Seismic Source Location Research Articles

Back‐Projection Imaging of Extended, Diffuse Sources of Volcano‐Tectonic Seismicity During June 2010 at Sierra Negra Volcano, Galápagos

AbstractSeismic signals generated by subsurface volcano‐tectonic processes commonly have low signal‐to‐noise ratios that make it difficult to resolve both 3‐D subsurface structures and seismic source locations using established seismic imaging methods (e.g., phase picking, template matching). High‐amplitude bursts within the noise (i.e., eruptions) are sometimes suitable for traditional seismic imaging but occur infrequently relative to the duration of a systems eruption cycle and offer minimal insight into mechanisms that occur during more common intereruption periods. Prevalent noise recorded near volcanic systems is often nearly continuously generated by many overlapping low‐magnitude displacements at depth that can be directly linked to magma, fluid, and volatile transport. This noise contains valuable information about subsurface processes that occur between volcanic eruptions and may provide critical information for mitigating risks from volcanic hazards. We present a new computationally efficient method to comprehensively study temporal and spatial variations in extended, diffuse, low‐magnitude volcano‐tectonic seismicity by applying a back‐projection search algorithm to analyze coherent seismic energy arriving from potential subsurface source locations to available pairs of broadband seismometers. We apply this method to Sierra Negra Volcano for a 2.5‐week study period coinciding with a dike intrusion in June 2010. Through averaging (stacking), the back‐projection algorithm illuminates the most likely source locations of low‐magnitude, high‐frequency volcano‐tectonic seismicity before, during, and after previously identified volcano‐tectonic earthquakes. Our results corroborate known volcano‐tectonic earthquake locations and support the interpretation of a small‐volume dike intrusion at ~5–8 km below sea level along the south‐southeastern flanks of the Sierra Negra caldera in June 2010.

Coseismic displacements on Ischia island, real-time GPS positioning constraints on earthquake source location

We use GNSS data to simulate the early seismic source location of a M 4 event occurred in the island of Ischia, Italy. The study suggests that real-time GNSS data can support the seismic location system in the early stage of the emergency phase. We demonstrate that this very shallow earthquake, triggered significant displacements at a few stations in less than half an hour. Using exclusively GPS data, the first location of the hypocenter was possible with a latency of only 20 minutes. Early upgrades of the offset field in the first two hours confirm the source location confined within 1-2 km in the horizontal plane and less than 1 km depth.

Seismic signature of turbulence during the 2017 Oroville Dam spillway erosion crisis

Abstract. Knowing the location of large-scale turbulent eddies during catastrophic flooding events improves predictions of erosive scour. The erosion damage to the Oroville Dam flood control spillway in early 2017 is an example of the erosive power of turbulent flow. During this event, a defect in the simple concrete channel quickly eroded into a 47 m deep chasm. Erosion by turbulent flow is difficult to evaluate in real time, but near-channel seismic monitoring provides a tool to evaluate flow dynamics from a safe distance. Previous studies have had limited ability to identify source location or the type of surface wave (i.e., Love or Rayleigh wave) excited by different river processes. Here we use a single three-component seismometer method (frequency-dependent polarization analysis) to characterize the dominant seismic source location and seismic surface waves produced by the Oroville Dam flood control spillway, using the abrupt change in spillway geometry as a natural experiment. We find that the scaling exponent between seismic power and release discharge is greater following damage to the spillway, suggesting additional sources of turbulent energy dissipation excite more seismic energy. The mean azimuth in the 5–10 Hz frequency band was used to resolve the location of spillway damage. Observed polarization attributes deviate from those expected for a Rayleigh wave, though numerical modeling indicates these deviations may be explained by propagation up the uneven hillside topography. Our results suggest frequency-dependent polarization analysis is a promising approach for locating areas of increased flow turbulence. This method could be applied to other erosion problems near engineered structures as well as to understanding energy dissipation, erosion, and channel morphology development in natural rivers, particularly at high discharges.

