a new map of the geographic extent of the socorro mid crustal magma body for 35 years strong reflected phases from the sill like mid crustal socorro magma body smb have been observed on microearthquake seismograms recorded in the central rio grande rift new mexico in 1979 the lateral extent of the smb was estimated at 1700 km2 by mapping reflection points for 220 observed szs arrivals we have remapped the magma body using 697 pzp 2169 szp and 2589 szs reflections observed on seismograms recorded between 1975 and 1995 by new mexico tech seismic networks these data indicate that the smb covers an area greater than 3400 km2 by comparing the distribution of observed reflection points with the distribution of all possible reflection points limits can be set for the northern and southern boundaries of the smb the 80 km north south extent of the smb falls just inside a 100 km north south range of enhanced seismicity near socorro this area the socorro seismic anomaly ssa covers 5000 km2 and has the same elliptical shape as the outline of the smb based on observed reflection points the close spatial relation between the ssa and the mid crustal magma body along their northern and southern boundaries suggests that the observed seismicity can be used to place limits on the eastern and western boundaries of the magma body using the distribution of earthquakes within the ssa over the last 34 years the maximum east west extent of the smb is 60 km our reflection based value is 50 km our data indicates that the upper surface of the magma body displays no regional dip considering timing errors maximum possible relief on the magma body surface is 0 50 km

incorporating socorro magma body reflections into the earthquake location process when network station spacing is large relative to event depths constraining focal depths using direct arrivals is difficult to help overcome this problem we incorporated the szs szp and pzp socorro magma body reflections into the earthquake location process to find a velocity model appropriate for combining direct and reflected arrivals we jointly inverted for 75 hypocenters and a one dimensional velocity model we used over 1000 direct arrivals and over 400 reflected arrivals solving for p velocity vp and poisson s ratio v above the approximate base of the seismogenic zone vp and v below the base of the seismogenic zone depth to the upper surface of the magma body and station corrections high velocity and low velocity initial models converged to the same solutions without damping we found that vp and v may both decrease slightly below about 10 km depth magma body depth is near 18 75 km a close fit between observed and modeled reflected phase arrival times and an even lateral distribution of positive and negative reflected phase residuals indicate that the magma body upper surface is nearly flat in the socorro area combining direct and reflected phases reduces average focal depth error by more than a factor of 3 and average origin time error by a factor of 2 compared with using only direct phases this is because reflected arrivals are more sensitive to changes in focal depth than direct arrivals and combining upgoing direct and downgoing reflected times reduces the trade off between focal depth and origin time for this method to be successful phases must be carefully identified to help identify secondary phases we used relative arrival time curves plotting phase minus p times versus s minus p times picking reflection pairs szp and szs szp and pzp or szs and pzp identified on a single seismogram significantly reduces the chances of phase misidentification the szp szs and szp pzp pairs also help prevent trade offs between reflector depth and focal depths

comparison of vibroseis and explosive source methods for deep crustal seismic reflection profiling in the basin and range province direct comparison of low fold high energy explosive and high fold lower energy vibroseis methods for acquiring deep crustal seismic reflection data in the basin and range province suggests that the high fold common midpoint cmp method there does not provide the best possible image of lower crustal structure during the recent acquisition of a vibroseis profile in the basin and range province we fired single deep shot holes to obtain a coincident single fold explosive section within the upper crust upper 3 s the explosive source and vibroseis records are nearly equivalent for record times below 3 s however comparison of the explosive source gathers and the coincident final 60 fold vibroseis section demonstrates that low fold explosive profiling provides a higher quality image of the midcrust to lower crust 3 10 s the higher record quality of the explosive sources results primarily from the larger seismic energy levels produced by the explosives making them less sensitive to common noise sources whereas deeper than 4 5 s the vibroseis energy levels on individual source efforts fall to that of ambient noise levels the explosions provide signal generated energy exceeding ambient noise levels down to 18 19 s although individual reflections can be correlated on explosion and vibroseis shot gathers reflection events on the 60 fold vibroseis stack do not correlate to those on the single fold explosion profile suggesting that the high fold cmp method in our study did not maintain the integrity of the weak lower crustal reflected arrivals reasons why the high fold cmp method apparently failed include complex even time varying statics nonhyperbolic moveout at long offsets and the difficulty in resolving stacking velocities with data having low signal to noise ratios reflections on the explosion section are longer and imply a greater degree of layering than one would infer from the lower energy vibroseis section

evolving geographic patterns of cenozoic magmatism in the north american cordillera the temporal and spatial association of magmatism and metamorphic core complexes four maps are presented here that show the location and extent of magmatic fields between eastern alaska and northern mexico during the successive time intervals of 55 40 40 25 25 10 and 10 0 ma and four others show the distribution of metamorphic core complexes during the same cenozoic time intervals the maps are based on u s geological survey and canadian cordilleran data bases contining about 6000 isotopic dates and extensive literature review for nearly 60 ma the development of metamorphic core complexes has coincided with the locus of a really extensive and voluminous intermediate composition magmatic fields the association is suggestive of a close link between magmatism and core complex formation namely that magma directly and indirectly lowers the strength of the crust magmatism thus controls the location and timing of core complex formation the stresses responsible may be inherited from mesozoic crustal thickening locally created by uplift and magmatic thickening of the crust and imposed by the global pattern of plate motions and driving forces since the miocene rates of magmatism extension and core complex formation have declined the modern basin and range province is not a suitable model for the situation that existed during major magmatic culminations the singular event of early miocene time the merging of two large magmatic fields extinguishing the laramide magmatic gap explains several disconnected observations the hyperextension episode of the colorado river corridor rapid reorientation of stress patterns across much of western north america and subsequent rapid tectonic movements in california magma triggered breakup of western north america lithosphere coincided with development of the san andreas transform system thermal destruction of the laramide magmatic gap created a california microplate about 22 ma ago that moved rapidly away from north america thus two plate tectonic processes thermal destruction of the lithosphere bridge and northward growth of a transform system interacted to produce miocene and later tectonic patterns and events

deep earthquakes beneath central taiwan mantle shearing in an arc continent collision the island of taiwan owes its existence to the collision of an island arc on the philippine sea plate with the eurasian continental shelf most of the earthquakes in the mantle beneath taiwan clearly are related to one of these plates subducting beneath the other but beneath the central part of the island there is a zone of subcrustal seismicity that is not obviously related to any subduction zone we investigated the uncertainties in the locations of these anomalous earthquakes by rereading p and s wave arrival times and computing information density functions for several of these events the seismic zone itself is then defined through a joint probability function incorporating the entire data set individual information density functions show that several events are likely to be located at depths exceeding 60 km with two events exceeding 80 km depth the joint probability function shows a narrow zone of seismicity about 20 km wide along strike plunging to the east combining these results with evidence from surface geology focal mechanisms and subsurface tomography we suggest that these events are caused by an eastward displacement of the eurasian mantle north of 24 n which juxtaposes it against colder subducted crust to the south moreover the heat produced by this type of shearing in the mantle must be insignificant for brittle failure to exist at these depths

stress sensitivity of fault seismicity a comparison between limited offset oblique and major strike slip faults we present a new three dimensional inventory of the southern san francisco bay area faults and use it to calculate stress applied principally by the 1989 m 7 1 loma prieta earthquake and to compare fault seismicity rates before and after 1989 the major high angle right lateral faults exhibit a different response to the stress change than do minor oblique right lateral thrust faults seismicity on oblique slip faults in the southern santa clara valley thrust belt increased where the faults were unclamped the strong dependence of seismicity change on normal stress change implies a high coefficient of static friction in contrast we observe that faults with significant offset 50 100 km behave differently microseismicity on the hayward fault diminished where right lateral shear stress was reduced and where it was unclamped by the loma prieta earthquake we observe a similar response on the san andreas fault zone in southern california after the landers earthquake sequence additionally the offshore san gregorio fault shows a seismicity rate increase where right lateral oblique shear stress was increased by the loma prieta earthquake despite also being clamped by it these responses are consistent with either a low coefficient of static friction or high pore fluid pressures within the fault zones we can explain the different behavior of the two styles of faults if those with large cumulative offset become impermeable through gouge buildup coseismically pressurized pore fluids could be trapped and negate imposed normal stress changes whereas in more limited offset faults fluids could rapidly escape the difference in behavior between minor and major faults may explain why frictional failure criteria that apply intermediate coefficients of static friction can be effective in describing the broad distributions of aftershocks that follow large earthquakes since many of these events occur both inside and outside major fault zones

