earthquake multiplets in the southeastern solomon islands the 350 km stretch of the solomon islands trench that lies adjacent to the islands of guadalcanal and san cristobal has been the site of 10 large shallow earthquakes since 1966 june 15 1966 doublet m s 7 7 7 3 may 20 21 1977 quadruplet m s 6 7 7 5 7 5 7 5 november 4 5 1978 doublet m s 6 9 7 1 and two single events on october 23 1979 m s 7 1 and february 7 1984 m s 7 7 analyses of p wave first motions aftershock distributions p wave forms recorded at epicentral distances of 30 to 90 on the wwssn and 256 s period rayleigh wave spectra obtained from the ida and gdsn networks are used to examine the source characteristics of the events the close temporal association and large size of earthquakes precluded collection of useful seismograms for the latter of the two 1966 events and the second and third events of the 1977 sequence p waveforms analyzed show smooth traces that can be modeled with simple trapezoidal source time functions located at shallow 15 45 km depths focal mechanisms are consistent with oblique subduction of the indian plate beneath the pacific plate at about n73 e estimates of seismic moment for individual events range from 0 2 to 2 3 10 27 dyne cm average stress drops of the sequences are estimated to range between 10 and 40 bars this section of island arc may have ruptured in a similar sequence of earthquakes during the 1930 s conversion of the cumulative seismic moment to displacement and averaging over 40 years suggests a convergence rate of about 4 5 cm year 1 about one half of the rates that have been assessed on the basis of seafloor magnetic lineations

broadband time domain modeling of earthquakes from friuli italy short period sp and long period lp seismograms written by the main shock and two principal aftershocks of the 1976 friuli italy earthquake sequence are modeled in the time domain using synthetic seismograms the main shock occurred on 6 may 1976 20h 00m m s 6 5 and both aftershocks on 15 september 1976 03h 15m m s 6 0 and 09h 21m m s 5 9 source models were determined initially by trial and error and then refined using a waveform inversion program two point sources of radiation are required to adequately model the aftershock short period records for the 09h 21m aftershock the model derived from short period records also produces good fits to the long period data the seismic moment of this earthquake is found to be 0 8 to 1 0 10 25 dyne cm the sp model for the 03h 15m aftershock on the other hand predicts long period synthetics which do not agree with the observations in particular the sp moment 0 37 10 25 dyne cm is about formula times smaller than the lp moment 1 10 25 dyne cm adding a long period component to the sp model considerably improves lp waveform and moment agreement in the case of the main shock a reasonable fit to the observed sp data is obtained using three point sources of radiation however lp synthetics computed using this model do not agree with the observations and the sp moment 0 65 10 25 dyne cm is a small fraction of the lp moment 3 to 5 10 25 dyne cm time function durations indicate that the individual events inferred from the sp records are radiated from patches of the fault having radii of 2 to 4 km and stress drops in the range 35 to 276 bars in comparison stress drops estimated from lp data are found to be 12 bars main shock and 24 bars 09h 21m aftershock these observations suggest that the short period instruments are sensitive to the high frequency radiation emitted from small high stress drop areas on the fault plane whereas the long period instruments record the overall motion during the earthquake

seismotectonics of southern iran the oman line seismotectonic characteristics of the transition area between the zagros continental collision zone and the makran ocean continent convergence zone the oman line are carefully examined a northeast trending zone of earthquakes terminates the zagros belt of seismicity just west of the oman line this seismic zone extends northeastward beyond the main zagros thrust and coincides with a surface escarpment clearly visible on landsat imagery events with accurately determined depths along this seismic zone occurred in the upper 20 km of the crust and possibly show a shallow 10 northeastward dip east of the oman line the zendan minab fault system is relatively aseismic the oman line region may be characterized by underthrusting of a wedge of arabian shelf edge beneath iranian crust or by an indentation of arabia into the iranian crustal block as a promontory available seismological and geological data cannot uniquely distinguish between these two possible tectonic settings the area located to the northwest of the oman line region now a zone of continental collision appears to have been characterized by an episode of neogene subduction a well located intermediate depth earthquake occurred in this area to the northeast of the main zagros thrust on november 9 1970 it was accurately located at 107 km depth beneath a line of quaternary volcanoes its focal mechanism may indicate downdip tension with nodal planes that strike closely parallel to the trend of the zagros arc comparison of this event and other neotectonic features in this part of iran with those in other active convergent zones suggests that a descending oceanic lithosphere beneath the zagros volcanic arc may still be attached to the colliding arabian plate