Seismological investigation of September 09 2016, North Korea underground nuclear test

On Sep. 9, 2016, a seismic event of mb 5.3 took place in North Korea. This event was reported asa nuclear test. In this study, we applied a number of discriminant techniques that facilitate the ability to distinguish between explosions and earthquakes on the Korean Peninsula. The differences between explosions and earthquakes are due to variation in source dimension, epicenter depth and source mechanism, or a collection of them. There are many seismological differences between nuclear explosions and earthquakes, but not all of them are detectable at large distances or are appropriate to each earthquake and explosion. The discrimination methods used in the current study include the seismic source location, source depth, the differences in the frequency contents, complexity versus spectral ratio and Ms-mb differences for both earthquakes and explosions. Sep. 9, 2016, event is located in the region of North Korea nuclear test site at a zero depth, which is likely to be a nuclear explosion. Comparison between the P wave spectra of the nuclear test and the Sep. 8, 2000, North Korea earthquake, mb 4.9 shows that the spectrum of both events is nearly the same. The results of applying the theoretical model of Brune to P wave spectra of both explosion and earthquake show that the explosion manifests larger corner frequency than the earthquake, reflecting the nature of the different sources. The complexity and spectral ratio were also calculated from the waveform data recorded at a number of stations in order to investigate the relation between them. The observed classification percentage of this method is about 81%. Finally, the mb:Ms method is also investigated. We calculate mb and Ms for the Sep. 9, 2016, explosion and compare the result with the mb: Ms chart obtained from the previous studies. This method is working well with the explosion.

Source and dynamics of a volcanic caldera unrest: Campi Flegrei, 1983\u201384

Despite their importance for eruption forecasting the causes of seismic rupture processes during caldera unrest are still poorly reconstructed from seismic images. Seismic source locations and waveform attenuation analyses of earthquakes in the Campi Flegrei area (Southern Italy) during the 1983–1984 unrest have revealed a 4–4.5 km deep NW-SE striking aseismic zone of high attenuation offshore Pozzuoli. The lateral features and the principal axis of the attenuation anomaly correspond to the main source of ground uplift during the unrest. Seismic swarms correlate in space and time with fluid injections from a deep hot source, inferred to represent geochemical and temperature variations at Solfatara. These swarms struck a high-attenuation 3–4 km deep reservoir of supercritical fluids under Pozzuoli and migrated towards a shallower aseismic deformation source under Solfatara. The reservoir became aseismic for two months just after the main seismic swarm (April 1, 1984) due to a SE-to-NW directed input from the high-attenuation domain, possibly a dyke emplacement. The unrest ended after fluids migrated from Pozzuoli to the location of the last caldera eruption (Mt. Nuovo, 1538 AD). The results show that the high attenuation domain controls the largest monitored seismic, deformation, and geochemical unrest at the caldera.

Testing seismic amplitude source location for fast debris-flow detection at Illgraben, Switzerland

Abstract. Heavy precipitation can mobilize tens to hundreds of thousands of cubic meters of sediment in steep Alpine torrents in a short time. The resulting debris flows (mixtures of water, sediment and boulders) move downstream with velocities of several meters per second and have a high destruction potential. Warning protocols for affected communities rely on raising awareness about the debris-flow threat, precipitation monitoring and rapid detection methods. The latter, in particular, is a challenge because debris-flow-prone torrents have their catchments in steep and inaccessible terrain, where instrumentation is difficult to install and maintain. Here we test amplitude source location (ASL) as a processing scheme for seismic network data for early warning purposes. We use debris-flow and noise seismograms from the Illgraben catchment, Switzerland, a torrent system which produces several debris-flow events per year. Automatic in situ detection is currently based on geophones mounted on concrete check dams and radar stage sensors suspended above the channel. The ASL approach has the advantage that it uses seismometers, which can be installed at more accessible locations where a stable connection to mobile phone networks is available for data communication. Our ASL processing uses time-averaged ground vibration amplitudes to estimate the location of the debris-flow front. Applied to continuous data streams, inversion of the seismic amplitude decay throughout the network is robust and efficient, requires no manual identification of seismic phase arrivals and eliminates the need for a local seismic velocity model. We apply the ASL technique to a small debris-flow event on 19 July 2011, which was captured with a temporary seismic monitoring network. The processing rapidly detects the debris-flow event half an hour before arrival at the outlet of the torrent and several minutes before detection by the in situ alarm system. An analysis of continuous seismic records furthermore indicates that detectability of Illgraben debris flows of this size is unaffected by changing environmental and anthropogenic seismic noise and that false detections can be greatly reduced with simple processing steps.

