defining seismogenic sources from historical earthquake felt reports abstract we present a method that uses macroseismic intensity data to assess the location physical dimensions and orientation of the source of large historical earthquakes intensity data contain a great deal of information that can be used to constrain the essential characteristics of the seismic source in particular both the seismological theory and its practice suggest that the orientation of the source of significant earthquakes is reflected in the elongation of the associated damage pattern a plausible and easily manageable way of describing a seismic source is by representing it as an oriented rectangle the length and width of which are obtained from moment magnitude through empirical relationships this rectangle is meant to represent either the actual surface projection of the seismogenic fault or at least the projection of the portion of the earth crust where a given seismic source is likely to be located the systematic application of this method to all the m 5 5 earthquakes that occurred in the central and southern apennines italy in the past four centuries returned encouraging results that compare well with existing instrumental direct geological and geodynamic evidence the method is quite stable for different choices of the algorithm parameters and provides elongation directions that in most cases can be shown to be statistically significant in particular the resulting pattern of source orientations is rather homogeneous showing a consistent appennines parallel trend that agrees well with the ne sw extension style of deformation active in the central and southern portions of the italian peninsula

geologic evolution of the gulf of maine region abstract in this study we reconstruct the evolution of the northern new england passive margin whose development has been influenced by pleistocene glaciations the morphology of the northern new england shelf is rather unique consisting of a inner lowland the gulf of maine with an average depth of 150 m and an area of 90 700 km2 and georges bank a high whose crest is less than 40 m deep and has an area of 27 000 km2 the bank s northern slope facing the gulf of maine has a maximum relief of 377 m on the seaward side of georges bank is the 2000 m high continental slope deeply cut by canyons two channels northeast and great south channels east and west of georges bank provide passageways from the gulf of maine to the open sea this morphology was acquired by a combination of tertiary fluvial erosion pleistocene glacial erosion deposition and pleistocene holocene marine processes fluvial glacial erosion in the gulf of maine was so extensive as to expose basement thus making it possible to map the various terranes making up this foundation these terranes include the pre carboniferous avalon and meguma units a carboniferous permian rift basin formed by the oblique continental collision during the closure of the paleozoic proto atlantic and a late triassic early jurassic rift system created during the opening of the present atlantic basement in the gulf of maine remained above sea level from the opening of the atlantic 190 ma early jurassic to the eocene 55 ma that the gulf of maine remained a high for so long may have been due to igneous activity along the northwest trending boston ottawa lineation extending from the vicinity of the st lawrence river canada to gulf of maine from late triassic to early cretaceous the northwest trending new england seamounts south of georges bank may represent a seaward extension of this lineation on georges bank rising hundreds of meters above the gulf of maine the basement exposed in the gulf is mantled by sediments thousands of meters thick included in these sediments are early jurassic to earliest cretaceous reefs along the continental slope and carbonates north of the reefs grading landward into continental sediments cretaceous cenozoic continental marine terrigenous sediments and pleistocene glacial deposits the continental slope on the seaward flank of georges bank has a complex history of early to mid mesozoic carbonate accretion mid to late mesozoic and cenozoic calcareous terrigenous sediments and canyon erosion burial and exhumation going back to early cretaceous

elongated depressions associated with pockmarks in the western slope of the norwegian trench abstract terraces or elongated depressions have been surveyed in the western slope of the norwegian trench in origin the depressions seem to be closely related to the formation of pockmarks in the norwegian trench a study area d has been selected in the western slope for the description of the elongated depressions and their associated features an area of similar size e has also been selected in the norwegian trench in order to compare the recent geology of the two areas it is found that the elongated depressions probably are caused by a combination of shallow gas seepage the geometry of subsurface bedding and the action of bottom currents furthermore examples of faulting within soft silty unconsolidated clay is documented by use of deep towed boomer data the existence of such faults could explain how gas migrates through the upper layers

stochastic finite fault ground motion simulation in a wave field diffusive regime case study of the mt vesuvius volcanic area abstract the main aim of the present work consists in the validation of stochastic method for simulating weak ground motion in a diffusive regime due to low to moderate magnitude earthquakes and in particular in its application to a volcanic area we simulated the peak ground acceleration and the response acceleration spectra caused by two earthquakes scenarios m d 4 3 and m d 5 4 at mt vesuvius volcanic area by using the stochastic finite fault simulation method we validated the stochastic methodology by combining source path and site parameters of the investigated area considering the time duration parameter t rms calculated on the study seismograms the values of time durations are confirmed by calculating the same parameter t rms on the seismogram energy envelope described by multiple scattering models in terms of scattering and the intrinsic dissipation coefficient initially the simulations were evaluated for 10 local earthquakes 1 7 m d 3 6 that occurred at mt vesuvius in 1999 and are then compared with the observed data the comparison between simulated and observed seismograms has been used to calibrate the stochastic procedure and has been considered as the starting point for simulating ground motion for the scenario earthquake m d 3 6 that could occur in the study area the scenario earthquake and the relative fault features were chosen on the base of statistical tectonic structural and historical studies of the study area we simulated ground motions for a maximum magnitude value m max of 4 3 determined from examination of the gutenberg richter law for the study area and also for an m max 5 4 a magnitude that is associated with the earthquakes that struck the ancient town of pompei 17 yr before the eruption of mt vesuvius that occurred in a d 79 the largest values of a max for the m d 4 3 seismic event are in the range of 0 140 g 0 029 g in the case of m d 5 4 we obtain peak ground acceleration values in the range between 0 17 g and 0 55 g

new insights on 3 d plates interaction near taiwan from tomography and tectonic implications abstract recent tomographic results are used to trace the south china sea and philippine sea subducting slabs south and northeast of taiwan along the manila and ryukyu trenches respectively in particular the 3 d plates interaction beneath taiwan is discussed based on a close up view of the tomographic sections together with earthquake hypocenters distribution our study indicates that 1 the east dipping south china sea slab can be followed to the north up to the latitude of hualien 2 the eurasian plate subducts beneath most part of the taiwan island down to the 670 km depth discontinuity 3 the north dipping philippine sea slab can be followed slightly west of the longitude of hualien both plates thus interact beneath northern taiwan where the arc continent collision is paroxysmal 4 slab detachment is envisaged at the northern edge of the subducted eurasian plate beneath the coastal range of taiwan which may facilitate the northwestward motion of the philippine sea plate with respect to eurasia slabs geometries obtained from tomographic sections allow us to reconstruct the late neogene plate kinematics and dynamics in this region our main conclusions are 1 the size of the original south china sea was about twice its present size 2 the subducted part of the west philippine basin i e the largest oceanic basin of the philippine sea plate extends only 400 km north of the ryukyu trench 3 slab detachment might have occurred 3 5 my ago beneath the central and northern ryukyu arc along a weak zone that is aligned with the gagua ridge an ancient plate boundary 4 the ryukyu trench has propagated westward from 126 e of longitude southeast of miyako island to the locus of the present arc continent collision along a major lithospheric tear that cut through the continent ocean boundary first and then through the continental lithosphere as a consequence the southern ryukyu margin developed progressively from east to west as a subduction zone during the last 8 my it has been elaborated onto a passive margin which corresponded to the edge of the eurasian continental shelf bordering the south china sea prior to pliocene 5 the southern okinawa trough rifted during the last 2 1 ma following the propagation of the subduction zone 6 subduction of the eurasian margin probably stopped recently beneath northern taiwan and a flip of subduction from west to east is being initiated near hualien

forecasting seismic scenarios on etna volcano italy through probabilistic intensity attenuation models a bayesian approach abstract in this paper we apply a probabilistic procedure to model the attenuation of the macroseismic intensity in the mt etna region which allows estimating probabilistic seismic scenarios starting from the local earthquake catalogue we select a dataset of 47 events having epicentral intensity i0 from vi to ix x ems and update the model parameters previously achieved for italy according to the bayesian paradigm for each class of epicentral intensity i0 we then estimate the probability distribution of the intensity at a site conditioned on the epicentre site distance through a binomial beta model under the assumption of a point seismic source and isotropic decay circular the mode of the distribution is taken as the expected intensity is at that site since the strongest earthquakes show a preferential propagation of shaking along the fault strike and a rapid decrease in the perpendicular direction we also consider the anisotropic decay elliptical of the intensity due to a linear source finite fault we therefore transform the plane so that the ellipse has the length of the fault rupture as maximum axis and its strike as azimuth is changed into a circle with fixed diameter then we apply the probabilistic model obtained for the isotropic case to the modified data the entire calculation procedure is implemented in the software proscen which given the location and the epicentral intensity and eventually the fault parameters of the earthquake to be simulated generates the probabilistic seismic scenario according to the isotropic and anisotropic models of attenuation the results can be plotted on grid maps representing 1 the intensity that can be exceeded with a fixed probability or 2 the probability of exceeding a fixed intensity value the first representation may also find application in seismic monitoring at etna volcano in order to produce real time intensity shakemaps based on the instrumental parameters calculated by the automatic earthquake processing system

intensity versus ground motion a new approach using italian data abstract the correlations between macroseismic intensity and peak ground acceleration and between macroseismic intensity and arias intensity are compared the data base consists of 56 records 168 time histories related to ns ew and z components derived from nine italian earthquakes that occurred between 1980 and 1990 the intensity data have been determined using two approaches 1 the standard approach which is a general description of the effects on a certain village town general intensity and 2 a localized approach based on a description of effects in the immediate vicinity and in the same lithological morphological and hydrogeological conditions as the recording instrument itself local intensity this second approach to determining intensity could more closely approximate the ground motion parameters obtained from the accelerograms indeed the results of the regressions that were performed show a clear improvement in the correlation when using the local instead of the general intensity in a comparison with peak ground acceleration arias intensity

the relationship between seismic velocity and density in the continental crust a useful constraint summary plots of seismic velocity and density of rock samples show that a range of densities is possible for rocks of each seismic velocity and vice versa although a single linear relationship is often assumed in crustal gravity calculations because of the scatter whenever rocks of known seismic velocity are converted to density using this relationship a reduction is made to the resolving power of the resulting gravity calculation if these rocks reach thicknesses of more than a few kilometres then the uncertainties become significant when compared with the size of commonly observed gravity anomalies examples are considered from the north sea mississippi and carolina trough it is concluded that the use of a seismic velocity measurement as the only indication of rock density does not provide a useful constraint when attempting to reproduce observed gravity variations an appropriate model for isostatic compensation is probably the most important factor for successful predictions of crustal structure on the basis of gravity data

