dynamical fault rupture processes in heterogeneous media abstract the fault rupture processes in a horizontally layered medium and also in a three dimensionally heterogeneous structure are investigated on three dimensional spontaneous dynamic shear crack models the wave equations for three dimensional space are solved numerically by a finite difference scheme under the appropriate boundary conditions and the finite stress fracture criterion the heterogeneous properties of the elastic medium particularly the existence of low velocity zones give remarkable effects on the rupture process yielding appreciably decelerated rupture velocities and large fault displacements in and around the zones and strong motions in the near field the large fault displacements are enhanced when the rupture breaks the ground surface the static seismic moment in the case with a low velocity zone is essentially the same as in a homogeneous half space an attempt is made to simulate the rupture process of a moderate size earthquake by applying the above shear crack model in a heterogeneous medium with depth dependent and laterally heterogeneous stress drop the model appears to explain the observed features to a satisfactory degree

finite faults and inverse theory with applications to the 1979 imperial valley earthquake abstract using a representation theorem from elastodynamics subsurface slip on a known fault is formulated as the solution to an inverse problem in which recorded surface ground motion is the data two methods of solution are presented the least squares method which minimizes the squared differences between theory and data and the constrained least squares method which simultaneously maintains a set of linear inequalities instabilities in the solution are effectively eliminated in both methods and the sensitivity of the solution to small changes in the data is quantitatively stated the inversion methodology is applied to 77 components of near field ground acceleration recorded during the 15 october 1979 imperial valley earthquake the faulting is constrained to propagate bilaterally away from the epicenter at an average velocity of 90 per cent of the shear wave speed on a vertical fault plane extending from the surface to 10 km depth inequality constraints are used to keep the faulting sequence physically reasonable by maintaining right lateral motion and positive slip velocity the preferred solution is stable and provides a good fit to the data it is also realistic and consistent with observed surface offsets and independent estimates of seismic moment

dynamic rupture of asperities and stress change during a sequence of large interplate earthquakes in the mexican subduction zone we investigate the spatial and temporal variations of shear stress due to the successive failures over an extensive segment of the mexican subduction zone during a sequence of large interplate earthquakes that occurred over a period of 13 yr for this purpose we develop 3d dynamic rupture models incorporating a shal lowly dipping fault located above the subducting plate the spatial distribution of dynamic stress drop over the fault has been estimated for each of the events through an inversion procedure using some of the previously derived kinematic fault param eters as observational constraints the results revealed quite heterogeneous stress changes during these earthquakes coming from medium to high dynamic stress drop due to the rupture of a few patch like asperities and from stress increase in between and around them two weak asperities located southeast of the michoacan segment were ruptured first by the 1979 petatlan event the 1981 playa azul event ruptured two asperities in the central zone with a stress drop higher than 80 bars the largest 1985 michoacan earthquake resulted from the rupture of two large size strong asperities located at both sides of the 1981 fault zone with high stress drop of 80 to 100 bars and from another two asperities at depth two days after this largest event two asperities were broken during the zihuatanejo aftershock in the southeastern adjacent zone many after shocks of these large events tend to be distributed in the zones of stress increase outside the asperities while only small numbers of aftershocks have been observed within these asperity zones it appears that several major asperities that existed in this extensive segment have been ruptured successively so as to fill unbroken gaps on the plate interface thus the stress change left over from the previous earthquake has dominant effects on the next event in this subduction zone

rupture process of the 1980 izu hanto toho oki earthquake deduced from strong motion seismograms abstract the 1980 izu hanto toho oki earthquake is studied in detail using near field strong motion seismograms recorded at japan meteorological agency stations a seismogram inversion method is applied to deduce the dislocation distribution and the character of rupture propagation during this earthquake this earthquake involves left lateral strike slip motion on the almost vertical plane with a strike of n10 w the fault plane is shallower than about 12 km in depth and the length is about 20 km the large dislocation large seismic moment occurs near the hypocenter and at the southern end of the fault plane the rupture propagates southward from the central part of the fault plane and spreads to the shallow area of the northern part of the fault plane after a delay of about 5 sec relative to the initiation of this earthquake the total seismic moment is about 7 10 25 dyne cm the aftershocks of magnitude equal to or greater than 4 0 take place in the areas where high stresses are expected to remain after this earthquake the mechanical weakness of small submarine monogenetic volcanoes which are located above the source region seems to affect the rupture process of this earthquake

inversion of strong motion seismograms for the source process of the naganoken seibu earthquake of 1984 abstract the source process of the noganoken seibu earthquake of 1984 m 6 8 has been investigated in detail by using strong motion seismograms recorded at stations within 100 km of the epicenter a new inversion method was applied to analyze the strong motion seismograms we parameterized the problem by using a model in which the fault was composed of a certain number of subfaults which started rupturing with arbitrary magnitude at arbitrary time and we have successfully obtained the dislocation distribution and the characteristics of rupture propagation on the fault the rupture starts at the shallowest point on the central part of the fault and then propagates over the whole source region within about 3 s the total seismic moment estimated from the final solutions is 3 10 25 dyne cm the dislocation is inhomogeneously distributed on the fault plane and reaches a maximum of about 1 5 2 m the largest dislocation occurs in the near surface region on the deeper part of the fault the dislocation seems to be small there were few aftershocks at the center and on the shallow part of the fault most of the large aftershocks m 4 0 were located on the deeper part of the fault these results suggest that the aftershock activity was highest on the unruptured regions of the fault where stresses were concentrated from the distribution of dislocations on the fault the size of the source area is estimated to be approximately 12 km length 6 km depth the main shock ground motions having periods longer than l s were estimated at sites where no recordings exist by using the final source model at the summit of mt ontake the maximum displacement and velocity are 15 and 12 cm s respectively the maximum short period acceleration at the period of 0 1 s is about 300 gal at the summit of mt ontake

dynamic rupture propagation during the 1891 nobi central japan earthquake a possible extension to the branched faults the dynamic rupture process of the 1891 nobi japan earthquake was closely reinvestigated by using a boundary integral equation method based on currently available observations we discussed mainly a possibility of the rupture propagation on branched faults including a buried fault called the gifu ichinomiya line several numerical simulations were made incorporating the information on the fault geometry based on the active fault traces on the ground surface and the maximum principal strain direction observed by the triangular surveys for the past 100 years the results suggest that the rupture should propagate along the buried fault during the nobi earthquake if a weak geological discontinuity exists along the gifu ichinomiya line and is connected to the neodani fault which is one of the major surface faults appearing during the nobi earthquake

a possible rupture process of slow earthquakes on a frictional fault summary a possible mechanism for the occurrence of slow earthquakes is investigated by calculating numerical solutions for the dynamical rupture process on a quasi three dimensional fault with heterogeneous frictional strengths experimental friction laws for the dependence of sliding frictional stress on slip velocity which are based on the cohesive properties of fault asperities are taken into considerations it is found that the applied stress does not drop very rapidly with time and the rupture velocity remarkably decreases as the dependence on slip velocity becomes smaller these deceleration effects for the rupture propagation are greatly enhanced with increasing heterogeneities in the distribution of frictional strength and as the initial shear stress has lower levels with respect to the average strength for these cases the growth of rupture is extremely slow in a nucleus region with the dimension as large as 10 times the initial rupture length and gains a terminal velocity dependent on the above factors the displacement time function becomes noticeably extended in these cases and indicates a stick slip like phenomena in the extended time interval for a strongly heterogeneous fault it seems that these results could explain the characteristic features of slow earthquakes

contribution of radar interferometry to a two step inversion of the kinematic process of the 1992 landers earthquake we study the rupture process of the 1992 landers earthquake to limit the trade off between slip amplitude and rupture time that affects solutions using only seismological data we adopt a two step approach we first constrain the slip distribution and its uncertainty by independent geodetic data to recover in the second step the temporal details of the rupture propagation the first step consists of an inversion of interferometric data and global positioning system measurements both independently and together to constrain slip distribution on a three segment fault model along both strike and dip direction we use a genetic algorithm to test the uniqueness of the solution and a least squares formulation to find the model which best fits the data we conclude from the results of these geodetic inversions that interferometric data are rich enough to access the slip distribution in the case of the landers earthquake since the surface deformations are more sensitive to shallow slip in our configuration the slip amplitude is better resolved near the surface than at depth the resulting slip distribution is in agreement with geological observations at the surface and confirms the heterogeneous nature of the landers earthquake most of the slip occurs at shallow depths on the homestead valley fault second segment with a maximum value of around 7 m another high slip zone is observed on the johnson valley fault first segment at 8 km depth in the second step we invert strong motion data with the a priori final slip amplitude and its uncertainty deduced from geodetic data to constrain the time history of the rupture process this second step emphasizes a strong variation of the temporal development of the earthquake fast rupture front velocities appear within high slip zones and the rupture slows when it encounters a resistance along the fault on average the rupture front propagates with velocities close to the s wave velocity and terminates about 20 s after initiation the large variations in both slip amplitude and rupture velocity suggest that the rupture process is better described by successively breaking asperities than by a pulse propagating with constant velocity

roughness at the base of the seismogenic zone contributing factors the cutout depth of microseismic activity in continental fault zones appears to correspond to the onset of greenschist metamorphic conditions at about 300 c it can generally be modeled as the transition from frictional to quasi plastic behavior in quartzofeldspathic crust shear resistance increases with depth through the frictional regime to peak at the transition beneath which it falls off exponentially with increasing temperature larger earthquake ruptures ml 5 5 nucleate around this transition depth where the highest concentrations of strain energy may accumulate varying depth and amplitude of the peak shear resistance along strike induce fluctuations in strain energy concentration at the base of the seismogenic zone factors affecting the depth of the transition include crustal composition geometry and mode of faulting fluid pressure levels in the frictional regime and water content in the quasi plastic regime quasi plastic strain rate and geothermal gradient evaluation of their relative importance is complicated because several are interdependent however compositional change may cause abrupt irregularities in seismogenic depth and peak shear resistance while regional variations in heat flow look to be particularly effective in creating long wavelength heterogeneities in strain energy concentration affecting faulting style

dynamic faulting studied by a finite difference method we have developed a finite difference method that is especially adapted to the study of dynamic shear cracks we studied a number of simple earthquake source models in two and three dimensions with special emphasis on the modeling of the stress field we compared our numerical results for semi infinite and self similar shear cracks with the few exact solutions that are available in the literature we then studied spontaneous rupture propagation with the help of a maximum stress criterion from dimensional arguments and a few simple examples we showed that the maximum stress criterion depended on the physical dimensions of the fault for a given maximum stress intensity the finer the numerical mesh the higher the maximum stress that had to be adopted a study of in plane cracks showed that at high rupture velocities the numerical results did not resolve the stress concentration due to the rupture front from the stress peak associated with the shear wave propagating in front of the crack we suggest that this is the reason why transonic rupture velocities are found in the numerical solutions of in plane faulting when the rupture resistance is rather low finally we studied the spontaneous propagation of an initially circular rupture two distinct modes of nucleation of the rupture were studied in the first a plane circular shear crack was formed instantaneously in a uniformly prestressed medium after a while once stress concentrations had developed around the crack edge the rupture started to grow in the second type of nucleation a preexisting circular crack became unstable at time t 0 and started to grow the latter model appeared to us as a more realistic simulation of earthquake triggering in this case the initial stress was nonuniform and was the static field of the preexisting fault

