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Why Are Large Earthquakes Rarely Overdue for New Zealand Faults?

Session: Overdue?

Type: Oral

Date: 4/21/2021

Presentation Time: 09:45 AM Pacific

Description: 

The timing and size of successive prehistoric earthquakes on individual faults provides key information for seismic hazards in the coming years. Future earthquakes can be considered overdue when the elapsed time since the last earthquake exceeds the average recurrence interval for each fault. In New Zealand paleoseismic data for over 50 faults with slip rates <~30 mm/yr indicate that future earthquakes are rarely overdue. A similar conclusion can be reached for New Zealand’s historical earthquakes where the duration of the most recent recurrence interval on each fault is typically less than the mean recurrence interval from paleoseismic trenching. Paleoseismic data on New Zealand faults may be incomplete, however, comparable measured and calculated mean recurrence intervals for individual faults are inconsistent with the relatively long mean recurrence intervals being entirely due to earthquakes missing from the record. We propose that the rarity of overdue earthquakes could also be partly attributed to a combination of earthquake ‘clustering’ on individual faults and sampling bias towards those faults presently in active phases of their earthquake histories. Sampling biases partly arise due to our tendency to collect paleoseismic data from clearly defined active fault traces that have been most active in the recent past (e.g., <2 kyr) and, in many cases, are experiencing shorter than average recurrence intervals. Such sampling bias is less important for high slip-rate faults (e.g., > 10 mm/yr) for which well-defined fault scarps will be observed independent of temporal earthquake clustering and large earthquakes are more likely to be overdue. Independent of the number of seismically overdue faults, dating uncertainties are too large to test the hypothesis that the timing of large earthquakes was non-uniform across New Zealand in the Late Holocene, although the historical record appears to show both triggered slip and multi-fault ruptures associated with elevated numbers of large magnitude earthquakes every 70-100 years.

Presenting Author: Andy Nicol

Student Presenter: No

Authors