runoff from snowmelt and streamflow was set to 100mg liter-1, about half the concentration in five mod-em spring-fed streams in the upland drainage. Inground water, TDS was set to 2100 mg liter-1, on

the basis of an average of five water wells nearestthe zone of ground-water discharge. The ground-water discharge rate was set to 1.25 x 108 m3year-'. This value was reached on the basis of theaverage rate of accumulation of calcium carbonateand calcium sulfate in the lake bed and on theassumption that there is a long-term balance be-tween the input of dissolved solids in ground waterand their removal to sediments. This value is onthe same order of magnitude as an estimate ofground-water discharge for the modern hydro-logic system by B. E. DeBrine [thesis, New Mexico

Spatiotemporal Patterns in the Energy Releaseof Great Earthquakes

Barbara RomanowiczFor the past 80 years, the energy released in great strike-slip and thrust earthquakes hasoccurred in alternating cycles of 20 to 30 years. This pattern suggests that a global transfermechanism from poloidal to toroidal components of tectonic plate motions is operating ontime scales of several decades. The increase in seismic activity in California in recent yearsmay be related to an acceleration of global strike-slip moment release, as regions of sheardeformation mature after being reached by stresses that have propagated away fromregions of great subduction decoupling earthquakes in the 1960s.

The contributions of toroidal and poloidalcomponents of global tectonic plate mo-tions are about equal (1). The poloidalcomponent, associated with upwelling anddownwelling currents, is directly related todensity variations in the deep mantle. Thetoroidal component, which represents hor-izontal shearing, exists primarily because ofthe presence of the rigid lithospheric platesand the associated heterogeneity in rheol-ogy. In addition to aseismic motions, whichare difficult to observe, the poloidal compo-nent is expressed in thrust and normalfaulting and the toroidal one, in strike-slipearthquakes. Yet the temporal pattern ofseismic energy release is dominated by thatof great thrust earthquakes, which are morethan one order of magnitude larger than thelargest strike-slip earthquakes. The contri-bution of the latter has therefore largelybeen ignored in analyses of temporal pat-terns of global seismic moment release.

In this report, I examine the spatiotem-poral distribution of great shallow earth-quakes since 1920 by taking into accountthe fault geometry and, as a first approxi-mation, by considering two classes of earth-quakes separately: strike-slip and subduc-tion zone thrust earthquakes. Data are tak-en from the CMT (Centroid Moment Ten-sor) catalog (2) for the past 15 years, as wellas from catalogs of great earthquakes in theSeismographic Station and Department of Geologyand Geophysics, University of California, Berkeley, CA94720.

last century (3, 4) and a recently publishedcatalog of large earthquakes by Pacheco andSykes (PS) (5).

,E

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Institute of Mining and Technology (1971)].L. B. Leopold, Am. J. Sci. 219, 152 (1951).E. Antevs, J. Geol. 62, 182 (1954).J. E. Kutzbach and H. E. Wright, Quat. Sci. Rev. 4,147 (1985).S. Hostetler and L. E. Benson, Clim. Dyn. 4, 207(1 990).W. S. Broecker and G. H. Denton, Geochim.Cosmochim. Acta 53, 2465 (1989).P. M. Grootes and M. Stuiver, Eos 73, 258 (1992).We thank F. W. Bachhuber for providing severalradiocarbon dates and L. V. Benson and P. DeDeckker for comments. Supported by NationalScience Foundation grant EAR-9019134.

21 December 1992; accepted 23 March 1993

The total seismic energy release is dom-inated by that of the largest or "great"earthquakes (3, 6). The temporal and spa-

tial patterns observed for these earthquakesare different and more regular than those ofmoderate to large ones. Several interestingtemporal patterns have been recognized. Inthe record of the past 50 years, a strikingfeature is a peak in the earthquake energy

release around 1960 (3) when several greatearthquakes occurred within a few years.