regional difference in scaling laws for large earthquakes and its tectonic implication we compiled 67 large earthquakes which occurred at and around plate boundaries for the last 140 yrs and classified them into four groups interplate strike slip events intraplate strike slip events underthrust events at island arc subduction zones and underthrust events at continental margin subduction zones for each group of earthquakes we examined relations between seismic moment m 0 fault length l fault width w and average fault slip d and found the following scaling laws in the case of interplate strike slip events the well known l cubed dependence of seismic moment breaks down when l exceeds 30 km because the extent of the seismogenic zone is limited in depth 12 km for large events l 30 km d and m 0 increase with l as d overline delta tau l mu left alpha l beta right and m 0 overline delta tau w l 2 left alpha l beta right respectively where the mean fault width bar w is 12 km and the mean stress drop overline delta tau is 1 8 mpa here and are structural parameters for intraplate strike slip events we obtained nearly the same relations except for significantly higher stress drop 3 1 mpa the difference in stress drop between interplate and intraplate events may be ascribed to the difference in stress accumulation rates and thus the recurrence time of earthquakes in the case of underthrust events at island arc subduction zones we also found the saturation of fault width bar w 120km and the breakaway from the l cubed dependence of m 0 for events larger than l 200 km if we consider the average dip angle of plate boundaries at island arc subduction zones to be 20 30 this indicates that the extent of the seismogenic zone in depth is limited to 40 60 km in the case of continental margin subduction zones on the other hand we could not find the saturation of fault width nor the breakaway from the l cubed dependence of m 0 from the analysis of the present data set w 200 km l 1000 km for sufficiently large earthquakes in general the downward rupture growth is limited to a certain depth due to the existence of a ductile unstressed region which extends under the brittle seismogenic zone since the brittle ductile transition occurs at 300 400 c the difference in the lower limit of the seismogenic zones between tectonically different regions may be attributed to the difference in thermal state there

terrestrial heat flow in the malawi rifted zone east africa implications for tectono thermal inheritance in continental rift basins abstract rift basins show the widest range of heat flow values compared to passive continental margins and intracratonic sag basins elevated heat flow is often used to infer asthenospheric upwelling or subsurface magmatic activity for magma assisted rifts despite the lack of surface volcanism for magma poor rifts it is possible that magmatic bodies will be present at lower and mid crustal levels but yet to breach the surface thus necessitating consideration of heat flow from tectono thermal inheritance in addition to more recent magmatism we use aeromagnetic data to investigate the terrestrial heat flow distribution in the malawi rifted zone mrz in east africa defined as the neogene malawi rift and surrounding permo triassic karoo rift basins we use the two dimensional power density spectrum technique to estimate the curie point depth cpd geothermal gradient and heat flow beneath the mrz our results reveal predominance of shallow cpds 18 20 km high geothermal gradients 29 32 c km and elevated heat flow 70 82 mw m 2 within the karoo rift basins along the malawi rift geothermal gradient 25 27 c km and heat flow 60 66 mw m 2 are generally lower except pronounced high heat flow 70 82 mw m 2 at the rungwe volcanic province rvp in the north and at the central rift segment where the rift overprints karoo rift basins the surrounding cratonic blocks show deeper cpds 24 27 km lower geothermal gradients 22 24 c km and lower heat flow 53 63 mw m 2 we interpret that the elevated heat flow in the karoo rift basins are related to residual heat flow from the permo triassic rifting phase replenished by later jurassic cretaceous diking events apart from the thermal anomaly beneath the rvp which is due to neogene magmatism other important targets for geothermal explorations in the mrz are the residual thermal anomalies in the karoo rift basins we infer that in areas of active magma poor rifting pronounced heat flow may not only occur in regions of asthenospheric upwelling but may concentrate in the ancient magmatic rift segments our findings have implications for crustal stretching tectono thermal inheritance and geothermal energy potentials in east africa and other segments of magma poor rifting

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

3 d simulations of surface wave propagation in the kanto sedimentary basin japan part 2 application of the surface wave bem abstract the 3 d surface wave boundary element method is applied to the same set of strong motion records as those of part 1 kato et al 1993 namely the long period strong motion with periods around 8 sec recorded at the jma japan meteorological agency stations in yokohama and tokyo during the 1980 izu hanto toho oki earthquake this method simulates exact love and rayleigh waves in a laterally heterogeneous medium including 3 d basin structures the needed computer memory and cpu time are made manageable by assuming the inside and outside of the basin to consist of horizontal flat layers in addition vertical interfaces are assumed between the basin and the outside bedrock for simplicity in this study after checking the range of validity of the flat layer approximation for the kanto sedimentary basin the generation and propagation of surface waves in the basin are numerically investigated in particular our investigation places special emphasis on the cause of the observed long duration which could not be explained in part 1 using the surface wave gaussian beam method when the bedrock outside the western margin of the kanto basin is incorporated into the model the observed long duration at the tokyo station is successfully reproduced this can be explained by the strong surface waves generated by the surface wave to surface wave conversion at the margin and appearing in tokyo after the direct surface waves from the epicenter this result agrees with kinoshita et al 1992 who studied records from an array of seismographs in tokyo moreover when a sedimentary basin of sagami bay is included as was done in part 1 the simulations are greatly improved particularly at the yokohama station which corresponds to the node in the radiation pattern of the source this is due to the fact that the radiation pattern is strongly affected by the near source heterogeneity of the sagami basin all these results together with direct comparisons to 2 d modelings clearly show the strong 3 d effects of the basin structures on the generation and propagation of surface waves for this earthquake

slip history of the 1994 sanriku haruka oki japan earthquake deduced from strong motion data abstract we analyzed the seismic waves of the 1994 sanriku haruka oki earthquake m w 7 7 which occurred in the aftershock area of the 1968 tokachi oki earthquake m w 8 2 applying a multiple time window inversion scheme to near source strong motion data we obtained a detailed spatiotemporal rupture process and compared it with that of the tokachi oki earthquake the fault geometry is constructed based on the aftershock distribution the obtained rupture model is consistent with the cmt solution even for a non double couple component the total seismic moment is 4 0 10 20 n m large slips are concentrated in three asperities the first asperity centers about 40 km south and 50 km west from the hypocenter with a maximum slip of 4 4 m the second one centers about 60 km west from the hypocenter with a maximum slip of 2 2 m and the third one lies about 110 km west from the hypocenter with a maximum slip of 2 6 m the obtained moment rate and the duration on the first and second asperities are lower and much longer than those on the third asperity respectively the first asperity does not overlap with an area of large slip during the tokachi oki earthquake but the second or third seem to overlap with or be adjacent to the asperity of the tokachi oki earthquake our inversion result also shows an abrupt change of the rupture velocity from 1 8 to 3 0 km sec at the central part of the fault plane a difference of the seismic coupling between the oceanic and the continental lithospheres at the trenchward side and at the landward side of the 143 e meridian seems to affect the rupture process of this earthquake

estimation of high frequency wave radiation areas on the fault plane by the envelope inversion of acceleration seismograms we have developed an inversion method to estimate high frequency wave radiation areas on the fault plane it is very difficult to perform a source inversion for seismic waveforms whose frequencies are higher than 1 hz because of two problems one is the difficulty in obtaining green s functions that are accurate enough for such high frequencies and the other is that involved in doing a waveform inversion for such oscillatory data we overcame these problems by using aftershock records as empirical green s functions and by using the envelopes of acceleration seismograms as the inversion data instead of waveforms envelopes were calculated by smoothing the acceleration seismograms with a running root mean square time window we applied this method to the 1993 kushiro oki japan earthquake mw 7 6 which occurred on a horizontal fault plane inside the pacific plate subducting beneath the north american plate we adopted the mjma 4 9 aftershock as the empirical green s function event we bandpass filtered 2 0 10 0 hz the strong motion acceleration seismograms at three stations and calculated the envelopes by smoothing with a 5 0 s window length the 40 km 60 km fault plane was divided into 4 6 meshes and the acceleration radiation intensity and rupture time of each mesh were obtained by the inversion the results are as follows 1 high frequency waves radiated weakly around the rupture starting point but radiated mainly near the periphery of the fault plane an area with particularly strong radiation intensity of high frequency waves is seen at the edge of the fault plane about 30 km southwest of the rupture starting point its size is about 20 km 20 km 2 the distribution of high frequency radiation intensity and slip distribution are complementary to each other that is the radiation of high frequency waves was strong in those areas where that of low frequency waves was weak

appropriate boundary conditions for modeling very long earthquakes and physical consequences we have studied two sets of models to find appropriate boundary conditions for the theoretical modeling of very long earthquakes i e those for which the length l is much larger than the width w of the rectangular aftershock area and the resulting physical consequences in the first set we study the conventional rectangular crack model for l w 2 and 4 i e the stress drop is confined to a rectangular region and there is no rupture outside this region w model in the second set of models which is suggested by geologic data we constrain the stress drop to a rectangular zone seismogenic layer but we do not have any constraint on the rupture above and below this layer the applied stress in this region above and below the seismogenic layer is taken to be exactly equal to the yield stress of the material so that no stress drop occurs when it yields this implies that the material on either side of the seismogenic layer is shearing aseismically just before the earthquake occurs and also implies that the material in these regions is purely plastic the major conclusion of our work is that the second set of models the rupture propagates dynamically for some distance into these nonbrittle zones above and below the seismogenic layer this result is supported by geological evidence and also by recent seismic evidence that moments of some large earthquakes increase with increasing wavelength at very long periods the slip in large parts of these zones is found to be nonnegligible the slip is still confined to a finite width which may be considered as an effective width but this width is larger than the width of the seismogenic brittle layer this effective width increases as the length of the region of stress drop increases and may be larger than this length for long faults thus for the second set of models the aftershock area would underestimate the actual rupture area the moment and the slip at the center for the second set of models are larger than for the first set for the same l w ratio of the stress drop region even if we assume that there is no radiated energy from the regions above and below the seismogenic zone for l w 2 the duration of slip at interior points on the fault for the two models are the same but for l w 4 the duration of slip at a given interior point is much larger for the second set of models however the particle velocity during this added time of slip for the second class of models is small and almost constant so that at least for l w up to 4 this difference may not be significant for strong ground motion near such a fault the rise times and slips at any interior point of the fault is mainly width controlled for the first set of models and mainly controlled by the length of the elastic region or the effective width for the latter set since longer faults are observed to have larger slips than shorter ones of the same width the first set of models would predict that stress drops increase with rupture length for the same fault width however this observation can be explained by the second set of models even for constant stress drops for the second set of models we find that a fault twice the length of another has much less than double the slip for the same average stress drop we suggest that both classes of models studied are realistic models of earthquake sources but for different cases

