Episodic Tremor And Slip Events Research Articles

Cascading rupture of patches of high seismic energy release controls the growth process of episodic tremor and slip events

Slip phenomena on plate interfaces reflect the heterogeneous physical properties of the slip plane and, thus, exhibit a wide variety of slip velocities and rupture propagation behaviors. Recent findings on slow earthquakes reveal similarities and differences between slow and regular earthquakes. Episodic tremor and slip (ETS) events, a type of slow earthquake widely observed in subduction zones, likewise show diverse activity. We investigated the growth of 17 ETS events beneath the Kii Peninsula in the Nankai subduction zone, Japan. Analyses of waveform data recorded by a seismic array enabled us to locate tremor hypocenters and estimate the migration patterns and spatial distribution of the energy release of tremor events. Here, we describe three major features in the growth of ETS events. First, independent of their start point and migration pattern, ETS events exhibit patches of high seismic energy release on the up-dip part of the ETS zone, suggesting that the location of these patches is controlled by inherent physical or frictional properties of the plate interface. Second, ETS events usually start outside the high-energy patches, and their final extent depends on whether the patches participate in the rupture. Third, we recognize no size dependence in the initiation phase of ETS events of different sizes with comparable start points. These features demonstrate that the cascading rupture of high-energy patches governs the growth of ETS events, just as the cascading rupture of asperities governs the growth of regular earthquakes.

Cascadia Convergent Zone: An Example of Primary Convergent Seismogenic Structure

In this article, a case is made for very-large or primary seismogenic structures in convergent margins, based on anomalous large earthquake magnitudes (Mw 8 - 9) relative to rupture lengths. Out of 56,293 earthquakes (magnitudes ≥ 5) cataloged worldwide, the 10 largest events in transform, divergent, and interior settings average magnitudes of 7.3 - 7.6. But in convergent margins, the average magnitude of the 10 largest events is 8.5, roughly 32 times more energy than the other neotectonic settings. The large anomalous magnitudes of energy release in convergent margins are attributed to the transfer of inter-plate stress to the upper-plate, where convergent elastic strain is accumulated during interseismic intervals. The large volumes of rock that accumulate the elastic strain in the upper-plates of convergent zones are defined here as primary seismogenic structures. Several datasets of 1) modern upper-plate convergent strain, 2) historical earthquakes, 3) modern upper-plate vertical displacements, and 4) recent inter-plate events of Episodic Tremor and Slip (ETS) are compared to establish the extent of the primary seismogenic structure in the Cascadia convergent zone. The across-margin extents of 1) significant convergent strain, 2) margin-parallel bands of vertical displacement, 3) historical seismicity and 4) ETS events, representing inter-plate coupling and shear stress transfer to strain accumulation in the upper-plate, are used to map the width of the primary seismogenic structure. The across-margin width of the primary seismogenic structure in the central Cascadia margin ranges from 300 km in the south-central margin to 450 km in the north-central margin, as mapped landward from the buried trench. A broad source region of coseismic energy release in the Cascadia primary seismogenic structure (300 - 450 km width) could yield stronger shaking in interior metropolitan centers from a future major rupture of the mega-thrust than has been modeled from a narrow “locked” zone located offshore under the outer continental shelf. Despite low dip angle and associated wide inter-plate coupling, the Cascadia margin likely serves as an example of inter-plate shear stress transfer to elastic strain accumulation in the upper-plate of some other well-coupled convergent margins worldwide.

Spatial Comparisons of Tremor and Slow Slip as a Constraint on Fault Strength in the Northern Cascadia Subduction Zone

AbstractWe measured displacement vectors from horizontal components of 80 Global Positioning System stations to analyze six major episodic tremor and slip (ETS) events from 2007 to 2016 in northern Cascadia and inverted for slip on a realistic plate interface. Our results indicate that slow slip of up to 2 cm extends updip of tremor by about 15 km beneath the Olympic Peninsula. In these ETSs, slow slip extends from the downdip portion of the tremorgenic region beyond the updip extent of tremor, although still downdip of the inferred locked megathrust. Slip updip of tremor is a persistent feature of all six ETS events. Inversions that restrict slip to occur only in regions that generated tremor produced slip distributions with unphysical characteristics, such as 8‐cm slip concentrated at the updip part of the tremor footprint. Updip slow slip without tremor may suggest that the gap between stress and strength widens updip above the observed limit of tremor. In these ETSs, the region updip of tremor may have undergone only limited ductile failure surrounding potentially tremorgenic patches. A widening gap between stress and strength in the updip direction is consistent with an observed along‐dip dependence of low‐frequency earthquake occurrence and numerical simulations of slow slip. Alternatively, rheological properties in the region updip of tremor may favor stable slip and not permit seismic slip (i.e., tremor). Additionally, we find that along‐strike variations in the amount of slow slip updip of tremor correspond to changes in lithology of the overlying crust.

