Lynn R. Sykes

Tectonics of the Caribbean and Middle America Regions from Focal Mechanisms and Seismicity

Seismic data strongly support recent theories of tectonics in which large plates of lithosphere move coherently with respect to one another as nearly rigid bodies, spreading apart at ocean ridges, sliding past one another at transform faults, and underthrusting at island arcs. Boundaries between adjacent plates of lithosphere are defined by belts of high seismic activity. Redetermination of more than 600 hypocenters in the Middle America region and previous studies in the Galapagos and Caribbean regions define the boundaries of two relatively small, nearly aseismic plates in the region of interest. The first, the Cocos plate, is bordered by the East Pacific rise, the Galapagos rift zone, the north-trending Panama fracture zone near 82° W., and the Middle America arc; the second, the Caribbean plate, underlies the Caribbean Sea and is bounded by the Middle America arc, the Cayman trough, the West Indies arc, and the seismic zone through northern South America. Focal mechanisms of 70 earthquakes in these regions were determined to ascertain the relative motion of these two plates with respect to the surrounding regions or plates. The results show underthrusting of the Cocos plate beneath Mexico and Guatemala in a northeasterly direction and beneath the rest of Central America in a more north-northeasterly direction. The Cocos plate is spreading away from the rest of the Pacific floor at the East Pacific rise and at the Galapagos rift zone. Motion is right-lateral strike-slip along the Panama fracture zone, a transform fault connecting the Galapagos rift zone and the Middle America arc. At the same time, the Caribbean plate is moving easterly with respect to the Americas plate, which is here taken to include both North and South America and the western Atlantic. Left-lateral strike-slip motion along steeply dipping fault planes is observed on the Cayman trough. The Americas plate is underthrusting the Caribbean in a westerly direction at the Lesser Antilles and near Puerto Rico. Unlike the Lesser Antilles, however, motion at present is not perpendicular to the Puerto Rico trench but instead is almost parallel to the trench along nearly horizontal fault planes. Computations of rates of motion indicate that underthrusting is at a higher rate in southeastern Mexico and Guatemala than in western Mexico and that the Caribbean is moving at a lower rate relative to North America than is the Cocos plate.

Nature of seismic coupling along simple plate boundaries of the subduction type

The downdip width of the seismogenic zone is defined for 19 subduction zones. This width is measured from the base of the accretionary prism to the maximum depth of nucleation of thrust events along the plate boundary. Those two points are taken to define the upper and lower depth transitions from stable to unstable frictional sliding. The lower depth transition is found to be between 35 and 70 km. The dip angle of the thrust zone is also reevaluated. We find a linear increase in the dip angle as a function of depth, the slope of which varies between 0.2° and 0.6° km−1. The downdip width obtained, which is generally narrower than previously determined by most other authors, varies from about 50 to 150 km. We also determine the ratio of the rate of slip that occurs in earthquakes to the rate of relative plate motion. This ratio is defined as the seismic coupling coefficient (a). We obtain two different estimates of the seismic coupling coefficient: an average value from 90 years of seismicity and a value obtained using the slip-predictable recurrence model for large earthquakes. We find a large variation in the computed values of a along and among subduction zones. For most of the subduction zones a is much less than 1.0; for several it is less than a few percent. Worldwide, we find no significant correlation between either the seismic coupling coefficient or the width of the seismogenic zone and subduction parameters such as the age of the oceanic lithosphere that is being subducted, plate convergence rates or absolute velocity of the upper plate in the hot spot reference frame. Such correlation exists only for a few individual subduction zones where other parameters do not vary as much. The observed variations in seismic coupling could be explained as differences in the frictional behavior of materials at the plate interface. Some of these differences may be attributed to the subduction of large bathymetric features, the roughness of topography, the presence of unstable triple junctions and active-spreading ridges, and sediment composition.

