precursory seismic quiescence before the 1994 kurile earthquake mw 8 3 revealed by three independent seismic catalogs we have found that the m w 8 3 kurile earthquake on october 4 1994 followed an outstanding seismic quiescence starting 5 6 years before the mainshock near the ruptured area we have analyzed three independent seismic catalogs institute of seismology and volcanology hokkaido university isv japan meteorological agency jma and international seismology center isc in spite of selecting different magnitude bands and time windows all three catalogs presented the common feature of the seismic quiescence this fact strongly suggests that the seismic quiescence should not be a man made change but actually occurred moreover we have confirmed that the seismic quiescence was the most significant and the earthquake was the largest in the past twenty five years in this region therefore we confidently interpret this seismic quiescence as an indication of a preparation process for the m w 8 3 kurile earthquake

implications of pre eruptive magmatic histories of zircons for u pb geochronology of silicic extrusions zircons in silicic magmas begin to crystallize 10 s to 100 s of thousands of years ka prior to their eruption u pb and u th dating for zircons from 32 young extrusions indicate a range of mean pre eruption ages from 33 to 247 ka with a median of 92 ka explosive large volume widespread eruptions show a narrower range and a younger median of 70 ka these results indicate that the difference between initial zircon growth and eruption age measured by other e g 40ar 39ar techniques is in excess of the achievable analytical uncertainty for eruptions 30 million years ma and likely for eruptions of up to 300 ma protracted pre eruption zircon growth is well established by an increasing number of studies on magma process time scales but this source of eruption age bias has been largely ignored in chronostratigraphic applications of high precision u pb data until intrinsic properties of zircons or their host deposits enable a priori estimates of pre eruption age bias it is prudent to expand the uncertainties attending all u pb zircon ages interpreted as deposition ages in e g chronostratigraphic studies by 0 200 ka

aftershock sequence of the 1994 mw 6 8 liwa earthquake indonesia seismic rupture process in a volcanic arc we present the aftershock activity following the february 15 1994 mw 6 8 earthquake which was strongly felt in southern sumatra indonesia near the great sumatran fault gsf at this place the slip rate is supposed to be low neverthless three m 6 events occurred along this segment during this century no significant instrumental microseismicity has ever been recorded there we use data from both the regional indonesian network and a local seismic array operating 11 days after the mainshock during one month aftershocks mostly locate in a broad zone of 55 20 km near two active nw trending strike slip segments of the gsf separated by a recent caldera suwoh during the experiment the nw segment from suwoh up to 15 km se of the ranau lake caldera an old right stepover pull apart was very active as first suggested by the aftershock distribution and the lack of coseismic rupture at the surface the 20 focal mechanisms determined provide evidence for various post seismic stress adjustments on secondary faults located in the ranau suwoh paleo pull apart graben less than 20 of the aftershocks are directly linked to the main rupture a nearly pure right lateral strike slip faulting reaching 25 km depth a narrow seismic gap underlines the active volcanic area of suwoh we conclude that the rupture process along the gsf is controlled both by volcanism and structures and that the volcanic activity affects the mechanical properties of the crust only in a narrow zone

source spectral characteristics of two large intra slab earthquakes along the southern kurile hokkaido arc abstract in 1993 and 1994 two large earthquakes occurred along the southern kurile hokkaido arc the 1993 kushiro oki earthquake 15 january mw 7 6 and the 1994 shikotan earthquake 4 october mw 8 2 these were intra slab earthquakes that ruptured through a substantial part of the subducting oceanic lithosphere we study source spectral characteristics of these large intra slab earthquakes based on strong motion records for the kushiro oki earthquake the s wave spectra on three rock site stations are directly compared with the theoretical spectra based on the omega squared source model and the source parameters estimated from long period seismic waves the observed spectra are roughly consistent with the theoretical ones at low frequencies

aftershocks of the andreanof islands earthquake of june 10 1996 and local seismotectonics three questions have been addressed in connection with an mw 7 7 earthquake that occurred in the andreanof islands alaska on june 10 1996 first was there any variation in seismic activity rate during the previous five years that might have been diagnostic of the imminent occurrence of this event the answer based on the usgs pde catalog is no second though there were abundant pre event earthquakes and aftershocks on both sides of adak canyon why was there very little activity under the canyon itself a property that has been observed previously it is not possible to resolve the cause of the behavior with existing seismological data finally how does one explain the eastern group of aftershocks which fell within the zone of abundant aftershocks of the 1986 mw 8 andreanof islands earthquake when very little of the 1996 mainshock moment was released under or to the east of adak canyon one explanation of these aftershocks is that the recent aftershocks are late aftershocks of 1986 whose occurrence within a short time was induced by the stress pulse but not seismic slip from the 1996 earthquake though large earthquakes in the aleutians occur in distinct tectonic blocks the boundaries of these blocks may not always be defined reliably by aftershock zones

40ar 39ar dating of mineral separates and whole rocks from the western ghats lava pile further constraints on duration and age of the deccan traps abstract there has been an ongoing debate for almost two decades on the age and duration of deccan trap volcanism some of the best available ages have been determined using the 40 ar 39 ar method but differences in sample location laboratory procedure type of analyzed material and ages of monitors may have been responsible for much of the apparent spread in the data we have collected and analyzed samples from close to the bottom and top of the main lava pile in the western ghats near bombay we confirm previous findings that alteration and ar recoil strongly affect these rocks particularly measurements made on whole rocks when mineral separates mostly plagioclase are analyzed the spread is much reduced we find a mean age of 65 4 0 7 ma for five flows near the base of the section jahwar and igatpuri formations and 65 2 0 4 ma for a dyke cross cutting the poladpur formation below the topmost mahabaleshwar formation this implies that at least 1800 m out of the 2500 m composite trap section were erupted in a short interval less than 1 ma long and close to 65 5 ma i e close to the cretaceous tertiary boundary we find that older ages near 67 ma found by several authors are essentially based on whole rock samples which display disturbed spectra when this can be checked i e when the spectra are published

