simulation of seismicity due to ruptures on noncoplanar interactive faults we study the statistical property and temporal variation of activity due to ruptures on two noncoplanar interactive faults each fault is assumed to consist of cracks and intervening asperities the ruptures are assumed to occur in the form of catastrophic coalescence of the coplanar cracks on each fault we apply fracture mechanics to the analysis of the interactions among cracks and to the consideration of fracture condition our fault model gives rise to a complex frequency distribution of event sizes even if no externally imposed spatial inhomogeneity is assumed for the distributions of cracks and asperities and a homogeneous statistical distribution is assumed for fracture strengths for the smaller events the frequency distribution of the event sizes obeys the gutenberg richter magnitude frequency law it is also shown that the number of smaller events increases as the contribution of noncoplanar interactions increases we generally observe the periodic recurrence of events after transient change at the beginning of each sequence of activity the length of the transient part increases considerably as the contribution of noncoplanar interactions increases the recurrence period at the steady periodic state also lengthens with increasing contribution of noncoplanar interactions hence if the temporal variation of activity is observed in a fixed length of time then the activity appears to be much more complex when the contribution of noncoplanar interactions is larger it is also observed that events with smaller recurrence intervals are more abundant when the influence of noncoplanar interactions is large

stress slip and earthquakes in models of complex single fault systems incorporating brittle and creep deformations numerical simulations of slip evolution along a cellular vertical strike slip fault in an elastic half space are performed for several models representing discrete fault systems embedded in three dimensional elastic continua the geometry and imposed boundary conditions correspond approximately to the central san andreas fault the simulations incorporate brittle and creep deformations in series the net fault zone deformation rate is the sum of creep rate and frictional slip rate brittle fault properties are given by various distributions of earthquake stress drops on failing segments numerical cells the assumed distributions represent two idealized situations corresponding to different extreme states along an evolutionary path of a fault 1 a strongly disordered state characterized by a wide range of size scales representing immature fault zones and extended spatial domains and 2 a relatively regular state having a narrow range of size scales representing mature highly slipped faults the assumed creep properties are identical in all cases these are prescribed in terms of coefficients characterizing a power law dependency of creep slip rate on stress the combined brittle creep process and employed parameters lead to an overall pine tree stress depth profile with a brittle ductile transition depth of about 12 5 km and variable stress along strike profiles with brittle creep transition around 65 km nw of the 1857 rupture the spatial patterns of simulated hypocenters are statistically similar to observed data the results indicate that the range of size scales characterizing strong fault zone heterogeneities has important manifestations on the seismic response of a fault system a narrow range of size scales leads to frequency size statistics of earthquakes resembling the characteristic earthquake distribution and quasi periodic temporal distribution of large events as in the seismic gap hypothesis on the other hand a wide range of size scales leads to gutenberg richter power law frequency size statistics and random or clustered temporal distribution of large events the simulations demonstrate that treatment of the various observed forms of frequency size and temporal statistics of earthquakes can be unified through the concept of range of size scales characterizing fault zone heterogeneities this has a clear physical interpretation in terms of structural properties of a given fault zone or broad lithospheric domain and is supported by observed earthquake and fault data in some simulated cases the frequency size statistics of small earthquakes fall sharply below the self similar gutenberg richter line the results indicate that small earthquakes prepare the fault for the occurrence of a large event by smoothing during gradual tectonic loading the long wavelength components of stress on the fault this is done through short wavelength stress roughening associated with the numerous ruptures of the small events the above pattern of smoothing roughening of long short wavelengths of stress on the fault is reversed during large scale ruptures of the big events

