Small Seismic Array Research Articles

Seismic Anisotropy From 6C Ground Motions of Ambient Seismic Noise

AbstractWe propose a new approach capable of measuring local seismic anisotropy from 6C (three‐component translation and three‐component rotation) amplitude observations of ambient seismic noise data. Our recent theory demonstrates that the amplitude ratio of 6C cross‐correlation functions (CCFs) enables retrieving the local phase velocity. This differs from conventional velocity extraction methods based on the travel time. Its local sensitivity kernel beneath the 6C seismometer allows us to study anisotropy from azimuth‐dependent CCFs, avoiding path effects. Such point measurements have great potential in planetary exploration, ocean bottom observations, or volcanology. We apply this approach to a small seismic array at Pion Flat Observatory (PFO) in southern California, array‐deriving retrieves rotational ground motions from microseismic noise data. The stress‐induced anisotropy is well resolved and compatible with other tomography results, providing constraints on the origin of depth‐dependent seismic anisotropy.

Monitoring induced microseismicity in an urban context using very small seismic arrays: The case study of the Vendenheim EGS project

Monitoring the seismicity induced by fluid injections in deep geothermal reservoirs is often limited by the significant anthropogenic ambient noise level inherent to most enhanced geothermal system (EGS) projects in urban contexts. We report on the performance of a monitoring network made of three small aperture (72 m) seismic arrays composed of 21 3D nodes. We test the setup for four months (from 12 December 2020 to 8 April 2021) of the Strasbourg sequence of induced earthquakes (2019–2022) related to the EGS Georhin project (Vendenheim, France). The deployment starts a few days after the MLv = 3.6 induced earthquake of 4 December 2020 and covers the early shut-in period of the wells. We use a beamforming technique to characterize the main noise sources, which consist of slow apparent velocities of surface waves emitted from mobile anthropogenic sources (motorway and railway traffic). We detect events with a phase-weighted stacking method, which is efficient when wavefronts illuminate the arrays with a high apparent velocity. Earthquakes associated with these detections are located using a matched field processing technique. The obtained catalog includes 216 seismic events, which represent four times more events than the reference catalog from Le Bureau Central Sismologique Français–Renass (the national academic agency in charge of seismicity monitoring in France), and a reduction of the completeness magnitude from 0.3 to −0.5. The clustering of the seismicity is analyzed using waveform correlation. The enriched catalog reveals intermittent seismic activity during the slow and continuous decrease in fluid pressure after shut-in.

Maintenance process for the joint seismic-and-electromagnetic monitoring in low-ambient noise areas: the seismic gradient system

The project of the joint seismic-and-electromagnetic monitoring in low-ambient noise areas is aimed at studying the phenomena associated with the deformation of the Earth’s crust. The arrangements set of the geophysical monitoring in low-ambient noise areas is proposed. The paper describes in detail the technological scheme being of monitoring of the modern geodynamic activity with a magnetotelluric soundings method and a small seismic array presented as the seismic gradient system (SGS). Technical explanations, safety rules, equipment calibration and maintenance, and the technological schemes of the method of obtaining and archiving data are given. Maintenance procedures using SGS with geophones in Tien Shan to determine the endogenous component of the seismic field are described. We investigate a common workflow chart for SGS and main programs for the data caption and data processing. The proposed scheme has been tested and requires the application and adoption to the passive geophysical monitoring routine.

Improvement of Frequency–Bessel Phase-Velocity Spectra of Multicomponent Cross-Correlation Functions from Seismic Ambient Noise

ABSTRACT The frequency–Bessel (F–J) transformation method is effective for the extraction of multimode surface wave dispersion data from ambient noise cross-correlation functions (CCFs). Recently, this method has been improved in terms of increasing resolution and reducing artifacts (or cycle skipping) in Rayleigh wave dispersion measurements. However, these advances are restricted to the ZZ-component F–J method, which is only applicable to Rayleigh waves. In contrast, they have not been extended to Love waves, which are fundamental for determining the horizontally polarized shear-wave velocity and the radial anisotropy associated with it. Furthermore, there is still a lack of a methodology for combining these advances. In this study, we propose a modified multicomponent F–J (MMFJ) method to extract high-quality dispersion data of both the Rayleigh and Love waves. To achieve high resolution, we derive formulas to optimize the MMFJ spectra. With synthetic ambient noise data and USArray data, we demonstrated the effectiveness of the MMFJ method in eliminating “crossed” artifacts and enhancing resolution. In particular, the “crossed” artifacts are greatly reduced using the MMFJ with small seismic arrays when the interstation distances in the seismic array are not dense enough. As such, the new MMFJ method has significant potential for handling seismic arrays with a limited number of receivers and the subsequent tomography of radial anisotropies at high precision.

