a comparison between the tectonic stress measured in situ and stress parameters from induced seismicity at monticello reservoir south carolina during may and early june 1979 five three component digital seismographs were deployed near the monticello reservoir south carolina to determine the stress drops of reservoir induced earthquakes and compare them with earlier hydrofracture stress measurements of zoback and hickman this issue records for 327 events were obtained 10 of these events each recorded on four or more stations were analyzed to determine stress drops depths for the 10 events ranged from 0 5 to 1 4 km fault plane orientations were calculated from an inversion of the body wave displacements individual radiation pattern corrections were applied in calculating the moments at each station final estimates of seismic moment were obtained by averaging logarithmically the estimates from each component for each event an arithmetic moment was obtained from the individual estimates of the source radius and stress drop was then calculated once from the final estimates of seismic moment and source radius for each event moments of the 10 digitally recorded earthquakes ranged from 2 5 1017 to 2 7 1018 dyne cm for the p wave data and from 4 6 1017 to 4 6 1018 dyne cm for the s wave data the stress drops s wave data only generally ranged from about 0 2 to 4 bars four earthquakes were recorded by a strong motion instrument at the dam site magnitudes m for these events ranged from 2 8 to 3 0 moments taken as averages of the values from each component ranged from 1 7 to 3 4 1020 dyne cm and the stress drops from 13 to 92 bars large differences between the corner frequencies of the two components factors of 2 5 and 4 for two of the events correspond to a large relative uncertainty for these stress drop values nevertheless tens of bars of stress are apparently released for the largest events in the monticello sequence but only a few bars are released for the smaller magnitude events in comparison the in situ stress data measured in two holes give an estimate of shear stress of about 45 bars at 0 2 km depth and 30 to 70 bars at 1 km depth in the northern hole and from 15 20 bars near the surface to almost 70 bars at depth in the southern hole the variation in shear stress inferred from in situ measurements at the deeper levels indicates an inconsistency between the thrust faulting mechanisms of the earthquakes and the state of stress which is more consistent with thrust faulting at 0 2 km depth and strike slip or normal faulting below thus the estimates of shear stress from in situ measurements are of the same order as the stress drops of the m 2 8 3 0 events but about 1 order of magnitude larger than the stress drops obtained for the smaller events

the denver earthquakes of 1967 1968 abstract a detailed study of the earthquakes associated with the rocky mountain arsenal disposal well is presented long period surface wave studies are used together with p wave first motions to show that the 10 april 9 august and 27 november 1967 earthquakes occurred at depths of 3 to 5 km and were characterized by normal faulting along a northwest striking fault plane a joint hypocenter relocation of 103 microearthquakes of a data set of 279 recorded between 1967 and 1968 shows a hypocenter pattern striking n50 w with most of the events located about 5 km northwest of the disposal well at depths between 3 to 8 km a fault plane dipping southwest is tenuously suggested by those earthquakes with depths less than 5 km modeling of near field seismoscope observations lend support to the focal mechanisms derived

fossil strain from the 1811 1812 new madrid earthquakes observations of postseismic deformation generally suggest that the effects of postseismic viscoelastic relaxation may persist for many decades after an earthquake in particular strain effects from the three great earthquakes that occurred in the new madrid seismic zone in 1811 1812 may influence present day measurements of ground deformation in this active seismic region of the central united states forward calculations using an earth rheology that may be appropriate for a continental intraplate region and a simple fault model for the 1811 1812 sequence suggest that postseismic relaxation may be an important factor in driving the unexpectedly high rate of shear deformation recently observed in the southern new madrid seismic zone

selection of earthquake scaling relationships for seismic hazard analysis a fundamentally important but typically abbreviated component of seismic hazard analysis is the selection of earthquake scaling relationships these are typically regressions of historical earthquake datasets in which magnitude is estimated from parameters such as fault rupture length and area the mix of historical data from different tectonic environments and the different forms of the regression equations can result in large differences in magnitude estimates for a given fault rupture length or area we compile a worldwide set of regressions and make a first order shortlisting of regressions according to their relevance to a range of tectonic regimes plate tectonic setting and fault slip type in existence around the world regression relevance is based largely on the geographical distribution age and quantity quality of earthquake data used to develop them our compilation is limited to regressions of magnitude or seismic moment on fault rupture area or length and our shortlisted regressions show a large magnitude range up to a full magnitude unit for a given rupture length or area across the various tectonic regimes these large differences in magnitude estimates underline the importance of choosing regressions carefully for seismic hazard application in different tectonic environments

hydroacoustic monitoring of seismicity at the slow spreading mid atlantic ridge 1 in february 1999 long term hydroacoustic monitoring of the northern mid atlantic ridge mar was initiated six autonomous hydrophones were moored between 15 n and 35 n on the flanks of the mar results from the first year of data reveal that there is significant variability in along axis event rate groups of neighboring segments behave similarly producing an along axis pattern with high and low levels of seismic activity at a wavelength of 500 km this broad scale pattern is likely influenced by the axial thermal regime several earthquake sequences with variable temporal characteristics were detected suggesting fundamental differences in the cause of their seismicity off axis most seismic faulting occurs within a zone 15 km from the axis center

an empirical estimate of near source ground motion for a major mb 6 8 earthquake in the eastern united states abstract three empirical techniques are used to estimate ground motion amplitudes in the near source region of a major earthquake hypothesized to occur along the east coast of the united states these techniques include the estimation of strong motion parameters from the statistical analysis of selected strong motion records from empirical and theoretical attenuation relations and from modified mercalli intensity source parameters for the hypothesized earthquake were selected to be similar to those inferred for the 1886 charleston south carolina earthquake the epicentral region is characterized by a modified mercalli intensity of x from which magnitudes of m b 6 8 m s 7 2 and m 7 2 were estimated inferred fault mechanisms for the hypothesized event vary from strike slip on a vertical fault to reverse or reverse oblique slip on a fault dipping as shallow as 45 the median estimate of peak horizontal acceleration for a point located 15 km from the surface projection of fault rupture is 0 30 g independent of site geology corresponding estimates of peak horizontal velocity are 35 cm sec for soil and soft sedimentary rock sites and 18 cm sec for crystalline basement rock sites strong motion parameters for a point located 15 km from the epicenter corresponding to a point located directly above the rupture zone of the hypothesized earthquake are estimated to be approximately 80 per cent higher than these estimates or about 0 55 g for peak acceleration and about 63 and 33 cm sec for peak velocity on soil soft rock and crystalline rock respectively corresponding velocity acceleration ratios are 115 cm sec g for soil soft rock sites and 60 cm sec g for crystalline rock sites standard deviations associated with the natural logarithm of peak horizontal acceleration and velocity are estimated to be 0 5 and 0 7 respectively representing an 84th percentile estimate or equivalently an upper 68 per cent confidence limit that is approximately 65 per cent higher than the median for acceleration and 100 per cent higher than the median for velocity while these strong motion values are generally higher than those currently used in the seismic design of most structures in the eastern united states their impact on the seismic safety of specific sites in the region will depend on an assessment of the probability of occurrence of an earthquake of this size within the near source region of these sites

scaling law and its applications to earthquake statistical relations abstract the spectra of the far field body wave displacement for earthquakes of various magnitudes have been obtained using a two dimensional faulting model on a rectangular fault there are three corner frequencies f c 1 f c 2 and f c 3 in the displacement spectra f c 1 and f c 2 relate to faulting times for the long and wide directions on the rectangular fault respectively and f c 3 corresponds to the rise time of the source time function according to these three corner frequency values four spectral regions are defined u u 0 f 0 f f c 1 in the first region u 1 f f c 1 f f c 2 in the second region u 1 f 2 f c 2 f f c 3 in the third region and u 1 f 3 f f c 3 in the fourth region this results in different source parameter relations for different magnitude ranges i e different seismic moment ranges because the attenuation rates depend on frequency it is supposed that earthquake faulting is satisfied by three conditions of geometrical similarity stress environment and dynamic similarity therefore seismic moment faulting time along the length and width of the fault and rise time can all be scaled with fault length the constants in the scaling relations can be determined from seismic moment data for about 800 earthquakes that occurred during 1981 1983 and are given by dziewonski and woodhouse 1983 and m s and m b data from the isc bulletin thus the source parameter relations between m s and m b m s and m l m b and m l log m o and log s fault area log s and m s log l fault length and m s and log d average dislocation and m s can be deduced from the scaling law rather than from pure statistical relations these relations are statistically consistent with the observed data the average rupture velocity of the fault can also be estimated from the scaling law it is 2 65 km s the total time of rupture is 0 35 s km l km which is close to that estimated by kanamori 1981 and the dislocation slip rate is 287 1143 cm s

source parameters and effects of bandwidth and local geology on high frequency ground motions observed for aftershocks of the northeastern ohio earthquake of 31 january 1986 abstract a 10 station array geos yielded recordings of exceptional bandwidth 400 sps and resolution up to 96 db for the aftershocks of the moderate m b 4 9 earthquake that occurred on 31 january 1986 near painesville ohio nine aftershocks were recorded with seismic moments ranging between 9 10 16 and 3 10 19 dyne cm m w 0 6 to 2 3 the two largest aftershocks depth 5 3 5 6 km oblique right slip rake 30 strike n25 e yielded seismic signals above background noise at frequencies as high as 130 hz at epicentral distances up to 17 km the aftershock recordings at a site underlain by 8 m of lakeshore sediments show significant levels of high frequency soil amplification of vertical motion at frequencies near 8 20 and 70 hz viscoelastic models for p and sv waves incident at the base of the sediments yield estimates of vertical p wave response consistent with the observed high frequency site resonances but suggest additional detailed shear wave logs are needed to account for observed s wave response peak acceleration values obtained from the broadband recordings are about two and four times as large as those that would be recorded on strong motion recorders or short period networks with upper bandwidth limits of 30 and 15 hz respectively attenuation corrected acceleration spectra are used to reduce the influence of high frequency up to 100 hz local site effects on corner frequency estimates the moment versus source radius trend inferred for events with moments as small as 9 10 16 dyne cm based on the brune source model extends previous relations inferred for the central united states shows little evidence for a minimum source radius and suggests that stress drops for the smaller events m 0 19 dyne cm decrease with decreasing moment

