seismic source study of the racha dzhava georgia earthquake from aftershocks and broad band teleseismic body wave records an example of active nappe tectonics summary the racha dzhava earthquake ms 7 0 that occurred on 1991 april 29 at o9 12 48 1 gmt in the southern border of the great caucasus is the biggest event ever recorded in the region stronger than the spitak earthquake ms 6 9 of 1988 a field expedition to the epicentral area was organised and a temporary seismic network of 37 stations was deployed to record the aftershock activity a very precise image of the aftershock distribution is obtained showing an elongated cloud oriented n105 with one branch trending n310 in the western part the southernmost part extends over 80 km with the depth ranging from o to 15 km and dips north the northern branch which is about 30 km long shows activity that ranges in depth from 5 to 15 km the complex thrust dips northwards a stress tensor inversion from p wave first motion polarities shows a state of triaxial compression with the major principal axis oriented roughly n s the minor principal axis being vertical body waveform inversion of teleseismic seismograms was performed for the main shock which can be divided into four subevents with a total rupture time duration of 22s the most important part of the seismic moment was released by a gentle northerly dipping thrust the model is consistent with the compressive tectonics of the region and is in agreement with the aftershock distribution and the stress tensor deduced from the aftershocks the focal mechanisms of the three largest aftershocks were also inverted from body wave records the april 29th ms 6 1 and may 5th ms 5 4 aftershocks have thrust mechanisms on roughly e w oriented planes similar to the main shock surprisingly the june 15th ms 6 2 aftershock shows a thrust fault striking n s this mechanism is explai ned by the structural control of the rupture along the eastdipping geometry of the dzirula massif close to the borzhomi kazbeg strike slip fault in fact the orientation and shape of the stress tensor produce a thrust on a n s oriented plane nappe tectonics has been identified as an important feature in the caucasus and the source mechanism is consistent with this observation a hidden fault is present below the nappe and no large surface breaks were observed due to the main shock the epicentral region is characterized by sediments that are trapped between two crystalline basements the dzirula massif which crops out south of chiatoura and the caucasus main range north of oni most if not all of the rupture is controlled by the thrusting of overlapping deformed and folded sediments over the dzirula massif this event is another example of blind active faults with the distinctive feature that the fault plane dips at a gentle angle the racha range is one of the surface expressions of this blind thrust and its growtb is the consequence and evidence of similar earthquakes in the past ii 5

westward propagation of latitudinal asymmetry in a coupled ocean atmosphere model abstract hemispheric asymmetries of continental geometry have long been speculated to be the cause of the northern hemisphere position of the intertropical convergence zone over the central and eastern pacific it is unknown however how the effects of continental asymmetries are transmitted to and felt by the central pacific thousands of kilometers away this paper proposes a transmitter mechanism by investigating the response of a coupled ocean atmospheric model to a symmetry breaking force by the american continents the model treats land forcing implicitly as an eastern boundary condition in the absence of oceanic feedback the model response to the eastern boundary forcing is tightly trapped and confined to a small longitudinal extent off the coast whereas the climate over the interior ocean is symmetric about the equator ocean atmosphere coupling greatly enhances the transmissibility of the effects of the land forcing establishing large latitudinal asymmetry over a great zonal extent a westwar

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

the annual cycle in equatorial convection and sea surface temperature abstract the coupled atmosphere ocean system in the equatorial eastern pacific and atlantic exhibits a distinct annual cycle that is reflected in contrasting conditions at the times of the two equinoxes the contrasts are so strong that they dominate the annual march of zonally averaged outgoing longwave radiation for the equatorial belt the march equinox corresponds to the warm season when the equatorial cold tongues in the eastern pacific and atlantic are absent with the onset of summer monsoon convection over colombia central america and west africa in may june northward surface winds strengthen over the eastern pacific and atlantic the equatorial cold tongues reappear and the marine convection shifts from the equatorial belt to the intertropical convergence zones itczs along 8 n as the northern summer program the itczs remain strong and shift northward to new 10 n while sea surface temperature sst continues to drop over the cold tongues and the southern tropics perhaps in response to the

origin and emplacement of mariana forearc seamounts large seamounts occur on the outer half of the mariana forearc they represent a new class of seamounts consisting of horsts and diapirs of metamorphosed forearc material the degree of metamorphism in this material depends on the amount of water available and the pressure temperature regime of the forearc wedge the major source for the water involved in the metamorphism is most likely the descending slab theoretical models for thermal regimes in convergent margins suggest that the lower grade metamorphic facies will be restricted to the outermost part of a forearc zeolite and chlorite facies rocks predominate in dredge hauls from horsts on the outer 50 km of the mariana forearc thermal models indicate that higher grade greenschist facies should occur farther from the trench seamounts that were probably formed in response to diapiric emplacement of serpentinite predominate from 50 to 120 km from the trench uplift of the horsts and emplacement of the serpentinite diapirs were probably facilitated by vertical tectonic movement in response to subduction of plate seamounts and by fracturing of the mariana forearc

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

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

response of the eastern tropical pacific to meridional migration of the itcz the generation of the costa rica dome abstract a regional ocean circulation model with fine horizontal resolution has been developed in order to obtain a coherent seasonal picture of the eastern tropical pacific off central america the costa rice dome with a cyclonic circulation grows rapidly in late spring off the gulf of papagayo and matures in summer and early fall in accord with the northward migration of the itcz in winter strong northern converging in the southernmost itcz from three passes in central america excite warm anticyclones nonlinear eddies confined in the upper layer those anticyclones are identified as the intermediate geostrophic eddies by matsuura and yamagata the costa rica dome is eroded in winter and early spring by the westward propagating warm anticyclones but at the same time a new embryo of the dome begins to evolve from the westward propagating cold cyclonic eddy excited off the gulf of papagayo by the northers the costa rica dome can be maintained by the winds with the cyclonic wind stress curl in summer on

shear wave splitting and anisotropy from the aftershocks of the nahanni northwest territories earthquakes two large earthquakes ms 6 6 6 9 occurred in 1985 near the north nahanni river in the northwest territories canada the first on october 5 and the second on december 23 the earthquakes were followed by long aftershock sequences aftershocks between m 1 0 and 3 9 were recorded by five to eight three component stations during three field experiments in october 1985 and in january and september 1986 two distinct shear waves were generally observed in september 1986 the plane of polarization of the first arriving shear wave was well defined and averaged 156 8 measured clockwise from north the stress direction was inferred from the initial shear wave polarizations recorded at one station from a widespread aftershock distribution but was limited to no more than 10 km around it the delay between first and second arriving shear waves was up to 190 ms assuming that the two shear waves result from anisotropy and that the entire ray paths varying in length from 7 to 23 km contributed the maximum velocity anisotropy is 7 however the distribution of the anisotropy is not uniform the pressure axes determined from fault plane solutions for the two main shocks are nearly e w normal to the compressive direction found in the september 1986 field survey from shear wave splitting data four out of seven of the aftershocks for which focal mechanism orientations could be determined showed pressure axis orientations that agreed with the stress direction inferred from the initial shear wave polarizations from the field survey

azimuthal anomalies of short period p wave arrivals from nahanni aftershocks northwest territories canada and effects of surface topography abstract following two large earthquakes in the northwest territories of canada in 1985 and one in 1988 four field surveys were undertaken to study the extensive aftershock activity some of the seismographs employed recorded three component digital data which allowed the directions of approach of short period p wave arrivals to be analyzed at three stations observed azimuthal deviations reached as much as 40 from theoretically expected azimuths as computed from the inferred hypocenters for the events theoretical results are presented that support the hypothesis that the observed azimuthal deviations are caused principally by local topographic slopes in the vicinity of the recording instruments similar theoretical results indicate that effects of local surface topography on arrival azimuths of sh and sv waves are nearly an order of magnitude less than for p waves so that such effects will generally be unimportant in shear wave splitting studies

dependency of near field ground motions on the structural maturity of the ruptured faults little work has been undertaken to examine the role of specific long term fault properties on earthquake ground motions here we empirically examine the influence of the structural maturity of faults on the strong ground motions generated by the rupture of these faults and we compare the influence of fault maturity to that of other source properties slip mode and blind versus surface rupturing we analyze the near field ground motions recorded at rock sites for 28 large mw 5 6 7 8 crustal earthquakes of various slip modes the structural maturity of the faults broken by those earthquakes is classified into three classes mature intermediate and immature based on the combined knowledge of the age slip rate cumulative slip and length of the faults we compare the recorded ground motions to the empirical prediction equation of boore et al 1997 at all frequencies earthquakes on immature faults produce ground motions 1 5 times larger than those generated by earthquakes on mature faults the fault maturity appears to be associated with larger differences in ground motion amplitude than the style of faulting factor of 1 35 between reverse and strike slip earthquakes and the surface rupture occurrence factor of 1 2 between blind and surface rupturing earthquakes however the slip mode and the fault maturity are dependent parameters and we suggest that the effect of slip mode may only be apparent actually resulting from the maturity control we conclude that the structural maturity of faults is an important parameter that should be considered in seismic hazard assessment

