Motoo Ukawa

Crustal deformation caused by magma migration in the northern Izu Islands, Japan

Intense crustal activity including earthquake swarms, eruptions, and a caldera formation in the northern Izu Islands started on June 26, 2000, accompanied with large crustal deformation. Permanent GPS data reveals the spatial pattern and time evolution of ground deformation. The observations reveal shrinking and subsidence of Miyakejima and extension between Kouzushima and Niijima. We constructed a source model to explain the observed displacements during the period between June 26 and the end of August. The model consists of a deflation source (0.12km³) beneath Miyakejima, tensile faults (1.04km³) located between Miyakejima and Kouzushima, and several shear faults. Mass balance considerations suggest that a large amount of magma migrated 30km from Miyakejima toward Kouzushima.

EARTHQUAKE SWARMS AND DIKE INTRUSIONS OFF THE EAST COAST OF IZU PENINSULA,CENTRAL JAPAN

Abstract Relationships between earthquake swarms and dike intrusions are investigated in the monogenetic volcano region off the east coast of the Izu Peninsula, central Japan. Hypocentral distribution of an earthquake swarm associated with the 1989 Ito-oki submarine eruption and those of eight major earthquake swarms for 7 years before the eruption in the same region are precisely relocated. Comparison of the hypocentral distribution with a dike model obtained from crustal movement measurements by Okada and Yamamoto (1991) ensures that hypocentral areas of the earthquake swarms can be used as reliable indicators of intruded dikes. The epicentral area of each earthquake swarm shows an elliptic shape with a 3–5-km-long axis and a 2–4-km-short axis, the long axis trending from a NW-SE to WNW-ESE direction. The total hypocentral distribution of the nine swarms forms a seismic zone of about 20 km long and about 5 km wide, with each of the swarm areas overlapping. Focal migrations from deep (∼ 10 km) to shallow (∼ 3 km) depths are found in the two largest events in 1988 and 1989, indicating that dikes propagated from deep to shallow areas in the upper crust. These features of the hypocentral distributions suggest that the seismic swarms were induced by dike intrusions which ascended from a deep-seated magma reservoir with dike-like geometry located beneath the seismic zone. Solidification time of a dike with thickness comparable to that of the dike model of Okada and Yamamoto (1.45 m at the bottom of the dike) is several days, indicating that the dikes inducing the earthquake swarms solidified almost during the swarm period. The physical mechanisms of the dike-induced earthquake swarms are investigated for the Izu region. The focal depth migration and fault type of the earthquake swarm suggest that stress change in the region surrounding the dike due to dike emplacement is the most important factor for the cause of the earthquakes.

The 2000 Miyakejima eruption: Crustal deformation and earthquakes observed by the NIED Miyakejima observation network

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.

Collision and fan‐shaped compressional stress pattern in the Izu Block at the northern edge of the Philippine Sea Plate

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.

The South Fossa, Magna, Japan, revealed by high-resolution P-and S-wave travel time tomography