dynamic rupture analysis inversion for the source process of the 1990 izu oshima japan earthquake m 6 5 a waveform inversion has been applied to strong motion data using a dynamic shear crack model we studied the 1990 izu oshima earthquake mjma 6 5 which has vertical strike slip faulting with unilateral rupture propagation the inversion has two steps a waveform inversion and a crack inversion that are applied iteratively a waveform inversion is used to determine the distribution of rupture starting times and slip dislocations using the slip functions calculated by the initial crack model or by previous crack inversion a crack inversion is used to calculate dynamic crack propagation that explains the results of the above inversion in this step we use the estimated rupture times as a locking fracture criterion the maximum shear stress attained before a fault segment breaks gives a lower bound estimate of the peak shear strength at each fault segment then the dynamic stress drop distribution is estimated from the slip distribution obtained from waveform inversion assuming a dynamic crack model from the results we determine the rise time distribution and the distribution of a dimensionless stress ratio s defined as strength excess stress drop our analysis gives the following picture of the rupture process of the 1990 izu oshima earthquake 1 an asperity type faulting having large slip and high stress drop was detected in the region around the initiation point of rupture 2 south of the asperity zone barrier type faulting characterized by incoherent propagating rupture small slip long rise time and high strength excess was detected this zone corresponds to the intersection of the fault with the 1978 earthquake mjma 7 0

dynamic rupture processes on a dipping fault and estimates of stress drop and strength excess from the results of waveform inversion summary the dynamic rupture processes on a dipping fault in a horizontally layered medium are investigated on 3 d spontaneous shear crack models the wave equations for the 3 d space are solved numerically by a finite difference method under appropriate boundary conditions the displacement components right above and right below the dipping fault plane have been obtained by solving the boundary conditions across the fault it was found that the final fault slip on the hanging wall side is appreciably larger than that on the foot wall side and that their amount is quite sensitive to the fault depth and the elastic heterogeneities of the medium particularly of the existence of low velocity surface layers on the basis of the above model the dynamic rupture process of the 1961 kita mino earthquake in central japan has been investigated with constraints of the fault parameters derived from the results of waveform inversion analysis for this purpose a locking fracture criterion was introduced for rupture propagation from which a lower bound of the maximum shear stress before the rupture and hence of the strength excess has been estimated the dynamic stress drop has also been evaluated by repeated iterations to minimize the difference between the dynamic and kinematic fault slips the results revealed quite heterogeneous distribution of strength excess indicating large values near the sw section of the fault at shallow depths and in the central bottom section in contrast to small values at mid depths in the ne section around the rupture nucleus zone the distribution of stress drop was also found to be heterogeneous with the existence of high stress drop zones in the ne shallow and bottom sections and a low stress drop zone at most of mid depths except in the sw section these results suggest that the dynamic rupture of this earthquake initiates at a small nucleus zone with low strength excess and small stress drop then broke the shallower and deeper fault sections with moderate strength excess and high stress drop and finally ruptured barrier zones with high strength excess these zones may be interpreted as weak and strong asperities and barriers respectively

breaking of a single asperity rupture process and seismic radiation the problem of spontaneous shear rupture of a single circular asperity on an infinite fault plane is studied initially the fault plane is broken everywhere except at a circular asperity an applied displacement at infinity results in a stress concentration along the bounding edge of the asperity the frictional stress on the broken part of the fault plane is taken to be a constant once a point on the asperity breaks the stress there drops to the same value as on the main fault surface the rupture is started by relaxing the shear stress at a point on the asperity edge and is then allowed to propagate spontaneously using a critical stress level fracture criterion the rupture process is calculated numerically it is found that for asperities of constant strength the rupture first propagates around the edge of the asperity and then inward a phenomenon best described by the well known term of classic military maneuver the double encircling pincer movement in the appendix the expressions for the far field seismic radiation due to the rupture of such an asperity are derived it is shown that the nth cartesian component of the far field displacement at x t for p sv and sh waves using the notation of aki and richards 1980 is given by un x t dni 4 c2r s0 i3 t r c ds thus the far field pulses can be directly found from the stress drops on the fault plane this formula is also true for crack or dislocation problems the directivity function dni for displacement for the asperity problem is found to be that for the double couple modified by some factor in particular the fault plane is a nodal plane for sv waves for the rupturing of asperities on a finite fault these directivity functions are applicable only to the initial part of observed pulses at a receiver provided the receiver is not located on the fault plane outside the broken part of the main crack edge in which case it is inapplicable for all time

depth distribution of coseismic slip along the nankai trough japan from joint inversion of geodetic and tsunami data coseismic slip distribution on the fault plane particularly in the downdip direction associated with large subduction earthquakes can be estimated by joint inversion of geodetic and tsunami data two large earthquakes the 1944 tonankai earthquake mw 8 1 and the 1946 nankaido earthquake mw 8 3 occurred on the nankai trough southwestern japan where the philippine sea plate is subducting beneath the eurasian plate the source areas of these events extended over both land and ocean coseismic crustal movements on land were measured by leveling while those in ocean were recorded as tsunami waveforms on tide gauges the coseismic slip distribution inverted from these data shows that the slip on the shallower part of the fault plane is comparable to that on the deeper parts this indicates that large coseismic slip can occur beneath an accretionary wedge where current seismicity is low the result has implications for other subduction zones having a similar tectonic environment such as for the pacific northwest region of the united states although the possibility of a large earthquake there is still debated should a large subduction earthquake occur in this region coseismic slip on the shallow part could be large and the potential for large tsunamis is high

analysis and interpretation of the heterogeneous rupture process application of the empirical green s function method and nonlinear inversion technique to large earthquakes abstract heterogeneous rupture processes were revealed using an empirical green s function egf method and a nonlinear inversion technique three events the 1980 east izu peninsula earthquake the 1983 japan sea earthquake and the 1987 east chiba earthquake are studied kernel events used as empirical green s functions were selected among the foreshocks and aftershocks the dimension and orientation of the fault plane were evaluated from the aftershock distribution and main shock fault plane solution a moment ratio between the main shock and the kernel event was estimated in order to identify the number of subfaults strong motion waveforms recorded at near distances were inverted to estimate moment release rupture time and rise time at each subfault with the stress drop calculated from the moment release and rise time distributions of these faulting parameters can be related to the aftershock distribution and geological structures general features are that the moment release was large where the aftershock activity was low that the irregular rupture propagations detected here were related to geological structures such as a ridge junction seamount chains and other tectonic plate boundaries and that stress drop distributions show some relation to the pattern of aftershocks an exception is the small moment release at the bending corner of the 1983 japan sea earthquake where the aftershock activity was low with this method it is possible to extract not only the detailed source history but also information on geophysical structures

3 d simulations of surface wave propagation in the kanto sedimentary basin japan part 1 application of the surface wave gaussian beam method abstract the goal of this study is to simulate the displacement waveforms of 8 sec period love waves observed at tokyo and yokohama stations in the kanto sedimentary basin during the izu hanto toho oki earthquake of 29 june 1980 m 0 7 10 25 dyne cm the surface wave gaussian beam method is applied for this purpose on the basis of the 3 d seismic velocity and density structure of the kanto basin and assuming that the earth medium is laterally homogeneous outside the kanto basin waveforms of love waves are synthesized the synthesized seismograms underestimate the observed peak amplitudes at yokohama station this is primarily because the station is located in the direction of the nodal plane of the love wave radiation as indicated by yamanaka et al 1992 a quaternary basin exists in the sagami bay between the source location and the kanto basin we include the sagami basin in our model by the approximation of a circular low velocity region excellent agreement between observed and synthesized waveforms was achieved not only for amplitude but also for phase for the early parts of the wave trains at both stations we conclude that the low velocity quaternary basin in the sagami bay acts like a lens to focus surface wave energy resulting in high amplitudes the later arriving waves in particular the long duration observed at tokyo station however cannot be adequately explained by this method one possible reason for the failure of simulating the later phases is that this method disregards the secondary love waves converted from s waves and or surface waves at a laterally discontinuous boundary although the surface wave gaussian beam method cannot adequately predict the duration of observed seismograms it provides us with a satisfactory prediction of amplitudes and phases for early arrivals in laterally slowly varying media at drastically lower computation costs and less memory requirements than does other methods

coherence of mach fronts during heterogeneous supershear earthquake rupture propagation simulations and comparison with observations 1 we study how heterogeneous rupture propagation affects the coherence of shear and rayleigh mach wavefronts radiated by supershear earthquakes we address this question using numerical simulations of ruptures on a planar vertical strike slip fault embedded in a three dimensional homogeneous linear elastic half space ruptures propagate spontaneously in accordance with a linear slip weakening friction law through both homogeneous and heterogeneous initial shear stress fields in the 3 d homogeneous case rupture fronts are curved owing to interactions with the free surface and the finite fault width however this curvature does not greatly diminish the coherence of mach fronts relative to cases in which the rupture front is constrained to be straight as studied by dunham and bhat 2008a introducing heterogeneity in the initial shear stress distribution causes ruptures to propagate at speeds that locally fluctuate above and below the shear wave speed calculations of the fourier amplitude spectra fas of ground velocity time histories corroborate the kinematic results of bizzarri and spudich 2008a 1 the ground motion of a supershear rupture is richer in high frequency with respect to a subshear one 2 when a mach pulse is present its high frequency content overwhelms that arising from stress heterogeneity present numerical experiments indicate that a mach pulse causes approximately an 1 7 high frequency falloff in the fas of ground displacement moreover within the context of the employed representation of heterogeneities and over the range of parameter space that is accessible with current computational resources our simulations suggest that while heterogeneities reduce peak ground velocity and diminish the coherence of the mach fronts ground motion at stations experiencing mach pulses should be richer in high frequencies compared to stations without mach pulses in contrast to the foregoing theoretical results we find no average elevation of 5 damped absolute response spectral accelerations sa in the period band 0 05 0 4 s observed at stations that presumably experienced mach pulses during the 1979 imperial valley 1999 kocaeli and 2002 denali fault earthquakes compared to sa observed at non mach pulse stations in the same earthquakes a 20 amplification of short period sa is seen only at a few of the imperial valley stations closest to the fault this lack of elevated sa suggests that either mach pulses in real earthquakes are even more incoherent that in our simulations or that mach pulses are vulnerable to attenuation through nonlinear soil response in any case this result might imply that current engineering models of high frequency earthquake ground motions do not need to be modified by more than 20 close to the fault to account for mach pulses provided that the existing data are adequately representative of ground motions from supershear earthquakes

why large earthquakes do not nucleate at shallow depths although by definition tectonic earthquakes may nucleate anywhere within the seismogenic layer in almost all cases those earthquakes that become large nucleate near the base of the seismogenic layer because frictional strength and stress drop should increase with depth and in situ stress measurements suggest that the ambient stress increases with depth we have examined spontaneous rupture models with gradients of both average stress drop and average strength relative to ambient stress to determine whether this explains the observed phenomena we report here that ruptures that are nucleated with in the low stress drop shallow regions of the model are inhibited from propagating but those that are nucleated within the high stress drop regions can propagate over the entire fault plane

evidence for self similar triangular slip distributions on earthquakes implications for earthquake and fault mechanics we characterize average slip distributions on earthquakes beyond their individual heterogeneity for that we analyze a large number of seismic slip distributions both measured at the surface after earthquakes 44 profiles and derived from slip inversion models 76 models investigating the overall shape of these slip profiles we find that they are roughly triangular both along strike and dip and most of them 70 80 are asymmetric long linear slopes and high slip gradients therefore are the key ingredients to describe earthquake slip profiles the scaling relations between maximum displacement and length or width suggest furthermore that the triangular slip profiles are self similar such slip patterns make earthquakes dominated by one major zone of maximum slip hence one major asperity analyzing the position of hypocenters with respect to these asperities we find that earthquakes nucleate at a distance from them that averages 20 30 of their total length compiling observations on 56 earthquakes we show that this distance i e the asperity size is structurally defined we then compare the earthquake slip profiles to cumulative slip profiles measured on long term faults of various ages and sizes and find that all profiles have a similar shape triangular and asymmetric hence combining data for a large number of earthquakes leads to point out average generic characteristics of the coseismic slip that are similar to those that emerge from the accumulation of events with time on a single fault this suggests that these characteristics result from robust physical properties