the 1994 northridge earthquake 3 d crustal structure in the rupture zone and its relation to the aftershock locations and mechanisms a detailed 3 d p wave velocity structure of the crust in the epicentral area of the 17 january 1994 northridge earthquake is determined by using 104 709 arrival times from 1673 northridge aftershocks and 2948 other local earthquakes a test performed using the data from the nearby portable stations suggests that the aftershock hypocenters relocated with the obtained 3 d model are accurate to about 2 km we found that regions with high aftershock activity are generally associated with faster p wave velocities the velocity is high around the main south dipping fault of the 1994 northridge earthquake and the north dipping fault of the 1971 san fernando earthquake a linear distribution of strike slip aftershocks was found along a ne sw boundary between high velocity and low velocity structures to the west of this boundary a cluster of large shallow aftershocks with mixed mechanisms occurred in or near the border of a low velocity area while to the east aftershocks with thrust mechanisms occurred in a high velocity area these observations suggest that lateral variations of crustal properties are closely related to the fault segmentation in the transverse ranges a better understanding of these features is important for long term seismic hazard assessment in the los angeles area

crustal extension and magmatic processes cocorp profiles from death valley and the rio grande rift new crustal scale information on the interaction between normal faulting and magmatic activity is provided by recent cocorp deep seismic reflection profiles in death valley california and by reprocessing of the cocorp data from the socorro area of the rio grande rift new mexico the most striking feature on the seismic sections from these areas is a prominent subhorizontal series of reflectors at mid crustal depth previous studies have suggested that thin tabular magma bodies lie within the mid crustal reflective zones in addition because they are traced without apparent offset beneath faults both mapped at the surface and interpreted from the cocorp data these mid crustal horizons are here inferred to be detachments or zones of tectonic decoupling upper crustal cenozoic faults do not appear to penetrate deeper than 15 km in death valley and 13 km in the rio grande rift in death valley these faults are relatively planar with moderate dips 20 to 35 and appear to bound large basement rocks one such zone of normal faults can be traced from the magma body inferred at 15 km depth beneath central death valley to the surface location of a 690 000 yr old basaltic cinder cone listric and low to moderate angle normal faults are evident on the reprocessed new mexico data and constitute the structural component of upper crustal extension in particular a listric master fault traceable to depths as great as 13 km is inferred to underlie the albuquerque basin unlike the death valley data no faults are observed to merge with the socorro magma body per se rather subhorizontal to moderately west dipping packages of reflections are imaged between the base of the faulted upper crust 13 km depth and the mid crustal magma body about 20 km depth the middle crust marks a major rheological boundary between the faulted upper crust and a ductile lower crust extending by penetrative flow and intrusion events seen in the middle and lower crust are generally subhorizontal and prominent layering is observed a band of reflections attributed to the crust mantle boundary is evident on most seismic sections the upper mantle appears seismically transparent on some of the profiles the events attributed to the base of the crust are the deepest in a series of strong and continuous reflections at least one of which is a layer of magma this association supports the suggestion that magmatic intrusions are a probable cause for the high reflectivity observed in the deep crust of many extensional terranes

basin inversion in central taiwan and its importance for seismic hazard proyectos intramurales 2006 301010 ministerio de ciencia e innovacion cgl2009 11843 bte and the csic predoctoral program junta para la ampliacion de estudios jae predoc

reflection polarity of the midcrustal surrency bright spot beneath southeastern georgia testing the fluid hypothesis a small reflection seismic experiment with an explosive source was conducted in southeastern georgia to determine the polarity of an unusually strong midcrustal reflector the surrency bright spot sbs which was found at a depth of approximately 16 km during earlier cocorp profiling in the region in addition to being very bright the sbs is notable for being unusually flat and horizontal for about half of its 4 km length as these characteristics are similar to those of fluid caused reflections at shallow depths it has been suggested that the sbs may be caused by in situ midcrustal fluids if caused by fluid enclosed in fracture porosity in solid rock the reflection would be expected to exhibit a negative polarity from the top of the porous zone the vibroseis data however gave ambiguous results with regard to polarity due to the limited bandwidth and inherent uncertainties about the phase of the source signal the new experiment consisted of four dynamite shots each recorded at three receiver stations by seismic group recorders sgr borrowed from amoco production company comparison of the dynamite records with geophone polarity tests indicate that the sbs is characterized by a positive reflection coefficient at its top this result itself does not negate the fluid hypothesis a fluid fluid interface could cause the positive reflection as well as the flat spot nature of the reflector waveform modeling shows that the sbs is caused by two or more thinly spaced reflectors a relatively high impedance layer about 120 m thick provided a good match to the observed dynamite data but requires a lower boundary having a slightly smaller reflection coefficient than the upper boundary 0 7 versus 1 0 a 150 m thick porous zone model also provided a relatively good fit to the observed dynamite data but in this model the polarity test requires that the initial sbs reflection of positive polarity be caused by a fluid fluid interface within the porous zone and that the top of the porous zone be relatively non reflective though these observations do not preclude the fluid hypothesis they certainly make the high impedance model the simpler of the two alternatives presented here the additional constraints imposed by the modeling also suggest that the sbs is more complex than these simple two interface models in either of these cases wavelet tuning contributes in part to the unusually large amplitude of the sbs reflection

regional mapping of the crustal structure in southern california from receiver functions lateral variations of the crustal structure in southern california are determined from receiver function rf studies using data from the southern california seismic network broadband stations and los angeles regional seismic experiment surveys the results include crustal thickness estimates at the stations themselves and where possible cross sections are drawn the large scale moho depth variation pattern generally correlates well with the current status of the mesozoic batholith deep moho of 35 39 km is observed beneath the western peninsula ranges sierra nevada and san bernardino mountains where the batholith is relatively intact and shallow moho of 26 32 km is observed in the mojave desert where the batholith is highly deformed and disrupted high resolution lateral variations of the crustal structure for individual geographic provinces are investigated and distinctive features are identified the crustal structure is strongly heterogeneous beneath the central transverse ranges and deep moho of 36 39 km is locally observed beneath several station groups in the western san gabriel mountains moho is relatively flat and smooth beneath the western mojave desert but gets shallower and complicated to the east anomalous rfs are observed at two stations in the eastern mojave desert where a moho step of 8 10 km is found between the nw and se back azimuthal groups of station dan in the fenner valley asymmetric extension of the salton trough is inferred from the moho geometry depth extension of several major faults such as the san andreas fault and san gabriel fault to the moho is inferred

calculable fluid rock interactions this paper introduces a model of rock deformation anisotropic poro elasticity or ape where the response of fluid saturated rock to changing conditions prior to fracturing can be calculated the driving mechanism for deformation is fluid migration along pressure gradients between neighbouring intergranular microcracks and pores at different orientations to the stress field the parameters that control changes to microcrack geometry also control the splitting birefringence of seismic shear waves so that changes in deformation can be directly monitored by analysing the shear wave splitting which is observed in almost all rocks analysis of shear wave splitting in the earth9s crust and ape modelling show that distributions of mostly intergranular cracks in the crust are always geometrically close to fracturing with the implication that shear wave splitting is sensitive to comparatively minor changes of stress and minor changes of in situ conditions this has important implications for the state of criticality of the rockmass and as a consequence changes in shear wave splitting have been observed before larger earthquakes on those few occasions when suitable source receiver geometry coincides with appropriate seismic activity ape also has implications for monitoring changing conditions in reservoirs during hydrocarbon recovery

the 1995 kobe earthquake seismic image of the source zone and its implications for the rupture nucleation in order to understand what may have triggered the 1995 kobe japan earthquake and how the rupture proceeded after initiation we determined high resolution three dimensional 3 d p and s wave velocity and poisson s ratio structures in the kobe epicentral area and relocated the aftershocks with the obtained 3 d velocity model we used 64 337 p and 49 200 s wave high quality arrival times from 3634 kobe aftershocks and local microearthquakes recorded by both permanent seismic networks and portable stations that were set up following the kobe mainshock significant velocity variations of up to 6 are revealed in the aftershock area we found that areas with high aftershock activity are generally associated with low poisson s ratio which may be the strong and competent parts of the fault zone and were apt to generate aftershocks the kobe mainshock hypocenter is located in a distinctive zone characterized by low p and s wave velocities and a high poisson s ratio this anomaly exists in the depth range of 16 to 21 km and extends 15 to 20 km laterally this anomaly may be due to a fluid filled fractured rock matrix that contributed to the initiation of the kobe earthquake our interpretation has been supported by many pieces of evidence from hydrological geochemical seismological and geophysical investigations conducted at the kobe earthquake region

the opening of the gulf of california near loreto baja california mexico from basin and range extension to transtensional tectonics abstract detailed structural field analyses of faults and fractures have led to the reconstruction of the tectonic evolution of the loreto region baja california mexico during the opening of the gulf of california three main tectonic events have been recognized i formation of large scale radial structures along the future main gulf escarpment here interpreted as associated with emplacement of magmatic bodies at shallow levels during the beginning of extension processes in early middle miocene ii ne sw extension basin and range event active since 15 ma producing nw se normal dip slip faults in the oligo miocene volcaniclastic basement and indicating high extension rates iii transtensional tectonics recorded especially in the plio quaternary sediments and associated with the activation of nw se dextral strike slip faults and n s normal faults the tectonic structures related to the basin and range extension are sealed by the plio quaternary sediments of the loreto basin and by the volcanics of the mencenares volcanic complex a huge volcanic field interfingering with the sedimentary succession of the basin