Sensitivity of Seismic Velocity Changes to the Tidal Strain at Different Lapse Times: Data Analyses of a Small Seismic Array at Izu‐Oshima Volcano

AbstractWe investigate seismic velocity changes in response to the tidal strain at Izu‐Oshima volcano, Japan, by analyzing the data of permanent seismic stations and a small seismic array to evaluate the characteristics of strain sensitivity of velocity changes. We estimate the seismic velocity changes by phase differences between cross‐correlations functions of ambient noises at the frequency of 2–4 Hz stacked for time periods with different tidal strain amplitudes. The seismic velocity changes decrease and increase during dilatation and contraction periods, respectively, when analyzing the cross‐correlations functions at early lapse times ranging from 2 to 7 s. The strain sensitivity of seismic velocity changes is estimated to be at the early lapse times. However, we find that strain sensitivity of the seismic velocity changes decreases when analyzing the cross‐correlation functions at later lapse times from 7 s to 35 s. Applying an array analysis to the cross‐correlation functions, we observe apparent velocities of about 1 km/s at the early lapse times and those of higher than 1 km/s at the late lapse times. Since the group velocity of Rayleigh waves is 1.1 km/s at Izu‐Oshima volcano, the apparent velocities at the late lapse times may indicate the scattered or reflected body waves incident from a deeper region. Decrease of strain sensitivity with the lapse times therefore results from the emergence of body waves on the late lapse times. These results highlight the need to pay attention to wave types of cross‐correlation functions and their paths to interpret seismic velocity changes.

Seismic monitoring of soft-rock landslides: the Super-Sauze and Valoria case studies

This work focuses on the characterization of seismic sources observed in clay–shale landslides. Two landslides are considered: Super-Sauze (France) and Valoria (Italy). The two landslides are developed in reworked clay–shales but differ in terms of dimensions and displacement rates. Thousands of seismic signals have been identified by a small seismic array in spite of the high-seismic attenuation of the material. Several detection methods are tested. A semi-automatic detection method is validated by the comparison with a manual detection. Seismic signals are classified in three groups based on the frequency content, the apparent velocity and the differentiation of P and S waves. It is supposed that the first group of seismic signals is associated to shearing or fracture events within the landslide bodies, while the second group may correspond to rockfalls or debris flows. A last group corresponds to external earthquakes. Seismic sources are located with an automatic beam-forming location method. Sources are clustered in several parts of the landslide in agreement with geomorphological observations. We found that the rate of rockfall and fracture events increases after periods of heavy rainfall or snowmelt. The rate of microseismicity and rockfall activity is also positively correlated with landslide displacement rates. External earthquakes did not influence the microseismic activity or the landslide movement, probably because the earthquake ground motion was too weak to trigger landslide events during the observation periods.

Direct observation of rupture propagation during the 2011 off the Pacific coast of Tohoku Earthquake (M w 9.0) using a small seismic array

A great earthquake of Mw 9.0 occurred on March 11, 2011 off the coast of Tohoku region, Northeast Honshu, Japan. Strong ground motions from the earthquake were recorded at 4 stations of a small seismic array, with an aperture of about 500 m, located 120 km away from the epicenter. Peak ground acceleration exceed the full scale of 2g on the horizontal components, and was larger than 1g even on the vertical component. Two prominent bursts and at least two following smaller bursts are identified on the strong-motion records which lasted for longer than 200 s. We have performed semblance analysis to estimate the rupture propagation during the earthquake using coherent seismograms at frequencies of 0.5–2 Hz. The rupture seems to consist of at least four stages. Rupture propagated in a northerly direction in the beginning 50 s forming the first burst, then proceeded to the southwest from the epicenter in the next 50 s during the second burst. The rupture further extended southwests in the following 40 s, and finally migrated to the south for about 30 s. A small seismic array makes it possible to observe rupture propagation during a large earthquake even with a small number of stations.

