seismic tomography a window into deep earth abstract the goal of this paper is to provide an overview of the current state of the art in seismic tomography and trace its origins from pioneering work in the early 1970s to its present status as the pre eminent tool for imaging the earth s interior at a variety of scales due to length limitations we cannot hope to cover every aspect of this diverse topic or include mathematical derivations of the underlying principles rather we will provide a largely descriptive coverage of the methodology that is targeted at readers not intimately familiar with the topic the relative merits of local versus global parameterization ray tracing versus wavefront tracking backprojection versus gradient based inversion and synthetic testing versus model covariance are explored a variety of key application areas are also discussed including body wave traveltime tomography surface wave tomography attenuation tomography and ambient noise tomography established and emerging trends many of which are driven by the ongoing rapid increases in available computing power will also be examined including finite frequency tomography full waveform tomography and joint tomography using multiple datasets several practical applications of seismic tomography including body wave traveltime attenuation and surface waveform are presented in order to reinforce prior discussion of theory

intermediate depth seismicity in the western mediterranean unrelated to subduction of oceanic lithosphere there are three zones in the gibraltar area where intermediate depth earthquakes occur down to 150 km two of these the gulf of cadiz and the western alboran sea are oceanic areas which could be associated with the subduction of oceanic lithosphere our present measurements of arrival times of p and s waves show however that the third zone beneath the high atlas is a typical intracontinental chain where it is not likely that oceanic lithosphere has been subducted during the past 20 million years

anomalously deep earthquakes beneath the new hebrides trench abstract the central new hebrides island arc is a morphologically complex area where the island blocks of santo and malekula replace the trench and override the d9entrecasteaux fracture zone dfz a major aseismic ridge on the subducting plate earthquakes recorded by a local network have been located beneath the trench southwest of malekula at depths of 60 to 90 km because these locations are unusual and because tests with hypothetical data showed that two minima one for shallow depths and one for deep depths exist on the travel time surface for events in this area we investigated these hypocenters in more detail location quality was judged primarily by the fit to p and s wave arrival times calculated in a variety of velocity models we found that while the standard deviations of the travel time residuals from some of the events were nearly the same for shallow and deep locations they were significantly lower for several events at the deeper minimum moreover the marginal a posteriori density functions for depth indicate that the deeper depths are significantly more probable activity in this area appears to be sporadic most of the events were aftershocks of a large earthquake m w 7 1 in 1981 and a cluster of events was reported to have occurred in the same area over a period of one month in 1986 the existence of deep earthquakes in this area may be due to interactions of the dfz with santo and malekula because their locations follow a trend in the seismicity that parallels the western coast of the island blocks they also occur directly north of a sharp bend in the trench and near a major seismic discontinuity and may be a manifestation of recent en echelon subduction if these earthquakes are not occurring along the boundary of a new subduction zone the inferred existence of brittle failure at depths usually associated with a ductile rheology brings into question our understanding of the mechanics of the oceanic lithosphere

tectonics and topography for a lithosphere containing density heterogeneities the purpose of this paper is to clarify the dynamic role of lithospheric density heterogeneities in particular with respect to mountain building and other processes of intraplate deformation density anomalies within or just beneath the lithosphere constitute major sources for tectonic stress fields the product of their magnitude by their depth is shown to characterize their ability to induce deformation this rule of the density moment directly yields the lithospheric thickening or thinning rate when applied to structures of large lateral extent for anomalies of lateral extent that is small in comparison with their depth the deformation is vertically inhomogeneous and has been computed with the help of simple physical models of a stratified viscous newtonian lithosphere the analytical treatment is based on fourier transform continent continent collision thickens not only the crust but the entire lithosphere the cold root underlying a mountain chain induces strong regional compressive stresses able to sustain the mountain bulding process without further help from forces transmitted from far away thus the continental lithosphere is in a somewhat metastable mechanical state adiabatic i e rapid thickening or thinning tends to grow further once initiated tectonic phases of strong compression correspond to the climax of such instabilities the response of models with cold lithospheric roots of various intensities has been computed both in two and three dimensions they yield velocity distributions and stress fields instructive comparisons are made with earthquake focal mechanisms and in situ stress measurements in the alpine and appalachian regions in the presence of lateral variations of the mechanical properties of the lithosphere the tectonic style is not only function of the local topography and of the nature of its compensation deformations in neighbouring provinces are coupled as shown by 3 dimensional models for example thickening sustained by a cold lithospheric root may generate extension in peripheral zones of weakness these last results illustrate the point that the computation of regional tectonic stresses requires the knowledge of the density anomalies within the lithosphere on the one hand and of geometrical constraints related to lateral mechanical heterogeneities on the other

