the 9 june 94 bolivian deep earthquake an exceptional event in an extraordinary subduction zone we investigate the physical setting of the bolivian shock based on the history of the subducting nazca plate intraslab seismicity deep seismic moment release and seismic tomography south america has two broad regions of reverse arc curvature subduction constrained to this unique geometry produces slab kinking contortions that may cause unusual slab thickening as they sink to the bottom of the transition zone and encounter resistance to penetration into the lower mantle such contortions are observed at intermediate depths upstream from the slab source regions of both the great 1970 colombian and 1994 bolivian events thickening helps explain how the nazca slab accommodates large seismic source dimensions at depths of 625 650 km

finite fault source study of the great 1994 deep bolivia earthquake we apply an empirical green s function method to recordings of teleseismic body waves from the june 9 1994 deep focus bolivia earthquake the far field source time functions reveal that rupture occurred on a sub horizontal fault plane the main rupture began some 25 km to the east of the hypocenter consists of three large subevents and covers a total area of about 60 40 km the static stress drop is quite large more than 150 mpa and the obtained rupture velocity is less than 2 km sec while the total dimensions of the earthquake are large compared to estimates of the width of a metastable olivine wedge at this depth the moment release is confined largely to a narrow band along an ew axis and may be compatible with phase transitions within a thickened slab core

mantle phase changes and deep earthquake faulting in subducting lithosphere inclined zones of earthquakes are the primary expression of lithosphere subduction a distinct deep population of subduction zone earthquakes occurs at depths of 350 to 690 kilometers at those depths ordinary brittle fracture and frictional sliding the faulting processes of shallow earthquakes are not expected a fresh understanding of these deep earthquakes comes from developments in several areas of experimental and theoretical geophysics including the discovery and characterization of transformational faulting a shear instability connected with localized phase transformations under nonhydrostatic stress these developments support the hypothesis that deep earthquakes represent transformational faulting in a wedge of olivine rich peridotite that is likely to persist metastably in coldest plate interiors to depths as great as 690 km predictions based on this deep structure of mantle phase changes are consistent with the global depth distribution of deep earthquakes the maximum depths of earthquakes in individual subductions zones and key source characteristics of deep events

characteristics of seismic radiation during the 1994 bolivian earthquake and implications for rupture mechanisms 1 the pkp waves of the 1994 deep bolivian earthquake mw 8 3 recorded by the dense short period seismic array in taiwan were used to make direct measurements of seismic source radiation employing array stacking technique those data were used to reconstruct the time function of the source rupture the well resolved frequency content of this source time function was from the period of 1 to 10 seconds and to resolve the rupture properties it provided source information at higher frequencies than that in previous studies from global seismic network data rupture time analysis identifies four asperities in the array observations and the initial rupture time of each asperity correlates well with results from previous studies using global network data the analysed results of this study also indicate that unlike other asperities the two major ones did not release as much high frequency energy however based on the limited broadband observations in taiwan the long period energy was normally released this study concludes that this can be well explained by the lubrication of fault during the great earthquake several deep earthquake mechanisms have been discussed for this special rupture characteristic and none may be fully ruled out

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

area selection for diamond exploration using deep probing electromagnetic surveying abstract previously proposed methods of area selection for diamond prospective regions have predominantly relied on till geochemistry airborne geophysics and or an appraisal of tectonic setting herein we suggest that a novel deep probing geophysical technique electromagnetic studies using the natural source magnetotelluric mt method can contribute to such an activity essentially diamondiferous regions must have 1 old lithosphere 2 thick lithosphere and 3 lithosphere that contains high concentrations of carbon deep probing mt studies are able to address all three of these the second and the third of these can be accomplished independently using mt but for the first the geometries produced from modelling the mt observations must be interpreted with appropriate interaction with geologists geochemists and other geophysicists examples are given from the slave and superior cratons in north america with a brief mention of an area of the rae craton and general speculations about possible diamondiferous regions

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

empirical green s function study of the january 17 1994 northridge california earthquake the january 17 1994 northridge main shock was analyzed using an empirical green s function approach to estimate the spatial and temporal distribution of fault slip far field source time functions obtained from seismograms recorded by the berkeley digital seismic network bdsn and terrascope stations reveal the distributed nature of the source and the presence of a second large subevent the location of the fault beneath the san fernando valley as well as the updip directivity of the rupture contributed to the locally heavy damage due to this earthquake

tectonic setting of kimberlites kimberlites can be viewed as time capsules in a global plate tectonic framework their distribution illustrates clustering in time and space kimberlite ages span the assembly and break up of a number of supercontinents such as rodinia and gondwana these supercontinents show time lines with i broad periods devoid of kimberlite magmatism corresponding to times of continent stability and ii narrow kimberlite emplacement windows corresponding to times of fundamental plate reorganizations this episodicity implies that kimberlite emplacement events are intrinsically related to particular stages in the life cycle of supercontinents the onset of kimberlite magmatism is closely associated with thermal perturbations thermal insulation mantle upwelling beneath a stagnant or sluggish supercontinent these perturbations may have caused uplift and the onset of continental break up through fracture zones propagating into the supercontinent subsequent spreading and ocean floor development is marked by apparent cusps and jogs in plate motion paths resultant strain is accommodated along trans lithospheric corridors with episodic uplift and erosion and focused kimberlite melt migration the corridors are manifest as discontinuities in the lithosphere mantle measured as geophysical gradients and as changes in mantle lithosphere composition within the crust these corridors are expressed as a terrane boundaries b incipient continental rifts c fracture zones or d major dyke swarms some kimberlite populations are clustered along parallel sets of corridors widely distributed across a large part of a subcontinent and repeated magmatism is seen within many of the clusters the association of kimberlite occurrences with discontinuities may be ascribed to favorable conditions for melt production and to resultant melt focusing along high strain zones that contain fractures and faults such conditions may be attained during different stages in the evolution of continents a supercontinent formation b incipient rifting driven by far field stresses and onset of continental break up and c strain accommodation along the continental continuation of oceanic fracture zones during spreading type c may show concomitant kimberlite magmatism in separate continents after break up

the june 9 bolivia and march 9 fiji deep earthquakes of 1994 i source processes we examined the source characteristics of the 1994 deep earthquakes of bolivia june 9 and fiji march 9 over a period range of 1 1000 s using moment tensor and body wave analyses moment tensor inversions from 100 1000 s reveal that these two earthquakes were essentially point sources at very long period with insignificant isotropic and non double couple components with scalar moments mo 2 56 1021 nm for the bolivia and mo 0 26 1021 nm for the fiji earthquakes body wave analysis for the bolivia event reveals a 45 s duration a nearly linear e ne 1 km s rupture on the subhorizontal focal mechanism plane giving a stress drop 114 mpa for the fiji earthquake we find a change in focal mechanism and a linear 3 km s rupture plunging shallowly to the north on the steep dipping nodal plane with a duration of at least 20 s and 14 mpa both ruptures propagated parallel their null 2 axis and suggests that their width is controlled by the thickness of the seismogenic volume in deep slabs

srndpb isotopic evidence for crustal contamination in the rhum intrusion abstract sr nd and pb isotope ratios measured on cumulates from the eastern layered series els of the rhum intrusion indicate that the rhum magma chamber was contaminated by upper crustal amphibolite facies lewisian gneiss this is in marked contrast to the skye main lava series of skye which shows extensive contamination by lower crustal granulite facies lewisian gneiss 3 the amount of contamination involved was about 7 cryptic sr isotope profiles through the els show that contamination was extremely variable and may be controlled by mixing between fractionated contaminated magma and periodic injections of primitive magma into the chamber

