Earthquake Mechanisms and Plate Tectonics

7 Earthquake Mechanisms and Plate Tectonics

Seth Stein and Eryn Klosko Northwestern University, Evanston, Illinois, USA

1. Introduction Thus, at spreading centers plates move away from the boundary, whereas at subduction zones the subducting plate Earthquake seismology has played a major role in the devel- moves toward the boundary. At the third boundary type, opment of our current understanding of global plate tectonics transform faults, plate motion is parallel to the boundary. The and in making plate tectonics the conceptual framework used slip vectors of the earthquakes on plate boundaries, which to think about most large-scale processes in the solid Earth. show the motion on the fault plane, re¯ect the direction of During the dramatic development of plate tectonics, discussed relative motion between the two plates. from the view of participants by Uyeda (1978, and this volume), The basic principle of plate kinematics is that the relative Cox (1973), and Menard (1986), the distribution of earthquakes motion between any two plates can be described as a rotation on provided some of the strongest evidence for the geometry of a sphere about an Euler pole (Fig. 2). Speci®cally, at any point plate boundaries and the motion on them (e.g., Isacks et al., along the boundary between plates i and j, with latitude and 1968). More than thirty years later, earthquake studies retain a longitude , the linear velocity of plate j with respect to plate i is central role, as summarized here. vji !ji Â r 1 Because earthquakes occur primarily at the boundaries between lithospheric plates, their distribution is used to map the usual formulation for rigid body rotations in mechanics. plate boundaries and their focal mechanisms provide infor- The vector r is the position vector to the point on the boundary, mation about the motion at individual boundaries. Plate boundaries are divided into three types (Fig. 1). Z Oceanic lithosphere is formed at spreading centers, or mid- N ocean ridges, and is destroyed at subduction zones, or trenches. v12

Oceanic plate Ridge Trench Euler vector Greenwich r 12 Meridian Fracture zone Continental Transform Y plate fault Magnetic Euler pole Lithosphere anomalies

Asthenosphere FIGURE 2 Geometry of plate motions. At any point r along the boundary between plate i and plate j, with geopraphic latitude and FIGURE 1 Plate tectonics at its simplest. Plates are formed at ridges longitude , the linear velocity of plate j with respect to plate i is and subducted at trenches. At transform faults, plate motion is parallel vji !ji Â r. The Euler pole at latitude and longitude is the to the boundaries. Each boundary type has typical earthquakes. intersection of the Euler vector !ji with the Earth's surface.

INTERNATIONAL HANDBOOK OF EARTHQUAKE AND ENGINEERING SEISMOLOGY, VOLUME 81A ISBN: 0 -12- 440652-1 Copyright # 2002 by the Int'l Assoc. Seismol. & Phys. Earth's Interior Committee on Education. All rights of reproduction in any form reserved. 69 70 Stein and Klosko

and !ji is the rotation vector or Euler vector. Both are de®ned and are thus spreading centers. Figure 3b shows an alternative from an origin at the center of the Earth. case. The pole here is for plate 1( j 1) with respect to plate 2 The direction of relative motion at any point on a plate (i 2), so plate 1moves toward some segments of the boundary is a small circle, a parallel of latitude about the Euler boundary, which are subduction zones. Note that the ridge pole (not a geographic parallel about the North Pole!). For and subduction zone boundary segments are not small circles. example, in Figure 3a the pole shown is for the motion of The magnitude, or rate, of relative motion increases with plate 2 with respect to plate 1. The ®rst-named plate ( j 2) distance from the pole, since moves counterclockwise about the pole with respect to the second (i 1). The segments of the boundary where relative jvjijj!jijjrj sin 2 motion is parallel to the boundary are transform faults. Thus, where is the angle between the Euler pole and the site (cor- transforms are small circles about the pole and earthquakes responding to a colatitude about the pole.) Thus, although all occurring on them should have pure strike-slip mechanisms. points on a plate boundary have the same angular velocity, the Other segments have relative motion away from the boundary, linear velocity varies. If we know the Euler vector for any plate pair, we can write Rotation pole the linear velocity at any point on the boundary between the plates in terms of the local E±W and N±S components by a coordinate transformation. With this, the rate and azimuth of 21 plate motion become q Plate 1 NS 2 EW 2 rate jvjij vji vji 3 ! NS Plate 2 v azimuth 90 À tanÀ1 ji 4 Spreading vEW ridge ji such that azimuth is measured in degrees clockwise from North. Given a set of Euler vectors with respect to one plate, those with respect to others are found by vector arithmetic. For example, the Euler vector for the reverse plate pair is the Transform negative of the Euler vector (a) !ij À!ji 5

