aftershocks of the 2010 mw 7 2 el mayor cucapah earthquake reveal complex faulting in the yuha desert california 1 we detect and precisely locate over 9500 aftershocks that occurred in the yuha desert region during a 2 month period following the 4 april 2010 mw 7 2 el mayor cucapah emc earthquake events are relocated using a series of absolute and relative relocation procedures that include hypoinverse velest and hypodd location errors are reduced to 40 m horizontally and 120 m vertically aftershock locations reveal a complex pattern of faulting with en echelon fault segments trending toward the northwest approximately parallel to the north american pacific plate boundary and en echelon conjugate features trending to the northeast the relocated seismicity is highly correlated with published surface mapping of faults that experienced triggered surface slip in response to the emc main shock aftershocks occurred between 2 km and 11 km depths consistent with previous studies of seismogenic thickness in the region three dimensional analysis reveals individual and intersecting fault planes that are limited in their along strike length these fault planes remain distinct structures at depth indicative of conjugate faulting and do not appear to coalesce onto a throughgoing fault segment we observe a complex spatiotemporal migration of aftershocks with seismicity that jumps between individual fault segments that are active for only a few days to weeks aftershock rates are roughly consistent with the expected earthquake production rates of dieterich 1994 the conjugate pattern of faulting and nonuniform aftershock migration patterns suggest that strain in the yuha desert is being accommodated in a complex manner

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

importance of small earthquakes for stress transfers and earthquake triggering 1 we estimate the relative importance of small and large earthquakes for static stress changes and for earthquake triggering assuming that earthquakes are triggered by static stress changes and that earthquakes are located on a fractal network of dimension d this model predicts that both the number of events triggered by an earthquake of magnitude m and the stress change induced by this earthquake at the location of other earthquakes increase with m as similar to 10 dm 2 the stronger the spatial clustering the larger the influence of small earthquakes on stress changes at the location of a future event as well as earthquake triggering if earthquake magnitudes follow the gutenberg richter law with b d 2 small earthquakes collectively dominate stress transfer and earthquake triggering because their greater frequency overcomes their smaller individual triggering potential using a southern california catalog we observe that the rate of seismicity triggered by an earthquake of magnitude m increases with m as 10 alpha m where alpha 1 05 0 05 we also find that the magnitude distribution of triggered earthquakes is independent of the triggering earthquake s magnitude m when alpha approximate to b small earthquakes are roughly as important to earthquake triggering as larger ones we evaluate the fractal correlation dimension d of hypocenters using two relocated catalogs for southern california the value of d measured for distances 0 1 r 5 km is d 1 54 for the shearer et al catalog and d 1 73 for the hauksson et al catalog the value of d reflects both the structure of the fault network and the nature of earthquake interactions by considering only those earthquake pairs with interevent times larger than 1000 days we can largely remove the effects of short term clustering then d approximate to 2 close to the value d 2 alpha 2 1 predicted by assuming that earthquake triggering is due to static stress the value d approximate to 2b implies that small earthquakes are as important as larger ones for stress transfers between earthquakes and that considering stress changes induced by small earthquakes should improve models of earthquake interactions

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

the association of micro earthquake clusters with mapped faults in the dead sea basin about 1100 micro earthquakes that occurred along the dead sea basin during the past 25 years express ongoing seismic activity along mostly obscured fault segments we apply seismological and statistical methods in order to associate seismic activity with geological and geophysical data and to locate and characterize active fault segments in the basin three regional 1 d velocity models and the double difference method were used for the relocation of seismic events geostatistical analysis shows that in first order the micro earthquakes reflect faulting along segments of the main longitudinal fault while seismic quiescence along historically active zones indicates locked segments and stress concentration based on waveform similarity spatial and temporal proximity we define earthquake clusters that comprise about 30 of the relocated seismicity and two thirds of the total seismic moment about 80 of the clusters are associated with n s trending faults in two cases the clusters indicate a migration of the seismicity in time and space one of which preceded the largest earthquake in our catalog m5 2 in addition to the associated longitudinal activity several clusters appear to represent faulting in east west orientation which is interpreted here as a secondary fault system