morphology and processes associated with the accumulation of the fine grained sediment deposit on the southern new england shelf abstract a 13 000 km2 area of the southern new england continental shelf which is covered by anomalously fine grained sediment has been surveyed by means of high resolution seismic reflection and side scan sonar techniques to map its morphology and structure and a near bottom instrument system contributed to understanding present activity of the deposit seismic reflection profiles show that the fine grained deposit which is as much as 13 m thick has accumulated during the last transgression because it rests on a reflector that is geomorphically similar to and continuous with the holocene transgressive sand sheet still exposed on the shelf to the west the ridge and swale topography comprising the sand sheet on the shelf off new jersey and long island are relict in origin as the e same features are found buried under the fine sediment deposit southwestward migrating megaripples observed on the sonographs in the eastern part of the deposit are evidence that sediment is still actively accumulating in this area in the western part of the deposit where surface sediment is composed of silt plus clay evidence of present sediment mobility consists of changes in the near bottom suspended matter concentrations primarily associated with storms nantucket shoals and georges bank are thought to be the sources for the fine textured sediment storms and strong tidal currents in these shoal areas may still erode available fine grained material which then is transported westward by the mean drift to the southern new england shelf where a comparatively tranquil environmen permits deposition of the fine material

seismic activity of the san francisco bay region moment magnitude m with objective confidence level uncertainties are estimated for felt san francisco bay region earthquakes using bakun and went worth s 1997 analysis strategy for seismic intensity observations the frequency magnitude distribution is well described for m 5 5 events since 1850 by a gutenberg richter relation with a b value of 0 90 the seismic moment rate m yr since 1836 is 2 68 1018 n m yr 95 confidence range 1 29 1018 n m yr to 4 07 1018 n m yr the seismic moment rate since 1850 is nearly the same m yr in the 56 years before 1906 is about 10 times that in the 70 years after 1906 in contrast m yr since 1977 is about equal that in the 56 years before 1906 80 1 14 of the plate motion moment accumulation rate is available for release in earthquakes the historical m yr and the portion of the plate motion moment accumulation rate available for release in earthquakes are used in a seismic cycle model to estimate the rate of seismic activity in the twenty first century high and low rates of future seismic activity are both permissible given the range of possible seismic cycle recurrence times t and the uncertainties in the historical m and in the percentage of plate motion available for release in earthquakes if the historical seismic moment rate is not greater than the estimated 2 68 1018 n m yr and the percentage of the plate motion moment accumulation available for release in earthquakes is not less than the estimated 80 then for all t the rate of seismic moment release from now until the next 1906 sized shock will be comparable to the rate from 1836 to 1905 when m 6 1 2 shocks occurred every 15 to 20 years

attenuation of intensities and effect of local site conditions on observed intensities during the corinth greece earthquakes of 24 and 25 february and 4 march 1981 abstract field investigations conducted immediately after the gulf of corinth earthquake of 24 february 1981 provide an extensive set of data on intensity at different sites for the main shock and two of its principal aftershocks each site of intensity observation was characterized by the local site geologic condition the hypocenters of the three shocks were relocated by using the joint epicenter location technique with the aftershock of 12 march 1981 01 49 utc recorded by six portable stations as well as stations of the greek national seismographic network as the calibration event isoseismal maps prepared by using all intensity data for the three shocks were used to derive new attenuation relations analysis of the intensity residuals observed calculated intensity shows that local site geologic condition has a significant effect on the intensity observed at a site on an average under similar circumstances sites located on soil foundations experience about one intensity unit more shaking than sites located on rock foundations likewise sites located on neogene sediments intermediate foundation condition experience about 1 2 unit greater intensity effect than sites located on rock foundations attenuation relations derived by using intensity data for the three february march 1981 earthquakes show lower attenuation than the relation derived by using published isoseismal maps for 19 earthquakes which occurred in the corinth saronikos region during 1861 to 1975

three dimensional vp and vs structural models associated with the active subduction and collision tectonics in the taiwan region summary 3 d vp and vs models for the crust and upper mantle beneath the taiwan area have been determined using selected high resolution earthquake data from an island wide seismic network and two local seismic arrays lateral structural variations in the upper crust as also evident from surface geology are responsible for the observed large traveltime residuals or station corrections prior shallow velocity information inferred from traveltime residuals and joint hypocentral determination jhd station corrections for the uppermost crust is essential to facilitate a reliable tomographic inversion a finite difference method that is efficient and accurate for a highly heterogeneous velocity structure is applied to calculate p and s wave traveltimes from the source to receiving stations all earthquakes in the taiwan central weather bureau s catalogue are then relocated using the resultant 3 d vp and vs models the depth of the moho varies significantly especially along the east west direction in the western coastal plain and western foothills the depth of the moho is around 35 km which deepens gradually eastward reaches a maximum depth of 55 km beneath the eastern central mountain range shallows up rapidly beneath the longitudinal valley and coastal range and merges with the thin philippine sea plate offshore of eastern taiwan in central taiwan the central mountain range is bounded to the east and west by two steeply westward dipping active faults from the upper crust to a depth of about 30 km therefore the uplifted and thickened central mountain range serves as a backstop for the converging eurasian and philippine sea plates the crust beneath the central mountain range is characterized by a brittle high velocity and seismically active upper crust 15 km and a ductile low velocity and aseismic mid to lower crust below 15 km most probably due to the high geothermal activity from the excess heat supplied from the hot upper mantle beneath the thin oceanic crust to the east from the surrounding hotter upper mantle beneath the thickened continental crust and from shear heating during active collision the collision zone in eastern taiwan is characterized by an active and steeply eastward dipping seismic zone along a region of low vp and high vp vs ratio near the taitung region in southeastern taiwan it transforms into an active westward steeply dipping seismic zone along a transition zone between the high vp and vs oceanic crust and the low vp and vs continental crust near hualien region in central eastern taiwan there is no apparent seismicity within many sedimentary basins imaged from the tomographic inversion however af ew basins are either bounded on one side by an active fault or are characterized by blind faults beneath the geometry of the subduction zone in northeastern taiwan can be clearly imaged from the relocated earthquake locations behind the subduction a region of low vp and high vp vs ratio at depths of 5 to 10 km can be identified beneath the tatun chilung volcano group indicating a potential magma reservoir two steeply dipping linear seismic zones in the volcano region may mark the upward escape paths of the magmatic materials in the region

three dimensional density structures of taiwan and tectonic implications based on the analysis of gravity data abstract taiwan is located in a collision and subduction area and has a complex tectonic history to better understand the complicated structure beneath taiwan gravity studies in addition to seismic and geological studies provide useful geophysical information for studying shallow depths previous gravity studies of taiwan in the last 30 years focused on local regionalized explanations and 2 d profile modeling this study is the first to complete a 3 d gravity inversion of taiwan and it provides a more comprehensive and large scale tectonic analysis following 3 d gravity inversion using the least squares method we sliced horizontal and vertical profiles from the 3 d density model to visualize tectonic changes the low bouguer anomaly was caused by thick sediment and crust layers the high density layers are located in special tectonic areas such as the peikang and kuanying basement highs the deepest moho depth beneath the middle of the central range is 45 50 km the high gradient changes of the eastern section of the moho relief are shown by the complex mechanism of plate collision the geometry of plate subduction is apparent in northeastern taiwan and the oceanic crust is observable under eastern taiwan showing arc collision boundaries our 3 d density model when combined with updated gravity data and seismic tomography offers better resolution for deep structures than the previous 2 d forward results and serves as a physical property reference to better understand the tectonic structure beneath taiwan

the crust and upper mantle structure beneath southeastern china we analyzed teleseismic waveforms recorded by 44 stations in the fujian and taiwan provinces of china and obtained 5344 high quality receiver functions the crustal thickness h and average crustal vp vs ratio k beneath every station were estimated using the h k stacking method crustal thicknesses near the fujian province range from 28 3 to 32 8 km with an average of 31 1 km and the corresponding vp vs ratios vary from 1 70 to 1 84 with a mean of 1 76 from inland to offshore of the fujian province the crustal thicknesses decrease and poisson s ratios increase these may indicate decreasing sio2 and increasing calcalkaline contents in the crust the discontinuity structures such as the moho subducting slab the 410 and 660 km discontinuities hereafter we call them the 410 and the 660 are also studied using common converted point ccp stacking of receiver functions along two nw se lines of central and northern taiwan the ccp stacking results show a western dipping structure at depths above 50 km suggesting that the philippine sea plate is probably subducting beneath the eurasian continent plate near the central and northern taiwan the ccp stacking results show sharp and flat 410 and 660 km discontinuities and the transition zone thickness tzt is the same as that of ambient mantle beneath fujian and taiwan strait but thickens in the east of taiwan these results suggest that 1 the subducting eurasian continent plate is confined to the depths above 410 km beneath fujian and taiwan strait and 2 the south china sea slab may reach the transition zone beneath the east of taiwan 2007 elsevier b v all rights reserved

attenuation of modified mercalli intensity with distance in mexico abstract two relations are proposed to predict the attenuation of modified mercalli intensity i with distance d for mexican earthquakes i e ln i b 0 b 1 ln d d 9 b 2 d d 9 b 3 ln m s ln i b 0 b 1 d d 9 b 2 ln d d 9 b 3 ln m s m s is the earthquake surface wave magnitude d is a distance related to the maximum i mapped for an earthquake i or to m s the coefficients b i i 0 1 2 3 were obtained by fitting in a least square sense the information contained in the intensity maps of 32 events to the relations those events were classified in three groups according to their epicentral location focal mechanism and depth i e events related to the subduction zone intermediate depth earthquakes in south central mexico and to shallow crustal events along the trans mexican volcanic belt the i predicted by the proposed relations compare well with the i observed for historical earthquakes not included in the fitting results obtained from a parametrical study showed that the attenuation of i with d is different for each of the three types of earthquakes for distances of less than about 200 km the earthquakes associated with the subduction zone have a larger attenuation than the ones originating in the south central region of mexico for greater distances d 200 km the opposite behavior is observed the events located in the trans mexican volcanic belt have a larger attenuation with distance than that of events in the other two regions from these results it seems advisable in mexico to use several attenuation relations to estimate the seismic hazard at a site depending on the particular tectonic setting and the path of the events under consideration

taiwan orogeny thin skinned or lithospheric collision abstract the taiwan orogeny is young and presently very active it provides an excellent environment for studying ongoing orogenic processes especially since the region is monitored intensively with dense seismological and geodetic networks and new studies aiming at deciphering shallow and deep structures in and around taiwan have been recently conducted or are being planned the available data can be used continually to test critically hypotheses of the taiwan orogeny hypotheses dealing with the mechanics of mountain building are basic to the understanding of taiwan orogeny and are particularly amenable to testing the widely cited thin skinned tectonics hypothesis was formulated to explain mainly the geologic and relatively shallow