Rotation pole Euler vectors for other plate pairs are found by addition

!jk !ji !ik 6 12 so, given a set of vectors all with respect to plate i, any Euler vector needed is found from Plate 1 Plate 2 !jk !ji À !ki 7

Subduction For further information on plate kinematics see an intro- zone ductory text such as Cox and Hart (1986). As discussed there, motions between plates can be determined by combining three different types of data from different boundaries. The rate of spreading at ridges is given by sea-¯oor magnetic anomalies, and the directions of motion are found from the orientations Transform of transform faults and the slip vectors of earthquakes on (b) transforms and at subduction zones. As is evident, earthquake slip vectors are only one of three types of plate motion FIGURE 3 Relationship of motion on plate boundaries to the Euler pole. Relative motion occurs along small circles about the pole; data available. Euler vectors are determined from the relative the rate increases with distance from the pole. Note the difference the motion data, using geometrical conditions. Since slip vectors and transform faults lie on small circles about the pole, the pole sense of rotation makes: !ji is the Euler vector corresponding to the rotation of plate j counterclockwise with respect to i. must lie on a line at right angles to them (Fig. 3). Similarly, the Earthquake Mechanisms and Plate Tectonics 71 rates of plate motion increase with the sine of the distance from Ridge the pole. These constraints make it possible to locate the poles. Strike-slip fault Determination of Euler vectors for all the plates can thus (left lateral) Normal be treated as an overdetermined least-squares problem, and fault the best solution found using the generalized inverse to derive global plate motion models (Chase, 1972; Minster and Jordan, Fracture zone 1978; DeMets et al., 1990, 1994). Because these models use magnetic anomaly data, they describe plate motion averaged Transform over the past few million years. New data have become available in recent years due to the No seismicity No seismicity rapidly evolving techniques of space-based geodesy. These techniques (Gordon and Stein, 1992) (very long baseline radio Transform interferometry (VLBI), satellite laser ranging (SLR), the global positioning system (GPS), and DORIS (similar to GPS, but using ground transmitters)) use space-based technologies Normal to measure the positions of geodetic monuments to accuracies fault of better than a centimeter, even for sites thousands of kilo- Strike-slip fault meters apart. Hence measurements of positions over time yield (right lateral) Ridge relative velocities to precisions almost unimaginable during the early days of plate tectonic studies. A series of striking FIGURE 4 Possible tectonic settings of earthquakes at an oceanic results, ®rst with VLBI and SLR (e.g., Robbins et al., 1993), spreading center. Most events occur on the active segment of the and now with GPS (Argus and He¯in, 1995; Larson et al., transform and have strike-slip mechanisms consistent with transform 1997), show that plate motion over the past few years is faulting. On a slow spreading ridge, like the Mid-Atlantic, normal generally quite similar to that predicted by global plate motion fault earthquakes occur. Very few events occur on the inactive fracture zone. model NUVEL-1A. This agreement is consistent with the prediction that episodic motion at plate boundaries, as re¯ected in occasional large earthquakes, will give rise to steady north±south trending ridge segments, offset by transform motion in plate interiors due to damping by the underlying faults, such as the Vema Transform, which trend approxi- viscous asthenosphere (Elsasser, 1969). As a result, the mately east±west. Both the ridge crest and the transforms are earthquake mechanisms can be compared to the plate motions seismically active. The mechanisms show that the relative predicted by both global plate motion models and space-based motion along the transform is right±lateral. Sea-¯oor spreading geodesy. on the ridge segments produces the observed relative motion. For this reason, earthquakes occur almost exclusively on the active segment of the transform fault between the two ridge 2. Oceanic Spreading Center Focal segments, rather than on the inactive extension, known as a Mechanisms fracture zone. Although no relative plate motion occurs on the fracture zone it is often marked by a distinct topographic fea- Earthquake mechanisms from the mid-ocean ridge system ture, due to the contrast in lithospheric ages across it. Unfor- re¯ect the spreading process. Figure 4 schematically shows a tunately, some transform faults named before this