Non-volcanic Tremor Research Articles

Mélange rheology and seismic style

Shear displacements in crustal fault zones are accommodated by a range of seismic styles, including standard earthquakes, non-volcanic tremor, and continuous and transitory aseismic slip. Subduction channel shear zones, containing highly sheared, fluid-saturated trench-fill sediments intermingled with fragments of oceanic crust, are commonly inferred to occur along active subduction megathrusts. If this interpretation is correct, these plate boundary faults are not discrete planes, but may resemble the melange shear zones commonly found in exhumed subduction-related rock assemblages. Melange deformation appears to depend critically on the ratio of competent to incompetent material, with shear surfaces localized along lithological contacts or within competent domains, while matrix flow accommodates shearing by distributed strain. If the style of strain and/or displacement accommodation in a melange reflects the partitioning between aseismic and seismic slip, the proportion of competent material seems likely to be a significant factor affecting seismic style within subduction channel shear zones, and along comparable mixed-lithology fault zones.

Quantifying the time function of nonvolcanic tremor based on a stochastic model

Nonvolcanic tremor is the seismically observable component of slow earthquakes, which have been recently discovered in subduction zones around the world, including major study areas at the Nankai and Cascadia subduction zones. Although tremor appears to occur randomly in both areas, Cascadia tremor has a longer duration than Nankai tremor. In the present study, this difference in tremor duration is quantified using a Brownian slow earthquake model that explains several features of tremor and slow earthquakes. A previous Brownian model is improved and applied to explain the cumulative distribution function of tremor amplitude, which is approximated by a χ2 distribution. The model also shows that the power spectrum of tremor amplitude has a simple analytic formula, including a characteristic time. An inversion method is developed to measure the characteristic time from the tremor spectrum in the presence of non‐Gaussian background noise. The method is applied to several tremor sequences in the Nankai and Cascadia subduction zones to quantitatively confirm the apparent differences in tremor behavior between the two areas. The constants for Cascadia and Nankai tremor are 1000–3000 s and 100–1000 s, respectively, with temporal increases in these values observed over the course of 1‐day records of activity. The difference in characteristic time between the two areas may reflect geometric constraints such as the width of the tremor region.

Locating nonvolcanic tremors beneath the San Andreas Fault using a station‐pair double‐difference location method

It has been a challenging task to locate nonvolcanic tremors because of the lack of impulsive wave arrivals. A station‐pair double‐difference (DD) location method is developed to determine absolute tremor locations by directly using the station‐pair travel time differences measured from cross‐correlating tremor waveform envelopes. Multiple tremors are located together for inverting for station corrections to take into account velocity model inaccuracy. The new method is applied to tremors in the Parkfield region of central California between 27 July 2001 and 21 February 2009. Compared to the tremor catalog locations determined from a grid search location method, most of the newly located tremors are located at depths between 20 and 35 km, well below the seismogenic zone in the area. The tremors beneath Cholame, CA are more clearly separated into two zones laterally distributed across the San Andreas Fault, with most tremors occurring to the southwest and exhibiting a periodic pattern of occurrence. The new tremor locations help better delineate the spatial and temporal distribution of tremor activity and therefore are helpful for better understanding tremor origin and process.

