a teleseismic analysis of the new brunswick earthquake of january 9 1982 the analysis of the new brunswick earthquake of january 9 1982 has important implications for the evaluation of seismic hazards in eastern north america although moderate in size mb 5 7 it was well recorded teleseismically source characteristics of this earthquake have been determined from analysis of data that were digitally recorded by the global digital seismograph network from broadband displacement and velocity records of p waves we have obtained a dynamic description of the rupture process as well as conventional static properties of the source the depth of the hypocenter is estimated to be 9 km from depth phases the focal mechanism determined from the broadband data corresponds to predominantly thrust faulting from the variation in the waveforms the direction of slip is inferred to be updip on a west dipping nne striking fault plane the steep dip of the inferred fault plane suggests that the earthquake occurred on a preexisting fault that was at one time a normal fault from an inversion of bodywave pulse durations the estimated rupture length is 5 5 km average properties of the rupture process were examined by a moment tensor analysis of long period p and sh body waves the long period moment of this earthquake was 5 3 1024 dyne cm the static and dynamic stress drops are 41 and 65 bars respectively similar to those of many earthquakes with similar moment in regions that are more seismically active the joint epicenter determination algorithm was used to locate relative to the mainshock the three teleseismically recorded aftershocks that occurred through march 31 1982 the relocated hypocenters of the aftershocks are significantly different from each other and from that of the mainshock they provide additional support for the source dimensions inferred from the waveform analysis

broadband analysis of the extended foreshock sequence of the miyagi oki earthquake of 12 june 1978 the miyagi oki earthquake of 12 june 1978 a large ms 7 8 interplate thrust event occurred in a region which had not experienced earthquakes of magnitude greater than 7 since 1938 a sequence of four moderate sized 5 4 mb 6 1 earthquakes encircled the rupture zone of the miyagi oki earthquake over a period of 2 yr before the main shock broadband displacement and velocity records of body waves recorded digitally by stations of the global digital seismograph network are analyzed to determine the static and dynamic characteristics of the sequence these characteristics include moment radiated energy dynamic and static stress drop and apparent stress inversions of duration measurements made on the velocity waveforms permit quantifying the complexity of an event as well as constraining its rupture geometry intervals of 7 to 8 months separated the first three events the main shock occurred 4 months after the third event the rupture process of the third event was relatively complex the event also had a substantially higher dynamic stress drop 175 bars than did the stress drops of the first two events 9 and 10 bars respectively these observations suggest that the third event was an interme diate term precursor to the main shock the fourth event a short term precursor to the main shock occurred about 8 min before the main shock its dynamic stress drop 20 bars was lower than that of the third event but higher than that of the first two events

broad band deconvolution of gdsn data summary advances in the theory of wave propagation and source mechanism have coincided with the recent deployment of a global network of digitally recording seismographs the gdsn global digital seismograph network we demonstrate that broad band records of body wave ground displacement and velocity with spectral information from several hertz to tens of seconds can be obtained by the simultaneous deconvolution of the long and short period components of the gdsn seismographs frequency dependent effects in body waves that may arise from source complexity or from propagation through the earth are much more apparent and measurable in the broad band records than in the original seismograms

comparative quality and fidelity of deep sea and land based nannofossil records variation in sampling intensity of the geological record has long been suspected to distort our view of the history of life when the taxonomic diversity of the same widespread group of marine nannoplankton coccolithophorids is estimated conventionally and separately from published land and deep sea fossil records the two curves are very different they track sampling intensities from their respective rock records more closely than they do each other however when sampling intensity is corrected for using two recently developed independent techniques a common underlying signal emerges from the two records this shows diversity rising steadily throughout the mesozoic with a marked drop after the cretaceous 64 58 ma and a long term decline beginning in the late eocene and extending to the early miocene we conclude that the observable fossil record is strongly shaped by sampling bias and that this is a significant confounding factor for biodiversity analysis furthermore based on currently available sources we find that the raw deep sea record is a worse estimator of diversity than the land based record

