Tremor Locations Research Articles

Is essential tremor a risk factor for carpal tunnel syndrome? A prospective study excluding the most common comorbid conditions

Aims: Carpal tunnel syndrome (CTS) is the most common entrapment neuropathy of the upper extremity, resulting from compression of the median nerve. Tremor, a rhythmic and involuntary movement of any part of the body, is the most common movement disorder, affecting millions of people worldwide. There is limited research on the coexistence of CTS and ET (essential tremor). Methods: This prospective study included patients aged 18-65 who presented to the neurology outpatient clinic and were diagnosed with ET. Participants supplied informed consent, a sociodemographic form, and the Fahn-Tolosa-Marin Clinical Tremor Rating Scale (FTM TRS). Patients were evaluated for the presence and severity of CTS with electromyography (EMG). The EMG data were categorized as normal, mild, moderate, and severe CTS. Results: In the study, the average age of the 50 ET patients was 56.0 (35.0-64.0) years. The gender distribution comprised 22 (44.0%) males and 28 (56.0%) females. The following tremor locations were reported: 41 (82.0%) in both upper extremities, 3 (6.0%) in the right upper extremity, 2 (4.0%) in the left upper extremity, 1 (2.0%) in both upper extremities, head tremor, and voice, and 3 (6.0%) in both upper extremities and head tremor. A high prevalence of CTS was detected in ET patients (46%). Additionally, a positive correlation was found between CTS in ET patients, long disease duration, and female gender. No significant relationship was found between FTM TRS Part A, Part B, Part C, total scores, and median sensory and motor nerve amplitude and velocity values in ETS patients. Conclusion: CTS is more common in patients with ET. This suggests that ET may be a risk factor for CTS, independent of other factors. Early diagnosis and treatment of CTS in patients with ET is important to improve their quality of life.

Overpressure Estimation Based on Velocity Data Analysis in The Subduction Zone of Hyuga-nada at Western Nankai Trough, Japan

Subduction zone is one of the areas which has great potential of earthquake. Nankai trough in Japan is such an area with subduction zone where earthquake records are well archived, despite its complexity. Also, slow earthquakes have been detected in this zone over the past 20 years, so that there are many opportunities to understand the dynamics and deformation in such an active setting. To expand the knowledge about slow earthquake which might occur and trigger a bigger earthquake, this study wants to infer the relationship between overpressure, one of the states and properties of seismogenic zone, and tremors. To carry out this research, 4 velocity models of seismicity line taken in Hyuga-nada are used. And the recorded tremor data placed around the lines came from Yamashita. In correlating the overpressure and tremors, this research is started by converting the Vp model to porosity model using Wyllie equation to predict the pore pressure conditions in the area. Then, the model will be used for fitting the actual data. Assuming there is an exact location following the normal compaction curve as a function of effective stress, it can be used for estimating the overpressure. Overpressure is economized by comparing the observed porosity and the reference curve at the same depth below seafloor. Potential overpressure occurred may be associated with areas of low Vp. The estimated overpressure is in the range of 1.2 to 12 MPa in the depth range of 700 to 4000mbsf. This study also concludes that the overpressure zone overlaps with the tremor location, although the overpressure value estimated is also still uncertain due to the uncertainty of the density value used.

Locating tectonic tremors with uncertainty estimates: time- and amplitude-difference optimization, wave propagation-based quality control and Bayesian inversion

