2. Principal Stress rotates to EW direction

3. 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:
Better locations – s-wave picks
SH-wave polarity
Generate a more complete catalogue of earthquake focal mechanisms
Seismotectonic model

4. Study Area New Madrid Seismic Zone •Seismicity
Northern Mississippi embayment
Structure
Embayment structures
Focal mechanisms available
Herrmann - 16
Lui - 54
Chiu - 9
Allison Shumway - ?

6. Data Pre-1995: single component data and PANDA data
: New Madrid seismic network updates
• 96 three-component stations and 13 broadband seismometers
data set: 1567 events
md ≥ events
3.0 ≤ md < events
2.0 ≤ md < events
1.0 ≤ md < events
0 < md < events

7. Method - Generation of earthquake focal mechanisms
FPFIT
Use only P-wave Polarity
Parameter Estimate:
Strike = 200° ± 15
Dip = 85° ± 28
Rake = ° ± 35
Constrained FM when uncertainty ≤ 20°
FOCMEC
Combine SH- first motion polarity observations
Parameter Estimate
Strike = 180° ± 6
Dip = 40° ± 5
Rake = 90° ± 8

8. Method - Mapping focal mechanisms
1) Distribution of focal mechanisms on
segments
2) Categorize type by rake
3) Characterization

9. Seismic Cross Sections

10. Cartoon

11. 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
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
Reverse slip: 41%
Strike slip: 55%
Normal slip: 4%

12. Figure 1-1. Map showing focal mechanisms from past studies
Figure 1-1. Map showing focal mechanisms from past studies. Refer to Table 1 for information on each one. The light gray circles are locations of historic seismicity from 1974 to Locations of earthquake focal mechanisms from this study are represented by the dark gray circles. The thick gray line is an outline of the Mississippi Embayment.

14. Nodal plane strike

15. Determining the stress field from focal mechanism solutions
Objective is to find a uniform stress tensor that minimizes the discrepancy
between the predicted shear stress direction and the observed slip
direction on each fault plane.
Start with the regional stress field from the world stress map.
For each fault plane in the set of focal mechanism solutions, determine the
predicted shear stress direction – the rake.
Invert for the stress field that minimizes (observed rake – predicted rake) for
all fault planes.
Inversion yields an estimate of the best-fitting principal stress directions
and the ratio R=(σ2 – σ1)/(σ3 – σ1)
R < 0.5 σ1 and σ2 are similar magnitudes
R > 0.5 σ3 and σ2 are similar magnitudes

16. R < 0.5 σ1 and σ2 are similar magnitudes
σ1/σ2/σ3
dip strike/dip strike/dip strike
Below each histogram
Strike-slip
Thrust faults