on the interpretation of subevents in teleseismic waveforms the 1994 bolivia deep earthquake revisited we evaluate the methodologies that aim at resolving the space time slip distribution during large earthquakes in one common approach the source process of an event is assumed to be composed of a series of subevents critical points in teleseismic body waves such as peaks and coherent arrivals are viewed as discrete points in the spatiotemporal history of an earthquake we show that these features can be interpreted as the instantaneous centroid locations of the moment release history we demonstrate this result with an analysis of the mw 8 2 bolivia 1994 deep event we reconstruct the slip distribution in two contrasting ways first broadband p waves are interpreted as being composed of subevents five peaks in the moment release rate are located relative to the onset of the rupture using the azimuthal variations of onset to peak times second we invert broadband p waves for the slip distribution on a subhorizontal fault we use a nonlinear tomographic imaging technique called simulated annealing and an l1 misfit norm rupture velocities appear to be 2 km s 1 and largest slip values are found to the east and north of the onset in a 40 by 50 km region the instantaneous centroids of the moment release history correspond closely to the locations found using the subevent analysis however especially near the middle and the end of the rupture the instantaneous centroid locations are in regions of little moment release we discuss how the subevent analysis and the instantaneous centroid approach lead to dissimilar interpretations of rupture processes especially for events with length to width ratios near unity another result of this study is that the rupture of the 1994 bolivia event appears to end 50 km to the northwest of the onset implying that the slip area is larger that previously thought comparing the slip distribution with the aftershock distribution suggests that more than one physical mechanism is necessary to account for the rupture process of this great deep event

asthenospheric and lower lithospheric mantle contributions to continental extensional magmatism an example from the british tertiary province abstract the compositions of palaeocene basic magmas in skye and mull british tertiary igneous province btip shed light on the general problem of the relative importance of lithospheric and asthenospheric mantle as sources of continental magmatism during extension after the effects of crustal contamination in some magmas have been detected and the samples showing it eliminated from consideration the remainder show a significant diversity of trace element compositions basic lavas from skye and mull which lack crustal contamination have contents of compatible and moderately compatible elements e g ni cr y yb and also of moderately incompatible elements e g ti p zr hf sr mree which are within the range shown by the tholeiitic and mildly alkalic ocean island basalts oib as from iceland in contrast their contents of strongly and very strongly incompatible elements e g k rb ba u th nb ta lree are within the range shown by normal mid ocean ridge basalts these btip magmas are interpreted as second stage melts from mantle which had already lost a very small melt fraction the latter is thought to be represented by the strongly alkalic permo carboniferous dyke swarms associated with rifting at that time in nw scotland residual uppermost asthenosphere from the late palaeozoic magma genesis event is postulated to have become attached to the thickening lithosphere beneath nw scotland and thus to have remained coupled to it throughout the mesozoic until reactivated by palaeocene stretching occasional hawaiites in the top third of the skye lava pile remnant and also basalt dykes of the swarm cutting the granites of the central complex the last significant magmatic event at this centre have the elemental characteristics of typical north atlantic oib notably la ta 10 12 these compositions are thought to demonstrate input from asthenospheric source basic magmas throughout the lithospheric source dominated skye and mull magmatism it is suggested that all the basic magmas in this province are ultimately derived from asthenospheric sources but that the larger magma batches became contaminated during uprise by melting and assimilating the most fusible parts of overlying lithospheric mantle followed by lower crust in many cases smaller magma batches traverse the overlying lithosphere with little or no contamination perhaps the difference between the two lies in their flow behaviour laminar or turbulent during uprise

isotope and trace element geochemistry of proterozoic natkusiak flood basalts from the northwestern canadian shield abstract the proterozoic continental tholeiites of natkusiak province 723 ma old of the northwestern canadian shield are part of the franklin magmatic event they resemble phanerozoic flood basalts in their incompatible trace element and sr nd pb isotopic variations as well as in their large volumes and a relatively short period of eruption the wide variations in sr nd pb isotopic and incompatible trace element ratios e g initial nd 5 6 to 5 7 in the natkusiak basalts together with the correlations between the isotopic and trace element ratios imply the involvement of a crustal component in their genesis the natkusiak basalts are inferred to be derived from the subcontinental upper mantle and contaminated by precambrian basement during their ascent

geochemistry of the little dal basalts continental tholeiites from the mackenzie mountains northwest territories canada analyses of the little dal lavas and the 779 ma tsezotene sills both of the mackenzie mountains supergroup northwest territories canada show them to be continental tholeiites that are geochemically related the plagioclase and clinopyroxene phyric lavas are geochemically evolved and enriched in iron up to 20 2 wt as fe2o3 two magmatic lineages are identifiable and may represent different degrees of partial melting in the same source region but cannot be related by fractional crystallization within each lineage geochemical variation can be explained by fractional crystallization involving up to 60 crystallization of the original magma the most evolved lavas occur at the base of the pile less fractionated lavas occur toward the top of the sequence the nd isotopic composition of the little dal lavas averages e 1 4 at 780 ma trace element and isotopic compositions are permissive of contamination by continental crust but do not require a crustal component the preserved volume of the littl

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

a cascading failure during the 24 may 2013 great okhotsk deep earthquake on 24 may 2013 the largest ever recorded deep earthquake occurred beneath sea of okhotsk a multiple point source inversion procedure is applied to constrain source process of this earthquake based on waveform modeling of both direct p and sh waves and near surface reflected pp and ssh waves our results indicate that the earthquake consists of six major subevents separated in space and time encompassing a horizontal dimension of 64 4 km along n160 e and a downward depth extension of 35 4 km the geographic distribution and focal mechanisms of the inferred subevents and foreshock aftershock locations do not fit into plane rupture we suggest that the earthquake can be best explained by a cascading failure of shear instability within preexisting weak zones in the region with the perturbation of stress generated by a shear instability triggering another

global large deep focus earthquakes source process and cascading failure of shear instability as a unified physical mechanism abstract we apply a multiple source inversion method to systematically study the source processes of 25 large deep focus depth 400 km earthquakes with mw 7 0 from 1994 to 2012 based on waveform modeling of p pp sh and ssh wave data the earthquakes are classified into three categories based on spatial distributions and focal mechanisms of the inferred sub events 1 category one with non planar distribution and variable focal mechanisms of sub events represented by the 1994 mw 8 2 bolivia earthquake and the 2013 mw 8 3 okhotsk earthquake 2 category two with planar distribution but focal mechanisms inconsistent with the plane including eighteen earthquakes and 3 category three with planar distribution and focal mechanisms consistent with the plane including six earthquakes we discuss possible physical mechanisms for earthquakes in each category in the context of plane rupture transformational faulting and shear thermal instability we suggest that the inferred source processes of large deep focus earthquakes can be best interpreted by cascading failure of shear thermal instabilities in pre existing weak zones with the perturbation of stress generated by a shear instability triggering another and focal mechanisms of the sub events controlled by orientations of the pre existing weak zones the proposed mechanism can also explain the observed great variability of focal mechanisms the presence of large values of clvd compensated linear vector dipole and the super shear rupture of deep focus earthquakes in the previous studies in addition our studies suggest existence of relationships of seismic moment source duration 3 and moment source dimension 3 in large deep focus earthquakes

