Triggered Tremor Research Articles

Tectonic tremors in the Northern Mexican subduction zone remotely triggered by the 2017 Mw8.2 Tehuantepec earthquake

Surface waves from the 2017 Mw8.2 Tehuantepec earthquake remotely triggered tectonic tremors in the Jalisco region, approximately 1000 km WNW in the northern Mexican subduction zone. This is the first observation of tremor triggering in this region and one of the largest known examples of a triggered tremor in the world. Although prior studies have found tectonic tremors triggered by teleseismic waves in subduction zones and plate boundaries, further investigation of tremor triggering is crucially important for understanding the causative mechanism. We calculate the stress and strain changes across the three-dimensional plate interface attributable to seismic waves from the earthquake by full wavefield simulation. The maximum magnitude of the dynamic strain tensor eigenvalues on the plate interface, where tremors likely occur, is approximately 10–6. The subducting slab geometry effectively amplifies triggering waves. The triggering Coulomb failure stress changes resolved for a thrust fault plane consistent with the geometry are estimated to be approximately 10–40 kPa. The relationship between the triggering stress and triggered tremor amplitude may indicate that the aσ of the rate–state-dependent friction law is 10–100 kPa.

Does Deep Tectonic Tremor Occur in the Central‐Eastern Mediterranean Basin?

AbstractTectonic tremor has been observed at the roots of many fault systems around the Pacific rim, including convergent and transform plate boundaries. The extent to which deep tremor signals are prevalent along fault systems elsewhere, including the Mediterranean basin, has not yet been documented in detail. A body of evidence suggests that tremor triggered during the surface waves of teleseismic events may commonly occur where ambient tremor during episodic tremor and slip episodes occur, suggesting triggered tremor provides a useful tool to identify regions with ambient tremor. We perform a systematic search of triggered tremor associated with large teleseismic events between 2010 and 2020 at four major fault systems within the central‐eastern Mediterranean basin, namely the Hellenic and Calabrian subduction zones, and the North Anatolian and Kefalonia transform faults. In addition, we search for ambient tremor during a slow slip event in the eastern Sea of Marmara along a secondary branch of the North Anatolian Fault, and two slow slip events beneath western Peloponnese (Hellenic Subduction Zone). We find no unambiguous evidence for deep triggered tremor, nor ambient tremor. The absence of triggered tremor at the Hellenic and Calabrian subduction zones supports an interpretation of less favorable conditions for tremorgenesis in the presence of old and cold slabs. The absence of tremor along the transform faults may be due to an absence of the conditions commonly promoting tremorgenesis in such settings, including high‐fluid pressures and low‐differential stresses between the down‐dip limit of the seismogenic layer and the continental Moho.

Isolated Triggered Tremor Spots in South America and Implications for Global Tremor Activity

ABSTRACT We report new observations of triggered tectonic tremor in three regions in South America along the plate boundary between the Nazca and South America plates: southern Chile, Ecuador, and central Colombia. In these regions, tremor was observed during the passage of large‐amplitude surface waves of recent large earthquakes, which occurred in South America and around the world. In southern Chile, triggered tremor was observed around an ambient tremor active zone in the Chile triple junction region. In Ecuador and central Colombia, only one seismic station in each region recorded triggered tremor. With a single‐station approach, we are able to estimate potential tremor sources in these regions. Triggered tremor in Ecuador is likely associated with an inland fault near the volcanic region. In central Colombia, triggered tremor may be associated with the Romeral fault system rather than the subduction zone interface. In addition, we summarize global observations of tremor‐triggering stress and background ambient tremor activity in 24 tremor‐active regions. Based on the global summary of triggered and ambient tremor activity, the relative lack of triggered tremor in central and northern Chile and Peru is consistent with the lack of background tremor activity in these regions, suggesting tectonic tremor occurs only in isolated regions along major faults.

