tsunami deposits in a lacustrine sequence of the sanriku coast northeast japan abstract lacustrine deposits of the pacific coast of northeast japan consisting mostly of black organic mud contain intercalated thin beds of well sorted medium sand examination of the deposits from a coastal site of sanriku has revealed that sand grains are of marine origin and are fractions of deposits in marshy ponds transported from the littoral environment by a great flooding of seawater the mode of sedimentation shown in the thin beds of sand implies that they were deposited by tsunamis each with a maximum rise of 1 m or more above sea level

tsunami history of an oregon coastal lake reveals a 4600 yr record of great earthquakes on the cascadia subduction zone bradley lake on the southern oregon coastal plain records local tsunamis and seismic shaking on the cascadia subduction zone over the last 7000 yr thirteen marine incursions delivered landward thinning sheets of sand to the lake from nearshore beach and dune environments to the west following each incursion a slug of marine water near the bottom of the freshwater lake instigated a few year to several decade period of a brackish 4 salinity lake four additional disturbances without marine incursions destabilized sideslopes and bottom sediment producing a suspension deposit that blanketed the lake bottom considering the magnitude and duration of the disturbances necessary to produce bradley lake s marine incursions a local tsunami generated by a great earthquake on the cascadia subduction zone is the only accountable mechanism extreme ocean levels must have been at least 5 8 m above sea level and the cumulative duration of each marine incursion must have been at least 10 min disturbances without marine incursions require seismic shaking as well over the 4600 yr period when bradley lake was an optimum tsunami recorder tsunamis from cascadia plate boundary earthquakes came in clusters between 4600 and 2800 cal yr b p tsunamis occurred at the average frequency of 3 4 every 1000 yr then starting 2800 cal yr b p there was a 930 1260 yr interval with no tsunamis that gap was followed by a 1000 yr period with 4 tsunamis in the last millennium a 670 750 yr gap preceded the a d 1700 earthquake and tsunami the a d 1700 earthquake may be the fi rst of a new cluster of plate boundary earthquakes and accompanying tsunamis local tsunamis entered bradley lake an average of every 390 yr whereas the portion of the cascadia plate boundary that underlies bradley lake ruptured in a great earthquake less frequently about once every 500 yr therefore the entire length of the subduction zone does not rupture in every earthquake and bradley lake has recorded earthquakes caused by rupture along the entire length of the cascadia plate boundary as well as earthquakes caused by rupture of shorter segments of the boundary the tsunami record from bradley lake indicates that at times most recently 1700 yr b p overlapping or adjoining segments of the cascadia plate boundary ruptured within decades of each other

anomalous low velocity zone and linear alignment of seismicity along it in the subducted pacific slab beneath kanto japan reactivation of subducted fracture zone 1 a detailed investigation of the hypocenter distribution beneath kanto japan reveals a nw se trending linear alignment of seismicity within the subducted pacific slab we estimate the 3d seismic velocity structure in the pacific slab to understand the factors controlling the genesis of such intraslab earthquakes a narrow low velocity zone is imaged within the subducted slab over a length of 150 km which partly penetrates into the mantle portion of the slab the low velocity zone correlates in space with the nw se trending earthquake cluster a reactivation of hydrated fracture zone formed prior to subduction is probably related to the low velocity anomaly dehydration reactions of the hydrated oceanic mantle as well as the oceanic crust might lower the seismic velocity along the fossil fracture zone accompanied by intraslab earthquakes these observations support the hypothesis of dehydration embrittlement as the most viable mechanism for generating intraslab earthquakes

nontrivial decay of aftershock density with distance in southern california the decay of the aftershock density with distance plays an important role in the discussion of the dominant underlying cause of earthquake triggering here we provide evidence that its form is more complicated than typically assumed and that in particular a transition in the power law decay occurs at length scales comparable to the thickness of the crust this is supported by an analysis of a very recent high resolution catalog for southern california sc and surrogate catalogs generated by the epidemic type aftershock sequence etas model which take into account inhomogeneous background activity short term aftershock incompleteness anisotropic triggering and variations in the observational magnitude threshold our findings indicate specifically that the asymptotic decay in the aftershock density with distance is characterized by an exponent larger than 2 which is much bigger than the observed exponent of approximately 1 35 observed for shorter distances ranging from the main shock rupture length up to a length scale comparable to the thickness of the crust this has also important consequences for time dependent seismic hazard assessment based on the etas model

foreshock occurrence before large earthquakes rates of foreshock occurrence involving shallow m 6 and m 7 mainshocks and m 5 foreshocks were measured in two worldwide catalogs over 20 year intervals the overall rates observed are similar to ones measured in previous worldwide and regional studies when they are normalized for the ranges of magnitude difference they each span the observed worldwide rates were compared to a generic model of earthquake clustering based on patterns of small and moderate aftershocks in california the aftershock model was extended to the case of moderate foreshocks preceding large mainshocks overall the observed worldwide foreshock rates exceed the extended california generic model by a factor of 2 significant differences in foreshock rate were found among subsets of earthquakes defined by their focal mechanism and tectonic region with the rate before thrust events higher and the rate before strike slip events lower than the worldwide average among the thrust events a large majority composed of events located in shallow subduction zones had a high foreshock rate while a minority located in continental thrust belts had a low rate these differences may explain why previous surveys have found low foreshock rates among thrust events in california especially southern california while the worldwide observations suggests the opposite california lacking an active subduction zone in most of its territory and including a region of mountain building thrusts in the south reflects the low rate apparently typical for continental thrusts while the worldwide observations dominated by shallow subduction zone events are foreshock rich if this is so then the california generic model may significantly underestimate the conditional probability for a very large m 8 earthquake following a potential m 7 foreshock in cascadia the magnitude differences among the identified foreshock mainshock pairs in the harvard catalog are consistent with a uniform distribution over the range of observation

quantitative earthquake forecasts resulting from static stress triggering 1 in recent years the triggering of earthquakes has been discussed controversially with respect to the underlying mechanisms and the capability to evaluate the resulting seismic hazard apart from static stress interactions other mechanisms including dynamic stress transfer have been proposed to be part of a complex triggering process exploiting the theoretical relation between long term earthquake rates and stressing rate we demonstrate that static stress changes resulting from an earthquake rupture allow us to predict quantitatively the aftershock activity without tuning specific model parameters these forecasts are found to be in excellent agreement with all first order characteristics of aftershocks in particular 1 the total number 2 the power law distance decay 3 the scaling of the productivity with the main shock magnitude 4 the foreshock probability and 5 the empirical bath law providing the maximum aftershock magnitude which supports the conclusion that static stress transfer is the major mechanism of earthquake triggering

a study on the background and clustering seismicity in the taiwan region by using point process models 1 this paper investigates the shallow seismicity occurring in the taiwan region during the 20th century using a stochastic declustering method that has been developed on the basis of the theory of the epidemic type aftershock sequence model it provides a probability based tool to objectively separate the space time occurrences of earthquakes into a background and a clustering component on the basis of the background and clustering seismicity rates we discuss the correlation between the distribution of the cluster ratio and the regional seismotectonic structures specifically we find that the areas of the highest clustering ratio correspond to the major strike slip fault traces in and around taiwan additionally in the taiwan inland region during the period 1960 1990 the outputs for the stochastically declustered catalogue show a clear quiescence in background seismicity preceding the recovery of activation and the occurrences of the 1999 chi chi earthquake of ml7 3 while the other active regions show stationary background activity this could be interpreted as an effect of the aseismic slip in the chi chi rupture fault whereby the inland region around the chi chi source becomes a stress shadow

tsunami deposits from the 1993 southwest hokkaido earthquake and the 1640 hokkaido komagatake eruption northern japan the southwest hokkaido tsunami of july 12th 1993 left continuous onshore sand deposits along the west coast of oshima peninsuka hokkaido northern japan we investigated spatial distribution and lithofacies of the new tsunami deposits for its identification of ancient tsunami deposits an eyewitness acount and bent plants helped our interpretation of the onshore tsunami behavior we regard the following properties as typical of the coastal tsunami sand deposits 1 the deposits cover the surface almost continuously on gentle topography 2 deposit thicknesses and mean grain sizes descrease with distance from the sea 3 deposit thicknesses and lithofacies vary greatly across local surface undulation 4 graded bedding reflecting tsunami runup and backwash is present in thick deposits 5 the deposits are widely distributed along the coast and extend inland several tens of meters to 100 m we examined a candidate for the paleo tsunami deposits associated with the 1640 komagatake eruption and confirmed that the similar patterns are typical of ancient tsunami deposits

fundamental questions of earthquake statistics source behavior and the estimation of earthquake probabilities from possible foreshocks abstract estimates of the probability that an m l 4 8 earthquake which occurred near the southern end of the san andreas fault on 24 march 2009 would be followed by an m 7 mainshock over the following three days vary from 0 0009 using a gutenberg richter model of aftershock statistics reasenberg and jones 1989 to 0 04 using a statistical model of foreshock behavior and long term estimates of large earthquake probabilities including characteristic earthquakes agnew and jones 1991 i demonstrate that the disparity between the existing approaches depends on whether or not they conform to gutenberg richter behavior while gutenberg richter behavior is well established over large regions it could be violated on individual faults if they have characteristic earthquakes or over small areas if the spatial distribution of large event nucleations is disproportional to the rate of smaller events i develop a new form of the aftershock model that includes characteristic behavior and combines the features of both models this new model and the older foreshock model yield the same results when given the same inputs but the new model has the advantage of producing probabilities for events of all magnitudes rather than just for events larger than the initial one compared with the aftershock model the new model has the advantage of taking into account long term earthquake probability models using consistent parameters the probability of an m 7 mainshock on the southernmost san andreas fault is 0 0001 for three days from long term models and the clustering probabilities following the m l 4 8 event are 0 00035 for a gutenberg richter distribution and 0 013 for a characteristic earthquake magnitude frequency distribution our decisions about the existence of characteristic earthquakes and how large earthquakes nucleate have a first order effect on the probabilities obtained from short term clustering models for these large events

fault slip and seismic moment of the 1700 cascadia earthquake inferred from japanese tsunami descriptions 1 the 1700 cascadia earthquake attained moment magnitude 9 according to new estimates based on effects of its tsunami in japan computed coseismic seafloor deformation for hypothetical ruptures in cascadia and tsunami modeling in the pacific ocean reports of damage and flooding show that the 1700 cascadia tsunami reached 1 5 m heights at seven shoreline sites in japan three sets of estimated heights express uncertainty about location and depth of reported flooding landward decline in tsunami heights from shorelines and post 1700 land level changes we compare each set with tsunami heights computed from six cascadia sources each source is vertical seafloor displacement calculated with a three dimensional elastic dislocation model for three sources the rupture extends the 1100 km length of the subduction zone and differs in width and shallow dip for the other sources ruptures of ordinary width extend 360 670 km to compute tsunami waveforms we use a linear long wave approximation with a finite difference method and we employ modern bathymetry with nearshore grid spacing as small as 0 4 km the various combinations of japanese tsunami heights and cascadia sources give seismic moment of 1 9 1022 n m equivalent to moment magnitude 8 7 9 2 this range excludes several unquantified uncertainties the most likely earthquake of moment magnitude 9 0 has 19 m of coseismic slip on an offshore full slip zone 1100 km long with linearly decreasing slip on a downdip partial slip zone the shorter rupture models require up to 40 m offshore slip and predict land level changes inconsistent with coastal paleoseismological evidence