depth resolution of earthquakes in central asia by moment tensor inversion of long period rayleigh waves effects of phase velocity variations across eurasia and their calibration the moment tensor inversion method for long period rayleigh waves is discussed from the point of view of event depth resolution in this method depth is usually determined by finding the minima in appropriate least squares residuals versus depth curves we show that depth cannot be resolved if the data are sampled only at one frequency and a choice of frequencies spanning a wide range of values yields better resolution we also show that the variance reduction at the minimum depends strongly on biases in the data provided they are not simple periodic functions in azimuth from the source since such biases are introduced in particular when the propagation corrections are inaccurate this provides a means by which to study regional trends of phase velocities this method is then applied to obtain an estimate of lateral heterogeneity in central asia starting with a reference point in the pamir mountains and a set of accurately known phase velocities on paths to eurasian wwssn stations we study the rayleigh waves at these stations in the period range 30 to 90 s for eight events located in or near the tibet plateau at stances from the reference point heterogeneity in phase velocity is thus found to be about 1000 to 1500 km in this period range constraining the source parameters of the most distant events by body wave modeling we determine new phase velocities on paths to wwssn stations from the center of the tibet plateau the variation of phase velocity with source location reaches 1 to 2 and the low velocity within tibet clearly has a dominant effect we then discuss the regionalization of phase velocities as a possible way to determine a calibration curve for a given source to receiver path we find that although the regionalized phase velocities in eurasia are stable to first order they do not seem accurate enough to be used for moment tensor inversions with arbitrary source locations this may be due to non ray theoretical effects causing the breakdown of phase velocity additivity when higher order accuracy is needed on the other hand reference phase velocities on source receiver paths can be obtained reasonably fast for events large enough so that body wave modeling is possible to constrain depth and source mechanism we demonstrate that these phase velocity curves are accurate enough for the calibration of propagation effects for another event located as much as 400 to 500 km away this provides an encouraging approach to study both structure and source mechanisms of smaller events within a highly heterogeneous continental region the only constraint is to find several events with the right magnitude of both long period body waves and surface waves

seismicity of the forearc marginal wedge accretionary prism three different types of seismic data have been examined for seismic events occurring within the zone called the accreted wedge or forearc marginal wedge that underlies the inner trench wall of some arcs these types of data are 1 teleseismically recorded earthquakes that have been reported in the literature as occurring in major arc trench regions these events fail to demonstrate that earthquakes occur within the accreted wedge because the uncertainty of focal depth usually exceeds the depth dimension of the accreted wedge these data include many tsunamigenic earthquakes 2 local earthquakes located by combined ocean bottom seismograph and land networks in the arc trench region in the new hebrides and the central and eastern aleutian trench none of the more reliable of these hypocenters lies within the accreted wedge 3 s p intervals measured at stations on islands located on the outer ridge or at ocean bottom seismograph stations on the forearc marginal wedge these data do not show the existence of events occurring within the accreted wedge e g from 18 ocean bottom seismograph stations with a cumulative operation time of about 1 year the smallest s p time is about 2 5 s for events in the new hebrides and about 4 s for events in the adak and kodiak regions we found no s p time smaller than 2 s from 6 years of seismograms recorded at middleton island alaska and no s p time smaller than 4 s from 25 years of seismograms recorded on barbados all of the events could have occurred outside the forearc marginal wedge although we have located no events with hypocenters which are situated unambiguously within the accreted wedge we have located many events which occurred in the vicinity of the accreted wedge some occur in the upper portion of the benioff zone shallower than about 40 km and others occur in the leading edge of the overriding plate these locations reported in this paper which are within or close to the seismic networks comprise some of the most reliably located shallow events ever reported in convergent margins of island arcs the absence of seismic activity within the accreted wedge in any island arc that we studied suggests that the deformation evident in this region occurs aseismically

deflection of oceanic transform motion at a continental margin as deduced from waveform inversion of the 1939 accra ghana earthquake abstract waveform inversion of teleseismic p waves of the 1939 accra ghana earthquake m 6 5 suggests that the earthquake was composed of two events the smaller event m 6 1 occurring 9 5 s before the onset of the larger m 6 4 event the smaller event has a focal mechanism that suggests it occurred immediately north of the intersection of the akwapim and coastal boundary fault inversion results also indicate that the larger event could not have occurred on the reactivated akwapim fault system as suggested by previous researchers instead the larger event appears to have occurred along a strike slip fault that parallels the coastline of ghana these results along with the trend of historic earthquakes indicate that not all the motion along the romanche or chain fracture zones is being transmitted inland along the akwapim fault system but that some is being deflected tangentially along a fault that parallels the ghana coastline

