chemical heterogeneity in diffusion dominated aquitards 1 high resolution measurements of the three dimensional 3 d variability in pore water electrical conductivity ec were obtained at a clay rich aquitard research site the king site 120 m 70 m area in southern saskatchewan canada using direct push probing technology n 35 boreholes these measurements revealed complex lateral heterogeneity in the subsurface salt distributions which raised questions on the validity of the commonly used approach of transposing chemical data from distributed piezometers onto 1 d depth profiles for interpretation and solute transport modeling two dimensional numerical modeling simulations demonstrated that distributed nonuniform salt inputs have existed at the top of the unoxidized till aquitard for the past 3 5 kyr the source of the nonuniform salt inputs is unclear but may be due to either variations in microtopography 0 5 m or variable penetration depth of surface fractures a suite of generic solutions was developed to estimate the critical depth required to obtain chemical homogeneity in aquitards with nonuniform solute inputs and diffusion as the dominant transport mechanism these solutions are presented for a range of sinusoidal input functions with varying amplitudes magnitudes and wavelengths the solutions indicate ec distributions at the king site should be laterally uniform below a depth of around 20 m bg which is consistent with observed ec values at the site this study shows that aquitards with nonuniform salt inputs at their upper boundary are chemically heterogeneous above some critical depth and thus require careful attention when modeling solute transport processes within them

inversion of complex body waves we have developed a method that inverts seismic body waves to determine the mechanism and rupture pattern of earthquakes the rupture pattern is represented as a sequence of subevents distributed on the fault plane this method is an extension of our earlier method in which the subevent mechanisms were fixed in the new method the subevent mechanisms are determined from the data and are allowed to vary during the sequence when subevent mechanisms are allowed to vary however the inversion often becomes unstable because of the complex trade offs between the mechanism the timing and the location of the subevents many different subevent sequences can explain the same data equally well and it is important to determine the range of allowable solutions some constraints must be imposed on the solution to stabilize the inversion we have developed a procedure to explore the range of allowable solutions and appropriate constraints in this procedure a network of grid points is constructed on the i plane where and i are respectively the onset time and the distance from the epicenter of a subevent the best fit subevent is determined at all grid points then the correlation is computed between the synthetic waveform for each subevent and the observed waveform the correlation as a function of and i and the best fit mechanisms computed at each i grid point depict the character of allowable solutions and facilitate a decision on the appropriate constraints to be imposed on the solution the method is illustrated using the data for the 1976 guatemala earthquake

the 2013 mw 7 7 balochistan earthquake seismic potential of an accretionary wedge great earthquakes rarely occur within active accretionary prisms despite the intense long term deformation associated with the formation of these geologic structures this paucity of earthquakes is often attributed to partitioning of deformation across multiple structures as well as aseismic deformation within and at the base of the prism davis et al 1983 we use teleseismic data and satellite optical and radar imaging of the 2013 m w 7 7 earthquake that occurred on the southeastern edge of the makran plate boundary zone to study this unexpected earthquake we first compute a multiple point source solution from w phase waveforms to estimate fault geometry and rupture duration and timing we then derive the distribution of subsurface fault slip from geodetic coseismic offsets we sample for the slip posterior probability density function using a bayesian approach including a full description of the data covariance and accounting for errors in the elastic structure of the crust the rupture nucleated on a subvertical segment branching out of the chaman fault system and grew into a major earthquake along a 50 north dipping thrust fault with significant along strike curvature fault slip propagated at an average speed of 3 0 km s for about 180 km and is concentrated in the top 10 km with no displacement on the underlying decollement this earthquake does not exhibit significant slip deficit near the surface nor is there significant segmentation of the rupture we propose that complex interaction between the subduction accommodating the arabia eurasia convergence to the south and the ornach nal fault plate boundary between india and eurasia resulted in the significant strain gradient observed prior to this earthquake convergence in this region is accommodated both along the subduction megathrust and as internal deformation of the accretionary wedge

the 2012 sumatra great earthquake sequence the equatorial indian ocean is a well known place of active intraplate deformation defying the conventional view of rigid plates separated by narrow boundaries where deformation is confined on 11 april 2012 this region was hit in a couple of hours by two of the largest strike slip earthquakes ever recorded moment magnitudes mw 8 6 and 8 2 broadband seismological observations of the mw 8 6 mainshock indicate a large centroid depth 30 km and remarkable rupture complexity detailed study of the surface wave directivity and moment rate functions clearly indicates the partition of the rupture into at least two distinct subevents to account for these observations we developed a procedure to invert for multiple point source parameters the optimum source model at long period consists of two point sources separated by about 209 km with magnitudes mw 8 5 and 8 3 to explain the remaining discrepancies between predicted and observed surface waves we can refine this model by adding directivity along the wnw ese axis however we do not exclude more complicated models to analyze the mw 8 2 aftershock we removed the perturbation due to large surface wave arrivals of the mw 8 6 mainshock by subtracting the corresponding synthetics computed for the two subevent model analysis of the surface wave amplitudes suggests that the mw 8 2 aftershock had a large centroid depth between 30 km and 40 km this major earthquake sequence brings a new perspective to the seismotectonics of the equatorial indian ocean and reveals active deep lithospheric deformation

searching for hidden earthquakes in southern california earthquakes follow a well known power law size relation with smaller events occurring much more often than larger events earthquake catalogs are thus dominated by small earthquakes yet still missing a much larger number of even smaller events caused by signal fidelity issues to overcome these limitations we applied a template matching detection technique to the entire waveform archive of the regional seismic network in southern california this effort resulted in a catalog with 1 81 million earthquakes a factor of 10 increase which provides important new insights into the geometry of fault zones at depth foreshock behavior and nucleation processes and earthquake triggering mechanisms the extraordinary detail resolved in this type of catalog will facilitate a new generation of analyses of earthquakes and faults

