Borehole Station Research Articles

Regional study of site effects on the high-frequency spectral-decay parameter

The high-frequency spectral-decay parameter κ has been used in the field of engineering seismology for several decades. The zero-distance κ value (κ0) is a site parameter in ground motion models and strong motion synthesis. Using surface-borehole pairs of strong motion records from the strong-motion seismograph network KiK-net, we explored the effect of site soil on κ0, specifically the contribution from ground motion directionality. In the target area (138°E−143°E, 36°N–40°N), and records from earthquakes at 50 pairs of stations were collected. All horizontal orthogonal records were rotated from 0° to 180° with an increment of 10° to capture the difference between the average κ0 on two horizontal components and the median value of the rotated κ0 at surface and borehole stations. The same process was applied to Δκ0, the difference between surface station κ0 and borehole station κ0, to measure the contribution by soil column. The nonlinear behavior of soils led to large shear strains, reduced stiffness, and decreased resonance frequency. Thus, the effect of soil nonlinearity on κ is investigated. In the same area, strong ground motion records with peak ground acceleration >100 cm/s2 were selected to identify the soil nonlinearity by the moving time window deconvolution method, and the temporal variation of time-average shear wave velocity and κ at the 50 surface stations were assessed.

A New Posthole Seismometer at Concordia Permanent Research Facility in the Heart of the Icy East Antarctic Plateau

Abstract In the Southern Hemisphere, the prevalence of oceans and the difficulty of access to land result in reduced coverage of seismological stations, limiting our detailed knowledge of Earth’s structures and of large earthquakes sources. This situation is exacerbated inside the antarctic continent, where only two permanent seismic stations are currently available (IU.QSPA at South Pole and G.CCD). The CCD station, built in early 2000s with state-of-the-art surface instrumentation and located at the French–Italian Concordia base (75° S, 123° E), has been providing seismological data since 2008. However, it suffers from several problems: the vault is deformed by the hydrostatic pressure of the snow, the firn waveguide traps anthropogenic noise from the base causing strong noise below 1 s, and a coupling defect limits the performance above 30 s on the horizontal channels. To ensure the continuity of CCD and to improve its overall performance, we started in 2014 to plan the installation of a borehole seismometer at the site. In this article, we describe in detail this renovation of CCD and some examples of data analysis. The new borehole sensor shows that short-period disturbances are largely attenuated (−20 dB at 0.1 s) compared to the surface installation and that the horizontal channels have a lower noise level at long periods (−8 dB at 100 s). Data for all components are below the standard noise model between 0.1 and 0.2 s, which makes this sensor one of the quietest installations in the world for this bandwidth. For periods >600 s we observe atmospheric pressure-related perturbations on the vertical component. Despite this problem, the new CCD borehole station is a success with better-than-expected performances at all periods <600 s. The data produced are now distributed in the world’s data centers as G.CCD.20 and we encourage the scientific community to use the data for all studies requiring seismograms from Antarctica.

Underground Water Level, Strain, and Tilt Changes Observed Around the Fault Near Tono Research Institute of Earthquake Science

Near the Tono Research Institute of Earthquake Science (TRIES), Mizunami City, Gifu, two 500 m- deep shafts with diameters of 4.5 m and 6.5 m were excavated by the Japan Atomic Energy Agency (JAEA). The 6.5 m shaft is crossed by an NNW-trending geological fault (NNW fault), which has a right-lateral strike-slip component and a dip-slip component. Since before the excavation TRIES has been performing observations by a comprehensive set of borehole crustal activity observation devices in deep boreholes and extensometers in a vault near the shafts to observe variations in, for example, strain, tilt, and groundwater level continuously. During the shaft excavation, groundwater inflow to the shafts was encountered. The water was twice pumped out. During the pumping and non-pumping periods, TRIES’s observation data such as the groundwater level, the strain, and the tilt showed anomalous variations. The analysis of them revealed relationships among the groundwater level, the fault, the strain and the tilt. Strain analysis of TGR350 borehole station showed that the direction of the maximum shear strain was parallel to strike of the NNW fault. In addition, when the water in the shafts was being pumped out, the groundwater level at TGR350 declined and the maximum shear strain showed a right-lateral strike-slip movement in the NNW direction. When pumping was paused, the groundwater level increased, and the maximum shear strain showed a left-lateral strike-slip movement in the NNW direction. On the other hand, strain analysis of TGR165 borehole station showed different behavior from that of TGR350 borehole station deeper than TGR165. Tilt analysis of TGR165 borehole station showed tilt changes nearly perpendicular to strike of the NNW fault corresponding to the pumping and the non-pumping periods. Tilt analysis of TGR350 borehole station showed tilt changes of E-W direction with much smaller amplitude than that of TGR165. Behavior of the strain and the tilt during the pumping and non-pumping periods can be attributed in part to differences in the response of the medium to groundwater fluctuations, due to differences in the number and distribution characteristics of the fractures in the medium where the observation instruments are installed. We also discussed the observation results from the viewpoint of poroelasticity.