faulting style controls for the space time aftershock patterns abstract to understand the complexity and thus to predict earthquake occurrence in size time and space seismicity patterns are characterized by two robust empirical laws the exponential distribution of magnitude and the power law decrease of aftershock rate over time these laws are known as the gutenberg richter law and the omori law respectively using global earthquake catalogs we resolve that on average the k aftershock productivity and the p value exponent of the power law decrease of aftershock rate over time are dependent on the mainshock faulting style strike slip events have a lower aftershock rate n and k values and a larger p value than thrust and normal events respectively within the epidemic type aftershock sequence model strong k n values are driven by a high branching ratio value n within the same framework a relatively higher n value for the thrust events also predicts the lower p value we observe for thrust events as compared to strike slip and normal faulting events respectively furthermore we observed that earthquake interactions through time and space are a function of the faulting style when measured by t the exponent of the power law decrease of earthquake density over space the t values of thrust events for different time windows always remain smaller than those of the strike slip events when changes in faulting styles are driven by stress pattern the anderson faulting theory predicts thrust faulting that requires somewhat larger stresses in absolute magnitude than do normal and strike slip faulting within the framework of rate and state friction law changes in the stress heterogeneity patterns reproduce the p value changes we observe our results suggest that only stress perturbations associated with mainshock rupture affect the productivity and decay rate over time of aftershocks

are spontaneous earthquakes stationary in california 1 aftershocks and some main shocks are triggered with timing controlled by preceding events the remaining spontaneous earthquakes presumably respond to tectonic stresses we consider whether triggered events can be reliably identified whether the rest are stationary and whether external phenomena control them to all three questions some studies of earthquake physics and hazard assume answers many suggest that stress changes from large distant earthquakes can alter the local spontaneous earthquake rate we demonstrate significant differences in the apparent earthquake rate after declustering by different methods and we present criteria for assessing the influence of such distant events the estimated spontaneous earthquake rate depends on the lower magnitude threshold of included events whether spontaneity is treated in binary or probabilistic form as well as assumptions about catalog completeness different statistical tests give different answers to the question of stationarity we examine a reported rate change in southern california and the suggestion that it might have resulted from the 1960 chile and 1964 alaska earthquakes the rate change itself is questionable if it has occurred it was probably not caused by those distant events because the rate change is not present in northern california or in all parts of southern california

x ray tomography of tsunami deposits towards a new depositional model of tsunami deposits x ray computed microtomography is used to obtain high resolution imagery of a historical tsunami deposit in andalusia spain 1755 lisbon tsunami the technique allows characterization of grain size distribution structures component analysis and sedimentary fabric of fine grained unconsolidated tsunami deposits at resolutions down to particle scale the results are validated by comparing to data obtained using other techniques such as laser diffraction anisotropy of magnetic susceptibility and x ray microfluorescence on the same deposits specific technical details such as sampling scanning and image processing methods and further improvements are addressed the use of x ray computed microtomography provides new insights into the stratigraphy of the deposits and gives access to significantly more detailed view of key sedimentary features such as mudlines rip up clasts crude laminations convolutions floating outsized clasts and contacts between successive units this analysis of the 1755 tsunami deposits using x ray computed microtomography allows the proposal of new hypotheses for the sedimentary processes forming tsunami deposits deposition by settling is limited and the section analysed here is dominated by a high shear stress leading to the development of traction carpets with laminated mudlines corresponding to the basal frictional region of these carpets the onset of the tsunami backwash is marked by a micro vortex resembling kelvin helmoltz instabilities

a method of studying variations in the clustering property of earthquakes application to the analysis of global seismicity we propose a formal algorithm for identification of aftershocks following a main event our approach is based on the analysis of the local statistics the number of events following the initial earthquake displayed as a function of distance and time by comparing this distribution with the random catalog we establish for several magnitude ranges spatiotemporal thresholds for detection of aftershocks high confidence thresholds correspond to the limit at which the number of events derived from the actual catalog corrected for the background seismicity exceeds by a factor of 10 the expected level of background seismicity marginal confidence thresholds correspond to the spatiotemporal boundary along which these two values are approximately equal after several iterations the final thresholds are derived the swarming parameter is defined as the number of aftershocks with a cut off lower magnitude range following an initial earthquake of certain magnitude if a correction for the recurrence law is applied we find that the swarming parameter is magnitude independent this allows us to apply this technique to the analysis of global seismicity using a catalog derived from the isc bulletin for the years from 1964 to 1975 we determine for shallow earthquakes the average values of the swarming parameter in 9 9 cells a global map produced in this manner shows a very wide range of variations in the swarming property undoubtedly reflecting variations in the local geological conditions triple junctions are among the areas with the highest swarming values at the present time there is an insufficient amount of data to study regional variations in the swarming property of deep earthquakes considering the clustering as a function of depth alone we find detectable swarming for two depth ranges from 100 to 250 km and from 450 to 650 km not a single aftershock has been detected among nearly 200 earthquakes withmb 5 in a depth range from 250 to 450 km the average swarming parameter for earthquakes deeper than 100 km is 10 times smaller than that for the shallow earthquakes

micromorphological analysis of coastal sediments from willapa bay washington usa a technique for analysing inferred tsunami deposits tsunami deposits are provisionally distinguished in the field on the basis of anomalous sand horizons fining up and fining landward coupled with organic rich fragmented backwash sediments in this paper micromorphological features of a sediment sequence previously interpreted as being of tsunami origin are described these characteristics are shown to be consistent with the macro scale features used elsewhere but show additional details not seen in standard stratigraphies including possible evidence for individual waves possibly wave magnitude progression organic fragment alignment and intraclast microstructures although replication and more complete studies are needed this analysis confirms the identification of a tsunami in willapa bay in ca 1700 ad while demonstrating a widely applicable technique for confirming or refuting possible tsunami deposits

2d spatial distribution of reflection intensity on the upper surface of the philippine sea plate off the boso peninsula japan abstract the region off the boso peninsula japan is a tectonically complex area where the pacific plate is subducting beneath both the landward plate and the philippine sea plate phs from the japan trench and the izu bonin trench as the phs is subducting under the landward plate from the sagami trough it is important to better determine the structure of this region to deepen our understanding of its seismicity previous seismic reflection studies have shown that reflections from the upper surface of the phs vary with depth being stronger in the main slip area of the slow slip events beneath the boso peninsula boso sses however the spatial relationship between the reflective area and the sses is poorly constrained this study mapped the distribution of the reflective area using data recorded by ocean bottom seismometers during an active source seismic experiment we constructed a 3d p wave velocity structure by using traveltimes of first arrivals from 18 ocean bottom seismometer records we also adapted the traveltime mapping method to reflection traveltimes projecting them to the depth distance domain to map the 2d distribution of strong reflections from the top of the phs these reflections were concentrated in two areas one near the main slip area of the boso sses and the other about 60 km to the east in the first area the absence of strong velocity contrasts near the top of the phs suggests that the reflections were generated by a thin low velocity layer in contrast the structure of the second area has a convex shape of high velocity with a high velocity gradient near the top of the phs this structure may represent boninitic material of the outer arc high partially serpentinized peridotite or gabbro displaced by intraoceanic reverse faults

anomalous deepening of a belt of intraslab earthquakes in the pacific slab crust under kanto central japan possible anomalous thermal shielding dehydration reactions and seismicity caused by shallower cold slab material 1 a belt of intraslab seismicity in the pacific slab crust parallel to iso depth contours of the plate interface has been found beneath hokkaido and tohoku hypocenter relocations have shown that this seismic belt does not run parallel to but obliquely to the iso depth contours beneath kanto deepening toward the north from 100 km to 140 km depth the depth limit of the contact zone with the overlying philippine sea slab is located close to and parallel to this obliquely oriented seismic belt suggesting that the deepening of the seismic belt there is caused by the contact with the overlying slab the contact with this cold slab hinders the heating of the pacific slab crust by hot mantle wedge which would cause delay of eclogite forming phase transformations and hence deepening of the seismic belt there the depth limit of the subducting low velocity crust also deepens toward the north supporting this idea

a review of geological records of large tsunamis at vancouver island british columbia and implications for hazard abstract large tsunamis strike the british columbia coast an average of once every several hundred years some of the tsunamis including one from alaska in 1964 are the result of distant great earthquakes most however are triggered by earthquakes at the cascadia subduction zone which extends along the pacific coast from vancouver island to northern california evidence of these tsunamis has been found in tidal marshes and low elevation coastal lakes on western vancouver island the tsunamis deposited sheets of sand and gravel now preserved in sequences of peat and mud these sheets commonly contain marine fossils and they thin and fine landward consistent with deposition by landward surges of water they occur in low energy settings where other possible depositional processes such as stream flooding and storm surges can be ruled out the most recent large tsunami generated by an earthquake at the cascadia subduction zone has been dated in washington and japan to ad 1700 the spatial distribution of the deposits of the 1700 tsunami together with theoretical numerical modelling indicate wave run ups of up to 5 m asl along the outer coast of vancouver island and up to 15 20 m asl at the heads of some inlets the waves attenuated as they moved eastward along juan de fuca strait and into puget sound and the strait of georgia no deposits of the 1700 event or for that matter any other tsunami have yet been found in the strait of georgia suggesting that waves were probably no more than 1 m high in this area if a tsunami like the 1700 event were to occur today communities along the outer pacific coast from southern british columbia to northern california would be severely damaged there would be little time to evacuate these communities because the tsunami would strike the outer coast within minutes of the first ground shaking fortunately such tsunamis are infrequent perhaps as few as seven have occurred in the last 3500 yr other tsunamis that are much smaller and more localized although probably more frequent are caused by local crustal earthquakes and landslides along the british columbia coast two such tsunamis have occurred in british columbia in recent years one in 1946 in the strait of georgia and another in 1975 at the head of a fiord on the northern mainland coast

