an algorithm for automated tsunami warning in french polynesia based on mantle magnitudes we have developed a new magnitude scale mm based on the measurement of mantle rayleigh wave energy in the 50 to 300 sec period range and directly related to the seismic moment through mm log10 m 20 measurements are taken on the first passage of rayleigh waves recorded on scale on broadband instruments with adequate dynamical range this allows estimation of the moment of an event within minutes of the arrival of the rayleigh wave and with a standard deviation of 0 2 magnitude units in turn the knowledge of the seismic moment allows computation of an estimate of the high seas amplitude of a range of expectable tsunami heights the latter combined with complementary data from t wave duration and historical references have been integrated into an automated procedure of tsunami warning by the centre polynesien de prevention des tsunamis cppt in papeete tahiti

reassessment of the 1907 sumatra tsunami earthquake based on macroseismic seismological and tsunami observations and modeling on 4 january 1907 an earthquake occurred off the west coast of sumatra indonesia with an instrumental surface wave magnitude ms in the range of 7 5 8 0 at periods of 40 s the tsunami it generated was destructive on the islands of nias and simeulue where it killed hundreds and gave rise to the legend of the s mong this tsunami was also observed in other parts of the indian ocean basin relative to its instrumented magnitude the size of the tsunami was anomalous qualifying the event as a tsunami earthquake however unusually for a tsunami earthquake the shaking on nias was severe 7 ems we revisit the 1907 earthquake with a multidisciplinary approach by extracting evidence describing shaking effects or the tsunami from written documents and by acquiring new seismograms combining these we discriminate two large earthquakes within an hour of each other with clear differences in seismological character the first we interpret to be a tsunami earthquake with characteristic low levels of shaking an estimated average seismic moment m0 of 2 5 1028 dyn cm mw 8 2 in the frequency band 6 8 mhz and an epicentral location close to the front of the sunda megathrust the seismograms we analyzed also document a regular growth of moment with period approaching mw 8 4 at the longest resolvable period 170 s for the second earthquake that caused damage on nias we estimate ms 7 based on seismograms and phase data we also identify two ms 6 aftershocks within 24 h of the mainshock additionally we present a dataset of 88 locations within the indian ocean basin where the tsunami was observed using a subset of these we forward modeled the tsunami to propose a seismic rupture model extending along the sunda megathrust for about 220 km 94 7 e to 97 e with a maximum modeled slip of 21 m our new rupture model provides an acceptable fit to our new dataset of tsunami runup and inundation values from 88 local and far field locations in the indian ocean basin we also urge caution against an over reliance on the s mong legend for tsunami evacuation as its premise that a tsunami will only follow an earthquake with very severe ground motions is rendered ineffective for tsunami earthquakes

total moment spectra of fourteen large earthquakes total moment spectra mt m where m is the moment rate tensor are computed for 14 large earthquakes recorded by the international deployment of accelerometers ida network using the scalar moment retrieval method proposed by silver and jordan 1982 for each event we obtain estimates of mt averaged over the 10 disjunct 1 mhz intervals in the low frequency band 1 11 mhz typical ida record sets from events with mt 0 2 a 1 a 1027 dyne cm yield standard errors on the 1 mhz averages that are generally less than 20 our multiple band estimates of mt are usually consistent with comparable single band values found by other investigators from the total moment spectra we derive the zero frequency static moment mt 0 and the characteristic source duration c 2 var f t where f t is the time function of the moment rate tensor assumed to be the same for all components the parameter c formally depends on the second degree temporal spatial and mixed spatial temporal moments of an extended source but calculations with realistic source geometries indicate that the interpretation of tc strictly in terms of the second temporal moment leads to very little error a plot of c against shows considerable scatter some events lie significantly above the empirical scaling curve of kanamori and given 1981 slow earthquakes and some below fast earthquakes examples of the former include all three deep focus events analyzed here as well as the 1970 colombia and 1963 peru bolivia deep focus earthquakes whose total moment spectra are calculated from the moment tensor solutions of gilbert and dziewonski 1975 an example of the latter is the great sumbawa earthquake of august 9 1977 24 3 a which is inferred to have a relatively shallow spatial centroid 20 km and a nearly flat moment spectrum suggesting that the faulting was confined to the upper more brittle portion of the oceanic lithosphere three other large earthquakes tonga june 22 1977 23 2 a kuril islands december 6 1978 3 6 0 2 a and tumaco december 12 1979 25 2 a exhibit moment spectra which significantly decrease toward higher frequencies indicating larger values of c the aftershocks of both the tonga and kuril islands events are distributed over a considerable range of depths consistent with rupture into the lower more ductile portions of the oceanic lithosphere we speculate that the characteristic source duration may be causally related to the depth of coseismic rupturing within the lithosphere

sources of tsunami and tsunamigenic earthquakes in subduction zones we classified tsunamigenic earthquakes in subduction zones into three types earth quakes at the plate interface typical interplate events earthquakes at the outer rise within the subducting slab or overlying crust intraplate events and tsunami earthquakes that generate considerably larger tsunamis than expected from seismic waves the depth range of a typical interplate earthquake source is 10 40km controlled by temperature and other geological parameters the slip distribution varies both with depth and along strike recent examples show very different temporal change of slip distribution in the aleutians and the japan trench the tsunamigenic coseismic slip of the 1957 aleutian earthquake was concentrated on an asperity located in the western half of an aftershock zone 1200km long this asperity ruptured again in the 1986 andreanof islands and 1996 delarof islands earthquakes by contrast the source of the 1994 sanriku oki earthquake corresponds to the low slip region of the previous interplate event the 1968 tokachi oki earthquake tsunamis from intraplate earthquakes within the subducting slab can be at least as large as those from interplate earthquakes tsunami hazard assessments must include such events similarity in macroseismic data from two southern kuril earthquakes illustrates difficulty in distinguishing interplate and slab events on the basis of historical data such as felt reports and tsunami heights most moment release of tsunami earthquakes occurs in a narrow region near the trench and the concentrated slip is responsible for the large tsunami numerical modeling of the 1996 peru earthquake confirms this model which has been proposed for other tsunami earthquakes including 1896 sanriku 1946 aleutian and 1992 nicaragua

