stuck in the mud earthquake nucleation and propagation through accretionary forearcs 1 subduction zone earthquakes can propagate to the surface causing large seafloor displacements resulting in tsunamis this requires the earthquake to rupture through clay rich sediments of the accretionary wedge which are largely aseismic as found previously the frictional properties of a range of wet clays at low slip velocity are velocity strengthening thus inhibiting earthquake nucleation however at high slip velocity the same materials weaken almost immediately resulting in a negligible critical slip weakening distance and fracture energy we interpret this behaviour as rapid thermal pressurization of the pore fluid within the clay gouge the lack of fracture energy can explain how a large rupture propagating from depth might not be arrested by clay rich velocity strengthening sediments as is commonly seen the results suggest that generally earthquakes may be difficult to nucleate on mature faults dominated by clay but the propagation of earthquakes through these zones is energetically very favourable

testing the ability of surface arrays to monitor microseismic activity recently there has been much interest in the use of data from surface arrays in conjunction with migration based processing methods for passive seismic monitoring in this study we use an example of this kind of data recorded whilst 18 perforation shots with a variety of positions and propellant amounts were detonated in the subsurface as the perforation shots provide signals with known source positions and origin times the analysis of these data is an invaluable opportunity to test the accuracy and ability of surface arrays to detect and locate seismic sources in the subsurface in all but one case the signals from the perforation shots are not visible in the raw or preprocessed data however clear source images are produced for 12 of the perforation shots showing that arrays of surface sensors are capable of imaging microseismic events even when the signals are not visible in individual traces we find that point source locations are within typically 45 m laterally of the true shot location however the depths are less well constrained 150 m we test the sensitivity of our imaging method to the signal to noise ratio in the data using signals embedded in realistic noise we find that the position of the imaged shot location is quite insensitive to the level of added noise the primary effect of increased noise being to defocus the source image given the migration approach the array geometry and the nature of coherent noise during the experiment signals embedded in noise with ratios 0 1 can be used to successfully image events furthermore comparison of results from data and synthetic signals embedded in noise shows that in this case prestack corrections of traveltimes to account for near surface structure will not enhance event detectability although the perforation shots have a largely isotropic radiation pattern the results presented here show the potential for the use of surface sensors in microseismic monitoring as a viable alternative to classical downhole methods

seismic arrival enhancement through the use of noise whitening abstract a constant feature in seismic data noise is particularly troublesome for passive seismic monitoring where noise commonly masks microseismic events we propose a statistics driven noise suppression technique that whitens the noise through the calculation and removal of the noise s covariance noise whitening is shown to reduce the noise energy by a factor of 3 5 resulting in microseismic events being observed and imaged at lower signal to noise ratios than originally possible whilst having negligible effect on the seismic wavelet the procedure is shown to be highly resistant to most changes in the noise properties and has the flexibility of being used as a stand alone technique or as a first step before standard random noise attenuation methods

3 d rotation of double couple earthquake sources summary we discuss 3 d rotations by which one double couple earthquake source can be rotated into another arbitrary double couple due to the symmetry of double couple sources there are four such rotations an algorithm is obtained in analytical form which is also available as a computer program solving the inverse problem of 3 d rotation of double couple earthquake sources i e for each pair of focal mechanisms or seismic moment tensor solutions the program finds all four rotations which rotate one mechanism into another this algorithm may be used in a wide variety of studies of stress field causing earthquakes investigations of the relationship between the focal mechanisms and the tectonic features of a seismogenic region etc the same inversion algorithm can be used to study the 3 d rotation of any symmetric second rank tensor such as the stress or strain tensor