source parameters from locally recorded aftershocks of the 9 january 1982 miramichi new brunswick earthquake abstract source parameters are derived from 111 locally recorded three component seismograms of 40 aftershocks magnitude 1 0 to 3 5 of the 9 january 1982 miramichi new brunswick earthquake this data set greatly expands the set of three component data recorded at small source to receiver distances in eastern north america preliminary analysis showed that some aftershock spectra were modified by strong site effects the need to understand and account for site effects when analyzing the aftershock data in terms of simple source models led to a two part data analysis first site effects were modeled as a seismic wave resonance in a thin layer 0 to 10 m of low seismic impedance glacial till over a high seismic impedance bedrock half space second we calculated source parameters from sh wave seismograms and amplitude spectra incorporating what had been learned about site resonances correcting for obvious site effects partially succeeded in reducing scatter in the source parameter determinations but some site dependent discrepancies remain seismic moments range from 1 10 18 to 5 10 20 dyne cm and corner frequencies from 5 to 45 hz brune model stress drops primarily range between 5 and 300 bars stress drop apparently increases with increasing seismic moment trends in apparent stress calculated from radiated energy and seismic moment parallel trends in stress drop corner frequencies from these locally recorded data are greater than those derived from regionally recorded l g waves for earthquakes of comparable moment this result contradicts a recently proposed m 0 4 constant source scaling law based primarily on l g data

intraplate seismicity and stresses in young oceanic lithosphere we have conducted a systematic study of oceanic intraplate seismicity in the period 1964 1983 occurring in lithosphere younger than 35 million years focal mechanisms determined for 26 events using first motions p wave modeling and surface wave amplitude radiation patterns along with 13 previously determined mechanisms indicate a diversity of faulting styles and stress orientations in young oceanic lithosphere there is no evidence for a worldwide transition from extension to compression at a specific lithospheric age tensional axes of normal faulting events are oriented oblique to the spreading direction and do not show extension in the spreading direction congressional axes of thrust events show a weak preferred orientation in the spreading direction presumably due to ridge push stresses however stress axes of thrust events in very young lithosphere age 9 ma do not show a correlation with the spreading direction suggesting that the ridge push force is small near the ridge these observations indicate that tensional stress in the spreading direction is concentrated in a very narrow zone at the ridge axis in agreement with models which assume that the axial region has very little tensile strength normal faulting events occur at greater depths and temperatures than thrust events possibly because rocks under tensional stress have strength maxima at higher temperatures alternatively the depths of the normal faulting events may be a result of the higher thermal gradients at depth in young lithosphere if thermoelastic stresses are important in generating this seismicity differential thermal contraction along fracture zones does not seem to be a significant source of near ridge intraplate seismicity high seismicity rates in young lithosphere noted previously on a global scale seem characteristic of all three major ocean basins additionally most of the total oceanic intraplate moment release takes place in young lithosphere however most of the events and more than 96 of the total moment release of the near ridge intraplate seismicity are contained in five areas two zones in the indian ocean one near chagos bank and the other in the amsterdam st paul region indicate tension parallel to the trend of the plate boundary a region of high seismicity near the east pacific rise between 3 and 7 n shows both normal and thrust faulting while another region on the cocos plate west of the panama fracture zone is characterized by strike slip faulting thrust strike slip and normal faulting events are found in a small seismically active zone of the south central pacific the extremely uneven distribution of seismicity if not a result of the relatively short time period studied 20 years indicates local perturbations strongly affect near ridge seismicity

source parameters and scaling relations for reservoir induced seismicity in the longtan reservoir area we investigate the relationship between the impoundment and seismicity in the longtan reservoir southwestern china and find evidence that the seismicity was reservoir induced after the reservoir impoundment a pronounced increase in seismicity was observed in five clusters mainly concentrated in the areas where few earthquakes had occurred before the first filling the observed induced seismicity shows a strong correlation with the filling cycles the activity levels in the five clusters are different due to differences in the structures and permeabilities of the faults source parameters for 1 616 earthquakes with m l 0 1 4 2 recorded by 24 fixed and temporary stations deployed around the reservoir were calculated after applying corrections for geometrical spreading frequency dependent q and site effects the static stress drop and apparent stress in this area both appear to increase with increasing seismic moment over the entire magnitude range our results show that reservoir induced earthquakes have ten times lower average stress drop than natural tectonic earthquakes these results may indicate that the reservoir induced seismicity can occur with a lower tectonic stress due to the high pore pressures of the underground medium and that the effect of the water decreases the coefficient of friction

amplitude corrections for regional seismic discriminants a fundamental problem associated with event identification lies in deriving corrections that remove path and earthquake source effects on regional phase amplitudes used to construct discriminants our goal is to derive a set of physically based corrections that are independent of magnitude and distance and amenable to multivariate discrimination by extending the technique described in taylor and hartse 1998 for a given station and source region a number of well recorded earthquakes is used to estimate source and path corrections the source model assumes a simple brune 1970 earthquake source that has been extended to handle non constant stress drop the discrimination power in using corrected amplitudes lies in the assumption that the earthquake model will provide a poor fit to the signals from an explosion the propagation model consists of a frequency independent geometrical spreading and frequency dependent power law q a grid search is performed simultaneously at each station for all recorded regional phases over stress drop geometrical spreading and frequency dependent q to find a suite of good fitting models that remove the dependence on m b and distance seismic moments can either be set to pre determined values or estimated through inversion and are tied to m b through two additional coefficients we also solve for frequency dependent site phase excitation terms once a set of corrections is derived effects of source scaling and distance as a function of frequency are applied to amplitudes from new events prior to forming discrimination ratios thus all the corrections are tied to just m b or m 0 and distance and can be applied very rapidly in an operational setting moreover phase amplitude residuals as a function of frequency can be spatially interpolated e g using kriging and used to construct a correction surface for each phase and frequency the spatial corrections from the correction surfaces can then be applied to the corrected amplitudes based only on the event location the correction parameters and correction surfaces can be developed offline and entered into an online database for pipeline processing providing multivariate normal corrected amplitudes for event identification examples are shown using events from western china recorded at the station makz

the 2002 m5 au sable forks ny earthquake sequence source scaling relationships and energy budget 1 we find invariant high stress drops and radiated energies for a sequence of intraplate earthquakes we estimate source parameters of the au sable forks ny earthquake m5 2002 and aftershocks this intraplate earthquake was the largest to occur in eastern north america since 1988 and the largest to be recorded by regional broadband networks we use the empirical green s function egf method and define a set of qualitative and quantitative rules for the selection of egf earthquake pairs and for the quality verification of the obtained egf spectral ratio we use a multitaper code that performs the complex spectral division with minimum frequency leakage and allows transformation back to the time domain to check the validity of the egf event we estimate source parameters for 22 earthquakes m1 m5 in the sequence the median stress drop of 104 mpa using madariaga source model and 19 mpa for a brune model is significantly larger than estimates for interplate earthquakes the lower crustal strain rates and longer fault healing times in intraplate environments may be responsible for this high average value we find constant stress drop between m2 and m5 up to the bandwidth resolution limit 80 hz of the study and no evidence of stress drop breakdown for m2 to m1 earthquakes we find consistently high radiated seismic energy and apparent stress and a median radiated energy to seismic moment ratio of 9 10 5 this is significantly larger than estimates for interplate earthquakes 2 10 5 and consistent with higher stress drops and stronger faults

spectral characteristics of the excitation and propagation of lg from underground nuclear explosions in central asia the nonlinear inverse method of xie 1993 is applied to analyze lg spectra from many recent underground nuclear explosions in central asia the analysis simultaneously determines lg seismic moments m0 and corner frequencies fc of events as well as path variable lg q values and their frequency dependences at 1 hz lg q at 1 hz estimated for many paths from the lop nor and balapan test sites to various broadband incorporated institution for seismology chinese digital seismic network and kyrghizstan network stations agree well with path averaged lg coda q values from previously obtained tomographic maps for eurasia this suggests that the lg q values obtained in this inversion have not been significantly affected by focusing defocusing or other effects of three dimensional structural complexities the logarithms of lg m0 values obtained in this study correlate linearly with a slope of about 1 2 with international seismological centre isc body wave magnitudes mb the linear relationship is consistent with a previous empirical relationship found between the logarithm of p wave seismic moments and isc mb values the slope of the linear relationship also suggests that the logarithm of lg m0 is a more direct measure of event size than is mb which for larger events is affected by high frequency amplitude falloff another important finding of this study is that the m0 values for the explosions scale with fc 4 rather than with fc 3 as would be expected if stress drops were constant this is consistent with previous scaling relations derived using lg from earthquake sources and suggests that the difference between lg and local s excitation by the same source types explosion or earthquake may overwhelm that between the excitation of lg by different source types a comparison between lg m0 and fc values obtained using an explosion source model with those obtained using an earthquake source model suggests that the m0 values estimated using the explosion source model are systematically lower by a factor of 0 27 than those estimated using the earthquake source model this occurs even though the data used are the same in the inversions with the two types of source model the factor of 0 27 is therefore due to the source model dependence of the m0 fc estimates the overshoot and lower spectral levels at lower frequencies in the explosion source model tend to reduce both the estimated m0 and fc values this suggests that in any nuclear monitoring program the same type of source model either an earthquake or explosion should always be used to discriminate explosions using lg the source model dependence also suggests that more effort should be made to estimate the amount of overshoot in the lg excitation caused by various source types