SUMMARY The accurate location of tectonic tremors helps improve understanding of their underlying physical processes. However, current location methods often do not statistically evaluate uncertainties to a satisfactory degree and do not account for potential biases due to subsurface structures not included in the model. To address these issues, we propose a novel three-step process for locating tectonic tremors. First, the measured time- and amplitude differences between station pairs are optimized to obtain station-specific relative time and amplitude measurements with uncertainty estimates. Secondly, the time– and amplitude–distance relationships in the optimized data are used to roughly estimate the propagation speed (i.e. shear wave velocity) and attenuation strength. Linear regression is applied to each event, and the resulting velocity and attenuation strength are used for quality control. Finally, the tremor location problem is formulated within a Bayesian framework where the model parameters include the source locations, local site delay/amplification factors, shear wave velocity and attenuation strength. The Markov chain Monte Carlo algorithm is used to sample the posterior probability and is augmented by a parallel tempering scheme for an efficient global search. We tested the proposed method on ocean-bottom data indicating an intense episode of tectonic tremors in Kumano-nada within the Nankai Trough subduction zone. The results show that the range of the 95 per cent confidence interval is typically <7 km horizontally and <10 km vertically. A series of experiments with different inversion settings reveals that adopting amplitude data and site correction factors help reduce random error and systematic bias, respectively. Probabilistic sampling allows us to spatially map the probability of a tremor occurring at a given location. The probability map is used to identify lineaments of tremor sources, which provides insights into structural factors that favour tremor activity.

Intermittence of transient slow slip in the Mexican subduction zone

In the Mexican subduction zone, slow slip events of various duration, from a few days to several months have previously been identified in the regions of Guerrero and Oaxaca. They occur along with microseismicity: low-frequency earthquakes and tectonic tremors. In this study, we analyzed a 10-year long tremor catalog covering multiple SSE cycles, to identify periods of high tremor activity. We then used the temporal information of these tremor bursts to decompose GNSS time series. Surface velocities estimated for tremor and non-tremor periods are then inverted to locate slip rates on the plate interface. Combining the detected microseismicity information and GNSS time series, we show that large slow slip events are made of clusters of short-lasting slip events occurring at tremor times and located updip of the tremor area. This analysis also allowed us to unveil low amplitude transients in between large slow slip events, that last 8−9 days, occurring at tremor times and locations, in both Guerrero and Oaxaca regions. Finally, we show that the plate locking amplitude in between short and large slow slip events increases by a factor up to 0.5 at the location of tremors.

Rapid Tremor Migration During Few Minute‐Long Slow Earthquakes in Cascadia

AbstractSlow earthquakes are now commonly found to display a wide range of durations, moments, and slip and propagation speeds. But not all types of slow earthquakes have been examined in detail. Here we probe tremor bursts with durations between 1 and 30 min, which are likely driven by few minute‐long bursts of aseismic slip. We use a coherence‐based technique to detect thousands of tremor bursts beneath Vancouver Island in Cascadia. Then we examine 17 of the ruptures by tracking their evolving tremor locations over an 8‐km region. We find that tremor migrates at rates of 3–25 m/s: faster than longer tremor bursts. Though some observational biases persist, the short events' speeds appear to fill a gap in the spectrum of observed slow earthquakes. They may provide further evidence that whatever fault zone process creates slow earthquakes, it must allow for faster slip and propagation in smaller ruptures.

Integrated rockburst hazard estimation methodology based on spatially smoothed seismicity model and Mann-Kendall trend test

Rock burst is one of the most severe dynamic hazards in the underground mining industry, and it is essential to establish a reliable method to achieve accurate prevention and control of it. This paper proposes a comprehensive spatial and temporal rockburst warning method based on the Mann-Kendall trend test (MKT) and spatially smoothed seismicity model, which can evaluate rockburst risk in real-time and quantitatively. The method uses real-time values of rockburst warning indicators as input, uses MKT to determine the temporal trend of each indicator and takes whether the trend conforms to the rockburst precursor law as the criterion for triggering the alarm. Then combined with the confusion matrix to evaluate the warning effectiveness of each indicator and optimize the indicators, and then obtain the real-time temporal rockburst hazard index Q of the mining area through data fusion. Finally, the spatial rockburst hazard index S contour map is drawn using the spatially smoothed seismicity model to identify local rockburst hazard zones and assist mine personnel in taking preventive and control measures of rockburst in time. The results show that the proposed model considers the precursor information of rockburst evolution from “time-space-strength” multi-dimensionally. The warning effectiveness of Q reaches 0.431, which is higher than any single indicator. The local rockburst hazard zone shown in the contour map of S is consistent with the actual location of strong tremors and rockburst events. The model has been successfully applied in the Wudong coal mine in Xinjiang, China, which can assist mine personnel in making efficient and accurate rockburst prevention decisions and effectively reduce their prevention costs.

