Classifying Earthquakes in the NGAW2 Catalog

A Methodology for Classifying Earthquakes for Ground Motion Estimation

Katie Wooddell10/25/2011

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

PLACEHOLDER FOR FIGURES FROM NAA

PGA Event Terms based on Crude Classification (Used by AS08)

Period Dependence of Aftershock Term in AS08 NGA model

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

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

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)

MAG DIST (km) TIME (days)

2.5 19.5 6.0

3.0 22.5 11.5

3.5 26.0 22.0

4.0 30.0 42.0

4.5 35.0 83.0

5.0 40.0 155.0

5.5 47.0 290.0

6.0 54.0 510.0

6.5 61.0 790.0

7.0 70.0 915.0

7.5 81.0 960.0

8.0 94.0 985.0

2008 Wenchuan, China65 Earthquakes

Declustered Results:39 Class 2 Events Associated with the2008 Wenchuan, ChinaEarthquake

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 ΔRJB to account for healing in the crust

Identifying Class 1 and Class 2 Events

ΔRJB = 0 km

65 Events Total1 Cluster35 Clustered Events

Identifying Class 1 and Class 2 Events

ΔRJB = 2 km

65 Events Total3 Clusters41 Events-Cluster 12 Events-Cluster 23 Events-Cluster3

Identifying Class 1 and Class 2 Events

ΔRJB = 5 km

65 Events Total3 Clusters48 Events-Cluster 15 Events-Cluster 22 Events-Cluster3

PLACEHOLDER FOR FIGURES FROM NAA

Event Terms for Class 1 & Class 2 Earthquakes

PGA Event Terms for Class 2 Earthquakes

Issue: Slope Differences manifest as event terms

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)