Complex rupture process of the 2014 Thailand Mw 6.2 earthquake on the conjugate fault system of the Phayao fault zone

The earthquake with a moment magnitude 6.2 that occurred in northern Thailand on 5 May 2014 is the largest recorded in Thailand by modern seismographs; the source is located in the multi-segmented complex fault system of the Phayao fault zone in the northern Thai province of Chiang Rai. This geological setting is appropriate environment for investigating a compound rupture process associated with a geometrically complex fault system in a magnitude-6-class earthquake. To understand in detail the rupture process of the 2014 Thailand earthquake, we elaborate the flexible finite-fault inversion method, used it to invert the globally-observed teleseismic P waveforms, and resolved for the spatiotemporal distribution of both the slip and the fault geometry. The complex rupture process consists of two distinct coseismic slip episodes that evolved along two discontinuous fault planes; these planes coincide with the lineations of the aftershock distribution. The first episode originated at the hypocenter and the rupture propagated south along the north-northeast to south-southwest fault plane. The second episode was triggered at around 5 km north from the epicenter and the rupture propagated along the east-northeast to west-southwest fault plane and terminated at the west end of the source area at 4.5 s hypocentral time. The fault system derived from our finite-fault model suggests geometric complexities including bends. The derived spatiotemporal orientation of the principal stress axis shows different lineations within the two rupture areas and heterogeneity at their edges. This geological setting may have caused the perturbation of the rupture propagation and the triggering of the distinct rupture episodes. Our source model of the 2014 Thailand earthquake suggests that even in the case of small-scale earthquakes, the rupture evolution can be complex when the underlying fault geometry is multiplex.

Revealing new hydrocarbon potential through Q-compensated prestack depth imaging at Wenchang Field, South China Sea

Wenchang Field in the South China Sea contains a well-developed fault system, resulting in complex subsurface geology. Imaging the complex fault system plays an important role in hydrocarbon exploration in this area since the fault system forms a link between the source rocks and reservoirs. However, it is difficult to obtain a high-quality depth image of the fault system due to the effects of complex velocity and seismic absorption. Inaccurate depth velocities lead to fault shadows and structure distortions at the target zone. Absorption effects further deteriorate seismic imaging as they cause amplitude attenuation, phase distortion, and resolution reduction. We demonstrate how a combination of high-resolution depth velocity modeling and Q imaging work together to resolve these challenges. This workflow provides a step change in image quality of the complex fault system and targeted source rocks at Wenchang Field, significantly enhancing structure interpretation and reservoir delineation. A couple of commercial discoveries have been made, and several other potential hydrocarbon reservoirs have been identified based on the reprocessed data, which reveal new hydrocarbon potential in this region.