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

a mid ordovician temperate carbonate shelf the black river and trenton limestone groups of southern ontario canada abstract mid ordovician limestones of southern ontario are usually considered to be tropical shelf limestones and interpreted using facies models derived from recent tropical carbonate environments nevertheless depositional rates grain types faunas erosion surfaces mineralogy and geochemistry are more compatible with a temperate or even cold shelf environment in view of the low latitudes 20 s indicated by paleomagnetism this suggests deposition in relatively cold seas deposition below a tropical thermocline or that the paleomagnetic data are wrong similar features in other caradocian limestones of eastern north america show that these inferred cooler conditions were widespread if deposition occurred below a tropical thermocline most shallow water caradocian limestones must be re interpreted as deeper water deposits more likely in view of the evidence for temperate conditions in shallow water is that the whole eastern seaboard of north america was affected by cold currents arising from southern polar ice caps and thus was a temperate shelf environment

aeromagnetic mapping of precambrian geological structures that controlled the 1968 meckering earthquake ms 6 8 implications for intraplate seismicity in western australia abstract occurring in the southwest of western australia the 1968 meckering earthquake ms 6 8 resulted in the formation of an extensive surface rupture complex comprising faults with a range of orientations and reverse and dextral lateral offsets modeling of the seismological characteristics of the source showed that reverse failure occurred on a north south striking east dipping surface but how this is related to the local precambrian bedrock geology is not clear interpretation of new aeromagnetic data has allowed concealed bedrock lithology and structure to be mapped in previously unachievable detail these data show that the surface faulting correlates closely with linear magnetic anomalies interpreted as dykes faults and lithological contacts the complicated pattern of surface faulting contrasts with the more simple seismological fault model but can be explained in terms of the reactivation of northeasterly dykes and faults and northwesterly stratigraphic trending features in a stress regime with an east west oriented maximum principal stress space problems created where these two trends converge led to the creation reactivation of a linking north south trending thrust fault which accommodated the greatest displacements recorded for the 1968 event the district scale distribution of epicentres in the 3 years encompassing the meckering event shows the same northeasterly and northwesterly trends as seen in the aeromagnetic data the implied basement features controlling the seismicity will be prone to strike slip failure in the regional east west oriented stress field it is speculated that smaller events in the meckering area will tend to be strike slip and these account for most of the strain larger events all known examples of which involve predominantly thrusting are caused by stress build up where strike slip faults with the two orthogonal trends intersect this hypothesis provides an explanation for the lack of topography in the region which is incompatible with the high level of seismic activity and a predominance of thrust faulting

the 1971 may 12 burdur earthquake sequence sw turkey a synthesis of seismological and geological observations summary it is now widely accepted that the rapid extension observed in western turkey is mainly accommodated by large active normal faults which control the geomorphology the ne sw trending burdur acigol and baklan basins bounded by large faults form a system of half graben whose orientation is evident in both the topography and the tilting of neogene sediments adjacent to them we used long period p and sh waveforms to determine the source parameters of the two largest earthquakes of the 1971 may 12 sequence that occurred in the burdur region the main shock was followed by many aftershocks which were distributed in a broad zone elongated ne sw parallel to the long axes of the basins in the region documented surface breaks of the 1971 event suggest that the northwest dipping faults along the southern margin of the burdur basin are those which moved well exposed fault planes are found dominantly to the south of burdur lake the surface dips of these faults are greater than that of the nw dipping nodal plane obtained for the main shock combined seismological and geological observations suggest that these faults have listric geometries

