Are normal fault earthquakes due to elastic rebound or gravitational collapse?

Christian Bignami,Carlo Doglioni,Eugenio Carminati,Emanuela Valerio,Emanuela Valerio,Patrizio Petricca,Riccardo Lanari,Pietro Tizzani

doi:10.4401/ag-8455

Abstract

We discuss two competing models for explaining the ground deformation associated with normal faulting earthquake in the brittle elastic upper crust. The classic elastic rebound theory is usually applied for all tectonic settings. In normal fault earthquakes, this model would predict a horizontal stretching eventually responsible for the elastic rebound at the earthquake. However, volumes mostly subside vertically during an extensional earthquake and the collapsed ground in the hanging wall is about one order of magnitude larger than the uplifted volumes of the surrounding hanging wall and footwall. The elastic rebound model would explain this asymmetry with a high horizontal elastic compressibility of the hanging wall and footwall absorbing the coseismic push. We rather suggest that the force activating normal fault earthquakes is mostly dictated by the sliding of the hanging wall, owing gravitational potential. The much larger coseismic subsidence with respect to the uplift can be explained by the closure at depth of a diffuse network of microfractures developed during the interseismic period. Since the horizontal stretching does not exist below ~1 km of depth, with the minimum horizontal stress tensor becoming positive below that depth, the development of a normal fault can be activated only by the vertical maximum stress tensor, i.e., the lithostatic load. The common fluids expulsion at the coseismic stage requires diffuse secondary permeability in the upper crust, in agreement with the presence of a diffuse network of microfractures.

Highlights

Valerio et al [2018] demonstrated that a possible interpretation of the observed volumetric asymmetry related to the Mw 6.5 central Italy 2016 earthquake can be given through a numerical analysis showing the role of the hanging wall gravitational sliding. They proposed a 2D numerical model of the 2016 earthquake developed in a Finite Element (FE) environment under a structural-mechanic physic context
This approach takes into account the linear elasticity behavior of the medium by considering the plain strain approximation mode. This aims to reduce the differences among the modelled vertical movements and those computed with the available ALOS-2 DInSAR measurements
Bignami et al [2019] suggest that the force is rather dictated by the gravitational potential, whereas the much larger coseismic subsidence with respect to the uplift can be explained by the abrupt closure at depth of a diffuse network of fractures, which progressively developed during the interseismic period

Summary

Introduction

The brittle upper crust can be assumed as an elastic medium at the timescale of earthquakes, delivering elastic waves when perturbed instantaneously [e.g., Stein and Wysession, 2009]. Segall and Heimisson [2019] recently suggested that the elastic dislocation theory in extensional tectonic settings does not require a volume collapse at depth, as proposed by Bignami et al [2019]. Segall and Heimisson [2019] indicate that the common theory of the horizontal elastic rebound reasonably reproduces the observed geodetic data, and infer how the extra coseismicallyChristian Bignami et al subsided volume is accommodated by compressibility in the hanging wall. Segall and Heimisson [2019] recently suggested that the elastic dislocation theory in extensional tectonic settings does not require a volume collapse at depth, as proposed by Bignami et al [2019]. Segall and Heimisson [2019] indicate that the common theory of the horizontal elastic rebound reasonably reproduces the observed geodetic data, and infer how the extra coseismically. Christian Bignami et al subsided volume is accommodated by compressibility in the hanging wall. Their analytical solution fails to answer two fundamental questions about the ground deformation associated with earthquakes in extensional tectonic settings. We investigate here the energy source and kinematics of normal fault earthquakes, keeping in mind that the fault plane is only the passive discontinuity along which part of the energy accumulated by the crustal volume during the interseismic period is dissipated at the coseismic stage

Results

Discussion

Conclusion