Search

The occurrence of faulting in subducting plates is a major process that changes the mechanical properties of the subducting lithosphere and carries surface materials into mantle wedges. Two ocean-bottom seismometer networks deployed on the frontal accretionary wedge of the northern Manila trench in 2005 and on the outer slope of the trench in 2006 were used to detect earthquakes in the subducting plate. All available P and S manually picked phases and the waveforms of 16 short-period, three-component stations were used. Relocation was performed using the double-difference method with differential times derived from the phase-picked data. Two intraplate earthquake sequences of small-to-moderate magnitudes in the northern Manila subduction system were investigated in this study. The results revealed distinct fault planes, but a contrasting seismogeny over the northern Manila Trench. The seismicity in the frontal wedge (as measured in 2005) was mainly contributed by a fluid overpressure sequence, whereas that in the incoming plate (as measured in 2006) was contributed by the aftershocks of an extensional faulting sequence. The obtained seismic velocity models and Vp/Vs ratios revealed that the overpressure was likely caused by high pore-fluid pressure within the shallow subduction zone. By using the near-field waveform inversion algorithm, we determined focal mechanism solutions for a few relatively large earthquakes. Through the use of data obtained from global seismic observations, we determined that stress transfer may be responsible for the seismic activity in the study area during the period of 2005–2006. In late 2005, the plate interface in the frontal wedge area was unlocked by the overpressure effect due to a thrusting-dominant sequence. This event changed the stress regime across the Manila Trench and triggered a normal fault extension at the outer trench slope in mid-2006. However, in the present study, a hybrid focal mechanism solution indicating reverse and strike–slip mechanisms was implemented, and it revealed that the plate interface locked again in late 2006. [ABSTRACT FROM AUTHOR]

Graphical abstract Highlights • Seismogenic characteristics near the NE rifted margin of the South China Sea are revealed. • NW-SE tensional stress controls the Taiwan collision-Manila subduction transition. • Intraslab pre-existing faults affect the seismogenic behavior of a convergent system. Abstract Based on the distribution of an earthquake swarm determined from an ocean bottom seismometer (OBS) network deployed from 20 August to 5 September 2015 in the northernmost part of the South China Sea (SCS) and data from inland seismic stations in Taiwan, we resolved a M L 4.1 earthquake occurring on 1 September 2015 as a NE-SW-trending left-lateral strike-slip event that ruptured along the pre-existing normal faults generated during the SCS opening phases. The direction of the T-axes derived from the M L 4.1 earthquake and the background seismicity off SW Taiwan present high consistency, indicating a stable dominant NW-SE tensional stress for the subducted Eurasian Plate (EUP). The distinct stress variations on the two sides of these reactivated NE-SW trending features suggest that the presence of pre-existing normal faults and other related processes may lessen the lateral resistance between the Taiwan collision and Manila subduction system, and facilitate the slab-pull process for the subducting portion, which may explain the NW-SE tensional stress environment near the transition boundary in the northernmost part of the SCS. [ABSTRACT FROM AUTHOR]

The Eurasian Plate (EP) subducts beneath the Philippine Sea Plate (PSP) along the Manila subduction zone. The northern portion of this subduction system is located between the Taiwan and Luzon Island. Since the 2004 great Sumatra-Andaman earthquake, the tsunami generation potential and the hazard risk of the Manila subduction zone has been underlined. Although some main tectonic structures have been imaged based on geophysical approaches, the incomplete earthquake records caused by the poor seismic station coverage still limits our understanding for the seismogenic characteristics of the area. In order to obtain more information about the seismotectonics of the northern Manila subduction zone, two temporal seismic arrays composed of 8 ocean bottom seismometers (OBSs) were deployed along the northern Manila trench from June 22 to July 12, 2016 and from June 27 to July 20, 2017, respectively. During this recording period, 1437 earthquakes were determined and relocated. Less than 15 earthquakes have been reported in the international or local catalogue for the same period, showing the efficiency and necessity of OBSs networks for the seismictoctonic studies for the marine area. The result shows that the earthquakes appear to be segmented and divided into different groups. Furthermore, the hypocenters distribution reveals that most of the events occurred in the subducting plate to the west of the trench with depth shallower than 40 km and magnitude lower than 2, which infers the seismogenic behavior of the northern Manila Subduction system should be mainly controlled by the plate bending or/and the reactivation of tectonic structures in the subducting plate. Relatively less earthquakes are located to the east of the trench. Instead of spreading along the plate interface, they demonstrate a spatial concentration distribution. Further investigation is needed for the understanding of the mechanism. [ABSTRACT FROM AUTHOR]