an application of synthetic seismicity in earthquake statistics the middle america trench this paper demonstrates how synthetic seismicity calculations which are based on the concept of fault segmentation and incorporate the physics of faulting through static dislocation theory can improve earthquake recurrence statistics and hone the probabilities of hazard compared to forecasts constructed from a handful of earthquake recurrence intervals forecasts constructed from synthetic seismicity are more robust in that they embody regional seismicity information over several units of magnitude they can extrapolate seismicity to higher magnitudes than have actually been observed and they are formulated from a catalog which can be extended as long as needed to be statistically significant synthetic seismicity models can also be used to judge the stability of common rate estimates and the appropriateness of idealizations to the earthquake cycle i find that estimates of fault slip rate are unbiased regardless of sampling duration while estimates of earthquake recurrence time are strongly biased recurrence intervals estimated from seismicity samples less than about 10 times the actual recurrence interval will almost certainly be too short for the middle america trench mat it would take 200 and 400 years of monitoring to constrain slip and recurrence rates to 10 events m 6 have as much as a 60 probability of recurrence within 5 years due to the clustering of small earthquakes in foreshocks and aftershocks this probability drops to less than 15 for m 7 events increasing gap time generally increases conditional probability of earthquake occurrence but the effect is weak for the mat the spread parameters of the best fitting lognormal or weibull distributions 0 75 are much larger than the 0 21 intrinsic spread proposed in the nishenko buland hypothesis stress interaction between fault segments disrupts time or slip predictability and causes earthquake recurrence to be far more aperiodic than has been suggested

the earthquake prediction experiment at parkfield california since 1985 a focused earthquake prediction experiment has been in progress along the san andreas fault near the town of parkfield in central california parkfield has experienced six moderate earthquakes since 1857 at average intervals of 22 years the most recent a magnitude 6 event in 1966 the probability of another moderate earthquake soon appears high but studies assigning it a 95 chance of occurring before 1993 now appear to have been over simplified the identification of a parkfield fault segment was initially based on geometric features in the surface trace of the san andreas fault but more recent microearthquake studies have demonstrated that those features do not extend to seismogenic depths on the other hand geodetic measurements are consistent with the existence of a locked patch on the fault beneath parkfield that has presently accumulated a slip deficit equal to the slip in the 1966 earthquake a magnitude 4 7 earthquake in october 1992 brought the parkfield experiment to its highest level of alert with a 72 hour public warning that there was a 37 chance of a magnitude 6 event however this warning proved to be a false alarm most data collected at parkfield indicate that strain is accumulating at a constant rate on this part of the san andreas fault but some interesting departures from this behavior have been recorded here we outline the scientific arguments bearing on when the next parkfield earthquake is likely to occur and summarize geophysical observations to date

precursory seismic quiescence seventeen cases of precursory seismic quiescence to mainshocks with magnitudes fromml 4 7 toms 8 0 are summarized the amount of rate decrease ranges from 45 to 90 the significance of these changes varies between 90 and 99 99 the assumption that the background rate is approximately constant is fulfilled in most crustal volumes studied all quiescence anomalies seem to have abrupt beginnings and the rate during the anomalous period is fairly constant the duration of the precursors ranges from 15 to 75 months and it is not clear what factors determine that time at least three successful predictions have been based on seismic quiescence these cases have shown that mainshocks can be predicted based on quiescence but they have also shown that the interpretation of the data in real time is difficult and nonunique if a false alarm is defined as a period of quiescence with a significance level larger than a precursory quiescence in the same tectonic area then we estimate based on searches in four areas that the false alarm rate may be on the order of 50 failure to predict may be expected in perhaps 50 of mainshocks even in carefully monitored areas quiescence cannot be used as a precursor in tectonic environments with low seismic activity most characteristics of the phenomenon are still poorly defined but data exist which probably permit at least a doubling of the presently available data on case histories

