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Group 1 – ShakeOut | Poster Abstracts
Attenuation (NGA) empirical relations very closely (i.e., within roughly their epistemic
uncertainty) over the distance range 2-200 km, and intra-event standard deviations are very close
to the empirical values over the distance range 0 and 50 km. The simulations predict entrainment
by basin structure of a strong directivity pulse, with long-period spectral accelerations in Los
Angeles and Ventura basins significantly larger than those predicted by the empirical relations.
The ShakeOut-D ground motion predictions differ in some important respects from a recent
kinematically parameterized simulation of a geometrically similar scenario: (1) the kinematic rocksite
predictions depart significantly from the common distance-attenuation trend of the NGA and
ShakeOut-D results, and (2) ShakeOut-D predictions of long-period spectral acceleration within the
basins of the greater Los Angeles area, including those with concentrations of high-rise buildings,
are lower by factors of 2-3 than the corresponding kinematic predictions. The latter result agrees
with the results from a previous comparison of kinematically and dynamically parameterized
simulations of Mw7.7 San Andreas scenarios (TeraShake). As in the previous study, we attribute
the difference to reduced forward directivity due to the less coherent wavefield excited by the
spontaneous-rupture sources.
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IMPLICATIONS OF THE SHAKEOUT SOURCE DESCRIPTION FOR RUPTURE
COMPLEXITY AND NEAR-SOURCE GROUND MOTION Dalguer LA, Day SM, Olsen KB,
Cruz-Atienza VM, Cui Y, Zhu J, Gritz A, Okaya DA, and Maechling PJ
With the goal to evaluate the implications of the source description of the SoSAFE ShakeOut
scenario for the rupture and ground motion, we developed a diversity of large-scale dynamic
models in the Southern San Andreas fault. Four classes of models are considered: Each of the first
three classes incorporates stochastic irregularities in the stress drop compatible with seismological
observations, combined with some degree of constraint on the long-wavelength component of slip:
(i) Models that have stress drop preconditioned such that Mw and final surface slip approximate
the corresponding moment and slip-profile (or background slip) distribution along the strike of the
fault specified for ShakeOut by Hudnut et al. (2008). (ii) Models preconditioned so as to the depthaveraged
slip matches the slip-profile specified by Hudnut et al. and such that they match Mw as
well (iii). Models that match the ShakeOut Mw only. The constraints were imposed using a slipmatching
technique that iteratively performs kinematic and dynamic rupture simulations to find
stress drop distribution that yields the prescribed ShakeOut static slip characteristics. We also
investigated a fourth class of model based on simple asperities following the rules proposed by
Dalguer at al., 2008, BSSA, and matching only the fault dimensions of ShakeOut (notice that by
simply using this rule, the Mw also approximates the ShakeOut Mw). The class (i) and class (ii)
models each have two well-defined patches of high stress drop at the extremes of the fault,
corresponding to the a priori slip constraints from the ShakeOut scenario. All the fault models are
planar faults, and each dynamic model is the first step of a two-step procedure to calculate ground
motion for the ShakeOut scenario (see the poster of Olsen et al). Here we examine the rupture
complexity and the qualitative ground motion characteristics of these models, referring to the
poster of Olsen et al. for quantitative details of the ground motion estimates. All the models
generate the strongest ground motion next to the fault, driven mainly by deep patches of high
stress drop. Qualitatively, we note that the class (iii) and (iv) models are apparently more efficient
in exciting the San Gabriel and Los Angeles basin areas than are the slip-constrained class (i) and
(ii) models, probably because the slip distribution imposed on the latter models is not optimal for
exciting the guided-wave channel between San Andreas fault and Los Angeles basin.
2008 SCEC Annual Meeting | 93