Seismic Gaps and Plate Tectonics: Seismic Potential for Major Boundaries

The theory of plate tectonics provides a basic framework for evaluating the potential for future great earthquakes to occur along major plate boundaries. Along most of the transform and convergent plate boundaries considered in this paper, the majority of seismic slip occurs during large earthquakes, i.e., those of magnitude 7 or greater. The concepts that rupture zones, as delineated by aftershocks, tend to abut rather than overlap, and large events occur in regions with histories of both long- and short-term seismic quiescence are used in this paper to delineate major seismic gaps.In detail, however, the distribution of large shallow earthquakes along convergent plate margins is not always consistent with a simple model derived from plate tectonics. Certain plate boundaries, for example, appear in the long term to be nearly aseismic with respect to large earthquakes. The identification of specific tectonic regimes, as defined by dip of the inclined seismic zone, the presence or absence of aseismic ridges and seamounts on the downgoing lithospheric plate, the age contrast between the overthrust and underthrust plates, and the presence or absence of back-arc spreading, have led to a refinement in the application of plate tectonic theory to the evaluation of seismic potential.The term seismic gap is taken to refer to any region along an active plate boundary that has not experienced a large thrust or strike-slip earthquake for more than 30 years. A region of high seismic potential is a seismic gap that, for historic or tectonic reasons, is considered likely to produce a large shock during the next few decades. The seismic gap technique provides estimates of the location, size of future events and origin time to within a few tens of years at best.The accompanying map summarizes six categories of seismic potential for major plate boundaries in and around the margins of the Pacific Ocean and the Caribbean, South Sandwich and Sunda (Indonesia) regions for the next few decades. These categories range from what we consider high to low potential for being the site of large earthquakes during that period of time. Categories 1, 2 and 6 define a time-dependent potential based on the amount of time elapsed since the last large earthquake. The remaining categories, 3, 4, and 5, are used for areas that have ambiguous histories for large earthquakes; their seismic potential is inferred from various tectonic criteria. These six categories are meant to be interpreted as forecasts of the location and size of future large shocks and should not be considered to be predictions in which a precise estimate of the time of occurrence is specified.Several of the segments of major plate boundaries that are assigned the highest potential, i.e., category 1, are located along continental margins, adjacent to centers of population. Some of them are hundreds of kilometers long. High priority should be given to instrumenting and studying several of these major seismic gaps since many are now poorly instrumented. The categories of potential assigned here provide a rationale for assigning prorities for instrumentation, for future studies aimed at predicting large earthquakes and for making estimates of tsunami potential.

Great thrust earthquakes and aseismic slip along the plate boundary of the Makran Subduction Zone

The Makran subduction zone of Iran and Pakistan exhibits strong variation in seismicity between its eastern and western segments and has one of the worlds largest forearcs. We determine the source parameters for 14 earthquakes at Makran including the great (Mw 8.1) earthquake of 1945 (the only instrumentally recorded great earthquake at Makran); we determine the loci of seismic and aseismic slip along the plate boundary, and we assess the effects of the large forearc and accretionary wedge on the style of plate boundary slip. We apply body waveform inversions and, for small-magnitude events, use first motions of P waves to estimate earthquake source parameters. For the 1945 event we also employ dislocation modeling of uplift data. We find that the earthquake of 1945 in eastern Makran is an interplate thrust event that ruptured approximately one-fifth the length of the subduction zone. Nine smaller events in eastern Makran that are also located at or close to the plate interface have thrust mechanisms similar to that of the 1945 shock. Seaward of these thrust earthquakes lies the shallowest 70–80 km of the plate boundary; we find that this segment and the overlying accretionary wedge remain aseismic both during and between great earthquakes. This aseismic zone, as in other subduction zones, lies within that part of the accretionary wedge that consists of largely uconsolidated sediments (seismic velocities less than 4.0 km/s). The existence of thrust earthquakes indicates that either the sediments along the plate boundary in eastern Makran become sufficiently well consolidated and de watered about 70 km from the deformation front or older, lithified rocks are present within the forearc so that stick-slip sliding behavior becomes possible. This study shows that a large quantity of unconsolidated sediment does not necessarily indicate a low potential for great thrust earthquakes. In contrast to the east, the plate boundary in western Makran has no clear record of historic great events, nor has modem instrumentation detected any shallow thrust events for at least the past 25 years. Most earthquakes in western Makran occur within the downgoing plate at intermediate depths. The large change in seismicity between eastern and western Makran along with two shallow events that exhibit right-lateral strike-slip motion in central Makran suggest segmentation of the subduction zone. Two Paleozoic continental blocks dominate the overriding plate. The boundary between them is approximately coincident with the transition in seismicity. Although relative motion between these blocks may account for some of the differing seismic behavior, the continuity of the deformation front and of other tectonic features along the subduction zone suggests that the rate of subduction does not change appreciably from east to west. The absence of plate boundary events in western Makran indicates either that entirely aseismic subduction occurs or that the plate boundary is currently locked and experiences great earthquakes with long repeat times. Evidence is presently inconclusive concerning which of these two hypotheses is most correct. The presence of well-defined late Holocene marine terraces along portions of the coasts of eastern and western Makran could be interpreted as evidence that both sections of the arc are capable of generating large plate boundary earthquakes. If that hypothesis is correct, then western Makran could produce a great earthquake or it could rupture as a number of segments in somewhat smaller-magnitude events. Alternatively, it is possible that western Makran is significantly different from eastern Makran and experiences largely aseismic slip at all times. A knowledge of the velocity structure and nature of the state of consolidation or lithification of rocks at depth in the interior portion of the forearc of western Makran should help to ascertain whether that portion of the plate boundary moves aseismically or ruptures in large to great earthquakes. A resolution of this question has important implications for seismic hazard not only for western Makran but also for other margins, such as the Cascadia subduction zone of western North America, where historical thrust events have not occurred.