Interactive Tremor Monitoring

The purpose of this article is to explore new Web-based resources for disseminating data in an interactive and intuitive way that is accessible and engaging to the public without sacrificing scientific utility. This article highlights these new opportunities and describes their synthesis with recently developed automated tremor monitoring methodologies. The resulting product is a website that the reader may find useful to explore in tandem with this article, http://www.pnsn.org/tremor. In this article, the data dissemination example is specifically applied to Cascadia tremor, but the utilization of freely accessible Web applications to provide an interactive Web experience for the public and scientific community alike could easily be applied to many different aspects of seismology. Since their discovery nearly a decade ago, advances in both instrumentation and methodology in subduction zones around the world have brought the causal connection between seismically observed tectonic tremor (Obara 2002) and geodetically observed slow slip (Dragert et al. 2001) into sharper focus. In addition to the strong spatio-temporal correlation between the separately observed phenomena (Wech et al. 2009), mounting evidence indicates that deep, non-volcanic tremor represents slow shear (Ide et al. 2007; Wech and Creager 2007) occurring at the interface (Shelly et al. 2006, 2007) between the subducting oceanic and overriding continental plates, suggesting the two phenomena are different manifestations of the same process. Of course, there is still ongoing debate over the depth and mechanism of tectonic tremor (Kao et al. 2005; McCausland et al. 2005), but what is clear is that tremor serves as a proxy for slow slip. Considering geodesy's lower limits in spatial resolution and slow slip detection together with the abundance of low-level, ageodetic tremor, this connection makes tremor a key component for monitoring when, where, and how much slip is occurring. Because slow slip transfers stress …

Recurrence behavior of short‐term slow slip and correlated nonvolcanic tremor episodes in western Shikoku, southwest Japan

From the National Research Institute for Earth Science and Disaster Prevention Hi‐net tiltmeter data, we determine time‐dependent source processes of seven repeating short‐term slow slip events (SSE) in the western Shikoku region, southwest Japan, over the period from 2002 to 2007. The tiltmeters record clear signals of SSEs characterized by a slow rise, a week‐long duration, and a magnitude on the order of 10−2−10−1μrad. We apply a time‐dependent slip inversion method to the tilt records for the seven SSEs. For each SSE, a slip area migrates in the strike direction of the subducting slab at a rate of ∼10 km d−1. Comparing the slow slip area with epicenters of accompanying nonvolcanic tremors in subday time resolution, we find that the two phenomena are located on a smaller area than previously reported and migrate as a group. A moment rate function for each SSE exhibits good correlation with the temporal change in tremor activity. A comparison of the slip distributions of all of the analyzed SSEs shows that there is a patch of slow slip area (∼30 km in diameter) that is shared by all of the episodes. In addition, the location of the slip patch is in good agreement with the epicenters of tremors and very low frequency earthquakes. These lines of evidence suggest that the identified slip patch is one of the main slow slip patches in the region and a recurrence of SSEs may be controlled by the patch and that these “slow earthquakes” might be different manifestations of a single slip process on the deep plate interface.

Array analysis and precise source location of deep tremor in Cascadia

We describe a new method to estimate the S‐P time of tremor‐like signals and its application to the nonvolcanic tremor recorded in July 2004 by three dense arrays in Cascadia. The cross correlation between vertical and horizontal components indicates that very often the high‐amplitude tremor signal contains sequences of P and S waves characterized by constant S‐P times (TS‐P) in the range 3.5–7 s. A detailed observation of the three component seismograms stacked over the array stations confirms the presence of P and S wave sequences. The knowledge of the TS‐P poses a strong constrain on the source‐array distance, which dramatically reduces the uncertainty on source locations when used with more traditional array processing techniques. Data were analyzed using the zero lag cross‐correlation technique (ZLCC) to estimate the propagation properties of the most correlated phases in the wavefield. Detailed polarization analyses were computed using the covariance matrix method in the time domain. Polarization parameters, joint with the results of ZLCC, allows for the discrimination between P and S coherent waves. Results show that the tremor wavefield is composed mostly by shear waves, although a consistent amount of coherent P waves is often observable. The comparison of the back azimuth at the three arrays indicate that the source of deep tremor migrates over a wide area, and often many independent sources located far from each other are active at the same time. The tremor source was located by a probabilistic method that uses the results of ZLCC, given a velocity model. When available, the inclusion of the TS‐P time in the location procedure strongly reduces the depth range, with a distribution of hypocenters very near the subduction interface. This result, significantly different compared with previous less precise locations, makes the Cascadia nonvolcanic tremor more similar to the nonvolcanic tremor recorded in Japan, at least in cases of measurable TS‐P. The polarization azimuth aligned with the slow slip direction and the source located on the plate interface indicate that deep tremor and slow slip are two different manifestations of a common phenomenon related with the subduction dynamics.