a microphysical model for fault gouge friction applied to subduction megathrusts a microphysical model is developed for the steady state frictional behavior of illite quartz fault gouge and applied to subduction megathrust p t conditions the model assumes a foliated phyllosilicate supported microstructure which shears by rate independent frictional slip on the aligned phyllosilicates plus thermally activated deformation of the intervening quartz clasts at low slip rates or high temperatures the deformation of the clasts is easy accommodating slip on the foliation without dilatation with increasing velocity or decreasing temperature the shear of the clasts becomes more difficult increasing bulk shear strength until slip is activated on inclined portions of the phyllosilicate foliation where it anastomoses around the clasts slip at these sites leads to dilation involving clast matrix debonding balanced at steady state by compaction through thermally activated clast deformation model predictions taking pressure solution as the thermally activated mechanism show three regimes of velocity dependent frictional behavior at temperatures in the range of 200 500 c with velocity weakening occurring at 300 400 c in broad agreement with previous experiments on illite quartz gouge effects of slip rate normal stress and quartz fraction predicted by the model also resemble those seen experimentally extrapolation of the model to earthquake nucleation slip rates successfully predicts the onset of velocity weakening behavior at the updip seismogenic limit on subduction megathrusts the model further implies that the onset of seismogenesis is controlled by the thermally activated initiation of fault rock compaction through pressure solution of quartz which counteracts dilatation due to slip on the fault rock foliation

the peak frequency of direct waves for microseismic events abstractdirect seismic waves p or s waves are used to locate and further characterize microseismic events the resolution of information obtained from direct waves depends on the peak frequencies of the waveforms the peak frequency results from combination of the source propagation and the receiver effects for frequencies below the corner frequency propagation effects control the peak frequency in observed seismograms of microseismic events the frequency dependence of direct body waves can be modeled by attenuation specifically the global attenuation factor this model is consistent with observed data along surface profiles explaining the difference between the peak frequencies of p and s waves in addition the model is consistent with the peak frequencies observed on downhole monitoring arrays this can be used to invert effective attenuation providing additional unique measurement from microseismic events the corner frequency can be estimated from the average stress drop and analytical sour

stability of source mechanisms inverted from p wave amplitude microseismic monitoring data acquired at the surface we study the stability of source mechanisms inverted from data acquired at surface and near surface monitoring arrays the study is focused on p wave data acquired on vertical components as this is the most common type of acquisition we apply ray modelling on three models a fully homogeneous isotropic model a laterally homogeneous isotropic model and a laterally homogeneous anisotropic model to simulate three commonly used models in inversion we use geometries of real arrays one consisting in surface receivers and one consisting in buried geophones at the nearsurface stability was tested for two of the frequently observed source mechanisms strike slip and dip slip and was evaluated by comparing the parameters of correct and inverted mechanisms we assume these double couple source mechanisms and use quantitatively the inversion allowing non double couple components to measure stability of the inversion to test the robustness we inverted synthetic amplitudes computed for a laterally homogeneous isotropic model and contaminated with noise using a fully homogeneous model in the inversion analogously amplitudes computed in a laterally homogeneous anisotropic model were inverted in all three models we show that a star like surface acquisition array provides very stable inversion up to a very high level of noise in data furthermore we reveal that strike slip inversion is more stable than dip slip inversion for the receiver geometries considered here we show that noise and an incorrect velocity model may result in narrow bands of source mechanisms in hudson s plots

frictional melting of gabbro under extreme experimental conditions of normal stress acceleration and sliding velocity 1 with the advent of high velocity shear apparatus several experimental studies have been performed in recent years improving our understanding of the evolution of fault strength during seismic slip however these experiments were conducted under relatively low normal stress 20 mpa and using small cylindrical samples where a large gradient in slip velocity exists across the sliding surface given the above limitations the extrapolation of these experimental results to natural conditions is not trivial here we present results from an experimental study on gabbroic rocks using a newly developed rotary shear apparatus capable of reaching higher normal stress up to 50 mpa on ring shaped samples 30 50 mm internal external diameter and allowing precise control of the imposed slip velocity function the results confirm that steady state shear stress during the melt lubricated phase of the experiment depends on normal stress in the form of a power law equation as predicted by theoretical models however the exponent appears closer to 0 5 contrary to the theoretical prediction of 0 25 we observe no systematic dependence of shear stress on acceleration but increasing deceleration drastically decreases the recovery of friction during final slip we find that the slip weakening distance decreases inversely with increasing normal stress in agreement with theoretical considerations and decreases with increasing slip rate extrapolation of the slip weakening distance to natural conditions predicts a slip velocity for ancient seismic events of 0 3 1 m s when compared with field estimates these values compare well with seismological estimates