distinction portion of a spreading ridge offset by transform faults. Because became clear, such as the Vema, are known as ``fracture zones'' new lithosphere forms at the ridges and then moves away, the along their entire length. Earthquakes also occur on the relative motion of lithosphere on either side of a transform is in spreading segments. Their focal mechanisms show normal opposing directions. The direction of transform offset, not the faulting, with nodal planes trending along the ridge axis. spreading direction, determines whether there is right or left The seismicity is different on fast spreading ridges. lateral motion on the fault. This relative motion, de®ned as Figure 5b shows a portion of the Paci®c±Antarctic boundary transform faulting, is not what produced the offset of the ridge along the East Paci®c Rise. Here, strike-slip earthquakes occur crest. In fact, if the spreading at the ridge is symmetric (equal on the transforms, but we do not observe the ridge crest rates on either side), the length of the transform will not change normal faulting events. These observations can be explained with time. This is a very different geometry from a transcurrent by the thermal structure of the lithosphere, because fast fault, where the offset is produced by motion on the fault and spreading produces younger and thinner lithosphere than slow the length of the offset between ridge segments would increase spreading. The axis of a fast ridge has a larger magma with time. chamber than the slow ridge, and the lithosphere moving away The model is illustrated by focal mechanisms. Figure 5a from a fast spreading ridge is more easily replaced than for a shows a portion of the Mid-Atlantic Ridge composed of slow ridge. Thus, in contrast to the axial valley and normal 72 Stein and Klosko faulting earthquakes on a slow ridge, a fast ridge has an axial 3. Subduction Zone Focal high and absence of earthquakes. Mechanisms The mechanisms are consistent with the predictions of plate kinematics. The area in Figure 5a is a portion of the boundary Both the largest earthquakes and the majority of large earth- between the South American and Nubian (West African) quakes occur at subduction zones. Their focal mechanisms plates. An Euler vector for Nubia with respect to South re¯ect various aspects of the subduction process. Figure 6 is a America with a pole at 62 N, 37.8 W and a magnitude of composite cartoon showing some of the features observed in 0.328 degrees MyÀ1 predicts that at 0 N, 20 W Africa is different subduction zones. moving N81E, or almost due East, at 33 mm yÀ1 with respect Most of the large, shallow, subduction zone earthquakes to South America. The Vema is a boundary segment parallel to indicate thrusting of the overriding plate over the subducting this direction, and so is a transform fault characterized by lithosphere. The best such examples are the two largest ever strike-slip earthquakes with directions of motion along the recorded: the 1960 Chilean (M 2.7 Â 1030, M 8.3) and 1964 trace of the transform. The short segments essentially at right 0 s Alaskan(M 7.5 Â 1029,M 8.4)earthquakes.Thesewereimpres- angles to the direction of relative motion are then spreading 0 s sive events; in the Chilean earthquake 24 m of slip occurred on ridge segments. The spreading rate determined from magnetic a fault 800 km long along-strike and 200 km long down-dip. anomalies, and thus the slip rate across the transform, is Smaller, but large, thrust events are characteristic. For example, described by the Euler vector. Figure 7a shows the focal mechanisms of large shallow earthquakes along a portion of the Peru±Chile Trench, where the NazcaPlate issubductingbeneaththeSouthAmericanPlate.The mechanisms along the trench show thrust faulting on fault planes 15 20Ј with a consistent geometry; parallel to the coast, which cor- 14° N responds to the trench axis, with shallow dips to the northeast. These thrust events directly re¯ect the plate motion. At a point on the trench (17 S, 75 W), global plate motion model Vema NUVEL-1A (DeMets et al., 1994) predicts motion of the Nazca À1 10° N plate with respect to South America at a rate of 68 mm y and an azimuth of N76E. The direction of motion is toward the Doldrums trench, as expected at a subduction zone. The major thrust earthquakes at the interface between subducting and overriding plates thus directly re¯ect the subduction, and slip vectors from 6° N (a) –45° W –40° W their focal mechanisms can be used to determine the direction of plate motion. The rate of subduction is harder to assess. –50° S Although the rate can be computed from global plate motion models or space geodesy, not all of the plate motion is always

