Poster #184, Seismology

Using foreshocks, aftershocks and sequences to probe earthquake nucleation

Rebecca L. Colquhoun, & Jessica C. Hawthorne
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Poster Presentation

2021 SCEC Annual Meeting, Poster #184, SCEC Contribution #11184 VIEW PDF
If earthquake nucleation is governed by single-mode triggering, a series of earthquakes occur, each triggering another event, one of which happens to grow much bigger. In this process, nucleation of each earthquake is rapid. However, nucleation may be more complex than this, such as if it involves interactions with external processes like aseismic slip or pore pressure changes. We do not yet know whether earthquake nucleation occurs through single-mode triggering or is a more complex, extended process, in part because we have a limited number of high-quality observations of foreshocks occurring in the seconds before a mainshock.

We used phase coherence based template matching...
to identify foreshocks and aftershocks in the hour surrounding thousands of earthquakes on the Hikurangi subduction zone between 2005 and 2020. We first examined the plausibility of the two models by analysing the numbers of foreshocks and aftershocks. If earthquakes nucleate via single-mode 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 around magnitude 1. Our 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, there are fewer foreshocks than we would expect given standard single-mode triggering. The disagreement is particularly pronounced for larger (M>4) mainshocks. Given limited observations, it is difficult to know why there are few foreshocks. However, the low foreshock-aftershock ratio, coupled with the observed Omori decay rate, could imply that single mode triggering does dominate but that the fault properties could be altered from before to after the earthquake.

Finally, we analysed our coherence-based detections to search for and assess the abundance of densely spaced foreshock sequences. Such tremor-like sequences have occasionally been observed in the seconds preceding a mainshock, which could suggest the presence of externally driven processes. We find no large-amplitude clusters like those observed before. However, we do find a potential increase in sequences before the mainshock, which further suggests that earthquake nucleation is more complex than single-mode triggering.