A Model for Low‐Frequency Earthquake Slip

AbstractUsing high‐resolution relative low‐frequency earthquake (LFE) locations, we calculate the patch areas (Ap) of LFE families. During episodic tremor and slip (ETS) events, we define AT as the area that slips during LFEs and ST as the total amount of summed LFE slip. Using observed and calculated values for AP, AT, and ST, we evaluate two end‐member models for LFE slip within an LFE family patch. In the ductile matrix model, LFEs produce 100% of the observed ETS slip (SETS) in distinct subpatches (i.e., AT ≪ AP). In the connected patch model, AT = AP, but ST ≪ SETS. LFEs cluster into 45 LFE families. Spatial gaps (∼10 to 20 km) between LFE family clusters and smaller gaps within LFE family clusters serve as evidence that LFE slip is heterogeneous on multiple spatial scales. We find that LFE slip only accounts for ∼0.2% of the slip within the slow slip zone. There are depth‐dependent trends in the characteristic (mean) moment and in the number of LFEs during both ETS events (only) and the entire ETS cycle ( and and and , respectively). During ETS, Mc decreases with downdip distance but NT does not change. Over the entire ETS cycle, Mc decreases with downdip distance, but NT increases. These observations indicate that deeper LFE slip occurs through a larger number (800–1,200) of small LFEs, while updip LFE slip occurs primarily during ETS events through a smaller number (200–600) of larger LFEs. This could indicate that the plate interface is stronger and has a higher stress threshold updip.

Single‐station automated detection of transient deformation in GPS time series with the relative strength index: A case study of Cascadian slow slip

AbstractThe discovery of slow‐slip events over the past decades has changed our understanding of tectonic hazards and the earthquake cycle. Proper geodetic characterization of slow‐slip events is necessary for studies of regional interseismic, coseismic and postseismic deformation, and miscalculations can affect our understanding of the regional stress field and tectonic hazard. Because of the proliferation of GPS data over the last two decades, an automated algorithm is required to analyze the signals and model the deformation on a station by station basis. Using the relative strength index (RSI), a financial momentum oscillator, we test the ability to detect events of various sizes and durations. We first determine the statistics of the RSI under different noise conditions and then use this information as the basis for the automated transient detection algorithm by testing different synthetic signals. We then apply the technique to daily GPS displacement time series from 213 stations along the Cascadia subduction zone to form a record of transient deformation between 2005 and 2016. Our estimates of the spatial extent, duration, and propagation of major episodic tremor and slip (ETS) events are consistent with previous studies. We use the automated detections to remodel the displacement time series and obtain transient deformation rates over the past decade and discuss the tectonic implications. Finally, we analyze the correlation between transient detections and tremor showing good agreement between the two at slab depths commonly associated with ETS events.

Episodic Tremor and Slip (ETS) as a chaotic multiphysics spring

Episodic Tremor and Slip (ETS) events display a rich behaviour of slow and accelerated slip with simple oscillatory to complicated chaotic time series. It is commonly believed that the fast events appearing as non volcanic tremors are signatures of deep fluid injection. The fluid source is suggested to be related to the breakdown of hydrous phyllosilicates, mainly the serpentinite group minerals such as antigorite or lizardite that are widespread in the top of the slab in subduction environments. Similar ETS sequences are recorded in different lithologies in exhumed crustal carbonate-rich thrusts where the fluid source is suggested to be the more vigorous carbonate decomposition reaction. If indeed both types of events can be understood and modelled by the same generic fluid release reaction AB(solid)⇌A(solid)+B(fluid), the data from ETS sequences in subduction zones reveal a geophysically tractable temporal evolution with no access to the fault zone. This work reviews recent advances in modelling ETS events considering the multiphysics instabilities triggered by the fluid release reaction and develops a thermal-hydraulic-mechanical-chemical oscillator (THMC spring) model for such mineral reactions (like dehydration and decomposition) in Megathrusts. We describe advanced computational methods for THMC instabilities and discuss spectral element and finite element solutions. We apply the presented numerical methods to field examples of this important mechanism and reproduce the temporal signature of the Cascadia and Hikurangi trench with a serpentinite oscillator.