depth estimation from the scaling power spectrum of potential fields summary depth estimation from potential field power spectra requires a realistic assumption of the statistical properties of the source distributions density and susceptibility distributions in the earth s crust exhibit a long range dependence which is adequately described by scaling random fields with a spectral density proportional to some power of the wavenumber the theoretical power spectrum for a half space model of scaling sources explains the shape of observed power spectra of real potential field data very well minimizing the misfit between the model and the observed power spectrum yields an estimate for the depth to the top of the sources after demonstrating this approach on synthetic magnetic data we reinterpret power spectra of gravity and aeromagnetic data from utah hawaii and saskatchewan finding depth values that differ significantly from earlier interpretations all three power spectra are best explained by source distributions starting at surface level even the power spectrum from an aeromagnetic survey of a sedimentary basin with virtually non magnetic basin fill in the latter case a priori information on the intensity and the scaling exponent of the field caused by the basement had to be included to obtain an approximate estimate of the basin depth in general potential field power spectra are dominated by scaling properties of their source distributions and contain only limited depth information

an inversion method to analyze the rupture processes of earthquakes using near field seismograms a new and very practical nonlinear least square inversion method has been developed to investigate the dislocation distribution and the character of rupture propagation on an earthquake fault plane the problem is parameterized using a model in which the fault is composed of subfaults which start rupturing with arbitrary magnitudes at arbitrary times the performance of the method is examined by numerical simulations and the method proves useful for the analysis of source processes of earthquakes including complicated cases such as multiple events the source process of the naganoken seibu earthquake of 1984 has been analyzed using strong motion seismograms the dislocation distribution and the character of rupture propagation have been successfully obtained

in situ determination of heat flow in unconsolidated sediments subsurface thermal measurements are the most effective least ambiguous tools for locating geothermal resources measurements of thermal gradient in the upper few tens of meters can delineate the major anomalies but it is also desirable to combine these gradients with reliable estimates of thermal conductivity to provide data on the energy flux and to constrain models of the heat sources responsible for the anomalies problems associated with such heat flow measurements include the economics of casing or grouting holes the long waits and repeated visits necessary to obtain equilibrium temperature values the possible legal liability arising from disturbance of aquifers the hazards presented by pipes protruding from the ground and the security problems associated with leaving cased holes open for periods of weeks to months

rupture propagation beyond fault discontinuities significance of fault strike and location summary earthquake rupture sometimes occurs on several faults and often decelerates or terminates at fault stepovers factors that control the rupture processes across stepovers are important for an understanding of earthquake growth and termination in this study we investigate such factors by calculating the spontaneous rupture processes of two non coplanar faults in a 3 d model dealing with two extreme models in which two strike slip faults are either parallel or perpendicular we show that the rupture processes beyond fault discontinuities are drastically different for the two models we find three factors influencing rupture processes beyond fault discontinuities depth of the upper edge of the two faults location of the edge of the first fault and geometry of the two faults these factors determine the time and location of rupture jumps to the second fault for rupture propagation to the second fault it is essential for rupture on the first fault to arrive at the edge of the fault in particular whether rupture on the first fault reaches the earth s surface or not controls the difficulty of rupture jumps and the locations where the rupture is triggered which is also related to the step direction of the two faults this is because the stress perturbation at the fault edge is affected by the earth s free surface

the rupture process of the 1989 offshore ito earthquakes preceding a submarine volcanic eruption an earthquake swarm at the eastern offshore region of ito the izu peninsula started on june 30 1989 and became more active on and after july 4 the largest earthquake among the swarm mjma 5 5 occurred at 1109 jst 0209 ut on july 9 1989 and a second significant event followed 45 s later a remarkable volcanic tremor started on july 11 and a submarine eruption occurred just above the source region of the largest earthquake on july 13 in this paper we present a new inversion method to derive the source time function of rupture process the source time function is divided into several sections following an automatic algorithm and is interpolated with a cubic spline function the parameters which are the values and first derivatives at knots of the spline function are determined by minimizing a squared error between the observed and synthetic seismograms to demonstrate the performance of this method some numerical experiments simulations are carried out on near field seismic records applying this method to analysis of the two large earthquakes the detailed rupture processes are obtained a heterogeneous crustal structure near the volcanic area affects the rupture processes of these earthquakes the large stress drop due to the largest earthquake just beneath the submarine eruption point would assist a magma ascending and may have triggered the eruption 4 days after these earthquakes

curie point depth geothermal gradient and heat flow estimation and geothermal anomaly exploration from integrated analysis of aeromagnetic and gravity data on the sabalan area nw iran prospecting the geothermal resources in northwest of iran conducted in 1975 revealed several promising areas and introduced the sabalan geothermal field as a priority for further studies the sabalan mt representing the sabalan geothermal field is a large stratovolcano which consists of an extensive central edifice built on a probable tectonic horst of underlying intrusive and effusive volcanic rocks in this study curie point depth cpd geothermal gradient and heat flow map were constituted from spectral analysis of the aeromagnetic data for the nw of iran the top of the geothermal resource i e the thickness of the overburden was evaluated by applying the euler deconvolution method on the residual gravity data the thickness of the geothermal resource was calculated by subtracting the euler depths obtained from the cpds in the geothermal anomalous region the geothermal anomalous region was defined by the heat flow value greater than 150 mw m2 cpds in the investigated area are found between 8 8 km in the sabalan geothermal field and 14 1 in the northeast the results showed that the geothermal gradient is higher than 62 c km and the heat flow is higher than 152 mw m2 for the geothermal manifestation region the thickness of the geothermal resource was also estimated to vary between 5 4 and 9 1 km these results are consistent with the drilling and other geological information findings indicate that the cdps agree with earthquake distribution and the type of thermal spring is related to the depth of the top of the geothermal resource

numerical simulations of large earthquakes dynamic rupture propagation on heterogeneous faults our current conceptions of earthquake rupture dynamics especially for large earthquakes require knowledge of the geometry of the faults involved in the rupture the material properties of the rocks surrounding the faults the initial state of stress on the faults and a constitutive formulation that determines when the faults can slip in numerical simulations each of these factors appears to play a significant role in rupture propagation at the kilometer length scale observational evidence of the earth indicates that at least the first three of the elements geometry material and stress can vary over many scale dimensions future research on earthquake rupture dynamics needs to consider at which length scales these features are significant in affecting rupture propagation

deep learning of aftershock patterns following large earthquakes aftershocks are a response to changes in stress generated by large earthquakes and represent the most common observations of the triggering of earthquakes the maximum magnitude of aftershocks and their temporal decay are well described by empirical laws such as bath s law1 and omori s law2 but explaining and forecasting the spatial distribution of aftershocks is more difficult coulomb failure stress change3 is perhaps the most widely used criterion to explain the spatial distributions of aftershocks4 8 but its applicability has been disputed9 11 here we use a deep learning approach to identify a static stress based criterion that forecasts aftershock locations without prior assumptions about fault orientation we show that a neural network trained on more than 131 000 mainshock aftershock pairs can predict the locations of aftershocks in an independent test dataset of more than 30 000 mainshock aftershock pairs more accurately area under curve of 0 849 than can classic coulomb failure stress change area under curve of 0 583 we find that the learned aftershock pattern is physically interpretable the maximum change in shear stress the von mises yield criterion a scaled version of the second invariant of the deviatoric stress change tensor and the sum of the absolute values of the independent components of the stress change tensor each explain more than 98 per cent of the variance in the neural network prediction this machine learning driven insight provides improved forecasts of aftershock locations and identifies physical quantities that may control earthquake triggering during the most active part of the seismic cycle neural networks trained on data from about 130 000 aftershocks from around 100 large earthquakes improve predictions of the spatial distribution of aftershocks and suggest physical quantities that may control earthquake triggering

determination of thermal conductivity for deep boreholes two methods for thermal conductivity determinations on rock cores and fragments were tested on a suite of samples from the kontinentales tiefbohrprogramm der bundesrepublik deutschland ktb superdeep drill hole in germany they were also compared with estimates of thermal conductivity using the mineral composition of the rock and physical well logs and with in situ thermal conductivity measurements laboratory methods provide reasonably precise determinations of the thermal conductivities of both solid core 5 and drill cuttings 10 at room temperature and pressure the most common methods presently used for crystalline rocks are the steady state divided bar db technique and the transient half space line source ls sample preparation and measurement times are comparable for the db and ls with sample preparation being more time consuming on average for isotropic rocks there is little to choose from between the two methods which both give reliable values of conductivity in the vertical direction the ls is easier to set up and use in field laboratory situations which renders it the preferred method for field reconnaissance the gneissic crystalline rocks penetrated by the ktb boreholes typically have anisotropy of the order of 10 20 the db provides unambiguous values of conductivity in a given direction so its use is preferable for obtaining both principal conductivities and the vertical component anisotropy can be estimated using ls measurements in many different directions but the potential for large random errors is much greater than with the more straightforward db approach for deep research wells the difficulties of extrapolating laboratory results to in situ conditions particularly temperature present additional obstacles to determining heat flow laboratory measurements of water saturated samples under in situ conditions combined with in situ measurements and judicious use of calculations based on mineralogy and well log derived physical properties can aid in the accurate characterization of thermal conductivity in deep wells the application of different methods helped to link variations of heat flow with depth in the ktb hole to the anisotropy of thermal conductivity or thermal refraction and thus allowed the calculation of background heat flux in this geologically complex area

strong motion seismology although this is the first review on this topic to appear in a quadrennial report the roots of strong motion seismology extend back to at least 1932 when far sighted engineers in the seismological field survey of the u s coast and geodetic survey installed rugged fieldworthy instruments designed to make on scale recordings of large earthquakes carder 1964 these instruments are called accelerographs for their output closely mimics ground acceleration hudson 1979 the original instruments and their offspring have provided a wealth of information about ground motions of direct use to engineers due in large part to the continued devotion of the seismological field survey now part of the branch of engineering seismology and geology at the u s geological survey the number of recordings has increased substantially particularly in the last two decades and multiple recordings of a few california earthquakes have provided the data necessary to begin to unravel the complexities of the ground motions and to predict these motions on an empirical basis although seismologists used accelerograph records in studies of earthquake energy and ground motion attenuation as long ago as 1942 gutenberg and richter 1942 1956 widespread seismological use of the records began with aki s 1968 analysis of the 1966 parkfield earthquake

curie point depths of the island of kyushu and surrounding areas japan as part of a comprehensive nationwide evaluation of geothermal resources for japan the first of the curie point depth maps covering the island of kyushu has been prepared the map was created by inverting gridded regional aeromagnetic data two satisfactory algorithms were developed to invert the gridded data based upon a distribution of point dipoles the first algorithm estimates x0 y0 and z0 the coordinates of the centroid of the distribution by computing a least squares fit to the radial frequency of the fourier transform the second algorithm estimates centroid depth only by computing a least squares fit to the squared amplitude of the frequency estimates the average depth to the top zt of the collection of point dipoles was estimated by a variation of the second algorithm the depth to the bottom of the dipoles inferred curie point depth is zb 2z0 zt the depth estimates are hand contoured to produce the final map the curie point depth map is then compared to regional geology and heat