stress drop and frictional heating during the 1994 deep bolivia earthquake we use the slip model of the bolivian earthquake obtained by ihmle 1998 to investigate the spatial distribution of stress drop and calculate the frictional heat generated locally on the fault we find that the stress drop associated with the earthquake averages between 5 and 10 m pa over the fault the total frictional energy is estimated at between 3 1017 and 2 1018j if the fault width is as thin as the one produced by transformational faulting in laboratory experiments where it is about two orders of magnitude smaller than the local shear displacement melting likely occurred on a significant portion of the fault in this event as proposed by kanamori et al 1998 once rupture is triggered shear melting can quickly promote extensive slip

shear wave splitting continental keels and patterns of mantle flow in this study we investigated the origin of seismic anisotropy in the mantle beneath north america in particular we evaluated whether shear wave splitting patterns in eastern north america are better explained by anisotropy caused by lithospheric deformation anisotropy due to mantle flow beneath the lithosphere or a combination of both we examined new measurements of shear wave splitting from the missouri to massachusetts broadband seismometer array moma the north american mantle anisotropy and discontinuity experiment nomad as well as splitting parameters from several previous studies we developed a simple finite difference model that approximates mantle flow around a complex three dimensional continental lithospheric keel to evaluate potential anisotropy from mantle flow beneath the lithosphere in eastern north america we compared shear wave splitting observations to predicted splitting parameters calculated using this mantle flow model our results indicate that a significant portion of observed shear wave splitting in eastern north america can be explained by mantle flow around the continental keel however shear wave splitting patterns in a few regions of eastern north america indicate that a component of lithospheric anisotropy must exist particularly in regions containing the largest keel thicknesses for eastern north america as well as for splitting observations in australia europe and south america we favor a model in which anisotropy is controlled by a combination of both lithospheric deformation and subcontinental mantle flow

higher degree moment tensor inversion using far field broad band recordings theory and evaluation of the method with application to the 1994 bolivia deep earthquake summary we present a method to estimate parameters of the extended earthquake source using higher degree moment tensors at 27 centroid locations we show that a taylor series expansion of green s functions around a single centroid is not accurate enough when working with seismic wave periods and wavelengths in the range of the rupture duration and spatial extent of the fault respectively introducing a grid of 27 centroid locations on the fault and using higher degree moment tensors we are able to model adequately body and surface waves with periods and wavelengths smaller than the rupture duration and fault dimensions under simplifying assumptions an iterative inversion scheme is coded to estimate parameters of planar haskell type faults realistic inversion examples for deep and shallow earthquakes show that uni and bidirectional rupture models rupture direction fault and auxiliary plane and kinematic source dimensions and can be constrained with teleseismic body and or surface waves the application to the deep bolivia event indicates a subhorizontal fault plane unidirectional rupture to the north is slightly preferred the rupture duration of 25 s and fault dimensions of 47 25 km agree well with the estimates for the main pulse moment release given in other studies

the 2013 okhotsk deep focus earthquake rupture beyond the metastable olivine wedge and thermally controlled rise time near the edge of a slab the 2013 m8 3 okhotsk earthquake involves two primary mechanisms of deep focus earthquake rupture mineral phase transformation of olivine to spinel and thermal shear instability backprojection imaging of broadband seismograms recorded by the north american and european networks indicates bilateral rupture toward ne and sse the rupture paths of the ne segment and other regional m7 earthquakes are confined in narrow regions along the slab contours consistent with the phase transformation mechanism however the sse rupture propagates a long distance across the slab and aftershocks are distributed across a 60 km wide zone beyond the plausible thickness of the metastable olivine wedge favoring thermal shear weakening while the ne rupture is only visible at high frequencies the sse rupture is consistently observed across a broad frequency range this frequency dependent rupture mode can be explained by lateral variations of rise time controlled by thermal thinning of the slab near its northern end

hotspot tracks and the early rifting of the atlantic abstract many hotspot tracks appear to become the locus of later rifting as though the heat of the hotspot weakens the lithosphere and tens of millions of years later the continents are split along these weakened lines examples are the west coast of greenland east coast of labrador madeira hotspot the south coast of mexico north coast of honduras guyana hotspot and the south coast of west africa north coast of brazil st helena hotspot a modern day analog of a possible future rift is the snake river plain where the north american continent is being pre weakened by the yellowstone hotspot track this conclusion is based on reconstructions of the motions of the continents over hotspots for the past 200 million years the relative motions of the plates are determined from magnetic anomaly isochrons in the oceans and the motion of one plate is chosen ad hoc to best fit the motions of the plates over the hotspots however once the motion of this one plate is chosen the motions of all the other plates are prescribed by the relative motion constraints in addition to the correlation between the predicted tracks and sites of later continental breakup exposed continental shields correlate with the tracks their exposure may be the result of hotspot induced uplift which has led to erosion of their former platform sediment cover

spatial patterns of aftershocks of deep focus earthquakes this paper presents precise relative relocations for aftershocks of 59 intermediate and deep earthquakes using p pp and pkp arrival times read by us or reported in the literature these aftershocks included 36 rupture subevents occurring within 1 min of the initial event and 71 true aftershocks occurring later and identified statistically as being related to the initial event we compare the relative locations with the orientations of the wadati benioff zones and with the available focal mechanisms using statistical methods not previously applied to earthquake data surprisingly neither true aftershocks nor rupture subevents cluster along the nodal planes of intermediate or deep earthquake focal mechanisms aftershocks more than 20 km from the initial events occur preferentially in the plane of the wadati benioff zone while those lying closer have isotropically distributed directions rupture subevents occur after the travel time of the s wave from the initial event larger magnitude earthquakes generally possess rupture subevents lying farther from the initial event whereas true aftershocks can occur 50 km or more from initial events having magnitudes as small as 5 3 except following a few unusually large earthquakes with focal depths of about 100 km we find no rupture subevents or true aftershocks more than about 80 km from the initial hypocenter the existence of many aftershocks far from nodal planes does not favor models in which deep earthquake failure is simply slip along a planar fault rather the aftershocks may occur in response to a general redistribution of stress caused either by the occurrence of the initial event or possibly by nearby continuing phase transitions alternatively the failure zones of deep earthquakes may be surfaces which are frequently curved by 40 or more from the orientation of the initial nodal plane perhaps due to inhomogeneous stress distributions

directivity analysis of the deep bolivian earthquake of june 9 1994 the june 9 1994 deep bolivian earthquake m w 8 3 d 636 km occurred on a nearly horizontal rupture plane with an approximate area of 2000 km 2 the rupture plane is constrained by directivity analysis of the p wave displacement pulses recorded at local regional and teleseismic distances the fact that there are local recordings of the displacement provides a strong constraint on the vertical extent of faulting which is less than 10 km the moment release had at least four distinct episodes or subevents the best overall explanation for the subevents is rupture which initiates on the western margin of the fault plane propagates to the east and then continues bilaterally along a 350 azimuth with most of the moment release occurring to the north the apparent rupture velocity between subevents is very low approximately 1 2 km s

