inversion of complex body waves we have developed a method that inverts seismic body waves to determine the mechanism and rupture pattern of earthquakes the rupture pattern is represented as a sequence of subevents distributed on the fault plane this method is an extension of our earlier method in which the subevent mechanisms were fixed in the new method the subevent mechanisms are determined from the data and are allowed to vary during the sequence when subevent mechanisms are allowed to vary however the inversion often becomes unstable because of the complex trade offs between the mechanism the timing and the location of the subevents many different subevent sequences can explain the same data equally well and it is important to determine the range of allowable solutions some constraints must be imposed on the solution to stabilize the inversion we have developed a procedure to explore the range of allowable solutions and appropriate constraints in this procedure a network of grid points is constructed on the i plane where and i are respectively the onset time and the distance from the epicenter of a subevent the best fit subevent is determined at all grid points then the correlation is computed between the synthetic waveform for each subevent and the observed waveform the correlation as a function of and i and the best fit mechanisms computed at each i grid point depict the character of allowable solutions and facilitate a decision on the appropriate constraints to be imposed on the solution the method is illustrated using the data for the 1976 guatemala earthquake

a new self organizing mechanism for deep focus earthquakes the mechanism of deep focus earthquakes has been a puzzle since their discovery almost 70 years ago1 2 because brittle fracture and frictional sliding at depths in excess of 100 200 km would require unrealistic rock strengths3 4 rock strength does increase with pressure but a few hundred mpa equivalent to 10 20 km depth suffices to inhibit most fracture and elevated temperature activates ductile mechanisms that operate at stresses less than the fracture stength a range of mechanisms has been proposed for deep earthquakes including plastic instabilities5 7 shear induced melting8 11 and instabilities accompanying recrystallization12 13 or polymorphic phase transformation14 23 each of these proposed mechanisms has exhibited certain inherent weaknesses see kirby22 for review and discussion here we report experimental observations of high pressure faulting of metastable mg2geo4 olivine as it undergoes incipient transformation to a spinel 24 25 structure we present a model in which this faulting arises from the generation propagation and linking up of spinel filled anticracks26 when applied to the olivine spinel transformation in the earth s mantle the anticrack model27 satisfactorily accounts for the similarities and differences between shallow and deep earthquakes and removes the problems associated with frictional sliding

rupture characteristics of the deep bolivian earthquake of 9 june 1994 and the mechanism of deep focus earthquakes the m w 8 3 deep 636 kilometers bolivian earthquake of 9 june 1994 was the largest deep focus earthquake ever recorded seismic data from permanent stations plus portable instruments in south america show that rupture occurred on a horizontal plane and extended at least 30 by 50 kilometers rupture proceeded at 1 to 3 kilometers per second along the down dip azimuth of the slab and penetrated through more than a third of the slab thickness this extent is more than three times that expected for a metastable wedge of olivine at the core of the slab and thus appears to be incompatible with an origin by transformational faulting these large events may instead represent slip on preserved zones of weakness established in oceanic lithosphere at the earth s surface

observations of exotic inner core waves the seismic structure of earth s inner core is highly complex displaying strong anisotropy and further regional variations however few seismic waves are sensitive to the inner core and fundamental questions regarding the origin of the observed seismic features remain unanswered thus new techniques to observe different types of inner core waves are imperative to improve data coverage here we detail our method for detecting exotic inner core phases such as pkjkp and pkiikp using inner core compressional waves as proof of concept we use phase weighted stacking on long period data from a global distribution of stations and employ several synthetic methods including normal mode summation and specfem to identify and confirm the inner core phases we present evidence for two observations of exotic inner core compressional waves and apply the technique to a previously detected inner core shear wave a possible new inner core shear wave remains unconfirmed additionally we show how our method is important for rejecting potential observations and distinguishing between waves with similar traveltime and slowness the method is most successful for detecting exotic inner core compressional waves and will provide a new approach for studying the compressional wave structures in the upper inner core

