Tremor-like Signals Research Articles

Sensitivity analysis of the backprojection imaging method for seismic event location

Accuracy of earthquake location methods is dependent upon the quality of input data. In the real world, several sources of uncertainty, such as incorrect velocity models, low Signal to Noise Ratio (SNR), and poor coverage, affect the solution. Furthermore, some complex seismic signals exist without distinguishable phases for which conventional location methods are not applicable. In this work, we conducted a sensitivity analysis of Back-Projection Imaging (BPI), which is a technique suitable for location of conventional seismicity, induced seismicity, and tremor-like signals. We performed a study where synthetic data is modelled as fixed spectrum explosive sources. The purpose of using such simplified signals is to fully understand the mechanics of the location method in controlled scenarios, where each parameter can be freely perturbed to ensure that their individual effects are shown separately on the outcome. The results suggest the need for data conditioning such as noise removal to improve image resolution and minimize artifacts. Processing lower frequency signal increases stability, while higher frequencies improve accuracy. In addition, a good azimuthal coverage reduces the spatial location error of seismic events, where, according to our findings, depth is the most sensitive spatial coordinate to velocity and geometry changes.

Towards wearable tremor suppression using dielectric elastomer stack actuators

Active wearable tremor suppression devices apply actuators to the human body to produce joint torques that reduce tremor motion. This potential alternative to medications and surgery has the advantage of providing robust tremor treatment that is non-invasive, but the bulkiness of typical engineering actuators currently prohibits clinical implementations. Dielectric elastomer stack actuators (DESAs) offer a potential pathway towards achieving soft, low-profile tremor suppression devices: DESAs have similar mechanical properties as human muscles and can conform to the human limb. However, low actuation levels and a lack of commercial availability limit the development of DESA-based orthoses. Employing a control approach that only suppresses tremor while allowing the actuators to follow voluntary motion passively significantly decreases actuation requirements to improve potential for clinical devices. Still, DESAs that may offer the necessary actuation characteristics require specialized equipment and techniques. This research advances DESA-based tremor suppression by experimentally demonstrating DESA-based suppression of tremor-like signals on a scaled system using easily manufactured DESAs. Further discussion quantifies the DESA parameters that will enable physical implementations of human-scale tremor suppression and identifies pathways towards achieving those parameters.

A phase coherence approach to identifying co-located earthquakes and tremor

We present and use a phase coherence approach to identify seismic signals that have similar path effects but different source time functions: co-located earthquakes and tremor. The method used is a phase coherence-based implementation of empirical matched field processing, modified to suit tremor analysis. It works by comparing the frequency-domain phases of waveforms generated by two sources recorded at multiple stations. We first cross-correlate the records of the two sources at a single station. If the sources are co-located, this cross-correlation eliminates the phases of the Green’s function. It leaves the relative phases of the source time functions, which should be the same across all stations so long as the spatial extent of the sources are small compared with the seismic wavelength. We therefore search for cross-correlation phases that are consistent across stations as an indication of co-located sources. We also introduce a method to obtain relative locations between the two sources, based on back-projection of interstation phase coherence. We apply this technique to analyse two tremor-like signals that are thought to be composed of a number of earthquakes. First, we analyse a 20 s long seismic precursor to a M 3.9 earthquake in central Alaska. The analysis locates the precursor to within 2 km of the mainshock, and it identifies several bursts of energy—potentially foreshocks or groups of foreshocks—within the precursor. Second, we examine several minutes of volcanic tremor prior to an eruption at Redoubt Volcano. We confirm that the tremor source is located close to repeating earthquakes identified earlier in the tremor sequence. The amplitude of the tremor diminishes about 30 s before the eruption, but the phase coherence results suggest that the tremor may persist at some level through this final interval.

