Episodic Tremor And Slip Research Articles

Seismic tremor in subduction zones: Rock physics evidence

Episodic tremor and slip (ETS) have been correlated with rupture phenomena in subducting oceanic lithosphere at 30–45 km depth, where high VP/VS ratios, which suggest high‐fluid pressures, have been observed. ETS, by accommodating slip in the down‐dip portion of the subduction zone, may trigger megathrust earthquakes up‐dip in the locked section. During dehydration experiments on serpentinite (typical rock of the oceanic lithosphere) at temperatures found in nature at 30–45 km depth (400–550°C), we observe seismic signals in the form of acoustic emissions that closely resemble low frequency earthquakes, seismic tremor and regular earthquakes. Our findings support the concept that water released during dehydration reactions increases the pore pressures and can trigger ETS and regular earthquakes by reducing slip resistance.

Seismic wave triggering of nonvolcanic tremor, episodic tremor and slip, and earthquakes on Vancouver Island

We explore the physical conditions that enable triggering of nonvolcanic tremor and earthquakes by considering local seismic activity on Vancouver Island, British Columbia during and immediately after the arrival of large‐amplitude seismic waves from 30 teleseismic and 17 regional or local earthquakes. We identify tremor triggered by four of the teleseismic earthquakes. The close temporal and spatial proximity of triggered tremor to ambient tremor and aseismic slip indicates that when a fault is close to or undergoing failure, it is particularly susceptible to triggering of further events. The amplitude of the triggering waves also influences the likelihood of triggering both tremor and earthquakes such that large amplitude waves triggered tremor in the absence of detectable aseismic slip or ambient tremor. Tremor and energy radiated from regional/local earthquakes share the same frequency passband so that tremor cannot be identified during these smaller, more frequent events. We confidently identify triggered local earthquakes following only one teleseism, that with the largest amplitude, and four regional or local events that generated vigorous aftershock sequences in their immediate vicinity. Earthquakes tend to be triggered in regions different from tremor and with high ambient seismicity rates. We also note an interesting possible correlation between large teleseismic events and episodic tremor and slip (ETS) episodes, whereby ETS events that are “late” and have built up more stress than normal are susceptible to triggering by the slight nudge of the shaking from a large, distant event, while ETS events that are “early” or “on time” are not.

Automated detection and location of Cascadia tremor

A new autonomous seismic location and detection methodology enables real‐time opportunities for high‐resolution spatio‐temporal monitoring of non‐volcanic tremor. Combining a unique cross‐correlation technique with epicenter clustering analysis in northern Cascadia automatically yields thousands of tremor epicenters from the May 2008 Episodic Tremor and Slip (ETS) event while routinely detecting and locating inconspicuous inter‐ETS tremor bursts. Although ETS events in this area produce about two weeks of continuous tremor, we find a nearly equal amount of tremor during the last 15‐month inter‐ETS period. The resulting ETS and inter‐ETS epicenters occur in the slow slip region where the plate interface is 30–45 km deep and have a sharp, well‐resolved updip boundary about 75 km east of the downdip edge of the seismogenic megathrust zone. This ability to track tremor with high spatio‐temporal resolution facilitates automatic tremor monitoring and the mapping of the transition zone and regions of locked zone stress accumulation.

Probable low-angle thrust earthquakes on the Juan de Fuca–North America plate boundary

In 2004, two clusters of earthquakes occurred in the central part of the Cascadia forearc, which displays several characteristics indicative of along-strike and downdip variations in plate coupling. Moment tensor analysis for the main shock in each cluster indicates that both events, which had magnitudes of 4.9 and 4.8, were compatible with low-angle thrust motion on fault planes dipping 6°–15° to the east, consistent with the plate boundary dip of ∼12°. By tracing rays through a high-resolution two-dimensional crustal velocity model to match arrival times of secondary arrivals (pP and PmP), we estimate that the source depth was 9–11 km for the M4.9 event and 15–17 km for the M4.8 event. We conclude that these earthquakes probably represent seismogenic thrust motion on the nominally locked or transitional part of the Cascadia megathrust, updip of where episodic tremor and slip (ETS) have been documented. Continued high-resolution observations of seismicity and improved crustal models are needed to confirm whether apparent temporal correlations between ETS and continued seismic activity in these clusters indicate stress transfer along the megathrust.