three dimensional spontaneous rupture propagation and implications for the earthquake source mechanism summary we consider spontaneous cracks spreading out areally over a fault plane in an infinite medium as a realistic earthquake source model the boundary integral equation technique described in das is used to determine the displacements and stresses everywhere on the crack plane for faults spreading out symmetrically in all four quadrants over infinite planes of constant yield strength we find that the terminal rupture velocity viiicr in the direction of purely anti plane rupture is given by viiicr in the purely in plane direction of rupture propagation we find the terminal rupture velocity viicr to be given by viicr or viicr 0 5 depending on the yield strength we also find that crack propagation in the purely antiplane direction does not influence crack propagation in the purely in plane direction and vice versa for infinitely long shear cracks of finite width the slip at a point is found to grow more slowly after the arrival of a shear wave diffracted from the nearer crack edge the slip virtually ceases after the shear arrival from the further crack edge this implies that for such faults the slip is controlled by the fault width for a rectangular fault in an infinite medium our final dynamic solution is in closer agreement with the static elliptical crack solution of eshelby than with the static rectangular dislocation model solution of chinnery we find the relationship between the average slip and average dynamic stress drop e on such a rectangular fault of half width w to be e c w where c 0 7 the slip at an interior point is again controlled by the fault width for faults that are much longer than wide so that the slip may stop in the region where the fault initiated before the completion of the rupture process this implies that two rectangular faults of varying lengths but of the same width have the same slip for the same average stress drop a fact clearly contradicted by observations that fault slip increases with earthquake size for great earthquakes occurring along the same fault this contradiction is resolved if stress drops are also bigger for bigger earthquakes

subsurface structural features of the basement complex and geothermal resources using aeromagnetic data in the bahariya oasis western desert egypt egypt gives priority to western desert in land reclamation and sustainable development projects the bahariya oasis a northernmost oasis of egypt is closer to the capital compared to other remote oases and represents the most interesting potential candidates for the sustainable development many researches provide insight into the hydrogeology and structural geology of the bahariya oasis however very few attempts have been done to investigate its geothermal potentiality the aim of present study is to reflect the geothermal resources of the bahariya oasis encountered with the predominant structures utilizing aeromagnetic and borehole data the main subsurface structures and the depth to basement map were retrieved from the 2d modelling of the aeromagnetic data the results of the 2d models were verified by well stratigraphic data spectral analysis of the aeromagnetic data was applied to estimate the curie point depth cpd and to produce geothermal gradient and heat flow maps for the bahariya oasis the results of the study indicated that the depth to the basement rocks in the area ranges from 0 6 to 4 0 km the cpd is 15 km in the central parts and increases to 24 km eastward the geothermal gradient varies from 20 to 35 c km with an average 30 c km however the heat flow changes from 60 to 100 m w m2 with an average of 90 m w m2 these results in addition to basement structures imply a high temperature gradient and heat flow correlating with shallow curie depths the results are in agreement with previous work in the same region

spatial temporal variation of seismicity and spectrum of the 1980 earthquake swarm near the izu peninsula japan abstract the spatial temporal variation of seismicity of the 1980 earthquake swarm off the east coast of the izu peninsula japan was investigated hypocentral distribution focal mechanism wave forms and spectra of seismic waves were studied the hypocenters were relocated by using the master event method the forerunning earthquakes which started about one week before the largest shock main shock the 1980 izu hanto toho oki earthquake m 6 7 occurred within the quiescent area of the earthquake activity for the preceding one year the swarm area migrated toward the south with time and triggered the main shock in june 1980 the fault dimension and geometry were estimated from the aftershock area the fault length and width are 14 km and 8 km the strike and dip angles of the fault are n15 w and 65 to n75 e locations of the events in an earlier earthquake swarm 1978 were also examined by using difference in the s p time at five selected stations distributed around the epicentral area the 1978 swarm events were found to have clustered within a very small area of 8 1 km 2 located about 2 km to the west of the 1980 swarm area the earthquakes which occurred after the main shock of the 1980 swarm were classified into two groups aftershocks and swarm events according to the location of epicenters wave forms and spectra of s waves the peak frequencies of spectra were distributed around 5 to 8 hz for the aftershocks and around 10 to 15 hz for the swarm events most of the aftershocks characterized by low frequency content occurred to the south of the main shock within 2 weeks after the main shock the number of aftershocks decayed following the modified omori9s formula with p 1 5 0 3 the swarm activity on the other hand continued intermittently for about 1 month after the main shock the 1980 seismic activity is interpreted as a complex of a foreshock main shock aftershock sequence and swarm activity the direction of the longer axis of the swarm area coincided with the direction of the maximum pressure axis of the main shock the trend of the aftershock zone coincided with the strike of the fault planes of the main shock and aftershocks this feature strongly suggests that tension cracks trending in the maximum stress direction opened prior to the occurrence of the main shock the opening of cracks may be accounted for by increasing of interstitial pore pressure associated with increase in regional stress

three stage rupture process of the 28 december 1994 sanriku oki earthquake we investigated the rupture process of the 28 december 1994 sanriku oki earthquake mw 7 7 using broadband seismograms recorded at local distances the earthquake rupture nucleated at the eastern end of the aftershock area as it propagated towards the west a major subevent occurred near the center of the aftershock area about 26 s after the initial rupture releasing most of the seismic energy of this earthquake about 24 s later another subevent followed at the western end of the aftershock area emitting large high frequency waves the whole rupture time is about 55 sec the three stage rupture process is very similar to the earlier stage rupture process of the 1968 tokachi oki earthquake mw 8 2 the difference is that the rupture of the tokachi oki earthquake further propagated towards the north in the following stage instead of terminating at the western end the occurrence of the sanriku oki earthquake is critical to the validity of the characteristic earthquake so far believed to exist in this part of the plate boundary

a review of the discrete wavenumber method we present a review of the discrete wavenumber dwn method the method introduced by bouchon and art 1977 allows the simple and accurate calculation of the complete green s functions for many problems in elastodynamics

the evolution of thermal structure and water chemistry in lake nyos abstract we collected a time series of physical and chemical data to gain a better understanding of the dynamics of lake nyos measurements of water and gas chemistry and temperature made during january march and may 1987 are compared to data taken in september 1986 just after the initial co 2 gas release there is no pattern of change in overall heat content of the lake although heat input to bottom waters 185 208 m has occurred at a rate of 1600 mw m 2 this increase in heat content translates to a change from 23 38 to 24 12 c at 200 m and can be explained by geothermal heat flow and addition of thermal spring water concentrations of ca 2 mg 2 na k fe 2 and alkalinity have increased only in bottom waters in situ lake processes such as sulfate and iron reduction are unable to account for the changes in alkalinity observed chemical changes are consistent with a scenario where slightly thermal soda water is being input to the bottom of the lake measurements of p co 2 at depth ranged from 18 to 28 of saturation and exhibited horizontal variability overall recharge of co 2 in bottom waters is negligible mainly because of increasing ion concentrations in bottom water total stability of the water column increased 33 from 48 800 j m 2 in september 1986 to 64 700 j m 2 in may 1987 as long as co 2 concentrations remain the same this level of stability is higher than could be disrupted by common limnologic or meteorologic processes there is thermal and chemical evidence that a buildup of dissolved iron and co 2 in bottom waters must have preceded the august 1986 gas release in addition a survey of all crater lakes in cameroon indicates that only lakes nyos and monoun contain high concentrations of dissolved iron and co 2 thus there is a low probability of any other cameroonian lake releasing a substantial volume of co 2

an inversion method to analyze rupture processes of small earthquakes using stopping phases 1 we propose a method to obtain source parameters most importantly the rupture velocity of small earthquakes on the basis of stopping phases it is assumed that a rupture begins at one focus of an ellipse and spreads radially at a constant rupture velocity on the fault surface eventually terminating on the elliptical boundary this model allows us to investigate a wide variety of rupture configurations ranging from a circular to a unilaterally propagating rupture in this model we find that two high frequency stopping phases hilbert transformations of each other are radiated and that the difference in arrival times between the two stopping phases is dependent on the average value of the rupture velocity the radius of the major semiaxes of the ellipse and its ellipticity these parameters can be estimated by a nonlinear least squares inversion method we perform simulation tests which allow us to detect a hilbert transform pair whose arrival times coincide with ones predicted by an assumed source model from noisy data these simulation tests show that our inversion method is useful since our method does not require an a priori assumption of the average rupture velocity the orientation of the fault plane and the circular crack model it provides us with a tool for estimating accurate source dimensions as well as rupture processes of small earthquakes

focal mechanism of an earthquake of baroque age in the regno delle due sicilie southern italy abstract a new kinematic function kf which incorporates fault dimension mean variation in velocity of rupture v r radiation pattern and mean depth from which the majority of the damaging energy is radiated is able to model the shape of isoseismals of the 1694 earthquake the focal mechanism of this earthquake is estimated to be a normal fault of apenninic direction dipping 60 70 towards the ne with a left lateral strike slip component due to the similarity with isoseismals of other earthquakes in the southern apennines it is argued that similar mechanisms could be responsible for other earthquakes in the region

a study of spectral methods of estimating the depth to the bottom of magnetic sources from near surface magnetic anomaly data summary based on a critical evaluation of several different spectral magnetic depth determination techniques on areally large synthetic layered and random magnetization models we recommend the following considerations in the usage of the methods as necessary prerequisites to successful bottom depth determinations 1 using windows with sufficient width to ascertain that the response of the deepest magnetic layer is captured and by verifying the spectra and computing the depth estimates with the largest possible windows 300 500 km 2 avoiding filtering to remove arbitrary regional fields accomplished by compiling magnetic anomalies derived from modern spherical harmonic degree 13 earth s main field models e g recent international geomagnetic reference field models igrf or comprehensive models cm 3 ascertaining the near circularity of the autocorrelation function to avoid analysing biased spectra containing strong anomaly trends and 4 avoid determining the slopes from the exponential low wavenumber part of the spectra in the cases of layered magnetization we also describe the details of the new spectral peak forward modelling method and discuss the conditions under which the method can lead to useful results we found that despite all these precautions in some cases the results can still be erroneous and therefore we recommend a critical evaluation of the results by modelling heat flow and taking into account seismic information on the crustal and lithospheric thicknesses and seismic velocities wherever possible in the southcentral us east of the rockies where the surface heat flow ranges between 40 and 65 mw m 2 w e obtained the magnetic bottom depth of 40 10 km using the approach of the forward modelling of the spectral peak this range is similar to the seismically derived crustal thickness of 45 50 km suggesting therefore that the entire crust may be magnetic in this region because of the uncertainties in the various heat flow contributing parameters such as the variations in thermal conductivity radiogenic heat and hydraulic regime we could not constrain the lithospheric thickness beyond an estimate ranging approximately from 100 to 200 km

