precursory seismic quiescence before the 1994 kurile earthquake mw 8 3 revealed by three independent seismic catalogs we have found that the m w 8 3 kurile earthquake on october 4 1994 followed an outstanding seismic quiescence starting 5 6 years before the mainshock near the ruptured area we have analyzed three independent seismic catalogs institute of seismology and volcanology hokkaido university isv japan meteorological agency jma and international seismology center isc in spite of selecting different magnitude bands and time windows all three catalogs presented the common feature of the seismic quiescence this fact strongly suggests that the seismic quiescence should not be a man made change but actually occurred moreover we have confirmed that the seismic quiescence was the most significant and the earthquake was the largest in the past twenty five years in this region therefore we confidently interpret this seismic quiescence as an indication of a preparation process for the m w 8 3 kurile earthquake

precursory seismic quiescence a preliminary assessment of the hypothesis numerous cases of precursory seismic quiescence have been reported in recent years some investigators have interpreted these observations as evidence that seismic quiescence is a somewhat reliable precursor to moderate or large earthquakes however because failures of the pattern to predict earthquakes may not in general be reported and because numerous earthquakes are not preceded by quiescence the validity and reliability of the quiescence precursor have not been established

precursory quiescence before the august 1982 stone canyon san andreas fault earthquakes the stone canyon earthquake sequence started during august 1982 and lasted for about four months it contained four mainshocks withm l 4 each with an aftershock zone about 4 km long these mainshocks progressing from southeast to northwest ruptured a segment of the fault approximately 20 km long leaving two gaps which were later filled by them l 4 6 mainshocks of january 14 and may 31 1986 the equivalent magnitude of the sequence ism l 5 0 precursory seismic quiescence could be identified in 1 the northernmost 10 km of the aftershock zone which contained three of the mainshocks and 2 the southern gap in the aftershock zone the fault segment containing the first mainshock and its aftershocks did not show quiescence this pattern of precursory quiescence is very similar to two cases in hawaii where the rupture initiation points of the mainshocks m s 7 2 and 6 6 respectively were located in volumes of constant seismicity rate surrounded by volumes with pronounced precursory quiescence the precursory quiescence before the august 1982 stone canyon earthquakes lasted for 76 weeks amounted to a reduction in rate of about 60 and could be recognized without any false alarms that is the anomaly was unique within the 60 km study segment of the fault and in the years 1975 through august 1982 eighteen foreshocks occurred between july 27 and august 7 1982 we conclude that the august 1982 mainshocks could have been predicted based on seismic quiescence and foreshocks

temporal characteristics of some earthquake sequences in greece latoussakis j stavrakakis g drakopoulos j papanastassiou d and drakatos g 1991 temporal characteristics of some earthquake sequences in greece in m wyss editor earthquake prediction tectonophysics 193 299 310 temporal features of the aftershock activities following some moderate shallow earthquakes in greece have been studied by making use of the modified omori formula and akaike s information criterion aic although the main shocks occurred in different seismogenic regions in greece the aftershock activities show almost the same anomalous pattern in most cases the secondary aftershocks of the aftershock activity of the main event are preceded by seismic quiescence also in some cases the aftershock activity recovers to a normal level or increases beyond it prior to the occurrence of the large aftershock the observed temporal pattern may be useful in predicting a large aftershock which might be as large and destructive as the main shock if the aftershock activity is monitored immediately after the occurrence of the main shock

precursory seismic quiescence before the january 1982 hilea hawaii earthquakes abstract two earthquakes of m l 5 4 occurred within half an hour of each other within the hilea area of southern hawaii on 21 january 1982 the aftershock distribution suggests that together they ruptured an area of approximately 5 km radius and their joint equivalent magnitude was 5 6 the first motions indicate faulting on a near horizontal plane at 10 km depth with the crust slipping to the southeast the seismicity rate in the source area was studied using the earthquake catalog of the hawaiian volcano observatory this catalog contains some reporting rate changes that affect the count of smaller earthquakes strongly with a period of low reporting during 1974 to 1977 although we have used 2 5 as the minimum magnitude of homogeneous reporting after 1971 some of the artificial rate changes are still present in the data the catalog was declustered using reasenberg9s algorithm and a magnitude correction of 0 1 was applied to the data between 1974 and 1984 the seismicity rate for the period of november 1971 through 1985 was examined in four adjacent regions one of these contained the aftershocks the aftershock volume and the 3 km annulus around it showed a period of 46 weeks of low seismicity rate immediately before january 1982 during which the rate was decreased by 87 per cent the seismicity rate in the other three volumes was normal during this time we conclude that this low reporting rate was not likely due to artificial changes in the catalog the fact that the quiescence anomaly coincided in space with the 1982 aftershock volume and that its termination coincided with the 1982 main shocks suggests that the quiescence was a precursor comparing the seismicity rate within all possible 46 week windows to the background rate we found that the precursory rate decrease was more significant than any other rate decrease in all volumes studied except artificial low rate periods thus this quiescence precursor could be recognized without false alarms however the statistical significance estimated by the z and tests was low 75 per cent and 49 per cent respectively more case histories are needed to determine empirically the thresholds of these tests for accepting precursory anomalies without too many false alarms the m 6 6 kaoiki earthquake that was located about 25 km north of the 1982 source area was preceded by 125 weeks of quiescence while the m 7 2 kalapana earthquake quiescence precursor lasted about 200 weeks these observations suggest that in hawaii quiescence precursor times may be a function of magnitude

geophysical and geochemical evidence for subsurface hydrocarbon leakage in the bering sea alaska abstract geophysical and geochemical surveys were conducted on the bering sea shelf to investigate whether hydrocarbons found in the surficial sediment particularly in association with certain geophysical events indicate the presence and nature of more deeply reservoired oil and gas deposits surface sediment samples were obtained at 1000 sites where high resolution seismic data indicated the presence of gas seepage and or surface faulting in addition sites for background measurements were occupied away from areas of gas seepage results of the geophysical survey show seismic wipeout zones to be present throughout the bering sea shelf sediment samples obtained within the shallow wipeout zones confirm that these features are caused by gas in addition compositional and isotopic analyses indicate that the gas is of biogenic and thermogenic origins the thermogenic leakage appears to be related to localized faulting and shallow structuring the high resolution seismic data therefore provide an excellent tool for locating leakage points thereby enhancing the opportunity of finding migrated thermogenic hydrocarbons in surficial sediments an examination of vertical variability shows the hydrocarbon concentrations to be strongly related to various processes occurring in the surficial sediments core samples must be taken from below the zone of maximum disturbance to obtain unaltered hydrocarbon samples

