late pleistocene glacial evolutionary stages in the gredos mountains iberian central system during the last glacial period late pleistocene the summits of the gredos mountains iberian central system were covered by plateau glaciers and later evolved into valley and slope glaciers the evolutionary sequence of these paleoglaciers has been obtained using geomorphological indicators and taking the principal moraine as reference these moraines show the limit between the glaciation and deglaciation periods in the glaciation period g four chronologically sequenced stages have been identified the local glacial maximum or maximum ice extent gm limited retreat lr re advance ra and finally major stabilization the steady state stage of greatest glacial stability ms in the deglaciation period d six chronologically sequenced stages have been identified an absolute retreat which involved regression and stabilization d1 two major retreat periods d2 and d4 and two stabilization periods d3 and d5 finally the last stage represents the complete disappearance of the glaciers d6 this evolutionary sequence allows more precise studies of the ages assigned through numerical dating procedures and of the correlation of the paleoglaciers in the gredos mountains with those of other regions

testing probabilistic seismic hazard estimates by comparison with observations an example in italy summary an evaluation of the actual reliability of probabilistic seismic hazard psh assessments provided by existing numerical techniques is mandatory to orientate new researches and improvements two procedures devoted to this task are proposed which are based on the comparison of the hazard estimates with empirical observations e g strong motion data these procedures have been applied to the estimates provided by the methodology adopted for most recent seismic hazard evaluations in italy the analysis shows that a significant mismatch exists between peak ground acceleration values characterized by an exceedence probability of 10 per cent in 30 yr and what has actually been observed at 68 accelerometric stations located on stiff soil where continuous seismicity monitoring has been performed in the last 30 yr although this finding should be considered with caution it suggests that a future re examination of the adopted psh computational model could be useful to avoid possible underestimates of seismic hazard in italy

global mantle heterogeneity and its influence on teleseismic regional tomography abstract a new global p wave tomography model is determined using a flexible grid parameterization this new model better reveals the mantle structure under the polar regions than the previous tomographic models the subducting slabs are generally imaged clearly as high velocity high v zones the young slabs are still subducting in the upper mantle and the mantle transition zone mtz whereas the old and ancient slabs are either stagnant in the mtz or have subducted down to the lower mantle even reaching the core mantle boundary low velocity low v anomalies are generally revealed in the mantle under the hotspot regions it seems that a variety of mantle upwelling plumes exist some strong plumes are visible in the whole mantle under the long living hotspots such as those in south central pacific africa hawaii and iceland whereas weak plumes are visible in only some depth range under the minor hotspots under the intraplate volcanoes in east eurasia bering sea and west alaska significant low v anomalies are revealed in the upper mantle which may reflect hot and wet upwelling associated with corner flows in the big mantle wedge bmw above the stagnant pacific slab in the mtz and perhaps deep slab dehydration as well the subduction triggered magmatism in the bmw may be a new class of mantle plumes we also used the new global model to investigate the influence of whole mantle heterogeneity on the determination of upper mantle tomography under japan with a teleseismic inversion method the results show that the mantle heterogeneities outside the target volume of regional tomography can cause significant changes 0 2 0 4 s to the observed relative travel time residuals of a distant earthquake the pattern of regional tomography remains the same even after correcting for the whole mantle heterogeneity but there are some changes in the amplitude of velocity anomalies in the regional tomography hence it is necessary to correct for the mantle heterogeneities outside the target volume so as to obtain a better regional tomography

deep structure of northern mississippi embayment in september 1980 the u s geological survey conducted a seismic refraction investigation of the northern mississippi embayment during the investigation 34 shots from nine shotpoints were recorded along a series of profiles the profiles were parallel to and across an inferred precambrian rift zone which is outlined by a series of magnetic anomalies and covers an area at least 200 km 125 mi long and 70 km 45 mi wide along the northeast southwest trending axis of the rift a 0 7 to 1 1 km 2 297 to 3 609 ft thick 1 8 km sec layer representing the unconsolidated tertiary and cretaceous sediments overlies a 2 km 6 562 ft thick 5 95 km sec layer representing a paleozoic carbonate sequence beneath the carbonates is a 3 km 9 843 ft thick low velocity layer which probably consists of late precambrian clastics the presence of this low velocity layer is indicated on the seismic profiles by an abrupt cutoff of the 5 95 km sec arrivals at about 20 km 12 mi from the shotpoints this cutoff is followed by a 30 to 40 km 20 to 25 mi long zone of no arrivals beyond which are seen delayed arrivals from a 6 2 km sec crystalline basement the 11 km 6 8 mi thick 6 2 km sec basement overlies a lower crus composed of a 6 6 km sec layer whose thickness decreases to the northeast owing to its replacement by an anomalous high velocity 7 3 km sec basal layer the base of the crust ranges from a 39 km 24 mi depth at the southern end of the profile to a 46 km 29 mi depth at the northern end where the 7 3 km sec layer is 20 km 12 mi thick below the base of the crust moho the upper mantle has a velocity of 8 0 km sec the velocity structure beneath the west flank of the embayment is simpler than that of the axis 5 95 6 2 6 6 7 3 and 8 0 km sec the low velocity layer is absent and the anomalous 7 3 km sec layer is thinner than along the axial profile the profiles perpendicular to the rift show that the low velocity layer is restricted to the axial zone and that the anomalous 7 3 km sec basal crustal layer reaches maximum thickness there the tectonic model proposed to explain the origin of this embayment velocity structure includes a late precambrian mantle plume that intruded the lower crust of the northern embayment causing uplift and or crustal stretching and subsequent rifting of the axial area this was followed by erosion subsidence and subsequent deposition of sediments in the resulting trough

including anisotropy in 3 d velocity inversion and application to marlborough new zealand summary two types of seismic velocity heterogeneity are often observed in a given region pronounced patterns of anisotropy shown by local earthquake shear wave splitting and seismic velocity structure imaged by local earthquake traveltimes we seek to combine these types of observations by including anisotropy in 3 d velocity inversion the fast polarization directions from local earthquake shear wave splitting are used to define an initial anisotropy model the ray paths are estimated for each of the shear wave splitting observations and the 3 d initial model is determined via an averaging process and a coherence analysis the initial magnitude of anisotropy at a node is dependent on the number of adjacent ray paths and the consistency of the shear wave splitting anisotropy measurements the traveltime inversion is parametrized with an isotropic component and two azimuthal anisotropy parameters for each node the ray paths are defined with approximate 3 d ray tracing and for each point along a ray path the velocity is defined as a function of the ray path azimuth at that point a blended approach is used to damp the perturbations to the magnitude of anisotropy and or perturbations to the fast direction tests with synthetic traveltime data show that the anisotropic velocity inversion method is reliable application to the marlborough region shows variations in anisotropy in the brittle crust ductile lower crust mantle and subducted slab the patterns are consistent with the shear wave splitting observations but show some additional features the maximum anisotropy is approximately 12 per cent oriented northeast between the awatere and wairau faults this high anisotropy may reflect deformation and extensive fracturing in a region of high total shear strain in the region of interaction of the ductile lower crust with the shallow subducting slab moderate anisotropy with east west orientation is imaged this is consistent with ductile deformation roughly parallel to the plate velocity

methodology for obtaining constraints on ground motion from precariously balanced rocks precariously balanced rocks provide constraints on the level of ground motion that could have occurred during the time the rocks have been in their current positions field measurements of the quasi static toppling accelerations of precariously balanced rocks provide information for computer modeling of the response of the rocks to given ground motion histories the quasi static toppling acceleration is determined by the ratio of the quasi static force through the center of mass whose moment about the rocking point counterbalances that of gravity and the mass of the rock for an estimate of dynamic toppling peak ground accelerations we use a finite difference numerical code to model the dynamic response of rocks to arbitrarily complex acceleration time histories we test our computational methodology and results using the university of nevada shake table the shake table tests of both idealized rectangular shapes and actual rocks confirm the accuracy of our methodology for estimating dynamic toppling accelerations of precarious rocks for different waveforms representing earthquake ground motion time histories a study of statistical variation of the dynamic toppling acceleration for a suite of seismograms provides constraints on the peak ground acceleration on hard rock sites in mojave desert about 15 km from the san andreas fault of about 0 5 g for synthetic seismograms with relatively high frequency content or about 0 4 g for seismograms with spectra similar to the recent izmit turkey earthquake a major source of uncertainty is the uncertainty in the spectrum of ground motion for large earthquakes waveforms with relatively more low frequencies can topple the rocks with lower peak ground accelerations we describe methods for reducing the uncertainties with further study and more data

tomographic pn velocity and anisotropy structure in the central and eastern united states abstract a total of 19 166 pn phase readings from the international seismological centre isc and the national earthquake information center neic catalogs as well as hand picked arrivals from the incorporated research institutions for seismology iris data management center were inverted to map the velocity and anisotropy structure of the lithospheric mantle in the central and eastern united states ceus our pn tomographic model shows a broad region of very fast velocity under the north american craton the northern ceus and significant lateral variations within the rest of the ceus the surface locations of the major intraplate seismic zones are near the edges of high velocity anomalies which is consistent with the notion that stress accumulation and hence focused deformation are likely to occur at the rheological boundaries around the rigid lithospheric root however the ancient rifts show no clear correlation to the low velocity anomalies in the lithospheric mantle our pn anisotropic model shows a complex pattern of fast directions with an overall north south trend in the ceus that may reflect the preserved fabrics of the cratonic lithosphere nonetheless high pn anisotropy seems to wrap around the high velocity blocks which may indicate local deformation around the rigid blocks

seismic velocity model of the central united states version 1 description and simulation of the 18 april 2008 mt carmel illinois earthquake abstract we have developed a new three dimensional seismic velocity model of the central united states cusvm that includes the new madrid seismic zone nmsz and covers parts of arkansas mississippi alabama illinois missouri kentucky and tennessee the model represents a compilation of decades of crustal research consisting of seismic aeromagnetic and gravity profiles geologic mapping geophysical and geological borehole logs and inversions of the regional seismic properties the density p and s wave velocities are synthesized in a stand alone spatial database that can be queried to generate the required input for numerical seismic wave propagation simulations we test and calibrate the cusvm by simulating ground motions of the 18 april 2008 m w 5 4 mt carmel illinois earthquake and comparing the results with observed records within the model area the selected stations in the comparisons reflect different geological site conditions and cover distances ranging from 10 to 430 km from the epicenter the results based on a qualitative and quantitative goodness of fit gof characterization indicate that both within and outside the mississippi embayment the cusvm reasonably reproduces 1 the body and surface wave arrival times and 2 the observed regional variations in ground motion amplitude cumulative energy duration and frequency content up to a frequency of 1 0 hz in addition we discuss the probable structural causes for the ground motion patterns in the central united states that we observed in the recorded motions of the 18 april mt carmel earthquake online material simulated and observed waveforms and response spectral acceleration ground motion prediction equation comparison

