Kinematic Modeling of the Denali Earthquake Regional distance seismic and GPS data were inverted for kinematic slip parameters (e.g. Dreger et al., 2004; Oglesby et al., 2005). Slip is mostly shallow with 4 primary asperities. Rupture velocity is fast on average. Simulated PGV correlates with the occurrence of liquefaction features (e.g. Kayen et al., 2004) Continuing efforts include updating the kinematic model using the recently released GPS data set, and examining implications for using finite source parameters in ShakeMap. Dreger 04HQGR0013

2003 San Simeon earthquake During the project period the San Simeon earthquake was also investigated (e.g. Hardebeck et al., 2004; Dreger et al., 2005) Finite-source models show a shallow rupture extending 25 km SE of the epicenter. The finite-source parameters were used to update ShakeMap the day of the earthquake greatly improving strong shaking estimates in the most heavily impacted areas. Dreger 04HQGR0013

2004 Parkfield Earthquake During the project period the 2004 Parkfield earthquake was also investigated (Langbein et al., 2005; Dreger et al., 2005). Seismic waveform and continuous GPS data were inverted for coseismic slip (middle panel) Continuous GPS data were inverted for postseismic slip (bottom panel) Fault slip rate inferred from repeating earthquake recurrence times (top panel) has an inverse relationship with the coseismic slip suggesting differing fault rheology. Postseismic slip occurs over the entire fault surface (depending on smoothing) but appears to be concentrated at shallow depths where coseismic slip was low. Ongoing work will update the coseismic kinematic model with near-fault seismic observations, and investigate relationships between coseismic, postseismic slip and repeating earthquake behavior. Dreger 04HQGR0013

Basin Modeling in the SF Bay Area Dolenc, Dreger and Larsen Teleseismic observations Near Coast of Central Chile, 07/29/98, Mw=6.4, Depth=58 km Dreger 04HQGR0062

Basin Modeling in the SF Bay Area H/V spectral ratio - results Correlations between teleseismic and local earthquake arrival time delays and amplitudes and microseism dominant period have been found for sites in the Santa Clara Valley These observations have positive correlation with basin depth in the USGS V2 model Continuing work on this project is focussing on inverting these data for basin velocity structure, and investigating 3d wave propagation in the Napa and Sonoma Valleys Dreger 04HQGR0062

Strong Ground Motions Derived from Geodetic Slip Models Junkee Rhie, Douglas S. Dreger, and Mark H. Murray Develop Method for near-realtime determination of fault orientation and slip from geodetic data New methodology to derive strong ground motions Recast geodetic slip models to kinematic slip models assume constant stress drop and therefore slip velocity assume constant rupture velocity (in typical range) define spatially variable slip time history using the constant slip velocity (larger slip takes longer to accumulate) Allows determination of strong motions such as peak ground velocity without seismic data or seismic inversion Advantage of new approaches Less computation time compared with seismic inversion Independent determination of fault orientation Redundancy if seismic data sets are sparse or inaccessible in real-time Means to combine seismic and geodetic data in a joint near-realtime inversion Murray and Dreger 04HQGR0043/44

PGV ShakeMaps for Mw 6.4 Northridge earthquake (B) (C) PGV ShakeMaps for three slip models (A) and (C) Uniform- and variable- slip model from GPS inversion Hudnut et al., 1996 (B) Reference slip model from seismic and GPS inversion Wald et al., 1996 Observation (red) and determined PGV (blue contour with black numbers) values in cm/s Uniform slip model severely underestimates observed PGV. Both the variable and reference slip model maps define the area of very strong shaking (extend of 10-20 cm/s contours), but there is a mismatch between were the largest observed and simulated motions are located. Murray and Dreger 04HQGR0043/44

Line-Source Solution Used to Update ShakeMap

Planar Finite-Source Model and Simulated Ground Motions Broadband seismic waveforms and nearby GPS data were used Elongated rupture area to the SE Simulated PGV(cm/s) correlates well observations at stations not used to find slip model (unfilled triangles)

San Simeon Earthquake Summary Both line- and plane-source models show rupture extends 25 km SE of epicenter. Dimensions of rupture from finite-source study was used to correct ShakeMap several hours after the event and before non-realtime near-fault data became available. Simulated PGV agree well with values at the two closest stations. This application demonstrates the utility of finite-source information in characterizing near-fault motions in situations of sparse coverage.