Nori Nakata

Body wave extraction and tomography at Long Beach, California, with ambient‐noise interferometry

We retrieve P diving waves by applying seismic interferometry to ambient-noise records observed at Long Beach, California, and invert travel times of these waves to estimate 3-D P wave velocity structure. The ambient noise is recorded by a 2-D dense and large network, which has about 2500 receivers with 100 m spacing. Compared to surface wave extraction, body wave extraction is a much greater challenge because ambient noise is typically dominated by surface wave energy. For each individual receiver pair, the cross-correlation function obtained from ambient-noise data does not show clear body waves. Although we can reconstruct body waves when we stack correlation functions over all receiver pairs, we need to extract body waves at each receiver pair separately for imaging spatial heterogeneity of subsurface structure. Therefore, we employ two filters after correlation to seek body waves between individual receiver pairs. The first filter is a selection filter based on the similarity between each correlation function and the stacked function. After selecting traces containing stronger body waves, we retain about two million correlation functions (35% of all correlation functions) and successfully preserve most of body wave energy in the retained traces. The second filter is a noise suppression filter to enhance coherent energy (body waves here) and suppress incoherent noise in each trace. After applying these filters, we can reconstruct clear body waves from each virtual source. As an application of using extracted body waves, we estimate 3-D P wave velocities from these waves with travel time tomography. This study is the first body wave tomography result obtained from only ambient noise recorded at the ground surface. The velocity structure estimated from body waves has higher resolution than estimated from surface waves.

Monitoring a Building Using Deconvolution Interferometry. II: Ambient‐Vibration Analysis

Abstract Application of deconvolution interferometry to earthquake data recorded inside a building is a powerful technique for monitoring parameters of the building, such as velocities of traveling waves, frequencies of normal modes, and intrinsic attenuation. In this study, we apply interferometry to ambient‐vibration data, instead of using earthquake data, to monitor a building. The time continuity of ambient vibrations is useful for temporal monitoring. We show that, because multiple sources simultaneously excite vibrations inside the building, the deconvolved waveforms obtained from ambient vibrations are nonzero for both positive and negative times, unlike the purely causal waveforms obtained from earthquake data. We develop a string model to qualitatively interpret the deconvolved waveforms. Using the synthetic waveforms, we find the traveling waves obtained from ambient vibrations propagate with the correct velocity of the building, and the amplitude decay of the deconvolved waveforms depends on both intrinsic attenuation and ground coupling. The velocities estimated from ambient vibrations are more stable than those computed from earthquake data. Because the acceleration of the observed earthquake records varies depending on the strength of the earthquakes and the distance from the hypocenter, the velocities estimated from earthquake data vary because of the nonlinear response of the building. From ambient vibrations, we extract the wave velocity due to the linear response of the building.

Body and surface wave reconstruction from seismic noise correlations between arrays at Piton de la Fournaise volcano

Body wave reconstruction from ambient seismic noise correlations is an important step toward improving volcano imaging and monitoring. Here we extract body and surface waves that propagate in Piton de la Fournaise volcano on La Reunion island using ambient noise cross correlation and array-processing techniques. Ambient noise was continuously recorded at three dense arrays, each comprising 49 geophones. To identify and enhance the Greens function from the ambient noise correlation, we apply a double beamforming (DBF) technique between the array pairs. The DBF allows us to separate surface and body waves, direct and reflected waves, and multipathing waves. Based on their azimuths and slownesses, we successfully extract body waves between all the combinations of arrays, including the wave that propagates through the active magmatic system of the volcano. Additionally, we identify the effects of uneven noise source distribution and interpret the surface wave reflections.

The Pawnee earthquake as a result of the interplay among injection, faults and foreshocks

The Pawnee M5.8 earthquake is the largest event in Oklahoma instrument recorded history. It occurred near the edge of active seismic zones, similar to other M5+ earthquakes since 2011. It ruptured a previously unmapped fault and triggered aftershocks along a complex conjugate fault system. With a high-resolution earthquake catalog, we observe propagating foreshocks leading to the mainshock within 0.5 km distance, suggesting existence of precursory aseismic slip. At approximately 100 days before the mainshock, two M ≥ 3.5 earthquakes occurred along a mapped fault that is conjugate to the mainshock fault. At about 40 days before, two earthquakes clusters started, with one M3 earthquake occurred two days before the mainshock. The three M ≥ 3 foreshocks all produced positive Coulomb stress at the mainshock hypocenter. These foreshock activities within the conjugate fault system are near-instantaneously responding to variations in injection rates at 95% confidence. The short time delay between injection and seismicity differs from both the hypothetical expected time scale of diffusion process and the long time delay observed in this region prior to 2016, suggesting a possible role of elastic stress transfer and critical stress state of the fault. Our results suggest that the Pawnee earthquake is a result of interplay among injection, tectonic faults, and foreshocks.