Geometry and seismic properties of the subducting Cocos plate in central Mexico

The geometry and properties of the interface of the Cocos plate beneath central Mexico are determined from the receiver functions (RFs) utilizing data from the Meso America Subduction Experiment (MASE). The RF image shows that the subducting oceanic crust is shallowly dipping to the north at 15° for 80 km from Acapulco and then horizontally underplates the continental crust for approximately 200 km to the Trans‐Mexican Volcanic Belt (TMVB). The crustal image also shows that there is no continental root associated with the TMVB. The migrated image of the RFs shows that the slab is steeply dipping into the mantle at about 75° beneath the TMVB. Both the continental and oceanic Moho are clearly seen in both images, and modeling of the RF conversion amplitudes and timings of the underplated features reveals a thin low‐velocity zone between the plate and the continental crust that appears to absorb nearly all of the strain between the upper plate and the slab. By inverting RF amplitudes of the converted phases and their time separations, we produce detailed maps of the seismic properties of the upper and lower oceanic crust of the subducting Cocos plate and its thickness. High Poisson's and Vp/Vs ratios due to anomalously low S wave velocity at the upper oceanic crust in the flat slab region may indicate the presence of water and hydrous minerals or high pore pressure. The evidence of high water content within the oceanic crust explains the flat subduction geometry without strong coupling of two plates. This may also explain the nonvolcanic tremor activity and slow slip events occurring in the subducting plate and the overlying crust.

A slip pulse model with fault heterogeneity for low‐frequency earthquakes and tremor along plate interfaces

Deep low‐frequency earthquakes (LFEs) and nonvolcanic tremor have distinctive characteristics unlike those of regular earthquakes, including strong anisotropy in their migration velocity and source spectra displaying 1/f decay. We show that a physical model can explain these features in a simple framework with slip pulses originating on fault heterogeneity and triggered by slow‐slip events. LFE/tremor source areas in the model consist of unstable patches sparsely and heterogeneously distributed following a Gaussian distribution. The difference in their migration speeds along dip and along strike was reproduced, without anisotropic rheological properties, by introducing alignments of their sources similar to observed streaks of LFEs/tremor. The key to reproducing inverse linear spectral decay is that the slip pulse has a constant mean moment rate. This model provides new insights into the physical source process of LFEs and tremor and should find practical use in assessing properties of deep plate interfaces.

Tremor bands sweep Cascadia

During the slow slip events in Cascadia and Japan, a well‐documented feature of nonvolcanic tremor (NVT) is its puzzling slow along‐strike migration. But the cause, possible implications, and underlying physics of this long‐term tremor migration and its relationship with slow slip remain elusive. Here, we use tremor data recorded by a dense seismic array in Cascadia, and apply a beam‐backprojection technique to map the spatiotemporal evolution of tremor with high resolution. It reveals that elongated slip‐parallel bands of tremor activity illuminate the slipping part of the plate interface over the time‐scale of several hours, and sweep Cascadia along‐strike from south to north. We propose that small changes in static stress due to slow slip in a section of the fault cause slip and associated NVT activity in the adjacent section, and the resulting progressive along‐strike transfer of stress is responsible for the long‐term tremor migration during a slow slip event.