upper mantle structure at walvis ridge from pn tomography abstract passive continental margins offer the unique opportunity to study the processes involved in continental extension and break up within the lispwal lithospheric structure of the namibian continental passive margin at the intersection with the walvis ridge from amphibious seismic investigations project combined on and offshore seismic experiments were designed to characterize the southern african passive margin at the walvis ridge in northern namibia in addition to extensive analysis of the crustal structures we carried out seismic investigations targeting the velocity structure of the upper mantle in the landfall region of the walvis ridge with the namibian coast upper mantle pn travel time tomography from controlled source amphibious seismic data was used to investigate the sub moho upper mantle seismic velocity we succeeded in imaging upper mantle structures potentially associated with continental break up and or the tristan da cunha hotspot track we found mostly coast parallel sub moho velocity anomalies interpreted as structures which were created during gondwana break up

regional gravity anomalies depth of the foreland basin and isostatic compensation of the new guinea highlands the new guinea highlands form an active thrust belt advancing southward over the australian continental margin surface geology indicates thin skinned deformation of sediments and continental basement building mountains 2 4 km in average elevation the depth of the adjacent foreland basin is no more than 1500 m in papua new guinea and in many places only 100 200 m indicating little deflection of the australian lithosphere beneath the load of the mountains calculations of elastic loading in which the topographic load of the entire mountain range is supported by australian lithosphere of uniform flexural rigidity d predict a basin 2 10 times deeper than observed in eastern new guinea for a wide range of flexural rigidities 1021 1025 nm bouguer gravity anomalies in the eastern highlands between 143 e and 144 e are consistent with local airy compensation of the mountains and preclude a continuous plate with d 1022 nm underlying any significant portion of the mountains in contrast the observed broad foredeep flexure in central new guinea near 141 142 e requires a strong lithosphere d 1024 1025 nm and limited gravity and sediment thickness observations farther west indicate similar flexural rigidities bouguer gravity measurements and observed foreland basin depths throughout new guinea can be explained by a strong and continuous lithosphere beneath the foreland basin extending no more than 20 60 km north of the range front implying that the plate has weakened beneath the thrust belt collateral evidence for plate weakening beneath the highlands may be found in earthquakes at 10 25 km depth with high angle thrust mechanisms and in widespread quaternary volcanism throughout the eastern thrust belt a 50 mgal positive residual anomaly over the northern 100 km of the eastern highlands cannot be explained by local compensation or mechanical support from the south indicating either a large excess mass at depth or some additional form of support for the northeastern highlands the gravity observations in eastern new guinea require 10 15 km of crustal thickening but observed sediment thicknesses can only account for 0 5 km necessitating 5 15 km of thickening in the basement

geodynamic evolution of the lithosphere and upper mantle beneath the alboran region of the western mediterranean constraints from travel time tomography an edited version of this paper was published by the american geophysical union copyright 2000 agu see also http www agu org pubs crossref 2000 2000jb900024 shtml http atlas geo cornell edu morocco publications calvert2000 htm

deep structure of an arc continent collision earthquake relocation and inversion for upper mantle p and s wave velocities beneath papua new guinea we examine mechanisms of lithospheric growth and modes accommodating subcrustal convergence following the cessation of subduction by analyzing the seismicity and upper mantle velocities in the new guinea arc continent collision earthquakes in the papua new guinea png region from 1967 to 1984 have been relocated in a joint inversion with seismic velocities using p and s wave arrival times recorded by seismographs in png a total of 957 well recorded earthquakes were chosen for use in three dimensional velocity inversions from an initial catalog of 12 960 events based on the stability of initial hypocenter relocations constant velocity blocks were defined as irregular polyhedra to give fine detail in heavily sampled areas without introducing a large number of poorly controlled parameters and to exploit a priori knowledge of the geometry of velocity anomalies spherical geometry was used to accurately calculate long ray paths up to 1500 km in length most of the 700 best located hypocenters from the png data set are in the upper mantle beneath the finisterre huon fh ranges the newly accreted island arc terrane beneath the papuan peninsula and in the new britain seismic zone an oceanic subduction system these hypocenters show a well defined seismic zone dipping vertically or steeply to the north beneath the northern fh ranges from 125 to 250 km depth continuous along strike with the new britain seismic zone to the east which shows earthquakes to 600 km depth the steeply dipping intermediate depth seismicity flattens near 100 km depth beneath the fh ranges and forms a subhorizontal seismic zone beneath the fh ranges by contrast seismicity continues upward to the surface beneath new britain where the arc has not yet collided with new guinea the 100 km deep flat zone does not continue south of the fh ranges but appears to be truncated below the eastern and southern boundaries of the island arc terrane and no clear evidence for a south dipping seismic zone could be found the relationship of the flat part of the slab 100 km beneath the fh ranges and surface faulting is difficult to discern the apparent surface suture bounding the south side of the fh ranges is 100 km directly above the southern termination of intermediate depth seismicity nowhere in png could clear evidence for arc polarity reversal be found from seismicity all of the well located subcrustal earthquakes beneath the papuan peninsula lie in a narrow horizontal line between 125 and 175 km depth that follows the center of the peninsula and are distinctly separated from the fh seismic zone to the north these events do not require a southward subducting plate beneath the papuan peninsula an alternate and reasonable explanation for these events is that they are a result of unstable thickening of the lithosphere information on lateral heterogeneity in the mantle is limited because large variations in crustal velocities dominate the observed pattern of travel time residuals allowing for only crustal heterogeneity reduces travel time residual variances by 40 from a simple one dimensional structure while allowing for additional mantle heterogeneity results in 10 further reduction for the various parameterization s attempted here inversions with several different block geometries show low velocities at 35 to 171 km km depth beneath the bismarck sea back arc basin and high velocities just south of the intermediate depth fh seismic zone the pattern of lateral heterogeneity supports the inference from seismicity of north dipping slabs beneath both northeast new guinea and new britain