Eltanin Small earthquakes Bending earthquakes - Few, small –54° S

Great thrust earthquakes Normal fault earthquakes - Often, but not always - Few, large - e.g., 1960 Chile, - e.g., 1933 Sanriku, 1965 Rat Island, 1964 Alaska 1977 Indonesia –58° S Udintsev Intermediate - Not observed everywhere 660 earthquakes (b) –145° W –140° W –135° W km Deep seismic zone - Near slab top FIGURE 5 Maps contrasting faulting on slow and fast spreading - Either single or double centers. (a) The slow Mid-Atlantic ridge has earthquakes both on the - Either downdip compression or downdip extension active transform and ridge segment. Strike-slip faulting on a plane - Dip may vary considerably parallel to the transform azimuth is characteristic. On the ridge seg- - Depth may vary considerably ments, normal faulting with nodal planes parallel to the ridge trend is “Composite” subduction zone seen. (b) The fast East Paci®c Rise has only strike-slip earthquakes on the transform segments. Mechanisms from Engeln et al. (1986), FIGURE 6 Schematic of some of the features observed at sub- Huang et al. (1986), and Stewart and Okal (1983). duction zones. Not all features are seen at all subduction zones. Earthquake Mechanisms and Plate Tectonics 73

Nazca–South America Plate Boundary Zone depth of 25 km and thrusting in its lower part, between 40 290° 300° and 50 km. These observations constrain the position of the . . neutral surface separating the upper extensional zone from . . . the lower ¯exural zone, and thus provide information on the South . . American mechanical state of the lithosphere. Occasionally, trenches are ° –10 S . . Plate the sites of large normal fault earthquakes (e.g., Sanriku 1933 . . and Indonesia 1977). There has been some controversy .

whether to interpret these earthquakes as bending events in . Fore land the upper ¯exural sheet or as ``decoupling'' events showing

. Andes . Thrust rupture of the entire downgoing plate due to ``slab pull.'' . .

. Belt The deeper earthquakes, which form the Wadati±Benioff . zone, go down to depths of 700 km within the downgoing slab. .

Their mechanisms provide important information about the

Nazca . . . physics of the subduction process. The essence of the process Plate . . is the penetration and slow heating of a cold slab of lithosphere –20° S GPS Site . in the warmer mantle. This temperature contrast has important Motion

. . consequences. The subducting plate is identi®ed by the loca- NUVEL-1A 77 mm/y tions of earthquakes in the Wadati±Benioff zone below the (a) zone of thrust faulting at the interface between the two plates. Earthquakes occur to greater depths than elsewhere because the slab is colder than the surrounding mantle. The mechanisms of earthquakes within the slab similarly re¯ect this phe- Forearc nomenon. The thermal evolution of the downgoing plate and South Nazca Alti Thru Foreland Trench Stable its surroundings is controlled by the relation between the rate

plano 5 Plate st America Plate – at which cold slab material is subducted and that at which it 10 mm/ Belt heats up, primarily by conduction as it equilibrates with the Motion surrounding mantle. In addition, adiabatic heating due to the y increasing pressure with depth and phase changes contribute. Numerical temperature calculations show that the downgoing plate remains much colder than the surrounding mantle 30–40 mm/y locked until considerable depths, where the downgoing slab heats up 18–33 mm/y stable sliding 10–15 mm/y shortening to the ambient temperature. Comparison of calculated tem- peratures, the observed locations of seismicity, and images from seismic tomography shows that the earthquakes occur in (b) 68–77 mm/yr net convergence the cold regions of the slab. The thermal structure also helps FIGURE 7 (a) GPS site velocities relative to stable South America explain their focal mechanisms. The force driving the sub- (Norabuena et al., 1998), and selected earthquake mechanisms in duction is the integral over the slab of the force due to the the boundary zone. Rate scale is given by the NUVEL-1A vector. density contrast between the denser subducting material and (b) Cross-section across Andean orogenic system showing velocity the density of ``normal'' mantle material outside. This force, distribution inferred from GPS data. known as ``slab pull,'' is the plate driving force due to subduction. Its signi®cance for stresses in the downgoing plate and for driving plate motions depends on its size relative to the released seismically in earthquakes (Kanamori, 1977). In this resisting forces at the subduction zone. There are several such case, the seismic slip rate estimated from seismic moments can forces. As the slab sinks into the viscous mantle, material must be only a fraction of the real plate motion. Nonetheless, it is be displaced. The resulting force depends on the viscosity of useful to determine the seismic slip rate to assess the fraction the mantle and the subduction rate. The slab is also subject to of seismic slip, as it re¯ects the mechanics of the subduction drag forces on its sides and resistance at the interface between process. It is also interesting to know how this seismic slip varies the overriding and downgoing plates. The latter, of course, is as a function of time and position along a subduction zone. often manifest as the shallow thrust earthquakes. Figure 6 also shows other types of shallow subduction zone One way to study the relative size of the negative buoyancy earthquakes. An interesting class of subduction zone earth- and resistive forces is to use focal mechanisms to examine quakes result from the ¯exural bending of the downgoing the state of stress in the downgoing slab. Earthquakes plate as it enters the trench. Precise focal depth studies show a above 300 km generally show stress axes corresponding to pattern of normal faulting in the upper part of the plate to a extension directed down the slab dip, whereas those below 74 Stein and Klosko