Time reversal seismic source imaging using peak average power ratio (PAPR) parameter

The time reversal method has become a standard technique for the location of seismic sources. It has been used both for acoustic and elastic numerical modelling and for 2D and 3D propagation models. Although there are many studies concerning its application to point sources, little so far has been done to generalise the time reversal method to the study of sequences of seismic events. The need to describe such processes better motivates the analysis presented in this paper. The synthetic time reversal imaging experiments presented in this work were conducted for sources with the same origin time as well as for the sources with a slight delay in origin time. For efficient visualisation of the seismic wave propagation and interference, a new coefficient—peak average power ratio—was introduced. The paper also presents a comparison of visualisation based on the proposed coefficient against a commonly used visualisation based on a maximum value.

Comprehensive evaluation of the stability of the left-bank slope at the Baihetan hydropower station in southwest China

The stability of the left-bank slope is a crucial geological engineering problem at the Baihetan hydropower station, China. Due to continuous excavations on the rock slope, different regions of the surrounding rock mass undergo varying degrees of unloading deformation. It is important to assess the stability of the rock slope from a macroscopic viewpoint by investigating its deformation characteristics and mechanisms. Therefore, in this work, microseismic (MS) monitoring was first employed to detect the progressive rock mass damage in the rock slope subjected to excavation, including the initiation, propagation, coalescence, and interaction of rock microfractures. Numerical modeling was subsequently performed to understand the deformation and failure mechanism of the rock slope. Moreover, traditional surveying approaches (i.e., multiple-point extensometers and inclinometers) and field observations were also used to analyze the deformation and failure characteristics of the rock slope. The MS monitoring results showed that spatiotemporal regularities in the evolution of seismic source locations were indicators of deformation failure and potential sliding surfaces. MS event clustering can be used to delineate activated pre-existing geological structures (i.e., LS331 and LS337). The simulation results show that the deformation and failure characteristics of the rock slope are mainly controlled by pre-existing weak structural planes (i.e., the intraformational faulted zones LS3319, LS331, and LS337 and fault F17). These results agree well with the results of geological data and conventional monitoring data. Our study reveals that an integrated approach combining MS monitoring, numerical modeling, traditional surveying, and field observations leads to a better understanding of the behavior of the rock slope under the influence of excavation as well as greater control of the working faces, ensuring safety under complex geological and excavation conditions.

Interpretation of the extent of hydraulic fracturing for rockburst prevention using microseismic monitoring data

As the mining depths increase, an increasing number of deep coal mines in China encounter frequent intense rockburst problems. Conventional destress measures, employed effectively in shallow mines to reduce rockburst risk, are not suitable to deep coal mines because they are labor-intensive and time-consuming and hence are costly. Hydraulic fracturing in coal seams before mining a particular area has been considered as an effective destressing method for rockburst prevention. This paper focuses on using microseismic data to evaluate the extent of hydraulic fracturing in coal seams. To that purpose, innovative methods are proposed to process and interpret microseismic monitoring data. The proposed methods consist of an improved HHT method for signal filtering, an improved time-window energy eigenvalue method for first arrival picking, and a four-channel combined algorithm for seismic source location determination. Using the elaborate signal processing and interpretation methods, high-precision source locations of microseismic events recorded in a field hydraulic fracturing test at Huafeng Coal Mine are obtained. Microseismic event frequency and energy contours are plotted to characterize the fracture development and propagation process. The interpretation method was successfully applied in the coal seam hydraulic fracturing tests. Direct field observation and stress monitoring were also conducted to verify the results by the microseismic data interpretation method. Compared with conventional monitoring techniques such as stress monitoring and direct field observation, microseismic monitoring can cover a large monitoring volume with a high response sensitivity and it can capture the spatial-temporal fracture evolution process easily. It provides a practical approach to quantify the extent of hydraulic fracturing in coal seams for rockburst prevention.

