Focusing seismic energy along faults through time-variable rupture modes: Wenchuan earthquake, China

Abstract

Field analysis and differential interferometry derived from Synthetic Aperture Radar (PALSAR) of the epicentral area of the May 12 2008 M _w 8.0 Wenchuan (汶川) earthquake zone reveal details about fault ruptures and ground displacements, with field-documented vertical offsets reaching 12 m along the trace of the main fault rupture, and PALSAR-measured horizontal displacements reaching 12.3 m. We document slip indicators on the fault scarp that show initially sub-horizontal then progressively steeper slip increments with time along the same fault during the 2 min 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. Field, seismic, and PALSAR-measured displacements are dominantly thrust near the epicentral region, and dextral in northeasterly areas, showing that slip was partitioned along the fault in space and time during the earthquake. The changing slip directions may correlate with three main pulses of energy release at 0, 23, and 57 s during the earthquake as recorded by seismograms. The earthquake generated massive landslides >100 km north of the epicenter along the trace of the fault. These massive sturzstroms formed where entire mountainsides collapsed and moved at high velocity down populated mountain valleys, jumping smaller mountains and ricocheting off steep canyon walls. These avalanches rode on cushions of water and compressed air that generated air blasts from beneath the debris, knocking down trees high on opposing mountain slopes. The concentration of massive landslides and ground deformation so far north of the epicenter was related to the Mode II-III fracture propagation focusing seismic energy along the fault towards the north, explaining why so much devastation occurred >100 km from the epicenter, such as in the destroyed city of Beichuan (北川). Understanding near-surface earthquake processes and geological hazards associated with massive earthquakes in populated mountainous areas has the potential to save many lives in the future.

Keywords

China / Wenchuan earthquake / Longmenshan / neotectonics / PALSAR

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