Amplitude and phase variations of surface waves in a laterally heterogeneous earth : ray- and beam-theoretical approach

Thesis (Ph. D.)--Massachusetts Institute of Technology, Dept. of Earth, Atmospheric, and Planetary Sciences, 1986.Microfiche copy available in Archives and ScienceBibliography: leaves 198-208.by Kiyoshi Yomogida.Ph.D

Observation of Microtremors in the Tsukuba Area, Japan, using a Portable Broadband Seismometer

Studies of microtremors have been advanced by different approaches, that is, a variety of observational studies and analyses, for both the short-period and the long-period ranges since the microtremors for each range have their own source and site characteristics in time and space. We, therefore, conducted microtremor observation around Tsukuba City, Ibaraki, Japan, in July and August, 1991, in order to clarify site characteristics of six locations around the Tsukuba Mountain, deploying a portable broadband seismometer (Streckeisen STS-2) in the field of subsurface structure and studying ground motion in both frequency ranges, simultaneously. By a comparison with an STS-1 seismometer, the STS-2 gives reliable frequency ranges higher than 0.09 Hz and 0.05 Hz in horizontal and vertical components, respectively. The correlation of the reference site and the other sites implies that the source of microtremors shares common characteristics for the lower range ( 1 Hz), particularly 1.2 ～ 2.5 Hz, in this area. Three types of dominant peak frequencies for the range of 0.1 ～ 1 Hz are revealed: (1) the peak frequencies ranging from 0.2 to 0.3 Hz observed near Mt.Tsukuba can be explained by the topographic high model of Bard (1982). Two frequency peaks ranging (2) from 0.2 to 0.4 Hz and (3) from 0.5 to 0.8 Hz, observed at the stations on alluvials, are related to any vertical resonance in soft layers, consistent with other geological information. Amplitude ratios at sedimentary sites with respect to TSK (Mt.Tsukuba), a rock site, are greater than unity over 5 Hz where the ratios are reported to be smaller than unity in many areas, which implies relatively hard sedimentary layers in the Tsukuba area

1994 Multi-site Broadband Seismic Observation at Sakurajima Volcano, Japan

Broadband observation of volcanic earthquakes has been recently conducted at many volcanoes. At the Sakurajima Volcano in Japan, one of the most active volcanoes all over the world, we conducted two broadband seismic observations successfully in last three years. However, they left some problems in terms of the number of observation sites and recording systems. This paper reports the outline of our third observation using three broadband seismometers (Streckeisen STS-II) with continuous recording at the Sakurajima Volcano from February 18 to March 28, 1994. This observation is distinguished from the previous two by the three stations operating continuously over the entire period. Although the Sakurajima Volcano had very low seismic activities during this observation period, we observed some kinds of volcanic earthquake, A-type and B-type earthquakes and volcanic tremors, particularly one series of interesting clone events

Physical properties of some unequilibrated Antarctic ordinary chondrites

The intrinsic and bulk densities, porosity, thermal diffusivity, and ultrasonic-wave velocities (V_p and V_s) were measured for four unequilibrated ordinary chondrites (Y-74156,-74191,-74647 and -75097). Thermal diffusivity was also measured for five equilibrated ordinary chondrites (ALH-77288,-77294,-78103,-78251 and MET-78003). V_p and V_s measurements were made for three mutually perpendicular directions at room temperature and one atmosphere pressure. Thermal diffusivity was measured under vacuum conditions (below (10)^ mmHg) in the temperature range of 100 to 500 K. V_p and V_s values are much smaller than those expected from their mineral compositions. The decrease in V_p and V_s is much greater than that estimated from the porosity value itself. It is suggested that many oblate pores, i.e. cracks, exist in the samples. Aspect ratio of cracks is estimated to be much smaller than 0.05. Such thin cracks are presumably produced by impact events, although we cannot observe any heavy shock induced features in thin sections. Velocity anisotropy between transmitting dircctions is observed in almost all samples. Thermal diffusivities are also much smaller than those expected from their mineral compositions. Existence of many cracks is also suggested from the thermal diffusivity-porosity relations. Relationship between the measured physical properties and the petrologic types was investigated. For H chondrites we can see a rough trend that porosity decreases with increase in petrologic type number. For L chondrites, however, we cannot see any clear correlation between the physical properties and the petrologic types