The 2004–2008 dome-building eruption at Mount St. Helens, Washington: epilogue

The 2004–2008 dome-building eruption at Mount St. Helens ended during winter 2007–2008 at a time when field observations were hampered by persistent bad weather. As a result, recognizing the end of the eruption was challenging—but important for scientists trying to understand how and why long-lived eruptions end and for public officials and land managers responsible for hazards mitigation and access restrictions. In hindsight, the end of the eruption was presaged by a slight increase in seismicity in December 2007 that culminated on January 12–13, 2008, with a burst of more than 500 events, most of which occurred in association with several tremor-like signals and a spasmodic burst of long-period earthquakes. At about the same time, a series of regular, localized, small-amplitude tilt events—thousands of which had been recorded during earlier phases of the eruption—came to an end. Thereafter, seismicity declined to 10–20 events per day until January 27–28, when a spasmodic burst of about 50 volcano-tectonic earthquakes occurred over a span of 3 h. This was followed by a brief return of repetitive “drumbeat” earthquakes that characterized much of the eruption. By January 31, however, seismicity had declined to 1–2 earthquakes per day, a rate similar to pre-eruption levels. We attribute the tilt and seismic observations to convulsive stagnation of a semisolid magma plug in the upper part of the conduit. The upward movement of the plug ceased when the excess driving pressure, which had gradually decreased throughout the eruption as a result of reservoir deflation and increasing overburden from the growing dome, was overcome by increasing friction as a result of cooling and crystallization of the plug.

Seismic responses to fluid pressure perturbations in a slipping fault

AbstractSeismicity induced by fluid injection in a natural fault is investigated in situ in the near field of the source. We present synchronous seismic and hydromechanical measurements directly recorded in the decametric injection zone. The three main types of seismic events were recorded during injection and shut‐in: high‐amplitude and short duration seismic events (SE) (i.e., microearthquakes), low to constant amplitude and 5 to 17 s long tremor‐like signals (TLS), and long period events (LP) with a narrow‐frequency band content. Seismicity first initiates with a sequence of SE and TLS, when pressure is high (~3.5 MPa), slip is activated on the fault, which experiences a twentyfold increase of permeability. Then LP events appear to be associated to fluid leakage in the fault caused by dilation during slip. During shut‐in, residual pressures as low as 0.6 MPa still trigger SE events. We show that the initial TLS sequence triggers when a progressive transition occurs from rupture controlled by effective stress variations close to the injection source to a large friction weakening‐dominated slip on the fault. We conclude that the combination of these different seismic signal types may be a proxy to monitor fault instability associated to fluid pressure perturbations.

Regional seismicity: A potential pitfall for identification of long-period long-duration events

Long-period long-duration (LPLD) events are tremorlike signals that have been observed during monitoring of hydraulic-fracture treatment programs. LPLD events have been interpreted to reflect slow deformation processes on fractures or faults that are misoriented for reactivation with respect to the present-day stress field. Regional earthquakes could easily be mistaken for LPLD events because both are characterized by similar frequency content ([Formula: see text]) and duration ([Formula: see text]). Using data from a 10.5-month continuous downhole deployment of a 15-Hz geophone array in a tight-sand gas field in western Canada, we compared recordings of small earthquakes with previously published LPLD events. We determined that regional earthquakes can show similar waveform characteristics to LPLD events, underscoring the importance of distinguishing regional earthquake signals from LPLD events to ensure robust interpretation of reservoir deformation processes.

Dynamic Ruptures on a Frictional Interface with Off-Fault Brittle Damage: Feedback Mechanisms and Effects on Slip and Near-Fault Motion

The spontaneous generation of brittle rock damage near and behind the tip of a propagating rupture can produce dynamic feedback mechanisms that modify significantly the rupture properties, seismic radiation, and generated fault zone structure. In this work, we study such feedback mechanisms for single rupture events and their consequences for earthquake physics and various possible observations. This is done through numerical simulations of in-plane dynamic ruptures on a frictional fault with bulk behavior governed by a brittle damage rheology that incorporates reduction of elastic moduli in off-fault yielding regions. The model simulations produce several features that modify key properties of the ruptures, local wave propagation, and fault zone damage. These include (1) dynamic generation of near-fault regions with lower elastic properties, (2) dynamic changes of normal stress on the fault, (3) rupture transition from crack-like to a detached pulse, (4) emergence of a rupture mode consisting of a train of pulses, (5) quasi-periodic modulation of slip rate on the fault, and (6) asymmetric near-fault ground motion with higher amplitude and longer duration on the side with reduced elastic moduli. The results can have significant implications to multiple topics ranging from rupture directivity and local amplification of seismic motion to near-fault tremor-like signals.

