ridge offsets normal faulting and gravity anomalies of slow spreading ridges we develop a model relating mid ocean ridge normal faulting and crustal structure by examining recently available high resolution gravity and multibeam bathymetric data of the mid atlantic ridge and other spreading centers results of the analysis reveal a consistent pattern of positive residual gravity anomalies along the crust paralleling all major atlantic offsets studied especially along the inside corner side of the offsets individual residual gravity anomaly spikes local peaks which have amplitudes of up to 20 mgal and typical across axis spacing of 10 30 km often coincide with individual major fault scarps suggesting crust thinned by normal faulting theoretical calculations indicate that the amplitude and spacing of the observed residual gravity spikes are consistent with the presence of successive ridge parallel low angle faults that originate episodically at inside corners of ridge offset intersections fault scarp heights and the amounts of crustal thinning as inferred from gravity anomalies are consistently larger at inside corners than at outside corners supporting a model in which tectonic extension near ridge offsets is asymmetric with low angle faults occurring preferentially at inside corners these results on spatial variations in seafloor morphology and gravity anomalies further support a three dimensional tectonic faulting model at oceanic spreading centers with three major characteristics 1 low angle faults form preferentially at inside corners where the mantle lithosphere is the strongest and the lithospheric plates are sufficiently decoupled across ridge axis offsets 2 low angle faults decrease in throw toward midpoints of long ridge segments where large low angle faults may not be sustained by a weak lithosphere and 3 the residual gravity peaks are statistically larger at transform than near nontransform offsets indicating that the length of a ridge offset and its tectonic style control the development of low angle faults

localized seismic deformation in the upper mantle revealed by dense seismic arrays seismicity along continental transform faults is usually confined to the upper half of the crust but the newport inglewood fault nif a major fault traversing the los angeles basin is seismically active down to the upper mantle we use seismic array analysis to illuminate the seismogenic root of the nif beneath long beach california and identify seismicity in an actively deforming localized zone penetrating the lithospheric mantle deep earthquakes which are spatially correlated with geochemical evidence of a fluid pathway from the mantle as well as with a sharp vertical offset in the lithosphere asthenosphere boundary exhibit narrow size distribution and weak temporal clustering we attribute these characteristics to a transition from strong to weak interaction regimes in a system of seismic asperities embedded in a ductile fault zone matrix

microearthquakes beneath median valley of mid atlantic ridge near 23 n tomography and tectonics data from a microearthquake experiment in the median valley of the mid atlantic ridge near 23 n in 1982 are used to measure earthquake source parameters to determine the laterally heterogeneous seismic velocity structure across the inner floor and to develop a kinematic tectonic model for this portion of the median valley fifty three microearthquakes occurred over a 10 day period beneath the median valley inner floor and eastern rift mountains twenty of 23 well located inner floor epicenters define a line of activity about 17 km long having a strike of n25 e and located near an along axis depression some 300 400 m deeper than surrounding regions earthquakes with well resolved hypocenters generally have focal depths of 4 8 km beneath the seafloor of both the inner floor and the rift mountains the hypocentral locations are robust with respect to plausible lateral variations in seismic velocity structure composite fault plane solutions for inner floor events indicate normal faulting on planes dipping at angles near 45 normal faulting mechanisms although poorly constrained are also indicated for the rift mountain microearthquakes seismic moments approximate fault dimensions and average stress drops for the largest events recorded are 1019 1020 dyn cm 200 400 m and 1 70 bars respectively a twodimensional tomographic inversion of p wave travel time residuals from microearthquakes and local shots indicates a well resolved lateral heterogeneity in crustal velocity structure across the median valley inner floor p wave velocities at 1 5 km depth within a zone less than 10 km wide beneath the central inner floor are lower by several percent than in surrounding regions the most likely explanation for the low velocities is that the region is the site of the most recent local magmatic injection and remains pervasively fractured as a result of rapid hydrothermal quenching of the newly emplaced crustal column by this view the seismic velocity structure at the ridge axis evolves probably by the sealing of cracks and pores within the first few hundred thousand years of crustal accretion consideration of the detailed sea beam bathymetry in this region of the inner floor the characteristics of large earthquakes that the region has experienced during the past 25 years and the results of the microearthquake and tomography analysis suggests that this section of the median valley has been undergoing continued horizontal extension and block faulting without significant crustal injection of magma for at least the past 104 years

ridge segmentation faulting and crustal thickness in the atlantic ocean the mid atlantic ridge mar between the kane and atlantis fracture zones consists of segments 20 85 km in length1 bull s eye patterns in the mantle bouguer gravity anomaly field2 centred on several segments associated with narrow rift valleys3 have been interpreted as centres of strong mantle upwelling and thick crust here i present a map of normal faults inferred from 5 104 km2of sea beam bathymetry4 along the mar between 28 n and the atlantis fracture zone faults are mapped both on and off ridge using a criterion that distinguishes them from volcanic topography abyssal hill lineations seem to form at the rift valley walls through the growth of the normal faults towards ridge segment ends these faults are more widely spaced with larger throws than those at segment centres and often define the boundaries of the gravity bull s eyes these two different faulting styles which probably reflect changes in lithospheric strength are preserved into the rift mountains large throw faults are highly correlated with the mantle bouguer highs suggesting that amagmatic extension on the large faults contributes to the crustal thinning inferred towards the segment ends

