Geoscience Reference
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and elastic waves in the crust and near-surface environment. Geologists provide a
framework for studying deformation near plate boundaries by documenting the style
and timing of faulting over geological time, and by examining exhumed faults to
study frictional characteristics and evolution of fault gouge. Seismologists use the
elastic wave energy radiated from dynamic fault ruptures to estimate the size of
earthquakes and to determine details of the rupture process, along with quantifying
seismic wave propagation and ground shaking effects. Geodesists measure
deformations of the rock around a fault zone both before (interseismic), during (co-
seismic), and after (postseismic) an earthquake, as well as stable sliding of some
aseismic faults. Rock mechanics researchers determine frictional mechanisms and
theory for rupture nucleation and arrest to guide understanding of the frictional
instabilities associated with earthquakes and stable sliding. The collective scientific
understanding of earthquake faulting from these endeavors feeds into earthquake
engineering and emergency response efforts to mitigate the impacts of fault ruptures.
The earthquake cycle notion provides a basic framework for understanding
deformation near a fault that is loaded by large-scale plate motions. Once an
earthquake has occurred and the postseismic period of stress and strain transients has
ended, the earthquake cycle begins anew with interseismic frictional locking of the
fault and onset of fault zone strain accumulation. Geological, seismic, and geodetic
data are used to evaluate the size and frequency of large earthquakes in a particular
region. A catalog of historical behavior of a fault is then used to assess how large and
how often fault ruptures can be expected statistically. Geodetically determined rates
of strain accumulation can be evaluated relative to total plate motions and stress drop
determinations for prior events on the fault to anticipate where and how much future
strain release will occur. Determining the statistical likelihood of earthquakes in a
region is of particular interest to society because engineering building codes are
guided by the probability of experiencing various levels of ground shaking within the
lifetime of a building. However, this earthquake cycle model is only useful to the
extent that we can fully understand how deformation accumulates and how faults fail.
The nonlinearity of frictional instabilities, the influence of dynamic and static stress
perturbations by other earthquakes, and the complexity of stress heterogeneity from
prior ruptures and non-uniformity of strain accumulation all add uncertainty to
forecasting future earthquake occurrence.
Recent Advances—The Wide Range of Slip Velocities
The scientific view of how faults slip has evolved dramatically in the past
decade. Developments in space geodesy—particularly the Global Positioning System
(GPS) and Interferometric Synthetic Aperture Radar (InSAR)—allow the interseismic
deformation phase of the earthquake cycle to be imaged with unprecedented temporal
(GPS) and spatial (InSAR) sensitivity. Prior to the development of large GPS arrays,
measurements of deformation in fault zones were either unavailable or sporadic. As
GPS resolution improved to the several millimeter level, it became clear that overall
strain accumulation does not always follow a simple linear model (see Figure 2.10).
Instead, observations show that steady accumulation of deformation in the volume
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