the physical basis of mb ms and variable frequency magnitude methods for earthquake explosion discrimination we use recent results from deterministic numerical modeling of earthquakes and explosions to develop an improved physical basis for earthquake explosion discrimination we use six three dimensional finite difference simulations of earthquakes for the earthquake source models and four empirical and four numerical models for explosions in different materials four factors contribute to the success of the mb ms discriminant source spectra focal mechanism near source elastic properties and interference of p spectral differences contribute about 0 5 magnitude unit to the separation between earthquake and explosion populations for large events but this difference vanishes for events with mb 4 0 using a frequency band of 0 5 3 0 hz for small events the vfm discriminant is complementary to mb ms in several ways vfm is improved by p interference while mb ms is degraded vfm is more effective for hard rock explosions while mb ms will preferentially discriminate explosions in low velocity materials and vfm is insensitive to fault orientation vfm is more sensitive to noise and regional attenuation differences than mb ms simultaneous use of both methods should allow improved discrimination of small events

gutenberg s surface wave magnitude calibrating function theoretical basis from synthetic seismograms the surface wave magnitude as originally defined by gutenberg 1945 appears to be the most stable quantity rating the strengths of seismic sources it is based on the ground displacement produced by a nearly monochromatic wave train the calibrating function underlying the magnitude definition was obtained from observations of the amplitudes as a function of epicentral distance for a wealth of earthquakes no distinction was made as to focal depths thus the calibrating function depends on one parameter the epicentral distance only subsequent attempts to introduce a focal depth correction were not successful in practice and one and the same calibrating function continues to be applied to events with focal depths ranging from 0 to about 70 km assuming a strike slip point source located inside a standard earth model synthetic seismograms of the fundamental mode sh love wave are computed for a range of focal depths and azimuths amplitude distance curves are obtained and averaged over the azimuths the curves are the basis for improved calibrating functions average depth corrections are obtained and applied to some populations of seismic events the corrected surface wave magnitudes are internally more consistent than the uncorrected ones usually published applying the depth correction to published surface wave magnitudes and considering themb ms relation for a sample of earthquakes and explosions it has been established that for a given intensity of short periodic radiation the 20 s radiation intensity can vary by orders of magnitude while it is known that the 20 s radiation intensity is generally lower for explosions than for earthquakes it turns out to be even lower if the depth correction is applied yielding a clearer separation between both classes of seismic events

on the geometric complexity of earthquake focal zone and fault systems a statistical study we discuss various methods used to investigate the geometric complexity of earthquakes and earthquake faults based both on a point source representation and the study of interrelations between earthquake focal mechanisms we briefly review the seismic moment tensor formalism and discuss in some detail the representation of double couple dc earthquake sources by normalized quaternions non dc earthquake sources like the clvd focal mechanism are also considered we obtain the characterization of the earthquake complex source caused by summation of disoriented dc sources we show that commonly defined geometrical fault barriers correspond to sources without any clvd component we analyze the cmt global earthquake catalog to examine whether the focal mechanism distribution suggests that the clvd component is likely to be zero in tectonic earthquakes although some indications support this conjecture we need more extensive and significantly more accurate data to answer this question fully

effect of source depth correction on the estimation of earthquake size the relationship between surface wave magnitude ms and seismic moment mo of earthquakes is essential for the estimation of seismic risk in any region in the hypothesis of constant stress drop theoretical models predict that log mo and ms are related by a linear law the slope most commonly found in the literature is around 1 5 here we show that the application to the ms values of the necessary correction for the focal depth gives a general increment of the correlation coefficient and that a slope around 1 0 is consistent with the global data while for regionalized data it can vary from about 1 0 to 2 0