estimating the elastic parameters of anisotropic media using a joint inversion of p wave and sv wave traveltime error a method is presented to estimate the elastic parameters and thickness of media that are locally laterally homogeneous using p wave and vertically polarized shear wave sv wave data this method is a layer stripping technique and it uses many aspects of common focal point cfp technology for each layer a focusing operator is computed using a model of the elastic parameters with which a cfp gather can be constructed using the seismic data assuming local homogeneity the resulting differential time shifts dtss represent error in the model due to anisotropy and error in thickness in the r p domain dtss are traveltimes that connect error in layer thickness z vertical slowness q and ray parameter p series expansion is used to linearize with respect to error in the elastic parameters and thickness and least squares inversion is used to update the model for stability joint inversion of p and sv data is employed and as pure sv data are relatively rare the use of mode converted psv data to represent sv in the joint inversion is proposed analytic and synthetic examples are used to demonstrate the utility and practicality of this inversion

a common origin for aftershocks foreshocks and multiplets we demonstrate that the statistics of earthquake data in the global cen troid moment tensor cmt and national earthquake information center neic catalogs and local california council of the national seismic system cnss catalog are consistent with the idea that a single physical triggering mechanism is responsible for the occurrence of aftershocks foreshocks and multiplets specifically we test the hypothesis that tectonic earthquakes usually show clustering only as a result of an initial earthquake triggering subsequent ones and that the magnitude of each trig gered earthquake is entirely independent of the magnitude of the triggering earth quake therefore a certain percentage of the time as determined by the gutenberg richter magnitude frequency relationship an earthquake should by chance be larger than or comparable in size to the earthquake that triggered it this hypothesis predicts that the number of times foreshocks or multiplets are observed should be a fixed fraction of the number of aftershock observations we find that this is indeed the case in the global cmt and neic catalogs the average ratios between foreshock aftershock and multiplet rates are consistent with what would be predicted by the gutenberg richter relationship with b 1 we give special attention to the solomon islands where it has been claimed that unique fault structures lead to unusually high numbers of multiplets we use monte carlo trials to demonstrate that the solomon islands multiplets may be explained simply by a high regional aftershock rate and earthquake density we also verify our foreshock results from the more complete recordings of small earthquakes available in the california catalog and find that foreshock rates for a wide range of foreshock and mainshock magnitudes can be projected from aftershock rates using the gutenberg richter relationship with b 1 and the relationship that the number of earthquakes triggered varies with triggering earthquake magnitude m as c10m where c is a productivity constant and is equal to 1 finally we test an alternative model that proposes that foreshocks do not trigger their mainshocks but are instead triggered by the mainshock nucleation phase in this model the nucleation phase varies with mainshock magnitude so we would expect mainshock magnitude to be correlated with the magnitude number or spatial extent of the foreshocks we find no evidence for any of these correlations

the 1995 november 22 mw 7 2 gulf of elat earthquake cycle revisited summary the 1995 november 22 mw 7 2 nuweiba earthquake occurred along one of the left stepping segments of the dead sea transform dst in the gulf of elat aqaba it was the largest earthquake along the dst in at least 160 yr the main shock was preceded by earthquake swarms north and south of its ne striking rupture since the early 1980s and was followed by about 6 months of intense aftershock activity concentrated mainly northwest and southeast of the main rupture in this study we re analyse ers 1 and ers 2 insar data for the period spanning the main shock and 5 post seismic years because the entire rupture was under the gulf water surface observations related to the earthquake are limited to distances greater than 5 km away from the rupture zone coseismic interferograms were produced for the earthquake 1 week 4 months and 6 months non linear inversions were carried out for fault geometry and linear inversions were made for slip distribution using an ascending descending 2 frame data set the moment calculated from our best fitting model is in agreement with the seismological moment but trade offs exist among several fault parameters the present model upgrades previous insar models of the nuweiba earthquake and differs from recent teleseismic waveform inversion results mainly in terms of slip magnitude and distribution the moment released by post seismic deformation in the period of 6 months to 2 yr after the nuweiba earthquake is about 15 per cent of the coseismic moment release our models suggest that this deformation can be represented by slip along the lower part of the coseismic rupture localised deformation along the gulf shores nw of the main rupture in the first 6 months after the earthquake is correlated with surface displacements along active gulf parallel normal faults and possibly with shallow m 3 9 d 6 km aftershocks the geodetic moment calculated by modelling this deformation is more than an order of magnitude larger than expected for a single m 4 aftershock but could be a result of a sequence of aftershocks and or aseismic slip the major aftershocks and the slip along gulf parallel normal faulting nw of the main rupture are associated with positive coulomb stress changes induced by the main event