p wave travel times stability and change within the source volume of a m 7 2 earthquake the dense seismograph network on the south flank of kilauea afforded a unique opportunity to study travel time changes as a function of space and time within the source volume of the hawaii earthquake ms 7 2 of 1975 careful analysis of more than 600 teleseismic p arrivals from deep fiji tonga earthquakes revealed the following out of eight stations studied six showed constant travel times to within 0 03 s for up to 10 years with the exception of some small changes of about 0 03 s at the time of the main shock one station ahu showed a highly significant and unique travel time decrease by 0 13 s during 1971 1972 at the beginning of this velocity anomaly a southwest rift intrusion caused closure of surface cracks associated with normal faults located near the station ahu also geodetic measurements revealed that between august and october 1971 compressive strain of 4 10 5 was accumulated perpendicular to the southwest rift in the area of ahu we conclude that these data show for the first time that in situ velocity increases occur due to the closure of cracks by tectonic forces the ahu residuals returned to normal approximately at the time of a major earthquake swarm on the fault zone near ahu during which surface cracks were observed to have opened again this travel time decrease was corroborated by residuals from novaya zemlya explosions but highfrequency local signals did not show it assuming that the stress sensitivity of the velocity was similar to that found by reasenberg and aki 1974 we have interpreted the teleseismic residual change at ahu as due to a p velocity increase of about 10 in the top 3 5 1 5 km of the crust the only station wha which showed a large 0 2 s and extended 1972 to 1975 travel time increase was located only 4 km from the main shock epicenter we interpret this velocity decrease as a precursor to the 1975 main shock and we hypothesize that a process reverse from that at ahu caused this anomaly by first opening and then closing cracks in the crust below wha both of these velocity anomalies were extremely localized in crustal volumes of dimensions of about 5 3 km

precursors to the kalapana m 7 2 earthquake the kalapana hawaii earthquake of november 1975 had a rupture length of 40 to 50 km and was located on the south flank of the active volcano kilauea the source mechanism was dip slip normal faulting on a plane dipping 20 degrees to the se with the greatest principal stress oriented in that direction and accumulated by volcanic intrusions into kilauea s rifts the source area of the 1975 earthquake was subject to intensive geological and geophysical research for many years before this earthquake because of its proximity to the volcano we studied the distribution of epicenters for small earthquakes from 1962 to 1975 seven seismographs located within 3 km of the aftershock area had been in operation for 3 to 10 years before the mainshock and geodetic triangulations and trilaterations in the source area had been carried out repeatedly since 1914 we found that precursory changes occurred throughout most of the rupture area but in two distinctly different patterns in the larger outer anomalous area the seismicity rate was decreased by 50 during the 3 8 years before the mainshock in addition several geodetic lines indicated anomalous strain release during this time within two inner areas the seismicity remained high then increased shortly before the mainshock in one of the inner areas a p wave travel time delay of 0 2 s could be detected which began about 3 5 years before the mainshock within the other inner anomalous area geodetic strain was accumulating until the first half of 1975 when 3 5 10 4 strain 35 bars was released aseismically by contrast the outer anomalous volume was experiencing strain softening from 1970 1971 on we interpret these observations as indicating that strain softening by fault creep in the outer anomalous area transferred stress into two major asperities locked portions of the fault a velocity decrease and foreshocks were observed in one asperity and high stress accumulation in the other implying that dilatancy of the crust probably occurred our model is qualitatively supported by the independent evidence of strong motion records which show that the kalapana earthquake was a complex multiple rupture we conclude that the kalapana earthquake was preceded by a preparatory process which lasted 3 8 0 3 years and which had dimensions of 45 10 km covering approximately the aftershock area

reporting history of the central aleutians seismograph network and the quiescence preceding the 1986 andreanof island earthquake the earthquake catalog of the central aleutian seismic network casn was searched systematically for significant rate changes in the period from 1976 through 1985 the magnitude signatures for the nine changes were modeled by synthetic signatures by this method six of the rate changes could be identified as artificial based on the criteria that the rate change was confined to small magnitude events and that magnitude shifts were present for each of these changes an artificial cause existed that affected the network within one to two months of the time of change as estimated from the catalog analysis one of these cases was a rate decrease by a factor of 2 associated with a storm that had put out of operation two thirds of the network another change coincided with the introduction of digital data processing the four remaining cases were artificial reporting rate increases due to installation of several new stations or to repair of several stations that had failed the three rate changes that could not be identified unambiguously as artificial or natural on the basis of magnitude signatures were all rate decreases by factors of approximately 2 two of these were clearly due to disasters that had put out of operation more than half of the network the third was the proposed precursory quiescence to the 1986 andreanof island m 7 9 earthquake from this we conclude that without knowledge of the network history many of the artificial reporting rate changes in earthquake catalogs can be identified as artificial and that their onset time can be estimated to within one to two months an earthquake count independent from the adak catalog was obtained from the helicorder records of the station adak for the years 1978 through 1985 for the vicinity of adak island of a total of 1831 events 335 were not in the adak catalog and 981 were located within the 1986 aftershock area the helicorder earthquake count showed a pronounced decrease by about 50 per cent similar in amount to that in the adak catalog however the decrease started in 1980 at a time when in the central subregion covered by the adak catalog only m 2 9 events decreased while the smaller magnitude bands show a maximum decrease in 1982 we conclude that the possibility could not be ruled out that electronics changes contributed to the reporting rate decrease starting in the years 1980 to 1982 however it seems very unlikely that a network that was at its healthiest during the precursor time would be reduced by an improvement in the electronics to a detection level approximately equal to that in two short periods when only one third of the stations were operating therefore the seismic quiescence reported and interpreted as a precursor by kisslinger et al 1985 was most likely a natural phenomenon

multiple asperity model for earthquake prediction large earthquakes often occur as multiple ruptures reflecting strong variations of stress level along faults dense instrument networks with which the volcano kilauea is monitored provided detailed data on changes of seismic velocity strain accumulation and earthquake occurrence rate before the 1975 hawaii 7 2 mag earthquake during the 4 yr of preparation time the mainshock source volume had separated into crustal volumes of high stress levels embedded in a larger low stress volume showing respectively high and low stress precursory anomalies