propagation of high frequency seismic waves inside the subducted lithosphere from intermediate depth earthquakes recorded in the vanuatu arc three seismic phases associated with the slab of subducted lithosphere are identified on seismograms of intermediate depth earthquakes depths 70 km recorded in the distance range up to 350 km by the vanuatu formerly new hebrides seismograph network they are interpreted as 1 a p wave refracting into the inner portion of the slab 2 a p wave traveling mainly through the outer upper portion of the slab and 3 a p to s converted wave in which the conversion occurs across the upper surface of the slab the existence of the two p arrivals from intermediate depth earthquakes suggests that these earthquakes are not located in the strong and cold inner core of the slab but within an outer zone where the velocity is lower than that within the inner portion of the slab the low frequency character of the shear wave converted from the p wave across the outer boundary of the slab indicates that either the boundary is a transition layer with a finite thickness or the material above the slab has high attenuation for high frequency shear waves large spatial variations in the transmission of high frequency shear waves also indicate that there is a localized zone of anomalous shear wave attenuation in the mantle wedge lying just above the subducted slab and beneath the active volcanos

quaternary uplift rates at a plate boundary lae urban area papua new guinea abstract uplift along the southern margin of the south bismarck plate in papua new guinea is a result of plate convergence terrane docking and triple junction migration lithofacies data combined with paleontological and 14 c dating yield uplift rates for the markham valley north flank and the lae urban area in the range 2 0 7 6 m 10 3 yr these rates are established for the past 21 000 years and have probably obtained for 10 6 yr depositional surfaces 8100 years old in the lae urban area are tilted the uplift may be occurring by steady creep or in episodic increments of 1 m with recurrence intervals of the order of 10 2 yr as seems to be the case for the northern huon peninsula terraces if episodic uplift is occurring earthquakes of magnitude 7 0 can be anticipated accompanied by slump induced tsunamis

seismicity of the indo australian solomon sea plate boundary in the southeast papua region abstract seismicity and earthquake focal mechanism plots of the southeast papua and woodlark basin region for the period january 1960 to may 1979 show that 1 a the west woodlark basin spreading centre extends from the deep west woodlark basin through dawson strait into goodenough bay southeast papua 2 b a southeast seismic trend in the west woodlark basin is associated with a left lateral transform fault but a gap exists between this zone and the seismic east woodlark basin spreading centre 3 c southeast papua seismicity divides into a shallow earthquake zone in which the earthquakes occur mainly in the northeast side of the owen stanley range and an intermediate depth southwest dipping benioff zone which extends almost from mt lamington to goroka the benioff zone indicates the presence of a southwest dipping slab of solomon sea plate beneath the indo australian plate in the southeast papua and ramu markham valley region this subduction zone has collided with the new britain subduction zone of the solomon sea plate along the ramu markham valley the solomon sea plate is now hanging suspended in the form of an arch beneath ramu markham valley inhibiting further subduction beneath southeast papua

shallow seismicity of arc continent collision near lae papua new guinea abstract in northeastern new guinea the narrow ramu markham valley rmv separates island arc rocks to the north from those of continental origin to the south and appears to be the western onland extension of the new britain trench to explore the tectonic processes at the leading edge of the island arc during collision we operated a portable seismic network for six weeks near the city of lae at the eastern end of the rmv we observed a narrow near vertical belt of seismicity between 10 and 30 km depth that we call the lae seismic zone lsz starting at the rmv in the southwest and trending northeasterly cutting across surface geologic structure the truncation of the lsz along a steep plane by the ramu markham fault zone rmfz and earthquake first motions suggest that the earthquakes occur in the hanging wall of a steep n dipping fault that crops out at the rmfz we also consider that the lsz is within the lower plate of a gently dipping thrust below 20 km depth the microearthquake zone is truncated by a gently ne dipping plane coinciding in depth and dip with nodal planes of recent large mb 5 6 and 6 0 thrust earthquakes we suggest that the huon peninsula is being emplaced onto the australian plate along a gently 25 dipping thrust fault that is 20 km deep beneath lae the rmfz may be a steeply dipping thrust fault that connects with this gently n dipping thrust but accommodates little convergence at present the lsz trends nearly perpendicular to an anticlinal range which appears to be sheared in a left lateral sense p wave first motions for earthquakes in the lsz with steep 70 to 90 dip nodal planes that strike parallel to the lsz suggest a component of south side up displacement also hence the crustal block south of the lsz may be rising relative to the huon peninsula and the rapid quaternary uplift rates estimated for the lae coastal region may be higher than the uplift rate of the huon peninsula as a whole we suggest that the lsz reveals a tear of small offset in the huon terrane but may be similar to a structure that produced a magnitude 7 earthquake near madang in 1970

