The prestack reverse-time depth migration is developed to estimate the reflection coefficient of the subsurface. To examine the capability of this method, a numerical experiment is implemented and it is shown that this method gives proper reflection coefficients even for a structure with 45-degree dips. It is also applied to real data of land survey. A distribution map of the reflection coefficient can be obtained after compensating the amplitude from the 3D to 2D wavefield including consideration for the Q value. However, extracting the velocity perturbation from the estimated reflection coefficient is hard due to inaccuracy of the estimated source strength, oscillatory be- havior of the image, and lack of information about the density. The waveform inversion method is compared with the present method and it is found that the first iteration of the former method is nearly identical to the present method. Problems which are inherent in these methods are discussed.

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A gravity survey of the island of Taiwan was carried out from 1980 to 1987. In a 7-year effort, 603 gravity stations, most of which are located on geodetic stations, have been established. The average spacing between the stations is about 7km. The Bouguer and terrain corrections were made over a distance range of 100km using an average density of 2.57g/cm³. After the reductions, the Bouguer anomaly map was constructed. In general, the isogals of the Bouguer anomaly map trend NNE in consonance with the overall structural trend of the island. Negative anomalies cover a major part of the island; positive anomalies dominate in the eastern part of the Central Range, the Coastal Range and the northern extremity of the island. A high gradient belt, parallel to the trend of the island, is observed in the eastern Central Range across the suture zone between the Philippine Sea and Eurasian plates. In addition, the gravity anomaly data suggests that the island of Taiwan is not in isostatic equilibrium.

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An imaginary empirical law to relate earthquake magnitude, M, hypocentral distance, R, and the amount of expected static deformation of the Earth's surface was derived by a simulation using the Monte Carlo method. Assuming a point double couple for each of a million seismic sources, the frequency distributions of displacement, strain and tilt were examined. The results are summarized as follows: 1) Expected maximum values of crustal deformation can be expressed by logUmax=1.5M-2logR(km)-6.0 for static displacement, U(cm), logFmax=1.5M-3logR(km)-10.7 for static strain and tilt, F. 2) Expected average values of crustal deformation are 1/3-1/4 of the expected maximum values stated above. 3) The whole distributions of deformation are confined within relatively narrow ranges, and most are between the maxima and 1/10 of the averages. Although the above results can be used only for rough estimation, the obtained empirical law is useful not only for designing an observation network of crustal deformation but also for evaluation of the influences of seismic events.

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Steady thermal field associated with magma mass intrusion is studied in two-dimensional (2D) and three-dimensional (3D) spaces. It is proved that the latter model leads to surface heat flow substantially smaller than that for the former. A numerical approach based on a relaxation technique for solving the Laplace equation is made use of. Transient heat conduction is solved when a lava dome makes an appearance on the earth's surface. Converting the differential equation that governs the problem into a difference equation, 2D and 3D solutions are obtained. It becomes apparent that the 2D analysis results in a cooling rate considerably smaller than that for the 3D analysis. A similar transient problem related to the filling of a reservoir with high-temperature magma is also studied. Even in this case, a 2D analysis leads to cooling appreciably slower than that for a 3D analysis. The mode of heat flow anomaly development at the earth's surface for the 2D analysis is noticeably different from that for the 3D analysis. In short, 3D analyses of the geothermal problems are important for obtaining realistic results although 2D analyses have so far been customary in many cases.

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This study presents a 500-km-long crustal transect across the Lofoten volcanic passive continental margin, N. Norway, by compiling the results of two successive Ocean Bottom Seismographic (OBS) experiments performed in 1988. The OBS profiles were acquired from the Norwegian mainland, across the continental shelf, over an area covered with landward flood basalts, to the Lofoten basin. The land side end of the crustal model represents a thinned continental structure. The crust in this part has strong structural complexity, mainly due to faulting during pre-Tertiary continental thinning phases. Between the continental shelf and the seaward dipping reflectors (SDR), the model represents an extremely thinned continental crust and ocean/continent transition zone. This region is interpreted to be dominated by an early Tertiary continental rifting phase that progressed until early Eocene. The observed lower crustal reflectors, which are interpreted as intrusions in the lower crust, as well as the landward flood basalts indicates an extensive magmatic activity during the continental rifting phase. Between the SDR and magnetic anomaly 21, an oceanic crust with thick lower crust and a high velocity layer at the bottom of the crust (7.3km/s) are obtained. This high velocity layer is believed to be created by anomalously hot asthenospheric material rising around a hot spot. The comparison of the crustal structure across the Lofoten margin with the structure of the Voring-More margin shows significant differences in the volume of the lower crustal high velocity layer, which can be interpreted in terms of a NE-ward decrease of the influence of the hot spot.

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