Coda Waves Research Articles

Laterally varying scattering properties in the North Anatolian Fault Zone from ambient noise cross-correlations

SUMMARY Intrinsic absorption and scattering properties provide us with information about the physical state and heterogeneity of the Earth’s crust. These properties are usually obtained by observing the energy decay of naturally occurring earthquakes, leading to sparse spatial sampling and therefore average scattering values over a large region. This study uses ambient noise cross-correlations to analyse the energy decay and scattering properties over a part of the North Anatolian Fault (NAF; Turkey) from the continuous records of the 73 stations of the DANA temporary array in the frequency band 0.1–0.5 Hz. The region covered by the stations has rapidly varying geological characteristics and is highly faulted around the northern strand of the NAF. We measured in the noise correlations the space–time evolution of the energy of the coda waves. We first perform measurements in separate subregions. The local scattering and attenuation properties are obtained by global optimization of a 2-D solution of the radiative transfer equation for surface waves. We found that the mean free path and attenuation coefficient are considerably varying laterally with strong scattering observed in the region lying along the northern strand of NAF. The optimization provides well-constrained values for the scattering mean free path on the order of 10 km in the fault region. The mean free path is much larger (>100 km) in the neighbouring regions. We compare our global observations with a phonon based Monte Carlo simulation of scattered energy in a laterally variable scattering model. These simulations confirm the large contrast of heterogeneity between NAF and the surrounding crust and provide further constraints on the lateral extent of NAF. When sources are located inside the fault zone, we find a signature of the actual non-uniform scattering properties, observed as a concentration of energy in the fault zone for a limited amount of time. This in turn suggests that lateral variations of scattering properties should be taken into account in future monitoring studies.

On The Efficiency of P‐Wave Coda Autocorrelation in Recovering Crustal Structure: Examples From Dense Arrays in the Eastern United States

AbstractDue to a sharp contrast in elastic properties across the basement rocks of sedimentary basins (SBs), strong reverberations are generated during the passage of seismic waves. Traditional receiver function methods become inadequate for imaging crustal structure due to the existence of these strong reverberations. We investigate the feasibility of an autocorrelation technique to extract vertical component receiver functions from teleseismic earthquake data and the efficiency of the method to image the crustal architecture in presence of a SB. The method involves spectral whitening followed by autocorrelation and stacking in the depth domain. We show promising results when using temporary seismic networks in the eastern United States. Using synthetic and field‐data examples, we demonstrate that vertical autocorrelations are more efficient than classical radial receiver functions for interpretation purposes in an SB context. We also perform a joint analysis of the amplitudes on radial and vertical receiver functions for characterizing the thickness of the Mohorovičić discontinuity (Moho). We find that the Moho in the eastern United States is a transitional layer (up to 5‐km thick) instead of a sharp boundary. Further, we point out that it is challenging to unambiguously pick and interpret reflected phases on autocorrelations because of the effects of reverberations, cross‐mode contaminations, and a narrow frequency band limiting the resolution of velocity gradients. We therefore send a message of caution for future interpretations based on this technique.

Effect of Random 3D Correlated Velocity Perturbations on Numerical Modeling of Ground Motion from the Source Physics Experiment

