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Testing Earthquake Nucleation Length Scale in North-Central Oklahoma With Pawnee Aftershocks

Description: 

The interpretation of precursory seismicity can depend on a critical nucleation length scale that has been documented in laboratory experiments but is largely unconstrained in the seismogenic crust. To probe the nucleation length scale and associated earthquake nucleation processes at 2–7 km depths in Oklahoma, we studied seismic activity occurring prior to nine M 2.5–3.0 earthquakes that are aftershocks of the 3 September 2016 M 5.8 Pawnee, Oklahoma, earthquake. We analyzed seismicity occurring prior to these larger aftershocks and estimated the static stress changes associated with each event of each sequence based on precise earthquake relocations and magnitude estimates. If the earthquakes are spaced close enough to prior events, they could be plausibly triggered through standard stress transfer, and this might be evidence for a small nucleation length scale. On the other hand, if the events are spaced very far apart relative to their size, then static stress transfer is an unlikely triggering mechanism and some other process, such as widespread aseismic slip, would be required to collectively trigger the seismicity. Five of the nine large aftershocks studied did exhibit foreshock sequences, and we found all five of them were plausibly triggered via static stress transfer from nearby earthquakes occurring hours to seconds earlier, consistent with the cascade nucleation model and a small nucleation length scale (≤ 1 m) in this region. The smallest earthquakes we could quantitatively study were M −1.5 events, which likely have 1–2 m rupture dimensions. The existence of these small events also supports a small nucleation length scale ≤ 1 m, consistent with laboratory experiments on flat, smooth, bare rock samples. The other four of the nine large aftershocks studied did not have detectable seismicity within a 2 km radius of their hypocenters in the preceding 16 hour time windows. While this does not constrain nucleation length, it indicates strong variability of fault properties even within our 15 km study region in north-central Oklahoma.

Session: Earthquake Preparation Across Scales: Reconciling Geophysical Observations With Laboratory and Theory

Type: Oral

Date: 4/20/2023

Presentation Time: 11:00 AM (local time)

Presenting Author: Gregory McLaskey

Student Presenter: No

Invited Presentation: 

Authors