HIGH PRECISION LOCATION OF MICRO‐SEISMIC SOURCE IN UNDERGROUND COAL MINE

AbstractAccurate location of micro‐seismic source in underground coal mine is of great significance for monitoring and early warning of dynamic disaster. Micro‐seismic source information is generally extracted through inversion of the data acquired by the underground sensors. The installation of the sensors is limited around underground roadway. The unreasonable arrangement of sensors along the roadway will greatly decrease the precision of the seismic location. As for the ill‐posed problems caused by the inversion for source location with sensor‐received information, high precision location algorithm of micro‐seismic source based on the arrangement optimization of monitoring points is presented in this paper. Firstly, the ill‐posed problems are judged by calculating the coefficient matrix condition numbers. The ill‐posed matrix is then pretreated by centralization and row balance jointly. The regularization parameters of pretreated matrix A and b are calculated by the L‐curve method. The seismic source coordinates regularization solution is obtained via Tikhonov regularization algorithm. The research shows that the matrix magnitude is decreased hugely with centralization method, ill‐posed condition numbers are reduced with row balance pretreatment. The minimum seismic source coordinate error of Tikhonov regularization solution after the pretreatment is 3.09 m which has been greatly decreased compared to the error by Gaussian elimination solution before the pretreatment. High precision seismic source location is realized in limited space of underground roadway by the above optimization processing.

Discontinuum Modelling Approach for Stress Analysis at a Seismic Source: Case Study

Rockbursts in underground mines can cause devastating damage to mine workings; hence, it is important to be able to assess the potential for their occurrence. The present study focuses on a large seismic event that took place at an underground base metal mine in Canada. The event took place in a dyke near the 100/900 orebodies on 3880 Level (1180 m below surface) of the Copper Cliff Mine in Sudbury, Canada. A 3D continuum stress analysis of the orebodies, i.e., 100 and 900, using an orebody-wide model encompassing the major geological structures and in situ stress heterogeneity in the mine shows low potential for rockburst at the seismic source location—a result which contradicts the fact that a large seismic event actually took place. A postulation is thus made that there had been highly stressed regions caused by geological disturbances at the source location before mining activities took place. In order to verify the postulation, a further study is undertaken with the discrete element modelling technique, whereby a cube-shaped model containing a fracture network is subjected to a stress state similar to that at the source location. A model parametrical study is conducted with respect to the distribution of the fracture (joint) network and its mechanical properties. The results reveal that when joints are densely distributed at the source location, the stress state becomes significantly burst prone. It is observed that the length, density, stiffness, and orientation of joints have a large influence on the stress state along the joints, while the friction angle, cohesion, and tensile strength do not influence the stress state. A cube-shaped model is constructed with joint sets actually mapped at the mine and a stress analysis is performed. The results demonstrate the generation of highly stressed regions due to the interaction of the joints with the applied in situ stress fields, thus leading to burst-prone conditions. The present study numerically confirms that a preexisting fracture network found within a stiff rockmass could lead to burst-prone stress state at relatively moderate mining depths.

Reverse time migration for microseismic sources using the geometric mean as an imaging condition

Time reversal is a powerful tool used to image directly the location and mechanism of passive seismic sources. This technique assumes seismic velocities in the medium and propagates time-reversed observations of ground motion at each receiver location. Assuming an accurate velocity model and adequate array aperture, the waves will focus at the source location. Because we do not know the location and the origin time a priori, we need to scan the entire 4D image (3D in space and 1D in time) to localize the source, which makes time-reversal imaging computationally demanding. We have developed a new approach of time-reversal imaging that reduces the computational cost and the scanning dimensions from 4D to 3D (no time) and increases the spatial resolution of the source image. We first individually extrapolate wavefields at each receiver, and then we crosscorrelate these wavefields (the product in the frequency domain: geometric mean). This crosscorrelation creates another imaging condition, and focusing of the seismic wavefields occurs at the zero time lag of the correlation provided the velocity model is sufficiently accurate. Due to the analogy to the active-shot reverse time migration (RTM), we refer to this technique as the geometric-mean RTM or GmRTM. In addition to reducing the dimension from 4D to 3D compared with conventional time-reversal imaging, the crosscorrelation effectively suppresses the side lobes and yields a spatially high-resolution image of seismic sources. The GmRTM is robust for random and coherent noise because crosscorrelation enhances signal and suppresses noise. An added benefit is that, in contrast to conventional time-reversal imaging, GmRTM has the potential to be used to retrieve velocity information by analyzing time and/or space lags of crosscorrelation, which is similar to what is done in active-source imaging.