application of pattern recognition techniques to earthquake catalogs generated by model of segmented fault systems in three dimensional elastic solids techniques and seismicity parameters described by eneva and ben zion 1997 are used to examine synthetic earthquake catalogs generated by ben zion 1996 for precursory patterns of large model events different model realizations represent various levels of fault zone disorder these include models with uniform properties u a parkfield type asperity a fractal brittle properties f and multi size scale heterogeneities m the seismicity parameters used are based on information contained in typical earthquake catalogs reflecting earthquake distribution in space time and size the analysis highlights the complexity of the information content of the synthetic earthquake catalogs simple repetitive precursory signals have not been found however local extrema in the examined parameters are found to have significant association in time with large events thus our techniques and parameters may be useful for intermediate term earthquake prediction especially when parameters are used in combinations some analysis results are the same for all model realizations and some depend on the model features characterizing all catalogs are as follows 1 large model events are statistically predictable on the basis of patterns in the distribution of smaller events 2 for a given parameter the type of precursory extrema maxima or minima is the same for all models 3 the interparameter correlation for any parameter pair has the same sign positive or negative in all models 4 the large events are neither slip nor time predictable based on previous large events results that differ from model to model include the following 1 the degree of predictability of large events correlates with the degree of regularity in the assumed fault properties following the order u f a and m 2 there is no one to one correlation between type of precursory extrema maxima or minima and type of precursory trends increase or decrease this produces great variations in observable trends for any given parameter both from model to model and for different events in the same model 3 the interparameter correlations vary among models with the highest correlations in model f most discussed patterns are in agreement with observations from seismically active zones laboratory models and mining induced seismicity

research on groundwater radon as a fluid phase precursor to earthquakes groundwater radon monitoring work carried out in southern california by the university of southern california since 1974 is summarized here this effort began with a sampling network over a locked segment of the san andreas fault from tejon to cajon and was later expanded to cover part of the southern transverse mountain ranges groundwater samples were brought back weekly to the laboratory for high precision scintillation counting needs for more frequent sampling and less labor prompted the development of an economical and field worthy instrument known as the continuous radon monitor about 10 have been installed in the network since early 1980 the groundwater radon content was found to show anomalous increases mostly at a single station before a number of moderate and nearby earthquakes our work is hampered by a lack of large earthquakes that may have a regional impact on radon anomalies and by the complexity of the underground hydrological regime to circumvent this difficulty we have chosen to monitor only deep artesian wells or hot spring wells

statistical analysis of the spatial distribution of earthquakes variation of the spatial distribution of earthquakes before and after large earthquakes abstract we have investigated the spatial distribution of earthquakes using the polya eggenberger model and the morishita index which is used for the analysis of the spatial dispersion of bio species first we found that the spatial distribution of earthquakes is characterised by clustering when we divide the certain area into equal area meshes the frequency distribution of meshes with same number of events fits well to the polya eggenberger model on the basis of this result we examined the degree of spatial clustering of earthquakes by using the morishita index defined by i q i 1 q n i n i 1 n n 1 where q n and ni are the numbers of meshes total events and events of the i th mesh respectively the index i corresponds to the contagious coefficient plus 1 in the polya eggenberger statistics the dependence of i on the mesh size provides information about clustering we analysed the actual distribution patterns of earthquakes before and after the miyagi oki 1978 m 7 4 the tokachi oki 1968 m 7 9 and the nihonkai chubu 1983 m 7 7 earthquakes large scale clusters of about 50 100 km are dominant in the aftershock sequences after the aftershock activity earthquakes tend to distribute randomly prior to large earthquakes small patches of 10 20 km are created this statistical feature of the earthquake distribution seems to be related to the phenomena of the precursory clusterings of earthquakes doughnut patterns preseismic quiescence and foreshocks prior to large earthquakes

man made changes of seismicity rates seismicity catalogs contain important information about processes which occur in seismically active regions of the earth many authors have examined these catalogs for patterns and variations in patterns which might reflect changes in these processes we have found that these catalogs include a complex mixture of real and man made changes one must identify and account for the man made changes before the real ones can be identified and understood many man made changes in seismicity catalogs are manifested as changes in seismicity rates and can therefore be identified through careful examination of these rates obvious effects include increases or decreases in the detection and reporting of smaller events which accompany the installation or closure of seismic stations these types of changes can be recognized by examining the distribution of seismicity rate changes in the magnitude domain they are characterized by increases or decreases in the number of small events in the catalog at times when the number of larger events remains constant systematic changes in the magnitudes assigned to events can also be identified by examining seismicity rates because they cause apparent changes in rates of data sets with magnitude cutoffs the sign of the apparent rate change depends on the sign of the magnitude change and the type of cutoff used the effects of detection changes can be easily remedied by using a magnitude cutoff which eliminates the smaller events from consideration the techniques we have developed allow one to determine the optimum cutoff magnitude corrections are necessary for remedying the effects of systematic changes in magnitude estimates these corrections can be determined through modeling of observed rate changes caused by these shifts man made changes are present in all seismicity catalogs whether local regional or teleseismic they must be accounted for if these catalogs are to provide meaningful information on real process changes in the earth