ABSTRACTExplosions are traditionally discriminated from earthquakes, using the relative amplitude of compressional and shear waves at regional and teleseismic distances known as the P/S discriminant. Pyle and Walter (2019) showed this technique to be less robust at shorter distances, in detecting small-magnitude earthquakes and low-yield explosions. The disparity is largely due to ground motion from small, shallow sources being significantly impacted by near-surface structural complexities. To understand the implications of wave propagation effects in generation of shear motion and P/S ratio during underground chemical explosions, we performed simulations of the Source Physics Experiment (SPE) chemical explosions using 1D and 3D velocity models of the Yucca Flat basin. All simulations used isotropic point sources in the frequency range 0–5 Hz. We isolate the effect of large-scale geological structure and small-scale variability at shallow depth (<5 km), using a regional 3D geologic framework model (GFM) and the GFM-R model derived from the GFM, by adding correlated stochastic velocity perturbations. A parametric study of effects of small-scale velocity variations on wave propagation, computed using a reference 1D velocity model with stochastic perturbations, shows that the correlation length and depth of stochastic perturbations significantly impact wave scattering, near-surface wave conversions, and shear-wave generation. Comparisons of recorded and simulated waveforms for the SPE-5 explosion, using 3D velocity models, demonstrate that the shallow structure of the Yucca Flat basin contributes to generation of observed shear motion. The inclusion of 3D wave scattering, simulated by small-scale velocity perturbations in the 3D model, improves the fit between the simulated and recorded waveforms. In addition, a relatively low intrinsic attenuation, combined with small-scale velocity variations in our models, can confirm the observed wave trapping and its effect on duration of coda waves and the spatial variation of P/S ratio at basin sites.

Teleseismic P-Wave Coda Autocorrelation Imaging of Crustal and Basin Structure, Bighorn Mountains Region, Wyoming, U.S.A.

ABSTRACT We demonstrate successful crustal imaging via teleseismic P-wave coda autocorrelation, using data recorded on a 261 station array of vertical-component high-frequency geophones in the area of the Bighorn Mountains, Wyoming, U.S.A. We autocorrelate the P-wave coda of 30 teleseismic events and use phase-weighted stacking to yield seismic profiles comparable to low-passed versions of those produced via controlled-source vertical seismic reflection. Our process recovers reflections from the bottoms of the Bighorn and Powder River basins that flank the Bighorn Mountains. We also identify a mid-crustal reflector that aligns with a region of increased reflectivity, previously interpreted as a Precambrian province boundary. Our results demonstrate the utility of crustal imaging with teleseismic P-wave coda energy using modern large-array seismic data, and they corroborate previous interpretations of crustal structures in the study area.

Acoustic wave propagation through solar granulation: Validity of effective-medium theories, coda waves

Context. The frequencies, lifetimes, and eigenfunctions of solar acoustic waves are affected by turbulent convection, which is random in space and in time. Since the correlation time of solar granulation and the periods of acoustic waves (∼5 min) are similar, the medium in which the waves propagate cannot a priori be assumed to be time independent. Aims. We compare various effective-medium solutions with numerical solutions in order to identify the approximations that can be used in helioseismology. For the sake of simplicity, the medium is one dimensional. Methods. We consider the Keller approximation, the second-order Born approximation, and spatial homogenization to obtain theoretical values for the effective wave speed and attenuation (averaged over the realizations of the medium). Numerically, we computed the first and second statistical moments of the wave field over many thousands of realizations of the medium (finite-amplitude sound-speed perturbations are limited to a 30 Mm band and have a zero mean). Results. The effective wave speed is reduced for both the theories and the simulations. The attenuation of the coherent wave field and the wave speed are best described by the Keller theory. The numerical simulations reveal the presence of coda waves, trailing the ballistic wave packet. These late arrival waves are due to multiple scattering and are easily seen in the second moment of the wave field. Conclusions. We find that the effective wave speed can be calculated, numerically and theoretically, using a single snapshot of the random medium (frozen medium); however, the attenuation is underestimated in the frozen medium compared to the time-dependent medium. Multiple scattering cannot be ignored when modeling acoustic wave propagation through solar granulation.

Revisiting Lunar Seismic Experiment Data Using the Multichannel Simulation with One Receiver (MSOR) Approach and Random Field Modeling

Major advancements in surface wave testing over the past 2 decades have led researchers to revisit and re-analyze archived seismic records, particularly those involving measurements on the Moon. The goal of such recent efforts with lunar seismic measurements has been to gain further insights into lunar geology. We examined the active seismic data from the Apollo 16 mission for their surface wave information using a multichannel approach. The inversion of Rayleigh surface waves provided a subsurface estimate for the uppermost 8 m of the lunar subsurface with the shear wave velocities varying from 40 to 50 m/s at the surface to velocities in the range of 95–145 m/s with an average of 120 m/s at a depth of about 7 m. Generally, the results from this inversion demonstrated good agreement with previous studies. Also, we carried out numerical modeling of wave propagation in a highly-heterogeneous domain to examine the effects of such anomalous features on the acquired seismograms. Results confirmed that a sharp-contrast bi-material domain can indeed produce significant coda wave as reflected on the lunar seismic traces.

