structural analysis and interpretation of the surface deformations of the el asnam earthquake of october 10 1980 the el asnam earthquake of october 10 1980 represents the major seismotectonic event of these last decades in the west mediterranean area it has induced a large amount of surface breaks this earthquake reveals the importance of compressional phenomena that characterize nowadays the tectonics of north africa the tectonic analysis of surface rupture shows the complexity of the deformation mechanism the principal mechanism consists of the activation of a nw se trending thrust fault accompanied with a left lateral motion and with an intense deformation of the northwestern overthrusting block an accurate description of these deformations shows that for a large part the breaks geometry depends closely on the substratum structure finally the analysis of the dislocations on irrigation ducts just above the fault traces allowed a quantitative study of the deformation

geology and tectonics of the betic zone se spain abstract the southern margin of the iberian continent comprises a complex geological history which can be traced back into the proterozoic when a gneissic basement was formed a cover of nevado filabride rocks was deposited onto this proterozoic basement during the palaeozoic the permian intrusion of the bedar granite suggests that the palaeozoic sedimentary rocks now exposed in the betic cordilleras have been affected by the hercynian orogeny similar to that of palaeozoic rocks in the adjoining ossa morena zone the hercynian basement was probably covered by alpujarride carbonates in the mesozoic when rifting opened tethys and intruded 140 ma old basic dyke swarms the palaeozoic basement of the betic cordilleras comprises a suture of late cretaceous subduction 80 85 ma b p this can be inferred from hp lt metamorphic facies obducted in coeval thrust slices of nevado filabride rocks the nevado filabride nappes include mesozoic carbonates transformed into marbles macael the suture was probably annnealed after the closure of tethys the mesozoic boundary between africa and europe and the rifting in tethys ceased during the palaeogene when the atlas mountains were formed 36 ma b p subsequently relative movements between africa and europe in the neogene were accommodated by strike slip faults these define the iberian microplate moving eastward by atlantic spreading and possibly subducting under italy neogene deformation of iberia is most intensive in southeastern spain due to emplacement of a mantle diapir under the alboran area this occurred early in the neogene aquitanian and caused nappe spreading involving alpujarride and malaguide rocks the subsequent neogene history has now been studied in great detail and is reviewed here it involves clear examples of calc alkaline volcanism most probably not related to subduction seismic motion along strike slip faults within the iberian microplate and syntectonic deposition in intermontane neogene basins

earthquakes faulting and stress in the los angeles basin since 1920 fourteen moderate sized m l 4 9 6 4 earthquakes have been reported in the los angeles basin these events are associated with both mappable surficial faults and concealed faults beneath the basin sediments to determine the style of faulting and state of stress in the basin single event focal mechanisms for 244 earthquakes of m 2 5 that have occurred during 1977 1989 have been calculated fifty nine percent of the events are strike slip and are mostly located near two of the major northwest striking right lateral strike slip faults in the basin the newport inglewood fault and the palos verdes fault the 1988 pasadena and the 1988 upland earthquakes showed left lateral strike slip on northeast striking faults numerous small earthquakes in the eastern part of the basin show left lateral strike slip faulting and form a northeast trend near yorba linda thirty two percent of the events have reverse mechanisms and are distributed along two broad zones the first the elysian park fold and thrust belt coincides with anticlines along the eastern and northern flank of the los angeles basin extending into santa monica bay the second the torrance wilmington fold and thrust belt coincides with anticlines mapped on the southwest flank of the basin and extends from offshore newport beach to the northwest into santa monica bay oblique faulting that could be inferred by the merging of strike slip and compressional tectonics does not occur in the basin instead the coexistence of zones of thrusting and large strike slip faults in the basin suggests that the thrust and strike slip movements are mostly decoupled a few normal faulting mechanisms appear to be related to faulting orthogonal to the axes of plunging anticlines the trend of the maximum horizontal stress varies from n1 w to n31 e across the basin and consistently forms high angles with the fold axes the stress field that exists along the flanks of the basin has a vertical minimum stress axis this stress field and ongoing folding and thrusting suggest that tectonic deformation is concentrated along the flanks of the deep central basin today the deformation of the basin consists of uplift and crustal thickening and lateral block movement to accommodate the north south compression across the basin

active faulting in the gulf of aqaba new knowledge from the mw 7 3 earthquake of 22 november 1995 abstract on 22 november 1995 the largest earthquake instrumentally recorded in the area with magnitude m w 7 3 occurred in the gulf of aqaba the main rupture corresponding to the strike slip mechanism is located within the gulf of aqaba which forms the marine extension of the levantine fault also known as the dead sea fault the levantine fault accommodates the strike slip movement between the african plate and the arabian plate the gulf of aqaba itself is usually described as the succession of three deep pull apart basins elongated in the n s direction concerning historical seismicity only two large events have been reported for the last 2000 years but they are still poorly constrained the seismicity recorded since installation of regional networks in the early 1980s had been characterized by a low background level punctuated by brief swarmlike activity a few months in duration three swarms have already been documented in the gulf of aqaba in 1983 1990 and 1993 with magnitudes reaching at most 6 1 m w we suggest that the geometry of the rupture for the 1995 event is related to the spatial distribution of these previous swarms body wave modeling of broadband seismograms from the global network along with the analysis of the aftershock distribution allow us to propose a well constrained model for the rupture process northward propagation of the rupture has been found we have demonstrated that three successive subevents are necessary to obtain a good fit between observed and synthetic wave forms the total seismic moment released was 7 42 10 19 n m the location of the subsevents shows that the three stages of the rupture involve three different segments within the gulf substantial surface breakage showing only normal motion up to 20 cm affecting beachrock was observed along the egyptian coast we show that these ruptures are only a secondary feature and are in no case primary ruptures the stress tensor derived from striations collected in quaternary sediments shows radial extension this result supports landsliding of the beach terraces under the action of the earthquake shaking

acceleration source spectra anticipated for large earthquakes in northeastern north america a set of 16 large intraplate earthquakes are considered on the basis of their tectonic settings compressive focal mechanisms and shallow focal depths as possible analogs for large earthquakes that may be anticipated to occur in northeastern north america the earthquakes range in seismic moment from 5 1023 to 3 1026 dyne cm and include the mainshocks and largest aftershocks of the 1982 miramichi canada the 1985 to 1988 nahanni canada and the 1988 tennant creek australia earthquake sequences the teleseismic recordings of these earthquakes are spectrally analyzed by correcting the logarithmically averaged acceleration spectra for the frequency dependent effects of attenuation and interference of the depth and direct phases the resulting acceleration source spectra are generally flat at high frequencies the mainshock spectra exhibit intermediate spectral falloffs that is u where 1 over frequencies that characteristically extend from 0 1 to 0 5 hz in contrast the largest aftershocks of the three sequences named above exhibit 2 spectral shapes the high frequency source spectral levels increase with seismic moment approximately as ru m 1 3 the 1988 saguenay canada earthquake which occurred significantly deeper than the other earthquakes exhibits an anomalous spectral shape combining the teleseismic data with strong motion recordings indicates that the intermediate spectral falloff for this earthquake extends from 0 2 to 3 hz the spectral amplitude at 3 hz is a factor of 3 higher than the amplitude anticipated for an earthquake of this size from the other 15 earthquakes

age of the peach springs tuff southeastern california and western arizona sanidine separates from pumice of the early miocene peach springs tuff are concordantly dated at 18 5 0 2 ma by two isotopic techniques the peach springs tuff is the only known unit that can be correlated between isolated outcrops of miocene strata from the central mojave desert of southeastern california to the western colorado plateau in arizona across five structural provinces a distance of 350 km thus the age of the peach springs tuff is important to structural and paleogeographic reconstructions of a large region biotite and sanidine separates from bulk samples of the peach springs tuff from zones of welding and vapor phase alteration have not produced consistent ages by the k ar method published ages of mineral separates from 17 localities ranged from 16 2 to 20 5 ma discordant 40ar 39ar incremental release spectra were obtained for one biotite and two of the sanidine separates ages that correspond to the last gas increments are as old as 27 ma the 40ar 39ar incremental release determinations on sanidine separated from blocks of peach springs tuff pumice yield ages of 18 3 0 3 and 18 6 0 4 ma laser fusion measurements yield a mean age of 18 51 0 10 the results suggest that sanidine and biotite k ar ages older than about 18 5 ma are due to inherited ar from pre tertiary contaminants which likely were incorporated into the tuff during deposition sanidine k ar ages younger than 18 ma probably indicate incomplete extraction of radiogenic 40ar whereas laser fusion dates of biotite and hornblende younger than 18 ma likely are due to postdepositional alteration laser fusion ages as high as 19 01 ma on biotite grains from pumice suggest that minerals from pre tertiary country rocks also were incorporated in the magma chamber

inversion of complex body waves we have developed a method that inverts seismic body waves to determine the mechanism and rupture pattern of earthquakes the rupture pattern is represented as a sequence of subevents distributed on the fault plane this method is an extension of our earlier method in which the subevent mechanisms were fixed in the new method the subevent mechanisms are determined from the data and are allowed to vary during the sequence when subevent mechanisms are allowed to vary however the inversion often becomes unstable because of the complex trade offs between the mechanism the timing and the location of the subevents many different subevent sequences can explain the same data equally well and it is important to determine the range of allowable solutions some constraints must be imposed on the solution to stabilize the inversion we have developed a procedure to explore the range of allowable solutions and appropriate constraints in this procedure a network of grid points is constructed on the i plane where and i are respectively the onset time and the distance from the epicenter of a subevent the best fit subevent is determined at all grid points then the correlation is computed between the synthetic waveform for each subevent and the observed waveform the correlation as a function of and i and the best fit mechanisms computed at each i grid point depict the character of allowable solutions and facilitate a decision on the appropriate constraints to be imposed on the solution the method is illustrated using the data for the 1976 guatemala earthquake

on the growth of normal faults and the existence of flats and ramps along the el asnam active fold and thrust system the combination of detailed topographic leveling on the southwest segment of the el asnam thrust fault with existing seismic and geologic data implies that the geometry of this fault involves shallow dipping flats and steep ramps the fault appears to be growing along strike toward the southwest end where the main shock initiated in 1980 from a depth of about 10 km the main thrust appears to ramp to the basement cenozoic cover interface on a plane striking n40 e and dipping 50 55 to the northwest along the southwest segment where folding has not yet developed the thrust continues steeply through the cenozoic cover to the near surface where it flattens causing normal faulting along the central and northeast segments which display a more evolved fold structure the deep thrust probably flattens at a depth of 5 6 km into a decollement along the cenozoic jurassic interface before ramping to the surface the sara el marouf and kef el mes anticlines have thus formed as fault propagation folds normal faults at beni rached probably branch with the thrust to maintain kinematic compatibility between the deep ramp and decollement the greater separation 7 km between the normal faults at beni rached and the thrust where it crosses oued cheliff than along the southwest segment 1 km reflects the greater depth of the ramp to flat bend we infer that the september 9 1954 earthquake activated only the central deep segment of the main thrust together with the beni rached normal faults while that of october 10 1980 activated the whole system of flat decollements ramp thrusts and compatibility normal faults further complexities of the faulting in map view are related to changes of strike of the thrust in particular north of oued cheliff

