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Katie Wooddell 10/ 25/ 2011

Classifying Earthquakes in the NGAW2 Catalog A Methodology for Classifying Earthquakes for Ground Motion Estimation. Katie Wooddell 10/ 25/ 2011. NGA Ground Motion Observations. Median ground motions from aftershocks are systematically lower than median ground motions from mainshocks

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Katie Wooddell 10/ 25/ 2011

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  1. Classifying Earthquakes inthe NGAW2 CatalogA Methodology for Classifying EarthquakesforGround MotionEstimation Katie Wooddell 10/25/2011

  2. NGA Ground Motion Observations • Median ground motions from aftershocks are systematically lower than median ground motions from mainshocks • Large variability in event terms for both mainshocks and aftershocks

  3. PGA Event Terms based on Crude Classification (Used by AS08) PLACEHOLDER FOR FIGURES FROM NAA

  4. Period Dependence of Aftershock Term in AS08 NGA model

  5. NGA Conclusions • Systematic difference in median short period spectral acceleration for mainshocks and aftershocks • Consistent with lower stress drops for aftershocks • Need to account for this difference in GMPEs • Issue: Definition of aftershock was not clear

  6. NGA West 2 Event Classifications • CLASS 1 EVENTS: Events with systematically higher median ground motions • CLASS 2 EVENTS: Events with systematically lower median ground motions • Subsequent events in a time window, X, following an earthquake • Events close to the rupture plane

  7. Identifying Class 1 and Class 2 Events GARDNER-KNOPOFF ALGORITHM: Dependent events (“aftershocks”) must fall within both a time and distance window determined by the magnitude of the mainshock • Pros: • Provides a good starting point and framework for distinguishing between Class 1 and 2 events. • Simple time and distance windows that are easy to apply and modify. • Does not require a complete catalog. • Cons: • Class 2 Events are misclassified as Class 1 Events when the rupture plane is long. • Class 1 Events (off-plane events) can be misclassified as Class 2 Events (on-plane events)

  8. 2008 Wenchuan, China 65 Earthquakes Declustered Results: 39 Class 2 Events Associated with the 2008 Wenchuan, China Earthquake

  9. Identifying Class 1 and Class 2 Events • Modifications to the Distance Window Criteria: • Fault geometries used for faults (if available) • Fault trace, depth to top of rupture, dip, and dip direction specified for each fault trace • Distance calculated between the surface projection of the Class 1 Event fault plane and potential Class 2 Events in the database (ΔRJB) • A maximum allowable ΔRJB is selected based on ground motion properties • Modifications to the Time Window Criteria: • Currently no modifications have been made • Proposed Modifications: • Time dependent ΔRJBto account for healing in the crust

  10. Identifying Class 1 and Class 2 Events ΔRJB = 0 km 65 Events Total 1 Cluster 35 Clustered Events

  11. Identifying Class 1 and Class 2 Events ΔRJB = 2 km 65 Events Total 3 Clusters 41 Events-Cluster 1 2 Events-Cluster 2 3 Events-Cluster3

  12. Identifying Class 1 and Class 2 Events ΔRJB = 5 km 65 Events Total 3 Clusters 48 Events-Cluster 1 5 Events-Cluster 2 2 Events-Cluster3

  13. Event Terms for Class 1 & Class 2 Earthquakes PLACEHOLDER FOR FIGURES FROM NAA

  14. PGA Event Terms for Class 2 Earthquakes

  15. Issue: Slope Differences manifest as event terms

  16. Conclusions • Systematic difference in ground motions from Class 1 & Class 2 events that needs to be included in the GMPEs • We propose the following classifications for the NGAW2 GMPEs: • Class 1 Events: Events (ΔRJB > ??) with systematically higher median ground motions including mainshocks, off-plane events, and triggered events. • Class 2 Events: Events (ΔRJB < ??) with systematically lower median ground motions (on-plane events)

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