Model of Deep Nonvolcanic Tremor Part II: Episodic Tremor and Slip

Bursts of tremor accompany a moving slip pulse in Episodic Tremor and Slip (ETS) events. The sources of this non-volcanic tremor (NVT) are largely unknown. We have developed a model describing the mechanism of NTV generation. According to this model, NTV is a reflection of resonant-type oscillations excited in a fault at certain depth ranges. From a mathematical viewpoint, tremor (phonons) and slip pulses (solitons) are two different solutions of the sine-Gordon equation describing frictional processes inside a fault. In an ETS event, a moving slip pulse generates tremor due to interaction with structural heterogeneities in a fault and to failures of small asperities. Observed tremor parameters, such as central frequency and frequency attenuation curve, are associated with fault parameters and conditions, such as elastic modulus, effective normal stress, penetration hardness and friction. Model prediction of NTV frequency content is consistent with observations. In the framework of this model it is possible to explain the complicated pattern of tremor migration, including rapid tremor propagation and reverse tremor migration. Migration along the strike direction is associated with movement of the slip pulse. Rapid tremor propagation in the slip-parallel direction is associated with movement of kinks along a 2D slip pulse. A slip pulse, pinned in some places, can fragment into several pulses, causing tremor associated with some of these pulse fragments to move opposite to the main propagation direction. The model predicts that the frequency content of tremor during an ETS event is slightly different from the frequency content of ambient tremor and tremor triggered by earthquakes.

Slow and delayed deformation and uplift of the outermost subduction prism following ETS and seismogenic slip events beneath Nicoya Peninsula, Costa Rica

Two ODP CORK (Ocean Drilling Program circulation obviation retrofit kit) borehole hydrologic observatory sites deployed in 2002 at the toe of the subduction prism off Nicoya Peninsula, Costa Rica were visited in December 2013. The five years of seafloor and formation fluid pressure data collected since the previous visit include clear signals associated with an episodic tremor and slip (ETS) event off the coast of Nicoya Peninsula in 2009, and a Mw 7.6 subduction thrust earthquake beneath the Peninsula in 2012. Formation pressure anomalies associated with the ETS event are similar to ones observed following ETS events observed previously here, as well as ones following very low frequency earthquake swarms within the Nankai accretionary prism off southwestern Japan. Positive and negative impulsive transients in the hanging wall and foot wall of the subduction thrust, respectively, suggest contractional and dilatational strain generated by local slip propagating up the thrust fault beneath the outermost prism. In the case of the 2009 event, the transients occurred roughly two weeks after the initiation of slip observed at GPS sites along the adjacent coast. At the same time, a decrease in seafloor pressure at the prism site relative to the subducting plate was observed, indicating concurrent uplift of the prism of 1.2 cm. Other events at the prism toe following ETS events closer to the coast are seen in 2006, 2007, 2008, 2010, and 2011. The time between the initiation of ETS slip constrained by GPS and the onset of the prism toe transients suggest up-dip “rupture” propagation along the seaward part of the subduction thrust at rates of a few km/day. In the case of the 2009 event, the slip at the prism toe (c. 11 cm), estimated from the 1.2 cm uplift and the local dip on the decollement (6°), is roughly a factor of 5 greater than the slip further landward estimated from GPS data by Dixon et al. (in press). In other cases, slip at the toe is less or unresolvable. Similar uplift and formation fluid pressure transients occurred two days after the 2012 Nicoya thrust earthquake, then episodically (unrelated to aftershocks) every 1–3 weeks over the next few months. Slip rates at the prism toe, estimated from the magnitude and duration of these and ETS-related uplift events, are roughly 5 cm/day. Such rates are much too slow for tsunamigenesis or seismogenesis. Total slip of the outermost prism after the 2012 earthquake, determined from the uplift accumulated until the end of the observation period and the estimated local fault dip, is roughly 75 cm, a significant fraction of the seismogenic slip of the Mw 7.6 earthquake itself (a maximum of over 3 m). Negative steps in formation pressure are observed at both the prism and subducting plate sites simultaneous with the 2012 thrust earthquake. The sign of these steps is inconsistent with the contractional deformation predicted by an elastic fault-dislocation model with slip limited to the seismogenic zone. Highly compliant (low shear modulus) sediment below the decollement, or very low-stress deformation along splay faults, may be responsible for the co-seismic dilatation observed in the outermost prism and incoming plate.

