Analyis of Bridge Structures Crossing Fault Rupture Zones: Seismic Ground Motion Simulation Douglas Dreger, Gabriel Hurtado, and Anil Chopra University of California, Berkeley

Objective Develop a set of characteristic near-fault (10s of meters distance) ground motions for engineering design. Explore ground motion sensitivity to faulting style and rupture behavior. In particular consider cases with surface rupture where ground motions have both dynamic and static components.

Elastic Dislocation of a Mw6.5 Earthquake

Computational Method 4th order staggered grid finite-differences Complete wavefield computation Arbitrary model complexity Velocity Structure Half-space, layered (1D) Source Complexity Uniform slip Variable slip, rise time and rupture velocity Computational requirements Grid spacing (20m), fmax (20 Hz), model size (2.3 Gp), requiring 119 Gbytes of memory Computations are performed on the LLNL MCR supercomputer using 512 processors

Finite-Difference Grid and Station Subarray Position FP FN

Considered Fault Geometries

Source Characteristics Mw 6.5 28.8km x 9.3km length by width fault from Wells and Coppersmith (1994) 0.71m of uniform slip variety of faulting style (strike-slip to thrust) constant 0.5 second rise time from Somerville et al. (1999) sensitivity to rise time is examined constant rupture velocity of 80% of the shear wave velocity sensitivity to rupture velocity is examined

Comparison of Displacement Results Strike-slip Oblique Reverse Thrust

Comparison of Velocity Results Strike-slip Oblique Reverse Thrust

Strike-Slip Results Fling Controlled Directivity controlled

Are The Time Histories Reasonable?

Dip-Slip Results Strike-Slip Case Dip=40; rake=110 case

Z component Displacement Motions Away from Fault

Rise Time Sensitivity - FP Component

Rise Time Sensitivity - FN Component rt=0.3s rt=0.5s rt=0.7s

Rupture Velocity Sensitivity

Variable Slip Models Kinematic Model Method of Mai and Beroza (2001) to obtain random slip distributions Assume constant rupture velocity Assume constant slip velocity - leads to variable rise time Models have variable variable static offset close to the fault, and degrees of rupture directivity Cumulative Slip

Conclusions A finite-difference code was used to simulate very near-fault (15m) strong ground motion time histories. These time histories show static fault offset as well as dynamic pulses. Simulations for different faulting styles shows the near-fault records as highly variable. These idealized motions are used in the study of fault-crossing bridge response.