an appraisal of aftershocks behavior for large earthquakes in persia abstract this study focuses on the distribution of aftershocks in both location and magnitude for recent earthquakes in iran 43 earthquakes are investigated using data from the global international seismological center isc seismic catalogue and from the regional earthquake catalogue of the institute of geophysics university of tehran igut between 1961 2006 and 2006 2012 respectively we only consider the earthquakes with magnitude greater than 5 0 the majority of these events are intracontinental occurring over four seismotectonic provinces across iran processing aftershock sequences reported by both catalogues with cut off magnitude of 2 5 and a sequence duration of 70 days leads us to define a spatial horizontal area a occupied with the aftershocks as a function of mainshock magnitude m for persian earthquakes isc log10 a 0 45ms 0 23 igut log10 a 0 25mn 1 7 also we found a range for the difference between the magnitudes of the strongest aftershock and of the mainshock in each sequence which varies from 0 9 isc to 1 27 igut finally the spacial distribution of the aftershocks corresponds reasonably well to the causative fault planes of earthquakes

assessment of teleseismically determined source parameters for the april 25 2015 mw 7 9 gorkha nepal earthquake and the may 12 2015 mw 7 2 aftershock on april 25 2015 a major m w 7 9 thrust earthquake ruptured the deeper portion of the seismogenic plate boundary beneath nepal along which india is underthrusting eurasia an m w 7 2 aftershock on may 12 2015 extended the eastern down dip edge of the rupture these destructive events caused about 9000 fatalities and 23 000 injuries the overall rupture zone is about 170 km long and 40 80 km wide this region of the plate boundary previously experienced a large earthquake in 1833 and in 1934 a larger m s 8 0 event located to the east ruptured all the way to the surface the main himalayan thrust mht on which slip occurred in 2015 has a very low dip angle of 6 and the depth of the mainshock slip distribution is very shallow extending from 7 to 18 km the shallow dip and depth present challenges for resolving faulting characteristics using teleseismic data we analyze global teleseismic signals for the mainshock and aftershock to estimate source parameters evaluating the stability of various procedures used for remotely characterizing kinematics of such shallow faulting back projection and finite fault slip inversion are used to assess the spatio temporal rupture history and evidence for frequency dependent radiation along dip slip zone width constraints from near field geodetic observations are imposed on the preferred models to overcome some limitations of purely teleseismic methods radiated energy stress drop and moment rate functions are determined for both events

global and local influences on the chemical composition of snowfall at dye 3 greenland the record between 10 ka b p and 40 ka b p abstract wisconsin age ice from dye 3 greenland shows a number of 18o level changes which indicate the occurrence of rapid climate transitions in order to study the effect of climate change on geochemical fluxes we have selected several of these transitions for chemical analysis at each transition we have measured chloride nitrate and sulfate concentrations with 5 cm depth resolution all three anionic species show significant variations which correlate with the measured 18o shifts in general periods of high 18o warm periods have lower anion concentrations than adjacent periods of low 18o cold periods however the relative concentration shifts are not the same for all species indicating that the concentration variations cannot be caused only by changes in snow accumulation rate acting on a constant anion flux in addition to these rapid concentration changes over the last 40 ka slower secular variations in baseline concentrations also occurred baseline chloride and sulfate concentrations reached maxima near time of maximum ice volume baseline nitrate on the other hand remained relatively constant until near the end of the glaciation when its concentration rose if a constant sulfate flux is assumed the measured sulfate concentrations imply a dependence of snow accumulation rate on 18o for the wisconsin which is similar to that presently observed in greenland the sulfate concentrations would then suggest that wisconsin snowfall rates were at times as much as eight times lower than today

evidence of a collision between the yucatan block and mexico in the miocene we present the evidence for an anomalous southwest dipping slab in southern mexico the main evidence comes from a clear receiver function image along a seismic line across the isthmus of tehuantepec and is also supported by a previous global tomographic model the slab dips at 35 is approximately 250 km in length and appears to truncate the cocos slab at about 120 km depth we hypothesize that the slab was created by subduction of oceanic lithosphere prior to the collision of the yucatan block with mexico at approximately 12 ma this scenario would explain the chiapas fold and thrust belt as the product of this collision and its age constrains the date of the event to be in the miocene

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

inversion of complex body waves ii abstract after our previous paper we developed a method to deconvolve body waves into a multiple shock sequence we first consider a general case where a rupture process consists of subevents with arbitrary source parameters and next a special case where a source process consists of subevents with two different fault mechanisms a simple criterion to check the significance of solution is given by a statistical consideration on the estimated values the method is applied to two large earthquakes the 1976 tangshan earthquake and the 1976 east turkey earthquake in both cases the earthquake sources consist of major strike slip events and minor thrust events the total seismic moments of the major events are comparable to the values obtained by surface wave analyses the 1968 borrego mountain earthquake is also analysed by the same method for comparison a single mechanism solution results in a good waveform match and no significant secondary mechanism is required for this event

