An improved method is introduced to analyze travel time data from a seismic array composed of stations at various elevations, and is applied to the data of volcanic earthquakes or tremors occurred in Sakurajima Volcano. By this method, arrival azimuths and apparent velocities of seismic waves, and wave velocities in the surface layer are estimated with their errors. Several events are observed with anomalously low wave velocity in the surface layer. This is probably attributed to apparent late arrival times at the station which is highest in altitude and is nearest to the Minami-dake Crater of the Volcano. From the analysis of explosion earthquakes, the existence of such low wave velocity is denied and P wave velocity in the surface layer is estimated as 500m/sec or over. Predominant frequencies of waves are 4 to 6 Hz for the events of which relatively dominant initial phases are recognized. 0n the other hand, so-called “B-type” earthquakes or continuous tremors have the frequency around 2.5 to 4 Hz.

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Reanalyses of shallow earthquakes in the Unzen volcanic area have proved that their focal mechanisms are well explained by mormal faulting, in contrast to the previously supposed strike-slip faulting. Tension axis is in the N-S direction. Predominace of active faults, striking in the E-W direction, and forming a graben-like topography, seems to support the present model. Geodetically, crustal deformation in this area is characterized by marked N-S extension and local subsidence in the graben zone. These evidences suggest the stress system of normal fault type (tension axis is almost horizontal in the N-S direction, and pressure axis is nearly vertical) in this volcanic area, which (1) accumulated the normal fault offsets in the late Quaternary, (2) controlled the crustal deformation in the last some decades of years, and (3) caused earthquakes cccasionally. The above-stated stress system would have been closely related to the volcanism of this area, too.

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As an attempt to demonstrate the regional difference in volcanism in terms of morphology and to find out a general rule of the developmental sequence of composite volcanoes, in this paper, volcano types and their population have been preliminarily studied, based on the data included in the List of the Wrorld Active Volcanoes (a draft edited by Katsui, 1971). Among total 830 volcanoes listed, 699 volcanoes are well described to study the volcano types. The 699 volcanoes are classified into 57 types according to their specific combination and developmental sequence of the simple volcanoes of which each volcano is composed. Each type is here called a volcano type and is expressed by the volcano-morphologic formula, which is deined as an arrangement of the abbreviations of simple volcanoes in order of their formation. The 57 volcano types are grouped into the following six volcano series according both to the type of the oldest simple volcano (usually the main cone) and to the existence of caldera: mixed (strato) volcano series [M--], mixed volcano with caldera series [MC--], shield volcano series [S--], shield volcano with caldera series [SC--], caldera volcano series [C--], and monogenetic volcano group series [G]. Global population percentages of the volcanoes included in each volcano series are 62%, 10%, 11%, 3%, 7%, and 6%, in the above order, respectively.

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Seismic observations of volcanic micro-tremors were carried with two small nets of three components seismograms, set temporarily on the east and west sides of the Sakura-jima-volcano, making up for seismic monitoring net arround the active crater. In the various type of occurrence of micro-tremors, the tremors of typical case, in which tremors occured succesively, immediately after the occurrence of shallow volcanic earthquake and followed by summit eruption, were mainly studied about the natures as seimic waves. Examining particle motion diagrams, phase velocities and Fourier spectra, we obtained the results as follows: (1) P and SV phases were identified from shallow earthquakes, and the origins were estimated beneath the summit crater of Minamidake. (2) Fourier spectra of micro-tremors showed regular peaks at some fundamental frequencies and at those of integer multiple of the fundamental ones. (3) Fundamental peak frequencies varied with different time of volcanic activity. (4) In the typical case, fundamental peak frequencies increased gradually from O.5 Hz to 1.3 Hz with short time between the occurrence of shallow earthquake and following summit emption. (5) Those observational facts suggest that dimension and/or physical boundary conditions of origins of micro-tremors can change easily. (6) Because of complexity of vibrational modes, neither wave type nor phase velocities of microtremors could be determined, and the location of origins were not obtained directly.

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