imaging continental breakup using teleseismic body waves the woodlark rift papua new guinea this study images the upper mantle beneath the d entrecasteax islands papua new guinea providing insight into mantle deformation beneath a highly rifted continent adjacent to propagating spreading centers differential travel times from p and s wave teleseisms recorded during the 2010 2011 cdpapua passive seismic experiment are used to invert for separate vp and vs velocity models of the continental rift a low velocity structure marks the e w axis of the rift correlating with the thinnest crust high heat flow and a linear trend of volcanoes this slow region extends 250 km along strike from the oceanic spreading centers demonstrating significant mantle extension ahead of seafloor breakup the rift remains narrow to depth indicating localization of extension perhaps as a result of mantle hydration a high vp structure at depths of 90 120 km beneath the north of the array is more than 6 5 faster than the rift axis and contains well located intermediate depth earthquakes these independent observations place firm constraints on the lateral thermal contrast at depth between the rift axis and cold lithosphere to the north that may be related to recent subduction although the polarity of subduction cannot be resolved this geometry is gravitationally unstable downwelling or small scale convection could have facilitated rifting and rapid lithospheric removal although this may require a wet mantle to be realistic on the required time scales the high v structure agrees with the maximum p t conditions recorded by young ultra high pressure rocks exposed on the rift axis and may be implicated in their genesis

continental breakup and uhp rock exhumation in action gps results from the woodlark rift papua new guinea we show results from a network of campaign global positioning system gps sites in the woodlark rift southeastern papua new guinea in a transition from seafloor spreading to continental rifting gps velocities indicate anticlockwise rotation at 2 2 7 myr relative to australia of crustal blocks north of the rift producing 10 15 mm yr of extension in the continental rift increasing to 20 40 mm yr of seafloor spreading at the woodlark spreading center extension in the continental rift is distributed among multiple structures these data demonstrate that low angle normal faults in the continents such as the mai iu fault can slip at high rates nearing 10 mm yr extensional deformation observed in the d entrecasteaux islands the site of the world s only actively exhuming ultra high pressure uhp rock terrane supports the idea that extensional processes play a critical role in uhp rock exhumation gps data do not require significant interseismic coupling on faults in the region suggesting that much of the deformation may be aseismic westward transfer of deformation from the woodlark spreading center to the main plate boundary fault in the continental rift the mai iu fault is accommodated by clockwise rotation of a tectonic block beneath goodenough bay and by dextral strike slip on transfer faults within and surrounding normanby island contemporary extension rates in the woodlark spreading center are 30 50 slower than those from seafloor spreading derived magnetic anomalies the 0 5 ma to present seafloor spreading estimates for the woodlark basin may be overestimated and a reevaluation of these data in the context of the gps rates is warranted

velocity structure in northern chile evidence of subducted oceanic crust in the nazca plate summary 2 d p wave velocity models were determined for the subduction zones near iquique and antofagasta in northern chile simultaneously inverting p and s wave arrival times from locally recorded earthquakes for velocity structure and hypocentral locations a 2 d parametrization was used because of the paucity of data but is justified by the lack of significant variations along the strike of the subduction zone observed from both refraction profiles and simple 3 d inversions the crust and upper mantle are parameterized by constant velocity regions of irregular shape with the size and boundaries of these regions governed by prior information about the structure and by the ability of the data to resolve p wave velocities beneath the antofagasta region there is evidencc of an approximately 10 km thick layer of oceanic crust attached to the top of the subducting nazca plate this crust has a p wave velocity of 7 3 f 0 1 km s and is observed down to a depth of 60 f 10 km this depth also corresponds to the maximum depth of seismogenic coupling in the chilean subduction zone the subducted oceanic crust overlies an oceanic upper mantle with a p wave velocity of 8 0 f 0 1 km s apparently oceanic crust is being subducted beneath iquique as well however this feature is less constrained by the data available from this region

