FOCUSING SEISMIC ENERGY ALONG FAULTS THROUGH TIME-VARIABLE RUPTURE MODES: WENCHUAN EARTHQUAKE, CHINA

Field analysis and RADAR differential interferometry derived from The Phased Array type L-band Synthetic Aperture Radar (PALSAR) of the epicentral area of the May 12 2008 Mw 8.0 Wenchuan earthquake zone reveals some of the largest-ever fault ruptures and ground displacments, with field-documented vertical displacements reaching 12 meters along the trace of the main fault rupture, and PALSAR-measured horizontal displacements reaching 12.3 meters. Slip indicators on the fault scarp show initially sub-horizontal then progressively steeper slip increments with time along the same fault during the 2 minute earthquake, suggesting that the rupture propagated as a mixed Mode II-III fracture, with strike-slip movement at the tip directing energy from the initial rupture to points north of the epicenter, followed by vertical displacement in the interior of the slipped region. The changing slip directions may correlate with three main pulses of energy release at 0, 23, and 57 seconds during the earthquake as recorded by seismograms. The earthquake generated massive landslides > 100 km north of the epicenter along the trace of the fault. The concentration of massive landslides and ground deformation so far north of the epicenter may be related to the Mode II-III fracture propagation focusing the energy along the fault towards the north. The focusing of energy explains why so much devastation occurred more than 100 km from the epicenter, such as in the destroyed city of Beichuan.