Automatic detection and characterization of seismic tremors in northern Cascadia

We develop an algorithm that automatically detects and characterizes seismic waveforms associated with episodic tremor and slip (ETS) events in northern Cascadia. For each hour‐long seismogram, the process begins with the calculation of moving average and scintillation index functions. A logic chart is designed to sort the observed waveform patterns into a set of classes based on these functions. An ETS event is always manifest by many station‐hours with coherent tremor (CT) patterns. Automatic analysis of seismic waveforms recorded in the past three years gives results consistent with the work done by tedious visual inspection of seismograms. Our algorithm, which is free from human oversight, is ideal for an automatic tremor monitoring system to give immediate notice of an ETS event.

Migration characteristics of seismic tremors in the northern Cascadia margin

Seismic waveforms from a dense regional array were analyzed to study episodic tremor‐and‐slip (ETS) events in northern Cascadia. The April 2005 episode occurred mostly in northern Vancouver Island (VI) where the Explorer plate subducts beneath the North America plate, while the May 2005 episode was mainly in middle VI. All tremor occurrences are positively correlated with the presence of subducted slabs. Besides the “steady” along‐strike migration and “halting” behavior found in previous ETS events, a new pattern (“jumping”) is found this time. Visual inspection of previous seismic records suggests that the jumping behavior is probably common to ETS tremors in all regions of VI. The majority of tremors occurred between 25 and 45 km depth in places where local seismicity is sparse or absent. Our observations are compatible with the interpretation that fluids released from dehydration of subducted materials are involved in tremor generation.

Segmentation in episodic tremor and slip all along Cascadia

The recent discovery of episodic tremor and slip (ETS) in subduction zones is based on slow slip episodes visible in global positioning system observations correlated with nonvolcanic tremor signals on seismometers. ETS occurs just inboard from a region capable of great megathrust earthquakes; however, whether there is any communication between these two processes remains unknown. In this study we use new single-station methods to compile an ETS catalog for the entire Cascadia subduction zone, offshore western North America, and compare the patterns with a variety of along-strike trends for the subducting and overriding plates. Correlated ground vibrations and strain observations are found all along the subduction zone, demonstrating that ETS is an inherent part of the subduction process. There are three broad (300–500 km), coherent zones with different recurrence intervals (14 ± 2, 19 ± 4, 10 ± 2 months), where the interval duration is inversely proportional to upper plate topography and the spatial extent correlates with geologic terranes. These zones are further divided into segments of ETS that occur at times typically offset from each other. The seven largest (100– 200 km) segments appear to be located immediately landward from forearc basins interpreted as manifestations of megathrust asperities, implying that there is a spatial link between ETS and earthquake behavior. It is not yet clear if any temporal link exists, but the regional time between ETS episodes could be controlled by strength variations due to composition of geologic terranes.

Short‐term slow slip and correlated tremor episodes in the Tokai region, central Japan

We observe the related phenomenon of short‐term slow slip event (SSE) and low‐frequency nonvolcanic tremor on the deeper extension of the 1944 Tonankai and the anticipated Tokai earthquake rupture areas, central Japan. Japan nation‐wide seismometer and tiltmeter array, NIED Hi‐net has recorded these correlated episodes of SSE and tremor, known as episodic tremor and slip (ETS) in 2004 and 2005. The events are located on two areas in the Tokai region, one is near the Shima peninsula, and the other is around the Aichi area. Both episodes have a typical duration of 2–3 days and released seismic moment corresponding to Mw ∼ 6.0. Moreover, these episodes repeatedly occur with a recurrence interval of about six months in both areas. This evidence suggests that the coincidence between the SSE and the tremor is a characteristic behavior in the whole tremor belt‐like zone in the Nankai trough subduction zone.