geometry and mechanism of faulting of the 1980 el asnam algeria earthquake from inversion of teleseismic body waves and comparison with field observations the el asnam earthquake of october 10 1980 ms 7 3 provided a wealth of geological and seismological data and is an ideal event for comparing geologically and seismologically derived models the event produced extensive surface faulting in addition to the main tectonic deformation which is clearly a thrust widespread secondary normal faulting was observed at the surface in the southwest region the surface break of a thrust fault could be traced for 24 km in the northeast part of the fault zone normal faults with throws of several meters were observed although no clear thrust type surface breaks were observed in this region geodetic measurements and aftershocks indicate that thrusting was nonetheless the main tectonic mode of deformation in this paper the rupture process of the el asnam earthquake is investigated by inversion of teleseismic p and sh waves the effort is concentrated on the late part of the waveforms which carries information about the faulting in the least understood north eastern segment of the fault zone during the earthquake rupture initiated at the southern terminus of the southwestern fault segment and propagated northward as indicated by the epicentral location and the observable azimuthal directivity of the body wave shapes and amplitudes the seismogenic faulting in the southwestern segment has a thrust mechanism with the following average parameters seismic moment 3 9 1019 n m depth 6 km strike 220 dip 46 and rake 72 the duration of rupture on this fault segment was approximately 10 s which together with the observed fault length of 24 km indicates a rupture velocity of 2 4 km s unconstrained inversions suggest normal faulting in the northeast part of the source region that began immediately following the arrival of the rupture to the northeast region the normal faulting mechanism is in agreement with that observed for the large surface fault scarps in this region guided by the evidence from aftershocks and geodetic measurements a solution with a thrust type mechanism was also sought using a priori constraints based on the field observations the observed strike and slip angle we obtain the following thrust source mechanism for the northeast segment seismic moment 3 6 1019 n m depth 6 km strike 230 dip 20 and rake 91 this solution requires that the thrusting in the northeast be initiated 10 s after the arrival of the rupture from the southwest suggesting that some time is required to overcome the geometric barrier marked by the abrupt change in the dip and azimuth inversions are nonunique with respect to the seismic moment associated with normal faulting the ambiguity being caused by the limited bandwidth of the observed body waves the moment estimated from the surface waves at periods about 300 s however favors thrust faulting to be dominant in the northeast region

source parameters for 11 earthquakes in the tien shan central asia determined by p and sh waveform inversion the tien shan mountain belt of central asia was formed in late paleozoic time and reactivated in tertiary time following the collision of india with eurasia more than 1500 km to the south to study the style and distribution of faulting occurring today in the tien shan we digitized long period world wide standard seismograph network p and sh waveforms of 11 of the largest tien shan earthquakes between 1965 and 1982 and then used a least squares inversion routine to constrain their fault plane solutions and depths four of the earthquakes occurred near the southern edge of the tien shan and two occurred in the intermontane fergana basin these earthquakes occurred at depths of 10 20 km and are associated with thrust faulting on east west to southwest northeast striking fault planes that dip moderately 35 55 the other five earthquakes in the northern tien shan were mostly deeper 15 44 km and the fault plane solutions of four of them are similar to the events farther south the exception an event on june 2 1973 probably occurred on a gently northward dipping east west striking fault plane all events occur within the basement at depths of 10 km or greater waveforms for earthquakes near the edges of the dzungarian fergana and kucha basins are fit best using velocity structures with thick sediment layers implying that sediments from these basins have been and are being underthrust beneath the neighboring mountains we examined only the relatively small mb 5 5 6 2 events after 1965 and did not study any of the largest events ms 8 that occurred in the tien shan earlier in this century still we can conclude that north south shortening is presently occurring in the tien shan with the formation of basement uplifts flanked by moderately dipping thrust faults the present day tectonics of the tien shan seem to be analogous to those of the rocky mountains in colorado wyoming and utah during the laramide orogeny in late cretaceous and early tertiary time