ocean continent boundary in the iberia abyssal plain from multichannel seismic data abstract the ocean continent boundary of the west iberia margin is marked by a basement ridge trending n s four segments of this ridge are recognized each of them being progressively offset westward from 40 n to 43 n because the setting and seismic character of the ridge in the iberia abyssal plain are similar to those of the galicia margin ridge which is made of serpentinized peridotite we think that the southern segments of the ridge are also made of the same mantle material the segmentation of the ridge suggests that the northward propagation of the continental break up during the north atlantic opening in early cretaceous times was discontinuous each segment possibly corresponding to a propagation step east of the ridge the basement of the whole iberia abyssal plain consists of highly thinned continental crust locally resting on a seismic reflector that resembles the s reflector previously recognized off galicia by analogy with the galicia margin we propose that the tilted crustal blocks lay on serpentinized peridotite derived from the upper mantle the s reflector corresponding to the contact between crustal rocks and serpentinite

geodetic inversion for the fault model of the 1994 shikotan earthquake the fault parameters of the 1994 shikotan earthquake were determined by inverting all available geodetic data using an iterative linearized least squares method a one fault inversion shows that a shallow dip fault plane model does not reproduce the minor deformation observed on shikotan island while a steep dip model can fit all the data well multifault inversion shows that a fault plane divided into two subfaults lengthwise is the best fitting model based on akaike s information criteria aic for shallow dip faulting mechanism this model yields a very high stress drop 40 mpa on the deeper subfault for a steep dip model a fault plane divided into two subfaults sideways gives the minimum aic all the computations for models consisting of two to eight subfaults show that the chi square values of the shallow dip models are about 10 higher than those of the steep dip models considering the high stress drop the larger chi square value and the requirements of a complex subfault system for the shallow dip model we reach the conclusion that the steep dip model is more suitable to explain the geodetic observations

the 1994 hokkaido toho oki earthquake sequence the complex activity of intra slab and plate boundary earthquakes abstract we study the complex activity of the 1994 hokkaido toho oki earthquake sequence based on strong motion records and teleseismic data first we examine s wave acceleration spectra of the aftershocks by comparison with that of the main shock the main shock m w 8 2 an intra slab earthquake has been characterized by strong radiation of high frequency seismic waves from different shapes of the spectral ratios of the aftershocks to the main shock the aftershocks can be classified into three types next we redetermine the source parameters focal depth seismic moment and source duration for larger events by waveform modeling of teleseismic p waves the hypocentral locations and stress drops estimated from these parameters show that the three aftershock types fall into different categories of subduction zone earthquakes intra slab relatively deep plate boundary and shallow plate boundary earthquakes finally we estimate source spectra for three different earthquake categories after correcting s wave acceleration spectra for the attenuation effect the source spectrum at high frequencies is modeled by using a high cut filter whose parameters are a cut off frequency f max and a decay rate the source spectrum of intra slab earthquakes has high f max and small decay rate due to this feature and the high stress drop intra slab earthquakes radiate strong high frequency seismic waves through this study we demonstrate that the 1994 hokkaido toho oki earthquake sequence has the complex activity of intra slab and plate boundary earthquakes

extensional tectonics in the gorringe bank rocks eastern atlantic ocean evidence of an oceanic ultra slow mantellic accreting centre the gorringe bank corresponds to an upper mantle peridotite ridge enclosing a 500 m thick 50 km long laccolith like body of gabbro locally cut and poorly covered by tholeiitic rocks strain and kinematic analysis of orientated gabbros and peridotites sampled during the gorringe diving cruise 1996 provides new kinematic constraints on extensional high temperature deformation recorded at deep levels during stretching near an accreting centre axis of a mantle dominant oceanic lithosphere it is argued that the gorringe bank lithosphere formed at an oceanic ultra slow n010 020 trending accreting centre mostly by passive tectonic denudation of the mantle without any synchronous large magmatism this peculiar lithosphere may be representative of the iberia oceanic domain located between the continent and the j anomaly ridge which likely marks the beginning of true spreading at an oceanic spreading ridge

40ar 39ar ages for the alkaline volcanism and the basement of gorringe bank north atlantic ocean abstract gorringe bank is situated on the europe africa plate boundary at the eastern end of the azores gibraltar fracture zone it has two summits gettysburg bank to the southwest and ormonde bank to the northeast we applied the 40 ar 39 ar stepwise heating method to date six samples of the alkaline volcanic rocks two gabbros from the ormonde bank and a dolerite from the gettysburg bank the results that the alkaline volcanism lasted probably for less than 6 ma 66 60 ma although the nature of this volcanism precludes any subduction feature during its setting the alkaline volcanism of ormonde is probably linked to upper cretaceous eocene compressive tectonic events the basement rocks of gorringe bank reveal distrubed 40 ar 39 ar age spectra one plagioclase and one biotite from a gabbro give evidence for a thermic event whose age is tentatively estimated at about 75 ma and related to a variation in the direction of the relative movement between europe and africa the more probable age given by a plagioclase of another gabbro and by a dolerite 110 ma corresponds to tilting northeastward of the gorringe massif

newly imaged shape of the deep seismic zone within the subducting pacific plate beneath the hokkaido corner japan kurile arc arc junction 1 the geometry of the deep seismic zone associated with the subducting pacific plate beneath the north american plate in the hokkaido corner japan kurile arc arc junction has been investigated based on the hypocenters accurately relocated by a dense local seismographic network with three dimensional p and s wave velocity structures the model suggests that the lateral changes in the dip of the subduction zone are more rapid than previously believed we found that the deep seismic zone in the pacific plate dips at 20 to 30 on the side of the japan arc the dip angle increased in a narrow region around 143 144 e by 10 to 20 and the dip was 40 to 50 on the side of the kurile arc in the transitional zone around 143 144 e an unusual distribution of microearthquakes has been discovered within the pacific plate which was located on a near vertical plane with an area of 50 100 km2 with strike that was perpendicular to the kurile trench axis this feature was named the tokachi oki slab cracking zone we suggest that this is a slab cracking zone torn by the extensional stress due to the rapid lateral changes in the dip of the subduction zone the slab cracking zone may possibly grow into the large disruption or tear of the slab in the future