This peak is correlated with a peak in theamplitude of the Chandler wobble, al-though the physical connection betweenthese two phenomena is still a matter ofdebate (7-10). Another feature is the pro-

gression of great earthquakes around boththe Pacific rim and the Alpide Belt between1935 and 1965 (1 1). The total energy

Fig. 1. (A) Cumulative moment released bystrike-slip earthquakes in the past 15 years, fromthe CMT catalog (2). In all figure parts, the unitsof cumulative moment are 1020 N-n. The calcu-lation is done with a yearly increment. All strike-slip earthquakes of moment larger than 0.1 x

1020 N-m have been included, where strike-slipis defined as corresponding to a mechanismwith minimum dip of both nodal planes of 550.The results are robust with respect to changesby 50 to 100 in this definition. For reference, thebest fitting linear curve (dashed line) and thebest fitting exponential curve (dotted line), where5M = Mo e+t/T, are shown; the rise time T for theexponential fit is about 5 years. If we remove theMacquarie Island earthquake of 23 May 1989from consideration, the rise time remains -5years. Also shown are separate linear fits for the

periods 1977 to 1986.(B) Cumulative moment

,(,' release by strike-slip|J,8 earthquakes in the PS

catalog (5). (C) Same as(B) for non-strike-slipearthquakes. Note thechange of scale from (B)to (C). Most of the greatstrike-slip earthquakeswould disappear into thebackground if both

types of earthquakesi93s 1960 1i85 were plotted together.

Year

SCIENCE * VOL. 260 * 25 JUNE 1993

ate and calcium sulfate, on the basis of analyses ofcontiguous sediment samples. Depth and surfacearea were recalculated for each step with morpho-metric data for the basin. The initial lake elevationbefore the rise during the glacial maximum was1852 m, on the basis of shoreline gravels linked tobasin-center stratigraphy. Before the rise in eleva-tion, TDS was set to 8000 mg liter-1, which issomewhat lower than previously estimated for thisunit by F. W. Bachhuber [Geol. Soc. Am. Bull. 101,1543 (1989)]. This lower TDS value was chosen as aconservative estimate of runoff and corresponds tothe upper part of the optimal range for C. rawsoni [D.R. Engstrom and S. R. Nelson, Palaeontol. Palae-olimnol. Palaeoceanogr. 83, 295 (1991)], which ispresent in small numbers in this interval. The TDS in

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release by earthquakes has decreased signif-icantly in the past 20 years (5).

Great earthquakes, as defined by theirmagnitude or seismic moment, are domi-nated by subduction zone decouplingevents. A more physical definition, fromthe point of view of plate tectonics, is toconsider all decoupling earthquakes, thatis, those that rupture the brittle zone in itsentire thickness. For strike-slip earth-quakes, the saturation of the downdipwidth of the fault occurs when it is com-mensurable with the thickness of the brittlezone (-10 to 25 km). The correspondingchange of scaling observed for this type ofearthquake is for moments of 0.6 x 1020 to1.0 x 1020 N-m (12). For subduction zoneevents, on the global scale, such a limit ismore difficult to define, in that saturationwidths vary from region to region (from 50km to more than 250 km) (13). On aver-age, however, the downdip width of adecoupling earthquake will be at least fiveto ten times that for a strike-slip event. Thetemporal pattern of moment release (MR)during the last century is dominated byeveN02N-i

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27/06/192910/08/1931

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\ / \ / \ _ _ 22/08/1949-10 _ _ / 17/12/1949

- - - -- - - -- - - - 17/1 2/1 949-20. w . w , . 15/08/1950

1920 1940 1960 1980 18/11/1951Year 04/11/1952

j. 2. Proportion of moment released in strike- 09/03/1957p earthquakes as compared to total moment 10/07/1958lease, calculated in 5-year bins for earth- 06/11/1958akes of moment greater than 0.1 x 10 0 N-m 22/051/960the period from 1920 to 1990. The solid 13/10/1963rizontal line is the average moment propor- 28/03/1964In for that period, and straight dashed lines 24/01/1965e 95% confidence limits calculated for the 04/02/1965tire period. Jagged broken lines are 95% 16/05/1968nfidence limits calculated on the basis of a 11/08/1969st of the null hypothesis that the yearly mo- 30/07/1972ant proportions are uncorrelated in each 26/05/1975year period. Both peaks in 1935 to 1940 and 04/02/1976'85 to 1990 are significant. All observed 19/08/1977,aks are the results of contributions from more 2/12/1981an one great earthquake. The periods of 25/0/1986west moment proportion in 1950 to 1955 and 30/11/1987360 to 1965 are also significant for the second 23/05/1989st (long dashed lines). This statistical argu- 03/03/1990ent is limited because quoted moments have 16/07/1990known uncertainties, which may be large.

definition for great events (listed in Table1). For thrust events, this definition is morerestrictive than in earlier studies (3).