source characteristics of the 12 november 1996 m w 7 7 peru subduction zone earthquake the 12 november 1996 m w 7 7 peru subduction zone earthquake occurred off the coast of southern peru near the intersection of the south american trench and the highest topographical point of the subducting nazca ridge we model the broadband teleseismic p waveforms from stations in the global seismic network to constrain the source characteristics of this subduction zone earthquake we have analyzed the vertical component p waves for this earthquake to constrain the depth source complexity seismic moment and rupture characteristics the seismic moment determined from the nondiffracted p waves is 3 5 1020n m corresponding to a moment magnitude m w of 7 6 7 7 the source time function for the 1996 peru event has three pulses of seismic moment release with a total duration of approximately 45 50 seconds the largest moment release occurs at approximately 35 40 seconds and is located 90km southeast of the rupture initiation approximately 70 of the seismic moment was released in the third pulse

source modeling of the hsingtai china earthquakes of march 1966 abstract the hsingtai china earthquakes of march 1966 were a series of destructive earthquakes associated with the shu lu graben five strong shocks of ms 6 occurred within a period of less than a month the largest of which was ms 7 2 body and surface waves over the period range from several to 100 s have been modeled for the four largest events using synthetic seismograms in the time domain and spectral analysis in the frequency domain data from ground deformation local geology regional seismic network and teleseismic joint epicenter determination have also been used to constrain the source model and the rupture process the fault mechanism of the hsingtai sequence was mainly strike slip with a small component of normal dip slip the strikes of the four largest shocks range from n26 to 30 e approximately along strike of the major faults of the shu lu graben and the aftershock distribution the source mechanisms can be explained with a nnw sse extensional stress and a nee sww compressional stress acting in the area the major shocks all had focal depths 10 km the four largest shocks in the sequence were characterized by a relatively simple and smooth dislocation time history the durations of the far field source time functions ranged from 3 5 to 5 s while the rise times were all 1 s the seismic moments of the four largest earthquakes ranged from 1 43 1025 to 1 51 1026 dyne cm 1 the fault sizes of the four events were very close assuming circular faults the diameters of the four events were determined to be between 10 and 14 km stress drops varied from 52 to 194 bars a trend of increasing stress drop with earthquake size was observed a survey of stress drop determinations for 15 major intraplate earthquakes shows that on the average the magnitude of stress drop of oceanic intraplate earthquakes and passive continental margin events is higher 200 to several hundred bars than that of continental intraplate earthquakes 100 bars or less

irpinia italy 1980 earthquake waveform modelling of strong motion data summary several strong motion records of the irpinia southern italy 1980 earthquake have been modelled for a total time duration of about 180 s with the use of the rnultimode summation of waves of p sv and sh type if waveform modelling is applied to unfiltered accelerations the best that can be done at present is amplitude envelope matching in time and frequency domain the early phase of the studied event can be synthesized by 12 main episodes of different sizes modelling the rupturing process the space time distribution of such episodes suggests that the rupturing process did propagate in a bilateral fashion this model of the seismic source gives rise to synthetic velocity records which match quite well the experimental ones to avoid the problems connected with the large instability of displacements obtained from direct numerical integration of observed accelerograms the synthetic signals can be first used to perform acceleration or velocity fitting of experimental data an estimate of the ground displacement above 1 hz can then be obtained by analytical integration of the synthetic velocities

a procedure for modelling shallow dislocation sources summary expressions for displacements on the surface of a layered half space due to an arbitrary oriented shear dislocation are given in terms of generalized ray expansions useful approximations of these expressions for shallow events as recorded at teleseismic distances for realistic earth models are presented the results of this procedure are used to generate synthetic p sv and sh waveforms for various assumptions of stress drop

the 1967 caracas earthquake fault geometry direction of rupture propagation and seismotectonic implications the fault plane orientation of the july 30 1967 caracas earthquake mw 6 6 has been a source of controversy for several years this strike slip event was originally thought to have occurred on an east west oriented fault plane reflecting the relative motion between the caribbean and south american plates more recently however the complex seismic radiation from this event was interpreted as being indicative of a north south striking fault that ruptured along three en echelon segments in this study we synthesize evidence based on the intensity and damage reports the distribution of aftershocks and the results of a joint formal inversion of the p and sh waves and show that these data clearly indicate that the rupture of the 1967 earthquake occurred on an east west trending fault system using a master event technique the largest aftershock which occurred 40 min after the main event is shown to lie 50 km east of the epicenter of the mainshock the epicentral distances of small aftershocks registered in caracas based on the s p arrival time differences and the polarizations of the p waves are also consistent with these events occurring on an east west oriented fault system north of caracas a joint inversion of the teleseismic p and sh waves recorded on long period seismographs of the world wide standardized seismographic network shows that in a time frame of 65 s four distinct bursts of seismic moment release subevents occurred with a total seismic moment of 8 6 1018 n m the first three subevents triggered sequentially from west to east in a direction that is almost identical to the east west trending nodal planes of the source mechanisms the average depth of these three subevents is 14 km the fourth and last identifiable subevent of the sequence shows a reverse faulting mechanism with the nodal planes oriented roughly east west it occurred at a 21 km depth about 50 km to the north of the fault zone defined by the strike slip subevents this fourth subevent appears to reflect compressional deformation of the southern caribbean possibly related to underthrusting along the proposed curacao trench the complexity of the fault system causing the 1967 earthquake suggests that the relative motion along the caribbean south america plate boundary in central venezuela is taken up over a broad highly faulted and highly stressed zone of deformation and not by a simple major throughgoing fault