criteria for the design of embankment dams founded on gypsiferous geological formations this article sets criteria for selecting the typology of embankment dam and the elements for the foundation to be impervious enough blanket cutoff when the dam site contains disseminated gypsum particles in limited quantities less than 40 and the flow is interstitial numerical modeling analysis of the soluble material dissolution of a dam foundation includes two coupled processes seepage and dissolution in a previous manuscript the mathematical model that simulates the progress of the dissolution of the gypsum particles in the foundation was implemented in the code disolucion2d also validated by experimental work this code simulates the advance of the dissolution front foundation area where the dissolution of soluble particles is taking place at each time step based on the flow velocities obtained from the seepage flow net calculation the dissolution process progressively increases the permeability of the foundation and therefore the flow rate under the dam jeopardising in some cases the safety of the dam based on the validated code a study covering 140 cases of dams and their foundations was carried out from the relevant data for each dam site height thickness of the gypsiferous layer gypsum percentage of the layer and the prescribed criteria for maximum permissible flow rate or maximum flow rate increase the various type sections of the dam that satisfy the required conditions can be identified from among these the dam designer will be able to choose the optimal solution taking into consideration the particular characteristics of the project available materials economic costs

deep structures along the boundary of a collisional belt attenuation tomography of p and s waves in the greater caucasus summary seismic attenuation q 1 o fp and s waves shows a major discontinuity from the russian platform to the tectonically active greater caucasus broadband records show this boundary over paths 4 long as revealed by the decay of amplitude spectra from a digital seismic network flanking the greater caucasus we measure attenuation from individual spectra using a non linear least squares procedure to determine an attenuation parameter t simultaneously with source parameters at frequencies between 1 and 15 hz the t measurements are then inverted for spatial variations of q 1 with parametrizations of varying complexity model variance for heterogeneous structures improves by more than 30 per cent compared with homogeneous parametrizations site corrections also significantly improve the fit in these inversion results mountainous regions exhibit q values 2 3 times lower q s 775 75 than do the adjacent shields q s 2060 315 showing that the discontinuity is large for both regions q p is roughly equal to q s comparison of body wave to coda spectra indicates that intrinsic absorption rather than scattering dominates the q 1 measurements at least beneath the mountains hence q 1 variations may give a reasonable proxy for temperature if so then temperature beneath the mountains exceeds that beneath the shield by 70 400 c these temperature increases may not be high enough to generate widespread partial melting beneath the mountains but could produce regional metamorphism and could contribute substantially to isostatic compensation of the mountains whatever their origin the boundary in seismic attenuation is abrupt and large between stable craton and an adjacent mountain belt demonstrating that q 1 is a sensitive indicator of tectonic process

simultaneous travel time inversion for earthquake location and subduction zone structure in the central aleutian islands a combined location and velocity inversion technique is applied to travel time data from 151 well recorded central aleutian earthquakes the data include p and s wave arrivals at stations of a local network and p pp and sp wave arrivals at teleseismic stations after correction for crustal structure at the reflection points the depth phases pp and sp provide important constraints on subduction zone structure not ordinarily resolved by other types of data the structure is assumed to vary only across the arc and in depth it is parameterized with cubic splines by specifying the velocity at each point of a gridded arc cross section to avoid ray tracing it is assumed that the velocity part of the problem is linear preliminary results show that deep earthquakes apparently occur within a narrow downdip zone near the upper surface of the descending slab in contrast with previous studies which have located them within the presumably stronger and colder inner core of the slab a slab thickness of 80 100 km and a downdip length of about 400 km well below the deepest seismic activity are indicated the slab is characterized by seismic velocities as much as 11 higher than the surrounding mantle in its upper portions and 4 6 higher at depth a sharp velocity gradient and lower velocities occur directly beneath the volcanic arc near the top of the slab the slab anomaly appears to spread out and fall off very slowly with depth these are probably not real effects but consequences of the omission of three dimensional ray tracing