strongly depth dependent aftershock production in deep earthquakes the mechanisms that permit earthquake rupture at the high pressures occurring below 100 km depth remain enigmatic and controversial the most distinct difference between deep and shallow earthquakes is a scarcity of aftershocks below 100 km we report strong changes in aftershock productivity with depth from 100 to 300 km aftershock productivity normalized to account for the effect of main shock magnitude is more than an order of magnitude lower than that of shallow events between 300 and 550 km aftershock productivity is particularly low falling to about one third the level between 100 and 300 km below 550 km however aftershock productivity abruptly increases to more than twice as great as that between 100 and 300 km a similar pattern is seen in alternate measures of aftershock productivity including the raw numbers of aftershocks the fraction of mainshocks that have aftershocks and the fraction of mainshock seismic moment released in aftershocks the depth dependence of aftershock productivity is consistent with a previously proposed change in earthquake generation mechanism near 300 km depth notably the abrupt increase in aftershock productivity below 550 km corresponds with abrupt decreases in earthquake duration and complexity and sug gests a major change in the rupture mechanism at that depth as well online material catalog of mainshocks and related aftershocks used in this study

intraplate and interplate faulting interactions during the august 31 2012 philippine trench earthquake mw 7 6 sequence on august 31 2012 a large m w 7 6 thrust earthquake occurred within the subducting philippine sea plate seaward of a low seismicity region of the plate boundary 9 5 n 11 5 n possibly as a result of horizontal compressional stress accumulation offshore of a locked megathrust the mainshock ruptured from 30 50 km depth with high radiated energy seismic moment ratio and enriched short period p wave radiation the nine largest aftershocks with global centroid moment tensor solutions m w 5 2 5 6 were shallow 10 13 km normal faulting outer rise events and a waveform template analysis using regional broadband data indicates many 48 110 similar normal faulting events m b 4 0 5 5 and a few 8 110 likely shallow thrust faulting events on the megathrust with additional very small unidentified events coulomb stress perturbations may contribute to the mix of intraplate and interplate faulting geodetic assessment of any slip deficit on the megathrust is essential for quantifying the potential for a future large interplate rupture in this region

spatiotemporal variation of stress drop during the 2008 mogul nevada earthquake swarm we estimate stress drops for 148 shallow 6 km earthquakes in the complex 2008 mogul nevada swarm using empirical green s function derived spectral ratios near source temporary broadband seismometers deployed before the mw4 9 main shock provide high quality records of many foreshocks and aftershocks and an ideal opportunity to investigate uncertainties in corner frequency measurement as well as stress drop variation related to space time depth mechanism and magnitude we explore uncertainties related to source model measurement approach cross correlation limit and frequency bandwidth p s wave results range from 0 2 0 15 0 3 0 15 to 36 20 58 7 mpa a variation greater than the error range of each individual estimate although this variation is not explained simply by any one parameter spatiotemporal variation along the main shock fault plane is distinct coherent clusters of high and low earthquakes are seen and high foreshocks correlate with an area of reduced aftershock productivity these observations are best explained by a difference in rheology along the fault plane average s of 3 9 1 1 4 0 1 1 mpa using p s are similar to those found for earthquakes in a variety of settings implying that these shallow potentially fluid driven earthquakes do not have systematically lower than average tectonic earthquakes 4 mpa and therefore have similar or higher due to proximity to the surface expected ground motions compared to typical earthquakes the unprecedented detail achieved for these shallow small magnitude earthquakes confirms that when measured precisely is a valuable observation of physically meaningful fault zone properties and earthquake behavior