triggering of earthquakes during the 2000 papua new guinea earthquake sequence 1 a sequence of large earthquakes occurred in the new britain new ireland region of papua new guinea in late 2000 the sequence started with a mw 6 8 earthquake along the new britain trench on 29 october about 20 days later a mw 8 2 earthquake occurred in the new ireland region on 16 november and produced large strike slip surface displacements and tsunamis following the mw 8 2 event two large earthquakes mw 7 5 occurred along the nearby new britain trench on 16 and 17 november furthermore small triggered events were observed over a wide area outside of the rupture zones of the large earthquakes with different focal mechanisms from those of the major events there is likely some mechanism s that triggered this remarkable sequence and we document the details of the spatial and temporal patterns of the events we investigated if static stress changes can explain the initiation of the large earthquakes and also some groups of triggered smaller events there are mixed results the occurrences of the two m 7 5 major events may be explained by the static stress changes however there are also some earthquakes that are not consistent with triggering by static stress changes there may be multiple mechanisms including static and dynamic stress changes that are needed to explain the complicated sequence of earthquakes

the campania lucania southern italy earthquake of 23 november 1980 abstract we present the main seismological results of our study of the campania lucania earthquake of 23 november 1980 a complete set of far field and local data has been analysed from long period body waves data we determine the fault plane solution 1 140 1 60 2 75 2 54 a depth of 15 km and calculate a seismic moment of 6 1025 dyne cm and a source duration of 6 s from data of a local network deployed immediately after the event we determine aftershock locations they are aligned in a direction nw se that fit extremely well with the focal solution determined above we can choose as fault plane the plane striking 140 and dipping at 60 and the event is a normal event with a large component of left lateral strike slip the source area evaluated from this aftershock distribution 14 km 40 km is quite suitable for an earthquake of a seismic moment of 6 1025 dyne cm

analysis of the 1986 mt lewis california earthquake preshock sequence mainshock aftershock sequence zhou y mcnally k c and lay t 1993 analysis of the 1986 mr lewis california earthquake preshock sequencemainshock aftershock sequence phys earth planet inter 75 267 288 the 1986 mt lewis earthquake m l 5 7 occurred on a right lateral fault northeast of and oblique to the calaveras fault in a region that had not experienced significant seismicity since 1943 data from the nearby lawrence livermore seismic network and selected u s geological survey stations are used to relocate events within 15 km of the mainshock epicenter during the period 1980 1987 using the master event method beginning 17 months before the mainshock 22 events ruptured in the depth range 5 9 km within 1 4 km and northwest of the mainshock epicenter in an area subsequently almost devoid of aftershocks this cluster of preshock activity is clearly separated both spatially and temporally from the background activity in the surrounding area composite focal mechanisms for the preshocks and for nearby aftershocks suggest that there are two slightly different focal mechanisms amongst the preshocks one being similar to the mainshock and aftershocks and one being rotated in strike cross correlations of digitally recorded short period waveforms of 10 of the clustered preshocks mu 1 5 2 5 reveal that the average inter event peak cross correlation between seismograms is 0 62 six nearby early aftershocks show an average inter event peak cross correlation between seismograms of 0 54 only a few aftershocks have cross correlations of 0 6 or higher which implies that the events were slightly further apart or multiple mechanisms were active during the early aftershock period no significant differences are observed between the spectra of the preshocks and aftershocks the aftershock area expanded along the strike with time ultimately defining a north south fault plane 11 km in length extending from 3 to 10 km in depth the mainshock appears to have originated at the base of the seismogenic zone and ruptured bilaterally along strike and updip a forward modeling technique is used to model teleseismic body waveform data of the mainshock the long period data p and sh are consistent with a point source strike slip earthquake with a teleseismic moment of 3 9 1017 n m we infer that the mainshock involved the rupture of an asperity in the central portion of the aftershock zone