source processes and seismotectonic implications of the 1995 and 1996 cyprus eastern mediterranean region earthquakes abstract the teleseismic body waves of two earthquakes were inverted for their sources to obtain a rupture process for each the february 23 1995 ms 5 8 and october 9 1996 ms 6 8 events took place to the northwest and southwest of cyprus both the earthquakes generated complex body waveforms which were inverted using the method of kikuchi and kanamori 1991 the inversion results suggest that the 1995 and 1996 events were multiple events associated with subevents taking place on different faults located nearby the source regions three subevents were necessary to explain the waveforms of the 1995 earthquake the first subevent showed a predominantly normal faulting mechanism while the second subevent located 20 km to the north of the first subevent had a strike slip mechanism with dip slip component despite the difference in mechanisms both the subevents had slip vectors oriented nearly e w the last subevent was located in the proximity of the first subevent and had a normal faulting mechanism with a slip vector in n s direction the different depths and slip vector directions of the first and third subevents may suggest strain partitioning with depth in the source region of the 1995 event the complex waveforms of the 1996 earthquake were modeled with three subevents too combining the hypocentral distribution of the aftershocks with the inversion results revealed that the first subevent took place on a fault plane dipping about 20 to the east southeast the mechanism of the first subevent together with the azimuthal pulse width distribution suggest that the sense of motion on the fault plane was right lateral strike slip and unilateral rupture propagation we estimate from the major subevent a slip rate of 10 mm year for the african plate the second and third subevents were located 35 km to the southeast of the main shock and had a predominantly normal faulting mechanism with slip vectors in north northwest direction the rupture process analysis of the 1995 and 1996 events suggests that the region to the west of cyprus experiences extensional tectonics

heat flow on the hawaiian swell and lithospheric reheating most oceanic volcanic centres caused by hotspot activity are surrounded by sea floor that is unusually shallow for its age1 2 there is increasing evidence that swells found on older sea floor like the hawaiian swell have formed from a broad scale reheating and thinning of the lithosphere as it passes over a mantle hotspot3 4 if the formation and subsequent disappearance of these swells are controlled by thermal processes they should have an above normal heat flux we present here the results from 95 new heat flow measurements on the hawaiian swell along the older part of the swell the heat flow is 20 25 higher than the normal heat flow for crust of this age this anomaly is consistent with the observed swell uplift the shape and amplitude of the heat flow anomaly require that the reheating be largely confined to the lower half of the lithosphere

teleseismic interpretation of the 1968 dasht e bayaz ne iran earthquake the 1968 dasht e bayaz earthquake is the oldest destructive earthquake in north east iran recorded by the wwssn network at teleseismic distances these records are well distributed in azimuth and allow a detailed study of the rupture process of the main shock we invert the wwssn long period body waves of the dasht e bayaz earthquake and its principal aftershock which occurred about 20 hours later to their sources to study the complexity of the sources the results of our analysis suggest that the main shock consists of three major subevents while the principal aftershock is a single event mechanisms of the subevents of the main shock are mainly strike slip and consistent with the observed fault movement the principal aftershock has a dip slip mechanism the sum of the seismic moments of the main shock is m0 4 5 1026 dyn cm the moment of the largest subevent of the main shock is m0 2 4 1026 dyn cm and its stress drop is about 200 bar this shows that the main shock has a high stress drop nature the largest subevent with shallow depth and large lateral displacement mostly caused the great destruction during the source process of the main shock the moment of the principal aftershock is m0 1 9 1025 dyn cm and its stress drop is about 40 bar mechanism of the earthquakes suggest that there is a shortening in north east iran mostly due to the north east compressional stress in the iranian plateau

high resolution ammonium ice core record covering a complete glacial interglacial cycle high resolution ammonium measurements were performed along the greenland ice core program grip deep ice core covering a complete climatic cycle no overall anthropogenic increase is observed over the last 300 years however springtime concentrations have roughly doubled since 1950 biomass burning is estimated to be a major source for ammonia emissions for preindustrial times it contributes between 10 to 40 to the total ammonium deposited on the central greenland ice sheet during the holocene no correlation is found between the ammonium summer concentrations recorded over the last 100 years and the area burned in northern north america which is considered to be the main source area for ammonium deposited on the central greenland ice sheet this suggests that the meteorological factor is predominant for the pattern of ammonium spikes observed in the ice core if unchanged meteorological conditions are assumed for the holocene as indicated by the 18o ice record a decreasing biomass burning activity toward present time can be derived from the ammonium ice record soil and vegetation emissions are responsible for the ammonium background concentrations in the ice the record therefore may be used to trace back the biomass history of the north american continent a pronounced decreasing trend in background ammonium is found during the holocene reflecting decreasing temperature and therefore lower nh3 emissions in the source region variations in the ammonium concentration during the glacial age are discussed in terms of changes in transport and deposition mechanisms and changes in source strength which can be related to the extent of the laurentide ice sheet the data suggest that the laurentide ice sheet was built up immediately after the last interglacial and went through several large fluctuations during the last ice age

impurity sources of f cl no3 and so4 2 in greenland and antarctic precipitation five polar impurity sources are proposed anthropogenic no3 and so42 in recent greenland but not antarctic snow marine aerosols the dominant cl source volcanic eruptions which produce detectable but variable peaks in so42 f and or cl concentrations biogenic sulfurous gases from soil and coastal waters the dominant source for so42 in nonvolcanic strata an unidentified source of background nitrate which is more active in the northern hemisphere and which displays a summer concentration maximum solar activity and supernova events do not modulate no3 concentrations in greenland snow