Tremor, Gait, and Balance Disorders in Essential Tremor and the Efficacy of Atenolol

ABSTRACT Background: To assess gait and balance in patients with essential tremor (ET), which is the most common tremor disorder characterized by bilateral action tremor of the hands with possible involvement of further tremor locations and to test the efficacy of atenolol on tremor and gait and balance. Methods: In this prospective study, gait and balance were evaluated by static posturography in addition to International Cooperative Ataxia Rating Scale (ICARS) in 22 ET patients without any additional neurologic signs on examination. Tremor severity was assessed by The Essential Tremor Rating Assessment Scale (TETRAS). The efficacy of atenolol 100 mg/day both on tremor and gait and balance was investigated. Posturographic recordings were performed in gender, age-, and height-matched healthy volunteers who constituted the control group. Results: The mean center of gravity sway velocity of the ET patients recorded on firm and foam surfaces with eyes open and closed was not significantly different from the healthy controls but sway velocity on tandem stance was significantly increased (P = 0.032). Gait speed (P = 0.015) was decreased and step width was increased (P = 0.001). An improvement in TETRAS scores was recorded with atenolol treatment (P = 0.046). An improvement not reaching statistical significance was present in ICARS scores (P = 0.059). However, posturographic parameters did not change significantly (P > 0.05). Conclusion: Increased sway velocity on tandem stance, decreased gait speed and increased step width are consistent with midline cerebellar dysfunction in ET patients with no obvious gait and balance deficits on clinical examination. Atenolol seems to be effective on tremor but does not cause a significant improvement in gait and balance deficits.

Seismic tremor location of 10 large paroxysmal eruptions of Tungurahua volcano, Ecuador

SUMMARYThe most recent eruptive period of Tungurahua volcano lasted 17 yr (1999–2016), generating strong eruptive phases with the release of large amounts of seismic and acoustic energies. We have selected 10 large eruptions and located their seismic sources. The location method involves applying a new method for correcting for site effects in the seismic records and identifying, in the frequency domain, the signals from the ground-coupled airwaves and pyroclastic flows. The locations are computed by minimizing the differences of the source energy rates in the [0.4, 2.5] Hz range. The results suggest that the first three eruptions have locations mainly concentrated at the [−1, 2] km depth range (asl), and the locations of the fourth eruption are mainly coincident with depths of a possible reservoir estimated from petrological studies. This fact strongly suggests that this eruption was responsible of changes in the dynamics of Tungurahua, making the volcano prone to produce Vulcanian eruptions.

Spatial variations of the effective elastic thickness and internal load fraction in the Cascadia subduction zone

SUMMARYThe effective elastic thickness (Te) can reflect lateral change of rheological properties from subducting plate to overlying plate, and aid in understanding the relationship between geodynamic evolution and regional tectonic response to accumulated forces by plate convergence. The internal load fraction (F), calculated simultaneously with Te, is an index for subsurface mass distribution. Here we introduce high-resolution maps of Te and F in the Cascadia subduction zone by analysing the coherence between gravity anomaly and topography using a fan wavelet method. The general Te pattern correlates well with the overall tectonic characteristics. The diverse Te pattern along the forearc zone offshore North America indicates different mechanical coupling states of the slabs between the northern and southern parts. The central subducting plate is locally decoupled from the overlying plate due to a group of subducted seamounts. The F pattern shows intriguing correlations with the full locked zone (high F) and the location of episodic tremor and slip (low F) along the forearc area. F may also be a good indicator for the location of a fluid/melt upwelling in the volcanic arc area. Seismicity is more likely to occur in areas of low Te or steep Te gradient. The long-term mechanical state of lithosphere will affect or even control the temporal variations of plates during multiple seismic cycles.