stress field constraints on intraplate seismicity in eastern north america focal mechanisms of 32 north american midplate earthquakes mo 3 8 6 5 were evaluated to determine if slip is compatible with a broad scale regional stress field derived from plate driving forces and if so under what conditions stress regime pore pressure and frictional coefficient using independent information on in situ stress orientations from well bore breakout and hydraulic fracturing data and assuming that the regional principal stresses are in approximately horizontal and vertical planes 10o the constraint that the slip vector represents the direction of maximum resolved shear stress on the fault plane was used to calculate relative stress magnitudes defined by the parameterb s2 s3 s s3 from the fault stress geometry as long as the focal mechanism has a component of oblique slip i e the b axis does not coincide with the intermediate principal stress direction this calculation identifies which of the two nodal planes is a geometrically possible slip plane gephart 1985 slip in a majority of the earthquakes 25 of 32 was found to be geometrically compatible with reactivation of favorably oriented preexisting fault planes in response to the broad scale uniform regional stress field slip in five events was clearly inconsistent with the regional stress field and appears to require a localized stress anomaly to explain the seismicity significantly all five of these events occurred prior to 1970 when many regional networks were installed and their focal mechanisms are inconsistent with more recent solutions of nearby smaller events the frictional likelihood of the geometrically possible slip on the selected fault planes was evaluated in the context of conventional frictional faulting theory the ratio of shear to normal stress on the fault planes at hypocentral depth was calculated relative to an assumed regional stress field regional stress magnitudes were determined from 1 s s3 ratios based on the frictional strength of optimally oriented faults the basis for the linear brittle portion of lithospheric strength profiles 2 the computed relative stress magnitude b values and 3 a vertical principal stress assumed equal to the lithostat two end member possibilities were examined to explain the observed slip in these less than optimally oriented fault planes first the frictional coefficient was held constant on all faults hydrostatic pore pressure was assumed regionally and the fault zone pore pressure was determined since pore pressure is a measurable quantity with real limits in the crust p0 s3 this end member case was used to determine which of the geometrically possible slip planes were frictionally likely slip planes alternately pore pressure was fixed at hydrostatic everywhere and the required relative lowered frictional coefficient of the fault zone was computed slip in 23 of the 25 geometrically compatible earthquakes was determined to also be frictionally likely in response to an approximately horizontal and vertical regional stress field derived from plate driving forces whose magnitudes are constrained by the frictional strength of optimally oriented faults assuming hydrostatic pore pressure regionally the conditions for slip on these 23 relatively well oriented earthquake faults were determined relative to this regional crustal strength model and require only moderate increases in pore pressure between about 0 4 0 8 of lithostatic hydrostatic is about 0 37 of lithostatic or alternately moderate lowering 50 of the frictional coefficient on the faults which slipped superlithostatic pore pressures are not required focal mechanisms for the two other earthquakes with slip vectors geometrically consistent with the regional stress field however did require pore pressures far exceeding the least principal stress or extremely low coefficients of friction these events may reflect either local stress rotations undetected with current sampling or poorly constrained focal mechanisms the analysis also confirmed a roughly north to south contrast in stress regime between the central eastern united states and southeastern canada previously inferred from a contrast in focal mecha nisms between the two areas most central eastern united states earthquakes occur in response to a strike slip stress regime whereas the southeastern canadian events require a thrust faulting stress regime this contrast in stress regime with a constant maximum horizontal stress orientation determined by far field plate driving forces requires a systematic lateral variation in relative stress magnitudes superposition of stresses due to simple flexural models of glacial rebound stresses are of the correct sense to explain the observed lateral variation but maximum computed rebound related stress magnitude changes are quite small about 10 mpa and do not appear large enough to account for the stress regime change if commonly assumed stress magnitudes determined from frictional strength apply to the crust at seismogenic depths