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

active tectonics of the algerian atlas mountains evidence from aftershocks of the 1980 el asnam earthquake summary the el asnam algeria earthquake of 1980 october is the largest earthquake to have occurred in north africa since instrumental records began ms 7 3 it was caused by movement on a segmented reverse fault which is part of the fault zone that separates the chelif alluvial plains from the coastal range of the atlas mountains the coastal range is the actively deforming plate boundary between africa and eurasia in this paper the results of an aftershock study carried out using a 28 station portable network in the epicentral area in the 5 weeks after the main shock are presented a total of 4517 aftershocks have been located using both p and s phases of these 4517 events 1279 pass a set of quality criteria and are used to discuss the detailed tectonics of the aftershock zone the southwestern half of the aftershock zone has a relatively restricted band of seismicity running parallel to the main fault in cross section this seismicity is observed to lie almost exclusively in the footwall of the main fault and indicates both antithetic reverse faulting and extension parallel to the fault zone the only exception to this pattern occurs at a junction of the main fault segments where a distinct cluster of events with very variable focal mechanisms is located in the hangingwall this cluster may be equated with a barrier that impeded the rupture of the main shock beyond the southwest end of the main fault a mixture of strike slip and reverse faulting occurred and suggests a mechanism whereby this end of the fault can act as an asperity for earthquake generation aftershocks in the northeastern half of the study area indicate movement on a stack of listric reverse faults the focal mechanisms of these events show that the n dipping nodal planes assumed to be the fault planes flatten with depth we suggest that the faults sole into a low angle 20 decollement at about 8 10 km depth at the base of the aftershock zone we argue that the northeastern part of the 1980 aftershock zone is typical of the actively deforming coastal belt of northwest algeria subduction of the western mediterranean basin beneath the north african margin does not seem to occur instead the margin is absorbing the motion with old normal faults being reactivated as thrusts

models of the complex source of the el asnam earthquake abstract the el asnam earthquake of 10 october 1980 was originated by a thrust across a sw ne oriented fault 36 km long and 15 km wide it is a complex event with a rupture propagating from the sw to the ne fault mapping hypocenter distribution and b factor studies indicate fault branching to the ne a dislocation model is used to explain the branch with normal faulting at beni rached as a consequence of the thrust across a main fault that changes orientation the mechanism of the 1954 earthquake must be similar even though only normal faulting was observed at the surface

relations between surface deformation fault geometry seismicity and rupture characteristics during the el asnam algeria earthquake of 10 october 1980 abstract the el asnam earthquake of october 10 1980 ms 7 3 produced surface faulting on a northeast trending thrust fault of 30 km length with displacements of up to 6 5 m though average displacements were about 3 m in addition widespread tensional features were formed some in clear association with folding above the thrust and others in an area beyond the exposure of the thrust at the surface which may be related to buried reverse faults the observed thrust fault is split into southern central and northern segments local and teleseismic data are examined to show that the main shock nucleated at the southwest end of the fault and propagated 12 km northeast where a second rupture of approximately equal moment occurred continuing the faulting a further 12 km northeast along the central segment both ruptures nucleated at about 8 10 km depth displacements were largest on the central segment where they were probably enlarged by aftershocks including one of mb 6 1 three hours after the main shock the northern segment was much shorter than the other two and showed smaller displacement the junctions between fault segments are marked by distinct geomorphological characteristics and a change in strike of the faulting as well as a sudden drop in the observed displacement it appears that the rupture development is influenced by the changes in fault geometry between segments and that such junctions or barriers have persisted through much of the late quaternary

source mechanism of the 1987 vatnafjoll earthquake in south iceland the may 25 1987 vatnafjoll earthquake occurred on the mid atlantic plate boundary at a junction between a transform the south iceland seismic zone and a spreading segment the eastern volcanic zone the south iceland seismic zone has had the most destructive earthquakes in the history of iceland with average intervals of 80 100 years and estimated maximum magnitudes around 7 the 1987 vatnafjoll earthquake was the largest earthquake mb 5 8 ms 5 8 mw 5 9 in south iceland since the 1912 magnitude 7 earthquake and is the first event sufficiently large to allow a meaningful study of source process in this geologically interesting region foreshocks and aftershocks of the vatnafjoll earthquake were located with relatively good accuracy they define an elongated n s epicentral area 12 km long and 4 km wide with hypocentral depths mostly between 6 and 13 km we determined centroid source parameters of the vatnafjoll earthquake by inverting long period teleseismic p and sh waves the centroid source mechanism and foreshock and aftershock distribution indicate n s right lateral strike slip faulting on a near vertical plane the rupture initiated near the base of the crust and propagated upward along an approximately 10 km long fault but did not reach the surface the centroid depth is well constrained and is between 5 and 8 km depth at the 95 confidence level the seismic moment is 9 1 1024dyn cm the source has relatively short duration of 3 s which may indicate higher stress drop than global average of earthquakes of this size the south iceland seismic zone is a 70 km long e w striking transform zone between two overlapping rift zones the eastern and western volcanic zones it is characterized by bookshelf type of tectonics that is there is no surface faulting in the direction of the transform instead n s right lateral strike slip faults are distributed along and perpendicular to the transform zone the vatnafjoll earthquake has characteristics of earthquakes in the south iceland seismic zone and is probably not related directly with magmatic activity in the eastern volcanic zone the transform zone thus extends farther into the rift zone than was previously recognized

the evolution of a thrust fault system processes of rupture initiation propagation and termination in the 1980 el asnam algeria earthquake summary we discuss work completed to date on the 1980 algerian earthquake and introduce a description of the earthquake in terms of rupture propagation using the nomenclature of fracture mechanics field and teleseismic studies have shown the earthquake fault to be divided into three major thrust segments rupture began at the sw end of the system and propagated along successive segments geological sections reveal that different fault segments have suffered different amounts of displacement in the past the amount of geological displacement increases systematically from almost zero at the sw end of the fault to several hundred metres further ne faulting at the sw end is very young and appears to be extending the thrust system along strike the aftershock data of ouyed et al reveal that strike slip aftershocks occur beyond the sw end of the thrust fault these are accommodating compression by conjugate strike slip faulting on north and west trending planes this network of conjugate faults plays a crucial role in the initiation of major eathquakes on the thrust system and may be regarded as a plastic zone beyond the end of the thrust fault increasing horizontal stress causes the strike slip system to evolve into a zig zag network of thrusts which is partly controlled by the pre existing strike slip faults finally a through going thrust can develop and leads to catastrophic rupture along the main fault plane to describe the propagation of the main shock we extend aki s concept of geometric barriers and divide these into two types in conservative barriers the slip vector is common to both fault planes whereas in non conservative barriers the slip vectors are different on the two fault planes that meet at the barrier although both types of barrier arrest dynamic rupture only nonconservative barriers need produce aftershocks as off fault deformation is required to accommodate the difference in slip vectors by considering the aftershock zones as areas of distributed deformation we show that fracture toughness in the earth is not a material constant but is related to fault geometry and therefore changes as fault systems evolve the current south westwards growth of the el asnam thrust system can be seen as the latest phase in an evolution consisting of periods of along strike extension alternating with periods of increasing net displacement during which no fault growth occurs these two types of behaviour allow a segmented fault system to be produced

tertiary basin development and tectonic implications whipple detachment system colorado river extensional corridor california and arizona a stratigraphic and structural record of synchronous extension and detachment faulting is preserved in predominantly miocene deposits of the whipple detachment system colorado river extensional corridor in the mohave mountains and aubrey hills of arizona and the eastern whipple mountains of california near parker dam these deposits comprise four unconformity bounded sequences composed of locally derived epiclastic and volcanic rocks and the peach springs tuff the three older sequences represent syntectonic units that were deposited coeval with detachment faulting and the fourth is interpreted to be postextensional the sequences are correlated between four fault bounded regions which are the remnants of four different depositional basins similar sequences can be correlated over broad areas of the extensional corridor despite the general lack of widespread units the basins developed in about the same positions relative to each other and to volcanic sources as they occupy at present this is shown by gradational changes of pre tertiary rock types between regions systematic variations in the abundance of magmatic units and correlative volcanic units that occur in two adjacent regions the basins formed in the early miocene from segmentation of the upper crust into blocks bounded by high angle faults that trended both parallel and perpendicular to the direction of extension and which were terminated at middle crustal depths by a low angle detachment fault extreme rotation of one large crustal block which constitutes the central mohave mountains is recorded by a major unconformity in the lower miocene section of one basin because coeval sections that formed in adjoining basins do not record this rotation the underlying crustal blocks must have been separated by transfer faults that allowed them to rotate independently these proposed transfer faults are represented at present by major faults with trends that parallel the direction of extension on the whipple detachment system

