Fourteen—
Hypocenter Mapping and the Extensibility of Seismotectonic Paradigms

James W. Dewey

Introduction

This paper will consider earthquake mapping from the viewpoint of scientific "paradigms," in the context of Kuhn's (1970) theory of the structure of scientific revolutions. Paradigms are "universally recognized scientific achievements that for a time provide model problems and solutions to a community of users" (Kuhn, 1970, p. viii ). A paradigm is a framework within which the community identifies and solves the puzzles of its discipline. It provides both a basis for interpreting old data and, more important in the long run, a rationale for acquiring new data. A paradigm may be gradually modified in order to account for new results of ongoing observations. From time to time, the community may face observations that either contradict the paradigm in a fundamental way or that are both inexplicable by the paradigm and too important for the community to ignore. The community ultimately resolves such a crisis by adopting a radically new paradigm that accounts for the observations that led to the old paradigm and that also explains the observations that could not be accounted for by the old paradigm. This radical change of paradigm, in Kuhn's model, constitutes a scientific revolution.

For the past two decades, most research in seismotectonics has been based on two paradigms. On a local scale, we have the earthquake/fault paradigm: societally and tectonically significant crustal earthquakes commonly occur as the result of shear displacement on preexisting geologic faults that are big enough to, in principle, be imaged by appropriate geologic or geophysical data. On a broad scale, there is what I will call the earthquake/plate-tectonics paradigm: the locations and focal mechanisms of many earthquakes are predictable from a knowledge of the relative motions of tectonic plates on the Earth's surface and by application of a few simple constitutive rules governing plate behavior. Seismotectonic research is commonly based on both

paradigms, but this is not always so. I find it convenient from an expository standpoint to consider the two paradigms separately.

At present, neither the earthquake/fault nor earthquake/plate-tectonics paradigm faces anomalies of the sort that imply an imminent revolution throughout seismotectonics. There are, however, some earthquake source regions that have been studied from the viewpoints of the two paradigms and that nevertheless remain poorly understood. Seismicity data from these regions do not strongly suggest slip on preexisting faults when interpreted by the current fault paradigm, or the working of a particular plate-tectonics process when interpreted by the current plate-tectonics paradigm. Moreover, there is a precedent for proposing locally applicable models that are essentially independent of the current earthquake/fault and earthquake/plate-tectonics paradigms. Individual earthquakes have, for example, been modeled as consequences of large landslides (Kanamori et al., 1984; Eissler and Kanamori, 1987) or of tensile failure under high fluid pressure (Julian, 1983). Seismologists who work in puzzling source regions therefore face three options. They can continue to apply current plate-tectonics and fault paradigms, hoping that more and higher quality data will reveal the nature of the source regions. Alternatively, they can try to explain the existing observations by modifying the current paradigms without abandoning the fundamental assumptions upon which the paradigms are based. Finally, they can abandon the paradigms altogether and, hoping to precipitate a scientific revolution in a subfield of seismotectonics, search for locally applicable seismotectonic models that do not involve plate-tectonics processes or slip on preexisting faults.

This paper will attempt to demonstrate the value of the two conservative options for solving local puzzles—the collection of more data in the framework of existing paradigms and the elaboration of existing paradigms. My intent is not to belittle the importance of contemplating radically new paradigms but rather to convey the extent to which recent observational studies and recent elaborations have strengthened the existing paradigms. A wide range of phenomena is already at least partially accounted for by the paradigms; the solution to some local puzzles may lie in a few more data points plus the application of hypotheses developed in other seismic zones. Furthermore, the paradigms have continued to be extensible to cover previously unexplained observations and are therefore likely to be extensible in the future to cover still more observations.