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

probabilities of activation of seismic faults in critical regions of the aegean area summary properties of the accelerating seismic crustal deformation pattern and of the intermediateterm seismic quiescence pattern have been combined to identify faults in the aegean area 34 n 43 n 19 e 30 e that have a considerable probability of generating strong earthquakes m 6 4 during the next 5 yr period eight groups of such faults have been identified and the probability of each fault being activated during this time period has been estimated three of these groups are located in southern greece hellenic arc two in central greece east central greece ionian islands two in northwestern turkey and one in the area of the borders of albania greece and the former yugoslav republic of macedonia the importance of these results for intermediate term seismic hazard assessment is discussed

seismic quiescence a test of the hypothesis and a precursor to the next parkfield california earthquake summary the parkfield segment of the san andreas fault shows seismic quiescence for m 2 0 earthquakes since early to mid 1986 the rate decreases by 45 to 70 per cent depending on the magnitude band we interpret this change as real because it is concentrated in the larger magnitudes and cannot be explained by a reasonable magnitude shift the rate decrease is present in the parkfield segment of the fault between 35 6 and 36 1 n and no other rate change as significant has been observed during the time when high quality data were available since 1975 we interpret this observation as a seismic quiescence precursor to the next parkfield mainshock although information on the expected duration of precursory quiescence is weak we estimate that the parkfield earthquake should occur in the interval 1990 february to 1992 february precursory seismic quiescence which lasted approximately 1 5 yr was found in and possibly around the source volume of an earthquake withm 3 6 which occurred on 1986 august 29 in the parkfield area this observation demonstrates that the quiescence hypothesis is applicable to the parkfield segment of the san andreas fault the false alarm rate was estimated in a detailed analysis as a function of alarm level for the fault segments surrounding the m 5 0 stone canyon 1982 august and m 3 6 parkfield 1986 august mainshocks for these two events the duration and statistical significance of false alarms as a function of space time and magnitude band was determined we propose that in this tectonic environment and with the data characteristics of these earthquake catalogues attempts to predict mainshocks based on the quiescence hypothesis may be successful and generate one to two false alarms covering about 10 per cent of the time space dimensions of the catalogue

spatial changes in volcanic and seismic activity prior to great earthquakes spatial relationship between volcanism and seismicity prior to the occurrence of several great interplate earthquakes in the circum pacific area has been examined in order to understand the process of underthrusting in detail the locations of the epicenters of the great earthquakes have been examined in terms of locations of concentrated volcanic and seismic activity in a short time period in order to determine if there is any relationship between these activities and whether such relationships can be used to refine the underthrusting model the study shows that in most cases 1 there is no volcanic activity in the vicinity of the epicenter of the great earthquake and 2 maximum volcanic activity is localized i e volcanoes adjacent to one another exhibit considerable activity in a short time period very little systematic spatial relationship between these three parameters is observed although in most cases there is no volcanic activity at the time of maximum earthquake activity locations of active volcanoes and earthquake activity during the five years prior to the occurrence of the great earthquake do not appear to be a guide to the epicenter of the great earthquake this study therefore suggests that although there is temporal relationship between the occurrence of maximum volcanic and seismic activity and the occurrence of great earthquake there appears to be no systematic spatial relationship between these three parameters

regular intervals between hawaiian earthquakes implications for predicting the next event during the years 1941 through 1983 five earthquake mainshocks of moderate magnitude occurred at regular intervals of 10 5 1 5 years within a 6 kilometer radius in hawaii it is proposed that these kaoiki earthquakes will continue to occur at regular intervals because the strain accumulation rate and the strained volume remain constant with appropriate instrumentation it may be possible to refine predictions of subsequent kaoiki earthquakes

classification of earthquake precursors from a fracture mechanics model abstract a new classification of earthquake precursors is proposed based on a fracture mechanics approach which includes the effects of stress and the acceleration of a fracture through an asperity two phases of earthquake warning associated with intermediate term and short term anomalies can be related in a simple manner to two peaks in the stress intensity the first broad peak is further split into two halves associated with precursory strain release strain hardening followed by precursory stress drop strain softening and the second peak responds to the transition from quasi static fracture and slip to dynamic rupture the model predicts a seismic b value anomaly which is in good agreement with laboratory experiment and some aspects of empirical observation of earthquakes in particular evidence is presented of the distinctive short term b value anomaly predicted by the second stress intensity peak and a critical foreshock related b value of 0 5 other earthquake precursors can also be classified into this unifying model and it is suggested that those anomalies which respond to the cumulative production of microcracks e g coda q 1 could produce a more distinctive two stage anomaly after differentiation with respect to time such a two stage anomaly especially if it were observed in several independent earthquake precursors could be a very useful method of deciding when a low level earthquake warning should be changed to a higher level warning

comparison of orientations of stress and strain tensors based on fault plane solutions in kaoiki hawaii the stress tensor orientation was estimated based on inversion from 238 first motion fault plane solutions of earthquakes with mostly m 3 5 0 6 located in the 10 km radius kaoiki crustal volume separate inversions for subvolumes containing 20 50 events yielded the same results in several adjacent volumes suggesting that the stress tensor is homogeneous in those parts of the kaoiki area and that the inversion results are stable and meaningful five spatial subsets of the data were found for which the orientation of at least one of the principal axes was different from that in the other sets by 20 80 and at confidence levels exceeding 95 the volcano summits of kilauea and mauna loa and their rift systems are identified as the source of stress in the kaoiki crust because the greatest principal stress points to kilauea and mauna loa in addition the strain tensor due to energy released by these 238 earthquakes was computed for the kaoiki area and several subvolumes of it by summing the moment tensors the moment tensor of each earthquake was constructed from the individual fault plane solutions and from an estimate of the scalar moment derived from the moment magnitude relationship a comparison of the directions of strain and stress tensors showed close agreement for subvolumes with predominantly strike slip faulting in these volumes the inversion process for stress directions led to misfits of approximately equal size for the conjugate near vertical nodal planes these observations are interpreted to show that in the strike slip regime of the upper part of the crust neither of the nodal planes is preferred for faulting rupture probably occurs along the nw and along the nh striking nodal planes in separate earthquakes subvolumes with more decollement faulting showed significant differences of 30 40 between the principal strain and stress directions in these volumes the near horizontal nodal planes showed noticeably smaller misfits in the inversion for the stress directions these facts are interpreted to indicate that the decollement plane is weak allowing slip on it even if the principal stresses are inclined at a large angle to it it is proposed that comparison of strain and stress tensor calculations may be able to differentiate between tectonic regimes uniform in strength no well developed fault plane and regimes in which a fault with low frictional strength dominates as a function of time significant rotations of the strain tensor by approximately 45 can be observed which seem to be related to the occurrences of kaoiki mainshocks during three background periods of about 7 years each the average strain tensor showed an orientation typical for predominant decollement faulting while two premainshock periods of 2 5 years each showed an orientation closer to strike slip faulting it is proposed that this pattern may be repeated before the next kaoiki mainshock the strain released seismically is less than the geodetically observed strain by approximately an order of magnitude stress strain fault plane solutions hawaii