the detection of low magnitude seismic events using array based waveform correlation summary it has long been accepted that occurrences of a known signal are most effectively detected by cross correlating the incoming data stream with a waveform template such matched signal detectors have received very little attention in the field of detection seismology because there are relatively few instances in which the form of an anticipated seismic signal is known a priori repeating events in highly confined geographical regions have been observed to produce very similar waveforms and good signals from events at a given site can be exploited to detect subsequent co located events at lower magnitudes than would be possible using traditional power detectors even greater improvement in signal detectability can be achieved using seismic arrays running correlation coefficients from single sensors can be stacked over an array or network to result in a network correlation coefficient displaying a significant array gain if two events are co located the time separating the corresponding patterns in the wave train as indicated by the cross correlation function is identical for all seismic stations and this property means that the correlation coefficient traces are coherent even when the waveforms are not we illustrate the power of array based waveform correlation using the 1997 august 16 kara sea event the weak event that occurred 4 hr after the main event was barely detected using an sta lta detector on the spits array but is readily detected by signal matching on a single channel the main event was also recorded by the far more distant norsar array but no conventional detection can be made for the second event a clear detection is however made when the correlation coefficient traces are beamformed over all sensors of the array we estimate the reduction in detection threshold of a test signal on a regional seismic array using waveform correlation by scaling down a master signal and immersing it into seismic noise we show that for this case waveform correlation using a single channel detects signals of approximately 0 7 orders of magnitude lower than is possible using an sta lta detector on the array beam waveform matching on the full array provides an additional improvement of approximately 0 4 magnitude units we describe a case study in which small seismic events at the barentsburg coal mine on spitsbergen were detected using the signals from a major rockburst as master waveforms many spurious triggers occurred in this study whereby short sections of signal exhibited coincidental similarity with unrelated incoming wave fronts we demonstrate how such false alarms can almost always be identified and screened out automatically by performing frequency wavenumber analysis upon the set of individual correlation coefficient traces

microearthquake characteristics and crustal vp vs structure at the mid atlantic ridge 35 n we report on the results of a microearthquake experiment conducted at the along axis bathymetric high of the slow spreading ridge segment near 35 n on the mid atlantic ridge mar of a total of 255 microearthquakes recorded during the 43 day experiment 31 were located near the oceanographer transform at the northern end of the segment 79 occurred at the nontransform offset at the southern end of the segment and 145 were at the segment center at the segment center earthquake epicenters lay within the median valley inner floor and formed a 12 km long trend paralleling a steep scarp bounding the western wall of the inner valley focal depths were 3 4 km below the seafloor most 80 of the focal mechanisms for 32 segment center earthquakes are consistent with normal faulting on faults approximately parallel to the axial trend from a joint inversion for hypocenters and p and s wave velocity structures we determined a horizontally averaged v p v s ratio that decreases from 2 9 in the shallowmost 300 m to 1 7 at 2 km depth and we interpret this decrease as indicating a decreasing contribution of thin cracks to fracture porosity with depth the maximum depth of seismicity 4 km is anomalously shallow compared with other mar segments at which microearthquake experiments have been carried out cross axis relief is also anomalously low for this segment s center and on the basis of this and other mar microearthquake experiments there appears to be a correlation between cross axis relief and maximum depth of seismicity from the correlation of cross axis relief and inferred crustal thickness we suggest a relationship between thick crust high crustal temperatures and low cross axis relief in qualitative agreement with thermomechanical models for the depth of the axial valley

roughness at the base of the seismogenic zone contributing factors the cutout depth of microseismic activity in continental fault zones appears to correspond to the onset of greenschist metamorphic conditions at about 300 c it can generally be modeled as the transition from frictional to quasi plastic behavior in quartzofeldspathic crust shear resistance increases with depth through the frictional regime to peak at the transition beneath which it falls off exponentially with increasing temperature larger earthquake ruptures ml 5 5 nucleate around this transition depth where the highest concentrations of strain energy may accumulate varying depth and amplitude of the peak shear resistance along strike induce fluctuations in strain energy concentration at the base of the seismogenic zone factors affecting the depth of the transition include crustal composition geometry and mode of faulting fluid pressure levels in the frictional regime and water content in the quasi plastic regime quasi plastic strain rate and geothermal gradient evaluation of their relative importance is complicated because several are interdependent however compositional change may cause abrupt irregularities in seismogenic depth and peak shear resistance while regional variations in heat flow look to be particularly effective in creating long wavelength heterogeneities in strain energy concentration affecting faulting style