a great earthquake rupture across a rapidly evolving three plate boundary on 1 april 2007 a great tsunamigenic earthquake moment magnitude 8 1 ruptured the solomon islands subduction zone at the triple junction where the australia and solomon sea woodlark basin plates simultaneously underthrust the pacific plate with different slip directions the associated abrupt change in slip direction during the great earthquake drove convergent anelastic deformation of the upper pacific plate which generated localized uplift in the forearc above the subducting simbo fault potentially amplifying local tsunami amplitude elastic deformation during the seismic cycle appears to be primarily accommodated by the overriding pacific forearc this earthquake demonstrates the seismogenic potential of extremely young subducting oceanic lithosphere the ability of ruptures to traverse substantial geologic boundaries and the consequences of complex coseismic slip for uplift and tsunamigenesis

spatial distribution of the difference between the magnitudes of the main shock and the largest aftershock in the circum pacific belt the distribution of the magnitude difference d 1 between the main shock and the largest aftershock for large circum pacific earthquakes is observed to be peaked at 1 2 magnitude units in accord with bath s law but also at 1 8 magnitude units the peak at 1 8 has been not noted before and is shown here with the t test to be statistically valid the observed spatial distribution of the sequences is the basis to suggest that along circum pacific convergent plate boundaries values of d 1 tend to be larger for earthquake sequences within back arc areas as compared to sequences closer to the plate interface although admittedly a good number of exceptions to this generalization are also observed to exist

earthquake hazard after a mainshock in california after a strong earthquake the possibility of the occurrence of either significant aftershocks or an even stronger mainshock is a continuing hazard that threatens the resumption of critical services and reoccupation of essential but partially damaged structures a stochastic parametric model allows determination of probabilities for aftershocks and larger mainshocks during intervals following the mainshock the probabilities depend strongly on the model parameters which are estimated with bayesian statistics from both the ongoing aftershock sequence and from a suite of historic california aftershock sequences probabilities for damaging aftershocks and greater mainshocks are typically well constrained after the first day of the sequence with accuracy increasing with time

the climate response of the indo pacific warm pool to glacial sea level growing climate proxy evidence suggests that changes in sea level are important drivers of tropical climate change on glacial interglacial time scales these paleodata suggest that rainfall patterns over the indo pacific warm pool ipwp are highly sensitive to the landmass configuration of the maritime continent and that lowered sea level contributed to large scale drying during the last glacial maximum lgm ca 21 000 years before present using the community earth system model version 1 2 cesm1 we investigate in detail the mechanisms by which lowered sea level influenced the climate of the ipwp during the lgm the cesm1 simulations show that in agreement with previous hypotheses changes in atmospheric circulation are initiated by the exposure of the sunda and sahul shelves ocean dynamical processes amplify the changes in atmospheric circulation by increasing the east west sea surface temperature sst gradient along the equatorial indian ocean the coupled mechanism driving this response is akin to the bjerknes feedback and results in a large scale climatic reorganization over the indian ocean with impacts extending from east africa to the western tropical pacific exposure of sahul shelf and the associated changes in surface albedo play a key role because it is more effective at exciting the positive feedback than exposure of the sunda shelf this mechanism could explain the pattern of dry wet eastern western indian ocean identified in climate proxies and lgm simulations however this response also requires a strengthened sst gradient along the equatorial indian ocean and it is currently unclear whether this is detectable in marine paleoreconstructions strategies to resolve this issue are discussed

slip kinematics and dynamics during and after the 1995 october 9 mw 8 0 colima jalisco earthquake mexico from gps geodetic constraints we use horizontal and vertical crustal displacements derived from gps measurements at 26 sites in western mexico to study the coseismic and post seismic kinematics and dynamics of the 1995 october 9 m w 8 0 colima jalisco earthquake along the middle america trench the measurements bracket the entire landward edge of the approximately 150 km long rupture zone and span a 4 yr period for most sites we solve for the temporal evolution of slip along the subduction interface by inverting gps displacements for the coseismic and four post seismic intervals march 1995 march 1999 subject to the assumption that the crust responds elastically to slip along a shallow dipping curved subduction interface coseismic rupture of up to 5 m was largely focused above depths of 20 km and was limited to a 120 140 km long segment of the subduction zone within one week of the earthquake post seismic slip migrated downdip to depths of 16 35 km where it has since decayed logarithmically we also find evidence for shallow aseismic slip during 1996 or early 1997 northwest of the coseismic rupture zone and increasingly widespread relocking of shallow regions of the subduction interface after early 1997 the relative lack of afterslip in shallow regions of the subduction interface suggests that the interface lies in the unstable frictional regime and hence is strongly coupled between earthquakes by 1999 the cumulative slip moment associated with post seismic slip equaled 70 per cent of the coseismic moment with nearly all of this slip occurring downdip from the coseismic rupture zone the migration of slip after the earthquake to a deeper and presumably velocity strengthening area of the subduction interface and the logarithmic decay of afterslip conform to the qualitative and quantitative predictions of a model in which the fault kinematics are prescribed by rate and state variable frictional laws however misfits to the geodetic displacements exceed the average displacement uncertainties for all epochs implying one or more of the following 1 the elastic response is heterogeneous due to slip along unmodelled upper crustal faults or variations in the elastic properties of the crust 2 other post seismic mechanisms such as viscoelastic or poroelastic effects contribute to or possibly dominate the post seismic response 3 we have underestimated the uncertainties in the gps displacements