On the correlation between earthquake coda horizontal-to-vertical spectral ratios and amplification functions at the KiK-net network

The Japanese KiK-net network comprises about 700 stations spread across the whole territory of Japan. For most of the stations, VP and VS profiles were measured down to the bottom borehole station. Using the vast dataset of earthquake recordings from 1997 to 2020 at a subset of 428 seismic stations, we compute the horizontal-to-vertical spectral ratio of earthquake coda, the S-wave surface-to-borehole spectral ratio, and the equivalent outcropping S-wave amplification function. The de facto equivalence of the horizontal-to-vertical spectral ratio of earthquake coda and ambient vibration is assessed on a homologous Swiss dataset. Based on that, we applied the canonical correlation analysis between amplification information and the horizontal-to-vertical spectral ratio of earthquake coda across all KiK-net sites. The aim of the correlation is to test a strategy to predict local earthquake amplification basing the inference on site condition indicators and single-station ambient vibration recordings. Once the correlation between frequency-dependent amplification factors and amplitudes of horizontal-to-vertical coda spectral ratios is defined, we predict amplification at each site in the selected KiK-net dataset with a leave-one-out cross-validation approach. In particular, for each site, three rounds of predictions are performed, using as prediction target the surface-to-borehole spectral ratio, the equivalent of a standard spectral ratio referred to the local bedrock and to a common Japanese reference rock profile. From our analysis, the most effective prediction is obtained when standard spectral ratios referred to local bedrock and the horizontal-to-vertical spectral ratio of earthquake coda are used, whereas a strong mismatch is obtained when standard spectral ratios are referred to a common reference. We ascribe this effect to the fact that, differently from amplification functions referred to a common reference, horizontal-to-vertical spectral ratios are fully site-dependent and then their peak amplitude is influenced by the local velocity contrast between bedrock and overlying sediments. Therefore, to reduce this discrepancy, we add in the canonical correlation as a site proxy the inferred velocity of the bedrock, which improves the final prediction.

Orientation Corrections of a Borehole Seismometer Network in Taiwan Using Teleseismic Earthquakes

Abstract The Central Weather Bureau, Taiwan, has deployed a borehole seismic network of 62 stations nationwide in Taiwan since 2008. Each borehole station includes a force-balance accelerometer on the surface, and a combination of an accelerometer and a broadband seismometer in the deep hole. The noise analyses have shown significant improvement in the performance of the borehole instruments compared with the instruments deployed on the surface. We propose a method to obtain robust orientations, as well as the uncertainty for both the borehole accelerometer and broadband seismometer using long-period waves (>2 s) after the S-wave arrival time of three mid-large teleseismic events and one local event. Overall, we successfully calibrated the orientations of the borehole sensors and found that the significant azimuth change for some borehole seismometers was always due to sensor maintenance. Based on the results of this study, we conclude that the long-period waveforms generated by a teleseismic event, wavelength of which is four times longer than the borehole depth, are useful for borehole orientation calibration. The correlation coefficients of the three components for both surface and borehole sensors larger than 0.9 are appropriate. The magnitude and epicenter distance of the event and on-site noise level are the factors that affect the quality of calibration. In addition, we conclude that the method of orientation calibration we propose is helpful for data quality control, including data timing and sensor failure. With the supplement of this azimuth estimation information, we expect that these high-quality borehole data in Taiwan will improve seismological research and will benefit the society.