source processes of three aftershocks of the 1983 goodnow new york earthquake high resolution images of small symmetric ruptures broadband large dynamic range geos data from four aftershocks m l 2 to 3 of the 1983 goodnow new york earthquake recorded at hard rock sites 2 to 7 km away from the epicentral area are used to study rupture processes of three larger events with an m l 1 6 event as the green s function event we analyze the spectra and spectral ratios of ground velocity at frequencies up to 100 hz and conclude that 1 there are resolvable p wave f max 51 and 57 hz at two sites 2 there is abundant information on sources of larger events up to frequencies of 50 to 60 hz and 3 there is an unstable nonlinear instrument resonance at about 90 hz we analyze the artifacts of low pass filters in the deconvolved rupture process including the limited time resolution and biases in the rise time measurement some extensions of the empirical green s function egf method are proposed to reduce these artifacts and to precisely estimate the relative locations of events that are close both in time and in space applying the egf method to three aftershocks we find that these events have ruptures that are simple crack like characterized by small fault radii 70 to 120 m and static stress drops that vary depending on the size of the events between about 5 to 16 bars we also find that two of the events with origin times differing by 0 60 sec are separated by 190 110 meters assuming a causal relationship between the two would result in a slow propagation velocity 0 05 0 03 time of the local shear wave velocity we therefore interpret the corresponding ruptures as being distinct the area between the rupturing patches having a characteristic length much smaller than those of the rupturing patches comparison of the results of this study with those obtained for the goodnow main shock and microearthquakes in california and hawaii suggests that the stress drops of the goodnow aftershocks decrease considerably by up to a factor of 25 or more from those estimated for the main shock even after the estimated large uncertainty in the latter estimates is considered this decrease is similar to those reported for the nahanni earthquakes m b 5 0 to 6 5 in northwest territories and for the north palm springs earthquake m l 5 9 sequence in california the stress drops of the simple crack like microearthquakes are significantly variable by factors between 3 and 27 within the single source areas the median of stress drops of the intraplate goodnow aftershocks is lower by factors of 2 to 4 than the medians calculated for the other interplate microearthquake sequences

seismic lg wave q tomography in and around northeast china 1 we investigate regional variations in the lg wave quality factor q in northeast china and its vicinity with a tomographic method digital seismic data recorded at 20 broadband stations from 125 regional events are used to extract lg wave spectra tomographic inversions are independently conducted at 58 discrete frequencies distributed log evenly between 0 05 and 10 0 hz we simultaneously invert for the lg wave q distribution and source spectra at individual frequencies without using any a priori assumption about the frequency dependence of the q model and source function the best spatial resolution is approximately 1 1 in well covered areas for frequencies between 0 4 and 2 0 hz the lg q shows significant regional variations and an apparent relationship with regional geology we use a statistical method to investigate the regional variations of lg q and their frequency dependence the average q0 1 hz lg q in the entire investigated region is 414 sedimentary basins are usually characterized by lower average q0 values from 155 to 391 while volcanic mountain areas have relatively high average q0 values from 630 to 675 lg q generally increases with increasing frequency however the frequency dependence has complex nonlinear features on a double logarithmic scale indicating that the commonly adopted power law relationship may be oversimplified in a broad frequency band the frequency dependence varies in different geological areas with larger variations seen at lower frequencies

source spectral properties of small earthquakes in the northern north sea abstract source parameters of 40 small shallow earthquakes m 2 5 3 5 in the northern north sea region were calculated using p wave spectra of digital records from the western norway network station hya the results show a difference in the source spectral properties of earthquakes generated in the north sea rift zone and the uplift zone of the norwegian coastal margins corner frequencies fo of p wave spectra are constant for the margin zone events for the seismic moment interval e12 e14 nm there is a tendency of decreasing fo with the seismic moment for rift zone events rift zone events have larger stress drops than margin zone events with the same magnitude increase of stress drop with moment was observed for both zones

comparison of dynamic source parameters for earthquakes in different tectonic regions of the northern cascadia subduction zone abstract based on characteristics of displacement amplitude spectra of near distant s waves from 52 earthquakes in the magnitude m l range 0 6 to 5 4 dynamic source parameters were computed and compared for three tectonic regions in the northern cascadia subduction zone a downgoing slab of the juan de fuca plate with earthquake depths of about 30 km the same slab further east with deeper events about 60 km and an overriding part of the north american plate no distance dependence of observed corner frequencies was obtained nor any significant regional difference in source parameters although there is an indication of smaller stress drop in the north american plate for larger events factors which may distort a regional comparison were examined but neither source radiation directivity nor propagation path and station site characteristics have significant influence the structural model has influence however and two different density velocity models were used for the deeper juan de fuca slab events seismic moments m 0 range from e 17 to 0 4 e 24 dyn cm source radii from 0 07 to 1 6 km stress drops from less than 0 1 to 40 bar and average dislocations from less than 0 1 to 7 cm in the range e 18 to e 21 dyn cm in which the vast majority of the events are confined both the stress drop and displacement increase rapidly with event size seismic moment or magnitude as the seismic moment increases rapidly with fault dimension the relation between seismic moment dyn cm and m l magnitude computed from data from all three regions is similar to relations presented for california and reads log m 0 17 65 1 02 m l there are however differences between events in the north american plate and the juan de fuca plate with larger m l for the former at equivalent seismic moment

source spectral scaling inversion for two earthquake sequences offshore western norway abstract short period and high frequency noress observations from two after shock sequences offshore western norway with epicentral distances of 424 and 405 km respectively have been used in testing a new hedgehog inversion procedure for determination of source spectral scaling characteristics based on a previously developed spectral ratio method this inversion procedure provides insight into resolution properties uncertainties and trade off considerations between source parameters the two main events analyzed have m l magnitudes of about 5 while the smallest aftershocks are about 1 5 with seismic moments in the range 10 17 to 10 12 nm the observed spectral ratios support 2 source models for both of the earthquake sequences but only if certain constraints are imposed on the scaling parameter that determines the stress drop sensitivity with respect to seismic moment in that case stress drop is found to be almost independent of seismic moment of 3 24 for one of the sequences and increasing with moment of 3 80 for the other without such constraints which may not be justifiable in the case of earthquake sequences equally good solutions are found for 3 models in which case attains values around 5 to 6 indicating strongly increasing stress drop external constraints on the source spectral slope and or corner frequency will similarly help in resolving the method which also simultaneously determines corner frequencies or stress drop if adopting some model relating this to corner frequency for all of the events albeit with a poorer resolution can either use independently determined seismic moments or it can determine these as part of the inversion quite similar results are obtained in those two cases reflecting essentially the quality of the initial moment determinations the observed variations in stress drop sensitivity between two earthquake sequences only 84 km apart and occurring in similar tectonic environments are consistent with the considerable variations that have been observed on a larger scale caution is therefore needed if the scaling relations derived in this study are applied to other events larger ones and or series of main events or to different seismic regions

on the origin of the southern permian basin central europe abstract a detailed study of the structural and stratigraphic evolution of the southern permian basin during latest carboniferous to early jurassic times supported by quantitative subsidence analyses and forward basin modelling for 25 wells leads us to modify the conventional model for the rotliegend zechstein development of this basin the late permian early jurassic tectonic subsidence curves are typical for a permian to early triassic extensional stage that is followed by thermal subsidence however a purely extensional model is extremely problematic because active faulting during this time is minor and generally hard to document using inverse techniques to model the subsidence curves we quantitatively show that a significant component of late permian and triassic tectonic subsidence can be explained by thermal relaxation of early permian lithospheric thinning and by delayed infilling of paleo topographic depressions that developed during the early permian in this interpretation stephanian autunian wrenching resulted in thermal destabilisation of the lithosphere deep fracturing of the crust disruption and erosion of its sedimentary cover and regional uplift of the area of the future southern permian basin upon termination of wrench tectonics and associated volcanism towards the end of the autunian the southern permian basin began to subside in response to thermal contraction of the lithosphere the evolving basin was isolated from the world oceans and had subsided possibly up to some 700 m below their level at the beginning of upper rotliegend sedimentation after catastrophic flooding of this paleo topographic depression at the beginning of the zechstein changing sea level sedimentation and subsidence rates remained essentially in balance although the effects of triassic rifting overprinted parts of the southern permian basin its overall subsidence pattern persisted well into the jurassic in contrast to the remainder of the southern permian basin permian and triassic crustal extension contributed significantly to the subsidence of the polish trough

rapid intraplate strain accumulation in the new madrid seismic zone remeasurement of a triangulation network in the southern part of the new madrid seismic zone with the global positioning system has revealed rapid crustal strain accumulation since the 1950s this area experienced three large moment magnitudes 8 earthquakes in 1811 to 1812 the orientation and sense of shear is consistent with right lateral strike slip motion along a northeast trending fault zone as indicated by current seismicity detection of crustal strain accumulation may be a useful discriminant for identifying areas where potentially damaging intraplate earthquakes may occur despite the absence of large earthquakes during historic time

scaling relations for composite earthquake models we establish relations between seismic scaling parameters for gen eral distributions of subevents in a composite multiple event model of faulting the radiation from the main seismic event is presumed to be produced by a multitude of smaller earthquakes subevents with a range of source sizes under the composite earthquake hypothesis the seismic moment scaling the depen dence of the seismic moment on the event s size is the same for the large earthquake and all its components if at lowest frequencies the subevent spectra add coherently and at higher frequencies above the corner frequency of the smallest subevent the spectra add incoherently then y 6 2 and a 1 where y is the high frequency spectral fall off 6 determines the scaling of seismic moment m0 with source size r such that mor is constant and o is the frac tion of the total rupture area occupied by subevents in each particular case these relations can be used to verify whether the studied large earthquake is a composite of smaller earthquakes the goal of this article is to establish relations be tween seismic scaling parameters for general distribu tions of subevents in a composite multiple event model of faulting joyner and boore 1986 boatwright 1988 the radiation from the main seismic event is presumed to be produced by a multitude of smaller earthquakes subevents with a range of source sizes under the com posite earthquake hypothesis the seismic moment scal ing the dependence of the seismic moment on the event s size is the same for the large earthquake and all its

some recent lg phase displacement spectral densities and their implications with respect to the prediction of ground motions in eastern north america abstract lg phase displacement spectral density corner periods of recent earthquakes in central new brunswick and central arkansas are used to point out a potential source of dispersion in the relative scaling of eastern north american strong motion data the corner periods of one of the central new brunswick and several of the larger central arkansas earthquakes are atypical of other events of similar magnitude and seismic moment in the region the maximum far field ground motion of an earthquake is a function of its corner period therefore strong motion data of earthquakes that exhibit atypical characteristics such as a displaced corner period are a source of dispersion in the scaling of that strong motion data if plotted as a function of magnitude or seismic moment

new brunswick strong ground motion records abstract although large earthquakes including several of magnitude 7 are known to have occurred in eastern canada since the arrival of european settlers in the 16th century no significant strong ground motion records were available until 1982 and earthquake engineering design relies on extrapolation of results from mostly california data modified to take into account the well known lower attenuation of felt intensities in the east in january 1982 a m b 5 7 earthquake occurred in the sparsely populated central region of new brunswick and was felt at ground level to distances of 400 km quickly installed strong motion recorders registered peak acceleration over 50 of gravity at high frequencies close to a m b 5 0 aftershock this is an order of magnitude higher than expected from the reported intensities and from the absence of significant damage however peak ground velocities as well as the low frequency fourier components of acceleration are consistent with accepted ground motion intensity relations this highlights again the need to go beyond the single parameter scaling of ground motion for purposes of the building code for ordinary buildings and for special engineering structures the large dynamic range of the digital records from the eastern canadian telemetered seismic network allows limited extrapolation of the strong motion records to the larger earthquakes and by inference to perhaps other eastern earthquakes in granitic environments such as charlevoix temiskaming and cornwall where large magnitudes appear to be generated in small source volumes resulting in explosion like high frequency but short duration near source ground motions