some features of aftershock patterns in greece summary a quantitative study of 12 aftershock sequences which occurred in greece and the surrounding regions from 1971 to 1980 has been made the aim of the study was to investigate the temporal pattern of each aftershock sequence and to examine the possibility of predicting the largest aftershock the magnitudes of the main shocks were ml 5 0 the minimum and maximum numbers of aftershocks were 25 and 191 respectively the temporal analysis was performed using the modified omori formula and akaike information criterion first we examine the omori law to see if holds for the whole aftershock sequence second we examine the same data set in order to find out if there is any possibility of distinguishing two aftershock sequences as follows 1 the first must include all the shocks since the occurrence of the main event until the occurrence of the largest aftershock and 2 the second must include all events following the occurrence of the largest aftershock until the end of the aftershock activity in 10 cases out of 12 the model of two different aftershock sequences provides the best fit to the data the aftershock activity prior to the large aftershocks was then examined although the main shocks occurred in different seismogenic regions an almost identical behaviour of aftershock activity was observed the activity shows an appreciable decrease from the level expected from the modified omori formula also in some cases the aftershock activity recovers to a normal level or increases beyond it prior to the occurrence of the main aftershock these results seem to be independent of the magnitude of the large aftershock if the aftershock activity is monitored in real time this observed temporal pattern may be useful in predicting a large aftershock

seismicity before and after large shallow earthquakes around the japanese islands abstract by establishing the space time distribution of certain features of seismicity before and after large shallow earthquakes occurring mainly along the deep sea trenches around the japanese islands these earthquakes can be classified into three types a b and c in type a seismic activity in the focal region becomes very low prior to a large earthquake creating a seismic gap of the second kind in type b seismic activity gradually increases in and around the focal region before a large earthquake and in type c seismic activity increases steadily without a predominant principal earthquake ever occurring i e a swarm pattern occurs observation of the locations where these three types of earthquakes occur indicates a close relationship with submarine topography type a earthquakes occur where the submarine topography is quite simple type b earthquakes where it is complex such as in a subduction zone disturbed by oceanic ridges or active tectonic lines and type c where seamounts subduct beneath the continental plate at the deep sea trench creating a very complex submarine topography where the submarine topography in the focal region is simple and flat the earthquake fault surface may be relatively homogeneous whereas the submarine topography is more complex the fault itself is also more complex thus it is deduced that the differences in seismicity patterns between these three types is due to the irregularity of their faults and a hypothesis to explain the mechanism of these three patterns is proposed

precursory seismic quiescence in the mudurnu valley north anatolian fault zone turkey the seismicity rate in the mudurnu valley of turkey was studied using an earthquake catalogue that reports events homogeneously down to magnitude 2 3 for the years 1985 1989 and covers the area between latitudes 40 2 and 41 0 n and longitudes 30 0 and 31 5 e during this period the only two main shocks m 4 0 and m 4 3 occurred on 1988 september 6 and 1988 december 9 within about 30km of each other a highly significant seismic quiescence is evident in the area surrounding these main shocks while the seismicity rate in the rest of the area covered by the catalogue remains constant the quiescence becomes more pronounced the smaller the area around the main shocks that is studied the smallest areas that can be studied contain about 60 earthquakes and have dimensions of approximately 25km on each side the decreases in seismicity rates are 50 80 per cent depending on the volume and period used for defining the quiescence the quiescence started in 1988 january and lasted about seven months with approximately 4 5 months of normal activity separating it from the main shock of december the precursor time of 12 months for an m 4 3 main shock is similar to those observed in california it is concluded that it is possible to resolve precursory quiescence before moderate and large earthquakes in the mudurnu area with the existing seismograph network

seismic quiescence at parkfield an independent indication of an imminent earthquake the parkfield segment of the san andreas fault midway between san francisco and los angeles has experienced six moderate sized earthquakes since 1850 with an average recurrence time of about 22 years based on this regular behaviour and the occurrence of the most recent earthquake in 1966 a shock of local magnitude ml 5 7 is expected to occur by 19931 2 here we suggest that a current lack of small earthquakes on this segment could be a precursory anomaly to the next characteristic parkfield earthquake only four earthquakes of m 2 5 have occurred on the segment since january 1986 whereas twenty would have been expected extrapolating from the mean background rate comparison with other cases of precursory quiescence suggests that the m 5 7 earthquake should occur in the next two years rupturing the same 35 km fault segment that ruptured in 19663

distribution of attenuation in the kaoiki hawaii source volume estimated by inversion of p wave spectra a new method to simultaneously invert for q structure and source parameters was used on a set of 635 microearthquakes 0 9 m 2 0 in the kaoiki area of southern hawaii approximately 2800 signals were analyzed which had been recorded by 6 short period vertical seismographs at epicentral distances of a few to 10 km the hypocentral depths ranged from o to 14 km with the bulk of the sources in the 7 5 10 5 km range the hypothesis to be tested was that the source volume of the m 6 6 kaoiki main shock of november 16 1983 may be heterogeneous in attenuation distribution we assumed that the observed p wave displacement spectra could be modelled by a source spectrum with an 2 high frequency decay a single layer resonance filler to account for local site resonances and whole path attenuation along the ray path in a next step the attenuation factor q was constrained by tomographically reconstructing the three dimensional q structure for the source region and using it as starting model for a nonlinear inversion of the corner frequency the seismic moment m0 and a new q value this process was iterated until the results changed less than 0 1 and were accepted as final the average q was approximately constant and very low 105 q 115 between o and 7 km depth it increased to approximately 160 between 9 and 11 km in the depth ranges were the spatial coverage was good 3 9 km the q values in the nw part of the 1983 aftershock volume were larger by approximately 10 15 compared to those in the se part this 12 contrast correlates with other evidence for heterogeneity the nw region shows high coda q no surface faulting medium seismicity rate and medium precursory quiescence while the se region shows low coda q pervasive faulting high seismicity rate strong precursory quiescence and possibly low b values we conclude that the kaoiki source volume is strongly heterogeneous with the contrasting density of cracks controlling the difference in crustal parameters the new simultaneous inversion technique is well suited to derive detailed q structure and source parameter information from microeaithquake spectra recorded at close epicentral distances

a reinterpretation of the precursory seismic b value anomaly from fracture mechanics summary a new model is presented which can explain the major temporal fluctuations in seismic b value in terms of the underlying physical processes of time varying applied stress and crack growth under conditions of constant strain rate the model predicts two minima in the b value separated by a temporary maximum of inflexion point a corollary being that a single broad maximum would be expected in scattering attenuation these fluctuations in b value are consistent with reported intermediate term and short term earthquake precursors separated by a period of seismic quiescence we present preliminary results from controlled laboratory experiments and report recent field observations both of which are consistent with the predicted form and in particular exhibit the distinctive second short term anomaly which reaches a value bc 0 5 during the critical phase

anomalous seismicity of the knin area yugoslavia prior to the m 5 5 earthquake of 1986 abstract a case of seismicity changes prior to the m 5 5 knin yugoslavia earthquake has been analysed in addition to the well expressed seismic quiescence significant b value strain release and seismicity rate temporal changes have been observed the estimated precursor time of t 6 8 yrs is longer than expected on the basis of published log t m relationships a much better agreement is obtained if log t is compared to the longest dimension of the aftershock