focal mechanisms of large earthquakes in the south island of new zealand implications for the accommodation of pacific australia plate motion b the plate motion model nuvel 1 predicts oblique convergence between the pacific and australian plates in the south island of new zealand we used p and sh body waveform analysis to constrain the focal mechanisms of the 15 largest earthquakes ms 5 8 that have occurred in this region since 1964 in order to see how the plate motion is accommodated at the southern end of the alpine fault convergence is achieved by oblique slip movement along a concentrated zone of deformation in the southern offshore region one event may be related to thrusting of the australian plate beneath the pacific plate and another strike slip event probably demonstrates movement on an active strike slip fault system parallel to but offset from the southern limit of the alpine fault this geometry provides a possible mechanism for the rapid uplift of the fiordland region deformation in the northern south island is more distributed in the south west marlborough region partitioning occurs between strike slip faulting in the se and reverse faulting farther nw in the buller region we suggest that the partitioning developed as a consequence of an increasing component of shortening that was accommodated by slip on reactivated pre existing normal faults in the buller region shortening in the buller region may have deflected the ne end of the alpine fault towards the nw forming the prominent bend the marlborough fault system with its youngest and most active faults to the se probably developed in an attempt to maintain a through going strike slip structure as each of the strike slip faults was transported towards the north west partitioning of the opposite polarity with reverse faulting se of the strike slip faulting occurs in north east marlborough the boundary between the two different styles of partitioning in ne and sw marlborough appears to coincide with a change in the nature of the downgoing slab and a change in strike of faults of the marlborough fault system a normal faulting earthquake on the northern edge of the chatham rise probably results from a complex interaction of the buoyant continental crust in that region with the subduction zone and the overlying marlborough fault system

the 1996 cyprus earthquake a large deep event in the cyprean arc summary we examine the source mechanisms and depths of the largest earthquake that has occurred in the vicinity of cyprus in the last 50 years the 1996 october 9 earthquake mw 6 8 and its principal aftershock on october 10 mw 5 8 being the first large event in the area for which seismic data from the global digital network are available it provides an excellent opportunity to study the complex tectonic processes of the eastern mediterranean we modelled the source mechanisms and depths of the earthquakes by a least squares body waveform fitting procedure the waveform fits resulting from our minimum misfit solutions are considerably better than those predicted by previous studies we show strong evidence that both events were at depths of 76 85 km rather than 20 30 km as has been suggested by other studies these earthquakes form a group indicating that the mediterranean lithosphere south of the hellenic and cyprean arcs is in east west compression as it is subducted to the north and northeast we additionally examine the source mechanism and depth of the 1999 august 11 earthquake mw 5 6 which occurred at about 11 km with a reverse dip slip mechanism it probably reflects crustal shortening between the african and turkish plates

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

the meckering earthquake of 14 october 1968 a possible downward propagating rupture average source parameters of the 1968 meckering australia earthquake are obtained by the inversion of body waves the objectives of the inversion are the elements of the moment tensor and the source time history an optimum source depth of 3 km is determined but because of source complexity the point source assumption fails and the moment tensor obtained at that depth has a large nondouble couple term compensated linear vector dipole 34 per cent the source parameters of the major double couple are strike 341 dip 37 rake 61 and seismic moment 8 2 1025 dyne cm the source time function is of approximately 4 sec duration with a long rise time and a sharp fall off the fault length is constrained on the surface by the observed surface break and results from vertical displacement modeling suggest a width of approximately 10 km in the middle assuming a dip of 37 that restricts the entire faulted area to lie above 6 km depth two finite fault models for the earthquake are presented with rupture initiating at a point 1 near the top of the fault and 2 at the bottom of the fault both models produce similar long period synthetics but based on the short period waveforms model 1 is favored it is argued that such a rupture process is the most reasonable in this cold shield region