A Lack of Dynamic Triggering of Slow Slip and Tremor Indicates That the Shallow Cascadia Megathrust Offshore Vancouver Island Is Likely Locked

AbstractGreat subduction zone earthquakes vary considerably in the updip extent of megathrust rupture. It is unclear if this diversity reflects variations in interseismic strain accumulation owing to the limited number of subduction zones with seafloor monitoring. We use a borehole seismic‐geodetic observatory installed at the updip end of the Cascadia fault offshore Vancouver Island to show that the megathrust there does not appear to slip in triggered tremor or slow‐slip events when subjected to moderate dynamic stress transients. Borehole tilt and seismic data from recent teleseismic M7.6–8.1 earthquakes demonstrate a lack of triggered slow slip above the Mw 4.0 level and an absence of triggered tremor despite shear‐stress transients of 1–10 kPa that were sufficient to trigger tremor on the downdip end of the interface. Our observations are most consistent with a model in which the Cascadia fault offshore Vancouver Island is locked all the way to the trench.

A MATLAB GUI for Examining Triggered Tremor: A Case Study in New Zealand

We introduce a user‐friendly MATLAB GUI (graphical user interface) software for searching for triggered tectonic tremor. This software includes the following features: (1) multiple panels, whose primary function screen shows seismograms filtered at various frequency bands, surface‐wave seismograms or spectrograms for comparison, and a station map; (2) a tag function with which users can classify each station by quality and/or group and whose results will show on the map; (3) the locations of tremor sources from which users can manually pick the arrival times of individual tremor bursts and quickly obtain preliminary locations; and (4) output visualization from which users can align triggered tremor as a move‐out plot or save snapshots to a PDF file. For demonstration, we apply this package to examine triggered tremor in New Zealand between 2004 and 2016. We analyze 5300 seismograms from 18 M_w ≥ 7.5 target earthquakes and find that 9 generate triggered tremors in six regions in the central and the northern Hikurangi subduction margin, the central strike‐slip Alpine fault, Kaikōura and Southland in the South Island, and Northland in the North Island. The minimum apparent tremor‐triggering stress observed at the surface stations in these regions varies from 5.0 to 16.6 kPa. This GUI‐based package can facilitate the process of analysis for identifying triggered seismic events from a sizeable seismic dataset.

Migration of Deep Low‐Frequency Tremor Triggered by Teleseismic Earthquakes in the Southwest Japan Subduction Zone

AbstractDeep low‐frequency tremor triggered by teleseismic surface waves is concentrated in several locations along the ambient tremor source region in the Nankai subduction zone, southwest Japan. We systematically investigated triggered tremor in northern Kii and western Shikoku, at 71 teleseismic earthquakes by applying a matched filter technique. We detected 9 and 17 events of triggered tremor in northern Kii and western Shikoku, respectively. In addition, we observed tremor migration episodes, whose directions were always along‐dip. In northern Kii, seven episodes showed downward migration at a speed of 5–20 km/hr. In western Shikoku, six episodes migrated at a speed of 10–100 km/hr, in which five episodes showed upward migration. Migration of the triggered tremor suggests that dynamic stress perturbation from distant earthquakes may trigger propagating creep events caused by fluid.

Tectonic tremor and LFEs on a reverse fault in Taiwan

AbstractWe compare low‐frequency earthquakes (LFEs) from triggered and ambient tremor under the southern Central Range, Taiwan. We apply the PageRank algorithm used by Aguiar and Beroza (2014) that exploits the repetitive nature of the LFEs to find repeating LFEs in both ambient and triggered tremor. We use these repeaters to create LFE templates and find that the templates created from both tremor types are very similar. To test their similarity, we use both interchangeably and find that most of both the ambient and triggered tremor match the LFE templates created from either data set, suggesting that LFEs for both events have a common origin. We locate the LFEs by using local earthquake P wave and S wave information and find that LFEs from triggered and ambient tremor locate to between 20 and 35 km on what we interpret as the deep extension of the Chaochou‐Lishan Fault.