300 km generally show downdip compression. A proposed 4. Diffuse Plate Boundary explanation is that there are two basic processes operating: Earthquake Focal Mechanisms near the surface the slab is being extended by its own weight; at depth the slab begins to ``run into'' stronger material and Although the basic relationships between plate boundaries and downdip compression occurs. Another crucial effect may be earthquakes apply to continental as well as oceanic lithosphere, buoyancy due to mineral phase changes that occur at different the continents are more complicated. The continental crust is depths in the cold slab and in the surrounding mantle. much thicker, less dense, and has very different mechanical Numerical models of stress in downgoing slabs, using these properties from the oceanic crust. Because continental crust and assumptions, can reproduce the shallow down-dip tension and lithosphere are not subducted, the continental lithosphere deep downdip compression (Vassiliou, 1984; Bina, 1996). records a long, involved tectonic history. In contrast, the oceans Finally, it is worth noting that not all features shown in the record only the past 200 million years. One major result of these schematic (Fig. 6) have been observed at all places. For factors is that plate boundaries in continents are often diffuse, example, the dips and shapes of subduction zones vary sub- rather than the idealized narrow boundaries assumed in the rigid stantially. Some show double planes of deep seismicity; some plate model, which are a good approximation to what we see in do not. Even the very large thrust earthquakes, considered the oceans. The initial evidence for this notion comes from the characteristic of subduction zone events, are not observed in distribution of seismicity and the topography, which often all subduction zones. In recent years, considerable effort has imply a broad zone of deformation between the plate interiors. been made to understand such variations.

JF NA

AR IN CO CA AF PH PA

NZ AU

FIGURE 8 Comparison of the idealized rigid plate geometry to the broad boundary zones implied by seismicity, topography, or other evidence of faulting. Fine stipple shows mainly subaerial regions where the deformation has been inferred from seismicity, topography, other evidence of faulting, or some combination of these. Medium stipple shows mainly submarine regions where the nonclosure of plate circuits indicates measurable deformation; in most cases these zones are also marked by earthquakes. Coarse stipple shows mainly submarine regions where the deformation is inferred mainly from the presence of earthquakes. These deforming regions form wide plate boundary zones, which cover about 15% of the Earth's surface. The precise geometry of these zones, and in some cases their existence, is under investigation. Plate motions shown are for the NUVEL-1global relative plate motion model. Arrow lengths are proportional to the displacement if plates maintain their present relative velocity for 25 My. Divergence across mid-ocean ridges is shown by diverging arrows. Convergence is shown by single arrows on the underthrust plate. (After Gordon and Stein, 1992.) Earthquake Mechanisms and Plate Tectonics 75