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

development of a time domain variable period surface wave magnitude measurement procedure for application at regional and teleseismic distances part ii application and ms mb performance the russell surface wave magnitude formula developed in part i of this two part article and the m s vmax measurement technique discussed in this article provide a new method for estimating variable period surface wave magnitudes at regional and teleseismic distances the m s vmax measurement method consists of applying butterworth bandpass filters to data at center periods between 8 and 25 sec the filters are designed to help remove the effects of nondispersed airy phases at regional and teleseismic distances we search for the maximum amplitude in all of the variable period bands and then use the russell formula to calculate a surface wave magnitude in this companion article we demonstrate the capabilities of the method by using applications to three different datasets the first application utilizes a dataset that consists of large earthquakes in the mediterranean region the results indicate that the m s vmax technique provides regional and teleseismic surface wave magnitude estimates that are in general agreement except for a small distance dependence of 0 002 magnitude units per degree we also find that the m s vmax estimates are less than 0 1 magnitude unit different than those from other formulas applied at teleseismic distances such as rezapour and pearce 1998 and van k et al 1962 in the second and third applications of the method we demonstrate that measurements of m s vmax versus m b provide adequate separation of the explosion and earthquake populations at the nevada and lop nor test sites at the nevada test site our technique resulted in the misclassification of two earthquakes in the explosion population we also determined that the new technique reduces the scatter in the magnitude estimates by 25 when compared with our previous studies using a calibrated regional magnitude formula for the lop nor test site we had no misclassified explosions or earthquakes however the data were less comprehensive a preliminary analysis of eurasian earthquake and explosion data suggest that similar slopes are obtained for observed m s vmax versus m b data with m b 5 thus the data are not converging at lower magnitudes these results suggest that the discrimination of explosions from earthquakes can be achieved at lower magnitudes using the russell 2006 formula and the m s vmax measurement technique

contour mapping of relic structures in the precambrian basement of the reelfoot rift north american midcontinent a new contour map of the basement of the reelfoot rift constructed from drill hole and seismic reflection data shows the general surface configuration as well as several major and minor structural features the major features are two asymmetric intrarift basins bounded by three structural highs and the rift margins the basins are oriented normal to the northeast trend of the rift two of the highs appear to be ridges of undetermined width that extend across the rift the third high is an isolated dome or platform located between the basins the minor features are three linear structures of low relief oriented subparallel to the trend of the rift two of these located within the rift basins may divide the rift basins into paired subbasins these mapped features may be the remnants of initial extensional rifting half graben faulting and basement subsidence the rift basins are interpreted as having formed as opposing half graben and the structural highs are interpreted as having formed as associated accommodation zones some of these features appear to be reactivated seismogenic structures within the modern midcontinent compressional stress regime a detailed knowledge of the geometries of the reelfoot rift s basement features therefore is essential when evaluating their seismic risk potential

estimation of intraplate strain accumulation in the new madrid seismic zone from repeat gps surveys the new madrid seismic zone nmsz a region within the eastern portion of the north american plate where great earthquakes occurred in 1811 1812 provides a type example of large earthquakes within a relatively stable continental interior assessing the scale and causes of deformation in such regions as well estimation of the recurrence interval for the large earthquakes within them has long been recognized as an important but challenging issue for which the advent of space based geodesy provides an important new tool here we report initial results from two global positioning system gps surveys of a geodetic network spanning the new madrid seismic zone because the expected strain signal is small the data were analyzed in three different ways first site velocities were estimated and analyzed for spatial gradients parallel and perpendicular to the trend of the major strike slip faults inferred from seismicity this analysis suggests but does not require a few millimeters per year of right lateral shear across the southern portion of the seismic zone consistent with the earthquake focal mechanisms second we divided the network into near field subnetworks and found possibly significant shear associated with approximately e w shortening within the southern nmsz and possible n s shortening across the left step connecting the presumed strike slip faults third we compared the strain observed along individual baselines to that predicted from the results of earlier near field surveys which combined gps and triangulation data in the southern part of the seismic zone we found general agreement with the earlier study although our data suggest lower strain rates hence after only 2 years of measurements the gps data suggest but do not yet require detectable strain accumulation however given that great earthquakes occur in the area and that our results are reasonably consistent with the results of other geodetic studies using different data and with earthquake mechanisms it seems likely that we are seeing strain accumulation in the southern nmsz a simple interpretation of the data as showing about 3 to 5 mm yr 1 of right lateral fault parallel motion implies a 1000 to 3300 year recurrence for great earthquakes with 5 to 10 m slip assuming all the accumulating strain is released seismically this recurrence interval may be an underestimate if the observed strains are largely postseismic effects of the 1811 1812 earthquakes a possibility suggested by the observation that plate wide data show less motion across the nmsz future surveys should resolve this issue as the longer measurement spans yield more precise velocity estimates

tectonic deformation in the new madrid seismic zone inferences from map view and cross sectional boundary element models the lack of instrumental recordings and obvious fault scarps associated with the 1811 1812 new madrid earthquakes necessitates examination of more subtle indicators of the geometry and type of faulting associated with these events morphologic and geologic features and the distribution of modern seismicity are used to infer the slip distribution and type of faulting strike or dip slip number strike length and width of the major faults in the new madrid seismic zone nmsz this is accomplished through two dimensional boundary element modeling of the strain field arising from slip on hypothetical faults that is driven by either coseismic or uniform regional strains tectonic deformation is reflected in 1 the seismicity 2 the lake county uplift 3 reelfoot lake 4 the fractured rocks of the blytheville arch and 5 the st francis sunk lands many of these features can be qualitatively explained as resulting from tectonic deformation due to slip on two left stepping right lateral strike slip faults that are coincident with the northeast trending zones of seismicity and the blytheville arch the morphology appears to be at least in part a consequence of major earthquakes that rupture these faults the locations of the 1811 1812 and largest post 1812 earthquakes and the models are consistent with a process in which the 1811 1812 earthquakes relieved accumulated regional shear strain causing the greatest post 1812 shear strains to exist at the ends of the fault zone modeling results also suggest that the numerous small earthquakes in the nmsz are not aftershocks of the 1811 1812 earthquakes but instead represent continuous localized adjustments to a uniform regional strain field the bootheel lineament does not appear to be significant in shaping the morphology geologic structure and pattern of seismicity in the nmsz the absence of fault scarps and coherence of shallow reflectors overlying the blytheville arch suggests that the major nmsz faults may not have ruptured to the surface during the 1811 1812 earthquakes the inferred lengths of the 1811 1812 earthquake ruptures also suggest that their sizes may have been overestimated model predicted subsidence within the st francis sunk lands indicates that tectonic deformation also may have influenced alluvial processes in the nmsz

seismic moment assessment of earthquakes in stable continental regions iii new madrid 1811 1812 charleston 1886 and lisbon 1755 summary the sizes of three major or great historical earthquakes are reassessed using the isoseismal area regression tools developed in parts i and ii of this study of stable continental region scr seismicity the earthquakes are 1811 new madrid central united states and its following sequence 1886 charleston coastal south carolina and 1755 lisbon oceanic intraplate off the continental shelf of portugal the analysis confirms the large size of these events and for the first time places constraints on the uncertainty of their seismic moment release because of the exceptionally low seismic wave attenuation of eastern north america ena a separate north american regression of seismic moment on isoseismal area was developed additionally the unknown western extents of the new madrid isoseismal areas were calibrated with the patterns of the m 6 3 6 6 1843 and 1895 earthquakes application of part ii analysis procedures with these corrections yields new madrid size estimates expressed as moment magnitude of m 8 1 0 31 for the 1811 december 16 m 7 8 0 33 for the 1812 january 23 and m 8 0 0 33 for the 1812 february 7 principal events the charleston earthquake s magnitude decreases from m 7 4 to m 7 3 0 26 after compensation for the effect of coastal plain sediments on its inner isoseismals intensity regressions for lisbon are calibrated against the isoseismal pattern of the nearly co located m 7 8 1969 st vincent earthquake which in this case increases the predicted size of lisbon from m 8 4 to m 8 7 0 39 these size estimates are supported by data from independent phenomena extent and severity of liquefaction the maximum distance of induced landslides and for lisbon tsunami wave amplitudes estimated source parameters are controlled by crustal or lithospheric temperature which governs the depth extent of brittle faulting using estimated continental and oceanic geotherms viable fault lengths are 30 80 km for charleston 120 180 km for 1811 new madrid and 180 280 km for lisbon for average displacements of 2 4 m 8 11 m and 10 14 m respectively and for average static stress and strain drops at the estimated seismic moments of this study the 1811 new madrid and the 1755 lisbon events are respectively the largest known scr and oceanic lithosphere earthquakes