the 2002 denali fault earthquake alaska a large magnitude slip partitioned event the mw moment magnitude 7 9 denali fault earthquake on 3 november 2002 was associated with 340 kilometers of surface rupture and was the largest strike slip earthquake in north america in almost 150 years it illuminates earthquake mechanics and hazards of large strike slip faults it began with thrusting on the previously unrecognized susitna glacier fault continued with right slip on the denali fault then took a right step and continued with right slip on the totschunda fault there is good correlation between geologically observed and geophysically inferred moment release the earthquake produced unusually strong distal effects in the rupture propagation direction including triggered seismicity

new paleoearthquake ages from the western garlock fault implications for regional earthquake occurrence in southern california abstract new paleoseismic investigations on the western segment of the garlock fault at twin lakes california reveal evidence for up to six surface ruptures in the past 5600 years calibrated radiocarbon dates from accelerator mass spectrometer analysis of detrital charcoal constrain the timing of three well defined events at twin lakes to post a d 1450 event a 720 395 b c event g and 3425 2200 b c event i two probable events are also constrained to 625 1525 a d event c and 155 b c a d 615 event e and a possible additional event to 3425 2200 b c and prior to event i our findings offer new insights into mid to late holocene behavior of the western and central segments of the garlock fault and regional fault interactions the timing of the most recent event mre on the western segment likely correlates with the mre at paleoseismic sites on the central segment suggesting that both segments do sometimes rupture together during large earthquakes evidence for events during periods of seismic quiescence on adjacent segments demonstrates that the western and central segments also sometimes rupture independently of one another the occurrence of event g during a lull in seismic strain release at 2 5 ka on faults in the eastern california shear zone ecsz contrasts with other studies that suggest the garlock fault ruptures in phase with ecsz faults our data suggest that seismic strain release on the garlock fault may actually be more in phase with moment release on the mojave section of the san andreas fault and the transverse ranges faults of the los angeles region online material two input files written for the computer program oxcal 4 1 7 and two output files from the oxcal analyses the first input file calibrates and re weights probability density functions pdfs for radiocarbon ages based on stratigraphic ordering and the second input file calculates pdfs for earthquake ages based on radiocarbon ages and stratigraphy bracketing the events

insar time series analysis of l band wide swath sar data acquired by alos 2 operating at l band 24 cm wavelength in wide swath modes is one of the characteristics of the new and next generation satellite synthetic aperture radar sar missions after 3 years of operation the japan aerospace exploration agency advanced land observing satellite 2 alos 2 satellite has acquired a wealth of l band wide swath sar data over many areas using its scansar mode we present interferometry sar insar time series analysis results from alos 2 scansar data we analyze the possible error sources in the insar and correct them if possible we present different time series analysis results including azimuth frequency modulation rate error line of sight los ionospheric phase azimuth shift caused by the ionosphere and los displacement processed using both full aperture and burst by burst workflows the final insar los displacement time series result reveals both large scale tectonic and small scale anthropogenic deformation components the results demonstrate the potential for measuring continental or even global scale tectonic deformation and illustrate the promise of upcoming l band wide swath sar missions such as the nasa isro sar mission

surface slip during large owens valley earthquakes data sets and expanded results contributing to this study are available in the supporting information the earthscope southern and eastern california lidar project available online at http opentopo sdsc edu involved data acquisition and processing for the plate boundary observatory pbo by ncalm http www ncalm org unavco operates the pbo for earthscope http www earthscope org supported by the national science foundation ear 0350028 and ear 0732947 funding for this study was provided by the southern california earthquake center scec project 12140 the geological society of america graduate student research fund the community foundation of san bernardino county and the western washington university geology department we thank g seitz m price and k morgan for assistance in the field and s bacon j arrowsmith r weldon k scharer j unruh c madden madugo and d haddad for helpful discussions constructive reviews by d schwartz r briggs e schermer d clark and one anonymous reviewer substantially improved the paper we also thank the staff at the uc white mountain research center for facilitating this work any use of trade firm or product names is for descriptive purposes only and does not imply endorsement by the u s government

crustal structure and seismicity distribution adjacent to the pacific and north america plate boundary in southern california new three dimensional 3 d v p and v p v s models are determined for southern california using p and s p travel times from local earthquakes and controlled sources these models confirm existing tectonic interpretations and provide new insights into the configuration of geological structures at the pacific north america plate boundary the models extend from the u s mexico border in the south to the southernmost coast ranges and sierra nevada in the north and have a 15 km horizontal grid spacing and an average vertical grid spacing of 4 km down to 22 km depth the heterogeneity of the crustal structure as imaged by v p and v p v s models is larger within the pacific plate than the north american plate similarly the relocated seismicity deepens and shows more complex 3 d distribution in areas of the pacific plate exhibiting compressional tectonics the models reflect mapped changes in the lithology across major geological terranes such as the mojave desert the peninsular ranges and the transverse ranges the interface between the shallow mono of the continental borderland and the deep moho of onshore california forms a broad zone to the north beneath the western transverse ranges ventura basin and the los angeles basin and a narrow zone to the south along the peninsular ranges the near surface increase in velocity from the surface to up to 8 km depth is rapid and has a logarithmic shape for stable blocks and mountain ranges but is slow with a linear shape for sedimentary basins at midcrustal depths a rapid increase in v p is imaged beneath the sediments of the large sedimentary basins while beneath the adjacent mountain ranges the increase is small or absent

