Tsunami hazard assessment along the Chinese mainland coast from earthquakes in the Taiwan region

Transcription

1 DOI /s ORIGINAL PAPER Tsunami hazard assessment along the Chinese mainland coast from earthquakes in the Taiwan region Hou Jingming 1,2 Li Xiaojuan 1 Yuan Ye 2 Wang Peitao 2 Received: 26 May 2015 / Accepted: 14 December 2015 Springer Science+Business Media Dordrecht 2015 Abstract Tsunami disasters have been documented several times in Chinese history, mostly in the Taiwan region. To assess the tsunami hazard along the coast of mainland China from sources in Taiwan, this study analyzed historical tsunamis and undersea seismic events around Taiwan and found that the frequency and magnitude of earthquakes around Taiwan are significantly higher than in the adjacent Ryukyu and Manila trenches. The probabilistic seismic hazard analysis method was adopted to estimate the maximum possible earthquake magnitude around Taiwan. Then, six tsunami sources were assumed in those places where both earthquakes and tsunamis have occurred previously. Numerical models of the tsunamis were used to calculate the probable maximum tsunami amplitude and tsunami arrival time. The largest tsunami amplitude and the shortest arrival time were drawn on a GIS map. The modeling results provided a summary of tsunami hazards along the coast of mainland China from tsunami sources in Taiwan. The results showed that tsunamis triggered by the maximum possible earthquakes in the Taiwan region would arrive first at Zhejiang, Fujian, Guangdong, and Hainan Provinces within 3 h; the largest tsunami amplitude was up to 3.3 m. Thus, parts of Zhejiang, Fujian, and Guangdong Provinces were identified as regions with the highest hazard levels. Keywords Hazard assessment Tsunami Taiwan Chinese mainland 1 Introduction Tsunami disasters have gained increased worldwide awareness in the aftermaths of the 2004 Indian Ocean tsunami and the 2011 tsunami in Japan. Tsunamis can be triggered by a variety of sources, but they are initiated mainly by shallow earthquakes in subduction & Hou Jingming houjingming1982@126.com 1 2 College of Resource Environment and Tourism, Capital Normal University, Beijing, People s Republic of China National Marine Environmental Forecasting Center, Beijing, People s Republic of China

2 zones. Globally, approximately 75 % of major tsunamis occur in the Pacific Ocean and its marginal seas (IOC 2013). China, because of its location to the west of the Pacific and near the Ryukyu and Manila trenches, also faces the threat of tsunamis. An Mw 7.0 earthquake that occurred in Pingtung, Taiwan (December 26, 2006) and the subsequent associated tsunami illustrate that constant awareness of the hazard of tsunamis is necessary along the coast of Southeast China (Liu et al. 2007). Tsunami disasters have occurred several times in Chinese history, mostly in the Taiwan region. Historical tsunami events and earthquakes with magnitudes [6.0 that have occurred in the Taiwan region between 1964 and 2014 are plotted in Fig. 1. These data were taken from the American National Geophysical Data Center (NOAA 2015) and the United States Geological Survey (USGS 2015). Figure 1 shows that both earthquakes and tsunamis around Taiwan are concentrated in the eastern waters of the Fig. 1 Historical earthquakes and tsunamis near Taiwan from 1964 to 2014