seismic moments local magnitudes and coda duration magnitudes for earthquakes in central california onscale seismograms recorded at stations in the u s geological survey s usgs central california seismic network calnet have been used to estimate the seismic moment m and local magnitude ml for earthquakes of 1 ml 4 located on the san juan bautista and parkfield sections of the san andreas fault the coyote lake section of the calaveras fault the sargent fault and near livermore these data together with m and ml estimates for 4 ml 6 earthquakes in these areas cannot be fit with a single linear log m versus ml relation rather the data are consistent with log m 1 5 ml 16 for 3 ml 6 with log m 1 2 ml 17 for 1 1 2 ml 3 1 2 and with a slope of to 1 fro 1 2 ml 1 1 2 whereas usgs coda duration magnitude md is consistent with ml for 1 1 2 ml 3 md is larger than ml at ml 1 1 2 and smaller than ml at ml 3 log m can be estimated to a precision of 0 2 for 1 md 3 1 2 earthquakes in central california by applying log m 1 2 md 17 to the md that have been routinely published by the usgs

west coast tides during cascadia subduction zone tsunamis low neap tides occurred along the west coast of the united states and canada around 0500 ut january 27 1700 a time for the last major cascadia earthquake tsunami based on teletsunami arrival times in japan satake et al 1996 however high range perigean spring tides occurred only a week later illustrating how accurately occurrence times must be known to constrain the region s background water levels during such events the ranges of background water levels increase northward from 3 m at monterey to 8 5 m at queen charlotte as estimated from the amplitudes of perigean spring tides and subtidal winter fluctuations

tsunamis and tectonic deformation at the northern cascadia margin a 3000 year record from deserted lake vancouver island british columbia canada three layers of sand occur within a sequence of muddy gyttja in a 3 m core recovered from deserted lake which lies at the head of a fjord on the central west coast of vancouver island british columbia canada the sedimentology and microfossil assemblages of these layers are consistent with deposition by tsunamis radiocarbon ages suggest that the tsunamis were generated by plate boundary earthquakes at the cascadia subduction zone about 2600 1600 and 300 years ago diatom and protozoan contents of the gyttja layers indicate that relative sea level fell in the periods between tsunamis the sea level changes are probably a product of interseismic uplift of the north america plate margin post tsunami diatom and protozoan assemblages are generally more marine than their pre tsunami counterparts suggesting that the study site subsided during earth quakes estimates of interseismic and coseismic deformation during one earthquake cycle indicate that less than half of the interseismic uplift was recovered during the earthquake leading to cumulative or permanent uplift of the area

triggering of seismicity at short timescales following californian earthquakes 1 a method for measuring seismicity rate changes caused by the occurrence of major earthquakes in the surrounding crust is proposed it is based on a nonstationary poisson modeling of earthquake activity an estimate of the seismicity rate change and a probability measure of seismicity triggering following a large shock are derived and the removal of the influence of independent aftershock sequences of other nearby previous earthquakes is attempted without using more traditional declustering techniques the seismicity rate change estimator is corrected for its natural bias in favor of positive changes at short timescales that has so far strongly hindered measures of seismicity quiescences three californian earthquakes are examined using this method the 1989 m7 1 loma prieta the 1992 m7 3 landers and the 1994 m6 7 northridge earthquakes positive triggering is very commonly observed as expected but quiescence is however more seldomly obtained for timescales up to 100 days after the main shock this relative absence of quiescence as compared to typical predictions of coulomb modeling for example is also independently observed through direct correlation analyses of californian seismicity the possibility that the stresses caused by the main shock are spatially highly variable may explain these observations

earthquake statistics at parkfield 2 probabilistic forecasting and testing construct two probabilistic forecasts h 1 with a spatially varying b value and h 2 with a uniform b value and test these hypotheses against each other both hypotheses use a spatially varying seismicity level a value determined from past seismicity we used a range of sampling parameters magnitude threshold cell size etc to assure robust results we found that in most of the tests hypothesis h 1 showed a higher likelihood than h 2 although both are a poor approximation to the seismicity data the most positive results for h 1 are obtained for testing magnitude ranges down to m 1 5 and with sampling radii as defined in paper 1 as appropriate for parkfield the superior performance of h 1 suggests that spatially varying b values should be considered in earthquake forecasts index terms 7215 seismology earthquake parameters 7223 seismology seismic hazard assessment and prediction 7230 seismology seismicity and seismotectonics keywords earthquake statistics probabilistic forecasting parkfield

imaging b value variation beneath the pamir hindu kush region the frequency magnitude distribution b value for the seismicity of the pamir hindu kush region from august 2008 to june 2010 is analyzed we used a very precisely located earthquake dataset containing 9532 events with a magnitude of completeness of 2 9 we attempted both 1d and 2d approaches to resolve the b value variation as a function of depth several pockets of high b value were detected during the cross sectional analysis that may reflect the changes in the physical properties of the subducting slab with depth under the hindu kush sector a high b value 1 2 is identified at the depth of 150 km but under pamir this zone b 0 9 is apparent at shallower depth an earlier detailed seismic tomography study revealed a low velocity zone beneath hindu kush where the high b value is imaged in the present study this observation is corroborated by the existence of fluid due to the dehydration and increase in pore pressure within the lower portion of the indian lithosphere the relatively high b value at shallower depth could be associated with the higher slab temperature as caused by the slower subduction under pamir

the september 16 2015 illapel tsunami chile sedimentology of tsunami deposits at the beaches of la serena and coquimbo abstract on september 16 2015 a mw 8 3 earthquake occurred off the coast of central chile 46 km west of the town of illapel the quake caused a tsunami impacting the coast mainly between the towns of caldera c 27 s and pichidangui c 32 s at coquimbo and la serena c 30 s the tsunami attained large wave heights on the order of 4 5 m leading to flooding and destruction of infrastructure including the coastal road we report the results of a post tsunami sedimentological field survey undertaken in october 2015 focussing on the beaches of coquimbo and la serena at playa changa a beach at coquimbo affected by the highest waves the tsunami destroyed large parts of the coastal road embankments on both the seaward and landward sides on the landward side the tsunami distributed triangular concrete construction elements of the embankment across the coastal plain on the seaward side granite boulders of up to 2 t were ripped out of the coastal road embankment and transported onto the beach sand used in construction of the embankment was redistributed across the beach as an outflow deposit at la serena we observed a laminated package of tsunami deposits along a 20 m long trench perpendicular to the coast the sediments have erosive basal surfaces with an amplitude of up to 50 cm the deposit thickness varies between 10 and 50 cm the sedimentary package consists of 6 mostly lenticular layers of variable thickness and spatial extent ranging between dark laminae a few millimeters thick and rich in heavy minerals and lighter colored sand layers up to 15 cm thick grain size distributions are moderately well to well sorted and unimodal with modes between 1 3 and 2 0 medium sand cross beds in the lower four layers indicate deposition from tsunami inflow cross bedding in the second layer from the top layer records outflow the abundance of cross beds and laminae suggests that deposition of sand from bed load traction currents was the prevailing mode of deposition water escape structures occur as small sand diapirs and volcanoes water escape through small volcanoes appears to have been coeval to formation of the overlying layer by traction deposition this synchroneity is indicated by sand sourced from the lower layer which has been preserved as a thin plume deformed in the downcurrent i e landward direction in the newly forming upper layer other sediment layers show sand diapirs intruding up to 15 cm into the overlying tsunami deposit the assemblage of sedimentary structures indicates rapid deposition from one or several waves associated with the tsunami event

mantle wedge flow pattern and thermal structure in northeast japan effects of oblique subduction and 3 d slab geometry abstract we develop a 3 d thermal model for the northeast japan subduction margin using a realistic slab geometry for the subducting pacific plate and investigate the effects of oblique subduction and 3 d slab geometry on the mantle wedge flow pattern and the thermal structure in the tohoku region the mantle wedge flow pattern is nearly two dimensional resulting in a thermal structure similar to those obtained by a 2 d model owing to the simple slab geometry and subduction nearly perpendicular to the margin however in hokkaido oblique subduction leads to 3 d mantle wedge flow with northerly inflow and west northwestward outflow and also results in lower temperatures in the shallow part of the mantle wedge than in tohoku due to lower sinking rate of the slab between hokkaido and tohoku the slab has a hinge like shape due to a relatively sharp change in the dip direction in this hinge zone northerly mantle inflow from hokkaido and westerly mantle inflow from tohoku converge discouraging inflow from northwest and resulting in a cooler mantle wedge the model predicted mantle wedge flow patterns are consistent with observed seismic anisotropy and may explain the orientations of volcanic cross chains the predicted 3 d thermal structure correlates well with the along arc variations in the location of the frontal arc volcanoes and help to provide new insights into the surface heat flow pattern and the down dip extent of interplate earthquakes

tidal marsh stratigraphy sea level change and large earthquakes i a 5000 year record in washington u s a abstract many of the estuaries of the pacific northwest of the u s a and canada contain stratigraphic sequences typified by alternating peat mud couplets recent studies in this region interpret such couplets as the product of repeated large magnitude s or 9 earthquakes on the cascadia subduction zone the resultant pattern of land level movements is described by a model the earthquake deformation cycle of coseismic land subsidence followed by land uplift during interseismic strain accumulation however peat mud couplets similar to those recorded in the pacific northwest are found on other less tectonically active temperate latitude coasts such as northwest europe and the atlantic coast of the u s a where they have been interpreted as the product of non seismic coastal processes in this paper we apply the methods and scientific framework common to sea level investigations in northwest europe to a sequence of peat mud couplets recorded in the lower johns river an estuary in southern washington to provide a test of the earthquake deformation cycle stratigraphic investigations of the intertidal sediments along the lower johns river using lithological pollen diatom and foraminiferal data show evidence for eight coastal submergence events during the last 5000 years to evaluate the earthquake deformation cycle we assess the lateral extent of peat mud couplets the synchroneity of submergence the presence of tsunami deposits accompanying submergence and the suddenness and amount of submergence each submergence is shown to be accompanied by changes in coastal sedimentation broadly commensurate with those predicted by the earthquake deformation cycle demonstrating the continued intermittent seismic activity of the cascadia subduction zone throughout the mid and late holocene quantitative analyses of contemporary and fossil biostratigraphic data using twinspan and detrended correspondence analysis enable us to estimate the magnitude of submergence accompanying each peat mud couplet one event was accompanied by submergence of about 1 5 m or more four events by intermediate submergence of about 1 0 5 m and a further three events by submergence of