deformation rates estimated from earthquakes in the northern cordillera of canada and eastern alaska 1 we estimate deformation rates from earthquake catalog statistics to investigate current strain distribution resulting from the collision of the yakutat block with north america the collision is accommodated over a broad region of alaska and nw canada in a variety of tectonic regimes our estimates have significant uncertainties but the results are generally in agreement with other deformation rate estimates gps and geological data and provide useful first order constraints on local long term tectonic deformation and seismic hazard we estimate that 50 75 of relative plate motion is taken up by earthquakes on the yakutat north america boundaries with lateral crustal extrusion other far field strain and possibly aseismic creep accommodating the remainder we calculate dextral strike slip of 4 5 10 2 mm a on the denali fault system in alaska remarkably similar to late pleistocene holocene rates from geomorphic offsets right lateral motion essentially bypasses the denali fault in easternmost alaska and westernmost yukon passing instead along the totschunda and duke river fault systems right lateral transpression of 5 10 mm a suggests that the alaskan fore arc does not have fully block like behavior between the denali and tintina kaltag faults in alaska 10 20 mm a of dextral shear is accommodated by several ne trending bookshelf faults significant seismic deformation in cases higher than expected from gps data is indicated in the ne cordillera where preexisting faults have a strong control on the pattern and mechanisms of current deformation

contribution to the circulation climatology of the eastern equatorial pacific lower atmospheric jets novel data sets have been evaluated to reappraise the annual cycle of atmospheric circulation over the pacific in the eastern pacific cross equatorial surface winds from the southern hemisphere generate an oceanic cold tongue immediately to the south of the equator the associated lower tropospheric thickness pattern entails a minimum of absolute topography in the midtroposphere near the equator and together with the small coriolis parameter this causes a jet stream of up to 10 ms 1 at 700 600 mbar centered at 0 2 n the jet is most intense in late boreal winter because then the near equatorial surface low pressure trough is flat the jet is fed by lower tropospheric convergence over equatorial south america and by upper tropospheric convergence and subsidence over a narrow equatorial zone of the eastern pacific over the central pacific a jet of up to 10 ms 1 is found at 10 12 n and 925 850 mbar while friction brakes the flow in the layer of steepest meridional pressure gradient near the surface this lower tropospheric jet as the surface pressure gradient is also strongest in late boreal winter overall the ncep analyses resolve the near surface circulation less well than the long term ship observations however the midtropospheric equatorial jet over the eastern pacific is also borne out by the 1967 1971 radiosoundings over galapagos and the lower tropospheric trade wind jet over the central pacific was also identified from the intense radiosonde program of the march april 1967 line islands experiment

seismicity in the vicinity of the snorcle corridors of the northern canadian cordillera the slave northern cordillera lithospheric evolution snorcle corridors of the northern cordillera sample some of the most and least seismically active regions of canada the earthquake history of this region is short precise determination of earthquake locations and depths is not possible even today nonetheless significant gains in our knowledge of the seismicity of this region have been made in recent years from studies of historic earth quakes microseismicity studies and advances in waveform modelling techniques combined with broadband data that allow for determination of focal mechanisms and depths for moderate earthquakes this article summarizes our current knowledge of the seismicity and seismic hazards across the region these detailed analyses have shown that i the largest historical earthquakes have occurred in regions of ongoing microseismicity ii the largest earthquakes have occurred in pairs or in swarms suggesting that stress triggering is important in this region iii the active faults are concentrated in the offshore region iv there is a concentration of seismicity in the fold and thrust belt several hun dred kilometres from the active plate margin and v there is no seismicity associated with the quaternary volcanic zone in northern british columbia potentially damaging magnitude m 5 earthquakes can be expected every few years in the vicinity of the northern cordillera the mw 7 9 denali alaska earthquake where mw is the moment magnitude was a good reminder that the effects of a large earthquake can be substantial even hundreds of kilometres from the epicentre detailed studies of seismicity earth structure and tectonics with the latter made possible in large part by the snorcle transect will allow for informed decision making for resource development and the design of safe structures and infrastructure in the northern canadian cordillera resume les corridors du projet snorcle de la cordillere septentrionale permettent d echantillonner quelques unes des regions sismiques les plus et les moins actives du canada l histoire des seismes de cette region est breve meme

seismicity and geometry of a 110 km long blind thrust fault 1 the 1985 kettleman hills california earthquake the august 4 1985 kettleman hills earthquake was the third in a sequence of moderate shocks to occur beneath the northern half of a 110 km long fold chain bounding the eastern california coast ranges the 1982 mw 5 4 new idria 1983 mw 6 5 coalinga and 1985 mw 6 1 kettleman hills events define a southward progression of seismic activity beneath the fold we use teleseismic waveforms geodetic modeling hypocenters relocated in a three dimensional velocity model and subsurface structural data to investigate the kettleman hills earthquake the main shock results from motion on a shallowly dipping thrust fault buried at 10 km depth aftershocks and coseismic fault slip extend 20 km along the fold axis nearly the full extent of the kettleman hills north dome anticline aftershocks occur primarily several kilometers in front of the fault tip and in the core of the anticline the main shock and several foreshocks occurred at a 2 km right step in the quaternary fold axis which also corresponds to the southern end of the 1983 coalinga and northern end of the 1985 kettleman hills aftershock zones from this we infer that the step in the fold is caused by an offset or tear in the underlying fault the scalar seismic moment is 1 6 1018 n m consistent with the geodetic deformation and the duration of rupture is 16 s 3 4 times greater than for the average earthquake with this scalar moment the slow rate of moment release provides an explanation for the low level of ground shaking and low local magnitude reported for the event the peak of the geodetic uplift is located 5 km perpendicular to the quaternary fold axis we argue that the fault is propagating northeast into the undeformed san joaquin valley sediments and that the overlying fold is growing at about 0 5 mm yr

architecture of the crust and uppermost mantle in the northern canadian cordillera from receiver functions the northern canadian cordillera ncc is an active orogenic belt in northwestern canada characterized by deformed autochtonous and allochtonous structures that were emplaced in successive episodes of convergence since the late cretaceous seismicity and crustal deformation are concentrated along corridors located far 200 to 800 km from the convergent plate margin proposed geodynamic models require information on crust and mantle structure and strain history which are poorly constrained we calculate receiver functions using 66 broadband seismic stations within and around the ncc and process them to estimate moho depth and p to s velocity ratio vp vs of the cordilleran crust we also perform a harmonic decomposition to determine the anisotropy of the subsurface layers from these results we construct simple seismic velocity models at selected stations and simulate receiver function data to constrain crust and uppermost mantle structure and anisotropy our results indicate a relatively flat and sharp moho at 32 2 km depth and crustal vp vs of 1 75 0 05 seismic anisotropy is pervasive in the upper crust and within a thin 10 15 km thick sub moho layer the modeled plunging slow axis of hexagonal symmetry of the upper crustal anisotropic layer may reflect the presence of fractures or mica rich mylonites the subhorizontal fast axis of hexagonal anisotropy within the sub moho layer is generally consistent with the se nw orientation of large scale tectonic structures these results allow us to revise the geodynamic models proposed to explain active deformation within the ncc

deep slab fluids fuel extremophilic archaea on a mariana forearc serpentinite mud volcano ocean drilling program leg 195 1 as the pacific plate subducts beneath the mariana forearc it releases water that hydrates the overlying mantle wedge converting it to serpentinite that protrudes to form mud volcanoes at the seafloor excess h2o ascends through these mud volcanoes and exits as cold springs at their summits the composition of this deep slab derived water has been determined by drilling on two of these seamounts it has a ph of 12 5 and relative to seawater is enriched in sulfate alkalinity na cl k rb b light hydrocarbons ammonia 18o and deuterium and depleted in chloride mg ca sr li si phosphate and 87sr within the upper 20 m below seafloor at south chamorro seamount a microbial community operating at ph 12 5 made up overwhelmingly of archaea is oxidizing methane from the ascending fluid to carbonate ion and organic carbon while reducing sulfate to bisulfide and probably dissolved nitrogen to ammonia