Site Characterization of Swiss Strong-Motion Stations: The Benefit of Advanced Processing Algorithms

ABSTRACTSince 2009, 91 new strong-motion stations were built for the renewal of the Swiss Strong Motion Network. Another nine stations will be installed until 2022. For each new station, an extensive site characterization study is performed to model the 1D seismic-velocity profile and, for some sites, the liquefaction potential. Geophysical (passive and active surface-wave methods) and geotechnical cone penetration test (CPT) with additional pore-pressure measurement (CPTu) and seismic CPT (SCPT) methods are used. Analyzing the passive and active recordings with a variety of established and advanced methods, the fundamental frequency of the site, the polarization of the wavefield, the Love- and Rayleigh-wave phase-velocity dispersion curves, and the Rayleigh-wave ellipticity function are retrieved. The liquefaction potential is assessed using CPTu. SCPT measurements are sometimes used to determine the shallow underground structure. The benefits of the combination of different appropriate methods are shown for two examples—the borehole station SBUS in Buochs and the upcoming borehole station SCME in Collombey-Muraz. At both the sites, the CPTu measurements show an elevated liquefaction potential. Combining the passive and active data, the dispersion curves for Love and Rayleigh waves and Rayleigh-wave ellipticity curves are retrieved over a wide-frequency range and inverted for the S-wave velocity profile, in which the shallow part is constrained by the active or SCPT data, the intermediate part by the dispersion curves of the passive methods, and the deepest part by the ellipticity information. For Buochs, the 1D SH-wave amplification functions modeled for the velocity profiles are compared with the empirical amplification for earthquake recordings. Finally, an overview of the site characterization results for 52 of the newly installed seismic stations is given.

Multi-Data Integration System to Capture Detailed Strong Ground Motion in the Tokyo Metropolitan Area

To accurately capture ground motion in the Tokyo metropolitan area, we have developed a multi-data integration system that combines a large amount of ground motion data gathered from nationwide strong-motion seismograph networks (K-NET and KiK-net); Metropolitan Seismic Observation network (MeSO-net), which covers the Tokyo metropolitan area with a high density of about 300 observation stations; observation equipment held by private companies; and smartphone-based seismographs. K-NET, KiK-net, and MeSO-net are operated by National Research Institute for Earth Science and Disaster Resilience. The seismic waveform data recorded by MeSO-net, which are based on borehole observations, are one of the most important data sets for this system. To ensure collection of the waveform data, we strengthened the data center functions and made the collected data available to the public. In addition, to estimate the ground motion at the surface, which is important for disaster prevention, from the waveform data of MeSO-net, we carried out temporary seismic and microtremor array observations on the ground surface at each MeSO-net borehole station, and estimated ground amplification characteristics and the S-wave velocity structure. We also developed a smartphone-based seismograph with the aim of realizing seismic observations for tens of thousands of sites in the future. We recruited monitors to deploy the smartphone seismometers in the Tokyo metropolitan area, and developed a function to notify monitors of the results of a rough evaluation of the soundness of buildings based on observation data acquired during an earthquake. Furthermore, we have developed a Tokyo metropolitan area version of Kyoshin Monitor, the strong motion monitor system, with which the current ground motion in the Tokyo metropolitan area can be captured in real time by integrating and visualizing observation data from K-NET, KiK-net, and MeSO-net on a map on the website. We can capture the propagation of the ground motion in detail directly from the high-density data set integrated from these three networks. In addition, we also integrated data from Super-Dense Real-time Monitoring of Earthquakes (SUPREME) network of Tokyo Gas Co., Ltd., which operates about 4,000 observation stations in the Tokyo metropolitan area, after applying a time correction. We verified the integration method by reproducing the ground motion in the Tokyo metropolitan area during the 2011 Tohoku earthquake. The study findings have made it clear that the ground motion in the Tokyo metropolitan area can be captured in more detail by the integration of data produced by the public and private sectors.