Determination of slow slip episodes and strain accumulation along the Cascadia margin

Continuous GPS stations in the Pacific Northwest Geodetic Array network clearly record subduction‐related strain accumulation and slow slip episodes along the Cascadia convergent margin. Many of the slow slip episodes have been correlated in time and space with seismic evidence for nonvolcanic tremor, leading to the previous discovery of episodic tremor and slip (ETS). In this study, we use a hyperbolic tangent curve fitting technique for the identification of slow slip times and displacement magnitudes within the GPS time series, independent of seismic tremor data. We then apply this technique to study the patterns of strain accumulation and release associated with ETS events and characterize patterns of coupling associated with the locked and transition zones of the plate interface. We demonstrate the effectiveness of this automated technique for both identification of slow slip observations and calculation of slow slip displacements. Recurrence patterns in the distribution of GPS observations demonstrate coherence among neighboring stations over time and apparent along‐strike segmentation of the subduction interface. When slow slip events are removed from the time series, we can estimate the total site velocities between slow slip events. These velocities decay as depth to the subduction interface increases, but they diverge from the long‐term trends expected from the interseismic cycle at ∼30–60 km above the interface, consistent with the location where slow slip displacements occur. Forward modeling of coupling on the plate interface reveals that in between slow slip events there is a patch of at least 30% coupling from 20 to 35 km depth, which is needed to produce the observed back slip displacements. Intriguingly, our best fitting models have a decrease in coupling down to ∼30% at ∼20 km depth followed by a peak of greater than 80% coupling at ∼30–35 km depth, suggesting the source zone for ETS events acts as a distinct locking zone that releases strain more frequently than the updip seismogenic locked zone, although a zone of constant ∼30% coupling cannot be ruled out with this data set. Such a scenario indicates that frictional behavior with depth follows a more complex model than a simple temperature controlled transition. We propose that coupling initially decreases with depth due to a decrease in strength of the overriding lower crust, but then coupling increases again when the subducting plate comes in contact with the stronger overriding mantle.

Slab morphology in the Cascadia fore arc and its relation to episodic tremor and slip

Episodic tremor and slip (ETS) events in subduction zones occur in the general vicinity of the plate boundary, downdip of the locked zone. In developing an understanding of the ETS phenomenon it is important to relate the spatial occurrence of nonvolcanic tremor to the principal structural elements within the subduction complex. In Cascadia, active and passive source seismic data image a highly reflective, dipping, low‐velocity zone (LVZ) beneath the fore‐arc crust; however, its continuity along the margin is not established with certainty, and its interpretation is debated. In this work we have assembled a large teleseismic body wave data set comprising stations from northern California to northern Vancouver Island. Using stacked receiver functions we demonstrate that the LVZ is well developed along the entire margin from the coast eastward to the fore‐arc basins (Georgia Strait, Puget Sound, and Willamette Valley). Combined with observations and predictions of intraslab seismicity, seismic velocity structure, and tremor hypocenters, our results support the thesis that the LVZ represents the signature of subducted oceanic crust, consistent with thermal‐petrological modeling of subduction zone metamorphism. The location of tremor epicenters along the revised slab contours indicates their occurrence close to but seaward of the wedge corner. Based on evidence for high pore fluid pressure within the oceanic crust and a downdip transition in permeability of the plate interface, we propose a conceptual model for the generation of ETS where the occurrence and recurrence of propagating slow slip and low‐frequency tremor are explained by episodic pore fluid pressure buildup and fluid release into or across the plate boundary.

Unusual Microseisms Seen in the Reelfoot Fault Zone, Northern Tennessee, from a Reflection Experiment

Abstract A swarm of microseisms with ground motions equivalent to earthquakes of M L -1 and smaller was fortuitously detected in 100 of 162, 14 sec-duration long-offset vibroseis shot gathers collected for a seismic reflection experiment near Mooring, Tennessee, directly over the Reelfoot fault zone on the afternoon of 16 November 2006. These natural events show up in the shot gathers as near-vertically incident P waves with a dominant frequency of 8–10 Hz and probably occurred at depths of greater than 10 km. The inferred seismicity rate of 250–1000 events per hour is 2–3 orders of magnitude higher than the background seismicity rate for the New Madrid seismic zone. This detection of microseismic swarms in the Reelfoot fault zone indicates active physical processes that may be similar to nonvolcanic tremor seen in the Cascadia and San Andreas fault zones and merits long-term monitoring to understand its source.