source parameters of some significant earthquakes near koyna dam india seismic moment stress drop source radius and fault dislocation have been determined for nineteen significant earthquakes in the konya area india using displacement spectra of shear waves computed from strong motion accelerograph records though the stress drop shows a definite increasing trend with the seismic moment its correlations with source radius and corner frequency indicate that a constant stress drop of 170 bars represents a good mean value to describe the source mechanism of the koyna dam earthquakes an empirical relationship also has been established between magnitude and seismic moment

neotectonics of the upper mississippi embayment abstract although the upper mississippi embayment is an area of low relief the region has been subjected to tectonic influence throughout its history and continues to be so today tectonic activity can be recognized through seismicity patterns and geological indicators of activity either those as a direct result of earthquakes or longer term geomorphic structural and sedimentological signatures the rate of seismic activity in the upper mississippi embayment is generally lower than at the margins of tectonic plates the embayment however is the most seismically active region east of the rocky mountains with activity concentrated in the new madrid seismic zone this zone produced the very large new madrid earthquakes of 1811 and 1812 geological and geophysical evidence of neotectonic activity in the upper mississippi embayment includes faulting in the benton hills and thebes gap in missouri paleoliquefaction in the western lowlands of missouri subsurface faulting beneath and tilting of crowley s ridge in northeastern arkansas and southeastern missouri subsurface faulting along the crittenden county fault zone near memphis tennessee faulting along the east flank of the tiptonville dome and numerous indicators of historic and prehistoric large earthquakes in the new madrid seismic zone paleoearthquake studies in the new madrid seismic zone have used trenching seismic reflection shallow coring pedology geomorphology archaeology and dendrochronology to identify and date faulting deposits of liquefied sand and areas of uplift and subsidence the cause of today s relatively high rate of tectonic activity in the mississippi embayment remains elusive it is also not clear whether this activity rate is a short term phenomenon or has been constant over millions of years ongoing geodetic and geological studies should provide more insight as to the precise manner in which crustal strain is accumulating and perhaps allow improved regional neotectonic models

simulation of ground motion in the zagros region of iran using the specific barrier model and the stochastic method abstract physically based ground motion prediction equations for soil and rock sites in the zagros region have been developed based on the specific barrier model sbm used within the context of the stochastic model instead of direct time domain simulation random vibration theory was used to estimate measures of peak motion in terms of the pseudospectral velocity of anelastic harmonic oscillator with 5 viscous damping to avoid the uncertainties calibration of the source model uses a database of carefully selected strong motion data without ambiguity about the site condition therefore only rock sites are selected for determining source parameters also to avoid any inconsistencies caused by magnitude conversion formulas we restricted the dataset only to events with available moment magnitudes regression analysis is performed using the random effects model that considers both interevent and intraevent variabilities to effectively deal with the problem of an unequal number of records from different earthquakes no sign of self similarity breakdown is observed between the source radius and its seismic moment the local and global stress drops derived for the zagros region 39 and 116 bars respectively are more consistent with the values obtained by other authors for an interplate regime than the values for an intraplate region however from the viewpoint of source heterogeneity as the ratio of the stress drops is an indicator of the complexity of the source and heterogeneity of slip on the fault the zagros events which have a stress drop ratio of about three are more homogeneous than other interplate events stochastic simulations are then implemented to predict peak ground motion and response spectra parameters for rock and soil site conditions online material catalog tables of earthquakes and attenuation tables

stochastic prediction of ground motion and spectral response parameters at hard rock sites in eastern north america empirical predictions of ground motions from large eastern north american earthquakes are hampered by a lack of data for such events for this reason most prediction techniques have been based at least in part on data from the seismically active and well instrumented western north america concentrating on the prediction of response spectra on hard rock sites we have used a relatively new theoretical technique that does not require western data to make ground motion predictions for eastern north america this method often referred to as the stochastic model has its origins in the work of hanks and mcguire who treat high frequency motions as filtered random gaussian noise for which the filter parameters are determined by a seismological model of both the source and the wave propagation the model has been successfully applied to the predictions of ground motions in the western united states and to short period magnitudes from large to great earthquakes worldwide for our application the essential parameters of the model are estimated by using existing data from small to moderate eastern north american earthquakes a crucial part of the model is the relation between seismic moment and corner frequency the relation proposed in 1983 by nuttli for mid plate earthquakes leads to predictions of ground motions that are lower than available data by a factor of about 4 on the other hand a constant stress parameter of 100 bars gives model predictions in good accord with the data to aid in applications the ground motion predictions are given in the form of regression equations for earthquakes of magnitude 4 5 to 7 5 at distances within 100 km of the source the explanatory variables are hypocentral distance and moment magnitude m because predictions are often required in terms of m g rather than m we have used the theoretical model to establish a relation between the two magnitudes the predicted relation agrees with the sparse data available although the large uncertainties in the observed magnitudes for the larger events as well as the sensitivity of the theoretical magnitude to the attenuation model make it difficult to discriminate between various source scaling models

an analysis of the 1985 nahanni earthquakes abstract two large magnitude earthquakes occurred in 1985 near the north nahanni river in the northeast cordillera northwest territories canada on 5 october m s 6 6 and 23 december m s 6 9 these events were unprecedented for this region of the cordillera and the december event is the largest earthquake ever known in the eastern part of the canadian cordillera this report presents detailed studies of the nahanni earthquakes their source parameters macroseismic effects aftershock sequences triggered rock avalanches their relationship to the geology of the epicentral area and their seismic hazard implications parameters calculated here for these two major events are parentheses indicate other studies the earthquakes occurred within a small relatively undeformed plateau the mackenzie plain in the foreland fold belt created along the northeastern cordillera during the columbian or laramide orogeny that fold belt is now subjected to northeast southwest horizontal compressive stresses possibly very similar to the original laramide stress regime and the fault plane implied by the seismic mechanisms is very similar to the laramide faults in the area however no surface fault break has been found and none of the mapped laramide faults in the epicentral area appears to be involved in the aftershock activity it is thus believed that the nahanni earthquakes occurred on a blind laramide thrust fault under the mackenzie plain the large magnitudes of the nahanni earthquakes show that the present seismic zoning for building code purposes in the eastern cordillera north of 60 n needs reassessment and suggest that the zoning in the eastern cordillera in british columbia and alberta may also need revision the felt areas of the earthquakes show that regional anelastic attenuation in the eastern canadian cordillera is significantly anisotropic and much lower along strike than across strike the felt areas also appear to scale with seismic moment rather than m b magnitude the thrust mechanisms regional stress regime shallow focal depth and high velocity sedimentary rocks in the focal region suggest that the nahanni earthquakes and particularly the near field strong ground motion recorded for the second event are ideal engineering design data for critical facilities in eastern north america

the intracratonic caraibas itacarambi earthquake of december 09 2007 4 9 mb minas gerais state brazil abstract on december 9 2007 a 4 9 mb earthquake occurred in the middle of the sao francisco craton in a region with no known previous activity larger than 4 mb this event reached intensity vii mm modified mercalli causing the first fatal victim in brazil the activity had started in may 25 2007 with a 3 5 magnitude event and continued for several months motivating the deployment of a local 6 station network a three week seismic quiescence was observed before the mainshock initial absolute hypocenters were calculated with best fitting velocity models and then relative locations were determined with hypodd the aftershock distribution indicates a 3 km long rupture for the mainshock the fault plane solution based on p wave polarities and hypocentral trend indicates a reverse faulting mechanism on a n30 e striking plane dipping about 40 to the se the rupture depth extends from about 0 3 to 1 2 km only despite the shallow depth of the mainshock no surface feature could be correlated with the fault plane aeromagnetic data in the epicentral area show short wavelength lineaments trending nne ssw to ne sw which we interpret as faults and fractures in the craton basement beneath the surface limestone layer we propose that the caraibas itacarambi seismicity is probably associated with reactivation of these basement fractures and faults under the present e w compressional stress field in this region of the south american plate

average seismic source parameter relations for plate margin earthquakes abstract empirical mb ms and mo data are used to develop an average spectral scaling relation for plate margin earthquakes using equations based upon a rectangular bilateral dislocation model with uniform rupture velocity the spectra give values of fault rupture length and width static stress drop and average fault displacement as a function of mb ms and mo compared to mid plate earthquakes of the same seismic moment the large average plate margin earthquake has a bigger rupture length rupture area and average fault displacement and a smaller rupture width and static stress drop

the sharpsburg kentucky earthquake of 27 july 1980 the sharpsburg kentucky earthquake was the second largest earthquake to have occurred in the united states east of the continental divide in the past 20 yr having a seismic moment of 4 1 1023 dyne cm a surface wave focal mechanism study defines a nodal plane striking n30 e dipping 50 se and a nearly vertical nodal plane striking n60 w p wave first motion data indicate right lateral motion on the nodal plane striking n30 e with the pressure axes oriented east west these angles can be varied by 10 without affecting the fit to the surface wave data the surface wave solution is reinforced by a modeling of long period seismograms at regional distances the p pp and sp polarities and amplitudes from the short period vertical component array stack at norsar are used together with six unambiguous short period p wave first motions recorded in north america to test whether it is possible to constrain focal mechanism solutions with such data these solutions are compatible with the surface wave solution waveform modeling of the norsar data suggests a source pulse duration of 1 0 sec and constrains the depth to 12 0 km to match mb estimates from norsar and canadian stations t for teleseismic p must be 0 7 and 0 5 respectively when the synthetics are scaled using the surface wave seismic moment in spite of extensive coverage of the epicentral zone fewer than 70 aftershocks were recorded the largest aftershock and an mblg 2 2 aftershock locations suggest that the nodal plane striking n30 e is the fault plane an aftershock area of 30 to 50 km2 implies a stress drop of 2 8 to 6 bars and a dislocation of 2 0 to 3 4 cm because of the variety of studies performed this earthquake is presently the best studied eastern north american seismic event with well constrained estimates of focal depth focal mechanism and seismic moment and indications of the duration of the source time function and upper mantle p wave t

