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Research: earthquake dynamics
The research in our team combines theoretical, computational and
seismological approaches to improve our understanding of the physics
of earthquakes.
Our research topics include:
Earthquake nucleation
How do earthquakes start? This is one of the fundamental questions of
earthquake dynamics, with implications on earthquake predictability,
early warning systems and time-dependent seismic hazard.
I have been investigating the different styles of earthquake nucleation
implied by usual fault friction laws. Recent results, in sustained
collaboration with Allan Rubin (Princeton), include the
characterization of quasi-static pulses during nucleation under
rate-and-state friction with the slip evolution law.
Dynamic nucleation under linear slip-weakening friction
(Ampuero, Vilotte and Sanchez-Sesma, 2002)
Earthquake nucleation under rate-and-state friction with the "aging" state evolution law
(Rubin and Ampuero, 2005)
Earthquake nucleation under rate-and-state friction with the "slip" state evolution law
(Ampuero and Rubin, 2008)
Complexity of 3D earthquake dynamics
Seismic ground motions in the vicinity of active faults are strongly affected
by the spatio-temporal details of the earthquake source.
Unfortunately, near-field recordings that can nourish empirical
approaches for ground motion prediction are notoriously scarce. An
emerging complementary approach is based on direct simulations of
earthquake dynamics scenarios, with the aim to integrate our best
current knowledge about source complexity and earthquake physics.
In collaboration with Martin Mai (ETH Zurich) and former graduate
student Johannes Ripperger (ETH Zurich, now in Oxford), I have been
studying the effect of initial stress heterogeneities on the
statistical properties of dynamic ruptures and the induced near-field
ground motions. Post-doc Javier Ruiz has recently joined the team to
work on this topic.
Rupture "percolates" through a heterogeneous initial stress
(Ampuero, Ripperger and Mai, 2006; Ripperger, Ampuero, Mai and Giardini, 2007)
(a) a set of stochastic initial stress realizations
(b) dynamic rupture fronts and final slip
(c) slip velocity at the center of the fault
(Ripperger, Mai and Ampuero, 2008)
Dynamic rupture on a heterogeneous initial stress
featuring dynamic triggering of secondary events ahead of the main rupture front
(courtesy of Johannes Ripperger)
Earthquake source seismology
Most of the information that can discriminate between different models
of earthquake physics is contained in high frequency waves.
Unfortunately these are hard to analyze, due to the opacity of the
Earth crust. We are exploring new techniques to extract deterministic
and statistical information from strong ground motion records, based
on wavelet and array techniques.
The very frequent small magnitude earthquakes can also provide a wealth
of information about earthquake physics. However they require
specific processing methods. We are applying empirical Green's
functions techniques to study very early aftershocks and to analyze
the directivity of small magnitude events. This research started in
collaboration with Allan Rubin (Princeton) and continues with
post-doc Javier Ruiz.
Detection of composite microearthquake in Parkfield,
based on empirical Green's function deconvolution
(Ampuero and Rubin)
Earthquakes on bimaterial faults
Mature faults usually juxtapose
rocks of different physical properties. Dynamic ruptures on such
bimaterial faults feature interesting phenomena induced by the
coupling between slip and normal stress changes: a tendency for
macroscopic source asymmetry, preferred rupture direction, preferred
aftershock triggering direction and asymmetric off-fault damage.
In collaboration with Allan Rubin
(Princeton) and with Yehuda Ben-Zion (USC), I have been investigating
the different styles of bimaterial ruptures predicted by various
fault friction laws. Recent findings include the macroscopic
asymmetry induced by the bimaterial effect on rupture pulses
generated by fast velocity-weakening friction, and the competing
effect of fault heterogeneities.
Dynamic rupture on a bimaterial fault with slip-weakening friction
(Rubin and Ampuero, 2007)
Dynamic rupture styles on a bimaterial fault with velocity-weakening friction
(Ampuero and Ben-Zion, 2008)
Earthquakes on non planar faults
Earthquakes are classically modeled as slip on faults that are planar
or have a few large-scale geometrical features like steps, jogs, kinks and
branches. However, the geometrical complexity of natural faults
extends over a broad range of length scales and this might have an
effect on the propagation of dynamic rupture fronts, and on
high-frequency seismic radiation.
In collaboration with Raul Madariaga (ENS Paris) I have been
investigating the dynamics of faults on rough faults.
