No abstract

The Mw8.4 Illapel earthquake occurred on 16 September was the largest global event in 2015. This earthquake was not unexpected because the hypocenter was located in a seismic gap of the Peru‐Chile subduction zone. However, the source model derived from 3‐D spectral‐element inversion of teleseismic waves reveals a distinct two‐stage rupture process with completely different slip characteristics as a composite megathrust event. The two stages were temporally separated. Rupture in the first stage, with a moment magnitude of Mw8.32, built up energetically from the deeper locked zone and propagated in the updip direction toward the trench. Subsequently, the rupture of the second stage, with a magnitude of Mw8.08, mainly occurred in the shallow subduction zone with atypical repeating slip behavior. The unique spatial‐temporal rupture evolution presented in this source model is key to further in‐depth studies of earthquake physics and source dynamics in subduction systems.

The February 5, 2016 (UT), Meinong, Taiwan, earthquake, brought extensive damage to nearby cities with significant velocity pulse-like ground motions. In addition to the spatial slip distribution determination using filtered strong motion data, we show that on the advantage of the densely distributed seismic network as a seismic array, we can project the earthquake sources (asperities) directly using nearly unfiltered data, which is crucial to the understanding on the generation of the velocity pulse-like ground motions. We recognize the moderate but damaging M L 6.6 Meinong earthquake was a composite of an M W 5.5 foreshock and M W 6.18 mainshock with a time delay of 1.8-5.0 s. The foreshock occurred in the hypocenter reported by the official agency, following by the mainshock centroid occurred 12.3 km to the north northwest of the hypocenter and at a depth of 15 km. This foreshock-mainshock events are non-distinguishable as it was buried as one event, while using low-frequency filtered seismic data for the finite-fault inversion. Our results show that the velocity pulse-like ground motions are mainly resulted from the source of mainshock with its directivity and site effects, resulting in the disastrous damages in Tainan City. Although finite-fault inversion using filtered seismic data for spatial slip distribution on the fault has been a classic procedure in understanding earthquake rupture processes, using a dense seismic network as a seismic array for unfiltered records helps us delineate the earthquake sources directly and provide more delicate information for future understanding on earthquake source complexity.

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.