Crustal Velocity Structures Imaged from Four-Component OBS Data Across the Southern Gagua Ridge in the Western Philippine Sea

  • Author(s): Jia-Ming Deng, Tan-Kin Wang, Ting-Ren Chen, Chao-Shing Lee, and Char-Shine Liu
  • DOI: 10.3319/TAO.2014.08.28.01(T)
  • Keywords: Crustal velocity structures, Four-component OBS data, Gagua Ridge, Poisson’s Ratio, Transpression

 

Abstract

Crustal structures near a linear oceanic ridge, the Gagua Ridge, between the West Philippine Basin and the Huatung Basin in the western Philippine Sea were imaged based on head-wave, refracted and reflected P-wave arrivals recorded from 24 ocean-bottom seismometers (OBS). Velocity anomaly zones, one below the Gagua Ridge summit and the others beneath two toes of the Gagua Ridge, imaged by large lateral variations in P-wave velocity of 5.5 - 6.4 km s-1 and low velocity of 4 - 5 km s-1 in the upper crust may have been generated when the Gagua Ridge was formed. East of the ridge, velocity anomaly zones, constrained by large lateral variations in P-wave velocity (4.8 - 6.4 km s-1), relatively low velocity (4 - 5 km s-1) and laterally high anomaly of Poisson¡¦s ratio (0.02 - 0.04) in the upper crust and abrupt crustal thickening (6 - 8.5 km) northward were obtained. West of the ridge, the velocity anomaly zones indicated by large lateral variations in P-wave velocity (5.2 - 6.2 km s-1) and laterally high anomaly of Poisson¡¦s ratio (0.02 - 0.04) in the upper crust and thick crust (thickening southward from 9 - 12 km) were found below the Huatung Basin and the Western Trough of the Gagua Ridge. Abrupt crustal thickening northward east of the ridge may be related to northwestward convergence of the Philippine Sea Plate. These velocity anomaly zones in the upper crust at both sides of the ridge might result from deformed, fractured or faulted zones. These zones support E - W compression, N - S shearing (or transpression) and uplifting that may have also created the Gagua Ridge and crustal thickening west of the ridge.

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