rupture process of a multiple main shock sequence analysis of teleseismic local and field observations of the tennant creek australia earthquakes of january 22 1988 on january 22 1988 three large intraplate earthquakes with ms 6 3 6 4 and 6 7 occurred within a 12 hour period near tennant creek australia these earthquakes which occurred over a small interval of time and within a small volume of space present a unique opportunity to study the rupture process of the class of intraplate earthquakes that occur as multiple main shocks broadband displacement and velocity records of body waves from teleseismically recorded data are analyzed to determine source mechanisms depths and complexity of rupture of each of the three main shocks hypocenters of an additional 150 foreshocks and aftershocks constrained by local arrival time data and field observations of surface rupture are used to complement the source characteristics of the main shocks in order to derive as complete a description of the rupture process as possible the interpretation of the combined data sets suggests that the overall rupture process involved unusually complicated stress release as locations of the main shock hypocenters progressively moved from west to east we infer that the first and third main shocks denoted as ms1 and ms3 produced the southeast northwest trending scarps observed at the western end the kunayungku fault and at the eastern end the east end of the lake surprise fault respectively of the rupture zone the epicenter of the only immediate foreshock was located in the gap between these two fault scarps ms1 nucleated near this epicenter and ruptured upward and to the northwest from a depth of 6 5 km ms3 ruptured predominantly to the se at a depth of 4 5 km the second main shock ms2 is inferred to have produced the deformation of the southwest trending central scarp segment the western end of the lake surprise fault from the sense of thrusting seen at the surface and from the distribution of aftershock hypocenters the south dipping nodal planes derived from waveform modeling are identified as the fault planes for earthquakes ms1 and ms3 in contrast the dip of the central fault scarp is reversed relative to the dips of the western and eastern fault scarps the rupture process of ms2 turns out to be commensurately complex and sufficiently explains the geological complexity ms2 consisted of three subevents the southeast dipping nodal plane of the first two subevents is coplanar with a southeast dipping plane implied by locations of aftershocks which did not break the surface choice of the north dipping plane as the rupture plane of the third subevent consistent with the surface deformation and coplanar with a second plane delineated by aftershocks would imply conjugate faulting the majority of the aftershocks are concentrated near the edges of the fault planes and there is an absence of activity in the center of the planes the areas of absent activity may represent the failed asperities of the main shocks in which substantial stress relief occurred the rupture process of each main shock is characterized by the rapid release of energy followed by a much slower release of moment and by aftershock zones whose dimensions exceed the inferred dimensions of the rupture these characteristics suggest that substantial slow slip occurred on each of the three fault interfaces that was not accompanied by major energy release the first main shock nucleated at the deepest part of its aftershock zone and ruptured upward in contrast ms2 and ms3 nucleated at depths that were one third to one half of the maximum depth of their aftershock zones this variation of focal depth and the strong increase of moment and radiated energy with each main shock imply that lateral variations of strength were more important than vertical gradients of shear stress in controlling the progression of rupture

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

source parameters of the 1980 mammoth lakes california earthquake sequence from the more than 1500 mammoth lakes earthquakes recorded on three component digital seismographs spudich et al 1981 150 were used in an analysis of the locations mechanism and source parameters a composite fault plane solution of nine earthquakes 3 9 m 5 1 defines a right lateral strike slip mechanism on a steeply dipping nearly east west plane striking s75 e or left lateral strike slip on a nearly north south plane striking n10 e vertical cross sections of well located aftershocks indicate possibly three east west planes that coincide with the locations of the four largest earthquakes with ml 6 0 using the spectral analysis of s waves brune 1970 source parameters for 67 earthquakes were determined forty eight had magnitudes greater than or equal to 3 0 seismic moments ranged from 9 20 1018 dyn cm to 2 33 1024 dyn cm earthquakes with seismic moment greater than about 1 0 1021 dyn cm had nearly constant stress drops 50 bars earthquakes with seismic moment less than about 1 0 1021 dyn cm had stress drops that apparently decrease as seismic moment decreases