3 island, in the vicinity of the boundary between the Eurasian and Philippine plates. The frequency and magnitude of earthquakes in Taiwan are both higher than in the Ryukyu and Manila trenches. Between 1964 and 2014, 91 earthquakes with magnitudes [6.0 occurred near Taiwan. About 87 % of the past earthquakes in this region occurred at focal depths of \60 km. The depth of the eastern Taiwan waters lies between 1000 and 5000 m. Thus, the magnitudes and focal depths of the earthquakes, and the local bathymetric conditions are all suitable for triggering a tsunami (Chen et al. 2007). In addition, most tsunamis in Taiwan are generated locally and this proximity to mainland China accentuates the risk of serious damage in that region. Tsunami amplitude and arrival time are important parameters for the assessment of tsunami hazard (Wang and Liu 2006). By analyzing previous tsunami and earthquake data, and by estimating the maximum possible seismic magnitude around Taiwan, this study calculated future tsunami amplitudes and arrival times at mainland China from earthquake sources near Taiwan. The hazard assessment was conducted primarily for mainland China and Hainan Island, not including Taiwan and the islands in the South China Sea. 2 Statistics of historic tsunamis There are many tsunamis recorded in ancient Chinese literature (Yu et al. 2001; Wang et al. 2006; Wong and Chan 2006; Lau et al. 2010); however, some of these might have actually been storm surges or other events that would require a corresponding seismic description to be characterized accurately as tsunamis. In addition to the collection of Chinese historical tsunami data, this research also analyzed tsunami data from the American National Geophysical Data Center. These tsunami data (2000 BC 2014 AD) contain the times of occurrence, latitudes and longitudes, tsunami type, level of confidence, and other relevant information. Following the careful comparison and analysis of these two data sets, a list of credible tsunamis was compiled (Table 1). The most devastating tsunami in Taiwan occurred in Keelung, which was triggered by an Mw 7.0 earthquake on December 18, This tsunami resulted in countless ships sinking, buildings collapsing, and hundreds of deaths (Wang et al. 2005). Two of the past tsunamis were recorded by modern instruments. During a tsunami near Taiwan on September 16, 1994, the Dongshan and Shantou gauge stations recorded a maximum tsunami of 26 and 47 cm, respectively. On December 26, 2006, an Mw 7.0 earthquake occurred off the coast of Taiwan at Pingtung and generated a tsunami. The tsunami propagated to Chongwu in 3.4 h where the maximum tsunami height was 7.8 cm. The maximum tsunami amplitude of the same tsunami at Dongshan was 10 cm. Table 1 Historical tsunamis in Taiwan Epicenter Time Coordinate Magnitude Pingtung, Taiwan May 22, 1781 (22.1 N, E) Unknown Chiayi, Taiwan August 9, 1792 (23.6 N, E) 6.8 Keelung, Taiwan December 18, 1867 (25.5 N, E) 7.0 Hualien, Taiwan November 14, 1986 (24.1 N, E) 7.4 Taiwan Strait September 16, 1994 (22.6 N, E) 6.7 Pingtung, Taiwan December 26, 2006 (21.8 N, E) 7.0

4 3 Geological analysis and maximum magnitude 3.1 Tectonic setting Taiwan lies at the junction of the Eurasian and Philippine plates, linking the Ryukyu and Manila trenches. The Philippine Plate has been moving to the northwest since the early Cenozoic Era (Cheng et al. 2007). Thus, the Philippine Plate subducts beneath the Eurasian Plate in the northeast of Taiwan, whereas in southern Taiwan, the Eurasian Plate lies under the Philippine Plate. Eastern Taiwan is the collisional forefront of the Luzon arc and the continental margin, where the plates collide and squeeze, making this region more geologically active. 3.2 Probabilistic seismic hazard analysis method The probabilistic seismic hazard analysis (PSHA) method has been applied to assess the maximum possible earthquake magnitude in Taiwan, based on the analysis of geological structures and existing seismic data. It is used to evaluate earthquake damage by considering the probability of a specified level of ground motion being exceeded at a specific location and linking this probability within a given period to the annual frequency of exceedance. PSHA was proposed by Cornell (1968) and improved upon by Coppersmith (1991). Prior to the development of the PSHA method, deterministic methods were often used to assess potential worst-case seismic hazards. The size and location of the seismic source generally correspond to the worst-case scenario. Compared with the conservative deterministic methods, PSHA is a more scientific method (Baker 2008). There are numerous factors that are difficult to determine in an earthquake disaster assessment, i.e., the earthquake s location and intensity. PSHA aims to quantify these uncertain factors and to combine them to draw a clear description of future earthquakes. With the PSHA method, the worst-case ground motion intensity is no longer used; instead, all possible seismic activities and their corresponding occurrence probabilities are considered. Cheng et al. (2007) analyzed the seismic characteristics of the Taiwan area and calculated the maximum possible magnitude using the PSHA method. Here, fault activity was analyzed using a revised PSHA method, and then, seismic hazard maps were drawn for Taiwan considering 475- and 2475-year return periods. Four major source types were studied, and a logical tree approach was used to handle the uncertainty of the parameters in the PSHA. The results of the maximum possible earthquake magnitude, given in Table 2, were used in this study. Table 2 Fault parameters of hypothetical earthquakes Number Longitude Latitude Magnitude Rupture length (km) Rupture width (km) Strike ( ) Dip ( ) Slip ( ) Focal depth (km)