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

seismicity and geometry of a 110 km long blind thrust fault 2 synthesis of the 1982 1985 california earthquake sequence during 1982 1985 three 5 4 mw 6 5 earthquakes migrated 65 km along the north half of a string of quaternary folds at the east front of the california coast ranges there is no surface fault associated with these earthquakes but the fold chain oriented parallel to the san andreas fault 30 km to the west takes up contraction normal to the san andreas fault identification of active blind thrust faults is hindered by the absence of a fault trace but may be revealed by the presence of growing folds at the surface and by earthquakes at depth because of excellent seismic geodetic and geologic data this earthquake sequence illuminates the geometry of the blind faults and enables us to probe their mechanics we relocate seismicity and compute focal mechanisms in a three dimensional velocity space and model the vertical deformation associated with the 1983 m 6 5 coalinga earthquake the aftershock zones abut at echelon offsets in the fold axes and the mainshocks display reverse slip perpendicular to the axes suggesting that the folds conceal a contiguous segmented thrust fault background seismicity concentrates at bends and breaks in the fold chain sites that may correspond to tears and ramps in the thrust fault at depth seismic reflection profiles reveal thrust and reverse faults dipping toward the san andreas fault at depths of 5 10 km with several kilometers of cumulative slip and high angle reverse faults in the anticlines with several hundred meters cumulative slip coseismic fold uplift accompanied the coalinga and kettleman hills north dome earthquakes suggesting that fold growth is episodic and coupled closely to repeated earthquakes on the underlying thrusts the north half of the fold chain has been the site of several 6 m 6 5 earthquakes since 1885 thus kettleman hills middle dome the next fold segment to the south has an elevated seismic potential the aftershock zones of the three main shocks are diffuse and occupy a region much larger than the site of seismic slip aftershocks occur 5 7 km from the fault regions where on the basis of a boundary element model the shear strain increase caused by the fault slip exceeds 20 ppm equivalent to about 0 7 mpa we argue that the broad aftershock zone is a product of high sustained off fault stress caused by repeated displacement on faults that do not cut the earth s surface the fault tip stresses lead to the formation of secondary faults which can become sites of aftershocks and postseismic creep

rupture histories of eastern north american earthquakes abstract digital worldwide records of p and sh waves are used to invert for rupture histories of the recent larger eastern north american earthquakes the data include a broad bandwidth to facilitate recovery of both source details and total moment the following events are studied 9 january 1982 new brunswick 5 october and 23 december 1985 nahanni 25 november 1988 saguenay and 25 december 1989 ungava earthquakes we employ a finite fault waveform inversion scheme that discretizes the slip history as a function of position on the fault this formulation allows estimation of rise time and slip velocity in addition to the spatial distribution of slip static stress drops averaged over the entire rupture surface range from a few tens of bars to just over 100 bars and are similar to values estimated for western united states earthquakes estimates of stress drop for spatially limited asperities are in the range of a few hundred bars with the value for the saguenay earthquake asperity being the largest approaching 1 kbar the variation in stress drop is considerable but no evidence is seen for a scaling relation in which stress drop increases with moment maximum slip velocities on the fault also have a wide range from 50 cm sec to perhaps greater then 200 cm sec with the saguenay and 5 october nahanni earthquakes lying at the upper end of this range of the events studied the saguenay earthquake is unique in terms of its greater depth spatially concentrated source and large asperity stress drop

blueschist metamorphism in an active subduction zone the high pressure low temperature metamorphic rocks known as blueschists have long been considered to form in subduction zones where the descent of a relatively cold slab leads to the occurrence of unusually low temperatures at mantle pressures until now however the link between blueschist facies rocks and subduction zones has been indirect relying on a spatial association of blueschists with old subduction complexes and estimates of the geothermal gradients likely to exist in subduction zones here we strengthen this link by reporting the discovery of blueschist facies minerals lawsonite aragonite sodic pyroxene and blue amphibole in clasts from a serpentinite seamount in the forearc of the active mariana subduction zone the metamorphic conditions estimated from the mineral compositions are 150 250 c and 5 6 kbar 16 20 km depth the rocks must have been entrained in rising serpentine mud diapirs and extruded from mud volcanoes onto the sea floor further study of these rocks may provide new insight into the tectonics of trench forearc systems and in particular the processes by which blueschist facies clasts come to be associated with forearc sediments in ancient subduction complexes

linking serpentinization hyperalkaline mineral waters and abiotic methane production in continental peridotites an integrated hydrogeological bio geochemical model from the cabeco de vide ch4 rich aquifer portugal abstract continental active serpentinization of ultramafic rocks is today recognized as a key process triggering a sequence of phenomena involving the passage from inorganic to organic and metabolic reactions these may have a role in the origin of life and may explain the occurrence of abiotic hydrocarbons on earth and other planets production of hyperalkaline waters and abiotic methane ch4 are two critical steps in this sequence they were described independently by specific hydrogeological and geochemical models here we update and combine these models into a unified scheme using and integrating geological hydrogeological hydrogeochemical gas geochemical and microbial analyses acquired from 2002 to 2014 in the cabeco de vide cdv study site portugal the hyperalkaline ph 10 5 na cl ca oh mineral water of cdv evolve from groundwater peridotite interaction serpentinization generating hydrogen h2 which according to multiple theoretical laboratory and field evidence likely reacted with co2 within metal catalyst rich rocks abiotically producing ch4 up to 1 2 mg l 24 4 oo

surface displacements in the 1906 san francisco and 1989 loma prieta earthquakes the horizontal displacements accompanying the 1906 san francisco earthquake and the 1989 loma prieta earthquake are computed from geodetic survey measurements the 1906 earthquake displacement field is entirely consistent with right lateral strike slip on the san andreas fault in contrast the 1989 loma prieta earthquake exhibited subequal components of strike slip and reverse faulting this result together with other seismic and geologic data may indicate that the two earthquakes occurred on two different fault planes

teleseismic estimates of the energy radiated by shallow earthquakes the energy flux contained in the p wave groups p pp sp or the s wave groups s ps ss radiated by a shallow earthquake is modeled assuming that the energy flux in the direct and depth phases adds incoherently by defining generalized radiation patterns which incorporate this neutral interference the wave groups are analyzed as though they were comprised of a single phase measurements of the energy flux in the wave groups are corrected in the frequency domain for both the body wave attenuation and the frequency band of the recording the corrected measurements are then used to estimate the seismic energy radiated by the earthquake this analysis is applied to digital recordings of the teleseismic wave groups radiated by the may 2 1983 coalinga california earthquake and the october 28 1983 borah peak idaho earthquake for the coalinga earthquake an estimate of es 1 6 0 4 1021 dyn cm was determined from six p wave groups while the sh wave group recorded at station col returned an estimate of es 1 2 1021 dyn cm for the borah peak earthquake an estimate ofes 3 2 0 5 1021 dyn cm was determined from seven p wave groups the distribution of the isoseismals for the borah peak earthquake indicate that the energy radiated by this event was focussed to the northwest in the direction of rupture propagation correcting the teleseismic estimate for this focussing gives es 3 6 0 5 1021 dyn cm combining these estimates of the radiated energy with broad band estimates of the seismic moment yields estimates of the apparent stress of 17 and 7 bars for the coalinga and the borah peak earthquakes respectively

the early earth geochemical and isotope record of serpentinizing environments from archean stichtite mg6cr2 oh 16 co3 4h2o abstract a record of the conditions and duration of archean serpentinization is preserved as chemical isotopic and textural properties of the magnesium chromium hydroxycarbonate mineral stichtite this provides a window into the conditions associated with a potentially habitable environment on early earth and mars stichtite is associated with archean chromite rich serpentinite rocks of south africa western australia and southern india stichtite compositions range within the cr fe al hydrotalcite group with trends unique to each discrete serpentinite host body elemental and textural analysis of stichtite associated chromite indicates that stichtite forms in fore arc layered intrusive boninite setting rocks through interaction of chromite and methane rich serpentinizing fluids the degree to which chromite is replaced by stichtite is inferred to correlate with the length of time that the host rocks spent within the stichtite window carbon stable isotope values of archean stichtite suggest carbon sourcing from marine kerogen with minor marine dissolved inorganic carbonate in most samples as opposed to the strongly kerogen dominated phanerozoic stichtite the carbon and hydrogen stable isotope profile of archean stichtite ranges from the methane fields of active serpentinizing zones to geothermal with trends suggestive of methane oxidation rather than the substrate depletion noted in phanerozoic stichtite thus isotope values of stichtite appear to record oceanic oxygenation levels the association of the stichtite 2h polytype nee barbertonite with aragonite antigorite suggests this is a higher pressure temperature polytype of stichtite reaction textures isotopic values and qualitative mineral thermobarometric indicators indicate that stichtite forms during serpentinization in a methane h2 rich environment within fluid conduits ranging from near the surface to depths where pressure is up to 2 8 gpa and temperature is up to 300 c