p wave anisotropic tomography of the nankai subduction zone in southwest japan 1 the active subduction of the young philippine sea phs plate and the old pacific plate has resulted in significant seismic heterogeneity and anisotropy in southwest sw japan in this work we determined a detailed 3 d p wave anisotropic tomography of the crust and upper mantle beneath sw japan using 540 000 p wave arrival times from 5 249 local earthquakes recorded by 1095 stations the phs slab is imaged clearly as a high velocity high v anomaly which exhibits considerable lateral variations significant low velocity low v anomalies are revealed above and below the phs slab the low v anomalies above the phs slab may reflect the upwelling flow in the mantle wedge and the phs slab dehydration and they form the source zone of the arc volcanoes in sw japan the low v zones under the phs slab may reflect the upwelling flow in the big mantle wedge above the pacific slab the anisotropy in the crust and upper mantle is complex in kyushu the p wave fast velocity direction fvd is generally trench normal in the mantle wedge under the back arc which is consistent with the corner flow driven by the phs slab subduction the fvd is trench parallel in the subducting phs slab under kyushu we think that the intraslab seismicity is a potential indicator to the slab anisotropy that is the phs slab with seismicity has kept its original fossil anisotropy formed at the mid ocean ridge while the aseismic phs slab has reproduced the anisotropy according to its current deformation

p and s wave tomography and anisotropy in northwest pacific and east asia constraints on stagnant slab and intraplate volcanism we determined three dimensional p and s wave velocities and p wave azimuthal anisotropic tomography of the northwest pacific subduction zones by inverting 1 225 086 p wave and 335 117 s wave arrival times from 13 413 earthquakes our results show some differences between p and s wave images for the stagnant pacific slab in the mantle transition zone mtz beneath northeast china the stagnant slab looks thicker in the p wave image than that in the s wave image which may reflects the effects of both hydration and lower temperature in the mtz though differences in the resolution of p and s wave tomography may also have some effects the changbai intraplate volcanism is caused by hot and wet upwelling in the big mantle wedge above the stagnant pacific slab our p wave anisotropy tomography shows that the fast velocity direction fvd in the subducting philippine sea plate beneath the ryukyu arc is ne sw trench parallel which is consistent with the spreading direction of the west philippine basin during its initial opening stage suggesting that it may reflect the fossil anisotropy a striking variation of the fvd with depth is revealed in the subducting pacific slab beneath the northeast japan arc which may be caused by slab dehydration that changed elastic properties of the slab with depth the fvd in the mantle wedge beneath the northeast japan and ryukyu arcs is trench normal which reflects subduction induced convection beneath the kuril and izu bonin arcs where oblique subduction occurs the fvd in the mantle wedge is nearly normal to the moving direction of the downgoing pacific plate suggesting that the oblique subduction together with the complex slab morphology have disturbed the mantle flow

uppermost mantle seismic velocity structure beneath usarray we apply pn tomography beneath the entire usarray footprint to image uppermost mantle velocity structure and anisotropy as well as crustal thickness constraints beneath the united states the sparse source distribution in the eastern united states and the resulting longer raypaths provide new challenges and justify the inclusion of additional parameters that account for the velocity gradient in the mantle lid at large scale pn velocities are higher in the eastern united states compared to the west but we observe patches of lower velocities around the new madrid seismic zone and below the eastern appalachians for much of the mantle lid below the central and eastern united states we find a moderate positive velocity gradient in the western united states we observe a moderate gradient in the region of the juan de fuca subduction zone but no significant gradient to the south and east of this region in terms of anisotropy we find that the pn fast axes generally do not agree with sks splitting orientations suggesting significant vertical changes in anisotropy in the upper mantle in particular the circular pattern of the fast polarization direction of sks in the western united states is much less pronounced in the pn results and in the eastern us the dominant pn fast direction is approximately north south whereas the sks fast polarizations are oriented roughly parallel to the absolute plate motion direction

seismic structure of the central us crust and shallow upper mantle uniqueness of the reelfoot rift abstract using seismic surface waves recorded with earthscope s transportable array we apply surface wave imaging to determine 3d seismic velocity in the crust and uppermost mantle our images span several proterozoic and early cambrian rift zones mid continent rift rough creek graben rome trough birmingham trough southern oklahoma aulacogen and reelfoot rift while ancient rifts are generally associated with low crustal velocity because of the presence of thick sedimentary sequences the reelfoot rift is unique in its association with low mantle seismic velocity its mantle low velocity zone lvz is exceptionally pronounced and extends down to at least 200 km depth this lvz is of variable width being relatively narrow 50 km wide within the northern reelfoot rift which hosts the new madrid seismic zone nmsz we hypothesize that this mantle volume is weaker than its surroundings and that the reelfoot rift consequently has relatively low elastic plate thickness which would tend to concentrate tectonic stress within this zone no other intraplate ancient rift zone is known to be associated with such a deep mantle low velocity anomaly which suggests that the nmsz is more susceptible to external stress perturbations than other ancient rift zones

shattered rock and precarious rock evidence for strong asymmetry in ground motions during thrust faulting recent thrust faulting models both physical and numerical have indicated very high ground motions on the hanging wall and relatively low motions on the footwall of thrust faults in this article recent field evidence supporting this strong asymmetry is described earlier field observations of objects thrown in the air also suggest that ground motions must have exceeded 1 g and 100 cm sec new shattered rock evidence on the hanging wall of thrust faults in southern california supports the occurrence of intense ground motions on the other hand the lack of shattered rock and the presence of precariously balanced rocks on the footwall of two thrust faults in southern california indicate relatively low ground motions thus indicating strong asymmetry in ground motions near the fault trace for thrust faults some current ground motion attenuation curves and hazard maps may give too low values on the hanging wall and too high values on the footwall this could be of critical importance to estimating earthquake hazard from potentially large thrust fault earthquakes in the los angeles basin and elsewhere in the world the new evidence also supports the idea that dynamic inertial detachment of the hanging wall from the footwall occurs in nature in the manner suggested by some recent physical and numerical models if such detachment occurs it could also have important implications for understanding thrust fault rupture mechanics for example the paradox of large overthrusts and the heat flow paradox for thrusts

fault slip rates and earthquake histories for active faults in southern california abstract within the context of the historical record of seismicity we review the geological data bearing on the quaternary slip rates and paleoearthquake histories of active faults in southern california

evaluating seismically induced mass movement hazard in caramanico terme italy abstract mass movements and earthquakes represent two major geological hazards in the municipal territory of caramanico terme south central apennines available records revealed the contemporaneous occurrence of earthquakes and slope failures on four occasions in the last four centuries 1627 1706 1933 and 1984 these events with local intensities ranging from vi to ix generated mass movements varying from a rotational slope failure to rock block falls all occurred in the southern periphery of the town and involved a thick carbonate megabreccia caprock and coarse colluvia which overlie a clayey substratum field investigation and review of historical records helped to delimit the areas susceptible to seismically triggered rockfalls the mapping of historic and pre historic rockfall deposits revealed their dispersal patterns and provided the basis for a determination of potential hazard zones we approximate the temporal hazard assessment by relating the rockfall occurrence to the probability of earthquake triggering considering the vi degree triggering threshold indicated by local historical data the statistical analysis of the regional seismic activity shows that events capable of inducing rockfalls have an approximately decennial recurrence in caramanico the approach presented could be readily applied to other potential risk areas of italy by exploiting the rich long term record of historical seismicity in general temporal hazard estimates at relatively low intensity levels will be possible even where the seismic history of the site is only well documented for a relatively limited time interval provided that this interval is much longer than the recurrence time of the events exceeding the threshold considered

site amplification factors in central and southern california determined from coda waves abstract site amplification factors at frequencies of 1 5 3 6 and 12 hz were determined for 158 stations of the southern california seismic network scsn from coda waves of 306 local earthquakes using a recursive stochastic inverse method our results for southern california are consistent with those of central california obtained by su et al 1992 in that the site amplification at a station is systematically related to the surface geology of the station site the site amplification is high for young quaternary sediments and decreases with increasing geologic age at all frequencies between 1 5 and 12 hz this decreasing rate is higher for younger rock sites gradually becomes lower with increasing geologic age and appears to be insignificant when the site reaches mesozoic age or older the relationship between site amplification and geologic age obtained for southern california agrees with that for central california amplification factors at 158 sites in southern california were combined with those at 132 sites in central california through a common reference site the result is a unique data base of site specific uniformly referenced amplification factors throughout central and southern california

a multidisciplinary approach to seismic hazard in southern california abstract a serious obstacle facing seismic hazard assessment in southern california has been the characterization of earthquake potential in areas far from known major faults where historical seismicity and paleoseismic data are sparse this article attempts to fill the voids in earthquake statistics by generating master model maps of seismic hazard that blend information from geology paleoseismology space geodesy observational seismology and synthetic seismicity the current model suggests that about 40 of the seismic moment release in southern california could occur in widely scattered areas away from the principal faults as a result over a 30 yr period nearly all of the region from the pacific ocean to 50 km east of the san andreas fault has a greater than 50 50 chance of experiencing moderate shaking of 0 1 g or greater and about a 1 in 20 chance of suffering levels exceeding 0 3 g for most of the residents of southern california thelion9s share of hazard from moderate earthquake shaking over a 30 yr period derives from smaller closer more frequent earthquakes in the magnitude range 5 m 7 rather than from large san andreas ruptures whatever their likelihood

upper crustal azimuthal anisotropy across the contiguous u s determined by rayleigh wave ellipticity constraints on upper crustal seismic anisotropy provide insight into the local stress orientation and structural fabric but such constraints are scarce except in areas with dense recordings of local seismicity we investigate directionally dependent rayleigh wave ellipticity or rayleigh wave h v horizontal to vertical amplitude ratios between 8 and 20 s period across usarray to infer azimuthal anisotropy in the upper crust across the contiguous u s to determine the h v ratios we use all available multicomponent ambient noise cross correlations between all usarray stations operating between 2007 and 2013 in many locations the observed h v ratios are clearly back azimuth dependent with a 180 periodicity which allows the fast directions and amplitudes of upper crustal anisotropy to be determined the observed patterns of anisotropy correlate well with both near surface geological features e g the intermountain seismic belt and appalachian ouachita collision belt and a previous stress model