seismic monitoring leveraging existing telecom infrastructure at the sdasa active passive and ambient noise analysis abstract we analyze active and passive seismic data recorded by the stanford distributed acoustic sensing array sdasa located in conduits under the stanford university campus for the active data we used low energy sources betsy gun and sledge hammer and recorded data using both the das array and 98 three component nodes deployed along a 2d line the joint analysis of shot profiles extracted from the two data sets shows that some surface waves and refracted events are consistently recorded by the das array in areas where geophone coupling was suboptimal because of surface obstructions das recordings are more coherent in contrast surface waves are more reliably recorded by the geophones than the das array because of the noisy environment and weak sources neither data set shows clear reflections we demonstrate the repeatability of das recordings of local earthquakes by comparing two weak events magnitude 0 95 and 1 34 with epicenters 100 m apart that occurred only one minute from each other an

investigation of microearthquake activity following an intraplate teleseismic swarm on the west flank of the southern east pacific rise between february 1991 and may 1992 33 intraplate earthquakes having body wave magnitudes between 4 3 and 6 0 were located on the west flank of the southern east pacific rise by the international seismological center seven months after the last teleseismic event we deployed four ocean bottom seismometers at the site of the teleseismic swarm one hundred and ninety two microearthquakes were located using p and s travel times of events recorded by three or more instruments during the 16 day deployment most of the microearthquakes were in a band about 30 km long and 6 km wide between and parallel to seamount chains in addition several events were distributed along a line perpendicular to the main seismicity band and parallel to the ridge axis the focal depths of the microearthquakes range from 1 to 15 km and most are between 5 and 12 km similar to the depth range of the teleseismic events hung and forsyth 1996 the composite p wave polarities indicate that the microearthquakes had a variety of focal mechanisms we developed a new grid search inversion technique that utilizes the p wave polarities and the empirically corrected ratios of p and s wave amplitudes to find the focal mechanisms of individual events within the acceptable travel time and amplitude misfits focal solutions are fairly stable normal faulting is found in the ridge parallel seismicity line the thrust and strike slip faulting in the main seismicity band is distinctly different from the exclusively normal faulting mechanisms of the teleseismic events there is no apparent depth dependence of fault types none of the existing models of the sources of stress ridge push thermoelastic stresses loading by local topographic features caldera collapse and north south extension of the pacific plate provides a satisfactory explanation for both the teleseismic swarm and microearthquakes we propose a new tectonic scenario in this scenario the lithosphere is prestressed by the cooling of the plate magma rising from the deeper mantle induces normal faulting ahead of the dike tips in the lower lithosphere which is already under extensional thermal stress producing the larger teleseismically detected events once the dikes propagate into the lithosphere the region surrounding the dikes behind the tips is compressed by the overpressure of magma depending on the geometry of the dikes the local orientations of the minimum principal stress and the local weaknesses in the lithosphere thrust or strike slip faulting the microearthquakes may occur

the brittle plastic transition and the depth of seismic faulting a simple rheological model of shearing of the lithosphere that has gained wide acceptance is a two layer model with an upper brittle zone in which deformation takes place by frictional sliding on discrete fault surfaces and a lower plastic zone in which deformation takes place by bulk plastic flow the two are separated by an abrupt brittle plastic transition which is assumed to be indicated by the lower limit of seismicity experimental studies however as well as the deformation structures of mylonites indicate that a broad transitional field of semi brittle behavior lies between these extremes this is a field of mixed mode deformation with a strength that can be expected to be considerably higher than that predicted from the extrapolation of high temperature flow laws for quartzofeldspathic rocks the semi brittle field lies between t1 the onset of quartz plasticity at about 300 c and t2 feldspar plasticity at about 450 c a model is presented in which the transition t1 does not correspond to a transition to bulk flow but to a change from unstable velocity weakening friction to stable velocity strengthening friction t1 thus marks the depth limit of earthquake nucleation but large earthquakes can propagate to a greater depth t3 t3 t2 which corresponds to the lower limit of dynamic frictional behavior in the semi brittle field and approximately to the peak in strength the zone between t1 and t3 is one of alternating behavior with flow occurring m the interseismic period and with co seismic dynamic slip occurring during large earthquakes this zone is characterized by mylonites interlaced by pseudotachylytes and other signs of dynamic faulting the transition t1 is also marked by a change in the generation mechanism of fault rocks from abrasive wear above which produces cataclastites to adhesive wear below which is proposed as an important generation mechanism of mylonites in the upper part of the semi brittle field

characterization of hydraulic properties of rocks using probability of fluid induced microearthquakes the use of borehole fluid injections is typical for exploration and development of hydrocarbon or geothermal reservoirs such injections often induce small magnitude earthquakes the nature of processes leading to triggering of such microseismicity is still not completely understood here we consider induced microseismicity using as examples two case studies of geothermal reservoirs in crystalline rocks and one case study of a tight gas sandstone reservoir in all three cases we found that the probability of induced earthquakes occurring is very well described by the relaxation law of pressure perturbation in fluids filling the pore space in rocks this strongly supports the hypothesis of seismicity triggered by pore pressure moreover this opens additional possibilities of using passive seismic monitoring to characterize hydraulic properties of rocks on the reservoir scale with high precision

