Seismicity of the Incoming Plate and Forearc at the Mariana Trench Recorded by Ocean Bottom Seismographs
Abstract
A yearlong ocean bottom seismometer (OBS) deployment spanning both the incoming Pacific Plate and the forearc is used to study the seismicity at the Mariana subduction zone. The 2012-2013 deployment consisted of 20 broadband OBS and 5 suspended hydrophones, with an additional 44 short period OBS and 15 suspended hydrophones for one month while the active source component of the deployment took place. Earthquakes are located using a nonlinear method (NonLinLoc, Lomax et al. 2000) that incorporates a 3D velocity model. The crustal model is based on active source profiles, and the mantle on Rayleigh wave tomography from earthquakes and ambient noise. 1,690 earthquakes have been located between February 2012 - January 2013, with a magnitude of completeness 2.5 ml. Of these events, the majority occurred in a large cluster beneath and just west of Big Blue Seamount, an active serpentine mud volcano. Preliminary focal mechanisms from regional waveform inversion indicate the occurrence of strike-slip and thrust earthquakes within the cluster, suggesting deformation along the megathrust as well as the forearc above. This is consistent with other evidence of a fractured and altered forearc wedge. A smaller cluster of events exists to the south, confirming results from an earlier Mariana seismic study that the forearc seismicity is not evenly distributed and reflects variable conditions along the trench.
Nearly 400 events occurred seaward of the trench and are associated with the bending of the incoming plate. Extensional plate-bending earthquakes may play a part in the hydration of the subducting oceanic crust and mantle by providing a pathway for water to reach mantle depths. These events have been located to depths of 35 km below the seafloor, or 28 km below the Moho, and preliminary focal mechanisms indicate normal faulting to at least 30 km depth below seafloor. Laterally, the furthest event is 173 km from the trench, although the majority of events occur within 70 km of the trench. The onset of earthquakes may indicate where water starts to infiltrate the plate, and is consistent with evidence of alteration to the subducting mantle from velocity models of the area. If alteration occurs to the depths indicated for normal faulting events, the amount of water subducted in the Mariana trench is greater than previously estimated.- Publication:
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AGU Fall Meeting Abstracts
- Pub Date:
- December 2018
- Bibcode:
- 2018AGUFM.T41H0405E
- Keywords:
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- 1242 Seismic cycle related deformations;
- GEODESY AND GRAVITYDE: 7240 Subduction zones;
- SEISMOLOGYDE: 8123 Dynamics: seismotectonics;
- TECTONOPHYSICSDE: 8170 Subduction zone processes;
- TECTONOPHYSICS