Shallow Basin Structure and Attenuation Are Key to Predicting Long Shaking Duration in Los Angeles Basin

AbstractGround motions in the Los Angeles Basin during large earthquakes are modulated by earthquake ruptures, path effects into the basin, basin effects, and local site response. We analyzed the direct effect of shallow basin structures on shaking duration at a period of 2–10 s in the Los Angeles region through modeling small magnitude, shallow, and deep earthquake pairs. The source depth modulates the basin response, particularly the shaking duration, and these features are a function of path effect and not site condition. Three‐dimensional simulations using the CVM‐S4.26.M01 velocity model show good fitting to the initial portion of the waveforms at periods of 5 s and longer but fail to predict the long shaking duration during shallow events, especially at periods less than 5 s. Simulations using CVM‐H do not match the timing of the initial arrivals as well as CVM‐S4.26.M01, and the strong late arrivals in the CVM‐H simulation travel with an apparent velocity slower than observed. A higher‐quality factor than traditionally assumed may produce synthetics with longer durations but is unable to accurately match the amplitude and phase. Beamforming analysis using dense array data further reveals the long duration surface waves have the same back azimuth as the direct arrivals and are generated at the basin edges, while the later coda waves are scattered from off‐azimuth directions, potentially due to strong, sharp boundaries offshore. Improving the description of these shallow basin structures and attenuation model will enhance our capability to predict long‐period ground motions in basins.

Method of S Wave Identification: Application to Measured blasting vibration signals

S wave identification is an indispensable step in the process of deriving the dynamic parameters of rock masses, which are highly significant in guiding the design and construction of hydraulic engineering projects. However, it is difficult to identify S waves accurately because of 1) the influence of overlapping within the P wave coda and 2) the incomplete separation between S and P waves owing to the characteristic short distances of blasting seismic wave propagation. This paper presents a modified method for accurately identifying S waves at the engineering scale using blasting vibration signals. The proposed method combines the application of the short-time average zero-over rate with polarization analysis. By using the short time average zero-over rate, it is possible to effectively reduce the computational time while improving the computational efficiency. Polarization analysis is used to improve the accuracy of the method. A comparison of numerical identification and theoretical results revealed that the improved method is clearly capable of S wave identification with errors of less than 2%. To demonstrate the capability and accuracy of the proposed method, measured vibration signals were introduced to the Fengning pumped-storage power station and the S wave velocities obtained from the first arrival times were compared to obtain an estimated S wave velocity.

Hydraulic Injection‐Induced Velocity Changes Revealed by Surface Wave Coda and Polarization Data at a Shale Play Site in Southwest China

AbstractWe investigated temporal variations of seismic wave velocity associated with hydraulic fracturing using Green's functions computed from ambient noise data. In October and November of 2014, we set up a broadband array at a shale play site inside the Sichuan basin where a pilot horizontal drilling and hydraulic injections were conducted. We first computed cross‐correlation functions using continuous data recorded by 21 three‐component broadband sensors deployed around the treatment well. We then employed a running window correlation‐based coda wave interferometry technique to measure apparent velocity changes from the daily Green's functions of all the station pairs in the frequency range of 1 to 3 Hz. We found significant velocity changes right after the hydraulic fracturing, which exhibited a clear direction‐dependent pattern. S wave velocity along raypaths parallel to the well trajectory showed a clear increase while those perpendicular exhibited a small decrease. The anisotropic changes in seismic velocity observed here were also confirmed from surface wave horizontal particle motion data. By comparing our observations with normal stress changes calculated with a half‐space elastic model, we speculate that stress changes induced by the hydraulic fracturing were likely to be responsible for the observed anisotropic changes in seismic velocity. Our results suggest that time‐lapse seismic imaging with ambient noise data provides a promising probe for monitoring geomechanical changes related to exploitation of unconventional oil and gas resources.