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

the 1996 earthquakes in sulawesi indonesia we study the rupture process of the 1 january mw 7 9 16 july mw 6 6 and 22 mw 7 july 1996 earthquakes in sulawesi indonesia a teleseismic body waveform inversion of very broadband records shows that these events occurred beneath the accretionary prism the main shock was due to a well determined fault with a strike of 53 n and a very shallow dip of 7 from its source time function duration 30 sec and the aftershock distribution we estimate a rupture area of 90 60 km2 and an average slip of 1 80 m the surface displacement computed with our best model fits well the displacement vector at the only available gps station at tomini the tsunami generated by the mainshock had an approximate source radius of 45 km which roughly agrees with the rupture size estimated above the earthquakes took place in a relay zone between the trench and the palu koro transcurrent fault they ruptured a shallow dipping thrust fault which corresponds to the subduction interface under the north sulawesi arm the slip vectors of all the events have a nnw orientation parallel to the direction of convergence between the north sulawesi arm and the celebes sea among the peculiarities of the rupture process we found that the july 22 aftershock was the only event to have a well defined precursor 1 8 sec before the main p wave onset

correlation of the peach springs tuff a large volume miocene ignimbrite sheet in california and arizona the peach springs tuff is a distinctive early miocene ignimbrite deposit that was first recognized in western arizona recent field studies and phenocryst analyses indicate that adjacent outcrops of similar tuff in the central and easten mojave desert may be correlative this proposed correlation implies that outcrops of the tuff are scattered over an area of at least 35 000 km2 from the western colorado plateau to barstow california and that the erupted volume allowing for posteruption crustal extension was at least several hundred cubic kilometres thus the peach springs tuff may be a regional stratigraphic marker useful for determining regional paleogeography and the time and extent of tertiary crustal extension

transform fault earthquakes in the north atlantic source mechanisms and depth of faulting we have determined the centroid depths and source mechanisms of 12 large earthquakes on transform faults of the northern mid atlantic ridge from an inversion of long period body waveforms the earthquakes occurred on the gibbs oceanographer hayes kane 15 20 and vema transforms we have also estimated the depth extent of faulting during each earthquake from the centroid depth and the fault width for five of the transforms earthquake centroid depths lie in the range 7 10 km beneath the seafloor and the maximum depth of seismic faulting is 14 20 km on the basis of a comparison with a simple thermal model for transform faults this maximum depth of seismic behavior corresponds to a nominal temperature of 900 100 c in contrast the nominal temperature limiting the maximum depth of faulting during oceanic intraplate earthquakes with strike slip mechanisms is 700 100 c the difference in these limiting temperatures may be attributed to the different strain rates characterizing intraplate and transform fault environments three large earthquakes on the 15 20 transform have shallower centroid depths of 4 5 km and a maximum depth of seismic faulting of 10 km corresponding to a limiting temperature of 600 c the shallower extent of seismic behavior along the 15 20 transform may be related to a recent episode of extension across the transform associated with the northward migration of the triple junction among north american south american and african plates to its present position near the transform the source mechanisms for all events in this study display the strike slip motion expected for transform fault earthquakes slip vector azimuths agree to within 2 3 of the local strike of the zone of active faulting the only anomalies in mechanism were for two earthquakes near the western end of the vema transform which occurred on significantly nonvertical fault planes secondary faulting occurring either precursory to or near the end of the main episode of strike slip rupture was observed for five of the 12 earthquakes for three events the secondary faulting was characterized by reverse motion on fault planes striking oblique to the trend of the transform in all three cases the site of secondary reverse faulting is near a compressional jog in the current trace of the active transform fault zone we find no evidence to support the conclusions of engeln wiens and stein that oceanic transform faults in general are either hotter than expected from simple thermal models or weaker than normal oceanic lithosphere

the geometry of aleutian subduction three dimensional seismic imaging we constrain the subduction geometry of the aleutian slab by the distribution of seismicity recorded at local seismic networks and by a teleseismic p wave residual sphere analysis of 11 earthquakes and one nuclear explosion the source depth and mechanism of each earthquake are determined by modeling teleseismically recorded long period p and sh waveforms we construct theoretical residual spheres by tracing rays through three dimensional thermal models of the subducting slab assuming a pressure independent thermal coefficient of velocity vp t rays are initiated at the estimated source depths and at epicenters whose relative locations are consistent with those obtained from the observed travel times after correction for slab effects for each event we determine the values of slab strike and dip that provide the best match to the observations average values of 67 confidence intervals determined by an f test are 5 in dip and 10 in strike strike varies along the arc by 60 for all but one of the modeled events the estimated orientation of the slab s aseismic extension agrees to within two standard deviations of that derived from seismicity distributions assuming a constant dip a slab penetration depth of 600 km fits the central aleutian events best this is at least 300 km below the deepest earthquakes slabs terminating above 400 km or below 1000 km provide significantly worse fits though there is a trade off between vp t and penetration depth the best fitting value of vp t using a 600 km deep slab is 0 5 0 1 ms 1 k 1 this is consistent with both ultrasonic laboratory measurements and values obtained for deeper earthquakes in several other subduction zones vp t estimated from shallow thrust events is 20 50 larger than that for intermediate focus events suggesting that a simple thermal model with constant vp t under predicts the velocity anomalies shallower than 100 km depth the fact that a 600 km deep slab provides better fits than a 1000 km deep slab implies that the downdip p rays exit the slab near a depth 600 km a slab bend to either a shallower or steeper dip would satisfy the p times hand picked pcp travel times are most consistent with the slab bending to a 20 steeper dip before plunging into the lower mantle in the western aleutians the relative plate motion becomes transform seismicity ceases at less than 100 km depth and volcanism is no longer active we image a slab like high seismic velocity zone apparently associated with the along arc lateral transport of cold subducted lithosphere from the east an earthquake whose location is known from local observations is analyzed in detail the velocity structure inferred using this location is nearly identical to that obtained using our scheme of annihilating location information from both theoretical and observed times a severe bias would result however from the commonly used procedure of analyzing travel times computed with respect to an incorrect event location such as that of the isc

multispectral remote sensing as stratigraphic and structural tool wind river basin and big horn basin areas wyoming stratigraphic and structural analyses of the wind river basin and big horn basin areas of central wyoming are in progress one result has been the development of an approach to stratigraphic and structural analysis that uses photogeologic and spectral interpretation of multispectral image data to characterize the attitude thickness and lithology of strata new multispectral systems that have only been available since 1982 are used with topographic data to map upper paleozoic and mesozoic strata exposed on the southern margin of the bighorn mountains landsat acquired thematic mapper tm data together with topographic data are used to map lithologic contacts measure dip and strike and develop a stratigraphic column that is correlated with conventional surface and subsurface sections aircraft acquired airborne imaging spectrometer ais and thermal infrared multispectral scanner tims data add mineralogic information to the tm column including the stratigraphic distribution of quartz calcite dolomite smectite and gypsum results illustrate an approach that has general applicability in other geologic investigations that could benefit from remotely acquired information about areal variations in attitude sequence thickness and lithology of strata exposed at the earth s surface application of our methods elsewhere is limited primarily by availability of multispectral and topographic data and quality of bedrock exposures

horizontal coseismic deformation of the 1999 chi chi earthquake measured from spot satellite images implications for the seismic cycle along the western foothills of central taiwan 1 the 1999 chi chi earthquake mw 7 6 broke a major thrust fault along the western foothills of the central range of taiwan we have measured the horizontal coseismic displacement field by correlating optical satellite images acquired before and after the earthquake these data reveal the fault trace and a clockwise rotation of surface displacements toward the north with much larger displacements and strain in the hanging wall at the surface coseismic slip increases from 5 6 m near the epicenter to 10 11 m to the north in the epicentral area we observe a left lateral strike slip zone trending n125 e and farther north a fault zone trending n s with a right lateral component the data were modeled using elastic dislocations the fault geometry consists of a shallow

seismological evidence for shallow thrusting north of the timor trough summary the eastern sunda arc in indonesia is the site of subduction of the continental crust of australia and its shallow seismicity is manifested by infrequent large ms 7 earthquakes that occur near the volcanic arc within the upper plate rather than as interplate events beneath the forearc in the same region volcanic activity has ceased and small shallow earthquakes are rare we examine two of the larger earthquakes the events of 1962 may 15 ms 7 2 and 1977 august 27 ms 7 0 the 1977 event occurred north of central timor and the inversion of long period p and sh waves indicates a nearly pure thrust mechanism at a depth of 10 km below sea level and a seismic moment of 4 1 1026 dyne cm the p wave nodal planes for this solution strike roughly parallel to the mapped trace of a thrust zone the wetar thrust zone at the southern margin of the backarc basin and the south nodal dipping plane is consistent with the sense of thrusting on this thrust zone a broad zone of compressional deformation is evident in seismic reflection profiles over the epicentral region of the 1977 event and gravity data suggest that a small amount of either isostatically uncompensated crustal thickening or flexure of the underthrusting plate has occurred the 1962 earthquake occurred approximately 300 km north east of the 1977 earthquake and near the eastern end of the volcanically quiet section of the banda arc the similarity of the few available waveforms to those at the same stations for the 1977 event suggests a similar source mechanism modelling of pdif waveforms for the 1962 event indicates a depth of less than 40 km and a preferred centroidal depth of 20 km for which a seismic moment of 5 1026 dyne cm is estimated these fault plane solutions indicate that most of the seismic energy released by the collision between australia and the sunda arc is generated by faulting within the overriding plate and we infer that a major change in the plate geometry is imminent we suggest that most of the relative convergence between the plates is accommodated by thrusting of the marginal basin the banda sea southward beneath the arc and that the present geometry represents the initial stage of the reversal of the arc s polarity

interpretation of seismic pulse duration in terms of a propagating rupture a method and case histories abstract extended earthquake source models in which a rupture migrates in a fault plane radiate signals whose duration show a directional variation analysis of spectral corner frequencies may be used to estimate the parameters of an extended source but an alternative method is by the direct measurement of teleseismic body wave pulse durations on broad band seismograms this time domain approach is shown to have certain advantages including easier estimation of confidence limits and treatment of interfering arrivals a method is described for testing the compatibility of observed pulse durations with possible source geometries derived from a propagating rupture model this establishes the identity of or non existence of compatible source geometries the results provide evidence to help discriminate between source directivity and anelastic attenuation and between the fault and auxiliary planes p pp pcp or other phases observed at only a small number of stations can provide sufficiently wide directional sampling of the focal sphere to place tight constraints on the parameters of commonly assumed rupture models more constraint is imposed if the s wave radiation is also sampled for example with sp or direct s accurate measurement of durations is neither realistic nor essential a confidence of 30 in pulse duration is typical but better measurements are possible if the instrument phase response is removed from the observed seismogram a synthetic example is used to demonstrate these points and real examples studied by previous authors are re examined in more detail to assess the value of this approach in the analysis of extended sources