omak rock and the 1872 pacific northwest earthquake abstract omak rock is an about 40 ton granite boulder balanced on a narrow support the rock is located in northern washington state in the general vicinity of the 1872 pacific northwest m 7 earthquake the survival of the balanced rock has in the past been used to place limits on peak acceleration and intensity at its location and thus possibly on the epicenter of the earthquake using dynamical analysis i find that the rock could have survived far greater peak ground motion than previously concluded and might well have survived within a few kilometers of the epicenter thus invalidating the earlier conclusions

active faulting and deformation of the coalinga anticline as interpreted from three dimensional velocity structure and seismicity this work gives a clear picture of the geometry of aftershock seismicity in a large thrust earthquake interpretation of hypocenters and fault plane solutions from the 1983 coalinga coast range california earthquake sequence in combination with the three dimensional velocity structure shows that the active faulting beneath the fold primarily consists of a set of southwest dipping thrusts uplifting blocks of higher velocity material above the main listric blind thrust there is a conjugate fault steeply northeast dipping that provides the western limit of the aftershocks within the coalinga anticline and that corresponds in location and spatial extent with the adjacent pleasant valley syncline the character of the seismicity varies with the degree of previous deformation on each section of the anticline where the previous uplift was largest the shallow seismicity shows secondary faulting on either side of the fold with orientations that correspond to the preexisting geologic structure diffuse seismicity characterizes the area with the least previous deformation the mainshock rupture terminated where the fold trend was no longer uniform but had competing north and west trending features the upward extent of the mainshock rupture ended at the approximate boundary between franciscan and great valley sequence rocks above that depth the main thrust appears to splay into a steeper segment and a near horizontal segment thus the extent of rupture area is limited by the area of uniform structural orientation and by the variation in the type of material with the three dimensional velocity model each individual hypocenter moved slightly 0 2 km in accord with the details of the surrounding velocity structure so that secondary features in the seismicity pattern are more detailed than with a local one dimensional model and station corrections the overall character of the fault plane solutions was not altered by the three dimensional model but the more accurate ray paths did result in distinct changes in particular the mainshock has a fault plane dipping 30 southwest instead of the 23 obtained with the one dimensional model

serpentinization carbon and deep life the aqueous alteration of ultramafic rocks through serpentinization liberates mantle carbon and reducing power serpentinization occurs in numerous settings on present day earth including subduction zones mid ocean ridges and ophiolites and has extended far into earth s history potentially contributing to the origins and early evolution of life serpentinization can provide the energy and raw materials to support chemosynthetic microbial communities that may penetrate deep into earth s subsurface microorganisms may also influence the composition and quantity of carbon bearing compounds in the deep subsurface however conditions created by serpentinization challenge the known limits of microbial physiology in terms of extreme ph access to electron acceptors and availability of nutrients furthermore the downward transport of surface carbon and subsequent mixing with calcium rich fluids at high ph contributes to the precipitation and immobilization of carbonate minerals the following chapter will explore the physiological challenges presented by the serpentinite environment data from studies of serpentinite hosted microbial ecosystems and areas in need of further investigation physical and chemical consequences of serpentinization serpentinization is an alteration process of low silica ultramafic rocks characteristic of the lower oceanic crust and upper mantle these rocks are rich in the minerals olivine and pyroxene water rock reactions result in the oxidation of ferrous iron from olivine and pyroxene resulting in the precipitation of ferric iron in magnetite fe3o4 and other minerals and in the release of diatomic hydrogen h2 at low temperatures 150 c the reaction results in extremely high ph commonly above 10 the combination of h2 and co2 or co under highly reducing conditions leads to formation of methane and other hydrocarbons through fischer tropsch type ftt synthesis mccollom and seewald 2001 charlou et al 2002 proskurowski et al 2008 mccollom 2013 serpentinization also results in volume changes in the altered materials making serpentinites less dense than

source parameters of the 29 may and 5 june 1940 richardson mountains yukon territory earthquakes the 29 may and 5 june 1940 richardson mountains earthquakes are the two largest events ms 6 2 and 6 5 respectively known to have occurred in the northern yukon territory near the boundary of the canadian cordillera and the stable continental craton the earthquakes were located 20 km apart at 66 87 n 135 33 w and 66 85 n 135 80 w respectively for the may event mb 6 3 ms 6 2 and m 3 5 1025 dyne cm and for the june event mb 6 4 ms 6 5 and m 7 5 1025 dyne cm modeling of teleseismic and regional body waves for these earthquakes indicates a complex rupture sequence for each consisting of two to three subevents focal depths ranged from 14 5 to 11 km for the may rupture sequence and 10 to 7 km for the june sequence the focal mechanisms determined from modeling body waves are primarily strike slip along either n s or e w trending planes the former is preferred based on mapped surface faults in this region and the relative locations determined for the subevents the mechanisms are similar to those of other earthquakes in the richardson mountains but different from the pure thrust events that are observed in the mackenzie mountains 200 km to the southwest the alignment of the near horizontal pressure axes for the richardson mountains earthquakes suggests reactivation of pre existing zones of weakness within the crust by the contemporary stress field stress drops for these earthquakes 22 to 28 bars are low for intraplate events but similar to those estimated for other earthquakes in the eastern canadian cordillera

oldest dated earthquake in yukon territory canada a strong earthquake was recorded by hudson s bay company trader explorer robert campbell on december 27 1850 at fort selkirk yukon territory a search of records from the region failed to find an

the 1954 and 1980 algerian earthquakes implications for the characteristic displacement model of fault behavior joint epicenter determination of earthquakes that occurred in northern algeria near ech cheliff named orleansville in 1954 and el asnam in 1980 shows that the earthquake of 9 september 1954 m 6 5 occurred at nearly the same location as the earthquake of 10 october 1980 m 7 3 the 1954 main shock and earliest aftershocks were concentrated close to the boundaries of segment b nomenclature of deschamps et al 1982 king and yielding 1984 of the 1980 fault system which was to experience approximately 8 m of slip in the 1980 earthquake later aftershocks of the 1954 earthquake were spread over a broad area notably in a region north of the 1980 fault system that also experienced many aftershocks to the 1980 earthquake the closeness of the 1954 main shock and earliest aftershocks to the 1980 segment b implies that the 1954 earthquake involved either 1 rupture of segment b proper or 2 rupture of a distinct fault in the hanging wall or footwall block of segment b the first interpretation cannot be accommodated in the characteristic displacement model of fault behavior but instead requires a model such as the time predictable model which would permit slip in 1954 to have released only a fraction of the elastic strain that was stored on segment b and which would later be released in 1980 the second interpretation specifically with rupture in the hanging wall block of segment b accounts more directly than the first interpretation for differences in the focal mechanisms and ground deformations associated with the two main shocks this interpretation is therefore preferred the second interpretation is consistent with a characteristic displacement model of the 1980 fault system no matter how it is modeled the algerian episode provides an example of an earthquake 1954 that was large enough to perhaps be judged the culminating event of the seismic cycle in a regionally important fault zone but that was followed instead by an even larger shock 1980 the space time characteristics of the episode are unusual enough that they cannot represent the most likely mode of strain release on reverse fault systems in general but the episode should be viewed as one possible scenario for the evolution of seismicity on other such fault systems

teleseismic and near field analysis of the nahanni earthquakes in the northwest territories canada the analysis of the nahanni earthquakes of october 5 1985 ms 6 6 and december 23 1985 ms 6 9 will have important implications for the assessment of seismic hazards in intraplate environments to maximize the information available to seismic engineers broadband data recorded teleseismically are analyzed jointly with strong motion data recorded in the near field the time domain analysis of teleseismic data yields the source mechanisms depths and complexities of rupture of each earthquake both earthquakes occurred as shallow thrusts with centroid depths 6 to 7 km and shallowly dipping fault planes that correspond well with the aftershock distributions obtained from a local survey run by the canadian geological survey the shallow nodal plane for the october 5 earthquake dips 30 to the wsw while the shallow nodal planes of the subevents for the december 23 earthquake dip an average of 23 to the wsw the october 5 earthquake has an impulsive initial rupture followed by a weak subevent of longer duration but smaller moment release the december 23 earthquake exhibits more complexity being comprised of three subevents of similar size the subevent delays derived from the teleseismic analysis are used to help interpret arrivals in records of ground velocity recorded in the near field of the december 23 earthquake the rupture geometries inferred from the joint near and far field analysis suggest that the rupture processes were unusually complicated and that the 2 g peak that occurs late in one of the near field records could be a localized phenomenon spectral analyses of the teleseismic p waves yield the following source parameters for the october 5 and december 23 earthquakes respectively the seismic moments are 1 2 and 1 8 1026 dyne cm the radiated energies are 1 8 and 2 8 1021 dyne cm and the dynamic stress drops are 65 and 50 bar the acceleration source spectra of both earthquakes exhibit an intermediate slope u from 0 03 to 0 3 hz suggesting that the earthquakes represent the failure of asperities extrapolating the teleseismic p wave spectra to estimate the near field s wave spectra yields good fits to the acceleration spectra from two strong motion records but underestimates the spectra from a third strong motion record with the strongest but possibly localized accelerations

