Triggered Tremor Research Articles

Infrequent Triggering of Tremor along the San Jacinto Fault near Anza, California

We examine the conditions necessary to trigger tremor along the San Jacinto fault (SJF) near Anza, California, where previous studies suggest triggered tremor occurs, but observations are sparse. We investigate the stress required to trigger tremor using continuous broadband seismograms from 11 stations located near Anza, California. We examine 44 Mw ≥7:4 teleseismic events between 2001 and 2011; these events occur at a wide range of back azimuths and hypocentral distances. In addition, we included one smaller-magnitude, regional event, the 2009 Mw 6.5 Gulf of California earthquake, because it induced extremely high strains at Anza. We find the only episode of triggered tremor occurred during the 3 November 2002 Mw 7.8 Denali earthquake. The tremor episode lasted 300 s, was composed of 12 tremor bursts, and was located along SJF at the northwestern edge of the Anza gap at approximately 13 km depth. The tremor episode started at the Love-wave arrival, when surface-wave particle motions are primarily in the transverse direction. We find that the Denali earthquake induced the second highest stress (∼35 kPa) among the 44 teleseismic events and 1 regional event. The dominant period of the Denali surface wave was 22.8 s, at the lower end of the range observed for all events (20-40 s), similar to periods shown to trigger tremor in other locations. The surface waves from the 2009 Mw 6.5 Gulf of California earthquake had the highest observed strain, yet a much shorter dominant period of 10 s and did not trigger tremor. This result suggests that not only the amplitude of the induced strain, but also the period of the incoming surface wave, may control triggering of tremors near Anza. In addition, we find that the transient-shear stress (17-35 kPa) required to trigger tremor along the SJF at Anza is distinctly higher than what has been reported for the well-studied San Andreas fault.

Statistical properties of low-frequency earthquakes triggered by large earthquakes in southern Taiwan

The recent discovery of triggered tremors (TTs) and low-frequency earthquakes (LFEs) in various tectonic environments provides an opportunity for studying the fundamental properties and physical mechanisms of deep tectonic tremor. Here, we quantify the relationship between TTs and LFEs beneath the Central Range in southern Taiwan and their statistical properties during the teleseismic waves of six large distant earthquakes. Using waveforms of 11 LFEs triggered by the 2005 Mw 8.6 Nias earthquake as templates, we scan through 12hours of waveform data around six mainshocks and identify a total of 783 LFEs. The LFEs were mainly located in a compact region between 12 and 36km in depth near the Chaochou–Lishan Fault. Most of LFEs occurred within TT during the passage of large-amplitude surface waves, and the increase of the LFE rate during the surface waves is statistically significant. The LFE rates do not follow an Omori's type decay, but rather abruptly return to the background rate immediately after the surface-wave passage. These findings suggest that LFEs do not trigger any additional LFEs at later times and are primarily driven by an external forcing. Our observations are consistent with the inference that TTs consist of many reoccurring LFEs.

A Global Search for Triggered Tremor Following the 2011 Mw 9.0 Tohoku Earthquake

Abstract The 2011 M w 9.0 Tohoku, Japan, earthquake triggered deep tectonic tremor and shallow microearthquakes in numerous places worldwide. Here, we conduct a systematic survey of triggered tremor in regions where ambient or triggered tremor has been previously identified. Tremor was triggered in the following regions: south‐central Alaska, the Aleutian Arc, Shikoku in southwest Japan, the North Island of New Zealand, southern Oregon, the Parkfield–Cholame section of the San Andreas fault in central California, the San Jacinto fault in southern California, Taiwan, and Vancouver Island. We find no evidence of triggered tremor in the Calaveras fault in northern California. One of the most important factors in controlling the triggering potential is the amplitude of the surface waves. Data examined in this study suggest that the threshold amplitude for triggering tremor is ∼0.1 cm/s, which is equivalent to a dynamic stress threshold of ∼10 kilopascals. The incidence angles of the teleseismic surface waves also affect the triggering potentials of Love and Rayleigh waves. The results of this study confirm that both Love and Rayleigh waves contribute to triggering tremor in many regions. In regions where both ambient and triggered tremor are known to occur, tremor triggered by the Tohoku event generally occurred at similar locations with previously identified ambient and/or triggered tremor, further supporting the notion that although the driving forces of triggered and ambient tremor differ, they share similar mechanisms. We find a positive relationship between the amplitudes of the triggering waves and those of the triggered tremor, which is consistent with the prediction of the clock‐advance model. Online Material: Table of measured parameters and other information related to triggering/nontriggering information, and figures of observed seismograms.