the opening of the woodlark basin subduction of the woodlark spreading system and the evolution of northern melanesia since mid pliocene time abstract magnetic anomaly and seismological data define segments of active seafloor spreading and associated magnetic lineations trending ene in the woodlark basin the total opening rate has been approximately 6 cm yr for the last 1 m y spreading rates diminish by over 10 from east to west along the woodlark spreading system implying a pole of current opening 15 20 to the west commencement of seafloor spreading in the basin has apparently been time transgressive beginning prior to 3 5 m y in the east and at successively later times to the west earthquake focal mechanisms and geological evidence suggest that the land areas bounding the western end of the woodlark basin are undergoing tensional deformation we believe that eventually the woodlark basin plate boundary will propagate westward through the d entrecasteaux islands into the papuan peninsula hitherto unreported shallow seismicity associated with the northern margin of the ne trending section of the woodlark rise probably reflects partial decoupling of the woodlark and solomon basins possibly due to mechanical difficulties in subducting the young woodlark lithosphere analysis of the relative motions between the solomon indo australian and pacific plates shows that the woodlark spreading system has been subducted at high rates 10 cm yr beneath the solomon islands during the opening of the woodlark basin several tectonic and geological features limited to the region of interaction of the woodlark basin with the solomon trench and arc may be symptomatic of ridge subduction these features include high heat flow in the solomon trench which shoals to 4 km low levels of seismicity and only shallow hypocenters and voluminous eruptions of high k 2 o tio 2 olivine basalts and basaltic andesites extremely close to the trench axis this close association in space and time of an unusual volcanic suite with ridge subduction implies a strong dependence of the petrogenesis on the tectonic regime a combination of this study of the woodlark basin and the previous study of the bismarck basin taylor 1979 provides a reconstruction of the positions of the continents ocean basins and island chains in northern melanesia for mid pliocene time in accepting the existence of a solomon plate we can explain the trench like structure off the trobriand margin of new guinea the occurrence of late cenozoic calc alkaline volcanism along the papuan peninsula and the presence of intermediate depth seismicity beneath the north papuan peninsula the rapid changes in relative motions along or across the new ireland solomons chain over the past 3 5 m y may explain the spatial and temporal changes in igneous activity observed on these islands

seismotectonics of new guinea a model for arc reversal following arc continent collision the structure and evolution of the northern new guinea collision zone is deduced from international seismological center isc seismicity 1964 1985 new and previously published focal mechanisms and a reexamination of pertinent geological data a tectonic model for the new guinea margin is derived which illustrates the sequential stages in the collision and suturing of the bewani toricelli adelbert finisterre huon new britain arc to central new guinea followed by subduction polarity reversal in the west east of 149 e the solomon plate is being subducted both to the north and south bringing the new britain and trobriand forearcs toward collision west of 149 e the forearcs have collided and together they override a fold in the doubly subducted solomon plate lithosphere which has an axis that is parallel to the strike of the ramu markham suture and that plunges westward at an angle of 5 beneath the coast ranges of northern new guinea active volcanism off the north coast of new guinea is related to subduction of the solomon plate beneath the bismarck plate active volcanism of the papuan peninsula and quaternary volcanism of the new guinea highlands are related to slow subduction of the solomon plate beneath the indo australian plate along the trobriand trough and the trough s former extension to the west respectively from 144 148 e seismicity and focal mechanisms reveal that convergence between the sutured bismarck and indo australian plates is accommodated by thrusting within the finisterre and adelbert ranges and compression of the new guinea orogenic belt together with basement involved foreland folding and thrusting to the south the finisterre block overthrusts the new guinea orogenic belt whereas the adelbert block is sutured to new guinea and overthrusts the oceanic lithosphere of the bismarck sea along the new guinea trench west of 144 e seismicity defines a southward dipping wadati benioif zone and focal mechanisms indicate oblique subduction only this oldest westernmost portion of the collision has progressed past suturing to a full reversal in subduction polarity

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

detection and location of earthquakes in the central aleutian subduction zone using island and ocean bottom seismograph stations a network of eight university of texas ocean bottom seismographs obs operated for 6 weeks in 1978 about 50 km offshore of adak island alaska and nearby islands in 1979 a similar network of nine instruments was deployed for 7 weeks farther offshore within and up to 100 km seaward of the aleutian trench for shallow earthquakes on the outer trench slope for shallow earthquakes in the thrust zone and for intermediate depth events we have analyzed the obs and island based network data and evaluated the island network s capabilities for earthquake detection and location and for focal mechanism determination our three major conclusions are presented the first concerns shallow earthquakes on the outer trench slope in 1979 about 30 earthquakes occurred within the aleutian trench and up to 60 km seaward of the trench axis the island network located none of these events and detected p phases for only three of them ray tracing shows that the islands lie in a geometric shadow zone for events on the outer trench slope the best located events are shallower than 20 km and exhibit first motions consistent with normal faulting several authors have suggested that these events are caused by bending of the oceanic lithosphere at the outer rise prior to subduction if so then the event locations reported here show that the bending stresses exceed the strength of lithosphere only in a narrow zone extending about 10 km landward and 60 km seaward of the trench axis the second conclusion concerns shallow earthquakes in the thrust zone epicenters determined by island stations alone are virtually identical to epicenters determined using data from both island and obs stations the locations are similar whether they are determined using flat layered velocity models or a more realistic model and a ray tracing scheme nearly all the earthquakes detected by the obs network are also recorded by island stations three composite focal mechanism solutions that use data from both island and obs stations have p axes parallel to the direction of plate convergence the thrust zone events are separated from events on the outer trench slope by a gap in seismicity about 50 km wide situated on the inner trench slope the third conclusion concerns earthquakes deeper than 70 km epicenters determined using island network stations alone lie 10 to 80 km south of those determined using obs and island stations with the differences between epicenters depending both on event depth and on the velocity model used ray tracing with a realistic velocity model shows that for events at 200 km depth locations are about 80 km north of locations determined from island data alone these results do not support the observed increase in the dip of the benioff zone beneath 100 km depth that is suggested by locations determined from island network data alone using a flat layered velocity model

