A numerical model for the simulation of the seismic cycle in tectonic
settings in favor or against gravity: examples from Italy
Abstract
According to the concept of the seismic cycle, earthquakes result from
the strain accumulation over a variable decade to millennial period,
i.e., the interseismic stage, followed by a sudden stress release, i.e.,
the coseismic stage, eventually evolving in the postseismic stage.
Common analytical and numerical approaches simulate interseismic,
coseismic and postseismic stages independently. Often, coseismic models
constrain the slip of single or multiple planar sources to fit the
available geodetic and InSAR measurements to reproduce fault geometry,
slip and regional deformation, regardless the origin of the interseismic
forces. We developed a numerical model linking the ongoing interseismic
viscous deformation at depth with the coseismic brittle episodic
behavior of the upper crust. Our model assumes a brittle upper crust
where the fault is locked, and a ductile lower crust, where the fault is
steadily shearing. This approach is developed to model typical
extensional and compressional earthquakes in Italy including the forces
acting during the interseismic period, i.e., the lithostatic load and
the horizontal stress field. We adjusted the setup of our model to
simulate the interseismic, coseismic and postseismic phases of three
seismic events in Italy, two extensional (2009 L’Aquila Mw 6.1 and 2016
Amatrice-Norcia Mw 6.5) and one contractional (2012 Emilia Mw 6). The
results of our analysis, compared with the available geodetic and InSAR
data, show that the proposed numerical model can reproduce the seismic
cycle associated with the investigated events. The modeling provides
evidence of interseismic dilatancy above the brittle-ductile transition
at the bottom of the locked fault plane in the extensional tectonic
setting; coseismic fault motion is triggered by the hangingwall
gravitational collapse that recovers most of the interseismic dilatancy
formed almost orthogonal to the fault. Vice versa, in contractional
tectonic settings, the interseismic horizontal stress accumulates
elastic energy in the crustal volume above the bottom of the locked
fault; coseismic deformation recovers the elastic energy stored in the
hangingwall. The two different settings generate a deformation in favor
of gravity in extensional tectonic environments and against gravity in
contractional tectonic environments.