Spatio-temporal variability of seismic noise above a geothermal reservoir

Abstract We report on the application of seismic noise investigations, including H/V (horizontal to vertical) spectral ratio and array techniques, to a shallow gas-rich geothermal reservoir in Heybeli, southwestern Turkey. Fundamental resonant frequencies were determined to estimate the sediment thickness. Using small-scale seismic arrays, phase velocity dispersion curves were derived by correlating noise recordings according to the extended spatial autocorrelation method. Improved shear wave velocity profiles were estimated by combining Rayleigh wave dispersion curves and horizontal to vertical spectral ratios in a joint inversion. We found that the velocities obtained for the reservoir site are higher than those for a location outside the reservoir. In addition to the fundamental resonant peaks in the spectra, a clear 6-Hz-signal could be identified originating from the center of the geothermal field, repeatedly observed in 2010 and 2011. It had been claimed that low frequency (1–10 Hz) seismic signal anomalies were correlated with the occurrence of hydrocarbons. One of the physical mechanisms under consideration to explain these tremor-like signals above such reservoirs is resonant amplification due to the oscillation of bubbles. Based on the signal similarity with volcanic tremors, it is not a priori given that the liquid phase must be oil for resonance effects to occur. We therefore applied array techniques to identify potential noise originating from the Heybeli reservoir. In fact, the frequency–wavenumber ( f – k ) method clearly indicated a noise source coming from the main production well of the reservoir. In 2011, as part of our assessment, the operators of the spa facility stopped the extraction of thermal water for 2 h: the 6-Hz-signal disappeared after the pump had been stopped and reappeared after the pump began operating again. Thus, the 6-Hz-signal is likely of artificial origin. In addition, no natural noise source inside the reservoir could be identified.

Waveform inversion of shallow repetitive long period events at Villarrica Volcano, Chile

[1] Villarrica volcano is presently characterized by an active lava lake in the summit crater, with mild strombolian style eruptions of basaltic-andesitic products. The lava lake acts as a permeable barrier where gas-rich magma releases volatiles and then descends through the process of conduit convection to a deep storage reservoir. We identified thousands of repetitive long-period (LP) seismic events within a three-year time period using a matched filter; each was discovered to be associated with an infrasonic emission, and hence outgassing from the lava lake. Using this catalog of events for timing, we stacked signals over the three-year deployment from 2010-2012 to produce representative seismic and infrasonic signatures, and performed a full-waveform seismic inversion using nine stations from the network. The data are well represented by a single horizontal force source-time function, oriented N75°E and dipping 7°W from horizontal, with a maximum amplitude of 34.4 MN. The source is stable in time, and can be explained by a force couple system produced by a bursting bubble in the lava lake and the responsive drag force of the lava sloshing to fill in the void produced by the gas emission in an asymmetrical crater. Path effects lengthen the coda of the events with increasing distance from the source, resulting in emergent tremor-like signals at distant seismometers. Since the occurrence of LP events correlates with tremor intensity, we suggest much of the tremor results from the superposition of discrete LPs.

Comparison of 2D and 3D time-reverse imaging—A numerical case study

Time-reverse imaging has become an efficient tool to detect the origin of passively acquired seismic tremor signals. Practical experience has mainly been developed for 2D applications. Three component signals are reduced to two components and reverse propagated on the plane vertically below a station line. The data used for time-reverse imaging are the vertical and the horizontal particle displacement parallel to the line. Dropping the horizontal component perpendicular to the line causes partial loss of information on particle motion and directivity of the recorded waves. We present a comparison of 2D and 3D time-reverse imaging for a specific site with small cross-line gradients and investigate how closely 2D imaging approximates 3D imaging. Our large-scale synthetic survey with different S/N-ratios demonstrates how a subsurface source of tremor-like signals is imaged in different vertical planes. An imaging condition based on the energy density gives best results. We observe higher sensitivity to noise and stronger out-of-plane focusing for 2D than for 3D imaging. We suggest normalized visualization of multiple planes from 2D imaging in one 3D display as an approach to reliably locate sources. Comparison with examples of full 3D time-reverse imaging shows that normalized visualization of multiple 2D planes with a proper imaging condition can adequately approximate the result from full 3D imaging for the particular model considered in this study.