rupture of the 4 february 1976 guatemalan earthquake abstract an expanded body wave data set for the 4 february 1976 guatemalan earthquake is analyzed by iterative multiple station inversion to provide enhanced resolution of the rupture process of this important strike slip event the body wave data set used in an earlier study of this event in which a predominantly unilateral westward propagating rupture was proposed actually does not have sufficient resolution to reliably map the spatial heterogeneity for the source process the expanded azimuthal coverage of the new data set provides better spatial resolution of the primary rupture characteristics the guatemalan earthquake can be modeled as an asymmetric bilateral rupture extending eastward and westward along the motagua fault with the largest episode of moment release occurring 90 km west of the epicenter near a change in strike in the surface faulting the overall rupture velocity is about 3 5 km sec and the total moment is 3 7 10 27 dyne cm inversions with composite fault models having a second focal mechanism appropriate for the western end of the fault are consistent with the data but are not clearly required the characterization of the guatemalan earthquake as an irregular bilateral rupture agrees with the variable co seismic surface displacements measured along the fault outcrop as well as with an earlier surface wave investigation and is consistent in a general sense with previous characterizations of the irregular rupture processes of other large strike slip earthquakes such as the 1972 sitka alaska and the 1980 eureka california events

oxygen isotope and total beta radioactivity measurements on 10 m ice cores from the antarctic peninsula as part of the glaciology of the antarctic peninsula gap programme the snow cover at 25 stations was sampled to 10 m depth for oxygen isotope and total radioactivity analysis the mean annual oxygen isotope ratio correlates satisfactorily with 10 m temperature despite the complex topography of the area and suggests on average that climatic trends in the region are fairly systematic the relationship with temperature is similar to that derived from a simple model in which an air mass initially of maritime subtropical characteristics is progressively cooled as it moves towards the region the detailed isotope profiles show that for future deep drilling the most easily interpretable climatic information will be found in the more continental areas on the cast coast and on the plateau in the south of the region the degree of continentality of particular sites is reflected in the amplitude of the annual wave in the upper portion of the isotope ratio profile as well as in the accumulation rate

fault process and broadband ground motion simulations of the 23 october 2011 van eastern turkey earthquake abstract on 23 october 2011 an m w 7 1 earthquake occurred in eastern turkey close to the towns of van and ercis causing more than 600 casualties and widespread structural damage the earthquake ruptured a 60 70 km long northeast southwest fault with a thrust mechanism in agreement with regional tectonic stress regime we studied the fault process of the event and the recorded ground motions using different sets of data regional records 0 005 0 010 hz are used to constrain the centroid moment tensor solution near regional data 100 200 km from the fault are used for relocation of the hypocenter and in the frequency range 0 05 0 15 hz for inversion of the rupture propagation by two methods multiple point source model isola and multiple finite extent mufex source model mufex also provides an estimate of the model uncertainty which is quite large due to unfavorable station distribution we arrive at several plausible scenarios equally well fitting the observed data including global positioning system coseismic displacements with different styles of the rupture propagation a few alternative source models are used for broadband 0 1 10 hz ground motion simulations by means of the hybrid integral composite source model only models comprising source complexities such as a delayed rupture of shallow asperities enable explanation of the acceleration record at the only available near fault station which exhibits a long duration and two prominent wave groups these complex rupture models are used to simulate the ground motion in the near fault area specifically at van and ercis where records of the mainshock were missing providing reasonable agreement with the observed spatial distribution of damage

the rupture process and asperity distribution of three great earthquakes from long period diffracted p waves moment tensor source mechanisms for the largest twenty nine shallow earthquakes from 1981 m s 6 5 have been determined using long period rayleigh wave spectra and p wave first motions recorded by the international deployment of accelerograms ida global digital seismic network gdsn and world wide standardised seismic network wwssn these are compared with the centroid moment tensor mechanisms presented by dziewonski and woodhouse which were obtained in a contrasting manner in most cases the source is well represented by a single double couple mechanisms with good agreement of scalar moment and orientation between the two studies apparent mismatches for other cases may be interpreted in terms of source complexity we emphasise the objectivity of comparing moment tensor elements apparent discrepancies in the moment and orientation of double couple fault plane solutions are usually due to differences in the mxz and myz moment tensor elements which are poorly constrained by long period data

temporal variations of mineral dust biogenic tracers and anthropogenic species during the past two centuries from belukha ice core siberian altai 1 in july 2001 a 140 m long ice core was recovered from the belukha glacier 49 48 26 n 86 34 43 e 4062 m a s l in the siberian altai the ion chemistry of the upper 86 m covering the last two centuries is characterized by biogenic emissions ammonium and formate aeolian dust calcium magnesium chloride and sodium and anthropogenic species sulfate nitrate and ammonium particularly high ammonium and formate concentrations indicate pronounced emissions from siberian forests the inferred fire frequency does not show a long term trend but distinct periods of enhanced activity sulfate has the highest industrial to preindustrial ratio and an anthropogenic contribution of more than 80 variations in this record reflect sulfur dioxide emissions in siberia and kazakhstan sulfate concentrations remained low until 1950 then sharply increased and peaked in the 1970s the decrease in the 1980s is attributed to the economic political and social crises and to the replacement of coal with gas rising nitrate concentrations since 1960 reflect traffic growth and enhanced fertilizer application increasing ammonium concentrations since the 1950s are attributable to population inflow in southern siberia with the associated enhancement of agricultural activity a nitrate peak of short duration in 1908 is thought to be the atmospheric signature from the tunguska event on 30 june 1908

a note on the isostatic compensation and origin of king s trough summary the spectral analysis of a 550 km long ship board gravity and topography profile across the central part of king s trough in the north eastern atlantic ocean shows that the topography is regionally compensated on lithosphere older than 20myr this feature with its enigmatic trend oblique to that of the mid atlantic ridge therefore dates from no earlier than the late oligocene and was formed by intra plate activity such a constraint to its age agrees with the recent analysis of geological samples taken on the northern flank of the trough close to the position of the profile it is not consistent with previous suggestions that king s trough was created either by the rotations of the iberian plate to form the pyrenees or by a readjustment in the strike of the mid atlantic ridge furthermore this analysis shows no evidence for either thin crust or high density mantle under the trough there is no structural evidence to suggest that it formed as part of a compressional boundary

