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° ± 15Dip = 85° ± 28Rake = - 5° ± 35
Constrained FM when uncertainty ≤ 20°
FOCMECCombine SH- first motion polarity observations
Parameter Estimate
Strike = 180° ± 6Dip = 40° ± 5Rake = 90° ± 8
New Madrid Seismic Zone •Seismicity
Northern Mississippi embayment
•Structure
Embayment structures
Focal mechanisms available•Herrmann - 16•Lui - 54•Chiu - 9•Allison Shumway - ?
Study Area
1) Distribution of focal mechanisms on segments
3) Characterization
2) Categorize type by rake
Method - Mapping focal mechanisms
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
• TotalsReverse 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
• TotalsReverse 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
• TotalsReverse 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