K. Tsuruga

Detection of Localized Small-Scale Heterogeneities in the Hanshin-Awaji Region, Japan, by Anomalous Amplification of Coda Level

We estimated the spatial distribution of small-scale heterogeneities in and around the Hanshin-Awaji region including the coseismic fault area of the 1995 Hyogo-ken Nanbu, Japan, earthquake, by using the amplification factor of the coda level as a measure of scattering strength. We analyzed 137 earthquakes recorded at 64 stations. Because the coda level is strongly affected by site amplification, the determination of the amplification factor of the coda level requires the following three major steps: (1) checking the stability of coda waves among different stations such as coda Q and lapse time, (2) estimating the site amplification of coda waves for events outside the fault region, and (3) taking the spectral ratio of coda waves for events inside the fault zone after correcting the site characteristics obtained in step 2. The coda decay rate, \batchmode \documentclass[fleqn,10pt,legalpaper]{article} \usepackage{amssymb} \usepackage{amsfonts} \usepackage{amsmath} \pagestyle{empty} \begin{document} \(Q_{\mathrm{c}}^{-1}\) \end{document}, is common among stations for each event. The site amplification factor at each site estimated from 25 events correlates with the surface geological setting. The most important result is that coda amplification anomalies exist as clusters along the coseismic fault area, depending on frequency: beneath Awaji Island in the range of 1-4 Hz and beneath the area southwest of Kobe, 8-16 Hz. These high coda levels imply that strong heterogeneities exist there. The scales of these heterogeneities are estimated to be about 0.2-1.4 and 0.06-0.18 km, corresponding to the observed dominant frequencies (1-4 and 8-16 Hz, respectively). Manuscript received 10 January 2000.

Evaluation and interpretation of the effects of heterogeneous layers in an OBS/air-gun crustal structure study

Abstract We present a method for interpreting seismic records with arrivals and waveforms having characteristics which could be generated by extremely inhomogeneous velocity structures, such as non-typical oceanic crust, decollement at subduction zones, and seamounts in oceanic regions, by comparing them with synthetic waveforms. Recent extensive refraction and wide-angle reflection surveys in oceanic regions have provided us with a huge number of high-resolution and high-quality seismic records containing characteristic arrivals and waveforms, besides first arrivals and major reflected phases such as PmP. Some characteristic waveforms, with significant later reflected phases or anomalous amplitude decay with offset distance, are difficult to interpret using only a conventional interpretation method such as the traveltime tomographic inversion method. We find the best process for investigating such characteristic phases is to use an interactive interpretation method to compare observed data with synthetic waveforms, and calculate raypaths and traveltimes. This approach enables us to construct a reasonable structural model that includes all of the major characteristics of the observed waveforms. We present results here with some actual observed examples that might be of great help in the interpretation of such problematic phases. Our approach to the analysis of waveform characteristics is endorsed as an innovative method for constructing high-resolution and high-quality crustal structure models, not only in oceanic regions, but also in the continental regions.

A New 3D Gravitational Tensor Inversion for Imaging of Density Distribution in the Ground

We present a general framework for selecting optimal measurement locations for applications. This framework incorporates the basic ingredients that are usually part of an inversion process: choice of regularisation scheme, nature and size of measurement error, and use of prior knowledge to constrain solutions. The basic idea of the framework is to minimise errors associated with the reconstruction of a given quantity of interest. We introduce five functional layers which reflect the structure of the framework. Further, with framework adaption we formulate an iterative algorithm which finds optimal measurement locations at each iteration step to obtain improved reconstructions of the inverse problem. Using initial reconstructions as input, framework adaption does not require prior knowledge of the source distribution. We illustrate the feasibility of framework adaption by solving an acoustic source problem. Numerical examples are given.

A Continuous Monitoring for Shallow/Deep Seismic Reflectors by Accurately-controlled Source and Multi-receivers

Seismic ACROSS (Accurately-Controlled Routinely-Operated Signal System) enables us to continuously monitor the temporal change of the amplitude and/or travel-time of particular seismic reflectors or diffractctors from a time-variant region (e.g., oil-gas /CO2/aquifer reservoirs and a focal region) because of very accurate-and-stable seismic signals. The seismic ACROSS controlled by GPS clock is fixed type source which can generate 10-50Hz and 40 ton-f at 50Hz. To evaluate the seismic characteristics before and after the change and optimized source-receiver array design and to image the time-variant target region, we investigated the following cases with a velocity change of (i) 10 % in a CO2 reservoir with a 300-m-wide and 100-m-thick at 200m-depth, and (ii)~30 % in a deep-slip zone with a 10-km-long and 200-m-thick at 30-km-depth, by applying a imaging method using FDM back-propagating residual waveforms. We can robustly obtain the location and shape of the targets by using the velocity structure before the change even if a single source or the change in a near surface layer. We finally concluded that ACROSS technology is very useful to monitor the temporal change of physical properties in the time-variant regions with variable scale and depth.

Secondary Calibration Method of Seismometers by Utilizing ACROSS Signal

Abstract We have developed a technology for active monitoring of seismogenic zones and/or volcanic areas. The technology utilizes Accurately-Controlled Routinely-Operated Signal System (ACROSS) and is capable to detect subtle changes in physical properties. It also requires optimal placement of source and receiver arrays containing many easy-to-use seismometers. However, any current calibration method determining a frequency response of the ground motions for such seismometers is not accurate enough. We developed a secondary calibration method for easy-to-use seismometers by utilizing seismic ACROSS signals, a vibration table and reference accelerometers. We present the basic concept together with technical steps, and demonstrate the experimental results for 52 target uni-axial sensors by using the more reliable accelerometers as reference. Despite the average accuracy of the frequency responses for the reference sensors, we could easily obtain a precise enough frequency response for each tested sensor using the Maximum Likelihood method. In experimental results, the accuracies of frequency response for sensors were approximately 10 2 -10 3 . Although these values are not accurate enough for active monitoring of the Earth’s interior, we can conclude that this method is useful enough to provide a simultaneous and precisely-determined frequency response for a large array of easy-to-use seismometers.

Chapter 20 – Automatic Travel Time Determination from a Frequency-domain Transfer Function: The Sompi Event Analysis

Abstract We have developed a method to extract “events” localized in a time domain from a transfer function in the frequency domain, which is a part of the basic analysis in ACROSS (Accurately-Controlled Routinely-Operated Signal System). In response to the limitations with respect to the practical application shown in the previous procedure, we designed a revised version of this method, based on maximum likelihood estimation. The basic theory, including the revision, is presented here, along with a practical procedure for automatic travel time determination. We then submitted this revised version to a numerical test, the results of which supported the validity of this method for analysis of transfer functions involving plural “events” in the time domain.