16:30 〜 16:45
[SSS14-23] 繰り返しアレイ測量によるフィリピン断層のクリープ変位速度分布と中規模地震の発生位置の関係
キーワード：フィリピン断層、クリープ変位速度、アレイ測量、中規模地震
The Philippine fault is one of the fastest-slipping faults in the world, with left-lateral slip rates up to ~30 mm/yr. Some of its central segments in Masbate, Leyte, and Mindanao Islands are known to creep aseismically. Tsutsumi and Perez (2024) presented the first verifiable evidence of surface creep on Leyte Island based on surveys of offset cultural features. Creep rates estimated from the offset cultural features in the past 10–35 years are 20.9 ± 4.3 mm/yr for northernmost Leyte and ~15 mm/yr for central Leyte. Since 2013, we installed alignment arrays across the Philippine fault at 16 sites on Masbate, Leyte, and Mindanao Islands. Following the methods used by Lienkaemper et al. (2001), an alignment array comprises three fixed points, usually nails driven into concrete or pavement. A high-precision total station is centered and leveled over an instrument point (IS) on one side of the fault, and a first target is centered and leveled over an orientation point (OS) on the same side of the fault as the total station. A second target is centered and leveled over an end point (ES) on the opposite side of the fault. The change in the angle between the IS-ES and IS-OS directions between successive surveys at the site determines the amount of creep. The precision of the survey is that we can confidently detect any movement greater than 1–2 mm between successive surveys. We surveyed each site on average every other year and obtained geodetic creep rates and their temporal variations. We measured the highest creep rates at ~35 mm/yr on the northernmost Leyte Island. In central Leyte, we obtained a ~25 mm/yr rate with a surprisingly constant rate through time. These rates are higher than those derived from offset cultural features, probably because of insufficient coupling with the ground surface and plastic deformation of the cultural features. At a site between these fast-creeping sites, we measured a creep rate of 6.1 mm/yr, considerably lower than the surrounding sites, which suggested a slip deficit. In 2017, an Mw 6.5 earthquake ruptured the segment where the low creep rate was measured, producing a ~26-km-long surface rupture with a maximum left slip of 1 m. This earthquake demonstrated the presence of a partially locked patch along the creeping segment of the Philippine fault. The surface rupture area coincides where the fault steps about 1 km to the left along an otherwise continuous and straight fault on Leyte Island. We also identified evidence of surface creep on Mindanao Island. Creep rates measured at 2 sites on northern Mindanao are 5–15 mm/yr, considerably lower than the rates on Leyte. The Philippine fault on northern Mindanao ruptured the surface during the 2017 Ms 6.7 Surigao earthquake and the 1879 Ms 6.9 earthquake. On Masbate Island to the north, 4 alignment array survey sites yielded 5–10 mm/yr creep rates, considerably lower than on Leyte. Two surface-rupturing moderate earthquakes occurred in this creeping section during this century. In 2003, a moderate earthquake (Ms 6.2) produced ~18 km long surface rupture precisely along the Philippine fault with a maximum horizontal displacement of 47 cm. This fault section ruptured again in 2020 with an Mw 6.6 earthquake. About 24-km-long surface ruptures appeared along the 2003 ruptures. The maximum left-lateral displacement was 30–40 cm. Considering the slip amount and time interval between the successive events, the 2020 earthquake released strains roughly comparable to those released by creeping since 2003. The left-lateral motion on the Philippine fault on Masbate Island is accommodated by both aseismic creeping and coseismic motion. Estimating the creep rate and the ratio between creep and coseismic displacement is essential to better characterize the Philippine fault's mechanics and seismic hazards.