Nonlinear attenuation from the interaction between different types of seismic waves and interaction of seismic waves with shallow ambient tectonic stress

Strong seismic waves bring rock into frictional failure at the uppermost few hundred meters. Numerous small fractures slip with the cumulative effect of anelastic strain and nonlinear attenuation; these fractures should not distinguish between remote sources of stress. Still, frictional failure criteria are not evident especially when seismic waves change the normal traction on fractures. We identify three earthquakes as examples where consideration of interaction among dynamic stresses from different wave types and ambient tectonic stress provides theoretical predictions of nonlinear attenuation that are potentially testable with single station seismograms. For example, because Rayleigh waves produce shallow horizontal dynamic tension and compression, frictional failure should preferentially occur on the tensile half-cycle if no shallow tectonic stress is present and on the compressional half-cycle if the tectonic stress is already near thrust-faulting failure. We observed neither effect on records from the 2011 Mw 9.0 Great Tohoku earthquake. However, Rayleigh waves from this event appear to have brought rock beneath MYGH05 station into frictional failure at ∼10 m depth and thus suppressed high-frequency S-waves. The tensile half-cycle of high frequency P-waves reduced normal traction on horizontal planes beneath station IWTH25 during the 2008 Mw 6.9 Iwate-Miyagi earthquake, weakening the rock in shear and suppressing high-frequency S-waves. The near-field velocity pulse from the 1992 Mw 7.3 Landers earthquake brought the uppermost few hundred meters of granite beneath Lucerne station into frictional failure, suppressing high frequency S-waves. These mildly positive examples support the reality of nonlinear wave interaction, warranting study future strong ground motions. This article is protected by copyright. All rights reserved.

Shallow VS Imaging of the Groningen Area from Joint Inversion of Multimode Surface Waves and H/V Spectral Ratios

The Groningen gas field in the northern Netherlands is subject to production-induced earthquakes and has quickly become one of the seismologically best-instrumented areas on Earth. Accurate quantification of seismic hazard from potential future earthquakes requires accurate shallow velocity structure for ground-motion prediction. Toward this end, we present a shear-wave velocity model developed through the joint inversion of multimode Loveand Rayleigh-wave dispersion curves (DCs) and H/V spectral ratio (HVSR) measurements. We obtain local DCs from azimuthally averaged frequency–time analysis of the cross correlation of the ambient seismic field (ASF) between pairs of stations. HVSR is measured at each station from the directional energy density, that is, the autocorrelation of the ASF for all components. We simultaneously fit these observables at each station of the dense Loppersum array to infer a 1D velocity model from the surface to a depth of ∼900 m. In the frequency range considered (∼1–7 Hz), Rayleigh-wave DCs show high modal complexity, which makes clear identification of the modes challenging and leads us to downweight their contribution to the result. Fundamentaland higher-mode Love-wave dispersion is much clearer. We find good agreement between our model and independently derived models of shallow structure, which validates our approach and supports the value of HVSR analysis as a tool to map subsurface properties. Electronic Supplement: Frequency–time diagrams, theoretical kx , omega diagrams, example joint inversion for site 235587, and example of horizontal-to-vertical (H/V) spectral ratio (HVSR) at station site 235587.