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

mechanics of wedge shaped fault blocks 2 an elastic solution for extensional wedges listric planar low angle and high angle normal faults are common in hanging walls of detachment faults an elastic model has been developed to evaluate the role of basal friction wedge geometry pore fluid pressure within the wedge and boundary conditions applied along the wedge rear in controlling the stress distribution in an extensional fault wedge this model assumes a stress free condition on the top and fractional sliding on the base of the wedge respectively a linear variation of stress components as a function of depth along the wedge rear and a uniform horizontal stress applied on the wedge toe the model predicts that 1 for the surface slope equal to zero a thin wedge favors development of high angle and planar faults whereas a thick wedge favors development of low angle and listric normal faults variation of pore fluid pressures along the basal detachment fault hardly affects the predicted fault geometry pore fluid pressure within the wedge is critical in controlling the state of stress in the wedge higher values of the internal pore fluid pressure promote low angle normal faulting and locally high angle reverse faulting 2 variation of surface slope and the uniform horizontal normal stress applied at the wedge toe does not affect the predicted fault geometry appreciably although the distribution of deviatoric stress magnitude changes for different cases and 3 listric normal faults are predicted in all computations and the fault curvature increases as the vertical gradient of the horizontal normal stress along the wedge rear the wedge angle surface slope dip angle and the internal pore fluid pressure increase the model provides a simple conceptual guide to deciphering the formation of complex fault geometries and cross cutting relationships as a function of mechanical parameters related to geologic processes and settings for example it provides an explanation for why low angle normal faults cut high angle normal faults in hanging walls of some detachment fault systems in the u s cordillera

active normal faulting and crustal extension a world wide review of fault plane solutions and focal depths for large normal faulting earthquakes on the continents shows that the overwhelming majority of such earthquakes nucleate in the depth range 6 15 km on faults dipping between 30 and 60 in the few cases where levelling or seismic data are good enough these normal faults are shown to be approximately planar from the surface to their nucleation depth at the base of the brittle crust there is evidence that in some cases as a result of the enormous transitory increase in strain rate during large earthquakes rupture continues into the normally ductile lower crust on surfaces with substantially gentler dips these low angle surfaces may be analogous to some of the detachments seen in metamorphic core complexes of the western usa but the nature of the motion on them depends on strain rate as well as on rheological contrasts between the detachments and the blocks on either side such contrasts however are unlikely to introduce substantial curvature to an originally planar shear zone if the observed spread in active normal fault dips is caused by rotation of the faults and the blocks they bound during extension then a maximum value of 1 7 can be accommodated by seismic activity on a single generation of normal faults with continued extension either a new generation of steeper faults cutting the rotated first faults is likely to form or the deformation will continue aseismically on faults dipping at less than 30 large earthquakes have not been observed to nucleate on very low angle 20 normal faults within the continental crust anywhere in the world such faults can of course move aseismically but are unlikely to do so on a large scale within the upper crust in areas where steep normal faults are seismically active thus extensional models that require concentrated simple shear on large sub horizontal faults within the brittle upper crust will also require a spatial separation between aseismic very low angle faulting and seismic highangle faulting since seismogenic high angle faults dominate the topography of extending regions upper crustal very low angle faulting if it occurs on a large scale presumably does so in areas that are relatively fiat

faulting process of the 1990 june 20 iran earthquake from broadband records summary we study the rupture process of the iranian earthquake of 1990 june 20 from broad band data we redetermined its moment tensor and source mechanism from long period surface waves then we studied the p and sh broad band recordings from the geoscope and iris seismic networks we inverted the body waves using a full waveform modelling for the inversion we used the gradient method of nab lek mixed with a limited exploration of the parameter space in order to control the resolution of inversion we proceeded by a stepwise procedure increasing the model complexity with every new inversion we used the reduction of cost functional as a criterion for the validity of the inverted model we first inverted the body waves considering a single point source model we found that body waves are dominated by a large energy release about 20 s from the onset of the signals in the next step fixing fault plane solution we inverted for the directivity of the source we found clear evidence of propagation towards the east of the epicentre we determined a rupture velocity of 2 5km s 1 the preceding source inversion reduced the variance of the residuals by about 30 per cent further improvement of the fit of the body wave signals was obtained using an extended line source with constant fault plane solution we obtained a very good fit with an asymmetrical model initially rupture is bilateral but north western propagation stops after 10 s later rupture continues unilaterally in the south eastern direction with a rupture velocity of the order of 2 5 km s 1 finally an inversion was attempted with a line source with varying fault mechanism the final solution obtained by this procedure reduces the variance of the body wave residuals by 60 per cent with respect to the variance of the observed body wave signals variations in fault plane solution along the fault are considered to be well resolved

local earthquakes on three component seismographs heterogeneity in the aftershock zone of el asnam algeria 10 10 1980 m 7 3 seismograms of aftershocks of the el asnam earthquake recorded by three component stations of a local array provide direct evidence of structural heterogeneity in the aftershock zone inversion of arrival times for three dimensional velocity structure is thus needed for the relocation of hypocenters these fall within a reduced depth interval from 6 to 12 km in the underthrust block

the el asnam 1980 october 10 inland earthquake a new hypothesis of tsunami generation summary the western mediterranean sea is not considered as a high seismic region only several earthquakes with magnitude above five occur each year and only a handful have consequences on human beings and infrastructure the el asnam algeria earthquake of 1980 october 10 with an estimated magnitude ms 7 3 is one of the most destructive earthquakes recorded in northern africa and more largely in the western mediterranean basin although it is located inland it is known to have been followed by a small tsunami recorded on several tide gauges along the southeastern spanish coast in 1954 a similar earthquake having occurred at the same location induced a turbidity current associated to a submarine landslide which is widely known to have cut submarine phone cables far from the coast this event was followed by a small tsunami attributed to the landslide thus the origin of the tsunami of 1980 was promptly attributed to the same kind of submarine slide as no evidence of such mass movement was highlighted and because the tsunami wave periods does not match with a landslide origin in both cases 1954 and 1980 this study considers two rupture scenarios that the coseismic deformation itself of about 10 cm off the algerian coast near tenes is sufficient to produce a low amplitude several centimetres tsunami able to reach the spanish southeastern coast from alicante to algeciras gibraltar strait to the west after a discussion concerning the proposed rupture scenarios and their respective parameters numerical tsunami modelling is performed on a set of bathymetric grids then the results of wave propagation and amplification maximum wave height maps are discussed with a special attention to alicante spain harbour where the location of two historical tide gauges allows the comparison between synthetic mareograms and historical records showing sufficient signal amplitude this study is part of the active tsunami hazard assessment in mediterranean sea especially concerning its occidental part that is the algerian spanish and french coasts

u th isotope constraints on gas hydrate and pockmark dynamics at the niger delta margin abstract the application of uranium thorium dating methods to authigenic carbonates provides unique constraints on the temporal evolution of methane seeps at ocean margins in this study we report u th isotope measurements for carbonate breccias collected from within a hydrate bearing pockmark located at the niger delta margin these concretions were extracted from a carbonate rich layer in the upper two meters of a sediment core n2 ks 44 1200 m water depth well above the present day sulphate methane transition zone about 3 m depth and the presence of gas hydrates in the sediment the stratigraphy of core n2 ks 44 was established by tuning its downcore al ti profile to a well dated nearby reference core and carbonate 230 th u ages were calculated using isochron methods our results indicate that a major event of aragonite precipitation occurred between about 13 and 2 5 ka at the studied location comparison of sediment accumulation rates at both core n2 ks 44 and the nearby reference site suggests that the initial stage of carbonate precipitation between 13 and 10 ka was associated with sediment winnowing probably related to intense fluid seepage in contrast our data indicate that sedimentation rates rapidly increased within the pockmark after 7 ka in agreement with the presence of carbonate breccias exhibiting u th ages older than their corresponding stratigraphic age this observation would suggest that sediment reworking took place after that time possibly caused by erosion of the surrounding sediment within the pockmark we hypothesize that the period of carbonate formation between 13 and 2 5 ka was related to an upward migration of gas hydrate reservoirs to the near seafloor environment after this pulse of enhanced fluid flow the diminution of methane fluxes at the base of the local gas hydrate occurrence zone would have led to hydrate dissolution in sub surface sediments and pockmark formation thereby explaining the progressive increase in sedimentation rates the absence of recent carbonate concretions and the deepening of the sulphate methane transition zone at site n2 ks 44 inferred from pore water data overall these results provide further constraints about the relationship between gas hydrate dynamics and the evolution of pockmarks at ocean margins through time

seismotectonics of the el asnam earthquake the mechanism of the el asnam earthquake of 10 october 1980 ms 7 2 the strongest recorded in northwestern africa is compatible with a convergence between the african and european plates the thrust is also consistent with field observations and with long period data from the global network aftershocks cluster around the fault trace with a maximum depth of 12 km maximum vertical displacement is 6 m and there is some left lateral motion the fault trace of the 1954 earthquake at beni rached was also reactivated

the el asnam algeria earthquake of 10 october 1980 multiple source mechanism determined from long period records we present a study of the el asnam algeria earthquake of 10 october 1980 from a large collection of long period surface and body wave records the focal mechanism of the main shock is well constrained by the p wave first motions at teleseismic distances and field observations it was a thrust event on a plane striking n45 e and a dip angle of 54 to the north it had a small component of left lateral motion 83 ouyed et al 1981 gaudemer et al 1981 this earthquake was very well recorded on wwssn stations and on gdsn and ida digital stations with a good azimuthal distribution from these records we confirm the focal solution and obtain a seismic moment m o 5 1026 dyne cm the p wave seismograms indicate a complex source we show that it is not sufficient to model the source by a multiple event but it is also necessary to include a propagation effect in order to explain accurately the waveform with assumptions based on field observation of the surface breaks we model the p waves including two discontinuities of the propagation along the fault plane and obtain a good agreement of the waveform

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

migration of seismicity and earthquake interactions monitored by gps in se asia triple junction sulawesi indonesia 1 global positioning system gps measurements made in sulawesi indonesia from 1992 to 1999 detected coseismic and transient postseismic deformation related to the 1 january 1996 mw 7 9 earthquake on the north sulawesi minahassa trench these motions are superimposed on the long term secular motion 40 mm yr of the left lateral palu fault in central sulawesi and continued for about 1 5 2 years following the earthquake a string of earthquakes of magnitude 6 migrated along the minahassa trench from west to east subsequently two earthquakes of magnitude 6 occurred on or near the palu fault migrating toward the south modeling the increase in coulomb stress generated by the successive earthquakes agrees with the hypothesis of interacting events an unclamping effect possibly due to fluid migration in the palu area is also suggested by the stress computations and the detected gps displacements