bayesian identification of soil stratigraphy based on soil behaviour type index cone penetration test cpt has been widely used to determine the soil stratigraphy including the number n and thicknesses hn of soil layers during geotechnical site investigation because it is rapid repeatable and economical for this purpose several deterministic and probabilistic approaches have been developed in the literature but these approaches generally only give the best estimates e g the most probable values of n and hn based on cpt data according to prescribed soil stratification criteria providing no information on the identification uncertainty degrees of belief in these best estimates this paper develops a bayesian framework for probabilistic soil stratification based on the profile of soil behaviour type index ic calculated from cpt data the proposed bayesian framework not only provides the most probable values of n and hn but also quantifies their associated identification uncertainty based on the ic profile and prior knowledge equations are derived for the proposed ap

the shikotan earthquake of october 4 1994 lithospheric earthquake a large m w 8 2 earthquake occurred off shikotan is one of the kurile is on october 4 1994 we inverted 32 body wave records to determine the rupture pattern using an iterative deconvolution method the mechanisms of the subevents were allowed to vary during rupture the source parameters obtained are the location of the initial break 43 48 n 147 40 e the centroid depth 56 km strike dip rake 49 75 125 for the total source the seismic moment m o 2 6 10 21 nm m w 8 2 source time duration t 42 s the average rupture velocity 2 5 km s we also determined the mechanism using long period love and rayleigh waves from 14 stations the solution for a finite source distributed over a depth range from 0 to 90 km is strike dip rake 54 76 129 with mo 2 3 10 21 nm in good agreement with that from body waves referring to the extent of the aftershock area and the subevent distribution we estimated the fault area s 120 60 km the average slip d 5 6m and the stress drop 11 mpa we computed synthetic waveforms as well as static displacements using either the steep or the low angle plane as the fault plane and found that the steep dip fault model fits the data better our result the mechanism large centroid depth high stress drop strongly suggests that the 1994 shikotan earthquake is a lithospheric earthquake an intra plate event that ruptures through a substantial part of the subducting oceanic lithosphere this type of lithospheric earthquake is relatively common

effect of spatial variability of shear strength parameters on critical slip surfaces of slopes abstract critical slip surface css of slopes is useful information in system reliability analyses and risk assessments of slopes this paper investigates the effect of spatial variability of shear strength parameters on the css distribution of a two dimensional slope using a finite element stress based slope stability method the failure mechanics for typical csss in a heterogeneous slope are analyzed and distributions of css for various combinations of horizontal and vertical scales of fluctuation sofs are evaluated the results show that local failures may occur in statistically homogeneous slopes which cannot be observed in deterministic slope stability analyses the distribution range of css first increases and then decreases with an increasing scale of fluctuation as the ratio of horizontal sof to vertical sof is large local failures have entry points located at the top of a slope by contrast as the ratio of horizontal sof to vertical sof is relatively small local failures have entry points located either at the top of a slope or on the slope surface the findings facilitate a more accurate evaluation of the system reliabilities or risks of slopes

evidence of multi phase cretaceous to quaternary alkaline magmatism on tore madeira rise and neighbouring seamounts from 40ar 39ar ages abstract the tore madeira rise is a seamount chain located 300 km off the portugal and morocco coasts attributed to hotspot activity u pb ages of lavas from the northern and central tore madeira rise range between 103 and 80 5 ma whereas 40 ar 39 ar ages from the central and southern tore madeira rise yield ages ranging from 94 5 to 0 5 ma we performed new 40 ar 39 ar measurements to better understand the geodynamic history of the tore madeira rise plagioclase ages from the bikini bottom and torillon seamounts suggest ages of 90 ma and 60 ma respectively amphiboles from the seine seamount yield an age of 24 0 0 8 ma biotites from lavas of the ashton seamount give ages of 97 4 1 1 ma and 97 8 1 1 ma the geochronological database available on the tore madeira rise has been filtered on statistical criteria to eliminate unreliable ages the resulting database reveals three pulses of alkaline magmatism on the tore madeira rise at 103 80 5 ma at c 68 ma and between 30 ma and the present the magmatism was continuous from 103 ma until c 68 ma and from c 30 ma until the present on the tore madeira rise the surrounding seamounts and the portugal coast we suggest that the space time distribution of this magmatism results from the interaction between a wide thermal anomaly emitting magmatic pulses and the complex motion of the iberian plate supplementary material a detailed ar measurements dataset is available at http www geolsoc org uk sup18359

automatic identification of the critical slip surface of slopes abstract although numerical methods based on strength reduction are becoming popular in slope stability analysis they fail to provide a distinct critical slip surface and only provide a shear band the widely used visualization techniques for defining the critical slip surface are susceptible to subjective judgment and are inefficient for batch analysis and three dimensional analysis when a slope fails the displacements on the two sides of the critical slip surface will be substantially different based on this observation an automatic identification method for locating the critical slip surface is proposed the k means clustering algorithm is first applied to automatically separate the nodal displacements into two categories representing the sliding mass and the stable block then the scatters near the separation surface are obtained by constructing the alpha shape of the sliding mass finally the critical slip surface is obtained by fitting the extracted scatters a homogeneous slope a slope with a thin weak layer and a real landslide are used to test the effectiveness of the proposed method the results show that the proposed method can automatically and accurately identify two dimensional and three dimensional critical slip surfaces

unbending and horizontal fracture of the subducting pacific plate as evidenced by the 1993 kushiro oki and the 1981 and 1987 intermediate depth earthquakes in hokkaido abstract a large number of aftershocks of the 1993 kushiro oki earthquake m jma 7 8 1987 hidaka earthquake m jma 7 0 and 1981 western hidaka earthquake m jma 7 1 have been observed by the seismic network of hokkaido university these three earthquakes are the largest events that have occurred in the depth range from 100 km to 200 km under the pacific side of hokkaido for 109 years they migrated from west to east in the last 12 years their focal mechanism solutions represent typical down dip extension in the lower zone of the double seismic zone in the descending pacific plate we investigated aftershock distributions of the three intermediate depth earthquakes to study physical properties of the descending slab aftershocks of the kushiro oki earthquake spread horizontally over an area of 50 km 40 km a vertical cross section of this aftershock distribution shows that it extends from the lower seismic zone almost to the upper seismic zone it shows a slightly concave shape of about 35 km in length aftershocks of the hidaka and western hidaka earthquakes also extend horizontally over areas of 10 km 10 km and 8 km 12 km respectively these main shocks occurred in the lower seismic zone the horizontal aftershock distributions of all three large earthquakes indicate that the horizontal nodal planes of the focal mechanisms rather than the vertical nodal planes are the fault planes such a characteristic suggests that the descending lithosphere under hokkaido has been unbent macroscopically over the last 12 years unbending and the long horizontal faults of the intermediate depth earthquakes may be caused by mantle flow in the asthenosphere beneath the pacific plate we suggest that the descending slab of about 100 km thickness constitutes two layers a seismic brittle upper layer and an aseismic ductile lower layer small ruptures may tend to occur easily along both boundary surfaces of the upper layer under the action of forces especially unbending and form the double seismic zone