The Ambient Seismic Field at Groningen Gas Field: An Overview from the Surface to Reservoir Depth

The long-term exploitation of the Groningen gas field led to compaction at reservoir depth, subsequent ground subsidence, and recently earthquakes. As part of an ongoing effort to quantify the hazard and risk in the region, several permanent and temporary seismic arrays have been deployed. As a result, the Groningen area is one of the seismologically best-instrumented areas worldwide. In this article, we describe several seismic experiments that were conducted in the region and take advantage of the numerous possibilities they offer to characterize the ambient seismic wavefield at the surface, in the shallow subsurface, and at reservoir depth. By means of beamforming, analysis of cross-correlation functions, surface-wave eigenfunction analysis, and correlations of neighboring frequencies, we are able to determine the main characteristics of the ambient seismic field (ASF), including the predominant propagation modes and phases. We retrieve clear multimode Rayleigh and Love waves, as well as and P waves, from cross correlations of the ASF. At reservoir depth, we show that the wavefield is largely trapped and reflected between geologic boundaries above and below the reservoir. This article reviews the characteristics of ASF observations with the goal of guiding future investigations of shallow structure of the Groningen area. Electronic Supplement: Figure showing cross-correlation envelope functions between the two deep borehole arrays.

Nonlinear Suppression of High‐Frequency S Waves by Strong Rayleigh Waves

Strong Rayleigh waves are expected to bring the shallow subsurface into frictional failure. They may nonlinearly interact with high‐frequency S waves. The widely applied Drucker and Prager (1952) rheology predicts that horizontal compression half‐cycle of strong Rayleigh waves will increase the strength of the subsurface for S waves and predicts that S waves with dynamic accelerations >1 g will reach the surface. We did not observe this effect. Rather, we observed that strong high‐frequency S waves arrived at times of low Rayleigh‐wave particle velocity. Physically, high‐frequency S waves cause failure on horizontal fractures in which Rayleigh waves do not change the normal traction. Failure then may depend on the ratio of the shear invariant to the ambient vertical stress. The shear invariant is the square root of the sum of the squares of terms proportional to the resolved horizontal velocity from Rayleigh waves and to the resolved high‐frequency dynamic acceleration from S waves. That is, an ellipse should bound resolved dynamic acceleration versus resolved particle velocity. Records from seven stations from the 2011 Tohoku earthquake and El Pedregal station during the 2015 Coquimbo Chilean earthquake exhibit this expected effect of this nonlinear interaction.

Combination of Hi‐net and KiK‐net Data for Deconvolution Interferometry

Abstract Application of deconvolution interferometry to wavefields observed by KiK‐net, a strong‐motion recording network in Japan, is useful for estimating wave velocities and S ‐wave splitting in the near surface. At the location of the borehole accelerometer of each KiK‐net station, a velocity sensor is also installed as a part of a high‐sensitivity seismograph network (Hi‐net). I present a technique that uses both Hi‐net and KiK‐net records for computing deconvolution interferometry. The deconvolved waveform obtained from the combination of Hi‐net and KiK‐net data is similar to the waveform computed from KiK‐net data only. This similarity in the waveforms indicates that one can use Hi‐net wavefields for deconvolution interferometry. Because Hi‐net records have a high signal‐to‐noise ratio (SNR) and high dynamic resolution, SNR and quality of amplitude and phase of deconvolved waveforms can be improved with Hi‐net data. These advantages are especially important for short‐time moving‐window seismic interferometry and deconvolution interferometry using coda waves.

Nonlinear Attenuation of S Waves by Frictional Failure at Shallow Depths

Abstract Strong S waves produce dynamic stresses, which bring the shallow subsurface into nonlinear anelastic failure. The construct of coulomb friction yields testable predictions about this process for strong‐motion records. Physically, the anelastic strain rate increases rapidly with increasing dynamic stress, and dynamic stress is proportional to the difference between total strain and anelastic strain. Nonlinear models of vertically propagating S waves in layered media confirmed and illustrated analytical inferences. The effective coefficient of friction bounds (clips amplitude) the resolved horizontal acceleration normalized to the acceleration of gravity. There is a tendency for the random signal from vertically propagating S waves to become transiently circularly polarized at the maximum (clipped) resolved acceleration, as the acceleration component perpendicular to the current acceleration adds weakly the resolved acceleration. Frictional attenuation does not preferentially suppress high‐frequency signal; it cannot be modeled by increasing ordinary linear attenuation. In addition, an effect of shallow cohesion is to allow brief pulses of strong high‐frequency acceleration to reach the surface. Frictional attenuation within deep overpressured aquifers suppresses shaking recorded at the surface, but does not simply clip amplitude at a given resolved acceleration. The anelastic strain rate increases slowly with stress within shallow muddy sediments. The accelerations from reverberations within such layers can exceed 1 g .