in the footsteps of pliny tracing the sources of garamantian carnelian from fazzan south west libya abstract references in the ancient sources indicate that the libyan desert was a source of carbunculi semi precious red stones and gemstones variously interpreted as ruby garnet and spinel amongst others while gemstones are not attested in the geological strata of fazzan south west libya a range of silica based stones including chert chalcedony agate and carnelian are known to originate in this area linked to an early civilisation known as the garamantes it has been long proposed that the geochemical signature and the variations in the relative proportions of quartz moganite phases can be used to distinguish between groups of stones of different origin the proposed methodology was tested on a number of archaeological samples from the garamantian sites of jarma ancient garama and saniat jibril in fazzan fragments of chert carnelian and amazonite found at the two sites have been identified as raw materials associated with beadmaking trace elemental data obtained by la icp ms were combined with mineralogical data obtained by x ray powder diffraction and raman spectroscopy on the same samples and a group of reference samples the dataset has been compared with the available literature and data from other localities around the world to this purpose a preliminary database of silica based materials was established for provenance work based on the scarce data available in the literature the importation of these stones from eastern localities such as india may be ruled out the measured data on archaeological samples and debitage allow us to define a reliable reference group of parameters for materials from fazzan which are likely to be derived from a unique geological source the methodology should be extended and compared with cherts and carnelians from a range of mediterranean and sub saharan sites this characterisation work is a tool of high potential utility for a new investigation of ancient contact and trade across the trans saharan zone

the 16 august 1931 valentine texas earthquake evidence for normal faulting in west texas the 1931 valentine texas earthquake occurred in a region near the transition from the southern basin and range rio grande rift physiographic province to the southern great plains province previous studies of this earthquake have presented focal mechanisms from first motion data supporting either pure normal or strike slip faulting with extension oriented in a southwest northeast direction nearly perpendicular to the physiographic boundary between the provinces this study presents the results of an inversion of regional and teleseismic body wave phases to determine earthquake source parameters results of the inversion suggest that the earthquake occurred at a depth of 10 km along a fault dipping 54 to the east with a strike of 10 to 20 and a rake of 155 the strike is similar to that of the southern segment of the nearby mayfield fault a fault that shows evidence for repeated movement during the quaternary seismic and geodetic information suggest unilateral rupture toward the south during the earthquake the t axis for the earthquake shows extension oriented in a northwest southeast direction a direction in agreement with the direction given by geologic stress indicators

possible focal mechanism change during reactivation of a previously ruptured subduction zone and stress tensor implications summary body waveform inversion methods usually assume a constant double couple source or a combination of such subevents previous studies suggest that the source rupture of mature sources in terms of coupling along smooth subduction zones are usually resolved on this assumption however for events where the source area overlaps previously ruptured zones the conventional inversion methods fail to resolve the source process for such events the subduction coupling may vary between the mature source area and the previously ruptured zone causing a variation in the focal mechanism too we study the implications on the stress tensor of the coexistence of two such focal mechanisms using a major aftershock of the 1985 chilean earthquake m 6 0 the 1978 kurile m 7 6 and 1980 santa cruz islands m 7 7 earthquakes for the first event we use the two different sets of focal parameters obtained by a study which describes a focal mechanism change during the process the epicentres of the latter two large events are located in the spots which had remained unbroken in the preceding large events e g the 1963 and 1969 kurile earthquakes and the 1966 santa cruz is earthquake respectively for each event one of the focal mechanism solutions shows a substantial strike slip component the stress tensors are examined under the constraints that the shear stress is parallel to the slip vector and a friction law is satisfied two particular families of stress tensors examined are 1 the tensors that are optimally orientated with respect to the mature source area and 2 the tensors that have a vertical principal stress direction the first type of tensors can fully account for the observed slip orientation only for the chile aftershock but it comes close to the observed one in the case of the kurile island event and would indeed reactivate the decoupled area at a reasonably low stress level the second type of tensors can be compatible with the two observed slip orientations in all cases however for the santa cruz is and kurile is earthquakes the friction law which should be satisfied on each plane requires too high a stress deviation