observational constraints on the fracture energy of subduction zone earthquakes we relate seismologically observable parameters such as radiated energy seismic moment rupture area and rupture speed to the dynamics of faulting to achieve this objective we computed the radiated energy for 23 subduction zone earthquakes recorded between 1992 and 2001 most of these earthquakes have a magnitude m w 7 5 but we also included some smaller m w 6 7 well studied subduction zone earthquakes and six crustal earthquakes we compiled static stress drop estimates for these 29 earthquakes from literature and used a slip weakening model to determine the radiation efficiency of these earthquakes we also determined the rupture speed of these earthquakes from literature from fracture mechanics fracture energy and hence radiation efficiency can be related to the rupture speed the radiation efficiencies estimated from the partitioning of energy as given by the slip weakening model are consistent with the rupture speed estimated for these earthquakes most earthquakes have radiation efficiencies between 0 25 and 1 and are hence efficient in generating seismic waves but tsunami earthquakes and two deep earthquakes the 1994 bolivia and the 1999 russia china border earthquakes have very small radiation efficiencies 0 25 and hence dissipate a large amount of energy during faulting we suggest that differences in the radiation efficiencies of different types of earthquakes could be due to fundamental differences in their rupture mechanics in deep events the energy is probably dissipated in thermal processes in the fault zone while it is possible that the morphology of the trench causes large energy dissipation during the rupture process of tsunami earthquakes

the 1996 june 17 flores sea and 1994 march 9 fiji tonga earthquakes source processes and deep earthquake mechanisms summary source parameters of the 1996 flores sea and 1994 fiji tonga deep earthquakes are derived from teleseismic body waves recorded by the global seismic network of broad band seismograph stations both events consisted of several subevents models to approximate the spatial and temporal extent of the source process include point sources propagating point sources and a combination of these for the flores sea event rupture lasted about 23 s and terminated some 70 km east of the nucleation point as inferred from the duration of p wave pulses in agreement with the findings reported by other investigators our preferred model suggests bilateral rupture propagation it consists of four point sources that have variable double couple radiation patterns and source time histories and explains well the large compensated linear vector dipole clvd component inferred from the harvard centroid moment tensor cmt solution the main moment release in the fiji tonga event lasted only about 15 s our best model consists of two point sources with a total moment release of 2 8 1020 n m rupture propagated subhorizontally from the nucleation point to the north the termination of rupture was located about 40 km to the north of rupture initiation the inferred velocity of moment release in the flores sea and fiji tonga events was 3 5 km s 1 a value which is higher than that inferred for the great bolivian earthquake of june 1994 other derived source parameters static stress drop radiated seismic energy and maximum seismic efficiency are also significantly different from those inferred for the 1994 bolivian event suggesting that deep earthquake processes do not follow an easily detectable common mechanism

source and aftershock properties of the 1996 flores sea deep earthquake the june 17 1996 mw 7 8 flores sea event is one of the largest deep earthquakes and also produced an energetic aftershock sequence allowing study of the spatial relationships between moment release aftershock occurrence and slab geometry the locations of the mainshock rupture sub events aftershocks and background slab seismicity are determined using a relative location algorithm the event occurred at the edge of a 150 km wide aseismic region that represents a tear in the lowermost part of the indonesian slab the aftershocks define a 90 km long region parallel to slab strike two other aftershocks locate 10 25 km outside the previously active slab the mainshock propagated about 75 km eastward along an e w striking southward dipping plane with a rupture velocity of 3 5 km s there was a substantial change in focal mechanism during the rupture with the fault plane strike changing by about 25 overall the flores sea deep earthquake shows numerous large aftershocks and a moderate rupture velocity 3 5 km s and stress drop 7 20 mpa these characteristics are similar to the 1994 tonga deep earthquake but different from the 1994 bolivia event suggesting they may be typical of large deep earthquakes in colder slabs systematic differences in rupture parameters for deep earthquakes in warm and cold slabs suggest that the rupture mechanism is highly sensitive to temperature

the complex rupture process of the 1996 deep flores indonesia earthquake mw 7 9 from teleseismic p waves from an analysis of broadband teleseismic p waveforms we resolve the complex rupture characteristics of the 17 june 1996 flores earthquake mw 7 9 depth 590 km this earthquake was unusual because of its large rupture length and the variation in rupture velocity and stress drop the 20 s rupture propagated westward at 2 3 km s to a distance of 20 30 km and eastward at 4 km s over 75 km average static stress drop was about 15 mpa but the stress drop during the episode of main moment release was a factor of four higher the oblique normal faulting mechanism is consistent with the stress expected as the slab descends along a highly curved trench significant rupture complexity is also reported for the deep 1994 mw 7 7 fiji and mw 8 2 bolivia earthquakes and is evident in the source time functions of many smaller earthquakes at depths 300 km thus source complexity of deep earthquakes appears no less common than for shallow earthquakes and requires the existence of significant heterogeneity in the physical properties that control deep earthquake rupture

