Abstract
Active faults commonly repeat cycles of sudden rupture and subsequent silence of hundreds to tens of thousands of years, but some parts of mature faults exhibit continuous creep accompanied by many small earthquakes. Discovery and detailed examination of creeping faults on land have been in a rapid progress with the advent of space-borne synthetic aperture radar interferometry. In this study, we measured the spatial variation of the creep rate along the Philippine fault on Leyte Island using ALOS/PALSAR data acquired between October 2006 and January 2011. Prominent creep of 33pm {11} mm/year was estimated in northern and central parts of the island except for a locked portion around latitude 11.08–11.20^circ N. We compared the creep rate distribution along the fault with the slip distribution of the 2017 M_w 6.5 Ormoc earthquake which occurred in northern Leyte, estimated from the displacements mapped by ALOS-2/PALSAR-2 interferometric data. The estimated slip of the 2017 earthquake amounted up to 2.5 m and to moment magnitude of 6.49, with the dominant rupture area coinciding with the locked portion identified from the interseismic coupling analysis. Teleseismic waveforms of the 2017 earthquake and another event that occurred in 1947 (M_s 6.9) exhibit close resemblance, indicating two ruptures of rather similar locations and magnitudes with a time interval of 70 years.
Highlights
It has been known that faults accommodate a spectrum of fault slip from fast rupture of regular earthquakes to stable sliding, and slow earthquakes in between (e.g., Peng and Gomberg 2010; Avouac 2015)
Large repeaters of M≥ 6 are more difficult to find than smaller ones because of smaller number of samples (Uchida and Bürgmann 2019), large repeaters would be valuable for studying earthquake physics because the rupture characteristics can be known in detail compared to smaller ones, for example by examining the microearthquakes that occur around large earthquake ruptures (Uchida et al 2012)
Seismic potential in northern and central Leyte In the previous sections, we showed that (1) the Philippine fault in northern and central Leyte has been creeping with a rate of 33 ± 11 mm/year, with the exception of a locked segment within a major 15-km-long step-over section (Box B, latitude 11.08◦ N–11.20◦ N, Fig. 10), (2) the 2017 earthquake occurred along the Philippine fault and the rupture extent well corresponds to the locked section (Fig. 12), and (3) the 1947 earthquake ruptured a similar area on the fault
Summary
It has been known that faults accommodate a spectrum of fault slip from fast rupture of regular earthquakes to stable sliding, and slow earthquakes in between (e.g., Peng and Gomberg 2010; Avouac 2015). As summarized by Harris (2017), other examples of relatively large earthquakes that occurred on faults with spatial variations in the degree of fault locking include 1868 Mw 6.8 Hayward earthquake (Bakun 1999), a few earthquakes in 1970s–1980s along Eureka Peak, Imperial, San Andreas, and Superstition Hills faults in Southern California (Louie et al 1985), and multiple Mw > 6.8 events along the Longitudinal Valley Fault in Taiwan (Thomas et al 2014a). We claim that the 2017 Mw 6.5 Ormoc earthquake, that ruptured the Philippine fault in the northern part of Leyte Island and caused damages, was close to a first-class characteristic earthquake that ruptured an isolated locked segment within creeping sections. The results from two descending data sets were merged after obtaining the mean velocity (Additional file 1: Figure S2)
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