modeling near field and teleseismic observations from the amchitka test site during the amchitka testing program recordings were made of the near field ground velocity for events milrow and cannikin these records exhibited a p arrival a feature associated with near field pp and a developing rayleigh wave in this investigation simple models were found which match the complete seismograms they simultaneously predict the amplitude wave shape timing and periodicity of both the body waves and the surface waves models for both the amchitka crustal structure and the source time functions were developed the crustal model is similar to previous models in its average properties so travel time constraints are still satisfied small modifications have been made to match the waveform data the events on amchitka can apparently be represented as isotropic point sources with the types of simple source time histories predicted by classical yield scaling laws a source model for event longshot was developed on the basis of the yield scaling laws since no near field records are available for it the source models were tested by comparing the teleseismic body waveforms they predict with observations short period p waves long period p waves and long period ps waves were considered the short period p waves provided the best constraint to match them it was necessary both to develop a model for teleseismic pp since it arrives later than the near field model would predict and to estimate the average value of t the milrow and cannikin teleseismic short period p waves indicate that the average effective t for teleseismic ray paths from amchitka is 0 9 s the scatter about this value however is very substantial the accuracy of the scaling laws was tested by comparing synthetics for longshot to teleseismic observations the predicted amplitudes and wave shapes fall within the scatter in the observations though again this scatter is substantial

are asperity patterns persistent implication from large earthquakes in papua new guinea 1 we studied the distribution of asperities for large recent earthquakes along the new britain trench papua new guinea to investigate if they are the same for repeated ruptures of the subduction boundary we determined the slip distributions of two earthquakes mw 8 in 1971 using pdiff waveforms and an earthquake mw 7 9 in 1995 using direct teleseismic p waves combining these findings with previous results for two earthquakes mw 7 5 in 2000 we compared the source areas and asperity distributions for this region of the new britain trench our results show that the locations of the asperities for the individual earthquakes did not significantly overlap although the same portion of the subduction zone seems to have reruptured this fact supports the idea that asperities are not persistent features when portions of the new britain subduction zone slip in large earthquakes

the 1971 san fernando earthquake a double event evidence is presented which suggest that the 1971 san fernando earthquake may have been a double event that occurred on two separate subparallel thrust faults it is postulated that the initial event took place at depth on the sierra madre fault zone which runs along the base of the san gabriel mountains rupture is postulated to have occurred from a depth of about 15 km to a depth of about 3 km a second event is thought to have initiated about 4 sec later on another steeply dipping thrust fault which is located about 4 km south of the sierra madre fault zone the surface trace of this fault coincides with the san fernando fault zone which was the principal fault associated with surface rupture it is postulated that rupture propagated from a depth of 8 km to the free surface the moments of the first and second events are approximately 0 7 10 26 dyne cm and 1 0 10 26 dyne cm respectively this model is found to explain the combined data sets of strong ground motions teleseismic p and s waveforms and static offsets better than previous models which consist of either a single fault plane or a plane having a dip angle which shallows with decreasing depth nevertheless many features of the observed motions remain unexplained and considerable uncertainty still exists regarding the faulting history of the san fernando earthquake

waveform modelling of the may 20 1990 sudan earthquake abstract very broad band seismic waveforms were modelled to derive the moment tensor the source time function and the depth of the may 20 1990 sudan earthquake long period body waves surface waves and broad band displacement recorded by the m ed n et regional network were merged with digital data from global seismic networks to compare fault geometries obtained by different methods and assess associated uncertainties our analysis indicates that the may 20 earthquake had a seismic moment of 5 3 10 26 dyne cm and a depth of about 10 km the focal mechanism shows a mixture of transcurrent geometry with one plane striking nw se and a smaller dip slip component with the t axis oriented in the n s direction the fault geometry agrees with models calling for the recent re activation of the nw se trending aswa transform zone marking the northern termination of the western branch of the rift in contrast the n s tensional component which also dominates the focal mechanisms of the aftershocks dziewonski et al 1991 supports the evidence for the rotation of the regional quaternary stress field to the north in contrast with simple two plate models of rifting in east africa the juba event allowed us to test the capability of the m ed n et regional seismic network to provide an accurate real time description of the seismic source

east african earthquake body wave inversion with implications for continental structure and deformation summary source parameters for five african intraplate events are obtained from body wave inversion for the moment tensor parameters for the events are as follows location date mb strike dip rake depth ethiopia 4 05 69 5 8 327 1 69 2 45 0 4 zaire 3 20 66 6 3 44 6 44 9 81 5 10 zambia 12 2 68 5 9 20 3 40 2 264 8 6 zambia 5 15 68 5 7 49 1 39 8 263 2 28 s africa 9 29 69 5 6 123 5 89 4 0 58 4 the ethiopia and zaire events exhibit normal faulting as would be expected for their rift zone locations the 1968 may 15 zambia event is unusual in that p pp separation times place its focus in the lower crust 28 km which may be below the brittle ductile transition zone implying high strain rates inversion yielded a high angle normal fault the depth and focal mechanism of this event along with the mechanism for the 1968 december 2 zambia event suggest southwestward rift extension teleseismic long period vertical p waves from the south africa event exhibit distortions that may be attributed to lateral inhomogeneities in the source structure

