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The Seismological Signature of Earthquake Nucleation

Description: 

Frictional theory and laboratory experiments indicate that earthquakes initiate gradually over a finite fault region, with a dimension related to local elasto-frictional properties. Estimating this nucleation dimension on natural faults could therefore provide constraints on in-situ frictional properties and stress state; however, direct observation of the nucleation phase has proved challenging, also because the effect of nucleation on far-field seismological observables was not quantified.

Here I introduce a source model for earthquake nucleation and discuss the resulting scaling relations between source properties (far-field pulse duration, seismic moment, stress drop). I derive an equation of motion based on fracture mechanics for a circular rupture obeying rate-state friction and a simpler model with constant stress drop and fracture energy. The latter provides a good approximation to the rate-state model, and leads to analytical expressions for far-field displacement pulses and spectra. The onset of ground motion is characterized by exponential growth with characteristic timescale t₀=R₀/v_f, with R₀ the nucleation dimension and v_f a limit rupture velocity. Therefore, normalized displacements have a constant duration, proportional to the nucleation length rather than the source dimension. For ray paths normal to the fault, the exponential growth results in a Boatwright spectrum with corner frequency ω_c=1/t₀. For other orientations, the spectrum has an additional sinc(.) term with a corner frequency related to the travel time delay across the asperity. Seismic moments scale as M₀ ~ R(R-R₀)R₀, where R is the size of asperity, becoming vanishingly small as R approaches R₀. Therefore, stress drops estimated from M₀ and f_c are smaller than the nominal stress drop, and they increase with magnitude up to a constant value, consistent with some seismological studies. The constant earthquake duration is also in agreement with reported microseismicity: for 0 < M_W < 2 events studied by Lin et al (2016) in Taiwan, the observed durations imply a nucleation length between 45-80m.

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

Type: Oral

Date: 4/20/2023

Presentation Time: 09:00 AM (local time)

Presenting Author: Camilla Cattania

Student Presenter: No

Invited Presentation: Yes

Authors