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To analyze the source characteristics of the 2016-2017 central Italy seismic sequence, source spectra of 78 earthquakes of M L = 3.5-6.1 were separated from the S wave Fourier spectra using the two-step nonparametric generalized inversion technique. Source parameters (e.g., stress drop) were estimated from the source spectra following the ω-square model. Stress drops were found mainly in the range 0. 113-12.190 MPa. The significant dependence of stress drop on magnitude indicates the breakdown of earthquake self-similar scaling in this sequence. The low stress drops for small events following the release of high stress by the mainshock might have led to stress accumulation on the unruptured fault, which could explain the subsequent occurrence of multiple major events. We investigated the source rupture directivity for 36 events through the azimuthal variation of apparent source spectra. Significant variation was observed at specific frequency bands (generally, over corner frequencies) in 10 events caused by rupture directivity, which was verified by the stable estimation of rupture plane. The rupture parameters confirmed unilateral ruptures predominantly on the NNW-SSE striking fault with fast rupture velocity (2.52-2.84 km/s) for the 10 events. Rupture parameters for an additional four events with stable estimated rupture plane were also analyzed. These were characterized by slow rupture velocity causing weak directivity effects. According to the rupture parameters for the 14 events, prevailing SSE or SEE (NNW or NNE) rupture directivity was a common feature for events to the north (south) of the mainshock in this earthquake sequence.

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The South China Sea is recognized as an area at high risk of tsunamis. The Manila Trench has long been considered as the regional source of tsunamis that might affect Chinese coastal areas, and considerable analysis of the tsunami hazard has been conducted in this area. However, there has been no consideration of other potential local sources near the coastal area. Thus, the locations of local potential tsunami sources (PTSs) along the southern coast of China and the evaluation of their impact on tsunami hazard assessment require investigation. We identified eight local PTSs for given seismic activity parameters. For the probabilistic tsunami hazard analysis (PTHA), the lower-limit earthquake magnitude was determined as 7.0, based on numerical simulation of tsunami scenarios. Six measured sites in the Pearl River Estuary and Taiwan Strait were selected for PTHA, which were referenced to Macao, Hong Kong, Daya Bay, Shantou, Xiamen, and Quanzhou. The annual rate of tsunami waves exceeding a given height [Formula: see text] was calculated for each site. The results show that the upper-limit earthquake magnitude and the relative geographical positions of the measured site of interest and the PTS are two of the most important factors in the PTHA computation. The probabilities of tsunami waves exceeding a given height [Formula: see text] within 100 years and their return periods were calculated for each site. The results show that the probability of [Formula: see text][Formula: see text]m within 100 years is 30–40% in Xiamen and Quanzhou but 5–10% in Macao and Hong Kong. If the Manila Trench were considered as a regional source, these probabilities would be higher. It is concluded that the tsunami hazard on the southern coast of China is very high and that both regional and local PTSs should be included in any future PTHA.

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The consideration of soil nonlinearity is important for the accurate estimation of the site response. To evaluate the soil nonlinearity during the 2008 Ms8.0 Wenchuan Earthquake, 33 strong-motion records obtained from the main shock and 890 records from 157 aftershocks were collected for this study. The horizontal-to-vertical spectral ratio (HVSR) method was used to calculate five parameters: the ratio of predominant frequency (RF p ), degree of nonlinearity (DNL), absolute degree of nonlinearity (ADNL), frequency of nonlinearity (fNL), and percentage of nonlinearity (PNL). The purpose of this study was to evaluate the soil nonlinearity level of 33 strong-motion stations and to investigate the characteristics, performance, and effective usage of these five parameters. Their correlations with the peak ground acceleration (PGA), peak ground velocity (PGV), average uppermost 30-m shear-wave velocity (V S30 ), and maximum amplitude of HVSR (A max ) were investigated. The results showed that all five parameters correlate well with PGA and PGV. The DNL, ADNL, and PNL also show a good correlation with A max , which means that the degree of soil nonlinearity not only depends on the ground-motion amplitude (e.g., PGA and PGV) but also on the site condition. The fNL correlates with PGA and PGV but shows no correlation with either A max or V S30 , implying that the frequency width affected by the soil nonlinearity predominantly depends on the ground-motion amplitude rather than the site condition. At 16 of the 33 stations analyzed in this study, the site response showed evident (i.e., strong and medium) nonlinearity during the main shock of the Wenchuan Earthquake, where the ground-motion level was almost beyond the threshold of PGA > 200 cm/s 2 or PGV > 15 cm/s. The site response showed weak and no nonlinearity at the other 14 and 3 stations. These results also confirm that RF p , DNL, ADNL, and PNL are effective in identifying the soil nonlinearity behavior. The identification results vary for different parameters because each parameter has individual features. The performance of the PNL was better than that of DNL and ADNL in this case study. The thresholds of ADNL and PNL are proposed to be 2.0 and 7%, respectively.