a hybrid method for calculating the radiated wave energy of deep earthquakes abstract the energy radiated as waves is a parameter vital to the understanding of earthquake rupture physics calculations of this energy however are difficult and wrought with uncertainty we developed a new hybrid method for the calculation of radiated wave energy and other key seismic parameters the method calculates these parameters in an internally consistent and objective manner allowing for robust comparison between parameters and across earthquakes in addition the method does not require deconvolution or filtering of data thus results contain all recorded high frequencies we use the method to calculate source time functions seismic moment duration and radiated wave energy for 19 recent deep depth 292 km earthquakes ranging in size from m w 6 5 to 8 2 we compute effective seismic efficiency for the three largest events the principle result is that the effective seismic efficiencies are less than 0 3 if earthquake rupture adhered to the orowan model then the average frictional stress during rupture is equal to the final shear stress on the fault plane and the seismic efficiency is 1 but since the measured seismic efficiency is less than 1 the rupture process of these large deep earthquakes partitions more energy into friction and the average frictional stress during rupture is higher than the final fault plane stress

localized polymorphic phase transformations in high pressure faults and applications to the physical mechanism of deep earthquakes earthquake mechanisms based on frictional instabilities are widely accepted for relatively shallow earthquakes such mechanisms in unmodified form are highly unlikely for deep earthquakes because of the expected high frictional shear stresses at depths of 300 700 km several alternative mechanisms have been proposed here i discuss a new type of shear instability observed in experiments on ordinary polycrystalline ice and polycrystalline tremolite by my colleagues and me these shear instabilities lead to rapid stress drops and sound emissions and have the following characteristics 1 a single sharp shear zone forms in the maximum shear stress orientation 2 confining pressure does not suppress the instability and under some conditions raising confining pressure pc lowers the differential stress at which it occurs 3 stress drops can occur repeatedly after reloading at about the same differential stress 4 the mean stress vs temperature conditions at which the instabilities occur in ice and tremolite place them in the fields of thermodynamic stability of ice ii and diopside talc respectively although i have not as yet confirmed the existence of these high pressure phases in the shear zones transformation in the shear zone is implicated because such shear instabilities do not occur in similar materials in which polymorphic phase changes do not occur reconstructive polymorphic phase changes localized along shear zones may be a general phenomenon in crystalline materials stressed non hydrostatically near transition boundaries moreover by analogy the reconstructive phase transformations that are thought to occur in deeply subducted lithosphere may lead to localized shear instabilities manifested by deep earthquakes and produce the observed double couple first motion distribution finally if this is the earthquake source mechanism in deeply subducted lithosphere then earthquakes should shut off when all of the polymorphic phase changes have occurred this may explain the maximum depth of earthquakes at about 680 km

a broadband p wave analysis of the large deep fiji island and bolivia earthquakes of 1994 the complex broadband p wave displacement recordings of the 570 km deep march 9 1994 mw 7 6 fiji island and the 660 km deep june 9 1994 mw 8 2 bolivia earthquakes are analyzed to estimate the spatial and temporal characteristics of their ruptures we model relative arrival times and amplitudes of coherent subevents in p waveforms and perform waveform analyses we resolve 1 short rupture durations relative to their seismic moments 12 and 40 s for the fiji and bolivia earthquakes respectively 2 small areas of main moment release for both earthquakes 30 40 km 3 high stress drops compared to shallow earthquakes with the stress drop of the bolivia event 283 mpa being significantly higher than the stress drop of the fiji event 26 mpa 4 small changes in mechanism up to 10 changes in fault plane orientation and 5 a steeply dipping rupture plane for the fiji earthquake and a shallowly dipping rupture plane for the bolivia earthquake details of the ruptures are difficult to resolve using teleseismic p waveforms

detection of pkjkp at intermediate periods by progressive multi channel correlation abstract we have identified the seismic phase pkjkp traveling through the inner core of the earth as a shear wave in intermediate period 2 10 s records of the deep 1996 flores sea earthquake at eight stations of the french seismic network this constitutes direct evidence of the solidity of the inner core which while generally recognized was until now only inferred from indirect evidence the arrival times on stacked seismograms require a shear wave velocity at the top of the inner core icb 3 65 km s in agreement with values suggested from normal mode observations julian et al s 1972 observation of a lower velocity 2 95 km s is easily reconciled with our result if interpreted as the surface reflection ppkjkp the high poisson ratio 0 44 can be reconciled with a normal crystalline structure without invoking partial melting on account of the overwhelming pressure at the center of the earth