Thermo‐poro‐mechanics of chemically active creeping faults: 3. The role of serpentinite in episodic tremor and slip sequences, and transition to chaos

AbstractDuring the last decade, knowledge over episodic tremor and slip (ETS) events has increased dramatically owing to the widespread installation of GPS and seismic networks. The most puzzling observations are (i) the periodic nature of slow seismic events, (ii) their localization at intermediate depths (estimated 15–40 km), and (iii) the origin of the nonvolcanic fluids that are responsible for the tremor activity. We reconcile these observations using a first principles approach relying on physics, continuum mechanics, and chemistry of serpentinite in the megathrust interface. The approach reproduces the GPS sequences of 17 years of recording in Cascadia, North America, as well as over 10 years in the Hikurangi Trench of New Zealand. We show that strongly endothermic reactions, such as serpentinite dehydration, are required for ETS events. We report that in this tectonic setting, it is its chemical reaction kinetics, not the low friction, that marks serpentinite as a key mineral for stable, self‐sustained oscillations. We find that the subduction zone instabilities are driven from the ductile realm rather than the brittle cover. Even when earthquakes in the cover perturb the oscillator, it relaxes to its fundamental mode. Such a transition from stable oscillations to chaos is witnessed in the ETS signal of NZ following the M6.8, 2007 seismic event, which triggered a secondary mode of oscillations lasting for a few years. We consequently suggest that the rich dynamics of ductile modes of failure may be used to decipher the chaotic time sequences underpinning seismic events.

Geodetic evidence on segmentation of Cascadia subduction zone based on episodic tremors and slips using multivariate harmonic analysis of GPS time series

Episodic tremor and slip (ETS) events with different recurrence intervals have been observed in abundance all along the Cascadia subduction zone margin. Analysis of seismic records as well as Global Positioning System (GPS) time series of the Pacific Northwest Geodetic Array (PANGA) has suggested three distinct coherent zones for the occurrence of these events. In this paper multivariate harmonic estimation has been deployed for further analysis of the segmentation in this area. Raw time series of 43 permanent GPS stations have been used for this purpose. The GPS stations have been geographically divided into three distinct groups including those in the northern, middle and southern parts of the study area. After the reduction of time series for the linear trend as well as annual and semiannual effects, the data series of each group has been analyzed using the multivariate harmonic estimation technique. Subsequently, different combinations of GPS stations including the stations located in the southern, northern and middle zones have been analyzed. Furthermore, the northern and middle, southern and middle as well as the northern and southern zone pair combinations have also been analyzed. The statistical measure devised for identifying the significant frequencies suggests common periods that are consistent with the recurrence intervals of the ETS events already reported for each of the above three geographic zones. Moreover, the method can provide geodetic evidence, in addition to geophysical ones, on the segmentation of ETSs, provided that the adopted time series are of a sufficient length. The geodetic evidence obtained in this research is consistent with the recurrence intervals as well as the boundaries obtained by the analysis of seismic records. Contrary to univariate harmonic estimation, multivariate approach using spatio-temporal correlation of the GPS time series is capable to detect those ETSs whose impacts on the time series are weak.

Propagation dynamics of seismic and aseismic slip governed by fault heterogeneity and Newtonian rheology

Slow earthquakes called episodic tremor and slip (ETS) events propagate over 100 km at low average velocities, ∼10 km per day, along several plate interfaces accompanying seismic and aseismic slip. These low propagation velocities differentiate slow earthquakes from ordinary earthquakes, and thus understanding their propagation processes is fundamental to understanding the poorly constrained physics governing the diversity and universality of earthquakes. We show that rheological heterogeneity on faults primarily governs ETS propagation on the basis of comprehensive modeling and observations that correlate migration patterns with the energetics of tremor on a plate‐bounding fault in southwest Japan. The fault has persistent small‐scale segmentation, in which ETS events started propagating energetically in relatively brittle sections and decelerated generally with a parabolic pattern in relatively ductile sections. Simulated spontaneous ruptures that are based on these parabolic tremor migration patterns constrain the cause of ductility to Newtonian plastic flow or perhaps dilatant strengthening, but reject large‐scale fluid flows. We discuss possible elementary processes underlying the Newtonian rheology. This model is also consistent with the observed seismicity migration pattern before the 2011Mw9.0 Tohoku‐oki earthquake, suggesting delayed triggering by theMw 7.3 foreshock.