comparison of a complex rupture model with the precursor asperities of the 1975 hawaii m s 7 2 earthquake a simplified multiple source model was constructed for the 1975 hawaiims 7 2 earthquake by matching synthetic signals with three component accelerograms at two stations located approximately 45 km from the epicenter six major subevents were identified and located approximately the signals of these are larger by factors of 1 4 to 3 2 than that of theml 5 9 foreshock which occurred 70 minutes before the main rupture and also triggered the sam 1 recorders at the two stations dividing the rupture length 40 km by the duration of strong ground shaking 50 sec an average rupture velocity of 0 8 km sec about 25 of s velocity is obtained thus it is likely that the rupture stopped between subevents the approximate epicenters of the 6 major subevents and of the foreshock support the hypothesis that they were located in high stress asperities which rupture during the main shock except for the last events which is interpreted as a stopping phase generated at a barrier these asperities have been previously defined on the basis of differences in the precursor pattern before the mainshock thus it appears that both the details of the precursors and of the main rupture depended critically on the heterogeneous tress distribution in the source volume this suggests that main rupture initiation points and locations of high rupture accelerations may be identified before the mainshock occurs based on precursor anomaly patterns a satisfactory match of synthetic signals with the observations could be obtained only if the aximuth of the fault plane of subevents was rotated from n60 e to n90 e and back to n30 e these orientations are approximately parallel to the nearest kilauea rift segments hence the slip directions and greatest principal stresses were oriented perpendicular to the rifts everywhere from this analysis and other work it is concluded that this fault surface consisted of three types of segments with different strength hard asperities radius 5 km soft but brittle segments between the asperities radius 5 km and a viscous half 10 40 km which slipped during the mainshock but where microearthquakes and aftershocks are not common

formation and tectonic evolution of southeastern china and taiwan isotopic and geochemical constraints abstract the southern part of china consists of the proterozoic yangtze craton and the phanerozoic south china foldbelts including the himalayan foldbelt of taiwan models for continental growth have been many and controversial isotopic and geochemical data from mesozoic and younger granitoids and sediments are used here to place constraints on the tectonic evolution of southeastern china and to evaluate whether the young phanerozoic foldbelts are representative of old rejuvenated landmass whether they characterize crustal accretion through successive subduction processes hence suggesting a net growth of continental mass or whether they represent some intermediate situation available sm nd isotopic data for phanerozoic granitoids and metasediments from the south china foldbelts and taiwan invariably show proterozoic model ages t dm ranging from 1 to 2 5 ga with a mean of 1 54 0 30 1 ga all rocks have negative nd t values 2 to 15 suggesting variable but important contributions of old continental materials to the sources of the phanerozoic rocks obviously they do not represent wholesale growth of continental mass the granitoids of the south china foldbelts have multiple origins as viewed from their chemical and isotopic characteristics secular geochemical variation has been established for inland granitoids of proterozoic to mesozoic age their high i sr values 0 710 0 737 suggest that most granitoids are essentially anatectic products of ancient continental crust this is compatible with remelting via microcontinental collision or terrane accretion repeated intracrustal reprocessing by partial fusion and differentiation has undoutedly contributed to important mineralization mesozoic granitoids excluding a type alkaline granites in the coastal region and taiwan show relatively low i sr 0 705 0 710 and high nd values implying that greater amounts of mantle components have been added to ancient continental material in the generation of these rocks this means that mesozoic magmatism in the coastal region resulted from subduction of a paleo pacific plate kula beneath the asian continent the overall geochemical evidence seems to be consistent with a model of serial collisions between microcontinental blocks for the bulk of the south china foldbelts the latest arc continent collision began about 4 ma ago in eastern taiwan

spatial seismicity variations along convergent plate boundaries summary five major convergent plate boundaries south america izu bonin marianas new hebrides tonga kermadec and indonesia show strong variations in levels of background seismicity on scales ranging from tens to thousands of kilometres these variations were tested using two statistical approaches and we conclude that small earthquakes are not distributed randomly along these zones two types of large scale seismicity variations termed first order were recognized first regions with dimensions on the order of 100 km with extremely high seismicity levels first order actives one such region was recognized in each of the zones studied second large scale 500 to several thousand kilometres differences in the level of background seismicity along a given plate boundary regions with consistent levels of background seismicity are termed first order segments we examined each first order segment for smaller scale variations ten regions ranging in length from 40 to 170 km with anomalously high seismicity levels were recognized fifty three regions ranging in length from 25 to 355 km were found to have anomalously low seismicity levels thus areas with anomalously high levels of activity are rare in subduction zones these observations indicate that background seismicity in subduction zones is not randomly distributed along the strike of the zones it seems likely that the observed variations reflect tectonic differences in fact many of the seismicity variations which we observed appear to be spatially related to features on the seafloor or on the overriding plate if this is so then they may provide a powerful tool for characterizing subduction zones and understanding the mechanisms of earthquake generation

seismic activity changes progressing simultaneously with slow slip in the tokai area abstract anomalous movements were detected simultaneously in both the seismic and the gps observations in the tokai area the central part of the japanese islands from the late 1990s to 2000 the anomalies are of great concern since the pending risk of a large megathrust earthquake in this area has been predicted for more than 20 years the gps data revealed that a slow slip on the plate interface had commenced beneath lake hamana the center of which is positioned around the edge of the assumed focal zone on the other hand the seismic data indicated that a delicate but clear quiescence appeared over a wide area that spreads into the main focal zone analyses of the seismicity changes in space and time confirmed that the contrast in the seismicity rate is distinct inside the focal zone while the integrated seismicity indicated quiescence some locations were distinguished as activated zones possibly indicating the appearance of asperities the rise of the seismicity rate in a quasi stationary manner suggests an increase in the stress rate at that location the following hypothesis is proposed based on the simultaneously detected evidences the slow slip progressing beneath lake hamana will induce a stress shift that invades the interior of the main locked zone which will increase the contrast of the seismicity rate possibly reflecting inhomogeneity in the locking strength even in this stage the activated zones still maintain a locked state to prevent overall breakage investigations of the b value changes and of tidal dependence in seismicity that reveal the stress concentrated state also support the hypothesis if this is the case the observed change in seismicity would indicate the process of stress redistribution in the locking state which represents the preparatory process toward final breakage tracking such seismicity changes would yield valid information for predictions of the next tokai earthquake