resolution of non double couple mechanisms simulation of hypocenter mislocation and velocity structure mismodeling mechanisms of earthquakes and induced seismic events are frequently found using the moment tensor description rather than the traditional double couple dc as it allows for nonshear source phenomena however non dc source components are sensitive to event mislocations and inexact velocity models of the crust inaccuracies such as these can generate spurious non dc components in the mechanism which should be taken into account during the interpretation we perform a synthetic case study simulating seismic observations at soultz sous forets alsace hot dry rock hdr site synthetic p and s amplitudes for several shear tensile source models are inverted assuming hypocenter mislocation and velocity structure mismodeling in several types of station coverage satisfactory reconstruction of the source mechanism is achieved except for rather extreme model simplification and extremely poor station coverage thus our results suggest that non dc mechanisms can be successfully resolved in local studies with reasonable station configuration when errors in event location and velocity profile are realistic

on the mechanical role of smectite in subduction zones smectites a group of hydrated phyllosilicate minerals are often a main component of sediments entering a subduction zone they are common in hemipelagic sediments and consist of an authigenetic component that arises from the alteration of volcanic ash intercalated in the sediment and a detrital component smectite rich strata are structurally weak retain water and are highly porous a review of sediment lithology in subduction zones around the world suggests that decollement zones may preferentially form in smectite rich horizons one consequence of this observation is that the assignment of high frictional resistance to materials in a decollement may be unwarranted at deeper structural levels smectite is transformed to illite a mechanically stronger mineral estimates of the depth of this transition coincide with the depth at which earthquakes are first detected along plate interfaces i speculate that the smectite illite transition in subduction zones may be an important component of sediment consolidation processes that trigger interplate seismicity

comparison of migration based location and detection methods for microseismic events microseismic monitoring in the oil and gas industry commonly uses migration based methods to locate very weak microseismic events the objective of this study is to compare the most popular migration based methods on a synthetic dataset that simulates a strike slip source mechanism event with a low signal to noise ratio recorded by surface receivers vertical components the results show the significance of accounting for the known source mechanism in the event detection and location procedures for detection and location without such a correction the ability to detect weak events is reduced we show both numerically and theoretically that neglecting the source mechanism by using only absolute values of the amplitudes reduces noise suppression during stacking and consequently limits the possibility to retrieve weak microseismic events on the other hand even a simple correction to the data polarization used with otherwise ineffective methods can significantly improve detections and locations a simple stacking of the data with a polarization correction provided clear event detection and location but even better results were obtained for those data combined with methods that are based on semblance and cross correlation

joint location and source mechanism inversion of microseismic events benchmarking on seismicity induced by hydraulic fracturing summary seismic monitoring can greatly benefit from imaging events with a low signal to noise ratio snr as the number of the events with a low signal grows exponentially one way to detect weaker events is improvement of a snr by migration type stacking of waveforms from multiple stations we have developed a new method of location of seismic events that involves stackingofseismicphasesandamplitudesalongdiffractiontraveltimecurvestosuppressnoise and detect seismic events with a snr lower than that on individual receivers the stacking includes polarity correction based on a simultaneous seismic moment tensor inversion and detection algorithm on the stack function we applied this method to locate microseismicity induced by hydraulic fracturing first we calibrated the velocity model by locating perforation shots at known locations then we processed 3 d of data from microseismic monitoring of shale stimulation and benchmarked migration type locations of the largest events that were manually located the detected and located events induced by hydraulic fracturing in this case study are mostly shear events forming narrow bands along the maximum horizontal stress direction approximately 100 m above the injection intervals the proposed technique is fully automated and feasible for real time seismic monitoring