bedform distribution and inferred sand transport on georges bank united states atlantic continental shelf four bedform provinces have been identified on georges bank using sidescan sonar and echo sounding techniques large sand waves superimposed on sand ridges small sand waves megaripples and featureless seafloor the large sand waves and sand ridges are found on the bank crest where the surface tidal currents are strongest areas of small sand waves and megaripples formed where tidal currents are moderate in strength border the area of large sand waves to the north and south featureless seafloor is found farthest from the bank crest where surface tidal currents are weakest sand wave asymmetry and surface sediment texture have been used to infer bedload transport paths on georges bank in the large sand wave area bedforms indicate a clockwise transport around each of the linear north west striking sand ridges with slight convergence of the sand waves on the ridge crests this transport pattern implies erosion from the troughs and accumulation on the sand ridges the asymmetry of the small sand waves along the south side of georges bank indicates that sand is also transported southward away from the linear sand ridges on top of the bank although the asymmetry of megaripples could not be determined the occurrence of megaripples between the small sand wave province and areas of featureless seafloor suggests a decreasing effectiveness of sand transport away from the bank crest this sand dispersal pattern is further supported by the surface sediments which become progessively finer to the north and sw away from the crest of georges bank

characteristics of pockmarks in the norwegian trench a 3 km wide corridor across the norwegian trench was surveyed in detail with high resolution side scan sonar and subbottom profiler boomer in order to study pockmarks the pockmarks in the norwegian trench are dish shaped depressions found in the soft silty clays they range from 2 m to 300 m in diameter and appear with a maximum density of 40 50 per km2 lineated depressions and strings of small pockmarks were discovered the strings are generally composed of individual pockmarks 10 15 m in diameter each string can be up to 400 m long some medium sized pockmarks 40 50 m in diameter were found in groups often associated with a large parental pockmark the deep towed subbottom profiler records indicated columnar disturbances fountains in the sediments below some of the pockmarks these fountains are interpreted as vents or fissures through which gas or liquids migrate buried pockmarks and anomalies interpreted as boulders possibly gas bubbles trapped in the substrata were also found the recordings bear evidence of gas or liquid migration originating from layers lying beneath the soft clay sequence a large hydrocarbon gas reservoir has recently been found during exploration drilling in the immediate vicinity of the studied pockmark area pockmarks may be indicators of deep lying gas or oil reservoirs and therefore prove interesting features in relation to hydrocarbon prospecting

the role of erosion by fish in shaping topography around hudson submarine canyon abstract an 800 km2 area of rough topography around the head of hudson canyon off the eastern united states is attributed to erosion by tilefish lopholatilus chamaeleonticeps and associated species of crustaceans the rough topography has a relief of 1 10 m occurs in water depths of 120 500 m and has been cut into a semilithified silty clay substrate since the onset of the holocene transgression commercial fishing activity indicates that a large population of tilefish which dig burrows in the sea floor occupy the area of the rough topography average tilefish burrows are 1 6 m in diameter and 1 7 m in depth they have a clustered not uniform distribution and their average density is 2 500 per km2 the close match of areas of rough topography and high til fish populations the active burrowing of the sea floor and the clustered distribution of the burrows suggest that the hummocky topography in this area may be the result of continuous erosion by tilefish and associated crustaceans during the holocene an erosion rate of 13 cm per 1 000 years is necessary to create this topography during the past 13 000 years and 18 cm per 1 000 years if as is more likely based on the depths at which tilefish presently are found the erosion started 9 000 years ago

subsurface imaging taiger experiments and tectonic models of taiwan abstract the seismicity deformation rates and associated erosion in the taiwan region clearly demonstrate that plate tectonic and orogenic activities are at a high level major geologic units can be neatly placed in the plate tectonic context albeit critical mapping in specific areas is still needed but the key processes involved in the building of the island remain under discussion of the two plates in the vicinity of taiwan the philippine sea plate psp is oceanic in its origin while the eurasian plate eup is comprised partly of the asian continental lithosphere and partly of the transitional lithosphere of the south china sea basin it is unanimously agreed that the collision of psp and eu is the cause of the taiwan orogeny but several models of the underlying geological processes have been proposed each with its own evolutionary history and implied subsurface tectonics taiger taiwan integrated geodynamics research crustal and mantle imaging experiments recently made possible a new round of testing and elucidation the new seismic tomography resolved structures under and offshore of taiwan to a depth of about 200 km in the upper mantle the steeply east dipping high velocity anomalies from southern to central taiwan are clear but only the extreme southern part is associated with seismicity toward the north the seismicity disappears the crustal root under the central range is strongly asymmetrical using 7 5 km s as a guide the steep west dipping face on the east stands in sharp contrast to a gradual east dipping face on the west a smaller root exists under the coastal range or slightly to the east of it between these two roots lies a well delineated high velocity rise spanning the length from hualien to taitung the 3 d variations in crustal and mantle structures parallel to the trend of the island are closely correlated with the plate tectonic framework of taiwan the crust is thickest in the central taiwan collision zone and although it thins toward the south the crust is over 30 km thick over the subduction in the south in northern taiwan the northward subducting psp collides with taiwan and the crust thins under northern taiwan where the subducting indenter reaches 50 km in depth the low vp vs ratio of around 1 6 at a mid crustal depth of 25 km in the central range indicates that current temperatures could exceed 700 c the remarkable thickening of the crust under the central range its rapid uplift without significant seismicity its deep exhumation and its thermal state contribute to make it the core of orogenic activities on taiwan island the expanded network during the taiger deployment captured broadband seismic data yielding enhanced s splitting results with mainly sks skks data the polarization directions of the fast s waves follow very closely the structural trends of the island supporting the concept of a vertically coherent taiwan orogeny in the outer few hundred kilometers of the earth

focal mechanism determination in taiwan by genetic algorithm we determined the focal mechanism solutions for earthquakes with mag nitude ml 4 0 that occurred in the taiwan region between 1991 and 2005 first motion polarities of p waves recorded at over 700 seismic stations in taiwan were used because of the large number of events and stations involved we implemented the genetic algorithm in a nonlinear global search for the focal mechanism solutions the algorithm was tuned and validated through synthetic tests we finally determined the focal mechanisms of 1635 events with good qualities among 4188 earthquakes focal mechanism solutions for a majority of the earthquakes display a dominant pat tern of thrust fault type reflecting the compressive stress field due to the plate colli sion normal fault events occurred at intermediate depths in subduction zones which is likely the result of the bending of the subducting slabs strike slip faults are also found within the eurasia plate around the peikang basement high and in collision zones near ilan where the geometry of the colliding plates is complex our study pro vides a database of focal mechanisms for studying seismogenic structures and plate tectonics this database can also be used by structural seismologists to compute syn thetics for waveform tomography studies

moho depth variation in taiwan from teleseismic receiver functions we determine lateral variation of moho discontinuity crustal thickness and vp vs ratios in the vicinity of taiwan by analyzing all the available teleseismic waveform data collected by the broadband array in taiwan for seismology network from 1998 to 2004 the crustal thickness and the average crustal vp vs ratio beneath each station are obtained by stacking ps ppps and ppss psps phases coherently the best estimated crustal thickness of taiwan from 27 broadband stations is 30 km on average the thinnest crust 11 15 km is found east of the longitude valley suture zone as part of the oceanic crust of the philippine sea plate in other places the crustal thickness varies from 17 19 km in northern taiwan to 32 39 km in the southwestern island the deepest moho is found to be 53 km beneath station sslb in central taiwan the average crustal vp vs ratio in taiwan is 1 74 with higher values of 1 74 1 99 in the north and lower values of 1 60 1 74 in the south the crustal thickness variation is supported by gravity measurements in taiwan and indicates that the collision between the philippine sea plate and eurasian plate in taiwan involves the whole crust the thin crust and high vp vs ratios in northern taiwan are believed to be related to the volcanism in tatun and keelung located in the southwestern extent of the okinawa trough

shallow water pockmark formation in temperate estuaries a consideration of origins in the western gulf of maine with special focus on belfast bay abstract a systematic mapping program incorporating more than 5000 km of side scan sonar and seismic reflection tracklines in the western gulf of maine has identified more than 70 biogenic natural gas deposits occupying 311 km2 in nearshore muddy embayments many of these embayments also contain pockmark fields with some exhibiting geologically active characteristics including the observance of plumes of escaping fluids and sediment pockmarks hemispherically shaped depressions of various size and depths formed through fluid escape of gas and or pore water are sometimes found within or outside gas fields although many gas fields lack pockmarks altogether although the origin of the natural gas remains unclear if coastal environments at times of lower sea level were similar to the present numerous lake wetland valley fill and estuarine sources of organic rich material may have formed on the inner shelf if these deposits survived transgression and remain buried they are potential gas sources intensive mapping of the belfast bay pockmark field in 1998 produced the first nearly continuous side scan sonar mosaic of a gulf of maine pockmark field with a corresponding 3 dimensional geological model generated from seismic data statistical analysis of pockmark geometry gas deposit loci and subsurface evidence for gas enhanced reflectors suggest that gas migration from deeper lateral sources along permeable subsurface strata may be the mechanism for pockmark formation in areas lacking gas curtain seismic reflections the coarse grained transgressive ravinement unconformity between pleistocene glacial marine mud and holocene mud may act as a conduit for distributing methane to the field s margins

attenuation of intensity with epicentral distance in the iberian peninsula we have classified the attenuation of the medveded sponheuer and karnik msk intensity into five types each as a function of the epicentral intensity i o based on the mean radii of 254 isoseismal maps mainly historical earthquakes in the iberian peninsula geographically representing each earthquake with its corresponding attenuation tendency it can be seen that those with low attenuation lie west of the peninsula and those with high attenuation in the south and east this regionalization seems to be due as much to the seismotectonic characteristics different crustal types and size of the earthquakes as to the different construction types in each region these attenuation values are similar to those of southern europe but much higher than those found in the united states from the point of view of seismic hazard evaluation these laws represent an improvement with regard to those used so far we have extended previous attenuation studies to the whole of the iberian peninsula and in some points differences of attenuation assignment of almost two degrees of intensity have been corrected