a lower crustal extension to a midcrustal magma body in the rio grande rift new mexico a prominent and spatially localized phase arriving approximately 4 s after the initial compressional arrival repeatedly observed on seismograms recorded in the central rio grande rift from intermediate and deep focus earthquakes at distances near 90 is identified as a probable compressional to shear wave conversion from a lower crustal root of the socorro midcrustal magma body smb this phase is inconsistent with previously determined crustal models containing a simple sill like midcrustal magma body with a negligible conduit system finite element synthetic seismogram modeling suggests that a partially molten root with a western boundary dipping steeply to the east extends several kilometers from the 19 km deep upper surface of the magma body down into the lower crust of the southwestern portion of the albuquerque belen basin the location and geometry of this feature suggest that the intracrustal intrusion of mantle magmas responsible for the ongoing inflation of the smb and associated seismicity is occurring via an off axis conduit system located beneath the active margin of the albuquerque belen basin

seismic depth imaging of normal faulting in the southern death valley basin motivated by the need to image faults to test cenozoic extension models for the death valley region of the western basin and range province an area of strong lateral velocity variations we examine the geometry of normal faulting in southern death valley by seismic depth imaging we analyze cocorp death valley line 9 to attain an enhanced image of shallow fault structure to 2 5 km depth previous work used standard seismic processing to infer normal faults from bed truncations displacement of horizontal reflectors and diffractions we obtain a detailed velocity model by nonlinear optimization of first arrival times picked from shot gathers examine the unprocessed data for fault reflections and use a kirchhoff prestack depth imaging procedure to handle lateral velocity variations and arbitrary dips properly fault plane reflections reveal the listric true depth geometry of the normal fault at the black mountains range front in southern death valley this is consistent with the concept of low angle ex

effect of anisotropic consolidation stress paths on the undrained shear behavior of reconstituted wenzhou clay abstract naturally deposited clays are generally k0 consolidated in some cases such as embankment construction and foundation pit excavation the loading or unloading will lead to the re consolidation of clays which can be simulated by a simplified consolidation stress path with a certain ratio of vertical to horizontal consolidation stresses in this study a series of triaxial shear tests was conducted on a reconstituted wenzhou clay to investigate the effects of consolidation stress paths on the stress strain pore water pressure strain and effective stress path behaviors of overconsolidated samples based on the test results the shear strength and pore water pressure coefficient at failure are found to grow and reduce linearly with the increase of the ratio of vertical to horizontal consolidation stresses respectively more strain softening and shear dilation are observed for the anisotropically consolidated samples compared to isotropically consolidated ones the slope of failure line is approximately 1 25 for all specimens tested in this study

three dimensional tomography of the 1992 southern california earthquake sequence constraints on dynamic earthquake rupture tomographic inversion of p wave arrival times from aftershocks of 1992 southern california earthquakes is used to produce three dimensional images of subsurface velocity the preliminary 1992 data set augmented by the 1986 m 5 9 north palm springs sequence consists of 6458 high quality events recorded by the permanent regional network providing 76306 raypaths for inversion the target area consisted of a 104 x 104 x 32 km 3 volume divided into 52 x 52 x 10 rectilinear blocks significant velocity perturbations appear to correlate with rupture properties of recent major earthquakes preliminary results indicate that a low velocity anomaly separates the dynamic rupture of the m 6 5 big bear event from the m 7 4 landers main shock a similar low velocity region separates the m 6 1 joshua tree sequence from the landers rupture high velocity anomalies occur at or near nucleation sites of all four recent main shocks north palm springs joshua tree landersbig bear a high velocity anomaly is present along the san andreas fault between 5 and 12 km depth through san gorgonio pass this high velocity area may define an asperity where stress is concentrated and where future large earthquakes may begin

an earthquake instability model based on faults containing high fluid pressure compartments it has been proposed that large strike slip faults such as the san andreas contain water in seal bounded compartments arguments based on heat flow and stress orientation suggest that in most of the compartments the water pressure is so high that the average shear strength of the fault is less than 20 mpa we propose a variation of this basic model in which most of the shear stress on the fault is supported by a small number of compartments where the pore pressure is relatively low as a result the fault gouge in these compartments is compacted and lithified and has a high undisturbed strength when one of these locked regions fails the system made up of the neighboring high and low pressure compartments can become unstable material in the high fluid pressure compartments is initially underconsolidated since the low effective confining pressure has retarded compaction as these compartments are deformed fluid pressure remains nearly unchanged so that they offer little resistance to shear the low pore pressure compartments however are overconsolidated and dilate as they are sheared decompression of the pore fluid in these compartments lowers fluid pressure increasing effective normal stress and shear strength while this effect tends to stabilize the fault it can be shown that this dilatancy hardening can be more than offset by displacement weakening of the fault i e the drop from peak to residual strength if the surrounding rock mass is sufficiently compliant to produce an instability slip will propagate along the fault until the shear fracture runs into a low stress region frictional heating and the accompanying increase in fluid pressure that are suggested to occur during shearing of the fault zone will act as additional destabilizers however significant heating occurs only after a finite amount of slip and therefore is more likely to contribute to the energetics of rupture propagation than to the initiation of the instability

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

strain offsets with monotonous damped oscillations during the 1986 izu oshima volcano eruption the sequential process of the 1986 izu oshima volcano eruption japan was recorded by a volumetric strain meter near the erupted summit the records show occasional strain offsets accompanying long period damped oscillations which we interpret as responses of the volcanic system to perturbations in magmatic pressure the polarities of strain offsets were contractive for all events before the enormous ground deformation of november 21 due presumably to the intensive dike intrusion on the contrary all events after that had expansive polarities the characteristic frequencies of oscillations were 42 6 mhz for the events before changing abruptly to half that after the ground deformation in our model which is a one dimensional magma filled conduit the boundary condition at the top of the conduit changed from a closed to an open end upon a void formation at the top of the magma head due to its descent immediately after the dike intrusion before the magma escape from the reservoir as a dike magma was episodically supplied from the reservoir to the conduit and thus contraction events were generated after the magma escape magma left in the conduit was episodically drained back to the reservoir to generate expansion events this episodic magma supply or drainage was modeled by a prescribed pressure change at the bottom of the conduit we solved the free oscillation problem of the conduit pipe with an elastic sidewall to calculate the resultant volumetric strain outside the pipe physical properties of the magma and the excitation mechanism are inferred from the application of our formulation to the observation the conduit length and fluid acoustic velocity are estimated to be 10 km and 430 m s respectively suggesting that the fluid is composed of a gas liquid mixture if the conduit radius is taken to be 40 m and if the damping of oscillation occurred by viscous dissipation the fluid viscosity is 108 109 pa s a reasonable estimate for the basaltic magma the applied pressure changes at the bottom of the conduit are in a range from 105 to 106 pa with risetimes of the order of 4 to 100 s

seismic reflection investigations of the mount cayley bright spot a midcrustal reflector beneath the coast mountains british columbia an unusually high amplitude midcrustal reflector was discovered by a lithoprobe seismic reflection survey crossing a predominantly plutonic region in southwestern british columbia canada the bright spot was observed adjacent to mount cayley a large quaternary volcanic center associated with the cascadia subduction zone magmatic arc characterization of the bright spot may advance our understanding of either the midcrustal structure of magmatic supply systems or the presence and storage of saline fluids in the midcrust the location and three dimensional geometry of the reflector were determined using travel time inversion the resulting suite of models consistently describe a northwest dipping reflector that lies 12 5 to 13 km beneath the mount cayley volcanic complex the areal extent of the reflective surface is 3 km by 1 km and the structure is less than 1 6 km thick the transition between the body and the country rock must be rapid 200 m and exhibit a large impedance contrast in order to generate reflections of such high frequencies 30 hz and relative amplitudes 8 0 db our preferred interpretation for the reflector is a fossil sill complex associated with the volcanic development of mount cayley however the data do not rule out the possibility that the reflective structure represents melt lenses or saline fluid produced by the dewatering of the subducted slab the mount cayley bright spot lies just below the brittle upper crust as do the midcrustal reflectors detected beneath japanese subduction arc volcanoes this correlation suggests that thermal and rheological properties just below the brittle ductile crustal transition may be favorable for reflector formation

neogene basins of the northern rio grande rift partitioning and asymmetry inherited from laramide and older uplifts abstract three asymmetric neogene basins in the northern rio grande rift of new mexico and colorado the san luis basin the upper arkansas river graben and the blue river graben are tilted toward large flanking normal faults and lie astride the similarly asymmetric late cretaceous early tertiary laramide san juan san luis sawatch and front range gore range uplifts respectively the steep thrust faulted side of each uplift is on the same side as the down rotated side of each of the neogene basins in addition the direction of stratal tilt changes northward across the villa grove accommodation zone from east in the san luis basin to west in the upper arkansas river graben this accommodation zone coincides approximately with the northward change from the east directed san juan san luis uplift to the west directed sawatch uplift these observations supported by seismic reflection studies across the san luis basin and studies of several other superimposed pairs of rift basins and laramide uplifts suggest that the basin bounding normal faults are listric and merge at depth with the older thrusts which are also listric and root into the crust at about 15 16 km the blue river graben is complicated by lack of basin fill and a thrust history along the west side of the gore range that is at least as old as late paleozoic nonetheless the neogene valley is demonstrably tilted west and lies astride an overall west directed thrust system similar to other thrust and basin relationships in the northern rio grande rift

