Electromagnetic emissions observed in a series of rock fracture tests are described. Four kinds of rocks, basalt, gabbro, granite and quartzite were pressed by uniaxial compression to fracture, for all of which many signals were detected at two microwave bands (2GHz and 300MHz). These detected signals consist of intermittent pulses of a short duration. Comparing the microwave records and the observation with a high-speed digital video camera, we found that the pulse signals were generated after the decrease of the axial load, and even after the macroscopic fracture (deformation) was completed. This differs from the occurrence of lower frequency emissions (0.3-300kHz) monitored as well, which became active and was strongest during the load decrease. The occurrence of signals at the two microwave bands did not always coincide, but a signal at 300MHz often followed a signal at 2GHz with a short interval of 50-100ns. An additional detector at 22GHz picked up emissions only for quartzite, which occurred exclusively during the decrease of axial load.

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A flight of late Holocene marine terrace fringes the central area of Puget Sound, and records uplift over an extensive area above the Seattle fault zone. The E-W trending blind thrust fault zone is a source of major seismic hazards in the Seattle metropolitan area. Gravity and seismic reflection surveys indicate a south- dipping fault plane, but its exact location and timing of past activities were unknown. LiDAR topographic mapping of the Puget lowland revealed several fault scarps on the glacial landscape hidden under the dense forest. We observed the fault, offset on the Holocene marine terrace surface and measured the former shoreline height at 97 locations using LiDAR DEM to map terrace deformation patterns and their relation to the faults. Studied areas include 1) Alki Point, 2) the southern part of Bainbridge Island, and 3) the southeastern Kitsap Peninsula near Port Orchard and southwestern Bainbridge Island. The height of the former shorelines marked by the Holocene terrace changes from ca. 10.7 to 7.3m a. s. l in the west to 12.2 to 10.1m in the east of the Toe Jam Hill fault, and 10.6 to 7.8m in the west to 9.7 to 7.9m in the east of the Waterman Point fault. These changes indicate differential uplift of the terrace surfaces across the faults. There are two newly identified faults in this study. One is the Point Glover fault that is marked by a scarp in the LiDAR map and associated 2m offset of the terrace surface. The other is the South Beach Point fault inferred by the northward tilt of the terrace surface. Because these faults strike E-W, parallel to the main Seattle Fault on its south side, and have south-facing scarps and north-dipping fault planes, they are probably back-thrsuts to the main Seattle Fault. The width of the backthrust zone is at least 4km. The age of the terraces approximately coincides with the most recent faulting event on the surface fault (at least for Toe Jam Hill Fault, ca. 1000yr BP), thus the differential uplift probably occurred simultaneously with fault movement. Although the surface backthrust scarps are less than a few kilometers long and vertical offset is 2-3m, the total amount of uplift reaches about 12m. Subtracting the effect of the vertical displacement and the amount of northward tilting, the uplift of several meters still remains on the Seattle fault zone of over about 4km wide. This broad zone of uplift is not due to the slip on the subsidiary backthrusts, but probably due to the blind thrust of the main Seattle fault. We infer that at least some of the coastal deformation is caused by broad surface upwarping above the Seattle fault and that the upwarping occurred at ca. 1000yrs BP, associated with ruptures on at least three of the backthrusts. The uplift and faulting may represent the largest earthquake in the Puget Sound area during the late Holocene.

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Areal strain increased noticeably in the region around the northern boundary of the Izu Peninsula in September to December 2000 when a lot of low-frequency earthquakes occurred beneath Mt. Fuji. In the same time the seismic activity in eastern Yamanashi Prefecture became low. Since increase of the areal strain indicates reduction of the pushing force of the colliding Izu block, the decrease of seismicity in eastern Yamanashi Prefecture is easily understood. Further, because diminution of the tectonic stress beneath Mt. Fuji implies reduction of the confining pressure in the magma reservoir, we think it is probable that degassing took place in the magma to build up high pressure in the focal region near the chamber which caused the remarkable activity of the low-frequency earthquakes. We suggest the noticeable increase of the areal strain in late 2000 might be produced by a mechanical separation of the Izu block from the Philippine Sea plate or detachment of the crust of the Izu block as proposed by Seno (2005).

