microscale evidence of liquefaction and its potential triggers during soft bed deformation within subglacial traction tills published conceptual models argue that much of the forward motion of modern and ancient glaciers is accommodated by deformation of soft sediments within the underlying bed at a microscale this deformation results in the development of a range of ductile and brittle structures in water saturated sediments as they accommodate the stresses being applied by the overriding glacier detailed micromorphological studies of subglacial traction tills reveal that these polydeformed sediments may also contain evidence of having undergone repeated phases of liquefaction followed by solid state shear deformation this spatially and temporally restricted liquefaction of subglacial traction tills lowers the shear strength of the sediment and promotes the formation of transient mobile zones within the bed which accommodate the shear imposed by the overriding ice this process of soft bed sliding alternating with bed deformation facilitates glacier movement by way of stick slip events the various controls on the slip events have previously been identified as i the introduction of pressurised meltwater into the bed a process limited by the porosity and permeability of the till and ii pressurisation of porewater as a result of subglacial deformation to which we include iii episodic liquefaction of water saturated subglacial traction tills in response to glacier seismic activity icequakes which are increasingly being recognized as significant processes in modern glaciers and ice sheets as liquefaction operates only in materials already at very low values of effective stress its process form signatures are likely indicative of glacier sub marginal tills

nucleation and seismic tremor associated with the glacial earthquakes of whillans ice stream antarctica 1 the ability to monitor transient motion along faults is critical to improving our ability to understand many natural phenomena such as landslides and earthquakes here we usedata from a gps and seismometer network that were deployed to monitor the regularly repeating glacial earthquakes of whillans ice stream west antarctica to show that a unique pattern of precursory slip precedes complete rupture along the bed of the ice stream additionally we show that rupture can be independently tracked by increased levels of microseismic activity including harmonic tremor that are coincident with the onset of slip at any location thus providing a remote means of monitoring stress and rupture propagation during the glacial earthquakes

testing for the predictability of dynamically triggered earthquakes in the geysers geothermal field abstract the geysers geothermal field is well known for being susceptible to dynamic triggering of earthquakes by large distant earthquakes owing to the introduction of fluids for energy production yet it is unknown if dynamic triggering of earthquakes is predictable or whether dynamic triggering could lead to a potential hazard for energy production in this paper our goal is to investigate the characteristics of triggering and the physical conditions that promote triggering to determine whether or not triggering is in anyway foreseeable we find that at present triggering in the geysers is not easily predictable in terms of when and where based on observable physical conditions however triggered earthquake magnitude positively correlates with peak imparted dynamic stress and larger dynamic stresses tend to trigger sequences similar to mainshock aftershock sequences thus we may be able to predict what size earthquakes to expect at the geysers following a large distant earthquake

kinematic and seismic analysis of giant tabular iceberg breakup at cape adare antarctica 1 satellite imagery reveals that a series of large icebergs b15b in april 2001 c19 in june 2003 and b15a in october 2005 broke up or fractured while exiting the ross sea in a narrowly defined area off cape adare antarctica examination of recent swath mapped bathymetric observations revealed that the principle agent of these breakups is a previously unknown 9 km long ridge with minimum depths of 215 m that we call davey shoal satellite imagery shows that the icebergs are driven into the shoal by coastal currents that converge over the narrow continental shelf one of the icebergs the 120 km by 30 km b15a was instrumented with a seismograph gps and fluxgate compass this instrumentation provided a unique opportunity to establish the details of the iceberg kinematics that were not revealed by satellite imagery alone and to correlate seismic events observed both on the iceberg and in the far field during breakup b15a fractured from multiple strikes against davey shoal and the adjacent possession islands these strikes were driven by the combination of tidal currents and the coastal mean flow the periods of iceberg sourced seismic radiation were correlated with the strikes the iceberg and land based seismic signals showed that the iceberg fracture its sliding across the shoals and the ice on ice stick slip contacts among the postbreakup iceberg fragments generated the strong chaotic and harmonic tremor episodes that were observed at distances as far as the south pole where these signals propagated as seismically coupled hydroacoustic t phases