Tremor asperities in the transition zone control evolution of slow earthquakes

Slow earthquakes, characterized by slow slip and associated seismic radiation called non‐volcanic tremor, have been observed in major subduction zones worldwide. They constitute an important mode of stress release for the fault's transition zone, which lies directly downdip of the locked segment, the nucleation zone of large damaging earthquakes. However, the depth of tremor in Cascadia, and the factors governing tremor generation and rupture propagation during slow quakes remain enigmatic. Here, we develop a novel multibeam‐backprojection (MBBP) method to detect and locate tremor using multiple mini seismic arrays. We apply this technique to image tremor activity during an entire ETS‐cycle including a large episodic tremor and slip (ETS) event in Cascadia with unprecedented resolution. Our results suggest that the majority of the tremor is occurring near the plate interface. We observe strongly heterogeneous tremor distribution with patches in the transition zone that experience repeated tremor episodes and produce most of the tremor. The patches, tens of kilometers in dimension, behave like asperities on the fault plane. During the large ETS event, rupture propagation velocity varies at least by a factor of five, and seems to be modulated by these tremor asperities. These observations support a model in which the transition zone is heterogeneous and consists of patches of asperities with surrounding regions slipping aseismically. The asperities fail quasi‐periodically releasing stress and appear to regulate rupture propagation and tremor generation during slow earthquakes. This study presents new observations revealing the tectonic characteristics of the transition zone controlling the generation and evolution of slow earthquakes.

Temporal evolution of an episodic tremor and slip event along the northern Cascadia margin

[1] Episodic tremor and slip (ETS) have now been observed along the northern Cascadia margin for over 15 years. Recent densification of GPS coverage and the introduction of Gladwin borehole strainmeters (BSMs) under the Plate Boundary Observatory have enabled the derivation of improved slip models and have allowed more detailed monitoring of the migration of the slip surface during prolonged ETS events. For this study we examine the along-strike migration of the May 2008 ETS as determined from the GPS and BSM observations. GPS sites overlying the 30 km depth contour of the subducting plate interface show that the slip initiates west of northern Puget Sound and propagates bidirectionally. Shear strain time series at regional BSM sites confirm this bidirectional expansion of the slip zone. In a simple model of along-strike propagation constrained by the GPS and BSM time series, the speed of the northwest propagation varies from 8, then to 2, and then to 15 km/d, but the speed of the south propagation stays at 6 km/d. The tremors observed for this ETS episode show a similar bidirectional migration pattern and similar changes in northwest migration velocity, but the migration front is slightly ahead of the slip propagation front. These results provide important information for understanding the mechanics of ETS.

Space-time correlation of slip and tremor during the 2009 Cascadia slow slip event

[1] Episodic Tremor and Slip (ETS), involving transient deformations accompanied by emergent, low-frequency tremor occurs in subduction zones around the world. ETS events increase the shear stress on locked megathrusts and may potentially trigger damaging earthquakes. Despite the clear association of tremor and slip the physical relationship between them is unresolved. Tremor appears to result from slip on small asperities on the plate interface due to either creep on the surrounding fault, or stress increases ahead of the propagating slow-slip front. Previous studies of migrating slow slip events have not had sufficient spatial and temporal resolution to differentiate between these two models. To address this, we invert GPS data from the August 2009 ETS event in central Cascadia for the space-time evolution of fault slip-rate. We find a correlation in both space and time between tremor epicenters and the independently determined position of high fault slip-rate. This supports the first hypothesis that tremor asperities are loaded directly by slow slip, rather than by stress increases ahead of the slip front, and provides new insights into the mechanics of ETS.

Episodic tremors and slip in Cascadia in the framework of the Frenkel-Kontorova model

Received 27 August 2010; revised 20 October 2010; accepted 28 October 2010; published 13 January 2011. [1] The seismic moment for regular earthquakes is proportional to the cube of rupture time. A second class of phenomena, collectively called slow earthquakes, has very different scaling. We propose a model, inspired from the phenomenology of dislocation dynamics in crystals, that is consistent with the scaling relations observed in the Cascadia episodic tremor and slip (ETS) events. Two fundamental features of ETS are periodicity and migration. In the northern Cascadia subduction zone, ETS events appear every 14.5 months or so. During these events, tremors migrate along‐strike with a velocity of 10 km/day and simultaneously zip back and forth in the relative plate‐ motion direction with at ypical velocity of 50 km/h. Our model predicts the formation of a sequence of slip pulses on the boundary of the plates, which describes the major features of fault dynamics, including periodicity and the migration pattern of tremors. Citation: Gershenzon, N. I., G. Bambakidis, E. Hauser, A. Ghosh, and K. C. Creager (2011), Episodic tremors and slip in Cascadia in the framework of the Frenkel‐Kontorova model, Geophys. Res. Lett., 38, L01309,