seismic quiescence and asperities in the tonga kermadec arc highly significant temporal and spatial seismicity rate changes were found along the tongaskermadec plate boundary the standard deviate z test was used to compare systematically the rates in different periods and different volumes and to test for uniqueness of anomalies found the noaa hypocenter data file showed a pronounced decrease in the number of small magnitude earthquakes reported after 1969 at the same time as such a reporting decline exists in the worldwide data therefore only earthquakes with mb 4 9 were studied a search for quiescence before recent mainshocks yielded two uniquely significant precursory anomalies one missed mainshock and one false alarm quiescence started 63 months before the january 1976 m 8 0 kermadec and 25 months before the june 1977 tonga m 7 2 earthquakes the december 1975 m 7 5 shock was not preceded by a seismicity rate change the seismic gaps in the tonga kermadec arc with sufficient seismicity for analysis 17 22 and 31 34 show constant rates up to the present therefore on the basis of the quiescence hypothesis earthquakes are not forecast for these segments the stress drops of 380 interface events were estimated using the mb ms method the average was uniform along the island arc with the exception of two segments with average of less than half the normal values these segments coincided with the two largest recent interface ruptures however in the northern tonga the was low before the 1975 rupture whereas in the kermadec the 1976 aftershocks showed low asperities could not be identified on the basis of high stress drops a systematic study of the seismicity rate as a function of space defined large variations along the arc the areas of highest and lowest seismicity rate correlate with the northern end of the trench and with the louisville ridge intersection respectively however other large rate contrasts exists between arc segments without obvious tectonic differences this implies that seismicity doughnut patterns do not reliably define precursors one segment of outstandingly high seismicity rate 30 31 s is identified as a major asperity because it contains a large number of m 6 events and a large number of recent mainshock aftershock sequences and because it stopped the great 1976 rupture we propose that the only other segment of high seismicity rate near 20 5 s also represents a major asperity

seismic quiescence before the m 7 1988 spitak earthquake armenia a detailed analysis of the 35 yr of seismicity between 1962 and 1997 using a gridding technique shows that the m7 spitak earthquake of 1988 december 7 was preceded by a quiescence anomaly that started at approximately 1984 0 5 and lasted about 5 0 5 yr up to the main shock this quiescence anomaly had a radius of about 20 3 km estimated from circular areas with 75 per cent rate decrease centred at the point of maximum significance of the anomaly the quiescence was clearly present in the aftershock volume during the 5 yr before the 1988 main shock but its statistically strongest expression was located 30 km nw of the epicentre this anomaly fulfills the association rules between precursory quiescence anomalies and main shocks even for a tight definition and is therefore proposed as a case of precursory quiescence the largest value of the standard deviate z found by random selection of samples by gridding was z 14 for a time window of tw 3 yr using a sample size of n 300 events this makes this anomaly the strongest observed so far and it is the first documented in an environment of continental collision there are no false alarms exceeding in significance the precursor the armenian earthquake catalogue used for this study had 4600 earthquakes with m mmin 2 2 in the area bounded by 39 5 to 42 n 42 5 to 47 e from the point of view of homogeneous reporting this is the best catalogue we have analysed so far the limits of the data used and the density of the grid are dictated by the data and have no influence on the results the choice of free parameters does not influence the results significantly within the following limits 100 n 500 2 tw 7 2 2 mmin 2 8

precursory seismic quiescence before the mw 8 3 tokachi oki japan earthquake on 26 september 2003 revealed by a re examined earthquake catalog 1 the 2003 tokachi oki earthquake mw 8 3 occurred on 26 september 2003 off the pacific coast of hokkaido japan in the present study an earthquake catalog is used that lists 2 000 earthquakes with m 3 3 all of the earthquake waveforms were recorded by the institute of seismology and volcanology hokkaido university in the present study these waveforms are manually re examined and hypocenters and magnitudes are re calculated a detailed analysis of the re determined earthquake catalog between 1994 and 2003 using a gridding technique zmap shows that the 2003 tokachi oki earthquake is preceded by two neighboring seismic quiescence anomalies that start around the beginning of 1999 and last about 5 years until the main shock occurs these quiescence anomalies are located around the asperity ruptured by the main shock and the z values are 3 9 and 4 0 for a time window of tw 4 years using a sample size of n 100 earthquakes the detected seismic quiescences can be interpreted as being caused by a decrease of 50 in the stressing rate based on dieterich s theory it is proposed that a quasi static pre slip occurs at the northeastern edge of the asperity ruptured by the main shock and lasts for five years until the main shock occurs by calculating the change in the coulomb failure stress cfs it is found that negative cfs areas are consistent with the two quiescence anomalies and a positive cfs area corresponds to the hypocenter of the main shock indicating that the quasi static pre slip model is a plausible one

phenomenon of local seismic reversal before strong earthquakes abstract we explore here the following hypothetical phenomenon the territorial distribution of low magnitude seismicity is reversed within several months prior to a strong earthquake in the area surrounding its source the diameter of this area is on the order of 100 km this phenomenon is investigated for the lesser antilles and southern california on that basis we suggest a hypothetical earthquake prediction algorithm to be tested by advance prediction retrospective application of the algorithm and the first three years of advance prediction in lesser antilles give encouraging results in comparison with well known intermediate term prediction algorithms m8 cn and mendocino scenario the sr algorithm gives smaller time space areas of the alarm it has a serious disadvantage however the catalogue should be complete for a longer period of time and for smaller magnitudes than are required in cn and m8

relative seismic quiescence before large aftershocks in terms of the modified omori s formula and akaike s information criterion thirteen aftershock sequences have been analysed quantitatively to check if any anomalous change of seismic activity took place before the occurrence of the largest aftershock the analysis follows the method proposed by the epicentre distribution of the main shocks covers the aegean sea greece area and the surrounding regions the earthquakes occurred from 1988 to 1998 although difficulties existed for a quantitative treatment of seismic quiescence the results of the temporal analysis of the aftershock activities seem to be promising in eight of thirteen cases the observed temporal pattern is similar the aftershock activity decreases significantly before the occurrence of the largest aftershock with respect to the level expected according to the modified omori s formula thereafter the activity recovers to its previous level just before or immediately after the occurrence of the main aftershock moreover an estimation of the reliability of the method is made based on the results of this study as well as of previous similar investigations from a total of 41 aftershock sequences which have been analysed following matsu ura s method 29 of them 71 show a remarkable relative quiescence before the occurrence of the main shock although it is difficult to construct a model which explains the precursory pattern the real time monitoring and check for seismic quiescence could at least contribute to a qualitative prediction of the largest aftershock which could at times be as large and disastrous as the main shock