Data-Driven Signal–Noise Classification for Microseismic Data Using Machine Learning

It is necessary to monitor, acquire, preprocess, and classify microseismic data to understand active faults or other causes of earthquakes, thereby facilitating the preparation of early-warning earthquake systems. Accordingly, this study proposes the application of machine learning for signal–noise classification of microseismic data from Pohang, South Korea. For the first time, unique microseismic data were obtained from the monitoring system of the borehole station PHBS8 located in Yongcheon-ri, Pohang region, while hydraulic stimulation was being conducted. The collected data were properly preprocessed and utilized as training and test data for supervised and unsupervised learning methods: random forest, convolutional neural network, and K-medoids clustering with fast Fourier transform. The supervised learning methods showed 100% and 97.4% of accuracy for the training and test data, respectively. The unsupervised method showed 97.0% accuracy. Consequently, the results from machine learning validated that automation based on the proposed supervised and unsupervised learning applications can classify the acquired microseismic data in real time.

Characteristics of Slow Slip Event in March 2020 Revealed From Borehole and DONET Observatories

We have detected an event of pore pressure change (hereafter, we refer it to “pore pressure event”) from borehole stations in real time in March 2020, owing to the network developed by connecting three borehole stations to the Dense Oceanfloor Network System for Earthquakes and Tsunamis (DONET) observatories near the Nankai Trough. During the pore pressure event, shallow very low-frequency events (sVLFEs) were also detected from the broadband seismometers of DONET, which suggests that the sVLFE migrated toward updip region along the subduction plate boundary. Since one of the pore pressure sensors have been suffered from unrecognized noise after the replacement of sensors due to the connecting operation, we assume four cases for crustal deformation component of the pore pressure change. Comparing the four possible cases for crustal deformation component of the volumetric strain change at C0010 with the observed sVLFE migration and the characteristic of previous SSEs, we conclude that the pore pressure event can be explained from SSE migration toward the updip region which triggered sVLFE in the passage. This feature is similar to the previous SSE in 2015 and could be distinguished from the unrecognized noise on the basis oft-test. Our new finding is that the SSE in 2020 did not reach very shallow part of the plate interface because the pore pressure changes at a borehole station installed in 2018 close to the trough axis was not significant. In the present study, we estimated the amount, onset and termination time of the pore pressure change for the SSE in 2020 by fitting regression lines for the time history. Since the change amount and duration time were smaller and shorter than the SSE in 2015, respectively, we also conclude that the SSE in 2020 had smaller magnitude that the SSE in 2015. These results would give us a clue to monitor crustal deformation along the Nankai Trough directly from other seafloor observations.

Peak Ground Velocity Spatial Variability Revealed by Dense Seismic Array in Southern California

AbstractUnderstanding and modeling variability of ground motion is essential for building accurate and precise ground motion prediction equations, which can net site‐specific characterization and reduced hazard levels. Here, we explore the spatial variability in peak ground velocity (PGV) at Sage Brush Flats along the San Jacinto Fault in Southern California. We use data from a dense array (0.6 × 0.6 km2, 1,108 geophones, station spacings 10–30 m) deployed in 2014 for ~1 month. These data offer an opportunity to study small‐scale variability in this region. We examine 38 earthquakes (2 ≤ ML ≤ 4.2) within 200 km of the array. Fault strands and a small basin impact the ground motions, producing PGV variations up to 22% of the mean and a 40% reduction in P and S wave near‐surface velocities. We find along‐fault rupture directivity, source, and path effects can increase PGVs by 167%. Surface PGV measurements exceed the colocated borehole station (depth at 148 m) PGV by factors of 3–10, confirming the impact on PGV from near‐surface fault structures, basins, topography, and amplifications from soft sediments. Consistently, we find high PGVs within the basin structure. A pair of colocated GaML2.6 events produce repeatable PGV values with similar spatial patterns. The average corner frequencies of these two events are 11–16 Hz, and viable measurements of stress drop can differ by 6.45 MPa. Within this small array, the PGV values are variable implying spatial extrapolation of PGV to regions of known faults and basins, even across a small area, should be done with caution.