b values of two tectonic parts in the circum pacific belt two different data sets have been applied to compute theb parameter of the magnitude frequency relation for two different areas in the circum pacific belt these areas are a south america middle america and mexico b all the remaining island arcs of the circum pacific belt starting from alaska and the aleutian islands and proceeding southeastwards the first data set spreads over 77 years while the second one covers a 90 year period in both cases the results clearly showed that theb values are lower in the first of the above mentioned areas while respectively higher in the second this is due to the tectonic conditions in these areas as estimated during the present work

the seismic coupling of subduction zones revisited 1 the nature of seismic coupling for many of the world s subduction zones has been reevaluated geodetic estimates of seismic coupling obtained from gps measurements of upper plate deformation during the interseismic period are summarized we compared those with new estimates of seismic coupling obtained from seismological data the results show that with a few notable exceptions the two methods agree to within about 10 the seismological estimates have been greatly improved over those made 20 30 years ago because of an abundance of paleoseismological data that greatly extend the temporal record of great subduction earthquakes and by the occurrence in the intervening years of an unusual number of great and giant earthquakes that have filled in some of the most critical holes in the seismic record the data also again with a few notable exceptions support the frictional instability theory of seismic coupling and in particular the test of that theory made by scholz and campos 1995 overall the results support their prediction that high coupling occurs for subduction zones subjected to high normal forces with a switch to low coupling occurring fairly abruptly as the normal force decreases below a critical value there is also considerable variation of coupling within individual subduction zones earthquake asperities correlate with areas of high coupling and hence have a semblance of permanence but the rupture zones and asperity distributions of great earthquakes may differ greatly between seismic cycles because of differences in the phase of seismic flux accumulation

the geodetic signature of the m8 0 oct 9 1995 jalisco subduction earthquake the october 1995 mw 8 0 jalisco subduction earthquake has provided a thorough geodetic observation of the coseismic subduction process an 11 station regional gps network located directly onshore of the rupture demonstrates consistent vertical subsidence verified by tide gauge data and southwest directed extension with measured displacements reaching 1 meter unusually shallow and non uniform faulting is required to explain the displacements we determine that up to 5 meters of slip occurred within the upper 15 km of the thrust fault zone and 2 meters possibly as shallow as 8 km and that slip was likely distributed in two main patches the paucity of continental sediments in this subduction zone could be responsible for the anomalously shallow faulting

rupture depth varying seismicity patterns for major and great mw 7 0 megathrust earthquakes large earthquakes on subduction zone plate boundary megathrusts result from intervals of strain accumulation and release the mechanism diversity and spatial distribution of moderate size aftershocks is influenced by the mainshock rupture depth extent mainshocks that rupture across the shallow megathrust to near the trench have greater intraplate aftershock faulting diversity than events with rupture confined to deeper portions of the megathrust diversity of intraplate aftershock faulting also increases as the size of the mainshock approaches the largest size event to have ruptured that region of the megathrust based on these tendencies we identify breakthrough ruptures as those involving shallow rupture of the megathrust with volumetrically extensive elastic strain drop around the plate boundary that allows activation of diverse intraplate faulting influenced by long term ambient deformation stresses in contrast homogeneity of the aftershock faulting mechanisms indicates only partial release of elastic strain energy and remaining potential for another large rupture