the march 9 1994 m w 7 6 deep tonga earthquake rupture outside the seismically active slab we investigate the rupture process of the march 9 1994 m w 7 6 deep tonga earthquake and its relationship to the background seismicity of the subducted tonga slab variations in observed p and s wave pulse duration indicate that the rupture propagated to the nne and extended well beyond the background seismicity we inverted 47 p and sh waveforms including regional broadband waveforms from the southwest pacific seismic experiment using a method that solves for the focal mechanism change during the rupture and the distribution of moment release along the fault plane the results indicate that significant moment release occurred in previously aseismic regions outside the active seismic zone and that the rupture terminated 10 20 km beyond the bounds of the previous seismic activity a significant change in focal mechanism occurred when the rupture propagated into the previously aseismic region rupture along the near vertical nne striking nodal plane provides a somewhat better fit to the body waveforms than rupture along the near horizontal nodal plane this result combined with the planar alignment of aftershocks and the general nne directivity of the waveforms provides strong evidence that the rupture occurred on the near vertical plane thermal modeling of the tonga slab indicates that the rupture terminated in material about 200 c warmer than the temperature that normally limits the occurrence of smaller earthquakes additionally aftershocks seem to be suppressed in the outer regions of the rupture which contain about half of the moment release but only 1 of the 15 well located aftershocks we suggest that slabs may be composed of an inner cold core where seismic rupture initiates and small earthquakes occur and a thermal halo of warmer material which can sustain rupture and only a few aftershocks the mechanism by which rupture propagates through the warmer material need not be similar to the process governing rupture nucleation in the cold slab core nucleation may occur through a process limited to the cold core such as transformational faulting and propagation through the warmer material may occur through ductile faulting or plastic instabilities isolated deep earthquakes in other subduction zones such as the 1994 bolivia event may occur almost completely within the warmer zone accounting for the lack of background seismicity and the dearth of aftershocks

rupture process of large earthquakes in the northern mexico subduction zone the cocos plate subducts beneath north america at the mexico trench the northernmost segment of this trench between the orozco and rivera fracture zones has ruptured in a sequence of five large earthquakes from 1973 to 1985 the jan 30 1973 colima event m s 7 5 at the northern end of the segment near rivera fracture zone the mar 14 1979 petatlan event m s 7 6 at the southern end of the segment on the orozco fracture zone the oct 25 1981 playa azul event m s 7 3 in the middle of the michoacan gap the sept 19 1985 michoacan mainshock m s 8 1 and the sept 21 1985 michoacan aftershock m s 7 6 that reruptured part of the petatlan zone body wave inversion for the rupture process of these earthquakes finds the best earthquake depth focal mechanism overall source time function and seismic moment for each earthquake in addition we have determined spatial concentrations of seismic moment release for the colima earthquake and the michoacan mainshock and aftershock these spatial concentrations of slip are interpreted as asperities and the resultant asperity distribution for mexico is compared to other subduction zones the body wave inversion technique also determines the moment tensor rate functions but there is no evidence for statistically significant changes in the moment tensor during rupture for any of the five earthquakes an appendix describes the moment tensor rate functions methodology in detail

seismicity shear failure and modes of deformation in deep subduction zones abstract planes of shear failure in deep seismic zones are identified as being parallel to focal mechanism nodal planes of deep focus earthquakes we relocate deep seismicity reported in the international seismological centre isc catalogue for earthquakes deeper than 400 km in the honshu bonin marianas java banda and south america subduction zones using the joint hypocentral determination method recent events with harvard centroid moment tensor cmt solutions and nearby seismicity are viewed from the direction of the cmt null axis comparing the two orthogonal nodal planes with the seismicity pattern planar features of seismicity parallel to cmt nodal planes are found in each deep seismic zone and are identified as planes of shear failure the sense of displacement on these planes is one of resistance to deeper penetration honshu and bonin show imbrication and horizontal displacement of the deepest part of the seismogenic slab while marianas and brazil show conjugate shear failure banda and argentina show patterns of vertical and horizontal shear planes for the western pacific slabs we find significant lateral displacements towards the south of the deepest part of the subduction flow tonga has been analyzed previously showing the same pattern as identified here in all benioff zones planes of shear failure are observed which act to shorten the slabs and displace matter horizontally normal to the slab moreover differential faulting acts to rotate the slab of subducted lithosphere to a more horizontal posture indicating that slabs experience resistance to penetration of the 670 km discontinuity