Estimation of High-Frequency Spectral Decay (κ) for the Gyeongju Area in South Korea

ABSTRACT The high-frequency spectral decay parameter kappa (κ) is estimated for the Gyeongju area using records of the temporary seismic array in the Gyeongju area (TSAG), which operated from 2010 to 2013. Gyeongju is a city located in the southeastern part of the Korean Peninsula. Moderate earthquakes frequently occurred around the Gyeongju area in the past; the area has experienced numerous earthquakes for more than a year since an earthquake of ML 5.8 occurred in Gyeongju on 12 September 2016. It is also an important place from an engineering point of view because nuclear facilities are located in the area. The high-frequency spectral decay model of Anderson and Hough (1984) and the standardized procedure for the κ-estimation suggested by Ktenidou et al. (2013) are applied to estimate κ for the Gyeongju area. The estimation gives rise to the equation, κ=0.0219(±0.00151)+7.56×10−5(±1.45×10−5)R indicating that the site-specific κ0-value and the slope κR are 0.0219 s and 7.56×10−5 s/km, respectively, for the Gyeongju area. The κ-estimation for individual stations of TSAG was done to obtain the site-specific κ0-values of stations. Despite the close distance between TSAG stations, the site-specific κ0-values of each station vary. Other than TSAG, κ is also estimated for the permanent borehole station HDB, which has been operated by the Korea Institute of Geoscience and Mineral Resources and is located in the Gyeongju area. The estimated κ0-value of 0.0178 s for HDB is smaller than that of TSAG by 0.0219 s. This is because HDB is a borehole station, but all stations of TSAG are installed at surface. The variation of the κ0-values among the individual TSAG stations and the difference between κ0 for TSAG and that for HDB indicate that spectral decay characteristic at high frequency is affected by surficial sediment layers.

Spectral Characteristics of Daily to Seasonal Ground Motion at the Piñon Flats Observatory from Coherence of Seismic Data

AbstractWe investigate coherences of seismic data recorded during three years (2015–2017) at the Piñon Flats Observatory (PY) array and a collocated 148 m deep borehole station B084, along with oceanic data from a buoy southwest of the PY array. Seismic and barometric recordings at PY stations are analyzed with a multitaper spectral technique. The coherence of signals from seismic sources is >0.6 at 0.05–8 Hz between closely spaced (<65 m) surface stations and decreases to ∼0.2 in frequency bands in which the wavelengths are smaller than interstation distances. There are several local coherence increases at 1–8 Hz between nearby (<65 m) surface stations, whereas large coherence values between a surface and 148 m deep borehole stations are only present at the secondary microseism (∼0.14 Hz). These points to significant modification of seismic recordings in the top crust, and those continual near‐surface failures might produce shallow rapidly attenuating signals at surface stations. Incoherent local atmospheric effects induce incoherent seismic signals in low‐ and high‐frequency ranges through different coupling mechanisms. Between 0.003 and 0.05 Hz, atmospheric loadings generate ground tilts that contaminate the two horizontal seismic recordings and decrease their coherence, whereas the vertical component is less affected. At 1–8 Hz, coupling of atmospheric pressure with surface structures transmits incoherent signals into the ground, degrading the seismic coherence in all three components. The two horizontal coherences show seasonal variations with extended coherent frequency bands in winter and spring, likely to be produced by seasonal variations in microseisms and local ground tilts. The coherences also contain high anomalies between 2 and 4 Hz resulting from anthropogenic activities. The results provide useful information on instrument characteristics and variations in the shallow crustal response to earthquakes, seasonal and ambient sources of seismic energy, along with atmospheric pressure–temperature changes and anthropogenic activities.

Feasibility study for the microseismic monitoring of the natural gas reservoir of "SANT'ALBERTO" (Po Plain, Italy)

In this paper we present a feasibility study for the surface seismic monitoring of the “Sant’Alberto” gas reservoir (Po Plain, Northern Italy). The project was commissioned to INGV in the framework of the activities planned by the oil & gas EP 2) the installation of the borehole station allows to improve the detection threshold everywhere within DI, and up to ML =-0.4 nearby the reservoir; and 3) localization thresholds obtained in the crustal volume included in DE, but not in DI, show values around ML = 1.0.