tomographic imaging of lithospheric structures under taiwan abstract tomographic images of the crustal and mantle velocity structures under taiwan are obtained by simultaneous inversion of local earthquake p wave arrival times for hypocenters and p wave velocity structures in northern taiwan a high velocity zone coinciding with the wadati benioff zone can readily be identified as the subducted philippine sea plate the imaged zone dips toward the north at an angle of 40 from a depth range of 20 55 to 100 130 km an upper mantle low velocity wedge ranging from 40 to 80 km in depth exists above the subducted slab above this wedge is the ilan plain of northern taiwan which lies at the west end of the okinawa trough a well recognized back arc basin the crustal velocities under the plain are also relatively low the well defined high velocity zone and the low velocity wedge provide some constraints on the thermal structures of the subduction system under northern taiwan in tomographic images across the central section of taiwan thickening of the crust and up arching of the lower crustal materials under the central range are commonly observed the crust under the western foothills region is clearly thinner and the near surface low velocity layers are well developed the structures under the central range show that although the taiwan orogeny is quite young a root deeper in the north and shallowing to the south has formed the results of our tomography show that a significant portion of the lithosphere is involved in the taiwan orogeny

seismic hazard maps for cuba and surrounding areas a seismic hazard assessment for cuba and the surrounding areas has been performed in response to a possible revision of the national building code the hazard assessment has been done according to the standard methodology adopted by the global seismic hazard assessment program and by introducing some computational techniques used for the seismic hazard map of italy problems of earthquake catalog treatment attenuation of peak ground acceleration and macroseismic intensity as well as seismic source definition have been rigorously analyzed thirty six seismogenic zones have been identified and characterized from a seismicity point of view the present study offers a picture of the seismic hazard on cuban territory based on historical seismicity and the benefits drawn from the most recent international investigations on the subject such as the logic tree approach used to represent the inevitable uncertainties encountered through the choice of attenuation relation the final results are maps of the expected shaking with a 475 year return period in terms of peak ground acceleration and macroseismic intensity which point out the high hazard along the southern coast of cuba where the expected ground motion without the aleatoric uncertainty in the attenuation relations is between 0 20 g and 0 30 g the rest of the island is characterized by values representing less severe shaking

estimating locations and magnitudes of earthquakes in eastern north america from modified mercalli intensities we use 28 calibration events 3 7 m 7 3 from texas to the grand banks newfoundland to develop a modified mercalli intensity mmi model and associated site corrections for estimating source parameters of historical earthquakes in eastern north america the model batchmode documentclass fleqn 10pt legalpaper article usepackage amssymb usepackage amsfonts usepackage amsmath pagestyle empty begin document mathrm mmi 1 41 1 68 times mathbf m 0 00345 times delta 2 08 mathrm log delta end document where is the distance in kilometers from the epicenter and m is moment magnitude provides unbiased estimates of m and its uncertainty and if site corrections are used of source location the model can be used for the analysis of historical earthquakes with only a few mmi assignments we use this model mmi site corrections and bakun and wentworth s 1997 technique to estimate m and the epicenter for three important historical earthquakes the intensity magnitude m i is 6 1 for the 18 november 1755 earthquake near cape ann massachusetts 6 0 for the 5 january 1843 earthquake near marked tree arkansas and 6 0 for the 31 october 1895 earthquake the 1895 event probably occurred in southern illinois about 100 km north of the site of significant ground failure effects near charleston missouri manuscript received 14 march 2002

attenuation and source parameters of shallow microearthquakes at mt etna volcano italy we estimated the attenuation laws of high frequency seismic waves in the shallow crust depth 5 km and earthquake source parameters by using a se lected data set of 320 shallow events 2 6 md 4 2 recorded at mt etna volcano during the last two flank eruptions occurring in 2001 and 2002 2003 the quality factor q was estimated from spectra of p and s waves for 24 stations of the local permanent network by applying a spectral ratio technique the results show varia tions in both qp and qs as a function of frequency according to the power law q q0f n with n ranging between 0 3 and 1 3 for p waves and between 0 2 and 0 9 for s waves as typical of volcanic environments strong azimuthal variations of qp were also found suggesting the presence of local strong lateral heterogeneities and or of fluid filled cracked volumes after correction for attenuation we estimated the source parameters seismic moment source radius and stress drop of a subset of 66 shallow events under the assumption of a circular dislocation the estimated seismic moments m0 range from 10 13 to 10 15 n m the source radii r are confined between 100 and 1000 m and stress drop dr ranges between 0 2 mpa and about 4 mpa combining the source parameters obtained in this study with those calculated by pataneet al 1997 for an old data set of smaller microearthquakes 10 9 m0 10 14 n m recorded in the same area we re evaluated the scaling relationship between seismic moment m 0 and corner frequency f c for the earthquakes with m 0 ranging between 10 13 and 10 15 n m we confirm that microearthquakes at mt etna seem not to obey a

taiwan mountain building insights from 2 d thermomechanical modelling of a rheologically stratified lithosphere summary the taiwan orogen has long been regarded as a case example for studying mountain building in association with subduction processes in this paper we present a fully coupled thermomechanical modelling of the taiwan collision based on a realistic viscous elastic plastic rheology it satisfactorily reproduces available thermochronometric data long short term deformation patterns heat flux and erosion sedimentation distribution across the taiwan orogeny we found that a deep seated flux of asian crustal material into the orogenic wedge should be invoked to counter balance observed exhumation and erosion in the central range however in contrast with recent thermokinematic models of exhumation and deformation suggesting that underplating plays a significant role we show that most constraints on exhumation and deformation can be more straightforwardly interpreted by the frontal accretion of the rheologically layered asian crust we finally infer that such a model is in better agreement with the basic expectation that the hot young and buoyant chinese continental margin should hardly be subducted beneath the cold old and dense oceanic plate of the philippines sea

a simple algorithm for tracing synthetic isoseismals probabilistic calculation of regional seismic hazard maps also requires the use of the so called attenuation relations which give the reference shake ability at certain distances from the earthquake sources this article achieves progress in this area in fact the present tests on a series of earthquakes in california san fernando 1971 whittier narrows 1987 northridge 1994 suggest that in some regions the areal shapes of the territories damaged by past earthquakes may be synthetically traced sometimes amazingly well with a simple algorithm that considers some gross features of the sources and this is compatible with theory it seems that this algorithm gives rather stable results moreover when the detailed modeling techniques available nowadays are inapplicable due to lack of data or for purpose of saving time and money it might be useable for improving seismic hazard calculations and conversely for retrieving information about sources of earthquakes from the preinstrumental era

the potential for a great earthquake along the southernmost ryukyu subduction zone interseismic gps data along the hualien suao coast ne taiwan shows a pattern of strain accumulation that is consistent with a potential future large shallow earthquake along the southernmost ryukyu subduction zone the measured shortening rate parallel to the ryukyu trench is 80 mm yr about twice of the shortening rate perpendicular to the ryukyu trench we invert for slip deficit rates and the geometric configuration of the plate interface our preferred fault model dips 10 northward and extends about 70 km from the ryukyu trench to a depth of 13 km the slip deficit rate exhibits a left lateral motion of 78 mm yr and a normal motion of 36 mm yr on a 290 trending fault the slip rate budget of the southernmost ryukyu subduction zone is close to the plate convergence rate suggesting the plate interface is fully locked assessments of seismic hazard in this region need to consider the potential threat from m w 7 5 8 7 tsunami earthquakes generated by shallow ruptures

the attenuation of seismic intensity in italy part i theoretical and empirical backgrounds abstract we critically analyze the results on seismic intensity attenuation in italy derived by albarello and d amico 2004 and gasperini 2001 we demonstrate that due to the inadequacy of certain underlying assumptions the empirical relationships determined in those studies did not best reproduce the decay of intensity as the distance from the source increases we reconsidered some of the relevant concepts and assumptions used in these intensity attenuation studies macroseismic epicenter epicentral intensity data completeness to suggest some useful recipes for obtaining unbiased estimates in particular we suggest that 1 data for distances from the source at which an intensity below the limit of diffuse perceptibility iv is expected should be excluded from attenuation computations because such data are clearly incomplete 2 attenuation equations that include a term proportional to the epicentral intensity i 0 with a coefficient different from 1 0 must not be used because they imply a variable offset between i 0 and the intensity expected at the epicenter and 3 epicentral intensities must be recomputed consistently with the attenuation equation because those reported by the italian catalog do not generally correspond with the intensity predicted at the epicenter by the attenuation relationships so far proposed following these suggestions produces a significant reduction in the standard deviation of the model that might lead to a corresponding reduction of the estimates of seismic hazard

detecting imminent eruptive activity at mt etna italy in 2007 2008 through pattern classification of volcanic tremor data abstract volcano monitoring aims at the recognition of changes in instrumentally observable parameters before hazardous activity in order to alert governmental authorities among these parameters seismic data in general and volcanic tremor in particular play a key role recent major explosive eruptions such as okmok aleutians and chaiten chile in 2008 and numerous smaller events at mt etna italy have shown that the period of premonitory seismic activity can be short only a few hours which entails the necessity of effective automatic data processing near on line here we present a synoptic pattern classification analysis based on self organizing maps and fuzzy cluster analysis which is applied to volcanic tremor data recorded during a series of paroxysmal eruptive episodes and a flank eruption at etna in 2007 2008 in total eight episodes were analyzed in six of these significant changes in the dynamic regime of the volcano were detected up to 9 h prior to the onset of eruptive activity and long before changes in volcanic tremor amplitude and spectral content became evident in classical analysis in two cases the state transition was