deep seismic reflection evidence for the role of extension in the evolution of continental crust summary deep seismic reflection profiling as well as geologic studies indicate that extensional basins are a common feature of continental crust the wide spread occurrence of extensional basins combined with published models on the effects of extension on lower crustal rocks suggests that extensional processes play an important role in the evolution of continental cmst extension is now recognized to have been the last major tectonic event to affect approximately 50 of the united states this observation and the preservation of extensional features in areas which have experienced subsequent episodes of compression suggests that extensional processes may lead to a strengthening of continental crust in areas of active extension such as the western united states seismic and petrologic data as well as theoretical modeling of heat flow data suggests that the lower crust may be predominantly intrusive igneous material emplaced during extension and that the present mohorovicic discontinuity formed during extension although the interpretation of the various data is somewhat speculative we suggest that the volume of continental material in some areas has doubled as a result of extension thus extension may result in the addition of a significant amount of new material to the continents

the deep structure of south central taiwan illuminated by seismic tomography and earthquake hypocenter data abstract the taiwan mountain belt is generally thought to develop above a through going basal thrust confined to within the sedimentary cover of the eurasian continental margin surface geology magnetotelluric earthquake hypocenter and seismic tomography data suggest however that crustal levels below this basal thrust are also currently being involved in the deformation here we combine seismic tomography and earthquake hypocenter data to investigate the deformation that is taking place at depth beneath south central taiwan in this paper we define the basement as any pre eocene rifting rocks and use a p wave velocity of 5 2 km s as a reference for the interface between these rocks and their sedimentary cover we found that beneath the coastal plain and the western foothills clustering of hypocenters near the basement cover interface suggests that this interface is acting as a detachment this detachment is located below the basal thrust proposed from surface geology for this part of the mountain belt inherited basement faults appear to determine the geometry of this detachment and their inversion in the alishan area result in the development of a basement uplift and a lateral structure in the thrust system above them however across the shuilikeng and the chaochou faults earthquake hypocenters define steeply dipping clusters that extend to greater than 20 km depth above which higher velocity basement rocks are uplifted beneath the hsuehshan and central ranges we interpret these clusters to form a deeply penetrating east dipping ramp that joins westward with the detachment at the basement cover interface it is not possible to define a basal thrust to the east beneath the central range

the structure and kinematics of the central taiwan mountain belt derived from geological and seismicity data 1 the structure of the taiwan mountain belt is thought to be that of an imbricate thrust and fold belt developed in a forward breaking sequence above a shallowly dipping basal detachment in recent years however a growing amount of seismicity data from the internal part of the mountain belt indicates the existence of widespread fault activity in the middle and lower crust suggesting that deeper levels of the crust must be involved in the deformation than predicted by the shallow detachment imbricate thrust belt model to address this issue we present new geological mapping together with earthquake focal mechanism and seismic energy release data from the central part of taiwan we concur with the interpretation that the foreland basin part of the western foothills comprises an imbricate thrust system that is developing as a forward breaking sequence that is structurally and kinematically linked to a basal detachment at between 7 and 10 km depth to the east of the foreland basin however in the hsuehshan and central ranges our data show the presence of two fault systems an earlier inactive thrust system with a well developed cleavage is cut by a system of steeply dipping active faults that penetrate to a depth of 25 to 30 km or more in the hsuehshan range the second fault system is best represented by a structural and kinematic model in which this part of the mountain belt forms a zone of transpression with a structural architecture similar to that of a crustal scale positive flower structure eastward in the central range mesozoic basement rocks are over thrusting strongly folded and cleaved deep water sediments of the first now inactive thrust system the involvement of deep crustal levels and mesozoic basement in the second fault system is suggestive of the reactivation of preexisting basin bounding faults that were located on the eurasian continental margin

preface to the 1999 chi chi taiwan earthquake dedicated issue the 1999 chi chi taiwan earthquake m w 7 6 struck central western taiwan at 1 47 a m local time on 21 september or at utc 17 47 20 september the causal fault was immediately identified to be the mapped active chelungpu thrust fault as it produced a remarkable 100 km long surface break with fault scarps as high as 8 m in places the nucleation point of this long rupture was defined by an epicenter at 120 82 e and 23 85 n with a focal depth of 8 km a first motion solution was reported to be striking n20 e dipping 30 se with an average rake of 85 chang et al 2000 the event was officially named after the nearest town chi chi by the central weather bureau cwb shin 2000 this was the most devastating earthquake to hit taiwan in modern times the chi chi earthquake and its energetic aftershock sequence inflicted a casualty toll of 2435 and an estimated us 4 billion property loss tsai et al this issue this catastrophe was nonetheless fortunate in several aspects the mainshock struck when schools were not in session the loss of student lives would have been unthinkable in view of the total collapse of hundreds of school buildings in emergency response it was fortunate that the cwb completed in 1995 the taiwan rapid earthquake information release system that was capable of electronically releasing pertinent information within a couple minutes of an earthquake occurrence rapid earthquake information greatly facilitated the disaster relief missions and it mitigated further secondary losses scientifically because of the completion of the taiwan strong motion instrumentation program tsmip the occurrence of the chi chi earthquake sequence has resulted in the richest seismological data recovery in the world to date in both quality and quantity these data were open to the world with a data cd of the

tectonic cycles in southern africa abstract the recurrence of major geological events within the 3600 ma stratigraphic record of southern africa suggests a periodicity of320 25ma for at least the last 3200 ma this periodicity is reflected in major episodes of continental volcanism granitoid plutonism orogeny reciprocity of terrestrial and marine dominated sequences platform carbonate sedimentation glaciation and first order changes in sea level these serve to define 11 completed megacycles each introduced by a major rifting event accompanied by listric faulting alluvial sedimentation in restricted fault bound troughs volcanism and a type granitoid plutonism prior to 3200 ma continental rift magmatism occurred subaqueously due to very high early archean sea levels resulting in pillow lavas and a lack of interbedded terrigenous siliciclastics the long term reciprocity of terrestrial and marine dominated sequences records eustatic first order changes in sea level with highstands characterized by generally humid warm continental climates low relief and tropical afforestation on the other hand lowstand climates were hot and arid with high relief and widespread desertification thick marine sequences accumulate in areally extensive epeirogenic basins without significant volcanism or faulting and commonly are divided into an early phase of open shelf sedimentation followed by deposition in a brackish marine foreland basin within openshelf environments an early phase of biochemical precipitation and or siliciclastic reworking along linear shorelines is separated from a later phase involving rapid deposition of immature siliciclastics along lobate shorelines evolution of the foreland basin is related to orogenic activity and the isolation of an epeiric sea which sometimes developed the characters of a barred basin marine dominated sequences display a deterministic stacking of lithofacies important glacial epochs commonly occur midway through first order regression sometimes with lesser events closely following peak transgression potentially they display 320 ma periodicity but with an unexplained mid proterozoic gap glaciation is related to the combined effect of climatic cooling associated with first order regression and depletion of atmospheric co2 by biological uptake associated with platform carbonate development terrestrial volcanism is believed to be the main contributor of atmospheric co2 climatic change is accompanied by cyclical patterns of cratonic weathering and pedogenesis

the alps 2 controls on crustal subduction and ultra high pressure rock exhumation in alpine type orogens building on our previous results we use 2 d upper mantle scale thermomechanical numerical models to explore key controls on the evolution of alpine type orogens and the alps per se focusing on ultra high pressure u hp metamorphic rocks the models show that uhp rocks form and exhume by burial and subsequent buoyant ascent of continental crust in the subduction conduit here we test the sensitivity of the models to surface erosion rate crustal heat production plate convergence divergence rates geometry of the subducting continental margin and strength of the retrocontinent surface erosion affects crustal exhumation but not early buoyant exhumation metamorphic temperatures increase with crustal radioactive heat production maximum burial depth prior to exhumation increases with plate convergence rates but exhumation rates are only weakly dependent on subduction rates onset of absolute plate divergence does not trigger exhumation in these models we conclude that contrasting peak pressures exhumation rates and volumes of u hp crust exhumed in the alps orogen primarily reflect along strike contrasts in the geometry thermal structure and or strength of the subducting microcontinent brianconnais and continental european margin crust the experiments also support the interpretation that the western alps u hp internal crystalline massifs exhumed as composite stacked plumes and that these plumes drove local crustal extension during orogen scale shortening for weak upper plate retrocrusts postexhumation retrothrusting forms a retrowedge overall these results are consistent with predictions using the exhumation number ratio of buoyancy to side traction forces in the conduit which expresses the combined parameter control of the depth volume of crustal subduction and the transition to buoyant exhumation

