Shuichiro Hori

Temporal changes in seismic velocity of the crust around Iwate volcano, Japan, as inferred from analyses of repeated active seismic experiment data from 1998 to 2003

We have examined temporal changes in seismic velocity of the crust through repeated active seismic experiments at Iwate volcano, Japan, where a significant volcanic activity and an M6.1 earthquake were observed in 1998. We apply a cross spectrum moving window technique to seismic data recorded at eight stations for the six explosions detonated from1998 to 2003. The seismic velocity at the frequency range of 3-9 Hz decreased by about 1% during the three months including the occurrence of M6.1 earthquake. The seismic velocity gradually increased, and about one third of the decrease was recovered by 2002. Then, the seismic velocity decreased again in 2003. Spatio-temporal changes in the volumetric strains predicted from the M6.1 fault mechanism and the volcanic pressure sources are well correlated with the seismic velocity changes observed in 1998. However, the predicted stress fields are not completely matched with the observed velocity changes from 1998 to 2003. This inconsistency may be due to unknown regional tectonic stress field and/or localized stress fields induced by volcanic pressure sources. It should be noted that the observed velocity changes indicate frequency dependent characteristics although the mechanism is not yet understood.

Distinct S-wave reflectors (bright spots) detected beneath the Nagamachi-Rifu fault, NE Japan

Distinct phases reflected from mid-crustal reflectors (SxS and PxP phases) were observed in seismograms of aftershocks of 1998 M5.0 Sendai earthquake at nearby stations. We estimated the locations of the reflectors (bright spots) by using arrival time differences between these phases and direct waves. A clear reflector is located in the depth range of 15 to 21 km just beneath the fault plane of the M5.0 event. It dips toward the NNE direction with a dip angle of about 25°. Other reflectors are also located beneath the fault plane of the event. Internal structure of the S-wave reflector was estimated from spectral amplitude ratios of reflected SxS-wave to direct S-wave. Observed spectral ratios show that S-wave velocity in the reflector body is ∼1.1 km/s, and its thickness is about 50 m. This suggests that a thin reflector body (bright spot) partially filled with fluids exists in the lower crust beneath the focal area of the 1998 event.

P-wave Velocity Structure of the Crust and Its Relationship to the Occurrence of the 1999 İzmit, Turkey, Earthquake and Aftershocks

We determined hypocenters and focal mechanism solutions of aftershocks, and three-dimensional P -wave velocity structure in and around the focal area of the 1999 Izmit earthquake ( M w 7.4). Aftershocks form a 170-km-long narrow zone trending in an east-west direction along the northern branch of the North Anatolian Fault Zone (NAFZ). They are not homogeneously distributed in the whole aftershock area, but consist of several clusters. There are three significant clusters in the aftershock activity: (1) near the hypocenter of the mainshock, (2) near 29.2° E in the Marmara Sea, and (3) east of 30.4° E. Focal mechanism solutions of aftershocks have various types. But they are similar to each other within each cluster. The P -wave velocity structure obtained has a distinct low-velocity area to the west of the mainshock hypocenter. The mainshock rupture area estimated by Yagi and Kikuchi (2000) lies outside this low-velocity area. There exists a high-velocity anomaly to the east of 30.4° E. This high-velocity area lies below the aftershock cluster distributed to the east of 30.4° E. This high-velocity anomaly extends to the shallower depth of the southern branch (Iznik-Mekece fault) of NAFZ. The Anatolian earthquake sequence that had migrated westward for the past 60 yr did not propagate into this southern branch. This suggests the possibility that the Anatolian earthquake sequence progressed to the west exploiting an area that might break more easily.

Aftershock Activity of the 1999 İzmit, Turkey, Earthquake Revealed from Microearthquake Observations

An accurate aftershock distribution of the 1999 Izmit, Turkey, earthquake was obtained by using the data from a local seismic network, IZINET, and 10 temporary seismic stations. More than 2000 aftershocks were relocated for the period of about 2 months following the mainshock. From this aftershock distribution we obtained several pieces of information on the characteristics of the mainshock. First, the mainshock initiated fault rupture from a place adjacent to an active swarm area where many microearthquakes had been occurring for more than 20 yr prior to the mainshock. Second, the aftershock region extended in the east-west direction along the North Anatolian Fault Zone (NAFZ). This confirms that the mainshock was caused by a slip on the NAFZ. Third, the western end of the rupture caused by the mainshock is likely to have reached up to about 29.2° E in the Izmit Bay, and hence the total length of the fault rupture caused by the mainshock amounts to about 150 km, as long as the estimate of the fault rupture length is based on the aftershock distribution. This information is important for the discussion on the possibility of future large earthquakes in the west of the source region of the Izmit earthquake. We also found a clear tendency that aftershocks occur in clusters, which implies strong heterogeneity in both the rupture process and the medium along the fault zone. Manuscript received 7 October 2000.

Hypocenter and focal mechanism distributions of aftershocks of July 26 2003 M6.4 northern Miyagi, NE Japan, earthquake revealed by temporary seismic observation

We conducted a temporary seismic observation just after the occurrence of July 26, 2003, M6.4 northern Miyagi earthquake, in order to precisely locate aftershock hypocenters. Thirteen portable data-logger stations and one satellite communication telemetry station were installed in and around the focal area of the M6.4 event. Hypocenters of aftershocks were located by using data observed at those temporary stations and nearby permanent stations of Tohoku University, National Research Institute for Earth Science and Disaster Prevention (NIED) and Japan Meteorological Agency (JMA). Obtained aftershock distribution clearly delineates the fault plane of this M6.4 event in the depth range of 3–12 km. The fault plane dips westward at an angle of ~50 degree in the northern part of the aftershock area and northwestward at ~40 degree in the southern part. Data observed at dense temporary stations just above the focal area and nearby permanent stations allowed us to determine focal mechanisms of many aftershocks. The results show that focal mechanism of reverse fault type is predominant in this aftershock sequence. Directions of P-axes, however, varies mainly with locations of hypocenters, and are classified into three groups. Aftershocks with P-axis of NW-SE direction occurred mainly in the southern part of the aftershock area where the M5.6 foreshock and the main shock ruptures were initiated. Many aftershocks with P-axis of east-west direction took place in the central part of the aftershock area where large amount of fault slips by the main shock were estimated from waveform inversions. Many aftershocks in the northernmost part of the aftershock area have focal mechanisms with P-axis of NE-SW direction, similar to that of the M5.5 largest aftershock. A few aftershocks with normal fault type occurred close to convex regions of the main shock fault plane or outside of it.