Shallow velocity model in the area of Pozzo Pitarrone, Mt. Etna, from single station, array methods and borehole data

<p>Seismic noise recorded by a temporary array installed around Pozzo Pitarrone, NE flank of Mt. Etna, have been analysed with several techniques. Single station HVSR method and SPAC array method have been applied to stationary seismic noise to investigate the local shallow structure. The inversion of dispersion curves produced a shear wave velocity model of the area reliable down to depth of about 130 m. A comparison of such model with the stratigraphic information available for the investigated area shows a good qualitative agreement. Taking advantage of a borehole station installed at 130 m depth, we could estimate also the P-wave velocity by comparing the borehole recordings of local earthquakes with the same event recorded at surface. Further insight on the P-wave velocity in the upper 130 m layer comes from the surface reflected wave observable in some cases at the borehole station. From this analysis we obtained an average P-wave velocity of about 1.2 km/s, compatible with the shear wave velocity found from the analysis of seismic noise.</p>

A Study on Kappa Value in Taiwan Using Borehole and Surface Seismic Array

Since the inception of the borehole seismic array deployed by the Central Weather Bureau (CWB) in Taiwan in recent years, a large quantity of strong‐motion records have been accumulated from frequently occurring earthquakes around Taiwan that provide the opportunity to investigate the spectral decay parameter kappa ( κ ) and to understand further the site effects of seismic stations. In this study, we used 133 earthquakes recorded at 30 borehole seismic arrays. Each array includes two force‐balance accelerometers, one at the surface and the other inside the borehole. Based on the regression analysis between κ ‐value and hypocentral distance for each surface–borehole station pairs, the resulting κ derived from surface stations are higher than results from borehole stations. The estimated κ ‐values are found to range from 0.032 to 0.097 s at surface and 0.012 to 0.078 s in borehole. In comparison with a study of site corrections for earthquake magnitude determination, these higher κ ‐values associated with negative station corrections can correspond to the effect of soil conditions. To evaluate station corrections of borehole seismic array, we collected records from the cosite stations of Taiwan Strong‐Motion Instrumentation Program (TSMIP) and Central Weather Bureau Station Network (CWBSN) and then derived a linear relationship between the κ ‐values and station corrections. Finally, both resulting station corrections and κ ‐values reveal high coherence in local site conditions.

Seismic Responses of Two RC Buildings and One Wood Building Based on an Input Wave Field

AbstractA recently proposed three-dimensional (3D) linear method for examining soil-building interactions based on an input seismic wave field is, after some improvements, applied to estimate seismic building responses in the reclaimed zone of Tokyo Bay, where ground motions include a considerable amount of surface waves, thus reconfirming the effects of the method in a different situation. A seismic wave field involves seismic waves propagating in a 3D medium. The proposed method was developed with the goal of adequately treating seismic surface waves trapped by a (several-kilometers) deep underground structure in a soil-building interaction system. Two simulations were carried out. The first simulation successfully reproduced surface, downhole, and building accelerograms that were recorded at one borehole station during two medium-sized earthquakes. In the second simulation, seismic responses of a midrise RC model building and a wood model building were favorably calculated at the other borehole station...

OGS improvements in 2012 in running the North-eastern Italy Seismic Network: the Ferrara VBB borehole seismic station

Abstract. The Centro di Ricerche Sismologiche (CRS, Seismological Research Centre) of the Istituto Nazionale di Oceanografia e di Geofisica Sperimentale (OGS, Italian National Institute for Oceanography and Experimental Geophysics) in Udine (Italy) after the strong earthquake of magnitude M=6.4 occurred in 1976 in the Italian Friuli-Venezia Giulia region, started to operate the North-eastern Italy Seismic Network: it currently consists of 17 very sensitive broad band and 18 simpler short period seismic stations, all telemetered to and acquired in real time at the OGS-CRS data centre in Udine. Real time data exchange agreements in place with other Italian, Slovenian, Austrian and Swiss seismological institutes lead to a total number of about 100 seismic stations acquired in real time, which makes the OGS the reference institute for seismic monitoring of North-eastern Italy. The south-western edge of the OGS seismic network (Fig. 1) stands on the Po alluvial basin: earthquake localization and characterization in this area is affected by the presence of soft alluvial deposits. OGS ha already experience in running a local seismic network in high noise conditions making use of borehole installations in the case of the micro-seismicity monitoring of a local gas storage site for a private company. Following the ML = 5.9 earthquake that struck the Emilia region around Ferrara in Northern Italy on 20 May 2012 at 02:03:53 UTC, a cooperation of Istituto Nazionale di Geofisica e Vulcanologia, OGS, the Comune di Ferrara and the University of Ferrara lead to the reinstallation of a previously existing very broad band (VBB) borehole seismic station in Ferrara. The aim of the OGS intervention was on one hand to extend its real time seismic monitoring capabilities toward South-West, including Ferrara and its surroundings, and on the other hand to evaluate the seismic response at the site. We will describe improvements in running the North-eastern Italy Seismic Network, including details of the Ferrara VBB borehole station configuration and installation, with first results.