traffic load induced permanent deformation of road on soft subsoil a method for predicting the traffic load induced settlement of road on soft subsoil with a low embankment is proposed the traffic load induced dynamic stress in subsoil is calculated by the multilayer elastic theory not covered in this paper then the plastic vertical strain in subsoil is calculated by an empirical equation in which constants are related to the physical and mechanical properties of subsoil the method was applied to analyze three different cases in saga japan comparisons of the calculated values with field data indicate that the proposed method can provide a reasonable prediction of traffic load induced permanent settlement of the road on soft subsoil with a low embankment the method is useful for designing the road with a low embankment on soft subsoil for the cases studied with embankment thickness of 0 75 to 2 7 m the depth significantly influenced by traffic load is about 6 m below the base of the embankments

one way cyclic triaxial behavior of saturated clay comparison between constant and variable confining pressure abstractthe one way cyclic triaxial test has been recognized as a useful tool for solving many engineering problems such as the prediction of deformation in subsoils under traffic loading most studies of the one way cyclic behavior of soils are performed in triaxial tests with constant confining pressure ccp which can apply cyclic compressive deviatoric stress only however the real stress field in subsoils under traffic loading includes varying normal stresses in three directions therefore it is more desirable to simulate the in situ loading conditions induced by passing vehicles through a combination of the varying deviatoric stress and varying confining pressure recognizing this two series of one way cyclic triaxial tests on remolded saturated clay were conducted in this study one in undrained conditions and the other in partially drained conditions in each series both tests with the ccp and variable confining pressure vcp were carried out comparisons between the results of the vcp and

fluid mediated influence of adjacent thrusting on the seismic cycle at parkfield the san andreas fault near parkfield california has experienced moderate to large earthquakes about every 20 years since 1881 the most recent of which occurred in 19661 3 statistical recurrence and precursory quiescence models1 4 5 led to a prediction of the next event for 1992 or before but this event has not yet occurred here i present a model6 that attributes the non occurrence of the expected earthquake to the influence of adjacent thrusting on the rate of fault weakening from increasing pore pressures within the fault zone reductions in fault normal strain rates7 9 and the onset of seismic quiescence along the san andreas fault both coincide with adjacent blind thrust faulting in the 1982 85 earthquake sequence of new idria coalinga and kettleman hills these events turned off a compaction induced weakening mechanism and extended the proposed seismic cycle at parkfield a model simulation of this effect shows a strong correlation in space and time of calculated and observed earthquake hypocentres and shows that observed seismicity patterns from 1969 95 can be explained by reduced compaction rates brought on by adjacent thrusting the model may have applications to other linked thrust strike slip fault systems such as are found in southern california and other transpressional regions

deep structure of the northern rio grande rift beneath the san luis basin colorado from a seismic reflection survey implications for rift evolution abstract a seismic reflection survey by chevron across the san luis basin northern rio grande rift and san juan volcanic field of southern colorado is reprocessed with extended correlation to search for basement structure the trace of the main bounding fault of the basin a high angle normal fault against the sangre de cristo range can be correlated to a wide zone of dipping reflection fabric and soles out at lower crustal depths 26 28 km the deeper reflection fabric represent either broad extensional strain or pre existing structure such as a laramide thrust system the sangre de cristo bounding fault in san luis basin does not sole out at mid crustal depths but continues into the lower crust with a shallower dip the basin architecture in the northern rio grande rift san luis basin provides little if any evidence that the sangre de cristo bounding fault should flatten in a shallow listric fashion this fault geometry is quite similar to the high angle bounding fault in the espanola basin but contrasts with less deeply rooted faults in the albuquerque basin in the central rio grande rift deeper soling out of the sangre de cristo bounding fault could be due to less extension in the northern rio grande rift and or greater strength of the lithosphere compared to the central rio grande rift unequivocal moho reflections beneath the san luis basin cannot be identified probably due to limited signal penetration or a gradational nature of the moho the majority of rift related movement observed on the sangre de cristo bounding fault is post eocene either the western margin of the basin is marked by a tight monocline or a low angle normal fault

a synthesis of the geologic evolution of taiwan abstract the island arc of taiwan is composed of cenozoic geosynclinal sediments more than 10 000 m thick lying on a pre tertiary metamorphic basement pleistocene to miocene andesitic islands surround the main island and are related mostly to arc magmatism the penghu island group in the taiwan strait is covered with pleistocene flood basalt neogene shallow marine clastic sediments are exposed mainly in the western foothills with pleistocene andesitic extrusives at the northern tip and the northeastern offshore islands a thick sequence of paleogene to miocene argillitic to slaty metaclastic rocks underlies the western central range and forms the immediate sedimentary cover on the pre tertiary metamorphic complex to the east which represents an older mesozoic arc trench system the coastal range in eastern taiwan is a neogene andesitic magmatic arc including also a large variety of volcaniclastic and turbiditic sediments cenozoic taiwan is the site of arc continent collision where the luzon arc on the philippine sea plate overrides the chinese continental margin on the eurasian plate east and northeast of taiwan the polarity of subduction changes whereby the oceanic philippine sea plate is subducting beneath the ryukyu arc system on the eurasian plate continent arc collision in taiwan island is anomalous and may occur in a broad belt of deformation rather than along a well defined plate boundary or subduction zone

curie point depth estimates from aeromagnetic data from death valley and surrounding regions california aeromagnetic data were analyzed to determine the curie point depth cpd by power density spectral and three dimensional inversion methods within and surrounding death valley in southern california we calculated the cpd for 0 5 regions using 2d power density spectral methods and found that the cpds varied between 8 and 17 km however the 0 5 region may average areas that include shallow and deep cpds and because of this limitation we used the 3d inversion method to determine if this method may provide better resolution of the cpds the final 3d model indicates that the depth to the bottom of the magnetic susceptible bodies varies between 5 and 23 km even though both methods produced roughly similar results the 3d inversion method produced a higher lateral resolution of the cpds the shallowest cpds occur within the central and southern death valley panamint valley coso geothermal field and the tecopa hot springs region deeper 15 km cpds occur over outcropping granitic and precambrian lithologies in the panamint range grapevine mountains black mountains and the argus range the shallowest cpd occurs within the central death valley over a possible seismically imaged magma body and slightly deeper values occur within the panamint valley southern death valley and tecopa hot springs the shallow cpd values suggest that partially molten material may also be found in these latter regions the cpd computed heat flow values for the region suggest that the entire area has high heat flow values 100 mw m 2 on the other hand locally extremely high values 200 mw m 2 occur within the panamint valley the southern and central death valley and tecopa hot springs region these locally high heat flow values may be related to midcrustal magma bodies but additional geophysical experiments are needed to determine if the magma bodies exist

limitations of quantitative analysis of deep crustal seismic reflection data examples from glimpce amplitude preservation in seismic reflection data can be obtained by a relative true amplitude rta processing technique in which the relative strength of reflection amplitudes is preserved vertically as well as horizontally after compensating for amplitude distortion by near surface effects and propagation effects quantitative analysis of relative true amplitudes of the great lakes international multidisciplinary program on crustal evolution seismic data is hampered by large uncertainties in estimates of the water bottom reflection coefficient and the vertical amplitude correction and by inadequate noise suppression processing techniques such as deconvolution f k filtering and migration significantly change the overall shape of amplitude curves and hence calculation of reflection coefficients and average reflectance thus lithological interpretation of deep crustal seismic data based on the absolute value of estimated reflection strength alone is meaningless the relative strength of individual events however is preserved on curves generated at different stages in the processing we suggest that qualitative comparisons of relative strength if used carefully provide a meaningful measure of variations in reflectivity simple theoretical models indicate that peg leg multiples rather than water bottom multiples are the most severe source of noise contamination these multiples are extremely difficult to remove when the water bottom reflection coefficient is large 0 6 a condition that exists beneath parts of lake superior and most of lake huron

mechanics of active magmatic intraplating in the rio grande rift near socorro new mexico 1 we investigate long term deformation due to the socorro magma body smb one of the largest active intrusions in the earth s continental crust using interferometric synthetic aperture radar insar observations and finite element simulations insar data spanning 15 years 1992 2007 indicate that the magma body is associated with a steady crustal uplift at a rate of about 2 mm yr 1 previous work showed that while the pattern of surface uplift is consistent with an elastic inflation of a large sill like magma body the smb could not have formed via steady elastic inflation because the latter would be outpaced by magma solidification we resolve this problem using coupled thermo visco elastic models and place constraints on the intrusion history as well as the rheology of the ambient crustal rocks we demonstrate that observations rule out the linear maxwell response of the ductile crust but are consistent with laboratory derived power law rheologies our preferred model suggests that the age of the smb is of the order of 103 years and that the apparent constancy of the present day uplift may be due to slow heat transfer and ductile deformation in a metamorphic aureole of a giant sill like magma intrusion rather than due to a steady increase in the magma overpressure the smb is a contemporaneous example of magmatic intraplating a process by which large volumes of mafic melt stall and spread at midcrustal depths due to density or rheology contrasts

