The Slapdown Phase in High-acceleration Records of Large Earthquakes

Abstract

The 2008 Iwate-Miyagi Nairiku earthquake (M_w 6.9, M_(jma) 7.2) produced strong shaking throughout northern Honshu, Japan, with severe damage to buildings and extensive landslides. The shallow event occurred in southwestern Iwate Prefecture (39.03°N, 140.88°E, depth 8 km) on 13 June 2008 at 23:43:45 GMT (Japan Meteorological Agency 2008). This earthquake produced relatively high-frequency ground motions, which resulted in large values of peak ground acceleration (PGA). The surface accelerometer of the station IWTH25 of KiK-net, located 3 km southwest of the epicenter, produced one of the largest strong-motion values of PGA (4,278 cm/s^2 for the vector sum of the three components) ever recorded (http://www.kik.bosai.go.jp/kik/index_en.shtml). The new accelerometers installed in KiK-net last year have a recording range up to 4,000 cm/s^2, which made it possible to record such large ground motions near the source (http://www.kik.bosai.go.jp/kik/index_en.shtml). The sampling rate of the record of IWTH25 is 100 Hz (http://www.kik.bosai.go.jp/kik/index_en.shtml). The surface acceleration record at station IWTH25 shows an asymmetric amplification in the vertical components (Aoi et al. 2008). The upward vertical acceleration is much larger than the downward direction, although in the borehole record at a depth of 260 m at the same site, the upward and downward accelerations have symmetric amplitudes (Figure 1). On the other hand, the horizontal components do not show this asymmetric effect. This difference between the surface and borehole recordings for the vertical component implies a strong nonlinear amplification. In this paper, we will analyze these records and propose a mechanism to produce the large vertical accelerations. The predominance of large upward acceleration spikes is not unique to the Iwate-Miyagi Nairiku earthquake, so our proposed mechanism may be applicable to a number of large vertical acceleration records.