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 regularization 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 regularization 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, that is, 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 favors 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 (a) aseismic slip might play a significant role during the initial stage and (b) 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.This research was supported by the Natural Environmental Research Council (NERC) through the Centre for the Observation and Modelling of Earthquakes, Volcanoes and Tectonics (UK) (GA/13/M/031) and the LiCS large grant (NE/K011006/1). This research was also supported by a Chinese Scholarship Council-University of Liverpool joint scholarship awarded to YJ (201706450071). PJG acknowledged the support of the Spanish Ministerio de Ciencia e Innovación research project, grant agreement number PID2019-104571RA-I00 (COMPACT) and the Beca Leonardo a Investigadores y Creadores Culturales 2020 from the Fundación BBVA. RADARSAT-2 images were acquired under SOAR-E project 28209 from the Canadian Space Agency, and ENVISAT images from ESA CAT1 project 6745. The manuscript was edited by Guido Jones, currently funded by the Cabildo de Tenerife, under the TFinnova Programme supported by MEDI and FDCAN funds. Thanks go to Mehdi Nikkhoo for providing the triangular dislocation functions for the displacement gradient tensor. The authors are also grateful to Tim Wright, Dan Faulkner, and Louisa Brotherson for insightful discussions of different aspects of this study, as well as to editor Rachel Abercrombie, associate editor, Eric Lindsey, and another anonymous reviewer for their helpful and constructive reviews, which greatly improved this manuscript.Peer reviewe