chemical and isotopic analyses of hydrocarbon bearing fluid inclusions in olivine rich rocks we examined the mineralogical chemical and isotopic compositions of secondary fluid inclusions in olivine rich rocks from two active serpentinization systems the von damm hydrothermal field mid

earthquake source spectra in eastern north america source spectra for earthquakes of 3 m 7 in eastern north america ena are determined from regional seismograph recordings strong motion records teleseismic data and modified mercalli intensity data the inclusion of mmi data is made possible by the finding that the felt area of an earthquake is an accurate measure of its high frequency spectral level at the source current ground motion relations for ena are based on a brune source model with a stress parameter of 100 bars this model is a poor fit to ena source spectra for events of m 4 at high frequencies 5 to 10 hz observed spectral amplitudes are more nearly matched by a brune stress parameter of 200 bars intermediate frequency amplitudes near 1 hz are matched by a stress parameter of approximately 50 bars an empirical two corner source model is proposed to describe ena source spectra for events of 4 m 7

emplacement growth and gravitational deformation of serpentinite seamounts on the mariana forearc summary serpentinite seamounts representing some of the first material outputs of the recycling process that takes place in subduction zones are found on the outer mariana forearc multichannel seismic mcs and bathymetric data collected in 2002 image the large scale structures of five seamounts as well as the pre seamount basement geometry and sediment stratigraphy we present data from three edifices that provide insights into seamount growth and internal deformation processes and allow us to support the interpretation that serpentinite mud volcanoes are formed by the episodic eruption of mud flows from a central region the presence of thrust faulting at the base of turquoise and big blue seamounts along with the low surface slopes 5 18 of all the seamounts studied lead us to infer that these edifices spread laterally and are subject to gravitational deformation as they grow numerical simulations using the discrete element method dem were used to model their growth and the origins of features that we see in mcs sections such as basal thrusts inward dipping reflections and mid flank benches the dem simulations successfully reproduced many of the observed features simulations employing very low basal and internal friction coefficients 0 1 and 0 4 respectively provide the best match to the overall morphology and structures of the serpentinite seamounts however the simulations do not capture all of the processes involved in seamount growth such as withdrawal of material from a central conduit leading to summit deflation compaction dewatering and degassing of mud flows mass wasting in the form of sector collapse and growth upon a dipping substrate a strong reflection beneath the summit of big blue the largest serpentinite seamount on the mariana forearc represents the floor of a summit depression that has been partially in filled by younger muds supporting the idea that serpentinite seamounts grow by episodic mud volcanism boundaries of mud flow units are visible in bathymetric data and as normal polarity subhorizontal reflections on seismic profiles big blue seamount displays complex nesting relationships as it merges with other seamounts to form a large composite edifice flank flows of serpentinite muds on big blue and celestial seamounts downlap pre existing forearc substrate the interface between serpentinite seamounts and the underlying forearc sediments is represented by a reverse polarity reflection beneath big blue and celestial seamounts suggesting that the substrate is undercompacted overpressured and may be a zone of fluid migration dem simulations suggest that this boundary represents a distinct decollement along which the seamounts slide laterally in contrast turquoise seamount grows laterally not by stable sliding along the top of forearc sediments but by incorporating them into large basal thrusts

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

lg and rg waves on the california regional networks from the december 23 1985 nahanni earthquake we investigate lg and rg propagation in california using the central and southern california regional networks approximately 550 stations constitute these two short period networks providing a dense coverage of almost the entire state the waveforms recorded from the december 23 1985 nahanni canada earthquake are used to construct three profiles along the propagation path almost n s and three perpendicular to the propagation path almost e w to look at the nature of propagation of these two types of surface waves groups of records from stations in various geological and tectonic provinces in california are also examined in order to establish regional characteristics of the surface waves we find that the propagation characteristics of lg differ from those of rg across california lg waves are apparently more sensitive to crustal heterogeneities the most striking observations are the similarity of coda for both the lg and the rg waves within geologic provinces and the marked difference in coda between regions these differences are seen in the amplitudes coda duration shape of the energy envelope frequency content and sharpness of the phase initiation in general a decrease in the moho depth near the pacific coast is correlated with a decrease in the surface wave amplitude especially at higher frequencies 0 15 0 2 hz most interesting is the association of the san andreas fault with abrupt changes in the wave train amplitudes the surface waves are amplified in the vicinity of the fault zone and then decrease in amplitude after the zone is crossed in the coast ranges amplitudes are low and waveform coherence is poor the rg phase dominates the record in the sierra nevada and both surface waves are amplified by the thick sedimentary sequence of the great valley

evaluation of models for earthquake source spectra in eastern north america there have been several relations proposed in the last few years to describe the amplitudes of ground motion in eastern north america ena these relations differ significantly in their assumptions concerning the amplitude and shape of the spectrum of energy radiated from the earthquake source in this article we compare ground motions predicted for these source models against the sparse ena ground motion database the source models evaluated include the two comer models of boatwright and choy 1992 atkinson 1993a haddon 1996 and joyner 1997a b and the one comer model of brune as independently implemented by frankel et al 1996 and by toro et al 1997 the database includes data from ena mainshocks of m 4 and historical ena earthquakes of m 5 5 for a total of 110 records from 11 events of 4 m 7 3 all recorded on rock we also include 24 available rock records from 4 large earthquakes in other intraplate regions con clusions are checked to determine whether they are sensitive to the addition of these non ena data the atkinson source model as implemented in the ground motion relations of atkinson and boore 1995 is the only model that provides unbiased ground motion predictions over the entire period band of interest from 0 1 to 10 sec the source models of frankel et al 1996 toro et al 1997 and joyner 1997a b all provide unbiased ground motion estimates in the period range from 0 1 to 0 5 sec but over estimate motions at periods of 1 to 10 sec the haddon 1996 source model over predicts motions at all periods by factors of 2 to 10 these conclusions do not change significantly if data from non ena intraplate regions are excluded although the ten dency of all models toward overprediction of long period amplitudes becomes more pronounced the tendency of most proposed ena source models to overestimate long period motions is further confirmed by an evaluation of the relationship between ms a measure of the spectrum at 20 sec period and moment magnitude a worldwide catalog of shallow continental earthquakes triep and sykes 1996 is compared to the ms m relations implied by each of the source models the atldnson source model is consistent with these data while other proposed ena models overpredict the average ms for a given m the implications of mmi data from historical earthquakes are also addressed by exploiting the correlation between felt area and high frequency source spectral level high frequency spectral amplitudes as specified by the atkinson and boore 1995 frankel et al 1996 toro et al 1997 and joyner 1997a b source models equal or exceed the levels inferred from the felt areas of most of the large ena events with the noteable exception of the saguenay earthquake by contrast high frequency spectral amplitudes specified by the haddon 1996 source model agree with the felt area of the saguenay earthquake but overpredict the felt areas of nearly all other large events in general models that fit the saguenay data be it intensity data strong ground motion data regional seismographic data or teleseismic data will not fit the data from the remaining earthquakes a source model derived from the california database suitably modified for re gional differences in crustal properties is also evaluated this model is not signifi