tomographic imaging of the shallow crustal structure of the east pacific rise at 9 30 n compressional wave travel times from a seismic tomography experiment at 9 30 n on the east pacific rise are analyzed by a new tomographic method to determine the three dimensional seismic velocity structure of the upper 2 5 km of oceanic crust within a 20 18 km2 area centered on the rise axis the data comprise the travel times and associated uncertainties of 1459 compressional waves that have propagated above the axial magma chamber a careful analysis of source and receiver parameters in conjunction with an automated method of picking p wave onsets and assigning uncertainties constrains the prior uncertainty in the data to 5 to 20 ms the new tomographic method employs graph theory to estimate ray paths and travel times through strongly heterogeneous and densely parameterized seismic velocity models the nonlinear inverse method uses a jumping strategy to minimize a functional that includes the penalty function horizontal and vertical smoothing constraints and prior model assumptions all constraints applied to model perturbations are normalized to remove bias we use the tomographic method to reject the null hypothesis that the axial seismic structure is two dimensional three dimensional models reveal a seismic structure that correlates well with cross and along axis variations in seafloor morphology the location of the axial summit caldera and the distribution of seafloor hydrothermal activity the along axis segmentation of the seismic structure above the axial magma chamber is consistent with the hypothesis that mantle derived melt is preferentially injected midway along a locally linear segment of the rise and that the architecture of the crustal section is characterized by an en echelon series of elongate axial volcanoes approximately 10 km in length the seismic data are compatible with a 300 to 500 m thick thermal anomaly above a midcrustal melt lens such an interpretation suggests that hydrothermal fluids may not have penetrated this region in the last 103 years asymmetries in the seismic structure across the rise support the inferences that the thickness of seismic layer 2 and the average midcrustal temperature increase to the west of the rise axis these anomalies may be the result of off axis magmatism alternatively the asymmetric thermal anomaly may be the consequence of differences in the depth extent of hydrothermal cooling

imaging widespread seismicity at midlower crustal depths beneath long beach ca with a dense seismic array evidence for a depth dependent earthquake size distribution we use a dense seismic array composed of 5200 vertical geophones to monitor microseismicity in long beach california poor signal to noise ratio due to anthropogenic activity is mitigated via downward continuation of the recorded wavefield the downward continued data are continuously back projected to search for coherent arrivals from sources beneath the array which reveals numerous previously undetected events the spatial distribution of seismicity is uncorrelated with the mapped fault traces or with activity in the nearby oil fields many events are located at depths larger than 20 km well below the commonly accepted seismogenic depth for that area the seismicity exhibits temporal clustering consistent with omori s law and its size distribution obeys the gutenberg richter relation above 20 km but falls off exponentially at larger depths the dense array allows detection of earthquakes two magnitude units smaller than the permanent seismic network in the area because the event size distribution above 20 km depth obeys a power law whose exponent is near one this improvement yields a hundred fold decrease in the time needed for effective characterization of seismicity in long beach

lithospheric flexural strength and effective elastic thicknesses of the eastern anatolia turkey and surrounding region abstract the lithospheric structure of eastern anatolia and the surrounding region including the northern part of the arabian platform is investigated via the analysis and modeling of bouguer anomalies from the earth gravitational model egm08 the effective elastic thickness of the lithosphere eet that corresponds to the mechanical cores of the crust and lithospheric mantle is determined from the spectral coherence between bouguer anomalies and surface elevation data its average value is 18 7 km from the logarithmic amplitude spectra of bouguer anomalies average depths of the lithosphere asthenosphere boundary lab moho conrad and basement in the study area are constrained at 84 km 39 km 16 km and 7 km respectively the geometries of the lab and moho are then estimated using the parker oldenburg inversion algorithm we also present a lithospheric strength map obtained from the spatial variations of eet determined by yield stress envelopes yse the eet varies in the range of 12 23 km which is in good agreement with the average value obtained from spectral analysis low eet values are interpreted as resulting from thermal and flexural lithospheric weakening according to the lithospheric strength of the eastern anatolian region the rheology model consists of a strong but brittle upper crust a weak and ductile lower crust and a weak lower part of the lithosphere on the other hand lithosphere strength corresponds to weak and ductile lower crust a strong upper crust and a strong uppermost lithospheric mantle for the northern part of the arabian platform

field testing of fiber optic distributed acoustic sensing das for subsurface seismic monitoring distributed acoustic sensing das is a relatively recent development in the use of fiber optic cable for measurement of ground motion discrete fiber optic sensors typically using a bragg diffraction grating have been in research and development and field testing for more than 15 years with geophysical applications at least 12 years old bostick 2000 and summary in keul et al 2005 however developments in recent years have sought to remove the need for point sensors by using the fiber cable itself as a sensor mestayer et al 2011 miller et al 2012

