Earthquake nucleation and fault slip complexity in the lower crust of central Alaska
Abstract
Earthquakes start under conditions that are largely unknown. In laboratory analog experiments and continuum models, earthquakes transition from slow-slipping, growing nucleation to fast-slipping rupture. In nature, earthquakes generally start abruptly, with no evidence for a nucleation process. We report evidence from a strike-slip fault zone in central Alaska of extended earthquake nucleation and of very-low-frequency earthquakes (VLFE). Standalone VLFEs occurred in the Minto Flats fault zone in 2013 (Mw 3.5, depth 23 km) and 2015 (Mw 3.8, depth 21 km), and both mechanisms were consistent with strike-slip faulting. In 2016 a VLFE transitioned into an earthquake of magnitude 3.7 and was preceded by a 12-hour-long accelerating foreshock sequence. Benefiting from 12 seismic stations deployed within 30 km of the epicenter, we identified coincident radiation of distinct high-frequency and low-frequency waves during 22 s of nucleation. Estimation of the source time function revealed several pulses concentrated over a 10-second time period. Phase coherence analysis indicated that high-frequency nucleation signals originated within 1 km of the subsequent 3.7 earthquake. The power-law temporal growth of the nucleation signal is quantitatively predicted by a model in which high-frequency waves are radiated from the vicinity of an expanding slow slip front. The observations reveal the continuity and complexity of slip processes near the bottom of the seismogenic zone of a strike-slip fault system in central Alaska.
- Publication:
-
AGU Fall Meeting Abstracts
- Pub Date:
- December 2018
- Bibcode:
- 2018AGUFM.T43B..03T
- Keywords:
-
- 1242 Seismic cycle related deformations;
- GEODESY AND GRAVITYDE: 7230 Seismicity and tectonics;
- SEISMOLOGYDE: 8118 Dynamics and mechanics of faulting;
- TECTONOPHYSICSDE: 8163 Rheology and friction of fault zones;
- TECTONOPHYSICS