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The Mid Niigata prefecture Earthquake occurred on October 23, 2004. Suzuki et al. (2004) and Maruyama et al. (2005a, 2005b) found small surface ruptures at Obirou and Aoshima areas. Suzuki et al. (2004) also argued that the earthquake was caused by movements of the Obirou Fault and the northern part of the Muikamachi-Bonchi-Seien fault. The aim of this report is to examine active geological structures and landforms in epicentral region of the earthquake. We carried out aerialphoto interpretation, geological survey, and geomorphometry by airborne laser scanning. As a result of our survey, the Suwatoge Flexure was found to be an active structure in the region, together with Obirou Fault and the Muikamachi-Bonchi-Seien fault. The Suwatoge Flexure is recognized as the fold on Uonuma formation (Plio-Pleistocene) and uplifts terraces in west/northwest direction; this is generally correspond with the earthquake source direction. The amount of vertical displacement in each terrace is as follows; more than 70m on Higher terrace (H2 surface), about 40m on Middle terrace (M1 surface) and about 10m on Lower terrace (L1 surface). The mean rate of vertical displacement is estimated 0.3m/10³ year based on M1 Terrace surface covered by In-Kt Tephra [≥130, 000 year: Machida and Arai (2003)]. The Suwatoge Flexure is located at the eastern margin of the distributed area of aftershock epicenters, and its south end corresponds with that of the distribution of aftershock epicenters. The largest vertical movement of 71.5cm uplifts (GSI, 2004) as the co-seismic crustal deformation is located on the west side of the Suwatoge Flexure. Therefore, the formation of the Suwatoge Flexure were probably caused by activities of earthquake source faults.

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The 2004 Mid Niigata Prefecture earthquake (M_J6.8) that occurred on October 23, 2004 is the 2nd largest intra-plate earthquake after F-net, the broadband seismograph network of the National Research Institute for Earth Science and Disaster Prevention (NIED) was established with a dense and homogeneous distribution all over Japan. We determined moment tensor solutions of the main shock, aftershocks, and earthquakes occurred around mid Niigata prefecture from January 1997 to October 22, 2004, using a surface wave with an extended method of the NIED F-net routine processing. The horizontal distance to the station is rounded to the nearest interval of 1km, and the variance reduction approach is applied to a focal depth from 2km with an interval of 1km. We obtain the moment tensors of 117 events with M_J exceeding 2.8 and spatial distribution of these moment tensors. The focal mechanism of main shock, aftershocks, earthquakes before main shock, is mainly of the reverse fault type with P axes trending WNW-ESE. But the focal mechanism of 15 percent aftershocks is the strike-slip type. There are high dip angle reverse faults, in deep part, and low dip angle reverse faults in shallow part.

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The Mid Niigata Prefecture Earthquake (October 23, 2004, 17:56 (JST), M_w 6.6) that was preceded by a couple of foreshocks, produced a number of aftershocks including six events with a moment magnitude (M_w) of 5.5 or greater. We evaluated the relationship between seismic moment (M₀) and corner frequency (f_c), and identified spectral characteristics in the aftershock area. A spectral ratio method was employed to determine f_c of 57 smaller events (3.5≤M_w≤4.0). These events with similar focal mechanisms are located within 20km from the mainshock epicenter, and were recorded at four common stations. In the spectral ratio method, the mainshock spectrum is divided by the spectrum of a foreshock or an aftershock. By taking the averages of the spectral ratios calculated at four common stations, propagation path effects can be reduced. The obtained f_c-M₀ relation shows substantial scatter relative to a straight line of M₀∝f_c^-3. The scatter in the narrow range of M_w suggests the variation of radiated seismic energy E_R of small earthquakes, and deviation from an ω^-2 model based on the assumption of constant rupture velocity. The moment-rate spectra become more complex in the vicinity of f_c than those expected from a typical ω^-2 model. We also classified the aftershock area into five zones based on the estimated faults by a previous study, and investigated spectral ratio characteristics in the frequency band from 1 to 7Hz. In particular, the slopes of spectral ratios vary in the high frequency band, which may reflect the difference in relative quantity of the radiated energy among the zones. Our results suggest clues that could be useful in evaluating the seismic energy release in a complex manner during the aftershock sequence.

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The 2004 Mid Niigata Prefecture Earthquake (M_JMA 6.8) occurred on 23 October 2004. The mainshock was followed by four aftershocks with M_JMA≥6.0. This earthquake is located in the Niigata-Kobe Tectonic Zone in which large strain rates (>0.1ppm/y contraction) have been observed by GPS data. We deployed three temporary online seismic stations in the aftershock area. Combining data from the temporary stations and from permanent stations around the aftershock area, we determined aftershock locations, and estimated the structures and the stress change in and around the aftershock region. Based on these results, we suggested a generating process of the 2004 Mid Niigata Prefecture Earthquake supposing that a very weak region exists in the weak zone in the lower crust just beneath the seismogenic fault.

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