regional spectral scaling relations of source parameters for earthquakes in the baltic shield abstract spectral analysis of regionally recorded lg waves is performed to determine source parameters such as seismic moment source radius and stress drop of earthquakes in the baltic shield and to derive regional spectral scaling relations the data consist of about 350 lg phases on short period vertical component analog seismograms from earthquakes with magnitude m l ranging from 2 to 5 2 source radii estimated from the corner frequency show only a slight increase with increasing seismic moment in the range 3 10 18 to 5 10 20 dyn cm while the increase is more rapid for larger events baltic shield earthquakes show increasing stress drops ranging from about 0 1 to 10 bars for increasing seismic moment the relatively low stress drops could in part be explained by bias due to band limited analog data the slope of the curve relating seismic moment to corner frequency is steeper than 3 which suggests a departure from a constant stress drop scaling relation proposed for large earthquakes the derived relationship between the seismic moment m o and magnitude m l is log m o 16 93 1 01 m l for 2 m l 5 2

regional variation of the short period 1 to 10 second source spectrum we determined m b the body wave magnitude calculated from the maximum amplitude of short period p waves of 38 large earthquakes in various tectonic provinces the data are divided into three groups group 1 subduction zone thrust events group 2 nonsubduction zone dip slip and oblique slip events and group 3 strike slip events groups 2 and 3 include intraplate earthquakes comparison of m b values for these three groups of events suggests that the source spectral amplitudes of intraplate events at a period of about 1 4 sec is 2 to 5 times larger than those of subduction zone events with the same m w we also determined the source spectra of 28 large earthquakes m w 6 5 to 7 7 directly from global digital seismographic network gdsn data over a period range from 1 to 10 sec at periods from 1 to 2 sec the source spectral amplitudes of intraplate earthquakes are 2 to 4 times larger than those of subduction zone events with the same m w the difference decreases as the period increases to 10 sec

determination of earthquake focal mechanisms and crustal structure in eastern australia using surface waves abstract the surface waves from two medium sized m 5 5 earthquakes in southeast australia 1966 mt hotham and 1982 wonnangatta were examined to determine average s wave crustal structure for the region and the focal mechanisms of the two earthquakes the s wave models were determined from an analysis of the group velocities of the rayleigh waves for all paths a two layer crust overlying a moho at about 40 km is required to fit the observations in the upper layer down to about 25 km the velocity gradients are very small and the s wave velocities are in the range 3 4 3 6 km s in the lower crust the s wave velocities increase gradually to about 4 6 km s at the moho where the velocity gradients are again very small the region where the s wave velocities increase significantly with depth corresponds to the region of the lower crust which generates strong seismic reflections mathur 1983 the thin crust postulated beneath bass strait and tasmania johnson 1973 is not substantiated by these data and a crustal thickness of about 40 km is also required for this region for the longer profiles 1800 km to the north and northwest of the earthquakes the velocity step at the moho is smaller than for the shorter profiles 700 km however because the surface waves are less dispersed for the shorter profiles the resolution is poorer and the models less reliable for the 1982 wonnangatta earthquake the surface wave synthetics from a focal mechanism similar to that obtained by denham et al 1985 fit the observations at five regional stations very well and confirm the nw se compressive stress regime found by other workers however for the 1966 mt hotham earthquake the surface wave synthetics generated from the normal faulting solution which was obtained from an analysis of p wave first motions denham et al 1985 do not agree with the rayleigh wave trains observed at australian stations rather the surface wave observations support a thrust fault type mechanism with the pressure axis having a similar direction to that obtained from the 1982 wonnangatta earthquake and other earthquakes in the region

mode of strain release of three graben associated earthquakes the nature of graben associated earthquakes has been investigated through a study of three earthquake sequences using various seismological geophysical and geological data the three earthquake sequences are the hsingtai china earthquakes maximum magnitude ms 7 2 of march 1966 the nevada earthquakes maximum m 7 1 of 1954 and the new madrid earthquakes maximum ms 8 8 of 1811 1812 these earthquake sequences represent a unique mode of strain release away from plate margins and have bearings on earthquake hazard evaluation a number of similarities have been observed during these earthquake sequences for instance all the three sequences are characterized by large multiple shocks or swarms which last from several weeks to several months among the large multiple shocks in each sequence the largest one usually is not the first one migrations of epicenters were observed during the hsingtai and the new madrid earthquakes significant amount of strike slip motion was associated or inferred with all these three sequences in particular for the hsingtai earthquakes the swarm of multiple shock activities is probably due to induced instability or strong coupling and interactions among a series of adjacent faults in a heterogeneous graben the strike slip motion can be explained by a change or a rotation of stress orientation after the formation of a graben the graben events are similar to the earthquake swarms associated with spreading oceanic ridges but with significant differences available data indicate that the recurrence intervals for the three graben earthquake sequences studied are of the order of thousands of years

the tasman sea earthquake of 25 november 1983 and stress in the australian plate abstract the tasman sea earthquake of 25 november 1983 was large enough m 0 1 1 10 18 nm to be recorded world wide and provide information on the state of intraplate stress in the lithosphere beneath the tasman sea the earthquake occurred beneath the abyssal plain at a depth of about 25 km and was associated with almost pure dip slip faulting the direction of the pressure axis of the focal mechanism is similar 139 degrees e of n to those observed from the nearby australian mainland hence both the oceanic tasman sea and continental australia appear to be part of the same stress regime however the direction of stress in this part of the australian plate does not coincide with the north south direction of motion of the plate and therefore forces other than the ridge push must be invoked to generate the stresses observed

aftershock sequences of the 1982 miramichi new brunswick earthquakes the mb 5 7 miramichi new brunswick earthquake of 9 january 1982 was located by comparison to the aftershock distribution in central new brunswick canada at 47 00 n 66 60 w with a focal depth of 7 km and an uncertainty of 3 km on each of the spatial coordinates the main shock was followed by a long and complex aftershock sequence with four events m 4 5 and more than 800 events m 1 0 until 30 june 1982 the aftershocks were studied by three temporary field networks set up in the epicentral areas from 10 to 20 january 2 to 7 april and 17 to 23 june 1982 and by the permanent canadian seismograph networks the aftershock activity was concentrated in a volume approximately 6 km ns 6 km ew 8 km deep and arranged in a north trending conjugate v pattern the miramichi earthquakes were predominatly thrust earthquakes occurring on north trending mid to high angle faults in the upper 8 km of the crust in the presence of an east west compressive horizontal regional stress field detailed field surveys have not yet been able to associate the earthquakes with any known preexisting faults the main shock had a moment of 2 2 0 7 1024 dyne cm and a stress drop of 35 to 70 bars the rupture started at 7 km depth on a plane striking 195 dipping 50 down to the west and propagated up to the east with a rake angle of 120 aftershocks deeper than 3 5 km had composite mechanisms very similar to the main shock s but shallower events took place on more steeply dipping planes up to 78 dip the rupture planes of the main shock and its principal aftershocks may have been similarly curved the magnitude frequency relationship b 0 8 of the aftershocks was comparable to the regional one the aftershock activity decayed as t 0 8 and had the highest rate of activity following the principal aftershock on 11 january that aftershock mb 5 4 appears to have ruptured the east dipping plane conjugate to the main shock s plane two other principal aftershocks mb 5 1 on 9 january and mb 5 0 on 31 march were associated with the main shock rupture plane while the fourth large event mb 4 7 on 16 june occurred 30 km west of the other activity and was seemingly unrelated however all the earthquakes were apparently associated with the same granitic terrain the miramichi earthquakes did not produce any significant surface faulting one small 25 mm surface displacement was found but it was identified as a secondary stress induced movement along a preexisting joint

modeling of strong ground motions from the 16 september 1978 tabas iran earthquake we have examined the adequacy of the published ks3 and l1 slip distribution models developed for the m s 7 4 16 september 1978 tabas earthquake hartzell and mendoza 1991 by modeling the high frequency accelerograms recorded at dayhook boshrooyeh and tabas stations the agreement in peak ground acceleration pga and duration between data and simulated accelerograms was chosen as the criterion for the model adequacy the fault was specified with a seismogenic rupture area 95 km long and 45 km wide we used a semi empirical simulation method in which accelerograms from an aftershock from the 1979 imperial valley earthquake were used to represent the radiation pattern of p sv or sh waves for a source receiver geometry the fault surface is divided into many subfaults the contributions from these subfaults weighted by the slip amounts were lagged and summed to simulate the accelerograms of the main event we computed accelerograms assuming constant rake and variable rake on the fault plane the former was simulated using the sum slip model i e the slip model representing the vector sum of the slip components at every point on the fault and the latter was simulated by summing the accelerograms generated by the slip models of the dip slip and strike slip faults the influence of the rise time of the main event was investigated the ks3 asperity model produces results marginally better than the l1 asperity model the duration is predicted consistent with the data the peak amplitudes of the simulated ground motions remain within a factor of 1 5 and 3 of the recorded data at dayhook and boshrooyeh respectively for the variable rake and within a factor for 2 5 at both stations for the fixed rake angle ks3 model the large pga recorded on the tabas accelerograms could not be reproduced which warrants additional study a notable agreement was obtained in the response spectra at dayhook this study demonstrates that accelerograms generated by an earthquake in one tectonic region can be transported to another tectonic region so that they can be used to represent the empirical radiation pattern of the subfaults in simulating ground motions for a large event

self consistent earthquake fault scaling relations update and extension to stable continental strike slip faults using seismic moment m length l data for stable continental region scr faults augmented by data not included in previous studies this article reassesses and confirms the bilinear scaling relation for the dip slip faults of leonard 2010 furthermore using the new data i propose a separate bilinear relation for estimating magnitude from surface rupture length there are now a small number of scr strike slip earthquakes for which the fault dimensions have been estimated i use these to constrain a trilinear scaling relation for this class of fault for the four fault types interplate dip slip interplate strike slip scr intraplate dip slip and scr intraplate strike slip the scaling relations for the 10 permutations of m d l w and a are given