On the Origin of Orphan Tremors and Intraplate Seismicity in Western Africa

On September 5–7, 2018, a series of tremors were reported in Nigeria’s capital city, Abuja. These events followed a growing list of tremors felt in the stable intraplate region, where earthquakes are not expected. Here, we review available seismological, geological, and geodetic data that may shed light on the origin of these tremors. First, we investigate the seismic records for parent location of the orphan tremors using a technique suitable when a single-seismic station is available such as the Western Africa region, which has a sparse seismic network. We find no evidence of the reported tremors within the seismic record of Western Africa. Next, we consider the possibility of a local amplification of earthquakes from regional tectonics, reactivation of local basement fractures by far-field tectonic stresses, post-rift crustal relaxation, landward continuation of oceanic fracture zones, or induced earthquakes triggered by groundwater extraction. Our assessments pose important implications for understanding Western Africa’s intraplate seismicity and its potential connection to tectonic inheritance, active regional tectonics, and anthropogenic stress perturbation.

Complex Migration of Tremor Near Cholame, CA, Resolved by Seismic Array Analysis

AbstractTectonic tremor observed along the Cholame and Parkfield sections of the San Andreas Fault has previously been associated with transient slip beneath the seismogenic zone. To study tremor and associated Low Frequency Earthquakes (LFEs), we deployed three dense near‐fault seismic arrays for a period of 3 months in 2018. In early August 2018, the arrays recorded a strong 4‐day‐long deep transient that nucleated in Cholame and propagated toward the northwest. The initiation area is characterized by tremor that persists throughout the transient, while adjacent fault portions located to the northwest host more intermittent tremor. From the rate and location of tremor and LFEs, we infer a deep slip transient, whose along‐strike propagation velocity is ∼8 km/day. Secondary tremor fronts are observed to travel ahead of the main slow slip front at speeds that are times faster. These rapid migrations propagate in slow slip events (SSE) slip‐parallel direction, and are sometimes observed to back‐propagate into previously ruptured segments. High‐resolution images of secondary fronts obtained by using near‐fault borehole stations indicate the active tremor band takes the shape of a narrow strain pulse that is bounded from above by the lower edge of the seismogenic zone. We estimate the stress drops associated with the secondary slip fronts are of the order of a few kPa. Frequency‐dependent analysis suggests the tremor signal is coherent at frequencies as high as 16 Hz, and that the spatial distribution of high‐ and low‐frequency tremor radiators is sometimes complementary.

Tracking the summit activity of Mt. Etna volcano between July 2019 and January 2020 by integrating petrological and geophysical data

The evolution of volcanic activity observed at the New South East Crater (NSEC) and Voragine (VOR) between July 2019 and January 2020 has been deciphered by taking into account the changes of textures and chemical zoning of plagioclase and olivine crystals from the erupted lavas and tephra. The petrological observations have been integrated with analyses of the amplitude and source location of volcanic tremor and infrasound data. Characteristics of crystals erupted on July 2019 at the NSEC reflect protracted intrusions of magma into the mid-upper section of the plumbing system, approximately within 290–120 MPa, which acted as the main zone of magma accumulation and crystallization before the beginning of the eruptive activity. Textures and compositions of crystals erupted at VOR emphasize the beginning of volcanic activity driven by recharge/discharge phases that mostly affect the shallowest portion of the Mt. Etna plumbing system (<40 MPa). At the end of 2019, mineral compositions and zoning patterns changed again in accordance with eruption dynamics. The observed changes reflect the transition from an early phase, between November and December 2019, characterized by substantial equilibrium during magma storage and transport toward higher disequilibrium conditions and eruptive frequency, in January 2020. This has been associated to episodes of deep replenishment of mafic magmas displacing the resident one. Diffusion chronometry applied to zoned olivines shows that most of the episodes of magma intrusion correlate temporally with changes in the features of both volcanic tremor and infrasonic events in terms of amplitude and source location, providing evidence that such geophysical signals are directly related to the magma dynamics in the upper plumbing system.