earlier history of the 70 ma old canary hotspot based on the temporal and geochemical evolution of the selvagen archipelago and neighboring seamounts in the eastern north atlantic major element trace element and sr nd pb isotope data combined with 40ar 39ar age determinations of volcanic rocks from the selvagen islands and neighboring seamounts in the eastern north atlantic reveal the earlier history of the 70 ma old canary hotspot a basanitic to phonolitic late shield stage intrusive complex 29 ma is exposed on selvagem pequena the evolution of selvagem grande can be divided into three magmatic phases a tephritic to phonolitic late shield stage intrusive complex 24 26 ma and two rejuvenated or post erosional stages 8 12 and 3 4 ma consisting of alkali basalt basanite and rare phonolite during the early to mid miocene volcanic hiatus 12 24 ma the top of the volcano was beneath sea level as evidenced by marine carbonate sediments 13 24 ma dated through correlation of 87sr 86sr with the seawater sr isotope curve the geochemistry of the shield stage lavas indicates that they derive from plume sources whereas the post erosional lavas are derived from metasomatized lithospheric sources five sampled seamounts to the east and northeast of the islands range in composition from alkali basalt and basanite to phonolite samples from dacia conception bank and lars were dated at 9 17 and 68 ma respectively geochemical data suggest that the dredged samples come from the post erosional stage of volcanism and therefore the dates represent minimum ages for the seamount volcanoes the elevation of erosional platforms formed at wave base decrease from selvagen grande 100 m above sea level to lars seamount 900 m below sea level suggesting a southwest to northwest age progression and that all of these seamounts are older than the selvagen islands trace element and sr nd pb isotopic composition of the selvagen islands and neighboring seamounts are consistent with their origin from the canary plume interaction of the weak canary mantle plume with a slow moving plate appears to be responsible for generating a 450 km wide irregular hotspot track extending 800 km from the youngest canary island of hierro in the southwest to lars seamount in the northeast

138 121 ma asthenospheric magmatism prior to continental break up in the north atlantic and geodynamic implications along the galicia and gorringe banks and in the iberia abyssal plain of the north atlantic unroofed sub continental mantle fills the gap between true oceanic crust and the continental crust margin these lithospheric peridotites are intruded by gabbros and dolerites and locally covered by basalts primary magmatic zircons extracted from gabbros and meta gabbros of the two banks were dated by the u pb chronometer and initial hafnium isotope signatures hfi were determined on the same grains for mt gettysburg at gorringe gabbro emplacement ages of 137 5 0 5 2 ma and 135 7 0 8 ma are obtained and corresponding hfi lie at 20 5 0 3 2 and 19 5 0 4 substantiating magma formation from severely lile depleted mantle domains gabbro zircons from mt ormonde at gorringe yield a much younger age of 77 1 0 4 ma and the hf isotopes document an intermediately lile depleted mantle source having a hfi of 7 6 0 4 given its age and hf signature emplacement of this rock can be ascribed to the alkaline magmatic event that also affected the iberian continent in upper cretaceous time concerning the galicia section zircons from a meta gabbro yield an emplacement age of 121 7 0 4 ma and a hfi of 14 0 0 2 and a hfi of 14 6 0 2 is obtained for zircons from a previously dated meta gabbro of identical age these results indicate magma extraction from mantle reservoirs that are slightly less lile depleted than those sampled by the about 20 myr older gorringe gabbros the data demonstrate that magmatism occurring prior to complete separation of europe from america was essentially of asthenospheric origin both the 138 135 ma ages for the gorringe gabbros and 122 ma ages for the galicia gabbros are at least 5 myr older than the oldest sediments on gorringe and the break up unconformity at the galicia bank respectively magma source signatures of the syn rift gabbros are in agreement with values expected for differently depleted cretaceous morb type mantle reservoirs and the age difference for magmatism between the gorringe and galicia banks suggests a rate of 4 4 0 3 cm yr for northward progression of continental rifting based on the new results a structural model for iberia america rifting is discussed putting forward the idea that magma emplacement produces a level of weakness and decollement between the rifting crust and its underlying lithospheric mantle

long term seismic quiescences and great earthquakes in and around the japan subduction zone between 1975 and 2012 an earthquake catalog created by the international seismological center isc was analyzed including 3898 earthquakes located in and around japan between january 1964 and june 2012 shallower than 60 km with the body wave magnitude of 5 0 or larger clustered events such as earthquake swarms and aftershocks were removed from the isc catalog by using a stochastic declustering method based on epidemic type aftershock sequence etas model a detailed analysis of the earthquake catalog using a simple scanning technique zmap shows that the long term seismic quiescences lasting more than 9 years were recognized ten times along the subduction zone in and around japan the three seismic quiescences among them were followed by three great earthquakes the 1994 hokkaido toho oki earthquake m w 8 3 the 2003 tokachi oki earthquake m w 8 3 and the 2011 tohoku earthquake m w 9 0 the remaining seven seismic quiescences were followed by no earthquake with the seismic moment m 0 3 0 1021 nm m w 8 25 which are candidates of the false alarm the 2006 kurile islands earthquake m w 8 3 was not preceded by the significant seismic quiescence which is a case of the surprise occurrence as a result when limited to earthquakes with the seismic moment of m 0 3 0 9 1021 nm four earthquakes occurred between 1976 and 2012 in and around japan and three of them were preceded by the long term seismic quiescence lasting more than 9 years