quantifying seismicity parameter uncertainties and their effects on probabilistic seismic hazard analysis a case study of iran probabilistic seismic hazard analysis psha can accommodate various sources of uncertainties and it provides a rational framework for the precise portrayal of the hazard of a given region often the information used in the psha is incomplete and uncertain therefore the question arises how the uncertainty of the input data affects the estimated hazard characteristics in this study sensitivity analysis sa was conducted to identify the most dominant inputs affecting the assessment of the key seismicity parameters sps including the mean seismic activity rate b value of gutenberg richter and the maximum possible earthquake magnitude m hbox max the study was applied in five areas of iran for which such analyses have not been conducted in previous studies subsequently monte carlo simulation was employed to determine the effects of the uncertain input parameters on psha relevant to spectral accelerations corresponding to 10 and 2 probability of exceedance at least once in 50 years for this purpose a unified and declustered earthquake catalogue was used for the five major seismotectonic provinces of iran alborz azarbayejan zagros central east iran koppeh dagh and makran the results showed that the last complete part of the catalogue has a significant effect on the estimated value of seismic activity and the b value in contrast its influence is insignificant on the area characteristic maximum possible earthquake magnitude for which the most influential inputs are the maximum observed earthquake and its uncertainty furthermore the uncertainties of the input sps affected the seismic hazard estimates substantially and led to significant variability in the estimated ground motion characteristics

tsunami sedimentary facies deposited by the storegga tsunami in shallow marine basins and coastal lakes western norway sedimentary successions in small coastal lakes situated from 0 to 11 m above the 7000 year bp shoreline along the western coast of norway contain a distinctive deposit very different from the sediments above and below the deposit is interpreted to be the result of a tsunami inundating the coastal lakes an erosional unconformity underlies the tsunami facies and is traced throughout the basins with most erosion found at the seaward portion of the lakes the lowermost tsunami facies is a graded or massive sand that locally contains marine fossils the sand thins and decreases in grain size in a landward direction above follows coarse organic detritus with rip up clasts here termed organic conglomerate and finer organic detritus the tsunami unit generally fines and thins upwards the higher basins 6 11 m above the 7000 year shoreline show one sand bed whereas basins closer to the sea level 7000 years ago may show several sand beds separated by organic detritus these alternations in the lower basins may reflect repeated waves of sea water entering the lakes in basins that were some few metres below sea level at 7000 years bp the tsunami deposit is more minerogenic and commonly present as graded sand beds but also in some of these shallow marine basins organic rich facies occur between the sand beds the total thickness of the tsunami deposit is 20 100 cm in most studied sites an erosional and depositional model of the tsunami facies is developed

seismotectonics beneath the tokyo metropolitan area japan effect of slab slab contact and overlap on seismicity 1 we first determine the configuration of the upper surface of the pacific pac slab beneath kanto japan from the distribution of interplate earthquakes relocated by an appropriate 1 d velocity model then traveltime tomography is carried out to estimate three dimensional seismic velocity structures around kanto using 735 520 p wave and 444 049 s wave arrival times from 6508 local earthquakes the obtained results suggest that the philippine sea phs slab is subducting to depths of 130 140 km without a gap even to the northwest of the izu collision zone we subsequently define the lateral extent of the contact zone between the bottom of the phs slab and the upper surface of the pac slab phs pac interface and reveal that the slab contact zone underlies a wider area beneath kanto in harmony with the kanto plain the downdip limit of interplate thrust type earthquakes on the pac slab is deepened by 30 km locally under the slab contact zone this deepening is probably caused by a lower temperature environment in the pac slab resulting from the overlap with the phs slab subducting above and consequent thermal shielding by the phs slab from the hot mantle wedge we detect an extremely low velocity anomaly in the easternmost portion of the phs slab which is probably attributable to serpentinization of mantle peridotite interplate earthquakes are almost absent along the phs pac interface overlain by the serpentinized mantle in the phs slab suggesting that ductile deformation takes place along the interface because of low viscosity of the serpentine

megathrust asperities and clusters of slab dehydration identified by spatiotemporal characterization of seismicity below the andean margin summary the space time size distributions of seismicity during the last four decades 1973 2010 are used to identify megathrust asperities the degree of plate coupling hence potential future large earthquakes and slab dehydration below the andean margin 15 48 s this seismicity displays globally a typical magnitude distribution with a b value of 1 04 0 024 for a magnitude of completeness of 4 5 and without considering aftershock sequences however this unitary b value of the entire catalogue masks large variations 0 6 1 7 in space and time interpreted as related to different tectonics contexts or different states of the seismic cycle the study has been performed for different regions upper continental plate upper and lower parts of the subducting oceanic plate with respect to a 3 d slab geometry model all along the andean margin low b values 0 6 0 8 are observed in areas that were ruptured by large mw 7 5 megathrust earthquakes a temporal analysis of these features show that b values where low before these earthquakes and remain low after the relaxation of the aftershock phase suggesting that they are related to time persistent asperities within the seismogenic zone of the megathrust interestingly a persistently low b value was observed from 2001 to the end 2009 within the southern limit of the maule mw 8 8 earthquake 2010 february 27 our results also identify other low b asperities below the chilean margin that have not ruptured during the last 40 yr and are places of potentially large megathrust earthquakes in the future in contrast large b values 1 1 1 7 are observed at seismicity clusters occurring inside the subducted slab beneath the central volcanic zone cvz of the andes and below the forearc near the subduction point of aseismic ridges and fracture zones clusters below the cvz are spatially related to relative large intermediate depth earthquakes like the mw 7 8 tarapaca 2005 earthquake and may reveal extensive thermally driven dehydration of the oceanic lithosphere that favours wet mantle melting and therefore magmagenesis feeding the volcanic chain

spatial and temporal patterns of earthquake distribution in soviet central asia application of pair analysis statistics we have examined the soviet regional earthquake catalog for central asia for the period 1962 to 1983 to characterize the spatial and temporal structure of the earthquake process in this highly active continental collision zone three areas of apparently homogeneous earthquake distribution were identified as characteristic of distinct seismic regimes 1 a segment of the range bounding gissar kokshal fault zone 2 the interior of the crystalline massif of the tien shan mountain range and 3 the tadjik depression fold thrust belt the pair analysis technique applied in our study compares the spatial and temporal distribution of pairs of earthquakes with those determined by uniformly distributed events in the same area and time interval we have found the earthquake distribution in this region to be dominated by significant nonrandomness in both space and time the spatial nonrandomness is manifested in two major ways spatial clustering appears to characterize the overall structure of the earthquake process in the region as it is observed regardless of the scale considered and even upon removal of foreshocks aftershocks and swarms from the catalog anomalies at larger distances could also be distinguished suggesting the existence of a regular spatial alternation of subareas of higher and lower levels of seismic activity the nonrandomness in temporal distribution of earthquakes is also characterized by clustering at short time intervals in contrast with the spatial patterns earthquakes appear to be randomly distributed in time after removal of foreshocks aftershocks and swarms

diatoms as an aid in identifying late holocene tsunami deposits diatoms bacillariophyta help identify the onshore deposits of tsunamis from earthquakes on the cascadia subduction zone along the pacific coast of oregon washington and british columbia and on faults high in the north american plate in the puget sound area of washington at the copalis river washington diatom analyses suggest that a tsunami deposit about 300 calendric years old 300 yr bp originated from sandy shoals of the lower estuary rather than nearby beaches or coastal dunes at cultus bay and west point washington well preserved benthic estuarine diatoms in sand sheets overlying tidal marsh peat indicate that the deposits came from intertidal or nearshore areas of puget sound on an abruptly uplifted mudflat at the landward end of hood canal at lynch cove washington tidal flat diatoms refute the possibility of a terrestrial source for the sand diatoms in 300 yr bp tsunami deposits on the niawiakum river washington confirm that the sand in these deposits had a marine source and help

inflow and outflow facies from the 1993 tsunami in southwest hokkaido abstract we describe in detail the sedimentary facies provenance and grain size distributions of the 1993 tsunami deposits in the lower reaches of usubetsu river only two lithofacies were recognized a gravel lobe facies glf and a sand sheet facies ssf the gravel lobe facies was deposited rapidly by a hydraulic jump in contrast the sand sheet facies which contains both current ripples and dunes was deposited by a traction current under subcritical flow conditions we identified four stratigraphic units unit 1 unit 2 unit 3 and unit 4 in ascending order we interpreted units 1 and 2 as having been deposited by the inflow and outflow events of the first run up and units 3 and 4 by the inflow and outflow events of the second run up respectively units 3 and 4 were more widely distributed and coarser grained than units 1 and 2 because the second run up was larger than the first run up furthermore unit 4 was more widely distributed than unit 3 because the outflow eroded the inflow deposits grain size distributions of the inflow and outflow deposits were clearly different unit 4 deposits had a peak at 2 5 phi p 1 population in contrast unit 3 deposits had a bimodal distribution with peaks at 0 5 1 5 phi p 2 population and at 2 5 phi p 1 population a comparison of the tsunami deposits with other sedimentary deposits near the study area showed that the tsunami deposits were mainly derived from the seabed at depths below 5 5 m in the offshore area where the marine sands are about 2 2 5 phi in size and thus are the source of the p 1 population the p 2 population is derived from coarse 0 5 2 phi fluvial materials eroded during the tsunami run up according to our calculations benthic foraminiferal tests were entrained from the comparatively deep seabed at depths up to 100 m during the tsunami run up and included in the tsunami deposits

inadvertent changes in magnitude reported in earthquake catalogs their evaluation through b value estimates a simple procedure is presented for analyzing magnitudes and seismicity rates reported in earthquake catalogs in order to discriminate between inadvertently introduced changes in magnitude and real seismicity changes we assume that the rate and the frequency magnitude relation of the independent background seismicity do not change with time observed differences in the frequency magnitude relation a and b values between data from two periods are modeled as due to a transformation of the magnitude scale the transformation equation is found by a least squares fitting process based on the seismicity data for earthquakes large enough to be reported completely and by comparing the linear relation of one period to the other for smaller events an additional factor accounting for increased decreased detection is allowed this fitting technique is tested on a data set from parkfield for which two types of magnitudes amplitude and duration were computed for each earthquake we found that the b value fitting technique yielded virtually the same result as a linear regression assuming the same errors in the two magnitudes the technique is also applied to interpret the nature of reporting rate changes in a local guerrero mexico and a regional italy earthquake catalog in guerrero a magnitude change in 1991 37 can be modeled about equally well by m new m old 0 5 or by m new 1 02 m old 0 38 but residuals with the latter transformation are smaller in italy a magnitude change in 1980 21 cannot be modeled satisfactorily by a simple magnitude shift but is well described by a compression of the magnitude scale given by m new 0 67 m old 1 03 the proposed b slope fitting method provides a means to interpret quantitatively and in some cases correct for artificial reporting rate changes in earthquake catalogs

some premonitory phenomena of the 1995 hyogo ken nanbu kobe earthquake seismicity b value and fractal dimension abstract this paper presents a detailed and complex analyses of the spatio temporal evolution of seismic activity between 1976 and 1998 in a broad area surrounding the epicentre of the 1995 hyogo ken nanbu kobe earthquake as well as in the source area various precursory changes in seismicity such as quiescence followed by increased seismic activity b value and fractal dimension correlation dimension d2 anomalies appear approximately two years before the occurrence of the major event an increased level of background seismicity is clearly observed after the occurrence of the kobe earthquake particularly in the northeast extension of the source area all these anomalous changes in seismic activity before and after the occurrence of the 1995 hyogo ken nanbu earthquake appear in a relatively large region surrounding the epicentre of the main shock as well as in the source area

