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
• Large variability in event terms for both
mainshocks and aftershocks
PGA Event Terms based on Crude Classification (Used by AS08)
PLACEHOLDER FOR FIGURES FROM NAA
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, China
65 Earthquakes
Declustered Results:
39 Class 2 Events
Associated with the
2008 Wenchuan, China
Earthquake
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 Total
1 Cluster
35 Clustered Events
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
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
Event Terms for Class 1 & Class 2 Earthquakes
PLACEHOLDER FOR FIGURES FROM NAA
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)