Local site effects in Kumamoto City revealed by the 2016 Kumamoto earthquake

Abstract

Abstract:To evaluate local site effects in Kumamoto City, we installed six temporary seismic stations along a 6-km north-south survey line in the city immediately after the 2016 Kumamoto earthquake foreshock (Mj 6.4), which occurred on April 14, 2016. Seismic data from the 2016 Kumamoto earthquake (Mj 7.3), which occurred on April 16, 2016, were successfully recorded at two sites and indicated large amplitudes in the frequency range of 0.5-3 Hz. Site amplifications estimated from weak ground motion data, with a station at Mt. Kinbo used as a reference, are relatively variable along this survey line; however, site amplification factors in the frequency range of 0.5-3 Hz are not large enough to explain the amplitudes produced by the main shock. Nevertheless, site amplifications estimated from strong ground motion data recorded at the two sites during the main shock are large in the frequency range of 1-3 Hz. These findings reveal that the strong ground motions in the frequency range of 1-3 Hz were enhanced by nonlinear behavior of the subsurface soil in Kumamoto City. Moreover, it is observed that the frequency contents of the main shock data in the frequency range of 0.7-3 Hz differ significantly between the two sites, despite the proximity of these sites (600-m interval). Therefore, we also performed single-station microtremor measurements with an interval distance of approximately 100 m between these two sites. We confirmed that the peak frequencies of the horizontal-to-vertical spectral ratios of microtremors have trends that are similar to those of the site amplification factors between the two sites. However, these results could not explain the differences in strong ground motions observed at the two sites during the 2016 Kumamoto earthquake.

Is nonlinearity needed to predict the Kumamoto Mj 6.5 and Mj 7.3 mainshocks at KUMA site?

“…10 Hz). The PGA produced by Step3 simulation is about 447 gal, and this value is higher than the PGA limit (150 gal) indicated byTsuno et al (2017) as the threshold between linear and nonlinear behavior during Kumamoto sequence.…”

“…Kinbo and Mt. Aso pyroclastic sequence of ow deposits (Tsuno et al 2017), such as Aso-1 to Aso-4 which are younger from the Aso-1 to the Aso-4. Between the Aso-4 and Aso-3 deposits the Miyuki Formation has been found , Nakazawa et al 2018, with lithofacies ranging from basal gravel to a middle horizon of marine clay to upper gravel, interpreted as buried marine terrace deposits .…”

“…The wide magnitude range associated with the blind test target events allow Tsuno et al (2017) to verify the presence of nonlinear soil behavior in ground shaking. They reported nonlinearity to occur from aftershock spectra analyses when peak ground acceleration (PGA) is higher than the 150 gal and showa lowering of the peak frequency as reported in other earthquakes case study.…”

“…The prediction site (CPb) is located 3200 meters from the left edge of the section, at CMP 1000, where synthetic time histories of acceleration and strain have been evaluated. The KP plain model has plane parallel geometry around the target site, therefore it cannot reproduce the lateral variability observed using portable seismic stations(Tsuno et al 2017).…”

Special issue “2016 Kumamoto earthquake sequence and its impact on earthquake science and hazard assessment”

“…On the other hand, Kawase et al (2017) make a detailed survey of damages and reanalyze strong-motion records in the Mashiki town, attributing severe structural damages to a strong westward velocity pulse, rather than to site effects. Tsuno et al (2017) evaluate site amplification in Kumamoto based on dense observation of microtremor and conclude that site amplification is not enough to explain observed ground motion during the mainshock. Irikura et al (2017) examine three rupture models of the mainshock of the Kumamoto earthquake sequence and verify the effectiveness of source scaling relations for crustal earthquakes.…”