displaced rocks strong motion and the mechanics of shallow faulting associated with the 1999 hector mine california earthquake the paucity of strong motion stations near the 1999 hector mine earthquake makes it impossible to make instrumental studies of key questions about near fault strong motion patterns associated with this event however observations of displaced rocks allow a qualitative investigation of these problems by observing the slope of the desert surface and the frictional coefficient between these rocks and the desert surface we estimate the minimum horizontal acceleration needed to displace the rocks combining this information with observations of how many rocks were displaced in different areas near the fault we infer the level of shaking given current empirical shaking attenuation relationships the number of rocks that moved is slightly lower than expected this implies that slightly lower than expected shaking occurred during the hector mine earthquake perhaps more importantly stretches of the fault with 4 m of total displacement at the surface displaced few nearby rocks on 15 slopes suggesting that the horizontal accelerations were below 0 2 g within meters of the fault scarp this low level of shaking suggests that the shallow parts of this rupture did not produce strong accelerations finally we did not observe an increased incidence of displaced rocks along the fault zone itself this suggests that despite observations of fault zone trapped waves generated by aftershocks of the hector mine earthquake such waves were not an important factor in controlling peak ground acceleration during the mainshock

laurentide and montane glaciation along the rocky mountain foothills of northeastern british columbia mapping the surficial geology of the trutch map area nts 94g provides new data on the timing of continental and montane glaciations along the foothills of northeastern british columbia striated surfaces on mountain crests were dated to the late wisconsinan substage by cosmogenic dating the striations were produced by eastward flowing ice emanating from the region of the continental divide this ice was thick enough to cross the main ranges and overtop the rocky mountain foothill summits at 2000 m above sea level asl it is argued here that such a flow unhindered by topography could only have been produced by the cordilleran ice sheet and not by local cirque glaciation during this time the cordilleran ice sheet dispersed limestone and schist erratics of western provenance onto the plains beyond the mountain front conversely the laurentide ice sheet did not reach its western limit in the foothills until after cordilleran ice retreated from the area during its maximum the laurentide ice penetra

sinking mafic body in a reactivated lower crust a mechanism for stress concentration at the new madrid seismic zone we propose a geodynamic model for stress concentration in the new madrid seismic zone nmsz the model postulates that a high density mafic body situated in the deep crust directly beneath the most seismically active part of the nmsz began sinking several thousands of years ago when the lower crust was suddenly weakened based on the fact that deformation rates in the nmsz have accelerated over the past 9 k y we envision the source of this perturbation to be related to the last north american deglaciation excess mass of the mafic body exerts a downward pull on the elastic upper crust leading to a cycle of primary thrust faulting with secondary strike slip faulting after which continued sinking of the mafic body reloads the upper crust and renews the process this model is consistent with the youth of activity the generation of a sequence of earthquakes and the velocity evolution during interseismic periods which depend upon the density contrast of the mafic body with respect to the surrounding crust its volume and the viscosity of the lower crust

late cretaceous and cenozoic geology of the new madrid seismic zone structure contour maps constructed from well seismic reflection and outcrop data of the tops of the paleozoic section upper cretaceous section paleocene midway group and eocene section illustrate the post paleozoic structure of the new madrid seismic zone region isopach maps of the late cretaceous section midway group and eocene section help constrain the timing of the structural events these maps which encompass much of the northern mississippi embayment reveal reactivation of the underlying late precambrian cambrian reelfoot rift during midway group deposition but no reactivation during late cretaceous or eocene deposition the structure contour maps also indicate a subtle south plunging depression on the tops of the paleozoic upper cretaceous and midway group along the axis of the northern mississippi embayment that we have called a trench this trench is 50 km wide has a maximum depth of 100 m and appears to have formed during the eocene the trench s western boundary coincides with the blytheville arch lake county uplift and its southeastern margin underlies memphis tennessee the blytheville arch lake county uplift is the structure responsible for the new madrid seismic zone and thus it is possible that the southeastern margin of the trench is also a fault zone northern mississippi embayment post paleozoic stratigraphy consists of sands silts and clays that thicken from 477 m at new madrid missouri to 987 m near memphis tennessee the uniformity of these sediments indicates their elastic properties and therefore seismic velocities are very similar however variations in cementation and unconformities within the section may influence seismic wave propagation manuscript received 17 june 1999

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

analysing the 1811 1812 new madrid earthquakes with recent instrumentally recorded aftershocks although dynamic stress changes associated with the passage of seismic waves are thought to trigger earthquakes at great distances more than 60 per cent of all aftershocks appear to be triggered by static stress changes within two rupture lengths of a mainshock1 2 3 4 5 the observed distribution of aftershocks may thus be used to infer details of mainshock rupture geometry6 aftershocks following large mid continental earthquakes where background stressing rates are low are known to persist for centuries7 8 and models based on rate and state friction laws provide theoretical support for this inference9 most past studies of the new madrid earthquake sequence have indeed assumed ongoing microseismicity to be a continuing aftershock sequence10 11 12 here we use instrumentally recorded aftershock locations and models of elastic stress change to develop a kinematically consistent rupture scenario for three of the four largest earthquakes of the 1811 1812 new madrid sequence our results suggest that these three events occurred on two contiguous faults producing lobes of increased stress near fault intersections and end points in areas where present day microearthquakes have been hitherto interpreted as evidence of primary mainshock rupture we infer that the remaining new madrid mainshock may have occurred more than 200 km north of this region in the wabash valley of southern indiana and illinois an area that contains abundant modern microseismicity and where substantial liquefaction was documented by historic accounts our results suggest that future large mid plate earthquake sequences may extend over a much broader region than previously suspected

distribution of precarious rocks at the nevada test site comparison with ground motion predictions from nuclear tests 1 precarious rocks have been proposed as ground motion seismoscopes that have been in place thousands of years thereby providing important constraints on seismic hazard estimates numerous precarious rocks exist in the region of large nuclear explosions at nevada test site nts ground motion recordings from nuclear tests have provided regression equations for ground acceleration and ground velocity as a function of distance and explosion yield for comparison with constraints from precarious rocks very near large nts explosions there are no precarious rocks or semiprecarious rocks cliff faces are shattered and numerous recent rockfalls have fresh white surfaces covered by caliche proof that the rockfalls have been caused by the explosions as the distance from the explosions increases and the contours for predicted ground motion decrease fewer and fewer rockfalls are observed some semiprecarious rocks begin to appear and finally numerous precarious rocks are found with no evidence of recent rockfalls to quantify the calibration of precarious rocks and ground motion we 1 do field tests on the rocks to determine the quasi static and dynamic toppling accelerations 2 carry out computer tests using estimates of waveforms from nts explosions and 3 compare the results with ground motions predicted by empirical regressions for nuclear explosions we conclude that these comparisons provide an important validation of the precarious rock methodology and an approximate validation of preliminary estimates of toppling accelerations

reconstructions of mountain glacier profiles northeastern united states ice surface profiles are reconstructed for 37 proposed mountain glacier localities in the northeastern united states including sites in the catskill and adirondack mountains new york the green mountains vermont the white mountains new hampshire and the longfellow mountains and mount katahdin maine the reconstructions determined numerically from the shear stress equation gh sin show estimated ice thickness as a function of valley position and slope in general ice thickness increases from zero at the position of the glacier terminus terminal position is a required input to the model to some equilibrium slab thickness in the upper valley where the slab thickness is inversely proportional to valley slope glacier reconstructions support the empirical evidence for former local glaciation in the catskill and white mountains and in some localities in the adirondack mountains in several proposed glacier locations in the adirondack and green mountains in very shallow basins and in very gently sloping valleys however estimated ice thicknesses exceed the depth of their respective valleys or the elevation of up valley cols suggesting a re evaluation of these localities as sites of local confined ice flow confirmed localities of local glaciation are mostly in valleys with mean gradients greater than about 5 where reconstructed ice thicknesses are less than about 200 m

significant motions between gps sites in the new madrid region implications for seismic hazard position time series from global positioning system gps stations in the new madrid region were differenced to determine the relative motions between sta tions uncertainties in rates were estimated using a three component noise model con sisting of white flicker and random walk noise following the methodology of langbein 2004 significant motions of 0 37 0 07 one standard error mm yr were found between sites ptgvand stle for which the baseline crosses the inferred deep portion of the reelfoot fault baselines between stle and three other sites also show significant motion site mcty adjacent to stle also exhibits significant motion with respect to ptgv these motions are consistent with a model of interseismic slip of about 4 mm yr on the reelfoot fault at depths between 12 and 20 km if constant over time this rate of slip produces sufficient slip for an m 7 3 earthquake on the shallow portion of the reelfoot fault using the geologically derived recurrence time of 500 years this model assumes that the shallow portion of the fault has been pre viously loaded by the intraplate stress a gps site near little rock arkansas shows significant southward motion of 0 3 0 4 mm yr 0 08 mm yr relative to three sites to the north indicating strain consistent with focal mechanisms of earthquake swarms in northern arkansas

source and site characteristics of earthquakes that have caused exceptional ground accelerations and velocities abstract this study investigates the characteristics of the free field strong motion records that have yielded the 100 largest peak accelerations and the 100 largest peak velocities in a relatively complete database of accelerograms recorded through the summer of 2007 the peak is defined as the maximum zero to peak amplitude of the acceleration or velocity vector during the strong shaking this compilation includes 35 records with peak acceleration greater than 1 g 980 cm sec 2 and 40 records with peak velocities greater than 100 cm sec the results sample an estimated 150 000 instrument years of strong motion recordings the geometric mean of the two horizontal components of acceleration or velocity as used in many ground motion prediction equations is typically 0 76 times the magnitude of this vector peak accelerations in the top 100 come from earthquakes as small as magnitude 4 8 while velocities in the top 100 all come from earthquakes with magnitude 5 7 or larger these records are dominated by crustal earthquakes with thrust oblique thrust or strike slip mechanisms normal faulting mechanisms in crustal earthquakes constitute under 5 of the records in the databases searched and an even smaller percentage of the 100 largest acceleration or velocity records all national earthquake hazard reduction program site categories have contributed exceptional records in proportions similar to the extent that they are represented in larger databases