probabilistic seismic hazard assessment methodology for distributed seismicity this study deals with probabilistic seismic hazard analysis psha based on earthquakes that cannot be assigned to specific geologic structures distributed seismicity to calculate seismic hazard from distributed seismicity the psha methodology is extended in numerous ways a simple quantitative seismotectonic model enables statistical incorporation of certain seismotectonic knowledge and the application of attenuation relationships that are based on fault distance the influence of a large historic earthquake on seismic hazard is taken into account with an energy based seismic activity rate obtained by spatial distribution of the released seismic energy it will be shown that for at least a rough seismic hazard assessment the type of geometrical seismicity modeling is not a deciding factor to illustrate this the classical seismic source zone approach and the improved spatially smoothed seismicity approach are studied in detail to determine their common and distinctive components except the geometry of modeling the seismic activity there is no essential difference between the two approaches seismic activity in slovenia can be treated as distributed seismicity for this reason the presented methodology is applied to the calculation of seismic hazard maps of slovenia as a case study manuscript received 22 august 2002

a previously unreported type of seismic source in the firn layer of the east antarctic ice sheet we identify a unique type of seismic source in the uppermost part of the east antarctic ice sheet recorded by temporary broadband seismic arrays in east antarctica these sources termed firnquakes are characterized by dispersed surface wave trains with frequencies of 1 10 hz detectable at distances up to 1000 km events show strong dispersed rayleigh wave trains and an absence of observable body wave arrivals most events also show weaker love waves initial events were discovered by standard detection schemes additional events were then detected with a correlation scanner using the initial arrivals as templates we locate sources by determining the l2 misfit for a grid of potential source locations using rayleigh wave arrival times and polarization directions we then perform a multiple filter analysis to calculate the rayleigh wave group velocity dispersion and invert the group velocity for shear velocity structure the resulting velocity structure is used as an input model to calculate synthetic seismograms inverting the dispersion curves yields ice velocity structures consistent with a low velocity firn layer 100 m thick and show that velocity structure is laterally variable the absence of observable body wave phases and the relative amplitudes of rayleigh waves and noise constrain the source depth to be less than 20 m the presence of love waves for most events suggests the source is not isotropic we propose the events are linked to the formation of small crevasses in the firn and several events correlate with shallow crevasse fields mapped in satellite imagery

transoceanic wave propagation links iceberg calving margins of antarctica with storms in tropics and northern hemisphere 1 we deployed seismometers on the ross ice shelf and on various icebergs adrift in the ross sea including b15a a large 100 km by 30 km fragment of b15 which calved from the ross ice shelf in march 2000 the data reveal that the dominant energy of these floating ice masses is in the 0 01 to 0 1 hz band and is associated with sea swell generated in the tropical and extra tropical pacific ocean in one example a strong storm in the gulf of alaska on 21 october 2005 approximately 13 500 km from the ross sea generated swell that arrived at b15a immediately prior to and during its break up off cape adare on 27 october 2005 if sea swell influences iceberg calving and break up a teleconnection exists between the antarctic ice sheet mass balance and weather systems worldwide

top cloud motion field of typhoon megi 2016 revealed by gf 4 images gaofen 4 gf 4 is the first high resolution geostationary satellite of china launched on december 29 2015 its visible near infrared optical sensor is capable of imaging the earth at 50 m resolution covering a 512 km times512 km area with a minimum imaging interval of 20 s more than 300 gf 4 images taken in four sequences with imaging rates of 36 and 69 s per scene captured the development of typhoon megi 2016 from its peak in the afternoon of september 26 2016 to its dispersion two days later on september 28 these consecutive images recorded the motion field of the typhoon s top clouds nearly continuously by using advanced image matching technology the motion has been estimated for every pixel at subpixel accuracy from image pairs with 179 and 206 s intervals the process has generated time series of atmospheric motion vector amv fields at 50 m spatial resolution for the whole imaged area it is the first time to our knowledge that such high resolution and nearly continuous amv data of a typhoon system have been produced the data provide accurate measurements of the typhoon top cloud motion speed and direction and reveal quantitative details of the motion field spatiotemporal evolution at high altitude the finding that the high altitude top cloud motion speed is significantly lower than that recorded at low altitude below the planetary boundary layer is of scientific value and further exploitation of the motion data can lead to a better understanding of typhoon dynamics and thus improve cyclone modeling