a pore pressure and stability model for a silty clay under repeated loading the behaviour of saturated clays subjected to low frequency cyclic loading is of considerable importance in the design of offshore structures monotonie strain controlled and low frequency 0 1 hz repeated load triaxial tests have been carried out on reconstituted samples of keuper marl a silty clay under undrained conditions the strain controlled tests were carried out at overconsolidation ratios of 1 4 10 and 20 and the data from these tests were used to establish the stable state boundary surface results from repeated load tests on samples with overconsolidation ratios of 1 and 4 were compared with this boundary surface previous investigations have shown that the dry side of the state boundary surface for clays sometimes known as the hvorslev surface forms an empirically determined stability criterion for cyclic effective stress paths of samples of all stress histories normally consolidated and lightly overconsolidated soils subjected to undrained cyclic loading become heavily overconsolidat

ten second love wave propagation and strong ground motions in taiwan applying the surface wave gaussian beam method to a three dimensional 3 d structure of taiwan we have studied the generation and propagation of short period 10 s surface waves in a region of pronounced crustal heterogeneity especially in a region defined to be at an epicentral distance larger than a few focal depths away where high frequency near field motions are largely attenuated and surface waves are adequately developed by perturbing the source model as well as the crustal model in an iterative inversion process we have achieved an excellent fit to the recent strong motion observations recorded during the tung ao earthquake june 5 1994 ml 6 0 in taiwan this allows us to 1 obtain a refined 3 d crustal model of taiwan from an initial model that was derived from recent tomographic results and more important 2 explain the distribution patterns of the strong shaking reported in terms of intensity maps for historical large earthquakes and predict the long period strong motion distribution patterns for future large earthquakes in taiwan with an improved 3 d structure we have gained better insight into short period surface wave propagations as they are modified by crustal lateral heterogeneity focusing and defocusing are clearly demonstrated of particular interest is the central mountain range a massive tertiary metamorphic body forming the backbone of taiwan that seems to serve as a divergent lens for the propagating surface waves whereas the large sedimentary basin in southwestern taiwan seems to focus the wavefronts instead the computed distortions of the propagating surface wave field due to lateral heterogeneity are correctly reflected by the observations

three dimensional vp vs variations for the coso region california recent seismological studies of the coso region of southeastern california document both low p wave velocities and abnormal sv attenuation in indian wells valley south of the pleistocene volcanics of the coso range in order to learn more about the physical nature of these colocated anomalies a tomographic inversion for the three dimensional variations of vp vs the ratio of compressional to shear velocity was performed iterative back projection of 2966 shear and compressional wave travel time residuals from local earthquakes recorded on vertical instruments reveals that vp vs is generally high at the surface and decreases systematically to 10 km depth superimposed on this trend are several large anomalies near devil s kitchen in the coso geothermal area vp vs values are very low near the surface consistent with measured values for steam dominated geothermal systems abnormally high values of vp vs are observed in portions of indian wells valley and also below the cactus peak rhyolite dome from 2 to 5 km in depth in indian wells valley the co occurrence of low p velocities low s velocities high vp vs and anomalous sv attenuation are indicative of subsurface partial melt because the inversion is based on vertical data only however the results cannot be considered conclusive only suggestive

three dimensional models of p wave velocity and p to s velocity ratio in the southern central andes by simultaneous inversion of local earthquake data the pisco 94 proyecto de investigation sismologica de la cordillera occidental 1994 seismological network of 31 digital broad band and short period three component seismometers was deployed in northern chile between the coastal cordillera and the western cordillera more than 5300 local seismic events were observed in a 100 day period a subset of high quality p and s arrival time data was used to invert simultaneously for hypocenters and velocity structure additional data from two other networks in the region could be included the velocity models show a number of prominent anomalies outlining an extremely thickened crust about 70 km beneath the forearc region an anomalous crustal structure beneath the recent magmatic arc western cordillera characterized by very low velocities and a high velocity slab a region of an increased vp vs ratio has been found directly above the wadati benioff zone which might be caused by hydration processes a zone of lower than average velocities and a high vp vs ratio might correspond to the asthenospheric wedge the upper edge of the wadati benioff zone is sharply defined by intermediate depth hypocenters while evidence for a double seismic zone can hardly be seen crustal events between the precordillera and the western cordillera have been observed for the first time and are mainly located in the vicinity of the salar de atacama down to depths of about 40 km

moho reflectivity and seismic signal penetration abstract analysis of amplitude decay and frequency change is applied to determine limits of seismic signal penetration on deep seismic reflection data collected by lithoprobe and the consortium for continental reflection profiling cocorp changes in spectral frequency content are efficiently monitored with averaged instantaneous frequency and bandwidth results from montana show pronounced increases in average frequency and slight drops in amplitude associated with a die out of reflections at the moho results from quebec show amplitude and frequency change occurring through the moho on lines with and without moho reflections these contrasting observations confirm both adequate signal penetration and that the differences in moho reflectivity are representative of geologic differences

tomographic image of crustal structures across the chelungpu fault is the seismogenic layer structure or depth dependent abstract in 1999 a large earthquake m w 7 6 occurred along the chelungpu fault in the fold and thrust belt of western taiwan to shed more light on the subsurface structures and the seismogenic layers three dimensional velocity structures were inverted by using the travel times of both p and s waves from 2391 aftershocks recorded by the central weather bureau during the 15 months that followed from tomography a typical image of the large scale thrusting structures in the upper crust across the chelungpu fault was obtained in general high velocities beneath the western foothills and central ranges are separated from low velocities beneath the coastal plain by an east dipping boundary that is roughly consistent with the chelungpu fault on the surface the contrast in velocity on either side of the chelungpu fault is indicative of about a 7 to 9 km vertical offset in the upper crust the relocated hypocenter for the chi chi earthquake shifts by 2 2 km toward the northwest and its focal depth decreases by 0 7 km a plot of focal depths versus rock velocities where the aftershocks occurred shows earthquakes are more inclined to occur in rock with a velocity of around 5 6 km s this strongly suggests the seismogenic layer in the fold and thrust belt of taiwan is more structure dependent than depth dependent

composition of the crust in the grenville and appalachian provinces of north america inferred from v p v s ratios we use the ratios between p and s wave velocities vp vs derived from seismic refraction data to infer the composition of the crust in the grenville and the appalachian provinces of north america the crust exhibits vp vs increasing with depth from 1 64 to 1 84 there is a clear distinction between the grenville province average vp vs 1 81 and the appalachian province average vp vs 1 73 which persists at all depths the boundary between these provinces is east dipping extending for 100 km east of the champlain thrust in the appalachian province the increase in vp vs ratios with depth from 1 67 to 1 74 0 02 may reflect a normal decrease of silica content in the continental crust in the grenville province beneath the central granulite terrane an anomalous vp vs ratio of 1 82 0 02 is observed extending to a depth of 10 km this correlates with the abundance of ca plagioclase in the marcy anorthosite at greater depth 15 20 km where seismic lamination and high electrical conductivity is observed vp vs is 1 84 0 02 and correlates with the tahawus complex a layered mafic intrusion within the 25 km thick lower crust of the grenville province the vp vs is 1 84 0 02 and p velocity is 7 0 0 1 km s which are typical for plagioclase bearing rocks gabbro norite the high vp vs ratio in the grenville province has not been reported in crust of any other age since the grenville province contains 75 of the world s known anorthosites high vp vs ratio is related to high plagioclase we suggest that the composition of the grenville lower crust was significantly modified by the emplacement of the anorthosites in the mid proterozoic

application of revised ray tracing migration to imagine lateral variations of seismic fabric corresponding to different tectonic styles in the northern apennines abstract migration of zero offset seismic sections of deep crust can be done with methods based on ray tracing we modify the classical ray tracing migration method rtm introducing a consistency check to control whether back propagated rays satisfy the condition of strict normal incidence at the migrated reflector a synthetic test shows the effectiveness of the method in particular the control of normal incidence allows elimination of physically inconsistent reflectors from the migrated section then rtm is applied to a crustal seismic profile acquired in central italy using a velocity model obtained from wide angle data that reproduces the gross structures of the apenninic crust the lateral variation of the seismic fabric shown from the migrated section reveals the presence of coexisting extensional and compressional tectonic regimes kinematic diffraction modelling gives additional information about both the distribution of seismic velocities and major active geodynamic processes in the upper lithosphere the migrated section supports the subdivision of northern apennines in two tectonic regions a stretched upper plate tuscany and northern thyrrenian supported by a rise of the asthenosphere and a downwarped lower plate adria subducted below the mountain belt