importance of model parameterization in finite fault inversions application to the 1974 mw 8 0 peru earthquake the spatial and temporal slip distributions for the october 3 1974 mw 8 0 peru subduction zone earthquake and its largest aftershock on november 9 ms 7 1 are calculated and analyzed in terms of the inversion parameterization and tectonic significance teleseismic long period world wide standard seismograph network p and sh waveforms are inverted to obtain the rupture histories we demonstrate that erroneous results are obtained if a parameterization is used that does not allow for a sufficiently complex source involving spatial variation in slip amplitude risetime and rupture time the inversion method utilizes a parameterization of the fault that allows for a discretized source risetime and rupture time well located aftershocks recorded on a local network have the same general pattern as teleseismically determined hypocenters and help to constrain the geometry of the subduction zone for the main shock a hinged fault is preferred having a shallow plane with a dip of 11 and a deeper landward plane with a dip of 30 the preferred nucleation depth lies between 11 and 15 km a bilateral rupture is obtained with two major concentrations of slip one 60 to 70 km to the northwest of the epicenter and a second 80 to 100 km to the south and southeast of the epicenter for these source regions risetimes vary from 6 to 18 s our estimates of risetimes are consistent with the time for the rupture to traverse the dominant local asperity the slip distribution for the november 9 aftershock falls within a conspicuous hole in the main shock rupture pattern near the hypocenter of the main shock the november 9 event has a simple risetime function with a duration of 2 s aftershocks recorded by the local network are shown to cluster near the hypocenter of the impending november 9 event and downdip from the largest main shock source region slip during the main shock is concentrated at shallow depths above 15 km and extends updip from the hypocenter to near the plate boundary at the trench axis the large amount of slip at shallow depths is attributed to the absence of any significant accretionary wedge of sediments and the relatively young age and high convergence rate of the subducted plate which results in good seismic coupling near the trench axis

subduction induced fragmentation of the nazca plate off peru mendana fracture zone and trujillo trough revisited the mendana fracture zone and the trujillo trough are two major intra oceanic features on the nazca plate off peru during the seaperc cruise of the r v jean charcot in 1986 sea beam bathymetric data single channel seismic records geomagnetic and gravity measurements and heat flow data have been obtained over these two areas in this paper we confirm that the mendana fracture zone is actively opening perpendicular to its trend resulting in the formation of new oceanic crust since about 3 5 ma we calculate that this new rift is propagating westward along the fracture zone at a velocity of about 10 cm yr 1 or 2 cm yr 1 with respect to the trench this extensional feature appears to be genetically linked with the trujillo trough located 200 km to the north this trough is bounded to the east by a left lateral transpressive fault which became active probably at the time when the mendana rift started we then discuss a possible mechanism to explain the origin of these two areas of intraoceanic deformation

intraplate seismicity of the pacific basin 1913 1988 we establish here a comprehensive database of intraplate seismicity in the pacific basin relocation and analysis of 894 earthquakes yield 403 reliable intraplate earthquakes during 1913 1988 these numbers do not include earthquake swarms which account for another 838 events most of the remainder 304 events are actually plate boundary earthquakes that have been erroneously located in intraplate regions a significant number occur in recent years when location capabilities should have guarded against this situation relocations involve a careful linear inversion of p and s arrivals accompanied by a monte carlo statistical analysis we have also attentively removed the high number of clerical errors and nuclear tests that exist in epicenter bulletins

the energy release in earthquakes energy calculations are generally made through an empirical application of the familiar gutenberg richter energy magnitude relationships the precise physical significance of these relationships is somewhat uncertain we make use here of the recent improvements in knowledge about the earthquake source to place energy measurements on a sounder physical basis for a simple trapezoidal far field displacement source time function with a ratio x of rise time to total duration t 0 the seismic energy e is proportional to 1 x 1 x 2 m 0 2 t 0 3 where m 0 is seismic moment as long as x is greater than 0 1 or so the effect of rise time is not important the dynamic energies thus calculated for shallow events are in reasonable agreement with the estimate e 5 10 5 m 0 based on elastostatic considerations deep events despite their possibly different seismological character yield dynamic energies which are compatible with a static prediction similar to that for shallow events studies of strong motion velocity traces obtained near the sources of the 1971 san fernando 1966 parkfield and 1979 imperial valley earthquakes suggest that even in the distance range of 1 to 5 km most of the radiated energy is below 1 to 2 hz in frequency far field energy determinations using long period wwssn instruments are probably not in gross error despite their band limited nature the strong motion record for the intermediate depth bucharest earthquake of 1977 also suggests little teleseismic energy outside the pass band of a long period wwssn instrument

teleseismic estimates of the energy radiated by shallow earthquakes the energy flux contained in the p wave groups p pp sp or the s wave groups s ps ss radiated by a shallow earthquake is modeled assuming that the energy flux in the direct and depth phases adds incoherently by defining generalized radiation patterns which incorporate this neutral interference the wave groups are analyzed as though they were comprised of a single phase measurements of the energy flux in the wave groups are corrected in the frequency domain for both the body wave attenuation and the frequency band of the recording the corrected measurements are then used to estimate the seismic energy radiated by the earthquake this analysis is applied to digital recordings of the teleseismic wave groups radiated by the may 2 1983 coalinga california earthquake and the october 28 1983 borah peak idaho earthquake for the coalinga earthquake an estimate of es 1 6 0 4 1021 dyn cm was determined from six p wave groups while the sh wave group recorded at station col returned an estimate of es 1 2 1021 dyn cm for the borah peak earthquake an estimate ofes 3 2 0 5 1021 dyn cm was determined from seven p wave groups the distribution of the isoseismals for the borah peak earthquake indicate that the energy radiated by this event was focussed to the northwest in the direction of rupture propagation correcting the teleseismic estimate for this focussing gives es 3 6 0 5 1021 dyn cm combining these estimates of the radiated energy with broad band estimates of the seismic moment yields estimates of the apparent stress of 17 and 7 bars for the coalinga and the borah peak earthquakes respectively

the 1996 peru tsunamigenic earthquake broadband source process we investigate the spatio temporal complexity of moment release of the february 21 1996 peru earthquake mw 7 5 we use a non linear source tomographic technique based on simulated annealing to invert surface wave source spectra for the slip distribution on a gently dipping fault plane the spectra 5 65 mhz are obtained using an empirical green s function egf method applied to first and second orbit fundamental mode rayleigh waves spectra are well fit by a 110 km bilateral rupture subparallel to the trench updip of the hypocenter with a total moment of 2 0 1020 nm the non linear inversion reveals a 30 30 km major slip patch south the onset where rupture velocities attain 1 5 2 0 km s inversions of teleseismic broadband p and sh waves 10 500 mhz indicate little or no directivity consistent with the surface wave data the average source time functions for both the surface and body wave data are similar in shape and in duration 50 s part of the slip during the 1996 peru event occurred in a region of reduced background seismicity as was the case for the 1992 nicaragua slow event suggesting that the seismogenic potential of low seismicity regions near the trench should be globally reassessed

tsunami earthquakes the quest for a regional signal using the technique developed by newman and okal j geophys res 103 1998 26885 a dataset of digital records from 84 earthquakes is analyzed to investigate their source slowness in the quest for a possible regional signal in three subduction zones which experienced recent tsunami earthquakes nicaragua 1992 java 1994 peru 1996 the dataset is augmented by analog seismograms from historical events including major tsunamigenic earthquakes of the past 65 years we fail to detect a regional trend for slowness which suggests that the latter may be controlled on a more local scale by morphological structures of the subducting plate no correlation is found between slowness and either depth focal mechanism or seismic moment in nicaragua we document two slow historical earthquakes located on the slab down dip from the 1992 shock the most interesting results are in peru where a local area of slowness is tentatively defined around the source of the 1960 tsunami earthquake and where both the 1996 and 1960 tsunami earthquakes occur at the intersection of the trench with major topographic features on the nazca plate the mendana fracture zone and the trujillo trough respectively 2001 elsevier science b v all rights reserved