Post-doc Jean Elkhoury joined us to work on this topic.
Mode III dynamic rupture on a fault with multiple kinks
(Madariaga, Ampuero and Adda-Bedia, 2006)
Earthquake rupture with off-fault damage
Earthquakes are classically modeled as frictional instabilities on fault
surfaces. However, natural faults are surrounded by damaged fault
zones characterized by dense micro-fracturing and low elastic moduli.
Dynamic rupture fronts concentrate stresses that can overcome the
yield strength of the surrounding rock and trigger anelastic processes.
In collaboration with Yehuda Ben-Zion (USC) and Vladimir Lyakhovsky (GS
Israel), I am exploring the interaction between frictional ruptures
and a continuum representation of off-fault damage.
Pulse-like dynamic rupture with velocity-weakening friction
and off-fault continuum damage
(Ampuero, Ben-Zion and Lyakhovsky, SSA 2008)
Slow slip and non-volcanic tremor
Recently discovered recurrent slow slip events, low and very low frequency
earthquakes and non-volcanic tremor, primarily in subduction zones,
extend the known spectrum of earthquake phenomena.
In collaboraton with Allan Rubin (Princeton) and with Hugo Perfettini
(IRD France) I have been studying the implications of rate-and-state
friction on slow slip transients. Our simplest models require fine
tuning to match the ensemble of observations, and we are now
exploring additional physical ingredients: the effect of fault
heteroegeneities and fluid-related processes. Post-doc Gregor Hillers
recently joined the team to work on this topic.
Slow pulse propagating on a rate-and-state fault
with the "slip" state evolution law
(Ampuero and Rubin, 2008)
A periodic slow slip event on a creeping fault (velocity-strengthening rate-and-state)
containing one brittle asperity (velocity-weakening rate-and-state)
(Ampuero and Perfettini, 2008)
A periodic slow slip event on a creeping fault (velocity-strengthening rate-and-state)
containing one large and several small scale brittle asperities (velocity-weakening rate-and-state)
The slow slip event triggers fast slip on the secondary asperities.
(Ampuero and Perfettini, 2008)
Numerical methods for earthquake dynamics
The emergence of physics-based ground motion prediction approaches, the
current trend towards dynamically-constrained source inversion and
our increasing interest on stochastic aspects of earthquake rupture
are imposing high demands on computational methods for large scale
earthquake dynamics simulations. Modern methods should reach high
performance and high accuracy, and be able to accommodate geometrical
complexity, heterogeneous media, non-linear off-fault rheologies and
multiple physics in the fault zone.
Owing to continued collaboration with Jean-Pierre Vilotte (IPG Paris)
and graduate student Yoshihiro Kaneko, the application of the Spectral
Element Method (SEM) to earthquake dynamics has reached maturity. The
SEM code SEM2DPACK has been applied to rupture on non-planar faults
and rupture with off-fault damage and plasticity. Fault dynamics
features are now implemented in the SPECFEM3D code.
Recent methodological contributions to SEM include the application of
high-order symplectic time integration schemes to improve the
accuracy of numerical wave propagation over long distances, developed
in collaboration with post-doc Tarje Nissen-Meyer.
In collaboration with Josep De La Puente (LMU Munich) we have recently
introduced fault dynamics in the ADER-DG (Discontinuous Galerkin)
method, a high-order method that can take advantage of automated
unstructured meshing tools to overcome a major bottleneck of SEM
simulations.
Horizontal slip rate in the SCEC TPV5 problem, solved by SPECFEM3D
Dynamic rupture on a rate-and-state fault with a shallow velocity-strengthening layer
(Kaneko, Lapusta and Ampuero (2008)
Seismic hazard along the Peruvian subduction zone
Peru is located along the Nazca-South America subduction zone, one of
the most seismically active regions in the world. Earthquakes are the
first cause of human and economic loss by natural disasters in Peru.
Lima, the capital city, lies closer to subduction seismicity than any
other megacity of the western hemisphere.
Efforts are being coordinated with institutions in Peru to create
a new generation of earthquake risk management tools.
Our team is contributing with a portable seismic network (15
Guralp CMG-40T sensors with Nanometrics Taurus recorders), operated
in collaboration with the Instituto Geofisico del Peru
as an "Earthquake Task Force Unit" for seismological
studies and earthquake engineering applications, including seismic
tomography, seismicity monitoring, aftershock recording, and small
array microtremor methods for site effect quantification.