solar wind as the origin of rapid reddening of asteroid surfaces a comparison of the laboratory reflectance spectra of meteorites with observations of asteroids revealed that the latter are much redder with the spectral difference explained by space weathering though the actual processes and timescales involved have remained controversial a recent study of young asteroid families concluded that they suffered only minimal space weathering here we report additional observations of those families revealing that space weathering must be a very rapid process the final colour of a silicate rich asteroid is acquired shortly after its birth within 10 6 years of undergoing a catastrophic collision this rapid timescale favours solar wind implantation as the main mechanism of space weathering as laboratory experiments have shown that it is the most rapid of several competing processes we further demonstrate the necessity to take account of composition when evaluating weathering effectiveness as both laboratory and asteroid data show an apparent dependence of weathering on olivine abundance the rapid colour change that we find implies that colour trends seen among asteroids are most probably due to compositional or surface particle size properties rather than to different relative ages apparently fresh surfaces most frequently seen among small near earth asteroids may be the result of tidal shaking that rejuvenates their surfaces during planetary encounters

rupture of the large mw 7 8 deep earthquake of 1973 beneath the japan sea with implications for seismogenesis the earthquake of 29 september 1973 beneath the western edge of japan sea is the largest deep earthquake along the western pacific m w 7 8 focal depth 575 km the source region is near the tip of the japan wadati benioff zone where a continuous belt of seismicity clearly defines a unique shallow dipping 30 slab down to depths about 600 km this unique geometry of slab predicts a limited vertical extent available for earthquake rupture in direct conflict with previous reports of a large subvertical fault for the 1973 earthquake using a comprehensive data set of nearly 20 p and sh waveforms recorded at teleseismic distances we investigate rupture associated with this event in some detail by inversion of waveforms in particular we use wave trains with all relevant depth phases pp sp and ss that are crucial for resolving the vertical extent of rupture in addition to a northward component of rupture as in previous reports we find clear evidence for a significant component of eastward rupture propagation that cannot fit within a subvertical north south striking nodal plane meanwhile all regions of major slip are at essentially the same depth thus the true fault plane appears subhorizontal with the entire region of slip extending only about 15 6 km across the slab9s thickness the spatio temporal distribution of slip is well explained by a simple circular rupture front propagating at a speed of 3 4 km sec a comparison among the largest deep earthquakes shows that most rupture planes are more or less dimensionally equal with high aspect ratios ranging approximately from 0 5 to 1 while some events have fairly restricted source regions others seem to extend more than 30 km into the thickness of the slabs

observed spectral properties of near earth objects results for population distribution source regions and space weathering processes abstract we present new visible and near infrared spectroscopic measurements for 252 near earth neo and mars crossing mc objects observed from 1994 through 2002 as a complement to the small main belt asteroid spectroscopic survey smass http smass mit edu combined with previously published smass results we have an internally consistent data set of more than 400 of these objects for investigating trends related to size orbits and dynamical history these data also provide the basis for producing a bias corrected estimate for the total neo population stuart and binzel 2004 icarus 170 295 311 we find 25 of the 26 bus 1999 phd thesis taxonomic types are represented with nearly 90 of the objects falling within the broad s q x and c complexes rare a and e types are more common in the mc than neo population about 5 compared to 3 orbits in terms of possible space weathering effects we see a size dependent transition from ordinary chondrite like q type objects to s type asteroids over the size range of 0 1 to 5 km where the transition is effectively complete at 5 km a match between the average surface age of 5 km asteroids and the rate of space weathering could constrain models for both processes however space weathering may proceed at a very rapid rate compared with collisional timescales in this case the presence or absence of a regolith may be the determining factor for whether or not an object appears space weathered thus 0 1 to 5 km appears to be a critical size range for understanding the processes timescales and conditions under which a regolith conducive to space weathering is generated retained and refreshed

rupture processes of large deep focus earthquakes from inversion of moment rate functions we determine the source time histories of five recent 1994 1996 large deep focus earthquakes using a method that inverts for fault slip from far field moment rate functions the moment rate functions are obtained through the deconvolution of multiple body wave phases using broadband records from global seismic network gsn and geoscope stations tests of this method on synthetic data indicate that it is successful in determining the low frequency rupture characteristics of deep earthquakes under a variety of complicating conditions we find that source parameters such as average rupture velocity and stress drop are highly variable among the events studied and that some unusual characteristics exhibited by the june 9 1994 great bolivian earthquake are also found for other events comparison of the slip distributions with background seismicity and aftershock locations indicates that most of the moment release for large deep focus earthquakes is probably occurring within the active slab interior this provides further evidence that temperature controlled mechanisms such as transformational faulting play a large role in deep earthquake faulting most of the events studied also show a tendency for horizontal rupture propagation suggesting that isobaric processes may be an important factor in controlling progression of the rupture large gaps in the slip distributions point to possible occurrence on multiple fault planes