structure and composition of the plate boundary slip zone for the 2011 tohoku oki earthquake the mechanics of great subduction earthquakes are influenced by the frictional properties structure and composition of the plate boundary fault we present observations of the structure and composition of the shallow source fault of the 2011 tohoku oki earthquake and tsunami from boreholes drilled by the integrated ocean drilling program expedition 343 and 343t logging while drilling and core sample observations show a single major plate boundary fault accommodated the large slip of the tohoku oki earthquake rupture as well as nearly all the cumulative interplate motion at the drill site the localization of deformation onto a limited thickness less than 5 meters of pelagic clay is the defining characteristic of the shallow earthquake fault suggesting that the pelagic clay may be a regionally important control on tsunamigenic earthquakes

subseismic to seismic slip in smectite clay nanofoliation smectite clays are the main constituent of slipping zones found in subduction zone faults at shallow depth e g 1 km depth in the japan trench and in the decollements of large landslides e g 1963 landslide vajont italy therefore deformation processes in smectite clays may control the mechanical behavior from slow creep to fast accelerations and slip during earthquakes and landslides here we use 1 laboratory experiments to investigate the mechanical behavior of partly water saturated smectite rich gouges sheared from subseismic to seismic slip rates v and 2 nanoscale microscopy to study the gouge fabric at all slip rates deformation localizes in volumes of the gouge layer that contain a nanofoliation consisting of anastomosing smectite crystals seismic nanofoliations produced at v 0 01 0 1 and 1 3 m s are similar to subseismic nanofoliations obtained at v 10 5 m s this similarity suggests that frictional slip along water lubricated smectite grain boundaries and basal planes may occur from subseismic to seismic slip rates in natural smectite rich faults thus if water is available along smectite grain boundaries and basal planes nanofoliations can develop from slow to fast slip rates still when nanofoliations are found highly localized in a volume they can be diagnostic of slip that occurred at rates equal or larger than 0 01 m s in such a case they could be markers of past seismic events when found in natural fault rocks

an experimental investigation into the role of phyllosilicate content on earthquake propagation during seismic slip in carbonate faults carbonate faults commonly contain small amounts of phyllosilicate in their slip zones due to pressure solution and or clay smear to assess the effect of phyllosilicate content on earthquake propagation in carbonate faults friction experiments were performed at 1 3 m s on end members and mixtures of calcite illite smectite and smectite gouge experiments were performed at 9 mpa normal load under room humidity and water saturated conditions all dry gouges show initial friction values i of 0 51 0 58 followed by slip hardening to peak values of 0 61 0 76 slip weakening then ensues with friction decreasing to steady state values ss of 0 19 0 33 within 0 17 0 58 m of slip contrastingly wet gouges containing 10 50 wt phyllosilicate exhibit i values between 0 07 and 0 52 followed by negligible or no slip hardening rather steady state sliding ss 0 2 is attained almost immediately microstructurally dry gouges show intense cataclasis and wear within localized principal slip zones plus evidence for thermal decomposition of calcite wet gouges exhibit distributed deformation less intense cataclasis and no evidence of thermal decomposition it is proposed that in wet gouges slip is distributed across a network of weak phyllosilicate formed during axial loading compaction prior to shear this explains the 1 subdued cataclasis and associated lack of slip hardening 2 distributed nature of deformation and 3 lack of evidence for thermal decomposition due to low friction and lack of slip localization these findings imply that just 10 phyllosilicate in the slip zone of fluid saturated carbonate faults can 1 dramatically change their frictional behavior facilitating rupture propagation to the surface and 2 significantly lower frictional heating preventing development of microscale seismic markers

reservoir characterization using surface microseismic monitoring microseismic monitoring of reservoir processes can be performed using surface or near surface arrays we review the published technical basis for the use of the arrays and the historical development of the method beginning with locating earthquakes through geothermal exploration to the growing field of hydraulic fracture monitoring practical considerations for the array deployment and data processing are presented the road ahead for the technology includes a move toward life of field buried arrays as well as opportunities for extended interpretation of the data particularly inversion for source mechanism estimation and measurement of anisotropy in the monitored subsurface