source mechanisms of three moderate california earthquakes of july 1986 abstract we studied the source processes of three moderate size earthquakes in california the m s 6 0 event of july 8 1986 near palm springs the m s 5 8 shock of july 13 1986 offshore of oceanside and the m s 6 2 event of july 21 1986 in the chalfant valley on the east side of the sierra nevada the centroidal source parameters are estimated by inversion of long period p and sh waves recorded at teleseismic distances by seismographs of worldwide networks a more detailed description of the source is then obtained by analyzing broadband seismograms the north palm springs earthquake had a simple rupture process most of the seismic moment was released in the first 3 sec in a single step the strike of the source fault plane follows the average trend of the banning fault and its dip agrees with the dip of the fault plane delineated by the aftershocks the faulting mechanism of the main shock is right lateral strike slip with a sizable reverse component that is consistent with uplift north of the banning fault the earthquake offshore of oceanside had a slightly more complex rupture process the far field source time function consists of two pulses that represent the release of most of the moment in the first 3 sec the source mechanism indicates reverse faulting with the compressional axis trending northeast almost perpendicular to the trend of mapped faults offshore the event does not appear to be associated with major mapped faults in the area but the strike of one of the nodal planes agrees with the trend of the major mapped faults offshore and its mechanism is consistent with the missing component of plate convergence not taken up by the onshore strike slip fault systems and the idea of a weak san andreas fault system the rupture process of the chalfant valley earthquake was yet more complicated the source time function indicates that in a 4 sec time window three asperities were broken the source mechanism indicates right lateral strike slip faulting with a small normal faulting component on a fault that dips approximately 50 southwest earthquake activity suggests that presently the deformation in the northern part of the owens valley which includes the chalfant valley is dominated by conjugate strike slip faults which strike obliquely to the main range bounding normal faults the waveforms indicate the presence of a low velocity zone at a depth of about 20 km in the crust in the chalfant valley a synthesis of information from waveform modeling first motion fault plane solutions and aftershock distributions indicates that the nucleation and or termination of rupture for the north palm springs and the chalfant valley events were controlled by preexisting fault geometry

use of landsat thematic mapper images in regional correlation of syntectonic strata colorado river extensional corridor california and arizona this study utilizes enhanced landsat thematic mapper tm images in concert with field work to solve problems in regional correlation of miocene rocks in the colorado river extensional corridor of california and arizona these syntectonic volcanic and sedimentary deposits preserve a dateable record of crustal extension although regional correlation and analysis of the deposits are difficult because of the small sizes of the sedimentary basins in which they accumulated and disruption by postdepositional faulting based on field investigations four sequences of sedimentary and volcanic strata distinguished by stratigraphic and structural relations can be recognized in the mohave mountains arizona and the eastern whipple mountains california although the sequences are found in each of four structural regions the relative volumes and organization of lithologies within the sequences differ considerably between regions these sequences have distinctive appearances on the tm image the volcanic component is particularly important in distinguishing between regions and sequences because differences in color and textural patterns in the image are usually related to variations in the proportion and composition of the volcanic material and in the geometry of the volcanic bodies the recognition criteria derived from field mapping and image interpretation in the mohave and whipple mountains weremore applied to an adjacent area in which stratigraphic affinities were less well known stratigraphic and structural relations suggested by the tm image were confirmed by subsequent field work less

miocene synextension sedimentation patterns whipple mountains southeastern california implications for the geometry of the whipple detachment system a sedimentary basin formed in the southern and eastern whipple mountains during early to middle miocene detachment faulting stratigraphic relations and facies patterns indicate that the shape and size of the sedimentary basin was controlled both by fault activity and by sedimentary processes high angle normal faults formed the southwestern and perhaps also the northeastern boundary of the half graben whereas a transfer fault probably formed the southeastern boundary the transfer fault influenced the earliest synextension sedimentation in the basin the location of this fault and similar faults in the mohave mountains and aubrey hills of arizona is inferred to have been controlled by the position of primary irregularities or corrugations of the detachment fault shifts in facies patterns particularly a shift in the position of the basin depocenter reflect changes in basin geometry comparison with adjacent areas is necessary to distinguish between tectonic and nontectonic causes a shift in depocentral position parallel to regional transport direction in the whipple mountains basin is interpreted as resulting from a change in the location of the high angle normal fault forming the southwestern margin of the basin whereas a shift perpendicular to transport direction probably resulted from overtopping of topographic barriers within a fault block the most active phase of faulting occurred prior to deposition of stratigraphic units younger than the 18 5 ma peach springs tuff relative uplift of the core of the whipple mountains probably as a result of isostatic rebound following tectonic denudation influenced sedimentation patterns after deposition of the peach springs tuff

rupture process of the andreanof islands earthquake of may 7 1986 the andreanof islands earthquake of may 7 1986 ms 7 7 is the largest earthquake to have occurred along the aleutian arc in the past 20 yr source parameters of the mainshock three large aftershocks 6 1 ms 6 7 and a foreshock ms 6 0 are determined by inverting digitally recorded teleseismic waveforms and amplitudes long period p and sh waves are inverted simultaneously to determine the strike dip rake centroid depth source time function and seismic moment of each earthquake complexities in the rupture process of the mainshock are modeled using broadband 5 to 0 01 hz p wave displacement waveforms modeling the mainshock as a propagating line source the long period and broadband waveforms indicate that the locus of largest moment release propagated bilaterally along the arc with a well resolved velocity of 1 5 km sec the source mechanism is consistent with faulting on a low angle 23 1 thrust fault the fault s strike 260 10 and the rake of the slip vector on the fault 93 8 are less well resolved and trade off with each other acceptable centroid depths range between 10 and 25 km within this range of depths the misfit of the synthetic to the observed long period and broadband waveforms varies by only 6 per cent while moment estimates vary by as much as 64 per cent for a centroid depth of 20 km the estimated seismic moment is 1 1 1028 dyne cm 85 per cent of the moment estimated from surface wave analysis ninety per cent of the moment release was generated by three patches on the fault surface all located within 90 km of the nucleation point very little moment however was released near the epicenter of the mainshock the epicentral locations source mechanisms and depths of the foreshock and one of the aftershocks modeled by us are consistent with faulting on the same thrust plane as the mainshock the other two modeled aftershocks are located north of the island arc and have source depths of less than 15 km indicating that they occurred within the overriding plate the focal mechanisms of these events indicate almost pure strike slip faulting but the waveform data do not contain sufficient information to distinguish which of the couples of the double couple mechanism corresponds to the fault plane joint relocations of several of the aftershocks from one of these strike slip earthquakes suggests that right lateral strike slip faulting occurred on a northeasterly trending fault plane

source scaling of earthquakes in the shumagin region alaska time domain inversions of regional waveforms the scaling of pulse duration to seismic moment is estimated for earthquakes along an interplate thrust zone from digital waveforms recorded by short period and broad band instruments of the east aleutian shumagin seismic network we measure pulse duration using an empirical green s function technique based on damped time domain deconvolution from several thousand events 22 earthquakes with magnitudes 3 0 7 0 and depths 23 56 km are found to give reliable estimates of pulse duration durations are also determined directly from one parameter nonlinear inversions for a variety of simple functional forms of source time functions symmetric source pulses boxcar or triangle shapes fit waveforms better than an asymmetric model t exp 2t d for most 62 per cent of the waveform pairs while the asymmetric model fits best for only 8 per cent of the data pulse duration increases with the size of events from 0 1 to 10s over the seismic moment m0 range of 1014 to 3 1019 nm when normalized by the cube root of seismic moment pulse durations show 8 x variation comparable static stress drop estimates range from 0 2 to 135 mpa contrary to predictions of some laboratory and theoretical studies earthquakes at the deepest part of the thrust zone do not show significantly higher stress drops than do shallower events rupture properties however show a strong dependence on earthquake size the three largest events m0 5 1018nm have the three longest normalized durations on average 3 8 times longer than those for smaller events the durations require smaller events to have 10 100 x larger static stress drops or 4 x faster rupture velocities or some combination of the two possibly the largest events rupture both strong and weak patches while smaller events just rupture strong patches on the fault surface the characteristic dimension that separates large from small events 3 15 km is comparable to characteristic wavelengths of pacific basin bathymetry and may reflect the influence of the subducted sea floor upon fault zone heterogeneity

an aftershock study of the el asnam algeria earthquake of 1980 october 10 summary an array of 28 portable seismic stations was operated in the region of el asnam following the magnitude 7 3 ms thrust earthquake of 1980 october 10 locations of 494 events are presented in this paper and provide an indication of the overall form of the aftershock distribution tests to establish location accuracy particularly depth reduce this set to 277 events which it is argued are well constrained p waves alone are used in this study as a consequence of a debate about the reliability of reading s phases from the reduced set of 277 events 81 events provide well constrained focal mechanisms the locations are presented in the form of maps and cross sections and discussed in relation to information already derived from field mapping of surface breaks and teleseismic studies of the waveforms of the main event the zone of surface faulting including secondary normal faulting extended for 35 km but the aftershock distribution extends for twice this distance along the part of the fault which experienced substantial displacement in the main shock the fault plane itself appears to be devoid of aftershocks although many lie in the footwall beneath the fault at junctions between segments of thrust faulting strike slip motion occurs this is apparent in the aftershock focal mechanisms and in the surface ruptures in one place the large number of aftershocks in the north east area appears to be due to the reactivation of a fan like system of smaller reverse faults associated with surface folding activity at the south west end is considerably less than that in the north east and is not obviously associated with recognizable geological or morphological features

the ms 7 3 1992 suusamyr kyrgyzstan earthquake 1 constraints on fault geometry and source parameters based on aftershocks and body wave modeling we investigated the suusamyr kyrgyzstan earthquake of 19 august 1992 using aftershock data teleseismic body wave modeling and field observations aftershocks were recorded by the iris kyrgyzstan broadband network a temporary six station aftershock network and a regional network operated by the kyrgyz institute of seismology the aftershocks which range in depth from the surface to 18 km defined a 50 10 km long rupture zone that dips 50 13 to the south and strikes roughly east west the base of the eastern end of the aftershock zone shallowed to the east along strike and may represent a lateral ramp the surface ruptures also had an east west strike and dipped south but the total length less than 4 km was much shorter than the aftershock zone a teleseismic body wave inversion using a point source and a directivity correction yields a focal mechanism with a strike of 221 dip of 46 and a slip of 43 we obtained a moment of 4 1 1019 n m with a centroid depth between 5 and 21 km the rupture propagated along an azimuth of 330 60 which matches the relative location of the mainshock with respect to the aftershock zone the results of the aftershock study and teleseismic inversion yield a clear picture of the fault geometry of a large thrust earthquake