a procedure for modelling shallow dislocation sources summary expressions for displacements on the surface of a layered half space due to an arbitrary oriented shear dislocation are given in terms of generalized ray expansions useful approximations of these expressions for shallow events as recorded at teleseismic distances for realistic earth models are presented the results of this procedure are used to generate synthetic p sv and sh waveforms for various assumptions of stress drop

strong motion and teleseismic waveform inversions for the source process of the 2003 bam iran earthquake the bam earthquake m w 6 5 occurred on 26 december 2003 in southeastern iran causing a major disaster in the city of bam a remarkable vertical peak ground acceleration value of 988 cm s2 and two horizontal pulses were recorded inside the damaged city previous analyses showed that the earthquake was caused by a subsurface rupture on an unknown strike slip fault in this study we attempt to determine the precise fault location and source process of the 2003 bam earthquake by performing a multiple time window linear waveform inversion of teleseismic and strong motion data both individually and jointly we examined the general features of the fault location and the source process by analyzing the teleseismic displacement waveforms and determined the precise features and fault geometry by inverting the three components of strong motion velocity records the final estimate of the source process of the 2003 bam earthquake was determined by joint inversion of the datasets our results show that a single fault model characterized by the appropriate location of the hypocenter rake angle variations and the rayleigh like speed of the rupture front can satisfactorily explain the three components of the strong motion records at bam station

simultaneous inversion of waveform and geodetic data for the rupture process of the 1984 naganoken seibu japan earthquake summary a simultaneous inversion method of strong motion seismograms and geodetic data for inferring earthquake rupture process was developed we assumed a source model composed of many point sources of which slip vectors and rupture times are unknown parameters in addition to observed data we introduced smoothing constraints to determine many parameters without instability since the leastsquares solution depends on the relative weights of the smoothing constraints to a data misfit function we determined their values by minimizing akaike s bayesian information criterion abic by using abic we can objectively choose a unique model this inversion method was applied to the naganoken seibu japan earthquake of 1984 dividing a 15 x 10 km fault into 150 subfaults we obtained the minimum abic model with a seismic moment of 2 7 x los dyne cm this model has a slip peak in the eastern and shallow area of the assumed fault the high slip area is just under the areas where thrown out boulders were found by umeda et al 1987 and iio yoshioka 1988 comparing the aftershock distribution with the estimated slip distribution we found that the high slip area had few aftershocks this suggests that aftershocks are caused by the stress generated in the marginal region around the main shock slip the inversion analysis also revealed that there was a slight change in slip angle during the earthquake the high slip area has some amount of dip slip component while the slips in the other areas are almost purely right lateral

western extension of the iberian european plate boundary during the early cenozoic pyrenean convergence a new model abstract a dense seismic net work allows of drawing the western prolongation of the northern boundary of the iberian plate this early cenozoic deformed boundary is disrupted by two prominent nw se transform faults a kinematic reconstruction with a nw late cretaceous lower eocene motion of the iberian plate is proposed the azores biscay rise is interpreted as a short lived subduction zone whereas king s trough could be a transform fault

crustal structure and isostatic compensation near the kane fracture zone from topography and gravity measurements i spectral analysis approach summary six gravity and bathymetry profiles perpendicular to the kane fracture zone each more than 300 km long were gathered to study the variation in crustal structure in the vicinity of a major fracture zone and the gravitational edge effect at the contact between lithosphere of two different ages a spectral analysis of the gravity and bathymetric series as a function of wavelength shows that the gravitational edge effect is only significant at the longest wavelengths for remaining wavelengths the admittance the ratio of the amplitude of the gravity anomaly to the amplitude of the bathymetry is best explained by a model of isostasy in which topographic loads are partially supported by the flexural rigidity of an elastic plate about 6 km in thickness after subtracting the gravitational attraction of the bathymetry and its compensation substantial isostatic anomalies remain we interpret these anomalies as being caused by variations in crustal thickness which have little correlation with surface topography except at very long wavelengths the apparent crustal thickness varies by as much as a factor of 2 but there is no evidence indicating systematic thinning of the crust beneath the fracture zone our data do suggest that such density variations within the plate are also compensated by the isostatic response of an elastic plate but with very different effect from those at the surface this indicates that there are two different modes of crustal formation with different gravity and topographic signatures effusive volcanism which loads the surface of the elastic plate producing both topographic relief and coherent gravity anomalies and intrusive volcanism or underplating producing gravity anomalies but little topographic relief