kinematics and shear heat pattern of ductile simple shear zones with slip boundary condition extrusion by poiseuille flow and simple shear of hot lower crust has been deciphered from large hot orogens and partial slip boundary condition has been encountered in analogue models shear heat and velocity profiles are deduced from a simplified form of navier stokes equation for simple shear together with extrusive poiseuille flow and slip boundary condition for newtonian viscous rheology a higher velocity at the upper boundary of the shear zone promotes higher slip velocity at the lower boundary the other parameters that affect the slip are viscosity and thickness of the shear zone and the resultant pressure gradient that drives extrusion in the partial slip case depending on flow parameters resultant pressure gradient density and viscosity and thickness of the shear zone the velocity profiles can curve and indicate opposite shear senses the corresponding shear heat profiles can indicate temperature maximum inside shear zones near either boundaries of the shear zone or equidistant from them

deep focus earthquakes and recycling of water into the earth s mantle for more than 50 years observations of earthquakes to depths of 100 to 650 kilometers inside earth have been enigmatic at these depths rocks are expected to deform by ductile flow rather than brittle fracturing or frictional sliding on fault surfaces laboratory experiments and detailed calculations of the pressures and temperatures in seismically active subduction zones indicate that this deep focus seismicity could originate from dehydration and high pressure structural instabilities occurring in the hydrated part of the lithosphere that sinks into the upper mantle thus seismologists may be mapping the recirculation of water from the oceans back into the deep interior of our planet

the isolated 680 km deep 30 may 2015 m w 7 9 ogasawara bonin islands earthquake deep focus earthquakes located in very high pressure conditions 300 to 700 km below the earth s surface within sinking slabs of relatively cold oceanic lithosphere are mysterious phenomena the largest recorded deep focus earthquake m w 7 9 in the izu bonin slab struck on 30 may 2015 beneath the ogasawara bonin islands isolated from prior seismicity by over 100 km in depth and followed by only a few small aftershocks globally this is the deepest 680 km centroid depth event with m w 7 8 in the seismological record seismicity indicates along strike contortion of the izu bonin slab with horizontal flattening near a depth of 550 km in the izu region and rapid steepening to near vertical toward the south above the location of the 2015 event this event was exceptionally well recorded by seismic stations around the world allowing detailed constraints to be placed on the source process analyses of a large global data set of p sh and pp seismic phases using short period back projection subevent directivity and broadband finite fault inversion indicate that the mainshock ruptured a shallowly dipping fault plane with patchy slip that spread over a distance of 40 km with a multi stage expansion rate 5 km s down dip initially 3 km s up dip later during the 17 s total rupture duration the radiated energy was 3 3 10 16 j and the stress drop was 38 mpa the radiation efficiency is moderate 0 34 intermediate to that of the 1994 bolivia and 2013 sea of okhotsk m w 8 3 deep earthquakes indicating that source processes of very large deep earthquakes sample a wide range of behavior from dissipative more viscous failure to very brittle failure the isolated occurrence of the event much deeper than the apparently thermally bounded distribution of bonin slab seismicity above 600 km depth suggests that localized stress concentration associated with the pronounced deformation of the izu bonin slab and proximity to the 660 km phase transition likely played a dominant role in generating this major earthquake

space weathering on eros constraints from albedo and spectral measurements of psyche crater we present combined multi spectral imager msi 0 95 m and near infrared spectrometer nis 0 8 2 4 m observations of psyche crater on s type asteroid 433 eros obtained by the near earth asteroid rendezvous near shoemaker spacecraft at 5 3 km in diameter psyche is one of the largest craters on eros which exhibit distinctive brightness patterns consistent with downslope motion of dark regolith material overlying a substrate of brighter material at spatial scales of 620 m spectrum psyche crater wall materials exhibit albedo contrasts of 32 40 at 0 946 m associated spectral variations occur at a much lower level of 4 8 2 4 we report results of scattering model and lunar analogy investigations into several possible causes for these albedo and spectral trends grain size differences olivine pyroxene and troilite variations and optical surface maturation we find that the albedo contrasts in psyche crater are not consistent with a cause due solely to variations in grain size olivine pyroxene or lunar like optical maturation a grain size change sufficient to explain the observed albedo contrasts would result in strong color variations that are not observed olivine and pyroxene variations would produce strong band correlated variations that are not observed a simple lunar like optical maturation effect would produce strong reddening that is not observed the contrasts and associated spectral variation trends are most consistent with a combination of enhanced troilite a dark spectrally neutral component simulating optical effects of shock and lunar like optical maturation these results suggest that space weathering processes may affect the spectral properties of eros materials causing surface exposures to differ optically from subsurface bedrock however there are significant spectral differences between eros proposed analog meteorites ordinary chondrites and or primitive achondrites and eros freshest exposures of subsurface bright materials after accounting for all differences in the measurement units of our reflectance comparisons we have found that the bright materials on eros have reflectance values at 0 946 m consistent with meteorites but spectral continua that are much redder than meteorites from 1 5 to 2 4 m most importantly we calculate that average eros surface materials are 30 40 darker than meteorites