An earthquake‐like magnitude‐frequency distribution of slow slip in northern Cascadia

Major episodic tremor and slip (ETS) events with Mw 6.4 to 6.7 repeat every 15 ± 2 months within the Cascadia subduction zone under the Olympic Peninsula. Although these major ETS events are observed to release strain, smaller “tremor swarms” without detectable geodetic deformation are more frequent. An automatic search from 2006–2009 reveals 20,000 five‐minute windows containing tremor which cluster in space and time into 96 tremor swarms. The 93 inter‐ETS tremor swarms account for 45% of the total duration of tremor detection during the last three ETS cycles. The number of tremor swarms, N, exceeding duration τ follow a power‐law distribution N ∝ τ−0.66. If duration is proportional to moment release, the slip inferred from these swarms follows a standard Gutenberg‐Richter logarithmic frequency‐magnitude relation, with the major ETS events and smaller inter‐ETS swarms lying on the same trend. This relationship implies that 1) inter‐ETS slip is fundamentally similar to the major events, just smaller and more frequent; and 2) despite fundamental differences in moment‐duration scaling, the slow slip magnitude‐frequency distribution is the same as normal earthquakes with a b‐value of 1.

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.

Northern Cascadia episodic tremor and slip: A decade of tremor observations from 1997 to 2007

We analyze continuous seismic and GPS records collected in the last decade (1997–2007) to establish the most comprehensive observational basis for northern Cascadia episodic tremor and slip (ETS) events. A simple “ETS scale” system, using a combination of a letter and a digit, is proposed to quantitatively characterize the spatial and temporal dimensions of ETS events. Clear correlation between GPS and tremor signals is observed for all A/B class episodes, but the GPS signature is less obvious for minor ones. Regular ETS recurrence can be established only for A/B class episodes in southern Vancouver Island. Halting and jumping are very common in ETS migration patterns, and along‐strike migration can happen in both directions. A prominent tremor gap is observed in midisland around 49.5°N. This gap coincides with the epicenters of the only two large earthquakes beneath Vancouver Island. ETS tremors also tend to occur in places where the local seismicity is relatively sparse. The tremor depth distribution shows a peak in the 25–35 km range where strong seismic reflectors (i.e., the E layer) are documented. Detailed waveform analysis confirms the existence of shallow tremors above the currently interpreted plate interface. Our results suggest that a significant portion of the tremor activity and perhaps associated shearing are taking place along well‐developed structures such as the E layer, while fewer tremor bursts are generated elsewhere in response to the induced stress variation throughout the source volume.

Seismic and geodetic constraints on Cascadia slow slip

Automatically detected and located tremor epicenters from episodic tremor and slip (ETS) episodes in northern Cascadia provide a high‐resolution map of Washington's slow slip region. Thousands of epicenters from the past four ETS events from 2004 to 2008 provide detailed map‐view constraints that correlate with geodetic estimates of the simultaneous slow slip. Each of these ETS events exhibits remarkable similarity in the timing and geographic distribution of tremor density and geodetically inferred slip. Analysis of the latest 15‐month inter‐ETS period also reveals ageodetic tremor activity similar both in duration and extent to ETS tremor. Epicenters from both ETS and inter‐ETS tremor are bounded between the 30‐ and 45‐km plate interface depth contours and locate approximately 75 km east of previous estimates of the locked portion of the subducting Juan de Fuca plate. Inter‐ETS tremor overlaps but is generally downdip of ETS tremor and does not yet correlate with geodetically observed slip, but this is likely because the slip is below current GPS detection levels. Based on the tremor and slip correlation and the tremor‐duration and slip magnitude relationship, we suggest that the well‐resolved, sharp updip edge of tremor epicenters reflects a change in plate interface coupling properties. The region updip of this boundary may accumulate stress with the potential for coseismic shear failure during a megathrust earthquake. Alternatively, plate convergence in this region could be accommodated by continuous slow slip with no detectable tremor or by slow slip events with sufficiently long recurrence intervals that none have been detected during the past 10 years of GPS observations.

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.