active arc continent collision earthquakes gravity anomalies and fault kinematics in the huon finisterre collision zone papua new guinea the huon finisterre island arc terrane is actively colliding with the north edge of the australian continent the collision provides a rare opportunity to study continental accretion while it occurs we examine the geometry and kinematics of the collision by comparing earthquake source parameters to surface fault geometries and plate motions and we constrain the forces active in the collision by comparing topographic loads to gravity anomalies waveform inversion is used to constrain focal mechanisms for 21 shallow earthquakes that occurred between 1966 and 1992 seismic moment 1017 to 3 1020 n m twelve earthquakes show thrust faulting at 22 37 km depth the largest thrust events are on the north side of the huon peninsula and are consistent with slip on the ramu markham thrust fault zone the northeast dipping thrust fault system that bounds the huon finisterre terrane thus much of the terrane s crust but little of its mantle is presently being added to the australian continent the large thrust earthquakes also reveal a plausible mechanism for the uplift of pleistocene coral terraces on the north side of the huon peninsula bouguer gravity anomalies are too negative to allow simple regional compensation of topography and require large additional downward forces to depress the lower plate beneath the huon peninsula with such forces plate configurations are found that are consistent with observed gravity and basin geometry other earthquakes give evidence of deformation above and below the ramu markham thrust system four thrust events 22 27 km depth directly below the ramu markham fault outcrop are too deep to be part of a planar ramu markham thrust system and may connect to the north dipping highlands thrust system farther south two large strike slip faulting earthquakes and their aftershocks in 1970 and 1987 show faulting within the upper plate of the thrust system the inferred fault planes show slip vectors parallel to those on nearby thrust faults and may represent small offsets in the overriding plate these faults along with small normal faulting earthquakes beneath the huon finisterre ranges and a 25 along strike rotation of slip vectors demonstrate the presence of along strike extension of the accreting terrane and along strike compression of the lower plate

source parameters of the 1933 long beach earthquake regional seismographic network and teleseismic data for the 1933 m l 6 3 long beach earthquake sequence have been analyzed both the teleseismic focal mechanism of the main shock and the distribution of the aftershocks are consistent with the event having occurred on the newport inglewood fault the focal mechanism had a strike of 315 dip of 80 to the northeast and rake of 170 relocation of the foreshock main shock aftershock sequence using modern events as fixed reference events shows that the rupture initiated near the huntington beach newport beach city boundary and extended unilaterally to the northwest to a distance of 13 to 16 km the centroidal depth was 10 2 km the total source duration was 5 sec and the seismic moment was 5 10 25 dyne cm which corresponds to an energy magnitude of m w 6 4 the source radius is estimated to have been 6 6 to 7 9 km which corresponds to a brune stress drop of 44 to 76 bars both the spatial distribution of aftershocks and inversion for the source time function suggest that the earthquake may have consisted of at least two subevents when the slip estimate from the seismic moment of 85 to 120 cm is compared with the long term geological slip rate of 0 1 to 1 0 mm yr along the newport inglewood fault the 1933 earthquake has a repeat time on the order of a few thousand years

faulting mechanism of the el asnam algeria 1954 and 1980 earthquakes from modelling of vertical movements abstract since 1980 most of the papers published on the el asnam earthquake concern the geological and seismological aspects of the fault zone only one paper published by ruegg et al 1982 constrains the faulting mechanism with geodetic measurements the purpose of this paper is to reexamine the faulting mechanism of the 1954 and 1980 events by modelling the associated vertical movements for this purpose we used all available data and particularly those of the levelling profiles along the algiers oran railway that has been remeasured after each event the comparison between 1905 and 1976 levelling data shows observed vertical displacement that could have been induced by the 1954 earthquake on the basis of the 1954 and 1980 levelling data we propose a possible model for the 1954 and 1980 fault systems our 1954 fault model is parallel to the 1980 main thrust fault with an offset of 6 km towards the west the 1980 dislocation model proposed in this study is based on a variable slip dislocation model and explains the observed surface break displacements given by yielding et al 1981 the dewey 1991 and avouac et al 1992 models are compared with our dislocation model and discussed in this paper

mapping in the oman ophiolite using enhanced landsat thematic mapper images abstract the large areal extent and complex nature of the terrain has meant that previous largely field based mapping programmes in the oman ophiolite have produced maps of internally inconsistent quality recognition of subtle changes and minor outcrops across the area of a 1 1 map sheet is possible in the field only by unrealistically exhaustive mapping by using parts of the short wavelength infrared spectrum landsat thematic mapper data have the capacity to capitalize on distinctions in the spectral reflectances of different rock types and their weathering products and to allow consistent mapping across the whole ophiolite decorrelation stretching of these data produces images in which it is possible to recognize variations in gabbro composition to identify small acidic gabbroic and ultramafic intrusions to discriminate the uppermost mantle from the deeper mantle to locate the moho precisely and to delineate gossans and areas subject to chloritic epidotic alteration off axis plutonism is more widespread and dispersed than previously appreciated the significance of certain faults is reassessed in the light of new insights provided by the imagery such data could be a valuable mapping tool in similarly exposed terrains in arid and semi arid regions even where already mapped at the 1 100 000 scale

lithologic mapping in arid regions with landsat thematic mapper data meatiq dome egypt digital landsat thematic mapper tm data were evaluated for lithologic mapping capabilities over the meatiq dome area in the hyper arid eastern desert of egypt bi directional spectral reflectance data 0 4 2 5 m for powders of the major rock types exposed in the dome and published spectral reflectance data were used as guides in selecting tm band reflectance ratios that maximize discrimination of individual rock types on the basis of their respective mineralogies comparison of tm data with field and petrographic observations shows 1 increasing amounts of magnetite and other opaque minerals with low flat spectral reflectances decrease the ratio of tm band 5 1 55 to 1 75 to band 1 0 45 to 0 52 m 2 increasing amounts of hydroxyl bearing minerals with hydroxyl ion vibrational absorptions in tm band 7 wavelength region 2 08 to 2 35 m increase the ratio of tm band 5 to band 7 3 increasing amounts of fe bearing aluminosilicates that absorb in the band 4 wavelength region 0 76 to 0 9 m increase the product of the following two tm ratios band 5 to band 4 and band 3 0 63 to 0 69 m to band 4 and 4 thin 5 m desert varnish that covers many outcrops modulates but does not obscure the spectral reflectance signatures of the meatiq rocks the varnish consists of amorphous to poorly crystalline dioctahedral smectite iron oxides and or oxyhydroxides serpentinite mafic mylonite massive amphibolite quartzofeldspathic mylonite biotite schist and quartz phyllonite were mapped on the basis of their unique values in one or more of the three ratio images whereas coarse and fine grained granites granite gneiss and tonalite with similar mineralogies and tm band ratios were mapped as a group finer subdivisions were made where field traverses provided local verification results demonstrate that appropriate processing and presentation of landsat tm data can significantly augment field observations for lithologic mapping of large areas in arid regions

focal depths and mechanisms of large earthquakes on the arctic mid ocean ridge system as part of a global study of the source characteristics and tectonic implications of large earthquakes on mid ocean ridges we report on the focal depths and mechanisms of the six largest earthquakes that have occurred in the last 20 years on the arctic mid ocean ridge system for each earthquake we invert the long period p and sh waveforms to estimate the parameters of the best fitting point source including seismic moment centroid depth double couple source orientation and source time function three of the earthquakes occurred on the oceanic spreading center in the eurasian basin along ridge segments spreading at half rates of 4 6 mm yr these events have mechanisms very similar to those of ridge crest earthquakes on the mid atlantic ridge almost pure normal faulting on planes that dip at approximately 45 and strike parallel to the rift axis moments of 4 5 1024 dyn cm centroid depths of 1 2 km beneath the seafloor and water depths inferred from the predominant period of water column reverberations appropriate to epicentral locations within the median valley the remaining three earthquakes also characterized by normal faulting are associated with the continuation of the divergent plate boundary 2 3 mm yr half rate onto the continental shelf of the laptev sea where the crust becomes transitional in nature one of the largest known spreading center earthquakes august 25 1964 m0 1 1026 dyn cm occurred where the oceanic ridge intersects the outer edge of the continental slope waveform inversion for this event can resolve unilateral rupture from north to south landward along a fault at least 30 km in length the preferred centroid depth is 5 km beneath the seafloor in crustal material with an unusually low shear velocity but a centroid depth as great as 15 km cannot be ruled out two earthquakes beneath the continental shelf have significantly greater centroid depths 10 20 km than mid ocean ridge earthquakes indicating a thicker brittle regime and a cooler thermal structure than are typical of oceanic spreading centers the tectonic environment of these events is more representative of rifted continental lithosphere than of a mid ocean ridge

control of rupture by fault geometry during the 1980 el asnam algeria earthquake summary the el asnam earthquake of 1980 october 10 ms 7 3 occurred on a segmented reverse fault in the atlas mountains of algeria this report re examines the teleseismic data for the main shock and major aftershocks in the light of detailed studies of the surface breaks geodetic changes and aftershock distribution the observed thrust fault is split into southern central and northern segments by distinct offsets and changes in trend the southern and central segments are each about 12 km long but the northern segment showed only 3 4 km of surface thrust breaks however widespread tensional faulting on ridges in the north east area together with focal mechanisms of locally recorded aftershocks indicates that a series of imbricate listric thrusts exists to the north of the northern fault segment using isc arrival time lists a relative relocation scheme is applied to the main shock and major aftershocks to improve their locations with respect to the mapped faults the main shock epicentre was at the south west end of the fault system in a zone where the thrust has a zig zag surface trace by contrast most of the major aftershocks were located further north east and were associated with the northern thrust segment and the imbricate thrusts to the north the largest aftershock mb 6 1 which occurred about 3 hr after the main shock was located to the east of the main thrust fault beneath the chelif alluvial basin first motion fault plane solutions are presented for the main shock and largest aftershock the main shock solution indicates a fault plane dipping 54 nw with strike 040 the aftershock solution however has a more e w trend 080 in agreement with the trends of the basin edges near its location the rupture history of the main shock is investigated by forward modelling of the long period p waves four subevents are clearly distinguishable the first three subevents occurred in rapid succession and contributed to the first cycle of the p wave radiation these three subevents represent the successive rupturing of the southern central and northern fault segments in order to match the observed waveforms these three ruptures must have different mechanisms and moments the fault segments have successively shallower dipts from south to north 54 45 and 40 respectively and the central segment had the largest moment release in agreement with the geodetic data of ruegg et al the delay times between the three sources suggest a rupture velocity of about 3 5 km s 1 and their nucleation depths were 7 km the junctions between the fault segments acted as geometric barriers to the rupture propagation but were broken by its passage the fourth subevent of the main shock produced the second cycle of the p wave radiation it began about 18 s after the first i e several seconds after the end of the third rupture nabelek demonstrates that the fourth subevent occurred on a shallow angle 20 thrust plane examination of the locally recorded aftershocks suggests that this plane is a decollement surface underlying the imbricate thrusts of the north east area the total seismic moment found from the body wave modelling is 7 7 1026 dyne cm in good agreement with geodetic and surface wave studies