possible focal mechanism change during reactivation of a previously ruptured subduction zone and stress tensor implications summary body waveform inversion methods usually assume a constant double couple source or a combination of such subevents previous studies suggest that the source rupture of mature sources in terms of coupling along smooth subduction zones are usually resolved on this assumption however for events where the source area overlaps previously ruptured zones the conventional inversion methods fail to resolve the source process for such events the subduction coupling may vary between the mature source area and the previously ruptured zone causing a variation in the focal mechanism too we study the implications on the stress tensor of the coexistence of two such focal mechanisms using a major aftershock of the 1985 chilean earthquake m 6 0 the 1978 kurile m 7 6 and 1980 santa cruz islands m 7 7 earthquakes for the first event we use the two different sets of focal parameters obtained by a study which describes a focal mechanism change during the process the epicentres of the latter two large events are located in the spots which had remained unbroken in the preceding large events e g the 1963 and 1969 kurile earthquakes and the 1966 santa cruz is earthquake respectively for each event one of the focal mechanism solutions shows a substantial strike slip component the stress tensors are examined under the constraints that the shear stress is parallel to the slip vector and a friction law is satisfied two particular families of stress tensors examined are 1 the tensors that are optimally orientated with respect to the mature source area and 2 the tensors that have a vertical principal stress direction the first type of tensors can fully account for the observed slip orientation only for the chile aftershock but it comes close to the observed one in the case of the kurile island event and would indeed reactivate the decoupled area at a reasonably low stress level the second type of tensors can be compatible with the two observed slip orientations in all cases however for the santa cruz is and kurile is earthquakes the friction law which should be satisfied on each plane requires too high a stress deviation

the 2004 earthquakes offshore of the kii peninsula japan hypocentral relocation source process and tectonic implication on 5 september 2004 two major earthquakes of mw 7 2 and mw 7 3 referred to as the foreshock and the mainshock hereafter occurred close to the nankai trough in southwest japan this earthquake sequence is located within the subducting philippine sea plate phs with an aftershock distribution that overlapped a portion of the source region of the 1944 tonankai earthquake mw 7 9 we relocate 53 events with magnitudes greater than 4 2 by combining a single event location method for depth and a multiple event location method for epicenter and origin time source mechanisms and rupture histories of the foreshock and the mainshock are investigated by using a waveform modeling method which allows for mechanism changes during the rupture process the mw 7 2 foreshock and the mw 7 3 mainshock are relocated in the uppermost mantle of the phs on primary high angle reverse faults focal mechanisms are consistent with the horizontal and vertical projects of the distribution of aftershocks we speculate that this earthquake sequence was due to the collision reaction between the phs and the zenisu ridge

iterative deconvolution of complex body waves from great earthquakes the tokachi oki earthquake of 1968 abstract a method of body wave inversion is developed in an attempt to extract the information about asperities or barriers in a fault zone a sequence of point sources each being characterized with the seismic moment the onset time and the location are iteratively derived from observed records at multi stations where the two dimensional extent of the source location is taken into account a modification is made of the iterative method of kikuchi and kanamori on the formulation of inversion procedure to facilitate the computation using this method we analyse long period p waves of the tokachi oki earthquake of 1968 mw 8 2 and obtain several significant subevents with time durations of 10 s their spatio temporal distribution shows that the rupture process consists of three characteristic stages a a stage of introductory rupture b a stage of main rupture and c a stage of aftershocks the main rupture takes place in the form of clustering around a few sites of the fault plane the largest subevent occurs in the northwestern corner the stress drop associated with this event is estimated to be 200 bars one order of magnitude higher than the stress drop averaged over the entire fault plane the sum of the seismic moments of the individual subevents amounts to 2 3 1028 dyn cm which approximately coincides with the one estimated from the analysis of long period surface waves this implies that the source of the tokachi oki earthquake consists of several major subevents with time durations of 10 s in addition to other minor subevents

moment tensor inversion of complex earthquakes summary moment tensor inversion methods can be applied with success in the determination of source properties of simple earthquakes however these methods utilize the assumption of a point source which is inadequate for modelling many complicated shallow earthquakes for complex earthquakes an inversion using finite faulting models is desirable but the number of parameters involved requires that a good starting model be found or that independent constraints be placed on some of the parameters a method is presented for low pass filtering both the data and green s functions passing only signals with wavelengths greater than the dimension of the entire fault the filter tends to smooth complications in the waveforms and allows application of the point source moment tensor inversion this method is applied to body waves from the 1978 thessaloniki greece earthquake the 1971 san fernando earthquake and to a multiple point source synthetic model of the san fernando event for the thessaloniki event although a multiple source mechanism has been suggested inversion results before and after filtering were essentially identical indicating that a point source mechanism is sufficient in modelling the long period teleseismic body waves in the case of the san fernando earthquake the point source green s functions were incapable of simultaneously modelling the p and sh waves inversion of p waves alone resulted in extreme parameter resolution problems but allowed constraint in one axis of the moment tensor and suggested an overall source time function inversion of a synthetic san fernando data set yielded similar results but allowed an investigation of the shortcomings of the method under controlled circumstances although the results may require substantial interpretation the method presented represents a simple first step in the analysis of complex earthquakes

nitrate in the greenland ice sheet in the years following the 1908 tunguska event abstract the tunguska event on 30 june 1908 has been subjected to much speculation within different fields of research publication of the results of the 1961 expedition to the tunguska area florensky 1963 supports that a cometary impact caused the event based on this interpretation calculations of the impactor energy release and explosion height have been reported by ben menahem 1975 and velocity mass and density of the impactor by petrov and stulov 1975 park 1978 and turco et al 1981 turco et al 1982 used these numbers to calculate a production of ca 30 106 tons of no during atmospheric transit this paper presents a high resolution study of nitrate concentration in the greenland ice sheet in ca 10 years covering the tunguska event no signs of excess nitrate are found in three ice cores from two different sites in greenland in the years following the tunguska event by comparing these results with results for other aerosols generally found in the ice the lack of excess no3 following the tunguska event can be interpreted as indicating that the impactor nitrate production calculated by park 1978 and turco et al 1981 turco et al 1982 are 1 2 orders of magnitude too high to explain this it is suggested from other lines of reasoning that the impactor density determined by petrov and stulov 1975 probably is too low