Contemporary Compressive Stress and Seismicity in Eastern North America: An Example of Intra-Plate Tectonics

A large region of high horizontal compressive stress is delimited in eastern North America from a combination of fault plane solutions of earthquakes, in situ stress measurements, and geologic observations. Each of these methods, including in situ stress determination by both overcoring and hydrofracturing, yields nearly identical directions for the principal stresses. The maximum compressive stress trends east to northeast over an area extending from west of the Appalachian Mountain system to the middle of the continent, and from southern Illinois to southern Ontario. In eastern North America, intra-plate earthquakes appear to occur in areas of high stress along unhealed fault zones of late Paleozoic or younger age. An example of this is the seismic belt trending from Boston to the northwest through Ottawa. This seismic zone appears to be located along a continental extension of the Kelvin seamount chain which is postulated by others to be a transform fault related to the early opening of the North Atlantic. Similarly, the 1929 Grand Banks earthquake and the Charleston, South Carolina, seismic trend appear to be located along extensions of other oceanic fracture zones. The relation between high stress and unhealed fault zones may provide a means to assess the earthquake risk within plates. The observed pattern of stresses appears to be post-Mesozoic in origin. This work supports Voight9s hypothesis that the compressive stress observed within the North American plate may be generated by the same mechanism that drives the movements of large lithospheric plates. If this is indeed the case, stress measurements may furnish one of the best clues to the driving mechanism of plate tectonics.

Evolution of the stress field in southern California and triggering of moderate-size earthquakes: A 200-year perspective

Changes in stress in southern California are modeled from 1812 to 2025 using as input (1) stress drops associated with six large (7.0 7.5) earthquakes through 1995 and (2) stress buildup associated with major faults with slip rates > 3 mm/yr as constrained by geodetic, paleoseismic, and seismic measurements. Evolution of stress and the triggering of moderate to large earthquakes are treated in a tensoffal rather than a scalar manner. We present snapshots of the cumulative Coulomb failure function (ACFF) as a function of time for faults of various strike, dip, and rake throughout southern California. We take ACFF to be zero everywhere just prior to the great shock of 1812. We find that about 95% of those well-located M > 6 earthquakes whose mechanisms involve either strike-slip or reverse faulting are consistent with the Coulomb stress evolutionary model; that is, they occurred in areas of positive ACFF. The interaction between slow-moving faults and stresses generated by faster-moving faults significantly advanced the occurrence of the 1933 Long Beach and 1992 Landers events in their earthquake cycles. Coulomb stresses near major thrust faults of the western and central Transverse Ranges have been accumulating for a long time. Future great earthquakes along the San Andreas fault, especially if the San Bernardino and Coachella Valley segments rupture together, can trigger moderate to large earthquakes in the Transverse Ranges, as appears to have happened in the Santa Barbara earthquake that occurred 13 days after the great San Andreas shock of 1812. Maps of current ACFF provide additional guides to long-term earthquake prediction.

Focal Mechanisms of Deep and Shallow Earthquakes in the Tonga-Kermadec Region and the Tectonics of Island Arcs

