11:00 〜 11:15
[S07-8-03] Peculiar strong ground motions from the very deep (h=680 km) Mw 7.9 Ogasawara Islands earthquake of 2015 May 30
The Mw 7.9 earthquake beneath the Ogasawara Islands on 2015 May 30 at 6780 km depth, produced an anomalously large shock over Japan. This event is about 100 km deeper than other seismicity in the vicinity and produced a distinctive pattern of ground motion.
Usually deep earthquakes in the subducting Pacific slab near Ogasawara develop anomalously large ground motions along the east coast of Honshu due to an efficient slab waveguide effect for high-frequency (f > 1 Hz) signals. However, the waveforms of regional distances (1000 - 2000 km) for the very deep event indicate that the large ground acceleration arises from relatively low-frequency (f < 1 Hz) S-wave pulses and following long-period (f < 0.1 Hz) signals with long tails. The arrival of the slab-guided high-frequency signal was very late and weak compared with ordinary slab events.
Numerical simulation of seismic wave propagation shows how the deep source outside the main slab affects the wavefield. S waves from the source travel upwards and impinge on the crust at around 1000 km epicentral distance with similar slowness to P in the crust and produce strong S-to-P conversions at the free surface. The converted P waves are trapped in the crust with multiple Moho and surface reflections interfering to produced a long-period PL wave. The S-PL wave can travel substantial distances in the crust. Weak slab-guided S waves from the very deep event are also transferred injected into the crust and continue as an Lg wave in the crustal waveguide.
The very deep event produces a broader distribution of strong ground motion, but much stronger concentrated effects with potentially disastrous consequences can arise due to much enhanced slab waveguide effect when a very large event lies in the core of the slab.
Usually deep earthquakes in the subducting Pacific slab near Ogasawara develop anomalously large ground motions along the east coast of Honshu due to an efficient slab waveguide effect for high-frequency (f > 1 Hz) signals. However, the waveforms of regional distances (1000 - 2000 km) for the very deep event indicate that the large ground acceleration arises from relatively low-frequency (f < 1 Hz) S-wave pulses and following long-period (f < 0.1 Hz) signals with long tails. The arrival of the slab-guided high-frequency signal was very late and weak compared with ordinary slab events.
Numerical simulation of seismic wave propagation shows how the deep source outside the main slab affects the wavefield. S waves from the source travel upwards and impinge on the crust at around 1000 km epicentral distance with similar slowness to P in the crust and produce strong S-to-P conversions at the free surface. The converted P waves are trapped in the crust with multiple Moho and surface reflections interfering to produced a long-period PL wave. The S-PL wave can travel substantial distances in the crust. Weak slab-guided S waves from the very deep event are also transferred injected into the crust and continue as an Lg wave in the crustal waveguide.
The very deep event produces a broader distribution of strong ground motion, but much stronger concentrated effects with potentially disastrous consequences can arise due to much enhanced slab waveguide effect when a very large event lies in the core of the slab.