source parameters of montana earthquakes 1925 1964 and tectonic deformation in the northern intermountain seismic belt waveform modeling and first motion analysis are used to determine the source parameters of six 5 8 m 6 8 earthquakes that occurred between 1925 and 1964 within the northern intermountain seismic belt of montana results of this study suggest that the 1925 clarkston earthquake occurred along an oblique normal fault with a trend similar to the southern end of the clarkston valley fault the two largest earthquakes of the 1935 helena sequence occurred along right lateral strike slip faults with trends similar to the bald butte and helena valley faults the 1947 virginia city earthquake occurred along a northwest southeast trending segment of the madison fault movement at depth was along a fault with strike similar to that of the 1959 hebgen lake main shock a reanalysis of a m 6 0 aftershock of the 1959 hebgen lake sequence suggests the earthquake occurred at a depth of 8 km along a fault that is not seen at the surface an m 5 8 earthquake in 1964 located about 10 km from the 1959 aftershock may have occurred along steeply dipping fault planes 48 to 80 at depths of 8 to 14 km most events could be modeled as simple ruptures

single station cmt application to the michoacan mexico earthquake of september 19 1985 observations of three dimensional ground motion at a single point on the surface of the earth following an earthquake can be sufficient to determine the zeroth order moment tensor of the event we adapt the centroid moment tensor cmt method dziewonski et al 1981 of analysis to single station data and apply it to the september 19 1985 michoacan earthquake the obtained mechanism indicates faulting on a shallow plane consistent with the ongoing subduction of the cocos plate the scalar moment is 1 3 1028 dyne cm a direct deconvolution of the observed p wave train indicates that the moment release occurred in two subevents each of 20 seconds duration with a peak to peak separation in time of 25 seconds

field and teleseismic observations of the 1981 golbaf sirch earthquakes in se iran summary in 1981 june and july two large earthquakes occurred on the gowk fault system in south east iran both earthquakes were associated with surface faulting showing a combination of reverse and right lateral strike slip motion on parallel adjacent faults striking north south and dipping both east and west such motion can be seen to have occurred in the recent past and is responsible for the formation of elongated alluvium filled depressions in the footwall region common to both the east and west dipping faults the earthquake of june 11 involved motion along about 15 km of the gowk fault system with a moment of about 1 0 1026 dyne cm the earthquake of july 28 was larger and occurred farther north with movement along at least 65km of the gowk fault system and a moment of about 7 1026 dyne cm surface faulting in both earthquakes was complex and spread over several kilometres width on en echelon fault segments as well as on faults dipping both east and west the long period wwssn seismograms of these two mainshocks are unusually complicated and clearly made up of several individual subevents sensible interpretation of these waveforms without guidance from the surface faulting and the long period sro moment tensor inversion of dziewonski woodhouse would be impossible however a combination of these three data sources allows a geologically plausible interpretation of the rupture processes in these earthquakes to be made although the faulting in both mainshocks was on the same large scale feature the gowk fault the rupture zones of the two earthquakes did not apparently overlap and were separated by a gap of 5 km along strike in which both coseismic and recent geological displacements are very small rupture in both mainshocks appears to have nucleated near this gap and to have propagated away from it other such gaps are visible along the irregular displacement of the northern fault break and may have been the nucleation positions for individual subevents of the july 28 earthquake recognition of such nucleation sites is of obvious importance to earthquake prediction efforts

rupture process of the 1944 tonankai earthquake ms 8 1 from the inversion of teleseismic and regional seismograms 1 we digitized teleseismic and regional records of the 1944 tonankai earthquake we used the multiple time window method to invert these records for the spatial and temporal distribution of slip and rake we assume a 220 140 km fault with a spacing of 20 20 km and a maximum rupture velocity of 2 5 km s the inversion resolved an asperity along the accretionary wedge and under the shima and atsumi peninsulas the rupture propagated from southwest to northeast and there was little slip near the hypocenter consistent with the absence of uplift in the overlying submarine forearc basin the peak slip is 2 3 m and the total seismic moment is 2 4 1028 dyn cm mw 8 2 we compared the predicted and observed vertical geodetic displacements and inferred that the limit of rupture is consistent with the change from subsidence to uplift west of the atsumi peninsula this suggested that the earthquake did not rupture the plate interface in the tokai gap we modeled the tsunami using sea bottom displacements generated from this slip model the tide gauge amplitudes and frequency content are consistent with those predicted resolution tests from inverting synthetic seismograms computed using a simplified slip model indicated that the combination of teleseismic and regional data sets best recovers the asperity locations and peak slips given similar station geometry and degrees of freedom an inversion of the synthetic regional data set indicates that slip artifacts can occur early in the rupture process due to many clipped records while the inversion of synthetic teleseismic data set had less resolution and recovered only 50 of the peak slip