linear and nonlinear computations of the 1992 nicaragua earthquake tsunami numerical computations of tsunamis are made for the 1992 nicaragua earthquake using different governing equations bottom frictional values and bathymetry data the results are compared with each other as well as with the observations both tide gauge records and runup heights comparison of the observed and computed tsunami waveforms indicates that the use of detailed bathymetry data with a small grid size is more effective than to include nonlinear terms in tsunami computation linear computation overestimates the amplitude for the later phase than the first arrival particularly when the amplitude becomes large the computed amplitudes along the coast from nonlinear computation are much smaller than the observed tsunami runup heights the average ratio or the amplification factor is estimated to be 3 in the present case when the grid size of 1 minute is used the factor however may depend on the grid size for the computation

modeling the seismic source and tsunami generation of the december 12 1992 flores island indonesia earthquake on december 12 1992 a large earthquake m s 7 5 occurred just north of flores island indonesia which along with the tsunami it generated killed more than 2 000 people in this study teleseismicp andsh waves as well aspp waves from distances up to 123 are inverted for the orientations and time histories of multiple point sources by repeating the inversion for reasonable values of depth time separation and spatial separation a 2 fault model is developed next the vertical deformation of the seafloor is estimated from this fault model using a detailed bathymetric model linear and nonlinear tsunami propagation models are tested the data consist of a single tide gauge record at palopo 650 km to the north as well as tsunami runup height measurements from flores island and nearby islands assuming a tsunami runup amplification factor of two the two fault model explains the tide gauge record and the tsunami runup heights on most of flores island it cannot however explain the large tsunami runup heights observed near leworahang on hading bay and riangkroko on the northeast peninsula massive coastal slumping was observed at both of these locations a final model which in addition to the two faults includes point sources of large vertical displacement at these two locations explains the observations quite well

the mantle magnitude mm and the slowness parameter θ five years of real time use in the context of tsunami warning we study a database of more than 119 000 measurements of the mantle magnitude mm introduced by okal and talandier 1989 obtained since 1999 as part of the operational procedures at the pacific tsunami warning center the perfor mance of this method is significantly affected by the seismic instrumentation at the recording station with the very broadband sts 1 and ks54000 systems offering the lowest residuals between measured values of mm and those predicted from the har vard centroid moment tensor cmt catalog and also by the period at which spec tral amplitudes are measured with the best results between 70 and 250 sec with such mild restrictions estimates of seismic moments can be obtained in real time by retaining either the maximum value of mm measured on each record or its average over the various mantle frequencies with the resulting residuals on the order of 0 1 0 2 moment magnitude units mm deficiencies in the case of the two large earthquakes of peru 2001 and hokkaido 2003 are attributed to azimuthal bias from an excess of stations principally in north america in directions nodal for the focal mechanism and directivity patterns we further study a group of more than 3000 measurements of the energy to moment ratio h introduced by newman and okal 1998 which allows the real time identification of teleseismic sources violating scaling laws and in particular of so called tsunami earthquakes the use of a sliding window of analysis in the computation of h allows the separation of late earthquakes characterized by a delayed but fast moment release from truly slow earthquakes many such events are recognized notably on major oceanic and con tinental strike slip faults introduction and background we present detailed analyses of a large dataset resulting from implementation as part of the operational procedures of the pacific tsunami warning center ptwc of the real time calculation of the mantle magnitude mm and of the slowness parameter h as defined and introduced by okal and talandier 1989 and newman and okal 1998 re spectively during the past five years more than 119 000 individual measurements of mm and 3000 values of h have been computed through automated algorithms these values contribute to the estimation of the source characteristic of distant earthquakes in the framework of the real time as sessment of their tsunamigenic potential this study offers a progress report on the performance of the algorithms and in particular discusses the influence of instrumentation at the various reporting stations as the final revision of this article was being prepared the occurrence of the great sumatra earthquake on 26 de cember 2004 provided an opportunity to extend the concepts of mm and h to a range of magnitudes unexplored for the

assessment of the size of large and great historical earthquakes in peru a critical reappraisal of great historical earthquakes in peru is made in order to obtain quantitative estimates of rupture length seismic moment tsunami magnitude mt and moment magnitude mw newly collected information concerning several historical earthquakes permits the evaluation of macroseismic zones and from the size of these zones the estimation of length of rupture seismic moment and magnitude mw it is found that local tsunami data provide good quantitative estimates of mt and therefore an independent way of verifying macroseismic estimates of rupture length and seismic moment a space time diagram of great historical earthquakes shows the existence of three zones roughly corresponding to the segmentation of the nazca plate hasegawa and sacks 1981 subducting under peru the northern zone looks aseismic the central one is very complex it breaks either completely or in smaller segments and it may go through long quiescence periods the recurrence time for the southern segment the more regular one is of the order of one century

historical 1942 ecuador and 1942 peru subduction earthquakes and earthquake cycles along colombia ecuador and peru subduction segments two large shallow earthquakes occurred in 1942 along the south american subduction zone inclose proximity to subducting oceanic ridges the 14 may event occurred near the subducting carnegie ridge off the coast of ecuador and the 24 august event occurred off the coast of southwestern peru near the southern flank of the subducting nazca ridge source parameters for these for these two historic events have been determined using long periodp waveforms p wave first motions intensities and local tsunami data we have analyzed thep waves for these two earthquakes to constrain the focal mechanism depth source complexity and seismic moment modeling of thep waveform for both events yields a range of acceptable focal mechanisms and depths all of which are consistent with underthrusting of the nazca plate beneath the south american plate the source time function for the 1942 ecuador event has one simple pulse of moment release with a duration of 22 suconds suggesting that most of the moment release occurred near the epicenter the seismic moment determined from thep waves is 6 8 1020n m corresponding ot a moment magnitude of 7 8 7 9 the reported location of the maximum intensities ix for this event is south of the main shock epicenter the relocated aftershcks are in an area that is approximately 200 km by 90 km elongated parallel to the trench with the majority of aftershocks north of the epicenter in contrast the 1942 peru event has a much longer duration and higher degree of complexity than the ecuador earthquake suggesting a heterogeneous rupture seismic moment is released in three distinct pulses over approximately 74 seconds the largest moment release occurs 32 seconds after rupture initiation the seismic moment as determined from thep waves for the 1942 peru event is 10 25 1020n m corresponding to a moment magnitude of 7 9 8 2 aftershock locations reported by the iss occur over a broad area surrounding the main shock the reported locations of the maximum intensities ix are concentrated south of the epicenter suggesting that at least part of the rupture was to the south we have also examined great historic earthquakes along the colombia ecuador and peru segments of the south american subduction zone we find that the size and rupture length of the underthrusting earthquakes vary between successive earthquake cycles this suggests that the segmentation of the plate boundary as defined by earthquakes this century is not constant