how strong is an asperity a recent study of repeating earthquakes on the san andreas fault in central california by nadeau and johnson 1998 found that the smallest events occurred on patches having a linear dimension of the order of 0 5 m displacements of about 2 cm and stress drops of the order of 2000 mpa roughly 10 times larger than rock strengths measured in the laboratory the stress drop for larger events was observed to decrease as a power law of the seismic moment reaching the commonly observed value of 10 mpa at about magnitude 6 these large strengths are shown here to be consistent with laboratory data if the preexisting microcracks are all healed a hierarchical fractal asperity model is presented which is based on recent laboratory observations of contact distributions in sliding friction experiments this cantor dust model is shown to be consistent with the observed power law decrease in stress drop and increase in displacement with increasing event size the spatial distribution of hypocenters in the parkfield area is shown to be consistent with this simple fractal model and with a hierarchical clustering of asperities having a fractal dimension of d 1 and discrete rescaling factor of about 20

relationships between crustal contamination and crystallisation in continental flood basalt magmas with special reference to the deccan traps of the western ghats india the upper part of the deccan traps sequence bushe to mahabaleshwar formations shows a statistically significant tendency for the most mafic lavas to be the most contaminated by crustal materials this is the reverse of the relationship shown by suites evolving by contamination accompanied by fractional crystallisation afc the observed correlations e g between mg number and sr isotope initial ratios are partly an accidental consequence of the fact that the most mafic lavas are more abundant in the lower part of the sequence while contaminant availability declines in the upper part it is probable however that the correlations are augmented by increased contamination of hotter magma batches during ascent through dykes a process during which fractional crystallisation is suppressed by magmatic turbulence the absence of afc relationships suggests that most of the contamination took place during the ascent stage rather than in a magma chamber other continental flood basalt provinces such as the parana and etendeka do show afc relationships and it is speculated that this may be a result of magma chamber contamination coupled with flow rates which prevent contamination during ascent

origin of the silicic volcanic rocks of the early permian panjal traps kashmir india abstract the panjal traps of kashmir india represent a significant outpouring of mafic and silicic volcanic rocks during the early permian and are synchronous with the opening of the neotethys ocean previous studies have suggested that the silicic volcanic rocks are derived by differentiation of mafic magmas dacites and rhyolites collected from the lower portions of the volcanic pile near pampore kashmir are peraluminous anck 1 0 in composition their calculated whole rock i sr values are variable i sr 0 69307 to 0 71825 and indicate open system behavior of either rb or sr or both whereas their nd isotope compositions end t 8 6 to 8 9 are nearly uniform the end t values and trace element th nb pm 4 nb u 8 ratios suggest the rocks are derived partially or wholly from the crust least squares and melts modeling indicate that the likely source of the silicic rocks is the middle crust rather than the lower or upper crust furthermore the silicic panjal traps have trace element compositions similar to some felsic volcanic rocks and a type granitic rocks from other large igneous provinces e g karoo parana and camp the paleoproterozoic t dm ages t dm 1836 to 1937 ma indicate that the rocks were likely derived from an ancient crustal source which experienced multiple episodes of recycling in contrast to silicic volcanic rocks from other lips the silicic panjal traps are unusual in that they did not involve mantle source material the heat required to melt the crust was likely due to the continuous injection of contemporaneous basaltic magmas which formed the majority of the mafic panjal traps

the large aftershocks of the northridge earthquake and their relationship to mainshock slip and fault zone complexity the distribution of fault slip for the northridge mainshock and two mw 6 aftershocks is estimated using an approach that inverts the far field momentrate functions derived by empirical green s function deconvolution the slip for these events was found to have a strong negative correlation with the locations of small aftershocks and there is a notable clustering of aftershocks near the edges of slip suggesting that these events are triggered by a redistribution of stress following the mainshock rupture the two largest aftershocks ruptured the updip regions adjacent to the leading edge of the mainshock rupture one aftershock 11 hr after the main event initiated 8 km west of the mainshock slip terminus and ruptured east to the western edge of mainshock slip this event was found to have ruptured the same south dipping fault as the mainshock the two events however are separated by a lateral offset in the depth of seismicity that serves as a geometrical barrier to through going rupture an mw 6 2 aftershock approximately 1 min after the main event is inferred to have ruptured an updip region of the mainshock fault that is relatively devoid of small aftershock seismicity this event was located at the eastern edge of the sequence that overlaps with the north dipping fault responsible for the 1971 san fernando earthquake although the south dipping plane is favored for this aftershock it is not possible to rule out that it occurred on the north dipping fault structure or within the hanging block of the mainshock

geochemistry and petrogenesis of basaltic rocks from coppermine river area northwest territories proterozoic basaltic flows 2000 m thick and associated dykes and sills from the coppermine river area northwest territories have chemical compositions typical of continental tholeiites the low mg fe ratio and abundances of ni and cr indicate that the lavas were extensively fractionated prior to extrusion the variations of incompatible elements such as k rb ree y zr nb and th suggest that the rocks were affected by interaction with continental crust the samples least affected by contamination have trace element compositions very similar to those of p type mid ocean ridge basalts it is suggested that continental tholeiites have been generated from the same source as p type oceanic tholeiites and geochemical features such as the enrichment of some lithophile elements in many of these rocks may be related to crustal contamination the variations within the volcanic pile of the coppermine river area are related to those of an exposed part of the muskox layered intrusion

seismological constraints on the mechanism of deep earthquakes temperature dependence of deep earthquake source properties abstract seismological observations are of vital importance for understanding the mechanism of deep earthquakes most of the seismological observables for earthquakes deeper than 300 km are similar to shallow earthquakes deep earthquakes clearly represent shear failure on a planar surface as shown by their double couple mechanisms deep earthquake aftershock sequences show temporal decay rates and magnitude frequency relations b values that are similar to shallow earthquakes with the aftershocks occurring preferentially along the mainshock fault planes in addition deep earthquake rupture velocities are similar to those of shallow earthquakes however there are also some observations that are clearly distinctive relative to shallow earthquakes stress drops of deep earthquakes show a large variation but are larger on average than shallow earthquakes different deep seismic zones show very different b values in contrast to shallow earthquakes which show similar b values worldwide in addition deep earthquakes show fewer aftershocks than shallow earthquakes a variety of observations suggest that deep earthquakes are highly sensitive to the temperature of the slab both deep earthquake b values and the rate of deep earthquake aftershock occurrence are inversely correlated with the temperature of the deep slabs suggesting that these factors are temperature controlled to an extent much greater than with shallow earthquakes large deep earthquakes in warm slabs show slower rupture velocities larger stress drops and lower seismic efficiencies than similar earthquakes in cold slabs the width of deep seismic zones is also probably temperature controlled but deep earthquake rupture can propagate outside the normal limits of benioff zone seismicity simple thermal models for the tonga slab near the 1994 deep earthquake suggest that the temperature at the rupture termination point was at least 200 c warmer than the temperature that limits smaller earthquakes in the slab these observations can be used to evaluate physical models for deep earthquakes including brittle slip along fluid weakened faults transformational faulting and thermal and perhaps melt lubricated shear instabilities it is difficult to explain the large fault widths of some deep earthquakes using a fluid weakening model since hydrated materials are expected in only a narrow depth zone at the top of the slab and the fault planes do not have the expected orientations for reactivated faults the lateral extent of the largest deep earthquakes cast doubt on the transformational faulting model in which events should be confined within a narrow metastable wedge seismological studies have also failed to find evidence for the existence of metastable olivine in slabs the temperature dependence of deep earthquakes argues in favor of a temperature activated phenomenon such as thermal shear instabilities and perhaps fault zone melting

