Abstract
Despite a moderate magnitude, M_w = 6.4, the 5 February 2016 Meinong, Taiwan, earthquake caused significant damage in Tainan City and the surrounding areas. Several seismograms display an impulsive S-wave velocity pulse with an amplitude of about 1 m s-1, which is similar to large S-wave pulses recorded for the past several larger damaging earthquakes, such as the 1995 Kobe, Japan, earthquake (M_w = 6.9) and the 1994 Northridge, California, earthquake (M_w = 6.7). The observed PGV in the Tainan area is about 10 times larger than the median PGV of M_w = 6.4 crustal earthquakes in Taiwan. We investigate the cause of the localized strong ground motions. The peak-to-peak ground-motion displacement at the basin sites near Tainan is about 35 times larger than that at a mountain site with a similar epicentral distance. At some frequency bands (0.9 - 1.1 Hz), the amplitude ratio is as large as 200. Using the focal mechanism of this earthquake, typical “soft” and “hard” crustal structures, and directivity inferred from the observed waveforms and the slip distribution, we show that the combined effect yields an amplitude ratio of 17 to 34. The larger amplitude ratios at higher frequency bands can be probably due to the effects of complex 3-D basin structures. The result indicates that even from a moderate event, if these effects simultaneously work together toward amplifying ground motions, the extremely large ground motions as observed in Tainan can occur. Such occurrences should be taken into consideration in hazard mitigation measures in the place with frequent moderate earthquakes.
Highlights
The 5 February 2016 Meinong earthquake (U.S Geological Survey hypocenter parameters: 19:57:27 UTC, Mw = 6.4, 22.94°N, 120.60°E, 23.0 km) which occurred near the City of Tainan, Taiwan, caused severe damage with 117 fatalities and collapse of tall buildings despite its moderate magnitude (Fig. 1a)
Since the events in the future are unlikely to occur in exactly the same way as the events in the past, we believe that a good understanding of the special circumstance which caused the observed strong-motion hot spot is critically important for implementing comprehensive hazard mitigation measures in the future
If we combine the effects of the three factors, we get a range of amplification factor of 17 to [radiation pattern (2.4 to 3.4) × path-site effect (3.5 to 5 ) × directivity (2)], which is comparable to the observed ratio
Summary
The 5 February 2016 Meinong earthquake (U.S Geological Survey hypocenter parameters: 19:57:27 UTC, Mw = 6.4, 22.94°N, 120.60°E, 23.0 km) which occurred near the City of Tainan, Taiwan, caused severe damage with 117 fatalities and collapse of tall buildings despite its moderate magnitude (Fig. 1a). It is the most damaging earthquake in Taiwan since the 1999 Mw = 7.6 Chi-Chi earthquake. As will be shown later, the records at a few other stations, like TAI1 (CHY078) and CHN3 (CHY089) (locations are shown in Fig. 1a) near W21B display impulsive large velocity pulses. This study is motivated by these unusual observations, and discusses the hazard implications of ground motions from moderate earthquakes in urban environments
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