a procedure for the joint inversion of regional and teleseismic long period body waves a procedure for the joint inversion of regional and teleseismic long period waveforms for source parameters is presented three test cases demonstrate that the inclusion of regional distance seismograms to a data set of teleseismic observations stabilizes the inversion process and adds valuable constraints to the source parameter estimates particularly when the teleseismic green s functions are inadequate the added stability and constraints from regional waveforms can be attributed to the robust nature of regional distance green s functions as well as the added coverage of the focal sphere that regional data provide singular value decomposition is a useful inversion method that is well suited for this study because it provides a measure of the solution dependence on individual observations

alaska megathrust 2 imaging the megathrust zone and yakutat pacific plate interface in the alaska subduction zone we image the slab underneath a 450 km long transect of the alaska subduction zone to investigate 1 the geometry and velocity structure of the downgoing plate and their relationship to slab seismicity and 2 the interplate coupled zone where the great 1964 earthquake mw 9 2 exhibited the largest amount of rupture the joint teleseismic migration of two array data sets based on receiver functions rfs reveals a prominent shallow dipping low velocity layer at 25 30 km depth in southern alaska modeling of rf amplitudes suggests the existence of a thin layer vs of 2 1 2 6 km s that is 20 40 slower than underlying oceanic crustal velocities and is sandwiched between the subducted slab and the overriding plate the observed megathrust layer with vp vs of 1 9 2 3 may be due to a thick sediment input from the trench in combination with elevated pore fluid pressure in the channel our image also includes an unusually thick low velocity crust subducting with a 20 dip down to 130 km depth at 200 km inland beneath central alaska the unusual nature of this subducted segment results from the subduction of the yakutat terrane crust our imaged western edge of the yakutat terrane aligns with the western end of a geodetically locked patch with high slip deficit and coincides with the boundary of aftershock events from the 1964 earthquake it appears that this sharp change in the nature of the downgoing plate could control the slip distribution of great earthquakes on this plate interface

multi trace deconvolution with unknown trace scale factors omnilinear inversion of p and s waves for source time functions seismic source and wave propagation theories allow seismologists to compute long period synthetic seismograms that commonly match even subtle details of observed waveshapes but the overall trace amplitudes must be scaled by arbitrary factors to emphasize the excellent waveshape correspondence this unexplained amplitude scatter causes considerable difficulties when formally inverting observed waveforms for seismic source parameters omnilinear inversion simultaneously determines the linear model parameters and trace scaling factors to minimize the mismatch between observed and synthetic seismograms omnilinear inversion is applied to source time function deconvolution from long period p and sh waves a simulation with synthetic data shows that omnilinear inversion finds the proper scaling factors and the source time function and focal depth are better determined than for standard linear inversion omnilinear inversion is then applied to a data set of seven p waves and one sh wave from the underthrusting earthquake of july 5 1968 ms 6 6 in northern honshu japan omnilinear inversion produces a best fit focal depth of 36 km with a single pulse time function of 6 s duration and seismic moment of 4 1018 n m mw 6 4 this focal depth supports the notion that the seismically coupled plate interface extends no deeper than 40 km in northern honshu

evidence of tectonic release in long period s waves from underground nuclear explosions at the novaya zemlya test sites abstract evidence of tectonic release produced by underground nuclear explosions at novaya zemlya is present in teleseismic long period s waves sh waveforms from representative events at the southern and northern novaya zemlya test sites were modeled to determine the sh radiation patterns long period p and sv waves were also investigated to further constrain the equivalent double couple orientation of the tectonic release events within each test site have similar faulting mechanisms but there are definite differences in the tectonic release orientation between the two sites the southern test site double couple orientation is either vertical strike slip or 45 dipping thrust while the northern test site has an oblique normal dip slip orientation the limited resolution of sh signals and complexity of the observed sv waveforms along with uncertainties in the absolute explosion source strengths and t prevent unique solutions for both test sites assuming a strike slip orientation the 27 october 1973 southern novaya zemlya event has the largest tectonic release moment 1 6 10 24 dyne cm and 14 october 1969 northern event has the smallest moment 0 9 10 23 dyne cm of the 12 events studied the ratio of tectonic release moment to explosion strength varies by as much as a factor of 3 at the northern test site and by a factor of 2 at the southern site the variability of the tectonic release orientation and strength at the two test sites allows us to address the question of whether tectonic release affects yield estimation