constraining p wave velocity variations in the upper mantle beneath southeast asia we have produced a p wave model of the upper mantle beneath southeast se asia from reprocessed short period international seismological centre isc p and pp data short period p data of the annual bulletin of chinese earthquakes abce and long period pp p data we used 3d sensitivity kernels to combine the datasets and mantle structure was parameterized with an irregular grid in the best sampled region our data resolve structure on scale lengths less than 150 km the smearing of crustal anomalies to larger depths is reduced by a crustal correction using an a priori 3d model our tomographic inversions reveal high velocity roots beneath the archean ordos plateau the sichuan basin and other continental blocks in se asia beneath the himalayan block we detect high seismic velocities which we associate with subduction of indian lithospheric mantle this structure is visible above the 410 km discontinuity and may not connect to the remnant of the neo tethys oceanic slab in the lower mantle our images suggest that only the southwestern part of the tibetan plateau is underlain by indian lithosphere and thus that the upper mantle beneath northeastern tibet is primarily of asian origin our imaging also reveals a large scale high velocity structure in the transition zone beneath the yangtze craton which could have been produced in multiple subduction episodes the low p wave velocities beneath the hainan island are most prominent in the upper mantle and transition zone they may represent counter flow from the surrounding subduction zones and may not be unrelated to processes beneath eastern tibet 2005 elsevier b v all rights reserved

holocene sea levels in oceanic islands implications for the rheological structure of the earth s mantle abstract relative sea levels in oceanic islands that are situated sufficiently far from glaciated regions were studied in relation to the rheological structure of the earth s mantle the local hydro isostatic adjustment associated with the mantle flow from the oceanic side to an inland side depends on the size of the island the effective upper mantle viscosity and the elastic plate thickness the relative sea level has a significant dependence on the upper mantle rheology for islands with a radius larger than 10 km but it is almost independent of the upper mantle rheology for islands with a radius less than 10 km in other words the observed sea level changes in such small islands follow the global isostatic adjustment which depends strongly on the lower mantle viscosity in fact the relative sea levels in small oceanic islands impose important constraints on the lower mantle viscosity on the other hand the effective upper mantle viscosity and the elastic plate thickness are determined by analyzing the relative sea levels for islands greater than 10 km the observed sea level curves in japan and new zealand are consistent with the effective viscosity of the upper mantle equal to 2 to 6 10 21 poises the effective elastic plate thickness for japan and new zealand is estimated to be less than 50 km and greater than 100 km respectively the thickness of the lithosphere obtained by sea level curves in japan and new zealand is compatible with the results obtained by seismological studies

seismicity and p wave velocity image of the southern tyrrhenian subduction zone summary this paper focuses on the deep earth structure and earthquake distribution within the southern tyrrhenian subduction zone we discuss seismological observations which provide insight into the mechanics of subduction processes for an unusual compressional margin the motivation for this work derives from the recognition that little is known about one of the most intriguing geodynamic features of the italian peninsula and the fact that a large amount of information can be extracted from data collected by the istituto nazionale di geofisica national seismic network we show that the hypocentres of intermediate and deep earthquakes that have occurred since 1988 image a nw dipping plane continuous from the ionian sea to the tyrrhenian down to 450 km depth we also present a 3 d model of the deep structure beneath southern italy derived using deep earthquake seismic tomography tomographic images reveal a high velocity body dipping from the apulian and iblean forelands towards the tyrrhenian basin in the crust and upper mantle at least down to 320 km depth this high velocity body may be related to the cold subducted ionian lithosphere earthquake hypocentres are confined within the high velocity body delineating the subduction beneath the calabrian arc

distribution of magma beneath the toba caldera complex north sumatra indonesia constrained by three dimensional p wave velocities seismicity and gravity data 1 we estimate the one and three dimensional p wave velocity structure beneath the toba caldera complex a 30 100 km topographic depression in north sumatra using arrival time data of local earthquakes recorded by a 40 station seismic network that operated for 4 months inversions reveal the presence of p velocities up to 37 below normal that likely map the distribution of magma within this subduction related volcanic system considered the world s largest in the upper 10 km of crust the largest low velocity region underlies the southern two thirds of the depression and coincides with a gravity low centered over the resurgent dome a smaller volume of low velocities is observed in the upper crust under the north end of the depression separating the two regions is a zone of locally high velocities indicating that the shallow subcaldera magma system is composed of two separate reservoirs not a single one that extends the entire length of the caldera complex above each low velocity region is a postcollapse volcano that erupted mostly mafic lavas after the last major caldera collapse 74 kyr ago a low velocity column below one of these volcanoes can be traced into the uppermost mantle and corresponds with a planar distribution of low frequency earthquakes in the 20 to 40 km depth range the low frequency earthquakes apparently record the migration of melt in the mafic roots of this large volume crustal magma system