characterization of seasonal glacial seismicity from a single station on ice record at holtedahlfonna svalbard glacial seismicity provides important insights into glacier dynamic processes we study the temporal distribution of cryogenic seismic signals icequakes at holtedahlfonna svalbard between april

detecting remotely triggered microseismicity around changbaishan volcano following nuclear explosions in north korea and large distant earthquakes around the world we conduct a systematic survey on locally triggered earthquakes by large distant earthquakes in changbaishan volcano an active intraplate volcano on the border between china and north korea we examine waveforms of distant earthquakes recorded at broadband station changbaishan cbs near the volcano with estimated dynamic stresses over 5 kpa between 2000 and 2016 out of 26 selected distant earthquakes three of them show positive evidence of triggering during large amplitude surface waves the earthquakes that had positive or possible evidences of triggering generated larger long period surface waves indicating that they are more efficient in triggering microseismicity in addition since 2006 north korea has conducted five underground nuclear explosion une tests only 140 km away from changbaishan volcano by systematically examining waveforms of these unes recorded at station cbs we find that none of them have triggered microearthquakes in changbaishan volcano

glacial earthquakes in greenland and antarctica glacial earthquakes are a new class of seismic events first discovered as signals in long period seismograms recorded on the global seismographic network most of these events occur along the coasts of greenland where they are spatially related to large outlet glaciers glacial earthquakes show a strong seasonality with most events occurring during the late summer the rateofglacial earthquakeoccurrenceincreasedbetween2000and2005 with a stabilization of earthquake frequency at 2003 2004 levels in 2006 2008 recent observations establish a strong temporal correlation between the distinct seismic signals of glacial earthquakes and large ice loss events in which icebergs of cubic kilometer scale collapse against the calving face linking the seismogenic process to the force exerted by these icebergs on the glacier andtheunderlyingsolidearth asuddenchangeinglacierspeedresultsfrom these glacial earthquake calving events seasonal and interannual variations in glacier terminus position account for general characteristics of the temporal variation in earthquake occurrence glacial earthquakes in antarctica are less well studied but they exhibit several characteristics similar to glacial earthquakes in greenland

global ubiquity of dynamic earthquake triggering surface waves that were generated by 12 out of 15 earthquakes with magnitudes greater than 7 since 1990 led to a global increase in the number of small earthquakes this suggests that dynamic triggering of earthquakes is common and is independent of the tectonic environment earthquakes can be triggered by local changes in the stress field static triggering1 2 3 4 5 6 7 due to nearby earthquakes or by stresses caused by the passage of surface rayleigh and love waves from a remote large earthquake dynamic triggering8 9 10 11 12 13 14 15 16 17 18 however the mechanism frequency controlling factors and the global extent of dynamic triggering are yet to be fully understood because rayleigh waves involve compressional and dilatational particle motion volumetric changes as well as shearing whereas love waves only involve shearing triggering by either wave type implies fundamentally different physical mechanisms here we analyse broadband seismograms from over 500 globally distributed stations and use an automated approach to systematically identify small triggered earthquakes the low amplitude signals of such earthquakes would normally be masked by high amplitude surface waves our analysis reveals that out of 15 earthquakes studied of magnitude m greater than 7 0 that occurred after 1990 12 are associated with significant increases in the detection of smaller earthquakes during the passage of both the love and rayleigh waves we conclude that dynamic triggering is a ubiquitous phenomenon that is independent of the tectonic environment of the main earthquake or the triggered event