earthquake source mechanisms from body waveform inversion and intraplate tectonics in the northern indian ocean double couple point source parameters for 11 of the largest intraplate earthquakes in the northern indian ocean during the last 20 y were determined from a formal inversion of long period p and sh waveforms nine of the events have centroid depths at least 17 km below the seafloor well into the upper mantle two have centroid depths as great as 39 km using the source mechanisms of these earthquakes we distinguish two major intraplate tectonic provinces in the northern indian ocean to the west of the ninetyeast ridge in the southern bay of bengal intraplate earthquakes have thrust faulting mechanisms with p axes oriented n s the centroid depths of these earthquakes range from 27 to 39 km below the seafloor lithospheric shortening in this region is thus accomplished by thrust faulting in the strong core of the oceanic upper mantle while other geophysical evidence suggests that shallow sedimentary and crustal layers apparently deform predominantly by folding in the immediate vicinity of the ninetyeast ridge earthquakes display strike slip mechanisms with left lateral motion on planes parallel to the ridge this type of faulting occurs from at least 10 s to the northern end of the ninetyeast ridge near 10 n where the ridge meets the sunda arc seismic activity diminishes to the east of the ninetyeast ridge but is also characterized by strike slip faulting despite these variations in deformational style the inferred orientation of greatest compressive stress in the northern indian ocean displays a consistent long wavelength pattern over a large portion of the indian plate varying smoothly from nearly n s in the bay of bengal to nw se in the northeastern indian ocean this plate wide stress pattern and the high level of intraplate seismicity in the northern indian ocean are likely the results of substantial resistance along the himalayan continental collision zone to the continued northward motion of the western portion of the indian plate oceanic intraplate earthquakes in other regions where the level of deviatoric stress associated with the long wavelength part of the stress field is likely to be smaller need not be comparably reliable indicators of the plate wide stress field

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

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

dip range for intracontinental reverse fault ruptures truth not stranger than friction abstract histograms of fault dips have been compiled for moderate to large m 5 5 reverse slip intracontinental earthquakes with the slip vector raking 90 30 in the fault plane the principal data set is restricted to earthquakes where the fault plane in the focal mechanism can be unambiguously distinguished from the auxilliary plane the reverse fault dips are bracketed within the range 12 1 the observed dip range is consistent with reactivation of faults possessing rock friction coefficients within byerlee9s 1978 range 0 85 s 0 6 undergoing frictional lockup at dips approaching 60 the broad 25 to 35 peak may arise from progressive domino steepening of imbricate reverse faults above the optimal dip for reactivation in regions undergoing bulk shortening paucity of very low angle thrusts implies that it is generally the steeper ramps within ramp flat assemblages that fail in moderate to large earthquakes the subsidiary peak at 45 to 55 likely results from compressional reactivation of former normal faults in areas undergoing tectonic inversion requiring some degree of fluid overpressuring the results are consistent with previous studies on the dip range for active normal faults that again demonstrate frictional lockup at reactivation angles approaching 60 together these analyses suggest that byerlee friction coefficients apply to faults with displacements of up to a few kilometers

the source parameters of the cubukdag w turkey earthquake of 1986 october 11 summary long period p and sh waveforms and first motion polarities of p waves were used to determine the source parameters of the cubukdag in the eastern section of the buyuk menderes graben earthquake of 1986 october 11 the waveforms used in this event include the wwssn and the digitally recorded seismograms of the dwwssn sro and asro networks the minimum misfit solution obtained by the inversion of teleseismic body waveforms are better constrained that those of the previously published harvard centroid moment tensor solution and are in good agreement with field observations and the local geometry of the graben

uppermost mantle structure beneath eastern china and its surroundings from pn and sn tomography the pn and sn residuals from regional events provide strong constraints on the structure and lithological characteristics of the uppermost mantle beneath eastern china and its surroundings with the dense chinese digital seismic network in eastern china separate pn and sn tomographic inversions have been exploited to obtain p and s velocities at a resolution of 2 2 or better the patterns of p velocities are quite consistent with the s velocities at depth of 50 and 60 km but the amplitude of p wave speed anomalies are a little larger than those of s wave speed the low p wave speed high s wave speed and low vp vs ratio beneath the northern part of ordos basin are related to upwelling hot material abrupt changes in material properties are indicated from the rapid variations in the vp vs ratio