shallow intraplate seismicity related to the illapel 2015 mw 8 4 earthquake implications from the seismic source abstract the september 16 2015 mw 8 4 illapel chile earthquake is the first large event occurring in north central chile after the 1943 earthquake filling a known seismic gap in the region the earthquake took place in a complex tectonic region nearby an area where transition from erosive to accretionary margin occurs due to the collision of juan fernandez ridge jfr along the chilean margin we inverted the kinematic rupture process of the 2015 mw 8 4 illapel earthquake from the joint inversion of teleseismic body waves and near field data the relative weighting between datasets and the weighting of spatial temporal constraints are objectively estimated by applying the akaike s bayesian information criterion the coseismic slip model yields a total seismic moment of 4 92 1021 nm occurred over 120 s the rupture shows both downdip and updip propagation with slip extending along the thrust interface from 50 km depth to shallow near trench depths

marine tectonics processes at mid ocean ridges the use of multibeam echo sounders and various side looking sonar systems has greatly increased our knowledge and understanding of mid ocean ridge mor processes since the last iugg review on mid ocean ridge research macdonald 1983 the extensive use of swath mapping tools such as sea beam deep tow seamarc i and ii the application of multichannel seismic reflection techniques to the detection of axial magma chambers and the use of satellite data in the study of oceanic lithosphere have rejuvenated marine tectonics and have generated and stimulated a large number of important contributions this review will focus on what we feel were the major developments in marine tectonics with an emphasis on mid ocean ridges in the last four years certain topics such as marine seismicity and satellite studies are only briefly mentionned for reference and will be covered in greater detail in other reviews because of the exponential growth in the number of publications related to mors during 1982 1986 only a few references pertinent to each of the major research topics will be quoted in the text as examples but the reader will find an extensive bibliography at the end of the text references published prior to 1982 were added only where appropriate

p wave complexity and fault asperities the borrego mountain california earthquake of 1968 results from a synthetic seismogram analysis of the short period p waves from the borrego mountain earthquake of 9 april 1968 m l 6 4 are used to model the strong motion recording at el centro a short period long period deconvolution analysis of the teleseismic p waves suggested that a two source model would fit the data much better than the single source model presented by burdick and mellman 1976 forward and inverse modeling of the data demonstrated that two sources each of less than 2 sec duration the second occurring 2 2 sec after the first and both being at about 8 km depth best fit the short period waveforms from this model long period synthetics were generated which were found to be quite compatible with the data this source model was also used to synthesize the strong motion sh displacement velocity and acceleration records from el centro california the close match of synthetics and data is used to argue that short period waveforms contain much information about asperities which play a crucial role in the near source strong motions from an earthquake the borrego mountain event probably began with the failure of a fault asperity the evidence for this is the several hundred bars stress drops of the two short period sources and the probable location of these sources in a place where there was almost no aftershock activity or postseismic creep on the fault

revisiting the 1991 uttarkashi and the 1999 chamoli india earthquakes implications of rupture mechanisms in the central himalaya abstract the central himalaya has attracted much attention due to its potential to host a great earthquake an inference based on the history of earthquakes and the geodetic slip estimates the largest and earliest historically documented earthquake in the central himalaya occurred in 1803 m 7 7 near uttarkashi on its western part much before the great event of 1934 mw 8 2 located farther to the east both of which were felt severely in the gangetic plain south of the mountain front the seismotectonic model by seeber and armbruster 1981 predicts that large great earthquakes originate on the main himalayan thrust mht and their ruptures propagate southward to the main frontal thrust mft the rupture of the april 25 2015 mw 7 8 gorkha nepal earthquake however was southeast directed with a southwest directed slip and its mw 7 3 aftershock on may 12 also showed a similar slip direction post 2015 we revisited the 1991 uttarkashi mw 6 7 and the 1999 chamoli mw 6 5 earthquakes in india both of which had originated on the mht flat but show ssw directed slips as reported in this study ruptures of both these earthquakes fell short of reaching the mft just as the historically documented 1803 earthquake but their shaking effects were felt in the southern plains these observations compel us to conclude that the varying rupture modes exhibited by the himalayan earthquakes are controlled primarily by the geometry of the flat ramp flat system

imaging the 2016 mw 7 8 kaikoura new zealand earthquake with teleseismic p waves a cascading rupture across multiple faults the 13 november 2016 mw 7 8 kaikoura new zealand earthquake was investigated using teleseismic p waves backprojection of high frequency p waves from two regional arrays shows unilateral rupture of at least two southwest northeast striking faults with an average rupture speed of 1 4 1 6 km s and total duration of 100 s guided by these backprojection results 33 globally distributed low frequency p waves were inverted for a finite fault model ffm of slip the ffm showed evidence of several subevents however it lacked significant moment release near the epicenter where a large burst of high frequency energy was observed a local strong motion network recorded strong shaking near the epicenter hence for this earthquake the distribution of backprojection energy is superior to the ffm as a guide of strong shaking for future large earthquakes that occur in regions without strong motion networks initial shaking estimates could benefit from backprojection constraints