Micromechanics of asperity rupture during laboratory stick slip experiments

[1] Acoustic emissions and tremor-like signals are widely recorded in laboratory experiments. We are able to isolate the physical origins of these signals using high resolution nanoseismic analysis. The use of a picometer-sensitive, wide-band sensor array permits us to determine force-time functions and focal mechanisms for discrete events found amid the “noise” of friction, similar to how low frequency earthquakes are found buried within tremor. We interpret these localized events to be the rupture of μm-sized contacts, known as asperities. We performed stick-slip experiments on plastic/plastic and rock/rock interfaces and found a systematic difference between the nano earthquakes: the rock interface produces very rapid (<1 μs) implosive forces indicative of brittle failure and fault gouge formation, while rupture on the plastic interface releases only shear force and produces a nano quake more similar to earthquakes commonly recorded in the field. The difference between the mechanisms is attributed to the vast differences in the hardness and melting temperatures of the two materials, which affect the distribution of asperities as well as their failure behavior. With proper scaling, the strong link between material properties and laboratory earthquakes will aid in our understanding of fault mechanics and the generation of earthquakes and tectonic tremor.

Slip acceleration generates seismic tremor like signals in friction experiments

[1] Since their discovery nearly a decade ago, the origin of seismic tremor remains unclear. Recent studies indicate that various driving phenomena such as Earth and ocean tides, regional and teleseismic earthquakes enhance tremor activity. Observations of the coincidence with slow-slip events and of fast migrations of tremors have led frictional slip to be considered as the possible source of tremors. Indeed, laboratory friction experiments succeeded in generating and recording tremor like signals (TLS). Here we show a systematic correlation between the onset of slip acceleration and the emission of TLS in a laboratory friction experiment. TLS are generated when the shear stress reaches the peak static resistance and the dilatancy meets its maximum that is when the mature interface is close to failure. This robust result provides a comprehensive image of how natural seismic tremors might be generated and/or triggered by passing seismic waves, tides or even slow slip events.

Pitfalls of tremor-like signals for hydrocarbon exploration in producing oil fields in Potiguar Basin, northeast Brazil

The Earth's ambient noise has been used over the past four decades for sedimentary basin studies, but only in the past ten years have geophysicists begun to apply ambient seismic noise, a technique known as passive seismic, to hydrocarbon exploration.

Phenomenology of tremor-like signals observed over hydrocarbon reservoirs

We have observed narrow-band, low-frequency (1.5–4 Hz, amplitude 0.01–10 μm/s) tremor signals on the surface over hydrocarbon reservoirs (oil, gas and water multiphase fluid systems in porous media) at currently 15 sites worldwide. These ‘hydrocarbon tremors’ possess remarkably similar spectral and signal structure characteristics, pointing to a common source mechanism, even though the depth (some hundreds to several thousands of meters), specific fluid content (oil, gas, gas condensate of different compositions and combinations) and reservoir rock type (such as sandstone, carbonates, etc.) for each of those sites are quite different. About half of the sites are fully explored or even developed and producing fields, and hard quantitative data on the reservoirs are available (well data, reservoir monitoring data, seismic surveys, etc.). The other areas are essentially either explored prospect areas where we did not have access to hard reservoir data or (in only one case) areas where no exploration wells have been drilled at all. The tremor signal itself was observed over ALL locations investigated so far. The signals weaken at the rim of the reservoirs and are not observed outside the reservoir area. There is a strong correlation of the tremor power with the thickness of the hydrocarbon-bearing layers (‘pay zone thickness’) determined by borehole log measurements. The overall correlation between surface tremor measurements and accessible subsurface well data is higher than 90%. The phenomenological comparison of hydrocarbon tremor signals with volcanic tremor signals from Stromboli and Arenal volcanoes using both conventional spectral analysis tools and non-linear dynamics methods reveals fundamental similarities between those two phenomena as well as their close relation to bandpass filtered noise. Nevertheless, the specific signal sources are expected to be different for volcanoes and hydrocarbon reservoirs. Using the currently available data we present possible concepts (active or passive mechanisms) on the nature of the hydrocarbon tremor source. Our data lead us to conclude that we are most likely observing a characteristic filtering/mixing effect, with the energy input supplied by the natural seismo-acoustic background. The reservoir, i.e. the hydrocarbon-water-multifluid system contained in a porous medium, is expected to be the oscillatory element able to act as a filter/mixer (resembling essentially a in-reservoir path effect) for the natural seismo-acoustic background. Most intriguing seems the application aspect, i.e. the practical usability of this spectroscopic approach as a direct from-the-surface, non-invasive hydrocarbon indicator.