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.

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A M7.1 intra-slab earthquake occurred on May 26, 2003 at a depth of 68 km off-shore Miyagi Prefecture, northeastern Japan. The epicenter of this event was located about 80 km NNE from the epicenter of the 1978 offshore Miyagi earthquake with magnitude of 7.4, whose possible recurrence in near future is a source of concern. We relocated the 2003 M7.1 event and its aftershocks by the homogeneous station method. Aftershocks are distributed along a plane dipping steeply to the WNW, which coincides with one of the nodal planes of the moment tensor solution. They are distributed within both the subducting oceanic crust and the mantle of the slab. The mainshock is located near the center of the aftershock zone and probably occurred near the bottom of the oceanic crust. We also determined the slip distribution of the 2003 M7.1 event by a waveform inversion method based on the empirical Green's function. We inverted broad-band waveform data observed at stations with epicentral distance less than about 100 km. Results show that the spatial extent of the rupture area is almost consistent with that of the aftershock area. There exist two areas where large amount of slip occurred (asperities). One is located along the northeastern edge of the rupture area and the other is located in the southwestern portion. Aftershock activity is low within those asperities.

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A shallow M6.4 inland earthquake occurred on July 26, 2003 in the northern part of Miyagi Prefecture, northeastern Japan. We precisely located hypocenters of this event, its foreshocks and aftershocks by applying the double-difference method to data from temporal seismic stations and the seismic stations of Tohoku Univ., JMA and Hi-net. Aftershocks thus located are distributed in an area with about 15 km × 15 km and their depths range from 2 to 13 km. They are distributed along a curved plane dipping westward to north-westward with a dip of about 50 degrees. Its strike is about N-S in the northern part and about NNE-SSW in the southern part. The M5.6 foreshock, which occurred 7 hours before the main shock, is located near the center of the whole aftershock area at which the strike of the aftershock alignment changes abruptly. Aftershocks for the M5.6 foreshock are distributed in the southern part of the whole aftershock area. The main shock is located in the southern part of the aftershock area. It occurred near the edge of the area of aftershocks for the M5.6 foreshock. Aftershocks following the main shock are mainly distributed in the northern part of the aftershock area. The largest aftershock (M5.5) is located at the northern end of the whole aftershock area. After the occurrence of the main shock, few aftershocks occurred in the area of aftershocks for the M5.6 foreshock. We also determined fault plane solutions for the events that occurred during the sequence from the M5.6 foreshock to the largest (M5.5) aftershock. The spatial variation of focal mechanism is consistent with the curved geometry of the fault plane estimated from the aftershock distribution.

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A shallow M6.4 earthquake with a M5.6 foreshock and the M5.5 largest aftershock occurred in northern Miyagi Prefecture, northeastern Japan on July 26, 2003. The coseismic displacement was observed not only by GEONET (continuous GPS) but also by campaign GPS, leveling, and InSAR. We have plenty of displacement data by multiple geodetic measurements, which is unusually abundant as the M6-class earthquakes. The leveling route in the epicentral area was surveyed on the day just before the earthquake by chance, and that it was resurveyed just after the earthquake. RADARSAT SAR interferograms provide high resolution of surface displacements which reach 240 mm. We construct coseismic rectangular fault model by inversion of the multiple geodetic data. The number of observation points used in the inversion are 23, 17, 49 and 1601 for continuous GPS, campaign GPS, leveling, and InSAR, respectively. We found that two thrust-type fault segments whose strike differs by 50° were necessary to reproduce the observed deformation. Seismic moment for both segments is 1.8 × 10¹⁸ N·m (M_w 6.1), assuming rigidity of 30 GPa. Because rake angle of two segments is different by only 15°, azimuth of slip vectors is significantly different between two faults. Two segments having different orientation can explain the apparent disagreement of focal mechanisms estimated from initial phase of P wave and CMT waveform inversion.

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On September 26, 2003, a large earthquake with a magnitude of 8.0 occurred along the Kuril trench off Tokachi, Hokkaido, Japan. We investigated the source process by using teleseismic P- and SH-wave data. The main source parameters are as follows: the seismic moment 1.0×10²¹ Nm (Mw = 8.0); (strike, dip, rake) = (230°, 20°, 109°); the depth of initial break point 25 km; source duration 40 sec; and the maximum slip 5.8 m. We estimated the fault area to be 90 × 70 km², the average slip 2.6 m, and the stress drop 5.0 MPa. This earthquake was an interplate earthquake associated with the subduction of the Pacific plate. The rupture propagated northward from a shallow to a deep region. In this area, a great earthquake (magnitude 8.2) occurred in 1952. We also made limited inversion of nearfield records of the 1952 event and found that the 2003 asperity was also ruptured in 1952. Our result suggests that the 2003 Tokachi-oki earthquake was a recurrent event of the 1952 Tokachi-oki earthquake.

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A quasi-14 month fluctuation in the atmospheric excitation function for the Earth's wobble is discussed by using the re-analysis data of the European Center for Medium-range Weather Forecast for the 14 years between 1980 and 1993. The spectrum of the atmospheric wind excitation function shows a striking peak near the 14-month period. As a result, the atmospheric (wind plus pressure) excitation function shows exactly the same power as that of the geodetic excitation function inferred from the observed wobble at the Chandler wobble frequency (about 14 months), suggesting that the atmosphere excited the Chandler wobble between 1980 and 1993. The wind fluctuation comes mostly from the tropospheric wind.

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