a detailed subduction structure in the kuril trench deduced from ocean bottom seismographic refraction studies abstract in 1983 we conducted an ocean bottom seismograph obs experiment in the southernmost part of the kuril trench beneath which the pacific plate is subducting under hokkaido island japan the aim of this experiment was to determine crust and upper mantle structure from the oceanic basin to the continental slope by dense seismic refraction profiling using explosives and airguns the total length of the profiles was 560 km along which ten obs s were deployed the observed data were of good quality which enabled us to obtain a detailed velocity structure of the active margin down to 20 30 km we constructed a velocity structure model by ray tracing and amplitude modeling in the oceanic basin the crust has a typical oceanic structure characterized by three layers with p wave velocities of 1 8 3 8 6 5 and 6 5 7 0 km s the velocity gradient in layer 3 increases downward from 0 075 to 0 10 s 1 the moho discontinuity was well constrained by clearly observed p n and p m p phases the total thickness of the oceanic crust was determined to be 8 km and almost constant in the oceanic basin the p wave velocity is 7 9 km s beneath the moho discontinuity which increases downward with a rather small velocity gradient 0 015 0 03 s 1 beneath the continental slope we found relatively low velocity 2 5 to 5 5 5 8 km s material the oceanic moho discontinuity associated with the subducting plate was traced down to a depth of 25 km our seismic data strongly suggest that oceanic layer 2 is smoothly subducting and does not break up to form a wedge structure this result is in remarkable contrast with the velocity structure in the active margin of the ryukyu trench area about 1000 km south of the present experimental area where we found a 12 km thick prominent low velocity wedge is situated 50 150 km landward from the trench axis on the seaward side of the wedge the surface of the igneous basement undulates severely the wedge was probably formed by materials of oceanic origin such a difference in velocity structure suggests that the subduction mechanism in the trench area differs from region to region in the northwestern pacific

jasper seamount seven million years of volcanism jasper seamount is a young mid sized 690 km sup 3 oceanic intraplate volcano located about 500 km west southwest of san diego california reliable sup 40 ar sup 39 ar age data were obtained for several milligram sized samples of 4 to 10 ma plagioclase by using a defocused laser beam to clean the samples before fusion gee and staudigel suggested that jasper seamount consists of a transitional to tholeiitic shield volcano formed by flank transitional series lavas overlain by flank alkalic series lavas and summit alkalic series lavas twenty nine individual sup 40 ar sup 39 ar laser fusion analyses on nine samples confirm the stratigraphy 10 3 10 0 ma for the flank transitonal series 8 7 7 5 ma for the flank alkalic series and 4 8 4 1 ma for the summit alkalic series the alkalinity of the lavas clearly increases with time and there appear to be 1 to 3 m y hiatuses between each series the age data are consistent with the complex magnetic anomaly of jasper however the dominant reversed polarity inferred from the anomaly suggests that most of the seamount formed at ca 11 ma prior to the onset of chron c5n the duration of volcanism of jasper seamount is slightly longer than the duration of volcanism at hawaiian volcanoes suggesting thatmore individual age data from seamounts may constrain the age of a seamount only to within about 7 m y unless the stage of volcanism can be unambiguously determined extrapolating from the results of our study similar precision in age determinations should be possible on 50 mg of 1 ma plagioclase from mid ocean ridge basalt opening new possibilities in the geochronology of young low potassium volcanic rocks less

tikhonov s regularization for deconvolution in the empirical green function method and vertical directivity effect abstract application of tikhonov s technique using input errors for the parameter of regularization estimation enhances the accuracy and stability of the reconstruction of a source time function stf by the empirical green function egf method that gives us an opportunity to use simultaneously for analysis body and surface waves data and to estimate the horizontal and vertical directivity effects knowledge of the last is particularly useful for the choice of an active nodal plane of earthquakes with the dip slip fault orientation that allows us to classify these earthquakes to the interplate or intraplate types and thereby to reach the better understanding of tectonic processes in the region of interest by way of illustration an attempt to estimate average parameters of faulting in a first approximation is made herein for two russian far east large events with opposite types of focal mechanism orientation strike slip and dip slip the former is not a matter of interest in the context of vertical directivity effect but enables us to test the method the directivity analysis of pulse durations and inverse amplitudes of the relative source time functions rstfs restored at eight globally distributed stations iris indicates that the destruction in the source of the neftegorsk earthquake 05 27 1995 m w 7 1 propagated roughly horizontally in the direction 8 11 during 19 2 0 4 s along the rupture extending 35 5 4 9 km the calculated slip distribution along the rupture coincides within the error with the results of field geological measurements on the causal surface fault that proves that the neftegorsk earthquake source is well described by the model of the linear unilateral fault and gives a good assessment of the method applied the average parameters of faulting in the kamchatka earthquake 03 08 1999 m w 6 9 have been determined from data of 13 station iris it was shown that the destruction in its source propagated downward at an angle of about 60 with horizon in the direction about s156 e during 13 4 0 2 s along the rupture totaling 25 5 2 3 km in length therefore the nodal plane steeply dipped to the se was active and this event can be regarded as an intraplate type two asperities can be selected the first with the maximum slip 3 3 m located at a distance of about 7 km from the onset of rupture and the second with the maximum slip about 0 9 m centered at approximately 19 km from that

high precision 40ar 39ar dating of oligocene rhyolites from the mogollon datil volcanic field using a continuous laser system replicate 40ar 39ar analyses of sanidine from seven oligocene 28 2 ma rhyolite flows using a continuous laser dating system routinely yield analytical precisions of less than 0 45 1s on samples smaller than 0 5 mg this is as good or better than has been obtained by any other method even though the sample mass is several orders of magnitude smaller the system has the potential of resolving differences in apparent age of as little as 0 2 for high k ca samples as young as a few ma the analyses suggest that the seven volcanic units analyzed were erupted in an interval of about 0 1 m y or less

source models of two large intraslab earthquakes from broadband strong ground motions we construct source models of two large intraslab earthquakes the 1993 kushiro oki and 1994 hokkaido toho oki earthquakes along the southern kurile hokkaido arc based on strong motion records the source models well explain observed broadband strong ground motions we assume a distribution of rectangular shaped asperities on a fault plane and apply the empirical green9s function method to the synthesis of strong ground motions from the asperities the asperity parameters area and stress drop are estimated by matching the synthetic waveforms displacement velocity and acceleration time histories and spectra to the observed ones our source models have the following characteristics compared with those of inland and plate boundary earthquakes having a comparable seismic moment 1 the total asperity area is much smaller and 2 the stress drop is extremely high about 200 400 mpa these indicate that the large intraslab earthquakes radiate seismic energies from a small area in a short time this rupture process results in extremely strong radiation of short period seismic waves finally we confirm a correlation between outer entire fault size seismic moment and short period level of the acceleration source spectrum and inner fault parameters areas and stress drops of asperities for the two intraslab earthquakes