focal mechanisms of small earthquakes in the southeastern brazilian shield a test of stress models of the south american plate summary focal mechanisms of small earthquakes with magnitudes of about 3 in the se brazilian shield are calculated using s p amplitude ratios low attenuation q p from 400 to 800 in the shield upper crustal layers allowed sharp s arrivals to be recorded up to distances of 100 km besides p wave polarities sh wave first motions were also used to constrain the nodal plane orientations normal and reverse faulting mechanisms with strike slip components were found the inversion of four mechanisms to estimate the stress tensor indicated a strike slip stress regime with roughly e w orientated s 1 and n s s 3 both the orientations and the shape factor q 0 7 of the inverted stress are in excellent agreement with theoretical predictions for that part of brazil from the driving force model of coblentz richardson 1996 good agreement with the nature of the stress as well as its orientation was also found for the model of meijer 1995 both of these theoretical models include spreading stresses along the continent ocean lithospheric transition because the earthquakes are more than 300 km from the continental shelf they should not be avected by the local flexural forces caused by sediment load in the marginal basins the agreement between observed and theoretical stresses then confirms the importance of continental spreading forces in modelling intraplate stresses

source mechanisms of moderate size earthquakes and stress orientation in mid plate south america summary fault plane solutions of six mid plate earthquakes with magnitudes ranging from 4 8 to 5 5 mb were determined using p wave first motion data and the relative amplitudes of short period teleseismic depth phases pp and sp these results together with previous studies show a predominance of reverse and strike slip faulting indicating horizontal compressional stresses in mid plate south america the orientation of the p axes however is not uniform in western and southwestern brazil the p axes are oriented approximately e w probably due to the nazca south america plate convergence in eastern brazil the direction of the p axes show more scatter but an average e w direction is consistent with the available data on the contrary in northern brazil the p axes of two events in the amazon basin trend approximately n s a direction perpendicular to the direction of absolute plate motion as well as to the relative plate motions nazca south america and south america africa

forecasting damaging earthquakes in the central and eastern united states analysis of seismograph network data earthquake catalogs from 1727 to 1982 and paleoseismic data for the central and eastern united states indicate that the poisson probability of a damaging earthquake magnitude 6 0 occurring during the next 30 years is at a moderate to high level 0 4 to 0 6 when differences in seismic wave attenuation are taken into account the central and eastern united states has approximately two thirds the likelihood of california to produce an earthquake with comparable damage area and societal impact within the next 30 years

average seismic source parameter relations for mid plate earthquakes mid plate earthquakes which are defined as having their epicenters at least 500 km from plate margins represent a small portion of the total global seismicity however because those occurring within continental interiors are characterized by anomalously large damage areas they are of particular interest empirical mb ms and m o data are used to develop scaling relations for mid plate earthquakes from them it is found that the corner period t o2 of their far field displacement spectra varies as the one fourth power of the seismic moment which indicates that average stress drop increases as seismic moment increases by use of savage s 1972 rectangular bilateral dislocation model the derived spectra of mid plate earthquakes yield estimates of fault rupture length and width average fault displacement and source rise time as a function of seismic moment these relations show that large mid plate earthquakes such as the 1886 south carolina and 1811 and 1812 new madrid events do not require large fault rupture length

age dependence of oceanic intraplate seismicity and implications for lithospheric evolution we have determined depths for 16 oceanic intraplate earthquakes using body wave modeling a data set composed of these and 11 other well constrained depths shows that the maximum depth of seismicity deepens with increasing lithospheric age and appears to be bounded by a 700 800 c isotherm the maximum depth to which intraplate earthquakes occur is approximately equal to the flexural elastic thickness but much less than the seismic thickness from surface wave dispersion the maximum faulting depth is consistent with the predictions of rapid weakening at high temperatures for dry olivine rheologies determined by experimental rock mechanics oceanic intraplate earthquakes can occur at greater depths and temperatures than has previously been reported for continental crustal events presumably because oceanic lithosphere is stronger at high temperatures both the number of earthquakes and the cumulative seismic moment decrease with lithospheric age the cause of this trend is unclear possibilities include nonrandom location of preexisting weak zones with respect to the age provinces fault strengthening with age or a decrease in the stress level a decrease in intraplate stress with age is opposite that expected from ridge push stresses which increase with age but may result either from thermoelastic stresses which are greatest in young lithosphere or near ridge stress concentrations due to local heterogeneities in the spreading process

a continuing intraplate earthquake sequence near joao câmara northeastern brazil preliminary results shallow seismicity along a well defined section of reactivated precambrian basement near joao c mara brazil 5 5 s 33 7 w has continued since august 1986 a magnitude 4 3 event on 21st august with a decaying aftershock sequence and another main shock aftershock sequence which started in september were followed by a magnitude 5 1 event on 30th november and 11 events of magnitude 4 or above up to february 1987 a network of up to four vertical component short period seismometers with smoked paper recorders has been deployed since late august 1986 several thousand events have been recorded above magnitude 0 and activity was still being recorded about 10 events per day in may 1987 simple velocity structure good transmission and impulsive s arrivals on almost all seismograms combine to facilitate reliable hypocentral locations even for this sparse network and epicenters define a 25 km linear pattern of seismicity with northeasterly strike approximately coincident with a precambrian structural trend a composite fault plane solution shows dextral shear along this strike with a small normal component fault plane solutions and epicenter distribution in northeastern brazil suggest an overall picture of e w regional compression

comparison of source scaling relations of eastern and western north american earthquakes source scaling relations have been obtained for earthquakes in eastern north america and other continental interiors and compared with a relation obtained for earthquakes in western north america the scaling relation for eastern north american earthquakes was constructed from measurements of seismic moment and source duration obtained by the waveform modeling of seismic body waves the events used include nine events of m blg magnitude 4 7 to 5 8 that occurred after 1960 and four earlier events with magnitudes between 5 5 and 6 6 the scaling relation for events in other continental interiors was used for comparative purposes and to provide constraints for large magnitudes detailed analysis of the uncertainties in the scaling relations has allowed the resolution of two important issues concerning the source scaling of earthquakes in eastern north america first the source characteristics of earthquakes in eastern north america and other continental interiors are consistent with constant stress drop scaling and are inconsistent with nonconstant scaling models such as that of nuttli 1983 second the stress drops of earthquakes in eastern north america and other continental interiors are not significantly different from those of earthquakes in western north america and have median values of approximately 100 bars the source parameters of earthquakes in eastern north america are consistent with a single constant stress drop scaling relation whereas the source parameters of earthquakes in western north america are much more variable and show significant departures from an average scaling relation in which stress drop decreases slightly with seismic moment

seismic moment assessment of earthquakes in stable continental regions ii historical seismicity summary this part i study in conjunction with part 11 develops a method to determine within specified uncertainty bounds the seismic moment and thus moment magnitude of all earthquakes of stable continental regions scr for which instrumental or intensity data exist its basis is polynomial regression analysis using a database of scr earthquakes with direct seismic moment determination the independent variables include modern teleseismic magnitudes and regional magnitudes part i and isoseismal areas or number of recording stations part 11 part i11 is an application of the methodology of parts i and i1 to several major historical earthquakes all data used in the regressions are assigned individual uncertainties estimated from the literature or from experience formal confidence limits 68 per cent or 95 per cent on both the regression formulas and the predicted seismic moment values are then possible via error propagation analysis the most complete development is for the teleseismic magnitudes m and mb for both the final regression for log m is a quadratic formula that closely emulates the relationship between amplitude magnitudes and log m expected from dislocation theory and source scaling arguments regressions are also derived for the regional magnitudes mlg and m because there are many scr events mainly pre 1964 that have no teleseismic magnitudes prediction uncertainties from teleseismic magnitudes in moment magnitude units are in the ao 18 0 28 range and from regional magnitudes in the ko 23 0 38 range over a wide magnitude band finally the methodology developed here is generic even though the database is specific application to plate boundary oceanic intraplate or active continental intraplate regions should be straightforward

the dependence of peak horizontal acceleration on magnitude distance and site effects for small magnitude earthquakes in california and eastern north america abstract one hundred and ninety free field accelerograms recorded on deep soil 10 m deep were used to study the near source scaling characteristics of peak horizontal acceleration for 91 earthquakes 2 5 m l 5 0 located primarily in california of the six attenuation relationships developed in this study the one considered most reliable is given by the expression ln p h a 2 5 0 1 0 6 2 3 m l 1 0 ln r 7 2 8 where pha is the mean of the two horizontal components of peak acceleration in g m l is local magnitude r is epicentral distance in kilometers and is a random error term with mean of zero and standard deviation of 0 506 an analysis of residuals based on an additional 171 near source accelerograms from 75 earthquakes indicated that accelerograms recorded in building basements sited on deep soil have 30 per cent lower accelerations and that free field accelerograms recorded on shallow soil 10 m deep have 82 per cent higher accelerations than free field accelerograms recorded on deep soil accelerations recorded on rock were found to be similar in amplitude to those recorded on deep soil a separate analysis of site effects for the imperial valley and oroville regions of california indicated that peak accelerations from individual sites can depart significantly from predictions based on broad site classifications an analysis of residuals based on 27 selected strong motion recordings from 19 earthquakes in eastern north america indicated that near source accelerations associated with frequencies less than about 25 hz are consistent with predictions based on attenuation relationships derived for california once site effects and differences in regional magnitude scales are taken into account

strong motion seismology strong motion seismology is concerned with earthquake ground motion in the amplitude range that poses the threat of human injury or property damage because the occurrence of strong motion cannot be predicted in advance and because the time between significant events at any one place may range from tens to thousands of years strong motion data is recorded on triggered instruments that typically must be maintained in readiness for years between periods of significant recording as a consequence the strong motion data set is sparse because of the engineering need for estimates of future ground motion the scope of the field of strong motion seismology encompasses seismic source theory and any other aspect of seismology that can provide insights helpful in making the best possible estimates given the limited data this relationship with the rest of seismology is a reciprocal one in that strong motion data gives perhaps the best information available on earthquake source processes