a fault model for the nahanni earthquakes from aftershock studies abstract relative locations of 323 large aftershocks m 3 0 or greater in the period from 5 october 1985 to 25 march 1988 show that the m s 6 6 event on 5 october 1985 initiated at 62 208 n 124 217 w about 2 5 km northeast of the m s 6 9 main shock on 23 december 1985 the overall aftershock distribution suggests the october rupture was primarily a west dipping low angle thrust in subsequent aftershock activity the main rupture plane was marked by a distinct quiescent area of about 200 km 2 that persisted until the 23 december event most of the stress drop and slip occurred in this area following the 23 december rupture a similar sized quiescent zone was also observed however it was only evident during the first 24 hr of the aftershock sequence and the area was about 50 per cent too small to yield the overall stress drop the additional area appeared to come from secondary rupture zones that developed coincident with the main shock rupture precise locations of 182 small m 3 0 or less aftershocks recorded during a third field survey from 12 to 21 september 1986 indicated at least one and probably three high angle faults composite mechanism solutions showed thrust faulting except in a region directly south of the main shock rupture areas where there is a bend in one of the secondary fault zones and a concentration of aftershock activity mechanism solutions calculated for five of the largest aftershocks in the same region also indicated a similar variability development of secondary fault zones explained the increased complexity of the december event and may also provide an explanation for the vertical peak acceleration exceeding 2 g that was recorded about 10 sec after the december rupture initiated

problems of seismic hazard estimation in regions with few large earthquakes examples from eastern canada abstract seismic hazard estimates and seismic zoning maps are based on an assessment of historical and recent seismieity and any correlations with geologic and tectonic features that might define the earthquake potential evidence is accumulating that the large earthquakes in eastern canada m 7 may be associated with the rift systems hat surround or break the integrity of the north american craton the problem for seismic hazard estimation is that the larger historical earthquakes are not uniformly distributed along the paleozoic st lawrence ottawa rift system and are too rare on the mesozoic eastern margin rift to assess the overall seismogenic potential multiple source zone models for hazard estimation could include hypotheses of future m 7 earthquakes at any location along these rift systems but at a moderate probability such as that used in the canadian zoning maps the resultant hazard will be so diluted that it will not result in adequate design against the near source effects of such earthquakes the near source effects of large rare earthquakes can however be accommodated in conservative codes and standards for critical facilities if society is willing to pay the price

the fate of subducted oceanic slabs in the shallow mantle insights from boron isotopes and light element composition of metasomatized blueschists from the mariana forearc abstract serpentine muds from south chamorro seamount scs drilled during odp leg 195 at site 1200 contain metamafic clasts that experienced blueschist facies metamorphism including the critical mineral assemblage pumpellyite na amphibole epidote these schists represent fragments from the actual slab mantle interface at 27 km depth their heterogeneous lithology with a metasomatic character indicates significant mobility of major elements in the mariana forearc a region of melange formation as it can also be observed in onland exposures such as the catalina schist as the mariana forearc blueschists show no late stage alteration they permit the direct study of material transfer during the subduction processes at an active convergent margin this study presents the first data of detailed b isotope 11 b and light element variations in blueschist facies minerals from the mariana arc system the primary foci are b and li concentrations and 11 b values analyzed by sims and tof sims techniques minerals such as na rich amphibole phengite and chlorite are found to be strongly enriched in li up to 70 g g be up to 8 g g and b up to 35 g g and with 11 b values of 6 4 these new data are consistent with isotopically heavy b being released into the mariana forearc mantle wedge serpentinization of dry mantle peridotite after interaction with b rich slab released high ph fluids and confirm models of significant b loss and b isotope fractionation during forearc shallow slab dehydration the elevated li be and b concentrations in minerals that comprise the bulk of the rocks namely amphibole phengite and chlorite bear a strong potential to further transport li and b as well as the isotopically light component of b to greater depths in the mantle where ongoing metamorphism is responsible for further isotope and elemental fractionation and the formation of distinct mantle reservoirs e g volcanic arc and oceanic intra plate oib magmas

inhibition of subduction thrust earthquakes by low temperature plastic flow in serpentine abstract serpentine is considered a candidate hydrous mineral in explaining aseismic zones along faults however recent frictional experiments have shown a marked increase in its shear strength with increasing depth implying its deformation via unstable sliding at a plate interface in subduction zones here we report the results of a series of simple shear experiments designed to determine the dominant deformation style of low temperature serpentine species under p t conditions that correspond to the mantle wedge corner in cool subduction zones ca p 1 gpa and t 200 c we found that with increasing shear strain the dominant flow mechanism of the serpentinite evolves from 1 inhomogeneous semi brittle flow by strain localization into sample scale shear bands to 2 homogeneous ductile flow by intracrystalline deformation within individual serpentine grains with the development of a planar shape fabric and a strong crystallographic preferred orientation tem observations revealed that grain size reduction in lizardite crystals occurs due to 001 interlayer glide represented by the occurrence of tiny sheet like sectors oriented parallel to 001 liz lattice fringes which evolved from large elongated sectors with long axes oriented normal to 001 liz given that a low viscosity serpentinized layer upon a subducting plate interface produces strain localization within the layer and subsequent large bulk shear strains the above results indicate that the presence of low temperature serpentine species at the plate interface in cool subduction zones inhibits the initiation of subduction thrust earthquakes as stress is preferentially accommodated by plastic flow this hypothesis explains the occurrence of an anomalous non seismic region devoid of large thrust earthquakes in locally hydrated forearc mantle within northeast japan

mariana blueschist mud volcanism implications for conditions within the subduction zone several recently discovered active mud volcanoes on the nonaccretionary mariana convergent plate margin are erupting slab derived fluids serpentine mud and metamorphosed rocks from depths of as great as 25 km blueschist materials from the metamorphosed subducted plate are contained in the muds pore fluids indicate a depth dependence for decarbonation the mud volcanoes record in situ conditions along the decollement including pressure and temperature conditions and physical properties within the subduction zone similar mud flow material occurs worldwide as sedimentary serpentinite deposits in accreted fragments of former convergent margins making this kind of mud volcanism a more important phenomenon in convergent margins than previously recognized

a rock avalanche triggered by the october 1985 north nahanni earthquake district of mackenzie n w t a major rock avalanche estimated volume 5 7 106 m3 was triggered by the october 1985 north nahanni earthquake ms 6 6 the landslide is located at 62 27 n 124 18 w 23 km southwest of carlson lake in an uninhabited part of the district of mackenzie n w t it occurred within 10 km of the epicentre the rock avalanche detached from the northeast flank of a ridge located along the axis of the english chief anticline and involved massively bedded limestones of the middle devonian nahanni formation whilst most of the landslide mass was deposited at the foot of the original slope a mobile tongue of debris turned at right angles to the direction of initial sliding and travelled down valley coming to rest on a slope of about 5 the distal margin of the mobile tongue is approximately 1 55 km from the top of the rupture surface the fahrboschung of the landslide is estimated to be 14 5 and the rock avalanche exhibits exceptional mobility in relation to its volume the rupture surface is well expose

wind field differences between three meteorological reanalysis data sets detected by evaluating atmospheric excitation of earth rotation 1 we compared atmospheric excitations of the varying earth rotation calculated from three different meteorological reanalysis data sets ncep ncar ncep doe and era 40 over 23 years from 1979 to 2001 atmospheric excitations are evaluated by atmospheric angular momentum aam functions we found that differences in aam functions are primarily caused by differences in the wind data especially the northward wind data between the three reanalysis data sets differences between the ncep doe and era 40 aam functions are larger than those between the ncep ncar and ncep doe ones the northward wind differences that have a large effect on the equatorial aam functions originate from the upper troposphere at the tropics and midlatitudes in the southern hemisphere especially in three regions east pacific off latin america africa and the indian ocean the eastward wind differences are seen at midlatitudes in the southern hemisphere these differences are not confined to a single frequency the difference spectra show a reddish frequency dependence with some peaks in seasonal bands and tidal diurnal bands since the eastward and northward winds over the tropics can most efficiently change the earth s spin rate and polar motion respectively the wind accuracy over the tropics directly determines the accuracy in evaluations of the atmospheric excitations

geochemical bioenergetics during low temperature serpentinization an example from the samail ophiolite sultanate of oman abstract various classes of microbial and biomolecular evidence from global studies in marine and continental settings are used to identify a set of reactions that appear to support microbial metabolism during serpentinization of ultramafic rocks geochemical data from serpentinizing ecosystems in the samail ophiolite of oman are used to evaluate the extent of disequilibria that can support this set of microbial metabolisms and to provide a ranking of potential metabolic energy sources in hyperalkaline fluids that are direct products of serpentinization results are used to construct hypotheses for how microbial metabolism may be supported in the subsurface for two cases ecosystems hosted in rocks that have already undergone significant serpentinization and those hosted by deeper active serpentinization processes