Spatial‐temporal patterns of seismic tremors in northern Cascadia

We study in detail the two consecutive episodic tremor‐and‐slip (ETS) events that occurred in the northern Cascadia subduction zone during 2003 and 2004. For both sequences, the newly developed Source‐Scanning Algorithm (SSA) is applied to seismic waveform data from a dense regional seismograph array to determine the precise locations and origin times of seismic tremors. In map view, the majority of the tremors occurred in a limited band bounded approximately by the surface projections of the 30‐km and 50‐km depth contours of the plate interface. The horizontal migration of tremor occurrence is from southeast to northwest with an average speed of 5 km/d. In cross section, tremors in both sequences span a depth range of over 40 km across the interface, with the majority occurring in the overriding continental crust. In particular, 50–55% of them are located within 2.5 km from the strong seismic reflector bands above the plate interface. The lack of vertical migration implies that a slow diffusion process in the vertical direction cannot be responsible for tremor occurrences. The source spectra of tremors clearly lack high‐frequency content (>5 Hz) relative to local earthquakes. We propose two possible models to explain the relationship between slip and tremors. The first one regards ETS tremors as the manifestation of hydroseismogenic processes in response to the temporal strain variation associated with the episodic slip along the lower portion of the plate interface downdip from the locked zone. In the second model, tremors and slip are associated with the same process along the same structure in a distributed deformation zone across the plate interface. Neither model can be dismissed conclusively at this stage.

Geodetic and seismic signatures of episodic tremor and slip in the northern Cascadia subduction zone

Slip events with an average duration of about 10 days and effective total slip displacements of severalc entimetres have been detected on the deeper (25 to 45 km) part of the northern Cascadia subduction zone interface by observing transient surface deformation on a network of continuously recording Global Positioning System (GPS) sites. The slip events occur down-dip from the currently locked, seismogenic portion of the subduction zone, and, for the geographic region around Victoria, British Columbia, repeat at 13 to 16 month intervals. These episodes of slip are accompanied by distinct, low-frequency tremors, similar to those reported in the forearc region of southern Japan. Although the processes which generate this phenomenon of episodic tremor and slip (ETS) are not well understood, it is possible that the ETS zone may constrain the landward extent of megathrust rupture, and conceivable that an ETS event could precede the next great thrust earthquake.

The Source-Scanning Algorithm: mapping the distribution of seismic sources in time and space

SUMMARY We introduce a new method, which we call the Source-Scanning Algorithm (SSA), for imaging the distribution of seismic sources in both time and space. Using trial locations and origin times, the method calculates the ‘brightness’ function by summing the absolute amplitudes observed at all stations at their respective predicted arrival times. The spatial and temporal distribution of sources is then identified by a systematic search throughout the model space and time for the maximum brightness. The greatest advantages of this method are that: (1) it exploits waveform information (both arrival times and relative amplitudes) without the need to calculate highfrequency synthetic seismograms; and (2) it requires neither pre-assembled phase-picking data nor any a priori assumptions about the source geometry. A series of tests using synthetic data have shown that this method is robust and can faithfully recover the input source configuration to within 1 grid interval. Finally, we demonstrate the value of the algorithm by locating a typical tremor event with emergent waveforms that occurred during the recent episodic tremor and slip (ETS) sequence in the northern Cascadia subduction zone.

Episodic tremor and slip on the Cascadia subduction zone: the chatter of silent slip.

We found that repeated slow slip events observed on the deeper interface of the northern Cascadia subduction zone, which were at first thought to be silent, have unique nonearthquake seismic signatures. Tremorlike seismic signals were found to correlate temporally and spatially with slip events identified from crustal motion data spanning the past 6 years. During the period between slips, tremor activity is minor or nonexistent. We call this associated tremor and slip phenomenon episodic tremor and slip (ETS) and propose that ETS activity can be used as a real-time indicator of stress loading of the Cascadia megathrust earthquake zone.