radiocarbon evidence for extensive plate boundary rupture about 300 years ago at the cascadia subduction zone the cascadia subduction zone a region of converging tectonic plates along the pacific coast of north america has a geological history of very large plate boundary earthquakes1 2 but no such earthquakes have struck this region since euro american settlement about 150 years ago geophysical estimates of the moment magnitudes mw of the largest such earthquakes range from 8 ref 3 to 9 1 2 ref 4 radiocarbon dating of earthquake killed vegetation can set upper bounds on earthquake size by constraining the length of plate boundary that ruptured in individual earth quakes such dating has shown that the most recent rupture or series of ruptures extended at least 55 km along the washington coast within a period of a few decades about 300 years ago5 here we report 85 new 14c ages which suggest that this most recent rupture or series extended at least 900 km between southern british columbia and northern california by comparing the 14c ages with written records of the past 150 years we conclude that a single magnitude 9 earthquake or a series of lesser earthquakes ruptured most of the length of the cascadia subduction zone between the late 1600s and early 1800s and probably in the early 1700s

particle and aggregate mobility in till microscopic evidence of subglacial processes abstract microscopic studies of tills show that the deforming bed of glaciers is made up of minute rotational elements or wheels consisting of a nucleus alone or a nucleus with spiralling arms of fine grains these fine grains can move from one wheel to the next providing a process of high grain mobility within a deforming subglacial till recognition of this process explains a variety of glacial distribution patterns i e the mineralogical reflection of subjacent lithologies in the till matrix the side by side occurrence of far travelled erratics and locally derived matrix and the shape of geochemical anomalies

case for very low coupling stress on the cascadia ssubduction fault a fundamental problem in plate tectonics is the shear strength of major plate boundary faults this translates to the question whether the generally observed small earthquake stress drops of 3 10 mpa on major faults release most of the accumulated stress or only a small fraction of it there is strong evidence that the san andreas fault a major transform plate boundary is weak 20 mpa shear resistance it is not yet clear whether subduction thrust faults are also weak we present two types of evidence from the northern cascadia subduction zone that indicate very low coupling shear stress on that plate interface and hence very low strength of the subduction thrust fault comparable to that estimated for the san andreas fault first the well defined surface heat flow and heat generation allow negligible frictional heating on the plate interface the average shear stress on the fault must thus be very low over a time scale of a few million years second focal mechanism solutions for small crustal earthquakes in the southern vancouver island area indicate that the horizontal stress in the direction of plate convergence has a similar magnitude to the vertical stress this inferred stress state requires the present tectonic stress coupled across the subduction thrust fault to be very low one explanation for the weakness of the fault is the presence of near lithostatic pore fluid pressure in the region of the fault zone for which there is independent evidence the conclusion of a weak subduction thrust fault does not conflict with geodetic observations of contemporary surface deformation which indicate that the fault is currently locked accumulating strain energy toward a future great earthquake the surface deformation responds to the small 20 mpa temporal changes of the stress field associated with the subduction earthquake cycle this transient stress is superimposed on the larger background regional stress field in which the maximum compression is parallel to the margin the weakness of the cascadia subduction thrust fault and the unusual stress state of the forearc region have important implications for earthquake hazards for example a subduction earthquake may induce large strike slip earthquakes in the forearc that affect a large area

aftershocks of the coyote lake california earthquake of august 6 1979 a detailed study aftershock hypocenters and focal mechanism solutions for the coyote lake california earthquake reveal a geometrically complex fault structure consisting of multiple slip surfaces the faulting surface principally consists of two right stepping en echelon northwest trending partially overlapping nearly vertical sheets and is similar in geometry to a slip surface inferred for the 1966 parkfield california earthquake the overlap occurs near a prominent bend in the surface trace of the calaveras fault at san felipe lake slip during the main rupture as inferred from the distribution of early aftershocks appears to have been confined to a 14 km portion of the northeastern sheet between 4 and 10 km depth focal mechanisms and the hypocentral distribution of aftershocks suggest that the main rupture surface itself is geometrically complex with left stepping imbricate structure seismic shear displacement on the southwestern slip surface commenced some 5 hours after the mainshock aftershocks in this zone define a single vertical plane 8 km long between 3 and 7 km depth within the overlap zone between the two main slip surfaces the average strike of aftershock nodal planes is significantly rotated clockwise relative to the strike of the fault zone in close agreement with the stress perturbations predicted by crack interaction models aftershock activity in the overlap zone is not associated with a simple dislocation surface space and time clustering within the entire aftershock set suggest an alternation of seismic displacement between the component parts of the fault zone this alternation is consistent with local stress perturbations predicted by crack interaction models we conclude that the fault structure is geometrically complex and that the displacements that occur on its component surfaces during the aftershock process dynamically interact by generating perturbations in the local stress field which in turn control the displacements table 5 is available with entire article on microfiche order from american geophysical union 2000 florida avenue n w washington d c 20009 document j82 006 1 00 payment must accompany order

objective estimation of spatially variable parameters of epidemic type aftershock sequence model application to california the etas model is widely employed to model the spatio temporal distribution of earthquakes generally using spatially invariant parameters we propose an efficient method for the estimation of spatially varying parameters using the expectation maximization em algorithm and spatial voronoi tessellation ensembles we use the bayesian information criterion bic to rank inverted models given their likelihood and complexity and select the best models to finally compute an ensemble model at any location using a synthetic catalog we also check that the proposed method correctly inverts the known parameters we apply the proposed method to earthquakes included in the anss catalog that occurred within the time period 1981 2015 in a spatial polygon around california the results indicate significant spatial variation of the etas parameters we find that the efficiency of earthquakes to trigger future ones quantified by the branching ratio positively correlates with surface heat flow in contrast the rate of earthquakes triggered by far field tectonic loading or background seismicity rate shows no such correlation suggesting the relevance of triggering possibly through fluid induced activation furthermore the branching ratio and background seismicity rate are found to be uncorrelated with hypocentral depths indicating that the seismic coupling remains invariant of hypocentral depths in the study region additionally triggering seems to be mostly dominated by small earthquakes consequently the static stress change studies should not only focus on the coulomb stress changes caused by specific moderate to large earthquakes but also account for the secondary static stress changes caused by smaller earthquakes

space clustering properties of the betic alboran earthquakes in the period 1962 1989 abstract in order to obtain a way to quantify the spatial dependence between earthquakes in the betic alboran south spain region the single link clustering slc method has been applied to a catalogue of earthquakes with magnitude greater or equal to 4 0 that occurred in this region from 1962 to 1989 the significant distances of links obtained which show alignments whose direction is similar to the direction of some surface faults are 15 30 and 40 km and the limit of inconsistency appears at distances greater than or equal to 70 km most of the historical destructive earthquakes have taken place within the clusters obtained and the majority within the dc 30 km cluster this implies that the most relevant seismic activity occurs in some source areas where there is a strong nesting of activity

mantle wedge structure in the miyagi prefecture forearc region central northeastern japan arc and its relation to corner flow pattern and interplate coupling 1 we applied three dimensional 3 d seismic tomography to both active source data and local earthquake data recorded at onshore and offshore seismic stations in the miyagi prefecture forearc region of central northeastern japan to obtain a detailed seismic velocity model of the mantle wedge the velocity model showed a clear boundary that divides the mantle wedge into an oceanward high velocity toe and a landward low velocity region that extends trenchward from beneath currently active arc volcanoes the top of the seaward dipping boundary is about 20 30 km landward from the coastline and its bottom is approximately beneath the coastline we consider that this velocity boundary represents the trenchward limit of corner flow and the high velocity toe represents a stagnant mantle wedge the high spatial resolution of the tomographic image we produced allowed us to identify a spatial correlation between velocity variations in the mantle wedge immediately above the plate boundary and the spatial extent of the rupture areas of the large interplate earthquakes that occur repeatedly in the region our results suggest that variations of the volume fraction of serpentinization in the mantle wedge cause the irregularity of rupture patterns of megathrust earthquake sequences we also identified a swarm of active microseismicity in the mantle wedge well above the seismogenic plate boundary this activity may be related to dehydration of a detached seamount

active stress field and fault kinematics of the greater caucasus abstract this work contributes to depict the current seismicity fault kinematics and state of stress in the greater caucasus territories of georgia azerbaijan and russia we merged and homogenized data from different earthquake catalogues relocated 1000 seismic events created a database of 366 selected focal mechanism solutions 239 of which are new and performed a formal stress inversion preferential alignments of crustal earthquake foci indicate that most seismic areas are located along the southern margin of the belt and in the north eastern sector this is consistent with the presence of dominant active wnw ese faults parallel to the mountain range in the entire greater caucasus a dominant nne ssw oriented greatest principal stress 1 controls the over all occurrence of earthquakes of minor and major magnitude main earthquakes are characterized by a vertical least principal stress 3 corresponding to reverse kinematics reverse slip is more common along the southwestern and north eastern foothills of the greater caucasus although in these areas there are also scattered strike slip events this suggests the presence of local stress fields with horizontal 1 and 3 in the central southern part of the mountain belt in correspondence of the local collision between the lesser and the greater caucasus 1 rotates to nnw sse the strike slip events instead dominate along the southern flank of the central eastern mountain range this is interpreted as the effect of the collision that promotes eastward escape of the tectonic blocks located to the east

subduction of a wedge shaped philippine sea plate beneath kanto central japan estimated from converted waves and small repeating earthquakes 1 we estimate the configuration of the philippine sea plate phs subducted beneath kanto to understand the seismotectonics in the tokyo metropolitan area in addition to small repeating earthquakes that delineate both the upper and lower boundaries of the phs previously unrecognized converted waves at its upper boundary from lower boundary repeater sources provide a new constraint on the position of the boundary the results from 794 s to p and 212 p to s converted waves together with 200 repeater locations show that 1 the phs has a wedge like shape decreasing in thickness from 50 km beneath tokyo to zero at its northeastern limit and 2 the phs is relatively flat in the eastern part of kanto and bends upward near its northeastern limit from the fact that the phs in the izu bonin forearc also has a similar wedge like shape we infer that its shape was formed before its subduction on the other hand the relatively flat phs at its northeastern limit and its upward bend are probably a consequence of ongoing deformation from an interaction with the underlying thick pacific plate pac the history of the phs as a forearc before subduction and its contact with the pac after subduction suggests that the temperature of this part of the phs is colder than other regions which probably causes anomalously deep seismicity on the phs in kanto colder phs temperatures imply that the source regions of large earthquakes may extend deeper than expected an important fact when assessing the locations of disastrous earthquakes for tokyo

