On August 2, 2007, an M_JMA 6.4 moderate earthquake occurred off the southwest coast of Sakhalin Island, far eastern Russia. In the city of Nevelsk, situated just in the aftershock region, two people were killed and more than 240 buildings were damaged (we call this event as the 2007 Nevelsk earthquake). We have operated a temporal seismological network in southern Sakhalin consisted in ten stations with short-period sensors and 16-bit recorders since 2003. This dense network well recorded this earthquake and the following aftershock sequence. Initial aftershock hypocenters were calculated using a 1-D P-wave velocity structure that was estimated from 746 P-wave arrival times, including data from the aftershocks. To obtain a more precise aftershock distribution, we applied a double-difference hypocenter determination method. The well-determined aftershock hypocenters shows the following features; (1) in the northern and central parts of aftershock region, epicenters were distributed in the sea along the coast with NE-SW direction, (2) while in the southern part of aftershock region trend of epicenters changed toward land with NNW-SSE direction. This southern most part intersects the city of Nevelsk where severe damage occurred. On August 17, 2006, the 2006 Gomozavodskoe earthquake (M_JMA 5.9) occurred near the southeastward aftershock region of the 2007 Nevelsk earthquake. The 2006 earthquake sequence was occurred deeper area as compared to the 2007 event. We examine the change in Coulomb failure function produced by the 2006 event and conclude that the 2006 event has less influence on occurrence of the 2007 event.

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The M_jma 7.2 (M_w 6.9) Iwate-Miyagi Nairiku earthquake struck mountainous regions east of volcanic front in northern Honshu. To understand the relation between coseismic surface deformation and the seismogenic faulting and to have lessons in the long-term earthquake forecasting, we have performed urgent field investigations immediately after the main shock, while fragile structure and surface geomorphic features were fresh. More than 13 fault-rupture observations suggest that the estimated total length of the tectonic ground breakages reaches ∼20 km even though their locations are spotty rather than continuous along the entire trend. Contractional features such as thrust fault exposures, flexure, tilting, and buckling deformations predominate on the rupture zone, which is consistent with the reverse faulting under the WNW-ESE compressional stress field in northern Honshu. Such shortening features as well as vertical displacements were visible on cultural features such as concrete, asphalt paved roads, sidewalks, guardrails, drainage ditches, and rice paddies. Amounts of vertical offset and horizontal shortening measured using such cultural piercing points are mostly smaller than 50 cm (∼1 m of net slip). Fractures with such small slip, in turn, would not have been noticeable and may reflect the spotty distribution of the ruptures. Meanwhile, near the southern end of the rupture zone, fault structure and slip sense become complex and measured offsets are exceptionally large. We found a E-W-striking ∼1-km-long continuous rupture involved with 4-to-8-m dextral and 2-to-4-m vertical offsets of a paved road, trails, and rills near a massive giant landslide at the northern rim of the Aratozawa dam reservoir. Terrestrial LiDAR (Light Detection and Ranging) measurements together with our field observations reveal typical features of strike-slip faulting such as mole tracks, fissures, pressure ridges, bulges, and shutter ridges as well as the offset rills and ridges. Detail mapping of the ruptures suggests that this strike-slip dominant fault is a lateral ramp or tear fault that connects two distinct NNE-trending thrust faults, although we cannot rule out the possibility of a large mass movement due to gravitational force to induce such large displacements without suffcient geodetic and geologic data. The mapped zone of the ruptures approximately locates along the central part of the surface projection of a ∼40-km-long west-dipping source fault and associated aftershock zone. It also well corresponds to an asperity estimated from seismic and geodetic inversions, particularly southern end of the ground breakage zone. However, from the viewpoint of the long-term predictability, the surface fracturing occurred where none of active faults was previously mapped. Although several active geomorphic strands are likely to have reoccupied with the 2008 event, they are more spotty than the 2008 ruptures. Thus it would not have allowed us to properly evaluate size of the shock and entire extension of the rupture.

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We studied the effect of body waves on short period vertical ground motion based on both a numerical simulation and field array observations. The vertical components generated by the vertical point force within a close distance are targeted throughout the study. Firstly, we carried out a numerical simulation and confirmed that the effect of body waves on the full-wave field was generally limited to a close distance from the source with frequency dependence. In a frequency range lower than 5 Hz and at around the 1st predominant frequency (5.8 Hz), the effect appears beyond a far distance; in a range of about 6 to 10 Hz, it still remains up to 10 m or more; in a range higher than 10 Hz, it almost vanishes. Secondly, we analyzed the observed array data excited by a 30-kg sandbag falling 10, 20, and 40 m from the array center. The array configuration was a circle, 1 m in radius, consisting of seven sensors placed in the center and around the circumference at equal intervals. To obtain a better understanding of the results of the array analyses, simulated array data, based on the Green's functions for the full-wave field and for the Rayleigh wave, was produced and analyzed in the same manner as the observed field array data. The phase velocity results were calculated with three different methods, namely, the spatial autocorrelation (SPAC) method, a method based on the spatial 1st Fourier coeffcients in the azimuthal direction with respect to the cross spectral amplitude between the center and one of circumferential stations, and a technique which combined these two methods. In the case of an offset of 10 m (Case 1), the detected phase velocities from the observed data and the simulated data on the full-wave field were much lower than the theoretical phase velocity or that from the Rayleigh wave array data in a frequency range of 5 to 10 Hz, suggesting the strong effect of body waves. In the case of an offset of 20 m (Case 2), the estimated phase velocities from the observed data and the two other simulated array data coincided with the theoretical phase velocity, suggesting the weak effect of body waves. We conclude that in order to properly estimate an underground structure from the array measurement data at a close distance from the source, the analytical technique based on the full-wave Green's function should be applied. Taking a look at the choice of analytical methods for the array data, it can be pointed out that for our interest the two methods which use the spatial 1st Fourier coeffcients produce higher accuracy compared with the SPAC method in a relatively low frequency range. In addition, the phase velocity results were estimated from the CCA (centerless circular array) method by Cho et al. (2004). The CCA method provided similar accuracy compared with the above two methods based on the spatial 1st Fourier coeffcients. The CCA method seems to be applicable to highly directional wave fields located nearby the source.

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We found eight M 5.1 characteristic earthquakes regularly occurring since 1966 on the plate boundary between the Eurasian plate and the Philippine Sea plate near Miyakojima Island, the Ryukyu Arc, Japan. The quake recurrence interval was 5.89 years in average, and the standard deviation was only 0.73 years. The accumulating stress presumably ruptured the same asperity enclosed by the creeping zone repeatedly. Also, we found three other groups of small repeating earthquakes of M 4, which occurred close to the hypocenters of the M 5 events. Those groups also occurred regularly and we can consider them to be ‘characteristic’ earthquake sequences. Now, we called those groups A, B, and C. It is not clear whether groups A and B had an intrinsic recurrence interval or if they influenced each other. However, two events of group C occurred within one week after the M 5 quakes, indicating that the M 5 events triggered the group C events whose asperity had suffcient strain energy. No earthquake exceeding M 7, which could change the recurrence intervals, has been observed on the subduction zone around the Ryukyu Islands. Therefore, there should be numerous characteristic earthquake sequences in other areas of the Ryukyu district. We expect that the next M 5 earthquake at 50 km depth on the plate boundary near Miyakojima Island will occur between September 2012 and July 2014 with 70% probability, using the small-sample theory with a log-normal distribution model. Moreover, the M 5 event may be accompanied by an M 4 quake that could rupture the asperity of group C within one week.

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