dense lower crust elevates long term earthquake rates in the new madrid seismic zone knowledge of the local state of stress is critical in appraising intraplate seismic hazard inverting earthquake moment tensors we demonstrate that principal stress directions in the new madrid seismic zone nmsz differ significantly from those in the surrounding region faults in the nmsz that are incompatible with slip in the regional stress field are favorably oriented relative to local stress we jointly analyze seismic velocity gravity and topography to develop a 3 d crustal and upper mantle density model revealing uniquely dense lower crust beneath the nmsz finite element simulations then estimate the stress tensor due to gravitational body forces which sums with regional stress the anomalous lower crust both elevates gravity derived stress at seismogenic depths in the nmsz and rotates it to interfere more constructively with far field stress producing a regionally maximal deviatoric stress coincident with the highest concentration of modern seismicity moreover predicted principal stress directions mirror variations observed independently in moment tensors at the nmsz and across the region

age of the subducting pacific slab beneath east asia and its geodynamic implications abstract we study the age of the subducting pacific slab beneath east asia using a high resolution model of p wave tomography and paleo age data of ancient seafloor our results show that the lithosphere age of the subducting slab becomes younger from the japan trench 130 ma to the slab s western edge 90 ma beneath east china and the flat stagnant slab in the mantle transition zone mtz is the subducted pacific plate rather than the proposed izanagi plate which should have already collapsed into the lower mantle the flat pacific slab has been in the mtz for no more than 10 20 million years considerably less than the age of the big mantle wedge beneath east asia 110 million years hence the present flat pacific slab in the mtz has contributed to the cenozoic destruction of the east asian continental lithosphere with extensive intraplate volcanism and back arc spreading whereas the destruction of the north china craton during the early cretaceous 140 110 ma was caused by the subduction of the izanagi or the paleo pacific plate

precarious rock constraints on ground motion from historic and recent earthquakes in southern california new examples of precarious rocks are presented these rocks provide constraints on ground motion for historical and recent large earthquakes an approximate field calibration of the precarious rock methodology was provided by the m 7 1 hector mine earthquake of 16 october 1999 previously documented rocks at granite pass near kelso california brune 1996 were overturned by the earthquake and a nearby strong motion station indicated ground motions of about 0 2 g consistent with the toppling accelerations estimated in the published article the time of the last earthquake for the section of the fault nearest the precarious rocks has recently been determined to be more than 10 ka consistent with previously estimated ages for precarious rocks this first example of an actual earthquake field test lends strong support to the precarious rock methodology the other rocks discussed here give constraints on ground motions for historic earthquakes such as the 1812 and 1857 san andreas fault earthquakes the 1899 and 1912 san jacinto fault earthquakes the 1952 kern county earthquake and possibly recent but prehistoric earthquakes on the banning and garlock faults the ground motion constraints for these earthquakes are lower than predicted by some recent ground motion attenuation curves which are extrapolations to near fault distances from a data set dominated by data from larger distances but are generally consistent with peak ground accelerations observed from the recent large turkey and taiwan earthquakes and provide important additional information for seismic hazard analysis on the other hand there are areas where precarious rocks would be expected on the basis of previous studies but are apparently not found this suggests possible earthquakes on previously unrecognized or only recently recognized faults one such area is in northwestern san diego and southwestern orange county between the elsinore and newport inglewood faults the lack of precarious rocks in this area might be attributed to recent earthquakes on the blind thrust faults proposed in the area by grant et al 1999 2002 and rivero et al 2000

earthquakes quaternary faults and seismic hazard in california data describing the locations slip rates and lengths of quaternary faults are the primary basis in this work for constructing maps that characterize seismic hazard in california the expected seismic moment m0e and the strength of ground shaking resulting from the entire rupture of each mapped fault or fault segment are estimated using empirical relations between seismic moment m0 rupture length source to site distance and strong ground motions assuming a fault model whereby the repeat time t of earthquakes on each fault equals m0e m0g where the moment rate m0g is proportional to fault slip rate it is observed that the moment frequency distribution of earthquakes predicted from the geologic data agrees well with the distribution determined from a 150 year historical record the agreement is consistent with the argument that the geologic record of quaternary fault offsets contains information sufficient to predict the average spatial and size distribution of earthquakes through time in california the estimates of t for each fault are the foundation for constructing maps that depict the average return period of 0 1g peak horizontal ground accelerations and the horizontal components of peak acceleration peak velocity and the pseudovelocity response at 1 period and 5 damping expected to occur at the level of 0 1 probability during a 50 year period of time a map is also formulated to show the probability that 0 1g horizontal ground accelerations will occur during the next 50 years the maps serve to illustrate the potential value of quaternary fault studies for assessing seismic hazard interpretation of available slip rates indicates that the largest and most frequent occurrence of potentially destructive strong ground motions are associated principally with the san andreas san jacinto calaveras hayward and ventura basin fault zones other regions of similarly high hazard may yet remain unrecognized this inadequacy results primarily from an incomplete data set numerous faults for example are mapped along the coastal region of northern california and within the modoc plateau but relatively few studies relating to fault slip rate are reported a similar problem exists for other stretches of coastal california where marine reflection studies provide evidence of active faulting offshore yet yield little or no information of fault slip rate geological and geophysical field studies can work to remove these deficiencies a concerted effort to locate and define rates of activity on all faults in california is the most promising means to further quantify present levels of seismic hazard in california

rocking and overturning of precariously balanced rocks by earthquakes abstract precariously balanced rocks in seismically active regions may be used to provide constraints on probabilistic estimates of strong ground motion a computer program is developed to study the rocking response of models of precariously balanced rocks when subjected to various levels of ground accelerations the computer code validated by comparing the numerical and experimental results for simple shaped rigid blocks is used to study the rocking response of rectangular blocks with various aspect ratios to the el centro strong ground motion recording of the 18 may 1940 imperial valley earthquake results show that during earthquakes the horizontal components of ground motion play a dominant role in overturning rocking objects the rederived and corrected housner9s equation 1963 could be used to estimate the minimum overturning acceleration of precarious rocks at different frequencies by measuring the quasi static toppling acceleration in the field

methodology for using precarious rocks in nevada to test seismic hazard models abstract fields of precariously balanced rocks indicate that strong earthquake motions have not occurred at that site since the precarious rocks developed these fields can be characterized with an estimate of the peak acceleration that would be sufficient to topple the rocks and an estimate of how long the rocks have been precarious this article uses this information to test the input to probabilistic seismic hazard analysis the fundamental assumption is that the probability of exceeding a ground motion capable of toppling a precarious rock during a time period equal to the age of the rock is equal to the confidence level at which the inputs to the probabilistic seismic hazard analysis can be rejected we performed a probabilistic seismic hazard analysis for 26 sites of observed precarious rocks in nevada using preliminary estimates of the toppling acceleration and the age of the features two standard models are rejected with over 95 confidence by most of the precarious rock observations we consider several possible explanations for the inconsistency a probabilistic seismic hazard analysis that eliminates the area sources and only includes faults is consistent with the precarious rock observations at most of the sites spatial clustering of small magnitude events could maintain this effect without contradicting the precarious rock observations however it may not be necessary to completely reject the area sources from our probabilistic seismic hazard analysis the physics of rock stability may allow increasing the minimum magnitude to 6 0 in the area sources because the short duration of high frequency accelerations in smaller events may not topple all precarious rocks the precarious rocks could generally have small site effects attenuation models may overestimate ground motions on the foot wall of normal faults and a different attenuation model in general might decrease the ground motion predictions the ergodic assumption that is made in estimating the uncertainty in attenuation models may be contributing to overestimate the hazard individually each of these effects might allow more of the precarious rock sites to be consistent with the area source zones

precariously balanced rocks and ground motion maps for southern california groups of precariously balanced rocks are effectively low resolution strong motion seismoscopes that have been operating on solid rock outcrops for thousands of years and once the methodology has been developed can provide important information about seismic risk in one zone near victorville only 30 km from the nearest point on the san andreas fault more than 50 precarious rocks have been documented widespread rock varnish suggests that many of these rocks have been in their current unstable positions for thousands of years we have established the mechanical basis for rough estimates of the horizontal accelerations necessary to topple these rocks using field observations and numerical and physical modeling to verify that zones of precarious rocks do not occur near historic earthquakes searches using binoculars were made along roads with occasional foot surveys near large earthquakes based on these reconnaissance searches we conclude that no precarious rock zones are found within 15 km of zones of high energy release of historic large earthquakes to document the occurrence of precarious rocks in southern california road surveys were carried out along major roads four zones of precarious rocks and seven other zones of somewhat less precarious rocks have been documented published probabilistic ground motion maps for southern california are compared with the occurrence of zones of precarious and semi precarious rocks the results are encouraging and suggest that eventually studies of precarious rocks will provide important constraints on the assumptions on which the maps are based results from studies of precarious rocks may eventually provide important information for siting and design of sensitive structures such as hospitals and power plants precarious rocks give a direct indication of past ground shaking in contrast to the indirect inference provided by fault trenching studies which may be subject to uncertainties in the actual time history of slip due to the fault e g fault creep slow earthquakes or unknown dynamic stress drop it is concluded that precarious rocks warrant further study and quantitative analysis

comparison of recent probabilistic seismic hazard maps for southern california abstract probabilistic seismic hazard psh maps for southern california produced from the models of ward 1994 the working group on california earthquake probabilities 1995 and the u s geological survey and california division of mines and geology frankel et al 1996 petersen et al 1996 show the peak ground accelerations predicted with each model to occur at 10 probability in 50 years and the probability that 0 2 g will occur in 30 years for rock site conditions differences among the maps range up to 0 4 g and 50 respectively we examine the locations and magnitudes of the differences as a basis to define the issues and avenues of research that may lead to more confident estimates of psh in the future our analysis shows that three major factors contribute to the observed differences between the maps they are the size of maximum magnitude assigned to a given fault the proportion of predicted earthquakes that are distributed off the major faults and the use of geodetic strain data to predict earthquake rates

between event variance for large repeating earthquakes we estimate the variance in ground motions related to repeated large earthquakes occurring on the same fault segment with similar magnitudes we find eight earthquake pairs for which suitable strong motion records exist two are crustal strike slip earthquakes from california and six are subduction zone earthquakes from japan we consider only large earthquakes and deal with frequencies greater than the earthquake corner frequency so the variability that is considered here is related to smaller scale differences in the rupture process particularly on the part of the fault nearest the station we find that the variance of the 5 damped spectral accelerations of these pairs termed graphic 1 averages to about 45 and 80 of 2 for the crustal and subduction zone earthquakes respectively in which 2 is the contribution of source variability to the total variability of ground motion estimated by some recent ground motion prediction equations we suggest that graphic 2 is lower than 2 for the frequencies at which graphic 3 is estimated because it depends primarily on only local physical properties of a fault that are the same in repeated earthquakes we therefore suggest that at sites where the hazard is controlled by a single rerupturing source one could potentially use a between event variance that is smaller than 2 in seismic hazard calculations thus these results may help to resolve the inconsistencies that are now present between the national hazard maps and some precariously balanced rocks in southern california 1 embed inline graphic 1 gif 2 embed inline graphic 2 gif 3 embed inline graphic 3 gif