reevaluation of plate motion models based on hotspot tracks in the atlantic and indian oceans abstract plate motion models based on hotspot tracks in the atlantic and indian oceans predict minimal movement less than a few millimeters per year between these hotspots and their counterparts in the pacific ocean for the past 100 m yr whereas plate circuit exercises indicate relative motions of 20 mm yr hotspot based models also suggest that the rajmahal traps india were located 1000 km away from the kerguelen hotspot at 115 ma and the deccan traps india were located a similar distance from the reunion hotspot at 65 ma this is at odds with conclusions derived from paleomagnetism plate circuits and geochemical parameters that suggest a genetic link between flood basalt provinces in india and hotspots in the indian ocean these divergent views may be explained by plume action 1000 km from its center or errors in the hotspot motion models the latter hypothesis is scrutinized in this article by examination of the radiometric ages for hotspot tracks in the atlantic and indian oceans the

undercrusting by serpentinite beneath rifted margins the west galicia passive margin spain is bounded by a continuous belt of serpentinized peridotite here we present the results of petrological studies of the peridotite 39ar 40ar dating of high temperature ductile shearing of a dioritic dyke crossing the peridotite and regional seismic stratigraphy which demonstrate that the uplift ductile shearing and pervasive serpentinization of mantle rocks occurred at early cretaceous time during the rifting stage of the margin the serpentinite layer resting on fresh peridotite is probably several kilometres thick at present seismic data and gravity data cannot distinguish serpentinite from crustal material but our results from the galicia margin indicate that the seismic crust beneath rifted margin may include mantle material transformed into serpentinite by syn rift hydrothermal activity in some cases this undercrusting process might explain the young age and mobility of the moho beneath stretched continental basement

statistical characterization of shear strength parameters of rock mass for hydropower projects in china abstract this study performs a statistical analysis on shear strength parameters of rock mass for hydropower projects in china first a large database comprising 1139 groups of field direct shear tests on rock mass from 100 hydropower projects in china is compiled second the best fit probability distributions of shear strength parameters are identified using akaike information criterion aic finally new characteristic and design values of shear strength parameters of rock mass are suggested the results indicate that shear strength parameters increase with increasing quality of rock mass in particular the shear strength parameters of discontinuities in rock mass are significantly smaller than those of rock mass itself the discontinuities in rock mass should be paid more attention during geological investigations there is a large variation in shear strength parameters and the variability of cohesion is larger than that of tangent of friction angle the derived statistics of shear strength parameters of rock mass in this study provide an update to the values suggested in chinese codes and serve as a reference for hydropower projects with few site specific test data and similar geological conditions for partial factor design of dams

influence of spatial variability of soil strength parameters on probabilistic seismic slope displacement hazard analysis abstract permanent sliding displacement analysis is commonly used to evaluate the seismic performance of slopes specifically the fully probabilistic seismic slope displacement hazard analysis pssdha has attracted increasing attention recently since it appropriately accounts for various sources of uncertainties associated with the estimation of sliding displacement within this analysis the uncertainty of soil strength parameters e g effective cohesion c and effective internal friction angle is commonly quantified by a logic tree method in which the spatial variability of c and is simply ignored this study aims at proposing an extended pssdha framework by rigorously accounting for the spatial variability of c and and then investigating its influence on the resultant slope displacement hazard curves based on the newmark rigid block model two different approaches namely an analytical based and a simulation based method are proposed separately the results of the analytical based method are found to be consistent with those obtained by the simulation based one it is also shown that the displacement hazard would be underestimated if the spatial variability of c and is ignored such underestimation is more significant for slopes with soil parameters exhibiting weak c correlation and strong spatial variability besides it suggests that the proper configuration of logic tree framework plays an important role in developing displacement hazard curves and a relatively large range of discrete strength values is recommended to better account for the effect of the spatial variability of soil strength parameters

magmatisme fissural triasico liasique dans l ouest du massif armoricain france petrologie geochimie âge et modalites de la mise en place the dolerite dykes at the western end of britanny france which is located on the north atlantic margin were emplaced in a nw se field of faults at the trias lias time limit between 210 and 195 ma these rocks are quartz tholeiites enriched in tio2 with a trace element composition close to that of the deccan basalts the initial concentrations of ba k and rb though slightly modified by the alteration and of nb seem to have been affected by crustal contamination during the magma transfer these dykes are comparable in age and composition to the tholeiitic dykes exposed in the iberian peninsula in the moroccan atlas and in the northern appalachians they however have a different orientation furthermore these various domains differ in their postintrusion history the rifting leading to the opening of the central atlantic occurred soon after the intrusions of the dykes whereas the opening of the north atlantic began several tens of millions of years later following the direction of the armoric

earthquake distribution in the subduction zone off eastern hokkaido japan deduced from ocean bottom seismographic and land observations summary in 1986 observations of microearthquakes were conducted with the use of five ocean bottom seismometers obss on the continental slope east of the kushiro submarine canyon ksc off hokkaido island japan the observation area is located about 50 100 km landward of the southern kuril trench beneath which the pacific plate is subducted toward hokkaido island the obs data were analysed together with land data on hokkaido island to determine the precise earthquake distribution and the geometry of the subducted plate the obs data revealed for the first time the seaward seismicity east of ksc high and shallow seismicity is concentrated in an area between the coast line of hokkaido island and the 2000 m isobath a region between the 2000 and 4000 m isobaths corresponds to the source area of an interplate earthquake occurring in 1973 the nemuro hanto oki earthquake m 7 4 the obs array detected more than 10 events in this region most of them however are classified as earthquakes occurring within the subducted plate and the remarkable aftershock activity of the 1973 event is not recognized the seismic activity is extremely low between the 4000 and 6000 m isobaths as is commonly seen in subduction zones the seismicity is also very low beneath the trench in contrast with the adjacent trench areas in a vertical section taken perpendicular to the trench axis most of the located events form a seismic plane dipping landward the dip angle of the plane is 20d 25d beneath the continental slope while 35d 40d under hokkaido island the change in dip angle occurs 150 km landward of the trench axis this seismic plane is located 30 40 km below the plate boundary defined from the aftershock distribution of the nemuro hanto oki event the seismicity on the plate boundary is correlated with the geometry of the subducted plate the high activity occurs in a restricted region where the subducted plate shows a remarkable bend the seismic activity obtained is different from those in the adjacent regions along the southern kuril trench in particular the regional difference in seismicity is remarkable between the western and eastern sides of ksc our result suggests that the subduction zone along the kuril trench is divided into several tectonic blocks and ksc is one of their boundaries