Shallow episodic tremor near the Nankai Trough axis off southeast Mie prefecture, Japan

AbstractWe analyzed long‐term continuous seismic records (from September 2015 to April 2016) of Dense Ocean‐floor Network System for Earthquake and Tsunamis, an ocean‐floor observation system deployed around the fore‐arc slope of the Nankai subduction zone to investigate shallow tremor near the trough axis. We found that the activity of shallow tremor was concentrated in two time periods: 6 days in October 2015 and 2 weeks in April 2016. During the episode in April 2016, migration and triggering of tremor were observed. These characteristics are similar to those of tremor in the deeper part of the subduction zone. The triggering of tremor indicates that the tremor activity is very sensitive to nearby stress perturbation in the area of this study, which is near the initiation points of past large earthquakes along the Nankai Trough. Therefore, it is very important to monitor tremor activity in this region for understanding the stress accumulation process of megathrust earthquakes.

Exploration of remote triggering: A survey of multiple fault structures in Haiti

Triggering studies provide an important tool for understanding the fundamental physics of how faults slip and interact, and they also provide clues about the stress states of faults. In this study, we explore how seismic waves from the 27 February 2010 Mw8.8 Maule, Chile mainshock interact with the left lateral strike-slip Enriquillo–Plantain Garden Fault (EPGF) and surrounding reverse faults in the southern Haiti peninsula. The Chile mainshock occurred 6,000 km away and just 46 days after the 12 January 2010 Mw7.0 Haiti earthquake, a tragic event which activated multiple faults in the southern Haiti peninsula. During the surface waves of the Chile mainshock, several tectonic tremor signals were observed, originating from south of the EPGF trace. Cross-correlation of the triggered tremor and transient stresses resolved onto to the EPGF indicates that the Love wave of the Chile mainshock was the primary driving mechanism of the triggered deep shear slip and tremor signals, as opposed to dilatational stress changes generated by the Rayleigh wave. We also searched for any influence of transient stresses on Haiti aftershock activity by applying the matched filter technique to multiple days of seismic data around the time of the Chile mainshock. While we identified a slight increase in Haiti aftershock activity rate, the rate changes were significant only when small magnitude events were included in the significance tests. These observations are generally consistent with recent inferences that deep tectonic tremor is more sensitive than shallow earthquakes to external stress perturbations.

Triggered tectonic tremor in various types of fault systems of Japan following the 2012 Mw8.6 Sumatra earthquake

AbstractTectonic tremor, an extremely stress‐sensitive seismic phenomenon occurring in the brittle‐ductile transition section of a fault, is associated with the shearing mechanism of slow slip. Observations of triggered tremor can facilitate the evaluation of the existence of background ambient tremor and slow‐slip events. This paper presents widespread triggered tremor sources in Japan initiated by the surface waves of the 2012 Mw8.6 Sumatra earthquake on strike‐slip and thrust faults, in the deep volcanic low‐frequency earthquake active area, in the shallow tectonic tremor and very low frequency earthquake active regions, and in the subduction zone. In most regions, the amplitudes of triggered tremor are generally logarithmically proportional to the dynamic stress caused by various triggering earthquakes. Our observations suggest that triggered tremor in the newly discovered sources is the result of a more rapid rate of background ambient tremor, and evidence has suggested the existence of ambient tremor in some regions.