The Earthquake/Fault Paradigm

The hypothesis that earthquakes result from faulting developed near the turn of the century (Dutton, 1904; Reid, 1910) and evolved with contributions from many scientific disciplines. Although different communities of earth sci-

entists have different studies of seismogenic faulting as models, and their paradigms accordingly differ somewhat, I would suggest that the basic earthquake/fault paradigm for mapping crustal shocks comprises the following theses: (1) most crustal earthquakes result from the release of tectonic strain energy by sudden shear fracture along preexisting faults; (2) high-quality seismicity data from an active crustal source region will define two-dimensional planar or slightly curved zones of hypocenters, corresponding to faults, and focal mechanisms will have nodal planes parallel to the local orientations of the hypocentral surfaces; and (3) large crustal earthquakes occur on large faults, which therefore may extend from seismogenic depths to the near-surface and be mappable with geologic or geophysical data. The preceding theses are hypotheses and expectations that appear inherent in the ways in which observational seismologists design seismicity studies and in the ways in which they seem most naturally to interpret the resulting hypocenters and focal mechanisms.

A suite of observations that exemplify each of the theses in the basic earthquake/fault paradigm is found in the Parkfield, California, section of the San Andreas fault (fig. 1). Five earthquakes of magnitude about 6 have taken place since 1857 on apparently the same segment of the fault; the coseismic displacement seismologically inferred for each shock appears equal to the displacement that would be expected to accumulate elastically across the fault between shocks (Bakun and McEvilly, 1984). The most recent such shock occurred in 1966; geodetic data are consistent with the segment of fault that ruptured in 1966 being currently locked at depth and accumulating strain (Harris and Segall, 1987). Small and moderate earthquakes occur on a nearly vertical plane beneath the surface trace of the fault and have mechanisms consistent with slip on the fault. The fault zone is conspicuous in regional geology and geomorphology.

In a region in which seismogenic faults are not revealed in seismographic or geologic data as directly as the San Andreas fault is revealed near Parkfield, the earthquake/fault paradigm may be elaborated as in studies of the Coalinga earthquake of 2 May 1983 (fig. 1). Hypocenters and focal mechanisms in the Coalinga region do not define a single plane, and the causative fault of the main shock of 2 May 1983 does not outcrop at the surface. The hypocenters and focal mechanisms are, however, well accounted for in terms of slip on intersecting faults (Eaton, 1985b ). Stein and King (1984) have shown that the seismogenic reverse or thrust faulting that occurred at depth appears to manifest itself as folding in the weak near-surface sedimentary rock. They suggest that, in regions in which surface rock is sedimentary, the presence of reverse faulting at depth may often be found more easily by searching for associated folds rather than outcrops of fault planes. In this case, the apparent failure of the Coalinga earthquake to conform to the third thesis of the earthquake/fault paradigm led to a richer version of the para-

Figure 1
Seismicity of central California in the region of Parkfield and Coalinga
for the period 1975–1984 (from Dewey et al., in press, after Bakun and
Lindh, 1985). Focal mechanisms of the 1966 Parkfield and 1983 Coalinga
earthquakes are shown. N35° W is the direction of pure transform motion
between the Pacific and North American plates.

digm, in which active surface folds are added to active fault traces as possible indicators of potentially dangerous earthquake sources.

The seismicity of central Idaho (fig. 2) provides examples of several phenomena not yet, but probably soon to be, incorporated into the earthquake/fault paradigm. These phenomena are the quiescence of major faults at the small magnitude level for long periods between the generation of large earthquakes, the occurrence of small and moderate earthquakes away from major faults, and the occurrence of aftershocks on faults other than those of the corresponding main shocks.

Central Idaho was the site of the magnitude (M_s ) 7.3 Borah Peak earthquake of 28 October 1983. The cause of that earthquake was slip on a major preexisting fault, the Lost River fault (Crone et al., 1987), most of which had been quiescent down to magnitude 3.5 during the previous two decades (fig. 2). From 1963 until the 1983 Borah Peak main shock, small and moderate earthquakes in central Idaho had occurred most frequently several tens of kilometers west and north of the Borah Peak source. Although aftershocks in the first ten days following the main shock were located close to the coseismic fault surface defined from geologic and geodetic data, later aftershocks occurred in a zone twice as long as the main-shock rupture (fig. 2).