Application of teleseismic tomography to the study of shallow structure beneath Shizigou in the western Qaidam basin

Teleseismic body wave traveltime tomography is used to inverse the three-dimensional seismic velocity structure beneath Shizigou in the western Qaidam basin. The travel time are picked from the continuous observation data on a small seismic array of stations deployed during 2004–2007. The tomographic results obtained indicate that a NW-trending low velocity anomaly just beneath the target region insert northeastwards with a high dip angle. In the north, northeast and east of the low velocity anomaly, some high-velocity anomalies distribute with the same strike and coverage as those of Shizigou anticline.

On bias and noise in passive seismic data from finite circular array data processed using SPAC methods

The finite nature of typical small seismic arrays used in conjunction with spatial autocorrelation (SPAC) processing for observing the microtremor wavefield causes predictable perturbations of the SPAC spectrum when sources of seismic noise are confined to a restricted range of azimuths. Such perturbations are especially evident at higher frequencies where wavelengths are on the order of the array radius. The effects are readily modeled and show that the triangular array geometries commonly used for microtremor studies require azimuthal distributions of wave energy on the order of [Formula: see text] or greater to have a high probability of being free of such perturbations. The imaginary component of the SPAC spectrum, which is ideally zero for a sufficiently dense circular array and/or a sufficiently isotropic wavefield, is in practice often nonzero and provides three quality-control indicators: (1) an indication of insufficient spatial averaging, (2) an empirical measure of the level of statistical uncertainty in SPAC spectral estimates, and (3) an indication of departures from plane-wave stationarity of the seismic noise wavefield.

Observations of short period seismic scattered waves by small seismic arrays

The most recent observations of well correlated seismic phases in the high frequency coda of local earthquakes recorded throughout the world are reported. In particular the main results, obtained on two active volcanoes, Teide and Deception, using small array are described. The ZLC (Zero Lag Cross-correlation) method and polarization analysis have been applied to the data in order to distinguish the main phases in the recorded seismograms and their azimuths and apparent velocities. The results obtained at the Teide volcano demonstrate that the uncorrelated part of the seismograms may be produced by multiple scattering from randomly distributed heterogeneity, while the well correlated part, showing SH type polarization or the possible presence of Rayleigh surface waves, may be generated by single scattering by strong scatterers. At the Deception Volcano strong scattering, strongly focused in a precise direction, is deduced from the data. In that case, all the coda radiation is composed of surface waves.

Coherent processing of regional signals at small seismic arrays

Abstract Regional arrivals, similarly to short-period teleseismic P waves, have a spectral structure that the Fourier transforms of individual sensors at arrays for events located in limited source regions can be decomposed into source and site spectral factors. This structure has been demonstrated to be valid for Pn and Lg arrivals at NORESS. This property of regional arrivals can be exploited for: (a) Grouping events with respect to relative location; (b) Identifying events with differing source mechanisms; and (c) Finding differences in the source time functions between closely spaced events. Potentially useful techniques for accomplishing such analyses consist of computing site-averaged interevent coherences, interevent and intersite equalization methods combined with correlation techniques. The advantage of such approaches is that we need not know much about the propagation characteristics (Green's functions) along the paths to an array.

A microearthquake survey in the Abu-Simbel area in Egypt

From the middle of January to the middle of March, 1981 a microearthquake survey was conducted jointly by Polish and Egyptian seismologists in the Abu-Simbel area, about 230 km upstream from the Aswan High Dam. A small seismic array composed of six stations and a magnetic-tape digital recording system was operated during the survey. Altogether 63 microearthquakes at epicentral distances of up to 50 km and 12 local earthquakes at distances of up to 200 km were recorded, but only 16 microearthquakes could be properly located. All of them originated either directly under the High Dam Reservoir or close to its shores, thus implying their association with its water level fluctuations. It was deduced that under the Abu-Simbel array a deep fracture zone or deep fault extending in the NNE—SSW direction, 1–3 km wide and several kilometers deep, must be present to explain the observed effects of heavy attenuation and scattering of seismic waves under the array. This fracture zone might be a deep expression of the western edge of the Nile Valley in the Abu-Simbel area.