Poster of the Taiwan Region Earthquake of 26 December 2006 - Magnitude 7.1

Tectonic Summary

The southwestern Taiwan earthquakes of 2006 December 26, 12:26 UTC (M = 7.1) and 12:34 UTC (M = 6.9) occurred in a zone of transition along the north-south boundary between the Eurasian plate and the Philippine Sea plate. The Eurasian plate is moving east-southeast with respect to the Philippine Sea plate at a velocity of about 80 mm/y. A long the plate-boundary south of Taiwan, the Eurasian plate is oceanic lithosphere, and convergence is mostly accommodated by the subduction of the Eurasian plate beneath the Philippine Sea Plate. The subducted Eurasian plate is seismically active to depths of about 200 km offshore of southeastern Taiwan, to the east-southeast of the December 26 shocks. North along the plate boundary from southwestern Taiwan to northern Taiwan, by contrast, the Eurasian plate is buoyant continental lithosphere that resists subduction, and a significant fraction of plate convergence is accommodated by intense compressional deformation of the earths crust rather than by subduction of one plate beneath the other.

Preliminary focal-mechanism solutions indicate that the earthquake of 12:26 UTC occurred as the result of normal faulting and that the earthquake of 12:34 UTC occurred as the result of predominantly strike-slip faulting.

The normal-faulting focal-mechanism of the shock of 12:26 UTC suggests that this shock occurred as the result of intraplate stresses within the subducting Eurasian plate. Normal-faulting focal-mechanisms are commonly observed in the shallow parts of subducting plates; the causative stresses are generated by the bending of the subducting plates. A normal-fault focal-mechanism is not consistent with the earthquake having occurred as the result of shallow compressional deformation between two converging plates which each consist of buoyant lithosphere.

Presently available evidence does not permit a confident statement on whether the earthquake of 12:34 UTC occurred as the result of shallow deformation caused by convergence between two plates consisting of buoyant lithosphere or instead occurred as the result of deeper deformation within a subducted and deformed Eurasian plate. The style of faulting preliminarily inferred for the shock of 12:34 UTC, right-lateral strike-slip faulting on a northeast striking fault or left-lateral faulting on a north-northwest striking fault, would be consistent with the style of faulting that has been observed at the surface in southwestern Taiwan and that helps accommodate the mutual convergence of buoyant Eurasian lithosphere and Philippine Sea lithosphere. It is possible, however, that this style of faulting could also occur as the result of intraplate stresses within the subducting Eurasian plate, beneath its boundary with the Philippine Sea plate.