source complexity of the 1987 whittier narrows california earthquake from the inversion of strong motion records strong motion records for the whittier narrows earthquake are inverted to obtain the history of slip both constant rupture velocity models and variable rupture velocity models are considered the results show a complex rupture process within a relatively small source volume with at least four separate concentrations of slip two sources are associated with the hypocenter the larger having a slip of 55 90 cm depending on the rupture model these sources have a radius of approximately 2 3 km and are ringed by a region of reduced slip the aftershocks fall within this low slip annulus other sources with slips from 40 to 70 cm each ring the central source region and the aftershock pattern all the sources are predominantly thrust although some minor right lateral strike slip motion is seen the overall dimensions of the whittier earthquake from the strong notion inversions is 10 km long along the strike and 6 km wide down the dip the preferred dip is 30 degree and the preferred average rupture velocity is 2 5 km s moment estimates range from 7 4 to 10 0 times 10 sup 24 dyn cm depending on the rupture model

unraveling the sequence of serpentinization reactions petrography mineral chemistry and petrophysics of serpentinites from mar 15 n odp leg 209 site 1274 1 the results of detailed textural mineral chemical and petrophysical studies shed new light on the poorly constrained fluid rock reaction pathways during retrograde serpentinization at mid ocean ridges uniformly depleted harzburgites and dunites from the mid atlantic ridge at 15 n show variable extents of static serpentinization they reveal a simple sequence of reactions serpentinization of olivine and development of a typical mesh texture with serpentine brucite mesh rims followed by replacement of olivine mesh centers by serpentine and brucite the serpentine mesh rims on relic olivine are devoid of magnetite conversely domains in the rock that are completely serpentinized show abundant magnetite we propose that low fluid flux serpentinization of olivine to serpentine and ferroan brucite is followed by later stages of serpentinization under more open system conditions and formation of magnetite by the breakdown of ferroan brucite modeling of this sequence of reactions can account for covariations in magnetic susceptibility and grain density of the rocks citation bach w h paulick c j garrido b ildefonse w p meurer and s e humphris 2006 unraveling the sequence of serpentinization reactions petrography mineral chemistry and petrophysics of serpentinites from mar 15 n odp leg 209 site 1274 geophys res lett 33 l13306

southern surface rupture associated with the m 7 3 1992 landers california earthquake although most evidence suggests that the 28 june 1992 m 7 3 landers earthquake ruptured unilaterally north significant surface rupture was mapped on the eureka peak and burnt mountain faults to the south of the landers epicenter an eyewitness account reports that surface rupture occurred on the northern eureka peak fault within approximately 35 sec of the mainshock initiation array analysis of the landers mainshock provides evidence in support of this report a significant southern subevent in the early mainshock coda i also analyze dense array recordings of a m 5 6 aftershock that occurred 3 min after the mainshock at 34 7 65 n 116 23 82 w and show that there is strong evidence that this event was also associated with significant rupture on the eureka peak fault this analysis thus suggests that the eureka peak fault rupture was not caused by direct bilateral mainshock rupture but instead was initially triggered less than a minute after the mainshock and reruptured by the m 5 6 aftershock results for the evolution of the landers sequence suggest that mainshock subevents may in some cases be accurately described as aftershocks i e disjoint triggered events that occur within the duration of mainshock strong ground motion

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

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

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

structure and fabric of the crust and uppermost mantle in the northern canadian cordillera from rayleigh wave tomography abstract the seismic structure and fabric of the lithosphere and underlying mantle beneath the northern canadian cordillera provides important constraints on its evolution and current tectonics however it is poorly characterized due to historically sparse networks of seismic instruments we use data from past and recently deployed networks of broadband seismic stations in northwestern canada and measure rayleigh waves propagating between all available pairs of seismic stations using two complementary techniques ambient noise and teleseismic two station interferometry the rayleigh wave data are processed to obtain phase velocity dispersion curves that are inverted for phase velocity maps at periods between 8 and 80 s to first order these maps show high velocity anomalies within the canadian shield and low velocity anomalies within the cordillera at all periods at short periods 30 s mostly sensitive to uppermost mantle depths high velocity anomalies of the canadian shield extend west past the cordilleran deformation front and suggest the presence of cratonic lithosphere beneath the cordillera whereas the lowest velocities underlie the allochtonous terranes anisotropy within the crust and uppermost mantle exhibits fast axis orientations aligned with the major faults and fabric of the cordillera and show evidence for vertical changes in anisotropy these results provide new constraints on geodynamic models proposed to explain neotectonic deformation in this area

the 1987 whittier narrows earthquake sequence in los angeles southern california seismological and tectonic analysis the october 1 1987 whittier narrows earthquake m l 5 9 was located at 34 2 96 n 118 4 86 w at a depth of 14 6 0 5 km in the northeastern los angeles basin the focal mechanism of the mainshock derived from first motion polarities shows pure thrust motion on west striking nodal planes with dips of 25 5 and 65 5 respectively the aftershocks define an approximately circular surface that dips gently to the north centered at the hypocenter of the mainshock with a diameter of 4 6 km hence the spatial distribution of the mainshock and aftershocks as well as the focal mechanisms of the mainshock indicate that the causative fault was a 25 north dipping thrust fault striking west and is confined to depths from 10 to 16 km although most of the 59 aftershock focal mechanisms presented here document a complex sequence of faulting they are consistent with deformation of the hanging wall caused by the thrust faulting observed in the mainshock a cluster of reverse faulting events on north striking planes occurred within hours after the mainshock 2 km to the west of the mainshock the largest aftershock m l 5 3 occurred on october 4 and showed mostly right lateral faulting on the same north northwest striking plane within the hanging wall similarly several left lateral focal mechanisms are observed near the eastern edge of the mainshock rupture the earthquake and calibration blast arrival time data were inverted to obtain two refined crustal velocity models and a set of station delays when relocating the blast using the new models and delays the absolute hypocentral location bias is less than 0 5 km the mainshock was followed by nearly 500 locatable aftershocks which is a small number of aftershocks for this magnitude mainshock the decay rate of aftershock occurrences with time was fast while the b value was low 0 67 0 05 for a los angeles basin sequence

the prince george british columbia earthquake of 21 march 1986 on 21 march 1986 a widely felt earthquake occurred near 54 n 122 w adjacent to the rocky mountains in east central british columbia in an area of low historic seismicity the earthquake had a body wave magnitude of 5 4 a surface wave magnitude of 5 2 and a seismic moment of 2 4 1024 dyne cm the stress drop is estimated as 3 bars from the aftershock area the epicentral region is sparsely populated and only minor damage occurred although the earthquake was felt strongly modified mercalli intensity of v in the nearby city of prince george within 48 hours of the main shock portable short period seismographs began to be deployed in the epicentral region and were maintained there for eight days few aftershocks were recorded they were confined to a depth range of 9 to 16 km and the largest was magnitude 2 5 no foreshocks were detected by the canadian seismograph network the focal mechanism of the earthquake was thrust faulting on a northwest southeast plane dipping about 40 to the southwest this is similar to the structural trends mapped in the region the occurrence of this earthquake and several other moderate to large earthquakes in the foreland fold and thrust belt raises the question of whether there is a potential seismic hazard along the entire eastern margin of the canadian cordillera the lower than expected attenuation for seismic waves observed for this earthquake also has implications for seismic hazard assessment in the cordillera

seismic velocity model of the crust in the northern canadian cordillera from rayleigh wave dispersion data models of the seismic velocity structure of the crust in the seismically active northern canadian cordillera remain poorly constrained despite their importance in the accurate location and characterization of regional earthquakes on 29 august 2014 a moderate earthquake with magnitude 5 0 which generated high quality rayleigh wave data occurred in the northwest territories canada 100 km to the east of the cordilleran deformation front we carefully selected 23 seismic stations that recorded the rayleigh waves and divided them into 13 groups according to the azimuth angle between the earthquake and the stations these groups mostly sample the cordillera in each group we measured rayleigh wave group velocity dispersion which we inverted for one dimensional shear wave velocity models of the crust we thus obtained 13 models that consistently show low seismic velocities with respect to reference models with a slow upper and lower crust surrounding a relatively fast mid crustal layer the average of

large deformation finite element modelling of earthquake induced landslides considering strain softening behaviour of sensitive clay large scale landslides in sensitive clays cannot be explained properly using the traditional limit equilibrium or lagrangian based finite element fe methods in the present study dynamic fe anal

ocean atmosphere interaction and the tropical climatology part ii why the pacific cold tongue is in the east abstract the influence of coupled process on the climatology of the tropical pacific is studied in a model for the interaction of equatorial sst the associated component of the walker circulation and upper ocean dynamics in this part the authors show how different physical mechanisms affect the spatial pattern of the pacific warm pool and cold tongue in this coupled climatology when model parameters give a suitable balance between effects of upwelling and thermocline depth on sea surface temperature and for suitable atmospheric parameters a good prototype for the observed cold tongue configuration is produced this is largely determined by coupled ocean atmosphere processes within the basin presence of an easterly wind s component produced by factors external to the pacific basin can be important in setting up a cooling tendency but this is magnified and modified by a chain of nonlinear feedbacks between trade winds and ocean dynamics affecting the sst gradient within the basin these feedbacks d

