that have occurred on segments of the wasatch fault zone during the past 8000 years an average recurrence interval of 400 666 years with a preferred average of 444 years is calculated for the entire zone geologic data on the distribution of slip associated with prehistoric earthquakes and slip rates along the south central segment of the san andreas fault suggest that the m 8 1857 earthquake is a characteristic earthquake for this segment comparisons of earthquake recurrence relationships on both the wasatch and san andreas faults based on historical seismicity data and geologic data show that a linear constant b value extrapolation of the cumulative recurrence curve from the smaller magnitudes leads to gross underestimates of the frequency of occurrence of the large or characteristic earthquakes only by assuming a low b value in the moderate magnitude range can the seismicity data on small earthquakes be reconciled with geologic data on large earthquakes the characteristic earthquake appears to be a fundamental aspect of the behavior of the wasatch and san andreas faults and may apply to many other faults as well

universality of the seismic moment frequency relation we analyze the seismic moment frequency relation in various depth ranges and for different seismic regions using flinn engdahl s regionalization of global seismicity three earthquake lists of centroid moment tensor data have been used the harvard catalog the usgs catalog and the huang et al 1997 catalog of deep earthquakes the results confirm the universality of the values and the maximum moment for shallow earthquakes in continental regions as well as at and near continental boundaries moreover we show that although fluctuations in earthquake size distribution increase with depth the values for earthquakes in the depth range of 0 500 km exhibit no statistically significant regional variations the regional variations are significant only for deep events near the 660 km boundary for declustered shallow earthquake catalogs and deeper events we show that the worldwide values have the same value of 0 60 0 02 this finding suggests that the value is a universal constant we investigate the statistical correlations between the numbers of seismic events in different depth ranges and the correlation of the tectonic deformation rate and seismic activity the number of earthquakes above a certain threshold level per year the high level of these correlations suggests that seismic activity indicates tectonic deformation rate in subduction zones combined with the universality of the value this finding implies little if any variation in maximum earthquake seismic moment among various subduction zones if we assume that earthquakes of maximum size are similar in different depth ranges and the seismic efficiency coefficient is close to 100 for shallow seismicity then we can estimate for deeper earthquakes for intermediate earthquakes 5 and 1 for deep events these results may lead to new theoretical understanding of the earthquake process and better estimates of seismic hazard

a physical basis for time clustering of large earthquakes we develop a theory that links stress interaction between earthquakes and the occurrence of temporal clustering coseismic static stress change in the vicinity 50 km of large earthquakes suggests that perturbations of 0 1 to 1 bars may affect the occurrence of other earthquakes at larger distances interactions also seem to exist four m 8 earthquakes have occurred in mongolia on distant faults 400 km during the last century also paleoseismic observations documenting much longer time periods display a time clustering of major events we demonstrate with simple mechanical concepts that postseismic stress relaxation magnifies the coseismic stress change and has a major effect on fault interaction during the seismic cycle in the simple case where two distant faults are coupled the probabilistic occurrence of triggered earthquakes may increase dramatically due to long range postseismic coupling

a systematic test of the hypothesis that the b value varies with depth in california we spatially map the shallow to deep b value ratio in the crust in california previous studies of the frequency magnitude distribution as a function of depth for selected crustal regions indicated that b decreases from b 1 1 in the 0 5 km depth range to b 0 8 in the depth range 7 15 km our detailed mapping confirms that this pattern can be established at the 99 significance level for about 32 of the entire seismically active crust about 2 of the crust displays the opposite b gradient one such area is the san francisco bay area we speculate that differences in stress levels are the main factor controlling the depth dependency of b these results confirm that the b value should not always be considered a constant in studies such as seismic hazard estimations

