Earthquake Focal Mechanisms from the New Madrid Seismic Zone
Thesis Proposal Brown Bag
Greg Johnson
Outline
1) Purpose
2) Study area
3) Data & Method
4) Preliminary Results
5) Future Work
Purpose
History: Limited number of available focal mechanisms
•Herrmann: Surface wave studies of larger events
•Lui: Microearthquake focal mechanisms using
waveform modeling and PANDA data
•Chiu: P-wave first motion study from PANDA
recorded events
New three component data: 2000-2007
•Better locations – s-wave picks
•SH-wave polarity
Generate a more complete catalogue of earthquake focal
mechanisms
Seismotectonic model
Data
Pre-1995: single component data and PANDA data
1995-1999: New Madrid seismic network updates
• 96 three-component stations and 13 broadband
seismometers
2000-2007 data set: 1567 events
md ≥ 4.0 4 events
3.0 ≤ md < 4.0 13 events
2.0 ≤ md < 3.0 - 284 events
1.0 ≤ md < 2.0 - 1190 events
0 < md < 1.0 76 events
Method - Generation of earthquake focal mechanisms
FPFIT
Use only P-wave Polarity
Parameter Estimate:
Strike = 200° ± 15
Dip = 85° ± 28
Rake = - 5° ± 35
Constrained FM when uncertainty ≤ 20°
FOCMEC
Combine SH- first motion
polarity observations
Parameter Estimate
Strike = 180° ± 6
Dip = 40° ± 5
Rake = 90° ± 8
Study Area
New Madrid Seismic Zone
•Seismicity
Northern Mississippi
embayment
•Structure
Embayment structures
Focal mechanisms available
•Herrmann - 16
•Lui - 54
•Chiu - 9
•Allison Shumway - ?
Method - Mapping focal mechanisms
1) Distribution of focal mechanisms on
segments
2) Categorize type by rake
3) Characterization
Seismic Cross Sections
Cartoon
Preliminary Results
• Central segment: 28% of focal mechanisms identified with this segment show
reverse slip on a nodal plane striking ~N21W with ~45º dip to the southwest
• Totals
Reverse slip: 50%
Strike slip: 21%
Normal slip: 29%
• Southern segment: 19% of the focal mechanisms identified with this segment
show right-lateral strike-slip on a nearly vertical nodal plane striking ~N50E
• Totals
Reverse slip: 35%
Strike slip: 49%
Normal slip: 16%
• Northern segment: 45% of the focal mechanisms identified with this segment
show right-lateral strike-slip on a nearly vertical nodal plane striking ~N30E
• Totals
Reverse slip: 41%
Strike slip: 55%
Normal slip: 4%
Future work
• Complete 2007 dataset
• Evaluate and archive focal mechanisms
• Update catalog and map
• Seismotectonic Model (time permitting)
Thanks to my graduate committee for their help and support
throughout this study.
Mitch M. Withers
Stephen P. Horton
Randy T. Cox