application of fracture mechanics to the simulation of seismicity and recurrence of characteristic earthquakes on a fault we present a fault model consisting of a large number of collinear cracks and intervening asperities for the simulation of seismicity and recurrence of large earthquakes on a fault we apply fracture mechanics to the analysis of the interaction of cracks and to the consideration of fracture condition our fault model gives rise to a complex frequency distribution of event sizes even if no externally imposed spatial inhomogeneity is assumed for the distribution of cracks and asperities and a homogeneous statistical distribution is assumed for fracture strengths the frequency distribution of event sizes is found to be bimodal with no intermediate size event in all the calculations for the smaller events the frequency distribution shows a power law behavior whereas the larger size events denote ones that rupture the whole fault statistically the same frequency distributions are obtained for inhomogeneous distributions of crack lengths and fracture strengths the occurrence of events that rupture the whole fault is shown to be periodic when the fault is isolated the magnitudes of these large events generally increase with increasing recurrence period for given fault length and rate of remote loading the events having a longer recurrence period have a shorter fault length for given magnitude and rate of remote loading if there are fault interactions the periodicity in the recurrence of the large events that would be observed for the isolated fault disappears and the recurrence of the large events becomes complex

millisecond accurate monitoring of seismic travel times over 13 and 18 kilometer baselines using a repeatable airgun source we have monitored the travel times of several seismic phases along 13 and 18 km baselines from bouquet reservoir to boreholes on the san andreas fault near palmdale california the first arriving phases refract at a depth of 2 3 km while one later arrival is possibly a reflection from 10 km the majority of some 200 travel time determinations for the most prominent phases were made between august 1979 and march 1980 isolated measurements were made in november 1978 and march 1981 there exists ample geodetic and other evidence for significant strain changes during this period allowing for source variation due to fluctuating reservoir water levels we observed no travel time changes to the limit of our precision of 1 2 ms for the first arrivals and 3 5 ms for secondary arrivals or about 0 1 of the total travel time an earlier study which monitored rays penetrating only 500 m into the crust found that travel times varied by up to 0 3 over a period of several days the variations showed some correlation with strain changes due to local seismic activity and were not permanent to understand this different behavior of crustal rocks between 500 and 2000 m we investigate the depth dependence of their velocity strain sensitivity using measured p and s velocity profiles to gauge the rate at which confining pressure increases velocity we find that 1 bar of overpressure at 500 m can vary the p velocity by 0 1 while 1 bar at 2000 m will vary the velocity by only 0 01 applying the o connell budiansky physical model for cracked rock to the velocity depth profiles we can further characterize the local crust as having a high crack density a mobile fluid component and an average crack aspect ratio of 2 5 10 3 the combination of a mobile fluid content and high crack density suggests that fluids play a role in the transient velocity anomalies observed at a depth of 500 m and argues against significant undersaturation phenomena at seismogenic depths on the san andreas fault

an experimental study of complex response in river channel adjustment downstream from a reservoir an experimental study is described which attempts to reveal the complex response process of river channel adjustment downstream from a reservoir a descriptive model has been established which identifies three adjustment stages in stage i following the change of water and sediment input the width depth ratio of river channel w d and the river channel gradient s both decrease but the river channel sinuosity r increases in stage ii because of feedback from the changing boundary conditions w d starts to increase r decreases and the rate of gradient decrease becomes very slow in stage iii equilibrium is reestablished w d r and s tend to constant values and the system recovers its stable state