seismic strain rates in the central and eastern united states seismic strain rates for the entire central and eastern united states are estimated from historical seismicity and from several seismicity models typical strain rates are estimated to be the on the order of 10 12 to 10 11 per year except near historical sites of large earthquakes where higher rates pertain uncertainties in strain rates are large deformation caused by strain rates less than 10 10 yr 1 is very small compared to average denudation rates denudation and deposition may control the rate of earthquake occurrence in some parts of the central and eastern united states by modifying the vertical stress due to load a simple quantitative model predicts the earthquake occurrence rate in the appalachian mountains from an estimate of the regional denudation rate

empirical magnitude and spectral scaling relations for mid plate and plate margin earthquakes abstract published values of body wave magnitude m b surface wave magnitude m s and seismic moment m o for mid plate and plate margin earthquakes are analyzed to determine their inter relationship and to develop scaling laws for earthquake spectra all of the plate margin earthquakes used in the study occurred near the border of the pacific ocean the mid plate earthquakes occurred in both continental plate interiors and oceanic plate interiors for mid plate earthquakes the m b m s relation can be represented by two straight lines of slope one and two the intersection of the two lines corresponds to the body wave magnitude for which the corner period of the spectrum is at 1 sec the m b m s relation for plate margin earthquakes is more complicated because the derived spectra exhibit two corner periods a principal feature of the m b m o and m s m o relations is the fact that the large plate margin earthquakes have a higher m o value than the mid plate earthquakes for the same m b or m s because m b is a measure of the spectral amplitude for frequencies at which damaging ground motion occurs and m 0 is a measure of the spectrum at very long periods it follows that mb is a good estimator of strong ground motion whereas a moment derived magnitude is not the latter however is a good measure of fault rupture area the derived spectra of the mid plate earthquakes are flat at the long periods and have a slope of two at the short periods their seismic moment varies as the fourth power of the corner period implying that the stress drop increases as the moment increases for the large plate margin earthquakes the derived spectra are flat at the long periods have a slope of one at intermediate periods and a slope of two at short periods the stress drops are almost independent of moment for the larger plate margin earthquakes the moment magnitude relations for mid plate earthquakes indicate that an m s 8 7 event requires a fault rupture length of only 60 km for a fault width of 20 km a plate margin earthquake of m s 8 7 and fault width of 20 km would require a fault length of 6000 km obviously impossible assuming a fault width of 20 km an m s 8 2 earthquake has a rupture length of about 850 km which is as large as can be expected for strike slip faults such as the san andreas of california subduction zone earthquakes on the other hand if they have fault widths as great as 200 km can give rise to m s 8 7 earthquakes

a generalization of the stochastic summation scheme of small earthquakes to simulate strong ground motions a generalization of the stochastic summation scheme of small earthquakes following a general form of the source spectrum n model is developed to estimate the proper probability density functions for rupture propagation over the fault and to simulate site specific strong ground motions in two independent stages the probability density functions for rupture propagation are first developed for a single corner frequency source model and then extended to multiplicative and additive double corner frequency source models to partially account for the effects of finite fault rupture such as fault geometry rise time and rupture time on ground motions and to better match observed fourier amplitude spectra fas the generalized two stage stochastic summation scheme developed in this study can be used to simulate realistic strong ground motion time histories from small events represented as empirical green s functions egfs by specifying only two source parameters of the seismic moment and stress drop for a given site the proposed two stage stochastic summation schemes are tested using numerical examples following the theoretical source models as well as observed ground motions of the 2011 mw 9 0 tohoku earthquake in japan and the 1994 mw 6 7 northridge earthquake in california to verify the applicability of the approach comparisons between the observed and simulated time histories for these two earthquakes their response spectra and fas show satisfactory performance of the proposed approach for engineering purposes the results of these comparisons indicate that the proposed stochastic summation of small events as egfs can adequately replicate observed ground motions and evaluate uncertainties in ground motions with the generation of many time history realizations corresponding to a multitude of possible rupture processes the proposed approach can also be used to generate multiple realistic acceleration time histories matched on average to the target response spectra for use in earthquake performance design and to develop ground motion models for low seismicity regions where there is a lack of recording of strong ground motions

excitation and propagation of lg from earthquakes in central asia with implications for explosion earthquake discrimination broadband lg records from 52 shallow earthquakes that occurred near the balapan and lop nor nuclear test sites have been used to study spectral characteristics of the excitation and propagation of the lg phase in central asia using a source model that falls off as 2 at high frequencies and a nonlinear inversion method we simultaneously inverted for lg seismic moments m0 corner frequencies fc and path variable values of q0 and qlg at 1 hz and its power law frequency dependence the logarithms of m0 obtained using lg correlate linearly with logarithms of fc the slope being about 3 56 when all data are used and 4 04 when data were sufficient that full inversions could be performed these slopes differ only slightly from the slope of 3 83 obtained in our earlier studies using underground nuclear explosions the fc values in this study are systematically lower than those found for explosions at the same m0 level a comparison of corner frequencies and lg moment values for unknown events with the plots for known events of this study therefore provides means to discriminate between earthquakes and explosions the mb values are systematically lower for earthquakes of this study than for previously studied explosions at the same seismic moment moreover the logarithms of lg moments obtained in this study correlate linearly with a slope of 1 04 with mb values when all data are used and 1 21 when only well recorded events are used these slopes are close to the slope of 1 19 obtained for explosions the average qlg values along the paths used in this study agree well with those obtained using explosion data in central asia and with reported lg coda q values

spectral scaling of the 1985 to 1988 nahanni northwest territories earthquakes abstract the sequence of earthquakes that began in october 1985 near the nahanni river northwest territories canada provides an exceptional opportunity to study the variation with magnitude of the source spectra from intraplate earthquakes we study the spectral scaling for the seven largest events using p waves recorded at stations of the western canada telemetred network wctn the eastern canada telemetred network ectn and the global digital seismograph network gdsn these earthquakes range in size from m 4 8 to m 6 8 we compare predictions from various source scaling models with observed spectral ratios and source acceleration spectra for the suite of events the spectral ratios show a strong azimuthal dependence which is probably due to differences in the focal mechanisms or rupture characteristics of the events taking the average of the ratios from individual gdsn stations as the best representation of the spectral ratios an squared model with a constant stress parameter or one that increases slightly with moment magnitude gives a simultaneous fit to both the spectral ratios and to the absolute spectra the high frequency spectral levels by themselves imply a somewhat stronger dependence of the stress parameter on moment than found from fitting the overall spectra the stress parameters for the scaling laws are less than 35 bars for earthquakes as large as m 7 recently proposed scaling laws in which the corner frequency is proportional to the inverse quarter root of seismic moment for an squared model are not consistent with the data and neither are cubed models the spectral scaling for the sequence of nahanni earthquakes may not be representative of intraplate magnitude scaling in general since there is some evidence that aftershocks do not exhibit the same scaling as main shocks the high frequency spectral levels of the aftershocks of both the nahanni and the 9 january 1989 miramichi new brunswick earthquakes imply stress parameters that increase with magnitude by contrast the spectral levels for a series of main shocks in eastern north america show less magnitude dependence of the stress and generally higher stress levels when compared to aftershocks of the same magnitude

an investigation into earthquake ground motion characteristics in eastern north america abstract a random vibration model of the hanks mcguire type is used to predict peak ground motions at rock sites in eastern north america the assumed geometric decay and distance dependent duration approximate the propagation of direct body waves at short distances and lg waves at regional distances the model predicts peak acceleration and velocities as well as response spectra and magnitude for a given seismic moment and corner frequency to predict ground motions for a given lg magnitude m lg the model is first used to calculate the seismic moment corresponding to that magnitude assuming a source scaling law then knowing the moment and corner frequency the model is used to calculate peak ground motions available data from strong motion recordings and from the ectn and lrsm networks modified to estimate horizontal ground motions on rock where appropriate are used to verify the model9s assumption and predictions modified mercalli intensity data are also used ground motions predicted by the model with a stress drop of 50 to 200 bars agree with the ground motion data but the m lg values computed by the model for given seismic moments do not agree with nuttli9s 1983 moment versus m lg data for large earthquakes resolution of the latter disagreement awaits the collection of instrumental data from large events in eastern north america

high frequency spectral scaling of a main shock aftershock sequence near the norwegian coast a high frequency seismic element was recently added to the noress regional array in norway this system can monitor seismic signals at frequencies up to 50 hz in february 1986 the high frequency seismic element recorded an mblg 4 7 main shock and several aftershocks which occurred 420 km northwest of noress off norway s western coast these events produced high frequency signals which were well above the background noise at the station p wave spectra of these events scale in a manner consistent with the square constant stress drop source model the data do not require any change in this scaling to magnitudes mblg below 2 in contrast to previous reports that constant stress drop scaling breaks down at smaller magnitudes

depth variation of seismic source scaling relations implications for earthquake hazard in southeastern australia abstract advances in earthquake data acquisition and processing techniques have allowed for improved quantification of source parameters for local australian earthquakes until recently only hypocentral locations and local magnitudes ml had been determined routinely with little attention given to the inversion of additional source parameters the present study uses these new source data e g seismic moment stress drop source dimensions to further extend our understanding of seismicity and the continental stress regime of the australian landmass and its peripheral regions earthquake activity within australia is typically low and the proportion of small to large events i e the b value is also low it is observed that average stress drops for southeastern australian earthquakes appear to increase with seismic moment to relatively high levels up to approximately 10 mpa for ml 5 0 earthquakes this is thought to be indicative of high compressive crustal stress coupled with strong rocks and fault asperities furthermore the data indicates that shallow focus earthquakes shallower than 6 km appear to produce lower than average stress drops than deeper earthquakes between 6 and 20 km with similar moment recurrence estimates were obtained for a discrete seismogenic zone in southeastern australia decreasing b values with increasing focal depth for this zone indicate that larger earthquakes with high stress drops tend to occur deeper in the crust this may offer an explanation for the apparent increase of stress drop with hypocentral depth consequently earthquake hazard estimates that assume a uniform gutenburg richter distribution with depth i e constant b value may be too conservative and therefore slightly overestimate seismic hazard for surface sites in southeastern australia