field trial of seismic recording using distributed acoustic sensing with broadside sensitive fibre optic cables distributed acoustic sensing is an emerging technology using fibre optic cables to detect acoustic disturbances such as flow noise and seismic signals the technology has been applied successfully in hydraulic fracture monitoring and vertical seismic profiling one of the limitations of distributed acoustic sensing for seismic recording is that the conventional straight fibres do not have broadside sensitivity and therefore cannot be used in configurations where the raypaths are essentially orthogonal to the fibre optic cable such as seismic reflection methods from the surface the helically wound cable was designed to have broadside sensitivity in this paper a field trial is described to validate in a qualitative sense the theoretically predicted angle dependent response of a helically wound cable p waves were measured with a helically wound cable as a function of the angle of incidence in a shallow horizontal borehole and compared with measurements with a co located streamer the results show a similar behaviour as a function of the angle of incidence as the theory this demonstrates the possibility of using distributed acoustic sensing with a helically wound cable as a seismic detection system with a horizontal cable near the surface the helically wound cable does not have any active parts and can be made as a slim cable with a diameter of a few centimetres for that reason distributed acoustic sensing with a helically wound cable is a potential low cost option for permanent seismic monitoring on land

seismicity around parkfield correlates with static shear stress changes following the 2003 mw6 5 san simeon earthquake 1 earthquakes trigger other earthquakes but the physical mechanism of the triggering is currently debated most studies of earthquake triggering rely on earthquakes listed in catalogs which are known to be incomplete around the origin times of large earthquakes and therefore missing potentially triggered events here we apply a waveform matched filter technique to systematically detect earthquakes along the parkfield section of the san andreas fault from 46 days before to 31 days after the nearby 2003 mw6 5 san simeon earthquake after removing all possible false detections we identify 8 times more earthquakes than in the northern california seismic network catalog the newly identified events along the creeping section of the san andreas fault show a statistically significant decrease following the san simeon main shock which correlates well with the negative static stress changes i e stress shadow cast by the main shock in comparison the seismicity rate around parkfield increased moderately where the static stress changes are positive the seismicity rate changes correlate well with the static shear stress changes induced by the san simeon main shock suggesting a low friction in the seismogenic zone along the parkfield section of the san andreas fault

geothermal production and reduced seismicity correlation and proposed mechanism abstract at brady hot springs a geothermal field in nevada heated fluids have been extracted cooled and re injected to produce electrical power since 1992 analysis of daily pumping records and catalogs of microseismicity between 2010 and 2015 indicates a statistically significant correlation between days when the daily volume of production was at or above its long term average rate and days when no seismic event was detected conversely shutdowns in pumping for plant maintenance correlate with increased microseismicity we hypothesize that the effective stress in the subsurface has adapted to the long term normal operations deep extraction at the site under this hypothesis extraction of fluids inhibits fault slip by increasing the effective stress on faults in contrast brief pumping cessations represent times when effective stress is decreased below its long term average increasing the likelihood of microseismicity

crustal earthquake instability in relation to the depth variation of frictional slip properties recent stability studies using constitutive relations of the type found by dieterich ruina and others to describe frictional slip of rocks in the laboratory have provided a new explanation of the depth cutoff of shallow crustal earthquakes the class of friction laws discussed has the property that the sliding stress depends on normal stress temperature slip rate and slip history for sliding at a fixed slip rate v and fixed environment e g normal stress temperature etc the shear strength evolves toward a steady state value ss v stability analysis show that for d ss v dv 0 velocity strengthening implies stable steady state sliding at least for a broad class of constitutive laws more fully described in the paper experiments by dieterich and by tullis and weeks on westerly granite with mature sliding surfaces indicate that d ss v dv is negative at room temperature whereas higher temperature experiments by stesky show that d ss v dv becomes positive above approximately 300 c therefore in the case of the earth where temperature increases with depth the above observations seem to suggest that the depth cutoff of crustal earthquake activity can be understood in terms of the variation of the frictional response with depth from a regime with d ss v dv 0 this is not inconsistent with but rather refines the suggestion by sibson and others that the depth cutoff is due to a transition from brittle friction to ductile flow further our results show definitively that the shallow depth confinement of seismicity is compatible with a model in which deformation is localized to a fault zone extending well below the seismogenic depth the depth confinement does not require a model showing transition to a zone of broadly distributed creep flow beneath the seismogenic zone following mavko a two dimensional quasi static strike slip fault model is analyzed but using different numerical procedures and a depth variation of frictional properties based on the laboratory data mentioned above and the lachenbruch sass depth variation of temperature for the san andreas fault the resulting predictions of such features as the confinement of crustal earthquakes to shallow depths the development of locked patches the recurrence time for the seismic cycle the seismic stress drop and displacement etc are generally in agreement with the observed characteristics of large scale strike slip earthquakes along the san andreas fault calculations based on the laboratory data predict for example that stick slip nucleates around 5 7 km that large seismic motion occurs around the nucleation depth and above but diminishes gradually to zero at 13 15 km depths that rapid postseismic creep occurs over another 3 4 km depth and that at greater depths steady slip consistent with the average plate velocity is only modestly perturbed by the earthquakes occurring above the most uncertain parameter is the slip weakening distance l for evolution of fault surface state we illustrate how features of the predicted earthquake cycles vary with l over the range for which calculations are feasible