stratigraphic and microfossil evidence for late holocene tsunamis at swantown marsh whidbey island washington abstract four muddy sand sheets occur within a tidal marsh peat at swantown on the west coast of whidbey island washington the two largest sand sheets pinch out about 100 m inland and became thinner and finer grained landward all four sand sheets contain marine microfossils and have internal stratification they record repeated inundation of the marsh over a short time period by distinct pulses of sediment laden ocean water consistent with deposition by a tsunami wave train the layers have been radiocarbon dated to 1160 1350 1400 1700 1810 2060 and 1830 2120 cal yr b p the overlap in age between the two youngest layers and inferred great earthquake events at the cascadia plate boundary some 250 km to the west suggests they were emplaced by tsunamis from this source area the two older layers do not correlate with plate boundary events they may be products of tsunamis caused by earthquakes on local faults in the strait of juan de fuca or by submarine landslides in this area

a slab fragment wedged under tokyo and its tectonic and seismic implications a three dimensional evaluation of earthquake hypocentres beneath the kanto basin in japan reveals the presence of a distinct 25 km thick and 100 km wide body its fast seismic velocity and the presence of a double seismic zone suggest that it is a fragment of the pacific slab rather than an extension of the philippine sea slab this implies that the penetration of the philippine sea slab is much shallower beneath the kanto basin than was previously thought the philippine sea and pacific plate slabs both subduct beneath tokyo and so their configuration and seismic potential have been subject to intensive study1 2 3 4 5 6 7 8 previous work suggests that the philippine sea slab extends up to 100 km northwest of tokyo and subducts to a depth of 90 km beneath the kanto basin where it is folded against the underlying pacific slab here we evaluate seismic data in three dimensions and delineate a distinct 25 km thick and 100 km wide body beneath the kanto basin that has hitherto been considered to be part of the philippine sea slab we find that several of its characteristics such as its high seismic velocity and the presence of a double seismic zone point instead to an affinity with the pacific slab implying that the philippine sea slab penetrates to depths of only 35 40 km beneath the kanto basin we propose that the body is a fragment of pacific lithosphere that formed by the collision of two intersecting seamount chains with the japan trench 2 3 million years ago we suggest that such slab fragments may not be uncommon especially where seamount subduction deforms trenches and near triple junctions

geomorphological effects of tsunami run up and backwash abstract geomorphological processes associated with tsunami run up and backwash are highly complex to date most studies have considered the sedimentology of palaeo tsunamis while almost no detailed studies have been undertaken in areas known to have been affected by tsunamis in recent times research on contemporary tsunamis show that coastal landscapes may be greatly altered not only by direct tsunami run up orthogonal to the shoreline but also by episodes of vigorous backwash and by water flow sub parallel to the coastline the combined effect of these processes produces hitherto unrecognised coastal landforms that are dominated by the effects of high magnitude erosion and deposition deposition of large boulders is also associated with severe tsunamis while in adjacent areas run up processes often result in the deposition of continuous and discontinuous sediment sheets in many instances the upper limit of sediment deposition lies well below the upper limit of wave run up which is frequently marked by a well defined zone of stripped vegetation and soil the processes described have considerable implications for models of coastal evolution it may even be the case that the evolution of particular stretches of coastline are dominated by such high magnitude low frequency events

what controls interplate coupling evidence for abrupt change in coupling across a border between two overlying plates in the ne japan subduction zone article i nfo interplate coupling in the southernmost extent of the northeast japan subduction zone east off kanto japan is investigated in detail using a large data set of small repeating earthquakes in this region of the trench trench trench type triple junction the subducting pacific plate is in contact with two different overlying plates the north american and philippine sea plates the border between the two overlying plates extending northwestward from the triple junction is identified along the upper surface of the pacific plate based on the slip vectors of many interplate events including small repeating earthquakes the interplate coupling coefficient estimated from the cumulative slip of small repeating earthquakes for the period from 1993 to 2007 reveals a distinct change from south ca 0 3 to north ca 0 7 across the border this border also corresponds to the southern limit of the source areas of mn7 interplate earthquakes in the last 80 years along the japan trench again indicating the stronger coupling to the north we also investigate hypocenter distribution and seismic velocity structure along the plate boundary from a large number of travel time data obtained from the nationwide seismograph network the results reveal a distinct low velocity zone just above the pacific plate and low seismicity along the plate boundary in the region overlain by the philippine sea plate whereas there are no low velocity zones and the seismicity is high in the region overlain by the north american plate these observations imply that the geological difference of overlying plate controls large scale coupling at the plate interface

structural characteristics off miyagi forearc region the japan trench seismogenic zone deduced from a wide angle reflection and refraction study abstract the japan trench subduction zone located east of ne japan has regional variation in seismicity many large earthquakes occurred in the northern part of japan trench but few in the southern part off miyagi region is in the middle of the japan trench where the large earthquakes m 7 with thrust mechanisms have occurred at an interval of about 40 years in two parts inner trench slope and near land a seismic experiment using 36 ocean bottom seismographs obs and a 12 000 cu in airgun array was conducted to determine a detailed 2d velocity structure in the forearc region off miyagi the depth to the moho is 21 km at 115 km from the trench axis and becomes progressively deeper landward the p wave velocity of the mantle wedge is 7 9 8 1 km s which is typical velocity for uppermost mantle without large serpentinization the dip angle of oceanic crust is increased from 5 6 near the trench axis to 23 150 km landward from the trench axis the p wave velocity of the oceanic uppermost mantle is as small as 7 7 km s this low velocity oceanic mantle seems to be caused by not a lateral anisotropy but some subduction process by comparison with the seismicity off miyagi the subduction zone can be divided into four parts 1 seaward of the trench axis the seismicity is low and normal fault type earthquakes occur associated with the destruction of oceanic lithosphere 2 beneath the deformed zone landward of the trench axis the plate boundary is characterized as a stable sliding fault plain in case of earthquakes this zone may be tsunamigenic 3 below forearc crust where p wave velocity is almost 6 km s and larger this zone is the seismogenic zone below inner trench slope which is a plate boundary between the forearc and oceanic crusts 4 below mantle wedge the rupture zones of thrust large earthquakes near land e g 1978 off miyagi earthquake are located beneath the mantle wedge the depth of the rupture zones is 30 50 km below sea level from the comparison the rupture zones of large earthquakes off miyagi are limited in two parts plate boundary between the forearc and oceanic crusts and below mantle wedge this limitation is a rare case for subduction zone although the seismogenic process beneath the mantle wedge is not fully clarified our observation suggests the two possibilities earthquake generation at the plate boundary overridden by the mantle wedge without serpentinization or that in the subducting slab

onshore tsunami deposits caused by the 1993 southwest hokkaido and 1983 japan sea earthquakes onshore tsunami deposits resulting from the 1993 southwest hokkaido and 1983 japan sea earthquakes were described to evaluate the feasibility of tsunami deposits for inferring paleoseismic events along submarine faults tsunami deposits were divided into three types based on their composition and aerial distribution a deposits consisting only of floating materials b locally distributed siliclastic deposits and c widespread siliclastic deposits the most widely distributed tsunami deposits consist of the first two types type c deposits are mostly limited to areas where the higher tsunami runup was observed the scale of tsunami represented by vertical tsunami runup is an important factor controlling the volume of tsunami deposits the thickest deposits about 10 cm occur behind coastal dunes to produce thick siliclastic tsunami deposits a suitable source area such as sand bar or dune must be available in addition to sufficient vertical tsunami runup estimation of the amounts of erosion and deposition indicates that tsunami deposits were derived from both onshore and shoreface regions the composition and grain size of the tsunami deposits strongly reflect the nature of the sedimentary materials of their source area sedimentary structures of the tsunami deposits suggest both low and high flow regimes consequently it seems very difficult to identify tsunami deposits based only on grain size distribution or sedimentary structure of a single site in ancient successions

correlating seismicity parameters and subsidence in the tokai region central japan 1 we demonstrate that transients in subsidence rate in the tokai area of central japan correlate with changes in the rate and size distribution of earthquakes three largely independent seismic catalogs cover this region national research institute for earth science and disaster prevention nied japanese meteorological agency jma and japan university network catalog junec comparing seismicity patterns between these three data sets offers a unique opportunity to verify seismicity anomalies based on independent sources a study of data quality magnitude reporting consistency and consistency across the three data sources reveals that the nied data for the period 1981 2002 are best suited for seismicity analysis we spatially and temporally map seismicity rates finding that a significant decrease in the earthquake rate of m 2 0 events coincides with an accelerated subsidence period however this anomaly disappears when including smaller magnitudes in the analysis this relative quiescence of larger events can readily be explained when interpreting the transient in seismicity in the framework of a change in the earthquake size distribution or b value the background b value of about 0 8 increased in the period 1987 5 1989 5 to a value of b 1 2 a highly significant change that can be confirmed in all three data sets the correlation between subsidence and b value change is significant stress tensor inversions of focal mechanism data for the anomalous period show an increase in thrusting type earthquakes while a unique interpretation of the relationship between subsidence and b value is not possible we propose that an increase in the locking stress a slow stick event is consistent with all observations

a decade of passive seismic monitoring experiments with local networks in four italian regions we report on four seismic monitoring experiments that in the past ten years we carried out with dense local networks in seismically active italian areas where for at least a year tens of three component seismic stations were set up to record microseismicity the areas observed are alpago cansiglio located in the venetian alps citta di castello in the northern apennines marsica in the central apennines and val d agri located in the southern apennines we produced homogeneous catalogues regarding earthquake locations and local magnitudes to investigate seismicity patterns during an inter seismic period the four regions are characterised by different kinematics strain rates and historical recent seismicity we investigate earthquake distribution in space time and size obtaining reference seismic rates and parameters of the gutenberg and richter law we declustered the catalogues to look for coherent signs in the background seismic activity despite a difference in the catalogues magnitudes of completeness due both to the diverse detection threshold of the local networks and different seismic release we detect and observe two common main behaviours a the alpago cansiglio and marsica regions are characterised by a relatively lower rate of seismic release associated to the episodic occurrence of seismic sequences with the largest event being 3 ml 4 in these areas the seismicity is not localised around the main faults b the citta di castello and val d agri regions have a relatively high rate of seismicity release almost continuously with time and the increase in earthquake production is not clearly related to seismic sequences in these areas the seismicity nucleates around defined fault systems and is usually lower than ml 3 we suggest that the presence of over pressured fluids in the citta di castello and val d agri uppermost crustal volume may favour and mould the higher rate of microseismic release

