Aseismic Fault Slip during a Shallow Normal-Faulting Seismic Swarm
Constrained Using a Physically-Informed Geodetic Inversion Method
Improved imaging of the spatio-temporal growth of fault slip is crucial
for understanding the driving mechanisms of earthquakes and faulting.
This is especially critical to properly evaluate the evolution of
seismic swarms and earthquake precursory phenomena. Fault slip inversion
is an ill-posed problem and hence regularisation is required to obtain
stable and interpretable solutions. An analysis of compiled finite fault
slip models shows that slip distributions can be approximated with a
generic elliptical shape, particularly well for M≤7.5 events. Therefore,
we introduce a new physically-informed regularisation to constrain the
spatial pattern of slip distribution. Our approach adapts a crack model
derived from mechanical laboratory experiments and allows for complex
slipping patterns by stacking multiple cracks. The new inversion method
successfully recovered different simulated time-dependent patterns of
slip propagation, i.e., crack-like and pulse-like ruptures, directly
using wrapped satellite radar interferometry (InSAR) phase observations.
We find that the new method reduces model parameter space, and favours
simpler interpretable spatio-temporal fault slip distributions. We apply
the proposed method to the 2011 March-September normal-faulting seismic
swarm at Hawthorne (Nevada, USA), by computing ENVISAT and RADARSAT-2
interferograms to estimate the spatio-temporal evolution of fault slip
distribution. The results show that (1) aseismic slip might play a
significant role during the initial stage, and (2) this shallow seismic
swarm had slip rates consistent with those of slow earthquake processes.
The proposed method will be useful in retrieving time-dependent fault
slip evolution and is expected to be widely applicable to studying fault
mechanics, particularly in slow earthquakes.