anomalous mantle at 45 n mid atlantic ridge the mid atlantic ridge at 45 n has a ridge crest depth close to normal for slow spreading ridges and a crustal thickness of 6 km however the free air gravity over the region is higher than expected for such axial depths and erupted basalts are enriched in incompatible trace elements and radiogenic isotopes which could be taken to indicate the presence of a mantle plume beneath the spreading axis but the ridge is at normal depths and the crust is of normal thickness we model free air gravity across and parallel to the strike of the ridge a density structure derived from a cooling model provides a good fit for gravity across the ridge axis a gravity model along the ridge axis indicates that most of the gravity signal over the azores and reykjanes ridge is related to thicker crust while the gravity signal over 45 n can be modeled by a density anomaly which extends along the spreading axis from about 43 n to 50 n and from zero age to 35 m y such a mass excess can be explained by an increase in the garnet content of the mantle within the stability field of garnet peridotite the enriched basalts occur in the same region as the mass anomaly we interpret them as originating from cool hotspot mantle this anomalous mantle may derive from a hotspot that existed at the north america africa eurasia triple junction when it was at 45 n between 59 and 26 m y the king s trough complex is the remnant of that hotspot we consider that this cool hotspot mantle still underlies the spreading axis and contributes to crustal construction

illuminating the near sonic rupture velocities of the intracontinental kokoxili mw 7 8 and denali fault mw 7 9 strike slip earthquakes with global p wave back projection imaging 1 the denali and kokoxili strike slip earthquakes are two of the longest recent intracontinental ruptures previous studies report a range of rupture velocities here we image these earthquakes by reverse time migration of the intermediate frequency p wave train recorded by global broadband seismometers this technique permits a relatively direct measure of rupture velocity speed and direction as constrained by the radiated seismic energy free from restrictive assumptions or rupture speed bounds placed on the solution we compare our results with published seismic gps displacement and surface slip inversion results both ruptures were initially subshear and transitioned over a distance no longer than 40 km to supershear speeds close to the p wave speed of 5 6 km s we investigate the accuracy of our results with synthetic data and experiment with using different imaging parameters and seismic subnetworks these tests allow us to rule out the possibility of subshear speeds along the supershear segments although we cannot exclude supershear speeds of 4 5 6 5 km s our most reliable rupture velocities of 5 6 km s are close to the local p wave speeds we hypothesize that these intracontinental faults have weak shear strengths or high breakdown slips or crustal rigidities and experience at least moderate slip or slip rate weakening our observations and previous published results lead us to speculate that very long surface extending faults with general homogeneity in prestress and fault strength together with smaller adjacent fault segments to provide triggering may be necessary ingredients for the sub rayleigh to supershear rupture speed transition in strike slip earthquakes

high stress drops of short period subevents from the 1968 tokachi oki earthquake as observed on strong motion records strong motion records of the 1968 tokachi oki earthquake were examined and two very high stress drop subevents were identified the first subevent had been previously located by nagamune 1969 and the second subevent was located in this study using p waves recorded on short period wwssn records estimates of source parameters revealed small source dimensions 1 per cent of the aftershock area and very high dynamic and static stress drops in the kilobar range for both of the subevents it is suggested that these subevents are important in driving the main rupture of this earthquake the two subevents also produced the dominant accelerations on the strong motion records and it is shown that high peak accelerations 150 to 200 cm sec2 were recorded even at relatively large distances 100 to 200 km

faulting within the eastern baikal rift as characterized by earthquake studies earthquake relocations focal mechanism determinations waveform modeling and stress analyses using focal mechanism information are used to examine the faulting processes within the western baikal rift 95 to 108 5 results of the study show that high angle dip 70 faulting is most likely to occur in the more youthful parts of the rift and is most likely to be associated with m 4 5 events like the eastern rift nearly all earthquakes occurred on faults dipping 30 suggesting that in no part of the rift is significant deformation occurring on low angle structures focal mechanism information does not indicate there is a sharp boundary to the western rift as observed in the eastern rift less normal faulting rakes 60 to 120 occurs in the western rift than the eastern rift waveform modeling of the two largest earthquakes 1950 mondy and 1959 srednebaikal in the western rift gives focal depths of 10 to 15 km the 1959 event appears to have occurred along the eastern boundary fault on lake baikal just to the north of proval bay a feature created a m 7 1 earthquake in 1862 the 1950 event may have occurred along the southern boundary fault of the tunka depression stress orientation information indicates a near horizontal nw se to wnw ese striking 3 in the eastern part of the western rift

inversion of strong ground motion and teleseismic waveform data for the fault rupture history of the 1979 imperial valley california earthquake a least squares point by point inversion of strong ground motion and teleseismic body waves is used to infer the fault rupture history of the 1979 imperial valley california earthquake the imperial fault is represented by a plane embedded in a half space where the elastic properties vary with depth the inversion yields both the spatial and temporal variations in dislocation on the fault plane for both right lateral strike slip and normal dip slip components of motion inversions are run for different fault dips and for both constant and variable rupture velocity models effects of different data sets are also investigated inversions are compared which use the strong ground motions alone the teleseismic body waves alone and simultaneously the strong ground motion and teleseismic records the inversions are stabilized by adding both smoothing and positivity constraints the moment is estimated to be 5 0 10 25 dyne cm and the fault dip 90 5 dislocation in the hypocentral region south of the united states mexican border is relatively small and almost dies out near the border dislocation then increases sharply north of the border to a maximum of about 2 m under interstate 8 dipslip motion is minor compared to strike slip motion and is concentrated in the sediments the best fitting constant rupture velocity is 80 per cent of the local shear wave velocity however there is a suggestion that the rupture front accelerated from the hypocenter northward the 1979 imperial valley earthquake can be characterized as a magnitude 5 earthquake at the hypocenter which then grew into or triggered a magnitude 6 earthquake north of the border