short slip duration in dynamic rupture in the presence of heterogeneous fault properties recent studies of strong motion data consistently show that the risetime duration of slip at particular locations on the fault is significantly shorter than the overall rupture duration the physical explanation for this observation and its implications have become central issues in earthquake source studies two classes of mechanisms have been proposed to explain short risetimes one explanation is that velocity weakening frictional behavior on the fault surface causes the fault to self heal this possibility is suggested by rate dependent friction observed in laboratory experiments and by some two dimensional dynamic numerical simulations of earthquake rupture it has recently been demonstrated however that the velocity dependence of friction observed in the laboratory is too weak to cause faults to self heal an alternative explanation for short risetimes is that spatially heterogeneous fault strength e g barriers limit the slip duration in this paper we investigate this second explanation for short risetimes by constructing a three dimensional dynamic rupture model for the 1984 morgan hill california earthquake mw 6 2 using a kinematic model previously obtained from waveform inversion of strong motion data we assume velocity independent friction and a critical stress fracture criterion and derive a dynamic model specified by the spatial distribution of dynamic stress drop and strength excess that reproduces the slip and rupture time of the kinematic model the slip velocity time functions calculated from this dynamic model are then used in a subsequent inversion to fit the strong motion data by alternating between dynamic and kinematic modeling we obtain a dynamic model that provides an acceptable fit to the recorded waveforms in this dynamic model the risetime is short over most of the fault which is attributable entirely to the short scale length slip stress drop heterogeneity required by the strong motion data a self healing mechanism such as strongly velocity dependent friction is not required to explain the short risetimes observed in this earthquake

analysis of the 2001 geiyo japan earthquake using high density strong ground motion data detailed rupture process of a slab earthquake in a medium with a large velocity contrast 1 detailed rupture process of the 2001 geiyo japan earthquake mw 6 8 which is a slab earthquake in the philippine sea slab is investigated from the inversion of strong motion waveforms in the inversion a realistic curved fault model is assumed and the one dimensional underground structure models considering the deep slab structure and the shallow surface layers are used the estimated seismic moment and the maximum slip are 2 1 1019 n m and 2 4 m respectively the total rupture duration is 10 s the obtained rupture process is very complex the rupture changed gradually from a normal fault type into a more strike slip type during its propagation the slip distribution is heterogeneous and the rupture area extends over both of the slab oceanic crust and the slab oceanic mantle in the medium of the source area there exists a large velocity contrast at the slab oceanic crust mantle boundary the largest slip is seen in the low velocity low rigidity oceanic crust it is reasonable from the viewpoint of rupture dynamics that large slip occurs in the low rigidity layer which is easier to deform thus the obtained slip model suggests that the rupture process of the 2001 geiyo earthquake was strongly influenced by the heterogeneity of the background medium

effects of supershear rupture speed on the high frequency content of s waves investigated using spontaneous dynamic rupture models and isochrone theory 1 in this paper we achieve three goals 1 we demonstrate that crack tips governed by friction laws including slip weakening rate and state dependent laws and thermal pressurization of pore fluids propagating at supershear speed have slip velocity functions with reduced high frequency content compared to crack tips traveling at subshear speeds this is demonstrated using a fully dynamic spontaneous three dimensional earthquake model in which we calculate fault slip velocity at nine points locations distributed along a quarter circle on the fault where the rupture is traveling at supershear speed in the in plane direction and subshear speed in the antiplane direction this holds for a fault governed by the linear slip weakening constitutive equation by slip weakening with thermal pressurization of pore fluid and by rate and state dependent laws with thermal pressurization the same is also true even assuming a highly heterogeneous initial shear stress field on the fault 2 using isochrone theory we derive a general expression for the spectral characteristics and geometric spreading of two pulses arising from supershear rupture the well known mach wave and a second lesser known pulse caused by rupture acceleration 3 we demonstrate that the mach cone amplification of high frequencies overwhelms the de amplification of high frequency content in the slip velocity functions in supershear ruptures consequently when earthquake ruptures travel at supershear speed a net enhancement of high frequency radiation is expected and the alleged low peak accelerations observed for the 2002 denali and other large earthquakes are probably not caused by diminished high frequency content in the slip velocity function as has been speculated

depth to the bottom of magnetic sources dbms from aeromagnetic data of central india using modified centroid method for fractal distribution of sources abstract the depth to the bottom of the magnetic sources dbms has been estimated from the aeromagnetic data of central india the conventional centroid method of dbms estimation assumes random uniform uncorrelated distribution of sources and to overcome this limitation a modified centroid method based on scaling distribution has been proposed shallower values of the dbms are found for the south western region the dbms values are found as low as 22 km in the south west deccan trap covered regions and as deep as 43 km in the chhattisgarh basin in most of the places dbms are much shallower than the moho depth earlier found from the seismic study and may be representing the thermal compositional petrological boundaries the large variation in the dbms indicates the complex nature of the indian crust

gas buildup in lake nyos cameroon the recharge process and its consequences the gases dissolved in lake nyos cameroon were quantified recently december 1989 and september 1990 by two independent techniques in situ measurements using a newly designed probe and laboratory analyses of samples collected in pre evacuated stainless steel cylinders the highest concentrations of co2 and ch4 were 0 30 mol kg and 1 7 mmol kg respectively measured in cylinders collected 1 m above lake bottom probe measurements of in situ gas pressure at three different stations showed that horizontal variations in total dissolved gas were negligible total dissolved gas pressure near the lake bottom is 1 06 mpa 10 5 atm 50 as high as the hydrostatic pressure of 2 1 mpa 21 atm comparing the co2 profile constructed from the 1990 data to one obtained in may 1987 shows that co2 concentrations have increased at depths to below 150 m based on these profiles the average rate of co2 input to bottom waters was 2 6 108 mol a increased deep water temperatures require an average heat flow of 0 32 mw into the hypolimnion over the same time period the transport rates of co2 heat and major ions into the hypolimnion suggest that a low temperature reservoir of free co2 exists a short distance below lake bottom and that convective cycling of lake water through the sediments is involved in transporting the co2 into the lake from the underlying diatreme increased ch4 concentrations at all depths below the oxycline and a high14c content 41 modern in the ch4 4 m above lake bottom show that much of the ch4 is biologically produced within the lake the ch4 production rate may vary with time but if the co2 recharge rate remains constant co2 saturation of the entire hypolimnion below 50 m depth would require 140a given present day concentrations

iterative deconvolution of complex body waves from great earthquakes the tokachi oki earthquake of 1968 abstract a method of body wave inversion is developed in an attempt to extract the information about asperities or barriers in a fault zone a sequence of point sources each being characterized with the seismic moment the onset time and the location are iteratively derived from observed records at multi stations where the two dimensional extent of the source location is taken into account a modification is made of the iterative method of kikuchi and kanamori on the formulation of inversion procedure to facilitate the computation using this method we analyse long period p waves of the tokachi oki earthquake of 1968 mw 8 2 and obtain several significant subevents with time durations of 10 s their spatio temporal distribution shows that the rupture process consists of three characteristic stages a a stage of introductory rupture b a stage of main rupture and c a stage of aftershocks the main rupture takes place in the form of clustering around a few sites of the fault plane the largest subevent occurs in the northwestern corner the stress drop associated with this event is estimated to be 200 bars one order of magnitude higher than the stress drop averaged over the entire fault plane the sum of the seismic moments of the individual subevents amounts to 2 3 1028 dyn cm which approximately coincides with the one estimated from the analysis of long period surface waves this implies that the source of the tokachi oki earthquake consists of several major subevents with time durations of 10 s in addition to other minor subevents

stress breakdown time and slip weakening distance inferred from slip velocity functions on earthquake faults we estimate the critical slip weakening distance on earthquake faults by using a new approach which is independent of the estimate of fracture energy or radiated seismic energy the approach is to find a physically based relation between the breakdown time of shear stress tb the time of peak slip velocity tpv and the slip weakening distance dc from the time histories of shear stress slip and slip velocity at each point on the fault which can be obtained from dynamic rupture calculations using a simple slip weakening friction law numerical calculations are carried out for a dynamic shear crack propagating either spontaneously or at a fixed rupture velocity on a vertical fault located in a 3d half space and a more realistic horizontally layered structure with finite difference schemes the results show that tpv is well correlated with tb for faults even with a heterogeneous stress drop dis tribution except at locations near strong barriers and the fault edges we also inves tigate this relation for different types of slip weakening behavior we have applied the method to two recent strike slip earthquakes in western japan the 2000 tottori and the 1995 kobe events we integrated the slip velocity functions on the vertical fault obtained from kinematic waveform inversion of strong motion and teleseismic records from the arrival time of rupture tr to the time of the peak slip velocity tpv and we then corrected the slip obtained at tpv for the errors expected from the dynamic calculations it was found that the slip weakening dis tance d c estimated in the frequency window between 0 05 and 0 5 hz ranges between 40 and 90 cm on the two earthquake faults however if we consider the limited frequency resolution of the observed waveforms probable time errors in the slip velocity functions obtained from kinematic inversion and the uncertainty of the slip weakening behavior the above estimates may be those located between the minimum resolvable limit and the upper bound of their real values the estimated dc values do not necessarily seem to indicate larger values in the shallower part and smaller values in the deeper part of the fault but rather a spatially heterogeneous distribution that appears to be dependent on the local maximum slip this possible dependence might be interpreted by the frictional properties of the fault such as the degree of roughness or the thickness of gouge layers in addition to stress heterogeneities

on the inference of absolute stress levels from seismic radiation abstract spudich p k p 1992 on the inference of absolute stress levels from seismic radiation in t mikumo k aki m ohnaka l j ruff and p k p spudich editors earthquake source physics and earthquake precursors tectonophysics 211 99 106 this paper determines the conditions under which it is possible to learn the absolute stress level at a subset of points on a fault from observation of the fault kinematics specifically the points on a rupturing fault can be divided into two groups those points at which the rake rotates during the rupture process and those points at which the rake does not rotate if it is true that sliding frictional traction is collinear with the instantaneous velocity of one side of a fault then at the former group of points having rotating rakes there is a unique absolute stress consistent with the motion while at the latter group of points having rakes that do not rotate the absolute stress is unspecified by the motion published dislocation solutions for the 1979 imperial valley 1986 north palm springs 1987 whittier narrows and 1989 loma prieta california earthquakes are examined for evidence of temporal rotations of rake at the hypocenters and largest asperities the 1986 north palm springs earthquake shows evidence of a possible rake rotation at the hypocenter this may imply a low absolute stress level

formation and growth of normal faults at the divergent plate boundary in iceland the divergent plate boundary in iceland is characterized by 40 80 km long and 5 10 km wide swarms of tension fractures 102 m long and normal faults 103 m long the upper part of the crust is mainly composed of lava flows with abundant columnar joints that are mostly perpendicular to the lava contacts the lava flows are horizontal at the surface of the rift zone but become tilted at the rate of 1 for every 150 m depth in the crust at the surface of the rift zone the joints are vertical and parallel to the vertical principal stress because of tilting of the lava pile the columnar joints become oblique to this stress hence becoming potential shear fractures and form echelon sets at greater depths in the crust theoretical considerations suggest that normal faults start to nucleate on sets of en echelon columnar joints and or large scale tension fractures at crustal depths of 0 5 1 5 km the width depth must be the smallest controlling dimension of many faults nevertheless there is a positive linear relation r 0 91 between maximum throw and length of the holocene faults if the faults grow as self similar structures the throw length relationship can be explained by a similar relation between fault length and width