5 4 Numerical calculations A tsunami is a low-probability event, and observations of such events are rare; therefore, numerical simulations have become one of the primary means of tsunami research. In this study, tsunami amplitudes and arrival times were calculated using numerical models and the simulation results were analyzed. Six hypothetical tsunami sources were assumed, as shown in Fig. 2. The locations of the hypothetical earthquakes were set at sites of historical tsunami events and in earthquakeintensive areas; the magnitudes of the hypothetical earthquakes were determined according to Cheng et al. (2007). Hypothetical source 1 marks the intraslab source at the northern tip of Taiwan, which has a greater probability of earthquake occurrence according to Cheng et al. (2007). Historically, there have been deeper earthquakes with greater magnitudes at Fig. 2 Hypothetical seismic sources near Taiwan and their focal mechanisms

6 this location, but the sources were in open water. Hypothetical sources 2 6 are in locations where historical tsunamis have occurred. These sources were used to calculate the maximum possible tsunami amplitudes and earliest arrival times. Most of the focal depths of Taiwanese earthquakes with magnitudes [6 are between 0 and 60 km. The tsunami amplitude was calculated using the COMCOT model (see Sect. 4.1), and the tsunami travel time (TTT) model (see Sect. 4.1) was used to compute the arrival times. The seismic fault parameters used in the calculations, such as depth and strike, dip, and slip angles, were based on the parameters of the historical earthquakes closest to the hypothetical source. The historical focal mechanism data were from the Harvard University CMT Project (2015). The length (L) of the hypothetical fault was calculated using the following equation (Igarashi 2013), assuming fault width W = L/2 (M is the magnitude of the earthquake): log L ¼ 0:5M 1:9: ð1þ 4.1 Introduction of numerical models In this study, the tsunami amplitude was calculated using the COMCOT model, which was developed at Cornell University (Liu et al. 1998; Wang and Power 2011). The model has been used successfully for the simulation of many historical tsunami events (Liu et al. 1995; Wang and Liu 2006), and it has been applied to tsunami hazard studies by research institutes in many countries. This model has a standard modular design that can study the entire life span of a tsunami, from its generation to propagation and inundation. Multiple nested grids can be used in the model. Different coordinates (Cartesian/spherical) and equations (linear/nonlinear) can be flexibly configured according to demand. The tsunami propagation investigated in this study occurs in a coastal region, and therefore, nonlinear equations are adopted. The nonlinear equations in spherical coordinates are: og ot þ 1 R cos / þ op ow þ o cos /Q o/ ð Þ ¼ oh ot ; op ot þ g o P 2 þ g o PQ þ gh og R cos / ow H R ow H R cos / ow fq þ F x ¼ 0; ð2þ oq ot þ g o PQ þ g o Q 2 þ gh og R cos / ow H R o/ H R o/ þ fp þ F y ¼ 0 where R stands for the radius of the Earth, g is the free surface displacement relative to the mean sea level, h is water depth, H is the total depth (H = g? h), P and Q denote the volume flux components in the longitudinal w and latitudinal / directions, respectively, f is the Coriolis force coefficient, g represents gravitational acceleration, and F x and F y denote the bottom friction components in the longitudinal and latitudinal directions, respectively, which are evaluated using Manning s formula. Tsunami arrival time was calculated using the TTT model, which was developed by Paul Wessel, Geoware (Shokin et al. 1987). The TTT model can calculate the wave arrival time at each grid point. The model is based mainly on Huygens principle in which every point on a spherical surface is considered the source of secondary spherical waves (Shokin et al. 1987). The arrival times of all grid points around the focus are calculated, and the point with the shortest time is identified as the source. Each grid point is calculated in this way. Because the tsunami wave is long, the tsunami wave velocity can be computed using Eq. (3) (Murty 1977). Equation (4) is used to calculate the arrival time between grid points.