the landsat image mosaic of antarctica abstract the landsat image mosaic of antarctica lima is the first true color high spatial resolution image of the seventh continent it is constructed from nearly 1100 individually selected landsat 7 etm scenes each image was orthorectified and adjusted for geometric sensor and illumination variations to a standardized almost seamless surface reflectance product mosaicing to avoid clouds produced a high quality nearly cloud free benchmark data set of antarctica for the international polar year from images collected primarily during 1999 2003 multiple color composites and enhancements were generated to illustrate additional characteristics of the multispectral data including the true appearance of the surface discrimination between snow and bare ice reflectance variations within bright snow recovered reflectance values in regions of sensor saturation and subtle topographic variations associated with ice flow lima is viewable and individual scenes or user defined portions of the mosaic are downloadable at http lima usgs gov educational materials associated with lima are available at http lima nasa gov

seismic wave triggering of nonvolcanic tremor episodic tremor and slip and earthquakes on vancouver island 1 we explore the physical conditions that enable triggering of nonvolcanic tremor and earthquakes by considering local seismic activity on vancouver island british columbia during and immediately after the arrival of large amplitude seismic waves from 30 teleseismic and 17 regional or local earthquakes we identify tremor triggered by four of the teleseismic earthquakes the close temporal and spatial proximity of triggered tremor to ambient tremor and aseismic slip indicates that when a fault is close to or undergoing failure it is particularly susceptible to triggering of further events the amplitude of the triggering waves also influences the likelihood of triggering both tremor and earthquakes such that large amplitude waves triggered tremor in the absence of detectable aseismic slip or ambient tremor tremor and energy radiated from regional local earthquakes share the same frequency passband so that tremor cannot be identified during these smaller more frequent events we confidently identify triggered local earthquakes following only one teleseism that with the largest amplitude and four regional or local events that generated vigorous aftershock sequences in their immediate vicinity earthquakes tend to be triggered in regions different from tremor and with high ambient seismicity rates we also note an interesting possible correlation between large teleseismic events and episodic tremor and slip ets episodes whereby ets events that are late and have built up more stress than normal are susceptible to triggering by the slight nudge of the shaking from a large distant event while ets events that are early or on time are not

remotely triggered microearthquakes and tremor in central california following the 2010 mw 8 8 chile earthquake 1 we examine remotely triggered microearthquakes and tectonic tremor in central california following the 2010 mw 8 8 chile earthquake several microearthquakes near the coso geothermal field were apparently triggered with the largest earthquake ml 3 5 occurring during the large amplitude love surface waves the chile mainshock also triggered numerous tremor bursts near the parkfield cholame section of the san andreas fault saf the locally triggered tremor bursts are partially masked at lower frequencies by the regionally triggered earthquake signals from coso but can be identified by applying high pass or matched filters both triggered tremor along the saf and the ml 3 5 earthquake in coso are consistent with frictional failure at different depths on critically stressed faults under the coulomb failure criteria the triggered tremor however appears to be more phase correlated with the surface waves than the triggered earthquakes likely reflecting differences in constitutive properties between the brittle seismogenic crust and the underlying lower crust

motion of an antarctic glacier by repeated tidally modulated earthquakes subglacial seismicity reveals information about glacier behaviour analysis of repeated seismic events beneath an antarctic outlet glacier is consistent with sliding of debris laden ice over a bedrock asperity with an event frequency that is modulated by the ocean tides

surface wave potential for triggering tectonic nonvolcanic tremor corrected abstract source processes commonly posed to explain instances of remote dynamic triggering of tectonic nonvolcanic tremor by surface waves include frictional failure and various modes of fluid activation the relative potential for love and rayleigh wave dynamic stresses to trigger tectonic tremor through failure on critically stressed thrust and vertical strike slip faults under the coulomb griffith failure criteria as a function of incidence angle are anticorrelated over the 15 to 30 km depth range that hosts tectonic tremor love wave potential is high for strike parallel incidence on low angle reverse faults and null for strike normal incidence the opposite holds for rayleigh waves love wave potential is high for both strike parallel and strike normal incidence on vertical strike slip faults and minimal for 45 incidence angles the opposite holds for rayleigh waves this pattern is consistent with documented instances of tremor triggered by love waves incident on the cascadia megathrust and the san andreas fault saf in central california resulting from shear failure on weak faults apparent friction is 0 2 documented instances of tremor triggered by surface waves with strike parallel incidence along the nankai megathrust beneath shikoku japan however are associated primarily with rayleigh waves this is consistent with the tremor bursts resulting from mixed mode failure crack opening and shear failure facilitated by near lithostatic ambient pore pressure low differential stress with a moderate friction coefficient 0 6 on the nankai subduction interface rayleigh wave dilatational stress is relatively weak at tectonic tremor source depths and seems unlikely to contribute significantly to the triggering process except perhaps for an indirect role on the saf in sustaining tremor into the rayleigh wave coda that was initially triggered by love waves