Abstract Detailed tomographic images of the collision zone between the northern edge of the Philippine Sea and the Eurasian plates reveal a high correlation between tectonic features inferred from seismicity and P- and S-wave velocity structures. The linear tomographic inversion covered a 150 × 150 × 60 km region in the South Fossa Magna centered on the northern Izu Peninsula. Thirty-three three-component stations were located in the target region and 3823 high-quality earthquakes were selected from the catalogues of the NRIESDP, giving rise to 53,593 P- and 50,059 S-wave phase arrivals used in the inversion. The model was parameterized by 60 × 60 × 10 rectilinear blocks, measuring 2.5 km per side horizontally and 5–10 km varying thicknesses in depth. Three-dimensional perturbations from the one-dimensional, NRIESDP, layered model were derived by minimizing the squared misfit of the travel-time residuals. Regularization was employed to reduce the effects of noisy data by constraining the two-dimensional Laplacian of the model, within horizontal layers, to be small. The large sparse matrix was solved using the conjugate gradient algorithm LSQR. Reduction of misfit was 50% for the P-wave inversion and 57% for the S-wave inversion. There is a high correlation between the P- and S-wave inversions. Between 5–15 km depth both 3D images indicate low velocity anomalies trending northward in the west along the Fujikawa, from Suruga Bay to northwest of Mt. Fuji. North of Izu Peninsula there is a broad high-velocity zone. These results correlate with Bouguer gravity anomalies previously observed. Starting at 5 km depth, a high-velocity, northeast-dipping anomaly beneath Sagami Bay is inferred to be the shallow subducting PHS plate. To the north, a high-velocity zone in the vicinity of the East Yamanashi seismic swarm (15–32 km depth) is clearly observed on the P-wave inversion and to a lesser extent on the S-wave inversion. Because of the high seismicity in this region, we speculate this is the zone where the PHS plate is grazing the EUR plate to the northwest, and the high-velocity anomaly represents a large-scale asperity. To the west, below thMt. Fuji area, a broad low velocity is observed between 15 and 32 km depth. We interpret the deep, low-velocity structure, which exhibits little seismicity, to be either the subduction of the volcanic arc, or a region of elevated temperature associated with hot rising magma. Near the swarm area on the eastern edge of Izu Peninsula, low-velocity anomalies are observed near the surface on both P- and S-wave inversions. Along the eastern part of Izu Peninsula a prominent low-velocity anomaly is observed between 10 and 20 km on the S-wave image but is less pronounced on the P-wave inversion.

Poisson's ratios of the upper and lower crust and the sub-Moho mantle beneath central Honshu, Japan

Abstract Poissons ratios of the upper and lower crust and the sub-Moho mantle beneath central Honshu, Japan, are investigated using three independent methods that are based on S to P ratios of apparent velocities, the Wadati diagrams and an inversion of P and S arrivals. Shallow earthquakes at distances of 200—500 km from the Nagoya University Telemeter Network are used for the apparent velocity ratio method. Crustal and subcrustal earth-quakes under the network are used for the other two methods. The network consists of wide-band seismometers with three components which are particularly suitable for detecting S waves. The three different methods give a consistent result for Poissons ratio σ, that is, (1) σ = 0.23 ± 0.01 in the upper crust, (2) σ = 0.26−0.28 in both the lower crust and in the sub-Moho mantle. The result indicates a sharp contrast in σ between the upper and the lower crust rather than at the Moho. The low σ in the upper crust can only be explained by the presence of a substantial amount of free quartz, indicating granitic rocks. A higher σ in the lower crust suggests that this portion is presumably less saturated in silica and may be even undersaturated, pointing to intermediate to mafic rocks. The sub-Moho σ is almost equal to the σ averaged over the entire upper mantle that has been estimated from the Wadati diagrams of deep shocks beneath Japan but is higher than those calculated from Pn and Sn velocities in oceanic and stable continental regions.

Precise hypocenter locations of midcrustal low-frequency earthquakes beneath Mt. Fuji, Japan

Midcrustal low-frequency earthquakes (MLFs) have been observed at seismic stations around Mt. Fuji, Japan. In September–December 2000 and April–May 2001, abnormally high numbers of MLFs occurred. We located hypocenters for the 80 MLFs during 1998–2003 by using the hypoDD earthquake location program (Waldhauser and Ellsworth, 2000). The MLF hypocenters define an ellipsoidal volume some 5 km in diameter ranging from 11 to 16 km in focal depth. This volume is centered 3 km northeast of the summit and its long axis is directed NW-SE. The direction of the axis coincides with the major axis of tectonic compression around Mt. Fuji. The center of the MLF epicenters gradually migrated upward and 2–3 km from southeast to northwest during 1998–2001. We interpret that the hypocentral migration of MLFs reflects magma movement associated with a NW-SE oriented dike beneath Mt. Fuji.