teleseismic estimates of radiated seismic energy the e m 0 discriminant for tsunami earthquakes we adapt the formalism of boatwright and choy for the computation of radiated seismic energy from broadband records at teleseismic distances to the real time situation when neither the depth nor the focal geometry of the source is known accurately the analysis of a large data set of more than 500 records from 52 large recent earthquakes shows that this procedure yields values of the estimated energy ee in good agreement with values computed from available source parameters for example as published by the national earthquake information center neic the average logarithmic residual being only 0 26 units we analyze the energy to moment ratio by defining log10 ee m0 for regular earthquakes this parameter agrees well with values expected from theoretical models and from the worldwide neic catalogue there is a one to one correspondence between values of that are deficient by one full unit or more and the so called tsunami earthquakes previously identified in the literature as having exceedingly slow sources and believed due to the presence of sedimentary structures in the fault zone our formalism can be applied to single station measurements and its coupling to automated real time measurements of the seismic moment using the mantle magnitude mm should significantly improve real time tsunami warning

vulnerability of the dover strait to coseismic tsunami hazards insights from numerical modelling summary on 1580 april 6 a large earthquake shook the eastern english channel and its shores with numerous casualties and significant destruction documented some reports suggest that it was followed by a tsunami meanwhile earthquake magnitudes of mw 7 have been deemed possible on intraplate fault systems in neighbouring benelux this study aims to determine the possibilityofamw 5 5magnitudeearthquakegeneratingatsunamiinthedoverstrait oneof the world s busiest seaways in a series of numerical models focusing on sensitivity analysis earthquake source parameters for the dover strait are constrained by palaeoseismological evidence and historical accounts producing maps of wave heights and analysis of frequencies based on six strategically located virtual tide gauges of potential concern to engineering geologists a maximum credible scenario is also tested for mw 6 9 for earthquakes with mw of 5 5 none of the fault models we tested produced a tsunami on neighbouring shores however forearthquakeswithmw 6 9 bothextensionalandthrustingeventsproducedtsunami waves with open water amplitudes of up to 1 5 m and higher amplitudes might be expected in regions where waves are amplified by regional nearshore bathymetry sensitivity to parameter choice is emphasized but a pattern of densely inhabited coastal hotspots liable to tsunamirelated damage because of bathymetric forcing factors is consistently obtained

structure and seismic stratigraphy of the griptarane area more basin margin mid norway continental shelf abstract the mesozoic succession on the landward side of griptarane offshore mid norway reflects an upward change from lower jurassic alluvial deposits to middle jurassic marine continental deposits into upper jurassic lower cretaceous marine deposits in the latest jurassic there was deposition of submarine fans along faults associated with major extensional deformation and fault block rotation probably in volgian times this phase coincided with nw se extension and rifting in the slorebotn sub basin and at haltenbanken an onlap surface is interpreted to represent the kimmerian unconformity and to mark the approximate position of the jurassic cretaceous boundary some wnw ese to e w oriented faults exhibit reverse movements while e w trending folds indicate n s compression this compression is interpreted to coincide with a late early cretaceous phase of sinistral transpression in the gossa high and slorebotn sub basin and inversion and doming in the more and voring basins the predominance of wnw ese to nw se and ene wsw to ne sw trending structures indicates that the griptarane area has been influenced by the stress regimes of the more trondelag fault complex as well as the stress regimes in the regions further offshore

great earthquakes and subduction along the peru trench subduction along the peru trench between 9 and 15 s involves both large interplate underthrusting earthquakes and intraplate normal fault earthquakes the four largest earthquakes along the peru trench are from north to south the 1970 m 7 9 intraplate normal fault earthquake and the interplate underthrusting earthquakes in 1966 m 8 0 1940 m 8 and 1974 m 8 0 we have studied the rupture process of these earthquakes and can locate spatial concentrations of moment release through directivity analysis of source time functions deconvolved from long period p wave seismograms the 1966 earthquake has a source duration of 45 s with most of the moment release concentrated near the epicenter two intraplate normal fault events occurred in 1963 m 6 7 and 7 0 at the down dip edge of the 1966 dominant asperity the 1940 earthquake is an underthrusting event with a simple source time function of 30 s duration that represents the rupture of a single asperity near the epicenter the 1974 earthquake has a source duration of 45 50s and two pulses of moment release this earthquake has a bilateral rupture with the first pulse of moment release located northwest of the epicenter and the second pulse of moment release located southeast of the epicenter both pulses of moment release occur on the northern half of the aftershock area the 1970 earthquake is one of the largest intraplate normal fault earthquakes to occur in a subduction zone and has a moment release comparable with many large underthrusting events the aftershocks for the 1970 earthquake form two distinct clusters the smaller cluster near the epicenter has focal mechanisms characterized by down dip tension but the second aftershock cluster located 80 km southeast of the epicenter has focal mechanisms characterized by down dip compression the p waves for the main shock can be modeled as two sources with different focal mechanisms and depths similar to the two clusters of aftershocks the first event has a down dip tensional focal mechanism and is followed 40 s later by a distinct second event located 80 km southeast of the epicenter with a down dip compressional focal mechanism and a somewhat shallower depth than the first event the observable directivity indicates that the second source is located at the second cluster of aftershocks that have down dip compressional focal mechanisms the occurrence of both down dip tensional and compressional focal mechanisms may be explained by extreme unbending stresses associated with the anomalous slab geometry the unusually large size of the 1970 earthquake may also be related to the subduction of the mendana fracture zone the historic earthquake record along the peru trench indicates that the previous event in 1746 was much larger than any of the three underthrusting earthquakes this century the 1746 earthquake may have ruptured the entire segment in a multiple asperity earthquake thus the mode of rupture along the peru coast has changed between successive earthquake cycles

mm a variable period mantle magnitude this paper lays the theoretical groundwork for a variable period mantle magnitude mm based on the measurement of the spectral amplitude x of very long period rayleigh waves we retain the concept of magnitude by restricting ourselves to single station measurements ignoring the focal mechanism and the exact depth of the shallow earthquakes considered our measurements are made at a series of periods in all cases greater than 40 s and the largest value is retained this procedure effectively avoids the well known interference effects leading to saturation of magnitude scales defined at a fixed period two corrections are used a period dependent distance correction cd and a source correction cs also period dependent compensating for the variation of the excitation of rayleigh waves with period both of these corrections are fully predictable on the basis of standard surface wave excitation and dispersion theory the result is a formula of the type mm log10 x cd cs c0 in which all coefficients including the constant c0 are justifiable on sound theoretical grounds the analysis of a data set of 256 records from the broadband seismograph at papeete tahiti the ultra long period system at pasadena and stations of the geoscope network shows that the mean error in the estimation of the seismic moment is on the order of 0 1 0 2 units of magnitude with the standard deviation at each station also on the order of 0 2 units of magnitude no significant trend with either distance period or station can be identified the method can also be transposed to the time domain under some simple assumptions which are justifiable theoretically for typical teleseismic distances across the pacific basin both versions of the method lend themselves well to automation thus by providing a real time estimate of the seismic moment of distant earthquakes mm has considerable potential for tsunami warning purposes its concept can easily be extended to love waves and also to intermediate and deep earthquakes