Triggered Seismic Events along the Eastern Denali Fault in Northwest Canada Following the 2012 Mw 7.8 Haida Gwaii, 2013 Mw 7.5 Craig, and Two Mw>8.5 Teleseismic Earthquakes

We conduct a systematic search for remotely triggered seismic activity along the eastern Denali fault (EDF) in northwest Canada, an intraplate strike‐slip region. We examine 19 distant earthquakes recorded by nine broadband stations in the Canadian National Seismograph Network and find that the 2012 M w 7.8 Haida Gwaii and 2013 M w 7.5 Craig, Alaska, earthquakes triggered long duration (>10 s), emergent tremor‐like signals near the southeastern portion of the EDF. In both cases, tremor coincides with the peak transverse velocities, consistent with Love‐wave triggering on right‐lateral strike‐slip faults. The 2011 M w 9.0 Tohoku‐Oki and 2012 M w 8.6 Indian Ocean earthquakes possibly triggered tremor signals, although we were unable to locate those sources. In addition, we also identify many short‐duration (<5 s) bursts that were repeatedly triggered by the Rayleigh waves of the 2012 M w 7.8 Haida Gwaii earthquake. Although we were unable to precisely locate the short‐duration (<5 s) events, they appear to be radiating from the direction of the Klutlan Glacier and from a belt of shallow historical seismicity at the eastern flank of the Wrangell–St. Elias mountain range. The fact that these events were triggered solely by the Rayleigh waves suggests a different source mechanism as compared with triggered tremor observed along the EDF and other plate boundary regions. Online Material: Description of Rayleigh‐wave polarization detection; figures of spectograms, S ‐wave amplitudes, polarization analysis, and static stress changes; tables of earthquake catalogs and velocity model; and animations of tremors and bursts.

Triggering of tremor and inferred slow slip by small earthquakes at the Nankai subduction zone in southwest Japan

AbstractThe correlation of earthquakes with tremor and slow slip has not been clearly quantified. We investigate 12 year earthquake and tremor catalogs for southwest Japan and find that nearby small intraslab earthquakes are weakly correlated with tremor. In particular, the intraslab earthquakes with magnitudes ≥2.7 tend to be followed by tremor more often than expected at random by a factor of 2 to 6. The excess number of tremor before earthquakes is not as significant, although marginally more than expected. The underlying physical mechanism of the observed triggering of tremor and inferred slow slip by earthquakes is most likely to be the dynamic stress changes (several to several tens of kilopascals) rather than the much smaller static stress changes. The rate of triggering of tremor by earthquakes is similar to, although somewhat lower than, rates observed for similar amplitude stress changes due to the lower‐frequency teleseismic surface waves and tidal stressing.

S wave attenuation structure on the western side of the Nankai subduction zone: Implications for fluid distribution and dynamics

AbstractWe estimated the S wave attenuation structure in southwestern Japan and the western Nankai Trough by analyzing maximum S wave amplitudes at 4–8, 8–16, and 16–32 Hz with a correction term for apparent amplitude attenuation due to multiple forward scattering. Because the estimated attenuation (Q−1) in our tomographic study was much larger than Q−1 due to wide‐angle scattering, our estimated Q−1 was composed mainly of intrinsic attenuation. High‐attenuation areas (Q−1 &gt; 1/300 at 4–8 Hz) were imaged beneath Quaternary volcanoes and south off Shikoku. Low (&lt;1/1500 at 4–8 Hz) or moderate Q−1 (1/500–1/1000 at 4–8 Hz) was imaged beneath Shikoku and nonvolcanic areas of Chugoku. High and moderate Q−1 in and around Shikoku are located near the top of subducting Philippine Sea Plate. This correspondence implies that these high and moderate Q−1 reflect fluid in the subducting slab. By applying a theoretical model of attenuation in water‐saturated porous random media, we examined wave‐induced fluid flow induced by lower frequency (&lt;1 Hz) seismic waves that may be related with triggering of nonvolcanic tremor by surface waves. Even though Q−1 structure in this study cannot fully explain the tremor triggering by wave‐induced fluid flow, large uncertainties of Q−1 in tremor zone suggest that high resolution imaging of Q−1 and random inhomogeneities would give some constraints for the spatial variation of permeability and other medium properties.