teleseismic source parameters and rupture characteristics of the 24 november 1987 superstition hills earthquake long period body waves from the 24 november 1987 superstition hills earthquake are studied to determine the focal mechanism and spatial extent of the seismic source the earthquake is a complex event consisting of two spatially distinct subevents with different focal mechanisms two consistent models of rupture are developed for both models the second subevent begins 8 sec after the initiation of the first subevent and the preferred centroid depth lies between 4 to 8 km model 1 consists of two point sources separated by 15 to 20 km along strike of the superstition hills fault model 2 consists of one point source and one line source with a rupture velocity of 2 5 km sec with moment release distributed along strike of the focal plane at a distance of 10 to 22 km from the epicenter these moment release patterns show that a significant amount of long period energy is radiated from the southern segment of the fault total moment release for both models is approximately 8 10 25 dyne cm both models also suggest a change of dip from near vertical near the epicenter to steeply southwesterly dipping along the southern segment of the fault the difference in rupture characteristics and fault dips seen teleseismically is also reflected in aftershock and afterslip data and crustal structure underlying the two fault segments the northern segment had more aftershocks and a smaller proportion of afterslip than the southern segment the boundary between the two segments lies at a step in the basement that separates a deeper metasedimentary basement to the south from a shallower crystalline basement to the north

use of long period surface waves for rapid determination of earthquake source parameters 2 preliminary determination of source mechanisms of large earthquakes ms 6 5 in 1980 an inversion method for fast retrieval of earthquake source parameters developed earlier has been applied to 28 large m s 6 5 shallow earthquakes which occurred in 1980 rayleigh waves recorded by the ida international deployment of accelerographs network were used for the analysis for very shallow events the inversion is ill conditioned and two of the moment tensor elements m zx and m zy are unresolvable of the 28 events a solution is obtained for 25 with the constraints m zx m zy 0 although these constraints limit the solution to be either vertical strike slip or 45 dip slip the method provides a useful first approximation to the mechanism solution for deeper events all of the elements of the moment tensor can be determined after the seismograms have been retrieved the solution can be obtained in 10 min for most events the method would be adequate for preliminary cataloging purposes

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

teleseismic investigation of the january 22 1988 tennant creek australia earthquakes the three january 22 1988 earthquakes are the largest to occur within the australian continent in 20 years and produced two colinear but discontinuous wnw trending scarps enclosing a shorter ene trending scarp teleseismic long period p and sh and short period p waves are used to constrain the locations and source parameters for the three events the third earthquake has a seismic moment 1019nm equal to that of the 1968 meckering event and most likely ruptured the southern of the wnw trending faults relative relocations suggest that the two smaller earthquakes initiated north or nw of the third event centroid depths are less than 6 km but most likely 3 to 4 km and source durations are 4 to 6s so it is probable that each event broke the surface all three earthquakes have predominantly thrust mechanisms and nodal planes that strike west to wnw consistent with faults dipping southward at 30 to 40 only the first has a nodal plane that could support a north dip for the ene trending fault as proposed by other researchers on the basis of ground deformation and preliminary aftershock locations however source directivity evident in the short period p waves and relocations are interpreted better if this event like the others ruptured a sw dipping fault plane the ene trending scarp is interpreted to be a tear fault in the hanging wall that may have acted as a barrier to slip on a single continuous sw dipping fault resulting in this unusual at least for australia sequence of three large similar earthquakes

slab pull and slab push earthquakes in the mexican chilean and peruvian subduction zones we studied intermediate depth earthquakes in the chile peru and mexican subduction zones paying special attention to slab push down dip compression and slab pull down dip extension mechanisms although slab push events are relatively rare in comparison with slab pull earthquakes quite a few have occurred recently in peru a couple slab push events occurred in 1991 and one slab pull together with several slab push events occurred in 1970 near chimbote in mexico several slab push and slab pull events occurred near zihuatanejo below the fault zone of the 1985 michoacan event in central chile a large m 7 1 slab push event occurred in october 1997 that followed a series of four shallow mw 6 thrust earthquakes on the plate interface we used teleseismic body waveform inversion of a number of mw 5 9 slab push and slab pull earthquakes in order to obtain accurate mechanisms depths and source time functions we used a master event method in order to get relative locations we discussed the occurrence of the relatively rare slab push events in the three subduction zones were they due to the geometry of the subduction that produces flexure inside the downgoing slab or were they produced by stress transfer during the earthquake cycle stress transfer can not explain the occurence of several compressional and extensional intraplate intermediate depth earthquakes in central chile central mexico and central peru it seemed that the heterogeneity of the stress field produced by complex slab geometry has an important influence on intraplate intermediate depth earthquakes 2002 elsevier science b v all rights reserved

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

seismic and geodetic constraints on plate boundary deformation across the northern macquarie ridge and southern south island of new zealand summary in the southern new zealand region the boundary between the australian and pacific plates changes from oblique continental convergence in the central south island of new zealand to interoceanic transcurrent motion along the macquarie ridge source parameters of recent post 1964 upper crustal events mw 5 3 in the region 43 53 s 155 175 e are determined using p and sh waveform analysis in order to investigate the nature of strain accommodated seismically throughout the region repeated occupation of three geodetic networks across the southern south island provides estimates of contemporary crustal strain seismicity over the last 35 years shows uneven spatial and temporal distribution with clusters of events occurring in regions along the plate boundary that exhibit structural variation focal mechanisms of individual earthquakes vary widely but are consistent with the accommodation of long term estimates of the relative plate motion across the southern south island geodetically measured principal contraction directions are consistent with p axis orientations the mode of accommodation of the relative plate motion varies throughout the region influenced by the change in crustal composition on either side of the plate boundary the underlying structure as a relic of previous relative motions and the variation of obliquity of the relative motion with respect to these structures

earthquake induced flooding and paleoseismicity of the el asnam algeria fault related fold late quaternary activity of the el asnam thrust fault responsible for the october 10 1980 ms 7 3 earthquake and 36 km coseismic faulting is explored through diverse paleoseismological field methods we trenched through the main and secondary 1980 coseismic scarps made an analysis of earthquake induced flood deposits conducted a geomorphic study of leveling profiles and offset geological units across thrust and normal scarps and considered the geomorphology of the active fold twenty five radiocarbon ages from colluvial and alluvial deposits along with conventional fossils and artefacts dating of geological units show the timing of successive net vertical displacements on the thrust related fold and bending moment faults previous work showed that eight pre 1980 earthquakes occurred in clusters during the late holocene cumulative net vertical slip that amounts to 3 times the 1980 displacement is observed on multiple thrust fault scarps and offsets of geological units on bending moment faults maximum cumulative vertical displacements which are visible in different zones along the fault segments can reach 10 to 25 times the 1980 coseismic displacements the late quaternary uplift rate on the el asnam fault is 0 6 mm yr and taking into account slip variation along the fault this rate may range between 0 25 1 6 mm yr from which we obtain a shortening rate of 0 17 1 2 mm yr because the tell atlas mountains exhibit comparable seismogenic fault related folds with parallel and sequential structures of active faults a total average shortening rate of 2 05 mm yr during the late quaternary represents part of the convergence accommodated along the africa eurasia plate boundary

centroid depths of mid ocean ridge earthquakes dependence on spreading rate from inversions of teleseismic p and sh waveforms we have determined the source parameters of 50 large earthquakes that occurred during 1962 1983 on slowly spreading mid ocean ridges all events are characterized by predominantly normal faulting on planes that dip at approximately 45 and strike parallel to the local trend of the ridge axis centroid depths range from 1 to 6 km beneath the seafloor the p waves from these earthquakes show strong water column reverberations suggesting that fault rupture extended to the seafloor under the assumption that the centroid depth marks the mean depth of fault slip earthquake faulting extended to a depth of 2 10 km for these earthquakes the water depths inferred from the predominant periods of the water column reverberations constrain the epicenters of these earthquakes to lie within the relatively deep inner floor of the median valley the maximum centroid depths of ridge crest earthquakes decrease with increasing spreading rate and the maximum seismic moment may also decrease with increasing spreading rate these results indicate a general decrease with spreading rate in maximum thickness of the mechanically strong layer beneath the ridge axis region the concentration of large earthquakes within the deepest parts of the median valley and the depth extent of seismic faulting support lithospheric necking models for the origin of the median valley

the tangshan earthquake sequence and its implications for the evolution of the north china basin the 1976 tangshan earthquake sequence that occurred on the northern margin of the north china sedimentary basin is one of the strongest and most destructive intracontinental earthquake sequences ever recorded by worldwide long period seismograph networks we have studied the source process of the six largest events of this earthquake sequence by formally inverting long period p and sh wave seismograms from the world wide standardized seismographic network our analysis shows that the main shock was caused by motion on at least three fault segments each with a different orientation the initial and dominant faulting occurred on two segments of a north northeast trending right lateral strike slip fault system the last stage of faulting had a significant component of thrusting and occurred on an east northeast trending subsidiary fault at the southern end of the main fault the largest aftershock occurred at the northeastern end of the main fault and had a nearly pure normal faulting mechanism with east west striking nodal planes this event occurred in a pull apart region where the north northeast trending main strike slip fault system steps to the right this region subsided by more than 1 m during the earthquake sequence to the north of this step the main fault is delineated by numerous aftershocks whose mechanisms are by and large consistent with right lateral slip on this segment the large event in 1945 m 6 3 probably occurred on this part of the fault a region of substantial subsidence up to 1 5 m associated with the earthquake sequence was also observed in the south where the main fault system takes another step to the right two of the large aftershocks occurred nearby and had strike slip mechanisms the overall faulting process of the tangshan sequence is well characterized by right lateral slip on a set of right stepping faults the mechanisms and the points of nucleation of the strongest shocks and the extent of the ruptures appear to be closely tied to preexisting fault geometries a compilation of the mechanisms of large earthquake sequences in the north china basin strongly suggests that the present deformation of the entire basin is dominated by motions on right lateral strike slip systems trending north northeast the basin seems to have formed and evolved as a large transtensional structure at an en echelon step of a large scale strike slip system the deformation within the basin is both strike slip and extensional and appears to occur on smaller scales in an approximately selfsimilar fashion as evidenced by the deformation during the tangshan sequence this interpretation provides an explanation for the variable rate of subsidence in both space and time and the scatter in measured heat flow values as well as the overall shape of the north china basin it also provides an explanation for occasional thrust faulting earthquakes that occur apparently as a result of geometrical complexities of the strike slip faults transpression these observations are otherwise difficult to fit into a simple model of regional crustal extension and thermal subsidence

