Apr 23, 1999 - than for Los Angeles or San Francisco. [Frankel et al, 1996]. In contrast, our results. : imply that these largest earthquakes were. ⢠either far ...
Eos, Vol. 80, No. 17, April 27,1999 Geological Survey, Memphis, Tennessee, USA; and Martitia Tuttle, Department of Geology, University of Maryland, College Park, USA References Frankel, A., C. Mueller,! Barnhard, D. Perkins, E.V Leyendecker, N. Dickman, S. Hanson, and M. Hopper, National seismic-hazard maps: documentation, U.S. Geol. Surv. Open-File Rep. 96-532,69 pp., U.S. Gov.
Printing Office,Washington, D.C., 1996. Johnston, A. C, Seismic moment assessment of earthquakes in stable continental regions, III, New Madrid 1811-1812, Charleston 1886 and Lisbon 1755,Geophys. J.Int., 726,314-344,1996. Kerr, R. A., From Eastern Quakes to a Warmings Icy Clues, Science, 283,28-29,1999. Liu, L., M. D. Zoback, and PSegall, Rapid intraplate strain accumulation in the New Madrid seismic zone,Science, 257,1666-1669,1992. Newman, A. V, S. Stein, J.Weber, J. Engeln, A. Mao, and
Reply: New Results Justify Open Discussion of Alternative Models PAGES 197,199 A millennium ago, Jewish sages wrote that "the rivalry of scholars increases wisdom." In contrast, Schweig et al. (£bs, this issue) de mand that "great caution" be exercised in dis cussing alternatives to their model of high seis mic hazard in the New Madrid seismic zone (NMSZ).We find this view surprising; we have no objection to their and their coworkers' extensive efforts promoting their model in a wide variety of public media, but see no rea son not to explore a lower-hazard alternative based on both new data and reanalysis of data previously used to justify their model. In our view, the very purpose of collecting new data and reassessing existing data is to promote spirited testing and improvement of existing hypotheses. For New Madrid, such open reex amination seems scientifically appropriate, given the challenge of understanding intra plate earthquakes, and socially desirable because of the public policy implications. Specifically Schweig et al. favor a model in which the largest New Madrid earthquakes of 1811-1812 were magnitude 8 events, which : should recur every 500-1000 years. Using this . model, the assumed seismic hazard in parts of : the midcontinent is surprisingly high, in some • ways exceeding that in California. In particu" lar, incorporation of their model into the national seismic hazard maps, which are designed for application to building codes for earthquake-resistant construction, yields strik ing results.The predicted peak ground accel• eration expected in 50 years at 2% probability for the NMSZ exceeds that in San Francisco, and the predicted very high acceleration (exceeding 1.2 g) area for the NMSZ is larger than for Los Angeles or San Francisco [Frankel et al, 1996]. In contrast, our results imply that these largest earthquakes were • either far smaller or less frequent than they . assume and that the seismic hazard maps : should be revised to incorporate a lower haz, ard from large New Madrid earthquakes [Newman et al, 1999]. New Madrid is an intriguing situation for i exploration of alternative models because we ' have little data from any but small earthi quakes (owing to the low rate of seismicitity), , and we understand little about earthquakes within an essentially rigid plate (zeroth-order :
plate tectonics makes only the trivial predic tion that plate interiors are rigid so intraplate earthquakes and deformation should not occur). Moreover, it is unlikely that any model can be conclusively proved because one key issue (the size of earthquakes before the invention of the seismometer) can never be definitively resolved, and the other (when, if ever, such earthquakes will recur) cannot be resolved for hundreds or thousands of years. The alternative models under discussion here differ in the relative weights assigned to four lines of evidence: l.New Global Positioning System (GPS) measurements across the NMSZ and platewide continuous GPS data show little or no far-field motion across the NMSZ, in con trast to the apparently flawed earlier GPS results that Schweig et al. state support the seismic hazard maps. 2.Reestimation of the recurrence of future large earthquakes from the observed rate of smaller earthquakes yields recurrence times for M7 and 8 earthquakes of about 1400 ±600 and 14,000 ±7000 years, significantly longer than the earlier estimates Schweig et al.note were also incorporated in the hazard maps. 3. Paleoseismic studies imply that earth quakes comparable to the largest of those in 1811-1812 occurred in the past 1000 years, about 500 years apart. 4. Historic accounts of the largest 1811-1812 earthquakes have been interpreted as imply ing that these were M8 events. As summarized below, our model focuses on (1) and (2), is consistent with (3), and accepts that the techniques used in (4) have considerable uncertainties in estimating earthquake magnitude. In contrast, Schweig et al. focus on (4). Our key observation (1) is that the GPS data show little, if any, of the far-field motion across the faults expected between large earth quakes. We estimate such motion as 0±2 mm/yr, where the uncertainty is a 95% (two sigma) bound. Hence if the 1811-1812 earth quakes were M8 events with 5-10 m of hori zontal slip, their recurrence period should well exceed 2500 years.This period is a mini mum, because it is for the maximum rate rather than the best fitting near-zero value which predicts a much longer period, and for the assumption that all interseismic motion
T. H. Dixon, Slow deformation and low seismic haz ard at the New Madrid seismic zone, Science, 284, April 23,1999. Tuttle, M. P, R. H. Lafferty III, and E. S. Schweig III, Dating of liquefaction features in the New Madrid seismic zone and implications for earthquake hazard, U.S. Nuclear Reg. Com., NUREG/GR-0017,77 pp., 1998. Zoback, M. L., and M. D. Zoback, Rapid short-term rates of intraplate seismicity related to episodic strain release of high pore pressure? Geol. Soc.Am. Abstr. Programs, 24, A153,1992.
