Junzo Kasahara

Variation in crustal structure along the Kyushu-Palau Ridge at 15–21°N on the Philippine Sea plate based on seismic refraction profiles

We acquired coincident wide-angle and multi-channel seismic reflection data along four profiles perpendicular to the Kyushu-Palau Ridge (KPR) between 15°N and 20°N on the Philippine Sea plate. The crustal thickness beneath the KPR, which is a remnant arc created in the Late Eocene, varies along the strike from 8 to approximately 20 km and is always thicker than the adjacent oceanic crust of the West Philippine Basin to the west and the Parece Vela Basin to the east. The thickest crust among the four profiles, which is primarily due to a thickening of the lower crust, is found where the KPR adjoins Oki-no-Tori-Shima Island. There is no clear evidence of the thick (>5 km) middle crustal layer with a P-wave velocity of 6.0–6.5 km/s that has been inferred beneath the conjugate rifted counterpart of the Izu-Ogasawara(Bonin)-Mariana Island-arc. Our results suggest that the crust of the KPR at 15–21°N represents a less mature island arc crust relative to that further north along the ridge where a mid-crustal layer of 6 km/s has been reported.

A new method for determining OBS positions for crustal structure studies, using airgun shots and precise bathymetric data

Abstract Ocean-bottom seismometer (OBS) positions are one of the key parameters in an OBS-airgun seismic survey for crustal structure study. To improve the quality of these parameters, we have developed a new method of determining OBS positions, using airgun shot data and bathymetric data in addition to available distance measurements by acoustic transponders. The traveltimes of direct water waves emitted by airgun shots and recorded by OBSs are used as important information for determining OBS locations, in cases where there are few acoustic transponder data (<3 sites). The new method consists of two steps. A global search is performed as the first step, to find nodes of the bathymetric grid that are the closest to explaining the observed direct water-wave traveltimes from airgun shots, and acoustic ranging using a transponder system. The use of precise 2D bathymetric data is most important if the bottom topography near the OBS is extremely rough. The locations of the nodes obtained by the first step are used as initial values for the second step, to avoid falling into local convergence minima. In the second step, a non-linear inverse method is executed. If the OBS internal clock shows large drift, a secondary correction for the OBS internal clock is obtained, as well as the OBS location, as final results by this method. We discuss the error and the influence of each measurement used in the determination of OBS location.

Chapter 26 – Active Monitoring Using Submarine Cables—Leveraging Offshore Cabled Observatory for Passive Monitoring

Abstract Eight seafloor are observatories installed in Japanese water. Passive monitoring by these observatories has revealed a number of qualities that include detailed micro-earthquake activity before and after plate-boundary earthquakes, the existence of micro-tsunamis, the capability of early detection of tsunamis, the importance of seafloor geodetic observations, etc. These new findings were brought about by passive, real-time and continuous time series of earthquake and tsunami records acquired on the seafloor. Recently, our knowledge about earthquake source mechanisms has revealed the possibility of the detection of spatiotemporal changes in physical properties that may occur at inter-plate boundaries, and active seismic surveys have found reflection amplitude differences at seismically inactive plate boundaries or possible seismic asperities. We propose that the detection of such physical property changes becomes possible when we combine seafloor observations and active seismic sources dedicated to the precise monitoring of reflection amplitudes at plate boundary interfaces.

Interactive analysis tools for the wide-angle seismic data for crustal structure study (Technical Report)

Abstract The analysis of wide-angle seismic reflection and refraction data plays an important role in lithospheric-scale crustal structure study. However, it is extremely difficult to develop an appropriate velocity structure model directly from the observed data, and we have to improve the structure model step by step, because the crustal structure analysis is an intrinsically non-linear problem. There are several subjective processes in wide-angle crustal structure modelling, such as phase identification and trial-and-error forward modelling. Because these subjective processes in wide-angle data analysis reduce the uniqueness and credibility of the resultant models, it is important to reduce subjectivity in the analysis procedure. From this point of view, we describe two software tools, PASTEUP and MODELING, to be used for developing crustal structure models. PASTEUP is an interactive application that facilitates the plotting of record sections, analysis of wide-angle seismic data, and picking of phases. PASTEUP is equipped with various filters and analysis functions to enhance signal-to-noise ratio and to help phase identification. MODELING is an interactive application for editing velocity models, and ray-tracing. Synthetic traveltimes computed by the MODELING application can be directly compared with the observed waveforms in the PASTEUP application. This reduces subjectivity in crustal structure modelling because traveltime picking, which is one of the most subjective process in the crustal structure analysis, is not required. MODELING can convert an editable layered structure model into two-way traveltimes which can be compared with time-sections of Multi Channel Seismic (MCS) reflection data. Direct comparison between the structure model of wide-angle data with the reflection data will give the model more credibility. In addition, both PASTEUP and MODELING are efficient tools for handling a large dataset. These software tools help us develop more plausible lithospheric-scale structure models using wide-angle seismic data.

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.

Seafloor observatory for monitoring hydrologic and geological phenomena associated with seismogenic subduction zones

Recently, our seismological knowledge about source mechanisms of plate-boundary earthquakes has revealed the possibility in the detection of spatiotemporal changes in physical properties that may occur at interplate boundaries. Seismological interest is identified to be in seismogenic zones that are deeper than several kilometers from the surface.

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.