back propagating super shear rupture in the 2016 mw7 1 romanche transform fault earthquake how an earthquake rupture propagates strongly influences the potentially destructive ground shaking complex ruptures often involve slip along multiple faults which masks information on the frictional behaviour of fault zones geometrically smooth ocean transform fault plate boundaries offer a favourable environment to study fault dynamics because strain is accommodated along a single wide fault zone that offsets the homogeneous geology here we present an analysis of the 2016 mw 7 1 earthquake on the romanche fracture zone in the equatorial atlantic using data from both nearby seafloor seismometers and global seismic networks we show that this rupture had two phases 1 upward and eastward propagation towards a weaker region where the transform fault intersects the mid ocean ridge and then 2 an unusual back propagation westwards at a supershear speed towards the centre of the fault we suggest that deep rupture into weak fault segments facilitated greater seismic slip on shallow locked zones this highlights that even earthquakes along a single distinct fault zone can be highly dynamic observations of back propagating ruptures are sparse and the possibility of reverse propagation is largely absent in rupture simulations and unaccounted for in hazard assessments in one earthquake an oceanic transform fault ruptured in one direction and then backwards at a speed exceeding that of shear wave propagation according to an analysis of data recorded by nearby seafloor and global seismometers

surface deformation associated with fractures near the 2019 ridgecrest earthquake sequence contemporary earthquake hazard models hinge on an understanding of how strain is distributed in the crust and the ability to precisely detect millimeter scale deformation over broad regions of active faulting satellite radar observations revealed hundreds of previously unmapped linear strain concentrations or fractures surrounding the 2019 ridgecrest earthquake sequence we documented and analyzed displacements and widths of 169 of these fractures although most fractures are displaced in the direction of the prevailing tectonic stress prograde a large number of them are displaced in the opposite retrograde direction we developed a model to explain the existence and behavior of these displacements a major implication is that much of the prograde tectonic strain is accommodated by frictional slip on many preexisting faults

earthquake rupture through a step over fault system an exploratory numerical study of the leech river fault southern vancouver island the leech river fault lrf zone located on southern vancouver island is a major regional seismic source we investigate potential interactions between earthquake ruptures on the lrf and the neighb

spatio temporal foreshock evolution of the 2019 m 6 4 and m 7 1 ridgecrest california earthquakes abstract the 2019 m 6 4 and m 7 1 ridgecrest california earthquake sequence provides an ideal opportunity to study the seismicity evolution and interaction among multiple complex fault structures here we apply the matched filter detection method to obtain a relatively complete magnitude of completeness 0 9 and precisely relocated earthquake catalog the results show a short duration 31 minutes foreshock sequence with 28 events before the m 6 4 earthquake the foreshock sequence started with a m 4 0 event and was aligned along the nw se direction this implies that the m 6 4 rupture initiated on a nw trending fault segment before rupturing the primary sw trending fault repeating earthquakes before and after the m 7 1 event are separated in space and bound the areas of large coseismic slip in the m 6 4 and m 7 1 events this might reflect local slow slip acceleration near the edges of coseismic rupture asperities the nw striking fault zones illuminated by seismicity are separated into several sub regions with distinct pre m7 1 seismicity rate evolutions the m 7 1 event nucleated in a region of local seismicity concentration which intensified 3 hr before the m 7 1 mainshock the m 7 1 nucleation zone is characterized by a significantly low b value of events that occurred since the m 6 4 event which might indicate local failure conditions approaching a critical state

multiple cmt source analysis of the 2004 sumatra earthquake while it is agreed that the great sumatra earthquake of december 26 2004 was among the largest earthquakes of the past century there has been disagreement on how large it was which part of the fault ruptured and how the rupture took place we present a centroid moment tensor cmt analysis of the earthquake in which multiple point sources are used in the inversion to mimic a propagating slip pulse the final model consists of five point sources with the southernmost sources accounting for the majority of the moment release the presumed fault planes of the southern sources strike northwest while those in the north strike northeast consistent with the geometry of the subduction trench slip on the fault is found to be more oblique in the north than in the south the inversion with five sources leads to a moment magnitude for the sumatra earthquake of m w 9 3 consistent with estimates from long period normal mode amplitudes

hierarchical interlocked orthogonal faulting in the 2019 ridgecrest earthquake sequence a nearly 20 year hiatus in major seismic activity in southern california ended on 4 july 2019 with a sequence of intersecting earthquakes near the city of ridgecrest california this sequence included a foreshock with a moment magnitude mw of 6 4 followed by a mw 7 1 mainshock nearly 34 hours later geodetic seismic and seismicity data provided an integrative view of this sequence which ruptured an unmapped multiscale network of interlaced orthogonal faults this complex fault geometry persists over the entire seismogenic depth range the rupture of the mainshock terminated only a few kilometers from the major regional garlock fault triggering shallow creep and a substantial earthquake swarm the repeated occurrence of multifault ruptures as revealed by modern instrumentation and analysis techniques poses a formidable challenge in quantifying regional seismic hazards

superficial simplicity of the 2010 el mayor cucapah earthquake of baja california in mexico the geometry of faults is usually thought to be more complicated at the surface than at depth and to control the initiation propagation and arrest of seismic ruptures the fault system that runs from southern california into mexico is a simple strike slip boundary the west side of california and mexico moves northwards with respect to the east however the m w 7 2 2010 el mayor cucapah earthquake on this fault system produced a pattern of seismic waves that indicates a far more complex source than slip on a planar strike slip fault here we use geodetic remote sensing and seismological data to reconstruct the fault geometry and history of slip during this earthquake we find that the earthquake produced a straight 120 km long fault trace that cut through the cucapah mountain range and across the colorado river delta however at depth the fault is made up of two different segments connected by a small extensional fault both segments strike n130 e but dip in opposite directions the earthquake was initiated on the connecting extensional fault and 15 s later ruptured the two main segments with dominantly strike slip motion we show that complexities in the fault geometry at depth explain well the complex pattern of radiated seismic waves we conclude that the location and detailed characteristics of the earthquake could not have been anticipated on the basis of observations of surface geology alone