earthquake mechanisms in the hellenic trench near crete summary in the hellenic trench south of crete convergence between the southern aegean sea and africa occurs at a rate of at least 60 mm yr 1 previously published first motion fault plane solutions show a variety of different fault orientations and types but are not well constrained furthermore the lack of reliable focal depths for these earthquakes has obscured any simple pattern of deformation that might exist nonetheless the mechanisms of these earthquakes have strongly influenced views of the tectonics in the hellenic trench we have improved estimates of the fault parameters and focal depths for 14 of these earthquakes using long period p and sh waveforms the earthquake mechanisms fall into four groups a normal faults with a n s strike in the over riding material above the subduction zone b low angle thrusts with an e w strike at a depth of about 40 km c high angle reverse faults with the same strike but shallower focal depths than b d events within the suducting lithosphere with approximately e w p axes the thrusting in groups b and c is probably the mechanism by which the sediments of the mediterranean sea underplate and uplift crete these events have slip vectors in the direction 025 12 which represents the convergence direction between crete and africa along the sw facing boundary of the hellenic trench one of the events in group d occurred beneath the mediterranean ridge and involved high angle reverse faulting with a wnw ese p axis almost perpendicular to the direction of shortening deduced from folds at the surface the mediterranean sea floor in this region appears to be in a state of e w compression for reasons that are not clear

regional stress field of the indian plate we have calculated the regional stress field in the indian plate implementing dependence of slab pull and ridge push on the age of the oceanic lithosphere in the finite element procedure the high level of the calculated stress field order of a few kbar and the dominance of compression in the plate are consequences of the unique dynamic situation of the present day indian plate the calculated stress field explains the concentration of intraplate deformation in the region of the ninetyeast ridge and yields insight into the variations in stress directions in the australian continent

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

active tectonics of the eastern sunda and banda arcs the tectonics of the eastern sunda and banda arcs of eastern indonesia are inferred from the centroid depths seismic moments and fault plane solutions of 63 large earthquakes that occurred between 1962 and 1984 source parameters are estimated by inverting long period teleseismic p and sh waves earthquakes show that the collision of the australian continent with the banda arc shortens the upper plate beneath the forearc and back arc basin over the broad area between the timor and seram troughs along the eastern end of the sunda arc and along the southern banda arc earthquakes reveal that the arc and forearc respond to collision by shortening in the direction of convergence by elongating in the direction perpendicular to convergence and by thrusting over the back arc basin shallow thrust and strike slip earthquakes beneath the banda basin the back arc basin demonstrate that deformation within the back arc accommodates some of the northward motion of the australian continent the rate of north south shortening of the entire upper plate at the longitude of timor calculated from the seismic moments of the earthquakes over the 22 year period is roughly 20 of the predicted convergence rate between australia and southeast asia strike slip faulting within the banda basin results also in eastward motion of the banda arc with respect to western indonesia and australia eastward extrusion of the banda arc results in thrusting at the aru trough earthquakes due to thrusting of the back arc basin beneath the volcanic arc are too shallow to corroborate that the arc s polarity has reversed although the wetar back arc thrust zone at present has a high seismic slip rate earthquakes and their mechanisms provide evidence that the lithosphere of the weber basin a 7500 m deep forearc basin rests on the subducting lithosphere without intervening asthenosphere so that the weber basin subsides in response to sinking of the subducting lithosphere closure of the banda basin is geometrically incompatible with the australian plate subducting simultaneously beneath both the timor and seram troughs instead the bird s head which subducts beneath seram is decoupled from the australian plate in western new guinea and probably moves west or southwest with respect to australia in the eastern indonesian example strike slip faulting plays a major role in the evolution of the collision zone and clearly demonstrates the complexity in three dimensions of continental accretion and mountain building by arc continent collision

the geometry of back arc thrusting along the eastern sunda arc indonesia constraints from earthquake and gravity data the flores earthquake of december 23 1978 represents the first seismological evidence for active back arc thrusting behind the eastern sunda arc this mb 5 8 earthquake occurred north of flores island where seismic reflection profiling has revealed back arc thrusting interpreted to be a reaction to compression of the arc following collision with the australian continent we investigate the source mechanism and depth of the flores earthquake by inversion of long period p waveforms and relocate the earthquake s epicenter incorporating arrival time data from local stations we find that this event occurred beneath the accretionary prism south of the back arc thrust zone at a depth of 11 5 km 7 km below the seafloor the best fitting fault plane solution is consistent with active southward thrusting of the floor of the flores basin beneath the volcanic arc along a 30 dipping fault plane the morphology of the thrust zone and the free air gravity profile over the flores basin in the epicentral region resemble those of oceanic trenches and the observed gravity field is best interpreted if the crust of the flores basin dips smoothly below the accreted wedge at an average angle of about 6 the position of the earthquake hypocenter south of the flores thrust and the fault plane solution suggest that the 1978 earthquake is analogous to thrust events at subduction zones and represents slip between the subducting and overriding plates the flores thrust is thus the surface expression of a deep seated thrust zone and may represent the initial stage of polarity reversal of the eastern sunda arc