will be released seismically.The new GPS results appear more accurate than the earlier ones Schweig et al. cite in support of the seis mic hazard maps. Our new results do not sup port the previous finding [Liu et al., 1992] of surprisingly rapid strain accumulation, rough ly one third to two thirds of that for the San Andreas fault system, which we suspect result ed from limitations of the then-new GPS tech nique, possibly compounded by instability of shallow-rooted triangulation pillars they used. A similar conclusion (2) emerges from reestimating the recurrence of future larger earthquakes from the observed frequency of smaller earthquakes. An earlier such fre quency-magnitude analysis, which yielded a 550-1100-year recurrence for earthquakes with M >8.3 [Johnston and Nava, 1985], was incorporated into the hazard maps (see Schweig et al.). However, that analysis seems flawed by the assumption that all earth quakes with body wave magnitudes above 7 have surface wave magnitude 8.3. Without this assumption, reanalysis of these same data yields much longer recurrence times for M7 and 8 earthquakes of about 1400±600 and 14,000±7000 years.These values seem more plausible; since 1816, there are thought to have been 16 earthquakes with M >5 (about a 10-year recurrence), and two with M >6 (about a 100-year recurrence). Because earthquakes of a given size are approxi mately 10 times less numerous than those 1 magnitude unit smaller [Gutenberg and Richter, 1954; Okal and Romanowicz, 1994], M7 and 8 earthquakes should have about 1000-and 10,000-year recurrence. In hindsight, neither of our results seems sur prising. Because M8 earthquakes recur approximately every 200 years on segments of the San Andreas with slip rates of about 35 mm/yr, the simplest assumption for the midcontinent would be that the slip rates of less than 2 mm/yr imply correspondingly longer recurrence of thousands of years for earth quakes of comparable size. Similarly, if M8 earthquakes at New Madrid occurred every 500 years, we would expect an M7 event about every 50 years, and an M6 every 5 or so years. In fact, M6 earthquakes occur here only every 100 years or so, making it likely that the previously assumed 500-1000 year recurrence time for M8 earthquakes is too short. Hence, we believe that if the largest 18111812 earthquakes were M8 events, their recur rence interval is far greater than assumed by Schweig et al. Alternatively, these earthquakes may have been much smaller than they s
Eos,Vol. 80, No. 17, April 27,1999 assume.The latter possibility is suggested by the recurrence intervals of about 500 years estimated from the paleoseismic studies. These geological observations are consistent with the GPS and earthquake frequency-mag nitude data if the largest 1811-1812 shocks and the earlier large earthquakes were signifi cantly smaller than previously assumed, per haps M7 with slip of about 1-2 m. If so, 1-2 mm/yr of interseismic motion would corre spond to a 500-2000-year recurrence, consis tent with that expected for M7 earthquakes from the frequency-magnitude relation. Although this magnitude estimate is smaller than previously inferred from historical accounts of the shaking in the 1811-1812 earthquakes, those estimates have consider able uncertainty There is no direct way to esti mate earthquake magnitude from the descrip tions of shaking (intensity), and even qualita tive comparisons face the difficulty that the efficient transmission of seismic energy in sta ble eastern North America favors larger dam age and felt areas than elsewhere. Similarly, there is no direct way to relate liquifaction (sand blow) area and earthquake magnitude, and application of empirical relations faces the challenge that the New Madrid area con tains poorly consolidated and water-saturated sediment prone to pervasive liquifaction. Thus both the geodetic and frequency-mag nitude data imply that the largest earthquakes in the area are either smaller or far less fre quent than previously assumed. Both approaches rely on simple and standard assumptions. The assumption that steady farfield motion loads a fault prior to earthquake rupture is commonly used for plate bound aries, where space geodetic data show rates of motion consistent with those over millions of years, indicating that such steady motions give rise to episodic earthquakes [Gordon and Stein, 1992; Stein, 1993]. Similar consis tency is found for plate interiors [Dixon et al, 1996; Newman et al., 1999]; plates thought to have been rigid on geological timescales are
