Strong proximal earthquakes revealed by cosmogenic 3He dating of prehistoric rockfalls, Christchurch, New Zealand

Abstract

The 2011 rupture of previously undetected blind faults beneath Christchurch, New Zealand, in moment magnitude (M w ) 6.2 and 6.0 earthquakes triggered major rockfalls that caused fatalities and infrastructure damage. Here we use field, geospatial, seismologic, numerical modeling, and cosmogenic 3 He data to provide first evidence for prehistoric rockfall ca. 8– 6 ka, and a possible preceding event ca. 14– 13 ka, at a site where extensive rockfall occurred in the Christchurch earthquakes. The long (∼7 ± 1 k.y.) time intervals between successive rockfall events and the high peak ground velocity thresholds required for rockfall initiation at this site (∼20–30 cm/s) preclude earthquakes from major identified seismic sources, including the plate boundary Alpine fault, as likely rockfall triggering sources. Rockfalls were probably triggered by strong paleoearthquakes sourced from active faults proximal (i.e., <10–20 km) to Christchurch, including the sources of the 2011 Christchurch earthquakes and/or other currently unidentified faults. Given the inherent incompleteness of seismic source catalogues and challenges in obtaining earthquake chronologies for blind faults, high scientific priority should be given to the search for, and analysis of, geologic records of strong earthquake shaking near populated areas.