normal faulting in the simav graben of western turkey reassessed with calibrated earthquake relocations western turkey has a long history of large earthquakes but the responsible faults are poorly characterized here we reassess the past half century of instrumental earthquakes in the simav gediz region starting with the 19 may 2011 simav earthquake mw 5 9 which we image using interferometric synthetic aperture radar and regional and teleseismic waveforms this event ruptured a steep planar normal fault centered at 7 9 km depth but failed to break the surface however relocated main shock and aftershock hypocenters occurred beneath the main slip plane at 10 22 km depth implying rupture initiation in areas of low coseismic slip these calibrated modern earthquakes provide the impetus to relocate and reassess older instrumental events in the region aftershocks of the 1970 gediz earthquake mw 7 1 form a narrow band inconsistent with source models that invoke low angle detachment faulting and may include events triggered dynamically by the unilateral main shock rupture epicenters of the 1969 demirci earthquakes mw 5 9 6 0 are more consistent with slip on the south dipping akdag fault than the larger north dipping simav fault a counterintuitive aspect of recent seismicity across our study area is that the largest event mw 7 1 occurred in an area of slower extension and indistinct surface faulting yet ruptured the surface while recent earthquakes in the well defined and more rapidly extending simav graben are smaller mw 6 0 and failed to produce surface breaks though our study area bounds a major metamorphic core complex there is no evidence for involvement of low angle normal faulting in any of the recent large earthquakes

source characterization of two reykjanes ridge earthquakes surface waves and moment tensors p waveforms and nonorthogonal nodal planes well constrained fault plane solutions from p wave first motions for mid ocean ridge normal faulting earthquakes usually require nonorthogonal nodal planes local structural effects and or departures from a double couple source mechanism have been invoked to explain this phenomenon in order to obtain an independent determination of the source mechanisms for the april 24 1970 and april 3 1972 events on the southern reykjanes ridge we invert the rayleigh wave radiation pattern to obtain the source moment tensor the moment tensor formulation should be particularly well suited to this problem because it is not restricted a priori to a double couple source mechanism a potential drawback of the technique however is the requirement that phase velocities along the earthquake station paths be known very accurately in order to obtain the source phase from the observed phase and an objective of this study was to determine whether a regionalized phase velocity model compiled from published dispersion curves is adequate the results of the moment tensor inversion for both events indicate shallow normal faulting with the tension axis approximately horizontal and perpendicular to the local strike of the ridge apparent departures from a pure double couple source seem to result from errors in the data and the poor resolution of the mxz and myz components of the moment tensor for shallow sources after performing the inversion under a series of increasingly more stringent constraints we conclude that the data for both events are compatible with pure double couple sources with moments of 4 8 and 7 5 1024 dyn cm respectively we then show that interference between p pp and sp due to shallowness of the source can account for the observed nonorthogonality and match the observed p waveforms for the april 3 1972 event with theoretical seismograms calculated for a shear fault whose orientation is consistent with the surface wave solution the best fit to the data is obtained for a long narrow fault 13 km by 3 km with rupture initiating near the seafloor the moment indicated by the p waves is 7 5 1024 dyn cm these source parameters give an average displacement of about 60 cm and a stress drop of 30 60 bars taking into account various uncertainties although we might expect attentuation to be high in the mid ocean ridge environment the average attenuation required to fit the teleseismic data is not higher than normal t 1 s the p waves from the april 24 1970 earthquake were too small to be suitable for quantitative modeling by synthetic seismograms but are qualitatively consistent with a shallow fault model similar to that for the larger event we conclude that the faulting process described by these two earthquake mechanisms is directly related to the formation of rift valley topography