estimation of seismic moment and slip distribution of the april 1 1946 aleutian tsunami earthquake the 1946 aleutian earthquake produced a tsunami of tsunami magnitude mt 9 3 but the surface wave magnitude is only ms 7 4 making it a tsunami earthquake the discrepancy between the apparent size of the earthquake based on the seismic data and the size of the tsunami has been explained by several mechanisms including a landslide and a slow earthquake but there are few seismic data available to determine the correct mechanism we study the generating mechanism of the tsunami using tsunami waveforms recorded on tide gauges we have modeled the source of the 1946 aleutian tsunami as the result of an underthrusting earthquake we performed both forward and inverse modeling of the data using a finite difference calculation to compute synthetic tsunamis we include both vertical and horizontal deformation of the ocean bottom due to faulting in the computation the results of the inversion of the tsunami waveforms show that the slip on the fault is mainly concentrated in the shallow section of the fault near the epicenter there is little slip in the area of the aftershocks indicating that the aftershocks may represent an area of afterslip or triggered seismicity the moment estimate of the earthquake is 23 1020 n m or mw 8 2 the majority of the waveforms are well explained by this fault model and slip distribution including the honolulu waveform but the hawaiian data also suggest that other effects such as a landslide may be necessary for explaining the abnormal tsunami amplitudes in hawaii the 1946 earthquake is similar to other tsunami earthquakes such as the 1896 sanriku and 1992 nicaragua earthquakes indicating that slow shallow rupture may explain the disproportionally large tsunamis

subduction of the nazca plate beneath peru as determined from seismic observations a detailed reinvestigation of seismicity in the southern half of peru using data from a local seismic network and a focal mechanism study using p wave first motions reveal that for the first 100 km of descent the slab as defined by this seismicity enters at a normal dip angle near 30 and that below this depth it is bent to a nearly horizontal angle this horizontal slab extends eastward for about 300 km and then dips steeply below the thick continental lithosphere scsp observations are found to be consistent with the profile of the subducting plate as determined from the local seismicity beneath the region from southern peru to northern chile the nazca plate descends with an almost constant dip angle 30 down to at least 300 km depth between the 30 dipping plate in southern peru and the horizontal profile beneath central peru there is a contortion over a 100 km wide continuous lateral section of the subducting plate we infer that at least in the upper 150 km depth range the descending nazca plate is contorted rather than torn

variations in the mode of great earthquake rupture along the central peru subduction zone the historic record for the central peru subduction zone suggests significant variations in the earthquake size during the last 400 years during this century there have been four great underthrusting earthquakes along the central peru seismic zone from north to south these are the 17 october 1966 mw 8 1 24 may 1940 m 8 3 october 1974 mw 8 1 and 24 august 1942 m 8 2 earthquakes modified mercalli intensity data and tsunami observations for the earthquakes in this century are compared with the 29 october 1746 and 20 october 1687 earthquakes the 1746 earthquake had maximum intensity values between 9 and 13 s while the 1687 event had maximum values between 12 and 14 s suggesting that the two events failed different segments of the subduction zone we find that the 1746 event occurred along the segiment that includes both 1940 and 1966 earthquakes the size of the 1746 event is estimated to be mw 8 8 based on the ratio of near field tsunami heights for the 1746 and 1966 earthquakes the 1687 earthquake probably raptured the 1974 segment as well as the adjacent segment to the south where there is at present a gap between the 1942 and 1974 rupture zones the size of the 1687 event is estimated to be mw 8 7 based on both far field and near field tsunami height ratios of the 1687 and 1974 events both 1746 and 1687 earthquakes appear to be much larger than the events of this century in contrast to the simple single asperity nature of the 20th century earthquakes these older and larger events may represent multiple asperity ruptures along the peru subduction zone hence variations in the mode of earthquake rupture from cycle to cycle along the central peru seismic zone may explain the significant difference in earthquake size during die last 400 years

the november 20 1960 peru tsunami earthquake source mechanism of a slow event the november 20 1960 peru earthquake excited a tsunami that was anomalously large for an earthquake of moderate magnitude m 6 75 pas body wave first motions combined with observed love to rayleigh wave spectral ratios indicate slip along a shallowly dipping thrust fault rather than a single force mechanism suggestive of a landslide deconvolution of sh waveforms shows a time function consisting of 4 subevents with a total duration of 110 s using this time function body waveform inversion yields a very shallowly dipping thrust faulting mechanism strike 0 dip 6 slip 100 and a seismic moment of 1 9 1027 dyn cm however the time function obtained from the band limited body waveforms cannot fit the surface wave data which show a large increase in amplitude with period and a moment of 3 4 1027 dyn cm both types of data can be modeled by combining the body wave time function and a 130 s cosine time function representing a longer period component this event represents the largest earthquake to occur along this relatively aseismic portion of the peruvian coast the tsunami excitation is not anomalous relative to the measured seismic moment and moment magnitude mw 7 6 the disparity between tsunami height and surface wave magnitude results from underestimation of the earthquake size by conventional magnitude scales due to the long source duration

source mechanism of the magnitude 7 2 grand banks earthquake of november 1929 double couple or submarine landslide we have examined p s and surface waves derived from seismograms that we collected for the 1929 grand banks canada earthquake this event is noteworthy for the sediment slide and turbidity current that broke the trans atlantic cables and for its destructive tsunami both the surface wave magnitude m s and the body wave magnitude m b calculated from these seismograms are 7 2 fault mechanisms previously suggested for this event include a nw se striking strike slip mechanism and an approximately e w striking thrust mechanism in addition because of the presence of an extensive area of slump and turbidity current there exists the possibility that sediment slumping could also be a primary causative factor of this event we tested these fault models and a horizontal single force oriented n5 w model representing a sediment slide against our data among these models only the single force model is consistent with the p s and surface wave data our data however do not preclude fault models which were not tested from the spectral data of love waves at a 50 sec period we estimated the magnitude of the single force to be about 1 4 10 20 dynes from this value we estimated the total volume of sedimentary slumping to be about 5 5 10 11 m 3 which is approximately 5 times larger than a recent estimate of volume from in situ measurements the difference in estimates of overall volume is likely due to a combination of the inherent difficulty in estimating accurately the displaced sediments from in situ measurements and of inadequacy of the seismic model or perhaps because not only the slump but also a tectonic earthquake could have been the cause of this event and contributed significantly to the waveforms studied

