Abstract
Strong-motions from 79 moderate magnitude (5.9 ≥ Mw) earthquakes that caused various degrees of impact on humans and built-environment in Japan between 1996 and 2019, after the start of K-NET and KiK-net, are presented. As such, most of the earthquakes occurred beneath the land, and agencies had reported damages from events as small as Mw 4.1. Together, large peak ground accelerations exceeding 500 cm/s2were recorded during 15 earthquakes, reaching approximately 1128 cm/s2during one event. Similarly, large peak ground velocities exceeding 30 cm/s were recorded during ten earthquakes, reaching about 76 cm/s to the maximum. Most of the large values aforementioned were recorded within a hypocentral distance of approximately 30 km and on soil site conditions. Intermediate to long-period ground motions are of growing concerns in urban areas located on sedimentary basins with mid-rise and high-rise buildings. The threshold magnitude for the large ground motions at the periods of about 2–5 s is not well understood. In March 2013, the Japan Meteorological Agency introduced four levels of long-period ground motion intensity (LPGMI) based on absolute velocity response spectra (AVRS) between 1.6 and 7.8 s. In the present data set, LPGMI of level 3 (AVRS 50–100 cm/s) and level 4 (AVRS >100 cm/s) were observed each at a single site from shallow-focus earthquakes of Mw 5.8 and 5.9 at distances of approximately 12 and 15 km, respectively. However, the peak response periods were relatively short (1.6–1.8 s). The data showed that LPGM from moderate earthquakes is of lower concern regarding earthquake early warning at distances beyond 200 km. The 2018 Mw 5.5 North Osaka earthquake, despite the moderate magnitude, caused the loss of six human lives and brought significant damage to buildings and lifelines. Comparing the data with the ground motion prediction equations (GMPEs) and other events suggested that the North Osaka earthquake was probably a higher stress drop event. These moderate earthquakes’ observations hinted that the commonly used GMPEs in Japan may not sufficiently grasp these earthquakes' hazards.
Highlights
Big earthquakes are generally well reported and investigated in detail for disaster prevention measures for future earthquakes
At smaller distances (
The analysis of residuals for the PGA, PGV, and instrumental intensities based on the ground motion prediction equations (GMPEs) suggested that the observed values at the small distances are somewhat underestimated by the GMPEs
Summary
Big earthquakes are generally well reported and investigated in detail for disaster prevention measures for future earthquakes. The error is within the range of estimation errors, the negative value suggests that the median prediction curve tends to overestimate the northeast Kumamoto earthquake data This observation is in line with that the aftershocks tend to have a lower level of stress drops than the mainshocks, at least for smaller aftershocks (e.g., Nakano et al, 2015; Bindi et al, 2018), resulting in lower high-frequency ground motions. It is likely that the almost concurrent fault movements on two closely located faults with different orientations but with comparable magnitudes caused the larger ground motions during the North Osaka earthquake
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