Imaging the Subsurface Plumbing Complex of Steamboat Geyser and Cistern Spring With Hydrothermal Tremor Migration Using Seismic Interferometry

AbstractSteamboat Geyser in Yellowstone National Park is the tallest active geyser on Earth and is believed to have hydrologic connection to Cistern Spring, a hydrothermal pool ∼100 m southwest from the geyser vent. Despite broad scientific interest, rare episodic Steamboat eruptions have made it difficult to study its eruption dynamics and underground plumbing architecture. In response to the recent reactivation of Steamboat, which has produced more than 130 eruptions since March 2018, we deployed a dense seismic nodal array surrounding the enigmatic geyser in the summer of 2019. The array recorded abundant 1–5 Hz hydrothermal tremor originating from phase‐transition events within both Steamboat Geyser and Cistern Spring. To constrain the spatiotemporal distribution of the tremor sources, an interferometric‐based polarization analysis was developed. The observed tremor locations indicate that the conduit beneath Steamboat is vertical and extends down to ∼120 m depth and the plumbing of Cistern includes a shallow vertical conduit connecting with a deep, large, and laterally offset reservoir ∼60 m southeast of the surface pool. No direct connection between Steamboat and Cistern plumbing structures is found. The temporal variation of tremor combined with in situ temperature and water depth measurements of Cistern reveals interaction between Steamboat and Cistern throughout the eruption/recharge cycles. The observed delayed responses of Cistern Spring in reaction to Steamboat eruptions and recharge suggest that the two plumbing structures may be connected through a fractured/porous medium instead of a direct open channel, consistent with our inferred plumbing structure.

Influence of structure and pore pressure of plate interface on tectonic tremor in the Nankai subduction zone, Japan

Episodes of tectonic tremor are observed in the Nankai accretionary prism close to the trough axis; however, their non-impulsive signals and the distortion of seismic signals in the accretionary prism make it challenging to accurately estimate their locations. Reliable tremor locations would help to characterize geological conditions favourable for tremor generation and could offer insights into the up-dip limit of coseismic ruptures. Here we report a new approach to pinpoint tremor locations based on seismic phase polarization. The estimated epicentres of tremor distribution mostly coincide with low-velocity shear zone or thick underthrust sediments where high pore pressures are inferred, and the distribution of tremor appears to be controlled by characteristics of the shear zone. A tremor episode occurred a couple days after the off-Mie earthquake clearly shows migration at a velocity similar to that of very low frequency earthquakes. Migration of tremor initiated around the outer ridge where the estimated pore pressure is greatest. The spatio-temporal tremor distribution indicates that pore pressure relaxation could trigger the tremor migration. The occurrence of tremor thus may indicate a weak shallow plate interface that is prone to slip following large earthquakes. Based on the information of tremor migration, the permeability along the fault can be estimated as 3.7×10−12 m2.

High resolution location of deep low-frequency tremors beneath the Kii Peninsula, Nankai subduction zone, Japan, using data from a dense seismic array