Seismic Monitoring of Super High-Rise Building Using Ambient Noise with Dense Seismic Array

AbstractWe report the first thorough seismological study of the Shanghai Tower, the second tallest building in the world, by applying seismic interferometry to ambient noise recorded by a dense seismic array. We extract the building responses by deconvolution of the seismic vibrations for three components of the impulse response functions (IRFs), and retrieve two wave types corresponding to vertical and horizontal wave components that travel along the core tube of the tower. Five new resonant modes are identified here for the first time between 1 and 5 Hz. Displacements for the resonant modes around 1 Hz are strongly attenuated by the passive damper system installed on the top floor of the building. Refuge floors connected to the outrigger trusses system have unique structural properties, characterized by a specific resonant mode, which apparently shifts from 1.8 Hz at the base of the building, to ∼3 Hz at the top. The most interesting observation is the diurnal variation in both the seismic velocities and the mechanical response of the building, which correlates with temperature, and was detected by repeated measurement of coda waves for the IRFs and the horizontal-to-vertical spectral ratio. Seismic velocity appears to increase for the structure at midnight due to cooling that drives an increase in rigidity, which reaches a local maximum in the early morning and decreases after sunrise. In summary, our study demonstrates the effective application of seismic ambient-noise interferometry for structural health monitoring and for hazard assessment for super high-rise buildings worldwide.

Determination of high-frequency attenuation characteristic of coda waves in the central region of Nepal Himalaya

The high-frequency coda wave attenuation in the central region of Nepal, in and around the Kathmandu valley, is estimated using vertical component seismograms of local earthquakes recorded at 16 different seismic stations of NAMASTE array. The estimated result is expressed in terms of Qc, quality factor (inverse of coda wave attenuation). The value of coda quality factor (Qc) is estimated at eight central frequencies of 1.5, 3.0, 6.0, 9.0, 12.0, 15.0, 18.0, and 21.0 Hz through four different coda window length from 20 to 50 s at 10 s interval by using the single backscattering model. The value of coda Qc obtained from this study, shows a clear dependence on a frequency according to the power relation, Qc (f)= Q0 f n, where Q0 is Qc at 1 Hz, and f is frequency and n represents the degree of frequency dependence. The mean value of Qc of 16 different seismic stations was obtained as (110 ± 10.6) f 1.03±0.03 at 30 s coda window length, which represents the high attenuation characteristics of the study area, and attenuation decreases with increasing central frequency. Qo increases from 73.1 ± 10.1 to 156.1 ± 13.6 and n decreases from 1.12 ± 1.05 to 0.92±0.03 when the coda window length increases from 20 to 50 s. It is concluded that the study area is tectonically very active, highly heterogeneous, and heterogeneity decrease with depth. The coda Q obtained in this study is compatible with the result obtained in the region having a similar tectonic setting.

Role of site effect for the evaluation of attenuation characteristics of P, S and coda waves in Kinnaur region, NW Himalaya