Seismicity and state of stress in Guerrero segment of the Mexican subduction zone

We take advantage of a relatively dense network of seismic stations in the Guerrero segment of the Mexican subduction zone to study seismicity and state of stress in the region. We combine our results with recent observations on the geometry of the subducted Cocos plate imaged from receiver function (RF) analysis, an ultraslow velocity layer mapped in the upper crust of the subducted slab, and episodic slow slip events (SSEs) and nonvolcanic tremors (NVTs) reported in the region to obtain a comprehensive view of the subduction process. Seismicity and focal mechanisms confirm subduction of the Cocos plate below Mexico at a shallow angle, reaching a depth of 25 km at a distance of 65 km from the trench. The plate begins to unbend at this distance and becomes horizontal at a distance of ∼120 km at a depth of 40 km. Some of the highlights of the inslab seismicity are as follows: (1) A cluster of earthquakes in the depth range of 25–45 km, immediately downdip from the strongly coupled part of the plate interface, revealing both downdip compressional and extensional events. This seismicity extends from ∼80 to 105 km from the trench and may be attributed to the unbending of the slab. (2) The slab devoid of seismicity in the distance range of ∼105–160 km. (3) Sparse inslab seismicity revealing downdip extension in the distance range of 160–240 km. The NVTs are also confined to this distance range. The episodic SSEs occur on the horizontal segment, in the distance range of ∼105–240 km, where an ultraslow velocity layer in the upper crust of the slab has been mapped from waveform modeling of converted SP phases. Thus, in the distance range of ∼105–160 km, SSEs occur but NVTs and inslab earthquakes are absent. This suggests that metamorphic dehydration reactions in the subducting oceanic crust and upper mantle begin at a distance of ∼160 km, giving rise to both the inslab earthquakes and NVTs. No inslab earthquake occurs beyond 240 km. The receiver function images and P wave tomography suggest that the slab begins a steep plunge at a distance of ∼310 km, reaching a depth of 500 km around 340 km from the trench. The negative buoyancy of such a slab should give rise to large extensional stress in the slab. Yet inslab seismicity is remarkably low, which may be explained by a slab that is not continuous up to a depth of 500 km, but is broken at a shallower depth. The resulting slab window may permit subslab material to flow through the gap. This may provide an explanation for the recent rift‐related basalts found near Mexico City. The fore‐arc, upper plate seismicity, which during the period of study (1995–2007) consisted of a moderate earthquake (Mw5.8) near the coast (H = 12 km), its numerous aftershocks, and two shallow events farther inland, demonstrates a trench‐normal extension in the upper plate near this convergent margin, a state of stress that may be explained by tectonic erosion and/or seaward retreat of the trench. Seismicity, location of the mantle wedge, and rupture areas of Mexican earthquakes suggest that the downdip limit of rupture during large/great earthquakes in Guerrero may be 105 ± 15 km. Shallow‐dipping, interplate thrust earthquakes are not the only type of events that affect the seismic hazard in the region. The magnitudes of inslab downdip compressional and extensional earthquakes that occur within ∼20 km inland from the coast, in the depth range of 25–45 km, may reach 6.5 and 7.5, respectively. In addition, we now identify normal‐faulting earthquakes in the upper plate. These sources need to be taken into account in the hazard estimation.