rift structure of the northern mississippi embayment from the analysis of gravity and magnetic data gravity and magnetic data have been simultaneously inverted to derive a crustal model of the northern mississippi embayment which is compatible with geologic and seismic refraction data the results indicate that this region is a site of a late precambrian early paleozoic rift system an anomalously dense layer is present at the base of the crust and thickens beneath a broad northeast trending graben that formed during the initial phases of rifting the thickest part of anomalous crust occurs beneath the region of greatest seismic activity which lies within the geographic limits of the graben it is suggested from this correlation that strain accumulates within the anomalous crust layer which acts as an inhomogeneity in a relatively homogeneous lithosphere based on the results of this study and previous geophysical studies the complex tectonic evolution of the upper mississippi embayment since late precambrian time is rift formation possibly along a pre existing shear zone graben development in cambrian time reactivations in permian and cretaceous time and subsequent formation of the present mississippi embayment and fault zones with large influences from rift structures

dating offset fans along the mojave section of the san andreas fault using cosmogenic 26al and 10be analysis of cosmogenic 1 0 be and 2 6 al in samples collected from exposed boulders n 20 and from buried sediment n 3 from offset fans along the san andreas fault near little rock california yielded ages ranging from 16 to 413 ka which increase with dis lance from their source at the mouth of little rock creek in order to determine the age of the relatively younger fans the erosion rate of the boulders and the cosmogenic nuclide inheritance from exposure prior to deposition in the fan were established cosmogenic nuclide inheritance values that range between 8 5 x 10 3 and 196 10 3 atoms 1 0 be g 1 quartz were determined by measuring the concentrations and ratios of 1 0 be and 2 6 al in boulders n 10 and fine sediment n 7 at the outlet of the present active stream boulder erosion rate ranging between 17 and 160 mm k y 1 was estimated by measuring 1 0 be and 2 6 al concentrations in nearby bedrock outcrops n 8 since the boulders on the fans represent the most resistant rocks in this environment we used the lowest rate for the age calculations monte carlo simulations were used to determine ages of 16 5 and 29 7 ka for the two younger fan surfaces older fans older than 100 ka were dated by analyzing 1 0 be and 2 6 al concentrations in buried sand samples the ages of the three oldest fans range between 227 242 and 413 185 ka although fan age determinations are accompanied by large uncertainties the results of this study show a clear trend of increasing fan ages with increasing distance from the source near little rock creek and provide a long term slip rate along this section of the san andreas fault slip rate along the mojave section of the san andreas fault for the past 413 k y can be determined in several ways the average slip rate calculated from the individual fan ages is 4 2 0 9 cm yr 1 a linear regression through the data points implies a slip rate of 3 7 1 0 cm yr 1 a most probable slip rate of 3 0 1 0 cm yr 1 is determined by using a 2 test these rates suggest that the average slip along the mojave section of the san andreas fault has been relatively constant over this time period the slip rate along the mojave section of the san andreas fault determined in this study agrees well with previous slip rate calculations for the quaternary

an alternative approach to probabilistic seismic hazard analysis in the aegean region using monte carlo simulation abstract the aegean is the most seismically active and tectonically complex region in europe damaging earthquakes have occurred here throughout recorded history often resulting in considerable loss of life the monte carlo method of probabilistic seismic hazard analysis psha is used to determine the level of ground motion likely to be exceeded in a given time period multiple random simulations of seismicity are generated to calculate directly the ground motion for a given site within the seismic hazard analysis we explore the impact of different seismic source models incorporating both uniform zones and distributed seismicity a new simplified seismic source model derived from seismotectonic interpretation is presented for the aegean region this is combined into the epistemic uncertainty analysis alongside existing source models for the region and models derived by a k means cluster analysis approach seismic source models derived using the k means approach offer a degree of objectivity and reproducibility into the otherwise subjective approach of delineating seismic sources using expert judgment similar review and analysis is undertaken for the selection of peak ground acceleration pga attenuation models incorporating into the epistemic analysis greek specific models european models and a next generation attenuation model hazard maps for pga on a rock site with a 10 probability of being exceeded in 50 years are produced and different source and attenuation models are compared these indicate that greek specific attenuation models with their smaller aleatory variability terms produce lower pga hazard whilst recent european models and next generation attenuation nga model produce similar results the monte carlo method is extended further to assimilate epistemic uncertainty into the hazard calculation thus integrating across several appropriate source and pga attenuation models site condition and fault type are also integrated into the hazard mapping calculations these hazard maps are in general agreement with previous maps for the aegean recognising the highest hazard in the ionian islands gulf of corinth and hellenic arc peak ground accelerations for some sites in these regions reach as high as 500 600 cm s 2 using european nga attenuation models and 400 500 cm s 2 using greek attenuation models

lithospheric velocity structure of the new madrid seismic zone a joint teleseismic and local p tomographic study 1 the enigmatic seismicity in the new madrid seismic zone nmsz has been attributed to some abnormal lithospheric structure including the presence of dense mafic intrusions and a low viscosity lower crust however the area s detailed lithospheric structure remains unclear here we invert 2 056 teleseismic p and 12 226 local p first arrival times from a recent nine year dataset to infer the lithospheric velocity structure beneath the nmsz our results show that the seismically active zone is associated with a local ne sw trending low velocity anomaly in the lower crust and upper mantle instead of high velocity intrusive bodies proposed in previous studies the low velocity anomaly is on the edge of a high velocity lithospheric block consistent with the notion of stress concentration near rheological boundaries this lithospheric weak zone may shift stress to the upper crust when loaded thus leading to repeated shallow earthquakes

crust and upper mantle structure of the new madrid seismic zone insight into intraplate earthquakes we determine a 3 d p wave velocity model of the crust and upper mantle down to 400 km depth to investigate structural heterogeneity and its influences on the generation of intraplate earthquakes in the new madrid seismic zone we used 4871 high quality arrival times from 187 local earthquakes and 30 846 precise travel time residuals from 1041 teleseismic events recorded by the earthscope usarray transportable array our results show that beneath the reelfoot rift a significant low velocity low v zone exists in the upper mantle down to 200 km depth with a large volume of 200 x 200 x 150 km 3 the origin of the low v zone may be related to the passage of the bermuda hotspot and the stalled ancient farallon slab materials foundering in the mantle transition zone this low v zone may have relatively low shear strength and act as a viscously weak zone embedded in the lithosphere being apt to concentrate tectonic stress and transfer stress to the seismogenic faults in the upper crust leading to the large intraplate earthquakes in the new madrid seismic zone c 2014 elsevier b v all rights reserved

on the trade off between seismic anisotropy and heterogeneity numerical simulations and application to northeast japan p wave tomography has been recently used to study 3 d azimuthal and radial anisotropy of subduction zones and continental regions however the fundamental issue about the trade off between the isotropic and anisotropic structures is still unclear in this study we investigate this issue systematically with comprehensive synthetic tests our results indicate that good ray coverage in the azimuth for azimuthal anisotropy and incidence for radial anisotropy is required for determining reliable anisotropic models the isotropic and anisotropic structures are strongly coupled and smearing effects are significant when the rays used in the inversion are limited in a small range of azimuth or incidence we therefore plot ray azimuth and ray incidence ellipses at every grid nodes and propose to use the normalized length of the short axis i e the ratio of the short axis and long axis lengths for estimating the ray coverage quantitatively applying our novel approach to a large number of high quality arrival time data of local shallow and intermediate depth earthquakes we obtained new tomographic images of 3 d p wave azimuthal and radial anisotropy in northeast japan both the azimuthal and radial anisotropy results are determined reliably for the shallow parts of the study region whereas the smearing effects are significant in the deeper part of the mantle wedge and the subducting slab our results show dominant trench normal and vertical fast anisotropy in the mantle wedge while trench parallel and horizontal fast anisotropy in the subducting slab which indicates different dynamics in different domains of the subduction zone

pressure sensitivity of olivine slip systems and seismic anisotropy of earth s upper mantle the mineral olivine dominates the composition of the earth s upper mantle and hence controls its mechanical behaviour and seismic anisotropy experiments at high temperature and moderate pressure and extensive data on naturally deformed mantle rocks have led to the conclusion that olivine at upper mantle conditions deforms essentially by dislocation creep with dominant 100 slip the resulting crystal preferred orientation has been used extensively to explain the strong seismic anisotropy observed down to 250 km depth the rapid decrease of anisotropy below this depth has been interpreted as marking the transition from dislocation to diffusion creep in the upper mantle but new high pressure experiments suggest that dislocation creep also dominates in the lower part of the upper mantle but with a different slip direction here we show that this high pressure dislocation creep produces crystal preferred orientations resulting in extremely low seismic anisotropy consistent with seismological observations below 250 km depth these results raise new questions about the mechanical state of the lower part of the upper mantle and its coupling with layers both above and below

assessment of the site conditions of precariously balanced rocks in the mojave desert southern california we examine whether groups of precariously balanced rocks in the mo jave desert southern california may have remained standing for thousands of years because anomalous site conditions prevent the rocks from toppling during large earth quakes measurements of peak ground velocity at bandwidths of 0 5 1 1 2 2 4 4 8 and 8 16 hz for 56 earthquakes ml 3 5 4 8 at two sites of precarious rocks are compared with velocities recorded for the same earthquakes by three tri net stations located on engineering rock national earthquake hazard reduction program nehrp site class b we find that the residuals the logarithm of the ratio of the amplitudes recorded at the precarious rock sites to the trinet amplitudes for the same earthquake and epicentral distance at frequencies less than about 4 hz are negative i e deamplification of 50 250 whereas the residuals are slightly positive at the higher frequencies i e amplification of up to 25 50 high frequency ground motions e g peak ground acceleration may therefore be slightly amplified at the precarious rock sites which means that site conditions do not appear to explain the existence of the precarious rocks in areas where high peak ground accelerations are predicted in recent probabilistic seismic hazard psh models this discrepancy between the precarious rocks and the psh models should be urgently resolved