model for intermediate term precursory clustering of earthquakes a causal sequence of an earthquake swarm extended quiescence foreshocks and main shock can be understood in terms of an aggregation model of crack growth and ultimate fusion on a multiplanar earthquake fault system the rheological model is one of accelerated crack growth under conditions of slip weakening coupled with a slow recovery of static friction after fracture once a slip feature has been generated that spans a large part of the available geometry a stress shadow is cast on other neighboring cracks that strongly retards and inhibits crack growth thus initiating the quiescent phase abruptly the success of the model depends on the assumption that nonlinear rheology regulates the rate of slip and that earthquake faults are not simply connected surfaces but instead have many strands

low frequency magnetic field measurements near the epicenter of the ms 7 1 loma prieta earthquake the authors report the results of measurements of low frequency magnetic noise by two independent monitoring systems prior to the occurrence of the m sub s 7 1 loma prieta earthquake of 17 october 1989 their measurements cover 25 narrow frequency bands in the more than six decade frequency range 0 01 hz 32 khz with a time resolution varying from a half hour in the ulf range 0 01 10 hz to one second in the elf vlf range 10 hz 32 khz the ulf system is located near corralitos about 7 km from the epicenter the elf vlf system is located on the stanford campus about 52 km from the epicenter analysis of the elf vlf system is located on the stanford campus about 52 km from the epicenter analysis of the elf vlf data has revealed no precursor activity that they can identify at this time however the ulf data have some distinctive and anomalous features first a narrow band signal appeared in the range 0 05 0 2 hz around september 12 and persisted until the appearance of the second anomalous feature which consisted of a substantial increase in the noise background starting on 5 october and covering almost the entire frequency range of the ulf system third there was an anomalousmore dip in the noise background in the range 0 2 5 hz starting one day ahead of the earthquake finally and perhaps most compelling there was an increase to an exceptionally high level of activity in the range 0 01 0 5 hz starting approximately three hours before the earthquake there do not appear to have been any magnetic field fluctuations originating in the upper atmosphere that can account for this increase further while the systems are sensitive to motion seismic measurements indicate that there were no significant shocks preceding the quake thus the various anomalous features in the data and in particular the large amplitude increase in activity starting three hours before the quake may have been magnetic precursors less

hierarchy in earthquake size distribution abstract we propose a fault model in which there are a finite number of characteristic lengths between the smallest and the largest scales e g between a grain size and a size of great earthquakes a fault plane in one scale is segmented into blocks of statistically similar sizes by the barriers of statistically similar heights rupture expands by successively climbing the barriers the average barrier height is proportional to the average block size which defines the characteristic length of the relevant scale stresses before and after rupturing are assumed to be independent of earthquake size with these assumptions the probability for rupture to grow further is shown to be independent of the scale of barrier structure the rupture in one scale however tends to grow more easily as it grows so that once the rupture exceeds some critical size it no longer stops until it encounters the barriers in the next larger scale rupture grows by repeating this cycle which starts with rupturing of a unit block in one scale and ends with one in the next larger scale our model involves two parameters which must be determined empirically one is the length ratio r between two successive scales the other is the critical number of blocks n c which defines the critical size for unstable rupture growth in one scale we suggest values of 5 for both r and n c with these values our model explains a diversity of earthquake phenomena such as the magnitude frequency relation the seismic gap and the magnitude gap the magnitude difference between a main shock and its largest aftershock the occurrence of multiplet earthquakes the generation of high frequency seismic waves the incoherency of the near field acceleration and the recent observation by furumoto and nakanishi that the main rupture is often preceded by a precursory rupture whose time duration in general scales with seismic moment