Such observations might be viewed as already accounted for in an extended earthquake/fault paradigm, because they are considered possible outcomes of seismicity studies premised on the seismogenic-fault model. But I am not aware of an independently defined community of users that has reached consensus about the seismotectonic environments in which these phenomena should be considered expected outcomes of seismicity studies.

The phenomenon of fault quiescence is currently being studied using the hypothesis that individual segments of seismogenic faults slip by characteristic amounts in the coseismic phase of each seismic cycle, and that the characteristic displacement and interval between displacements of a given fault segment tend to increase with segment length (Allen, 1968, Sieh, 1978; Schwartz and Coppersmith, 1984). Faults that comprise many segments are most likely to experience frequent small and moderate shocks; faults that comprise a few large segments are most prone to long periods of quiescence separated by large earthquakes. This characteristic-displacement hypothesis would lead to an elaboration of thesis 2 of the earthquake/fault paradigm. Although seismographic recording in active source regions would still be expected to define planar faults, the largest faults in the region might not be revealed in a time period much shorter than the durations of the seismic cycles on the faults. Quiescence at small and moderate magnitudes would be expected in a short period of seismographic monitoring of a fault if geologic studies showed the fault to be geometrically simple with a recent history of large, episodic displacements.

The characteristic-displacement hypothesis also explains the observation

Figure 2
Seismicity of central Idaho in the vicinity of the Borah Peak earthquake of 28 October
1983. Epicenters are distinguished according to whether the shocks occurred before,
within ten days after, or between ten days and two years after the Borah Peak main
shock. Epicenters were computed by Dewey (1987). Borah Peak fault scarps are from
Crone et al. (1987), and the rupture surface is from Ward and Barrientos (1986).

that some large earthquakes are followed by aftershocks near, but not on, the main-shock rupture surface. Stress is effectively relaxed on the simple main-shock source by the occurrence of the main shock, and most of the main-shock surface will not rupture again until sufficient strain energy has accumulated to again produce the characteristic displacement. Aftershocks occur on the margins of the main-shock rupture, due perhaps to slip on smaller branch faults, which must accommodate some of the displacement of the main fault (King, 1983), or to slip on preexisting faults on which effective shear stress has increased as a consequence of main-shock faulting (Chinnery, 1966; Nur and Booker, 1972). Many aftershock studies are conducted on the assumption that aftershock hypocenters are distributed on the surface that slipped in the main shock. Under the characteristic-displacement hypothesis applied to aftershock sequences, the location of aftershocks of a major earthquake would be expected to define fault surfaces secondary to the fault surfaces on which most of the main-shock seismic moment was released.

Small earthquakes occurring kilometers to tens of kilometers away from major regional faults have traditionally been interpreted as due to minor faults slipping under the same regional tectonic stress causing slip on the major faults (Richter, 1958). Substantial elaboration of this interpretation has come with studies showing the wide range of preexisting fault orientations that may be favorable to slip under a given stress field (McKenzie, 1969; Angelier, 1984). It is also commonly accepted that some small earthquakes may reflect local stress fields that are either independent of the regional stress field or second-order consequences of large tectonic displacements induced by the regional stress field. Algorithms have been developed that permit extraction of several significantly different orientations of focal mechanisms from a suite of first motion data for a group of earthquakes (Brillinger et al., 1980) and that enable a search for a single orientation of the tectonic stress field that may be consistent with differently oriented focal mechanisms (Gephart and Forsyth, 1984).

