M 7.1 - 30 km SW of Hengchun, Taiwan

Tectonic Summary

The December 26, 2006, M 7.1 Taiwan region earthquake occurred in a zone of tectonic transition along the north-south boundary between the Eurasia plate and the Philippine Sea plate, off the south coast of Taiwan. Focal mechanism solutions indicate that rupture occurred as normal faulting on either steeply dipping fault or on a shallowly dipping fault. At the location of the earthquake, the Eurasia plate moves east-southeast with respect to the Philippine Sea plate at a velocity of about 80 mm/yr. Along the plate boundary south of Taiwan, the Eurasia plate is oceanic lithosphere, and convergence is mostly accommodated by the subduction of the Eurasia plate beneath the Philippine Sea plate. The subducted Eurasia plate is seismically active to depths of about 200 km offshore of southeastern Taiwan, to the east-southeast of the December 26th earthquake. North along the plate boundary from southwestern Taiwan to northern Taiwan, by contrast, the Eurasia plate is buoyant continental lithosphere that resists subduction, and a significant fraction of plate convergence is accommodated by intense compressional deformation of the Earth’s crust rather than by subduction of one plate beneath the other. A predominantly strike-slip faulting earthquake of M 6.9 (12:34 UTC) occurred only 8 minutes after this M 7.1 event (12:26 UTC).

The normal-faulting focal mechanism solution of the 12:26 UTC event suggests that it occurred as the result of intraplate stresses within the subducting Eurasia plate. Normal-faulting focal mechanism solutions 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 solution is not consistent with the earthquake resulting from the result of shallow compressional deformation between two converging plates which each consist of buoyant lithosphere. Slip on a fault aligned with either nodal plane of the focal mechanism solution is consistent with this intraplate setting.

Presently available evidence does not permit a confident statement on the whether the earthquake at 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 Eurasia plate. The style of faulting preliminarily inferred for the event at 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 Eurasia 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 Eurasia plate, beneath its boundary with the Philippine Sea plate.

The region surrounding Taiwan hosts frequent large earthquakes, with 10 earthquakes of magnitude 7+ within 400 km of the December 26th event over the preceding 40 years. The largest of these events was a M 7.7 earthquake in September 1999, roughly 200 km to the north, in the middle of the island of Taiwan (the Chi-Chi earthquake). That event resulted in at least 2,297 fatalities and 8,700 injuries, and left over half a million people homeless. Damage from large-scale subsidence and resulting landslides contributed to an estimated cost of $14 billion.

Hayes et al. (2016) Tectonic summaries of magnitude 7 and greater earthquakes from 2000 to 2015, USGS Open-File Report 2016-1192. (5.2 MB PDF)