block versus continuum deformation in the western united states abstract the relative role of block versus continuum deformation of continental lithosphere is a current subject of debate continuous deformation is suggested by distributed seismicity at continental plate margins and by cumulative seismic moment sums which yield slip estimates that are less than estimates from plate motion studies in contrast block models are favored by geologic studies of displacement in places like asia a problem in this debate is a lack of data from which unequivocal conclusions may be reached in this paper we apply the techniques of study used in regions such as the alpine himalayan belt to an area with a wealth of instrumental data the western united states by comparing plate rates to seismic moment release rates and assuming a typical seismogenic layer thickness of 15 km it appears that since 1850 about 60 of the pacific north america motion across the plate boundary in california and nevada has occurred seismically and 40 aseismically the san francisco bay area shows similar partitioning between seismic and aseismic deformation and it can be shown that within the seismogenic depth range aseismic deformation is concentrated near the surface and at depth in some cases this deformation can be located on creeping surface faults but elsewhere it is spread over a several kilometer wide zone adjacent to the fault these superficial creeping deformation zones may be responsible for the palaeomagnetic rotations that have been ascribed elsewhere to the surface expression of continuum deformation in the lithosphere our results support the dominant role of non continuum deformation processes with the implication that deformation localization by strain softening must occur in the lower crust and probably the upper mantle our conclusions apply only to the regions where the data are good and even within the western united states i e the basin and range deformation styles remain poorly resolved nonetheless we maintain that block motion is the deformation style of choice for those continental regions where the data are best

depth dependence of earthquake frequency magnitude distributions in california implications for rupture initiation statistics of earthquakes in california show linear frequency magnitude relationships in the range of m2 0 to m5 5 for various data sets assuming gutenberg richter distributions there is a systematic decrease in b value with increasing depth of earthquakes we find consistent results for various data sets from northern and southern california that both include and exclude the larger aftershock sequences we suggest that at shallow depth 0 to 6 km conditions with more heterogeneous material properties and lower lithospheric stress prevail rupture initiations are more likely to stop before growing into large earthquakes producing relatively more smaller earthquakes and consequently higher b values these ideas help to explain the depth dependent observations of foreshocks in the western united states the higher occurrence rate of foreshocks preceding shallow earthquakes can be interpreted in terms of rupture initiations that are stopped before growing into the mainshock at greater depth 9 15 km any rupture initiation is more likely to continue growing into a larger event so there are fewer foreshocks if one assumes that frequency magnitude statistics can be used to estimate probabilities of a small rupture initiation growing into a larger earthquake then a small m2 rupture initiation at 9 to 12 km depth is 18 times more likely to grow into a m5 5 or larger event compared to the same small rupture initiation at 0 to 3 km

evolution of moderate seismicity in the san francisco bay region 1850 to 1993 seismicity changes related to the occurrence of large and great earthquakes the rate of seismic activity of moderate size m 5 5 earthquakes in the san francisco bay sfb region has varied considerably during the past 150 years as measured by the rate of seismic moment release seismic activity in the sfb region is observed to accelerate prior to m 7 0 earthquakes in 1868 1906 and 1989 and then to decelerate following them we examine these seismicity changes in the context of the evolution of the stress field in the sfb region as a result of strain accumulation and release using a model of dislocations in an elastic halfspace we use a coulomb failure function cff to take into account changes in both shear and normal stresses on potential failure planes of varying strike and dip in the sfb region we find that the occurrence of a large or great earthquake creates a stress shadow a region where the stress driving earthquake deformation is decreased interseismic strain accumulation acts to reverse this process gradually bringing faults in the sfb region out of the stress shadow of a previous large or great earthquake and back into a state where earthquake failure is possible as the stress shadow generated by a large or great earthquake disappears it migrates inward toward the fault associated with that large or great event the observed changes in the rate of occurrence of moderate earthquakes in the sfb region are broadly consistent with this model in detail the decrease in seismicity throughout most of the sfb region and a localized increase in the monterey bay region following the great 1906 earthquake is consistent with our predicted stress changes the timing and location of moderate size earthquakes when the rate of seismicity increases again in the 1950s is consistent with areas in which the 1906 stress shadow had been eliminated by strain accumulation in the sfb region those earthquakes that are most inconsistent with our stress evolution model including the 1911 earthquake southeast of san jose are found to occur in regions where dip slip faulting is common in addition to strike slip the 1906 earthquake brought that zone of dip slip faulting closer to failure suggesting that the 1911 event may have been a reverse faulting earthquake rather than a strike slip one similar to the 1984 morgan hill earthquake the occurrence of activity on faults very close to the san andreas such as the lake elsman earthquakes of 1988 and 1989 appear to be associated with the last disappearence of the stress shadow on the loma prieta segment of the san andreas fault thus events of that type may represent an intermediate term precursor to a large earthquake such as the 1989 loma prieta event much of the moderate size earthquake activity in the sfb region appears to be modulated in time by the buildup and release of stress in large and great earthquakes a tensorial approach to earthquake prediction i e taking into account changes in the components of the stress tensor has several advantages over examining scalar changes such as those in seismic activity and moment release rates this tensorial approach allows for both activation and quiescence but in different subregions prior to as well as after large earthquakes