This effect is especially evident in continental interiors, such belt, and into the stable interior of the South American con- as the India±Eurasia collision zone in the Himalayas or the tinent. The GPS site velocities are relative to stable South Paci®c±North America boundary zone in the Western US. Plate America, so if the South American plate were rigid and all boundary zones (Fig. 8), indicated by earthquakes, volcanism, motion occurred at the boundary, they would be zero. Instead, and other deformation, appear to cover about 15% of the they are highest near the coast and decrease relatively Earth's surface (Gordon and Stein, 1992; Stein, 1993). smoothly from the interior of the Nazca plate to the interior of Insight into plate boundary zones is being obtained by South America. Figure 7b shows an interpretation of these combining focal mechanisms with geodetic, topographic, and data. In this, about half of the plate convergence (30± geological data. Although plate motion models predict only 40 mm yÀ1) is locked at the plate boundary thrust interface, the integrated motion across the boundary, GPS, geological, causing elastic strain that is released in large interplate trench and earthquake data can show how this deformation varies in thrust earthquakes. Another 18±30 mm yÀ1 of the plate motion space and time. Both variations are of interest. Possible spatial occurs aseismically by smooth stable sliding at the trench. The variations include a single fault system taking up most of rest occurs across the sub-Andean fold-and-thrust belt, causing the motion (e.g., Prescott et al., 1981), a smooth distribution permanent shortening and mountain building, as shown by the of motion (e.g., England and Jackson, 1989), or motion taken inland thrust fault mechanisms. Comparison of strain tensors up by a few relatively large microplates or blocks (e.g., Acton derived from GPS and earthquake data shows that the short- et al., 1991; Thatcher, 1995). Each of these possibilities ening rate inferred from earthquakes is signi®cantly less than appears to occur, sometimes within the same boundary zone. indicated by the GPS, implying that much of the shortening The distribution of the motion in time is of special interest occurs aseismically. The focal mechanisms also indicate some because steady motion between plate interiors gives rise to deformation within the high Andes themselves. There may be episodic motion at plate boundaries, as re¯ected in occasional some (at most 5±10 mm yÀ1) motion of a forearc sliver distinct large earthquakes, and in some cases steady creep (Fig. 9). The from the overriding plate, a phenomenon observed in some detailed relation between plate motions and earthquakes is areas where plate convergence is oblique to the trench, making complicated and poorly understood and hence forms a prime earthquake slip vectors at the trench trend between the trench- target of present studies. normal direction and the predicted convergence direction For example, Figure 7a shows focal mechanisms and vec- (McCaffrey, 1992). tors derived from GPS illustrating the distribution of motion Another broad plate boundary zone is the Paci®c±North within the boundary zone extending from the stable interior of America boundary in western North America. Figure 10 shows the oceanic Nazca plate, across the Peru±Chile trench to the the boundary zone, in a projection about the Euler pole. The coastal forearc, across the high Altiplano and foreland thrust relative motion is parallel to the small circle shown. Thus the

Plate boundary zone slip distribution

Time Time Displacement relative to Plate A

creep creep creep motion

plate (?) mic slip and mic slip and mic slip and

steady c seis dic seis sodi Major fault sodic seis

m (M.Y.)

Rigid Minor epi Major episo Minor epi Rigid

Long-ter plate Plate boundary plate interior zone of deformation interior

Seismic Plate B Plate A Aseismic

FIGURE 9 Schematic illustration of the distribution of motion in space and time for a strike- slip boundary zone between two major plates (Stein, 1993). 76 Stein and Klosko

° ° ° 60 ° ° 80 280 ° 220 240 260

. 80° N Trench

. ° 200 .

Alaska 1964

40° N

San Francisco 1906 60° N V JJdF-NAdF-NA 42 mm/y JdF Plate Borah Peak

° PA - NA 220 V Loma Prieta POLE

PA-NA PA-N Basin and A 59mm/y Parkfield range SAF Transform Landers

Northridge

40° N San Fernando

20° N .

