Abstract
We analyzed strong-motion and broadband recordings of two moderate-magnitude earthquakes that occurred in the Nankai Trough. The first event was the 2004 Mw 6.5 southeast off-Kii peninsula earthquake, an aftershock event inside the Philippine Sea Plate near the Nankai Trough axis. The second event was the 2016 Mw 5.8 southeast off-Mie Prefecture earthquake, an independent event in the rupture area of the 1944 Mw ~ 8 Tonankai earthquake. The centroid depths were 11 and 14 km for the 2004 and 2016 events, respectively. Despite a large difference in the moment magnitude between the two events, the JMA magnitude (Mj) was 6.5 for both the events. We found that the short-period ground motions (e.g., response spectra at periods < 1 s) as well as the much longer-period ground motions (> 20 s) for the 2016 event scaled generally well with the moment magnitude of the event. In contrast, the ground motions from the 2016 event were comparable to those for the larger-moment-magnitude 2004 event at equal distances at periods of about 2–20 s in wide areas and the observed acceleration response spectra at those periods were noticeably underestimated for the 2016 event by the ground motion prediction equation (GMPE) that employs Mw. An examination of the existing subsurface velocity model suggested that the difference in the relative location of the two events with respect to the thick accretionary prism of low seismic velocity most probably caused the comparable amplitude of the seismic waves at those periods. As a result, we posit that the values of Mj are equal for the two events because Mj is estimated using the displacement amplitude of ground motions at periods smaller than about 6 s. On the other hand, GMPE employing Mj generally described the observed data well. The results suggested that the plate boundary earthquakes in the Nankai Trough may excite strong long-period ground motions of engineering importance, and these ground motions appear to be better explained by Mj than by Mw in GMPEs for moderate-magnitude earthquakes in the Nankai Trough subduction zone.
Highlights
The Nankai Trough is a prominent geological structure resulting from the continuous subduction of the Philippine Sea Plate beneath the southwest Japan
We analyzed strong-motion and broadband ground motion data for the 2004 moment magnitude (Mw) 6.5 southeast off-Kii peninsula earthquake, an offshore aftershock event that occurred inside the Philippine Sea Plate near the Nankai Trough axis
We found that the peak ground acceleration (PGA) and acceleration response spectra (ARS) at periods smaller than about 0.5 s were noticeably underestimated by the ground motion prediction equation (GMPE) for the 2004 event, while the ARS at longer periods were generally well explained
Summary
As the original location of site AIC004 was on the 2-m-high cliff, topographic amplification of short-period components of ground motions may have partially contributed to the larger PGAs for the 2004 event Despite these differences, the coda parts for both events show similar amplitudes. Considering the faster estimation of the JMA magnitude, Dhakal et al. The observed AVRS values were adjusted using the same site parameters (AVS30 and depth of sediments) as used to predict the ARS in the previous section and are plotted in Fig. 11 at periods of 1, 2, 3, 5, 7, and 10 s. Comparison of PGVs from broadband recordings In the previous section, we described the results of some statistical analyses of the residuals between the observed strong-motion data and median predictions determined using GMPEs. The strong-motion data were recorded by K-NET and KiK-net and were mostly for distances within 400 km of the epicenters The similarities at periods of 5–20 s for these two events that had a significantly large difference in moment magnitude may be attributed to factors closely related to path effects
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