A delineation of the Nojima fault ruptured in the M7.2 Kobe, Japan, earthquake of 1995 using fault zone trapped waves

Yong-Gang Li, Keiiti Aki, & John E. Vidale

Published 1998, SCEC Contribution #449

We used four linear seismic arrays of portable seismometers at the northern Awaji Island, Japan, to record fault zone trapped waves from aftershocks of the 1995 M7.2 Hyogoken Nanbu (Kobe) earthquake from April to June 1996. Three arrays were deployed across the Nojima fault, which ruptured during the mainshock, while one array was deployed across the Higashiura fault, which did not break recently. We observed significant fault zone trapped waves with relatively large amplitudes and the long wave train following S waves only when both the stations and aftershocks were located close to the Nojima fault. The coda-normalized spectral amplitudes of trapped waves show a maximum peak at 4–7 Hz, which decreases rapidly with distance from the fault trace. The normalized amplitudes of trapped waves also show a decrease with hypocentral distance along the fault, giving an apparent Q of approximately ∼25 at 4–7 Hz. In comparison, the array across the Higashiura fault recorded much shorter wave trains with higher frequencies after S arrivals for the same events. We simulate these trapped waves as S waves guided in a low-velocity waveguide sandwiched between high-velocity wall rocks. We find an adequate fit by using a waveguide 60 m wide at the northern site and 30–40 m wide elsewhere along the Nojima fault, a waveguide S velocity of 1.5–1.7 km/s, and a Q value of 25. For the Higashiura fault, the S velocity is 2.5 km/s, and the Q value is 80. The locations of aftershocks for which we observed fault zone trapped waves show that the Nojima waveguide is 9 km long and dips southeastward at 80°–85° to a depth of ∼16 km. It extends 6 km farther south westward along the Asano fault, though there are no obvious surface breaks along it. However, the waveguide is disconnected from the Suma fault on the main island, which was also ruptured during the Kobe earthquake, possibly because of the existence of an offset between the Nojima fault and the Suma fault.

Key Words
California, ground motion prediction, guided waves, model

Li, Y., Aki, K., & Vidale, J. E. (1998). A delineation of the Nojima fault ruptured in the M7.2 Kobe, Japan, earthquake of 1995 using fault zone trapped waves. Journal of Geophysical Research, 103(B4), 7247-7263.