7 p s ¼ ffiffiffiffiffi gd ; ð3þ DtðrÞ ¼ Z r 0 dx sðxþ ; ð4þ where d denotes depth, s represents tsunami wave velocity, and r is the distance from the current node to another node that lies on a circle of radius r. This model has successfully calculated the propagation times of tsunamis, such as the Chilean tsunami in 2010 and the Japanese tsunami in Validation of numerical models On March 11, 2011, an Mw 9.0 earthquake occurred in the eastern waters of Honshu Island and triggered a large tsunami that caused major economic losses and casualties. This tsunami spread to China, first reaching the East China Sea, and then the Yellow and South Fig. 3 Computation domain and tide stations affected by Japan tsunami in 2011

8 China seas (Ren et al. 2013). Four hours after the earthquake, the tsunami reached Taiwan. Within 7 13 h, the tsunami had reached the provinces of Guangdong, Fujian, Zhejiang, Shanghai, and Jiangsu along the coast of mainland China. Gauge stations along the coast, including Dachen, Kanmen, Shipu, Dongshan, and Lvsi, detected tsunami amplitudes of cm (Wang et al. 2012). The locations of places and tide stations are shown in Fig. 3. This tsunami was used to verify the COMCOT and TTT models. Several gauge stations along the Chinese mainland coast were selected to assess the accuracy of the modeled tsunami amplitudes and arrival times. As given in Table 3, the length and width of the tsunami source were determined according to the empirical equation of the Japan Meteorological Agency (Igarashi 2013); other source parameters were obtained from the Harvard University CMT Project (2015). Figure 4 shows that the initial calculation of tsunami amplitude is in good agreement with the observational data, thereby suggesting that the model is credible. From Table 4, the relative error between the simulation and the observations of arrival time can be seen to be about 3 %. Table 3 Source parameters of the 2011 Japan tsunami Earthquake parameters Longitude Latitude Length Width Dip Slip Strike Depth Value E N km km km Fig. 4 Comparison between model results and measurements

9 In comparison with Fig. 4, it can be determined that the numerical results of COMCOT provide more accurate travel times. The calculation time of the TTT model is very short, which makes it suitable for the early warning and large-scale assessment of tsunamis. The arrival time is defined here as the minimum propagation time when the sea surface elevation exceeds 2 cm. 4.3 Computational domain As shown in Fig. 3, the computational domain covers most of the northwest Pacific Ocean (5 45 N, E). The model has a resolution of 1 min with a time step of 1 s; the calculations used nonlinear equations without grid nesting. Additionally, a vertical wall boundary was used at the water land boundary, and the Manning s roughness coefficient was Table 4 Comparison of numerical and observed arrival times Lvsi Shipu Kanmen Zhelang Observation 12.2 h 8.5 h 8.1 h 7.2 h Simulation 12.7 h 8.7 h 8.3 h 7.0 h Relative error 4.1 % 2.4 % 2.5 % -2.8 % Fig. 5 Distributions of maximum tsunami amplitudes

10 Fig. 6 Distributions of tsunami arrival times for each of the six modeled tsunamis (the interval of the arrival time contour is 30 min) 4.4 Numerical calculation The tsunami amplitude simulation was performed based on the parameters displayed in Table 1. The calculation results are shown in Fig. 5, from which it can be seen that a large tsunami could be generated if the maximum possible earthquake occurs off Taiwan. Because of the various fault parameters, the effect of each hypothetical tsunami differs from the others. For the Chinese mainland coast, the largest tsunami amplitudes of up to 3.3 m result from hypothetical tsunami source no. 2. The regions most affected extend from Fujian Province to Zhejiang Province. The second largest coastal tsunami amplitude, created by hypothetical tsunami source no. 6, can reach 3.1 m, and the areas most affected extend from southern Fujian Province to Guangdong Province. The modeled amplitudes of the other hypothetical tsunamis are smaller. Although hypothetical source no. 1 is in open water, the deeper focal depth makes the tsunami smaller. The tsunami generated by hypothetical source no. 4 is the smallest of those evaluated. This is primarily because of the nature of the strike slip fault, which only causes a small amount of vertical movement. According to the user s guide (IOC 2014) for the enhanced products of the Pacific Tsunami Warning Center, the levels of tsunami impact lie within four categories: (1)