The accuracy of moment estimates, es-pecially for historical earthquakes, has beenthe subject of debate (3, 5, 14). Even forthe recent largest earthquakes, a discrepan-cy in moment of a factor of 2 betweendifferent studies is not uncommon (5). Thisis, as discussed below, not a serious problemin my analysis. For earthquakes before the1950s, moment has often not been mea-sured directly but rather has been inferredfrom the reported magnitude. As for mech-anisms, they have been assigned on thebasis of indications given in the catalogs. Anumber of large earthquakes, primarily be-

fore the 1940s, have uncertain mecha-nisms. I arbitrarily assigned mechanisms tothem, according to the prevailing mecha-nisms in the region in which they occurredand the current plate tectonic configura-tion. More accurate estimates of momentand mechanism are available for the past 20years. For this reason, I initially analyzedthe CMT catalog, complementing it there-after with the PS catalog.

The cumulative MR by strike-slip earth-quakes in the past 15 years, as obtainedfrom the CMT catalog, shows an accelera-tion in MR since 1987 (Fig. 1A). This isdue to the contribution of eight strike-slipearthquakes of moment larger than 1 x1020 N-m that occurred between 1987 and

Table 1. Great earthquakes in the time period from 1920 to 1991. Moments are from (5); whenseveral determinations are available, they are indicated. Listed are earthquakes of moment greaterthan 3 x 1020 N-m with strike-slip and uncertain mechanisms and earthquakes of moment greaterthan 20 x 1020 N-m with thrust or normal mechanism; FZ, fault zone.

Date Moment Region Type of mechanism

69.0, 140.070.0, 44.0, 200.030.04.112.08.5, 24.940.014.55.952.0, 70.0, 27.228.5, 12.33.53.515.713.0, 44.525.0, 28.03.514.013.04.94.995.04.6350.018.0, 13.0585, 4005.3, 4.444.0, 40.01900,160067, 50, 75750.013.0, 24.048, 14028.022.03.07.03.7, 2.630.029.05.04.57.313.63.04.1

ChileKamchatkaMacquarieNinety East RidgeSouth SandwichAltaiJapanNew GuineaSumatraBanda SeaAlaskaCentral ChileAnatolian FaultAzoresPrince Edward FZChileBurmaPhilippinesQueen Charlotte I.North ScotiaNorth ScotiaTibetTibetKamchatkaAltaiAleutiansQueen Charlotte I.KurilesSouth ChileKurilesAlaskaBanda SeaAleutiansJapanKurilesQueen Charlotte I.AzoresGuatemalaSumbawaColumbiaMacquarieKermadecAlaskaMacquarieFijiPhilippines

ThrustThrustUnknownStrike-slipUnknownStrike-slipNormalUncertain (strike-slip)Strike-slipStrike-slipThrustUncertain (strike-slip)Strike-slipStrike-slipStrike-slipThrustUncertain (strike-slip)UnknownStrike-slipStrike-slipStrike-slipThrustStrike-slipThrustStrike-slipThrustStrike-slipThrustThrustThrustThrustThrustThrustThrustThrustStrike-slipStrike-slipStrike-slipNormalThrustStrike-slipStrike-slipStrike-slipStrike-slipStrike-slipStrike-slip

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1991, compared to only three in the previ-ous 10 years. The strongest contribution inthis 5-year period is from the MacquarieRidge earthquake of 23 May 1989, which isalso the largest earthquake (regardless ofmechanism) in the last 12 years. Thisearthquake alone is not the cause for theobserved pattern: the total MR in the5-year period from 1987 to 1991 is aboutfive times that in the preceding two 5-yearperiods; it is still three times larger if theMacquarie Ridge event is omitted. Also,the MR in each of the 4 years from 1987 to1990 is more than 2 SD larger than theaverage per year in the preceding 10 years.