source inversion of seismic waveforms the koyna india earthquakes of 13 september 1967 abstract the treatment of the seismic source inverse problem when diverse forms of waveform data are available is simple and elegant using a moment tensor formalism if earth structure is known and its effects predictable in terms of vertically inhomogeneous elastic layered models then all types of wave phenomena e g surface waves body waves leaky modes etc for a purely deviatoric moment tensor point source may be represented by at most a sum of three green9s functions for an arbitrary symmetric moment tensor point source one additional green9s function is needed for the p sv system however elegant this formalism may be for posing the linear inverse problem the major difficulties lie in earth structure unknowns and resultant nonlinearities in the green9s functions which can cause significant trade offs with source parameters a hybrid inversion procedure is set up to gain insight into the probable unknowns in particular problems by incorporating both a linearized least squares gradient method for the moment tensor or double couple and smoothed time function parameters and a nonlinear systematic trial and error search for moment tensor or double couple parameters for several assumptions of green9s function the inversion technique is applied to near regional waveform data from a small earthquake associated with the koyna reservoir which occurred 13 september 1967 as the second in a group of three events with very similar waveshapes but differing amplitudes the magnitudes of the second and third events are smaller by 0 2 and 0 8 units respectively compared to the first the absolute magnitude for the first event is poorly constrained but is estimated to be 4 0 to 4 5 rather than the previously published value of 5 5 to 6 0 from the similarity of waveshapes all three events are inferred to have the same mechanism and occurred within about 2 km of the same hypocenter the results from moment tensor and double couple inversions for event 2 data indicate that source depth was 5 km and that left lateral faulting occurred on a plane with a strike of n20 e 5 dip of 90 15 and a rake of 0 35 the inferred far field time function is approximately 3 sec in duration unusually long for the seismic moment of 9 10 22 dyne cm yielding a possible stress drop of about 0 05 bars a fault map was constructed from landsat image interpretation and shows a predominance of nnw to nne striking faults in the koyna area which is used to infer the appropriate nodal plane in the inversion results these faults tend to define a broad en echelon zone which parallels the western ghats in this area

source parameters of seven large australian earthquakes determined by body waveform inversion summary we determine source mechanisms and centroid depths for the seven largest earthquakes in and near the australian continent in the past 20yr using teleseismic body waves six of the earthquakes were thrusts and one was strike slip all five earthquakes that occurred beneath the continent were shallower than 10 km the ranges of centroid depths are constrained to be 1 5 2 4 0 3 5 11 and 6 10km and indicate nearly pure thrust faulting the three shallowest events were associated with surface faulting while the published first motion solutions for these three events included large amounts of strike slip our thrust mechanisms are more consistent with displacements observed at the surface of the earth the range of possible centroid depths for a strike slip earthquake beneath the continental shelf off the northwest coast is 23 29 km and that for a thrust earthquake in oceanic lithosphere beneath the tasman sea is 22 26 km below sea level the pressure axis is consistently near horizontal and in the western half of the continent five earthquakes indicate that the azimuth of the maximum compressive stress is e ene azimuthal range of 66 91 on the basis of one earthquake we conclude that the p axis is oriented nnw beneath the tasman sea

a model of reverse differentiation at dikii greben volcano kamchatka progressive basic magma vesiculation in a silicic magma chamber dikii greben volcano is the largest modern volcano with silicic rocks in the kurile kamchatka island arc it consists of many domes and lava flows of rhyodacite dacite and andesite which were erupted in a reverse differentiation sequence non equilibrium phenocryst assemblages quartz mg rich olivine an rich an poor plagioclase etc abundance of chilled mafic pillows in the dacites and andesites and linear variations of rock compositions in binary plots are considered as mineralogical textural and geochemical evidence for mixing mafic pillows in volcanics have a lower density because of high porosity and contain the same non equilibrium phenocryst assemblages as the host rocks their groundmass contains skeletal microlites of plagioclase and amphibole proving that the groundmass as well as the pillows themselves formed from a water rich basaltic magma at depth they are considered as supercooled vesiculated floating drops of a hot hybrid layer in the magma chamber which formed after refilling the lower density of the inclusions allows them to float in the host magma and to concentrate at the top of the chamber prior to eruption magma mingling was effected by mechanical disintegration of the inclusions in the host magma during eruption the rhyodacitic and basic end members of the mixing series cannot be linked by low p fractionation though high p amphibole rich fractionation is not excluded