foreshock occurrence before large earthquakes rates of foreshock occurrence involving shallow m 6 and m 7 mainshocks and m 5 foreshocks were measured in two worldwide catalogs over 20 year intervals the overall rates observed are similar to ones measured in previous worldwide and regional studies when they are normalized for the ranges of magnitude difference they each span the observed worldwide rates were compared to a generic model of earthquake clustering based on patterns of small and moderate aftershocks in california the aftershock model was extended to the case of moderate foreshocks preceding large mainshocks overall the observed worldwide foreshock rates exceed the extended california generic model by a factor of 2 significant differences in foreshock rate were found among subsets of earthquakes defined by their focal mechanism and tectonic region with the rate before thrust events higher and the rate before strike slip events lower than the worldwide average among the thrust events a large majority composed of events located in shallow subduction zones had a high foreshock rate while a minority located in continental thrust belts had a low rate these differences may explain why previous surveys have found low foreshock rates among thrust events in california especially southern california while the worldwide observations suggests the opposite california lacking an active subduction zone in most of its territory and including a region of mountain building thrusts in the south reflects the low rate apparently typical for continental thrusts while the worldwide observations dominated by shallow subduction zone events are foreshock rich if this is so then the california generic model may significantly underestimate the conditional probability for a very large m 8 earthquake following a potential m 7 foreshock in cascadia the magnitude differences among the identified foreshock mainshock pairs in the harvard catalog are consistent with a uniform distribution over the range of observation

dredged basalts from the western nazca plate and the evolution of the east pacific rise abstract ocean floor basalts and glasses were recovered from three stations along the western nazca plate from a sublinear topographic feature believed to represent the proto east pacific rise epr and include abyssal tholeiites feti basalts and glasses as well as transitional and little fractionated compositions when compared with their coexisting fresh glasses the feti basalts have higher total alkalies tio2 and mgo and lower feo suggesting that they have also been affected by non oxidative post magmatic alteration processes the feti glasses form a remarkably uniform compositional group through space and time a little fractionated composition having anmg number 73 similar to those reported from the mathematician ridge has higher na2o and tio2 and slightly lower cao than similar compositions from the slowly accreting mid atlantic ridge the basalts and glasses reported here exhibit the compositional diversity expected for propagating rifts and probably represent more than one volcanic episode both geochemical and geophysical interpretations support the inference that the epr grew from miocene times by the progressive growth and propagation of mantle perturbations leaving a remnant sublinear zone of rough topography characteristic of slower accretion as the trace of the proto epr continuing translations and rotations of axial segments are occurring along the epr probably in response to self reorganizations of mantle flow patterns arising from rapid melting and depletion of the source regions the data allow the inference that the youthful rift systems of the eastern pacific are far from thermodynamic equilibrium as might be expected if such systems were to drive fundamental life processes

study of the properties of seismicity in the mexico region summary new statistics of seismic properties are investigated in space and time for the mexico region they are main events seismicity number of earthquakes with aftershocks excluded swarming property average magnitude another way to study the slope of the recurrence time law maximum magnitude average focal depth and comparative efficiency of earthquakes in surface and body waves generation known as creepex quite a number of anomalies of the spatial distributions of the statistics were found some of them have evident geophysical meaning the explanation of others is rather controversial and some others have no interpretation as yet the most interesting phenomena are as follows 1 the average focal depth distribution indicates a different trend for the benioff zone west of approximately 96ow longitude due north which fits the position of the neovolcanic belt 2 creepex clearly distinguishes different tectonic areas i e subduction zones with low values transform faults medium and ridges high 3 high swarming is related to places with young sedimentary geology the time pattern of those properties in places of strong earthquakes is quite consistent and includes a phase of concentration of stress indicated by quiescence in average and maximum magnitude and main events seismicity and also more efficient body wave generation and anomalous swarming this phase is followed by an apparent weakening of the material indicated by a higher efficiency in surface wave generation