time intervals between characteristic earthquakes and correlations with smaller events an analysis based on a mechanical model of a fault certain correlations between characteristic events and smaller events are studied in the context of a spatially homogeneous mechanical model for a fault the model satisfies a one dimensional quasi linear wave equation in which the key nonlinearity is associated with the slipstick friction force at the interface between tectonic plates we find that the model gives rise to slipping events of all sizes the smaller events are consistent with the gutenberg richter statistical law whereas the characteristic events have a separate statistical distribution the crossover from smaller events to characteristic events coincides with a correlation length which sets the scale for clustering of smaller events before the characteristic rupture numerical results and scaling arguments relate the features of the distribution of characteristic events to the corresponding features of the distribution of smaller events as well as the length of the fault the distribution of time intervals between characteristic events has the distinguishing features of asymmetry and a finite upper cutoff the mean t and standard deviation of the time interval distribution are found to be related to features of the magnitude versus frequency distribution of smaller events and the coefficient of variation t is independent of the driving rate because this analysis relates properties of characteristic earthquakes to statistical features of smaller more numerous events as well as geometric features of the fault it may ultimately be useful for long term earthquake prediction

synthetic seismicity models of multiple interacting faults a synthetic seismicity model for multiple interacting faults of any strike and dip in a three dimensional elastic half space has been developed the ultimate purpose is to determine how hazard is modified by elastic interactions of events in complex fault systems and to examine the synthetic seismicity for unusual behavior before large events that might be observable in the real world each fault is subdivided into some number of equal sized patches and a coefficient of static friction random within a specified range is assigned to each the patch static strength is then the product of friction and differential normal stress stress both shear and normal accumulates due to some predefined tectonic driving force until one or more patches fail and slip instantaneously the slip can be in any direction within the fault plane and its magnitude is determined by a specified fractional stress drop this initial slip induces shear stress and strength changes on all other patches and faults and perhaps results in other patches failing patches can slip more than once once having failed their strength is reduced resulting in an overshoot that approximates the effect of dynamic stress enhancement an event is over when all patches are stable although present computational constraints do not allow models with more than about 1500 patches or a large number of faults experiments with simple models of two parallel strike slip faults 25 km length 15 km depth 694 m x 714 m patch size driven at different rates and separated by 10 or 15 km give some indicative results a range of friction between 0 1 and 0 2 with a stress drop of 10 about 3 mpa or 30 bars produces a b value 500 000 events close to 1 the models generate a distinct class of large characteristic events that rupture the full fault plane the distribution of interevent times for the background seismicity is very similar to that for a poisson process while the characteristic events are quasi periodic nearly half the larger events directly induce some activity on the other fault the effect of a characteristic event on the occurrence of another characteristic event on the opposite fault is small and depends on the separation at 10 km distance other events are retarded while at 15 km distance they are advanced at 10 km separation the probability of a pair of characteristic events within 1 year is reduced to 0 3 from 1 7 for the case with no interaction consideration of interactions between slip on the faults and the driving mechanism as well as higher stress drops and more overshoot would increase the magnitude of the interaction effects

gas geochemistry applied to earthquake prediction an overview terrestrial gases in groundwater and soil air have been extensively studied in recent years in seismically active areas especially in ussr china japan and united states in search of premonitory changes that might be useful for earthquake prediction concentrations of radon helium hydrogen mercury carbon dioxide and several other volatiles have been found generally to be anomalously high along active faults suggesting that the faults may be paths of least resistance for the terrestrial gases generated or stored in the earth to escape to the atmosphere temporal gas concentration changes with durations of a few hours to many months have been observed before many large earthquakes at a relatively small number of favorably situated stations at epicentral distances of up to several hundreds of kilometers these sensitive stations are generally located along active faults especially at intersections or bends of faults or some other structurally weak zones possibly because of tectonic strain concentration at such places however gas concentrations especially those measured at or near the ground surface may also be significantly affected by meteorological hydrological and other nontectonic changes in the environment the nontectonically induced variations must be carefully eliminated or recognized in the search of true earthquake related anomalies