The cumulative moment for strike-slipearthquakes from 1900 to 1989, from the PS

catalog (Fig. 1B), confirms the trend alreadynoted for the time period from 1977 to 1991based on the CMT catalog, the rate of MRbeing low from 1960 to 1986. Two earlierperiods of accelerated MR are evident, onearound 1940 and the other at the beginning ofthe century. The former is due to six strike-slip events of moment greater than 5 x 1020N-m that occurred from 1931 to 1942, where-as none occurred from 1907 to 1923 and nonefrom 1960 to 1974. The acceleration in 1905,dominated by the Altai events of July 1905,may have been largely overestimated. I con-clude that episodes of large and small MR instrike-slip earthquakes have alternated world-wide, at least in the last 80 years.

The cumulative MR by non-strike-slip

Fig. 3. Space-time distribution of large and great earthquakes in the period 1908 to 1991. Strike-slipearthquakes: (A) 1908 to 1928, (B) 1929 to 1949, (C) 1950 to 1970, and (D) 1971 to 1992. Thrustearthquakes: (E) 1908 to 1928, (F) 1929 to 1949, (G) 1950 to 1970; and (H) 1971 to 1992. For (A)to (D) squares are for strike-slip earthquakes with moment greater than 1 x 1020 N-n, triangles andyears of occurrence are for those with moment greater than 3 x 1 0 N-in. For (E) to (H) squaresare for thrust earthquakes with moment greater than 5 x 1020 N-n and less than 20 x 1020 N-m;triangles and years of occurrence are for those with moment greater than 20 x 1020 N-i. The mapsfor the earliest periods (A and E) are relatively uncertain and are given only for reference.

SCIENCE * VOL. 260 * 25 JUNE 1993

earthquakes, as given in the PS catalog (Fig.IC), shows one clear period of acceleratedMR around 1960. This period is well docu-mented (3, 11) and is due primarily to fivesubduction zone decoupling events, each ofMO > 100 x 1020 N-m (Table 1). Thisperiod falls largely between periods duringwhich strike-slip activity was enhanced.A plot (Fig. 2) of the ratio of MR in

strike-slip earthquakes with respect to thetotal MR, for the period from 1920 to 1991,further illustrates the alternation of MR ingreat strike-slip and thrust events. The twomain periods of accelerated MR in strike-slip earthquakes are also periods in whichthis ratio increased by a factor of 2 or more.Although the two great normal faultingevents (Table 1) were included in thiscomparison, the observations remain robustif these events are removed.

The geographical distribution of MR, forfour consecutive 20-year periods of strike-slip (Fig. 3, A to D) and thrust events (Fig.3, E to H), reveals several patterns. Theconcentration of great thrust events from1950 to 1970 primarily covers the Pacificplate boundary between Japan and Alaska.Since 1970, the occurrence of large strike-slip earthquakes may represent a continua-tion of Mogi's (11) progression patternalong the Pacific plate boundary, towardthe northeast and the southwest, to coverthe remaining nonspreading parts of thisboundary. This pattern is indicative of aprocess of stress transfer along adjacentplate boundaries with a rate of severalthousand kilometers in 10 to 20 years, ingood agreement with the concept of accel-erated plate tectonics (15, 16) in whichstresses can be transferred over large dis-tances in an elastic lithosphere underlain bya viscous asthenosphere.

The second trend evident in Fig. 3 is theoccurrence, during periods of acceleratedstrike-slip MR, of great earthquakes at lo-cations far removed from the western Pacif-ic plate boundaries, such as in the Azoresand the south Indian Ocean. These pat-terns are suggestive of a global couplingphenomenon involving stress transfer frompoloidal to toroidal motion. The transferwould have to be rapid and followed by aperiod of maturing in regions of toroidalenergy release. Whether the excitationmechanism is internal (ridge push, slabpull) or external [for example, Earth's rota-tion (3, 8)] and whether it has any period-icity remain to be determined, but theobserved time scale of the process, associ-ated with increased accuracy in the quanti-fication of earthquakes, raises the hope thatimproved understanding could be gained inthe next 20 years.