source characterization and fault plane determination for mblg 1 2 to 4 4 earthquakes in the charlevoix seismic zone quebec canada two earthquake doublets and two multiplets recorded by the charlevoix telemetered network cltn in the charlevoix seismic zone csz of southern quebec canada have been analyzed using an empirical green s function egf method to derive the relative source time functions rstf s of seven master events with mblg 1 2 to 4 4 we identified the doublets and multiplets using a waveform cross correlation and relative event location technique to verify that each earthquake pair had similar focal mechanisms and hypocentral locations three component s waveforms recorded by the high dynamic range 126 db instrumentation of the cltn were used to extract the rstf s the rstf s reveal that six of the seven events are simple with single source pulses having durations of 0 05 to 0 2 sec another earthquake 920310 0545 m 3 3 appears to be a double event with two episodes of rupturing azimuthal variations of the rstf pulse amplitudes and widths provide strong evidence for the rupture directivities of five of the earthquakes m 1 2 to 4 4 the azimuthal variations in the rstf pulse amplitudes were used to estimate the rupture directions and rupture velocities lower bound estimates of the rupture velocity range from 0 5 to 0 7 vs estimates of the rupture direction were combined with p wave focal mechanisms for the four largest events m 3 3 to 4 4 to identify the fault plane for these earthquakes source parameters were measured for the rstf s of the master events including seismic moments of 3 5 1018 to 5 3 1021 dyne cm fault radii of 100 to 330 m and static stress drops of 2 to 90 bars the fault radii and stress drop estimates for m 3 events agree well with estimates obtained by other researchers for m 3 to 4 5 earthquakes in the csz we also observed apparent scaling between the stress drop and the earthquake size which has been reported in other studies of stress drop in northeastern north america

high frequency observations and source parameters of microearthquakes recorded at hard rock sites we have estimated the source parameters of 53 microearthquakes recorded in july 1983 which were aftershocks of the miramichi new brunswick earthquake that occurred on 9 january 1982 these events were recorded by local three component digital seismographs at 400 sps component from 2 hz velocity transducers sited directly on glacially scoured crystalline basement outcrop hypocentral distances are typically less than 5 km and the hypocenters and the seven digital seismograph stations established all lie essentially within the boundaries of a granitic pluton that encompasses the faults that ruptured during the main shock and major aftershocks the p wave velocity is typically 5 km sec at the surface and at least 6 km sec at depths greater than about 1 km the events have s wave corner frequencies in the band 10 to 40 hz and the calculated brune model seismic moments range from 1015 to 1018 dyne cm the corresponding stress drops are generally less than 1 0 bars but there is considerable evidence that the seismic source signals have been modified by propagation and or site effects the data indicate a there is a velocity discontinuity at 0 5 km depth b the top layer has strong scattering attenuating properties c some source receiver paths differentiate the propagated signal d there is a hard rock site p wave f max between 50 and 100 hz and e some hard rock sites are characterized by p wave resonance frequencies in the range 50 to 100 hz comparison of this dataset with the january 1982 new brunswick digital seismograms which were recorded at sites underlain by several meters of low velocity surface sediments suggests that some of the hard rock site phenomena listed above can be explained in terms of a layer over a half space model for microearthquakes this result implies that spectrally determined source dimension scales with site dimension thickness of the layer more generally it emphasizes that it is very difficult to accurately observe the details of seismogenic processes from the earth s surface

single station fault plane solutions abstract fault plane solutions are derived from systematic trial and error grid testing of three component body waveform data from a single station modeling p and sh waveform data from five shallow events recorded teleseismically demonstrates that radiation pattern information contained within the interference of the direct wave and surface reflections and the overall relative amplitude between p and sh waveforms is sufficient to discriminate between fault type e g strike slip versus dip slip and often agrees with well constrained first motion studies events studied are the 9 april 1968 borrego mountain california 20 june 1978 thessaloniki greece 13 august 1978 santa barbara california 20 may 1979 alaska and 6 august 1979 coyote lake california earthquakes it is also shown using data from the 27 july 1980 sharpsburg kentucky earthquake that inclusion of pp p and sp p polarity and amplitude information to an otherwise unconstrained first motion study can significantly improve the quality of the fault plane solution although there are many potential problems source multiplicity directivity etc which can prohibit finding a good model with these techniques and inclusion of data from many stations is clearly desirable the results of this study suggest that sparse high quality waveform data sets may be as or more useful for obtaining source mechanisms than standard first motion studies at a minimum they should be performed together as a consistency check this procedure would be most useful in the common situation where only a few receivers are available for a particular event