slip patterns and earthquake populations along different classes of faults in elastic solids numerical simulations of slip instabilities on a vertical strike slip fault in an elastic half space are performed for various models belonging to two different categories the first category consists of inherently discrete cellular fault models such are used to represent fault systems made of segments modeled by numerical cells that can fail independently of one another their quasi independence is assumed to provide an approximate representation of strong fault heterogeneity due to geometric or material property disorder that can arrest ruptures at segment boundaries the second category consists of models having a well defined continuum limit these involve a fault governed by rate and state dependent friction and are used to evaluate what types of property heterogeneity could lead to the quasi independent behavior of neighboring fault segments assumed in the first category the cases examined include models of a cellular fault subjected to various complex spatial distributions of static to kinetic strength drops and models incorporating rate and state dependent friction subjected to various spatial distributions of effective stress normal stress minus pore pressure the results indicate that gradual effective stress variations do not provide a sufficient mechanism for the generation of observed seismic response strong and abrupt fault heterogeneity as envisioned in the inherently discrete category is required for the generation of complex slip patterns and a wide spectrum of event sizes strong fault heterogeneity also facilitates the generation of rough rupture fronts capable of radiating high frequency seismic waves the large earthquakes in both categories of models occur on a quasi periodic basis the degree of periodicity increases with event size and decreases with model complexity however in all discrete segmented cases the models generate nonrepeating sequences of earthquakes and the nature of the large quasi periodic events is highly variable the results indicate that expectations for regular sequences of earthquakes and or simple repetitive precursory slip patterns are unrealistic the frequency size fs statistics of the small failure episodes simulated by the cellular fault models are approximately self similar with b 1 2 and ba 1 where b and ba are b values based on magnitude and rupture area respectively for failure episodes larger than a critical size however the simulated statistics are strongly enhanced with respect to self similar distributions defined by the small events this is due to the fact that the stress concentrated at the edge of a rupture expanding in an elastic solid grows with the rupture size when the fault properties e g geometric irregularities are characterized by a narrow range of size scales the scaling of stress concentrations with the size of the failure zone creates a critical rupture area terminating the self similar earthquake statistics in such systems events reaching the critical size become on the average unstoppable and they continue to grow to a size limited by a characteristic model dimension when however the system is characterized by a broad spectrum of size scales the above phenomena are suppressed and the range of apparent self similar fs statistics is broad and characterized by average b and ba values of about 1 the simulations indicate that power law extrapolations of low magnitude seismicity will often underestimate the rate of occurrence of moderate and large earthquakes the models establish connections between features of fs statistics of earthquakes range of self similar regimes local maxima and structural properties of faults dominant size scales of heterogeneities dimensions of coherent brittle zones the results suggest that observed fs statistics can be used to obtain information on crustal thickness and fault zone structure

predictive modeling of the seismic cycle of the greater san francisco bay region the seismic cycle for the san francisco bay region is synthesized by a model combining the pre and post 1906 seismic histories the long term acceleration of seismic release seismic moment benioff strain release or event count in the seismic cycle and the shorter term accelerations preceding the larger earthquakes within that cycle are modeled using an empirical predictive technique called time to failure analysis in which rate of seismic release is proportional to an inverse power of the remaining time to failure the exponent of time to failure in the accelerating sequences appears to be scale invariant and the length of the full cycle is estimated at 269 50 years the 1989 loma prieta earthquake which is the culmination of the first subcycle in the present long term seismic cycle should have been predictable with an uncertainty of 2 years in time and 0 5 in magnitude although the specific location at loma prieta was not predictable by this technique if our model is correct and if the loma prieta earthquake is the culmination of a subcycle the san francisco bay region should be entering a relatively long 20 50 years period of seismic quiescence above magnitude 6 a great earthquake such as the 1906 san francisco event would appear to be more than a century in the future

active zone size versus activity a study of different seismicity patterns in the context of the prediction algorithm m8 a new precursory measure called active zone size azs based on the development of spatial correlations was recently introduced in the context of predicting large events in dynamical models of faults the new measure counts the fraction of the total seismic area which has exhibited earthquakes recently in each of the models tested azs was found to be more strongly correlated with an increase in the likelihood of a large event than the more conventional measure activity n which is defined to be the total number of earthquakes in this paper we report the first application of azs to seismicity catalogs we compare measurements of azs and n both individually and in the context of the intermediate term earthquake prediction algorithm m8 applied here specifically to data from the western united states to test the performance we replace functions in m8 which are based on n with similar functions based on azs when this is done at least on this limited data set the performance of ms is somewhat improved false alarm time is reduced and the landers earthquake is successfully predicted