equatorial mid tropospheric easterly jet over the eastern pacific comparison from the ecmwf and ncep ncar reanalyses 1 the upper air data set of the european centre for medium range weather forecasts ecm 1979 93 reanalysis is evaluated in comparison to the national centers for environmental prediction national center for atmospheric research ncp reanalysis for the same period attention is focused on the equatorial mid tropospheric easterly jet emtej over the eastern pacific first noted from a short series of rawin soundings at galapagos and then corroborated in its broad spatial context from the ncp 40 year reanalysis the present follow up study contributes a comparison of evidence from the two sources with different model assimilations the major characteristics of the emtej are confirmed although the winds from ecm are overall slower and those from ncp faster than the measurements over galapagos in both ecm and ncp the easterly speed maximum along the equator is over the eastern pacific located around 600 700 mb and further westward at progressively lower elevations divergence prevails in the realm of the speed maximum and is primarily fed by subsidence from aloft the ecm evidence differs from ncp by a more westward location of the easterly speed maximum and overall weaker flow

rheological properties of composite serpentine brucite suspensions implications for mudflow behavior on forearc seamounts abstract to better understand the factors that influence seafloor serpentinite mudflows such as those recently documented on the marianas seamounts we have conducted rheological measurements on composite serpentine brucite suspensions at 7 c and a salt concentration of 0 6 m the resulting flow curves were fitted by the bingham fluid model from which the bingham yield stress and plastic viscosity of each suspension was determined both the yield stress and plastic viscosity increase as the water content of the suspension decreases increasing the brucite fraction in the solid mixture results in an increase in yield stress and a decrease in viscosity of the suspension for the same water solid ratio physicochemical parameters such as ph temperature and electrolyte concentration also moderately affect the rheology of the suspension but the influence is not as significant as has been previously observed for suspensions containing smectite the results suggest that mineral composition of the serpentinite mud especially the abundance of brucite has as much of an impact as water content on variable mudflow behaviors such as those observed at the mariana seamounts

stress in western canada from regional moment tensor analysis more than 180 regional moment tensor rmt solutions for moderate sized earthquakes m 4 are used to examine the contemporary stress regime of western canada and provide valuable information relating to earthquake hazard analysis the overall regional stress pattern shows mainly ne sw oriented p axes for most of western canada with local variations in the northern cordillera the maximum compressive stress direction 1 varies from east west to north south to ne sw from south to north the stress direction 1 is consistent with the p axis direction for the largest earthquakes except in the central and northern mackenzie mountains where there is a 16 difference the yakutat collision zone shows a steady change in 1 from east west in the east to north south in the west in the canada united states border region rmt solutions suggest a north south compressional regime may extend through southern british columbia and northern washington to the eastern cordillera in the vancouver island puget sou

mechanics of back arc deformation in costa rica evidence from an aftershock study of the april 22 1991 valle de la estrella costa rica earthquake mw 7 7 the april 22 1991 mw 7 7 valle de la estrella costa rica earthquake represents back arc thrusting of the caribbean plate beneath the panama block along the north panama thrust belt large back arc thrusting events are quite rare occurring in only two other locations along the sunda arc and japan sea to better understand the mechanics of back arc thrusting we constrain the faulting geometry associated with the 1991 costa rica earthquake using aftershock locations and focal mechanisms obtained from a three component portable digital network deployed in and around the aftershock area following the mainshock the spatial distribution of aftershocks reveals a complicated faulting geometry in the rupture area focal mechanisms determined from inversion of p wave and tangentially and radially polarized s wave sh and sv respectively amplitudes recorded by this temporary network confirm fault complexity and indicate active thrust normal and strike slip faults in the back arc of costa rica most of the thrust events are confined to the southern portion of the aftershock zone in the vicinity of the mainshock their distribution suggests the existence of a near horizontal basal fault plane at a depth of about 15 km with many imbricate faults having steeper dips extending from the basal plane toward the surface events with strike slip mechanisms locate northwest of the thrust events and define a sw ne trending left lateral strike slip fault zone that represents the nw termination of the mainshock rupture and possibly the maximum nw extension of the panama block the superposition of the aftershock locations on a geologic map of the region shows that aftershocks are restricted to occur in the older more competent rock units volcanic and volcaniclastic rocks interbedded with carbonates of the back arc sedimentary basin shallow events depth 5 km occur only where these oldest units are exposed at the surface this suggests that 1 exposure of the lower units results from repeated earthquake slip on the shallow crustal faults imaged by the aftershocks and 2 as much as 7 km of basin fill material overlying the lower units northeast of the mainshock does not deform seismically but instead folds and possibly faults aseismically

paleogeographic reconstruction and origin of the philippine sea abstract our reconstruction of the philippine sea suggests that it formed by two distinct episodes of back arc spreading each of which resulted from seaward retreat of the trench in the first episode the protoizu bonin trench retreated northward and the west philippine basin formed behind the northern half of the palau kyushu ridge in the second episode the izu mariana trench retreated eastward and the shikoku and parece vela basins formed behind it during the last 17 ma the philippine sea basin has been moving northwestward with respect to eurasia shifting the ttt triple junction off central japan westward by about 50 km the motion of the philippine sea with respect to eurasia at the triple junction changed from north northwestward to west northwestward 10 5 ma ago for the period before 17 ma ago we construct two models retreating trench model and anchored slab model the izu bonin trench migrated from south to northeast rotating in a clock wise sense since 48 ma ago in the retreating trench model in the anchored slab model the trench has been fixed with respect to eurasian margin since 43 ma ago we prefer the retreating trench model because the deformation of the plate boundary along the eastern margin of eurasia during 30 17 ma ago is much simpler for this model than for the anchored slab model furthermore rotations of the bonin mariana islands are consistent with those predicted from the retreating trench model the 48 ma ages of the northern part of the palau kyushu ridge and of chichi jima of the bonin islands indicate that there was subduction beneath the northern half of the ridge beginning at least 48 ma ago from this and the subparallelism in trend between the northern part of the palau kyushu ridge and the central basin ridge we propose that the major part of the west philippine basin formed by back arc spreading in a n s direction behind the northern part of the palau kyushu ridge the pacific plate was moving northward with respect to hot spots from 48 to 43 ma ago which implies that the pacific plate is not likely to have been subducting beneath the west philippine basin during this time we speculate that another plate existed south of the pacific plate and thai it was subducting beneath both tile west philippine basin and australia the annihilation of this plate might he a cause for the sudden change of the pacific plate motion at 43 ma ago

the 1985 earthquake triggered north nahanni rockslide northwest territories canada the co seismic movement of a sedimentary rock mass conditioned by residual strength abstract the north nahanni rockslide volume 7 6 mm3 northwest territories canada was triggered by the mw 6 6 nahanni earthquake of october 5 1985 the earthquake was followed by a mw 6 8 earthquake at approximately the same location in december 1985 the landslide occurred in a palaeozoic carbonate sequence sliding along a thrust fault which partly follows bedding and partly cuts across bedding in the source rock mass the sliding surface thus consisted of two planes the lower plane dipped at 20 while the upper plane dipped at 35 tilting table test of block samples from the sliding surface and field measurements of large scale sliding surface roughness indicated that the friction angle of the sliding surface was 24 two dimensional 2d slope stability analysis was performed for the slope before and during the earthquake using a discontinuum numerical technique the distinct element udec code 2d static slope stability analyses indicate that the simple structured rock mass was marginally stable for sliding surface friction angles of 24 and was constrained by the residual shear strength of the pre sheared fault surface 2d dynamic analyses of the co seismic movements are conducted by comparing seismically induced slope displacements computed in both udec and slammer a sliding block analysis using real earthquake ground motions records of the december 1985 nahanni earthquake both results are in good agreement the dynamic analysis indicates that the slope becomes unstable for given seismic inputs that correspond closely to those estimated for the october earthquake at a friction angle of 24 for the sliding surface indicating that co seismic rock slope movement was only possible on low friction pre sheared surfaces at residual strength it also showed that the deformation behavior of the north nahanni rock masses was dependent on the frequency of the seismic signals because the static slope stability analysis showed that the slope was close to instability prior to seismic shaking the october 1985 nahanni earthquake operated as a trigger event that accelerated the occurrence of the slide