Kiyoshi Yomogida

SH-wave Green tensor for homogeneous transversely isotropic media by higher-order approximations in asymptotic ray theory

Tensor equations of the ray theory for homogeneous anisotropic elastic media are presented. For a point source, an explicit solution of the transport equation is obtained, thus the additional as well as principal components of the ray amplitudes for higher-order ray approximations are expressed only by differential operators of lower-order terms. Possibility of analytical calculation of higher-order approximations is exemplified for SH waves in a transversely isotropic medium. The ray series of the SH-wave Green tensor for the transversely isotropic medium involves only two non-zero terms and the complete ray solution coincides with an exact solution obtained by other complicated procedures.

Spatial and temporal variations of volcanic earthquakes at Sakurajima Volcano, Japan

Abstract We conducted broadband seismic observations at three sites of the Sakurajima Volcano, Japan, from December 1992 to March 1993 in order to clarify the spatial and temporal variations of spectral properties of volcanic earthquakes: B-type earthquakes and volcanic tremor episodes. We used three STS-2 seismometers recording in the frequency range from 0.03 to 6 Hz. Major spectral peaks of both B-type earthquakes and volcanic tremor episodes are located in the frequency range of 1.1–1.3, 2.3–2.5 and 3.4–3.6 Hz. From the similarities of temporal variations in spectra among all the stations and of body-wave propagation in an early part of seismograms, spectral peaks of B-type earthquakes and volcanic tremor episodes mainly reflect source characteristics rather than site and path effects. Temporal variations of the dominant peaks are classified into three types for B-type earthquakes and into four types for volcanic tremor episodes. Two types of B-type earthquakes are similar to two of volcanic tremor episodes in terms of temporal variations of spectral peaks and of polarization characteristics with the only difference in amounts of energy. The similarities in the temporal variation of spectral properties indicate that B-type earthquakes and volcanic tremor episodes share common source mechanisms with different energy magnitudes. Since volcanic tremor episodes tend to take place prior to the swarms of B-type earthquakes and explosion earthquakes associated with any summit eruptions, we conclude that volcanic tremor episodes occur at the early stage of low-energy radiation, followed by B-type earthquakes as higher energy is radiated.

Origin of short-period signals following P-diffracted waves: A case study of the 1994 Bolivian deep earthquake

Abstract Seismograms of the June 9, 1994, Bolivian deep earthquake recorded at epicentral distances from 100° to 122° show a train of signals with predominant frequencies between 1 and 2 Hz after the arrivals of short-period diffracted P-waves (P diff ). We investigate the origin of these signals following P diff by analyzing a total of 20 records from the IRIS broad-band network and the short-period network of New Zealand. The arrivals of late signals continue for over 100 s, that is two times longer than the estimated source duration of this event. Subsequent aftershocks, which cause the following signals, are not expected from the long-period records. These results indicate that the long continuation of short-period signals is not due to the source complexities. The signals following P diff have small incident angles, and their spectra show peaks at about the same frequencies. These characteristics of the following signals exclude the possibility that their origin is shallow structure such as the heterogeneities beneath the stations or upper mantle. P diff propagates a long distance within the heterogeneous region near the core-mantle boundary. We conclude that the short-period signals following the main P diff are scattered waves caused by small-scale heterogeneities near the core-mantle boundary.