Strong Ground Motion in the Source Region of the 1984 Western Nagano Prefecture Earthquake. Inferred from Displaced Boulders.

Abstract

Boulders displaced by the 1984 Western Nagano Prefecture Earthquake (MJMA=6.8) were investigated mainly in the eastern part of the source region which had not been fully surveyed. It was found that numerous boulders were displaced over the large extent of the eastern part of the source region. It is inferred that the boulders were not displaced by high-frequency seismic waves with frequencies between 5-10 Hz, but by low-frequency seismic waves of about 1 Hz, which radiated from the smooth dislocation on the fault plane. The boulder which experienced the largest ground motions was discovered in the eastern part. The ground motions estimated from the displacement of the boulder have a velocity of 5.5 m/s and an acceleration of 3.5 g in the vertical direction, and a velocity of 3.2 m/s and an acceleration of 2.0 g in the horizontal direction.

Seismically induced boulder displacement in the Port Hills, New Zealand during the 2010 Darfield (Canterbury) earthquake

“…3A), together with the general lack of clearly distinguishable boulder impacts on the edges of some major sockets, suggests that some boulders may have been ejected from sockets due to PVAs ] 1 g (Iio & Yoshioka 1992). Under such circumstances the mass of the boulder would be theoretically irrelevant to lateral transport distance, but shape, ground slope and transient vertical and horizontal ground motions at landing time of the boulder would be critical factors to determine the final displacement distances.…”

“…comm., 2012). Observed boulder displacements are therefore attributed to the dynamic phase of ground motion, occurring around the largest amplitude of the ground velocity (Iio & Yoshioka 1992) rather than the permanent tectonic deformation. Several measured boulder horizontal displacements, including boulders that show no geomorphic evidence for rolling or sliding ( Fig.…”

“…Other studies on seismically induced boulder displacements have indicated that topographic amplification may have been important because of the distribution of displaced boulders being concentrated on ridge crests (e.g. Umeda et al 1987;Iio & Yoshioka 1992). To investigate whether the topography of the Port Hills may have amplified the shaking response in the Darfield earthquake, we used 2D FLAC modelling.…”

“…In areas lacking dense seismometer arrays, it is necessary to use independent techniques to characterise earthquake ground motion. Coseismically displaced boulders may provide noninstrumental proxies of earthquake motion (Oldham 1899;Clark 1972;Bolt & Hansen 1977;Umeda et al 1987;Iio & Yoshioka 1992;Ohmachi & Midorikawa 1992;Bouchon et al 2000).…”

“…Umeda et al 1987;Iio & Yoshioka 1992;Bouchon et al 2000). Shaking table experiments and numerical arguments based on empirical data, on the other hand, suggest that boulder 'upthrow' can be produced by strong horizontal ground motion alone, due to the impact and dynamic interactions of boulders with the sidewalls of ground sockets (e.g.…”