Dynamic triggering of microearthquakes in three geothermal/volcanic regions of California

AbstractGeothermal/volcanic regions are most susceptible to local earthquake triggering by regional and remote earthquakes. Transient stresses caused by surface waves of these earthquakes can activate critically stressed faults. Though earthquakes can be triggered in geothermal/volcanic regions, it is less understood how these regions differ in their triggering responses to distant earthquakes. We conduct a systematic survey of local earthquakes triggered by distant earthquakes in three geothermal/volcanic regions of California: Long Valley Caldera, Coso Geothermal Field, and Geysers Geothermal Field. We examine waveforms of distant earthquakes with magnitudes ≥ 5.5 occurring between 2000 and 2012 and compute β statistics to confirm the significance of our findings. We find that Long Valley, Coso, and Geysers vary in triggering frequency—2.0%, 3.8%, and 6.8% in the 12 year period, respectively—and when compared to the triggering of deep tectonic tremors along the Parkfield‐Cholame section of San Andreas Fault (9.2% in the 12 year period). Stress triggering thresholds vary among the regions with Long Valley having the highest of ~5 kPa and ~1 kPa for the other regions. Because dynamic stresses from distant earthquakes are similar in these three regions, the varying triggering behavior likely reflects faults having a tendency to be at or near failure. This is compatible with Geysers having a higher a value in the Gutenberg‐Richter relationship and higher geothermal productivity than the other two regions. The observation of more frequent triggering of tremor than microearthquakes is consistent with recent laboratory studies on increasing triggerability with lower effective stress.

Remotely triggered nonvolcanic tremor in Sumbawa, Indonesia

AbstractWe present, for the first time, evidence for triggered tremor beneath the island of Sumbawa, Indonesia. We show triggered tremor in response to three teleseismic earthquakes: the Mw9.0 2011 Tohoku earthquake and two oceanic strike‐slip earthquakes (Mw 8.6 and Mw8.2) offshore of Sumatra in 2012. We constrain an apparent triggering threshold of 1 mm/s ground velocity that corresponds to about 8 kPa dynamic stress. Peak tremor amplitudes of about 180 nm/s are observed, and scale with the ground velocity induced by the remote earthquakes. Triggered tremor responds to 45–65 s period surface waves and predominantly correlates with Rayleigh waves, even though the 2012 oceanic events have stronger Love wave amplitudes. We could not locate the tremor because of minimal station coverage, but data indicate several potential source volumes including the Flores Thrust, the Java subduction zone, or Tambora volcano.

Model of Deep Nonvolcanic Tremor Part I: Ambient and Triggered Tremor

There is evidence of triggering of tremor by seismic waves emanating from distant large earthquakes. The frequency contents of triggered and ambient tremor are largely identical, suggesting that tremor does not depend directly on the nature of the source. We show here that the model of plate dynamics developed earlier by us is an appropriate tool for describing the onset of tremor. In the framework of this model, tremor is an internal response of a fault to a failure triggered by external disturbances. The model predicts generation of radiation in a frequency range defined by the fault parameters. Other specific features predicted are: the upper limit of the size of the emitting area is a few dozen km; tremor accompanies earthquakes and aseismic slip; the frequency content of tremor depends on the type of failure. The model also explains why a tremor has no clear impulsive phase, in contrast to earthquakes. A comparatively small effective normal stress (hence a high fluid pressure) is required to make the model consistent with observed tremor parameters. Our model indicates that tremor is not necessarily a superposition of low frequency earthquakes, as commonly assumed, although the latter may trigger them. The approach developed complements the conventional viewpoint which assumes that tremor reflects a frictional process with low rupture speed. Essentially our model adds the hypothesis that resonant-type oscillations exist inside a fault. This addition may change our understanding of the nature of tremor in general, and the methods of its identification and location in particular.