segmentation and crustal structure of the western mid atlantic ridge flank 25 25 27 10 n and 0 29 m y we conducted a detailed geological geophysical survey of the west flank of the mid atlantic ridge between 25 25 n and 27 10 n and from the ridge axis out to 29 ma crust acquiring hydrosweep multibeam bathymetry hawaii mr1 sidescan sonar imagery gravity magnetics and single channel seismic reflection profiles the survey covered all or part of nine spreading segments bounded by mostly nontransform right stepping discontinuities which are subparallel to flow lines but which migrated independently of one another some discontinuities alternated between small right and left stepping offsets or exhibited zero offset for up to 3 4 m y despite these changes the spreading segments have been long lived and extend 20 m y or more across isochrons a large shift 9 in relative plate motion about 24 22 ma caused significant changes in segmentation pattern the nature of this plate boundary response together with the persistence of segments through periods of zero offset at their bounding discontinuities suggest that the position and longevity of segments are controlled primarily by the subaxial position of buoyant mantle diapirs or focused zones of rising melt within segments there are distinct differences in seafloor depth morphology residual mantle bouguer gravity anomaly and apparent crustal thickness between inside corner and outside corner crust this demands fundamentally asymmetric crustal accretion and extension across the ridge axis which we attribute to low angle detachment faulting near segment ends cyclic variations in residual gravity over the crossisochron run of segments also suggest crustal thickness changes of at least 1 2 km every 2 3 m y these are interpreted to be caused by episodes of magmatic versus relatively amagmatic extension controlled by retention and quasiperiodic release of melt from the upwelling mantle detachment faulting appears to be especially effective in exhuming lower crust to upper mantle at inside corners during relatively amagmatic episodes creating crustal domes analogous to turtleback metamorphic core complexes that are formed by low angle detachment faulting in subaerial extensional environments

distributed acoustic sensing for reservoir monitoring with vsp 3d vsp has long been viewed as conceptually attractive for illuminating targets under complex overburden both for exploration purposes and for time lapse monitoring of reservoirs however the widespread use of 3d vsp has been hindered by the cost and risk of deploying geophones in a borehole and by the limited availability of accessible wells these hurdles are largely removed when acquiring downhole seismic with a new measurement called distributed acoustic sensing das

an improved method for determining the regional stress tensor using earthquake focal mechanism data application to the san fernando earthquake sequence the orientations of fault planes and slip directions indicated by a population of earthquake focal mechanisms can be used to determine best fit regional principal stress directions and r 2 1 3 1 a measure of relative stress magnitudes under the assumption of uniform stress in the source region this analysis allows for the possibility that failure occurs on preexisting zones of weakness of any orientation in the inversion we perform a grid search of stress models to find the one which requires the smallest total rotation of all the fault planes that is needed to match the observed and predicted slip directions the method allows for errors in orientations of both the fault planes and slip directions we have an objective means for identifying the more likely of the two possible fault planes from each focal mechanism relative to a given stress model thus we do not face the problem of ambiguity of nodal planes which plagues other analyses of this kind by using a grid search of stress models rather than a linearization scheme we are able to perform a realistic error analysis and thus establish confidence limits for the preferred regional stresses the method can be used to investigate possible stress inhomogeneities during earthquake sequences by analyzing subsets of the data population the technique has been applied to 76 events from the san fernando earthquake sequence for which we have found best fit stresses plunge and azimuth 1 7 187 2 27 281 3 62 84 and r 0 65 the average misfit between this stress model and all the data is about 8 and all but eight of the aftershocks have misfits of less than 20 these values are considerably less than the uncertainty of the focal mechanism determinations therefore significant stress inhomogeneities are not required by the data our analysis does not support the suggestion of a change in stresses during the aftershock sequence as proposed by others on the basis of an apparent change in focal mechanisms

fiber optic network observations of earthquake wavefields author s lindsey nj martin er dreger ds freifeld b cole s james sr biondi bl ajo franklin jb abstract 2017 american geophysical union all rights reserved our understanding of subsurface processes suffers from a profound observation bias seismometers are sparse and clustered on continents a new seismic recording approach distributed acoustic sensing das transforms telecommunication fiber optic cables into sensor arrays enabling meter scale recording over tens of kilometers of linear fiber length we analyze cataloged earthquake observations from three das arrays with different horizontal geometries to demonstrate some possibilities using this technology in fairbanks alaska we find that stacking ground motion records along 20nm of fiber yield a waveform that shows a high degree of correlation in amplitude and phase with a colocated inertial seismometer record at 0 8 1 6nhz using an l shaped das array in northern california we record the nearly vertically incident arrival of an earthquake from the geysers geothermal field and estimate its backazimuth and slowness via beamforming for different phases of the seismic wavefield lastly we install a fiber in existing telecommunications conduits below stanford university and show that little cable to soil coupling is required for teleseismic p and s phase arrival detection