the 1989 ms 6 3 ungava quebec earthquake a complex intraplate event on 25 december 1989 the largest earthquake ms 6 3 in northern quebec in at least 60 yr occurred in the ungava peninsula this earthquake was unique in that it marked the first time surface faulting could be confirmed for a historical eastern north american earthquake field observations indicated a combination of thrust and strike slip motion waveforms modeled in this study show that the earthquake consisted of two subevents a thrust subevent on a ne sw striking plane followed by a larger strike slip subevent on a nne ssw striking plane the surface waves give an equivalent double couple moment of 1 3 1025 dyne cm the sum of the subevent moments is greater than the simple source moment by almost a factor of 2 both subevents occurred at a depth of 3 km they were separated by 0 9 sec in time with the second subevent occurring 5 2 km southwest of the first the source parameters determined by modeling and those inferred from the ground observations generally agree but there are some differences with respect to the subevent offset these may be reconciled if the strike slip motion was distributed among the several small faults at the southwestern end of the rupture or if the complete surface rupture was not found the stress orientation nw compression inferred from the fault plane solution is consistent with the focal mechanisms of many other recent northeastern canadian events the ungava earthquake and many intraplate events from several continents share a number of characteristics including shallow hypocenters source complexity and occurrence on faults that showed no evidence for previous ruptures

application of waveform modeling to determine focal mechanisms of four 1982 miramichi aftershocks the objective of this study is to determine the focal mechanisms of small earthquakes at very near epicentral distances and use them to predict the observed waveforms to achieve our objectives the following steps were adopted before attempting to match the entire waveform a obtain an appropriate velocity model b calculate the green s functions and predict ground motions due to an earthquake specified by dip slip and strike of a fault c select all mechanisms which satisfy the first motion polarity at all stations d select a subset of focal mechanisms from those obtained in step c which produce p and s amplitudes and p sv and sv sh amplitude ratios correctly and e use focal mechanisms of step d to match waveforms as an alternative approach a least squares inversion for the moment tensor elements was performed and decomposition of moment tensor matrix to two double couples major and minor was performed green s functions were computed using the cagniard de hoop theory helmberger and harkrider 1978 we applied the above approaches to four aftershocks mblg 3 5 of the new brunswick earthquake of 9 january 1982 and succeeded in obtaining focal mechanisms and producing synthetic waveforms very similar to the observed waveforms two groups of focal mechanisms are found two of the earthquakes are found to have reverse fault geometry similar to that of the main event of 9 january 1982 mb 5 7 while the other two have significant components of strike slip motion on a dipping plane the results obtained from the least squares analysis support the results of the search method the fault geometries determined by the search method correspond well to the results of the major double couple of the least square inversion for completeness of the source characteristics investigation spectral analysis of the seismograms was performed to determine the source parameters corner frequencies fc seismic moments source dimensions and stress drops

source mechanism study of the 1982 new brunswick canada earthquake sequence using combined surface wave methods abstract a combined phase matched filtering and narrow bandpass filtering method and a surface wave love to rayleigh spectral amplitude ratio method are used to analyze the 1982 new brunswick earthquake sequence the analysis was carried out on the long period fundamental mode surface wave signals 10 to 50 sec for the main shock and its four largest aftershocks recorded at three gdsn stations scp lon and alq in north america estimates of source mechanism parameters focal depth dip rake and strike obtained for the 9 january 1982 main shock and the largest aftershock on 11 january 1982 are consistent with wetmiller et al 9s 1984 results for these events using local and regional body wave observations source mechanism parameters were also obtained for the 9 january 1982 the 31 march 1982 and the 16 june 1982 aftershocks which do not have independent estimates since these three events have low signal to noise ratios s n at some frequencies the source parameter estimates are not as well constrained as those of the main shock and the 11 january aftershock the inferred principal stress orientations are in all cases consistent with the prevailing ene maximum compressive stress characteristic of most areas in eastern north america among the principal aftershocks the most reliable source parameter estimates were for the 11 january aftershock because of its consistently high s n seismograms this aftershock consisted of thrusting on a fault plane conjugate to the main shock fault plane with dip of 40 to 55 to the northeast rake of 60 to 80 strike of 337 to 342 and a depth of 6 km this result also supports wetmiller et al 9s 1984 findings for this event the 9 january aftershock has a source mechanism similar to that of the main shock but a shallower depth of 6 km versus 7 km for the main shock the 31 march aftershock is associated with the upper portion of the main shock rupture with a much shallower depth of approximately 3 km the result for the 16 june aftershock is not as dependable but it appears to have a deeper depth of 8 km or more which suggests extension of the rupture zone to greater depth the results of this study indicate that surface wave data from only a few three component far field stations can be used to infer source parameters including depth for events as small as about m b 4 5 to the extent that eastern north america is typical of other continental interiors in terms of crustal structure and attenuation similar capabilities are anticipated for such areas