underdamped responses of a well to nearby swarm earthquakes off the coast of ito city central japan 1995 we observed coseismic and noncoseismic water level changes of an underdamped well aquifer system at epicentral distances of 5 km during the 1995 earthquake swarm activity off the coast of ito city shizuoka prefecture central japan each water level change consists of a static offset with a linear trend component followed by a damped harmonic oscillation the characteristic frequency and decay parameter of these oscillations are 18 3 s and 0 024 s 1 respectively we detected 14 such events within 8 days during the activity the offset values are mostly 2 mm or less but there were two exceptionally large offsets of 4 and 6 mm the observed waveforms are consistent with an analytical expression for the theoretical well level response to step like pressure head change in an underdamped well aquifer system this consistency allowed us to estimate the parameters of the well aquifer system and the offset component of pressure head changes from the observed hydrographs the seismic records at the nearest station indicate that among the 14 events only seven including the two exceptionally large events were apparently synchronous with swarm earthquakes among these seven events the largest two are quantitatively explained as the consequence of the coseismic volumetric strain changes the other five are interpreted as either the effect of coseismic strain change near a node of the volumetric strain field or some other coseismic effects the seven well events not synchronous with earthquakes indicate the occurrence of abrupt local pressure disturbances in the aquifer within a distance of a few tens of meters from the well

dynamic fault rupture and stress recovery processes in continental crust under depth dependent shear strength and frictional parameters abstract mikumo t 1992 dynamic fault rupture and stress recovery processes in continental crust under depth dependent shear strength and frictional parameters in t mikumo k aki m ohnaka l j ruff and p k p spudich editors earthquake source physics and earthquake precursors tectonophysics 211 201 222 the dynamic rupture process of large earthquakes in continental crust and the post seismic stress recovery process due to visco plastic behaviour are investigated by three and two dimensional numerical modelling taking into account the possible depth variations of the shear strength tectonic stress expected stress drop and of the critical weakening slip which are suggested by recent laboratory experiments on frictional parameters it was found that dynamic rupture can initiate at any depth in a brittle zone if high stress is concentrated with a dimensionless stress ratio s less than a certain limit but that the rupture cannot nucleate in a semi brittle zone with negative stress drop this suggests that the boundary between the fully brittle and semi brittle zones delineates the base of the seismogenic zone of background seismicity and aftershock distribution however the dynamic rupture of large earthquakes originating from the brittle zone can propagate into the semi brittle zone after some temporary arrest or with decelerated velocities which could generate low frequency seismic waves the depth extent of dynamic faulting and the maximum fault displacement are affected by the depth to the brittle semi brittle boundary and hence by the geothermal state within the crust the stress recovery due to visco plastic behaviour in the semi brittle and plastic regimes suggests that the rupture for eventual earthquakes tends to be nucleated near the upper boundary of the semi brittle zone

vertical crustal movement in the boso peninsula south kanto japan as deduced from the adjustment of a geodetic network with signals abstract in the adjustment of a leveling network that is frequently resurveyed we introduce corrections not only to the approximated heights of the stations but also to their rates of vertical movement which are linear over time as an unknown thus we can apply least squares collocation to the problem and have some cases corresponding to the rank deficiency of the design matrix in the observation equation we have applied this method to the study of the vertical crustal deformation in the kanto district particularly to the boso peninsula honshu japan rates of vertical crustal movement and their covariance matrices are deduced for two periods for the kanto district 1 1965 1978 and 2 1978 1989 and for three periods for the boso peninsula 1 1965 1972 2 1973 1976 and 3 1976 1988 the deduced rates range from 10 mm yr to 10 mm yr and their standard deviations amount to 11 mm yr for the kanto district for the boso peninsula they range from 22 mm yr to 6 mm yr and amount to 5 mm yr this peninsula is very close to the convergent plate boundary between the philippine sea plate and south kanto and the results of this analysis reveal the typical fluctuating behavior of the crustal movements associated with the subduction process during the interseismic period the author interprets this episodic crustal movement as being due to intermittent creep dislocation along the deeper part of the interface between the kanto district and the philippine sea plate

the bright spot of an earthquake abstract umeda y 1992 the bright spot of an earthquake in t mikumo k aki m ohnaka l j ruff and p k p spudich editors earthquake source physics and earthquake precursors tectonophysics 211 13 22 field surveys in the source region for some large shallow earthquakes reveal that there is a special spot characterized by high acceleration aftershock gap and parallel faults system this spot is called an earthquake bright spot which occupies a relatively small region compared to the final rupture region at least two kinds of phases p1 and p2 which are responsible for the rupture initiation and the bright spot formation can be identified on the broad band seismograms for large earthquakes duration times of preliminary waves p1 p2 are proportional to the earthquake magnitudes by using this evidence from the field and seismograms a model having three rupture stages is proposed for a growth process of large earthquakes the 1st stage is an initiation and propagation of rupture the 2nd stage is formation of the bright spot which produces the high frequency seismic waves in the 3rd stage smooth dislocations again propagate in bi laterally and radiate the long period seismic waves

estimation of radiated energy of finite source earthquake models we estimated seismically radiated energy for kinematic earthquake rupture models instead of integrating far field waveforms we used an expression for radiated energy with slip and stress on the fault plane using an analytic crack model of sato and hirasawa 1973 we show that the finite difference method can estimate the radiated energy fairly well however if the rupture propagation velocity is near the s wave velocity and the grid interval is coarse we may underestimate the radiated energy substantially we analyzed five models of three earthquakes of m w 5 7 for the 1995 kobe earthquake we used three different kinematic models and obtained consistent values of radiated energy of 2 4 3 6 1014 j there are common features among the three models 1 more efficient energy radiation from the deeper regions of the fault and 2 little energy radiation beneath the city of kobe the models of the kobe earthquake and the other two earthquakes show that energy radiation is concentrated near the hypocenter and the initiation point of asperities we also find the area of negative energy radiation energy absorption near the edge of large slipped areas although our estimates tend to be smaller by about a factor of 3 than previous estimates the difference is reasonable because the frequency range is not complete and we cannot deal with detailed rupture behavior such as rapid rupture acceleration and deceleration manuscript received 18 january 2002

reconstruction of dynamic rupture process of an earthquake with constraints of kinematic parameters in this paper a simple approximate method that converts the distribution of kinematic rupture parameters on a fault into that of physical or dynamic parameters is proposed the method in which the static stress drop assumed to be equal to the dynamic stress drop is applied to investigate rupture dynamics of the 1979 imperial valley california earthquake m 6 5 the dynamic rupture process is reconstructed by using a propagating crack which is a process of expanding area of stress release if we know the amount of stress drop and rupture time at each point on the fault dynamic rupture process can be simulated we use the kinematic parameters previously obtained by using waveform inversion techniques with constraints of the kinematic parameters the dynamic rupture process of the event can be reconstructed and several dynamic parameters stress drop and strength excess the difference between strength and applied initial stress can be estimated at each point on the fault by applying this method to the 1979 imperial valley earthquake we found that the rupture initiated at the low strength excess part first and then expanded toward the north and ruptured a large size high stress drop region located in the north part of the fault and finally decelerated and stopped at the north edge where the strength is high and stress drop is low these features are well explained by barrier model das and aki 1977 and are quite similar to those obtained by quin in 1990 who investigated the dynamic rupture process by the monte carlo technique with a large number of trials since repeated calculation is unnecessary our method is very effective and practical compared with his method

inversion for the physical parameters that control the source dynamics of the 2004 parkfield earthquake a fully dynamic inversion for the earthquake source process in which the geometry of the rupture area the stress conditions and frictional properties on the fault are obtained is carried out by inverting displacement records for the 2004 mw 6 0 parkfield california earthquake the rupture area of the earthquake is modeled using elliptical patches and seismograms from 10 near field digital stations are used synthetic tests to investigate the performance of the inversion in retrieving the rupture process demonstrate that we can reliably recover the large scale features of the spatiotemporal distribution of slip to investigate the stress conditions and frictional properties of the fault under which we produce a rupture model that fits the observed data we explore the parameter space using a monte carlo method and find an optimal region where the source models fit the data well the best fitting rupture process is shown to occur mainly within one horizontal elliptical region 22 km long along strike and 4 km wide along depth the seismic moment is 1 2 1018 n m and the stress drop over the ellipse is 4 mpa the rupture speed nearly constant during the entire rupture process is 2 9 km s the dimensionless quantity roughly the strain energy change per unit fault surface divided by the energy release rate which includes information on the stress and frictional properties on the fault is found to be 1 4 for the ellipse and strongly controls the rupture process along with the size of the initial circular patch that initiates the earthquake

slip along the san andreas fault associated with the great 1857 earthquake historical records indicate that several meters of lateral slip along the san andreas fault accompanied the great 1857 earthquake in central and southern california these records together with dendrochronological evidence suggest that the rupture occurred along 360 to 400 km of the fault including several tens of kilometers of the currently creeping reach in central california geomorphic expressions of late holocene right lateral offsets are abundant along the 1857 rupture along 300 kilometers of the 1857 rupture between cholame and wrightwood the youngest discernible offset ranges from 3 to 9 1 2 meters dormancy of the fault since 1857 almost certainly indicates that this latest offset was created in 1857 fault slip apparently associated with the 1857 earthquake varies in a broadly systematic way along the trace of the fault it is relatively uniform along each of several long segments but changes rather abruptly in value between these segments this nonuniform displacement pattern may imply that some segments of the fault rupture more frequently or experience a slower long term slip rate than others the 1857 offsets indicate a seismic moment m o between 5 3 and 8 7 10 27 dyne cm assuming a 10 to 15 km depth of rupture and relatively uniform slip as a function of depth a comparison with the rupture length average slip value and tectonic setting of the california earthquake of 1906 m s 8 1 4 indicates a value of m 8 1 4 for the 1857 event

evaluation of curie point depths geothermal gradients and near surface heat flow from high resolution aeromagnetic hram data of the entire sokoto basin nigeria abstract an evaluation of curie point depths geothermal gradients and near surface heat flow has been carried out from the spectral analysis of the recently acquired high resolution aeromagnetic hram data of the entire sokoto basin in northwestern nigeria the hram data was divided into twenty two 22 overlapping blocks and each block analyzed using the spectral centroid method to obtain depth to the top centroid and bottom of magnetic sources the depth values were subsequently used to evaluate the curie point depth cpd geothermal gradient and near surface heat flow in the study area the result shows that the cpd varies between 11 13 and 27 83 km with an average of 18 57 km the geothermal gradient varies between 20 84 and 52 11 c km with an average of 33 99 c km and the resulting heat flow varies between 52 11 and 130 28 mw m 2 with an average of 84 97 mw m 2 such heat flow values are suggestive of anomalous geothermal conditions and are recommended for detailed geothermal exploration in the basin

heterogeneity of the 1980 izu hanto toho oki earthquake rupture process summary the rupture process of the 1980 izu hanto toho oki japan earthquake is examined by inverting strong motion seismic waveform records synthetic seismograms for waveform inversion are calculated using the empirical green s function technique after selecting the optimum small events for the empirical green s function the rupture starting time moment release and rise time at each sub fault are estimated in this procedure the results are as follows in the northern part of the fault surface where the swarm activity occurred the rupture propagation is irregular less moment is released and the rise time is short the local stress drop distribution is also estimated using the results of the inversion it is found that the heterogeneities of the rupture propagation moment release distribution and stress drop distribution have some relation to the geological setting in this area or to the seismic swarm activity