ice flow of the antarctic ice sheet we present a reference comprehensive high resolution digital mosaic of ice motion in antarctica assembled from multiple satellite interferometric synthetic aperture radar data acquired during the international polar year 2007 to 2009 the data reveal widespread patterned enhanced flow with tributary glaciers reaching hundreds to thousands of kilometers inland over the entire continent this view of ice sheet motion emphasizes the importance of basal slip dominated tributary flow over deformation dominated ice sheet flow redefines our understanding of ice sheet dynamics and has far reaching implications for the reconstruction and prediction of ice sheet evolution

an integrated perspective of the continuum between earthquakes and slow slip phenomena slow slip a mechanism by which faults can relieve stress was thought to be distinct from earthquakes however a global review of slow slip phenomena suggests that instead there is a continuum between the two types of event

antarctic icequakes triggered by the 2010 maule earthquake in chile large earthquakes can trigger seismicity in remote regions analysis of seismic data from antarctica reveals ice quakes coincident with passing seismic waves from the 2010 chile earthquake suggesting that the ice sheet is sensitive to such triggers

temporal variation of tectonic tremor activity in southern taiwan around the 2010 ml6 4 jiashian earthquake deep tectonic tremor which is extremely sensitive to small stress variations could be used to monitor fault zone processes during large earthquake cycles and aseismic processes before large earthquakes in this study we develop an algorithm for the automatic detection and location of tectonic tremor beneath the southern central range of taiwan and examine the spatio temporal relationship between tremor and the 4 march 2010 ml6 4 jiashian earthquake located about 20 km from active tremor sources we find that tremor in this region has a relatively short duration short recurrence time and no consistent correlation with surface gps data we find a short term increase in the tremor rate 19 days before the jiashian mainshock and around the time when the tremor rate began to rise one gps station recorded a flip in its direction of motion we hypothesize that tremor is driven by a slow slip event that preceded the occurrence of the shallower jiashian mainshock even though the inferred slip is too small to be observed by all gps stations our study shows that tectonic tremor may reflect stress variation during the pre nucleation process of a nearby earthquake

antarctic ice shelf calving triggered by the honshu japan earthquake and tsunami march 2011 we use european space agency envisat data to present the first observational evidence that a northern hemisphere tsunami triggered antarctic ice shelf calving more than 13 000 km away the honshu tsunami of 11 march 2011 traversed the pacific ocean in 46 years this event further illustrates the growing evidence of ocean wave impact on antarctic calving and emphasizes the teleconnection between the antarctic ice sheet and events as far away as the northern hemisphere