scenarios for earthquake generated tsunamis on a complex tectonic area of diffuse deformation and low velocity the alboran sea western mediterranean the tsunami impact on the spanish and north african coasts of the alboran sea generated by several reliable seismic tsunamigenic sources in this area was modeled the tectonic setting is complex and a study of the potential sources from geological data is basic to obtain probable source characteristics the tectonic structures considered in this study as potentially tsunamigenic are the alboran ridge associated structures the carboneras fault zone and the yusuf fault zone we characterized 12 probable tsunamigenic seismic sources in the alboran basin based on the results of recent oceanographical studies the strain rate in the area is low and therefore its seismicity is moderate and cannot be used to infer characteristics of the major seismic sources these sources have been used as input for the numerical simulation of the wave propagation based on the solution of the nonlinear shallow water equations through a finite difference technique we calculated the maximum wave elevations and tsunami travel times using the numerical simulations the results are shown as maps and profiles along the spanish and african coasts the sources associated with the alboran ridge show the maximum potential to generate damaging tsunamis with maximum wave elevations in front of the coast exceeding 1 5 m the carboneras and yusuf faults are not capable of generating disastrous tsunamis on their own although their proximity to the coast could trigger landslides and associated sea disturbances the areas which are more exposed to the impact of tsunamis generated in the alboran sea are the spanish coast between malaga and adra and the african coast between alhoceima and melilla

focal mechanisms and size distribution of earthquakes beneath the krafla central volcano ne iceland seismicity was monitored beneath the krafla central volcano ne iceland between 2009 and 2012 during a period of volcanic quiescence when most earthquakes occured within the shallow geothermal field the highest concentration of earthquakes is located close to the rock melt transition zone as the iddp 1 wellbore suggests and decays quickly at greater depths we recorded multiple swarms of microearthquakes which coincide often with periods of changes in geothermal field operations and found that about one third of the total number of earthquakes are repeating events the event size distribution evaluated within the central caldera indicates average crustal values with b 0 79 0 04 no significant spatial b value contrasts are resolved within the geothermal field nor in the vicinity of the drilled melt besides the seismicity analysis focal mechanisms are calculated for 342 events most of these short period events have source radiation patterns consistent with double couple dc mechanisms a few events are attributed to non shear faulting mechanisms with geothermal fluids likely playing an important role in their source processes diverse faulting styles are inferred from dc events but normal faulting prevails in the central caldera the best fitting compressional and tensional axes of dc mechanisms are interpreted in terms of the principal stress or deformation rate orientations across the plate boundary rift maximum compressive stress directions are near vertically aligned in different study volumes as expected in an extensional tectonic setting beneath the natural geothermal fields the least compressive stress axis is found to align with the regional spreading direction in the main geothermal field both horizontal stresses appear to have similar magnitudes causing a diversity of focal mechanisms

contemporary crustal deformation within the pamir plateau constrained by geodetic observations and focal mechanism solutions we used an updated data set of 192 gps derived surface velocities and 393 earthquake focal mechanisms mw 3 0 hypocenter depths 30 km to evaluate the spatial variations in the surface strain rate and crustal stress regime throughout the pamir plateau and its surrounding regions the strain rate field was estimated using the spline in tension approach that solves for the surface velocity in a rectangular grid and the stress field was predicted from focal mechanism solutions using the damped regional scale stress inversion drssi method of hardebeck and michael journal of geophysical research https doi org 10 1029 2005jb004144 2006 the results show that the crustal stress field around the pamir plateau is predominantly characterized by nnw sse compression and e w extension which is consistent with the principal orientations of the two dimensional surface strain rate tensor this agreement supports the notion that the pamir and southwestern tien shan are uniformly strained blocks in particular the fan shaped rotational pattern between text shmax and the strain rate from the western pamir to the tajik basin shows that the counterclockwise rotation of the text shmax orientation is associated with vertical deformation which is consistent with the idea of schurr et al tectonics 33 8 2014tc003576 2014 concerning the gravitational collapse and westward extrusion of the crust in the western pamir we propose that such a stress strain pattern dominated by nnw ese oriented compression and e w trending extension originated from a combination of the northward push of the indian continent and the southward subduction of the tien shan

evidence of bias in estimations of earthquake size magnitudes of earthquakes determined using traditional methods show systematic deviations dependent on tectonic setting from accurate estimates of earthquake size magnitudes are overestimated for most of the continental earthquakes underestimated for mid oceanic ridges and differences of varying sign are obtained for subduction zones this is important in evaluating the seismic risk or consideration of effects such as the seismic versus aseismic strain release as seismic moments determined from magnitudes can be wrong by as much as a factor of four