glacimarine processes and inductive lithofacies modelling of ice shelf and tidewater glacier sediments based on quaternary examples abstract three variables control the type of glacial debris source and where and when debris is introduced to an ocean 1 type of glacial ice source ice sheet or valley glacier 2 condition of a grounded glacier melting freezing or frozen base and cold or warm internal ice and 3 type of glacier front ice shelf or tidewater using these variables in different combinations eight glacimarine regimes are postulated to exist by integrating information from holocene sediment and pleistocene glacimarine sequences with the eight inductive regimes lithofacies relationships are predicted and modelled the models show a complexity of glacimarine lithofacies the importance of recognizing this complexity lies in realistic interpretations of ancient sequences ice shelf sediment may be limited in the geologic record because of tectonic and climatic constraints required for ice shelf formation alternatively in some tidewater glacier regimes larger volumes of glacimarine sediment are produced than in ice shelf regimes ice proximal sediment is distinctive especially near subglacial meltwater stream effluxes ice shelves and tidewater glaciers may produce similar ice proximal lithofacies e g morainal bank associations and laminites in certain regimes however details of lithofacies may aid in distinguishing ice shelf from tidewater regimes ice shelf and tidewater iceberg zone sediment can be distinguished from each other if basal and high level englacial debris can be distinguished ice shelf zone sediment out from the grounding line has a basal debris source high level englacial debris is deposited in more distal ice shelf zones resulting in sediment by pass in corresponding positions of a tidewater glacier regime iceberg rafted debris comprises mixed basal and high level englacial debris deeper water zones receive large volumes of sediment when a melting freezing base grounding line is at the edge of a continental shelf when the grounding line retreats sediment supply may be drastically reduced because of a back sloping continental shelf and trapping by morainal banks cool water bioclastic carbonates may accumulate once a grounding line has retreated although ice shelf iceberg zones are more extensive than those in tidewater regimes ice shelf icebergs have low debris content and resulting ice rafted debris may be scarce the smallest volume of sediment is produced by a frozen base ice sheet ending as a tidewater front and the largest volume by a melting freezing base valley tidewater glacier glacimarine sediment has a relatively high preservation potential near continental interiors or margins however at continental margins mass flow olistostrome and tectonic melange deposits may be preserved with glacimarine diamictites these deposits may be difficult to distinguish from each other especially if metamorphosed such as at a convergent margin the models presented do not fully represent the glacimarine environment because many other variables are omitted however the principal concepts of generating glacimarine lithofacies associations are considered continued research will verify if each regime exists and will provide data on those regimes that exist today to compare and modify these models

evidence against a late wisconsinan ice shelf in the gulf of maine abstract proposals for the formation of a late wisconsinan ice shelf in the gulf of maine during the retreat of the laurentide ice sheet are considered to be inappropriate an antarctic type ice shelf does not fit the field data that indicate temperate glacial terrestrial and marine climates for the region between 18 ka and 12 ka a temperate ice shelf has no modern analogues and may be physically impossible the preponderance of stratified drift in the gulf of maine region supports temperate climates during late wisconsinan time it also indicates that glacial meltwater rather than ice in either an ice sheet or ice shelf was the primary transport mechanism of glacial sediment and the source for the glaciomarine mud for these reasons we have proposed glacial analogues for the deglaciation of the gulf of maine that consist of temperate or subpolar marine based glaciers characterized by depositional environments dominated by meltwater discharge directly to the sea or the sea by way of subaerial meltwater streams these analogues include alaskan fjord glaciers glaciers on the alaskan continental shelf that discharged meltwater directly into the sea in the not too distant past and austfonna nordaustandet svalbard norway that is presently discharging meltwater in the sea along a grounded ice wall this last example is the best modern day analogue for the depositional environment for most of the glaciomarine mud in the gulf of maine and deglaciation of the gulf

recent sedimentary processes in the prestige site area galicia bank nw iberian margin evidenced by high resolution marine geophysical methods an echo character analysis of the oil tanker prestige wreck area was undertaken using high resolution marine geophysical methods topas and airgun seismic reflection profiles multibeam echosounder and tobi sidescan sonar integration and comparison of the results using all these methods is presented given some practical applications for indirect near surface and seafloor interpretations ten different echo types were identified and grouped into four main classes i distinct ii indistinct iii irregular and iv hyperbolic echo character distribution enabled us to determine recent sedimentary processes in the area two major depositional systems can be found through the identification of these sedimentary processes a a slope depositional system sds located in the eastern and central area where mass movement processes are dominant and b a turbiditic main channel depositional system tds located in the western area where channel related processes are dominant both of these interact over the half graben structure of the southwestern sector of the galicia bank where the prestige wreck is located within the sds erosive and depositional mass movement processes characterised a complex depositional system erosive processes occur on the fault scarp channels head inter lobe channels and distal part of the sedimentary lobes moreover depositional processes take place on the top of the fault scarp sedimentary wedges sedimentary lobes and on the west flank of the main channel both depositional systems interact but the sds should be more active during fault scarp reactivation periods through relief rejuvenation and new exposed deposits microearthquake activity would favour the available materials being weathered eroded and transported by mass movements in such a situation the tds acts as the main collector of eroded sediment derived from the fault scarp throughout the sds being responsible for its evacuation into the iberian abyssal plain however outside of fault reactivation periods the sds is less active such as during the present situation

three dimensional p and s wave velocity structures beneath taiwan deep structure beneath an arc continent collision the island of taiwan is a byproduct of one of the few active collisions between a continent represented by the eurasian continental shelf and an island arc represented by the northern extension of the luzon arc on the philippine sea plate to understand better the evolution and current tectonics of this collision we selected 1260 well recorded earthquakes from an initial set of 50 000 located by the taiwan telemetered seismograph network for the determination of one and three dimensional p and s wave velocity structures beneath the island the results for both structures and earthquake relocations reveal that the island is divided tectonically into three distinct zones in the east the philippine oceanic plate is underthrusting the eurasian plate east of a north south boundary that is well defined by both seismic activity and a region of high velocity in the south the eurasian continental plate is underthrusting the philippine sea plate south of an east west boundary at about 23 n that is sharply defined by both subcrustal seismicity and a zone of relatively low velocities the dip of the subducted continent is shallow until it reaches the luzon island arc 50 km east of the main island structure under the main part of the island north of 230n reveals a shallow dipping zone of low velocities in the west above 25 km depth that narrows and steepens below that depth to at least 50 km beneath the central range the dip of this low velocity region is outlined by a narrow zone of seismicity that extends to depths of 100 km this seismic zone lies in a velocity saddle and marks an apparent offset in the central low velocity region at 24 n evidence for the subduction of the eurasian continent beneath taiwan therefore exists everywhere beneath the island and the low velocity regions in the mantle support but do not require the subduction of some 6 16 km of lower continental crust to depths of at least 50 km the distinct change in both the velocity structure and the seismic activity at 23 n and the offset of the low velocity regions at 24 n argue for abrupt changes in the nature of subduction across these latitudes these changes may be caused by an interaction of the luzon island arc with the subducted continental shelf that mimics a similar interaction evident at the surface finally the velocity structure of the subducting philippine sea plate suggests that the earthquakes beneath 70 km depth are not occurring within the highest velocity regions of the plate but are probably located near the upper edge of the plate there is also evidence from earthquake locations and velocity structure that suggests that the subducting plate is segmented and that subduction is presently occurring as far south as 22 n

two dimensional crustal structures of taiwan from gravity data to delineate the tectonic character of taiwan an island wide gravity survey of taiwan was conducted between 1980 and 1987 the bouguer anomaly map shows that in general isogals trend nne in consonance with the overall structural trend of the island with seismic and other geophysical data as constraints the subsurface density structures were modeled along three profiles across the major structural trends the gravity data are consistent with average continental moho depths of 26 km in the coastal plain and the western foothills 28 km underneath the coastal range in eastern taiwan and 33 km under the central range a lack of significant correlation of the bouguer anomalies with the topography implies dynamic rather than isostatic support of topography in the taiwan region

the role of long period ground motions on magnitude and damage of volcanic earthquakes on mt etna italy on october 2002 a seismic swarm occurred on mt etna one of the strongest events caused severe damage up to a european macroseismic scale inten sity of viii that contrasts with its local magnitude of 4 4 the occurrence of significant damage at such a small magnitude is repeatedly observed in the area and is tradition ally attributed to shallow source recorded strong motion accelerograms and broad band seismograms demonstrate that there is one more cause for the severe damage that is an anomalously strong low frequency 0 1 f 1 hz radiation deviating from the conventional brune 1970 spectral scaling therefore these earthquakes cause large ground displacements and long 20 sec durations of shaking the integration of digital accelerograms yields a maximum peak ground displacement as large as 1 8 cm at a distance of 18 km based on the sharp local attenuation of ground motion in the study area we infer that peak ground displacements near the epicenters did exceed 10 cm the occurrence of large displacements caused selective damage to medium rise 3 stories reinforced concrete buildings and elements like church facades the frequency cutoff below 1 25 hz in the wood anderson response attenuates the peak to peak amplitudes used to assess local magnitudes therefore ml values are not representative of the real strength of volcanic earthquakes because a prompt magnitude and damage potential assessment is crucial for civil protection actions a procedure is proposed which in near real time can be successful in identifying po tentially damaging earthquakes of mt etna through the computation of pseudove locity response spectra the procedure provides a magnitude value that is derived on a statistical basis from the housner 1952 spectral intensity computed in the low frequency band this parameter is a suitable near real time indicator of large earthquake induced building shaking and could also be applied for a preliminary es timate of the epicentral macroseismic intensity

microtopography and surficial sediment patterns in the central gulf of maine a 3 5 khz survey and interpretation abstract the seafloor of the gulf of maine is dominated by ice contact glaciomarine and postglacial sedimentary deposits based on 2085 km of 3 5 khz seismic profiling hummocky topography interpreted as morainal in origin is present over about half of the seafloor in water depths of 100 300 m small exposures 2 of overlying multilayered glaciomarine reflectors extend with the hummocky surface under much of an acoustically transparent layer covering about one third of the survey area this postglacial marine silty clay unit is up to 25 m thick other less common facies include an acoustically opaque smooth bed 10 and bedrock 6 the continuous seismic records reveal the sedimentary sequence and the complex patchy distribution of deposits accumulated during and after the retreat of the late wisconsinan laurentide ice sheet from the gulf of maine over the bedrock and morainal hummocks the meltwaters of glacial retreat locally deposited the glaciomarine multilayered acoustic reflectors the still younger acoustically transparent marine silts and clays display a smooth nearly level surface except for apparent scour around bedrock promontories and v shaped depressions suggestive of pockmark formation in four locations similarly postglacial to contemporary reworking of the seafloor produced the sand waves of georges shoals and smooth sandy seabed areas that are acoustically opaque

relationships between instrumental ground motion parameters and modified mercalli intensity in eastern north america a new relationship between modified mercalli intensity mmi and peak ground velocity pgv is developed by comparing instrumentally recorded and inferred historical pgv values to observed mmi for 18 significant earthquakes from eastern north america ena of moment magnitude 3 6 7 25 this pgv mmi relationship is implemented in shakemaps in ontario to estimate ground shaking intensity instrumentally derived mmi given instrumental recordings of pgv corresponding relationships for 5 damped pseudoacceleration psa at frequencies of 1 5 and 10 hz are also developed we compare the results with previous relationships between mmi and ground motion parameters developed for california by wald et al 1999a and atkinson and sonley 2000 we conclude that empirical relationships between mmi and ground motion are significantly different in eastern north america than in california thus such relationships should be assessed for each region where shakemaps are implemented manuscript received 24 october 2003