mainshock and aftershock analysis of the june 17 1996 deep flores sea earthquake sequence implications for the mechanism of deep earthquakes and the tectonics of the banda sea one of the largest mw 7 8 deep earthquakes 595 km in recorded history occurred beneath the flores sea on june 17 1996 we analyze broadband body waveforms to determine the seismic source characteristics of this important earthquake the source time function has a small initial subevent 5 s duration followed by three major subevents with a total source duration of 29 s directivity analysis indicates an unilateral rupture to the east southeast with a subhorizontal rupture length of 75 km a time independent moment tensor timt inversion gives a total seismic moment of 5 4 1020 n m mw 7 8 a time dependent moment tensor tdmt inversion indicates rupture on a near vertical fault strike of 130 dip of 80 sw rake of 166 during the first 5 s releasing 8 of the seismic moment the major part of the moment was released along a shallower dipping plane strike of 100 dip of 55 rake of 45 this change of faulting geometry may imply that the rupture initiated at a structural thermal or phase boundary of high stress concentration and then triggered the major moment release along a regional weak zone the mainshock produced at least 23 mb 2 9 aftershocks within the first 3 days seventeen of the largest aftershocks were located using the joint hypocenter determination algorithm the aftershocks define a plane dipping antithetically to the dip of the subducting slab with lateral and depth dimensions of 145 km and 76 km respectively the aftershock locations correlate with the southeast striking nodal plane of the moment tensor mechanism

high calcium pyroxene as an indicator of igneous differentiation in asteroids and meteorites our analyses of high quality spectra of several s type asteroids 17 thetis 847 agnia 808 merxia and members of the agnia and merxia families reveal that they include both low and highcalcium pyroxene with minor amounts of olivine 0 4 high calcium pyroxene is a spectrally detectable and petrologically important indicator of igneous history and may prove critical in future studies aimed at understanding the history of asteroidal bodies the silicate mineralogy inferred for thetis and the merxia and agnia family members requires that these asteroids experienced igneous differentiation producing broadly basaltic surface lithologies together with 4 vesta and its smaller vestoid family members and the main belt asteroid 1489 magnya these new asteroids provide strong evidence for igneous differentiation of at least five asteroid parent bodies based on this analysis of a small subset of the near infrared asteroid spectra taken to date with spex at the nasa irtf we expect that the number of known differentiated asteroids will increase consistent with the large number of parent bodies inferred from studies of iron meteorites

effect of slab temperature on deep earthquake aftershock productivity and magnitude frequency relations deep earthquakes generally show fewer aftershocks and a larger variability in magnitude frequency relations than shallow earthquakes1 5 these characteristics and the factors that control them may place constraints on the mechanism of deep earthquakes which is still uncertain6 8 here we show that systematic variations in aftershock productivity and magnitude frequency relations are related to the temperature of the downgoing slab as inferred from the vertical velocity and age of the subducting lithosphere deep earthquakes with substantial aftershock sequences are found only in colder slabs and earthquakes in warmer slabs consistently show few aftershocks magnitude frequency relations for deep earthquakes also show systematic variations as a function of slab thermal structure with warmer slabs showing fewer small earthquakes lower b values the statistical properties of deep earthquakes thus appear to be temperature sensitive to an extent not observed for shallow earthquakes and not adequately explained by simple geometrical limitations on fault width

the nature of deep focus earthquakes wadati 1928 first proved convincingly that some earthquakes occur at depths well beneath the earth s crust he essentially proposed the nomenclature used today by the international seismological centre isc i e earthquakes with focal depths exceeding 300 km are deep earthquakes and those with depths between 70 and 300 km are intermediate earth quakes however when there is little reason to distinguish between the two groups both types are called simply deep earthquakes to avoid confusion in this paper we refer to those with focal depths exceeding 300 km as very deep earthquakes and those with focal depths between 70 and 300 km as intermediate earthquakes deep earthquakes are of interest for at least four reasons first they are exceedingly common between 1964 and 1986 they constituted no less than 22 of all earthquakes having mb greater than 5 0 in the isc catalog second they most often occur in association with deep ocean trenches and volcanic island arcs in subduction zones one of the great achievements of twentieth century geophysics was the recognition that their occurrence in inclined planar groups or wadati benioff zones apparently delineates the cold down going cores of convection cells in the uppermost mantle third seismologists use body waves of deep earthquakes dispro portionately more often than those of shallow earthquakes to inves tigate core mantle and crustal structure this is because a deep earth quakes often possess relatively more impulsive sources b their body phases traverse the heterogeneous uppermost mantle only once on the ray path from hypocenter to station and c surface waves do not contaminate these body phase arrivals on seismograms finally deep earthquakes are