Multiband array detection and location of seismic sources recorded by dense seismic networks

We present a new methodology for detection and space–time location of seismic sources based on multiscale, frequency-selective coherence of the wave field recorded by dense large-scale seismic networks and local antennas. The method is designed to enhance coherence of the signal statistical features across the array of sensors and consists of three steps: signal processing, space–time imaging, and detection and location. The first step provides, for each station, a simplified representation of seismic signal by extracting multiscale non-stationary statistical characteristics, through multiband higher-order statistics or envelopes. This signal processing scheme is designed to account for a priori unknown transients, potentially associated with a variety of sources (e.g. earthquakes, tremors), and to prepare data for a better performance in posterior steps. Following space–time imaging is carried through 3-D spatial mapping and summation of station-pair time-delay estimate functions. This step produces time-series of 3-D spatial images representing the likelihood that each pixel makes part of a source. Detection and location is performed in the final step by extracting the local maxima from the 3-D spatial images. We demonstrate the efficiency of the method in detecting and locating seismic sources associated with low signal-to-noise ratio on an example of the aftershock earthquake records from local stations of International Maule Aftershock Deployment in Central Chile. The performance and potential of the method to detect, locate and characterize the energy release associated with possibly mixed seismic radiation from earthquakes and low-frequency tectonic tremors is further tested on continuous data from southwestern Japan.

Seismic network configuration by reduction of seismic source location errors

This article discusses the possibilities of improving the locations of seismic emission sources induced during underground mining works. The application of three-component seismic sensors made it possible to identify the location of sources using the propagation times of seismic waves and the directions of seismic rays. The article is a case study of Rudna, the largest underground Polish copper mine, and presents location methods that use times or/and directions of seismic wave propagation. For both methods, a discussion of location errors is also presented. The presented considerations are illustrated with a synthetic example that imitates the typical data recorded in Polish copper mines. A real example of source location for both the discussed methods of source location and error assessment is presented. The distributions of the location errors are presented for the two discussed methods for a sample panel of the Rudna mine. The best location for the three-component sensor that minimizes average location errors in the mining panels is presented. In the discussion, special attention is devoted to accurately determining the depth of seismic sources.

Weighted-elastic-wave interferometric imaging of microseismic source location

Knowledge of the locations of seismic sources is critical for microseismic monitoring. Time-window-based elastic wave interferometric imaging and weighted-elastic-wave (WEW) interferometric imaging are proposed and used to locate modeled microseismic sources. The proposed method improves the precision and eliminates artifacts in location profiles. Numerical experiments based on a horizontally layered isotropic medium have shown that the method offers the following advantages: It can deal with low-SNR microseismic data with velocity perturbations as well as relatively sparse receivers and still maintain relatively high precision despite the errors in the velocity model. Furthermore, it is more efficient than conventional traveltime inversion methods because interferometric imaging does not require traveltime picking. Numerical results using a 2D fault model have also suggested that the weighted-elastic-wave interferometric imaging can locate multiple sources with higher location precision than the time-reverse imaging method.

Estimates of seismic activity in the Cerberus Fossae region of Mars

The 2016 NASA InSight lander is the first planetary mission designed to study the deep interior of Mars. InSight's Seismic Experiment for Interior Structure (SEIS) package will quantify global and regional seismic activity and determine parameters like core properties, mantle composition, and Martian lithospheric thickness. An improved understanding of the location, magnitude, and frequency of potential seismic sources is essential for optimization of instrument design, sampling strategy, and interpretation of mission data. We focus on forecasting seismic activity for the Cerberus Fossae of the Elysium Planitia, chosen for their proximity to the proposed landing site and their recent formation and assuming these are active tectonic grabens. The minimum age we determine for the units around the Fossae, using Context Camera and High Resolution Imaging Science Experiment imagery for crater density surveying, is 10 Ma, placing them in the Late Amazonian. We are able to determine the rate of motion from measurements of observed throw, assuming that the faults remain active. Digital terrain models, made from stereo‐image pairs from the High Resolution Stereo Camera (HRSC), are used to determine the maximum throw on four graben systems. Using these measured throws to estimate a length‐averaged slip and assuming an inferred slip rate from surface age, we estimate an annual moment release of . From this we calculate an annual size‐frequency distribution of events using the Gütenberg‐Richter relationship. We estimate that between 1.5×100and 1.9×105events per year will have an amplitude greater than the peak band noise and so will be detectable at the InSight landing site.