evidence from gravity data for focused magmatic accretion along the mid atlantic ridge gravity data from the mid atlantic ridge between latitudes 27 50 n and 30 40 n show that the accretion of magma at the ridge is focused at discrete centres along the spreading axis large positive gravity anomalies equivalent to reductions of almost 50 in crustal thickness are observed over non transform discontinuities bounding spreading segments

pushing the limit of earthquake detection with distributed acoustic sensing and template matching a case study at the brady geothermal field template matching has been widely applied in the detection of earthquakes and other seismic events due to its power in detecting weak signals recent studies using synthetics have shown that application of template matching to large n arrays can potentially detect earthquakes substantially below the noise level here we apply template matching to the distributed acoustic sensing das data recorded in the brady hot springs geothermal field nevada using 5 catalogued events we detect 116 events and find 68 of them well below the noise level we confirm 112 events are true earthquakes by examining the patterns of their sensor to sensor cross correlation sections this demonstrates that the combination of das and template matching has capability to detect microseismicity below the noise level which is unusual for conventional seismic arrays and methods with the updated catalogue we observe a surge of earthquakes during the shutdown of a geothermal power plant nearby in addition the rapid increases in the downhole pressure record coincide with intense swarms of earthquakes these observations show a strong correlation between the seismicity frequencies and the downhole pressure changes finally we investigate several factors that may affect the detection performance and compare different strategies for spatial down sampling in order to provide helpful insights for future large n design and data processing

poisson s ratio structure of young oceanic crust we have applied full waveform inversion to wide aperture seismic reflection data from the southern east pacific rise near 14 s the data contain clear compressional wave and doubly converted shear wave arrivals that provide good constraints on the p and s wave velocities vp vs and hence poisson s ratio and seismic attenuation qp qs structure of seismic layer 2 layer 2a is highly attenuating qp 18 30 and qs 8 15 and layer 2b is moderately attenuating qp 30 50 and qs 20 25 our results show high at the seafloor and in layer 2a 0 48 across the top of the 2a b transition the rapid increase in vp is accompanied by a sharp drop in to 0 25 within just 200 m of the seafloor we perform simple calculations to gain an insight into the porosity and crack distribution with depth these calculations suggest that porosity is in excess of 30 in layer 2a but reduces to 6 7 at the top of the 2a b transition and to about 5 at a depth of 600 m below seafloor within layer 2b our results suggest that there is an increase in the average aspect ratio with depth across the 2a b transition the most likely explanation is that numerous thin cracks either mechanically close or are infilled at depth our results show there to be an abrupt change in the pore structure across the 2a b transition which is consistent with a lithologic transition from extrusives to dykes but is equally consistent with a transition mechanical or hydrothermal within the extrusive pile

models of ocean ridge lithospheric deformation dependence on crustal thickness spreading rate and segmentation we use three dimensional 3 d temperature and rheology models to investigate the effects of crustal thickness and ridge segmentation on mid ocean ridge lithospheric structure we find that crustal thickness variations associated with focused magma accretion at a slow spreading ridge segment can cause significant along axis gradients in crustal temperature and a pita pocket shaped weak zone in the lower crust that decouples brittle upper crust from upper mantle in contrast fast spreading ridge segments with little crustal thickness variation are found to be uniformly weak along axis the overthickened crust produced by ridge hotspot interaction alters the heat balance between magma emplacement and hydrothermal cooling creating an extremely weak lithosphere in a steady state model we apply a simple two dimensional 2 d cyclic faulting model to across axis sections of temperature and rheological structure slow spreading segments are predicted to have relatively large along axis variations in fault height and spacing in contrast fast spreading and hotspot affected segments are predicted to have much smaller variations in faulting styles except in the immediate vicinity of a major transform fault the presence or lack of a steady state axial rift valley is also predicted to depend on crustal thickness spreading rate and the ability of rift bounding normal faults to propagate along axis for fast spreading and hotspot affected segments the sizes of axial rift valleys are predicted to be small and may not be distinguishable from those of neovolcanic or magma chamber supported isostatic features this is in contrast to most slow spreading segments where axial rift valleys are predicted to be a dominant morphological feature for a slow spreading segment of great length however the axial rift valley size is predicted to diminish toward the segment center because large faults formed at segment distal ends cannot propagate through a large decoupling zone created by locally thickened crust at the segment center