distribution of kimberlite and aillikite in the diamond province of southern west greenland a regional perspective based on groundmass mineral chemistry and bulk compositions a regional study of dike and sill occurrences of kimberlitic rocks in terms of xenolith and megacryst assemblages from the diamond province in southern west greenland demonstrates very systematic variation in the composition of melts across the margin of the archaean core of the north atlantic craton melt derived from the diamond window in the lithospheric mantle below the archean core in greenland is kimberlitic toward the palaeoproterozoic nagssugtoqidian orogen melts also derived from the diamond window gradually change to ultramafic lamprophyre compositions evolving toward an end member akin to the aillikites of the penecontemporaneous torngat region in canada the identification of the types of melts is based on the groundmass paragenesis including the presence or absence of clinopyroxene and chromite and the compositions of groundmass ilmenite spinel and phlogopite the systematic regional variation is also shown although somewhat blurred by the bulk rock major and trace element compositions the change in melt composition is not a reflection of change in depth of initial melting but relates to the composition of the deep sclm kimberlite melts in the archaean craton are suggested to be reaction products between a carbonate rich astenospheric melt and an orthopyroxene bearing and phlogopite poor depleted sclm whereas the aillikite type melts across the craton margin are the products of reaction between a similar carbonate rich asthenospheric melts and a subducted sclm that had previously been subjected to metasomatism and veining

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

repeating earthquake finite source models strong asperities revealed on the san andreas fault 1 we investigate the rupture process of a sequence of repeating mw 2 1 earthquakes on the san andreas fault in parkfield spanning the occurrence of the september 28 2004 mainshock by inverting seismic moment rate functions obtained from empirical green s function deconvolution the results show that these events have extremely concentrated slip patches with radii on the order of 10 20 m with peak slip between 8 4 and 11 4 cm the rupture speed and rise time are consistent with values of larger earthquakes the spatial distribution of stress drop for the events shows low average values 2 5 5 6 mpa and very large peak values of 66 7 93 9 mpa the results show that strong asperities can exist at small scales on an otherwise weak fault and helps reconcile differences between traditional spectra based and tectonic loading methods for determining the stress drop of small repeating earthquakes

patterns and controls on the distribution of diamondiferous intrusions in australia abstract the distribution of kimberlite lamproite and related alkaline volcanism in australia can be broadly related to the structure of the australian continent and lithosphere diamondiferous kimberlites and lamproites with the apparent exception of the weakly diamondiferous orrorro kimberlites in the adelaide fold belt lie within the large precambrian shield where seismic tomographic models and heat flow data indicate the presence of relatively cold high seismic wave speed lithosphere tectosphere typically some 200 km thick or more beneath the archaean cratons and up to 300 km in parts of central australia many of the diamondiferous intrusions appear to lie at the margins rather than in the centre of the lithosphere domains the highest concentration of diamondiferous intrusions kimberlites and lamproites is on and around the kimberley craton where seismic data indicate crustal thicknesses of 35 40 km and a lithosphere up to 275 km thick that is distinct from proterozoic northern australia many but clearly not all of the intrusions show evidence of regional and local structural controls some are spatially associated with known crustal structures especially regional faults others are aligned either singly or in clusters along or near discontinuities and or gradients evident in regional scale potential field data especially the total horizontal gradients of gravity data continued upward tens to hundreds of kilometres many of these features are not evident in the original datasets as their signatures are masked by shorter wavelength near surface anomalies in some cases the kimberlites and associated rocks lie within crustal blocks and domains defined by discontinuities in the potential field data rather than at domain boundaries our overview suggests that analysis of potential field data especially horizontal gradients in upwardly continued potential field data at all scales can assist definition of crustal and potentially lithospheric structures that may influence the distribution of diamond pipes however more definitive mapping of australia s diamond prospective regions requires the integration of data on crustal structures especially trans lithospheric faults and geodynamic settings with high resolution tomographic models and other geophysical petrologic and isotopic information on the nature of the lithosphere beneath the australian continent

source inversion of the 1988 upland california earthquake determination of a fault plane for a small event abstract we examined short period p waves to investigate if waveform data could be used to determine which of two nodal planes was the actual fault plane for a small m l 4 6 earthquake near upland california we removed path and site complications by choosing a small aftershock m l 2 7 as an empirical green function the main shock p waves were deconvolved by using the empirical green function to produce simple far field displacement pulses we used a least squares method to invert these pulses for the slip distribution on a finite fault both nodal planes strike 125 dip 85 and strike 221 dip 40 of the first motion focal mechanism were tested at various rupture velocities the southwest trending fault plane consistently gave better fitting solutions than the southeast trending plane we determined a moment of 4 2 10 22 dyne cm the rupture velocity and thus the source area could not be well resolved but if we assume a reasonable rupture velocity of 0 87 times the shear wave velocity we obtain a source area of 0 97 km 2 and a stress drop of 38 bars choice of a southwest trending fault plane is consistent with the trend of the nearby portion of the transverse ranges frontal fault zone and indicates left lateral motion this method provides a way to determine the fault plane for small earthquakes that have no surface rupture and no obvious trend in aftershock locations

rupture velocity estimation of large deep focus earthquakes surrounding japan 1 rupture velocity is an important source parameter which is often difficult to determine especially for deep focus earthquakes where there is usually limited near source information to help overcome this problem we developed a new method to estimate rupture velocities of deep focus earthquakes with better resolution we first carry out teleseismic p waveform inversions to determine slip distributions for a range of rupture velocities on the two nodal planes then forward modeling of regional data is performed using the slip distributions determined in the teleseismic inversions to estimate the rupture velocity using this method we attempted to determine the rupture velocities of large deep focus earthquakes surrounding japan which are well recorded on teleseismic and regional networks empirical green functions are used for both the teleseismic and regional analyses although it is difficult to determine the rupture velocity from only the teleseismic data the analyses including regional data show clear difference which can resolve the rupture velocity and fault geometry for three deep earthquakes we obtained rupture velocities of about 1 2 km s which correspond to 20 40 of the shear wave velocity and are much slower than typical values for shallow earthquakes

source retrieval for deep local earthquakes with broadband records abstract body wave data recorded at a small network of broadband seismograph stations are analyzed to investigate local events with focal depths deeper than about 50 km for these events the initial portion of p wave displacement represents well the source time function with a scaler correction for the seismic moment the magnitudes of the analyzed earthquakes range from m w 3 1 to 6 5 it is shown that the seismic moment m 0 and the pulse width are well correlated as m 0 3 constant indicating that the stress drop is largely constant this dynamic similarity seems to be valid for a vast range of earthquake sizes m w 1 8 it is also shown that source complexity such as a multiple shock nature is not a characteristic of only large earthquakes but is often observed even for small earthquakes