Well-determined focal mechanisms based on reliable first motions of both compressional and shear waves are presented for 18 shallow, 6 intermediate, and 15 deep-focus earthquakes in the Fiji-Tonga-Kermadec region of the Southwest Pacific. The double-couple model is an adequate representation for earthquake mechanisms at all depths; most of the mechanisms are characterized by a predominance of dip-slip motions. The orientations of the mechanisms of deep and shallow earthquakes appear to be systematically and fundamentally different in respect to the orientation of the seismic zone. Whereas the shallow mechanisms all appear to accommodate movements between the adjacent sides of the seismic zone, the slip planes of the deep earthquake mechanisms are systematically nonparallel to the deep seismic zone. Hence, the deep zone of activity does not appear to be a simple thrust fault. The P, B, and T axes of the double-couple solutions tend to parallel the dip, strike, and normal directions, respectively, of the portions of the Tonga seismic zone deeper than about 80 km. The P axes tend to be more stable in orientation than the B and T axes. Large variations in the orientations of some of the deep mechanisms may reflect contortions of the deep seismic zone in a simple geometrical fashion. The shallow mechanisms indicate that thrust faulting is occurring beneath the inner (islandward) margins of the Tonga and Kermadec Trenches and that transform faulting is occurring at the northern end of the Tonga Arc. The over-all interpretation of the shallow mechanisms also includes hinge faulting south of Samoa at the juncture of the thrust and transform faults. These results, which are also in agreement with mechanism data for other regions such as Japan where both numerous and reliable data are available, are most simply interpreted by a tectonic model of an island arc in which (a) shallow earthquakes occur between a segment of lithosphere that moves downward into the mantle and the segments of lithosphere on the surface, and (b) deep earthquakes occur within the downgoing slab in response to a compressional stress within it.

Intraplate Earthquakes, Lithospheric Stresses and the Driving Mechanism of Plate Tectonics

Focal mechanisms of about eighty intraplate earthquakes and other information on in situ stress indicate that the interiors of many lithospheric plates are characterised by large horizontal compressive stresses. These stresses seem to be related to the driving mechanism of plate tectonics.

Evolution of moderate seismicity in the San Francisco Bay region, 1850 to 1993: Seismicity changes related to the occurrence of large and great earthquakes

The rate of seismic activity of moderate-size (M > 5.5) earthquakes in the San Francisco Bay (SFB) region has varied considerably during the past 150 years. As measured by the rate of seismic moment release, seismic activity in the SFB region is observed to accelerate prior to M > 7.0 earthquakes in 1868, 1906, and 1989, and then to decelerate following them. We examine these seismicity changes in the context of the evolution of the stress field in the SFB region as a result of strain accumulation and release using a model of dislocations in an elastic halfspace. We use a Coulomb failure function (CFF) to take into account changes in both shear and normal stresses on potential failure planes of varying strike and dip in the SFB region. We find that the occurrence of a large or great earthquake creates a “stress shadow”: a region where the stress driving earthquake deformation is decreased. Interseismic strain accumulation acts to reverse this process, gradually bringing faults in the SFB region out of the stress shadow of a previous large or great earthquake and back into a state where earthquake failure is possible. As the stress shadow generated by a large or great earthquake disappears, it migrates inward toward the fault associated with that large or great event. The observed changes in the rate of occurrence of moderate earthquakes in the SFB region are broadly consistent with this model. In detail, the decrease in seismicity throughout most of the SFB region and a localized increase in the Monterey Bay region following the great 1906 earthquake is consistent with our predicted stress changes. The timing and location of moderate-size earthquakes when the rate of seismicity increases again in the 1950s is consistent with areas in which the 1906 stress shadow had been eliminated by strain accumulation in the SFB region. Those earthquakes that are most inconsistent with our stress evolution model, including the 1911 earthquake southeast of San Jose, are found to occur in regions where dip-slip faulting is common in addition to strike-slip. The 1906 earthquake brought that zone of dip-slip faulting closer to failure, suggesting that the 1911 event may have been a reverse faulting earthquake rather than a strike-slip one similar to the 1984 Morgan Hill earthquake. The occurrence of activity on faults very close to the San Andreas, such as the Lake Elsman earthquakes of 1988 and 1989, appear to be associated with the last disappearence of the stress shadow on the Loma Prieta segment of the San Andreas fault. Thus events of that type may represent an intermediate-term precursor to a large earthquake, such as the 1989 Loma Prieta event. Much of the moderate-size earthquake activity in the SFB region appears to be modulated in time by the buildup and release of stress in large and great earthquakes. A tensorial approach to earthquake prediction, i.e., taking into account changes in the components of the stress tensor, has several advantages over examining scalar changes such as those in seismic activity and moment release rates. This tensorial approach allows for both activation and quiescence (but in different subregions) prior to as well as after large earthquakes.

Changes in State of Stress on the Southern San Andreas Fault Resulting from the California Earthquake Sequence of April to June 1992

The April to June 1992 Landers earthquake sequence in southern California modified the state of stress along nearby segments of the San Andreas fault, causing a 50-kilometer segment of the fault to move significantly closer to failure where it passes through a compressional bend near San Gorgonio Pass. The decrease in compressive normal stress may also have reduced fluid pressures along that fault segment. As pressures are reequilibrated by diffusion, that fault segment should move closer to failure with time. That fault segment and another to the southeast probably have not ruptured in a great earthquake in about 300 years.