systematic determination of earthquake rupture directivity and fault planes from analysis of long period p wave spectra summary if an earthquake has a primarily unilateral rupture the pulse width observed on seismograms will vary depending on the angle between the rupture direction and the takeoff vector to the station we have developed a method to estimate the amount of pulse broadening from the spectrum and apply it to a long period database of large globally distributed earthquakes that occurred between 1988 and 2000 we select vertical component p waves at epicentral distances of 20 98 w ecompute the spectrum from a 64 s long window around each p wave arrival each spectrum is the product of source receiver and propagation response functions as well as local source and receiver side effects since there are multiple receivers for each source and multiple sources for each receiver we can estimate and remove the source and receiverside terms by stacking the appropriate p log spectra for earthquakes deeper than 200 km source effects dominate the residual spectra we use our pulse width estimates to determine the best rupture direction and to identify which nodal plane of the harvard centroid moment tensor cmt solution is most consistent with this rupture direction for 66 events in about 30 per cent of the cases one of the two nodal planes produces a much better fit to the data and can be identified as the true fault plane when results from previous studies are available for comparison our rupture directions are usually consistent with their results particularly for earthquakes with simple rupture histories

the observation of inner core shear waves summary although the earth s inner core has long been thought to be solid there have not as yet been unequivocal observations of inner core shear waves here we present observations of the phases ppkjkp and skjkp for the flores sea event of 1996 june 17 the observations support the value of approximately 3 6 km s 1 for the inner core shear wave velocity and open up new possibilities for exploring the anisotropic structure and the attenuation properties of the inner core the analysis and validation of the observation is based on a new method that can be used in the search for and identification of inner core shear waves in this method normal mode synthetics for a solid inner core are compared with those for a fluid inner core it is only by such comparisons that it is possible to be certain that inner core shear waves rather than interferences of other phases are responsible for observed energy maxima in stacked seismic records

plastic instabilities implications for the origin of intermediate and deep focus earthquakes adiabatic or catastrophic plastic shear has been reported in metals polymers and metallic glasses the phenomenon is associated with rapid stress drops and audible pings or clicks as the material deforms in a plastic manner the driving force for the plastic instability is the stored elastic strain energy of the loading system and in many respects the behavior is reminiscent of the shear stress response arising from stick slip events during unstable frictional sliding although the precise mechanism is different it is shown here that adiabatic plastic shear is capable of explaining the detailed distribution of intermediate and deep focus earthquakes within subduction zones the earthquake events being the result of instabilities in material undergoing plastic flow it is argued that for a particular strain rate there exists a critical temperature tc which is depth dependent for temperatures below tc the material is strain rate softening and for a soft enough loading system may undergo catastrophic plastic shear for temperatures above tc the material is strain rate hardening and is always stable during plastic shear the cutoff depth for deep focus earthquakes then corresponds to the transition from strain rate softening to strain rate hardening material and for commonly accepted geothermal gradients within the slab corresponds to approximately 800 km the precise distribution of earthquakes within the slab is a function of the subtle interplay between the geothermal gradient and the tc gradient in particular a decrease in seismic activity is to be expected below about 300 km in the slab with total stress drops decreasing from a maximum of 700 mpa above 300 km to a maximum of 50 mpa below 300 km the differences in foci distribution between subduction zones such as tonga new hebrides and peru result from minor differences in the geothermal gradients within the slabs the model predicts the development of triple seismic zones high in the slab double seismic zones down to approximately 300 km and single seismic zones down to 800 km such a distribution is to be expected of relatively young cool slabs as the slab heats up the seismic activity retreats up the slab the paper only proposes a deformation mechanism for earthquake generation it does not address the stress field within the slab but only the distribution of strength thus the distribution of focal plane mechanisms is not considered only the locations where earthquakes due to plastic instabilities are possible the absence of earthquakes does not necessarily mean that the slab does not exist it only means that the slab is too hot to undergo plastic instability this means that aseismic subduction is a distinct possibility in many regions of high geothermal gradient within the slab i e circa 3 c km 1