petrogenesis of the 723 ma coronation sills amundsen basin arctic canada implications for the break up of rodinia the coronation mafic sills and associated natkusiak flood basalts in arctic canada are part of the 723 ma franklin igneous events a flood basalt and dyke swarm province that extends across the southern arctic of north america and greenland two representative sills 70 and 25 m thick respectively outcrop near coronation gulf in nunavut they are composed of clinopyroxene plagioclase olivine and fe ti oxides and have tholeiitic composition similar to the natkusiak basalts their end values range between 4 2 and 5 9 and have la yb n ratios of 1 5 2 these rocks are mantle derived and were affected by minor crustal contamination compositional variations indicate that the coronation sills have experienced extensive fractionation including flow differentiation during emplacement and show increased differentiation from west to east across the province a feature similar to the mackenzie dyke swarm however the mackenzie dyke swarm and the franklin igneous events have distinct chemical composition the franklin rocks are related to a mantle plume located near victoria island at the center of the dyke swarm and could represent an offshoot from the superplume related to the break up of rodinia the absence of such dykes and sills in siberia argues against locating siberia against northern laurentia in a rodinia reconstruction 2003 elsevier b v all rights reserved

source process of the 21 april 1997 santa cruz island earthquake mw 7 8 we apply an empirical green s function method to recordings of teleseismic body waves from the april 21 1997 santa cruz island earthquake the far field moment rate functions reveal that rupture occured on a shallow dipping fault plane dip 17 the main rupture consists of 6 large subevents and covers a total area of about 5000 km while the hypocenter is fixed at 33 km depth the main slip is located above 20 km depth within the accretionary wedge the source duration is about 70 sec and the average rupture velocity is found to be 1 9 km sec this event caused a local tsunami and although it is not possible to determine whether it was caused by tectonic faulting or submarine shaking the fact that most of the moment release occured at shallow depths is consistent with either of these possibilities and points to the utility of the procedure outlined here in possible rapid characterization of tsunami potential

kīholo bay hawai i earthquake sequence of 2006 relationship of the main shock slip with locations and source parameters of aftershocks 1 we study the source process of the k holo bay earthquake mw 6 7 which occurred beneath the northwest part of the island of hawai i on 15 october 2006 and static stress drops of small earthquakes that occurred in 2006 and 2007 around the main shock including aftershocks we relocate the aftershocks to determine the fault plane from the two nodal planes the relocated aftershocks define an e w trending plane that dips to the south in good agreement with one of the nodal planes given by the global centroid moment tensor solution waveform inversion is performed with multiple time windows to investigate the rupture speed and the slip distribution of the main shock waveforms of an aftershock with mw 5 2 are used to calculate empirical green s functions our results indicate that the rupture propagated unilaterally to the west with a rupture speed greater than 3 0 km s 63 of the shear wave velocity this westward rupture is consistent with the fact that aftershocks are distributed predominantly to the west of the main shock epicenter most aftershocks are located on the edge of patches with a large slip or asperities and some also occur inside the patches we also estimate static stress drops of 39 earthquakes 2 5 ml 3 5 that occurred in 2006 and 2007 near the source region of the k holo bay earthquake static stress drops range from 0 12 to 8 6 mpa and aftershocks around large slip patches of the main shock likely to have larger stress drops

instability of deformation this chapter addresses the mechanisms by which instabilities can develop in rocks in the nominally ductile regime under conditions that the authors believe are relevant to earth we will not address brittle failure or frictional sliding processes in detail but much of our discussion will be couched in terms of knowledge gained in studies of these processes as a consequence we will begin by discussing why brittle failure and frictional sliding cannot be the mechanism by which rocks become unstable at depth despite the strong evidence that they are the underlying mechanism of earthquakes at shallow depths e g scholz 1990 2002 brittle shear failure in the normal sense is fundamentally a tensile process tensile microcracks must first nucleate and self organize via interaction of the stress concentrations at their tips as a consequence brittle failure and frictional sliding are strongly inhibited by pressure because work must be done against the pressure to open the mode i cracks the inhibition is so strong that the stress required to create a fault or initiate sliding on an existing fault becomes greater than the room temperature flow stress of many rocks at pressures equivalent to only a few tens of kilometers in earth increasing temperature has little effect on brittle processes but in contrast the ductile flow stress of rocks falls exponentially with temperature because both pressure and temperature increase with depth in earth earthquakes by unassisted brittle fracture mechanisms or frictional sliding can only occur at depths less than 30 50 km nevertheless earthquakes occur to depths of almost 700 km in earth and seismological evidence demonstrates unequivocally that they occur by displacement across a surface or narrow zone i e they occur by faulting this conundrum can be explained qualitatively by certain specific types of mineral reactions occurring under certain restricted conditions the

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

implications of spatial and temporal development of the aftershock sequence for the mw 8 3 june 9 1994 deep bolivian earthquake on june 9 1994 the mw 8 3 bolivia earthquake 636 km depth occurred in a region which had not experienced significant deep seismicity for at least 30 years the mainshock and aftershocks were recorded in bolivia on the banjo and seda broadband seismic arrays and on the san calixto network we used the joint hypocenter determination method to determine the relative location of the aftershocks we have identified no foreshocks and 89 aftershocks m 2 2 for the 20 day period following the mainshock the frequency of aftershock occurrence decreased rapidly with only one or two aftershocks per day occuring after day two the temporal decay of aftershock activity is similar to shallow aftershock sequences but the number of aftershocks is two orders of magnitude less additionally a mb 6 apparently triggered earthquake occurred just 10 minutes after the mainshock about 330 km east southeast of the mainshock at a depth of 671 km the aftershock sequence occurred north and east of the mainshock and extends to a depth of 665 km the aftershocks define a slab striking n68 w and dipping 45 ne the strike dip and location of the aftershock zone are consistent with this seismicity being confined within the downward extension of the subducted nazca plate the location and orientation of the aftershock sequence indicate that the subducted nazca plate bends between the nnw striking zone of deep seismicity in western brazil and the n s striking zone of seismicity in central bolivia a tear in the deep slab is not necessitated by the data a subset of the aftershock hypocenters cluster along a subhorizontal plane near the depth of the mainshock favoring a horizontal fault plane the horizontal dimensions of the mainshock beck et al this issue silver et al 1995 and slab defined by the aftershocks are approximately equal indicating that the mainshock ruptured through the slab

shear promoted phase transitions in fe2sio4 and mg2sio4 and the mechanism of deep earthquakes phase transformations in end member olivines have been investigated in the temperature range comparable to the interior of subducting slabs this work constitutes the experimental evidence that the kinetics of transformation of silicate olivine phase to modified spinel and spinel phases is enhanced by shear deformation natural fayalite fe2sio4 subjected to a pressure gradient from 0 to 25 gpa at 380 c in the diamond anvil cell dac developed a ring of phase where the pressure was in the stability field of the phase and shear stress was large enough to promote the transition the sample inside the ring despite being at higher pressure remained dominantly as phase the outermost lower pressure region of the sample also remained as phase in the mg2sio4 system the transition from to was observed at 575 c in runs in which pressure covered the stability fields of phase phase and mixed oxides these results show that the characteristic transformation temperature tch can be lowered as much as 200 c by shear deformation on the basis of these observations we propose a nonhydrostatic kinetic boundary for the and transitions in mantle olivine for temperatures below this boundary the transformations are kinetically inhibited while above it the transformations can be promoted by shear deformation therefore olivine carried to a depth of several hundred kilometers in a subducting slab can remain as metastable phase until shear deformation causes its transformation we suggest that this region of shear promoted transformation in the cold interior of the subduction zone is responsible for the generation of deep earthquakes

