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Investigating Earthquake Nucleation: What Do Foreshocks, Aftershocks and Sequences Tell Us?

Earthquake nucleation has two end member models: externally-driven and single-mode. In externally driven nucleation, earthquakes are triggered by aseismic slip or pore pressure changes, but in cascade nucleation a series of earthquakes occur, each triggering another event, and one of these happens to grow much bigger. We don’t know whether earthquake nucleation cascades or is externally driven as we have a limited number of high quality observations of foreshocks and aftershocks. 

We used phase coherence based template matching to identify foreshocks and aftershocks of thousands of earthquakes on the Hikurangi subduction zone between 2005 and 2020. This is an ideal location to test the two models as it could plausibly host externally driven earthquakes, as it undergoes pore pressure changes and has experienced episodes of slow slip in the past. 

We examined the plausibility of 2 models by analysing the numbers of foreshocks and aftershocks. If earthquakes nucleate via cascade triggering, the relative ratios of foreshocks and aftershocks can be predicted using Omori’s law. With our new detections, we identified events surrounding the mainshock down to magnitude 1. Initial results indicate that both foreshocks and aftershocks decay as a power law with time from the mainshock, which is consistent with single-mode triggering. However, foreshocks have a lower productivity than mainshocks. This disagreement between time-dependent decay and productivity could imply that single mode triggering dominates, but that fault properties are externally altered from before to after the earthquake. 

Finally, we are currently further analysing the coherent detections to search for and assess the abundance of densely spaced sequences of foreshocks. Such sequences have occasionally been observed, and suggest the presence of externally driven processes, but we do not currently know how common foreshocks sequences are. 

Presenting Author: Rebecca L. Colquhoun

Student Presenter: Yes

Day: 4/19/2021

Time: 9:45 AM - 11:00 AM Pacific

Additional Authors