bedform and stratification characteristics of some modern small scale washover sand bodies washover sand bodies commonly develop along microtidal coastlines in beach barrier island or spit settings wave runup usually in conjunction with an abnormally high water level may overtop the most landward berm of the beach and the foredune crest if one exists to produce overwash and subsequent runoff across the more landward subaerial surface two main elements of the resulting deposit are the washover fan and runoff channel newly formed small scale washover deposits were examined along the outer banks north carolina near pt mugu california and at presque isle lake erie pennsylvania the fans were formed in response to unidirectional landward transport and the runoff channels in response to unidirectional flow usually in a landward direction but sometimes in shore parallel then seaward direction where overwash carried across the fan surface and entered a pond or lagoon a small scale delta microdelta developed in this case the washover fan consisted of two subfacies the wetted but subaerial part of the fan and the subaqueous washover delta flow associated with the development of the fan and runoff channel produced distinctive sets of bedforms and internal stratification high velocity discontinuous surges moving across the fan surface resulted in the development of a plane bed and subhorizontal to low angle landward dipping planar stratification which comprised the major part of the fan similarly rhomboid forms were produced by high velocity sheet flow across the fan surface where flow carried into a standing body of water delta type foreset strata developed for this case the lateral structural sequence was subhorizontal planar stratification merging landward into landward dipping delta tabular foreset strata in the runoff setting where flow became channelized and continuous both upper flow and lower flow regime currents were typical upper flow regime bedforms included antidunes standing waves and plane beds the most commonly observed lower flow regime bedforms included microdelta like bars low amplitude bars linguoid ripples and sinuous crested current ripple trains the sets of sedimentary structures comprising modern washover sand bodies provide criteria for the identification of similar deposits in ancient sediments and for more specific interpretation of the environment

three dimensional thermal model of the costa rica nicaragua subduction zone the thermal structure of a subduction zone controls many key processes including subducting plate metamorphism and dehydration the megathrust earthquake seismogenic zone and volcanic arc magmatism here we present the first three dimensional 3d steady state kinematic dynamic thermal model for the costa rica nicaragua subduction zone the model consists of the subducting cocos plate the overriding caribbean plate and a viscous mantle wedge in which flow is driven by interactions with the downgoing slab the cocos plate geometry includes along strike variations in slab dip which induce along strike flow in the mantle wedge along strike flow occurs primarily below costa rica with a maximum magnitude of 4 cm year 40 of the convergence rate for a mantle with a dislocation creep rheology an isoviscous mantle has lower velocities along margin flow causes temperatures variations of up to 80 c in the subducting slab and mantle wedge at the volcanic arc and backarc the 3d effects do not strongly alter the shallow 35 km thermal structure of the subduction zone the models predict that the megathrust seismogenic zone width decreases from 100 km below costa rica to just a few kilometers below nicaragua the narrow width in the north is due to hydrothermal cooling of the oceanic plate these results are in good agreement with previous 2d models and with the rupture area of recent earthquakes in the models along strike mantle flow is induced only by variations in slab dip with flow directed toward the south where the dip angle is smallest in contrast geochemical and seismic observations suggest a northward flow of 6 19 cm year we do not observe this in our models suggesting that northward flow may be driven by additional factors such as slab rollback or proximity to a slab edge slab window such high velocities may significantly affect the thermal structure especially at the southern end of the subduction zone in this area 3d models that include slab rollback and a slab edge are needed to investigate the mantle structure and dynamics

a new seismic hazard model for new zealand we present a new probabilistic seismic hazard analysis psha for new zealand an important feature of the analysis is the application of a new method for the treatment of historical distributed seismicity data in psha the psha uses the seismicity recorded across and beneath the country to define a three dimensional grid of a values i e parameter a of a gutenberg richter distribution log n yr a bm in which n yr is the number of earthquakes per year recorded inside each grid cell equal to or greater than magnitude m parameter b and the limiting maximum cutoff magnitude of the gutenberg richter distribution are defined from the surrounding region 14 crustal and 23 subcrustal seismotectonic zones are defined for the country and then smoothed across the boundaries of the zones the methodology therefore combines the modern method of defining continuous distributions of seismicity parameters frankel 1995 frankel et al 1996 with the traditional method of defining large area sources and the associated seismicity parameters e g algermissen et al 1990 the methodology provides a means of including deep subduction zone seismicity in a psha preserves the finer scale spatial variations of seismicity rates across a region avoids the undesirable edge effects produced in the traditional method when adjacent area sources enclose areas of significantly different seismicity rates and also enables parameters most reliably defined at a regional scale parameter b and maximum cutoff magnitude of a gutenberg richter distribution and slip type to be incorporated into the psha the psha combines the modeled seismicity data with geological data describing the location and earthquake recurrence behavior of 305 active faults and new attenuation relationships for peak ground acceleration and spectral acceleration developed specifically for new zealand different attenuation expressions are used for crustal and subduction zone earthquakes the resulting psh maps for a 150 year return period show the highest hazard to occur in the center and southwest of the country in the areas of highest historical crustal and deep subduction zone seismicity in contrast the longer return period maps 475 and 1000 year return period show the highest hazard to occur from the southwest to northeast ends of the country along the faults that accommodate the majority of the motion between the pacific and australian plates the maps are currently being used to revise new zealand s building code which has previously been based on pshas that did not explicitly include individual faults as earthquake sources manuscript received 10 april 2001

strength of stick slip and creeping subduction megathrusts from heat flow observations subduction faults called megathrusts can generate large and hazardous earthquakes the mode of slip and seismicity of a megathrust is controlled by the structural complexity of the fault zone however the relative strength of a megathrust based on the mode of slip is far from clear the fault strength affects surface heat flow by frictional heating during slip we model heat flow data for a number of subduction zones to determine the fault strength we find that smooth megathrusts that produce great earthquakes tend to be weaker and therefore dissipate less heat than geometrically rough megathrusts that slip mainly by creeping

common dependence on stress for the two fundamental laws of statistical seismology the gutenberg richter relation and the omori utsu law both power laws are two of the long standing relationships of statistical seismology here aftershock sequences are described according to the faulting style of their main shocks showing that the time delay before the onset of the power law aftershock decay rate is on average shorter for thrust main shocks than for normal fault earthquakes these similar dependences on the faulting style indicate that both the fundamental power laws are governed by the state of stress

second order moment of central california seismicity 1969 1982 the second order moment cross correlation function of earthquakes in the u s geological survey central california catalog between 1969 and 1982 was calculated with respect to a magnitude threshold m 4 0 over interevent distances up to 80 km and interevent times up to 320 days the statistical procedure results in a representation of the spatial temporal structure of the catalog associated with m 4 0 earthquakes and is capable of revealing patterns too weak to be detected in the space time distribution of seismicity for individual earthquake sequences a method is introduced for identifying aftershocks based on a physical two parameter model of the earthquake interaction process the results show that the aftershock process dominates the second order moment and may even obscure the statistical expression of a precursory process a concentration of foreshocks within 15 km and 3 days of m 4 0 main shocks exhibits an apparent migration toward the main shock loci with velocity 2 6 5 3 km d this concentration may be related to an observed tendency for m 4 0 events to cluster auto correlate over this interevent range with the identified aftershocks removed the residual catalog is poissonion in space and time when two m 4 0 earthquakes occur within 80 km and 40 days of each other aftershock productivity appears to be relatively enhanced in the earlier sequence this suggests that aftershock populations are not solely dependent on their main shocks and that unusually productive aftershock sequences may be predictors of future moderate earthquakes

seismic quiescence precursor to the 1983 kaoiki ms 6 6 hawaii earthquake a remarkable seismicity rate decrease of 65 per cent occurred in most of the aftershock volume 2 4 yr before the 1983 kaoiki ms 6 6 main shock if the background seismicity rate 1978 to mid 1981 is extrapolated to the time of the main shock more than 300 earthquakes of ml 1 8 are missing because of the quiescence subvolumes measuring 5 5 km on a side and located within the aftershock zone showed seismicity rate decreases ranging from 0 to 90 per cent the volume of no change had dimensions of approximately 10 3 km it contained the main shock hypocenter and was located near the center of the 10 km radius aftershock area the seismic quiescence in different subvolumes of the aftershock volume surrounding this central nonquiet zone varied somewhat in starting time amount of rate decrease and statistical significance according to the standard deviate z test the reported rate decrease is the most pronounced example of precursory quiescence defined to date the seismicity pattern before the 1983 kaoiki shock conformed to the quiescence hypothesis proposed on the basis of the precursors to the 1975 hawaii main shock no quiescence was observed within the immediate surrounding of the hypocenter while strong rate decreases occurred in most of the rest of the main shock source volume it is thus hypothesized that major asperities which contain high stress levels and from which main ruptures can emanate may be recognizable as volumes of constant seismicity rate within surrounding volumes of quiescence provided that decreases of seismicity rate as a function of time can be defined quantitatively

thermal structure of the kanto region japan using a 3 d numerical thermal model we investigate the thermal structure of the kanto region of japan where two oceanic plates subduct in a typical subduction setting with one subducting slab the motion of the slab drives solid state mantle flow in the overlying mantle wedge bringing in hot mantle from the back arc towards the forearc beneath kanto however the presence of the subducting philippine sea plate between the overlying north american plate and the subducting pacific plate prevents the typical mantle wedge flow pattern resulting in a cooler condition further frictional heating and the along margin variation in the maximum depth of slab mantle decoupling along the pacific slab surface affect the thermal structure significantly the model provides quantitative estimates of spatial variations in the temperature condition that are consistent with the observed surface heat flow patterns and distributions of interplate seismicity and arc volcanoes in kanto

geometry and relative motion of the philippine sea plate and pacific plate beneath the kanto tokai district japan the recent acquisition of high density and improved seismic data by the kanto tokai k t observational network of the national research institute of earth science and disaster prevention requires the revision of conventional plate configuration models for the k t district we propose a new interpretation of the configuration and relative motion of the philippine sea phs plate pacific pac plate and eurasian eur plate on the basis of the distribution of hypocenters velocity structure and focal mechanisms in the k t district the new model for the configuration of the phs and pac slabs clearly delineates the phs slab subducting northwestward beneath the k t district from the sagami and suruga troughs and the pac slab subducting westward beneath the phs slab from the japan trench the thickness of the descending phs slab is estimated to be 30 5 km the proposed configuration of the phs slab is significantly different from those obtained by other researchers including our own previous study the new model is fully consistent with global plate motion data the new model leads to an interpretation of the tectonic processes taking place beneath the k t district for the sagami trough wing of the phs slab the northeastern portion of the slab is dipping northwestward in direct contact with the upper surface of the pac slab the southwestern portion carrying the izu peninsula has been bent upward because of its buoyancy producing the observed contorted shape for the suruga trough wing the dip direction of the phs slab gradually changes from northwestward at the eastern portion to northward at the western portion the new model also indicates the presence of offset within the phs slab between the sagami and suruga trough wings this area is characterized by a low level of seismic activity the absence of a high velocity structure and the presence of quaternary volcanoes it seems reasonable to speculate that the distribution of quaternary volcanic edifice construction is controlled by the location of the offset within the phs slab in view of this new model the tectonic processes of large earthquakes which occurred recently in the studied area are also reexamined