The alternation, during the past 80 years,in MR between strike-slip earthquakes andgreat subduction zone earthquakes readily

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explains the decrease in total MR in the past20 years, in which strike-slip events havebeen active, producing moments that aresmaller (1/10 to 1/20) than those in periods ofaccelerated subduction zone activity. Thelarge contribution of aseismic energy releasein strike-slip systems provides an explanationfor the discrepancy inMR for the two types ofearthquakes. Mogi (17) proposed that differ-ent parts of plate boundaries go through alter-nating periods of high and low activity, on atime scale of tens of years. This notion andthe patterns revealed in the earthquake cata-logs imply that the increase in seismic activityat large magnitudes (M > 5) in Californiasince 1985 (18) or along the Anatolian faultin the 1940s (19) may be regional manifesta-tions of a larger scale phenomenon involvingstress transfer from subduction zones to zonesof toroidal energy release and resulting inactivation of specific parts ofplate boundaries.

Although the large-scale progressionsof great earthquakes around the Pacificand the Nazca plate have been noted, theimplications have not been incorporatedexplicitly in the identification of seismicpotentials of gaps (20). If the notion thatportions of plate boundaries are activatedin a nonrandom manner on time scales oftens of years and geographical scales ofseveral thousands of kilometers is valid, itmight be used to more accurately predictthe time of occurrence of great earth-quakes. The seismic gap theory has recent-ly been challenged on the basis of statis-tical analysis of event occurrence in thepast 20 years, primarily in subduction zoneareas (21). According to my study, theresolution of this question may have towait until the next period of increasedactivity in great subduction zone decou-pling earthquakes.

This study may provide the basis formore quantitative testing of several hypoth-eses concerning the stress transfer from theunderlying convective mantle to the tec-tonic plates. Data accumulated over anoth-er 10 or 20 years may be sufficient to sortout the causal relationships involving theEarth's rotation, internal processes, andearthquake energy release. In particular, wemay be able to monitor deformation relatedto plate tectonics, not only as averaged overgeological time scales but over several dec-ades-the scale of the Global Change Pro-gram. More precise knowledge of momentsand mechanisms of great earthquakes at theturn of the century would also be veryvaluable, if only to shed light on the possi-ble periodicity in the alternation of greatstrike-slip and thrust earthquakes.

REFERENCES AND NOTES

1. R. O'Connell, C. Gable, B. Hager, Glacial Isos-tasy, Sea Level, and Mantle Rheology (KluwerAcademic, Dordrecht, Netherlands, 1990).

1926

2. A. M. Dziewonski, G. Ekstrom, J. Woodhouse, G.Zwart, Phys. Earth Planet. Inter. 62, 194 (1990).

3. H. Kanamori, J. Geophys. Res. 82, 2981 (1977).4. K. Abe, J. Phys. Earth 23, 381 (1975).5. J. F. Pacheco and L. R. Sykes, Bull. Seismol. Soc.

Am. 82, 1306 (1992).6. J. Brune, J. Geophys. Res. 73, 777 (1968).7. F. A. Dahlen, Geophys. J R. Astron. Soc. 32, 203

(1973).8. D. L. Anderson, Science 186, 49 (1974).9. R. J. O'Connell and A. M. Dziewonski, Nature 262,

259 (1976).10. A. Souriau and A. Cazenave, Earth Planet. Sci.

Lett. 75, 410 (1985).11. K. Mogi, J. Phys. Earth 16, 30 (1968).12. B. Romanowicz, Geophys. Res. Lett. 19, 481

(1992).13. J. Kelleher, J. Savino, H. Rowlett, W. McCann, J.

Geophys. Res. 79, 4889 (1974).