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

continental subduction and three dimensional crustal structure the northern south island new zealand the three dimensional vp and vp vs structure of a region where subduction transitions to oblique transform faulting has been determined using arrival times from 579 local earthquakes recorded during a temporary deployment and 3146 earthquakes have been relocated between 40 km and 100 km depth the subducted plate is imaged as a relatively low velocity feature in the uppermost mantle reflecting the continental nature of the subducted crust in this region an increase in amplitude of this low velocity feature from northeast to southwest can be related to an increase in the thickness of the crust of the subducted plate in this direction velocity variations within the subducted and overlying plates show some spatial correlation this suggests an interaction between the plates which extends well beyond the plate interface and is consistent with other geophysical and geological evidence that the plate interface beneath marlborough is currently not accommodating much active subduction in the overlying plate the awatere fault is a major structural feature associated with a low velocity zone extending to 23 km depth there is a marked change in structure near this fault with seismic velocities being lower to the southeast a relatively high level of seismicity occurs in this region of lower seismic velocities suggesting a relationship between the two a possible explanation for this is elevated pore pressures caused by fluids derived from dehydration of the continental subducted crust the low velocity region in the overlying plate coincides with the region of most intense active deformation suggesting it is relatively weak

centrifuge modelling of a tetrapod jacket foundation under lateral cyclic and monotonic loading in soft soil the tetrapod jacket foundation is one of the most common types of foundations used for offshore structures although the static bearing capacity of the jacket foundation is important a safe design

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

modeling of highly anisotropic crust and application to the altiplano puna volcanic complex of the central andes 1 the altiplano puna volcanic complex apvc is located in the central andes and covers an area over 50 000 km2 the style of volcanism is predominately caldera forming ignimbrite eruptions active between 10 and 1 ma chmielowski et al 1999 used teleseismic events recorded from seven portable broadband seismic stations deployed in the apvc to identify a 1 km thick very low velocity zone at a depth of 19 km based on the correlation with surface volcanics the extremely low shear velocities of 1 km s or less and the depth of the layer the low velocity zone was interpreted to be a magma body using local events recorded during the same seismic deployment we have improved the crustal model for the apvc and detected seismic anisotropy in the crust above the low velocity zone using a hexagonal symmetry anisotropy code we computed synthetic receiver functions for many models of layered anisotropic media and found two different models consistent with the data in both models approximately 20 30 anisotropy is present in a 3 km thick surface low velocity layer and 15 20 anisotropy is present in the remaining crust above the buried low velocity zone the first model has the 1 km thick low velocity zone at 17 km while the alternate model has the low velocity zone at 14 km depth overlain by a 4 km thick transition zone of intermediate velocities this anisotropy may be due to the fracture system by which magma migrates to the surface from the midcrustal sill like magma body

tomographic inversion of local earthquake data from the hengill grensdalur central volcano complex iceland parameters is generally low however a quantitative analysis of resolution provides an unambiguous assessment of well resolved volumes within the better resolved regions of the model the averaging volumes are 1 2 km and 2 4 km in vertical and horizontal extent respectively results of tomographic inversion image three distinct bodies of anomalously high velocity two of these extend from near the surface to a depth of about 3 km these high velocity volumes are located directly beneath the surface expressions of the extinct grensdalur volcano and the extinct husmuli basalt shield the third high velocity structure occurs in the depth range of 3 4 km but does not extend to the surface these three high velocity bodies are interpreted to be solidified magmatic intrusions relatively low velocities underlay limited portions of the trace of the present accretionary axis and a low velocity body is imaged in the roots of the active hengill volcano the volume of lower velocities located beneath the surface expression of the hengill volcano is interpreted to be a region of partial melt

tomography of the source area of the 1995 kobe earthquake evidence for fluids at the hypocenter seismic tomography revealed a low seismic velocity 5 and high poisson s ratio 6 anomaly covering about 300 square kilometers at the hypocenter of the 17 january 1995 magnitude 7 2 kobe earthquake in japan this anomaly may be due to an overpressurized fluid filled fractured rock matrix that contributed to the initiation of the kobe earthquake

the alps 1 a working geodynamic model for burial and exhumation of ultra high pressure rocks in alpine type orogens abstract eocene ultra high pressure u hp rocks exposed in the western alps are generally interpreted to result from subduction of european continental crust beneath adria and its subsequent exhumation however the roles of extension either from plate divergence or internal to the orogen and erosion during exhumation remain controversial here we use 2d numerical geodynamic models to explain the formation and exhumation of u hp rocks in an alpine type orogen and interpret the results in the conceptual prowedge uplifted plug retrowedge conduit purc framework u hp metamorphism of oceanic and microcontinent crust in the models results from burial and accretion to a subduction channel conduit formed beneath an advancing retrocontinent rapid exhumation from u hp conditions is achieved by the buoyancy driven transport of a composite plume of stacked u hp oceanic and microcontinent crust from the subduction conduit to the overlying orogenic prowedge accommodated by coeval thrusting and normal sense shearing subsequent trans crustal exhumation is achieved by a combination of doming internal extension and later retrotransport of the u hp plume through the uplifted plug during underthrusting of the thick continental margin crust coupled with increased erosion our proposed mechanism implies that exhumation related normal sense shearing in the western alps per se was driven from below by the buoyancy of the ascending plume and that extension owing to plate divergence is not required to explain u hp rock exhumation the efficiency of the exhumation mechanism depends strongly on the buoyancy and strength of the u hp plume suggesting that in order to exhume rapidly it must achieve a critical size by implication the multiple small eocene u hp complexes within the internal crystalline massifs may have been exhumed as part of a single composite body comprising diverse units aggregated from different levels of the subduction conduit during burial or ascent rather than as individual small bodies exhumed in separate pulses

kinematic thermal and petrological model of the central alps lepontine metamorphism in the upper crust and eclogitisation of the lower crust abstract seismic and seismological studies as well as gravimetric models indicate that a slab of european lithospheric mantle and lower crust is currently underthrust below the apulian crust we assume a simple kinematic model in which the lower and upper subducted european crusts are decoupled along a decollement the lower crust goes into subduction without deformation the upper crust deforms by pure shear with a horizontal compressional axis the total erosional flux is adjusted to balance upper crust input so that the belt keeps the same geometry different distributions of erosion being used the computed temperature field is steady state if the kinematic model applies during a minimum time of 40 myr for a convergence rate of 8 mm yr equilibrium mineral assemblages and densities are determined from the computed p t conditions for a granodioritic chemical composition of the upper crust and an andesitic composition of the lower crust assuming local isostasy the density model fits the average topographic profile across the central alps the p t t paths obtained for the part of the upper crust initially at depths 10 to 16 km are compatible with the medium pressure oligocene metamorphism in the lepontine dome the peak calculated temperature for the deepest non subducted crustal rocks is 600 c for a pressure of 0 8 gpa near the lower limit of high pressure amphibolites we thus propose that the lepontine metamorphism corresponds to the steady state thermal regime however either faster erosion rates in the internal part of the belt or tectonic denudation are required for exhumation of the deeper portion of the belt the computed temperature field implies eclogitisation of the lower crust at a depth of 55 to 60 km we conclude that the moho limiting the deepest part of the root may correspond to the eclogitisation phase change lower crust eclogites have a density comparable to or higher than that of the mantle depending on their chemical composition 3 37 for andesitic eclogites 3 56 for gabbroic eclogites thus andesitic eclogites may stay in gravitational equilibrium in the mantle below the root whereas gabbroic eclogites are gravitationally unstable and should sink

three dimensional velocity structure seismicity and fault structure in the parkfield region central california this study examines the three dimensional velocity structure in a 60 by 80 km region containing the parkfield segment of the san andreas fault we use local earthquake and shot p arrival times in an iterative simultaneous inversion for velocity and hypocentral parameters using the three dimensional model we relocated 5251 events that occurred from 1969 to 1991 as well as the 1966 aftershocks and computed 664 fault plane solutions the san andreas fault saf characterized by a sharp across fault velocity gradient is the primary feature in the velocity solution there is a 5 20 lateral change in velocity over a 4 km width the contrast being sharper where there is better resolution the model also shows significant variations in the velocity and in the complexity of the velocity patterns along the saf the largest across fault velocity difference is below middle mountain where a large volume of low velocity material impinges on the saf from the northeast this material is inferred to be overpressured and may be key to understanding the unusual behavior in the parkfield preparation zone a 20 km long high velocity slice is imaged northeast of the saf near gold hill its along fault length corresponds to the length of the maximum slip in 1966 the relocated seismicity shows that the san andreas fault is a planar vertical fault zone at seismogenic depths ninety percent of the fault plane solutions that are on or near the saf were right lateral strike slip on subvertical fault planes that parallel the saf thus the surface fault complexities do not appear to extend to depth and therefore do not explain the rupture character at parkfield at parkfield variations in material properties play a key role in fault segmentation and deformation style our observations suggest that there may be a general relation between increasing velocity and increasing ability of the rocks to store strain energy and release it as brittle failure