Midplate regions, located far from the belts of earthquakes and late-Cenozoic deformation that define plate boundaries in the earthquake/plate-tectonic paradigm (see next section), pose special problems for the earthquake/fault paradigm. In most midplate areas monitored by regional or local networks of seismographs, the resulting hypocenters and focal mechanisms do not define planes of shear displacement coinciding with mapped crustal faults (for example, see Bollinger and Sibol, 1985; Wetmiller et al., 1984). It is possible that, in some of these areas, midplate earthquakes do not occur on preexisting faults but rather represent fractures of previously intact rock, being thereby natural analogs of some mine rockbursts (Evernden, 1975). In that case, the distribution of microcracks or the rheological properties of the unfractured rock, rather than the presence of geologically mappable faults, might determine the positions of earthquakes (McGarr et al.,

1975). Even if the resulting earthquakes involved the development of faults in previously intact rock, such earthquakes would not fulfill all of the expectations of the earthquake/fault paradigm. Earth scientists would lose one of the most important pratical consequences of the paradigm, that sites of future earthquakes are in principle identifiable by mapping of the faults on which they will occur. The apparent ineffectiveness of the paradigm in many midplate regions may, however, be due to the small sizes of the shocks for which the paradigm is being called upon to account. As just noted, even in areas with well-documented seismogenic faulting in the western United States, it may be difficult to associate many small earthquakes with individual faults. But in the active parts of the western United States, seismologists' confidence in the earthquake/fault paradigm does not depend on its making sense of the small shocks.

An observation that supports the appropriateness of the earthquake/fault paradigm for at least some midplate regions is that late Cenozoic reactivation of pre-Cenozoic faults has been identified at a number of sites in the central and eastern United States (Wentworth and Mergner-Keefer, 1983; Donovan et al., 1983). Most of these faults have been quiescent during the time period in which earthquakes might have been reliably located in their vicinities, but, as noted earlier in this section, quiescence of potentially seismogenic faults is quite commonly observed in regions of high Cenozoic tectonism. The crucial implication of the reactivation is that crustal faults can persist as sites of shear failure in tectonic environments that are far different from the environments in which the faults originally formed.

Results from a multidisciplinary investigation of the Mississippi Embayment seismic zone (fig. 3) have shown the value of the earthquake/fault paradigm in an important midplate seismic region. In addition, the Mississippi Embayment studies must be viewed as justifying the patience-trying accumulation of seismographic, geophysical, and geologic observations to solve a seismotectonic puzzle. The Mississippi Embayment source produced the New Madrid earthquakes of 1811 and 1812, the largest in the history of the central and eastern United States. The status of our knowledge of the distribution of earthquake epicenters in the Mississippi Embayment prior to mid-1974 is shown in the left panel of figure 3; the distribution does not suggest the presence of faults. In mid-1974, Saint Louis University installed a regional network in the Mississippi Embayment region (Stauder et al., 1976). This network greatly increased the number and accuracy of earthquake hypocenters in the Mississippi Embayment (right panel, fig. 3); the 1975–1985 data also permitted more accurate determination of hypocenters of pre-1975 shocks (Gordon, 1983). Geophysical studies conducted in the late 1970s and early 1980s suggested that the Mississippi Embayment is underlain by a late Precambrian–early Paleozoic rift (Hildenbrand, 1985), and geologic studies identified the late-Holocene Lake County uplift above the

Figure 3
Two views of seismicity of the Mississippi Embayment. At left, a pre-1975 view based on epicenters
routinely determined by the U.S. Geological Survey and its predecessors and published in catalogs such
as the "Preliminary Determination of Epicenters" (PDE). At right, a view based on epicenters recorded by
the St. Louis University Mississippi Embayment Network for 1975–1985 (small diagonal crosses) and on
pre-1975 earthquakes whose locations have been recomputed using calibration events from the post-1975
era (Gordon, 1983). The rift boundaries and outline of the Lake County Uplift, though plotted in the left
frame, are taken from the post-1975 work of Hildenbrand (1985) and Russ (1982).

most active part of the seismic zone. Epicenters, Paleozoic structure, and Quaternary data now fit a fault model quite nicely. The lineations suggested by recently recorded epicenters would correspond to individual faults being reactivated under a uniform stress system (Russ, 1982); focal mechanisms of the larger shocks of the past decade are consistent with this interpretation (Herrmann and Canas, 1978). The Lake County uplift probably represents the surface expression of a basement reverse fault (Russ, 1982; Nicholson et al., 1984) similar to the anticline that represents the surface effect of the reverse-slip fault at Coalinga, California.