S-Wave Triggering of Tremor beneath the Parkfield, California, Section of the San Andreas Fault by the 2011 Tohoku, Japan, Earthquake: Observations and Theory

Research Article| May 01, 2013 S‐Wave Triggering of Tremor beneath the Parkfield, California, Section of the San Andreas Fault by the 2011 Tohoku, Japan, Earthquake: Observations and Theory David P. Hill; David P. Hill U.S. Geological Survey, Volcano Science Center, MS 910, 345 Middlefield Rd., Menlo Park, California 94025hill@usgs.gov Search for other works by this author on: GSW Google Scholar Zhigang Peng; Zhigang Peng School of Earth and Atmospheric Sciences, Georgia Institute of Technology, Atlanta, Georgia 30332 Search for other works by this author on: GSW Google Scholar David R. Shelly; David R. Shelly U.S. Geological Survey, Volcano Science Center, MS 910, 345 Middlefield Rd., Menlo Park, California 94025 Search for other works by this author on: GSW Google Scholar Chastity Aiken Chastity Aiken School of Earth and Atmospheric Sciences, Georgia Institute of Technology, Atlanta, Georgia 30332 Search for other works by this author on: GSW Google Scholar Author and Article Information David P. Hill U.S. Geological Survey, Volcano Science Center, MS 910, 345 Middlefield Rd., Menlo Park, California 94025hill@usgs.gov Zhigang Peng School of Earth and Atmospheric Sciences, Georgia Institute of Technology, Atlanta, Georgia 30332 David R. Shelly U.S. Geological Survey, Volcano Science Center, MS 910, 345 Middlefield Rd., Menlo Park, California 94025 Chastity Aiken School of Earth and Atmospheric Sciences, Georgia Institute of Technology, Atlanta, Georgia 30332 Publisher: Seismological Society of America First Online: 14 Jul 2017 Online ISSN: 1943-3573 Print ISSN: 0037-1106 Bulletin of the Seismological Society of America (2013) 103 (2B): 1541–1550. https://doi.org/10.1785/0120120114 Article history First Online: 14 Jul 2017 Cite View This Citation Add to Citation Manager Share Icon Share Facebook Twitter LinkedIn MailTo Tools Icon Tools Get Permissions Search Site Citation David P. Hill, Zhigang Peng, David R. Shelly, Chastity Aiken; S‐Wave Triggering of Tremor beneath the Parkfield, California, Section of the San Andreas Fault by the 2011 Tohoku, Japan, Earthquake: Observations and Theory. Bulletin of the Seismological Society of America 2013;; 103 (2B): 1541–1550. doi: https://doi.org/10.1785/0120120114 Download citation file: Ris (Zotero) Refmanager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search Dropdown Menu toolbar search search input Search input auto suggest filter your search All ContentBy SocietyBulletin of the Seismological Society of America Search Advanced Search Abstract The dynamic stresses that are associated with the energetic seismic waves generated by the Mw 9.0 Tohoku earthquake off the northeast coast of Japan triggered bursts of tectonic tremor beneath the Parkfield section of the San Andreas fault (SAF) at an epicentral distance of ∼8200 km. The onset of tremor begins midway through the ∼100‐s‐period S‐wave arrival, with a minor burst coinciding with the SHSH arrival, as recorded on the nearby broadband seismic station PKD. A more pronounced burst coincides with the Love arrival, followed by a series of impulsive tremor bursts apparently modulated by the 20‐ to 30‐s‐period Rayleigh wave. The triggered tremor was located at depths between 20 and 30 km beneath the surface trace of the fault, with the burst coincident with the S wave centered beneath the fault 30 km northwest of Parkfield. Most of the subsequent activity, including the tremor coincident with the SHSH arrival, was concentrated beneath a stretch of the fault extending from 10 to 40 km southeast of Parkfield. The seismic waves from the Tohoku epicenter form a horizontal incidence angle of ∼14°, with respect to the local strike of the SAF. Computed peak dynamic Coulomb stresses on the fault at tremor depths are in the 0.7–10 kPa range. The apparent modulation of tremor bursts by the small, strike‐parallel Rayleigh‐wave stresses (∼0.7 kPa) is likely enabled by pore pressure variations driven by the Rayleigh‐wave dilatational stress. These results are consistent with the strike‐parallel dynamic stresses (δτs) associated with the S, SHSH, and surface‐wave phases triggering small increments of dextral slip on the fault with a low friction (μ∼0.2). The vertical dynamic stresses δτd do not trigger tremor with vertical or oblique slip under this simple Coulomb failure model. You do not have access to this content, please speak to your institutional administrator if you feel you should have access.

