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.

Abstract The eastern boundary of the North American Plate is marked by a narrow and continuous belt of seismic activity. The earthquake zone follows the crest of the Mid-Atlantic Ridge and its associated fracture zones (F.Z.) (Fig. 1; Plate 8A; Vogt, this volume, Ch. 12.) and is intimately related to the process of plate separation, transform faulting, and generation of oceanic crust. This part of the Mid-Atlantic plate boundary has the advantage of being relatively close to the dense seismograph networks of North America and Europe, and has therefore been the subject of more study than most other parts of the oceanic rift system. For this reason it has sometimes been taken as the type example of an oceanic rift, which can be misleading. Other segments of the ridge system, such as parts of the East Pacific Rise, have been shown to possess a distinctly different seismicity pattern (Stover, 1973), with most earthquakes occurring along the fracture zones and practically no seismicity along the ridge axes. In this paper we will describe the main features of the seismicity of the mid-oceanic ridge system in the North Atlantic and Arctic Oceans and how it relates to the physical state and processes near the plate boundary. Intraplate earthquakes, that is events within the North American Plate, are dealt with by Zoback and others (this volume). Because of the enormous amount of literature on the subject no attempt has been made to give complete reference to every paper. For more complete references

Abstract This volume is about the evolution of the North America Plate. It also briefly discusses the evolution of the Pacific Ocean and the Caribbean as they relate to North America. We will briefly sketch the present outlines of the North America Plate, review the plate-tectonic development of North America in a global context, and offer an overview of the major tectonic (Fig. 1) and geomorphic elements of our continent. Earthquakes are primary indicators of present plate boundaries and of plate motions that are related to these boundaries. The outline of the North America Plate is shown clearly on Figure 2, which is based only on the distribution of earthquakes occurring from 1977 to 1987. Note also the good definition of the Caribbean and Cocos Plates. In the Arctic regions (Fig. 3), earthquakes clearly outline the plate boundary along the mid-ocean ridge up to the north coast of Siberia. However, from there southward, across northeastern Siberia, the margin of the North America Plate is quite diffuse. Substantial intraplate earthquakes (Fig. 2) have occurred particularly within the United States. The 1811 to 1812 New Madrid (Missouri) and the 1866 Charleston (South Carolina) earthquakes exceeded magnitude 7 and occurred in areas that are still active today (Seeber and Armbruster, 1988; Hinze and Braile, 1988). New England is another area of intraplate seismic activity. According to Zoback and others (1986), the midplate stress of the North America Plate is compressive, with a maximum horizontal principal stress oriented northeast to east northeast. The stress field extends from the Rocky Mountain front to within 230 km of the Mid-Atlantic Ridge.

Abstract The Azores archipelago is located in the North Atlantic Ocean and consists of nine volcanic islands distributed along a general WNW–ESE trend, crossing the Mid-Atlantic Ridge (MAR) in the zone where three lithospheric plates (Eurasian, Nubian and North American) come across. To the west of the MAR, at the North American plate, the Corvo and Flores islands emerge from a present-day, relatively stable geological setting, while the other islands are located in an important seismically and volcanically active zone, corresponding to the boundary between the Eurasian and Nubian tectonic plates. Since the settlement of the Azores archipelago, in the mid-fifteenth century, several destructive earthquakes, such as the 1522 earthquake that severely affected São Miguel Island, and seismic crises with a significant destructive impact, have affected the region with intensities up to X (European Macroseismic Scale of 98 – EMS-98). The location of such events, either inland or offshore, is mainly related to the WNW–ESE structural system, which extends from the MAR to the Gloria Fault (GF). The instrumental seismic activity registered since 1997 has allowed improvements to the characterization of the present-day boundary between the Eurasian and Nubian plates.

Abstract A vital lesson of plate tectonics is that there is no validity to any assumption that the simplest and therefore most acceptable interpretation demands a proximal rather than a distant origin. (Coombs, 1997 , p. 763). This field trip steps back to provide the very long term and large-scale tectonic history that one might call the broader tectonic context of the 1906 San Francisco earthquake. In effect, the field trip follows a cross section of northern California, with stops that illustrate the geologic history of the region. The field guide also discusses several archaeological stops of significance to California's prehistory. The entire field trip is meant to be taken over a period of four days, with overnight stops in Davis and in Quincy. Day one comprises Stops 1–9; Day 2, Stops 10–18; Day 3, Stops 19–28; and Day 4, Stops 29–33. Northern California geology is the result of an extended history of active plate margin interactions spanning some 500 million years (m.y.). Over the course of this period, countless numbers of large earthquakes of different types no doubt accompanied tens of thousands of kilometers of movement between tectonic plates and microplates that eventually came together to form the rocks of northern California as we see them today. From ca. 500–18 million years ago (Ma), subduction, a process still active north of Cape Mendocino, dominated the geologic history of northern California. During this period, several subduction zones and volcanic arcs were active, and subduction zones, whose former positions we will see on our trip, consumed ocean basins thousands of kilometers wide, sweeping together a vast collage of rocks from far-flung locations in the process. Remnants of the most ancient of these subduction zones and collided blocks (typically called terranes) are preserved in the Sierra Nevada. The most recent subduction history along the North American margin involved the Farallon plate, a plate that lay east of the Pacific plate and was separated from it by a spreading mid-oceanic ridge. The products of this subduction episode are preserved in the rocks of the Coast Ranges, and in the granitic and younger volcanic rocks of the Sierra Nevada ((Figs. 1 ) and 2 ). For the past 18 m.y., the plate margin has been dominated by right-lateral faults of a transform plate margin, the most famous of which, the San Andreas fault, produced the 1906 earthquake. The present plate boundary between the Pacific plate on the west and stable North America on the east is a broad one consisting of two active zones: (1) the San Andreas fault system, whose right-lateral faults occupy the California Coast Ranges; and (2) a zone east of the Sierra Nevada including the right-lateral faulting associated with the Walker Lane and the Eastern California Shear Zone, and, east of the Walker Lane, the extensional faults of the Basin and Range province (Fig. 3 ). These zones of active faults, shown in Figure 3 , are responsible for generating most of the earthquakes in the area traversed by our field trip. The preexisting complexity of the crust resulting from the earlier tectonic history probably influenced the development and location of these more recent fault zones, in addition to giving us some exceedingly interesting and complex geology to examine on our trip. This long and complex history requires a great deal of discussion for complete understanding. In this guide, we present first the stops in sequence, including brief descriptions for each site. To augment these brief discussions, we follow the field trip guide with an Appendix in which we discuss the tectonic development of northern California more fully.