two current seismic quiescences within 40 km of tokyo summary two highly significant seismic quiescence anomalies that started in 1993 0 exist currently within about 40 km of tokyo they are located in central saitama province at 35 99 n 139 56 e and in tokyo bay south of funabashi at 35 59 n l39 94 e the radii of the anomalous volumes are approximately 15 km centred at about 25 and 20 km depth respectively the seismicity rate during the quiescence was less than 10 per cent background rate in both cases the significance of these quiescences measured by the standard deviate z test comparing the rate of the last three years to the backgound rate is higher than for any other rate changes present in the catalogue zmax 7 5 and 7 2 respectively however episodes of highly significant quiescences which could have been interpreted as precursors but were not followed by main shocks also exist in the data set a third seismic quiescence was found to be in progress since 1993 3 0 5 in nw saitama province at 36 18 n 139 22 e it has a radius of 14 km and is less significant than the other two anomalies zmax 6 8 in a polygon that covers the areas of all three quiescences defined above the seismicity rate since 1994 is only 30 per cent of the previous background rate i e 100 earthquakes that would normally have occurred did not this analysis is based on the earthquake catalogue of the national institute for earth science and disaster prevention for the years 1980 1996 2 and mmin 1 0 as well as for the years 1986 2 1996 2 with mmin 0 1 the anomalies are equally clear in the clustered and in the full catalogues to avoid contamination by explosions only nighttime data 19 00 to 7 00 hours local time were used the frequency magnitude distributions in the quiescent volumes at central saitama nw saitama and funabashi have b values of 1 3 1 5 and 1 1 respectively based on the hypothesis that crustal volumes with high b values may not be capable of m6 earthquakes these three quiescent volumes may be ruled out as source volumes for main shocks of these magnitudes using lilt seismic quiescence hypothesis alone it is a reasonable interpretation to suggest that m 6 5 0 5 earthquakes are expected to terminate each of the three quiescences within 1 year 1 year from now with a 50 per cent probability each the interpretation that all three quiet volumes could rupture together in a large earthquake seems less likely however based on the information that the quiescences are located in volumes with b values above the average b 0 8 it can be argued that the three current quiescences are false alarms the fact that these two lines of evidence allow opposing interpretations in these cases shows that neither hypothesis is sufficiently tested thus the quiescence hypothesis is not advanced enough for issuing earthquake predictions nevertheless it is important that scientific tests of prediction scenarios are formulated in real time so we can learn how to modify our hypotheses to make them useful for prediction in the future

background seismicity rates and precursory seismic quiescence imperial valley california the problem of recognizing precursory seismicity rate changes consists of two steps first a background or normal rate must be determined and second anomalous rates must be recognized in this paper we address the first of these steps determining a background seismicity rate two problems must be dealt with if a meaningful background rate is to be determined first dependent events must be recognized and removed from consideration and second detection or reporting changes must be accounted for a number of techniques are available for recognizing dependent events we used the one developed by mcnally 1976 in this work the technique we used for recgonizing detection and reporting changes was developed by habermann 1983 we demonstrate these techniques on seismicity from the imperial valley region of california after a reasonable background rate is determined we examine seismicity rates in this region prior to the imperial valley earthquake of 15 october 1979 the source area of the earthquake shows a possible precursory quiescence which started during july 1979 the time of this rate decrease is close to the time of a change in the seismic instrumentation in the imperial valley which makes it difficult to determine whether the decrease is real or related to detection several other seismicity rate changes observed in the imperial valley appear to be related to changes in the operation of the network there these include quiescent periods which occur only if magnitude cutoffs are applied to the data and periods of low reporting of events with m 2 5 these observations suggest that magnitudes were not reported for some events during several time periods all of these observations indicate that determination of detection histories is crucial for meaningful seismicity studies

quantitative mapping of a precursory seismic quiescence to the izu oshima 1990 m6 5 earthquake japan a highly significant seismic quiescence with a standard deviate z 10 1 corresponding to a 99 per cent confidence level lasted from 1987 7 up to the 1990 february 20 izu oshima m 6 5 earthquake the quiescent volume had dimensions of 30 km n s and 10 km e w and was centred below 14 km depth within the recently upgraded seismograph network of the earthquake research institute eri this main shock was the only one with a magnitude m 5 8 in the upper 30 km of the crust for which the precursory quiescence hypothesis could be tested within a radius of 50 km and during the observation period 1983 5 1995 9 there were no other 1 5 yr or longer periods of quiescence that were rated z 6 5 in the declustered earthquake catalogue except one that was associated with volcanic activity the total space time covered by alarms including the volcanic one was less than 1 per cent at the z 6 5 level the rarity of highly significant episodes of quiescence and the correlation in space and time suggest that a precursory seismic quiescence started 2 5 yr before the izu oshima 1990 earthquake in its source volume and to the north of it and that it can be recognized with an alarm level of z 6 0 generating no false alarms during the 1 5 yr quiescence window only 10 earthquakes occurred in the quiet volume whereas 50 events were expected based on the rate seen at other times in randomly selected volumes containing 50 100 and 200 events the anomaly scored z 6 1 to 10 1 on the basis of the data from may 1983 to 1995 there is no highly significant quiescence currently present in the izu oshima area

coda q before the 1983 hawaii ms 6 6 earthquake abstract coda q values were derived for more than 300 microearthquakes that occurred in a 6 yr period before the 16 november 1983 kaoiki hawaii earthquake m s 6 6 the sources were located within a 14 16 km rectangular region surrounding the main shock epicenter and most of them occurred at depths between 5 and 10 km digital recordings from three stations at epicentral distances ranging from 0 to 18 km were used coda q was calculated from the amplitude decay rate of the s wave coda in nine frequency bands from 4 5 to 27 hz the average q of the nw part of the studied area is about 15 per cent higher than that of the se part these two subregions also showed differences in seismicity b value and microearthquake source mechanisms in the high q volume the b value was 1 0 and the rate of earthquakes per unit volume was about 50 per cent of the rate in the low q volume where b 1 3 one interpretation of these observations is that more extensive faulting in the se kaoiki fault zone leads to lower q higher b value and a higher seismicity rate during the 5 to 6 yr before the mainshock the 1 yr average q values were stable no significant q change could be identified as a precursor to the main shock

second order moment of central california seismicity 1969 1982 the second order moment cross correlation function of earthquakes in the u s geological survey central california catalog between 1969 and 1982 was calculated with respect to a magnitude threshold m 4 0 over interevent distances up to 80 km and interevent times up to 320 days the statistical procedure results in a representation of the spatial temporal structure of the catalog associated with m 4 0 earthquakes and is capable of revealing patterns too weak to be detected in the space time distribution of seismicity for individual earthquake sequences a method is introduced for identifying aftershocks based on a physical two parameter model of the earthquake interaction process the results show that the aftershock process dominates the second order moment and may even obscure the statistical expression of a precursory process a concentration of foreshocks within 15 km and 3 days of m 4 0 main shocks exhibits an apparent migration toward the main shock loci with velocity 2 6 5 3 km d this concentration may be related to an observed tendency for m 4 0 events to cluster auto correlate over this interevent range with the identified aftershocks removed the residual catalog is poissonion in space and time when two m 4 0 earthquakes occur within 80 km and 40 days of each other aftershock productivity appears to be relatively enhanced in the earlier sequence this suggests that aftershock populations are not solely dependent on their main shocks and that unusually productive aftershock sequences may be predictors of future moderate earthquakes