14. G. Ekstrom and A. M. Dziewonski, Nature 332,319 (1988).

15. M. H. P. Bott and D. S. Dean, ibid. 243, 339(1973).

16. D. L. Anderson, Science 187, 1077 (1975).17. K. Mogi, Tectonophysics 22, 265 (1974).18. L. R. Sykes and S. C. Jaume, Nature 348, 595

(1 990).19. M. N. Toksoz, A. F. Shakal, A. J. Michael, Pure

Appl. Geophys. 117, 1258 (1974).20. J. Kelleher, L. R. Sykes, J. Oliver, J. Geophys.

Res. 78, 2547 (1973).21. Y. Y. Kagan and D. D. Jackson, ibid. 96, 21419

(1991).22. thank A. Dziewonski and C. Marone for discus-

sions and encouragement and J. F. Pacheco forproviding his catalog in digital form. D. Brillingerassisted with the statistical analysis.

19 January 1993; accepted 22 April 1993

Arabidopsis thaliana DNA Methylation Mutants

Aurawan Vongs,* Tetsuji Kakutani,t Robert A. Martienssen,Eric J. Richardstt

Three DNA hypomethylation mutants of the flowering plant Arabidopsis thaliana wereisolated by screening mutagenized populations for plants containing centromeric re-

petitive DNA arrays susceptible to digestion by a restriction endonuclease that was

sensitive to methylated cytosines. The mutations are recessive, and at least two are

alleles of a single locus, designated DDM1 (for decrease in DNA methylation). Amountsof 5-methylcytosine were reduced over 70 percent in ddml mutants. Despite thisreduction in DNA methylation levels, ddml mutants developed normally and exhibitedno striking morphological phenotypes. However, the ddm1 mutations are associated witha segregation distortion phenotype. The ddm1 mutations were used to demonstrate thatde novo DNA methylation in vivo is slow.

DNA methylation has been implicated inthe control of a number of cellular pro-cesses in eukaryotes, including transcrip-tion (1), imprinting (2), transposition(3), DNA repair (4), chromatin organiza-tion (5, 6), and a variety of epigeneticphenomena (7), although the function ofDNA methylation in eukaryotes remainsthe subject of continued debate. UsingDNA methylation mutants to study thefunctions of DNA modification in eukary-otes is an attractive approach that avoidssome of the limitations associated withcorrelative and methylation inhibitorstudies. Li et al. (8) recently reported thatmouse DNA methyltransferase mutants,generated by gene disruption, are unableto complete embryogenesis, although ho-mozygous mutant cell lines are viable andapparently normal. We report here theisolation and characterization of DNAmethylation mutants in the flowering

Cold Spring Harbor Laboratory, Cold Spring Harbor,NY 11724.

*Present address: Merck Research Laboratory, Rah-way, NJ 07065.tPresent address: Department of Biology, WashingtonUniversity, St. Louis, MO 63130.*To whom correspondence should be addressed.

SCIENCE * VOL. 260 * 25 JUNE 1993

plant Arabidopsis thaliana.We identified A. thaliana DNA meth-

ylation mutants by screening directly forplants that contained hypomethylated ge-nomic DNA using Southern (DNA) blotanalysis. Filters containing Hpa II-digest-ed genomic DNA prepared from leaves ofmutagenized A. thaliana plants were hy-bridized with a cloned 180-bp repeat foundin long tandem arrays at all five A. thalianacentromeres (9). Centromeric repeat ar-rays isolated from wild-type plants areresistant to Hpa II (CCGG) digestion (10,11) because of extensive methylation ofcytosines at the CpG and CpNpG sites,which is characteristic of plants (12)[5mCCGG and C'mCGG are not digestedby Hpa 11 (13)1. DNA methylation mu-tants, on the other hand, were predicted tocontain hypomethylated centromeric arrayssusceptible to Hpa II digestion. We exam-ined Southern filters containing DNA from79 pools representing approximately 2000plants from ethyl methanesulfonate-muta-genized M2 populations (ecotype Colum-bia) to look for centromere repeat multi-mers of low molecular weight that wereclipped from the long arrays by Hpa II.Individual plants from candidate pools were

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