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Seismic Link at Plate Boundary

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Seismic Link at Plate Boundary Seismic link at plate boundary Faical Ramdani∗, Omar Kettani and Benaissa Tadili Mohamed V University, Scientific Institute, Physics of the Earth Laboratory, Rabat, Morocco. ∗Corresponding author. e-mail: [email protected] Seismic triggering at plate boundaries has a very complex nature that includes seismic events at varying distances. The spatial orientation of triggering cannot be reduced to sequences from the main shocks. Seismic waves propagate at all times in all directions, particularly in highly active zones. No direct evidence can be obtained regarding which earthquakes trigger the shocks. The first approach is to determine the potential linked zones where triggering may occur. The second step is to determine the causality between the events and their triggered shocks. The spatial orientation of the links between events is established from pre-ordered networks and the adapted dependence of the spatio-temporal occurrence of earthquakes. Based on a coefficient of synchronous seismic activity to grid couples, we derive a network link by each threshold. The links of high thresholds are tested using the coherence of time series to determine the causality and related orientation. The resulting link orientations at the plate boundary conditions indicate that causal triggering seems to be localized along a major fault, as a stress transfer between two major faults, and parallel to the geothermal area extension. 1. Introduction (Krishna Mohan and Revathi 2011). This shows that the link between events is a complex feature Plate boundaries are the zones where most earth linked to varying network models (Abe and Suzuki dynamics are focussed. The complexity of tectonic 2006). Links limited to individual events have been boundaries draws attention to them as the largest related to low strain rate environments (Hough et al. earthquakes are felt in these areas and they elicit 2003), visco-elastic relaxation (Lorenzo-Martin et al. the natural hazard of seismic activity. However, the 2006), and solid Earth tides (Cochran et al. 2004). sequences of the main shocks and the triggering By analyzing earthquake pairs over a period of process constrain the seismic hazard assessment. many years, Wan et al. (2004) suggested that Some frequent issues remain as to whether the seis- Coulomb stress triggering may be observed for micity of the interplate zone with a high strain rate thrust earthquakes, while McKernon and Main has random aspects and may be associated with a (2005) limited triggering process to about 150 km. triggering mechanism, and how the seismicity can Extending earthquake activity to a spatial net- be associated with fault patterns and plate motion. work link shows that the alignment of the links is Triggering causes changes in the Coulomb stress on parallel to the Honshu Trench azimuth in Japan a specified fault, which is independent of regional (Tenenbaum et al. 2012) and the direction of the stress but which depends on the fault geometry, San Andreas Fault (SAF) (Jimenez et al. 2008). In the sense of slip, and the coefficient of friction the Ibero–Moroccan region, main shocks and their (King et al. 1994). Restricting the causality con- aftershocks are investigated as local features due to nection to a single predecessor or to an arbitrary fault mechanisms except for some historical event mainshock-aftershock scenario may not be enough studies related to triggering in the Catalan and NE Keywords. Seismic link; time series; distant earthquakes; causality; plate boundary. J. Earth Syst. Sci. 124, No. 4, June 2015, pp. 697–705 c Indian Academy of Sciences 697 698 Faical Ramdani et al. Iberian regions (Perea 2009). As zones of perma- zone along the plate boundary. In those particu- nent activity make it difficult to detect causal trig- lar areas, it is more convenient to introduce the gering, it is important to search for the statistical triggering flow direction that considers the link flow of triggering. The use of individual events is between active zones; then, when a threshold link complex as varying fault systems may be in a criti- is reached, the orientation of flow may be esti- cal state of failure before the passage of stress from mated to delimit the direction from the driver to the driver event. Thus, it is useful to consider a zon- the recipient zone in terms of statistical activity. ing characterized by a set of events over a period However, many plate boundaries include a consid- erable number of events that are difficult to pro- of many years before searching for triggering. This cess as we are limited by the running time. In this provides interdependent zones of coeval activity in regard, a specific time window was adopted for which causal triggering may be estimated. We test each of the plate boundary catalogs. Pacific zones a seismic link network from catalogs of Turkey and of subduction