dynamic rupture processes of inland earthquakes in japan weak and strong asperities reconstruction of dynamic rupture process of four large earthquakes in japan shows two types of high stress drop region on the fault planes i e high stress drop part with low strength excess and that with high strength excess where the strength excess is the difference between strength and applied initial stress here we call the first type of the above high stress drop part weak asperity or asperity with low strength and the second one strong asperity or asperity with high strength

heterogeneous distribution of dynamic stress drop and relative fault strength recovered from the results of waveform inversion the 1984 morgan hill california earthquake the dynamic rupture process of the 1984 morgan hill california earthquake m 6 2 has been investigated on the basis of a three dimensional 3d dynamic shear crack model from previous waveform inversion results for this purpose a locking fracture criterion is introduced for rupture propagation from which a lower bound of the peak shear stress just before rupturing and hence of the relative fault strength has been estimated at each fault segment the distribution of static and dynamic stress drops has also been independently evaluated from that of fault slips by linear and nonlinear inversion procedures respectively the results show that large slips located from the hypocenter to about 10 km and 14 to 17 km along the strike at depths between 8 and 12 km result from local dynamic stress drops exceeding 40 and 140 bars respectively negative stress drops down to 15 bars are required to explain very low slips over a shallow fault section if a non negative stress drop condition is imposed somewhat larger slips are obtained there than those from the waveform inversion the above results suggest that there could be a zone of velocity strengthening frictional behavior in the shallow crust which may have arrested slip motion during rupture propagation the strength excess is found to be generally small but somewhat larger at a small zone that has delayed rupture propagation the dynamic rupture initiated from a small nucleus zone with a low stress drop propagated southeastward breaking the deeper fault section with high stress drop and then broke a relatively high strength zone after a short time of arrest with highest stress drop

curie temperature depth estimation using a self similar magnetization model summary the earth s crust is magnetized down to the curie temperature depth at about 10 to 50 km this limited depth extent of the crustal magnetization is discernible in the power spectra of magnetic maps of south africa and central asia at short wavelengths the power increases as rapidly towards longer wavelengths as expected for a self similar magnetized crust with unlimited depth extent above wavelengths of about 100 km the power starts increasing less rapidly indicating the absence of deep seated sources to quantify this effect we derive the theoretical power spectrum due to a slab carved out of a self similar magnetization distribution this model power spectrum matches the power spectra of south africa and central asia for a self similarity parameter of 4 and curie temperature depths of 15 to 20 km

what can be learned from rotational motions excited by earthquakes summary one answer to the question posed in the title is that we will have more accurate data for arrival times of sh waves because the rotational component around the vertical axis is sensitive to sh waves although not to p sv waves importantly there is another answer related to seismic sources which will be discussed in this paper generally not only dislocations commonly used in earthquake models but also other kind of defects could contribute to producing seismic waves in particular rotational strains at earthquake sources directly generate rotational components in seismic waves employing the geometrical theory of defects we obtain a general expression for the rotational motion of seismic waves as a function of the parameters of source defects using this expression together with one for translational motion we can estimate the rotational strain tensor and the spatial variation of slip velocity in the source area of earthquakes these quantities will be large at the edges of a fault plane due to spatially rapid changes of slip on the fault and or a formation of tensile fractures

dynamic fault rupture process during the 1978 thessaloniki earthquake northern greece abstract recent observations of strong earthquakes that occurred in the aegean and surrounding areas during the last several years indicate rather low stress drop values an interesting example is the 1978 thessaloniki earthquake northern greece with ms 6 5 values between 6 6 and 12 bars have been determined for this earthquake by assuming unilateral or bilateral rupture propagation and circular or rectangular fault models these values are considerably lower than the average values calculated not only for intraplate earthquakes about 60 bars but also for interplate earthquakes about 30 bars in this paper an attempt is made to investigate the cause of this fact by studying the dynamic rupture process of this earthquake a three dimensional spontaneous shear crack model on a dipping fault in a layered medium was assumed and a finite difference approach has been adopted for numerically solving the wave equations in a three dimensional space it was found that such a low stress drop may be obtained in cases of small differences between ambient stress and fault strength and between tectonic shear stress and dynamic frictional stress such physical conditions in the focal region may result in an average fault displacement which is close to the observed one for this earthquake

slip weakening distance in dynamic rupture of in slab normal faulting earthquakes we apply the above method to three large in slab normal faulting earthquakes in the mexican subduction zone to do this we calculate the spatial distribution of slip velocity functions and final slip from kinematic waveform inversion of strong motion and teleseismic records and the stress history and final stress change from dynamic rupture calculations by integrating the slip velocity functions obtained from the inversion from the rupture arrival time to the time of peak slip velocity we obtain slip dat t pv and then correct it for dc at t b through dynamic calculations we also estimate the lowest resolvable limit and probable errors of dc from the slip velocity functions and its upper bound from a theoretical constraint between the dynamic stress drop and dc we found that the slip weakening distance dc estimated in the frequency band between 0 05 and 0 5 hz ranges between 40 and 120 cm on the in slab fault of the 1999 oaxaca earthquake mw 7 5 the largest dc is detected in the central fault and in part of the deeper sections and dc in the zone around the hypocentre ranges between 50 and 70 cm the estimated dc values appear to be less depth dependent but are rather more dependent on the local maximum slip this possible slip dependence might be interpreted by the degree of fault roughness in addition to stress heterogeneities the fracture energy g in the central section and in the hypocentral zone are roughly estimated to be of the order of 10 15 and 5 8 mj m 2 respectively both of the estimated dc and g values are somewhat larger than those on the vertical fault of two recent shallow strike slip earthquakes in western japan

determination of constitutive relations of fault slip based on seismic wave analysis constitutive laws define the boundary conditions on fault plane and govern many aspects of earthquake failure although several constitutive laws have been formulated based on laboratory rock experiments and applied to theoretical studies in various fields no actual relation during a natural earthquake has been determined the 1995 kobe earthquake is suitable for detailed kinematic analysis and this enables the first evaluation of constitutive relations for a natural earthquake in this study we determine spatiotemporal slip distribution on an assumed fault plane of the 1995 kobe earthquake by waveform inversion and then solve elastodynamic equations using a finite difference method to determine the stress distribution and constitutive relations on the fault plane an inversion method based on bayes theorem is employed to obtain a spatiotemporal slip distribution and enables us to ensure the objective uniqueness of the solution with numerous parameters and smoothing constraints this slip distribution is then used as part of the boundary condition in the finite difference calculation the time histories of slip and shear stress obtained then provide a constitutive relation at each point on the fault plane they show slip weakening relations almost everywhere on the fault plane while slip rate dependency is not clear the slip weakening behavior has a clear depth dependency indicating that the slip weakening rate d du is smaller in the shallow crust than that in the deep crust this may be associated with the paucity of shallow seismicity observed in the source region of this earthquake as reported for many mature fault systems

inversion of strong ground motion and teleseismic waveform data for the fault rupture history of the 1979 imperial valley california earthquake a least squares point by point inversion of strong ground motion and teleseismic body waves is used to infer the fault rupture history of the 1979 imperial valley california earthquake the imperial fault is represented by a plane embedded in a half space where the elastic properties vary with depth the inversion yields both the spatial and temporal variations in dislocation on the fault plane for both right lateral strike slip and normal dip slip components of motion inversions are run for different fault dips and for both constant and variable rupture velocity models effects of different data sets are also investigated inversions are compared which use the strong ground motions alone the teleseismic body waves alone and simultaneously the strong ground motion and teleseismic records the inversions are stabilized by adding both smoothing and positivity constraints the moment is estimated to be 5 0 10 25 dyne cm and the fault dip 90 5 dislocation in the hypocentral region south of the united states mexican border is relatively small and almost dies out near the border dislocation then increases sharply north of the border to a maximum of about 2 m under interstate 8 dipslip motion is minor compared to strike slip motion and is concentrated in the sediments the best fitting constant rupture velocity is 80 per cent of the local shear wave velocity however there is a suggestion that the rupture front accelerated from the hypocenter northward the 1979 imperial valley earthquake can be characterized as a magnitude 5 earthquake at the hypocenter which then grew into or triggered a magnitude 6 earthquake north of the border

higher order interrelations between seismogenic structures and earthquake processes abstract aki k 1992 higher order interrelations between seismogenic structures and earthquake processes in t mikumo k aki m ohnaka l j ruff and p k p spudich guest editors earthquake source physics and earthquake precursors tectonophysics 211 1 12 to advance our understanding of higher order smaller scale interrelations between the fault zone structure and the earthquake process we start with the observation that the source parameters of major california earthquakes deviate systematically from the self similarity this departure from the self similarity is interpreted in terms of both the specific barrier model and the asperity model the asperity model however is found to be inconsistent with the following observations 1 sharp impulsive displacements waveform in the direction perpendicular to the fault trace 2 the complementary relationship between fault slip and aftershock and 3 existence of barriers in the creeping segment of the san andreas fault the application of the specific barrier model on the other hand leads to a predominant subevent crack size of a few hundred meters in the creeping segment this result is consistent with the negative correlation between coda q 1 and b value observed in the stone canyon area for 1 5 3 temporal increase in coda q 1 is sometimes accompanied by an increase in the rate of surface deformation but not always so this is attributed to the erratic coupling between the creeping brittle region and the overlying brittle region the applicability of the conclusions obtained from california earthquakes to other regions is investigated using the observed scaling laws of seismic source spectra and it is suggested that the crack size of the creeping brittle zone may vary regionally from a few hundred meters to a few kilometers as an alternative to the widely held view that the aseismic creep in the plate boundary may occur as a giant slow earthquake our result suggests that it may be occurring as small fractures with scale lengths from a few hundred meters to a few kilometers

contemporary crustal deformations in the izu peninsula honshu japan and acceleration of plate bending in the northernmost part of the philippine sea plate since the beginning of the anomalous vertical crustal movement in the izu peninsul honshu japan many repeated precise levellings have been carried out by the geographical survey institute trilaterations covering the entire izu peninsula have also been carried out by the geographical survey institute a new technique is developed to adjust the results of levellings because they had been carried out for different epochs along each levelling route and because of rapid vertical crustal movements in conventional least squares adjustment of levelling network only corrections to the approximate height are assumed to be unknown while in the present analysis a special model in which rates of vertical deformation at any bench marks are also assumed to be unknown is adopted in addition tidal stations along the coast of the izu peninsula yield the rate of vertical crustal movement from analysis of tidal data independent of levelling data we select several special bench marks in which rates of vertical movement are determined by tidal analysis thereafter special adjustment is applied according to the type of network

a simplified hypocenter model based on the multi criterion principle for strong earthquakes abstract a simplified model and a tectonophysical method based on the multi criterion principle mcp for strong earthquakes are developed for estimating hypocenter parameters seismic energy and efficiency and earthquake magnitude and examined by means of calculations based on data for 28 strong earthquakes in china the results show that the deviations of the estimated magnitudes ms from those determined previously using conventional seismological methods are within 0 5 for 78 6 of the earthquakes examined mcp emphasizes that besides the failure mode of rock brittle or ductile which used to be considered in the study of hypocentral processes it is also necessary to take into consideration the instability mode gradual or sudden in determining whether earthquakes will occur or not a strong earthquake is primarily a sudden high pressure instability event in addition mcp may be used for elucidating relationships between stress level and stress drop and also indicates an approach to estimating seismic efficiency