three dimensional elastic wave velocity structure of the hualien region of taiwan evidence of active crustal exhumation the hualien region of taiwan is located at a complex transition of the boundary between the eurasian and philippine sea plates to the southwest the mountains of taiwan are uplifting rapidly as a consequence of an ongoing arc continent collision while to the east the oceanic philippine sea plate is subducting northward beneath eurasia we investigated the structure and dynamics of this region by analyzing seismograms of local earthquakes recorded during a deployment of the portable array for numerical data acquisition ii network p and s wave velocity structures deduced from travel time tomography analysis show that the collisional suture to the south of hualien is characterized by a narrow 10 km width near vertically dipping zone of low velocities that extends to depths in excess of 20 km velocities in the eastern central range west of the suture zone are significantly higher and define a feature 10 15 km wide that appears to be continuous from the near surface to depths as great as 40 km farther to the west beneath the western central range the velocities again decrease focal mechanisms of local earthquakes show that while thrust faulting is the predominate mode of deformation throughout the region normal faulting occurs as well beneath the eastern central range thus the rapid uplift of the mountains of taiwan may be a result not only of compressional shortening but also of an excess of positive buoyancy we suggest that the higher velocities and extensional mechanisms in the eastern central range are caused by the ongoing exhumation of previously subducted continental crust while the lower velocities to the west reflect continued underthrusting of the crust beneath the eastern central range

ground motion at the san francisco international airport from the loma prieta earthquake sequence 1989 abstract following the loma prieta earthquake the u s geological survey installed four portable digital seismic recorders at the san francisco international airport sfo for one week to study aftershock ground motion at this important bay area lifeline this study was motivated largely by the need to anticipate strong ground motion from future major earthquakes affecting the bay area and to a lesser extent by the fact that sfo was shut down for 13 hours owing to damage from the loma prieta shock accordingly the recording sites were chosen so as to elucidate the effects of varying thicknesses of low velocity surficial alluvium on the ground motion three large aftershocks with magnitudes ranging from 4 2 to 4 5 each produced ground motion that was recorded at all four sfo stations one of our stations was collocated with a permanent ground motion recorder that indicated a peak horizontal velocity of 29 cm sec and a peak horizontal acceleration of 0 33 g during the 18 october mainshock from the aftershock data and one mainshock record it is possible to extrapolate approximately the mainshock ground motion to other locations at sfo and more generally to assess the effects of low velocity sedimentary cover including artificial fill material on the character of the ground motion the main shock ground motion recorded at the permanent station was apparently typical for most of sfo where the near surface alluvium resulted in peak horizontal ground velocity in the frequency band 0 1 to 3 hz amplified by a factor of about 2 5 relative to that recorded at bedrock sites observations in the epicentral distance range 59 to 95 km including sfo of the moho reflected phases pmp and sms from the aftershocks support the hypothesis presented elsewhere that the phase sms accounted for much of the peak ground motion throughout most of the san francisco bay area

sedimentary framework of penobscot bay maine abstract analyses of seismic reflection profiles along with previously collected sediment samples and geologic information from surrounding coastal areas outline the characteristics distribution and history of the strata that accumulated within penobscot bay maine during the complex period of glaciation crustal movement and sea level change since late wisconsinan time sediments that overlie the rugged glacially eroded surface of paleozoic bedrock range in thickness from near zero to more than 50 m and consist of four distinct units 1 1 massive to partly stratified coarse grained drift forms thin 2 2 well stratified fine grained glaciomarine deposits are concentrated in bedrock depressions beneath the main passages of the bay during the period of ice retreat and marine submergence these sediments settled to the sea floor draped the irregular underlying surface of bedrock or drift and accumulated without disturbance by physical or biologic processes 3 3 heterogeneous fluvial deposits fill ancestral channels of the penobscot river beneath the head of the bay the channels were incised during a 40 m postglacial low stand of sea level due to crustal rebound and later were filled as base level was increased during holocene time 4 4 muddy marine sediments which are homogeneous to weakly stratified and rich in organic matter blanket older deposits within bathymetric depressions in the middle and lower reaches of the bay and cover a pronounced gently dipping erosional unconformity in the upper reach these sediments were deposited during the holocene transgression as sea level approached its present position and the bay became deeper late wisconsinan and holocene sedimentation in penobscot bay has smoothed the sea floor but it has not completely obscured the ice sculptured bedrock topography

a multiparameter approach to volcano monitoring based on 4d analyses of seismo volcanic and acoustic signals the 2008 mt etna eruption 1 volcanic tremor and low frequency events together with infrasound signals can represent important precursory phenomena of eruptive activity because of their strict relationship with eruptive mechanisms and with fluid flows through the volcano s feeding system important variations of these seismo volcanic and infrasound signals recorded at mt etna volcano occurred both in the medium and short term before the eruption that took place on 13 may 2008 the most significant changes were observed in the frequency content and location of lp events as well as in volcanic tremor location that allowed us to track the magma pathway feeding the 2008 eruptive activity the infrasound showed three different families of events linked to the activity of the three active vents north east crater south east crater and the eruptive fissure the seismic and infrasonic variations reported corroborated by ground deformations variations help to develop a quantitative prediction and early warning system for effusive and or explosive eruptions

the influence of critical moho reflections on strong ground motions recorded in san francisco and oakland during the 1989 loma prieta earthquake the amplitudes of strong ground motions from the loma prieta earthquake recorded in the san francisco and oakland areas exceeded the levels predicted by standard empirical attenuation relations preliminary analysis of accelerograms having known trigger times strongly suggests that the elevation of ground motion amplitudes in the distance range of approximately 40 to 100 km was due to critical reflections from the base of the crust these reflections which are identified on the basis of their arrival times and phase velocity and by comparison with simulated accelerograms were large and occurred at relatively close range because of the deep focal depth of the earthquake and the strong velocity gradient at the base of the crust these motions were further amplified presumably by impedance contrast effects at soft soil sites in san francisco and oakland the effect of the critical reflections in amplifying peak accelerations of the loma prieta earthquake in the san francisco and oakland regions was as large as the effect of soft soil site conditions focal depth has an important influence on strong motion attenuation at distances beyond about 40 km and empirical attenuation relations derived from shallow crustal earthquakes may underpredict the ground motions of deeper crustal events in this distance range further analyses using an expanded data base that includes recordings of aftershocks are required to rigorously test the proposed explanation of the ground motions recorded in san francisco and oakland and the conclusions drawn from that explanation

new gravity anomaly map of taiwan and its surrounding regions with some tectonic interpretations abstract in this study we compiled recently collected from 2005 to 2015 and previously reported published and open access gravity data including land shipborne and satellite derived data for taiwan and its surrounding regions based on the cross over error analysis all data were adjusted and new free air gravity anomalies were obtained shedding light on the tectonics of the region to obtain the bouguer gravity anomalies the densities of land terrain and marine sediments were assumed to be 2 53 and 1 80 g cm3 respectively the updated gravity dataset was gridded with a spacing of one arc minute several previously unnoticed gravity features are revealed by the new maps and can be used in a broad range of applications 1 an isolated gravity high is located between the shoushan and the kaoping canyon off southwest taiwan 2 along the luzon arc both free air and bouguer gravity anomaly maps reveal a significant gravity discontinuity feature at the latitude of 21 20 n 3 in the southwestern okinawa trough the ne sw trending cross back arc volcanic trail cbvt marks the low high gravity anomaly both free air and bouguer boundary

curie point depth from spectral analysis of magnetic data in taiwan abstract this study constructs an integrated magnetic anomaly map of taiwan and its vicinity that combines land magnetic data and marine magnetic data the reduction to pole rtp correction method was applied to the integrated magnetic anomaly data the corrected magnetic anomaly results for a region roughly 880 km in width and 660 km in length fit well with geological tectonic signatures and are used to estimate the curie point isotherm the adopted curie point depth method provides a relationship between the 2 d fft power spectrum of the magnetic anomalies and the depth of magnetic sources by transforming the spatial data into the frequency domain the basal depth of the magnetic sources is calculated from the magnetic data finally the curie point depth of taiwan is obtained the highest value of 17 km is located in northern taiwan and the lowest value of 6 km is located at southern taiwan and also has the highest thermal gradient of 88 c km the curie point depth is consistent with heat flow measurements with correlation coefficient 0 62

towards an automatic monitoring system of infrasonic events at mt etna strategies for source location and modeling active volcanoes characterized by open conduit conditions generate sonic and infrasonic signals whose investigation provides useful information for both monitoring purposes and studying the dynamics of explosive processes in this work we discuss the automatic procedures implemented for a real time application to the data acquired by a permanent network of five infrasound stations running at mt etna volcano the infrasound signals at mt etna consist in amplitude transients called infrasound events the adopted procedure uses a multi algorithm approach for event detection counting characterization and location it is designed for an efficient and accurate processing of infrasound records provided by single site and array stations moreover the source mechanism of these events can be investigated off line or in near real time by using three different models 1 strombolian bubble 2 resonating conduit and 3 helmholtz resonator the infrasound waveforms allow us to choose the most suitable model to get quantitative information about the source and to follow the time evolution of the source parameters

seafloor gas seeps triggered by a northern california earthquake abstract a series of marine surveys using high resolution seismic reflection and side scan sonar profiling was conducted on the klamath river delta from 1977 to 1985 in november 1980 near the middle of this 8 year time period a major earthquake magnitude 7 occurred in the area the earthquake caused liquefaction of shelf deposits off the klamath river and while mapping the sediment failures we observed an inordinate number of gas vents and acoustic anomalies in the sediment gas charging and water column gas clouds our analyses indicate that a change in gas seepage occurred after the earthquake there was a marked increase in gas abundance immediately after the earthquake a gradual decrease over the next year and stabilization at pre earthquake gas levels within five years or less the acoustic anomalies were observed in pre quaternary rocks as well as in surficial holocene deposits and gas seeps overlay truncated anticlines faults and syncline exposures thus the gas released may have been in part thermogenic in origin a large area 20 km 2 on the delta failed during the earthquake and the abundance of gas seeps and craters in the area suggests that sediment degassing may have been an important process contributing to failure