studies on precarious rocks in the epicentral area of the ad 1356 basle earthquake switzerland summary for the first time precarious rocks have been analysed in the epicentral area of the ad 1356 basle earthquake in northern switzerland several cliff sites in flat lying thickly bedded upper jurassic coral limestones in the jura mountains were investigated seven blocks are regarded as precarious with respect to earthquake strong ground motions the age of these precarious rocks could not be determined directly as for instance by radiometric dating methods however based on slope degradation processes it can be concluded that the formation of these blocks predates the ad 1356 basle earthquake the acceleration required to topple a precarious rock from its pedestal is estimated using geometrical data for individual block sections and earthquake strong motion records from stations on rock sites in the european strong motion database as input data for the computer program rocking v1 0 from the seismological laboratory university of nevada reno the calculations indicate that toppling of a precarious rock largely depends on earthquake strength but also on the frequency spectrum of the signal although most investigated precarious rocks are surprisingly stable for ground motions similar to those expected to have occurred during the ad 1356 basle earthquake at least two blocks are clearly precariously balanced with peak toppling accelerations lower than 0 3 g possible reasons why these blocks did not topple during the ad 1356 basle earthquake include incomplete separation from their base sliding of precarious rocks their size lower than assumed ground accelerations and or duration of shaking

seismic structure and crustal magmatism at the mid atlantic ridge 35 n we have determined the three dimensional p wave velocity structure and azimuthal anisotropy within the shallow crust of the inner valley at the mid atlantic ridge mar near 35 n the best fitting one dimensional model is 0 8 km s lower in velocity at a given depth than for fast spreading ridges while the isotropic three dimensional velocity structure shows a strong lateral heterogeneity up to 1 4 km s peak to peak variation at 1 km depth in both the along and cross axis directions an anomalous region with velocities as much as 0 8 km s lower than average and 3 5 km in lateral extent is located beneath a near axis seamount we interpret this feature as a region of near solidus temperatures with possibly a small degree of partial melt from the variation of travel time residuals with azimuth we determine a best fitting p wave anisotropy model that has 4 anisotropy from 500 m to 1 km depth 2 anisotropy in the depth range 1 1 5 km and no anisotropy at greater depth this finding is consistent with vertical cracks aligned parallel to the axial valley we find no evidence for the shallow high velocity ridge parallel feature commonly observed at the axis of fast spreading ridges and attributed to high velocity dikes beneath a thin extrusive layer the absence of this anomaly at the mar implies that the axis of crustal accretion is not at a stable location with respect to the inner valley walls on timescales greater than about 25 000 years an unstable axis of accretion will result in a thicker transition from extrusive rocks to dikes which may account for the generally lower velocities beneath the mar compared with the east pacific rise and a lateral variation in upper crustal lithology

erosion rates for pinnacles formed in soft sediments and implications for constraining seismic hazard models abstract we provide estimates of the rates of natural modification of badlands pinnacles formed in soft sediments at two sites in new zealand and assess the utility of pinnacles for constraining seismic hazard models one of the sites is in a humid temperate environment and the other is in a semiarid environment rates of pinnacle modification of 30 270 mm yr equivalent to 3 27 m century are estimated by comparison of images acquired one to two decades apart the primary mode of modification is the progressive narrowing of the pinnacle column by precipitation induced erosion along with consequent loss of height as the columns become thin and unstable additional damage may have been produced by earthquake shaking for specific pinnacles but in general this is likely to be trivial given that 1 estimates of the levels of peak ground acceleration pga required to shake down the pinnacles at the two sites are at least a factor of 5 13 greater than the pgas predicted or measured from regional earthquakes during the time periods of observation in other words they would only be significantly damaged by major local earthquakes and 2 a fragile pinnacle has actually formed in the time periods of observation the pinnacle modification rates are rapid enough that they would not have existed one to two centuries ago or would have been of substantially different geometries as such pinnacles are unlikely to be useful for constraining seismic hazard models for return periods greater than a few decades at least in the context of the new zealand environment

deep structure of japan subduction zone as derived from local regional and teleseismic events we have determined a detailed three dimensional p wave velocity structure of the japan subduction zone to 500 km depth by inverting local regional and teleseismic data simultaneously we used 45 318 p wave arrivals from 1241 shallow and deep earthquakes which occurred in and around the japan islands the arrival times are recorded by the japan university seismic network which covers the entire japan islands densely and uniformly we also used 4211 travel time residuals from 100 teleseismic events which are read from seismograms recorded by seismic stations in northeastern japan in comparison with the previous results obtained from only local and regional events the present result for the area around the lower plate boundary and the mantle below the plate is determined more reliably because of the addition of 7035 data from 100 teleseismic events and 41 very deep earthquakes in the crust and uppermost mantle low velocity zones are clearly visible beneath active volcanoes in the mantle wedge the low velocity zones generally parallel with the slab and exist continuously to a depth of about 200 km which is consistent with the petrological geochemical and geodynamic studies we consider that the existence of volcanism related low velocity anomalies in the mantle wedge is a general seismological characteristic of subduction zones in light of all the available tomographic results for many subduction zones in the world the pacific slab beneath japan is imaged more clearly than in previous studies as a high velocity zone with a thickness of 80 90 km and a p wave velocity 4 6 higher than the normal mantle lower velocity anomalies are found in the mantle below the slab

the plateau glacier in the sierra de bejar iberian central system during its maximum extent reconstruction and chronology abstract a detailed study of the glacial morphology in the sierra de bejar iberian central system provided a data set of geomorphic indicators to reconstruct the paleoglaciers developed in this mountain area during the last glacial cycle late pleistocene applying a physical based glacier model and using the geomorphic indicators a three dimensional reconstruction of the ice mass during the maximum extent of the glaciers has been carried out we used this reconstruction to project hypsometric curves over the former glaciers and to estimate the elas equilibrium line altitudes of the paleoglaciers for their stage of maximum extent at this stage the sierra de bejar hosted a plateau glacier considered as a dome shaped icecap around 57 km2 in area according to our estimations the maximum thickness of the ice was 211 m the minimum elevation of paleoglaciers 1210 m asl and the regional ela was at 2010 m asl during later stages reduction in ice mass due to deglaciation caused the icecap to evolve into an icefield and finally the main glacier was fragmented in valley and cirque glaciers the geochronological data obtained with 10be provides an age of 27 ka for the maximum extent of the glaciers gm whereas the global last glacial maximum lgm represents a younger stage of the sierra de bejar glacier evolution finally the new data obtained in the sierra de bejar allow evaluating the influence of some factors such as the continentality and latitudinal location in the development of glacial processes in these areas of the iberian central system

response of desert pavement to seismic shaking hector mine earthquake california 1999 1 the october 1999 mw 7 1 hector mine earthquake in the mojave desert california generated characteristic surface disturbances on nearby desert pavements these disturbances included 1 zones of wholesale gravel displacement interspersed with zones of intact pavement 2 displaced and rotated cobbles 3 moats around loosened embedded boulders 4 filling of abandoned cobble sockets boulder moats and other depressions with gravel and 5 formation of narrow subparallel linear strips of exposed fine grained subpavement matrix matrix lineations clasts displaced from matrix lineations and from cobble sockets tended to move downslope sharp boundaries of matrix lineations and slope controlled displacement directions on slopes of only a few degrees indicated that clasts remained close to the pavement surface during shaking the regular few decimeter spacing of matrix lineations suggests the presence of standing waves during seismic shaking boulder moats probably have good preservation potential and at some desert pavement locations might provide information on paleoseismic shaking although readily produced by coseismic shaking displaced cobbles are unreliable indicators of past earthquake activity because of potential multiple origins for an assumed earthquake recurrence interval of 10 ka seismically driven sediment fluxes similar to those generated by the hector mine earthquake at the lavic siding pavement study site may be marginally competitive with aseismic smoothing mechanisms driven by bioturbation rainbeat and wash for a 1 ka recurrence interval seismic smoothing is likely to play a significant role in pavement evolution

constraints on probabilistic seismic hazard models from unstable landform features in new zealand we undertake the first new zealand based pilot study to investigate the use of ancient precariously balanced rocks rocks that are unstably balanced on top of a pedestal as a criteria for testing estimates of earthquake shaking from probabilistic seismic hazard models for long return periods to date research to test seismic hazard models in new zealand has been restricted to the short historical record of earthquakes our survey of five sites in the south island of new zealand has yielded a total of 28 precariously balanced rocks which on the basis of established methodology are used to provide estimates of the maximum ground motions that could have occurred at the sites since the rocks became precarious age estimates for the precariously balanced rocks 40 400 to 55 300 years for central otago schist rocks and 77 300 years for northwest nelson granitic rocks are made from a limited number of cosmogenic dates obtained from bedrock removed from the pedestals of the rocks comparisons of the maximum peak ground accelerations and ages of the precariously balanced rocks with seismic hazard curves derived from the new zealand national seismic hazard model show that the rocks indicate considerably lower hazard than the seismic hazard model at sites located within 5 km of active faults whereas the agreement is favorable for sites located away from active faults the variability about the median estimates of peak ground acceleration for the fault sources and or the median accelerations for the fault sources assumed in the seismic hazard model may therefore be overestimated for the sites near active faults

precarious rock evidence for low near source accelerations for trans tensional strike slip earthquakes abstract this paper describes precarious rock evidence for low ground motions associated with extensional sections of strike slip faults recent evidence from physical and numerical models and data regressions has indicated that ground motion for extensional strike slip regions may be lower than for strike slip faults with a large fault normal tectonic stress component and for thrust faults in general data from compressional strike slip and thrust earthquakes dominates the database used in most regression curves for ground acceleration and in the calculation of current probabilistic seismic hazard maps therefore estimates of ground accelerations on these seismic hazard maps may be too high for sites near extensional sections of strike slip faults this paper discusses precariously balanced rock data from three areas near extensional sections of strike slip faulting 1 the region of the honey fault california with an active holocene fault 2 the red rock canyon region of the garlock fault near a dilatational step over and 3 the region just south of beaumont california near the hemet dilatational step over in the san jacinto fault these are all active strike slip faults with at least a few large earthquakes in the holocene and in the case of the san jacinto example historic large earthquakes m 7 thus the precarious rocks at these sites are evidence of relatively low ground motions associated with extensional strike slip faulting the results of this study could be very important in developing more detailed seismic hazard maps in the future