Generation and propagation of oceanic T-waves using elastic parabolic equation solutions

Oceanic T-waves are essential for location and identification of seismic sources since they travel long distances in the ocean and are typically the largest signals received at hydrophone arrays or coastal monitoring stations. T-waves either link directly into the SOFAR channel by conversion of elastic wave energy at a downward sloping interface between the elastic and fluid media, or are generated when elastic energy couples into low order acoustic modes due to bathymetric inhomogeneities. Elastic parabolic equation solutions will demonstrate generation and long range propagation of oceanic T-waves in the water column when the source is located in an elastic ocean bottom. Elastic parabolic equation solutions will be used to describe effects of ocean bottom parameters on transmission characteristics of a sloping boundary. The impact of small-scale bathymetric changes, for example due to a rough ocean bottom, will be characterized by averaging acoustic wavenumber spectra resulting from multiple bottom realizations. Favorable characteristics for T-wave generation will be determined. The impact of large scale bathymetry changes, such as a seamount or underwater ridge, will also be discussed. [Work supported by ONR.]

The plumbing of Old Faithful Geyser revealed by hydrothermal tremor

AbstractOld Faithful Geyser in Yellowstone National Park (USA) has attracted numerous scientific investigations for over two centuries to better understand its geological structure, the physics of its eruptions, and the controls of its intermittency. Using data acquired with a seismic array in 1992, we track the sources of hydrothermal tremor produced by boiling and cavitation inside the geyser. The location of seismic sources identifies a previously unknown lateral cavity at 15 m below the surface, on the SW side of the vent, and connected to the conduit. This reservoir is activated at the beginning of each geyser eruption cycle and plays a major role in the oscillatory behavior of the water level in the conduit before each eruption.

Solution and simulation algorithm of microseismic events location to three-dimensional model by comprehensive location method based on Matlab

The essential for microseismic monitoring is fast and accurate calculation of seismic wave source location. The precision of most traditional microseismic monitoring processes of mines, using TDOA location method in two-dimensional space to position the microseismic events, as well as the accuracy of positioning microseismic events, may be reduced by the two-dimensional model and simple method, and ill-conditioned equations produced by TDOA location method will increase the positioning error. This article, based on inversion theory, studies the mathematical model of TDOA location method, polarization analysis location method, and comprehensive difference location method of adding angle factor in the traditional TDOA location method. The feasibility of three methods is verified by numerical simulation and analysis of the positioning error of them. The results show that the comprehensive location method of adding angle difference has strong positioning stability and high positioning accuracy, and it may reduce the impact effectively about ill-conditioned equations to positioning results. Comprehensive location method with the data of actual measure may get better positioning results.

Excavation-induced microseismicity: microseismic monitoring and numerical simulation

The volume of influence of excavation at the right bank slope of Dagangshan Hydropower Station, southwest China, is essentially determined from microseismic monitoring, numerical modeling and conventional measurements as well as in situ observations. Microseismic monitoring is a new application technique for investigating microcrackings in rock slopes. A microseismic monitoring network has been systematically used to monitor rock masses unloading relaxation due to continuous excavation of rock slope and stress redistribution caused by dam impoundment later on, and to identify and delineate the potential slippage regions since May, 2010. An important database of seismic source locations is available. The analysis of microseismic events showed a particular tempo-spatial distribution. Seismic events predominantly occurred around the upstream slope of 1180 m elevation, especially focusing on the hanging wall of fault XL316-1. Such phenomenon was interpreted by numerical modeling using RFPA-SRM code (realistic failure process analysis-strength reduction method). By comparing microseismic activity and results of numerical simulation with in site observation and conventional measurements results, a strong correlation can be obtained between seismic source locations and excavation-induced stress distribution in the working areas. The volume of influence of the rock slope is thus determined. Engineering practices show microseismic monitoring can accurately diagnose magnitude, intensity and associated tempo-spatial characteristics of tectonic activities such as faults and unloading zones. The integrated technique combining seismic monitoring with numerical modeling, as well as in site observation and conventional surveying, leads to a better understanding of the internal effect and relationship between microseismic activity and stress field in the right bank slope from different perspectives.