Earthquake nucleation and fault slip complexity in the lower crust of central Alaska


Earthquakes start under conditions that are largely unknown. In laboratory analogue 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. Here we report evidence from a strike-slip fault zone in central Alaska of extended earthquake nucleation and of very-low-frequency earthquakes (VLFEs), a phenomenon previously reported only in subduction zone environments. 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 epicentre, we identify coincident radiation of distinct high-frequency and low-frequency waves during 22 s of nucleation. 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.

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Fig. 1: Minto Flats fault zone of central Alaska.
Fig. 2: Seismic waveforms for the 2015 and 2016 events observed at stations FTGH (red traces) and F6TP (black traces).
Fig. 3: Phase coherence between foreshock signal and mainshock signal for the 2016 event, filtered between 1 and 10 Hz.
Fig. 4: Alternative interpretations for the two stages of nucleation and rupture.
Fig. 5: Comparison between rate-state modelling and observations of power-law growth of the rupture stage interpreted as nucleation.

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Seismic instruments, data archiving and data access were supported by the U.S. National Science Foundation grants EAR-1352668 and EAR-1645313, the Alaska Earthquake Center, the IRIS Data Management Center and the PASSCAL Instrument Center. Y.K. was supported by Rutherford Discovery Fellowship from the Royal Society of New Zealand.

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All authors contributed to the manuscript. C.T., S.H. and V.S. performed the seismological analyses of the two events. J.H. performed the phase coherence analysis (Fig. 3). Y.K., J.P.A. and J.H. contributed the rate-and-state modelling and interpretation. C.J. performed the source time function estimation. C.T., N.R., K.S. and M.E.W. were responsible for the deployment of FLATS seismic stations. N.R. and S.H. discovered the high-frequency and low-frequency signals for the 2015 event. S.H. discovered the low-frequency foreshock (VLFE) of the 2016 event.

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Correspondence to Carl Tape.

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Tape, C., Holtkamp, S., Silwal, V. et al. Earthquake nucleation and fault slip complexity in the lower crust of central Alaska. Nature Geosci 11, 536–541 (2018).

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