micromechanisms of creep in clay rich gouge from the central deforming zone of the san andreas fault we report the strength and constitutive behavior of gouge sampled from the central deforming zone cdz of the san andreas fault layers of flaked cdz gouge were sheared in the triaxial saw cut configuration using the stress relaxation technique to measure the gouge strength over 4 orders of magnitude in shear strain rate and at rates as low as 5 10 10s 1 and within an order of magnitude of in situ rates deformation conditions correspond to the in situ effective normal stress 100 mpa and temperature 65 to 120 c at the sampling depth of 2 7 km gouge was sheared dry and with brine pore fluid at 25 mpa pore pressure dry gouge is stronger and more rate strengthening than brine saturated gouge brine saturated cdz gouge strengthens with increasing strain rate and decreasing temperature and the dependencies of strength on strain rate and temperature increase at rates below 5 10 9s 1 at strain rates greater than 5 10 9s 1 the rate dependence is consistent with previous studies on the cdz gouge conducted at even higher rates the increase in rate dependence below 5 10 9s 1 indicates a change in the rate controlling deformation mechanism the magnitude of the friction rate dependence parameter a and the temperature sensitivity of a are consistent with crystal plasticity of the phyllosilicates we hypothesize a micromechanical model for the cdz gouge whereby a transition from fracture and delamination accommodated frictional flow to crystal plasticity accommodated frictional flow occurs with decreasing strain rate

simultaneous microseismic event localization and source mechanism determination abstractmicroseismic monitoring has become a tool of choice for the development and optimization of oil and gas production from unconventional reservoirs the primary objective of micro seismic monitoring includes localization of micro seismic events and characterization of their source mechanisms most seismic events are of a nonexplosive nature and thus there are waveform polarity differences among receivers specifically double couple sources represented a challenge for migration based localization techniques we developed and applied a new migration type location technique combined with source mechanism inversion that allowed for constructive interference of signal in seismic waveforms the procedure included constructing image functions by stacking the amplitudes with compensated polarity changes the compensation weights were calculated by using moment tensor inversion this method did not require any picking of arrivals at individual receivers but it required receivers to be distributed i

petroleum reservoir characterization using downhole microseismic monitoring imaging of microseismic data is the process by which we use information about the source locations timing and mechanisms of the induced seismic events to make inferences about the structure of a petroleum reservoir or the changes that accompany injections into or production from the reservoir a few key projects were instrumental in the development of downhole microseismic imaging most recent microseismic projects involve imaging hydraulic fracture stimulations which has grown into a widespread fracture diagnostic technology this growth in the application of the technology is attributed to the success of imaging the fracture complexity of the barnett shale in the fort worth basin texas and the commercial value of the information obtained to improvecompletions and ultimately production in the field the use of commercial imaging in the barnett is traced back to earlier investigations to prove the technology with the cotton valley imaging project and earlier experiments at the m site in the piceance

application of the multichannel wiener filter to regional event detection using norsar seismic array data seismic arrays for detection of small earthquakes bene t from reduced noise levels by array processing methods we present a frequency dependent multi channel wiener l tering mcwf technique which employs an adaptive least squares method to remove coherent noise in seismic array data the noise records on a number of reference chan nels are used to predict the noise on a primary channel which can then be subtracted from the observed data a sequence of aftershocks caused by a feb 21 2008 main shock in spitsbergen was recorded in 2008 by the arces array in northern norway this aftershock sequence was ltered using the multi channel wiener lters on both triggered and continuous modes the spitsbergen spits array at a much closer dis tance to the source region provides reliable reference information on the true number of detectable aftershocks the triggered procedure detects 631 aftershocks at arces in total where conventional delay and sum beamforming combined with a band pass lter could detect 513 aftershocks with 181 false alarms using a series of constraints signal to noise ratio snr back azimuth and slowness the multi channel wiener l tered results found 577 aftershocks with 165 false alarms using the same constraints a complete automatic multi channel wiener procedure is developed for event detec tion on continuous data the appropriate snr threshold for aftershock detection 2 7 is suggested the mcwf also demonstrates the advantage of reducing false alarms compared to the beamforming method when detecting the same number of aftershocks