Simulations of tremor‐related creep reveal a weak crustal root of the San Andreas Fault

AbstractDeep aseismic roots of faults play a critical role in transferring tectonic loads to shallower, brittle crustal faults that rupture in large earthquakes. Yet, until the recent discovery of deep tremor and creep, direct inference of the physical properties of lower‐crustal fault roots has remained elusive. Observations of tremor near Parkfield, CA provide the first evidence for present‐day localized slip on the deep extension of the San Andreas Fault and triggered transient creep events. We develop numerical simulations of fault slip to show that the spatiotemporal evolution of triggered tremor near Parkfield is consistent with triggered fault creep governed by laboratory‐derived friction laws between depths of 20–35 km on the fault. Simulated creep and observed tremor northwest of Parkfield nearly ceased for 20–30 days in response to small coseismic stress changes of order 104 Pa from the 2003 M6.5 San Simeon Earthquake. Simulated afterslip and observed tremor following the 2004 M6.0 Parkfield earthquake show a coseismically induced pulse of rapid creep and tremor lasting for 1 day followed by a longer 30 day period of sustained accelerated rates due to propagation of shallow afterslip into the lower crust. These creep responses require very low effective normal stress of ~1 MPa on the deep San Andreas Fault and near‐neutral‐stability frictional properties expected for gabbroic lower‐crustal rock.

Tremors along the Queen Charlotte Margin triggered by large teleseismic earthquakes

AbstractWe conduct a systematic search of tectonic tremors along the Queen Charlotte Margin (QCM) in western Canada triggered by distant earthquakes. We identify triggered tremor as non‐impulsive, high‐frequency signals coherent among several stations and coincident with passing surface waves. So far, the 2002 Mw7.9 Denali Fault, the 2004 Mw9.2 Sumatra, and the 2011 Mw9.1 Tohoku‐Oki earthquakes have triggered clear tremor in this region. The 2010 Mw8.8 Maule, Chile and the 2012 Mw8.6 Sumatra earthquakes may have triggered, but tremors in these two cases did not meet all of our criteria. The triggered tremors are mostly located east of the Queen Charlotte Fault in the southern portion of Haida Gwaii, near the epicenter of the 28 October 2012 Mw7.7 earthquake. Similar to the observations in other regions, the triggered tremors were initiated by the Love waves and continued during the subsequent Rayleigh waves. Tremor bursts correlate with both the particle velocity and displacement of the Love waves, indicating they are triggered at either low‐angle thrust or vertical strike‐slip faults. In addition, we find that the triggering potential for the QCM is controlled by a combination of amplitude, period, and incident angles.