The site effect and attenuation studies are carried out for Kinnaur region of northwest Himalaya, India. A total of 109 local events happened in Kinnaur region of magnitude range 1.6–4.5, are utilized for present work. The earthquake records are influenced by the site effect depending on soft sediment thickness beneath the recording sites. Therefore, in the present study, records are corrected for site effects to estimate P (Qp), S (Qs) and coda (Qc) wave quality factor. The regional frequency dependent attenuation relations, i.e., $$ Q_{p} \left( f \right) = \left( {29 \pm 1} \right)f^{(1.01 \pm 0.05)} $$ , $$ Q_{s} \left( f \right) = \left( {38 \pm 5} \right)f^{(1.1 \pm 0.06)} $$ and $$ Q_{c} \left( f \right) = \left( {74 \pm 11} \right)f^{(1.17 \pm 0.01)} $$ are established for the Kinnaur region. The Kinnaur Himalaya mainly belongs to Higher Himalaya Crystalline (HHC) and Tethys Himalaya, where these two geological units are differentiated by the South Tibetan Detachment System (STDS). The resonance frequencies and attenuation characteristics are estimated for both regions, i.e., HHC and Tethys Himalaya. A comparison is made between HHC and Tethys Himalaya in the form of resonance frequencies and attenuation properties. The low value resonance frequency and high rate of attenuation towards the northern side of STDS, i.e., Tethys Himalaya support the presence of low-grade metasedimentary rocks. It suggests that Tethys Himalaya has high seismic hazard potential zone compared to HHC.

Lithological Interface Detection Using an Impact Source

Early detection of abnormal geological targets during drilling can enhance the safety of petroleum drilling. An impact source equipped with the advantage of long detection distances can recognize lithological interfaces. However, coda waves in the vibration waveform are significant to reflected waves, which are difficult to recognize in the time domain. This paper presents the design of an impact source that includes a hammer, impacted metal, Teflon, and a metal base. With the length and diameter of Teflon kept constant, the effect of the hammer, impacted metal, and metal base on the coda waves was experimentally investigated. According to the preferred metal materials, the effect of the length and diameter of Teflon on coda waves was also experimentally studied. A distance measurement experiment was implemented on 1.2 m sandstone on the basis of the preferred impact source design. The experimental results show that the coda waves are significantly attenuated by the preferred impact source. Moreover, the reflected waves are clearly identified in the time domain. Therefore, the preferred impact source can be used effectively in lithological interface detection.

Toward Source Region Tomography With Intersource Interferometry: Shear Wave Velocity From 2018 West Bohemia Swarm Earthquakes

AbstractThe concept of seismic interferometry embraces the construction of waves traveling between receivers or sources with cross‐correlation techniques. In the present study cross correlations of coda waves are used to measure traveltimes of shear waves between earthquake locations for five event clusters of the 2018 West Bohemia earthquake swarm. With the help of a high‐quality earthquake catalog, I was able to determine the shear wave velocity in the region of the five clusters separately. The shear wave velocities range between 3.5 and 4.2 km/s. The resolution of this novel method is given by the extent of the clusters and better than for a comparable classical tomography. It is suggested to use the method in a tomographic inversion and map the shear wave velocity in the source region with unprecedented resolution. Furthermore, the influence of focal mechanisms and the attenuation properties on the polarity and location of the maxima in the cross‐correlation functions is discussed. The intracluster ratio of P wave to S wave velocity is approximately fixed at 1.68.

Discrimination of the natural and artificial quakes in the Eastern Marmara Region, Turkey

The aim of this study is to determine the source of seismic quakes is whether natural or artificial in the Eastern Marmara Region which is also included two branches of western part of the North Anatolian Fault Zone. It is known as one of the most seismically hazardous region in Turkey and 1999 Izmit and Duzce Earthquakes were occurred inside this area. In addition, it has geothermal springs, lots of mining areas, tunnel, bridge and road construction points. Under these conditions, it is seismologically important to make a study about the discrimination of the natural and artificial events. 611 seismic events with Ml ≤ 2.5 which has occurred between 2017 and 2019, recorded by 11 seismic stations that are being operated by Republic of Turkey Prime Ministry Disaster and Emergency Management Authority Presidential of Earthquake Department were analyzed. Amplitude ratio, complexity and continuous wavelet transform (CWT) methods were performed; linear and quadratic discriminant functions (LDF, QDF) were used for statistical processes to all waveforms and their results. All waveforms were identified by significant manual investigation that P wave first motion polarity, no or low amplitude S wave recording, the vision of coda wave decay and Rg phase detection and it called as a first determination. The results were compared to first determination and the final decisions were gained with the combination of their all. Besides with the all methods, CWT analyze could give more reliable results than the others and amplitude ratio method can serve better results than complexity. Otherwise, LDF could classify the events better than QDF. Through the final decision, 395 earthquake and 182 blast recordings were clearly discriminated with the accuracy of 94.44% and the source types of 34 seismic events could not be identified. The obtained discriminant functions for each station are useful to determine the source types of the events and they could clarify the contaminated earthquake catalogues. Increasing the reliability of the final results would be related with using these methods together.