sedimentology of late glacial clays in lacustrine basins central ireland abstract distal bottom sediments of a large ice marginal interdomal lake that covered most of the central part midlands of ireland during the last deglaciation were poorly known recent work centred on two raised bog covered basins confirmed relatively thick distal glaciolacustrine sediments undisturbed samples taken in 100 mm cores enabled thin sectioning and micromorphological analysis to be carried out the upper part of the fine sediment mostly clay which is apparently homogenous at the macroscopic level is shown to have been bedded or laminated and subsequently homogenised by load casting with depth bedding or lamination becomes more common until the whole deposit is very finely and rhythmically laminated turbidites a consequence of density underflows together with coarse sediment rain out and dropstones confirm a likely ice marginal environment the finer sediment towards the top of the sequence reflects increasing distality synsedimentary folding and faulting load casting and water escape structures point to rapidly accumulating sediment in an unstable sedimentary environment the lack of bioturbation is not surprising and apparent minor traces towards the base of the rhythmites seems most likely to relate to synsedimentary faulting

high resolution analysis of a tsunami deposit case study from the 1755 lisbon tsunami in southwestern spain abstract a multi proxy approach using trench sediment analysis on the southwestern coast of spain los lances bay andalusia provides a high resolution record of the 1755 lisbon tsunami and offers new means to interpret tsunami deposits a combination of sedimentological grain size sorting ams micromorphological x ray tomography and radiography thin sections and geochemical analyses x ray microfluorescence icp aes combines to provide a comprehensive reconstruction of the different phases of tsunami deposition and flow dynamics vertical variations in sediment texture fabric and composition of the 1755 tsunami deposits in los lances are then discussed with reference to existing literature on diagnostic criteria commonly used for identifying and interpreting tsunami deposits finally possible leads for future investigation are proposed

the micromorphology of unconsolidated sediments this paper aims to describe the use of thin sections micromorphology in the sedimentology of unconsolidated sediments it provides examples of the use of thin sections in a variety of sedimentary environments from fault gouge through caves and volcanics to aeolian fluviatile marine periglacial and glacial it demonstrates that in the latter three fields the use of micromorphology is relatively widespread and that in glacial sedimentology it has revolutionised our way of thinking about subglacial sediments although micromorphology has been mainly descriptive so far methods of quantification observations are demonstrated some of the important aspects of micromorphology are its use for microstratigraphy and the possibility of relating observations to documented processes thereby allowing a more robust sedimentological interpretation of modern and ancient sediments

the 17 july 1998 tsunami papua new guinea evidence and initial interpretation abstract a m w 7 1 earthquake that occurred at 08 49 hours 17 july 1998 utc 6 49 pm local time off the north coast of papua new guinea generated a locally destructive tsunami more than 2189 people died and virtually no structures were left standing along 19 km of coast a fast moving wall of sand laden water left fishing nets and other detritus in trees up to 17 5 m above sea level concrete was stripped to the reinforcing metal and some trees were ripped out and carried more than a kilometre back into sissano lagoon the site is in an active sedimentary basin the aitape trough with 4500 m thickness of neogene sediments between the bewani fault zone and the wewak trench there is some debate about the form of tsunami propagation for this event however we present evidence for subsidence of 30 40 cm on the landward side of the spit fronting sissano lagoon furthermore the area may have subsided up to three times this century in situ stumps of drowned trees in the lagoon record at least one earlier event the low angle harvard university cmt solution m w 7 1 depth 6 0 km 10 landward dip on fault with northward displacement is consistent with the tectonic setting and pattern of aftershocks elastic modelling of the energy release suggests 2 m horizontal displacement over 600 km 2 40 cm subsidence landward and 60 cm uplift seaward convergent flow of the displaced water into the area of subsidence focused wave energy on the coast and generated the locally high wave the record of tsunami at sissano lagoon indicates that the spit fronting the lagoon is unsafe for habitation there is potential for similar coseismic coastal subsidence to focus tsunami in other areas with similar tectonic settings

time and size of a giant earthquake in cascadia inferred from japanese tsunami records of january 1700 geological evidence shows that great earthquakes have occurred in the recent prehistoric past in the cascadia subduction zone off the pacific coast of north america the most recent event or series of events is dated at about 300 years ago1 4 but the precise date and magnitude have not been determined geological investigations have not been able to distinguish a single giant earthquake from a series of great earthquakes occurring over a timespan of a decade or two4 although this information is important for the assessment of future hazard5 we have found several tsunami records in japan from ad 1700 with no indication of a local cause historical earthquake records and palaeoseismic evidence indicate the absence of a large earthquake in 1700 in south america alaska or kamchatka leaving cascadia as the most likely source of this tsunami the estimated time of the earthquake is the evening about 21 00 local time of 26 january 1700 the magnitude is estimated as 9 from the tsunami heights in which case the earthquake ruptured the entire length of the cascadia subduction zone2 these estimates are consistent with native american legends that an earthquake occurred on a winter night6

seismological evidence for the existence of serpentinized wedge mantle serpentinized peridotite is detected seismologically by mapping poisson s ratio beneath the kanto district central japan because serpentinite has a higher poisson s ratio than other rocks in the crust and upper mantle we determine three dimensional p and s wave velocity models using travel time tomography and then construct a three dimensional map of poisson s ratio the models show low velocity anomalies near the upper boundary of the slab in a depth range from 20 to 70 km in the low velocity region there is an area with poisson s ratio greater than 0 3 at depths of 20 45 km which we attribute to serpentinized peridotite the rest of the low velocity region is interpreted as descending oceanic crust with a poisson s ratio of about 0 25 the high poisson s ratio area is associated with low seismicity and weak interplate coupling which is consistent with the ductility of serpentinite

tsunami deposits in holocene bay mud in southern kanto region pacific coast of central japan abstract tsunami have probably deposited sand sheets that are intercalated in holocene bay floor mud exposed on the boso and miura peninsulas facing the convergent boundary of the pacific philippine sea and eurasian plates we have identified seven sand sheets at four drowned valleys and correlated these by 137 radiocarbon dates of shells and wood the sheets consist of poorly sorted muddy sand and well sorted sand layers in fining upward sequences containing abundant transported shells rip up clasts and wood fragments the sheets erosionally overlie bioturbated bay floor mud that contains molluscan shells in life position most of the sheets are less than 20 cm thick and rarely more than 50 cm thick some molluscan shells are older in these layers than in underlying mud both landward and seaward paleocurrents are shown in a few cases by imbrication of shells and by low angle wedge shaped lamination at least five of the sand sheets contain molluscan fossils derived from rocky coasts or shore platforms although they intercalated in mud deposited within bays at depths of 10 15 m two other sand sheets are dominated by open seashore ostracode assemblages although they were deposited in the brackish inner bay and muddy central bay five of the layers may correlate with emergences recorded by nearby holocene marine terraces these correlations suggest that great earthquakes triggered the inferred tsunami the tsunami occurred at intervals of 300 2000 years beginning about 10 000 years ago

comprehensive probabilistic seismic hazard analysis of the andaman nicobar regions an updated catalog of earthquakes has been prepared for the andaman nicobar and adjoining regions the catalog was homogenized to a unified magnitude scale and declustering of the catalog was performed to remove aftershocks and foreshocks eleven regional source zones were identified in the study area to account for local variability in seismicity characteristics the seismicity parameters were estimated for each of these source zones and the seismic hazard evaluation of the andaman nicobar region has been performed using different source models and attenuation relations probabilistic seismic hazard analysis has been performed with currently available data and their best possible scientific interpretation using an appropriate instrument such as the logic tree to explicitly account for epistemic uncertainty by considering alternative models source models maximum magnitude and attenuation relationships the hazard maps for different periods have been produced for horizontal ground motion on the bedrock level

evaluation of precursory seismic quiescence in sixteen subduction zones using single link cluster analysis this paper describes a new method single link cluster analysis slc to evaluate percursory quiescence for shallow earthquakes in sixteen subduction zones using data from the isc catalog to define quiescent regions we divided the catalog into time intervals with a durationt overlapping byt 2 we considered all earthquakes having magnitudes larger than some magnitudemmin lying within a specified distance of a great circle which is approximately coincident with the trench near a subduction zone within each time interval we connected or linked all earthquakes lying within some cutoff distanced of one another we then projected all these links onto the great circle and defined a region to be quiescent if it was not covered by the projection of any links for this study t was two years mmin wasmb 4 9 and we variedd from 100 to 400 km we defined an earthquake as following quiescence if it occurred within two years following and within 75 km of a quiescent zone as defined above the primary conclusion of this study was that earthquakes with surface wave magnitudes 7 2 and greater were about 5 15 more likely to follow quiescence than were the smaller background earthquakes withmb 4 9 a chi squared analysis shows that this result is significant at the 99 level in contrast earthquakes with surface wave magnitude of 6 7 to 7 1 were no more likely to follow quiescence than were background earthquakes of sixteen individual regions central america japan and peru chile were the only regions where large earthquakes were more likely to occur following quiescence than were background earthquakes for a cutoff link length of 300 km only in central america was the difference between large earthquakes and background earthquakes significant at the 95 level of significance for a cutoff link length of 250 km the significance level exceeded 95 only in japan the slc method is an objective quantitative method for evaluating large data catalogs or for monitoring quiescence in regions where quiescence is conjectured to precede large earthquakes

a constraint on the shear stress at the pacific australian plate boundary from heat flow and seismicity at the kermadec forearc new heat flow measurements and relocated hypocenters that constrain the subduction geometry of the pacific plate at the kermadec trench yield an estimate of shear stress on the thrust fault with the exception of a few relatively high values 60 mw m 2 on the upper forearc region near the active volcanic ridge most of the 64 heat flow values along two profiles across the forearc range from 20 to 40 mw m 2 corrections for bottom water temperature variations that caused nonuniform temperature gradients were required for most measurements the means of the most reliable values for the northern and southern profiles are 28 9 5 8 mw m 2 n 33 and 28 9 7 2 mw m 2 n 19 respectively and the measurements show no apparent systematic variation along the profiles over the range 50 to 150 km distance from the trench the means of the 10 values at each of two sites on the pacific plate seaward of the profiles are 57 2 6 3 and 60 2 6 6 mw m 2 redeterminations of focal depths and fault plane solutions of earthquakes in the vicinity of the heat flow profiles indicate thrust faulting on a plane dipping 17 2 calculated values of heat flow for a two dimensional analytical approximation to conduction through the upper plate diffusion into the downgoing slab and advection by that slab are consistent with either a uniform stress of 40 17 mpa along the thrust fault or stress increasing linearly at 0 5 0 2 mpa km 1 with distance from the trench axis the comparable scatter in the heat flow measurements about those calculated from these simple stress distributions does not show one to be a better approximation than the other