The Earthquake/Plate-Tectonics Paradigm

The earthquake/plate-tectonics paradigm emerged as one aspect of the global-tectonics revolution of the 1960s (Isacks et al., 1968). According to the paradigm, the Earth's crust and upper mantle comprise continent-sized slabs, or plates, of lithosphere that move with respect to each other and that are separated by boundaries along which their relative motions are accommodated. Over geologic time, the motions of the plates relative to each other are so much greater than their internal deformations that the plates are treated as rigid in global kinematic analysis of lithospheric displacements. Earthquakes occur on the plate boundaries. The type of displacement—normal, reverse, or strike-slip—producing the earthquakes on a given boundary depends on whether the relative motion of the adjacent plates is away from, toward, or parallel to the boundary.

An example of one type of refinement of the earthquake/plate-tectonics paradigm is given by ongoing studies of the ratio of seismic to aseismic slip across a plate boundary. Plate-tectonics models can predict the amount of relative motion between two plates that must be accommodated by some kind of slip on their mutual boundary, but the original earthquake/plate-tectonics paradigm did not provide a basis for estimating the percentage of relative plate motion across a particular boundary that would be accommodated by seismic slip. Recent studies have searched for systematic dependencies of this ratio on such plate-tectonics parameters as plate age and convergence rate (for example, Kanamori, 1986).

In the United States, the dependence of the seismic/aseismic ratio on plate age is at the heart of a controversy on the likelihood of great earthquakes in the Pacific Northwest subduction zone. The kinematics of global plate motions implies a convergence of 30 to 40 mm/year across the Pacific Northwest subduction zone. The thrust interface between the Juan de Fuca and North American plates has not, however, been seismogenic during the period in which it has been instrumentally monitored. Because the Pacific Northwest subduction zone involves the subduction of young oceanic plate, and because there is a worldwide tendency for the seismic/aseismic ratio to

increase as the age of subducting lithosphere decreases, it has been suggested that the plate interface is only temporarily quiescent and is storing energy for a future great earthquake (Heaton and Kanamori, 1984). But it is not clear if the tendency for seismogenic subduction of young lithosphere can be extrapolated to imply that the very young sediment-covered lithosphere of the Juan de Fuca plate should be seismogenic. Possibly underthrusting in the Pacific Northwest is accommodated aseismically, as would be suggested by extrapolation from the recent lack of plate interface earthquakes there. Both aseismic and seismic underthrusting seem consistent with the earthquake/plate-tectonics paradigm in its present form; resolution of the Pacific Northwest controversy is likely, however, to lead to the paradigm being modified or extended to more completely account for the behavior of young lithosphere in subduction zones.

The first versions of the earthquake/plate-tectonics paradigm did not account for the occurrence of earthquakes in plate interiors. Further development of the earthquake/plate-tectonics paradigm has relaxed the assumption of quasi-rigidity for broad regions of late-Cenozoic deformation in continental lithosphere adjacent to plate boundaries. However, the assumption of quasi-rigidity for the cratonic interiors of continental plates and for oceanic plates has been retained. Plate-tectonics kinematics rules are now applied to compute the motion of tectonically stable plate interiors from the orientations of and rates of slip along boundaries of oceanic plates (Minster and Jordan, 1978). The tectonically active belts on the margins of continental plates are modeled as buoyant lithosphere that deforms in response to the quasi-rigid motion of the plate interiors (Atwater, 1970; McKenzie, 1978).