loci and maximum size of thrust earthquakes and the mechanics of the shallow region of subduction zones earthquakes do not extend updip along the plate interface at most subduction zones all of the way to the plate boundary at the trench axis or deformation front rather the shallowest part of that interface moves mainly through stable aseismic slip that part of the plate boundary referred to here as the aseismic zone occurs along the base of an accreted wedge of young sediments the probable primary cause for the existence of this aseismic zone is the stable slip properties of the unconsolidated and semiconsolidated sediments in that zone subducted sediment is progressively dewatered and underplated to the base of the overriding plate through this process more consolidated rocks eventually come into contact at depth across both sides of the plate boundary from the point where sufficiently hard rock is found across the plate interface that interface will change its slip behavior to unstable stick slip sliding which is characteristic of consolidated material under most conditions this type of motion is accommodated seismically as episodic slip in large earthquakes the location of this transition to seismic behavior referred to as the seismic front marks the deep end of the aseismic zone and the top of the seismogenic zone i e that part of the plate interface that moves primarily in thrust earthquakes several convergent plate margins are discussed to illustrate the seismic front and the aseismic zone we find that the seismic front defined by smaller earthquakes that occur during the interval between large events is nearly the same as that for large events as inferred from the locations of their aftershocks the location of the seismic front is important for making estimates of the maximum possible size of thrust earthquakes along the plate boundary because it delimits the trenchward limit of the potential rupture area of these events the size of large thrust earthquakes is proportional to rupture area and to at least the cube of the downdip width w of that area thus by defining the location of the seismic front for convergent margins it is possible to make better estimates of w and hence to deduce the maximum size of future interplate earthquakes the average repeat time of such events is also related to w and so better estimates of average repeat time are possible from improved knowledge of w the hypothesis that the location of the seismic front is related to the maximum depth of subduction of unconsolidated sediment has implications for forearc mechanics we use a mechanical analysis laboratory modeling and multichannel seismic information to develop a simple model explaining the growth of forearcs the outer arc high or trench slope break that occurs arcward of most accretionary wedges represents an abrupt change in the critical taper of the accretionary wedge we argue that that change is caused by a large arcward increase in the strength of the material within the overriding plate that stronger material is called the backstop our laboratory modeling experiments indicate that a backstop with a trenchward dipping upper surface results in the development of an overlying structure that includes all of the primary morphologic features observed in modern forearcs in our modeling an accretionary wedge develops trenchward of the backstop an outer arc high develops above the trenchward toe of the backstop and farther arcward a passive forearc basin forms above the stronger material of the backstop this model is consistent with the observation that earthquakes do not extend updip along the plate interface all the way to the trench axis we hypothesize that plate motion is accommodated seismically along the base of the consolidated backstop and mostly or entirely aseismically trenchward of the toe of the backstop along the base of the accretionary wedge at several margins the seismic front is approximately coincident with the outer arc high supporting this interpretation

tsunami earthquakes slow thrust faulting events in the accretionary wedge the november 20 1960 peru october 20 1963 kurile and june 10 1975 kurile earthquakes are classified as tsunami earthquakes based on anomalously large tsunami excitation relative to earthquake magnitude long period surface wave analysis indicates double couple faulting mechanisms for all three events rather than single force mechanisms indicative of submarine landslides the earthquakes have shallow depths 15 km and are located near the trench axis and seaward of most other thrust zone events beneath the accretionary prism body waveform inversion indicates very shallowly dipping thrust faulting mechanisms for the three events with dip angles of 6 8 surface wave spectral amplitudes and deconvolution of sh waveforms suggests anomalously long source durations and large seismic moments relative to ms specifically the 1963 kurile event ms 7 2 shows a duration of 85 s and a moment of 6 0 1027 dyn cm mw 7 8 the 1975 kurile event ms 7 0 shows a duration of 60 s and a moment of 2 0 1027 dyn cm mw 7 5 and the 1960 peru event ms 6 75 shows a time function consisting of four subevents with a total duration of 110 130 s and a seismic moment of 3 4 1027 dyn cm mw 7 6 estimated rupture velocities are about 1 km s or less but there is no evidence of unusually low stress drops the august 1 1968 philippines event previously classified as a tsunami earthquake shows none of the anomalous source properties and teleseismic tsunami height measurements are sparse we do not consider this event a tsunami earthquake most of the anomalous tsunami excitation results from underestimation of earthquake size by ms due to the long source duration the tsunami heights are not significantly anomalous relative to seismic moment the slow nature of these events may result from rupture through the sedimentary rock along the basal decollement of the accretionary prism standard scaling laws when adjusted for the slow seismic velocity in the source region show an mw ms relationship similar to that observed for the tsunami earthquakes and predict ms saturation at about 7 3 rather than 8 0 for typical events

numerical simulation of the 1992 flores tsunami interpretation of tsunami phenomena in northeastern flores island and damage at babi island numerical analysis of the 1992 flores island indonesia earthquake tsunami is carried out with the composite fault model consisting of two different slip values computed results show good agreement with the measured runup heights in the northeastern part of flores island except for those in the southern shore of hading bay and at riangkroko the landslides in the southern part of hading bay could generate local tsunamis of more than 10 m the circular arc slip model proposed in this study for wave generation due to landslides shows better results than the subsidence model it is however difficult to reproduce the tsunami runup height of 26 2 m at riangkroko which was extraordinarily high compared to other places the wave propagation process on a sea bottom with a steep slope as well as landslides may be the cause of the amplification of tsunami at riangkroko the simulation model demonstrates that the reflected wave along the northeastern shore of flores island accompanying a high hydraulic pressure could be the main cause of severe damage in the southern coast of babi island

source characteristics of the 1992 nicaragua tsunami earthquake inferred from teleseismic body waves we analyzed the broadband body waves of the 1992 nicaragua earthquake to determine the nature of rupture the rupture propagation was represented by the distribution of point sources with moment rate functions at 9 grid points with uniform spacing of 20 km along the fault strike the moment rate functions were then parameterized and the parameters were determined with the least squares method with some constraints the centroid times of the individual moment rate functions indicate slow and smooth rupture propagation at a velocity of 1 5 km s toward nw and 1 0 km s toward se including a small initial break which precedes the main rupture by about 10 s we obtained a total source duration of 110 s the total seismic moment is m o 3 4 1020 nm which is consistent with the value determined from long period surface waves m o 3 7 1020 nm the average rise time of dislocation is determined to be 10s the major moment release occurred along a fault length of 160 km with the assumption of a fault width w 50 km we obtained the dislocation d 1 3 m from and d the dislocation velocity is d d 0 1 m s significantly smaller than the typical value for ordinary earthquakes the stress drop 1 1 mpa is also less than the typical value for subduction zone earthquakes by a factor of 2 3 on the other hand the apparent stress defined by 2 e s m o where and e s are respectively the rigidity and the seismic wave energy is 0 037 mpa more than an order of magnitude smaller than the nicaragua tsunami earthquake is characterized by the following three properties 1 slow rupture propagation 2 smooth rupture 3 slow dislocation motion