complex multifault rupture during the 2016 mw 7 8 kaikōura earthquake new zealand the 2016 moment magnitude m w 7 8 kaik ura earthquake was one of the largest ever to hit new zealand hamling et al show with a new slip model that it was an incredibly complex event unlike most earthquakes multiple faults ruptured to generate the ground shaking a remarkable 12 faults ruptured overall with the rupture jumping between faults located up to 15 km away from each other the earthquake should motivate rethinking of certain seismic hazard models which do not presently allow for this unusual complex rupture pattern science this issue p eaam7194 1 1 lookup doi 10 1126 science aam7194

characterization of fault zones there are currently three major competing views on the essential geometrical mechanical and mathematical nature of faults the standard view is that faults are possibly segmented and heterogeneous euclidean zones in a continuum solid the continuum euclidean view is supported by seismic gravity and electromagnetic imaging studies by successful modeling of observed seismic radiation geodetic data and changes in seismicity patterns by detailed field studies of earthquake rupture zones and exhumed faults and by recent high resolution hypocenter distributions along several faults the second view focuses on granular aspects of fault structures and deformation fields the granular view is supported by observations of rock particles in fault zone gouge by studies of block rotations and the mosaic structure of the lithosphere which includes the overall geometry of plate tectonics by concentration of deformation signals along block boundaries by correlation of seismicity patterns on scales several times larger than those compatible with a continuum framework and by strongly heterogeneous wave propagation effects on the earth s surface the third view is that faults are fractal objects with rough surfaces and branching geometry the fractal view is supported by some statistical analysis of regional hypocenter locations by long range correlation of various measurements in geophysical boreholes by the fact that observed power law statistics of earthquakes are compatible with an underlying scale invariant geometrical structure by geometrical analysis of fault traces at the earth s surface and by measurements of joint and fault surfaces topography

recurring large deep earthquakes in hindu kush driven by a sinking slab hindu kush subduction zone produces large intermediate depth earthquakes within a small volume every 10 15 years here we study the last three m 7 events within the cluster and find complex and diverse rupture processes however their main subevents appear to recur on the same fault patch dipping 70 to the south this recurrence requires an average of 9 6 cm yr slip rate on the patch much higher than the 1 cm yr surface convergence rate measured geodetically the high slip rate is likely caused by significant slab internal deformation such as localized slab stretching necking we infer that the hindu kush subducted slab below 210 km is sinking through the mantle at a vertical rate of 10 cm yr

slip on faults in the imperial valley triggered by the 4 april 2010 mw 7 2 el mayor cucapah earthquake revealed by insar 1 radar interferometry insar field measurements and creepmeters reveal surface slip on multiple faults in the imperial valley triggered by the main shock of the 4 april 2010 el mayor cucapah mw 7 2 earthquake co seismic offsets occurred on the san andreas superstition hills imperial elmore ranch wienert coyote creek elsinore yuha and several minor faults near the town of ocotillo at the northern end of the mainshock rupture we documented right lateral slip 40 mm on northwest striking faults and left lateral slip 40 mm on southwest striking faults slip occurred on 15 km and 20 km long segments of the san andreas fault in the mecca hills 50 mm and durmid hill 10 mm respectively and on 25 km of the superstition hills fault 37 mm field measurements of slip on the superstition hills fault agree with insar and creepmeter measurements to within a few millimeters dislocation models of the insar data from the superstition hills fault confirm that creep in this sequence as in previous slip events is confined to shallow depths 3 km

fault orientations in extensional and conjugate strike slip environments and their implications seismically active conjugate strike slip faults in california and japan typically have mutually orthogonal right and left lateral fault planes normal fault dips at earthquake nucleation depths are concentrated between 40 and 50 the observed orientations and their strong clustering are surprising because conventional faulting theory suggests fault initiation with conjugate 60 and 120 intersecting planes and 60 normal fault dip or fault reactivation with a broad range of permitted orientations the observations place new constraints on the mechanics of fault initiation rotation and evolutionary development we speculate that the data could be explained by fault rotation into the observed orientations and deactivation for greater rotation or by formation of localized shear zones beneath the brittle ductile transition in earth9s crust initiation as weak frictional faults seems unlikely

depth of seismicity in the imperial valley region 1977 1983 and its relationship to heat flow crustal structure and the october 15 1979 earthquake focal depths from over 1000 earthquakes occuring between 1977 and 1983 in the imperial valley southern peninsular ranges are used to study relationships between the depth of seismicity heat flow and crustal structure this study used relocated a and b quality events from the california institute of technology catalog that were carefully selected to insure focal depth precision of 2 km regional variations in focal depth appear to be related to regional heat flow variation whereas local variations in focal depth especially in the central imperial valley may be related to crustal structure these variations are studied by rheologic modeling a comparison of focal depths of earthquakes occurring before and after the october 15 1979 m 6 6 earthquake indicates that aftershocks during the first 2 months of the sequence were 2 3 km deeper than earthquakes occurring in other time periods the deepest earthquakes in imperial valley are spatially associated with a subbasement dome near the northern end of the imperial fault this dome coincides with the region where the imperial fault undergoes a transition from stick slip behavior to aseismic fault creep models of slip during the 1979 mainshock are also compared with premainshock and postmainshock seismicity a relocation of the 1940 m 7 1 mainshock suggests that this sequence began by rupturing the same portion of the fault that experienced maximum slip during the 1979 mainshock

