The 2010-2011 Canterbury Earthquake Sequence: From Paleoseismology to Policy
The 2010-2011 Canterbury earthquake sequence (CES), including the moment magnitude (Mw) 7.1 Darfield earthquake and Mw 6.2, 6.0, 5.9, and 5.8 aftershocks, occurred on a suite of previously unidentified, primarily blind, active faults, and is one of Earth's best recorded historical earthquake sequences.
The location of the CES proximal to and beneath a major urban centre enabled rapid and detailed collection of vast amounts of field, geospatial, geotechnical, hydrologic, biologic, and seismologic data, and allowed incremental and cumulative environmental responses to seismic forcing to be documented throughout a protracted earthquake sequence. The CES caused multiple instances of tectonic surface deformation (≥ 3 events), surface manifestations of liquefaction (≥ 11 events), lateral spreading (≥ 6 events), rockfall (≥ 6 events), cliff collapse (≥ 3 events), subsidence (≥ 4 events), and hydrological (10s of events) and biological shifts (≥ 3 events).
The terrestrial area affected by strong shaking (e.g. peak ground acceleration (PGA) ≥0.1-0.3 g), and the maximum distances between earthquake rupture and environmental response (Rrup), both generally increased with increased earthquake Mw, but were also influenced by earthquake location and source characteristics.
However, the severity of a given environmental response at any given site related predominantly to ground shaking characteristics (PGA, peak ground velocities) and site conditions (water table depth, soil type, geomorphic and topographic setting) rather than earthquake Mw. In most cases, the most severe liquefaction, rockfall, cliff collapse, subsidence, flooding, tree damage, and biologic habitat changes were triggered by proximal, moderate magnitude (Mw ≤ 6.2) earthquakes on blind faults. CES environmental effects will be incompletely preserved in the geologic record and variably diagnostic of spatial and temporal earthquake clustering.
Preliminary paleoseismic investigations in the CES region indicate recurrence of liquefaction in susceptible sediments of ~100 to 300 yr, recurrence of severe rockfall event(s) of ca. 6,000 to 8,000 yr, and recurrence of surface rupturing on the largest CES source fault of ca. 20,000 to 30,000 yr. These data highlight the importance of utilizing multiple proxy datasets in paleoearthquake studies. The severity of environmental effects triggered during the strongest CES earthquakes was greater than or equivalent to any historic or prehistoric effects recorded in the geologic record.
I suggest that the shaking caused by rupture of local blind faults in the CES comprised a 'worst case' seismic shaking scenario for parts of the Christchurch urban area. Moderate Mw blind fault earthquakes may contribute the highest proportion of seismic hazard, be the most important drivers of landscape evolution, and dominate the paleoseismic record in some locations on Earth, including locations distal from any identified active faults. This highlights the global value of improving 'off-fault' proxy records when characterizing the shaking hazard posed by strong earthquakes, particularly near major urban centres.