geochemical evolution of high pressure mafic granulites from the bacariza formation cabo ortegal complex nw spain an example of a heterogeneous lower crust the cabo ortegal complex northwestern iberian massif is a klippen formed of several structural units stacked during the hercynian collision all these units include ultramafic rocks metabasites and quartz feldspathic gneisses affected by different metamorphic conditions the bacariza formation is heterogeneous showing a conspicuous layering mainly defined by alternate high pressure ultrabasic to basic granulites retrogressed garnet amphibolites of intermediate composition and rare acid rocks forming garnet trondhjemitic gneisses this layering is inherited from a gabbroic protolith showing a composition rich in fe and ti major and trace elements of these rocks can be correlated to continental tholeiitic series of extensional settings these high pressure granulites are situated in normal contact between serpentinised ultramafic rocks and other high grade metabasites with lessevolved and more depleted composition comparable to t type and n type morb it is suggested that the layered gabbro type protolith was part of a continuous mafic crust this crust was initially formed during early ordovician in a continental extensional setting and progressively evolved to oceanic spreading

the mechanism of the deep bolivia earthquake of june 9 1994 we analyzed the iris broad band seismograms for the june 9 1994 bolivia earthquake the largest deep focus earthquake ever recorded we inverted the record sections with a duration of 330 sec which include p pp and sp phases and obtained a dip slip mechanism dip rake strike 13 4 1 or 89 103 95 with a scalar moment of mo 2 9 10 21 nm mw 8 2 this mechanism is similar to that obtained from long period 175 to 250 sec rayleigh and love waves 83 100 90 mo 3 0 10 21 nm mw 8 3 the main rupture was preceded by about 10 sec by a cluster of smaller events with a total moment of 1 2 10 20 nm mw 7 3 the main rupture starts at about 20 km east or northeast of the initial break and propagates horizontally in the east to northeast direction the source duration is 40 sec and the rupture dimension is about 40km if a fault area of 40 40km is assumed the stress drop is 110 mpa an inversion of body waves p pp and sp with 6 basis moment tensors yielded a best fit solution with a 10 implosive component this result is of marginal significance and its confirmation would require more detailed information on the structures near the source region pp bounce points and stations the observed amplitude of the radial mode 0s 0 1226 sec is consistent with a double couple model with less than 2 isotropic component

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

mapping the mantle lithosphere for diamond potential using teleseismic methods abstract recent developments in seismic magnetotelluric and geochemical analytical techniques have significantly increased our capacity to explore the mantle lithosphere to depths of several hundred kilometres to map its structures and through geological interpretations to assess its potential as a diamond reservoir several independent teleseismic techniques provide a synergistic approach in which one technique compensates for inadequacies in another shear wave anisotropy and discontinuity studies using single seismic stations define vertical mantle stratigraphic columns for example beneath the central slave craton seismic discontinuities at depths of 38 110 140 and 190 km appear to bound two distinct anisotropic layers tomographic 3 d inversions of seismic wave travel times and 2 d inversions of surface or scattered waves use arrays of stations and provide lateral coverage in combination and by correlation with electrical conductivity and xenolith petrology studies these techniques provide maps of key physical properties within parts of the cratons known to host diamonds beneath the slave craton the discontinuity at 38 km is the base of the crust the boundaries at 110 and 140 km appear to bound a layer of depleted harzburgite that is interpreted to contain graphite to date only some of these techniques have been applied to the slave and kaapvaal cratons so that the origin and geological history of the currently mapped mantle structures are not as yet generally agreed

gossan hill victoria island northwest territories an analogue for mine waste reactions within permafrost and implication for the subsurface mineralogy of mars abstract gossan hill is located within the minto inlier in central victoria island northwest territories n 71 36697 w 114 95155 a study of the mineralogical associations and geological setting of this deposit indicates that it is an arrested hydrothermal system frozen in permafrost from above the hill stands out because of the topographic relief of 75 m and the orange brown color of the surficial material the surface of the hill is marked by areas of concentric color zonation up to 3 m across with light gray centers surrounded by a yellow orange ring that is surrounded by an orange brown color that covers the rest of the surface of the hill trenches dug into these areas reveal that the central zone contains quartz and pyrite native sulfur in a loose aggregate of sand sized grains this central area is surrounded by a zone dominated by gypsum and quartz with some jarosite beyond this the surrounding surface consists of quartz hematite and amorphous iron oxides the radial arrangement of the mineral assemblage indicates an increase in oxidation of sulfur from the center outward analysis of isotopic composition of the sulfur indicates the source of sulfur could be the underlying strata the hill is underlain by inter bedded carbonate and sulfate evaporite sedimentary rocks of the kilian formation in the upper part of the neoproterozoic shaler super group the sedimentary rocks were intruded by diabase sills of the 720 ma franklin igneous event which crop out 2 km to the south of gossan hill the soft friable nature of the deposit and the topographic relief of the hill indicate a post glacial pleistocene age of formation permafrost has maintained the disequilibrium mineral assemblage since the cessation of fluid flow extraction of the permafrost ice from the central zone yields a liquid with a ph of 2 3 the observed long term persistence of pyrite encased within the acidic permafrost indicates that oxidation and dissolution reactions common in mine waste are slowed if not stopped in such an environment the predicted rise of arctic temperatures will cause the active layer to move deeper and result in the release of the acidic solutions frozen in the permafrost water ice or frozen co2 just below the martian surface would also preserve such mineral disequilibrium for very long periods of time no region exists on earth where ice has existed continuously for millions of years but the gossan hill deposit is an excellent terrestrial analogue on mars the subsurface ice may be very old ancient reactive martian mineral assemblages and the fluids associated with them will reflect conditions that existed in the past

supershear rupture in the 24 may 2013 mw 6 7 okhotsk deep earthquake additional evidence from regional seismic stations zhan et al 2014a reported supershear rupture during the m w 6 7 aftershock of the 2013 m w 8 3 sea of okhotsk deep earthquake relying heavily on the regional station pet which played a critical role in constraining the vertical rupture dimension and rupture speed here we include five more regional stations and find that the durations of the source time functions derived from these stations are consistent with zhan et al s supershear rupture model furthermore to reduce the nonuniqueness of deconvolution and combine the bandwidths of different stations we conduct a joint inversion of the six regional stations for a single broadband moment rate function mrf the best fitting mrf which explains all the regional waveforms well has a smooth shape without any temporal gaps the m w 6 7 okhotsk deep earthquake is more likely a continuous supershear rupture than a dynamically triggered doublet

ghosts of lithospheres past imaging an evolving lithospheric mantle in southern africa group ii 143 117 ma and group i kimberlites 108 74 ma intrude across the southwest boundary of the kaapvaal craton sampling the same volume of the subcontinental lithospheric mantle sclm in two time slices major and trace element analyses of 3699 peridotitic garnet xenocrysts were used to construct paleogeotherms for 17 kimberlite localities and to place each garnet at its depth of origin the ti contents of each garnet and the calculated x mg of its coexisting olivine were projected onto a southwest northeast section across the craton boundary and splines were used to interpolate between the virtual boreholes in each age group the sections show that the cratonic sclm extends at least 75 km southwest of the mapped craton boundary suggesting a dipping contact marked differences between the time slice sections show that between 117 and 108 ma the sclm on both sides of the craton boundary was heated and chemically refertilized by infiltrating asthenosphere derived melts thinning the depleted layer by 40 km the thermal and geochemical changes record a significant tectonothermal event that may be related to changes in the stress field associated with opening of the south atlantic or with mantle upwelling