Rapid ground deformation of the Miyakejima volcano on 26-27 June 2000 detected by kinematic GPS analysis

A kinematic GPS analysis of data from the Miyakejima volcano captured a fast developing deformation event on 26–27 June 2000 in unprecedented spatial and temporal detail. Initial ground deformation toward east and upward was observed in the southeastern part of the volcano at 18:00 on 26 June 2000, almost simultaneous with earthquake swarms. Some time after 21:30 on 26 June 2000 the displacements at these sites turned from eastward to westward. Three hours later the displacement rates increased gradually at GPS sites in the western part of Miyakejima as the seismicity migrated and approached the west coast, and reached a climax with submarine eruption at 09:00 on 27 June 2000. A Genetic Algorithm was used to explore the parameter space and to find the best fitting source geometry. This analysis leads to an interpretation that the 18:00 26 June earthquake swarm was caused by a dike intrusion near the Oyama crater. Starting from 21:30 this dike deflated and a new dike intruded near the west coast. Following the propagation of this dike to the offshore, a spherical source began deflating in the southwest of Oyama crater.

Microearthquakes and tectonics in an active back-arc basin: the Lau Basin

Abstract An Ocean Bottom Seismograph (OBS) array was deployed for 20–22 days in late 1984 to investigate the precise locations of microearthquakes and their tectonic implications for active back-arc opening in the northern Lau Basin. Using P- and S-wave arrival times from four or more OBSs, the hypocenters of ∼ 300 shallow earthquakes were located with a high confidence level. The magnitudes of most OBS-located earthquakes were estimated to be less than four. In the northern half of the survey area, a narrow, linear zone of microearthquakes, trending NNW-SSE, has been identified. The northern part of the narrow seismic zone is within a central axial depression at the southern end of the Peggy Ridge. Further south, the trend of the seismic zone becomes more N-S. The narrow seismic zone seems to be composed of at least six seismic segments, offset by short aseismic zones. Most of the seismic segments trend NNW-SSE, suggesting a system of left-stepping en echelon spreading ridges, where the spreading ridge segment is seismically inactive and the transform fault is active. The spreading ridges appear to strike N-S or NNW-SSE, but the direction of the back-arc opening is considered to be NW-SE. No hypocenters were located with a high level of precision in the area south of latitude 18°S, except a small isolated zone of shallow earthquakes at the southeastern part of the survey area. We suggest that the shallow earthquakes in this isolated seismic zone were intraplate events in the Tonga platelet. This platelet is separated from the major Indo-Australian plate by the back-arc opening system in the Lau Basin.

Excitation mechanism of large-amplitude volcanic tremor associated with the 1989 Ito-oki submarine eruption, central Japan

Abstract Unusually large-amplitude volcanic tremor was observed in association with the 1989 Ito-oki submarine eruption off the east coast of Izu Peninsula, central Japan. The largest-amplitude tremor occurred on July 13, 1989, and was studied by using digital seismic data (National Research Institute for Earth Science and Disaster Prevention), in the distance range of 6–250 km. Spectral analysis shows that the seismograms have two kinds of predominant spectral peaks, low-frequency peaks around 1 Hz and middle-frequency peaks between 2 and 7 Hz. The relative amplitude ratios of these peaks show distinctive spatial variation depending primarily on the distances from the crater. Low-frequency waves are predominant at close stations (distances less than 50 km from the crater) similar to typical low-frequency volcanic tremor, while at more distant stations the middle-frequency waves are more prominent than low-frequency waves. Propagation velocity analysis for narrow-frequency bands on the basis of correlation of envelopes of seismograms reveals that the middlefrequency waves are compressional body waves and the excitation of shear wave was ineffective, suggesting an explosive or implosive source as the simplest source model. In contrast to the middle-frequency waves, our analysis showed no evidence for the low-frequency waves to have propagated with any body wave velocity. Consideration of the spatial pattern of both waves suggests that surface waves were trapped in sedimentary layers from the shallow source and that no lowfrequency oscillations dominated at the source.