Vibrations of the TAIPEI 101 skyscraper caused by the 2011 Tohoku earthquake, Japan

The strong-motion recordings generated by the 11 March 2011, Tohoku, Japan earthquake recorded by the building seismic array in the TAIPEI 101, the second tallest building in the world, are more complicated than those at the free-field stations. Fundamental and higher-mode vibrations can be clearly seen on the spectra. The fundamental-mode frequency is about 0.15 Hz, which is the natural frequency of the skyscraper. Spectral ratios of ground accelerations at the building to those at a nearby borehole station increase from unity at the fifth floor underground to factors of 110 and 146, respectively, on the 74th and 90th floors above ground for the fundamental-mode vibrations. The frequency content of accelerograms, recorded on the fifth floor below ground from the great 2011 Tohoku-Oki earthquake, appears mainly in a frequency band of 0.015–0.1 Hz, leading to a fact that the ground motions did not cause high excitation in the TAIPEI 101 skyscraper.

Fault zone structure at depth from differential dispersion of seismic guided waves: evidence for a deep waveguide on the San Andreas Fault

SUMMARY Seismic guided wave dispersion can be used to image fault-zone structure at seismogenic depth. A two-station differential group velocity technique previously used for surface waves was adapted to solve for local fault-zone structure between two stations. This method was extended to solve for fault-zone structure between two earthquakes using differential group arrival times at a single station. The method was tested with finite-difference synthetic data for an inhomogeneous fault, as well as with a pair of shallow earthquakes recorded in the San Andreas Fault Observatory at Depth (SAFOD) borehole station. Results from a pair of deep earthquakes recorded in the SAFOD borehole station indicate that the low-velocity waveguide of the San Andreas Fault extends to >10 km depth. The waveguide at 10–12 km depth is 120–190 m wide and the velocity contrast is >20 per cent, similar to the values in the shallow subsurface. Multiple earthquakes and receivers could be used to map fault zone structure at seismogenic depth as a function of depth and strike.

Nonlinear Soil Response of a Borehole Station Based on One-Dimensional Inversion during the 2005 Fukuoka Prefecture Western Offshore Earthquake

The objective of this article is to present the nonlinear response of a soft sedimentary site based on a one-dimensional inversion by a genetic algorithm of the shear-wave velocity structure and damping factors of a borehole soil column during the 2005 Fukuoka Prefecture Western Offshore earthquake. First, we confirm that, according to the source rupture mechanism, the major and minor axes in the horizontal plane at the borehole station are the transverse and radial directions, respectively. Then, in order to corroborate in the linear domain the S-wave transfer function of the borehole's logging, we perform time-dependent spectral ratios analyses on small aftershocks. Finally, we show qualitative evidence of nonlinearity during the main- shock associated with a significant shift toward low frequencies of several resonant modes, and we evaluate the degree of nonlinearity by inverting the shear-wave veloc- ity structure and damping factors. Because of a directional effect present only in the major axis around 8 Hz, which prevents the use of the conventional objective function that minimizes the integrated residuals between observed and theoretical ratios, we introduce a simple objective function that depends only on peaks' frequency and amplitude. The efficiency of the objective function and the robustness of the inversion are shown by performing eight independent inversions converging to very similar minima.