the 40ar 39ar age dating of the madeira archipelago and hotspot track eastern north atlantic the 40ar 39ar ages for 35 volcanic rocks and 14c ages for two charcoal samples from the madeira archipelago and ampere seamount eastern north atlantic are presented the volcanic evolution of madeira can be divided into a voluminous shield stage 4 6 0 7 ma and a subsequent low volume posterosional stage 70 myr old hotspot which formed porto santo island 11 1 14 3 ma seine ampere 31 ma corral patch and ormond 65 67 ma feraud et al 1982 1986 seamounts and the serra de monchique 70 72 ma mcintyre and berger 1982 complex in southern portugal age and spatial relationships result in a calculated absolute african plate motion above the hotspot of 1 2 cm yr around a rotation pole located at 43 36 n 24 33 w

source depth dependence of micro tsunamis recorded with ocean bottom pressure gauges the january 28 2000 mw 6 8 earthquake off nemuro peninsula japan abstract micro tsunami waves with a maximum amplitude of 4 6 mm were detected with the ocean bottom pressure gauges on a cabled deep seafloor observatory south of hokkaido japan following the january 28 2000 earthquake m w 6 8 in the southern kuril subduction zone we model the observed micro tsunami and estimate the focal depth and other source parameters such as fault length and amount of slip using grid searching with the least squares method the source depth and stress drop for the january 2000 earthquake are estimated to be 50 km and 7 mpa respectively with possible ranges of 45 55 km and 4 13 mpa focal depth of typical inter plate earthquakes in this region ranges from 10 to 20 km and stress drop of inter plate earthquakes generally is around 3 mpa the source depth and stress drop estimates suggest that the earthquake was an intra slab event in the subducting pacific plate rather than an inter plate event in addition for a prescribed fault width of 30 km the fault length is estimated to be 15 km with possible ranges of 10 20 km which is the same as the previously determined aftershock distribution the corresponding estimate for seismic moment is 2 7 10 19 nm with possible ranges of 2 3 10 19 3 2 10 19 nm standard tide gauges along the nearby coast did not record any tsunami signal high precision ocean bottom pressure measurements offshore thus make it possible to determine fault parameters of moderate sized earthquakes in subduction zones using open ocean tsunami waveforms

a five years super slow aseismic precursor model for the 1994 m8 3 hokkaido toho oki lithospheric earthquake based on tide gauge data 1 here we present a super slow aseismic event model prior to the magnitude 8 3 kurile island hokkaido toho oki earthquake on 4 october 1994 based on data recorded by two tide gauges located 50 150 km from the earthquake epicenter both instruments recorded several cm of subsidence during a five year period prior to the earthquake the observed signals are consistent with a precursory quasi stable slip on the western half of the hokkaido toho oki fault plane when recognized such aseismic events can considerably improve our intermediate term several years prediction capability

the 2000 nemuro hanto oki earthquake off eastern hokkaido japan and the high intraslab seismic activity in the southwestern kuril trench 1 the 2000 nemuro hanto oki earthquake mw6 8 occurred in the southwestern part of the kuril trench the hypocenter was located close to the aftershock region of the great 1994 kuril earthquake mw8 3 named the 1994 hokkaido toho oki earthquake by the japan meteorological agency for which the fault plane is still in debate analysis of the 2000 event provides a clue to resolve the fault plane issue for the 1994 event the hypocenters of the 2000 main shock and aftershocks are determined using arrival times from a combination of nearby inland and submarine seismic networks with an improved azimuthal coverage they clearly show that the 2000 event was an intraslab event occurring on a shallow dipping fault plane between 55 and 65 km in depth the well focused aftershock distribution of the 2000 event the relative location of the 1994 event with respect to the 2000 event and the similarity between their focal mechanisms strongly suggest that the faulting of the great 1994 earthquake also occurred on a shallow dipping fault plane in the subducting slab the recent hypocenter distribution around the 1994 aftershock region also supports this result large intraslab earthquakes occurring to the southeast of hokkaido may occur due to a strong coupling on the plate boundary which generates relatively large stress field within the subducting pacific plate

quick aftershock relocation of the 1994 shikotan earthquake and its fault planes the 1994 shikotan earthquake of mw 8 2 occurred near the western kurile trench where the pacific plate is subducting beneath the north american plate although its mechanism is thrust type it is not a typical low angle thrust event with a nodal plane parallel to the plate boundary that occurs at the plate boundary in order to determine which of the two nodal planes of the main shock was the fault plane we relocated the main shock and all aftershocks with m 5 3 using a modified joint hypocenter determination method earthquakes on the northwestern side of the aftershock area including the main shock are located along a plane dipping east southeastward with a dip of 70 this plane is about 80 to 150 km long and 70 km wide the main shock occurred at the north northeastern bottom of this plane which is almost parallel to one of the two nodal planes of the main shock determined by harvard university which has a strike of n51 e and a dip of 76 thus we conclude that this plane is the fault plane that the fault broke at the northeastern bottom of the fault plane and that the rupture propagated to the surface and also to the southeast in addition to the aftershocks aligned with this fault plane there is a secondary alignment which is subparallel to the main fault plane and about 50 km east where the largest aftershock of mw 7 3 occurred on oct 9 since several aftershocks occurred on this secondary fault plane on the same day as the main shock it seems reasonable to suppose that this secondary fault plane was generated together with the main fault plane at the time of the main shock