11 Nat Hazards Fig. 7 Hazard assessments of tsunamis from Taiwan: a amplitude and b arrival time Table 5 Classification standard of tsunami hazard assessment Level Maximum wave amplitude Minimum arrival time T [ 9.0 h 1 H \ 0.3 m m B H \ 1.0 m 6.0 h \ T B 9.0 h m B H \ 3.0 m 3.0 h \ T B 6.0 h 4 H C 3.0 m T B 3.0 h \0.3 m, (2) m, (3) 1 3 m, and (4) [3 m. A level 4 tsunami warning should be issued if the tsunami amplitude is [3 m. The calculation results for tsunami arrival times are shown in Fig. 6, with a time interval of 0.5 h. In this figure, it is obvious that Fujian, Zhejiang, and Guangdong Provinces are the first locations to feel the effects of the tsunamis. The tsunamis can also spread quickly to Hainan Island, because the waters between Hainan and Taiwan are deep, which permit rapid propagation speeds. A tsunami can reach mainland China within 3 h, which presents a serious challenge for the prevention and mitigation of tsunami disasters. 5 Hazard assessment The maximum tsunami amplitudes and minimum arrival times for all six hypothetical tsunami sources were calculated and drawn on a GIS map to aid the clarification of the tsunami hazard. All prefectural-level coast lines of the Chinese mainland coast are shown in Fig. 7. The classification standard of the Pacific Tsunami Warning Center is listed in

12 Table 5. In Fig. 7, the color red (level 4) indicates the highest level of danger, followed by orange (level 3), yellow (level 2), and blue (level 1). Figure 7a shows that tsunami waves affect most of the east coast of mainland China, of which parts of Guangdong, Fujian, and Zhejiang Provinces are the areas at highest risk. From Fig. 7b, we can see that parts of Zhejiang, Fujian, Guangdong, and Hainan Provinces are first to experience the effects of the tsunamis (within 3 h). The cities of Ningde, Zhangzhou, Chaozhou, Shantou, Jieyang, and Shanwei are the conurbations most at risk of being affected by tsunamis, because they lie within the areas with the most dangerous levels of both tsunami amplitude and early arrival time. Assessment maps based on such results are helpful for local and regional governments in municipal planning and other relevant work. 6 Conclusions Based on the analysis of historical tsunami data and recent seismic data from Taiwan, the PSHA method was used to evaluate possible maximum earthquake magnitudes, and these results were used in numerical models to calculate the maximum possible tsunami amplitudes and minimum arrival times at mainland China. Numerical results of hypothetical tsunamis generated near Taiwan showed that mainland China is severely at risk from the effects of such physical events. In particular, the provinces of Zhejiang, Fujian, and Guangdong would be hardest hit, with maximum wave amplitudes as high as 3.3 m. A tsunami can arrive on the coast of mainland China within 3 h. However, the inundation process of coastal regions was not considered here. Because detailed local topography has strong influence on the process of tsunami inundation, the hazard assessment of tsunamis on small regions needs further research. In addition to development of a timely and effective warning system for tsunamis, early response work needs to be undertaken to cope with tsunami disasters. Acknowledgments This research was supported by the Chinese Public Science and Technology Research Funds Ocean Projects (Grant No ) and National Natural Science Foundation of China (Grant No ). We would like to thank Dr. Xiaoming Wang for providing help with the COMCOT model. References Baker JW (2008) An introduction to probabilistic seismic hazard analysis (PSHA). White paper, version 1:72 Chen Y, Chen Q, Zhang W (2007) Tsunami disaster in China. J Nat Disasters 16(2):1 6 Cheng CT, Chiou SJ, Lee CT, Tsai YB (2007) Study on probabilistic seismic hazard maps of Taiwan after Chi Chi earthquake. J GeoEngin 2(1):19 28 Coppersmith KJ (1991) Seismic source characterization for engineering seismic hazard analysis. In: Proceedings of the fourth international seismic zonation conference, vol 1. Earthquake Engineering Research Institute Stanford, California, pp 1 60 Cornell CA (1968) Engineering seismic risk analysis. Bull Seismol Soc Am 58(5): Harvard CMT Project (2015) Global CMT Catalog. Accessed 11 May 2015 Igarashi Y (2013) JMA tsunami assessment-travel time and wave forecasting including techniques, use and limitations. In: Regional training workshop on strengthening tsunami warning and emergency response standard operating procedures and the use of the ICG/PTWS PTWC new enhanced products, pp 8 9 IOC (2013) Exercise Pacific Wave 13. A Pacific-wide tsunami warning and communication exercise, 1 14 May 2013, vol 2. Summary Report, Paris IOC (2014) User s guide for the Pacific Tsunami Warning Center enhanced Products for the Pacific Tsunami Warning System. IOC Technical Series, Honolulu and Paris

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