strong proximal earthquakes revealed by cosmogenic 3he dating of prehistoric rockfalls christchurch new zealand the 2011 rupture of previously undetected blind faults beneath christchurch new zealand in moment magnitude m w 6 2 and 6 0 earthquakes triggered major rockfalls that caused fatalities and infrastructure damage here we use field geospatial seismologic numerical modeling and cosmogenic 3 he data to provide first evidence for prehistoric rockfall ca 8 6 ka and a possible preceding event ca 14 13 ka at a site where extensive rockfall occurred in the christchurch earthquakes the long 7 1 k y time intervals between successive rockfall events and the high peak ground velocity thresholds required for rockfall initiation at this site 20 30 cm s preclude earthquakes from major identified seismic sources including the plate boundary alpine fault as likely rockfall triggering sources rockfalls were probably triggered by strong paleoearthquakes sourced from active faults proximal i e 10 20 km to christchurch including the sources of the 2011 christchurch earthquakes and or other currently unidentified faults given the inherent incompleteness of seismic source catalogues and challenges in obtaining earthquake chronologies for blind faults high scientific priority should be given to the search for and analysis of geologic records of strong earthquake shaking near populated areas

precarious rock evidence for low ground shaking on the footwall of major normal faults preliminary interpretations of precarious rock observations suggest relatively low near fault 3 km footwall ground motions for normal faults relative to strike slip faults as reported for a dynamic foam rubber normal fault model by brune and anooshehpoor 1999 since for large normal fault earthquakes there are no instrumental data at such close distances from the fault trace precarious rocks may provide important data for estimating the ground motion for these earthquakes use of empirical curves in the literature based mainly on strike slip data may lead to overestimates of seismic hazard for the near fault footwall of normal faults these preliminary results may warrant more comprehensive and more quantitative followup studies

consistent slow exhumation in a late cenozoic glaciated landscape the presidential and carter ranges of the white mountains in new hampshire usa abstract to investigate the ability of the onset of quaternary glaciations to drive an acceleration in erosion rates we compare short timescale 103 104 yr erosion rates using the in situ terrestrial cosmogenic nuclide tcn 10be to longer time scale exhumation rates 106 107 yr using the apatite u th he method in the presidential and carter ranges of the white mountains in new hampshire usa our results are generally consistent with but refine previous studies using similar techniques resulting u th he ages and thermal modeling from mt washington in the presidential range n 4 91 7 to 117 3 ma suggest that exhumation rates from the late cretaceous to the present were slow

precarious rock and overturned transformer evidence for ground shaking in the ms 7 7 kern county earthquake an analog for disastrous shaking from a major thrust fault in the los angeles basin precariously balanced rocks and overturned transformers in the vicinity of the white wolf fault provide constraints on ground motion during the 1952 m s 7 7 kern county earthquake a possible analog for an anticipated large earthquake in the los angeles basin shaw et al 2002 dolan et al 2003 on the northeast part of the fault preliminary estimates of ground motion on the footwall give peak accelerations considerably lower than predicted by standard regression curves on the other hand on the hanging wall there is evidence of intense ground shattering and lack of precarious rocks consistent with the intense hanging wall accelerations suggested by foam rubber modeling numerical modeling and observations from previous thrust fault earthquakes there is clear evidence of the effects of rupture directivity in ground motions on the hanging wall side of the fault from both precarious rocks and numerical simulations on the southwest part of the fault which is covered by sediments the thrust fault did not reach the surface blind thrust overturned and damaged transformers indicate significant transfer of energy from the hanging wall to the footwall an effect that may not be as effective when the rupture reaches the surface is not blind transformers near the up dip projection of the fault tip have been damaged or overturned on both the hanging wall and footwall sides of the fault the transfer of energy is confirmed in a numerical lattice model and could play an important role in a similar situation in los angeles we suggest that the results of this study can provide important information for estimating the effects of a large thrust fault rupture in the los angeles basin specially given the fact that there is so little instrumental data from large thrust fault earthquakes

spatial distribution of glacial erosion rates in the st elias range alaska inferred from a realistic model of glacier dynamics glaciers have been principal erosional agents in many orogens throughout much of the recent geological past a modern example is the st elias mountains in southeastern alaska it is a highly convergent complex orogen which has been glaciated for much of its history we examine the seward malaspina glacier system which comprises two of the largest temperate glaciers in the world we focus on the pattern of erosion within its narrow passage through the st elias mountains the seward throat measured glacier surface velocities and elevations provide constraints for a full stress numerical flowband model that enables us to quantitatively determine the glacier thickness profile which is not easily measured on temperate glaciers and the basal characteristics relevant for erosion these characteristics at the bed namely the water pressure normal and shear stresses and sliding velocity are then used to infer the spatial variation in erosion rates using several commonly invoked erosion laws the calculations show that the geometry of the glacier basin exerts a far stronger control on the spatial variation of erosion rates than does the equilibrium line altitude which is often assumed to be important in studies of glaciated orogens the model provides a quantitative basis for understanding why erosion rates are highest around the seward throat which is generally consistent with local and large scale geological observations and thermochronologic evidence moreover model results suggest how glacier characteristics could be used to infer zones of active or recent uplift in ice mantled orogens 2012 american geophysical union all rights reserved

constraining regional paleo peak ground acceleration from back analysis of prehistoric landslides example from sea of galilee dead sea transform article i nfo accurate estimation of expected peak ground acceleration pga in seismically active regions is a challenging task the best way to estimate quantitatively expected pga is by investigating instrumental data of past strong earthquakes in a given area in some regions of the world however recorded data are scarce and if they exist they are typically available only since the late 19th century as such they are hardly representative of the true seismicity in the studied region we propose here an analytical approach to constrain the lower threshold of paleoseismic pga on the basis of back analysis of old landslides to perform the analysis we need a mapped landslide with geomorphic features that have been preserved in the field the slip surface a good reconstruction of the slope geometry and ground water level prior to failure and the mechanical properties of the sheared material we perform static and pseudo static limit equilibrium analyses using standard solution procedures to obtain lower bounds of paleoseismic pga back analyses of three different landslides around the sea of galilee sog return similar results that range between 0 15 and 0 5 g thus constraining the threshold paleoseismic pga range for this region the analytically inferred regional pga is supported by results of an independent numerical analysis of toppled columns in a nearby byzantine church using results from a recent paleoseismic trenching study performed on one of the studied landslides and a modified attenuation relationship for the study area we localize the loci of moment magnitude mw 7 0 earthquakes that can explain the studied failures along the boundaries of the sog and find that they coincide with traces of the eastern and western margin faults of the dead sea transform the temporal relationships between the observed failures are discussed on the basis of dated colluvial sediments geomorphologic constraints and archeological evidence

quantitative investigations of the missouri gravity low a possible expression of a large late precambrian batholith intersecting the new madrid seismic zone analysis of gravity and magnetic anomaly data helps characterize the geometry and physical properties of the source of the missouri gravity low an important cratonic feature of substantial width about 125 km and length 600 km filtered anomaly maps show that this prominent feature extends nw from the reelfoot rift to the midcontinent rift system geologic reasoning and the simultaneous inversion of the gravity and magnetic data lead to an interpretation that the gravity anomaly reflects an upper crustal 11 km thick batholith with either near vertical or outward dipping boundaries considering the modeled characteristics of the batholith structural fabric of missouri and relations of the batholith with plutons and regions of alteration a tectonic model for the formation of the batholith is proposed the model includes a mantle plume that heated the crust during late precambrian and melted portions of lower and middle crust from which the low density granitic rocks forming the batholith were partly derived the batholith called the missouri batholith may be currently related to the release of seismic energy in the new madrid seismic zone earthquake concentrations occur at the intersection of the missouri batholith and the new madrid seismic zone three qualitative mechanical models are suggested to explain this relationship with seismicity

the earthquake potential of the new madrid seismic zone the fault system responsible for new madrid seismicity has generated temporally clustered very large earthquakes in a d 900 100 years and a d 1450 150 years as well as in 1811 1812 given the uncertainties in dating liquefaction features the time between the past three new madrid events may be as short as 200 years and as long as 800 years with an average of 500 years this advance in understanding the late holocene history of the new madrid seismic zone and thus the contemporary tectonic behavior of the associated fault system was made through studies of hundreds of earthquake induced liquefaction features at more than 250 sites across the new madrid region we have found evidence that prehistoric sand blows like those that formed during the 1811 1812 earthquakes are probably com pound structures resulting from multiple earthquakes closely clustered in time or earthquake sequences from the spatial distribution and size of sand blows and their sedimentary units we infer the source zones and estimate the magnitudes of earth quakes within each sequence and thereby characterize the detailed behavior of the fault system it appears that fault rupture was complex and that the central branch of the seismic zone produced very large earthquakes during the a d 900 and a d 1450 events as well as in 1811 1812 on the basis of a minimum recurrence rate of 200 years we are now entering the period during which the next 1811 1812 type event could occur

faulting and effects of associated shaking at pisgah crater volcano caused by the 16 october 1999 hector mine earthquake mw 7 1 central mojave desert california a 240 m long zone of left stepping fractures and scarps was mapped on 23 and 24 october 1999 on the east flank of the main cinder cone at pisgah crater volcano about halfway between the north end of the surface rupture as mapped by other investigators and highway i 40 the pisgah crater fractures are faults not slump features although undoubted slumps and shallow slides are present and abundant nearby the free faces of the scarps range from 10 to 80 cm high evidence of strike slip was not observed the scarps are at the base of an erosion scarred 2 m high bench in the cone slope that suggests the cone has been faulted previously indicators of strong motion associated with the hector mine earthquake include shattered earth and tossed ground extensive collapse of oversteepened quarry slopes and slumping of waste dump slopes quarry cuts and gully banks a 1000 kg boulder that rested on a nearly flat surface was displaced 10 cm southward from its original position within its socket extensive sand slips on angle of repose slopes expulsion of dust and silt from compacting cinders local collapse of lava tubes and dislodgment of precariously balanced boulders from quarry faces were also seen taken together our observations suggest that strong motion associated with the hector mine earthquake on pisgah crater was nearly thrice the 0 33 g measured elsewhere by other investigators

low footwall accelerations and variable surface rupture behavior on the fort sage mountains fault northeast california the fort sage mountains fault zone is a normal fault in the walker lane of the western basin and range that produced a small surface rupture 20 cm during an m l 5 6 earthquake in 1950 we investigate the paleoseismic history of the fort sage fault and find evidence for two paleoearthquakes with surface displacements much larger than those observed in 1950 rupture of the fort sage fault 5 6 ka resulted in surface displacements of at least 0 8 1 5 m implying earthquake moment magnitudes m w of 6 7 7 1 an older rupture at 20 5 ka displaced the ground at least 1 5 m implying an earthquake of m w 6 8 7 1 a field of precariously balanced rocks pbrs is located less than 1 km from the surface rupture trace of this holocene active normal fault ground motion prediction equations gmpes predict peak ground accelerations pgas of 0 2 0 3 g for the 1950 rupture and 0 3 0 5 g for the 5 6 ka paleoearthquake one kilometer from the fault surface trace yet field tests indicate that the fort sage pbrs will be toppled by pgas between 0 1 0 3 g we discuss the paleoseismic history of the fort sage fault in the context of the nearby pbrs gmpes and probabilistic seismic hazard maps for extensional regimes if the fort sage pbrs are older than the mid holocene rupture on the fort sage fault zone this implies that current gmpes may overestimate near fault footwall ground motions at this site