slow earthquakes along oceanic fracture zones evidence for asthenospheric flow away from hotspots abstract the regime of strain release along transform faults of the mid oceanic ridge system is studied it is shown that earthquakes along certain fracture zones exhibit systematic discrepancies between values of their magnitudes measured at short and long periods implying a regime of slower strain release also observed in a pattern of complex body waveshapes these slow fracture zones do not correlate with simple geographic or kinematic properties but usually occur in the neighborhood of hotspot volcanism frequently also characterized by gravity and bathymetry anomalies we propose that regimes of slow strain release may be due to a partial lubrication of the fault along these fracture zones which may itself be due to asthenospheric flow from the nearby hotspots along the pattern of pipelines described by vogt and johnson and by morgan

imaging active faulting in a region of distributed deformation from the joint clustering of focal mechanisms and hypocentres application to the azores western mediterranean region abstract the matching between linear trends of hypocentres and fault planes indicated by focal mechanisms fms is frequently used to infer the location and geometry of active faults this practice works well in regions of fast lithospheric deformation where earthquake patterns are clear and major structures accommodate the bulk of deformation but typically fails in regions of slow and distributed deformation we present a new joint fm and hypocentre cluster algorithm that is able to detect systematically the consistency between hypocentre lineations and fms even in regions of distributed deformation we apply the method to the azores western mediterranean region with particular emphasis on western iberia the analysis relies on a compilation of hypocentres and fms taken from regional and global earthquake catalogues academic theses and technical reports complemented by new fms for western iberia the joint clustering algorithm images both well known and new seismo tectonic features the azores triple junction is characterised by fms with vertical pressure p axes in good agreement with the divergent setting and the iberian domain is characterised by nw se oriented p axes indicating a response of the lithosphere to the ongoing oblique convergence between nubia and eurasia several earthquakes remain unclustered in the western mediterranean domain which may indicate a response to local stresses the major regions of consistent faulting that we identify are the mid atlantic ridge the terceira rift the trans alboran shear zone and the north coast of algeria in addition other smaller earthquake clusters present a good match between epicentre lineations and fm fault planes these clusters may signal single active faults or wide zones of distributed but consistent faulting mainland portugal is dominated by strike slip earthquakes with fault planes coincident with the predominant nne ssw and wnw ese oriented earthquake lineations clusters offshore sw iberia are predominantly strike slip or reverse confirming previous suggestions of slip partitioning

spatial variations in the similarity of earthquake populations the case of the 1997 colfiorito sellano northern apennines italy seismic sequence abstract the spatial properties of events in the 1997 colfiorito sellano seismic sequence northern apennines italy were investigated using coherence a parameter derived from seismic moment tensors that quantifies the kinematic similarity between focal mechanisms the 1997 colfiorito sellano seismic sequence predominantly consists of normal faulting earthquakes with a few strike slip and reverse faulting episodes this kinematic heterogeneity is possibly related to the contemporaneous activity of two different sets of faults nw se normal faults and nne ssw sub vertical faults the latter inherited from the previous miocene compressional phase the study used two independently derived data sets of the same seismic sequence characterized by a different number of events and by different precision of spatial localisation their statistical significances assessed through a reshuffling procedure reveal that data sets with at least some hundreds of events and good positional precision are required to obtain significant results through coherence analysis results from the better quality data set indicate that this seismic sequence is characterized by a rapid decrease in the kinematic similarity between earthquake pairs within 2 km of separation particularly along directions sub perpendicular to the normal fault strike the decrease rate seems to be controlled by the geometric characteristics of the normal faults given that the mean along dip distance between fault segments is 2 km in proximity to pre existing tectonic lineaments the relative abundance of strike slip and reverse faults tends to decrease the kinematic similarity between events but does not influence the coherence decrease rate the presence of mixed focal mechanisms normal reverse and strike slip in a single seismic phase implies that mixed fault types are not restricted to polyphase tectonic histories such heterogeneous kinematics during a single phase may be induced by the presence of inherited discontinuities

