![]() ![]() Histogram of M w ≥ 3.5 events during 1 to 9. ![]() Histogram of M w ≥ 2.5 events during 1 to 9 and f. Histogram of the M w ≥ 3.5 earthquakes during 9 to 9 e. Histogram of the M w ≥ 2.5 earthquakes during 9 to 9 d. Blue and red bars show the histogram of negative and positive Coulomb stress changes respectively c. Accumulated event number (left y-axis) and magnitude distributions (right y-axis) during 1 to 9, associated with positive Coulomb stress changes (a) and negative Coulomb stress changes (b) c, d. Receiver fault of these local events are set the same as the sixth event (9, M w 6.2). Blue circles are the raw data, and black line is fitted flowing the Gutenberg-Richter law.įigure A3. The event distributions and their relationship with Coulomb stress changes of the first event at the regions plotted in Fig. Mechanism of the local events was assumed to be the same as the fifth event (a, c, and e: 1, M w 6.1) and the sixth event (b, d, and f: 9, M w 6.2).įigure A2. Number of earthquakes versus magnitude of events in the local catalog during 1 to 5. Solid circles show the local events ( M w ≥ 3.5) with Coulomb stress change $\Delta $ ≥ 5 kPa, within different depth range (a and b: 40–60 km c and d: 60–80 km e and f: 80–100 km). The Coulomb stress changes are computed at depth of 78 km and 63 km, receiver fault mechanism was set to be the fifth event (a, c, and e: 1, M w 6.1) and the sixth event (b, d, and f: 9, M w 6.2). ![]() The source fault model is based on the finite slip model from USGS website ( ). LL: Legaspi Lineament SVPF: Verde Passage Fault-Sibuyan Sea Fault SC: South China PSP: Philippine Sea Plate PP: Pacific Plate.įigure 2. Coulomb stress changes generated by the first event (9, M w 7.0). The red box in a marks the zoom-in region plotted in Fig. The focal mechanism of the six earthquakes and others (1976–2019) are obtained from the GCMT catalog. The main tectonic features are modified from Aurelio et al. The velocity ranges from 4.9 cm/a to 8.5 cm/a ( Yu et al., 2013). The purple lines and circles indicate the GPS velocity and errors, with respect to the Eurasian Plate. Hence, it is possible that several regions are relatively late in their earthquake cycles, which would enhance their susceptibility of being triggered by earthquakes at nearby and regional distances.įigure 1. The tectonic background (a) and focal mechanisms of earthquakes (b) at Philippines archipelago (PA). ![]() However, earthquakes with M w > 6.0 were relatively infrequent between 20 at PA. In the past 45 years, the released seismic energy shows certain peaks every 5–10 years. The dynamic stresses from these M w > 6 events are large enough (from 5 kPa to 3532 kPa) to trigger subsequent events, but a lack of seismicity and waveform evidence does not support delayed dynamic triggering among these events, even the shortest time interval is less than 24 hours. However, we cannot rule out the dynamic triggering mechanism, due to increased microseismicity in both positive and negative stress change regions, and an incomplete local catalog, especially right after the first M w 7.0 mainshock. We calculate the static Coulomb stress changes of the first five events, and find that the local seismicity after the 9 M w 7.0 earthquake is mostly associated with positive Coulomb stress changes, including the 1 M w 6.1 event, suggesting a possible triggering relationship. There are six earthquakes ( M w > 6) occurred between 2018/12//09/29 in PA, which provides an excellent opportunity to investigate the triggering relationship among these events. Philippine archipelago (PA) has strong background seismicity, but there is no systematic study of earthquake triggering in this region. ![]()
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