On the Influence of a Dilatant Asperity Patch on the Seismic Moment

Abstract

This paper proposes a novel procedure to examine the influences of friction, dilatancy, and normal stresses at fault zones on the estimation of seismic moment. For illustrative purposes, the study focuses on a circular frictional dilatant patch located within a frictionless pre-compressed fault zone undergoing relative shear. When dilatancy occurs, the interface beyond the dilatant region may experience separation due to the normal stresses acting on the fault plane, affecting the deformational response of the pre-stressed asperity. This approach allows for an evaluation of the normal stress on the dilatant region, leading to a re-interpretation of the conventional definition of seismic moment. We compare our model against a comprehensive catalog of earthquakes spanning 16 orders of magnitude, utilizing seismologically inferred source properties as well as data from two separate experimental studies that directly measure the shear-dilatant response of shear fractures in both laboratory and field settings. Our findings indicate that friction-induced dilatancy exerts minimal influence on the estimation of seismic moment. However, we emphasize that the discrepancies between our direct measurements and inferred estimates of seismic moment highlight the need for focused campaigns and in situ and on-fault assessments of earthquake mechanics. Plain Language summary. The conventional definition of the Seismic Moment has been central to unifying information from plate tectonics, geology, geodesy, and seismology. It looks at how the ground moves along a fault plane and the strength of the surrounding rocks. However, it often overlooks other factors that might affect this movement, such as the stress on the fault and the local topography that can induce additional physical responses. This study explores how these additional factors, particularly a process called dilatancy, can change our understanding of the seismic moment. The authors found that while these factors do play a role, they have a minimal impact on the traditional definition of seismic moment initially proposed by Aki in 1966.