Abstract The Meers fault scarp trends ESE-WNW along the northern edge of the Meers Valley in Commanche County, southwestern Oklahoma (Fig. 1 ). The impressively straight scarp can be traced for 16 mi (26 km). The southeastern two-thirds of the scarp intends across land that is under relatively intensive agricultural use, and the northwestern third extends across hilly open range. This land usage reflects the underlying Permian bedrock shales to the southeast and conglomerates to the northwest. The most visually impressive and instructive lengths of scarp are developed in the conglomerate terrain. Access procedure to a convenient length of the scarp in this terrain is as follows. First, contact Mr. Charlie Bob Oliver, manager of the Kimbell Ranch. The ranch house is located in Blue Creek Canyon on Oklahoma 58. Mr. Oliver has been closely involved in recent work on the fault and, if notified in advance, can make a key to the ranch tracks available. Oklahoma 58 must be followed south of Stumbling Bear Pass to a county road (Fig. 2). This road should then be taken for just over 1 mi ( 1.6 km) west to a group of houses north of the road. At this point a decision must be made with respect to the condition of a ranch track which trends north by the houses. This road is impassable for most vehicles after a rain (the alternative is a walk of about 1.5 mi; 2.4 km). The track is followed north for about 0.5 mi (0.8 km)

Abstract Problems with and approaches to seismic-hazard estimation in the midcontinent of the United States are evaluated by using recent data on stress regime, crustal age and structure, and seismicity of other stable continental regions. Evaluating earthquake hazard in the central U.S. is difficult because of the lack of identifiable seismogenic faults and because of the low rate of seismic activity. Furthermore, the recurrence intervals of large earthquakes are poorly known, in part because of the short historical record that spans only a fraction of the repeat times of these quakes. The seismotectonic regime of the central U.S. is dominated by the Reelfoot rift complex and the associated New Madrid, Missouri, seismic zone. However, there are other major tectonic structures in the region such as the Nemaha ridge, the Midcontinent rift system, and the Wichita-Ouachita orogenic belt; earthquakes generating damaging ground motion (approximately magnitude 5.0 or greater) have occurred in the states of Ohio, Illinois, Oklahoma, Texas, Kansas, Nebraska, Kentucky, Alabama, and Arkansas, as well as Missouri. Opinions vary widely about the best way to delineate seismic source zones in such a diffuse and varied seismotectonic environment. Moreover, detailed paleoseismic or neo-tectonic data that could improve hazard assessments are extremely sparse in the central United States. The Meers fault scarp in southwestern Oklahoma, with its evidence for Holocene displacement and its lack of background seismicity, highlights a new set of assessment problems. Development of site-specific probabilistic hazard curves are further hampered by the lack of strong ground-motion data and high-resolution attenuation data. We address aspects of the overall seismic-hazard assessment problem for which neotectonic information provides constraints. These include a seismic source zonation for the central U.S. and estimates of maximum possible earthquakes for these zones, especially for the New Madrid region.

Abstract The Meers fault in southwestern Oklahoma, with a prominent scarp resulting from late Holocene surface displacement, is the best-expressed late Quaternary surface fault known to occur in a “stable” continental interior (or mid-plate) region (i.e., regions far removed from areas of high tectonic rates). The Meers fault is part of a major fault system that has not been the locus of major tectonic activity since the Paleozoic, and although recent surface displacements have been sizable, average late Quaternary rates have been low, based on a lack of geomorphic expression indicating significant cumulative displacement. Activity of the Meers fault is unusual, because in mid-plate regions, few large historical earthquakes have occurred and recognized cases of late Quaternary surface faulting are very rare. Based on the extent of surface rupturing and amounts of displacement, the Meers fault appears capable of producing very large events (i.e., M > 7, or possibly even M > 7 ½). Recent events on the Meers fault produced surface displacements of a few to several meters. Such displacements are quite large, relative to the rupture length of about 40 km, and could result from a tendency for mid-plate or long-recurrence faults to rupture with higher stress drops than plate-margin or short-recurrence faults. Studies attempting to evaluate this possibility have produced conflicting results and may indicate this cannot be placed in as simple a context as plate-margin versus intraplate settings. A large earthquake on the Meers fault would produce strong ground motion throughout much of the south-central United States and could cause widespread damage. The existence of a potential source of large earthquakes in a region thought to be tectonically stable suggests that the seismic potential of this and other mid-plate regions may be underestimated.