Abstract.Intense crustal activity including earthquake In this study, we show the crustal deformation observed at the permanent GPS sites and construct a model based on the observed displacements. The dates and times in this article are based on Universal time(UT).

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Subcrustal earthquakes occur in a slab-like zone beneath SW Japan in association with the subduction of the Philippine Sea plate. They generate a distinct pair of P and S later phases with anomalously low apparent velocities of about 7 and 4 km s-', which cannot be explained by a laterally homogeneous velocity structure. These later phases are observed for particular sourcereceiver geometries: the sources at depths shallower than 50-60 km and the receivers beneath which the subcrustal seismic zone comes in contact with the bottom of the continental crust. We examined the nature of these later phases using two methods. One is to determine two types of apparent velocities, an 'event-common' apparent velocity and a 'station-common' apparent velocity, which may be different from one another for a laterally inhomogeneous velocity structure. The other is to trace seismic rays numerically for a laterally heterogeneous velocity model. An extensive analysis using these two methods leads to a conclusion that the slab-like seismic zone constitutes a low-velocity channel in the uppermost mantle with P and S velocities comparable to those of the lower continental crust beneath SW Japan. The waves trapped within the low-velocity channel escape from it through its contact with the continental Moho and are observed as a distinct pair of later phases. The head waves guided by the bottom boundary of the low-velocity channel are observed as weak arrivals of initial phases. The thickness (

After a quiescence of 12 years, the Izu‐Ooshima volcano, located about 30 km east of the Izu‐Peninsula in central Japan, erupted on November 15, 1986. On August 27, 1985, about 1 year before the eruption, a peculiar earthquake of magnitude 2.7 took place near the Moho boundary just beneath the Izu‐Ooshima volcano, where seismic activity is otherwise absent. Analysis of digital data obtained by the telemetered seismic network of the National Research Center for Disaster Prevention reveals the following distinctive features of the seismic waves: (1) The main part of the wave train is composed of S waves, and a small P phase precedes it. (2) The spectral density has a sharp peak at 1.0 Hz, which is independent of the epicentral distance, azimuth, and time, indicating a monochromatic spectrum at the source. (3) Ground motion for the initial part of the S wave is polarized in the N‐S direction at almost all the stations. The amplitude ratios of S to P waves and the polarization pattern of S waves suggest that a traction force caused by magma flow is a more probable source than a double‐couple mechanism, free oscillation of the magma reservoir or opening of a tensile crack. We propose a traction‐force model for this earthquake, in which magma flow through a conduit produces unidirectional viscous force on the conduit walls. The observations imply a N‐S force direction and a maximum force of about 2.8 × 1010 N. The pressure difference between the ends of the conduit is estimated as 6.1 × 107 Pa (610 bar), under the assumption of conduit dimensions of 2 km × 1 km. The monochromatic nature of the source spectrum is difficult to account for by the free oscillation of a magma reservoir. We tentatively suggest the intermittent opening of a barrier in the conduit as a plausible mechanism of the periodic oscillation.

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The Miyakejima observation network had been constructed by the National Research Institute for Earth Science and Disaster Prevention mainly until early 1999. This observation network has provided the crustal deformation data by tiltmeters and GPS and the seismic data by short-period and broadband seismometers in association with the 2000 Miyakejima eruption. The subsurface magma movement at the first stage of the present activity, during the period from June 26 to 27, was successfully detected mainly by the tilt measurements. The tilt change observed at five stations indicates the migration of magmas from the eastern part of Miyakejima to the western part. The most distinctive phenomenon appearing after the first stage is tilt steps, which started on July 8 with the first eruption from the summit crater. Each tilt step indicates an abrupt uplift of the summit area. These tilt steps continued until the eruption of August 18, which is the largest eruption up to early September, 2000. 45 tilt steps in total were observed in this period. The seismic data show a variety of seismograms including VT (volcano-tectonic) earthquakes, LF (low frequency) earthquakes and volcanic tremor. At the time of the tilt steps, very long period events with predominant periods of about 100 s were detected by the broadband seismometers. As the activity has still continued, this report summarizes the observation during June, July, and August, 2000.

SUMMARY The 2000 eruptive activity of Miyakejima island began with an earthquake swarm and crustal deformation that was clearly observed by continuous observations of ground tilt and GPS on Miyakejima. Based on the crustal deformation data, we estimated the magma migration process at the initial stage (18:30 LT on 2000 June 26—06:00 LT on 2000 June 27) of the activity. The activity in 2000 has been characterized by a collapse‐caldera formation and no fissure eruption on the flank of the island, which is markedly different from the style of the recent eruptions in 1940, 1962 and 1983. We constructed a source model that approximately explains the crustal deformation data. The model consists of four dykes, of which three are intruding dykes and the other is a contracting dyke. We can infer the magma migration process at the initial stage from the model as follows. At 18:30 LT on June 26, the magma began to ascend from a dyke‐shaped magma chamber and intrude at the southwestern flank of Miyakejima. In the intrusion, the total volume of intruded magma was ∼4 × 106 m3. At around 21:00 LT, a relatively large volume of magma began to intrude as a dyke beneath the west coast of Miyakejima. The dyke then propagated laterally towards the northwest. The amount of magma intruded from 21:00 LT to 01:00 LT was ∼40 × 106 m3. This large intrusion caused a discharge of magma from the magma chamber. The northwestward propagation of the dyke and the contraction of the chamber continued thereafter. The discharge of magma from the chamber beneath Miyakejima probably starved the first intrusion that had been ascending towards the southwestern flank of the island, resulting in the collapse‐caldera formation after the initial stage. The style of the 2000 eruptive activity relative to recent activity has primarily been changed by the subsurface discharge of magma towards the northwest.

The Izu block in central Japan, which is surrounded by the Nankai‐Suruga trough and the Sagami trough, is considered to be colliding with the Eurasian (EUR) plate at the northern edge of the Philippine Sea (PHS) plate. The stress state in the Izu block is investigated from focal mechanism data mainly obtained by the seismic network of the National Research Institute for Earth Science and Disaster Prevention. The predominant focal mechanisms in the central to eastern part of the Izu block indicate strike‐slip faulting with P axes oriented N–S to NW–SE and T axes oriented E–W to NE–SW, whereas focal mechanisms in the western part are strike‐slip and reverse faulting with P axes oriented N–S to NE–SW, indicating that the compressional axes remain horizontal throughout the region. The compressional stress trajectories estimated from the P axes show a fan‐shaped pattern, radiating from the northern edge of the Izu block where the collision is considered to be occurring. The fan‐shaped pattern of compressional stress trajectories is interpreted to be the result of the collision of the Izu block with EUR plate. Plane stress analysis by the finite element method supports this conclusion. The stable distribution of the T axis directions in the central to eastern part is basically interpreted as resulting from the slab pull force by the subducting PHS plate along the Sagami trough. Furthermore this model can explain the seismic quiescence in the northern part of the Suruga Bay region.