Tectonic Tremor beneath Cuba Triggered by theMw 8.8 Maule andMw 9.0 Tohoku‐Oki Earthquakes

We provide additional evidence of tectonic tremor in Cuba triggered by the 2010 M w 8.8 Maule, Chile, and the 2011 M w 9.0 Tohoku‐Oki, Japan, earthquakes. The high‐frequency tremor signals are modulated by long‐period surface waves, similar to triggered tremors observed in other tectonically active regions. We are able to locate two tremor sources triggered by the Tohoku‐Oki earthquake near the east–west trending Oriente fault around Guantanamo Bay. The tremor around Guantanamo Bay was triggered primarily by a Love wave of the Maule mainshock, and by both Love and Rayleigh waves of the Tohoku‐Oki mainshock. This is consistent with frictional failures at a vertical strike‐slip fault under a Coulomb failure criterion. Online Material: Figure of envelope functions of tremor records, and movies of spectogram and tremor correlation.

Triggering of tremors and slow slip event in Guerrero, Mexico, by the 2010 Mw 8.8 Maule, Chile, earthquake

We investigate the triggering of seismic tremor and slow slip event in Guerrero (Mexico) by the February 27, 2010 Maule earthquake (Mw 8.8). Triggered tremors start with the arrival of S wave generated by the Maule earthquake, and keep occurring during the passing of ScS, SS, Love and Rayleigh waves. The Rayleigh wave dispersion curve footprints the high frequency energy envelope of the triggered tremor, indicating a strong modulation of the source of tremors by the passing surface wave. This correlation and modulation by the passing waves is progressively lost with time over a few hours. The tremor activity continues during the weeks/months after the earthquake. GPS time series suggest that the second sub‐event of the 2009–2010 SSE in Guerrero is actually triggered by the Maule earthquake. The southward displacement of the GPS stations starts coincidently with the earthquake and tremors. The long duration of tremors indicate a continuing deformation process at depth, which we propose to be the second sub‐event of the 2009–2010 SSE. We show a quasi‐systematic correlation between surface displacement rate measured by GPS and tremor activity, suggesting that the NVT are controlled by the variations in the slip history of the SSE. This study shows that two types of tremors emerge: (1) Those directly triggered by the passing waves and (2) those triggered by the stress variations associated with slow slip. This indicates the prominent role of aseismic creep in the Mexican subduction zone response to a large teleseismic earthquake, possibly leading to large‐scale stress redistribution.

The role of velocity‐neutral creep on the modulation of tectonic tremor activity by periodic loading

Slow slip events and associated non‐volcanic tremors are sensitive to oscillatory stress perturbations, such as those induced by tides or seismic surface waves. Slow slip events and tremors are thought to occur near the seismic‐aseismic transition regions of active faults, where the differencea − b = ∂μ/∂lnVbetween the sensitivity of friction to slip rate and fault state, which characterizes the stability of slip, can be arbitrarily small. We investigate the response of a velocity‐strengthening fault region to oscillatory loads through analytical approximations and spring‐slider simulations. We find that fault areas that are near velocity‐neutral at steady‐state, i.e., ∂μ / ∂lnV ≈ 0, are highly sensitive to cyclic loads: oscillatory stress perturbations in a certain range of periods induce large transient slip velocities. These aseismic transients can in turn trigger tremor activity with enhanced oscillatory modulation. In this sensitive regime, a harmonic Coulomb stress perturbation of amplitude ΔS causes a slip rate perturbation varying as eΔS/(a−b)σ, where σ is the effective normal stress. This result is in agreement with observations of the relationship between tremor rate and amplitude of stress perturbations for tremors triggered by passing seismic waves. Our model of tremor modulation mediated by transient creep does not require extremely high pore fluid pressure and provides a framework to interpret the sensitivity and phase of tidally modulated tremors observed in Parkfield and Shikoku in terms of spatial variations of friction parameters and background slip rate.