structural and environmental controls on antarctic ice shelf rift propagation inferred from satellite monitoring 1 iceberg calving from ice shelves accounts for nearly half of the mass loss from the antarctic ice sheet yet our understanding of this process is limited the precursor to iceberg calving is large through cutting fractures called rifts that can propagate for decades after they have initiated until they become iceberg detachment boundaries to improve our knowledge of rift propagation we monitored the lengths of 78 rifts in 13 antarctic ice shelves using satellite imagery from the moderate resolution imaging spectroradiometer and multiangle imaging spectroradiometer between 2002 and 2012 this data set allowed us to monitor trends in rift propagation over the past decade and test if variation in trends is controlled by variable environmental forcings we found that 43 of the 78 rifts were dormant i e propagated less than 500 m over the observational interval we found only seven rifts propagated continuously throughout the decade an additional eight rifts propagated for at least 2 years prior to arresting and remaining dormant for the rest of the decade and 13 rifts exhibited isolated sudden bursts of propagation after 2 or more years of dormancy twelve of the fifteen active rifts were initiated at the ice shelf fronts suggesting that front initiated rifts are more active than across flow rifts although we did not find a link between the observed variability in rift propagation rate and changes in atmospheric temperature or sea ice concentration correlated with we did find a statistically significant correlation between the arrival of tsunamis and propagation of front initiated rifts in eight ice shelves this suggests a connection between ice shelf rift propagation and mechanical ocean interaction that needs to be better understood

triggered seismic events along the eastern denali fault in northwest canada following the 2012 mw 7 8 haida gwaii 2013 mw 7 5 craig and two mw 8 5 teleseismic earthquakes we conduct a systematic search for remotely triggered seismic activity along the eastern denali fault edf in northwest canada an intraplate strike slip region we examine 19 distant earthquakes recorded by nine broadband stations in the canadian national seismograph network and find that the 2012 m w 7 8 haida gwaii and 2013 m w 7 5 craig alaska earthquakes triggered long duration 10 s emergent tremor like signals near the southeastern portion of the edf in both cases tremor coincides with the peak transverse velocities consistent with love wave triggering on right lateral strike slip faults the 2011 m w 9 0 tohoku oki and 2012 m w 8 6 indian ocean earthquakes possibly triggered tremor signals although we were unable to locate those sources in addition we also identify many short duration 5 s bursts that were repeatedly triggered by the rayleigh waves of the 2012 m w 7 8 haida gwaii earthquake although we were unable to precisely locate the short duration 5 s events they appear to be radiating from the direction of the klutlan glacier and from a belt of shallow historical seismicity at the eastern flank of the wrangell st elias mountain range the fact that these events were triggered solely by the rayleigh waves suggests a different source mechanism as compared with triggered tremor observed along the edf and other plate boundary regions online material description of rayleigh wave polarization detection figures of spectograms s wave amplitudes polarization analysis and static stress changes tables of earthquake catalogs and velocity model and animations of tremors and bursts

deep tremor in new zealand triggered by the 2010 mw8 8 chile earthquake 1 deep non volcanic tremor nvt is usually associated with episodic slow slip events new zealand is one notable exception where numerous slow slip events have been identified yet nvt has remained undetected here we present the first known case of triggered nvt at new zealand s hikurangi subduction margin following the mw8 8 chilean earthquake of february 27 2010 we identify coherent high frequency tremor signals that are in phase with and modulated by the passing rayleigh waves this is consistent with the surface wave triggering potential for strike normal incidence on a low angle thrust fault after constraining the tremor depth on the plate interface we locate the tremor source within 20 km of the source area of episodic slow slip the tremor location is also near the edge of a region with high seismic attenuation that marks the boundary between dehydrated subducted slab and inferred hydrated underplated sediment we speculate that reduced interface friction and high fluid pressures provided by fluid rich underplated sediment facilitates the tremor generation

microearthquakes and subglacial conditions 1 ten passive seismic recording stations were deployed in a 9 km by 6 km array on rutford ice stream west antarctica to detect microearthquakes from the ice stream bed the array covered an area where a varying distribution of basal sedimentary conditions had been identified from seismic reflection surveys and interpreted as areas of both basal sliding and subglacial sediment deformation the passive seismic experiment was designed to give an independent confirmation of this interpretation as basal sliding is believed to be associated with the release of more acoustic energy than a deforming bed the results show that this is the case basal sliding producing six times more events this confirms that the spatial pattern of basal conditions does indeed reflect the distribution of different ice flow mechanisms it also shows that microearthquake monitoring can be a valuable new technique for mapping ice stream basal conditions over wide areas