Tremor Sweet Spots

Geophysics![Figure][1] CREDIT: © WIEDE, U. & M./ARCO IMAGES GMBH/ALAMY Seismic tremor is thought to be indicative of the slow release of small amounts of stress along plate boundaries, but it can also be triggered by large-amplitude seismic waves generated during large earthquakes. Gomberg and Prejean determined the distribution of tremor in Alaska after 11 of the largest earthquakes ( M < 7.2) around the globe between 2006 and 2012. As in previous observations, triggered tremor in the Aleutian Islands is related to the transition of friction along the subducting plate boundary from a locked state to a creeping state. However, tremor was also triggered in central mainland Alaska—a region far away from the subduction zone and devoid of any major crustal faults or appreciable seismic activity. Because there was no single characteristic of the triggering wave source or tectonic environment associated with the two regions, transient frictional processes at the plate interface may be responsible for tremor triggering. However, according to GPS data, neither zone shows any clear evidence of other concurrent related seismic processes such as slow slip events. J. Geophys. Res. 118 , 10.1002/2013JB010273 (2013). [1]: pending:yes

Triggered tremor sweet spots in Alaska

To better understand what controls fault slip along plate boundaries, we have exploited the abundance of seismic and geodetic data available from the richly varied tectonic environments composing Alaska. A search for tremor triggered by 11 large earthquakes throughout all of seismically monitored Alaska reveals two tremor “sweet spots”—regions where large‐amplitude seismic waves repeatedly triggered tremor between 2006 and 2012. The two sweet spots locate in very different tectonic environments—one just trenchward and between the Aleutian islands of Unalaska and Akutan and the other in central mainland Alaska. The Unalaska/Akutan spot corroborates previous evidence that the region is ripe for tremor, perhaps because it is located where plate‐interface frictional properties transition between stick‐slip and stably sliding in both the dip direction and laterally. The mainland sweet spot coincides with a region of complex and uncertain plate interactions, and where no slow slip events or major crustal faults have been noted previously. Analyses showed that larger triggering wave amplitudes, and perhaps lower frequencies (&lt;~0.03 Hz), may enhance the probability of triggering tremor. However, neither the maximum amplitude in the time domain or in a particular frequency band, nor the geometric relationship of the wavefield to the tremor source faults alone ensures a high probability of triggering. Triggered tremor at the two sweet spots also does not occur during slow slip events visually detectable in GPS data, although slow slip below the detection threshold may have facilitated tremor triggering.

Lack of Additional Triggered Tectonic Tremor around the Simi Valley and the San Gabriel Mountain in Southern California

Abstract We conduct a systematic search of tectonic tremor triggered by distant large earthquakes around the Simi Valley (SV) and the San Gabriel Mountain (SGM) in southern California. Out of 59 large earthquakes between 2000 and 2013, only the 2002 M w 7.9 Denali fault earthquake triggered clear tremor in the region. The observed travel times of the triggered tremors are consistent with theoretical predictions from tremor sources that are spatially clustered in the SV, close to the rupture zone of the 1994 M w 6.7 Northridge earthquake. We also estimate the triggering stress threshold as ∼12 kPa from measuring the peak ground velocities near the tremor source. The lack of clear tremor beneath the SGM provides a “negative” example for a region where tremor is expected to occur because of clear evidence of fluid‐rich zones at the middle crust. The results imply that the necessary conditions for tremor to occur are more than fluid‐induced low effective normal stress. Online Material: Figures of additional tremor inspection, tables of earthquake catalogs, and tremor locations.

Modeling dynamic triggering of tectonic tremor using a brittle‐ductile friction model

AbstractWe study the physics of dynamically triggered tectonic tremor by applying a brittle‐ductile friction model in which we conceptualize the tremor source as a rigid block subject to driving and frictional forces. To simulate dynamic triggering of tremor, we apply a stress perturbation that mimics the surface waves of remote earthquakes. The tectonic and wave perturbation stresses define a phase space that demonstrates that both the timing and amplitude of the dynamic perturbations control the fundamental characteristics of triggered tremor. Tremor can be triggered instantaneously or with a delayed onset if the dynamic perturbation significantly alters the frictional state of the tremor source.