Nobuyuki Morikawa

Ground motion and rupture process of the 2004 Mid Niigata Prefecture earthquake obtained from strong motion data of K-NET and KiK-net

The 2004 Mid Niigata Prefecture earthquake (37.289°N, 138.870°E, 13.1 km, MJMA 6.8; JMA), also known as the 2004 Niigata Prefecture Chuetsu earthquake, was a thrust type earthquake that occurred on October 23, 2004 at 17:56 (JST). Strong ground motions of PGA 800-1700 cm/s2 and PGV 60-130 cm/s were observed at stations located immediately above the source region. We deduced the rupture process of this earthquake with a multi-time-window linear waveform inversion procedure. We used near-fault strong ground motion data observed at nine K-NET and KiK-net stations within 50 km from the epicenter. In order to obtain appropriate Green’s functions for the waveform inversion, we constructed two velocity structure models for stations on the hanging wall and one structure model for stations on the footwall. The estimated total slip distribution contains three asperities: (a) around the hypocenter, (b) in the upper-middle section of the fault plane, and (c) southwest of the hypocenter. The maximum slip is 3.8 m at the hypocenter and the total seismic moment is 1.2 × 1019 Nm, which corresponds to Mw=6.7. The moment rate functions in asperities (a) and (c) have a short rise time, while those in asperity (b) have a longer rise time.

Three‐dimensional finite difference simulation of long‐period ground motions for the 2003 Tokachi‐oki, Japan, earthquake

[1]xa0To explore the generation and propagation mechanisms of long-period ground motions, we used a 3-D finite difference method to simulate wave propagation during the 2003 Mw 8.3 Tokachi-oki, Japan, earthquake. A large-scale computation with an inverted slip model and a geological- and geophysical-based velocity structure model successfully reproduced the overall characteristics of substantial observed velocity seismograms for a period range of 3.3 to 25 s. Significant long-period ground motions of extended duration were observed in the Yufutsu basin, where oil tanks were damaged at Tomakomai because of sloshing associated with the ground motions. Our simulation revealed that the combination of the large and shallow earthquake and the deep extent of the sedimentary basin generated long-period ground motions with a duration of several hundred seconds. The Yufutsu basin contains an internal subbasin that deepens from east to west, in contrast with the geometry of the overall basin that deepens from west to east. The long-period ground motions were amplified in both the eastern and western parts of the basin, with the durations being greatly extended in the western part. A series of computations with and without each of the velocity layers within the basin demonstrated that the basin response amplifies long-period ground motions as expected and that the subbasin response is effective in amplifying and extending long-period ground motions. Our quantitative 3-D finite difference simulations clarify the generation of disastrous long-period ground motions that occurred in the Yufutsu basin during the 2003 Tokachi-oki earthquake.

Rupture process of the 2005 West Off Fukuoka Prefecture earthquake obtained from strong motion data of K-NET and KiK-net

We have investigated the rupture process of the 2005 West Off Fukuoka Prefecture earthquake by the multitime- window linear waveform inversion method using the strong ground motion data recorded at 11 K-NET and KiK-net stations. From the waveforms of the P-wave portion, it is indicated that the energy release in the first few seconds was markedly lower than the subsequent part, and this causes difficulty in identifying onset of the S-wave. To decide an appropriate time window for the waveform inversion, we estimate the S-wave onset using aftershock records. The inverted slip distribution shows a single asperity of 8 km × 6 km and its center located 8 km to the southeast and 6 km above the hypocenter. The asperity explains most of the large-amplitude signals in the observed waveforms. The turning point from the initial low-energy-release rupture to the main high-energyrelease rupture is estimated from the spatial variation of the observed initial rupture phase. It is found 3.3 s after the initiation of the rupture at about 4 km to the southeast of the hypocenter. Stress drop during the initial rupture is estimated to be in the same order of those of moderate size aftershocks, which indicates that the initial rupture is an ordinary dynamic rupture.

Source estimates of the May 2006 Java earthquake

In the early morning of 27 May 2006, local time, central Java was jolted by strong seismic ground motion. In spite of the moderate size of the earthquake (Mw = 6.4), it caused severe damage nearYogyakarta city. According to a report from the Social Affairs Ministry of Indonesia, more than 5700 people were killed, 38,000 injured, and 423,000 evacuated. n nAs a result of the shaking, more than 126,000 buildings collapsed, and more than 392,000—including those of the famous Prambanan temple complex, located about 17 kilometers east of Yogyakarta—were severely damaged (Figure 1).