introduction to the special section on the 2019 ridgecrest california earthquake sequence following the 16 october 1999 m w 7 1 hector mine earthquake no earthquake larger than magnitude 6 0 occurred in southern california for nearly 20 yr that lull came to an end with the occurrence of the 4 july 2019 m w 6 4 earthquake at 22 40 utc 10 33 a m local time located in a relatively sparsely populated part of the state with most of its rupture within the china lake naval air weapons station nawscl adjacent to the civilian town of ridgecrest population 26 000 the earthquake was felt widely across most of southern california immediate response efforts were mounted to investigate the rupture and assess damage stewart et al 2019 and to calculate aftershock probabilities see hardebeck 2020 also see data and resources the earthquake occurred in a region historically characterized by higher than average aftershock productivity hardebeck et al 2019 leading to a higher than average probabilities for both large aftershocks and subsequent larger events the latter odds although still small were borne out with the occurrence of 6 july m w 7 1 earthquake at 03 19 53 utc 8 19 53 p m local time on 5 july 34 hr after the initial m w 6 4 arguably the m w 7 1 could be considered an aftershock but by convention this earthquake is regarded as the mainshock whereas the initial m w 6 4 is considered a foreshock

some thoughts on the use of insar data to constrain models of surface deformation noise structure and data downsampling repeat pass interferometric synthetic aperture radar insar provides spatially dense maps of surface deformation with potentially tens of millions of data points here we estimate the actual covariance structure of noise in insar data we compare the results for several independent interferograms with a large ensemble of gps observations of tropospheric delay and discuss how the common approaches used during processing of insar data affects the inferred covariance structure motivated by computational concerns associated with numerical modeling of deformation sources we then combine the data covariance information with the inherent resolution of an assumed source model to develop an efficient algorithm for spatially variable data resampling or averaging we illustrate these technical developments with two earthquake scenarios at different ends of the earthquake magnitude spectrum for the larger events our goal is to invert for the coseismic fault slip distribution for smaller events we infer the hypocenter location and moment we compare the results of inversions using several different resampling algorithms and we assess the importance of using the full noise covariance matrix

dynamic fault interaction during a fluid injection induced earthquake the 2017 mw 5 5 pohang event the november 15th 2017 mw 5 5 pohang earthquake south korea has been linked to hydraulic stimulation and fluid injections making this the largest induced seismic event associated with an enhanced geothermal system egs to understand its source dynamics and fault interactions we conduct the first 3d high resolution spontaneous dynamic rupture simulations of an induced earthquake we account for topography off fault plastic deformation under depth dependent bulk cohesion rapid velocity weakening friction and 1d subsurface structure a guided fault reconstruction approach that clusters spatio temporal aftershock locations including their uncertainties is used to identify a main and a secondary fault plane that intersects under a shallow angle of 15 based on simple mohr coulomb failure analysis and 180 dynamic rupture experiments in which we vary local stress loading conditions fluid pressure and relative fault strength we identify preferred two fault plane scenarios that well reproduce observations we find that the regional far field tectonic stress regime promotes pure strike slip faulting while local stress conditions constrained by borehole logging generate the observed thrust faulting component our preferred model is characterized by overpressurized pore fluids non optimally oriented but dynamically weak faults and a close to critical local stress state in our model earthquake rupture jumps to the secondary fault by dynamic triggering generating a measurable non double couple component our simulations suggest that complex dynamic fault interaction may occur during fluid injection induced earthquakes and that local stress perturbations dominate over the regional stress conditions these findings therefore have important implications for seismic hazard in active geo reservoir

the iquique earthquake sequence of april 2014 bayesian modeling accounting for prediction uncertainty the subduction zone in northern chile is a well identified seismic gap that last ruptured in 1877 on 1 april 2014 this region was struck by a large earthquake following a two week long series of foreshocks this study combines a wide range of observations including geodetic tsunami and seismic data to produce a reliable kinematic slip model of the mw 8 1 main shock and a static slip model of the mw 7 7 aftershock we use a novel bayesian modeling approach that accounts for uncertainty in the green s functions both static and dynamic while avoiding nonphysical regularization the results reveal a sharp slip zone more compact than previously thought located downdip of the foreshock sequence and updip of high frequency sources inferred by back projection analysis both the main shock and the mw 7 7 aftershock did not rupture to the trench and left most of the seismic gap unbroken leaving the possibility of a future large earthquake in the region

detailed 3d fault representations for the 2019 ridgecrest california earthquake sequence we present new 3d source fault representations for the 2019 m 6 4 and m 7 1 ridgecrest earthquake sequence these representations are based on relocated hypocenter catalogs expanded by template matching and focal mechanisms for m 4 and larger events following the approach of riesner et al 2017 we generate reproducible 3d fault geometries by integrating hypocenter nodal plane and surface rupture trace constraints we used the southwest northeast striking nodal plane of the 4 july 2019 m 6 4 event to constrain the initial representation of the southern little lake fault sllf both in terms of location and orientation the eastern little lake fault ellf was constrained by the 5 july 2019 m 7 1 hypocenter and nodal planes of m 4 and larger aftershocks aligned with the main trend of the fault the approach follows a defined workflow that assigns weights to a variety of geometric constraints these main constraints have a high weight relative to that of individual hypocenters ensuring that small aftershocks are applied as weaker constraints the resulting fault planes can be considered averages of the hypocentral locations respecting nodal plane orientations for the final representation we added detailed field mapped rupture traces as strong constraints the resulting fault representations are generally smooth but nonplanar and dip steeply the sllf and ellf intersect at nearly right angles and cross on another the ellf representation is truncated at the airport lake fault to the north and the garlock fault to the south consistent with the aftershock pattern the terminations of the sllf representation are controlled by aftershock distribution these new 3d fault representations are available as triangulated surface representations and are being added to a community fault model cfm plesch et al 2007 2019 nicholson et al 2019 for wider use and to derived products such as a cfm trace map and viewer su et al 2019