faulting associated with the olivine to spinel transformation in mg2geo4 and its implications for deep focus earthquakes samples of metastable mg2geo4 olivine fail by faulting when loaded at temperatures where the incoherent nucleation and growth of spinel the stable phase just becomes possible at lower temperatures where the transformation is inhibited metastable olivine is ductile and strong while at higher temperatures where the transformation happens very quickly samples are ductile and weak microfaults that have just been created and have slipped very little contain only very fine grained spinel those that have slipped more contain a mixture of fine grained spinel and larger broken olivine fragments a new type of transformation microstructure that develops only under nonhydrostatic stress occurs in all specimens that failed the spinel nucleates on grain boundaries and on subgrain boundaries within olivine and grows as fine grained lens shaped bodies with a strong preferred orientation normal to maximum compression the correlation between the onset of transformation the new microstructure and the mechanical instability suggests a cause and effect relationship the grain size of the spinel within the microfaults suggests that these zones have superplastic properties in this case the instability should not be suppressed by increasing pressure as normal frictional processes are suppressed analogous conditions exist in down going slabs of lithosphere in the mantle and we postulate that the same mechanism that causes faulting in our samples operates in the mantle causing deep focus earthquakes

nd isotopic evidence from wopmay orogen for 2 0 2 4 ga crust in western north america abstract nd isotopic data from the 1 9 ga wopmay orogen n w t canada strongly suggest the involvement of crust in the range of 2 0 2 4 ga remnants of three compositionally diverse magmatic arcs 1940 1902 ma 1890 1880 ma and 1875 1840 ma are preserved in wopmay orogen each of the three arcs originated west of the limit of exposed archean crust nineteen samples of plutonic and volcanic rocks from the three arcs yield a narrow range of initial e nd from 1 8 to 0 3 there is no apparent correlation between e nd and geographic distribution nor with whole rock chemistry sio 2 54 75 two basalt samples from west of the archean have initial e nd s between 2 and 3 two 1 87 ga granites which intrude through archean crust have much lower initial e nd s of 5 9 and 8 6 the data from the three magmatic arcs are best interpreted in terms of a source region which had a very limited range of e nd at 1 9 ga which we interpret as 2 0 2 4 ga crust presumably hottah terrane basement wopmay fault zone is therefore the surface manifestation of a suture between 2 0 2 4 ga hottah terrane and the archean slave craton there is a growing body of data which suggests that much of western north america may be underlain by 2 0 2 4 ga crust geochronologic and isotopic data from other precambrian cratons also suggest that 2 0 2 4 ga crust may be much more widespread than previously thought models of crustal growth which interpret 1 9 1 7 ga crust as mixtures of archean crust and proterozoic depleted mantle may be overestimating the amount of juvenile material added to the crust in this time period

subduction zone thermal structure and mineralogy and their relationship to seismic wave reflections and conversions at the slab mantle interface seismological studies of the interface between downgoing slabs of subducted lithosphere and the overriding mantle provide constraints against which models of mantle and slab temperature and bulk composition can be tested these studies employing waves reflected and converted at the slab mantle interface suggest that a velocity contrast of 5 10 occurs over a distance of 10 20 km to depths of at least 350 km the authors have investigated whether such sharp contrasts can result form thermal structure compositional differences phase changes or other effects such as preferred mineral orientation using a mantle and slab temperature model a petrologic model of mantle and slab mineralogy and a data base of elastic properties of mantle and slab phrases theoretical seismic velocities were calculated and compared with the observed velocity contrast and boundary thickness constraints at 9 6 gpa and 1000c conditions modeled at the slab mantle interface at approximately 280 km depth cooler temperatures in the slab interior produce at constant composition only approximately a 1 75 v sub p increase into the slab in any of the peridotitic and eclogitic mineralogies investigated variation in v sub p among the different bulk compositions is only approximately 0 5 these contrasts are substantially lower than those obtained from the seismological studies andmore indicate that temperature and bulk compositional differences between mantle and slab are inadequate to cause the observed velocity contrasts they propose that phase transformations in mantle and slab mineralogies notably elevation of the olivine spinel phase change normally occurring at 400 km depth are important factors in these sharp contrasts other contributing effects may include preferential mineral orientation near the slab surface and at shallower depths the presence of partial melt or hydrous fluid evolved from dehydration reactions along the interface less

petrogenesis of the natkusiak continental basalts victoria island northwest territories canada the natkusiak flows 1100 m thick sills and dykes of victoria island northwest territories are part of an extensive late hadrynian magmatic event in the northern canadian shield the flows are subaerially erupted plateau basalts and together with associated doleritic sills and dykes display many geochemical features typical of continental tholeiites in addition to fractional crystallization the rocks were affected by crustal contamination but to a smaller degree than usually encountered in continental tholeiites elsewhere the basalts were derived from a depleted upper mantle source very similar to that of n type mid ocean ridge basalts although it is suggested that the melting residue contained small amounts of garnet 2

shear instability in a viscoelastic material as the cause of deep focus earthquakes the mechanism of deep focus earthquakes has been examined by numerical and linear analysis of shear instability in subducting slabs we assume subducting slabs deform such that the spatially averaged strain rate becomes time independent furthermore we assume that rheology depends on temperature in the manner of an activation process using this model we investigate quantitatively the conditions under which shear instability takes place i e the deformation in the material concentrates in a thin layer whose temperature increases explosively until melting occurs first we study instability in a material whose rheological properties are spatially homogeneous shear instability takes place in a homogeneous material when exceeds a certain critical level e c which depends strongly on the stress in the slabs but does not depend on the detail of the creep law the value of the activation energy next we study instability in a material that contains shear zones similar to those often observed by field geologists i e a thin layer made of material with lower vicosity than the surrounding region we find that shear zones trigger shear instability even for the case of provided that g the ratio of characteristic time of thermal diffusion to that of shear heating in shear zones exceeds 1 and that f the ratio of stiffness of elastic deformation in the surrounding region to that of ductile deformation in shear zones becomes lower than 1 applying this condition to subducting slabs with shear zones we find that shear zones trigger shear instability even when the average strain rate is lower than by an order of magnitude from these results and from an estimate of in subducting slabs we conclude that shear instability is a mechanism likely to induce deep focus earthquakes

lead isotopic study of early proterozoic wopmay orogen nw canada role of continental crust in arc magmatism lead isotopes are sensitive indicators of crustal interaction in arc magmatism because of the large contrast in lead concentration between mantle derived magmas and continental crust and the three independent decay paths in the u th pb system preserved within wopmay orogen 1 94 to 1 84 ga are the remnants of three magmatic arcs the isotopic composition of lead in alkali feldspars separated from a diverse suite of samples from all three arcs fall in a relatively tight cluster with 206 pb 204 pb 15 59 15 88 207 pb 204 pb 15 27 15 36 and 208 pb 204 pb 35 15 35 54 except for samples from the only plutonic suite within wopmay orogen observed to intrude archean crust plutons intruding archean crust have 206 pb 204 pb 15 29 15 49 206 pb 204 pb 15 23 15 31 and 208 pb 204 pb 35 21 35 30 lead isotopic compositions do not correlate with rock composition both diorites and peraluminous granites have the same lead isotopic composition this isotopic composition