evidence suggesting fluid flow beneath japan due to periodic seismic triggering from the 2004 sumatra andaman earthquake 1 we show that very large surface waves from the 2004 sumatra andaman earthquake m9 2 triggered deep low frequency dlf seismic tremors beneath japan over a 500 km wide region the triggered seismicity was periodically excited synchronized with the amplitudes and phases of the oscillatory surface waves incident on the region periodic excitation implies that the specific phase of the surface waves plays an important role for the triggering estimation of stress changes in the source regions showed that the volumetric dilatation larger than 10 8 was the predominant factor for triggering thereby strongly supporting the hypothesis that dlf events are caused by processes related to fluid flow

tectonic tremor beneath cuba triggered by the mw 8 8 maule and mw 9 0 tohoku oki earthquakes we provide additional evidence of tectonic tremor in cuba triggered by the 2010 m w 8 8 maule chile and the 2011 m w 9 0 tohoku oki japan earthquakes the high frequency tremor signals are modulated by long period surface waves similar to triggered tremors observed in other tectonically active regions we are able to locate two tremor sources triggered by the tohoku oki earthquake near the east west trending oriente fault around guantanamo bay the tremor around guantanamo bay was triggered primarily by a love wave of the maule mainshock and by both love and rayleigh waves of the tohoku oki mainshock this is consistent with frictional failures at a vertical strike slip fault under a coulomb failure criterion online material figure of envelope functions of tremor records and movies of spectogram and tremor correlation

migration of early aftershocks following the 2004 parkfield earthquake a significant number of the early aftershocks following large shallow earthquakes are missing in existing earthquake catalogues an analysis of the waveforms of 3 647 relocated earthquakes along the parkfield section of the san andreas fault reveals 11 times more aftershocks within three days of the 2004 parkfield earthquake than listed in a standard catalogue large shallow earthquakes are immediately followed by numerous aftershocks a significant portion of these events is missing in existing earthquake catalogues mainly because seismicity after the mainshock can be masked by overlapping arrivals of waves from the mainshock and aftershocks1 2 3 4 however recovery of the missing early aftershocks is important for understanding the physical mechanisms of earthquake triggering2 3 4 and for tracking postseismic deformation around the rupture zone associated with the mainshock5 6 7 here we use the waveforms of 3 647 relocated earthquakes8 along the parkfield section of the san andreas fault as templates9 10 to detect missing aftershocks within three days of the 2004 magnitude 6 0 parkfield earthquake we identify 11 times more aftershocks than listed in the standard catalogue of the northern california seismic network we find that the newly detected aftershocks migrate in both along strike and down dip directions with logarithmic time since the mainshock consistent with numerical simulations of the expansion of aftershocks caused by propagating afterslip11 12 the cumulative number of early aftershocks increases linearly with postseismic deformation in the first two days supporting the view that aftershocks are driven primarily by afterslip13 14

seismic detection of an active subglacial magmatic complex in marie byrd land antarctica numerous volcanoes exist in marie byrd land a highland region of west antarctica high heat flow through the crust in this region may influence the stability of the west antarctic ice sheet 1 4 volcanic activity progressed from north to south in the executive committee mountain range between the miocene and holocene epochs but there has been no evidence for recent magmatic activity 5 7 here we use a recently deployed seismic network to show that in 2010 and 2011 two swarms of seismic activity occurred at 25 40 km depth beneath subglacial topographic and magnetic highs located 55 km south of the youngest subaerial volcano in the executive committee range we interpret the swarm events as deep long period earthquakes based on their unusual frequency content such earthquakes occur beneath active volcanoes are caused by deep magmatic activity and in some cases precede eruptions 8 11 we also use radar profiles to identify a prominent ash layer in the ice overlying the seismic swarm located at 1 400 m depth the ash layer is about 8 000 years old and was probably sourced from the nearby mount waesche volcano together these observations provide strong evidence for ongoing magmatic activity and demonstrate that volcanism continues to migrate southwards along the executive committee range eruptions at this site are unlikely to penetrate the 1 2 to 2 km thick overlying ice but would generate large volumes of melt water that could significantly affect ice stream flow