probabilistic seismic hazard at mt etna italy the contribution of local fault activity in mid term assessment abstract in this work we tackle the problem of seismic hazard at etna deriving from the recurrent seismogenic activity of local faults by adopting two independent methods based on probabilistic approaches we assess the hazard in terms of macroseismic intensity and represent the occurrence probability calculated for different exposure times both on maps and at fault scale seismic hazard maps obtained by applying the site approach through the sasha code and a new probabilistic attenuation model indicate the eastern flank of the volcano as the most hazardous with expected intensity i exp in 50 years i e the standard exposure time adopted in the seismic regulations ranging from degrees ix to x ems in shorter exposure periods 20 10 5 years values of i exp up to ix are also reached in the same area but they are clearly determined by the earthquakes generated by the timpe fault system in order to quantify the contribution of local seismogenic sources to the hazard of the region we reconstruct the seismic history of each fault and calculate with sasha the probability that earthquakes of a given intensity may be generated in different exposure times results confirm the high level of hazard due to the s tecla moscarello and fiandaca faults especially for earthquakes of moderate intensity i e vi i 0 vii with probabilities respectively exceeding 50 and 20 in 10 years and 30 and 10 in 5 years occurrence probability of major events i 0 viii at the fault scale has also been investigated by statistics on intertimes under stationary assumptions we obtain a probability of 6 8 in 5 years for each structure by introducing the time dependency time elapsed since the last event occurred on each fault through a bpt model we identify the moscarello and s tecla faults as the most probable sources to be activated in the next 5 years 2013 2017 this result may represent a useful indication to establish priority criteria for actions aimed at reducing seismic risk at a local scale

relationship between peak ground acceleration peak ground velocity and intensity in taiwan based on the strong motion data set from the 1999 chi chi taiwan earthquake and a shaking damage statistics database we investigated the correlations between strong ground motions and earthquake damage fatalities and building collapses through a regression analysis as a result the current earthquake intensity scale i t is placed on a more reliable instrumental basis this is necessary for the real time seismic monitoring operation in taiwan where programs for earthquake rapid reporting rrs and earthquake early warning ews are actively pursued it is found that the earthquake damage statistics give a much closer correlation with the peak ground velocity pgv than with the peak ground acceleration pga the empirical relationship between pgv and the intensity i t determined in this study can be expressed as i mathrm t 2 14 times mathrm log 10 mathrm pgv 1 89 this pgv based intensity is particularly useful in real time applications for damage prediction and assessment as the damage impact of high pgv is much more important for mid rise and high rise buildings that are characteristic of a modern society for smaller earthquakes m m

estimating earthquake location and magnitude from seismic intensity data analysis of modified mercalli intensity mmi observations for a training set of 22 california earthquakes suggests a strategy for bounding the epicentral region and moment magnitude m from mmi observations only we define an intensity magnitude m i that is calibrated to be equal in the mean to m m i mean m i where m i mmi i 3 29 0 0206 i 1 68 and i is the epicentral distance km of observation mmi i the epicentral region is bounded by contours of rms m i rms m i m i rms m i m i where rms is the root mean square rms m i m i is the minimum rms over a grid of assumed epicenters and empirical site corrections and a distance weighting function are used empirical contour values for bounding the epicenter location and empirical bounds for m estimated from m i appropriate for different levels of confidence and different quantities of intensity observations are tabulated the epicentral region bounds and m i obtained for an independent test set of western california earthquakes are consistent with the instrumental epicenters and moment magnitudes of these earthquakes the analysis strategy is particularly appropriate for the evaluation of pre 1900 earthquakes for which the only available data are a sparse set of intensity observations

crustal structure beneath taiwan using frequency band inversion of receiver function waveforms receiver function analysis is used to determine local crustal structure beneath taiwan we have performed preliminary data processing and polarization analysis for the selection of stations and events and to increase overall data quality receiver function analysis is then applied to data from the taiwan seismic network to obtain radial and transverse receiver functions due to the limited azimuthal coverage only the radial receiver functions are analyzed in terms of horizontally layered crustal structure for each station in order to improve convergence of the receiver function inversion frequency band inversion fbi is implemented in which an iterative inversion procedure with sequentially higher low pass corner frequencies is used to stabilize the waveform inversion frequency band inversion is applied to receiver functions at six stations of the taiwan seismic network initial 20 layer crustal models are inverted for using prior tomographic results for the initial models the resulting 20 1ayer models are then simplified to 4 to 5 layer models and input into an alternating depth and velocity frequency band inversion for the six stations investigated the resulting simplified models provide an average estimate of 38 km for the moho thickness surrounding the central range of taiwan also the individual station estimates compare well with the recent tomographic model of and the refraction results of rau and wu 1995 and the refraction results of ma and song 1997

a model for the termination of the ryukyu subduction zone against taiwan a junction of collision subduction separation and subduction boundaries 1 the nw moving philippine sea plate psp collides with the eurasian plate eup in the vicinity of taiwan and at the same time it subducts toward the north along sw ryukyu the ryukyu subduction zone terminates against eastern taiwan while the ryukyu trench is a linear bathymetric low about 100 km east of taiwan closer to taiwan it cannot be clearly identified bathymetrically owing to the deformation related to the collision making the location of the intersection of the ryukyu with taiwan difficult to decipher we propose a model for this complex of boundaries on the basis of seismicity and 3 d velocity structures in this model the intersection is placed at the latitude of about 23 7 n placing the northern part of the coastal range on eup as psp gets deeper along the subduction zone it collides with eup on the taiwan side only where they are in direct contact thus the eurasian plate on the taiwan side is being pushed and compressed by the nw moving philippine sea plate at increasing depth toward the north offshore of northeastern taiwan the wedge shaped eup on top of the ryukyu subducting plate is connected to the eup on the ryukyu side and coupled to the nw moving psp by friction at the plate interface the two sides of the eup above the western end of the subduction zone are not subjected to the same forces and a difference in motions can be expected the deformation of taiwan as revealed by continuous gps measurements geodetic movement along the east coast of taiwan and the formation of the hoping basin can be understood in terms of the proposed model

pockmarks and the recent geology of the central section of the norwegian trench a small area of 3 1 km2 in the norwegian trench has been selected for a detailed analysis of possible relationships between the recent geology and the formation of pockmarks two sets of side scan sonar and deep towed boomer dts data were used for the analysis the most recent deposit is unit 1 consisting of finely layered soft silty clays unit 1 has been divided into three seismic subunits 1a 1b and 1c based upon the boomer records the underlying unit 2 is seismically homogeneous consisting of stiff silty sandy clay with fragments of chalk most of the irregularities in the surface of unit 2 are found to be furrows traversing the study area in a n s general direction the seabed is heavily pockmarked mean density 30 km2 and some of the pockmarks are asymmetric with steep ese walls the maximum pockmark depth within the study area is 5 8 m and the maximum wall angle is about 12 in addition to describing the recent geology and surface pockmarks the following features are described and their relationship with pockmark formation discussed 1 sloping reflectors found below some of the pockmarks 2 columnar disturbances seen under many of the buried pockmarks and 3 masked reflectors intermittent strong reflectors found within subunit 1a which are probably caused by a raised gas concentration in porous layers pockmarks and associated features are assumed to be caused by gas escaping through the seabed a possible gas migration mechanism through the interface between units 1 and 2 and through unit 1 is also discussed

differences in attenuation among the stable continental regions 1 there are systematic differences in the attenuation of damaging earthquake ground motions between different stable continental regions scrs seismic intensity and weak motion data show that the attenuation in seismic waves for eastern north america ena is less than for india africa australia and northwest europe if ena ground motion attenuation relations are used in seismic hazard models for other scrs as is commonly done then the estimated ground motions and resulting hazard may be too large if an attenuation model that averages observations from ena and the other scrs is used to estimate the magnitudes of large historical earthquakes in ena as is the case for recent estimates of m for the 1811 1812 new madrid missouri and the 1886 charleston south carolina events then the magnitude estimates for these events will be too large as will be the resulting hazard

on the effect of synthetic and real data properties on seismic intensity prediction equations the present investigation focuses on the effect of input data properties on the estimation of seismic intensity prediction equation ipe coefficients emphasis is placed on small to moderate magnitude earthquakes synthetic intensity data points idps are created using a given ipe assuming independence of azimuth extensive simulations are performed for single earthquakes and a synthetic database tests of single earthquakes show that increasing the sample size narrows the range of obtained coefficients the larger the difference between the shortest and longest distance of idps from the epicentre the narrower is this range a short radius of perceptibility is more rapidly saturated with new data points than a long one the synthetic database is used to examine the effect of magnitude and depth errors the performance of synthetic data gives a model with which the real data can be compared the attenuation coefficient appears stable against magnitude errors of 0 2 units but starts to be overestimated as magnitude errors increase assuming an erroneous regional depth easily leads to intensity differences of 1 degree the mean coefficient values deviate from the correct ones and tend to increase with depth the results resemble the synthetic ones but imply larger uncertainties the attenuation coefficient appears to be the least sensitive coefficient to errors real data from seven post 1965 earthquakes in the magnitude range of 4 0 5 2 were retrieved from the intensity database of the united kingdom

three dimensional p velocity structures of the lithosphere beneath taiwan from the analysis of taiger and related seismic data sets 1 the convergence of the philippine sea and the eurasian plates in the taiwan region led to the formation of a young collisional mountain between two subduction zones of nearly orthogonal polarities the geological processes underlying the collision and its relation to subduction are the primary targets of taiger taiwan integrated geodynamic research project newly acquired passive and active sources data on land supplemented by ocean bottom as well as permanent seismic network data are used to derive a new 3 d tomographic velocity model using the 7 5 km sec contour as a marker for crustal deformation two trend parallel roots one under the central range with a maximum depth of 55 km and the other under the coastal range at 40 km are found to extend from southern to central 24 n taiwan between the two roots and lying approximately beneath the longitudinal valley the 7 5 km sec contour rises to 25 km depth in the upper mantle a high velocity zone east dipping and its upper surface coinciding with a wadati benioff zone in the south 22 8 n becomes near vertical and ill defined in the north 23 9 n the crustal deformation as defined by the roots and the along trend variations of the upper mantle structures under taiwan provide key information for the orogeny with the thickening of crust to 55 km or more processes such as eclogitization and delamination may come into play

the attenuation of seismic intensity in italy a bilinear shape indicates the dominance of deep phases at epicentral distances longer than 45 km the attenuation of seismic intensity with distance in italy is analyzed by using felt intensity report data obtained from two comprehensive historical databases recently made available the observed attenuation pattern that in the past was interpreted as a logarithmic or root square or cubic attenuation law shows quite clearly two different linear trends in the near and in the far field at distances shorter than 45 km the decrease of the intensity with distance is about one degree per 20 km while at longer distances the slope is about one degree per 50 km this is in agreement with some recent findings of realistic modeling of seismic ground motion that has been explained as the transition from upper crust direct sg phases to waves reflected at the moho controlling the energy main release the slope of the curve in the far field shows a regional dependence in agreement with recent works on the attenuation of pn and sn phases in italy if effective this correlation might allow us to discriminate the contribution of crustal and subcrustal paths in seismic intensity attenuation studies manuscript received 31 may 2000