insight on the crustal stress state in faial and pico islands azores from analysis of aftershocks of the 1998 earthquake the seismicity in the azores plateau can be of volcanic or tectonic origin the latter usually more destructive especially in the central group of islands in this study we aimed to obtain crustal stress indicators from seismic data collected in july 1998 following the july 9th m5 8 earthquake that struck near faial and pico islands the maximum horizontal stress shmax azimuth was obtained from two methods for the seismogenic volume the shmax was obtained from single and composite focal mechanisms whereas the state of stress beneath the seismic network was derived from shear wave splitting analysis of local earthquakes the composite focal mechanisms were obtained from a waveform clustering method applied to identify identical earthquakes the seismic anisotropy analysis could be applied to only six stations the polarization of the first s arrival being stable and with a secondary perpendicular pulse arriving 0 6 0 24 s later the focal mechanisms provided a nw se shmax with azimuth range of 96 114 and an associated transition of normal fault to strike slip dominant regime consistent with the estimated regional field of 110 120 and coherent with movement along three fault systems inland the shmax azimuths present greater variability in faial the stations located north of the caldeira present a stable azimuth of 151 153 and are consistent with the effect of a nnw sse fault system present in the islands rotating to 50 77 in the eastern sector of the pedro miguel graben probably due to a complex interaction of the wsw ene dominant fault system with old eroded volcanic complexes that segmented the island into several blocks in pico island the shmax direction azimuth ranges from 93 to 135 in a radial pattern around the island s main stratovolcano overlapping the regional stress field

a reassessment of the rupture characteristics of oceanic transform earthquakes 1 we investigate the long period source spectra of oceanic transform earthquakes and find that previously proposed slow rupture components can be explained as artifacts generated by the modeling procedure we use low frequency 20 mhz rayleigh and love waves to calculate the amplitude spectra of five earthquakes on the romanche and chain transform faults in the equatorial mid atlantic ocean we find that errors and approximations in the centroid depth focal mechanism and earth structure at the source have significant effects on the shape of the source spectra if global catalog values and an average crustal model are assumed the spectra exhibit apparent anomalous energy at long periods which has previously been interpreted as a result of slow rupture we recalculate the source spectra using precise independently determined depths and moment tensors and a more realistic oceanic crustal structure in the source region the resulting source spectra are flat at long periods with no indication of anomalous long period energy our results imply that oceanic transform earthquakes do not commonly have detectable slow rupture components

global scale analysis of seismic activity prior to 2004 sumatra andaman mega earthquake abstract following our experience in intermediate term monitoring of seismicity before large earthquakes worldwide and in some regions we apply a similar approach to the analysis of seismicity on global scale several decades before the december 26 2004 sumatra andaman mega earthquake the earth lithosphere is being considered as a single whole representing the ultimate level of the complex earth s hierarchy the study brings up the following questions are there any anomalies of the global seismicity behaviour observed in advance of the mega earthquake do these anomalies correspond to the global scale seismic patterns similar to those detected on the regional scale before great major and strong events in other words does the earth lithosphere considered as a single whole show up an approach of the mega earthquake in the way typical for events on the smaller levels of hierarchy the results of our investigation favour the positive answer to the questions specifically we found that during the decade before the sumatra andaman mega earthquake the earth lithosphere reveals at least in intermediate term scale classical symptoms of instability which can be depicted by known precursory seismic patterns these are i transformation of frequency magnitude distribution ii change in the rate of seismic activity and iii depth redistribution of activity moreover changes of dependencies between magnitudes of different types are detected at the same time the observed global scale patterns of collective behaviour of seismicity may indicate the state of criticality of the earth lithosphere before the sumatra andaman mega earthquake

display and quantitative assessment of distributions of earthquake focal mechanisms summary this paper describes quantitative methods for evaluating the distribution of focal mechanism orientations of groups of earthquakes and shows how to display this distribution on a triangle diagram we present a 2 based statistical test for determining whether two sets of focal mechanisms are drawn from distinct populations we apply these methods to 3625 better determined mechanisms for shallow earthquakes in the harvard centroid moment tensor cmt catalogue we describe the distributions of mechanisms in the whole catalogue and for catalogue subsets in some specific tectonic environments in addition we explore the geographical locations of mechanisms with orientations that occur relatively infrequently that is mechanisms that are unlike thrust normal or strike slip mechanisms such mechanisms are relatively rare along mid ocean ridges and in oceanic subduction zones the majority of these unusual mechanisms occur along plate boundaries where crustal thickness is highly variable and in regions where the plate convergence direction becomes oblique and thus relative motion changes from convergence to transform motion