three dimensional p wave velocity structure of the bear valley region of central california the three dimensional p wave velocity structure of the bear valley region of central california is determined by applying a circular ray tracing technique to 1735 p wave arrivals from 108 locally recorded earthquakes comparison of the results obtained from one dimensional and laterally varying starting models shows that many of the features in the structure determined are fairly insensitive to the choice of the starting model velocities associated with the gabilan granites southwest of the san andreas fault are slightly higher than those in the franciscan formation to the northeast and these two features are separated in the southern part of the region by a narrow fault zone with very low velocities in the southeastern part of the region where the gabilan granites do not abut the san andreas fault the low velocities of the fault zone cross over to the southwestern side of the fault they also appear to extend to depths of at least 15km thus locally reversing the contrast across the san andreas fault that prevails farther to the northwest in the northwestern part of the region the low velocities of the fault zone split and follow the surface traces of the san andreas and calaveras faults but do not appear to extend to depths much deeper than about 6km there also appears to be a well defined contrast in structure in the middle of the santa clara valley suggesting the existence of a fault in the basement of the valley that may be a southern extension of the sargent fault into this region relocated hypocenters beneath the san andreas fault cluster in a zone that dips about 80 southwest and intersects the surface trace of the fault in the southern part of region

stress magnitude strain rate and rheology of extended middle continental crust inferred from quartz grain sizes in the whipple mountains california knowledge of the magnitude of differential stress and strain rate during the formation of mylonitic shear zones in metamorphic core complexes provides constraints on the mechanical behavior of the middle continental crust during extension we analyzed the differential flow stress during the mylonitization of quartzofeldspathic rocks in the whipple mountains california using grain size piezometers and kinetic laws for grain growth mylonitic gneisses collected from two widely separated transects have grain sizes that cluster in a range from 32 to 61 m analysis of grain growth kinetics indicates that mylonitization of the gneisses continued during cooling to temperatures 500 c compatible with estimates from two feldspar thermometry quartz grain size piezometers suggest that the mylonitization occurred under differential stresses 1 3 of 40 150 mpa or maximum shear stresses of 20 75 mpa extrapolation of quartzite flow laws to 500 c indicates that the mylonitization occurred at strain rates faster than 10 14 s 1 these estimates suggest that the mylonitic zone within the whipple mountains had an effective viscosity of the order of 1018 4 1020 4 pa s these low viscosities and rapid strain rates combined with seismic reflection data showing that continental crust is layered suggest that more realistic physical models of extension of the continental lithosphere should treat the lithosphere as a heterogeneous distribution of high viscosity regions separated by low viscosity zones

relation of deformation and multiple intrusion in the death valley extended region california with implications for magma entrapment mechanism the crystalline core of the black mountains crustal section death valley california exposes a tremendous volume of miocene plutonic rock intruded at a depth of 10 13 km few plutons were intruded above 10 km prior to unroofing by tectonic denudation in the eastern part of the core a brittle detachment fault separates predominantly miocene hanging wall strata from the midcrustal miocene 11 6 and 8 7 ma plutonic complex and early proterozoic basement in the west mylonitic lineations and foliations locally well developed within the 11 6 ma intermediate mafic willow spring pluton are cut by dikes which emanate from an 8 7 ma silicic plutonic complex the younger silicic complex throughout the crystalline core exhibits few ductile deformational structures published thermal and barometric studies from both plutonic bodies indicate similar midcrustal 10 13 km emplacement depths at ambient temperatures just above 300 c the significant difference in densities of these magmas argues against a density control for magma entrapment also the country rock above and below the plutonic complex contains no apparent differences from field observations that would suggest a change in density the depth of entrapment corresponds well with the expected depth for the crustal strength maximum determined from laboratory experiments the similar emplacement depths but contrasting styles of deformation of the two plutonic bodies further suggests that entrapment may have been controlled by a high strength barrier represented by the brittle ductile transition late hypabyssal intrusions and associated volcanism are linked to diachronous rapid unroofing of the range block all show a northwest progression paralleling the regional extension direction thus when migration of magma through the high strength barrier did occur it was apparently related to increased strain rates which allowed magma ascent by fracture exploitation rheological stratification of the crust may have played an important if not major role in trapping magmas in the middle crust in this area

geometry of the garlock fault zone based on seismic relection data detailed examination of shot gathers from line 3 of the consortium for continental reflection profiling mojave experiment indicates a low velocity high amplitude seismic event that may originate from a south dipping portion of the garlock fault zone part of this event was previously interpreted as a subhorizontal surface located in the midcrust our analysis indicates however that the distance to this event varies consistently with the distance of the shot and receivers from the fault zone and that the velocities are too low for seismic waves traveling through predominantly crystalline rocks these characteristics are more consistent with the presence of a subvertical reflector located near the garlock fault zone than with the previously inferred subhorizontal reflector beneath the seismic line

3d velocity structure around the source area of the 1999 chi chi taiwan earthquake before and after the mainshock 3d v p and v p v s models for the source area before and after the chi chi earthquake are derived from a tomographic inversion of seismic travel time data from the taiwan central weather bureau seismic network cwbsn resulting 3d velocity models reveal a strong lateral variation in the crust particularly at a depth range of 5 to 25 km along the trend of the dipping chelungpu thrust in general low v p and high v p v s anomalies are found in and around the source volume of the chi chi earthquake a volume centered around the entire 100 km 40 km chelungpu rupture surface we pay special attention to the crustal properties and their changes due to the chi chi event and the association of the crustal anomalies with seismicity combining the results with those of other geological and geophysical surveys the crustal anomalies are interpreted as images of a highly fractured and fluid filled chi chi source region fluid overpressure is known to reduce the strength of the fault zone and in this case may have initiated the nucleation of the chi chi earthquake the v p models obtained before and after the chi chi event show some interesting heterogeneities regions of low v p anomaly appear to have further expanded with the occurrence of the chi chi earthquake the v p v s model shows high anomalies near the source rupture especially near where the rupture is believed to have nucleated after the chi chi event the v p v s model shows a substantial change that may reflect an outward shift of the fluid filled fractured source region this shift is shown by a general expansion in v p v s anomalies to a broader region e g where two intensive aftershock clusters occurred and a significant v p v s reduction to more normal values near the chelungpu rupture we attribute these changes in v p and v p v s anomalies as well as the expansion and shift of the anomalous regions to be a response to the localized stress change caused by the slip of the chi chi rupture the location of the seismic events is correlated with that of the crustal v p and v p v s anomalies large events appear to occur in the transition zones of both v p and v p v s anomalies this finding may be of use in the identification of weakened crustal regions in a seismic zone thus suggesting where impending large earthquakes are likely to occur all input data and initial and final models of this article are given as electronic files in the cd attached to the end of this issue

the shuilikeng fault in the central taiwan mountain belt for over 200 km along strike the shuilikeng fault of taiwan separates miocene rocks of the western foothills from the largely eocene and oligocene rocks of the hsuehshan range to the east despite its importance in the taiwan mountain belt the structure and kinematics of the shuilikeng fault are not well known here we present results from new geological mapping along 100 km of its strike length at the surface the shuilikeng fault is a steeply east dipping brittle fault with a series of splays and bifurcations along its southern part it cuts an earlier fold and fault system outcrop kinematic data vary widely from thrusting to strike slip the surface data are integrated with a relocated and collapsed seismicity database to interpret the fault location at depth these data indicate that the shuilikeng fault can be traced to greater than 20 km depth some 260 focal mechanisms from this dataset indicate that its kinematics is overall transpressive from a regional perspective we interpret the shuilikeng fault to reactivate a pre existing rift related basin bounding fault to the east of which rocks in the hsuehshan range are being exhumed

the mount perkins block northwestern arizona an exposed cross section of an evolving preextensional to synextensional magmatic system the steeply tilted mount perkins block northwestern arizona exposes a cross section of a magmatic system that evolved through the onset of regional extension new 4oar 39ar ages of variably tilted 0 90 o volcanic strata bracket extension between 15 7 and 11 3 ma preextensional intrusive activity included emplacement of a composite miocene laccolith and stock trachydacite dome complex and east striking rhyolite dikes related volcanic activity produced an 18 16 ma stratovolcano cored by trachydacite domes and flanked by trachydacite trachyandesite flows and 16 ma rhyolite flows similar compositions indicate a genetic link between the stratovolcano and granodioritic phase of the laccolith magmatic activity synchronous with early regional extension 15 7 14 5 ma generated a thick felsic volcanic sequence a swarm of northerly striking subvertical rhyolite dikes and rhyolite domes field relations and compositions indicate that the dike swarm and felsic volcanic sequence are cogenetic modes of magma emplacement changed during the onset of extension from subhorizontal sheets east striking dikes and stocks to northerly striking subvertical dike swarms as the regional stress field shifted from nearly isotropic to decidedly anisotropic with an east west trending horizontal least principal stress preextensional trachydacitic and preextensional to synextensional rhyolitic magmas were part of an evolving system which involved the ponding of mantle derived basaltic magmas and ensuing crustal melting and assimilation at progressively shallower levels major extension halted this system by generating abundant pathways to the surface fractures which flushed out preexisting crustal melts and hybrid magmas remaining silicic melts were quenched by rapid upper crustal cooling induced by tectonic denudation these processes facilitated eruption of mafic magmas accordingly silicic magmatism at mount perkins ended abruptly during peak extension 14 5 ma and gave way to mafic magmatism which continued until extension ceased