shallow subduction zone earthquakes and their tsunamigenic potential summary we have examined the source spectra of all shallow subduction zone earthquakes from 1992 to 1996 with moment magnitude 7 0 or greater as well as some other interesting events in the period range 1 20 s by computing moment rate functions of teleseismic p waves after comparing the source spectral characteristics of tsunami earthquakes earthquakes that are followed by tsunamis greater than would be expected from their moment magnitude with regular events we identified a subclass of this group slow tsunami earthquakes this subclass consists of the 1992 nicaragua the 1994 java and the february 1996 peru earthquakes we found that these events have an anomalously low energy release in the 1 20 s frequency band with respect to their moment magnitude although their spectral drop ov is comparable to those of the other earthquakes from an investigation of the centroid and body wave locations it appears that most earthquakes in this study conformed to a simple model in which the earthquake nucleates in a zone of compacted and dehydrated sediments and ruptures up dip until the stable sliding friction regime of unconsolidated sediments stops the propagation sedimentstarved trenches e g near jalisco can produce very shallow slip because the fault material supports unstable sliding the slow tsunami earthquakes also ruptured up dip however their centroid is located unusually close to the trench axis the subduction zones in which these events occurred all have a small accretionary prism and a thin layer of subducting sediment ocean surveys show that in these regions the ocean floor close to the trench is highly faulted we suggest that the horst and graben structure of a rough subducting oceanic plate will cause contact zones with the overriding plate making shallow earthquake nucleation and up dip propagation to the ocean floor possible the rupture partly propagates in sediments making the earthquake source process slow two factors have to be considered in the high tsunami generating potential of these events first the slip propagates to shallow depths in low rigidity material causing great deformation and displacement of a large volume of water second the measured seismic moment may not represent the true earthquake displacement because the elastic constants of the source region are not taken into account in the standard cmt determination

triple seismic zone and the regional variation of seismicity along the northern honshu arc the regional variation of seismicity along the northern honshu arc japan is studied using accurate focal depths and focal mechanism types we use focal depths determined from pp p time intervals reported in the isc bulletins for submarine earthquakes depths are corrected by considering the pp phase reported in the bulletins as the pwp phase the reflection from the ocean surface out of more than 600 well located earthquakes selected from the isc bulletins we determine the types of the focal mechanisms of 184 events using p wave first motion data based on historical seismicity of great and large earthquakes we divide the zone of thrust type earthquakes at the plate interface into two regions the shallow thrust zone 0 40kin where great earthquakes ms 8 0 occur and the deep thrust zone 40 60km where large earthquakes ms 7 4 occur the activity of great or large earthquakes shows a variation along the are in some regions both the shallow and deep thrust zones are active and in other regions only one of the thrust zones is active the seismicity of recent moderate size earthquakes mb 4 combined with the focal mechanism type shows a variation along the are which reflects the variation of the activity of great or large earthquakes where large earthquakes do not occur in the deep thrust zone neither thrust type nor down dip compression tension type events occur in and beneath the deep thrust zone where large earthquakes do occur in the deep thrust zone we find a number of thrust type earthquakes further in the latter case in some regions the down dip compression and tension type events of the double seismic zones extend seaward just beneath the deep thrust zone and form a triple planed structure of seismicity the triple seismic zone this study confirms the hypothesis of previous workers seno and pongsawat 1981 on the causal relation between the strong seismic coupling of two converging plates at the deep thrust zone and extension of the double seismic zone i e the presence or absence of activity within the slab beneath the deep seismic zone occurs when the deep thrust zone has a strong or weak coupling respectively here the weak coupling could be interpreted as either aseismic slip or as low stress buildup since the last large event occurred at the deep thrust zone triple seismic zones are found offshore of miyagi prefecture where the deep thrust zone has been broken recently in 1978 and offshore of fukushima prefecture we expect a future large earthquake at the deep thrust zone offshore of fukushima prefecture because the presence of the triple seismic zone suggests stress has been accumulating and 40 years have passed since the deep thrust zone was ruptured in 1938

monte carlo slip inversion in the frequency domain application to the 1992 nicaragua slow earthquake the spatio temporal complexity of moment release of the 1992 nicaragua slow earthquake mw 7 7 is estimated by simulated annealing contrary to linearized inversions monte carlo techniques do not make assumptions about a priori features of the model in particular large variations in rupture velocities can be imaged relative spectra between the main shock and an empirical green s function were obtained from 4 44 mhz using first and second orbit fundamental mode rayleigh waves the spectra were inverted for the moment distribution on a 2d horizontal fault under the physical constraints of slip positivity and causality the non linear inversion revealed two main regions of slip one centered 20 km to the nw of the epicenter the other 160 km to the se but delayed by 70 s rupture velocities in the slipping patches were found to be low around 1 km s large amount of slip occurred trench ward of the epicenter accounting for the large tsunami excitation analysis of the slip distribution suggests that the slow and smooth nature of the 1992 nicaragua event may be related to the frictional properties at the plate interface

analysis and automatic processing in near field of eight 1992 1994 tsunamigenic earthquakes improvements towards real time tsunami warning we study eight tsunamigenic earthquakes of 1992 1994 with data from single near field 3 component long period stations the analysis is made from the standpoint of tsunami warning by an automatic process which estimates the epicentral location and the seismic moment through the variable period mantle magnitude m m simulations of early warning based on the real time computation of the seismic moment are also tested with this system which would give a justified warning in each region of tsunami potentiality by exploiting the dependence of moment rate release with frequency the system has the capability of recognizing both tsunami earthquakes such as the 1992 nicaragua and 1994 java events as well as instances of the opposite case of low frequency deficiency interpreted as indicating a deeper than normal source 1993 guam event we report both the results of delayed time processing of the near field stations and the actual real time warnings at ppt which confirm the former

use of the mantle magnitude m m for the reassessment of the moment of historical earthquakes the mantle magnitudemm is used on a dataset of more than 180 wavetrains from 44 large shallow historical earthquakes to reassess their moments which in many cases had been previously estimated only on the basis of the earthquake s rupture area we provide 27 new or revised values ofmo based on the spectral amplitudes of surface waves recorded at a number of stations principally uppsala and pasadena among them and most significantly we document a large low frequency component to the source of the 1923 kanto earthquake the low frequency seismic moment is 2 9 1028 dyn cm in accord with geodetic observations on the other hand we revise downwards the seismic moment of the 1906 ecuador event which did not exceed 6 1028 dyn cm