how sandstone porosity and permeability vary with diagenetic minerals in the scotian basin offshore eastern canada implications for reservoir quality abstract the scotian basin is an under explored offshore basin that has produced gas and minor oil from jurassic cretaceous deltaic sandstones reservoir quality is an important exploration risk in the basin this study evaluates whether there are systematic stratigraphic or geographic variations in diagenetic mineral assemblage control of reservoir porosity and permeability three data sets are used a compilation of all available core plug porosity and permeability measurements 3271 in total a subset of 577 corresponding to thin sections with lithofacies assignments and a subset of 35 thin sections principally from reservoir sandstones in which modal abundance of diagenetic minerals has been determined by image analysis of scanning electron microscope backscattered electron images abundances above a 4 threshold of diagenetic kaolinite calcite and ankerite correlate inversely with permeability but chlorite and possibly siderite correlate positively diagenetic assemblages show no systematic variation with stratigraphy or geography both of which may involve variable detrital supply permeability shows the well known variation with grain size and depth of burial but after correcting for these effects it is greatest in estuarine channel and river mouth turbidite sandstones this is because these facies preferentially develop chlorite rims on detrital quartz grains and are also the sites of enhanced flow of late basinal fluids resulting in secondary porosity local effects such as facies distribution architecture of channel sandstones fluid pathways related to complex salt tectonics and the effects of overpressure are more important than predictable basin wide effects in controlling sandstone reservoir quality

temporal change in scattering and attenuation associated with the earthquake occurrence a review of recent studies on coda waves the study of coda waves has recently attracted increasing attention from seismologists this is due to the fact that it is viewed as a new means by which the stress accumulation stage preceding a large earthquake can be measured since the scattering paths nearly uniformly cover a fairly large region around the focus and observation stations compared with the direct ray paths to date we have had many reports on the temporal variation of the relation between coda duration and amplitude magnitude and that of the coda attenuationq 1 which is estimated from coda amplitude decay some of these have shown a precursor like behavior however others seem to have shown a coseismic change we have critically reviewed these reports and discussed what these observational facts tell us about the change in the heterogeneous crust we found significant temporal variations not only in the mean but also in the scatter ofq 1 associated with the mainshock occurrence the formation of new cracks the reopening and growing of existing cracks the interaction of these cracks and the pore water movement through these cracks might correspond to such variations in addition we may expect an inhomogeneous distribution of crack clusters in a fairly large region compared with the aftershock region the gradual appearance of such crack clusters seems to be the most plausible mechanism by which coda decay gradients are caused to largely scatter in the stress accumulation stage

global creepex distribution and its relation to earthquake source geometry and tectonic origin summary the ms mb ratio conventionally used for the discrimination of earthquakes and underground nuclear explosions has been studied for a pure earthquake catalogue in an attempt to classify earthquakes into two categories one related to extremely lowfrequency events of creep and the other to relatively high frequency explosions ms and mb measured at 20 s and 1 s respectively are considered as estimates of source spectra at two separate points thus allowing one to compare large samples of earthquakes from the spectral point of view creepex or the deviation of ms mfc points from the orthogonal regression of ms on mb is by definition a parameter independent of the size of seismic events it has been estimated from neic magnitudes and is studied in relation to parameters of central moment tensor solutions of the harvard group for this purpose the parameters of central moment tensors have been factorized to make classes of equivalence of events with the same dip and rake and arbitrary strike and size the resulting triangle representation of source mechanisms has been used to correct the global distribution of creepex for the type of source mechanism dip slip events have consistently lower creepex than strike slip ones however the global distribution of creepex either corrected or uncorrected for source mechanism has a clear tectonic pattern negative values in most of the subduction zones and positive ones in mid ocean ridges the dependence of creepex on focal depth is negligible according to empirical data for events shallower than 40 km limitation of the sample by considering only events in this depth range does not diminish considerably the amount of data and does not change the tectonic pattern of the creepex

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