an inter hemispheric volcanic time marker in ice cores from greenland and antarctica a strong volcanic acid signal is clearly registered using an acidity measuring technique in the a d 1259 ice layer in four different greenland ice cores camp century milcent crete and dye 3 this signal is similar in amplitude to the laki iceland a d 1783 volcanic event as recorded in the central and south greenland ice cores measurement of ice layers from corresponding age levels in antarctic ice cores byrd station south pole and j 9 on the ross ice shelf provides similar strong acid signals there is no historical record of a significant volcanic eruption for the period around a d 1260 in the northern hemisphere subsequent chemical analyses of all a d 1259 ice layers show similar compositions we suggest that the a d 1259 signals registered in both greenland and antarctica were caused by the same volcanic disturbance and that its epicenter was located at the earth s equatorial zone which enabled global distribution of the acid gases these results indicate that inter hemispheric dating of ice sheets is possible by the chemical identification of major eruptive volcanic events in the equatorial zone

inversion of teleseismic body waves for the moment tensor of the 1978 thessaloniki greece earthquake seismograms from wwssn and canadian network stations were modeled to determine the source parameters of the 20 june 1978 thessaloniki greece earthquake m s 6 4 the depth of the initial rupture was constrained to 11 1 km by comparison of the arrival times of surface reflections with synthetic short period seismograms a focal sphere plot of first motion polarities provided little constraint on other focal parameters except to indicate that predominantly normal faulting was involved a generalized inverse technique utilizing the moment tensor formalism was applied to teleseismic p and sh waves for six increments of depth the moment tensor obtained indicated a nearly horizontal n trending tension axis and a nearly vertical compression axis and yielded the following double couple source parameters strike 280 7 dip 55 3 rake 65 5 seismic moment 5 7 1025 dyne cm and a skewed triangular source time function with a rise time of about 1 sec and duration of 6 to 8 sec due to indications of multiple or finite source effects for this event and the assumption in the moment tensor formalism of a point source a low pass filter was applied to the data and the inversions were repeated the results were nearly identical with those of the original inversion suggesting that any individual sources had similar mechanisms or that the point source model is sufficient for this earthquake

king s trough reactivated pseudo fault of a propagating rift summary a compilation of magnetic data in the western north atlantic shows that seafloor spreading magnetic anomalies 26 to 18 are dislocated across a pseudo fault with a direction mirroring that of king s trough it is interpreted that this feature and king s trough were formed by northward propagation of the rift initiated by a hotspot that created the azores biscay rise the rift propagation stopped when king s trough became part of the plate boundary between eurasia and iberia at chron 17 most of the present day structure of king s trough was created by extensional motion across this plate boundary from chron 17 to 9 6c

a comparison of the isostatic response of bathymetric features in the north pacific ocean and philippine sea summary this paper concerns the calculation and analysis of admittance functions from large and uniform data sets of gravity and topography in four regions of the northern and western pacific ocean the purpose is to separate and describe possible differences in isostatic compensation between several type regions of oceanic crust a mid ocean ridge juan de fuca a mid plate seamount chain hawaiian ridge fracture zone topography on old crust north of hawaii and a marginal basin philippine sea results suggest that there are significant differences in the degree to which long wavelength topography has been compensated which can be distinguished between regions these differences are set in the perspective of three simple compensation mechanisms two of these consider local airy models in which raised topography is compensated at depth either by crustal roots or low density mantle a third considers the effects of an elastic plate of variable thickness supporting crustal variations conclusions are that a a thick plate possibly in excess of 30 km supports the hawaiian ridge b a much thinner plate of 5 to 15 km existed when the fracture zone topography was formed c the juan de fuca ridge is compensated either regionally by a plate 5 to 10 km thick or locally by sub crustal low densities at depths of 15 to 20 km and d the philippine sea shows no evidence for regional support ridges are compensated locally by differences in crustal thickness whereas the basins are underlain by density variations at depths comparable to those of the much younger juan de fuca ridge the major difference between admittance functions for the philippine sea and comparably aged regions of the north pacific ocean adds further new evidence of possible evolutionary differences between it and normal ocean basins

the geology and formation of king s trough northeast atlantic ocean king s trough comprises a 450 km long chain of roughly parallel basins and flanking ridges situated 700 km northeast of the azores its formation has been variously described as a short lived plate boundary a compressional boundary or a re orientated former bend in the mid atlantic ridge recent studies of seismic and gravity data concluded that the flanking ridges result from hot spot activity which began around 55 m y b p while the trough could have been produced by contemporaneous or later rifting or shearing the geological sampling described in this paper was planned to resolve the mode of formation of the trough a long range sidescan sonar survey of the region was used as control for new bathymetric and geologic maps of the area and for transponder navigated dredging and coring operations on the flanks of the trough the samples recovered show that basalt volcanism dated at over 52 m y b p was followed by the deposition of a chalk sequence until miocene times trachyte volcanism at 32 2 m y b p was followed by a period of tuff and ash volcanism near sea level in the late oligocene and miocene a middle miocene chalk sequence is at present exposed on the trough walls the detailed morphology and geological sequence at king s trough agrees best with a hypothesis which involves uplift by around 2 km of an aseismic ridge hot spot trace at about 32 m y ago which was followed by intra plate rifting at around 16 20 m y b p a number of interpretations are possible for the amounts of vertical displacement during this rifting we favour total vertical fault movements of the order of 2 5 to 3 5 km