Robert M. Hamilton

Recurrent Intraplate Tectonism in the New Madrid Seismic Zone

For the first time, New Madrid seismicity can be linked to specific structural features that have been reactivated through geologic time. Extensive seismic reflection profiling reveals major faults coincident with the main earthquake trends in the area and with structural deformation apparently caused by repeated episodes of igneous activity.

Diapiric origin of the Blytheville and Pascola arches in the Reelfoot rift, east-central United States: Relation to New Madrid seismicity

Most of the earthquakes in the New Madrid seismic zone correlate spatially with the Blytheville arch and part of the Pascola arch, which are interpreted to be the same structure. Both arches may have formed by diapirism along the axis of the Reeffoot rift. Seismic, geophys ical, and drill-hole data indicate that the rocks in the arches are highly deformed and fractured and have gross lithologic properties that make them weaker than rocks adjacent to the arches. The weaker rocks are inferred to fail seismically more readily than the stronger rocks adjacent to the arches.

Structure of the New Madrid seismic source zone in southeastern Missouri and northeastern Arkansas

New seismic reflection data in the southwestern part of the New Madrid seismic zone (NMSZ) show that a major zone of disrupted reflectors and a large antiform coincide with a prominent trend of earthquake epicenters between Caruthersville, Missouri, and Marked Tree, Arkansas. We speculate that the antiform may be caused by igneous intrusions. The strong correlation between the seismicity in this trend and the extent of the disrupted zone provides a geologic basis for defining the source zone for large earthquakes in this part of the NMSZ.

Seismic Activity and Faulting Associated with a Large Underground Nuclear Explosion

The 1.1-megaton nuclear test Benham caused movement on previously mapped faults and was followed by a sequence of small earthquakes. These effects were confined to a zone extending not more than 13 kilometers from ground zero; they are apparently related to the release of natural tectonic strain.

Cenozoic faulting in the vicinity of the Charleston, South Carolina, 1886 earthquake

Multichannel seismic-reflection profiles in the meizoseismal area of the 1886 Charleston earthquake and nearby offshore area show evidence of Cenozoic faulting. Deformation observed on two land profiles defines a northeast-striking reverse(?) fault zone named the Cooke fault. A Jurassic-age basalt layer at 750 m depth has 50 m vertical displacement, southeast side down. Overlying Upper Cretaceous and Cenozoic beds have decreasing displacement with decreasing depth. These displacements, coupled with larger (∼ 190 m) displacements of reflectors below the basalt, indicate continuing Cenozoic movement of a post–basalt-flow, pre–Late Cretaceous fault into Eocene time or later. The fault extends into a cluster of 1973–1978 epicenters; this suggests a possible causal relationship with seismicity. Marine multichannel seismic-reflection data combined with single-channel, high-resolution reflection profiles reveal Cenozoic faulting beneath the continental shelf near Charleston. A reverse fault 12 km offshore, named the Helena Banks fault, extends for at least 30 km and is observed as shallow as 10 m below the sea bottom, with the most recent movement in post-Miocene or Pliocene time. There is no known seismicity in the area of this fault. The reverse movement on the Helena Banks and Cooke faults is consistent with a northwest-southeast compressional stress regime. Elucidation of the relation, if any, between reactivated Triassic or older structures and/or a decollement (defined by diffractions at 11.4 ± 1.5 km depth) on the one hand and seismicity on the other will be important in assessing earthquake risk in many areas of similar tectonic setting outside of the Charleston region in the Atlantic coastal and continental margin area.

Seismic-wave attenuation associated with crustal faults in the new madrid seismic zone.

The attenuation of upper crustal seismic waves that are refracted with a velocity of about 6 kilometers per second varies greatly among profiles in the area of the New Madrid seismic zone in the central Mississippi Valley. The waves that have the strongest attenuation pass through the seismic trend along the axis of the Reelfoot rift in the area of the Blytheville arch. Defocusing of the waves in a low-velocity zone and/or seismic scattering and absorption could cause the attenuation; these effects are most likely associated with the highly deformed rocks along the arch. Consequently, strong seismic-wave attenuation may be a useful criterion for identifying seismogenic fault zones.

Seismology in Lebanon

During the last several years, while civil war raged in Lebanon, the Bhannes seismograph station near Beirut continued to operate and provide seismograms and phase readings on a regular basis. Seismologists around the world owe gratitude to Charles Tabet, an employee of both the Lebanon National Council for Scientific Research (NCSR) and the American University of Beirut, for his daily dedication to maintaining the stations operations. In addition, Iskandar Sursock, also of the NCSR, has provided essential support and personal resources to continue the seismological program in Lebanon.

Partners in the publication process

During the past year several questions concerning the publications process have come to the attention of the Publications Committee. These questions have made it clear to us that some members of AGU may not be familiar with some key elements of AGUs philosophy toward publication. Since the publications activities are so important to each member and to the Union as a whole, we believe that a brief overview of the responsibilities of the three primary groups involved in the publication process is in order. Most of what we say here would apply to any scientific journal or at least any scientific journal published by a learned society. Some aspects are specific to AGU.