large amplitude moho reflections sms from landers aftershocks southern california closely spaced aftershocks of the 28 june 1992 landers earthquake m w 7 3 were used to make event record sections that show the transverse components of s and sms arrivals at a distance of 70 to 170 km for the data recorded toward the north in the mojave desert large sms phases are observed with amplitudes 2 to 5 times greater than the direct s for similar distances to the south the sms arrival is comparable to or smaller than the s comparisons to synthetic seismograms indicate that the large amplitude sms phases are produced by the simple crustal structure of the mojave desert that allows a large moho reflection in contrast the more complex geologic structure to the south partitions the seismic energy into a more complicated set of seismic phases so that the moho reflection is diminished in amplitude the large sms phases observed in the mojave enhance the overall ground motions by a factor of 2 to 3 this suggests that when damaging earthquakes occur in other regions of simple crustal structures moho reflections will produce amplified strong motions at distance ranges around 100 km depending on the local structure

aftershocks driven by afterslip and fluid pressure sweeping through a fault fracture mesh a variety of physical mechanisms are thought to be responsible for the triggering and spatiotemporal evolution of aftershocks here we analyze a vigorous aftershock sequence and postseismic geodetic strain that occurred in the yuha desert following the 2010 m w 7 2 el mayor cucapah earthquake about 155 000 detected aftershocks occurred in a network of orthogonal faults and exhibit features of two distinct mechanisms for aftershock triggering the earliest aftershocks were likely driven by afterslip that spread away from the main shock with the logarithm of time a later pulse of aftershocks swept again across the yuha desert with square root time dependence and swarm like behavior together with local geological evidence for hydrothermalism these features suggest that the events were driven by fluid diffusion the observations illustrate how multiple driving mechanisms and the underlying fault structure jointly control the evolution of an aftershock sequence

high resolution interseismic velocity data along the san andreas fault from gps and insar 1 we compared four interseismic velocity models of the san andreas fault based on gps observations the standard deviations of the predicted secular velocity from the four models are larger north of the san francisco bay area near the creeping segment in central california and along the san jacinto fault and the east california shear zone in southern california a coherence spectrum analysis of the secular velocity fields indicates relatively high correlation among the four models at longer wavelengths 15 40 km with lower correlation at shorter wavelengths to improve the short wavelength accuracy of the interseismic velocity model we integrated interferometric synthetic aperture radar insar observations initially from advanced land observing satellite alos ascending data spanning from the middle of 2006 to the end of 2010 totaling more than 1100 interferograms with gps observations using a sum remove filter restore approach the final insar line of sight data match the point gps observations with a mean absolute deviation of 1 5 mm yr we systematically evaluated the fault creep rates along major faults of the san andreas fault and compared them with creepmeters and alignment array data compiled in uniform california earthquake rupture forecast version 2 ucerf2 moreover this insar line of sight dataset can constrain rapid velocity gradients near the faults which are critical for understanding the along strike variations in stress accumulation rate and associated earthquake hazard

ice sheet velocity mapping a combined interferometric and speckle tracking approach the first and second radarsat antarctic mapping missions amm 1 and 2 have now acquired interferometric synthetic aperture radar sar overmuch of the ice sheet the radarsat 24 day repeat cycle is nearly ideal for measuring slow ice motion e g 1 but application of sar interferometry is limited in faster moving areas with a 1day repeat period ers 1 2 tandem sar data are much better matched to fast motion but are not always available fortunately several authors have demonstrated the ability to measure velocity in fast moving areas by tracking sar speckle from image to image which works well even in the absence of visible features while these estimates have intrinsically lower resolution and poorer accuracy than direct phase measurements they serve well in areas where there are no data suitable for conventional interferometry this paper describes algorithms i have developed for merging interferometric and speckle tracking data from multiple swaths to form a single seamless mosaic of velocity at each point in the mosaic all the available data are combined to produce estimates of the velocity and the associated error this technique is demonstrated using radarsat data collected over lambert glacier antarctica during amm 1 and 2

absence of remote earthquake triggering within the coso and salton sea geothermal production fields geothermal areas are long recognized to be susceptible to remote earthquake triggering probably due to the high seismicity rates and presence of geothermal fluids however anthropogenic injection and extraction activity may alter the stress state and fluid flow within the geothermal fields here we examine the remote triggering phenomena in the coso geothermal field and its surrounding areas to assess possible anthropogenic effects we find that triggered earthquakes are absent within the geothermal field but occur in the surrounding areas similar observation is also found in the salton sea geothermal field we hypothesize that continuous geothermal operation has eliminated any significant differential pore pressure between fractures inside the geothermal field through flushing geothermal precipitations and sediments out of clogged fractures to test this hypothesis we analyze the pore pressure driven earthquake swarms and they are found to occur outside or on the periphery of the geothermal production field therefore our results suggest that the geothermal operation has changed the subsurface fracture network and differential pore pressure is the primary controlling factor of remote triggering in geothermal fields

