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Poster Abstracts
InSAR results based on C-band ERS data (Cakir et al., 2005) that suggested that the NAF segment near Ismetpasa may be only
partially locked. If so, the deeper locked portion of the fault must be characterized by a higher stressing rate, and presumably
shorter recurrence interval. We are modeling available InSAR and GPS data using numerical simulations of spontaneous
earthquake sequences that incorporate laboratory-derived rate-and-state friction laws. The goal of these simulations is to
constrain key parameters of fault friction such as the depth extent of the velocity-strengthening and velocity-weakening layers,
the long-term fault slip rate, and stress evolution in the seismogenic crust.
VARIABILITY OF SEISMIC SOURCE SPECTRA DERIVED FROM COHESIVE-ZONE MODELS OF A
CIRCULAR RUPTURE PROPAGATING AT A CONSTANT SPEED (A-085)
Y. Kaneko and P.M. Shearer
Static stress drop of earthquakes is often estimated from far-field body-wave spectra using measurements of corner
frequencies, together with seismic moment, which can be computed from the low-frequency part of the spectrum. Corner
frequencies are used to infer the source dimension based on a specific theoretical model. The most widely used model is from
Madariaga (1976), which considered a bilateral rupture expanding at a constant speed on a circular fault. This model assumes
that the rupture front is characterized by an abrupt change of fault strength from a uniform initial prestress to a kinetic
frictional stress, and hence the stress is singular at the rupture front. In this study, we investigate variability of source spectra
derived from dynamic models of expanding bilateral ruptures on a circular fault with a cohesive zone that prevents a stress
singularity at the rupture front. We study the dependence of far-field body-wave spectra on the rate of frictional weakening
(which controls the cohesive zone size), rupture speed, and dynamic stress drop. For each source model, we compute far-field
body-wave displacement synthetics for a homogeneous elastic space using the representation theorem (Aki and Richards,
2002). Our results show that P- and S-wave corner frequencies of displacement spectra are systematically larger than those
predicted by Madariaga (1976) and generally depend on the rate of frictional weakening in the source model, which affects the
fracture-energy-density distribution on the fault. For a given rupture speed, the average of corner frequencies over the focal
sphere is larger for source models with larger fracture energy. For ruptures propagating at 90 percent of the S-wave speed,
the azimuthal average of P-wave corner frequencies in the case with the smallest fracture energy is about 20 percent larger
than that of Madariaga (1976), which corresponds to about a factor of two difference in the inferred stress drop. Thus for these
ruptures, application of the Madariaga model overestimates stress drops by factors of two or more, depending upon the
fracture energy. We also find that the takeoff angle dependence of the corner frequencies for supershear ruptures is
systematically different from that for subshear ruptures due to the difference in radiated wave fields for supershear and
subshear ruptures.
THREE DIMENSIONAL ELASTIC RESPONSE OF COMPLIANT FAULT ZONES TO NEARBY
EARTHQUAKES: A THEORETIC STUDY (A-110)
J. Kang and B. Duan
Compliant fault zones have been detected along active faults by seismic
investigations (trapped waves and travel time analysis) and InSAR observations. However, the width and depth extent of
compliant fault zones are still under debate in the community. Numerical models of dynamic rupture build a bridge between
theories and the geological and geophysical observations. Theoretical 2D plane-strain studies of elastic and inelastic response
of compliant fault zones to nearby earthquake have been conducted by Duan [2010] and Duan et al [2010]. In this study, we
further extend the experiments to 3D with a focus on elastic response. We are specifically interested in how residual
displacements depend on the structure and properties of complaint fault zones, in particular on the width and depth extent.
We conduct numerical experiments on various types of fault-zone models, including fault zones with a constant width along
depth, with decreasing widths along depth, and with Hanning taper profiles of velocity reduction. . Our preliminary results
suggest 1) the width of anomalous horizontal residual displacement is only indicative of the width of a fault zone near the
surface, and 2) the vertical residual displacement contains information of the depth extent of compliant fault zones.
A UNIFIED FINITE ELEMENT METHOD FOR HETEROGENEOUS ELASTIC MEDIA WITH LOW TO HIGH
RATIO OF P- TO S-WAVE VELOCITIES (A-040)
H. Karaoglu and J. Bielak
This study reports on a mixed finite element formulation that overcomes certain limitations faced by low-order primal-based
formulations. A primal-based formulation with displacement as the variable is a widely used method to solve wave-
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