Geodynamics of the Sichuan earthquake region in May 12, 2008

Abstract

A disastrous earthquake with a magnitude of M 7.8‐ 7.9 occurred 93 km northwest of the administrative center Chengdu, Sichuan Province, China, on May 12, 2008. The hypocenter depth was about 30 km. Aftershocks with magnitudes up to M 6.1‐6.4 went on until the first half of June. Their epicenters were confined to the north-northwest‐trending zone extending for 315 km on each side of the site of the main event (http:// earthquake.usgs.gov/eqcenter/page). About 70 000 people were killed, tens of settlements were destroyed, and the total damage reached billions of dollars. The study of recent geodynamics of the earthquake region and adjacent territories allows us to suggest some considerations on the causes of increased seismicity in this part of Central Asia. The analysis of mechanisms in epicenters and GPS data revealed the direction of dislocations along faults bounding crustal blocks in the region, as well as the nature of stresses that emerged in these conditions. An increased liberation of energy comparable with its quantity in subduction and collision zones has been established for a series of interblock zones. Along with data on the deep structure of blocks and their boundaries, this provides a way of distinguishing risk zones in terms of seismicity in Central Asia. Vectors of horizontal crustal displacements based on the ITRF-system space geodesy investigations (http:// itrf.ensg.ign.fr/ITRF_solutions/2005/ITRF2005.php) and data on active faults [1] revealed in Central Asia. Vectors of horizontal crustal displacements in Central Asia revealed a series of blocks bounded by disjunctive dislocations. The blocks behave independently in the present-day geokinetic field [2‐4]. The May 12 event took place in the zone of the Longmen Shan major fault [5] representing a boundary between the Southeast China and Bayanhar blocks (Fig. 1). Such boundaries are, as a rule, expressed not by individual dislocations but wide interblock zones, within which an intense crushing of host rock takes place and a fair quantity of seismic energy is relieved. The position of hypocenters mainly at a depth of 20‐40 km corroborates the relatively shallow penetration of interblock zones into the lithosphere. The cited computations of seismic energy relieved along boundaries of the majority of blocks in Central and Eastern Asia, the Bayanhar block included, exhibit a very high seismic activity of these structures [6].