thermal modeling of continental subduction and exhumation constrained by heat flow and seismicity in taiwan abstract the thermal evolution of crustal exhumation subsequent to the subduction of the continental margin is modeled through numerical solutions to the two dimensional heat conduction equation the boundary conditions used in the modeling are basically constrained by available geophysical and geological observations in the taiwan area temperature distributions are calculated at one million year intervals over a 5 ma duration although this study presents a tentative estimation based on some reasonable assumptions the preliminary results not only explain the sharp change in seismicity within the crust but also agree with the general heat flow pattern on the surface of taiwan more specifically both the high heat flow and the aseismic belt beneath the eastern central range which are probably attributed to higher temperatures within the crust are modeled by crustal exhumation after the subduction of the continental margin however the occurrence of deeper earthquakes east of the central range is a result of the lower thermal gradient which is due to the subduction of the cold continental crust into the uppermost mantle in summary preliminary thermal modeling supports the possibility of a tectonic model of active continental subduction and crustal exhumation for the taiwan orogen

u s geological survey deep seismic reflection profile across the gulf of maine deep seismic reflection and magnetic data suggest that the gulf of maine is underlain by four crustal blocks of differing reflection and magnetic character two of these blocks the gulf of maine fault zone and adjacent central plutonic zone can be correlated with avalonian rocks in southern new england and new brunswick the boundary between them the fundy fault projects onshore near the ponkapoag fault in southeastern massachusetts in a region where no major crustal boundary has yet been identified a third block called the southern plutonic zone is interpreted as avalonian although the reflection and magnetic data are ambiguous the fourth block along the rifted continental margin is correlated with meguma rocks of nova scotia the late paleozoic variscan front crosses the gulf of maine within the gulf of maine fault zone and indicates significant compressional movement rather than strike slip the moho surface throughout the region is essentially flat and may have been produced by mesozoic crustal extension associated modification of the lower crust was minimal in the northern gulf and may have been moderate in the central and southern gulf the franklin rift basin formed by reactivation of the inferred avalon meguma boundary beneath georges bank as a low angle detachment

a preliminary study of crustal structure in taiwan region using receiver function analysis summary selected teleseismic data observed at temporary and permanent broad band stations have been analysed using the receiver function method in order to investigate the very complex crustal structure in taiwan region very significant azimuthal variations of radial and transverse receiver function responses from broad band stations could be attributed to among other things the sampling of incoming seismic waves across the nearby subduction zone a subsurface dipping interface or a localized anisotropic region a mid crust discontinuity interpreted as the conrad discontinuity can be identified at 18 20 km depth beneath tato and tpub stations in the western foothills but is absent beneath the two nearby stations sslb and tdcb in the central mountain range the separation of upper and lower crust beneath the western foothills and the steady increase in crustal velocity as a function of depth across the entire thicker crust beneath the central mountain range suggest that the tectonic evolution of the crust may be significantly different for these two adjacent regions although a thin skinned model may be associated with the tectonic evolution of the upper crust of the western foothills and western coastal plain a thick skinned or lithospheric deformation model can probably be applied to explain the crustal evolution of the central mountain range a trend of crustal thinning from east 50 52 km to west 28 32 km is in very good agreement with the results from two east west trending deep seismic profiles obtained using airgun sources the thinner crust 20 30 km beneath twb1 station in northeastern taiwan can be associated with the high heat flow backarc opening at the western terminus of the okinawa trough behind the subduction of the philippine sea plate the relatively simple crustal structure beneath kmnb station offshore southeastern china depicts typical continental crust with the moho depth at 28 32 km an apparent offset of the thickest moho beneath nacb station from the topographic high in the central central mountain range suggests that the taiwan orogeny has probably not reached its isostatic status

improved seismic tomography offshore northeastern taiwan implications for subduction and collision processes between taiwan and the southernmost ryukyu summary to improve the resolution in imaging the 3 d vp and vp vs structures in the taiwan region especially offshore eastern taiwan we combine the arrival times from eleven ocean bottom seismometers obss and from seismic stations of the japan meteorological agency jma with those from the permanent stations of taiwan central weather bureau seismic network cwbsn and taiwan strong motion instrumentation program tsmip by doing so we have obtained a new 3 d tomographic model for taiwan and its surrounding regions with a better resolution especially in the area offshore northeastern taiwan we also used this new tomography model to relocate the hypocentres of the earthquakes in northeastern taiwan and determined the focal mechanisms of relatively large events our tomography results indicate that in the region northeast of taiwan the subducting oceanic philippine sea plate beneath the eurasian continental lithosphere is characterized by a high vp layer surrounded by lower vp areas this p wave velocity characteristics of the subducting slab provides a better constraint on the geometry of the subduction interface especially in its shallower portion in the hangingwall eurasian block above the subduction interface a vertically elongated high vp vs body appears to originate from the interface at depths between 100 and 140 km we suggest that it represents the partially melted materials that are related to the magmatic activity of the ryukyu volcanic arc

three dimensional qp and qs tomography beneath taiwan orogenic belt implications for tectonic and thermal structure summary we determined the 3 d qp and qs structure of the taiwan orogenic belt to enhance understanding of the related tectonic and thermal structure beneath the collision zone the inversion used t values measured from the spectra of p and s waves from the dense taiwan strong motion network for moderate size earthquakes ml 4 5 5 5 to avoid source complexity the time period 1991 2007 includes the aftershock sequence of the 1999 chi chi earthquake that provides good ray coverage in central taiwan over 18 000 velocity spectra from 883 earthquakes were analysed a non linear least square technique is applied to the spectra for t determination by assuming a 2 source model for the frequency band of 1 30 hz a frequency independent q was assumed in this study the corner frequency of a specific event was fixed for the corresponding stations and a quality index was defined to assure good quality data for the inversion the results reveal the sharp variation of qp and qs across the recently ruptured chelungpu fault and the kaoping and chaochou faults in pingtung plain the q values in the hangingwall are smaller by about 85 and 110 for qp and qs respectively relative to the footwall the fault geometry is distinctly delineated by the contour of qp qs of 1 2 that extends to the depth of the geologically identified decollement structure beneath the central range the low qp low qs and high qp qs features coincide well with the aseismic zone comparison to the recent thermomechanical numerical models of taiwan shows that the low q zone corresponds to the exhumation of the lower crust the low qs regime high attenuation beneath the central ranges at the depth of 5 22 km coincides with predicted temperatures of 400 600 c the qs comparison with the major tectonic and thermal mechanical models of taiwan reveals that the shear wave attenuation model contains comprehensive rheological and thermal information of relevance to understanding mountain building processes this technique appears particularly useful for distinguishing strong and weak crustal regions in the absence of other constraints

estimates of site seismicity rates using ill defined macroseismic data a new approach to the problem of site seismic hazard analysis is proposed based on intensity data affected by uncertainties this approach takes into account the ordinal and discrete character of intensities trying to avoid misleading results due to the assumption that intensity can be treated as a real number continuous distribution estimators attenuation relationships etc the proposed formulation is based on the use of a distribution function describing for each earthquake the probability that site seismic effects can be described by each possible intensity value in order to obtain site hazard estimates where local data are lacking the dependence of this distribution function with the distance from the macroseismic epicenter and with epicentral intensity is examined a methodology has been developed for the purpose of combining such probabilities and estimating site seismicity rates which takes into account the effect of uncertainties involved in this kind of analysis an application of this approach is described and discussed

seismotectonics of taiwan abstract high quality seismicity data and focal mechanism solutions obtained during 1973 1983 by the permanent taiwan telemetered seismographic network and several temporary local seismographic networks are used for a detailed study of the seismotectonics of the taiwan area seismicity distribution in southern taiwan clearly reveals an east dipping benioff zone which has a thickness of about 30 km and begins to deepen along 121 e at a dip angle of 55 60 the leading edge of this benioff zone reaches a depth of about 180 km between 21 n and 22 n but tapers off to a shallower depth of about 100 km from 22 n to 23 n the presence of this seismic zone implies that subduction of the south china sea plate under the philippine sea plate extends from luzon northward to about 23 n the position of the northern boundary of the south china sea plate as tentatively defined according to the seismicity distribution passes through southern taiwan from the offshore area in the taiwan strait west of kaohsiung in an east northeast direction to the taitung area where a triple junction probably lies seismicity is found to disappear abruptly below a certain depth in many parts of taiwan this phenomenon may be attributed to the frictional to quasiplastic transition in the crust or upper mantle comparison of shallow seismicity with surface faults and fractures shows that all areas of active shallow seismicity are marked by densely developed faults and fractures however the converse is not necessarily true this may be partly due to the relatively short duration of seismicity data and partly due to excessive weakening of some of the severely faulted and fractured areas finally focal mechanism solutions for west central taiwan and the kuangfu fuli area in eastern taiwan predominantly show a maximum horizontal compression in the se nw direction which can be related to collision between the eurasian and philippine sea plates however focal mechanism solutions for both the hualien area in eastern taiwan and the tainan area in southwestern taiwan show remarkable irregularities which may result from local tectonic complexities

seismic tomography of taiwan improved constraints from a dense network of strong motion stations in this study a large collection of 41 141 s p times from the untapped records of the taiwan strong motion instrumentation program tsmip network is combined with the p and s wave arrival times from the taiwan central weather bureau seismic network cwbsn to image the v p and v p v s structures beneath taiwan the records from the 680 tsmip stations throughout taiwan in the past 15 years enhance the path coverage and the resolution in the tomography inversions tremendously our result for the v p structure largely confirms previous studies but brings better constraint on the v p v s structure the colliding luzon volcanic arc is characterized by a belt of high v p and high v p v s with high seismicity that includes the offshore islands of lutao and lanyu and the coastal ranges in eastern taiwan at the depth between about 13 and 25 km this high v p v s belt can be traced to the subduction zone in the region between hualian and ilan in the deeper portion the shallow portions of the southwestern coastal plain and the pingtung region are also characterized by a belt of high v p v s with lower seismicity most of the events occurred at the base of the high v p v s zones we suggest that material strength in those regions may be too low to accumulate stress which may indicate water saturated young sediments finally the central range region is characterized by a low v p v s belt