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Regional Dependence of Seismic Migration Patterns Terr. Atmos. Ocean. Sci., Vol. 23, No. 2, 161-170, April 2012 doi: 10.3319/TAO.2011.10.21.01(T) Regional Dependence of Seismic Migration Patterns Yi-Hsuan Wu1, *, Chien-Chih Chen 2, John B. Rundle 3, and Jeen-Hwa Wang1 1 Institute of Earth Sciences, Academia Sinica, Taipei, Taiwan, ROC 2 Department of Earth Sciences and Graduate Institute of Geophysics, National Central University, Jhongli, Taiwan, ROC 3 Center for Computational Sciences and Engineering, UC Davis, Davis, CA, USA Received 24 May 2011, accepted 21 October 2011 ABSTRACT In this study, we used pattern informatics (PI) to visualize patterns in seismic migration for two large earthquakes in Taiwan. The 2D PI migration maps were constructed from the slope values of temporal variations of the distances between the sites and hotspot recognized from the PI map. We investigated the 2D PI migration pattern of the 1999 Chi-Chi and 2006 Pingtung earthquakes which originated from two types of seismotectonic setting. Our results show that the PI hotspots mi- grate toward the hypocenter only when the PI hotspots are generated from specific depth range which is associated with the seismotectonic setting. Therefore, we conclude that the spatiotemporal distribution of events prior to an impending earthquake is significantly affected by tectonics. Key words: Earthquake forecast, Pattern informatics, Precursory seismicity, Seismic migration, Seismic nucleation Citation: Wu, Y. H., C. C. Chen, J. B. Rundle, and J. H. Wang, 2012: Regional dependence of seismic migration patterns. Terr. Atmos. Ocean. Sci., 23, 161- 170, doi: 10.3319/TAO.2011.10.21.01(T) 1. INTRODUCTION To mitigate seismic risks, predicting or forecasting an Some previous studies have investigated precursor ac- impending earthquake is a serious challenge in seismol- tivity before a large earthquake using the PI method (Chen ogy. To gain insight into earthquake physics, dynamical et al. 2005; Wu et al. 2008a). Before the occurrence of an models (cf. Main 1996; Wang 2008) and statistical physics earthquake, the hotspots, which will be explained below, can methods (cf. Main 1996) have been used to study seismic- be observed on or around the epicentral area. In addition, ity and related phenomena, including quiescence, activa- anomalous seismicity recognized from the PI map seems to tion, and migration, prior to large impending earthquakes. migrate toward the epicenter of an impending earthquake Without knowledge of the underlying dynamics, it is dif- (Wu et al. 2008a). This migration has also been observed in ficult to comprehensively understand such behavior. Study- simulated models and rock experiments (Ohnaka and Ku- ing anomalous seismicity related to precursor processes for wahara 1990; Rice 1992; Lapusta and Rice 2003). Due to large earthquakes using statistical methods can help under- a lack of systematic methods for investigating the temporal stand how large earthquakes are initiated. Among the sta- variation in earthquake migration patterns in the past, only a tistical physics methods (Bowman et al. 1998; Jaumé and few studies (Ando 1975; Mogi 1988) have been conducted. Sykes 1999; Hainzl et al. 2000; Zöller and Hainzl 2002; Wu et al. (2008b) improved the PI method to verify seis- Zöller et al. 2002; Chen 2003; Chen and Wu 2006; Wu et al. mic migration. They introduced the self-organizing spinodal 2008a, b), the pattern informatics (PI) method is a powerful (SOS) model to specify related time parameters. Using their candidate. Using the PI method, a dynamic pattern prior to a modified PI method, time-varying patterns of seismicity can large earthquake can be constructed from observed seismic- be produced in a progressive series of changing intervals. ity (Rundle et al. 2000a, b; Tiampo et al. 2002a). They observed a decreasing trend in the distances between the PI hotspots and epicenters, and also estimated the slope of the function to approach the decreasing trend. Wu (2010) expanded the technique to two dimensions, with the ca- * Corresponding author pability for calculating and imaging the slopes for all grid E-mail: [email protected] 162 Wu et al. sites in a study area. Wu (2010) also identified a donut-like 11 km on a side. The dimension of these boxes corresponds migration pattern where the PI hotspots migrate toward the roughly to the linear size of a M ≈ 6 earthquake (Wells and epicenter in both time and space domains. This approach Coppersmith 1994) which could have sufficient influence provides a method to image the nucleation of cracks and on the stress and seismicity within the entire box (Nanjo et accumulation of stresses around the epicenter until the fault al. 2006). After dividing the study region into grid boxes of breaks. 