age and tectonic implications of paleoproterozoic mafic dyke swarms for the origin of 2 2 ga enriched lithosphere beneath the ungava peninsula canada abstract mafic dyke swarms represent short lived magmatic events that carry important temporal and chemical constraints for the evolution of the lithospheric mantle five baddeleyite u pb isotopic analyses of paleoproterozoic mafic dyke swarms intruding the ungava peninsula yielded ages of 2508 6 2220 1 2212 3 2199 5 and 2149 3 ma these ages fall within the 2 5 2 0 ga period previously defined by five other swarms which together comprise a magmatic record of more than 500 ma the similar age composition and surface area of two 2 51 ga swarms along with the lack of coeval supracrustal rocks suggest they correspond to failed rifts an important magmatic hiatus of 280 ma follows but ends with the intrusion of numerous dykes between 2 23 and 2 20 ga in the thinnest lithosphere of the northern ungava peninsula these dykes exhibit chemical signatures that vary from enriched older 2229 2212 ma dykes nd t 4 to relatively depleted 2209 2199 ma voluminous younger dykes nd t 1 4 la yb ca 2 21 2 20 ga later dykes emplaced ca 2 17 ga within a rejuvenated paleoproterozoic lithosphere also have depleted chemical compositions and map the extent of rifting coeval with the emplacement of early basalts in the labrador trough dykes emplaced in the 2 15 2 00 ga period further to the south are proximal to coeval continental basalts with which they share enriched isotopic compositions nd t 1 5 to 7 and a positive correlation between la yb and zr nb la yb 4 10 zr nb 7 20 that suggest assimilation of an archean crustal component a progressive decrease in the nd t values of these magmas with decreasing age further implies increasing contamination until the archean lithosphere failed ca 2 0 ga and permitted the eruption of isotopically depleted basalts the ca 2 2 ga depleted northern dykes and the 2 1 1 9 ga povungnituk basalts of the cape smith foldbelt share a negative correlation between zr nb and la yb that is expected for the incorporation of an enriched lithospheric component this component is represented by the alkaline rocks comprised in the circum ungava supracrustal belts carbonatites lamprophyric lavas alkaline basalts and by the lac aigneau lamprophyric dykes 1941 3 ma intruding the ungava peninsula these alkaline magmas collectively have 2 2 2 1 ga depleted mantle nd model ages t dm similar to a 2 15 ga sm nd isochron defined by the basaltic rocks of the supracrustal belts although these nd t dm ages and isochron are 100 300 ma older than magmatic ages they are identical with the ca 2 2 ga mafic dykes of the ungava peninsula this time correlation along with the similar chemical signature of the northern dykes and younger povungnituk basalts suggest that the composition of younger lavas and alkaline magmas was influenced by an enriched component associated with the pervasive metasomatism of the lithosphere coeval with the emplacement of the many ca 2 2 ga dykes

numerical simulation of the earthquake generation process a numerical algorithm is proposed for the simulation of the earthquake process during a seismic cycle the algorithm is based on a heterogeneous discrete model of the fault plane and assumes there are two kinds of seismicity background crack like earthquakes and asperity like events an active zone of the fault contains an asperity distribution with a characteristic elementary area the background seismicity randomly develops shear stress free surfaces which tend to surround the asperities as in a 2d percolation process the model parameters are taken from observations on the vrancea romania intermediate depth seismic region the results emphasize the significant role of the geometry in the mechanism of the seismic failure the algorithm predicts the nonlinear behavior in the frequency magnitude distribution the decrease of theb slope associated with the asperity like events the magnitude range of major earthquakes and their recurrence times