Earthquake Focal Mechanisms from the New Madrid Seismic Zone Thesis Proposal Brown Bag Greg Johnson.

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

Focal mechanisms are a representation of the radiation pattern of seismic waves from which fault plane orientation can be inferred. It predicts the direction of motion and the amplitude of seismic waves determined by the orientation of a double couple point source

Each program utilizes different types of seismic waves for determining focal mechanisms. For this study, P-wave first motion polarity is only used for FPFIT, and both P- and SH-wave polarities are used for FOCMEC.

New Madrid Seismic Zone •Seismicity

Northern Mississippi embayment

•Structure

Embayment structures

Focal mechanisms available•Herrmann - 16•Lui - 54•Chiu - 9•Allison Shumway - ?

Study Area

is a shallow trough of poorly consolidated to unconsolidated Late Cretaceous through Quaternary sediments that plunge to the southwest (Stearns, 1957)

The trough sediments dip towards the south and towards the center of the trough, but the Paleozoic rocks below have variable dip (Ervin, 1975). The Paleozoic rocks lie unconfomably above the Precambrian crystalline basement (Bickford, 1986)

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%

28% of the total focal mechanisms in the central segment show reverse slip where a nodal plane has a similar strike and dip as that observed in the seismicity

19% of the focal mechanisms show right- lateral strike-slip on a nearly vertical nodal plane striking about N50E.

Earthquakes located within the northern segment illuminate a fault plane that strikes ~N30E with nearly vertical dip. Based on rake 41% of the earthquakes located in the northern segment have reverse slip, 55% have strike slip, and 4% have normal slip. Almost half (45%) of the focal mechanisms identified with this segment show right-lateral strike-slip on a nearly vertical fault striking close to N30E.

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

Earthquake Focal Mechanisms from the New Madrid Seismic Zone Thesis Proposal Brown Bag Greg Johnson.

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