Ridge

. GPS Site Motion

° ° °

240 260 280 FIGURE 10 Geometry and focal mechanisms for a portion of the North America±Paci®c boundary zone. Dot-dash line shows small circle, and thus direction of plate motion, about the Paci®c±North America Euler pole. The variation in the boundary type along its length from extension, to transform, to convergence, is shown by the focal mechanisms. The diffuse nature of the boundary zone is shown by seismicity (small dots), focal mechanisms, topography (1000 m contour shown shaded), and vectors showing the motion of GPS and VLBI sites with respect to stable North America (Bennett et al., 1999; Newman et al., 1999). boundary is extensional in the Gulf of California, essentially a motion and hence large earthquakes, seismicity extends as far transform along the San Andreas fault system, and convergent eastward as the Rocky Mountains. For example, the Landers in the eastern Aleutians. The focal mechanisms re¯ect these earthquake shows strike-slip east of the San Andreas, and the changes. For example, in the Gulf of California we see strike- Borah Peak earthquake illustrates Basin and Range faulting. slip along oceanic transforms and normal faulting on a ridge The diffuse nature of the boundary is also illustrated by vec- segment. The San Andreas has both pure strike-slip earth- tors showing the motion of GPS and VLBI sites with respect to quakes (Park®eld) and earthquakes with some dip-slip motion stable North America. Net motion across the zone is essen- (Northridge, San Fernando, and Loma Prieta) when it deviates tially that predicted by global plate motion model NUVEL- from pure transform behavior. The plate boundary zone is also 1A. The site motions show that most of the strike-slip occurs broad, as shown by the distribution of seismicity. Although the along the San Andreas fault system, but signi®cant motions San Andreas fault system is the locus of most of the plate occur for some distance eastward. Earthquake Mechanisms and Plate Tectonics 77

5. Intraplate Deformation and Bina, C.R. (1996). Phase transition buoyancy contributions to stresses in subducting lithosphere. Geophys. Res. Lett. 23, 3563±3566. Intraplate Earthquakes Chase, C.G. (1972). The n-plate problem of plate tectonics. Geophys. J. R. Astron. Soc. 29, 117±122. A ®nal important use of earthquake mechanisms is to study the Cox, A. (1973). ``Plate Tectonics and Geomagnetic Reversals.'' internal deformation of major plates. Although idealized plates W.H. Freeman, San Francisco. would be purely rigid, the existence of intraplate earthquakes Cox, A. and R.B. Hart (1986). ``Plate Tectonics: How it Works.'' re¯ect the important and poorly understood tectonic processes Blackwell Scienti®c, Palo Alto. of intraplate deformation. One such example is the New Madrid DeMets, C., R.G. Gordon, D.F. Argus, and S. Stein (1990). Current area in the central United States, which had very large earth- plate motions. Geophys. J. Int. 101, 425±478. quakes in 1811±1812. The seismicity of such regions is gen- DeMets, C., R.G. Gordon, D.F. Argus, and Stein, S. (1994). Effect of recent revisions to the geomagnetic reversal time scale erally thought to be due to the reactivation of preexisting faults on estimates of current plate motion. Geophys. Res. Lett. 21, or weak zones in response to intraplate stresses. Because 2191±2194. À1 motion in these zones are at most a few mm y , compared to Dixon, T.H., A. Mao, and S. Stein (1996). How rigid is the the generally much more rapid plate boundary motions, seis- stable interior of the North American plate? Geophys. Res. Lett. micity is much lower (Fig. 10). Similarly, major intraconti- 23, 3035±3038. nental earthquakes occur substantially less frequently than Elsasser, W.M. (1969). Convection and stress propagation in the plate boundary events; recurrence estimates for 1811±1812 upper mantle. In: ``The Application of Modern Physics to the Earth type earthquakes average 500±1000 years. Efforts are being and Planetary Interiors'' (S.K. Runcorn, Ed.), pp. 223±246. Wiley, made to combine geodetic data, which indicate deviations New York. from rigidity, to the earthquake data. For example, comparison Engeln, J.F., D.A. Wiens, and S. Stein (1986). Mechanisms and of the velocities for permanent GPS sites in North America depths of Atlantic transform earthquakes. J. Geophys. Res. 91, 548±577. east of the Rocky Mountains to velocities predicted by mod- England, P. and J. Jackson (1989). Active deformation of the eling these sites as being on a single rigid plate shows that the continents. Annu. Rev. Earth Planet. Sci. 17, 197±226. interior of the North American plate is rigid at least to the level Gordon, R.G. and S. Stein (1992). Global tectonics and space À1 of the average velocity residual, less than 2 mm y (Dixon geodesy. Science 256, 333±342. et al., 1996; Newman et al., 1999). Similar results emerge from Huang, P.Y., S.C. Solomon, E.A. Bergman, and J.L. 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