SUMMARY This study performs detection and location of deep low-frequency tremors beneath the Kii Peninsula, Nankai subduction zone, Japan, using data from a dense seismic array. We set a quantitative threshold for the tremor detection based on the Gaussian distribution of waveform coherence among stations to distinguish between tremor signal and noise. Then, we estimated slownesses and arrival azimuths by applying the Multiple Signal Classification method and performed hypocentre determination by backprojection onto the plate interface. During the analysed period of 2 yrs from July 2012 to July 2014, we detected 25155 tremor events which are approximately 2.2 times more than those in the AIST-tremor catalogue based on the conventional envelope correlation method ( 11475 events). This improvement is due to the detection of even weak amplitude tremors owing to the array analysis scheme, which uses recorded waveform’s phase information. Our high-resolution tremor catalogue leads to the following findings of the tremor activity in the Kii Peninsula. A few tremor episodes show continuous updip migration in the initial stage. The updip migration speed depends on whether the tremor migrates in the along-strike direction after the updip migration: approximately 1.0 km hr –1 for tremors that develop into the along-strike migration, and approximately 0.5 km hr–1 for those that do not. Estimated radiated energies are found to increase gradually with propagation into the updip direction. These findings suggest the depth dependence of the frictional strength on the plate interface. Furthermore, we noticed that accumulated radiated energies are positively correlated with the updip migration speed, and this relationship implies that fluid migration might play a role in the updip migration.

Origin and Properties of Hydrothermal Tremor at Lone Star Geyser, Yellowstone National Park, USA

AbstractGeysers are rare geologic features that intermittently discharge liquid water and steam driven by heating and decompression boiling. The cause of variability in eruptive styles and the associated seismic signals are not well understood. Data collected from five broadband seismometers at Lone Star Geyser, Yellowstone National Park are used to determine the properties, location, and temporal patterns of hydrothermal tremor. The tremor is harmonic at some stages of the eruption cycle and is caused by near‐periodic repetition of discrete seismic events. Using the polarization of ground motion, we identify the location of tremor sources throughout several eruption cycles. During preplay episodes (smaller eruptions preceding the more vigorous major eruption), tremor occurs at depths of 7–10 m and is laterally offset from the geyser's cone by ~5 m. At the onset of the main eruption, tremor sources migrate laterally and become shallower. As the eruption progresses, tremor sources migrate along the same path but in the opposite direction, ending where preplay tremor originates. The upward and then downward migration of tremor sources during eruptions are consistent with warming of the conduit followed by evacuation of water during the main eruption. We identify systematic relations among the two types of preplays, discharge, and the main eruption. A point‐source moment tensor fit to low‐frequency waveforms of an individual tremor event using half‐space velocity models indicates average VS ≳ 0.8 km/s, source depths ~4–20 m, and moment tensors with primarily positive isotropic and compensated linear vector dipole moments.

The Northern Terminus of Cascadia Subduction

AbstractThe Juan de Fuca (JDF) plate system is pivoting and breaking apart as a result of resistance to subduction. At the northern end, the Explorer (EXP) plate moves independently of the JDF plate along the Nootka Fault Zone (NFZ), which forms an unstable triple junction with the JDF ridge and the Sovanco Fracture Zone. We trace the subsurface extension of the NFZ using tomography by combining results from a new nearshore study with a previous ocean‐bottom study and other geophysical constraints. Cross sections from the offshore NFZ through central Vancouver Island suggest a strong fold in the subducting lithosphere (∼40°). Near the deformation front, the NFZ is defined by microseismicity that extends into oceanic mantle with low VP/VS (&lt;1.7) values. This anomaly extends to the NE below the continental shelf with a more northerly trajectory than common NFZ depictions through the seismicity concentration off Nootka Island. Tremor locations and receiver functions indicate that this more northerly trajectory persists across Vancouver Island. We propose a revised tectonic evolution for the NFZ that leads to (i) an EXP microplate confined to shallow depths below the convergent margin and which underthrusts North America no farther landward than Brooks Peninsula and (ii) a northern edge of Cascadia subduction that cuts between Brooks Peninsula and Nootka Island. The plate fold offshore Nootka Island and abundant seismicity there and at Brooks Peninsula are ascribed to stress concentrations on either side of the NFZ associated with deformation at the edges of more competent JDF, EXP, and North American plates.