Seismic Coda-Waves Imaging Based on Sensitivity Kernels Calculated Using an Heuristic Approach

In this paper we review and discuss the seismic method based on the analysis of seismic coda waves used in the last 10 years by the present authors and/or their co-workers, to produce separate images of intrinsic- and scattering attenuation in zones of peculiar geological interest (mainly volcanoes). Such separate attenuation images are considered by the scientific community as complementary to those from ordinary velocity-tomography and useful to improve the geological interpretation in volcanoes and in tectonically active zones. In this review we only list but do not discuss the most significative papers showing the images obtained, as we are focused to review the method and not the interpretation of data analysis. For sake of completeness, we anyway show also a new analysis applied to data from Stromboli volcano. We thus first introduce the physical model describing the seismogram Energy Envelope (derived from the solution of the Energy Transport integral Equation) and discuss its asymptotic approximations (Diffusion- and Single-scattering model). Then, we describe a numerical method to heuristically calculate the Sensitivity Kernels for the propagation of the scattered waves in the assumption of isotropic scattering. We attribute to these Sensitivity Kernels the physical meaning of probability that for a single source-receiver couple the measured attenuation parameters can be associated with the space coordinates. Based on this definition, the attenuation image can be obtained mapping the estimated attenuation parameters onto the zone under study weighting with the Sensitivity Kernels. We further discuss how to estimate the uncertainties associated with the results and report the list of the papers describing the (separated) scattering- and intrinsic-attenuation structures investigated using this approach.

Coda wave attenuation's dependency on Lapse time and frequency in west of Iran plateau using local earthquakes

In this study, the attenuation of coda waves has been estimated in the western part of Iran plateau using single backscattering method. Coda wave attenuation was estimated at seven central frequencies (fc = 1.5, 3, 4.5, 6, 9, 12, 18 Hz) and eight lapse times (tl = 10, 20, 30, 40, 50, 60, 70, 80s). More than 9100 local earthquakes with magnitude range 2-5.5 and epicentral distance less than 200 km have been used in this study. The frequency dependency relation of attenuation has been determined by using Q0f n where the Q0 and n are varying from 51 to 225 and from 0.71 to 1.12 for lapse times from 10 to 80 s. The obtained low Q0 and high n values indicate that the study region is characterized by high seismic attenuation and, therefore, it is comparable with other seismically active areas.In order to investigate the attenuation at different depths, two datasets with epicentral distance less than 100 km and between 100 and 200 km were separately used. In the next step by using ellipsoidal sampled region, the variation of coda wave attenuation with depth are explored form the first dataset (epicentral distance less than 100 km). A slight decrease in the value of estimated attenuation was observed at depth of ~74 km in Western Alborz (WA) which may be related to high dissipating medium. Also an attenuation anomaly was found at Zagros and Sanandaj-Siran Zone (SSZ) at depth of ~136 and ~145 km, respectively, which may be related to the low-velocity zone caused by partial melting of the remnant of the fossil Neo-Tethyan. Using the second dataset (epicentral distance between 100 and 200 km), a clear decrease in the value of Q0 observed at depth of 178 km in SSZ, which may coincide with the bottom of Arabian lithosphere.

