ALBERT

Description: The active collision between the Eurasia and Philippine Sea plates in eastern Taiwan has been explored from the recently determined 3D velocity images and relocated hypocenters. A north-northeast - south-southwest-trending high-velocity zone corresponding to the oceanic upper mantle is narrowly defined underneath the collision suture from Hualien to Taitung. This elevated and hot oceanic upper mantle must have played an important role in the tectonic evolution/mountain- building process of the adjacent continental crust. A northwest-dipping seismic zone can be identified in the northern collision zone extending from the surface to approximately 30 km depth, which can be correlated with the northern Longitudinal Valley Fault (LVF). This zone marks a transitional plate boundary separating the high VP and high VP/VS oceanic crust to the east and the high VP and VS upper crust and low VP and low VP/VS mid-to-lower continental crust to the west. A significant amount of plate convergence along the suture has been accommodated by the high-angle thrusting along the northern LVF. In contrast, a southeast-dipping seismic zone can be identified extending from the surface to approximately 25 km depth near Taitung in the southern collision zone. This zone coincides with a region of high VP and high VP/VS, suggesting that earthquakes occurred within a highly fractured or fluid-rich zone. The reverse polarity of active-plate boundary faults marks two distinguished transition boundaries, one from eastward subduction in southern Taiwan to east-west collision in the southern collision zone corresponding to the early phase of plate collision, and the other from east-west collision to northwest subduction in the northern collision zone corresponding to the advanced phase of plate collision. The central collision zone is creeping and aseismic, which can be attributed to the high heat flow and geothermal activity during an interseismic period since the 1951 Taitung earthquake.

Description: The 9 February 2010 M (sub w) 3.1 Siheung earthquake occurred close to the metropolitan Seoul City in the central Korean peninsula and was widely felt by residents in Seoul. The shock occurred at a depth of 12 km in Precambrian crystalline bedrock with predominantly strike-slip faulting on a near vertical fault plane striking east-southeast-west-northwest. A rupture radius of approximately 0.12+ or -0.01 km is estimated for the M (sub w) 3.1 shock, with a stress drop of 15.2 MPa. Sixteen small repeating earthquakes, spread in time up to about five and a half years, are detected based on their waveform similarities in a stable continental region setting. These shocks cluster along a trend striking approximately 110 degrees , which suggests a nodal plane of the mainshock focal mechanism striking N109 degrees is the likely fault plane. The repeating events around Siheung, Korea, show a recurrence interval of 0.77+ or -0.37 year/event for shocks of magnitude range M (sub w) 0.6-3.1. The waveform cross-correlation detector provided identification of small repeating earthquakes with similar waveforms on an expanded time window, which may suggest the existence of such repeating earthquakes in stable regions worldwide with relatively low seismicity. Such data may lead to improvements in delineating seismogenic features, understanding the earthquake source properties, and evaluating the earthquake hazards associated with infrequent occurrence of such earthquakes in stable regions.

Description: A moderate-sized earthquake (M (sub L) 4.8) occurred in the mideast Korea Peninsula on 20 January 2007. It was the largest inland earthquake to occur there since the inception of a modern seismic observation system. Although only four aftershocks were noticed in previous studies, a careful review of continuous data revealed that the main event was accompanied by at least 74 micro foreshocks and aftershocks. A subset of 25 events was selected for further analysis to determine precise earthquake locations, focal mechanism solutions, and the current status of regional tectonic stress, as well as to answer questions raised about the sequence. Earthquake hypocenters were seen to be more clustered after the HypoDD relocation. A source radius of 1 km for the main event was estimated based on the distribution of precisely determined aftershock locations. Focal mechanism solutions of larger events in the sequence suggest either a left-lateral strike-slip fault trending west-northwest-east-southeast or a right-lateral strike-slip fault trending north-northeast-south-southwest as the responsible structure. Although the Woljeongsa fault striking north-northeast-south-southwest in the local geological map matches one of the proposed trends, precise earthquake relocation results gave a contradictory result, showing that a previously unknown west-northwest-east-southeast striking fault was responsible for the earthquake sequence. We also observed an unusual lack of large-magnitude aftershocks, a relatively large stress drop during the main event, and no previous earthquake record in the region. Observations made in the study consistently indicate the sequence nucleated along a less-developed fault. Focal mechanism solutions suggest the current status of tectonic stress governing earthquake generation in Korea is east-northeast-west-southwest compression and north-northwest-south-southeast extension.

Description: High-quality first-arrival data collected with a high-density temporary seismic array and regional seismic network were used to construct a P-wave velocity model and identify the precise location of earthquakes in the active collision zone of southeastern Taiwan. A crustal-scale weak zone, defined by high seismicity, is characterized by a steeply east-dipping lower-velocity anomaly and represents a boundary between an uplifted upper-mantle and the Luzon volcanic arc. The main features of the weak zone, with its associated low-velocity zone and seismicity pattern, vary significantly along the collisional boundary of southeastern Taiwan. Along the weak zone, unlithified sediments or highly fractured materials are compacted as the Philippine Sea plate moves toward the Eurasian plate. In areas of more advanced collision, soft material is more compacted and can therefore store significantly greater amounts of strain energy. This energy is released through episodic earthquakes.