Automatic non-volcanic tremor detection in the Mexican subduction zone

Desarrollo de un algoritmo de detección de tremores no-volcánicos (TNV). Un TNV ocurre en el rango de frecuencia de 1 - 10 Hz, pero el ruido ambiental alto ocurre por encima de los 2 Hz en muchas de las estaciones del MesoAmerican Seismic Experiment (MASE), entonces los sismogramas se filtran con un paso banda de 1 - 2 Hz. Se determina el efecto de sitio comparando la ventana de las codas de terremotos regionales en todas las estaciones, y luego se remueve. Se aplica un filtro mediano a la amplitud absoluta de los datos filtrados con el filtro de paso banda para remover los picos de los terremotos locales. Se quita la tendencia y el valor mediano de los archivos de datos de un día para eliminar el efecto de las tormentas y el ruido local. Se obtiene el promedio de todos los datos por día para amplificar la energía coherente de los TNV y para disminuir el ruido local. Se determina empíricamente un límite de amplitud de los TNV y se aplica a todos los datos diariamente del periodo de estudio para generar el catálogo de los TNV. Se compara el catálogo determinado automáticamente con el catálogo creado por el análisis visual de los espectrogramas diariamente. Se hace un análisis preciso durante el mes con la mayor diferencia entre los dos catálogos (mayo de 2006). Encontrandose que el algoritmo de detección automática tuvo menos falsos positivos y menos TNV indeterminados que la detección inicial de TNV visualmente, por lo tanto el algoritmo automático desarrollado de detección de TNV tiene menos errores y puede usarse efectivamente para futuros análisis de la actividad de los tremores.

Tremor-tide correlations and near-lithostatic pore pressure on the deep San Andreas fault

Since its initial discovery nearly a decade ago, non-volcanic tremor has provided information about a region of the Earth that was previously thought incapable of generating seismic radiation. A thorough explanation of the geologic process responsible for tremor generation has, however, yet to be determined. Owing to their location at the plate interface, temporal correlation with geodetically measured slow-slip events and dominant shear wave energy, tremor observations in southwest Japan have been interpreted as a superposition of many low-frequency earthquakes that represent slip on a fault surface. Fluids may also be fundamental to the failure process in subduction zone environments, as teleseismic and tidal modulation of tremor in Cascadia and Japan and high Poisson ratios in both source regions are indicative of pressurized pore fluids. Here we identify a robust correlation between extremely small, tidally induced shear stress parallel to the San Andreas fault and non-volcanic tremor activity near Parkfield, California. We suggest that this tremor represents shear failure on a critically stressed fault in the presence of near-lithostatic pore pressure. There are a number of similarities between tremor in subduction zone environments, such as Cascadia and Japan, and tremor on the deep San Andreas transform, suggesting that the results presented here may also be applicable in other tectonic settings.

Complex nonvolcanic tremor near Parkfield, California, triggered by the great 2004 Sumatra earthquake

In several instances, the passing surface waves from large earthquakes have ignited nonvolcanic tremor (NVT) on major faults. Still, the mechanism of tremor and its reaction to the dynamic stressing from various body and surface waves is poorly understood. We examine tremor near Parkfield, California, beneath the San Andreas fault triggered by the Mw 9.2, 2004 Sumatra earthquake. The prolonged shaking produces the richest and the most varied observations of dynamically triggered tremor to date. The tremor appears in at least three distinct locations and shows activity pulsing with encouraging stress, as has been observed in other cases. The greatest amount of triggering and tremor modulation accompanies the long‐period Love waves. Rayleigh waves, on the other hand, appear to be less effective in exciting tremor sources. Also, at times, the tremor stops before the surface waves are complete, at other times it continues quivering after the waves have passed. While tremor is found to be sensitive to small stress changes, there are times when stresses of comparable magnitudes do not trigger noticeable tremor. Some tremors in this NVT sequence appear to be associated with the passage of P waves, which is unusual and surprising given the small stresses they impart.