Coda-wave decorrelation sensitivity kernels in 2-D elastic media: a numerical approach

SUMMARYProbabilistic sensitivity kernels based on the analytical solution of the diffusion and radiative transfer equations have been used to locate tiny changes detected in late arriving coda waves. These analytical kernels accurately describe the sensitivity of coda waves towards velocity changes located at a large distance from the sensors in the acoustic diffusive regime. They are also valid to describe the acoustic waveform distortions (decorrelations) induced by isotropically scattering perturbations. However, in elastic media, there is no analytical solution that describes the complex propagation of wave energy, including mode conversions, polarizations, etc. Here, we derive sensitivity kernels using numerical simulations of wave propagation in heterogeneous media in the acoustic and elastic regimes. We decompose the wavefield into P- and S-wave components at the perturbation location in order to construct separate P to P, S to S, P to S and S to P scattering sensitivity kernels. This allows us to describe the influence of P- and S-wave scattering perturbations separately. We test our approach using acoustic and elastic numerical simulations where localized scattering perturbations are introduced. We validate the numerical sensitivity kernels by comparing them with analytical kernel predictions and with measurements of coda decorrelations on the synthetic data.

Detecting Offshore Seismicity: Combining Backprojection Imaging and Matched‐Filter Detection

AbstractAn important manifestation of the background or coseismic deformation surrounding megathrust earthquakes is the offshore microseismicity, which are difficult to be directly detected by land‐based seismic instruments. Here, we improve the capability of detecting offshore events by combining two popular techniques: backprojection (BP) imaging and matched‐filter (MF) detection. The BP method is effective in retrieving offshore seismicity (Mw > 4.5) buried in the coda wave of large earthquakes. The capability of the MF method depends on the availability of the template pool; therefore, the BP‐inferred events can be used as additional templates to expand the MF detections (BP‐MF). We performed the BP‐MF approach in the period within 600 days after the 2011 Mw 9.0 Tohoku earthquake. We find overall 44.2% more offshore events than those listed in the Japan Meteorological Agency (JMA) catalog. In the near‐trench area, we detect 213% more events. Among the newly detected Mw > 4 events, the BP template contributions are twice more than those matched by the JMA templates. Based on the spatial consistency between aftershock‐depleted zones and large coseismic slip, we identify a possible large coseismic slip zone in the near‐trench region offshore Fukushima. Large b values (≥1.2) are found close to large aftershocks, possibly indicating localized pockets of small differential stresses. At several locations close to the trench, p values (0.93–1.11) are higher than those in the inland area (0.64–0.85). This may be due to the larger coseismic slip and hence larger stress drop of the outer‐rise normal‐faulting events compared to the deeper thrust‐faulting events.

Comparison of Experimentally Determined Two-Dimensional Strain Fields and Mapped Ultrasonic Data Processed by Coda Wave Interferometry.

Due to the high sensitivity of coda waves to the smallest structural alterations such as strain, humidity or temperature changes, ultrasonic waves are a valid means to examine entire structures employing networks of ultrasonic transducers. In order to substantiate this ex ante assessment, the viability of measuring ultrasonic waves as a valid point of reference and inference for structural changes is to be further scrutinized in this work. In order to investigate the influence of mechanical strain on ultrasonic signals, a four-point bending test was carried out on a reinforced concrete beam at Ruhr University Bochum. Thus, measurements collected from a network of selected transducer pairings arranged across the central, shear-free segment of the test specimen, were correlated to their respective strain fields. Detected ultrasonic signals were evaluated employing Coda Wave Interferometry. Such analysis comprised the initial non-cracked state as well as later stages with incremental crack depth and quantity. It was to ascertain that the test specimen can in fact be qualitatively compartmentalized into areas of compression and tension identified via Relative Velocity Changes presented in Attribute Maps. However, since results did not entail a zero crossing, i.e., neither positive nor negative values were to be calculated, only relative changes in this work displayed staggered over the height of the object under test, are discussed. Under the given methodological premises, additional information is currently required to make quantitative assertions regarding this correlation of ultrasonic and strain results. This holds true for the comparability of the ultrasonic and strain results for both non-cracked and even the cracked state.