on the mechanism of seismic decoupling and back arc spreading at subduction zones to address the problem of the great variability of the mechanical state of subduction zones we investigate the mechanics of back arc spreading and seismic decoupling back arc spreading is assumed to be due to rifting of the upper plate and hence occurs when trench normal tension reaches a critical value seismic decoupling is assumed to occur when the normal stress at the frictional interface is decreased by an amount sufficient to cross the friction stability transition two forces are important in this problem the first is a small component of the slab pull force which remains unbalanced by the subduction resistance and exerts a vertical suction force at the trench the second is a sea anchor force exerted on the slab that resists its lateral motion assumed to occur at the upper plate velocity both forces contribute to the coupling problem only the sea anchor force is responsible for back arc spreading the unbalanced slab pull force is determined from a force balance for subduction the sea anchor force is computed as the hydrodynamic resistance to the facewise translation of an elliptical disc through a viscous fluid the model predicts three regimes seismically coupled compressional arcs with advancing upper plates seismically decoupled extensional arcs with retreating upper plates and strongly extensional arcs which also have back arc spreading this model is applied in detail to the izu bonin marianas system it shows that back arc spreading occurs when the integrated tension in the upper plate exceeds a value of about 1 1013 n m 1 and requires a residual tension of about a third that to drive the back arc spreading once rifting is completed it shows why the plate interface near guam is seismically coupled while the plate boundary everywhere farther north is decoupled when applied globally it successfully predicts the state of seismic coupling and back arc spreading in more than 80 of the world s subduction zones of the remaining half can be seen to contain additional complications not included in the model about 10 of cases remain unexplained but some of these may have incorrectly determined seismic coupling coefficients

aftershock area expansion and mechanical heterogeneity of fault zone within subduction zones we investigate global variations in the pattern of aftershock area expansion associated with large subduction zone earthquakes the expansion of aftershock areas with time is relatively small in the subduction zones of alaska the aleutians mexico and parts of the kuriles and south america these subduction zones are interpreted to be characterized by moderate to strong interplate coupling in contrast the extent of aftershock expansion is greater for events within the subduction zones of northeast japan and western and south pacific where interplate coupling is inferred to be weak these results are interpreted in terms of an asperity model where a fault zone is represented by a distribution of strong spots asperities and weaker zones the immediate aftershock area is assumed to extend over an area occupied by asperities and the subsequent expansion to occur into the surrounding weaker zones if large asperities abut each other on a fault plane interplate coupling is strong and little expansion of aftershock area occurs smaller and more sparsely distributed asperities cause weak interplate coupling and significant expansion of aftershock area small and densely distributed asperities result in moderate interplate coupling and little aftershock expansion

regional and stress drop effects on aftershock productivity of large megathrust earthquakes the total number of aftershocks increases with mainshock magnitude resulting in an overall well defined relationship observed variations from this trend prompt questions regarding influences of regional environment and individual mainshock rupture characteristics we investigate how aftershock productivity varies regionally and with mainshock source parameters for large mw 7 0 circum pacific megathrust earthquakes within the past 25 years drawing on extant finite fault rupture models aftershock productivity is found to be higher for subduction zones of the western circum pacific than for subduction zones in the eastern circum pacific this appears to be a manifestation of differences in faulting susceptibility between island arcs and continental arcs surprisingly events with relatively large static stress drop tend to produce fewer aftershocks than comparable magnitude events with lower stress drop however for events with similar co seismic rupture area aftershock productivity increases with stress drop and radiated energy indicating a significant impact of source rupture process on productivity