A Long‐Term View of Episodic Tremor and Slip in Cascadia

AbstractEpisodic Tremor and Slip, or ETS, is a well‐recognized phenomenon in the Cascadia subduction zone. Constraining the precise location of geodetically measured slip during ETS is important for clarifying the relationship between slip and tremor during ETS, the location of ETS slip relative to the strongly locked zone, and the role of ETS in the overall slip budget. Here I present a new method for separating Global Navigation Satellite System time series in Cascadia into ETS and inter‐ETS components, deriving long‐term average ETS velocities for each site. These velocities are then inverted for a time‐averaged ETS slip rate on the plate interface. I find that ETS and its role in the slip budget are highly variable and segmented along strike. This inversion represents the highest‐resolution image of the ETS zone derived from geodetic data to date. Slip correlates well with tremor locations, and a possible second updip ETS zone is detected.

Reliability evaluation of volcanic tremor source location determination using cross-correlation functions

Summary Source location determination of volcanic tremor has been a challenge in seismology due to the waveform complexity and difficulties in reading P- and S-wave arrival times. We present a method for locating volcanic tremor recorded at a seismic network distributed around a volcano. The method combines the source-scanning algorithm and cross-correlation analysis. Tremor records are processed using a technique adopted from ambient seismic interferometry to obtain stacked cross-correlation functions (CCFs) for all station pairs, which are expected to show high amplitudes at the lag time that corresponds to the travel time difference between the stations. The best seismic source location is determined from the maximum of the sum of envelope amplitudes of CCFs at predicted travel time differences between all pairs of stations. This method does not compute theoretical amplitudes, assume an initial hypocenter location, or measure the arrival times. To quantitatively evaluate the accuracy of the location determination, we examine the method by using the vertical component seismic data of volcano-tectonic (VT) earthquakes recorded at six seismic stations at Izu-Oshima volcano. The VTs have been previously located by using arrival times of P- and S-waves, and the hypocenters are used as the reference for evaluation and error estimation of the method. The results show that the misfit, which is the distance between our estimated sources and the references, is about 2 km or less when using CCFs at the frequency band of 4–16 Hz which contains the dominant frequencies of direct S-waves. To test whether the method can be used for volcanic tremor, we simulate the tremors by combining the observed VTs that occurred randomly in time in a localized region. The simulated tremors are determined with location errors of approximately 1 km or less, when the sources of VTs are located within a distance of 1 km and CCFs are calculated for a minimum data length of about 2 minutes. The volcanic tremor location method we present here can be used as an alternative tool for volcano monitoring, especially to locate tremors and seismic events with no clear phase arrival.

Peak Tremor Rates Lead Peak Slip Rates During Propagation of Two Large Slow Earthquakes in Cascadia

AbstractWe explore the evolution of slow slip on the Cascadia megathrust during two large episodic tremor and slip events and compare stress changes to the spatial evolution of tremor from Pacific Northwest Seismic Network tremor locations. We used displacement time series from ~72 GPS stations, along with the Extended Network Inversion Filter to solve for the time‐dependent fault slip. The 2010 (Mw 6.8) and 2012 (Mw 6.8) events propagated northward and southward, respectively, allowing us to assess directional effects on slip behavior. We observed that tremor occurs on the leading edge of propagating slipping regions, well ahead of the highest slip rates, independent of the along‐strike propagation direction. Resolution tests using the actual tremor distributions to generate synthetic data show that our result of peak tremor rates leading peak slip rates is not due to biases introduced by temporal smoothing. Calculated stress changes due to the time‐dependent slip distributions imply that tremor is sensitive to kilopascals of stress, consistent with studies of tidally triggered tremor. Within the resolution of our model, our results are consistent with the hypothesis that significant tremor is triggered by stresses ahead of the highest slip rates. We also observe ongoing slip continuing several days after tremor has passed. Our observations are consistent with some numerical models of tremor patches that suggest that this behavior can be explained by densely packed asperities resulting in somewhat crack‐like propagation rather than a slip pulse that is as concentrated as the tremor activity.