Proposed Break in Magnitude Scaling of Earthquake Ground Motion and Source Dimensions for Subduction Mega-Earthquakes

Date: 4/24/2019

Time: 06:00 PM

Room: Grand Ballroom

In this study, I estimate the breakpoint magnitude (Mb) at which subduction megathrust earthquakes are expected to exhibit a break in scaling of both source dimensions and magnitude scaling rate (MSR) of radiated seismic ground motion for 79 global subduction zones identified by GEM. Reduced scaling of rupture length, rupture area, and MSR with moment magnitude is proposed to occur for events with magnitudes greater than Mb. Breakpoint magnitude is estimated from seismogenic fault widths, empirical source scaling relations, and aspect ratios of physically unbounded earthquake ruptures and their uncertainty. The concept stems from the well-established observation that source scaling and MSR of shallow continental strike-slip earthquakes exhibit a scaling break at approximately a magnitude of 6.5 and maximum seismogenic fault width of 15 km. A similar scaling break for megathrust earthquakes is difficult to observe because of the relatively large subduction interface widths and the limited number of events with ruptures that exceed these widths. However, I observe that all of the historical M > 8.7 mega-earthquakes in recent history have occurred on subduction zones for which the breakpoint magnitude is estimated to be approximately 8.5, consistent with the observed break in source scaling relations derived from these same subduction zones. Mean breakpoint magnitudes for selected global subduction zone interfaces are estimated to be 7.6 for Cascadia, 8.5 for Japan Trench, 7.5 for Nankai Trough, 8.2 for Hikurangi (North Island, New Zealand), 8.3 for Andaman, 8.0 for Sumatra, 7.6 for Java, 8.4-8.6 for South America, and 7.2-7.5 for Mexico-Central America. For all 79 subduction zones, the mean and median breakpoint magnitudes are 7.8 and 7.9, respectively, with standard deviations of 0.24. These mean and median global breakpoint magnitudes are consistent with the generic break in magnitude scaling defined in the updated (2018) BC Hydro subduction ground motion model.

Presenting Author: Kenneth W. Campbell

Authors