rift valley no rift valley transition at mid ocean ridges a simple mechanical model for the axial topographic and gravity variations with spreading rate is developed the model emphasizes the rapid transition from one mechanism to another with changing spreading rate our model has a brittle layer over a ductile layer which are being pulled by forces away from the ridge axis there exists a potential failure zone of width lf where the stresses within the brittle plate exceeds its yield strength if the yield strength of the brittle plate is nearly constant this width is largely determined by the strain rate and the viscosity of the strong mantle the high ductility of oceanic crust at high temperatures relative to that of mantle at the same temperatures leads to the concept of a decoupling region a critical feature in this model the size of the decoupling region ld is determined by the thickness of the oceanic crust and temperatures within the crust if the decoupling region is small ld lf as shown to be the case for fast spreading ridges then each of the two layers responds separately to the horizontal stretching the brittle plate will be broken only at the axis and any buoyant force of the region would lift it up to form the rise crest high as a result of local isostasy the changing size of the decoupling region gives rise to the transition from atlantic type axial topography to pacific type as well as the transition from a high amplitude highly variable gravity signature to a low amplitude very uniform gravity signature parameters can be chosen so that such a transition occurs at a half rate of 35 mm yr which is the critical half rate suggested by both topographic and gravity data

the seismic traffic footprint tracking trains aircraft and cars seismically although naturally occurring vibrations have proven useful to probe the subsurface the vibrations caused by traffic have not been explored much such data however are less sensitive to weather and low visibility compared to some common out of road traffic sensing systems we study traffic generated seismic noise measured by an array of 5200 geophones that covered a 7 10 km area in long beach california usa with a receiver spacing of 100 m this allows us to look into urban vibrations below the resolution of a typical city block the spatiotemporal structure of the anthropogenic seismic noise intensity reveals the blue line metro train activity departing and landing aircraft in long beach airport and their acceleration and gives clues about traffic movement along the i 405 highway at night as low cost stand alone seismic sensors are becoming more common these findings indicate that seismic data may be useful for traffic monitoring

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

postrifting seismicity and a model for the 1993 diking event on the coaxial segment juan de fuca ridge microearthquake data from two deployments of ocean bottom seismographs made shortly after the june july 1993 rifting episode on the coaxial segment of the juan de fuca ridge constrain the propagation path of the magmatic dike and provide insight into the effect of the dike injection on the axial stress field the first deployment consisting of five instruments in a small array aperture of 2 km at the eruption flow site 46 30 n was made 3 months after the episode the second deployment consisting of two larger arrays apertures of 9 km of five instruments each one at the eruption site and one at a site of vigorous venting of bacterial flocculent material floe site 46 18 n was made 11 months after the eruption epicenters from nine posteruption earthquakes near the floe site at the south end of the segment are confined to a narrow zone on the rift axis epicenters from 61 earthquakes observed near the flow site are located throughout the rift valley and include an event cluster just south of the observed lava flows we combine these posteruption earthquakes with t wave data recorded by sosus hydrophones during the rifting episode to develop a model for the diking event in this model magma propagated at depth along the axial rift zone until reaching the eruption site near 46 31 n during the next few days the dike swelled as magma accumulated behind the arrested tip elevating local stress levels and generating a swarm of seismic activity about 3 days after the magma reached the north end of the segment the top of the dike penetrated the seafloor surface resulting in eruptions along the rift zone at the flow site

age variations of oceanic crust poisson s ratio inversion and a porosity evolution model porosity in the oceanic crust is one of the most important factors influencing measured seismic velocities porosity is particularly important in the uppermost young crust where rapid variations in velocities with depth and crustal age are observed knowledge of the concentration and aspect ratios of inferred crack populations can be improved considerably if estimates of poisson s ratio are available from observations of compressional and shear seismic velocities p and s in this paper i present a joint seismic waveform inversion for p and s velocities are found while maximizing or minimizing poisson s ratio using a hypothesis testing mechanism i apply this method to ocean bottom hydrophone data in 140 ma atlantic crust the resulting solution corridor agrees with laboratory measurements without the low poisson s ratio anomalies at depths of 0 8 1 5 km found by spudich and orcutt 1980 and au and clowes 1984 on younger 15 ma pacific crust compiling other published p and s solutions an age dependent pattern emerges none of the solutions for crust older than 60 ma display the poisson s ratio anomaly i propose a simple crustal evolution model using thin and thick cracks to explain these observations thin cracks preferentially close at shallow depths in the crust producing the localized poisson s ratio anomaly sealing of all cracks by hydrothermal deposits as the crust ages restores the seismic velocities to consistency with laboratory measurements this model is consistent with similar models of crack populations and their evolution from shallow measurements

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

the potential of das in teleseismic studies insights from the goldstone experiment author s yu c zhan z lindsey nj ajo franklin jb robertson m abstract 2019 american geophysical union all rights reserved distributed acoustic sensing das is a recently developed technique that has demonstrated its utility in the oil and gas industry here we demonstrate the potential of das in teleseismic studies using the goldstone optical fiber seismic experiment in goldstone california by analyzing teleseismic waveforms from the 10 january 2018 m7 5 honduras earthquake recorded on 5 000 das channels and the nearby broadband station gsc we first compute receiver functions for das channels using the vertical component gsc velocity as an approximation for the incident source wavelet the moho p to s conversions are clearly visible on das receiver functions we then derive meter scale arrival time measurements along the entire 20 km long array we are also able to measure path averaged rayleigh wave group velocity and local rayleigh wave phase velocity the latter however has large uncertainties our study suggests that das will likely play an important role in many fields of passive seismology in the near future