PGS: Pratt on New Madrid

The February, 2014, meeting of the Potomac Geophysical Society will be held February 20th at the Fort Myer Officers’ Club in Arlington, VA (http://www.jbmhhmwr.com/index/Maps_and_Directions.html) in the Glassed-in room in the Fife and Drum (main dining room).

“The New Madrid seismic zone: Structure, earthquake potential and the debate over building codes” by Thomas L. Pratt, Ph.D., U.S. Geological Survey, Reston, VA,

The New Madrid Seismic Zone (NMSZ) area of the Central U.S. has seen Cambrian to Ordovician rifting and subsequent compression, subsidence and shearing. The southern arm of the NMSZ is coincident with the anticlinal Blytheville Arch (BA), which seismic reflection data presented here indicate is a pop-up structure within a strike-slip fault system along the Cambrian graben’s axial fault. The BA has perhaps the densest concentrations of faults in the region, but numerous faults that cut shallow (Eocene?) strata outside of the BA indicate that earthquakes have not been restricted to the NMSZ. Modern seismicity outlines the Reelfoot fault stepover in the strike-slip New Madrid fault system (NMFS), and comparison with the fault pattern in analog models of stepovers and restraining bends suggests that deformation caused by this stepover controls the extent of NMSZ seismicity. About 4.3 to 5 km of post-Eocene(?) slip on the southern arm of the NMSZ can be estimated from apparent shearing of the southern portion of Crowley’s Ridge, a north-trending, fault-bounded topographic ridge. This apparent displacement of Crowley’s Ridge suggests that slip extends south of the NMSZ. Only a small fraction of this long-term slip is expressed as uplift above the Reelfoot reverse fault, suggesting that strike-slip motion on the southern arm of the NMSZ continues north of the Reelfoot fault. The NMFS thus appears to be part of a broad set of strike-slip faults that spans the Reelfoot rift, with long-term slip continuing north and south of the modern seismic zone.

Meeting Room and Dining Arrangements: We now meet in the glassed-in room at the back of the main dining room—The Fife and Drum. We order individually from the Club menu, which has a nice variety of dinner offerings. We pay a single bill (I pay it with my Officers Club credit card), so we collect at least $25 from each diner with the agreement, that if one orders more than $20 in food and drink, he adds the amount over $20 to his contribution. The $5 overcharge goes to the Room Fee, Tax, Gratuity, and the Speaker’s Dinner. We collect on the Honor System. We did this throughout the last year, and it worked well. This room change and use of the menu have greatly reduced the loss that we have incurred in the past for having fewer than 20 people dining, and the change has preserved the viability of using the Officers Club for our meetings.

Reception at 6:00. Order Dinner at 7:00. Talk at 8:30 PM. Please note that the meal orders will be taken at 7:00. Allow 15 minutes for security entering Ft. Myer as all civilian vehicles are searched. To ensure access to and from Fort Myer use the Hatfield Gate, open 24 hours a day (http://www.jbmhhmwr.com/index/Maps_and_Directions.html). Reservations are not necessary, however, we need a head count, so, if you wish to attend dinner ($25), please inform Bob Fraser at 540-888-3001 or via E-mail at fraser.robert@comcast.net. If you wish, please feel free to attend the talk without dinner. Non-members and guests are welcome. Visit the PGS web site at http://www.potomacgeophysical.com for new meeting announcements, etc. Please send changes of address or email to fraser.robert@comcast.net.

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