An example of how modified plate tectonics principles may be applied to a region of deformed continental lithosphere adjacent to a major plate boundary is given by recent studies of the Coalinga, California, earthquake of 2 May 1983 (figure 1). The slip vector of the earthquakes is nearly orthogonal to the direction of relative motion between the North American and Pacific plates, so the shock cannot be viewed as accommodating slip between essentially rigid plates. This apparently places the earthquake outside the paradigm. However, Eaton (1985a ) and Minster and Jordan (1987) have noted that the strike of the San Andreas fault in central California is rotated several degrees counterclockwise from the trend of pure transform motion between the North American and Pacific plates predicted by assuming that oceanic plates and the interiors of continental plates are undeformed. In addition, the North American plate east of the San Andreas fault is experiencing west-northwest, east-southeast extension in the Basin and Range province. With relative motion between the undeformed interiors of the plates of about 56 mm/year, the strike of the fault and extension in the Basin and Range would result in about 10 mm/year of convergent plate motion orthogonal to the strike of the San Andreas fault, which must therefore be accommodated by

slip on some structure other than the San Andreas itself (Minster and Jordan, 1987). Some of this long-term motion would have been accommodated by slip on the fault that caused the Coalinga earthquake. Thus, the apparent failure of a crude application of the paradigm becomes a success with more sophisticated application, more data, and a broader context.

Many of the quantitative applications of the plate-tectonics paradigm cannot be used in the study of midplate earthquakes. For example, even the modified form of the paradigm applied to the study of the Coalinga earthquake cannot be used to account for the rake and rate of displacement on a fault in the interior of a plate experiencing negligible long-term deformation. Plate-tectonics models have been used, however, to account for midplate stress fields as due principally to plate-driving or plate-resisting forces on the boundaries of elastic lithospheric plates (Richardson et al., 1979). In addition, many students of midplate earthquakes see a correlation between midplate source regions and plate-boundary regions that were last active in the Paleozoic or Mesozoic.

Several characteristics of the midplate Mississippi Embayment seismic zone (figure 3) may be explainable by current or ancient plate-tectonics processes. The late Precambrian–early Paleozoic rift structure beneath the Embayment is thought to have developed at the edge of the North American craton at the opening of the proto–Atlantic Ocean but to have failed to develop into an oceanic spreading center (Hildenbrand, 1985). The focal mechanisms of most earthquakes in the Embayment imply an axis of maximum compressive stress approximately parallel to axes of maximum compressive stress throughout the interior of the North American plate. The source is viewed in the earthquake/plate-tectonics paradigm as an old plate-boundary structure now reactivated in a stress regime that, because it is platewide, is probably due to the forces that move plates or resist plate motion.

Discussion

The extensions of the earthquake/plate-tectonics and earthquake/fault paradigms cited in this paper frequently do more than enable the paradigms to cover, one by one, individual unexplained observations. Commonly, an extension to one paradigm aimed at explaining one observation resolves other puzzles in that paradigm or puzzles in the other paradigm. The interpretation of the Coalinga earthquake as accommodating plate motion normal to the San Andreas fault, for example, both explains the orientation of the slip vector in the earthquake and suggests that the systematic small discrepancy between the regional strike of the fault and the calculated direction of pure transform motion between the North American and Pacific plates is not due to errors in the plate-kinematics modeling. The fact that it is still

possible to clear up several puzzles with one hypothesis must be counted as evidence of the continuing vigor of the two paradigms.

Although the two paradigms have been most successful in regions of intense late-Cenozoic tectonism, one must be impressed that they also account for some of the most significant observations made over the past two decades in midplate regions. These observations include the nearly uniform orientation of the stress tensor across large areas of midplate North America, the evidence for seismogenic slip on preexisting faults in the Mississippi Embayment, and the discovery of late-Cenozoic slip on faults formed before the Cenozoic.

Acknowledgments

This paper is an outgrowth of a review of the seismicity of the contiguous United States that I have written with D. P. Hill, W. L. Ellsworth, and E. R. Engdahl (Dewey et al., in press). The reader is encouraged to consult Dewey et al., (in press) for more complete references to studies of the source regions and for a treatment of U.S. seismicity that does not once mention "paradigm."

I thank Bob Engdahl and Dave Perkins for their helpful reviews. Dave suggested a number of sentence rewrites, whose use I gratefully acknowledge.

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