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

decay of aftershock density with distance does not indicate triggering by dynamic stress the most predictable earthquakes are aftershocks which invariably follow mainshocks so what triggers aftershocks it was recently argued controversially that the decay of aftershocks with distance from the main earthquake could only be explained by dynamic triggering keith richards dinger and colleagues have tested this hypothesis and conclude that the observed decay can be better explained by static triggering resolving whether static or dynamic stress triggers most aftershocks and subsequent mainshocks is essential to understand earthquake interaction and to forecast seismic hazard it has recently been argued that the decay of aftershocks with distance from the main earthquake could be explained only by dynamic triggering this hypothesis has now been tested the conclusion being that the observed decay can be better explained by static triggering resolving whether static1 2 3 or dynamic4 5 6 7 8 stress triggers most aftershocks and subsequent mainshocks is essential to understand earthquake interaction and to forecast seismic hazard9 felzer and brodsky10 examined the distance distribution of earthquakes occurring in the first five minutes after 2 m 3 and 3 m 4 mainshocks and found that their magnitude m 2 aftershocks showed a uniform power law decay with slope 1 35 out to 50 km from the mainshocks from this they argued that the distance decay could be explained only by dynamic triggering here we propose an alternative explanation for the decay and subject their hypothesis to a series of tests none of which it passes at distances more than 300 m from the 2 m 3 mainshocks the seismicity decay 5 min before the mainshocks is indistinguishable from the decay five minutes afterwards indicating that the mainshocks have no effect at distances outside their static triggering range omori temporal decay the fundamental signature of aftershocks is absent at distances exceeding 10 km from the mainshocks finally the distance decay is found among aftershocks that occur before the arrival of the seismic wave front from the mainshock which violates causality we argue that felzer and brodsky10 implicitly assume that the first of two independent aftershocks along a fault rupture triggers the second and that the first of two shocks in a creep or intrusion driven swarm triggers the second when this need not be the case

regional variation in the number of aftershocks mb 5 of large subduction zone earthquakes mw 7 0 it is reasonable to expect that the number of aftershocks following a large earthquake should reflect the degree of heterogeneity on and near the fault plane to investigate this question we plot n the number of aftershocks with mb 5 0 occurring within the next 30 days after large subduction zone thrust faulting earthquakes mw 7 0 versus the magnitude mw of the main shock only earthquakes that occurred after the installation of the world wide standard seismographic network stations are considered because the detection threshold of aftershocks in many regions is greater than mb 5 0 n is obtained by fitting the data to gutenberg and richter s relation log n a b mb for the same mw n varies by a factor of about 40 in different regions n is less than expected in the eastern pacific including alaska and is greater than expected in the western pacific relative lack and excess of n correlates well with regions of supposedly strong and weak coupling respectively while small and large n also show some correlation with the expansion rate of aftershock area with time the variation in n is much larger than would be expected from the mere increase of aftershock areas in some cases it appears that the number of aftershocks n for earthquakes of similar mw may also reflect the complexity and enrichment of the p wave amplitude spectra from 1 to 10 sec for example the valparaiso chile earthquake of 1985 mw 8 0 generated n 46 earthquakes whereas for the michoacan mexico event of 1985 mw 8 1 n is only equal to 5 furthermore for the valparaiso earthquake p waveforms are much more complex and the spectra are more energetic at 1 to 10 sec as compared to the 1985 michoacan event

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

a constitutive law for rate of earthquake production and its application to earthquake clustering seismicity is modeled as a sequence of earthquake nucleation events in which the distribution of initial conditions over the population of nucleation sources and stress ing history control the timing of earthquakes the model is implemented using solutions for nucleation of unstable fault slip on faults with experimentally derived rate and state dependent fault properties this yields a general state variable constitutive formulation for rate of earthquake production resulting from an applied stressing history to illustrate and test the model some characteristics of seismicity following a stress step have been explored it is proposed that various features of earthquake clustering arise from sensi tivity of nucleation times to the stress changes induced by prior earthquakes the model gives the characteristic omori aftershock decay law and interprets aftershock parameters in terms of stress change and stressing rate earthquake data appear to support a model prediction that aftershock duration defined as the time for rates to return to the back ground seismicity rate is proportional to mainshock recurrence time observed spatial and temporal clustering of earthquake pairs arises as a consequence of the spatial depen dence of stress changes of the first event of the pair and stress sensitive time dependent nucleation applications of the constitutive formulation are not restricted to the simple stress step models investigated here it may be applied to stressing histories of arbitrary complexity the apparent success at modeling clustering phenomena suggests the possibil ity of using the formulation to estimate short to intermediate term earthquake probabil ities following occurrence of other earthquakes and for inversion of temporal variations of earthquake rates for changes in driving stress

earthquake size distribution in subduction zones linked to slab buoyancy some of the most devastating earthquakes are generated in subduction zones analysis of the stress state of subduction zones worldwide suggests that large earthquakes are generated more frequently where a young buoyant plate subducts