present abundant background seis- California, Gibraltar as zones of collision, and the micity ranging with various magnitudes, such as Philippines, New Zealand, and Japan as subduc- the Japan and New Zealand regions. We adopted tion zones. Short-time triggering and a long-lived a 10-year time window because it is a reasonable process of two decades are included, and the main period to search for a link between distant regions. objective is to first establish zones of spatial seismic The catalogs have to follow Gutenberg–Richter dependence. The seismic grids represented by time regression from a specific magnitude: the Magni- series are further tested to determine the drivers tude of Completeness (Mc). Mc is calculated from from recipient zones based on the coherence of Zmap by using maximum likelihood solution. How- the time series. Comparing high linked zones with ever, the use of Mc cut-off in these catalogs shows stress field azimuths and local tectonics provides that the remaining event number is considerable insight into some possible widespread behaviour of so we then increased our minimum magnitude to seismic triggering in the collision zones. 3 for Japan and New Zealand. In turn, Philippine regions show Mc4.5 which reduced the database too much, so in this case, the lower magnitude cut- off is reduced to 3. The data was obtained from 2. Dataset and methods the Japan University Network Earthquake Cata- logue (JUNEC) and GeoNet of New Zealand for the The databases used in this study include events period spanning from 1988–1998. For other regions that occurred in a relatively large area of the plate (the Philippines, Turkey, California), the catalogs boundary, but they are limited in space because were obtained from the US Geological Survey with linked earthquakes of major events may traverse a time window of 25 years (1988–2012) and the Isti- the entire earth. Since a set of shocks may trig- tuto Geogr´afico Nacional (Spain) for the Gibral- ger events far away from the main shock areas, tar region. The magnitude cut-off was adopted they must be compiled during a relatively impor- (table 1) as we investigated the long-distance link tant time window. In turn, triggered events may of the shocks, and the links related to the lower also trigger events in the main shock source zone. magnitudes were not included. Many earthquakes For this reason, the activity measured between may occur after wave arrivals, so we then enlarged zones represents a cross-correlation in both space the period necessary for triggering and we also and time. Sequences from aftershocks are not the made the distance as large as possible around only factor controlling the threshold link owing to the plate boundaries in order to include most of the wide time span and distant zones within the the events. The plate boundaries were partitioned plate boundaries. Individual shocks cannot repre- into a 1◦ × 1◦ grid. The grids are described by sent the source zones of the triggering in zones a time series of event numbers to each temporal of high activity where shocks occurred in every sample, and the activity within a grid covers a Table 1. The data used in the link network processing, the magnitude cutoff, score obtained by maximal coefficient R and related distance. Score Distance Region Period Nb events Mc cut-off Rmax PSI (km) Japan 1988–1998 39403 3 1 1.67 143 NZ 2002–2012 40091 3 1 3.13 1072 Gibraltar 1988–2012 6145 2.5 0.75 33.09 631 California 1988–2012 23350 2.8 1 22.19 620 Philippines 1988–2012 9831 3 0.8 35.52 332 Turkey 1988–2012 18136 2.7 0.7 31.11 88 Seismic link at plate boundary 699 period sample of 90 days, and the next sample (figure 1b). When the links of high correlated will cover the next 90-day non-intersecting time regions were established to each grid, we tested period. The prominent factor in determining a link the time series of the grid couple (i, j)inorder between distant regions is that there is at least one to estimate the direction of flow. The goal was period during a 25-year span where both cells are to obtain information on the causality between seismically active. The seismic dependence between the well-correlated grid couple, which shows the the cells increases linearly when the number direction of triggering. The method is based on a of simultaneous active periods increases. Standard frequency average of the slope of the phase coher- cross-correlation includes simultaneous passive ence with respect to the instantaneous mixture periods that may not fit the correlation, e.g., of the independent source (Nolte et al. 2008a). Pearson’s correlation; instead, we suggest a method This method, called the Phase Slope Index (PSI), to compute a coefficient based on cancelling peri- includes the non-linear interaction between the zones, which incorporates waves coming from other ods free from earthquakes and we consider only seismic sources independent from the two zones the number of events in the same temporal sam- under the link.
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