Modifying the intensity distribution by assessing the reliability

Lan-chi Kang 0 Xing Jin 0 Yong-xiang Wei 0 0 L. Kang (&) X. Jin Y. Wei Earthquake Administration of Fujian Province, Fuzhou 350003, China This article presents an application of a procedure to modify the intensity distribution by assessing the reliability. There are two potential possibilities that may influence the intensity distribution: (1) For the interpolation error, we generate a measured grid across the calculation region. When the point to station spacing is \5 km, we consider the results precise; however, some points have less precision because these are farther from the corresponding stations. When the spacing is between 5 and 50 km, we consider the results imprecise and define a reliability factor that correlates with the distance. (2) Some records may have errors that result from local site conditions, equipment problems, or some disturbance such as lightning stroke, which will lead to some grid points having an incorrect intensity. We regress the attenuation relation for sites with abnormal intensities and consider the results to be accurate when the standard deviation (STD) is \r and inaccurate when the STD is [ 2r. We then define a reliability factor to correlate with STD between r and 2r, such that the intensity distribution is in accord with both wave propagation theory and the investigation intensity. 1 Introduction For rapid emergency response, such as resource dispatch management, information regarding the spatial distribution of seismic intensity is particularly useful, and a near realtime or quick damage assessment in terms of seismic intensity is greatly in demand. One practical way to fulfill this demand is to correlate seismic intensity with peak ground motions because they can be acquired in near real time via a rapid reporting system (Lind et al. 1996; Yamazaki et al. 1997; David et al. 1999; Kanamori 1993; Ma et al. 2003; Jin et al. 2008; Wei et al. 2010). On May 12, 2008, the large magnitude Wenchuan earthquake struck the densely populated Sichuan Province of China (31.0 N, 103.4 E). More than 80,000 people were killed, *370,000 people were injured, and economic losses have been estimated to be in excess of 800 billion RMB. The large moment magnitude (MW 7.9) (Hao et al. 2009; Shen Sun et al. 2009; Xu et al. 2009; Zhang et al. 2009) of the event attests to it being a great intercontinental earthquake. The Wenchuan earthquake occurred on the Longmen Shan fault zone, which delineates the eastern margin of the Tibet Plateau. Located on the western circum-Pacific seismic belt, Taiwan is situated in the collision zone between the Philippine Sea plate (PSP) and the Eurasian plate (EP). As a result, seismicity in Taiwan is considerably high, as demonstrated by the catastrophic 1999 Jiji (ChiChi) earthquake. For rapid reporting of felt earthquakes within Taiwan, a real-time strong motion network, the Taiwan Rapid Earthquake Information Release System (TREIRS), has been operated by the Central Weather Bureau (CWB) since 1997 (Shin and Teng 2001; Wang et al. 2005). In addition to routine magnitude and hypocenter determinations following a felt earthquake, peak ground motions recorded by the TREIRS system are used to generate seismic intensity. In this paper, we use the records from the Wenchuan and Jiji (ChiChi) earthquakes to simulate the progress of rapid reporting of seismic intensity. After mesh generation in the calculation region, we apply the interpolation method to calculate the peak ground acceleration (PGA) for each grid. In accordance with the intensity scale, we transform the PGA to intensity. However, owing to some problems, following the analysis of the intensity distribution, we present a reliability index to modify the intensity distribution. 2 PGA and intensity In general, PGA is calculated in the reporting system of seismic intensity; however, estimating the intensity from PGA is a complex academic problem. Therefore, in this paper, we analyze the relation between intensity and PGA, which are given on the Chinese seismic intensity scale (GB/T 17742-2008). The relation can be written as where Am is the PGA, I is the intensity, and a and b are the regressed coefficients. Figure 1 shows the relation between Am and I. In the areas below I = IV, PGAs corresponding to intensity are not given, and the regression is shown as a dashdot line. Based on this result, we can directly calculate the intensity from PGA. 3 The intensity distribution In this study, we divide the calculated regions into 16,416 grids with a grid spacing of 0.02 and 21,091 grids with a grid spacing of 0.05 for the Jiji (ChiChi) and Wenchuan earthquakes, respectively. From the catalog, 379 records distributed across the whole of Taiwan have been selected for the ChiChi earthquake (Fig. 2b), and 87 records have been selected from the Wenchuan earthquake, including all stations in Sichuan Province and additional stations in Gansu and Shanxi provinces (Fig. 2a). Peak ground acceleration and peak ground velocity (PGV) grid points are calculated by an interpolation methodology as shown in Fig. 3 (Wei et al. 2010; Zhang 2008). In this interpolation methodology, the weighted is refined as the inverse square of the distance, as follows: >>8 D1ad 2 >>>>>>>>>>>>>>< Wa D1ad 2 DD11bbdd 22 D1cd 2 I 6 I=1.4417lnAm+0.039341 Fig. 1 The relation between Am and I. The PGA corresponding to intensity could be calculated by the relation if I \ IV Fig. 2 Distributions of epicenters (circle) and recording stations (triangle) for Wenchuan (a) and Jiji (ChiChi) (b) earthquake quality of the records, or site conditions. Two notable abnormal regions are observed in the intensity distribution. This may be due to a limited distribution of stations in these regions, so we could interpret that the interpolation precision is related to the distance from the stations to the grid points. Alternatively, local site conditions may influence PGA. Some grid points are controlled by abnormal PGA, for example, Fig. 5 showing three records, where the PGAs are higher contrast to some records with a closer epicentral distance; therefore, the region must be showing abnormal intensities. These two reasons lead to abnormal intensity estimates; therefore, we present a methodology to modify the intensity distribution by assessing the reliability. 4 Reliability assessments The methodology can be written as follows: A R WaAaf R; Ra WbAbf R; Rb WcAcf R; Rc where Wa; Wb; Wc is the weighted, Aa; Ab; Ac is the PGA of a, b, and c stations, and f R; Ra; f R; Rb; f R; Rc is the coefficient of attenuation relations. By transforming PGA or PGV at each grid point into intensity, we can generate an intensity contour (Fig. 4). Figure 4 depicts the spatial distribution of intensity for the calculated region. In general, the spatial distribution of intensity is closely related to station distribution, the 4.1 Reliability of the attenuation relation of PGA The most commonly used attenuation model in China that i (...truncated)