Microseismic Activity and Spatial Distribution of Coda-Q in the Westernmost Part of the North Anatolian Fault Zone, Turkey

We carried out a short-term observation of microearthquakes in the seismic gap region in the westernmost part of the North Anatolian fault zone. In this region, the North Anatolian fault zone branches into two fault zones: the Izmit-Sapanca fault zone on the north, and the Iznik-Mekece fault on the south. The results of our analyses are as follows: (1) Microseismic activity was much higher along the Izmit-Sapanca fault zone than along the Iznik-Mekece fault. (2) A fault plane solution of strike-slip type was obtained for a microearthquake along the Iznik-Mekece fault. On the other hand, the solutions of both strik-slip and normal-faulting types were determined for microearthquakes along the Izmit-Sapanca fault zone. (3) Relatively lower coda-Q values were estimated around the Izmit-Sapanca and Iznik-Mekece fault zones, compared with the values in the region north of the fault zones. This attenuation property is considered to be closely related to the inhomogeneity of the crust, probably due to the earthquake faulting along the North Anatolian fault zone. Seen from the above result (2), we may estimate the faulting type of the future large earthquake in the seismic gap region as follows: If it occurs along the Iznik-Mekece fault, it will have a strikeslip mechanism; but if it occurs along the Izmit-Sapanca fault zone, its faulting type may have two possibilities, strike-slip or normal-slip

Three-dimensional velocity structure of the upper crust in the Hida region, central Honshu, Japan, and its relation to local seismicity, Quaternary active volcanoes and faults

The three-dimensional (3-D) velocity structure of the upper crust in the central part of the Hida region, central Honshu, Japan, has been investigated by simultaneous inversion of travel time data for velocity and hypocentral parameters using Thurber's method (1983). The data used for this purpose were 2, 231 P-wave arrival times from 204 local earthquakes observed at 16 high-sensitivity seismograph stations. The iterative damped least-squares inversion used here provided reliable results with the diagonal elements of resolution matrix well exceeding 0.90 and standard errors less than 3% for the central area. The central part of the Hida region covering an extremely low seismicity area has high velocities probably extending down to a mid-crust. The high-velocity area composed of hard metamorphic and granitic complex extends from the western flank of the Hida mountains to the western section of the seismically active Atotsugawa-Ushikubi faults. The axial part of the Hida mountains and its southwestward area, on the other hand, is covered by remarkable low-velocities. The low velocities may be associated with a high thermal state due to active volcanism beneath the mountains involving volcanoes Mts. Tateyama, Yake, Norikura, and Ontake. The southwestern part of the Hida region is also partially covered by low-velocities beneath the Ryohaku mountains, which may also be related to volcanism of Mt. Hakusan. The obtained 3-D velocity structure appears to be qualitatively consistent with the Bouguer gravity anomalies and also with the large-scale 3-D structure so far obtained

Shallow Crustal Structure Beneath Taal Volcano, Philippines, Revealed by the 1993 Seismic Explosion Survey

We carried out an seismic explosion survey at Taal volcano in March, 1993. The explosions were done at the west of the volcanic island and digital event recorders were deployed along a fan-shooting survey line at the east of the volcanic island and also along a short straight survey line across the island. Small-aperture arrays were also operated at two sites west of the volcanic island. We found that P waves that traveled just beneath the main crater strongly attenuated and showed later arrivals. This result suggests the existence of a low-velocity and low-Q region around the main crater at a depth of about 1.5km, although the location and extent cannot be determined exactly. P wave velocity of this abnormal region may be eatimated to be lower than that of the surroundings by much greater than about 25%, if we assume the region is restricted just beneath the main crater. We applied the NMO correction to the later part of seismograms and found a reflector around the east of the main crater at a depth of about 6km, which may suggest the top surface of the magma reservoir

Anomalous depth dependency of the stress field in the 2007 Noto Hanto, Japan, earthquake: Potential involvement of a deep fluid reservoir

We have elucidated depth variations in the stress field associated with the 2007 Noto Hanto, Japan, earthquake by stress tensor inversion using high-quality aftershock data obtained by a dense seismic network. Aftershocks that occurred above 4 km in depth indicated a strike-slip stress regime. By contrast, aftershocks in deeper parts indicated a thrust faulting stress regime. This depth variation in the stress regime correlates well with that in the slip direction derived from a finite source model using geodetic data. Furthermore, the maximum principal stress (σ1) axis was stably oriented approximately W20°N down to the depth of the mainshock hypocenter, largely in agreement with the regional stress field, but, below that depth, the σ1 axis had no definite orientation, indicating horizontally isotropic stress. One likely cause of these drastic changes in the stress regime with depth is the buoyant force of a fluid reservoir localized beneath the seismogenic zone