six similar sequential ruptures of the san andreas fault carrizo plain california we document the precise sizes but not the dates of the six latest offsets across the san andreas fault at wallace creek california three and perhaps four of these including the latest in 1857 show dextral offset of 7 5 8 m the third and fourth offsets however are just 1 4 and 5 2 m the predominance of similar offsets for the latest six events suggests that the fundamental properties of the fault system that control slip size do not vary greatly from event to event the large offsets imply that ruptures involving this site are typically more than 200 km long

frequency of occurrence of moderate to great earthquakes in intracontinental regions implications for changes in stress earthquake prediction and hazards assessments we investigate the departure of shallow intracontinental earthquakes from the frequency magnitude relationship log n m a bm we consider seven large continental regions but exclude subduction zones in each active and stable areas are differentiated within active areas we separately categorize earthquakes on faults with high slip rates we use data sets from 1978 1994 and 1900 1994 which are complete for moment magnitudes mw 5 3 and 7 0 to calculate changes in b value with mw for all active intracontinental regions combined except asia b changes at 98 confidence from 0 90 to 2 1 at a corner magnitude mc of 6 9 to 7 0 the distribution for stable regions indicates a similar change values of mc are associated with downdip widths of rupture of about 23 to 27 km which are smaller than those for subduction zones in each case mc marks a transition from unbounded to bounded earthquakes in asia events from 1958 to 1994 also delineate a similar change near mc 7 0 earthquakes in asia of 7 0 mw 8 1 from 1900 to 1957 however define a b value of 1 0 and a moment release rate 5 6 times higher than that from 1958 to 1994 those anomalous values indicate that a large region of asia was close to failure i e near a self organized critical state during the earlier period we associate those changes with the giant himalayan earthquake in 1950 of mw 8 6 regional variations in seismic strain release rates correlate with the types of processes at adjacent plate boundaries extrapolating rates of activity of the past 95 years to mw 8 0 indicates that such events should be very rare in stable areas

long term seismic behavior of the focal and adjacent regions of great earthquakes during the time between two successive shocks we analyze the spatial temporal energy distribution of shallow depth h 70 km seismicity surface wave magnitude ms 7 occurring in the focal and adjacent regions of very great shallow earthquakes rupture lengths of the order of hundreds of kilometers in the circum pacific seismic belt for as many decades as possible before and after the occurrence of the main shock for this purpose we have revised the catalog of data for the world instrumental seismicity 1900 1995 by removing a series of heterogeneities in the earthquake reporting and magnitude determination this revised catalog is fairly complete for earthquakes with m 7 since early in the century our findings indicate that once a main shock aftershock sequence occurs the focal region enters a period of relative quiescence for large ms 7 earthquakes significant seismic activity ms 7 occurs during many decades prior to the next main shock this activity is fairly continuous or random in time and lasts until the onset of the main event but clusters in space around the ends of the forthcoming rupture and or in the vicinity of the principal epicenter except in the case of rare bilateral breaks the main epicenter is located at one of these ends thus major portions of the rupture zone experience no events with ms 7 during the seismic cycle larger aftershocks also cluster near the edges of the rupture and near the principal epicenter preshock activity minutes to weeks before the main shock if any occurs in general near the main epicenter we also find that the nucleation and ends of the rupture often coincide in space with recognizable geometrical and or geological inhomogeneities barriers existing in the lithosphere it appears that while strain energy accumulates high stress concentrations often occur at the ends of the future rupture zone which results in the long term foreshock activity and in the subsequent initiation of the main rupture from within those regions we suggest a simple mechanical model that explains the observed pattern of earthquake occurrence for both unilateral and bilateral ruptures in all cases of great earthquakes that have occurred this century for which a long and complete to a level of 2 to 3 orders of magnitude smaller than the moment of the main shock catalog of prior seismicity exists we can not recognize any statistically significant quiescent or doughnut patterns in the preceding seismicity at the ms 7 level