the 1356 basel earthquake an interdisciplinary revision summary within historical times one of the most damaging events in intra plate europe was the 1356 basel earthquake given its significance for assessing regional seismic hazard in central europe an interdisciplinary project was launched in 2005 to re explore this event our effort aimed to incorporate techniques from history seismology archaeology paleoseismology and engineering new and reinterpreted historical data from basel and its surroundings plus archaeological findings on buildings that survived the event and still exist enabled this macroseismic assessment palaeoseismological studies combined with historical evidence provided additional data for the surrounding areas archaeology offers sparse information on some castles and churches sometimes supported by historical records a contemporary source allows some reconstruction of the stronger fore and aftershocks this expanded information base improves our sense of the event s damage and consequences for the city of basel the relatively abundant archaeological data allowed us to assess statistically the macroseismic intensity at ix although the pattern of damage was scattered data points for the expected area of damage around basel are not distributed regularly the absence of historical and archaeological findings for southern germany might be due to archival problems future investigation may improve this situation our results confirm that the basel earthquake was the most destructive known for central europe intensities up to viii are found within a radius of about 30 km analysing the macroseismic field confirms our former assessment of the event and shows an epicenter located about 10 km south of basel the most probable range for the moment magnitude mw is between 6 7 and 7 1

comparison of the historical record of earthquake hazard with seismic hazard models for new zealand and the continental united states we compare the historical record of earthquake hazard experienced at 78 towns and cities sites distributed across new zealand and the continental united states with the hazard estimated from the national probabilistic seismic hazard psh models for the two countries the two psh models are constructed with similar methodologies and data our comparisons show a tendency for the psh models to slightly exceed the historical hazard in new zealand and westernmost continental united states interplate regions but show lower hazard than that of the historical record in the continental united states intraplate region factors such as non poissonian behavior parameterization of active fault data in the psh calculations and uncertainties in estimation of ground motion levels from historical felt intensity data for the interplate regions may have led to the higher than historical levels of hazard at the interplate sites in contrast the less than historical hazard for the remaining continental united states intraplate sites may be largely due to site conditions not having been considered at the intraplate sites and uncertainties in correlating ground motion levels to historical felt intensities the study also highlights the importance of evaluating psh models at more than one region because the conclusions reached on the basis of a solely interplate or intraplate study would be very different

dating precariously balanced rocks in seismically active parts of california and nevada precariously balanced boulders that could be knocked down by strong earthquake ground motion are found in some seismically active areas of southern california and nevada in this study we used two independent surface exposure dating techniques rock varnish microlamination and cosmogenic 36 cl dating methodologies to estimate minimum and maximum limiting ages respectively of the precarious boulders and by inference the elapsed time since the sites were shaken down the results of the exposure dating indicate that all of the precarious rocks are 10 5 ka and that some may be significantly older at victorville and jacumba california these results show that the precarious rocks have not been knocked down for at least 10 5 k y a conclusion in apparent conflict with some commonly used probabilistic seismic hazard maps at yucca mountain nevada the ages of the precarious rocks are 10 5 to 27 0 ka providing an independent measure of the minimum time elapsed since faulting occurred on the solitario canyon fault

mapping p wave azimuthal anisotropy of the new madrid seismic zone abstract we determine a detailed tomographic model of 3 d p wave azimuthal anisotropy in the crust and upper mantle beneath the new madrid seismic zone nmsz using a large number of high quality arrival time data of 877 local earthquakes and relative travel time data of 1511 teleseismic events recorded at 62 usarray stations the tomographic inversion is conducted by using a limited memory broyden fletcher goldfarb shanno algorithm for bound constrained optimization because damping regularization is not required in this inversion scheme the obtained results are less affected by human subjectivity a prominent low velocity zone with ne sw fast velocity directions of azimuthal anisotropy down to 160 km depth beneath the nmsz is imaged which may reflect upwelling mantle materials probably triggered by passage of the bermuda hotspot from the southwest of the mississippi embayment our results also reveal a subhorizontal high velocity anomaly around the moho discontinuity with a width of 150 200 km confirming the existence and defining the geometry of a lens shaped high density rift pillow near the moho which may result from the mantle upwelling nw se fast velocity directions of azimuthal anisotropy appear in the lower crust beneath the mississippi embayment this result may reflect recent weakening of a detachment fault above the rift pillow which drives recurrently sinking of the rift pillow and as a consequence causing large repeating intraplate earthquakes in the nmsz

band of precariously balanced rocks between the elsinore and san jacinto california fault zones constraints on ground motion for large earthquakes a spectacular band of precariously and semiprecariously bal anced rocks extends from riverside to near borrego valley cali fornia about halfway between the elsinore and san jacinto fault zones the rocks are distributed in a band a few kilometers wide midway between the san jacinto and elsinore fault zones an in dication that the distribution is caused by attenuation of strong ground motion from numerous large events along these two fault zones these rocks have apparently been in place for thousands of years and thus place important constraints on ground motions from earthquakes on the basis of field tests and photographic analysis the estimated quasi static toppling accelerations for the precariously balanced rocks are 0 32 6 0 10 g g 5 gravitational acceleration the dynamic toppling accelerations are within this range for earthquakes with waveforms similar to those recorded during the izmit and denali earthquakes these constraints are roughly consistent with the median predicted value of ground mo tion for m7 earthquakes but somewhat lower than 11s ground motion curves and much lower than the values from the 2 in 50 yr hazard maps the evidence from these rocks has importance for some of the assumptions that go into calculating probabilistic seismic hazard assessment including median and standard devia tion of ground motion attenuation curves especially for hard rock for which few instrumental data are available the possible exis tence of random background earthquakes and the smoothing dis tance for historical seismicity

toppling analysis of the echo cliffs precariously balanced rock toppling analysis of a precariously balanced rock pbr can provide insight into the nature of ground motion that has not occurred at that location in the past and by extension can constrain peak ground motions for use in engineering design earlier approaches have targeted 2d models of the rock or modeled the rock pedestal contact using spring damper assemblies that require recalibration for each rock here a method to model pbrs in 3d is presented through a case study of the echo cliffs pbr the 3d model is created from a point cloud of the rock the pedestal and their interface obtained using terrestrial laser scanning the dynamic response of the model under earthquake excitation is simulated using a rigid body dynamics algorithm the veracity of this approach is demonstrated through comparisons against data from shake table experiments fragility maps for toppling probability of the echo cliffs pbr as a function of various ground motion parameters rock pedestal interface friction coefficient and excitation direction are presented these fragility maps indicate that the toppling probability of this rock is low less than 0 2 for peak ground acceleration pga and peak ground velocity pgv lower than 3 m s 2 and 0 75 m s respectively suggesting that the ground motion intensities at this location from earthquakes on nearby faults have most probably not exceeded the above mentioned pga and pgv during the age of the pbr additionally the fragility maps generated from this methodology can also be directly coupled with existing probabilistic frameworks to obtain direct constraints on unexceeded ground motion at a pbr s location

new evidence for a large earthquake in the new madrid seismic zone between a d 1400 and 1670 in an integrated geological archaeological and geophysical study in the new madrid seismic zone of the southeastern united states we documented a prehistoric sand blow and related feeder dikes at an archaeological site near steele missouri archaeological analysis combined with radiocarbon dating suggest that the earthquake induced features formed between a d 1400 and 1670 this paleoseismic study provides the best evidence to date for a large earthquake occurring in the zone within 400 yr prior to the 1811 1812 new madrid earthquake sequence to determine an optimal location for excavating at the study site we mapped surficial artifact density and conducted geophysical surveys in doing so we were able to reveal critical relationships for constraining the age of the prehistoric earthquake with minimal impact to the archaeological site

stressing of the new madrid seismic zone by a lower crust detachment fault a new mechanical model for the cause of the new madrid seismic zone in the central united states is analyzed the model contains a subhorizontal detachment fault which is assumed to be near the domed top surface of locally thickened anomalous lower crust rift pillow regional horizontal compression induces slip on the fault and the slip creates a stress concentration in the upper crust above the rift pillow dome in the coseismic stage of the model earthquake cycle where the three largest magnitude 7 8 earthquakes in 1811 1812 are represented by a single model mainshock on a vertical northeast trending fault the model mainshock has a moment equivalent to a magnitude 8 event during the interseismic stage corresponding to the present time slip on the detachment fault exerts a right lateral shear stress on the locked vertical fault whose failure produces the model mainshock the sense of shear is generally consistent with the overall sense of slip of 1811 1812 and later earthquakes predicted rates of horizontal strain at the ground surface are about 10 7 year 1 and are comparable to some observed rates the model implies that rift pillow geometry is a significant influence on the maximum possible earthquake magnitude

age of unstable bedrock landforms southwest of yucca mountain nevada and implications for past ground motions we determine minimum exposure ages for unstable outcrops at three sites in amargosa desert southwestern nevada including a site at the southern end of yucca mountain varnish microlamination dating techniques provide minimum exposure ages of 12 5 36 k y for the unstable outcrops of welded tuff including a 24 k y age for the south yucca mountain site the youngest exposure age 12 5 k y is found at the site located only 10 km from the death valley furnace creek fault suggesting outcrops may be more frequently modified when close to major active earthquake sources a simplistic stability assessment of the south yucca mountain outcrops suggests peak ground accelerations pgas may not have exceeded about 1 g uncertainty bounds 0 5 2 g in at least 24 k y a pga of 1 g is consistent with the predicted 24 k y return period pgas from the near decade old yucca mountain probabilistic seismic hazard psh model except for the ninety fifth percentile and above we gain confidence in our interpretations by additionally observing 1 minimal damage to the south yucca mountain outcrops from a recent moderate earthquake that is estimated to have produced a pga of less than 0 1 g there i e motions less than 0 5 g do not significantly damage the outcrops and 2 severe damage to similar volcanic outcrops associated with pgas of the order 0 5 1 g near a nuclear blast site from the 1960s these observations support our suggestion that pgas greater than 0 5 1 g have not occurred at the south yucca mountain site for a time period of at least 24 k y significant seismic events that substantially modify the outcrops and produce associated rubble fields may therefore occur on longer time scales