0.1° × 0.1° in size and evaluating the values of time param- Seismicity and fault types often show regional depen- eters, the seismic intensity I(xi, tb, t), which is defined as the dence due to different tectonic conditions (cf. Isacks et al. average number of earthquakes with magnitude, M, larger 1968). In South California, most earthquakes show strike- than the cutoff magnitude, Mc, in a grid box xi and its eight slip faulting; while in central Japan, most of the earthquakes neighboring boxes (so-called the Moore neighbors) during show thrust faulting. Compared to earthquakes occurring tb and t, is evaluated. Here, tb is a sampling reference time in southern California and central Japan, the complex tec- that will be shifted from t0. The seismicity change between tonic background in Taiwan would make forecasting earth- t1 and t2 is: quakes more difficult. Taiwan is located at the junction of the collision zone between the Philippines Sea and Eurasian DIxib,,tt12,,tx=-I ibtt,,12Ixibtt, (1) ^^hh^ h plates (Seno 1977; Tsai et al. 1977; Wu 1978; Lin 2002). The tectonic conditions in this region are highly complex. and will be denoted as ∆I(xi, tb) hereafter. To show the change Seismicity in Taiwan also shows regional dependence (cf. in seismicity, background values are removed through nor- Wang 1988; Wang et al. 1994). Two large earthquakes, the malization, i.e., subtracting the temporal and spatial means 1999 Ms 7.6 Chi-Chi earthquake (Ma et al. 1999) and 2006 and dividing by their individual standard deviations. Seis- Ms 7.0 Pingtung earthquake (Ma and Liang 2008), occurred mic anomalies are shown by positive value for activation in different tectonic provinces in Taiwan. In this study, we and negative values for quiescence. To include the two used the 2D PI migration method to measure seismic activi- anomalies in the PI index, we take the absolute value and ties preceding the two earthquake sequences to examine the then estimate its temporal average and mean square. The regional dependence of dynamic patterns of seismicity. mean squared change indicates the occurrence probability of future threshold events, so it is plotted as hotspots on a map to show the areas at risk. 2. METHODOLOGY: PI METHOD AND MIGRA- To determine whether the hotspots migrate toward the TION PATTERN MEASUREMENT epicenter and to quantify the migration, Wu et al. (2008b) According to the concept of pattern dynamics, the PI defined the error distance, εn, to be the distance between the state vector can show seismicity changes of a system within nth earthquake and the nearest hotspot, as shown in Fig. 1a. an interval from t1 through t2, following the steps (Rundle et They also defined the integrated error distance, εarea(t1), to al. 2000a; Tiampo et al. 2002a, b; Chen et al. 2006; Holliday be the area below the curve of the mean error distance f et al. 2007; Wu et al. 2008a, b). The study region and time versus coverage ratio f. The coverage ratio f is defined as parameters must be determined first. The time parameters fA= H A , where A is the total area of the study region and include the beginning of data, t0, starting point of seismicity AH is the area covered by hotspots. The mean error distance change, t1, and end of seismicity change, i.e., the time just is calculated as before large earthquake, t2. The study region is discretized 1 N into grid boxes with a size of 0.1° × 0.1°, approximately ff= / n (2) N n = 1 (a) (b) Fig. 1. (a) Schematic diagram of the definition of error distance. The blocks represent the hotspots in a PI map and the circles the locations of large earthquakes occurred after t2. The error distance εn is the distance between the nth earthquake and the hotspot closest to it. (b) The relation between the mean error distance and the coverage fraction f of the PI hotspots. Regional Dependence of Seismic Migration Patterns 163 which varies with the fraction f. Figure 1b shows a decrease about 8 minutes apart, were located offshore southwestern in f with increasing f. The decrease is due to the gen- Taiwan (Yen et al. 2008). The focal depths were 44 and 50 eration of more hotspots in the PI map with a lower thresh- km, respectively, for the first and second earthquakes. The old, i.e., a higher f. Accordingly, the temporal evolution of focal plane solutions indicated normal faulting for the first anomalous seismic activity on every site can be examined event and strike-slip faulting for the second. They occurred by a plot of εarea(t1) versus t1 as shown in Fig. 2. A site with- in the area where the Eurasian plate starts to subduct east- out significant activity, i.e., a lack of hotspot, would have a ward beneath the Philippine Sea plate and thus in the South- flat trend, with a slope value close to 0, as in Fig. 2a. When western Seismic Zone defined by Wu and Chen (2007).
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