Comparisons of Triggered Tremor in California

Abstract We conduct a visual inspection of deep nonvolcanic tremor triggered by large teleseismic earthquakes around the Calaveras fault in northern California (NC) and the San Jacinto fault in southern California (SC). Out of the 42 large ( M w ≥7.5) earthquakes between 2001 and 2010, only the 2002 M w 7.9 Denali fault earthquake triggered clear tremor in these two regions. This is in marked contrast with the Parkfield–Cholame section of the San Andreas fault in central California (CC), where 12 earthquakes have triggered tremor in that region. The amplitude of the triggered tremor correlates with that of the triggering surface waves in CC and is consistent with the clock‐advance model. The lack of widespread triggered tremor in NC and SC is not simply a consequence of their different background noise levels from CC, but rather reflects different background tremor rates in these regions. Online Material: Tables of parameters for triggered/nontriggered teleseismic earthquakes and triggered tremors, and figures of band‐pass‐filtered seismograms, maximum vertical PGVs versus median amplitude of band‐pass‐filtered envelope functions, tremor amplitude versus PGV, and S ‐wave spectra (and corresponding noise) of local selected earthquakes.

Remote triggering of non-volcanic tremor around Taiwan

SUMMARY We perform a systematic survey of triggered deep ‘non-volcanic’ tremor beneath the Central Range (CR) in Taiwan for 45 teleseismic earthquakes from 1998 to 2009 with Mw≥ 7.5 and epicentral distance ≥1000 km to the broad-band station TPUB. Triggered tremors are visually identified as bursts of high-frequency (2–8 Hz), non-impulsive and long-duration seismic energy that are coherent among many seismic stations and modulated by the teleseismic surface waves. Out of the 45 earthquakes, we identified nine teleseismic events associated with nine tremor sources in the southern and five in the northern CR. Most of the tremor sources are located within the depth range of 15–25 km in the lower crust above the Moho. We find that the amplitudes of the surface waves play an important role in determining the triggering potential, and the apparent triggering threshold is ∼0.1 cm s−1, or 7–8 KPa. However, such threshold is partially controlled by the background noise level, which could prevent weaker tremor triggered by surface waves with smaller amplitudes from being identified. The amplitudes of the triggered tremor show a positive correlation with the amplitudes of the triggering surface waves, consistent with the predictions by the ‘clock-advance’ model. In addition to amplitudes, other factors, such as frequency contents and incidence angles, also affect the triggering potential. We find that intermediate-period (30–10 s) surface waves could trigger/modulate tremors, suggesting that long-period (>30 s) surface waves are not always required in long-range triggering. Tremors appear to be triggered by both Love and Rayleigh waves. When the incidence angles are parallel to the strike of the CR, all six events triggered tremor primarily during the Rayleigh waves. For strike normal incidence, only the 2001 Mw7.8 Kunlun earthquake showed predominant Love-wave triggering. This observation can be qualitatively explained by a simple Coulomb failure for a left-lateral shear on the low-angle detachment fault beneath the southern CR.

Triggered creep as a possible mechanism for delayed dynamic triggering of tremor and earthquakes

The passage of radiating seismic waves generates transient stresses in the Earth’s crust that can trigger slip on faults far away from the original earthquake source. The triggered fault slip is detectable in the form of earthquakes 1‐3 and seismic tremor 4‐7 . However, the significance of these triggered events remains controversial 8,9 , in part because they often occur with some delay, long after the triggering stress has passed. Here we scrutinize the location and timing of tremor on the San Andreas fault between 2001 and 2010 in relation to distant earthquakes. We observe tremor on the San Andreas fault that is initiated by passing seismic waves, yet migrates along the fault at a much slower velocity than the radiating seismic waves. We suggest that the migrating tremor records triggered slow slip of the San Andreas fault as a propagating creep event. We find that the triggered tremor and fault creep can be initiated by distant earthquakes as small as magnitude 5.4 and can persist for several days after the seismic waves have passed. Our observations of prolonged tremor activity provide a clear example of the delayed dynamic triggering of seismic events. Fault creep has been shown to trigger earthquakes 10‐12 , and we therefore suggest that the dynamic