crustal geophysics and seismicity in southern california the geographical distribution of the 1981 2005 seismicity in southern california forms a 150 km broad zone adjacent to the pacific north america plate boundary ranging from depths of 1 30 km with the bulk of the focal depths in the range of 2 12 km the distribution of the seismicity that includes both mainshock aftershock sequences and background events is affected by both static and kinematic geophysical parameters of the crust the static parameters include heat flow topography crustal density v p v s ratio hypocentral fault distance and crustal thickness from receiver functions the tectonic loading is represented by kinematic parameters such as the crustal shear strain rate field and the dilatational strain rate field in our analysis we normalize the seismicity relative to the areal density of the range of values of each of the parameters most of the seismicity occurs in areas of average heat flow low to intermediate topography average vp vs and high late quaternary fault density and forms seismogenic zones that extend through the brittle crust the location of late quaternary faults often described as zones of weakness influences the geographical distribution of seismicity more than any other parameter although above or below average crustal properties such as high heat flow thin crust or very low v p v s values exist these properties only influence the spatial distribution of seismicity in a minor way as an example the salton trough area of low topography high heat flow high v p v s high shear strain rate and thin crust has distributed seismicity within a thin seismogenic zone also somewhat surprisingly areas of high topography low heat flow low v p v s low shear strain rate and thick crust have low seismicity rates but a thin seismogenic zone we determine an empirical relationship between heat flow and crustal thickness to show how the 400 c temperature isotherm gradually deepens with crustal thickness and forms the base of the seismogenic zone for crustal thicknesses from 22 to 37 km for crustal thickness ranging from 37 to 43 km the 250 c isotherm forms the base of the seismogenic zone suggesting that seismic faulting in these regions is confined to the top of the upper crust 12 14 km and thus does not accommodate plate motion

chronology of tectonic geomorphic and volcanic interactions and the tempo of fault slip near little lake california new geochronologic and geomorphic constraints on the little lake fault in the eastern california shear zone reveal steady modest rates of dextral slip during and since the mid to late pleistocene we focus on a suite of offset fluvial landforms in the pleistocene owens river channel that formed in response to periodic interaction with nearby basalt flows thereby recording displacement over multiple time intervals overlap between 40 ar 39 ar ages for the youngest intracanyon basalt flow and 10 be surface exposure dating of downstream terrace surfaces suggests widespread channel incision during a prominent outburst flood through the little lake channel at ca 64 ka older basalt flows flanking the upper and lower canyon margins indicate localization of the owens river in its current position between 212 14 and 197 11 ka coupled with terrestrial light detection and ranging lidar and digital topographic measurements of dextral offset the revised little lake chronology indicates average dextral slip rates of at least 0 6 0 7 mm yr and 4 to 10 5 yr despite previous geodetic observations of relatively rapid interseismic strain along the little lake fault we find no evidence for sustained temporal fluctuations in slip rates over multiple earthquake cycles instead our results indicate that accelerated fault loading may be transient over much shorter periods 10 1 yr and perhaps indicative of time dependent seismic hazard associated with eastern california shear zone faults

deformation and seismicity in the coso geothermal area inyo county california observations and modeling using satellite radar interferometry interferometric synthetic aperture radar insar data collected in the coso geothermal area eastern california during 1993 1999 indicate ground subsidence over a 50 km 2 region that approximately coincides with the production area of the coso geothermal plant the maximum subsidence rate in the peak of the anomaly is 3 5 cm yr 1 and the average volumetric rate of subsidence is of the order of 10 6 m 3 yr 1 the radar interferograms reveal a complex deformation pattern with at least two irregular subsidence peaks in the northern part of the anomaly and a region of relative uplift on the south we invert the insar displacement data for the positions geometry and relative strengths of the deformation sources at depth using a nonlinear least squares minimization algorithm we use elastic solutions for a prolate uniformly pressurized spheroidal cavity in a semi infinite body as basis functions for our inversions source depths inferred from our simulations range from 1 to 3 km which corresponds to the production depths of the coso geothermal plant underpressures in the geothermal reservoir inferred from the inversion are of the order of 0 1 1 mpa except a few abnormally high underpressures that are apparently biased toward the small source dimensions analysis of the insar data covering consecutive time intervals indicates that the depths and or horizontal extent of the deformation sources may increase with time this increase presumably reflects increasing volumes of the subsurface reservoir affected by the geothermal exploitation we show that clusters of microearthquakes associated with the geothermal power operation may result from perturbations in the pore fluid pressure as well as normal and shear stresses caused by the deflation of the geothermal reservoir

earthquake in a maze compressional rupture branching during the 2012 mw 8 6 sumatra earthquake seismological observations of the 2012 moment magnitude 8 6 sumatra earthquake reveal unprecedented complexity of dynamic rupture the surprisingly large magnitude results from the combination of deep extent high stress drop and rupture of multiple faults back projection source imaging indicates that the rupture occurred on distinct planes in an orthogonal conjugate fault system with relatively slow rupture speed the east southeast west northwest ruptures add a new dimension to the seismotectonics of the wharton basin which was previously thought to be controlled by north south strike slip faulting the rupture turned twice into the compressive quadrant against the preferred branching direction predicted by dynamic coulomb stress calculations orthogonal faulting and compressional branching indicate that rupture was controlled by a pressure insensitive strength of the deep oceanic lithosphere