the 2010 mw 7 8 mentawai earthquake very shallow source of a rare tsunami earthquake determined from tsunami field survey and near field gps data 1 the mw 7 8 october 2010mentawai indonesia earthquake was a tsunami earthquake a rare type of earthquake that generates a tsunami much larger than expected based on the seismicmagnitude itproducedalocallydevastatingtsunami withrunupcommonlyinexcess of 6 m we examine this event using a combination of high rate gps data from instruments located on the nearby islands and a tsunami field survey the gps displacement time series are deficient in high frequency energy and show small coseismic displacements 16 m our modeling results show that the combination of the small gps displacements and large tsunami can only be explained by high fault slip at very shallow depths far from the islands and close to the oceanic trench inelastic uplift of trench sediments likely contributed to the size of the tsunami recent results for the 2011 mw 9 0 tohoko oki earthquake have also shown shallow fault slip but the results from our study which involves a smaller earthquake provide much stronger constraints on how shallow the rupture can be with the majority of slip for the mentawai earthquake occurring at depths of 6 km this result challenges the conventional wisdom that the shallow tips of subduction megathrusts are aseismic and therefore raises important questions both about the mechanical properties of the shallow fault zone and the potential seismic and tsunami hazard of this shallow region

tsunami generation by horizontal displacement of ocean bottom tsunami generation by an earthquake is generally modeled by water surface displacement identical to the vertical deformation of ocean bottom due to faulting the effect of horizontal deformation is usually neglected however when the tsunami source is on a steep slope and the horizontal displacement is large relative to the vertical displacement the effect becomes significant we show this for two recent earthquakes which generated much larger tsunamis than expected from seismic waves in the case of the 1994 june 2 java indonesia earthquake the focal mechanism was a very shallow dipping thrust and the source was near a very steep trench slope in the case of the 1994 nov 14 mindoro philippines earthquake strike slip faulting extended from ocean to land perpendicular to the coast line in both cases we found that the horizontal motion of slope had an important contribution to the tsunami generation

lithium isotopic composition of central american volcanic arc lavas implications for modification of subarc mantle by slab derived fluids abstract li contents and isotopic compositions were determined for a suite of well characterized basaltic lavas from the central american volcanic arc cava variable li y 0 2 0 5 li sc 0 1 0 4 and 6 li values 2 6 to 7 7 attest to significant compositional heterogeneity in the subarc mantle within specific arc segments these parameters correlate strongly with each other and with a number of other constituents e g k rb ba b la 10 be 9 be 87 sr 86 sr u ce and 230 th 232 th among others these correlations are particularly strong for nicaragua samples coupling of this particular set of constituents is best explained in terms of addition of subduction components to the subarc mantle moreover their selective enrichment with respect to relatively fluid immobile incompatible elements signifies the dominance of fluid vs silicate melt transport of slab components to the subarc mantle several interesting nuances are revealed by the li data first although li and b are strongly correlated in both costa rica and nicaragua there are systematic along strike variations in li b that are consistent with these elements having different fluid release patterns from subducted slab segments for example li b is highest in costa rica where auxiliary evidence indicates higher subduction zone temperatures apparently b is preferentially depleted and li retained in the slab under warmer conditions the same relations are reflected in li 10 be and other subduction tracer systematics all of which point to larger subduction contributions below nicaragua yet even nicaragua lavas vary widely in levels of subduction enrichment high ti basalts from nejapa are the least enriched and have the highest 6 li 1 4 to 2 6 these values are greater than in fresh morb ca 4 and are not easily explained by additions of subducted li because most oceanic crustal rocks and marine sediments have lower 6 li than morb with typical values between 8 and 20 thus it appears the nejapa data may be representative of isotopically light mantle domains relatively light 6 li values in an undepleted spinel lherzolite 11 3 from zabargad is red sea and in primitive backarc basalts 1 6 to 0 5 from lau basin support this conclusion considering representative fluid and mantle endmember compositions the cava data are consistent with limited up to a few percent additions of slab derived fluids to a heterogeneous mantle containing variably depleted and enriched domains to form the respective magma sources in our view the subarc mantle is heterogeneous on a small scale but some arc sectors clearly received greater slab inputs than others

metastable mantle phase transformations and deep earthquakes in subducting oceanic lithosphere earth s deepest earthquakes occur as a population in subducting or previously subducted lithosphere at depths ranging from about 325 to 690 km this depth interval closely brackets the mantle transition zone characterized by rapid seismic velocity increases resulting from the transformation of upper mantle minerals to higher pressure phases deep earthquakes thus provide the primary direct evidence for subduction of the lithosphere to these depths and allow us to investigate the deep thermal thermodynamic and mechanical ferment inside slabs numerical simulations of reaction rates show that the olivine spinel transformation should be kinetically hindered in old cold slabs descending into the transition zone thus wedge shaped zones of metastable peridotite probably persist to depths of more than 600 km laboratory deformation experiments on some metastable minerals display a shear instability called transformational faulting this instability involves sudden failure by localized superplasticity in thin shear zones where the metastable host mineral transforms to a denser finer grained phase hence in cold slabs such faulting is expected for the polymorphic reactions in which olivine transforms to the spinel structure and clinoenstatite transforms to ilmenite it is thus natural to hypothesize that deep earthquakes result from transformational faulting in metastable peridotite wedges within cold slabs this consideration of the mineralogical states of slabs augments the traditional largely thermal view of slab processes and explains some previously enigmatic slab features it explains why deep seismicity occurs only in the approximate depth range of the mantle transition zone where minerals in downgoing slabs should transform to spinel and ilmenite structures the onset of deep shocks at about 325 km is consistent with the onset of metastability near the equilibrium phase boundary in the slab even if a slab penetrates into the lower mantle earthquakes should cease at depths near 700 km because the seismogenic phase transformations in the slab are completed or can no longer occur substantial metastability is expected only in old cold slabs consistent with the observed restriction of deep earthquakes to those settings earthquakes should be restricted to the cold cores of slabs as in any model in which the seismicity is temperature controlled via the distribution of metastability however the geometries of recent large deep earthquakes pose a challenge for any such models transformational faulting may give insight into why deep shocks lack appreciable aftershocks and why their source characteristics including focal mechanisms indicating localized shear failure rather than implosive deformation are so similar to those of shallow earthquakes finally metastable phase changes in slabs would produce an internal source of stress in addition to those due to the weight of the sinking slab such internal stresses may explain the occurrence of earthquakes in portions of lithosphere which have foundered to the bottom of the transition zone and or are detached from subducting slabs metastability in downgoing slabs could have considerable geodynamic significance metastable wedges would reduce the negative buoyancy of slabs decrease the driving force for subduction and influence the state of stress in slabs heat released by metastable phase changes would raise temperatures within slabs and facilitate the transformation of spinel to the lower mantle mineral assemblage causing slabs to equilibrate more rapidly with the ambient mantle and thus contribute to the cessation of deep seismicity because wedge formation should occur only for fast subducting slabs it may act as a parachute and contribute to regulating plate speeds wedge formation would also have consequences for mantle evolution because the density of a slab stagnated near the bottom of the transition zone would increase as it heats up and the wedge transforms to denser spinel favoring the subsequent sinking of the slab into the lower mantle