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

radiocarbon dating of late quaternary fault segments and systems recent developments in radiocarbon dating contribute to the analysis of fault evolution both at the level of individual fault segments and with regard to entire fault systems 14 c assay by accelerator mass spectrometry and the use of high precision radiometric counting systems have both improved the resolution of fault chronologies in conjunction with other sources of numerical ages they make it possible to trace fault related folding warping and regional deformation and thus to analyse a fault assemblage in its geodynamic context at dungeness se england 14 c dating of redeposited shells gives a minimum age for a planation surface and a fault that predates it in the gulf of corinth greece dating of lithophaga shells in their borings reveals 2 3m of differential holocene uplift of the south shore of gulf relative to the north improved chronologies should make it possible to complement the search for seismic recurrence intervals with analysis of deformation processes

active folding in the algerian earthquake of 10 october 1980 a fold controlling the form of the local topography near el asnam algeria increased in amplitude at the time of the earthquake of 10 october 1980 the deformation occurred rapidly in an elastic brittle manner and not in the gradual ductile fashion in which folds are normally thought to develop

source parameters of the 1 october 1995 dinar turkey earthquake from sar interferometry and seismic bodywave modelling abstract the 1 october 1995 m s 6 1 dinar earthquake ruptured a 10 km section of the nw se dinar civril fault there are discrepancies between the published source parameters from seismic data with seismic moments in disagreement by over a factor of two we use both sar interferometry and seismic bodywave modelling to determine earthquake source parameters an interferogram generated from ers 1 2 sar imagery spanning the event and separated by 5 months is used to derive source parameters by a downhill simplex inversion with multiple monte carlo restarts we model the displacements in the satellite line of sight initially using uniform slip on a rectangular dislocation in an elastic half space the resultant model fault plane agrees in strike and location with the observed surface break but systematic residuals exist in the line of sight deformation field resulting in a r m s residual of 20 mm in the interferogram the residuals are reduced if the depth distribution of slip is allowed to vary spatially in four segments along a continuous fault plane our best fitting solution with a r m s misfit of 8 mm reveals two distinct areas of slip on the fault plane strike 145 dip 49 rake 270 a main rupture slipping by 1 44 m between depths of 1 and 8 km becoming deeper to the se and matching the observed surface rupture and an along strike continuation to the nw of the same fault plane but between depths of 8 and 13 km and not associated with a surface break the total geodetic moment 4 5 0 1 10 18 nm is more than twice as large as published seismic moments based on the inversion of p waveforms alone but close to the harvard cmt moment 4 7 10 18 nm we use sh waveforms in addition to the p waves used previously to determine an alternative seismic source mechanism sh waves constrain the depth to be shallower than solutions based on p waves alone agreeing with the depths from the interferometric inversion and resulting in a larger moment 3 1 0 4 10 18 nm than the previous bodywave estimates 2 2 1 10 18 nm the cmt moment reduces in magnitude to a similar size 3 3 10 18 nm if the centroid depth and fault dip are constrained to the values determined from bodywave modelling and interferometry thus the geodetic moment is 40 bigger than the moment determined from seismology

ten year recurrence time between two major earthquakes affecting the same fault segment earthquake ruptures stop when they encounter barriers impeding further propagation these barriers can theoretically originate from changes of geometry or nature of the seismic faults or from a strong lowering of the tectonic stresses typically due to the occurrence of a recent major earthquake we show here that this latter mechanism can be ineffective at stopping rupture expansion the 17 november 2013 magnitude 7 8 scotia sea earthquake has propagated into a 100 km long zone already ruptured 10 years ago by a magnitude 7 6 earthquake given the plate velocities between scotia and antarctic plates 8 9 mm yr simple recurrence models would have predicted that the segment affected by the 2003 earthquake could not be reruptured by a major earthquake during several hundreds of years this earthquake pair indicates that the variations of the tectonic stress during the seismic history of the fault are small compared to the stresses dynamically generated by a large earthquake

active tectonic studies in the united states 1987 1990 the techniques and instrumentation used in active tectonic studies are discussed and recent results are reviewed it is suggested that a critical mass of data on several particular regions has been accumulated making possible critical debates and attempts to assess earthquake hazards particular attention is given to studies of the pacific northwest region basin and range deformation studies and distributed deformation and hidden earthquake sources also included is a comprehensive bibliography for the period

comparison of centroid moment tensor and first motion solutions for western mediterranean earthquakes abstract comparison of centroid moment tensor and first motion solutions shows that the two different methods give compatible solutions for moderate sized earthquakes in the western mediterranean while the centroid moment tensor solution provides a good indication of the general style of deformation occurring in an area careful consideration of the first motion solution is still necessary for detailed study of source mechanics there is no obvious character of the centroid moment tensor solution such as small values of moment or large non double couple contribution that can be used as diagnostic of a significantly different first motion solution

deformations associated with the el asnam earthquake of 10 october 1980 geodetic determination of vertical and horizontal movements the resurvey of both a geodetic network and a leveling net was carried out in june 1981 8 months after the ms 7 3 el asnam earthquake of 10 october 1980 previous seismological and neotectonic studies ouyed et al 1981 yielding et al 1981 indicate that this event results from a northeast trending overthrust complex fault of about 40 km length which shows at least three principal segments with slightly different directions vertical movements evaluated by means of a trigonometric leveling method show an uplift of the northwestern side of the thrust fault of about 5 m and a depression of the southeastern edge of about 1 m these movements are progressively attenuated away from the fault trace horizontal movements have been evaluated by a classical first order triangulation method the resulting mean strain tensors calculated for different triangles of the geodetic network indicate a shortening of about 2 50 m which is consistent with the se nw direction of compression determined from neotectonic evaluations and focal mechanisms dislocation models are used to explain and discuss the observed deformations in light of the seismological data and observed ground breakages five segments are required to explain both horizontal and vertical deformations the magnitude of vertical displacements about 6 m at the junction between the southwest and the central segments of the fault argues for the breaking of this area during the main shock and for a slip vector of about 8 m at least in the central segment

source mechanism of the bartin earthquake of september 3 1968 in northwestern turkey evidence for active thrust faulting at the southern black sea margin abstract the bartin earthquake of september 3 1968 is the strongest instrumentally recorded earthquake to occur along the black sea margin in northwestern turkey the source mechanism of the bartin earthquake as determined in this study from long period p wave first motions and from the inversion of p and sh waves indicates thrust faulting with a strike of 28 dip of 38 and rake of 80 the average centroid depth is 4 km and the seismic moment is 3 9 10 25 dyn cm the bartin earthquake provides the first evidence for active thrust faulting at the southern margin of the black sea the black sea has probably evolved as a back arc basin resulting from the northward subduction of tethys seismicity fault plane solutions and other geophysical data suggest that at the present time the black sea is closing under north south compression

active thrust front of the greater caucasus the april 29 1991 racha earthquake sequence and its tectonic implications although fault bounded thrust sheest are common in the geological record seismic evidence for their motion is sparse the april 29 1991 racha earthquake m s 7 0 the largest instrumentally recorded earthquake in the greater caucasus is one of the largest recent earthquakes in continental thrust belts and provides evidence on mechanisms of thrust sheet motion using data from a deployment of program for array seismic studies of the continental lithosphere passcal digital seismographs and various other instrumental we locate 1952 aftershocks occurring between may 7 and june 30 1991 the aftershocks form a zone 70 km long and 10 25 km wide striking e w following the racha ridge at the southern boundary of the greater caucasus thrust system teleseismic body waves are inverted for source parameters of the mainshock and the two largest aftershocks the solutions show thrust faulting with centroid depths of 3 10 km comparable to depths of locally recorded aftershocks 2 12 km the shallow dipping nodal plane the aftershock distribution and surface geology demonstrate that the main event was caused by faulting on a thrust system dipping nne at 20 o 31 o bounding the southern slope of the greater caucasus this fault system thrusts the greater caucasus structures south over the dzhirula basement massif the inferred fault geometry suggests that the active fault is either a detachment between sediments and dzhirula basement or cuts through the basement at shallow depths the 1500 m high racha ridge overlies the aftershock zone and is a likely consequence of repeated similar earthquakes hence the 1991 earthquake sequence shows that the western greater caucasus is accommodating plate convergence at a rate possibly comparable to the eastern greater caucasus a few millimeters per year along strike geological discontinuities above and below the thrust surface correspond to the eastern end of the mainshock rupture area no strong evidence for transfer structures could be found along strike suggesting that differences in collisional style between the western and eastern greater caucasus may reflect differences in mechanical properties rather than differences in convergence rate a june 15 1991 event and its aftershocks southeast of the primary aftershock zone along strike show fault planes and slip vectors rotated 41 o clockwise from the mainshock this rotation is consistent with an along strike change in direction of the thrust front near 44 o e longitude and demonstrates strong local structural or topographic control on slip direction the rotation requires along strike shortening within the greater caucasus thrust system at a rate comparable to the rate of thrusting

earthquakes of the holocene geologic studies of earthquakes involve mapping of coseismic surface faulting and secondary deformation from historical events trenching and geomorphic analysis to define the timing and size of past earthquakes and investigations of fault zone structure and geometry in both unconsolidated sediments and bedrock this research is now being referred to as paleoseismology seismic geology and earthquake geology since the mid 1970s it has led to some of the most exciting and important contributions to the understanding of earthquake behavior in space and time hanks 1985 allen 1986 the present report is the first of what is hoped will become a regularly contributed summary of the geologic aspects of the study of earthquakes

an experiment in the retrieval of depth and source mechanism of large earthquakes using very long period rayleigh wave data abstract the possibility of obtaining an estimate of depth for large earthquakes from low frequency mantle wave data alone is investigated using moment tensor formalism after illustrating the sensitivity to depth of the eigenfunctions in the period range of 160 to 350 sec we give several examples of application of the two step inversion procedure proposed earlier for shorter period rayleigh wave data romanowicz 1982 we find that for earthquakes below a depth of about 50 km we are able to resolve depth even when a spherically symmetric average earth model is used for propagation corrections this is particularly interesting in the case of large subduction zone earthquakes for which an estimate of the vertical extent of faulting can thus be obtained independently of aftershock studies in a very fast and simple manner which does not involve any synthetic seismogram calculations or lateral heterogeneity modeling for shallower earthquakes especially those with one steeply dipping nodal plane the resolution of depth is less precise taking lateral heterogeneity into account by using available regionalized phase velocities improves the results marginally we anticipate that accounting more accurately for the low order harmonics in the worldwide phase velocity distribution should prove most helpful since these interfere the most with the theoretical radiation patterns of earthquakes the determination of depth depends more critically on the source process time which has to be accounted for especially for horizontally propagating faults