volcanic evolution in eastern papua abstract late mesozoic and cenozoic volcanic rocks in eastern papua record a complex series of volcanotectonic events which reflect interaction between the indo australian and solomon sea plates basement formations of upper cretaceous and eocene submarine basalt are comparable to those characteristic of sea floor spreading centers and are thought to have originated during volcanic activity associated with spreading in the coral sea basin arc trench type andesitic volcanism was prominent during the late cenozoic but shows no clear relationship to a subduction event an alternative explanation links the development of thickened crust and consequent crust mantle interaction with the generation of andesitic magmas the tectonic environment of eastern papua during the late cenozoic was one of block faulting and uplift associated with crustal tension the presence of quaternary peralkaline rhyolites suggests that this environment is now being replaced by active rifting

velocity structure of the pamir hindu kush region possible evidence of subducted crust the arrival times of compressional p and shear s waves from approximately 580 microearthquakes recorded by a temporary array in the pamir hindu kush region in central asia are used to deduce one and three dimensional velocity structures of this region the results for one dimensional structures imply that the moho is at 70 5 depth also there is a velocity reversal near 160 km depth which is inferred to be the beginning of the low velocity zone this reversal continues to depths of approximately 230 km below 230 km velocities are somewhat higher than those of normal mantle at similar depths 9 3 km s versus 8 4 km s for p waves the outstanding feature of the results for three dimensional velocity structures is a broad 40 km centrally located region with 8 to 10 lower velocities than those in the surrounding regions this low velocity region envelopes the seismic zone at depths between 70 and 150 km the region may actually extend beyond these depths but the results for shallower and deeper structure lack sufficient resolution to decide several tests using both hypothetical and real data were performed to estimate the reliability of the three dimensional solutions the results of these tests suggest that the inferred velocities are reasonably accurate representations of the average velocities in the blocks although one must be cautious of the effects of averaging in interpreting the solution the low velocity region is inferred to be a manifestation of substantial quantities of subducted continental crust therefore while subduction has occurred in the pamir hindu kush the results of the three dimensional inversions suggest that continental rather than only oceanic lithosphere has been subducted to depths of at least 150 km appendices are available with the entire article on microfiche order from american geophysical union 2000 florida avenue n w washington d c 20009 document j81 013 1 00 payment must accompany order

active deformation in the new guinea fold and thrust belt seismological evidence for strike slip faulting and basement involved thrusting the new guinea fold and thrust belt forms a 1000 km long mountain chain running the length of new guinea but its trend is oblique to the predicted convergence direction a large component of left lateral shear across the mountain belt is expected but geological evidence for such motion is ambiguous surface structures in the fold and thrust belt show mountain building by folding and thrusting supposedly thin skinned of pre pliocene continental shelf sediments although some evidence indicates that active faulting penetrates the basement we examine the active deformation of this mountain belt by using teleseismic long period p and sh waveforms short period p waveforms and p wave first motions to determine reliable focal mechanisms and source depths for the 18 largest earthquakes in the new guinea fold and thrust belt since 1964 eight thrust events with seismic moments m0 between 1017 and 2 1018 n m are distributed throughout the thrust belt at depths ranging from 0 5 to 45 km and have steeply dipping fault planes that strike parallel to surface structural trends six of the eight events are between 11 and 25 km deep the steep dips of nodal planes the depths of these earthquakes and the known thicknesses of the sedimentary section show that thrust faulting is not exclusively thin skinned i e confined above a shallow detachment surface but penetrates the crystalline basement at high angles most earthquakes in the western half of the thrust belt including the three largest earthquakes examined m0 5 1018 n m show strike slip faulting with the probable fault plane oriented east west so that displacement is left lateral the nine strike slip earthquakes account for almost 90 of the observed seismic moment in the period 1964 1985 and indicate a seismic slip rate of 5 25 mm a all but one of these strike slip earthquakes are shallower than the thrust earthquakes suggesting that east northeast convergence across the fold and thrust belt is accommodated by a decoupled system of strike slip and dip slip faults while the great height of the mountain belt is an obvious indication that crustal thickening is an important consequence of plate motion in new guinea seismological evidence suggests that translation by strike slip faulting may play a much greater role than previously recognized by analogy in older collisional orogens whose motions can be inferred only from remnant structures and not from seismicity strike slip faulting may have been equally important in accommodating convergence we estimate from seismic moments of earthquakes and from the volume of the mountains that deformation of the new guinea highlands may account for 5 20 of the total convergence between the pacific plate and australia in new guinea