Shear‐induced dilatancy of fluid‐saturated faults: Experiment and theory

Pore fluid pressure plays an important role in the frictional strength and stability of tectonic faults. We report on laboratory measurements of porosity changes associated with transient increases in shear velocity during frictional sliding within simulated fine‐grained quartz fault gouge (d50 = 127 μm). Experiments were conducted in a novel true triaxial pressure vessel using the double‐direct shear geometry. Shearing velocity step tests were used to measure a dilatancy coefficient (ɛ = Δϕ/Δln(v), where ϕ is porosity and v is shear velocity) under a range of conditions: background shearing rate of 1 μm/s with steps to 3, 10, 30, and 100 μm/s at effective normal stresses from 0.8 to 20 MPa. We find that the dilatancy coefficient ranges from 4.7 × 10−5 to 3.0 × 10−4 and that it does not vary with effective normal stress. We use our measurements to model transient pore fluid depressurization in response to dilation resulting from step changes in shearing velocity. Dilatant hardening requires undrained response with the transition from drained to undrained loading indexed by the ratio of the rate of porosity change to the rate of drained fluid loss. Undrained loading is favored for high slip rates on low‐permeability thick faults with low critical slip distances. Although experimental conditions indicate negligible depressurization due to relatively high system permeability, model results indicate that under feasible, but end‐member conditions, shear‐induced dilation of fault zones could reduce pore pressures or, correspondingly, increase effective normal stresses, by several tens of megapascals. Our results show that transient increases in shearing rate cause fault zone dilation. Such dilation would tend to arrest nucleation of unstable slip. Pore fluid depressurization would exacerbate this effect and could be a significant factor in generation of slow earthquakes, nonvolcanic tremors, and related phenomena.

An automatic monitoring system for nonvolcanic tremors in southwest Japan

The recent development of a system for exchanging and distributing seismic waveform data over high‐speed networks enables seismic events to be monitored in real time throughout Japan. In the present study, we have developed an automatic real‐time monitoring system for deep nonvolcanic tremors in southwest Japan. The system automatically detects the occurrence of nonvolcanic tremors and determines their hypocenters in real time. In addition, the system creates image files for the detected tremor activities and makes them accessible via the World Wide Web. To detect tremors we carry out a two‐step numerical hypothesis test, in which the first step is to test whether two given envelope seismograms are correlated and the second step is to test whether an event occurs using the results of the first test. This two‐step test is applied to real‐time data every 2 min. Once an event is detected, we can regard time lags that generate maximum cross correlations as the travel time differences, which we use for hypocenter determination. Although this procedure detects nontremor signals, most of them can be rejected using several criteria. Since the start of monitoring in 2006, the system has worked well for detecting a wide variety of tremor activities. Results from the present system will contribute to understanding the stress relaxation process in the transition zone between the locked and stable slip zones of the subducting Philippine Sea plate interface.

Tremor source location using time reversal: Selecting the appropriate imaging field

Studying triggered Non Volcanic Tremor (NVT) is important because it may help to map the depth of the locked zones of faults associated with high seismic risk. The success of this mapping depends on precisely locating the depth of tremor. Tremor, like other long‐lived signals (e.g., Earth hum) lacks distinct sharp timing features making it impossible to locate with classical approaches. Time Reversal has the advantage of exploiting the full waveform with no a priori assumption regarding the source or the observed signal. We perform a synthetic study of time reversal location of a long‐lasting source in the Los Angeles basin with a realistic 3D velocity model and sparse station set. We show that, the key to successfully locating NVT, is application of suitable imaging fields, such as the wave divergence, curl and energy current.

Tremor patches in Cascadia revealed by seismic array analysis

Episodic tremor and slip (ETS) events in Cascadia have recently been observed, illuminating the general area that radiates seismic energy in the form of non‐volcanic tremor (NVT). However, the picture of the ETS zone remains fuzzy because of difficulties in tremor detection and location. To observe the intimate details of tremor, we deployed a dense 84‐element small‐aperture seismic array on the Olympic Peninsula, Washington, above the tremor migration path. It recorded the main ETS event in May 2008, as well as a weaker tremor episode two months earlier. Using a beamforming technique, we are able to capture and track tremor activity with an unprecedented resolution from southern Puget Sound to the Strait of Juan de Fuca. The array technique reveals up to four times more duration of tremor compared to the conventional envelope cross‐correlation method. Our findings suggest that NVT is not uniformly distributed on the subduction interface, and unveils several distinct patches that release much of the tremor moment. The patches appear to be devoid of ordinary earthquakes, and may indicate the heterogeneity in fault strength that affects the modes of stress release within the ETS zone.