focal depth distribution of the 1982 miramichi earthquake sequence determined by modelling depth phases on 9 january 1982 in the miramichi region of new brunswick canada an earthquake with body wave magnitude mb 5 7 occurred and extensive aftershocks followed the mainshock was felt throughout eastern canada and new england usa the mainshock and several principal aftershocks were digitally recorded worldwide but smaller aftershocks were digitally recorded only at regional stations digital stations were not yet popular in 1982 therefore available regional digital waveform records for modelling are very limited fortunately two eastern canada telemetered network ectn stations ebn and kln produced excellent waveform records for most of the aftershocks until their closure at the end of 1990 the waveform records can be retrieved from the archive database at the geological survey of canada gsc since ebn had clear spmp records of the larger aftershocks with magnitude mn 2 8 we were able to determine focal depths for these larger events most of the focal depth solutions for the 113 larger

quasi dynamic models of simple earthquakes application to an aftershock of the 1975 oroville california earthquake a class of kinematic rupture models for simple earthquakes is proposed these models incorporate results from both theoretical and numerical solutions to the mixed boundary value problem of a three dimensional frictional rupture the analytic form for the slip velocity is divided into two phases the rupture growth during which the slip distribution is self similar kostrov 1964 and the healing during which the slip velocity decreases monotonically to zero the arrival of a p wave stopping phase propagating into the interior of the rupture area from the fault perimeter initiates the healing the duration of healing may be varied to produce a gradual or abrupt stopping of the rupture which permits modeling a wide range of high frequency spectral behavior these models are used to generate synthetics which are fit to the velocity and the velocity squared pulse shapes obtained from strong motion accelerograms of the 0103 aftershock of the 1975 oroville california earthquake the aftershock ml 4 6 has a moment of 1 8 1023 dyne cm and a total radiated energy of 2 7 1019 dyne cm the fitted source model propagates updip with a rupture velocity of 0 85 and a dynamic stress drop of 210 bars the arrival times of five stopping phases correlated across the suite of accelerograms are used to determine the relative locations of the hypocenter and five stopping events the description of rupture determined from these stopping events both corroborates the source model obtained from the waveform modeling and establishes an important counterpoint to the model by providing a dynamic interpretation of the unmodeled complexity of the waveforms

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

an experiment in systematic study of global seismicity centroid moment tensor solutions for 201 moderate and large earthquakes of 1981 data from the global digital seismograph network were used to obtain centroid moment tensor solutions using the method of dziewonski et al 1981 results were obtained for 201 earthquakes ranging in seismic moment from 7 1023 to 3 1027 dyne cm the wide dynamic range of the sro asro stations allows us to investigate using the same algorithm series of events among which the smallest and the largest may differ in moment by a factor as large as 1000 among the events studied is a particularly interesting series in january 1981 off the island of honshu the main shock was preceded by three foreshocks and followed by four aftershocks all falling within the range of our analysis there is an indication that the source mechanism changes with the size of the event statistical analysis of the results for all earthquakes reveals that the half duration of shallow and intermediate events is approximated by 1 6 10 8 m01 3 the shifts in origin time for deep focus earthquakes are indicative of the general complexity of these events which depends only weakly on the seismic moment the extreme departures from a double couple mechanism seem to depend both on the moment and on the focal depth while for shallow earthquakes these departures decrease from a maximum at about 1025 dyne cm with increasing moment the reverse appears to be true with respect to deep focus earthquakes shallow earthquakes north of new guinea and along the solomon islands show systematic and substantial departures from the double couple mechanism

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