effect of spatial variation of strength and modulus on the lateral compression response of cement admixed clay slab in deep excavation construction improved soil layers consisting of overlapping cement admixed columns formed by deep mixing method or jet grouting are often used to stabilise an excavation in soft soils the purpose of such soil layers is to resist lateral compression generated by movement of the retaining wall cement admixed soils are well known to have high heterogeneity in strength in this paper the heterogeneity in strength and young s modulus are studied using random finite element analyses considering three sources of variation namely a deterministic radial trend in strength and young s modulus a stochastic fluctuation component due to non uniform mixing and positioning errors arising from off verticality of the mixing shafts the results show that positioning errors have the largest effect on the strength of the slab as a whole whereas the radial trend has the smallest effect when normalised by the volume average strength based on the results obtained methods are proposed which allow e

the great kurile earthquake of october 4 1994 tore the slab the fault geometry depth and slip distribution of the kurile earthquake of oct 4 1994 are estimated using seismic waveforms aftershock distribution geodetic measurements and tsunami waveforms previous large earthquakes in the kurile arc had typical underthrusting focal mechanisms due to the local subduction seismic wave inversions of the 1994 event indicate a thrust type mechanism with a large strike slip component this does not represent an underthrust event at the subduction interface the focal depth is estimated at 50 kin just beneath the previously estimated subduction interface we calculate crustal deformation and tsunami waveforms from the two possible fault planes of the focal mechanism solution and find that both fault planes explain the observations we choose the north south striking plane because of the occurrence of a large m w 7 8 intermediate depth earthquake on dec 6 1978 northwest of the 1994 rupture area the lineation of aftershock distributions of these two events suggests that both earthquakes were due to a tearing of the slab perpendicular to the trench the joint inversion of geodetic and tsunami data shows that the largest slip about 17 m occurred at the upper end of the slab

refined coseismic displacement modeling for the 1994 shikotan and sanriku oki earthquakes we consider refined modeling of gps measurements of coseismic displacement for two large earthquakes occurring in the japan and kurile trenches calculations are performed for a layered sphere and the homogeneous half space more commonly used to model such data using fault geometry and slip distributions from previously published studies we show that the difference between the layered sphere and homogeneous half space calculations can be significant as expected from previous theoretical studies however it does not appear large enough to seriously affect results from previously published studies of these earthquakes nevertheless the fact that the effects of sphericity and especially layering are measurable in coseismic observations of great earthquakes at moderate distances indicates that these effects should be taken into account in order to fully exploit the precision of modern gps measurements

the october 4 1994 shikotan kurile islands tsunamigenic earthquake an open problem on the source mechanism on october 4 1994 an earthquake of magnitude m w 8 2 occurred in the western part of the kurile islands generating a tsunami that has been well recorded along the entire coast of japan previous works have shown that this earthquake does not represent a low angle thrust event normally expected in a subduction zone rather an intra plate event rupturing through the slab on the basis of the accepted mechanism two fault models representative of the nodal plane ambiguity have been suggested the goal of this work is to verify whether the tsunami simulations are able to rule out one of the two proposed fault models taking into account both fault models together with a heterogeneous slip along the fault we have performed numerical simulations of the tsunami all source models produce tide gauge records in agreement with the observed ones the limit of resolution of the performed simulations estimated by means of a perturbed bathymetry does not allow us to distinguish the best source model

influence of spatial variability of soil young s modulus on tunnel convergence in soft soils abstract field data have shown the fact that soil spatial variability could aggravate the uncertainty of tunnel convergence d a key indicator for serviceability and safety of tunnels this paper presents a detailed numerical analysis to investigate the probabilistic response of tunnel convergence in spatially varied soft soils the soil young s modulus es is highlighted and modeled with isotropic and horizontally stratified anisotropic random fields respectively the influence of scale of fluctuation sof of the es on convergence d is discussed in detail with respect to different directions i e the vertical and horizontal directions both for and d it is observed that ignoring the spatial variability of es i e disregarding the possibility of unfavorable soft soil low stiffness soil locally around tunnel can underestimate the mean value of d the horizontally stratified anisotropic random field is more appropriate than isotropic random field in the sense of an accurate prediction especially when extreme tunnel convergence occurs in horizontally stratified anisotropic random fields the influence of horizontal and vertical sof is different on tunnel convergence the surrounding soils near tunnel crown and invert or across tunnel horizontal diameter are very critical to the tunnel convergence in addition the effect of horizontal sof x on failure probability of the calculated d exceeding the specified allowable dlim is limited when the x is larger than 4 84 times of tunnel outer diameter

where and why do large shallow intraslab earthquakes occur we try to find how often and in what regions large earthquakes m 7 0 occur within the shallow portion 20 60 km depth of a subducting slab searching for events in published individual studies and the harvard university centroid moment tensor catalogue we find twenty such events in e hokkaido kyushu sw japan s mariana manila sumatra vanuatu n chile c peru el salvador mexico n cascadia and alaska slab stresses revealed from the mechanism solutions of these large intraslab events and nearby smaller events are almost always down dip tensional except for e hokkaido manila and sumatra the upper plate shows horizontal stress gradient in the arc perpendicular direction we infer that shear tractions are operating at the base of the upper plate in this direction to produce the observed gradient and compression in the outer fore arc balancing the down dip tensional stress of the slab this tectonic situation in the subduction zone might be realized as part of the convection system with some conditions as shown by previous numerical simulations 2003 elsevier b v all rights reserved

aftershock distribution of the october 4 1994 mw8 3 kurile islands earthquake determined by a local seismic network in hokkaido japan on october 4 1994 an earthquake with magnitude mw8 3 occurred in the western part of kurile islands at 43 42 n 146 81 e and 33 km in depth the hypocenter parameters were determined by hokkaido university in japan aftershocks following this remarkable event were located using data from a local seismic network operated by hokkaido university we found that most of the aftershocks occurred 1 on the fault plane of the mainshock 2 in the subducting plate around the fault plane of the mainshock and 3 in the focal area of the largest aftershock which occurred on october 9 with mw7 3 both 2 and 3 were not active immediately after the mainshock considering the time sequence of the aftershock activity we identified one of the nodal planes of the harvard quick cmt solutions as the fault plane of the mainshock the strike is almost parallel to the trench axis and the dip angle is near vertical it is obvious that this event is different from a low angle thrust type interplate earthquake the distribution of aftershocks strongly suggests that it is an intraplate event