seismic quiescence precursor to the 1983 kaoiki ms 6 6 hawaii earthquake a remarkable seismicity rate decrease of 65 per cent occurred in most of the aftershock volume 2 4 yr before the 1983 kaoiki ms 6 6 main shock if the background seismicity rate 1978 to mid 1981 is extrapolated to the time of the main shock more than 300 earthquakes of ml 1 8 are missing because of the quiescence subvolumes measuring 5 5 km on a side and located within the aftershock zone showed seismicity rate decreases ranging from 0 to 90 per cent the volume of no change had dimensions of approximately 10 3 km it contained the main shock hypocenter and was located near the center of the 10 km radius aftershock area the seismic quiescence in different subvolumes of the aftershock volume surrounding this central nonquiet zone varied somewhat in starting time amount of rate decrease and statistical significance according to the standard deviate z test the reported rate decrease is the most pronounced example of precursory quiescence defined to date the seismicity pattern before the 1983 kaoiki shock conformed to the quiescence hypothesis proposed on the basis of the precursors to the 1975 hawaii main shock no quiescence was observed within the immediate surrounding of the hypocenter while strong rate decreases occurred in most of the rest of the main shock source volume it is thus hypothesized that major asperities which contain high stress levels and from which main ruptures can emanate may be recognizable as volumes of constant seismicity rate within surrounding volumes of quiescence provided that decreases of seismicity rate as a function of time can be defined quantitatively

temporal variations in seismicity during quasi static and dynamic rock failure abstract a comprehensive model is presented which can explain temporal fluctuations in seismic b values in the period leading to mechanical failure in terms of the underlying physical processes of time varying applied stress and stress corrosion enhanced crack growth under conditions of constant strain rate the form of the b value anomaly in the period leading to failure depends on the form of the stress time relationship for the case where dynamic failure occurs at peak stress after a period of strain hardening the model predicts a single cusp like b value anomaly reaching a critically low value of 0 5 at failure for the physically most realistic case where dynamic failure is preceded by a period of precursory strain energy release during strain softening the model predicts two minima in the b value separated by a temporary maximum or inflection point these fluctuations in the b value are consistent with reported intermediate term and short term earthquake precursors separated by a period of seismic quiescence for the case of quasi static cataclastic flow the b value mirrors the stress and never falls to the critical value because there is no critical rupture new results from a series of controlled laboratory experiments are presented in which microseismic event rates and b value were monitored contemporaneously with stress time data for all variants of the stress time relationship recent field observations of temporal changes in seismicity rates and b value preceding major earthquakes are also reported both data sets exhibit b value anomalies which are consistent with the model predictions

occurrence of a predicted earthquake on the san andreas fault in may 1985 we predicted1 that an earthquake would occur on the san andreas fault near stone canyon california within a year the prediction was based on the observation of seismic quiescence defined as a significant decrease in the average occurrence rate of earthquakes within the source volume of the future mainshock a mainshock of magnitude ml 4 6 occurred on 31 may 1986 rupturing exactly the specified segment of the fault this is the first successful prediction of an earthquake along the san andreas fault and the probability to have come true by chance is 5 although the prediction addressed only a small main shock its success was significant because the location size and occurrence time were correctly specified for an earthquake in a populated area larger earthquakes will undoubtedly be successfully predicted by the same method in the future but the major segments of the san andreas fault near san francisco and los angeles have such a low background seismicity rate that the method will probably not be applicable there

apparent stresses stress drops and amplitude ratios of earthquakes preceding and following the 1975 hawaii ms 7 2 main shock abstract s wave spectra for approximately 300 earthquake signals 2 8 m l 3 9 were obtained from hand digitized wood anderson records from hilo hawaii all sources were located within part of the rupture volume of the 1975 hawaii m s 7 2 earthquake the epicentral distance to hilo was about 45 3 km the backazimuth was 180 10 south and the sources were located at depths of 9 1 4 km the apparent stress and brune stress drop based on 3 sec windows of the s waves were examined for possible temporal and spatial dependence most of the data showed a remarkable constancy of the average apparent stress and stress drop per event as a function of time no measurable change in average signal content took place at the time of the main shock a statistically significant difference in the average stress parameters per event was found between neighboring subvolumes radius 5 km of the 1975 aftershock zone a volume previously proposed as asperity based on seismicity pattern interpretation contained earthquakes with higher apparent stresses of 9 0 8 8 bars compared to the neighboring volume with 4 2 6 1 bars seventy eight spectra obtained from a second station showed the same difference between the two volumes as that found at station hilo suggesting that the signal differences should be interpreted in terms of source differences in addition amplitude ratios of sv p waves measured at hilo showed that a systematic difference of 10 in fault plane orientation existed during years before the 1975 main shock this agrees with the rotation of the fault plane during the main rupture reported elsewhere we hypothesize that heterogeneities along the fault plane with dimensions of about 5 km cause changes in fault plane dip differences in average stress release per earthquake strong contrasts in seismicity patterns and differences in aseismic strain release by fault creep these same heterogeneities in the fault plane apparently also control the location of major earthquakes and the complexity of their ruptures

the effect of uncertainties on estimates of background seismicity rate abstract identifying changes in the rate of background earthquakes is a common method to detect transients in seismic activity rates and these transients can often be linked to changes in physical properties of the crust or to the recording homogeneity of seismic networks in this article we consider the effect that various sources of uncertainty have on seismicity rate estimation and we describe a monte carlo method to compute the statistical significance of seismicity rate changes we consider real earthquake catalogs and synthetic catalogs that represent poissonian background seismicity we find that the choice of declustering algorithm and the variation in the related parameter values have the greatest impact on seismicity rate change calculations this sensitivity is a direct consequence of the fact that there is no unique or universally applied approach to separate aftershocks and foreshocks from background events hypocenter location uncertainty also influences the statistical significance of seismicity rate changes its relevance depends on the ratio of the location uncertainty to the extension of the seismicity rate anomaly we find that magnitude uncertainty and edge effects associated with seismicity declustering have less impact than the other factors but their influence is not negligible in an example application we suggest that when uncertainties are considered the reported seismicity rate changes prior to the 1992 landers and big bear mainshocks decrease significantly

shear wave polarization alignments recorded above the kaoiki fault zone hawaii eighty four shear wave records from the seismometer station ain at kaoiki hawaii covering the years 1979 1983 were examined for the presence of shear wave splitting and possible precursory change associated with a mainshock in 1983 time delayed arrivals with different polarizations were detected suggestive of shear wave splitting and the polarizations of the first arriving shear waves were aligned in the direction ese wnw this direction is consistent with the anisotropy being caused by cracks aligned approximately perpendicular to the direction of least principal stress which is uniform over the ray path in an otherwise heterogeneous stress field no time dependent variation of the polarization of the leading split shear wave was observed investigation of temporal variation of time delays between split shear waves was not possible due to the presence of scattered arrivals