Remotely triggered microearthquakes and tremor in central California following the 2010 Mw 8.8 Chile earthquake

We examine remotely triggered microearthquakes and tectonic tremor in central California following the 2010 Mw 8.8 Chile earthquake. Several microearthquakes near the Coso Geothermal Field were apparently triggered, with the largest earthquake (Ml 3.5) occurring during the large‐amplitude Love surface waves. The Chile mainshock also triggered numerous tremor bursts near the Parkfield‐Cholame section of the San Andreas Fault (SAF). The locally triggered tremor bursts are partially masked at lower frequencies by the regionally triggered earthquake signals from Coso, but can be identified by applying high‐pass or matched filters. Both triggered tremor along the SAF and the Ml 3.5 earthquake in Coso are consistent with frictional failure at different depths on critically‐stressed faults under the Coulomb failure criteria. The triggered tremor, however, appears to be more phase‐correlated with the surface waves than the triggered earthquakes, likely reflecting differences in constitutive properties between the brittle, seismogenic crust and the underlying lower crust.

Lessons from (triggered) tremor

I test a “clock‐advance” model that implies triggered tremor is ambient tremor that occurs at a sped‐up rate as a result of loading from passing seismic waves. This proposed model predicts that triggering probability is proportional to the product of the ambient tremor rate and a function describing the efficacy of the triggering wave to initiate a tremor event. Using data mostly from Cascadia, I have compared qualitatively a suite of teleseismic waves that did and did not trigger tremor with ambient tremor rates. Many of the observations are consistent with the model if the efficacy of the triggering wave depends on wave amplitude. One triggered tremor observation clearly violates the clock‐advance model. The model prediction that larger triggering waves result in larger triggered tremor signals also appears inconsistent with the measurements. I conclude that the tremor source process is a more complex system than that described by the clock‐advance model predictions tested. Results of this and previous studies also demonstrate that (1) conditions suitable for tremor generation exist in many tectonic environments, but, within each, only occur at particular spots whose locations change with time; (2) any fluid flow must be restricted to less than a meter; (3) the degree to which delayed failure and secondary triggering occurs is likely insignificant; and 4) both shear and dilatational deformations may trigger tremor. Triggered and ambient tremor rates correlate more strongly with stress than stressing rate, suggesting tremor sources result from time‐dependent weakening processes rather than simple Coulomb failure.

High‐frequency identification of non‐volcanic tremor triggered by regional earthquakes

Subsequent to the discovery of ambient “non‐volcanic” tremor activity along the Parkfield‐Cholame section of the San Andreas fault in central California, triggered tremors associated with the surface waves of large teleseismic earthquakes have been recognized. However, no evidence of triggered tremors from regional earthquakes has previously been found either here or in other tremor regions. By systematically filtering seismograms to higher frequencies (i.e., above 20 Hz) associated with 99 regional M5+ earthquakes since 2001, we identify four regional earthquakes that have triggered tremors in central California. Significant high‐frequency energy is also observed in previously identified teleseismically triggered and ambient tremors, suggesting a common mechanism. We find that long‐period and large‐amplitude surface waves from both regional and teleseismic events have a greater potential of triggering tremor in the same region, and that the inferred minimum triggering dynamic stress is ∼1 kPa.

Detecting low‐frequency earthquakes within non‐volcanic tremor in southern Taiwan triggered by the 2005 Mw8.6 Nias earthquake

We use a matched filter technique to detect 41 low‐frequency earthquakes (LFEs) within 700‐s of triggered tremor signals in the Southern Central Range in Taiwan during the surface waves of the 2005 Mw8.6 Nias earthquake off the coast of northern Sumatra. The depth distributions of LFEs after double‐difference relocations concentrate at the depth range of 12–38 km below the background seismicity and above the Moho depth inferred from receiver function studies. The locations of LFEs are close to the downward extension of the steep‐dipping Chaochou‐Lishan fault with only modestly high Vp/Vs ratios (1.75–1.85). Our observation indicates that at least portions of triggered tremor consists of many LFEs, similar to ambient tremor observed at other major plate boundary faults.