the february 6 2013 mw 8 0 santa cruz islands earthquake and tsunami the santa cruz islands region has high seismicity near a 90 bend in the boundary between the pacific and australian plates southward along the vanuatu island arc the australian plate under thrusts the pacific plate in a well defined subduction zone westward a transpressional predominantly transform boundary extends to the southern solomon islands subduction zone the santa cruz islands region has upper plate strike slip and normal faulting plate boundary under thrusting outer rise extensional faulting and intraplate compressional faulting on february 6 2013 the largest under thrusting earthquake mw 8 0 that has been instrumentally recorded in the region ruptured the megathrust the epicenter 10 738 s 165 138 e is about 1 north of epicenters of prior large shallow under thrusting events in vanuatu in 1934 m 7 8 and 1966 ms 7 9 and there is overlap of all three events aftershock zones but not their large slip regions at least 10 lives were lost with 6 more missing due to tsunami 1 5 m high striking the town of lata and several villages on the main santa cruz island of nendo ndeni and a nearby small island nibanga inundation of 500 m flooded the lata airport the tsunami was well recorded by dart buoys spanning an unusually wide three quadrant azimuthal aperture iterative modeling of teleseismic broadband p waves and the deep water tsunami recordings resolves the slip distribution there are two large slip patches with a southeastward rupture expansion at about 1 5 km s with the second patch appearing to have ruptured with large slip at the trench the event has relatively low short period seismic wave energy release but a typical overall moment scaled total energy the shallow rupture depth may be associated with the low level of short period energy and a near total stress drop may account for a lack of underthrusting aftershocks among a highly productive aftershock sequence that included three events with m w 7 0

anthropogenic seismicity rates and operational parameters at the salton sea geothermal field geothermal power is a growing energy source however efforts to increase production are tempered by concern over induced earthquakes although increased seismicity commonly accompanies geothermal production induced earthquake rate cannot currently be forecast on the basis of fluid injection volumes or any other operational parameters we show that at the salton sea geothermal field the total volume of fluid extracted or injected tracks the long term evolution of seismicity after correcting for the aftershock rate the net fluid volume extracted injected provides the best correlation with seismicity in recent years we model the background earthquake rate with a linear combination of injection and net production rates that allows us to track the secular development of the field as the number of earthquakes per fluid volume injected decreases over time

global survey of aftershock area expansion patterns we developed an objective method to define the aftershock areas of large earthquakes as a function of time after the main shock the definition is based upon the amount of energy released by aftershocks the spatial distribution of the energy release is first determined and is contoured the 1 day aftershock area is defined by a contour line corresponding to the energy release level of 10 15 6 ergs 100 km 2 day the 10 day 100 day and 1 y aftershock areas are similarly defined by contour lines corresponding to 10 14 8 10 14 0 and 10 13 5 ergs 100 km 2 day respectively we also define the expansion ratios at time t by the ratio of the aftershock area at t to that at 1 day using this method we study the aftershock area expansion patterns of 44 large m s 7 5 and five moderate shallow earthquakes which occurred from 1963 to 1980 each aftershock sequence is examined at four different times i e 1 day 10 days 100 days and 1 y after the main event we define the aftershock area expansion ratios and e by s 100 s 1 and l 100 l 1 respectively here s t and l t are the area and the length of the aftershock area respectively at time t our study suggests that a distinct regional variation of aftershock area expansion patterns is present it is strongly correlated with the tectonic environment in general the subduction zones of the mariana type have large expansion ratios and those of the chilean type have small expansion ratios some earthquakes that occurred in the areas of complex bathymetry such as aseismic ridges tend to have large expansion ratios these results can be explained in terms of an asperity model of fault zones in which a fault plane is represented by a distribution of strong spots called the asperities and weak zones surrounding the asperities the rupture immediately after the main shock mostly involves asperities after the main rupture is completed the stress change caused by the main shock gradually propagates outward into the surrounding weak zones this stress propagation manifests itself as expansion of aftershock activity in this simple picture if the fault zone is represented by relatively large asperities separated by small weak zones chilean type then little expansion of aftershock activity would be expected on the other hand if relatively small asperities are sparsely distributed mariana type significant expansion occurs the actual distribution of asperities is likely to be more complex than the two cases described above however we would expect that the expansion ratio is in general proportional to the spatial ratio of the total asperity area to the fault area