extreme multi millennial slip rate variations on the garlock fault california strain super cycles potentially time variable fault strength and implications for system level earthquake occurrence abstract pronounced variations in fault slip rate revealed by new measurements along the garlock fault have basic implications for understanding how faults store and release strain energy in large earthquakes specifically dating of a series of 26 0 3 5 2 5 m fault offsets with a newly developed infrared stimulated luminescence method shows that the fault was slipping at 14 0 2 2 1 8 mm yr approximately twice as fast as the long term average rate during a previously documented cluster of four earthquakes 0 5 2 0 ka this elevated late holocene rate must be balanced by periods of slow or no slip such as that during the ca 3300 yr long seismic lull preceding the cluster moreover whereas a comparison of paleoseismic data and stress modeling results suggests that individual garlock earthquakes may be triggered by periods of rapid san andreas fault slip or very large slip events the on off behavior of the garlock suggests a longer term mechanism that may involve changes in the rate of elastic strain accumulation on the fault over millennial time scales this inference is consistent with most models of the geodetic velocity field which yield slip deficit rates that are much slower than the average latest pleistocene early holocene post 8 13 ka garlock slip rate of 6 5 1 5 mm yr these observations indicate the occurrence of millennia long strain super cycles on the garlock fault that may be associated with temporal changes in elastic strain accumulation rate which may in turn be controlled by variations in relative strength of the various faults in the garlock san andreas eastern california shear zone fault system and or changes in relative plate motion rates

coseismic deformation from the 1999 mw 7 1 hector mine california earthquake as inferred from insar and gps observations we use interferometric synthetic aperture radar insar and global positioning system gps observations to investigate static deformation due to the 1999 m w 7 1 hector mine earthquake that occurred in the eastern california shear zone interferometric decorrelation phase and azimuth offset measurements indicate regions of surface and near surface slip which we use to constrain the geometry of surface rupture the inferred geometry is spatially complex with multiple strands the southern third of the rupture zone consists of three subparallel segments extending about 20 km in length in a n45 w direction the central segment is the simplest with a single strand crossing the bullion mountains and a strike of n10 w the northern third of the rupture zone is characterized by multiple splays with directions subparallel to strikes in the southern and central the average strike for the entire rupture is about n30 w the interferograms indicate significant along strike variations in strain which are consistent with variations in the ground based slip measurements using a variable resolution data sampling routine to reduce the computational burden we invert the insar and gps data for the fault geometry and distribution of slip we compare results from assuming an elastic half space and a layered elastic space results from these two elastic models are similar although the layered space model predicts more slip at depth than does the half space model the layered model predicts a maximum coseismic slip of more than 5 m at a depth of 3 to 6 km contrary to preliminary reports the northern part of the hector mine rupture accommodates the maximum slip our model predictions for the surface fault offset and total seismic moment agree with both field mapping results and recent seismic models the inferred shallow slip deficit is enigmatic and may suggest that distributed inelastic yielding occurred in the uppermost few kilometers of the crust during or soon after the earthquake

rupture complexity of the 1994 bolivia and 2013 sea of okhotsk deep earthquakes the physical mechanism of deep earthquakes depth 300 km remains enigmatic partly because their rupture dimensions are difficult to estimate due to their low aftershock productivity and absence of geodetic or surface rupture observations the two largest deep earthquakes the recent great 2013 sea of okhotsk earthquake m 8 3 depth 607 km and the great 1994 bolivia earthquake m 8 3 depth 637 km together provide a unique opportunity to compare their rupture patterns in detail here we extend a travel time sub event location method to perform full teleseismic p waveform inversion this new method allows us to explain the observed broadband records with a set of sub events whose model parameters are robustly constrained without smoothing we find that while the okhotsk event is mostly unilateral rupturing 90 km along strike with a velocity over 4 km s the bolivia earthquake ruptured about half this distance at a slow velocity about 1 5 km s and displayed a major change in rupture direction we explain the observed differences between the two earthquakes as resulting from two fundamentally different faulting mechanisms in slabs with different thermal states phase transformational faulting is inferred to occur inside the metastable olivine wedge within cold slab cores whereas shear melting occurs inside warm slabs once triggered

first observation of interhemispheric asymmetry in the epbs during the st patrick s day geomagnetic storm of 2015 in this work we have studied the characteristics of epbs such as their zonal drift and tilt from the low latitude and dip equatorial region in the indian longitude sector during the main phase of the 17 march 2015 storm all sky airglow imaging observations from tirunelveli 8 7on 77 8oe geographic 1 7on dip latitude and kolhapur 16 7on 74 3oe geographic 11 5on dip latitude are utilized here on 17 march 2015 epbs were observed to drift eastward during 14 30 16 30 ut between 3os and 15on dip latitudes a westward drift presumably under the influence of the disturbance dynamo electric field ddef initially appeared at higher dip latitudes almost 10 hours after the storm onset and subsequently the same was observed at lower dip latitudes the epbs attained a peak westward drift at 17 00 ut followed by a gradual decrease in their speed till 18 30 ut after regaining their westward speed the epbs continued to drift westward till 22 00 ut moreover a latitudinal gradient in the drift motion of the epbs was also observed on this night another interesting observation made from the images obtained from tirunelveli was the presence of a large westward tilt of the epbs the most intriguing finding of this study however was the asymmetry in the tilt of the epbs at conjugate points during the pre midnight hours on 17 march 2015 in this study the possible mechanisms that can explain these observations are discussed in the light of the current understanding of the equatorial electrodynamics and epbs

near field investigations of the landers earthquake sequence april to july 1992 the landers earthquake which had a moment magnitude m w of 7 3 was the largest earthquake to strike the contiguous united states in 40 years this earthquake resulted from the rupture of five major and many minor right lateral faults near the southern end of the eastern california shear zone just north of the san andreas fault its m w 6 1 preshock and m w 6 2 aftershock had their own aftershocks and foreshocks surficial geological observations are consistent with local and far field seismologic observations of the earthquake large surficial offsets as great as 6 meters and a relatively short rupture length 85 kilometers are consistent with seismological calculations of a high stress drop 200 bars which is in turn consistent with an apparently long recurrence interval for these faults