Remote triggering of tremor along the San Andreas Fault in central California

We perform a systematic survey of triggered tremor along the San Andreas Fault in central California for the 31 teleseismic earthquakes with Mw ≥ 7.5 since 2001. We identify 10 teleseismic events associated with clear triggered tremor. About 52% of the tremor is concentrated south of Parkfield near Cholame, where ambient tremor has been identified previously, and the rest is widely distributed in the creeping section of the San Andreas Fault north of Parkfield. Tremor is generally initiated and is in phase with the Love wave particle velocity. However, the pattern becomes complicated with the arrival of the Rayleigh waves, and sometimes tremor continues after the passage of the surface waves. We identify two cases in which tremor is triggered during the teleseismic PKP phase. These results suggest that while shear stress from the passage of the Love waves plays the most important role in triggering tremor in central California, other factors, such as dilatational stresses from the Rayleigh and P waves, also contribute. We also examine the ambient tremor occurrence rate before and after the teleseismic events and find a transient increase of stacked tremor rate during the passage of the teleseismic surface waves. This observation implies that the occurrence time of tremor is temporally advanced by the dynamic stresses of the teleseismic waves. The amplitude of the teleseismic waves correlates with the occurrence of triggered tremor, and the inferred tremor‐triggering threshold is ∼2–3 kPa. The relatively low triggering threshold indicates that the effective stress at the tremor source region is very low, most likely due to near‐lithostatic fluid pressure.

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.

Deep low‐frequency tremor that correlates with passing surface waves

The large surface waves from the 2004 Sumatra‐Andaman earthquake dynamically perturbed the upper mantle structure in Japan and triggered periodic deep low‐frequency seismic tremor in eastern and western Shikoku, western and central Tokai, and the Kii peninsula. We use the relationship between the amplitude of the triggered tremor and the stresses of the seismic waves to investigate the mechanism of deep low‐frequency seismic tremor. Volumetric strain changes from the 15–30 s Rayleigh waves play an important role in the strong triggering, likely via Coulomb failure stress changes. Building on previous results that the tremor signals become increasingly strong with increasing dilatation, we observe a clear increase in the triggered tremor with an increase in the dilatation due to the Rayleigh waves at the 30 km depth source regions. We also observe a correlation with the Coulomb failure stress change resolved on an appropriate plane. There is an exponential relationship between the signal amplitude from triggered tremor and both the dilatation and the Coulomb shear stress at the source region. This combined with the shape of the tremor packets implies that the tremor amplitude is predictable based on the amplitude of the incoming waves. The amplitude variations can be explained by a distribution of sources in the tremor source region.

Orthostatic tremor in monozygotic twins

Orthostatic tremor (OT) is characterized by a feeling of unsteadiness during stance that disappears when sitting or lying, associated with a fine-amplitude leg rippling while standing. Surface electromyography (EMG) recordings show a characteristic 13- to 18-Hz tremor.1 Tremor intensity is slowly progressive, sometimes spreading to other body regions, whereas the interval between standing and tremor triggering decreases over time. OT etiology remains unknown. Although most described cases of OT are sporadic, this disorder could be genetic, as suggested by few familial cases2–6 (see table E-1 on the Neurology Web site; go to www.neurology.org). We report two monozygotic twins with OT. The patients were two male monozygotic twins. Parents were not consanguineous, came from different villages, and remained healthy until their deaths (ages 76 and 87). Family history was negative for tremor or other neurologic disorders. The patients grew up together, lived in the same village, and worked as farmers until age 55, using agriculture chemical products. At age 68, in our outpatient clinic, they had tremor of both legs, occurring when standing and relieved …