quite rigid (to better than 1 mm/yr) on de cadal scales. Given that no alternative model has yet been posed or found superior, and that the earthquake frequency-magnitude data yield estimates of recurrence times simi lar to those from the GPS results, application of these steady motion ideas to intraplate set tings seems a natural starting point. Further geodetic and other research will likely clarify these issues.The accuracy of geo detic velocity estimates will improve owing to longer measurement intervals, making it pos sible to more precisely estimate both nearand far-field motion, and further improve models of the nature and causes of New Madrid seismicity In particular, if more accu rate future surveys continue to find essentially no interseismic motion, it will seem increas ingly likely that earthquakes like the largest 1811-1812 events will never recur in the NMSZ.This possibility is also suggested by the apparent decline in regional seismicity, the fact the paleoseismic studies do not find earthquakes prior to the past 2000 years, and the fact that topography in the New Madrid region is quite subdued, implying that the NMSZ is not a long-lived feature. Hence New Madrid seismicity may be a transient phe nomenon, the present locus of intraplate strain release which migrates with time between fossil weak zones. In summary, because both the geodetic and earthquake history data previously presumed to support a model of high seismic hazard instead favors a lower hazard, we are puzzled by the idea that it is somehow improper to discuss an alternative to the widely promoted high-hazard model. Given new data, we favor exploration of their implications both for the nature of intra plate seismicity and its seismic hazard. Speci fically, it appears that the model Schweig et al. favor implies seismic hazards significantly higher than we consider realistic, and that maps based on it may create undue public concern and unnecessarily inflate building
BOOK REVIEWS From Magma to Tephra (Developments in Volcanology 4) PAGE 196 A. Freundt and M. Rosi (Eds.) Elsevier Sci., New York, xv + 318 pp., ISBN 0444-92959-8,1998, $135.
Volcanology is a rapidly evolving science that has recently received a high media pro
file through Hollywood movies and television coverage of Montserrat in the West Indies and the Mexican volcanoes of Colima and Popocatepetl. One of the movies and all three of the real-life eruptions involve volcanoes that are subject to sudden, explosive erup tions that can cause many hazards. In the region of the volcanic vent, high alti tude eruption columns that resemble the mushroom cloud of an atomic blast can repeatedly disrupt regional air traffic. Within
costs. Furthermore, unrealistically high stan dards may actually reduce seismic safety by encouraging evasion of requirements which could be economically met if set at a more realistic lower level. In a democratic society, we see no reason such issues should not be discussed openly Authors Andrew Newman and Seth Stein, Department of Geological Sciences, Northwestern University, Evanston, Illinois, USA; John Weber, Department of Geology, Grand Valley State University, Allendale, Michigan, USA; Joseph Engeln, Department of Geological Sciences, University of Missouri, Columbia, USA;Ailin Mao and Timothy Dixon, Rosenstiel School for Marine and Atmospheric Sciences, University of Miami, Miami, Florida, USA References Dixon,T. H., A. Mao, and S. Stein, How rigid is the sta ble interior of the North American plate? Geophys. Res. Lett, 23,3035-3038,1996. Frankel, A., C. Mueller,! Barnhard, D. Perkins, E. Leyendecker, N. Dickman, S. Hanson, and M. Hopper, National seismic hazard maps: Documen tation, U.S. Geol. Surv. Open-File Rep. 96-532, U.S. Government Printing Office,Washington,D.C., 1996. Gordon, R. G., and S. Stein, Global tectonics and space geodesy, Science, 256,333-342,1992. Gutenberg, B., and C. FRichter,Se/s/?7/a7y of the Earth and Associated Phenomena, 2nd ed., Princeton University Press, Princeton, N.J., 1954. Johnston,A.C.,and S. J. Nava, Recurrence rates and probability estimates for the New Madrid seismic zone, J Geophys. Res., 90,6737-6753,1985. Liu, L, M. D. Zoback, and PSegall, Rapid intraplate strain accumulation in the New Madrid seismic Zone,Science, 257,1666,1669,1992. Newman, A.Y, S. Stein, J.Weber, J. Engeln, A. Mao, and T. H. Dixon, Slow deformation and low seismic hazard at the New Madrid seismic zone, Science, 284,April 23,1999. Okal, E. A., and B. Romanowicz, On the variation of b-values with earthquake size, Phys. Earth Planet. Inter, £7,55-76,1994. Stein, S., Space geodesy and plate motions, in Space Geodesy and Geodynamics, Geodyn. Ser, vol. 23, edited by D. E. Smith and D. L.Turcotte, pp. 5-20, AGU,Washington,D.C.,1993.
several kilometers of the volcanic edifice, fall out from the eruption can destroy roofs and hinder access while hot and ash-laden pyroclastic density currents can lay to waste any population centers that are encountered in their paths. Such events have claimed thou sands of lives in this century alone. Over time, the potential also exists for economic disrup tion and, on a global scale, climatic change. Scientists have begun to model many of the processes involved in explosive eruptions in an effort to gain an understanding and in turn improve hazard assessment.The fourth volume in a series on developments in vol canology, From Magma to Tephra, presents an overview of the physical principles, early research, and recent developments in model-