Physical properties of some Antarctic meteorites

The intrinsic and bulk densities, porosities and ultrasonic-wave velocities (V_p and V_s) in three mutually perpendicular directions were measured in five antarctic meteorites (ordinary chondrites; ALH-77288,-77294,-78103,-78251 and MET-78003). V_p and V_s measurements were made at the room temperature and under one atmospheric pressure. Measured V_p and V_s values are fairly smaller than those expected from their mineral compositions except for ALH-77288. High porosity values (∿10%) of the samples, except for that (∿2%) of ALH-77288,are consistent with such low elastic-wave velocities. Degree of decrease in velocity is, however, larger than that expected from spherical pores. Thus, V_p and V_s of chondrites may be controlled by the crack porosity. Very low transmitting efficiency of the shear-wave may also support this view. Velocity anisotropy between transmitting directions is observed in almost all samples. The thermal diffusivity of four antarctic meteorites (ordinary chondrites; ALH-769,-77231,Y-74191 and -74371) was measured under the vacuum condition (below (10)^ mmHg) in the temperature range of 100 to 500K. Thermal diffusivities of chondrites are also smaller than those expected from their mineral compositions and are ascribed to their high porosity values. The thermal diffusivities at 300K are 2.7∿5.0×(10)^m^2/s. The thermal diffusivity of ALH-77231 was measured for three mutually perpendicular directions. However, significant difference of thermal diffusivity between directions was not detected. We cannot find a correlation between the measured physical quantities and the petrologic types so far

Migration of tremor locations before the 2008 eruption of Meakandake Volcano, Hokkaido, Japan

We estimate the locations of three tremor sequences, denoted A, B, and C, that occurred before the 2008 eruption at Meakandake volcano, eastern Hokkaido, Japan, using the spatial distribution of seismic amplitudes of volcanic tremor. Although we used only five seismic stations, we could estimate the locations of three tremor sequences. We find two different tremor source areas: those of Tremor A located about 2 km southwest of the erupted crater (Area A) and the other about 1 km southeast to south of the crater as that of the Tremor C (Area B). The location of an early phase of the Tremor B is estimated in Area A while the location of its later phase appears to connect the two areas. This location migration during Tremor B occurred simultaneously with other important geophysical phenomena before the eruption event, as suggested by geodetic and geomagnetic studies. Our findings of the two tremor locations of the tremor sequences and location migration during Tremor Bmay be important for future monitoring activities at Meakandake volcano, particularly to forecast the location of a possible phreatic eruption

2-D Heterogeneous Structure in Eastern Hokkaido Inferred from S-wave Coda Amplification Factors

We examined 2-D heterogeneous structure in the eastern Hokkaido region, using the S-wave coda amplification factor (CAF) with seismic storong-motion date of Kyoshin Net(K-NET). CAF is defined as the spectral ratio of coda waves among different stations after corrections for source, site, and overall propagation.(i.e,cada-Q) effects. Confirming that code-Q values are nearly independent of both source and station, the site effect at each station was esetimated by the coda-normalization method. The source effect was removed by taking the spectral ratio of S-wave coda among stations for each earthquake. We evaluated the CAF value of each source-station pair for three components,using 95 seismograms for five local earthquakes recorded at 21 stations of K-NET. A map of CAF values at the frequency band of 16 Hz shows a systematic variation, implying that heterogeneities are concentrated at the central area (Shibeccha) in the eastern Hokkaido region, manifested locally as a large degree of scattering

Synthetic Seismograms Near a Finite Fault System

We developed computer codes to synthesize seismograms near a finite fault system with the discrete wave-number method. Besides we synthesized seismograms in two dimensional space, formulations in three dimension are also provided. The discrete wave-number method which regarded circular wave from epicenter as a sum of plane waves propagating any direction is useful in introducing a rectangular fault source and a stratified medium. 1

Back-Projection Imaging of a Tsunami Excitation Area With Ocean-Bottom Pressure Gauge Array Data

A back-projection method has been applied to many earthquakes in seismology due to its simple and low computational cost, and it can estimate complex fault rupture processes without any specific a priori information. In this study, we applied the back-projection method to the tsunami records observed using an ocean-bottom pressure gauge array and demonstrated it to be a powerful new tool other than the familiar waveform inversion. The obtained back-projection image was consistent with the initial tsunami height distributions estimated by previous waveform inversions, and its spatial resolution appeared to be even better. Our result suggests that the fault size of the 2016 Off-Fukushima earthquake was about half, different from the scaling law of standard earthquakes. The present tsunami back-projection analysis can also estimate the feature of early tsunami propagations. In addition, the estimated image seems to be reliable even 30 min after the origin time, so the back-projection analysis will be useful in an early detection of the location and spatial extent of a tsunami source. In the present case, the number of available stations in the analysis was found to be affected by the diffraction of tsunami propagation caused by the refraction by a high velocity zone near the Japan Trench. In other words, the further the source is from the coast, the more stations to be analyzed are available. Since most tsunami-generating earthquakes occur near the subduction axis or its outer-rise region, the back-projection analysis should be effective for source estimation of the majority of tsunami-generating earthquakes