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Approaches to Continental Intraplate Earthquake Issues

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Approaches to Continental Intraplate Earthquake Issues spe425-01 page 1 The Geological Society of America Special Paper 425 2007 Approaches to continental intraplate earthquake issues Seth Stein† Department of Earth and Planetary Sciences, Northwestern University, Evanston, Illinois 60208, USA “We choose to go to the moon in this decade and do the other things, not because they are easy, but because they are hard.”—John F. Kennedy, 1962 ABSTRACT The papers in this volume illustrate a number of approaches that are becom- ing increasingly common and offer the prospect of making signifi cant advances in the broad related topics of the science, hazard, and policy issues of large continental intraplate earthquakes. Plate tectonics offers little direct insight into the earthquakes beyond the fact that they are consequences of slow deformation within plates and, hence, relatively rare. To alleviate these problems, we use space geodesy to defi ne the slowly deforming interiors of plates away from their boundaries, quantify the associ- ated deformation, and assess its possible causes. For eastern North America, by far the strongest signal is vertical motion due to ice-mass unloading following the last glacia- tion. Surprisingly, the expected intraplate deformation due to regional stresses from plate driving forces or local stresses are not obvious in the data. Several approaches address diffi culties arising from the short history of instrumental seismology com- pared to the time between major earthquakes, which can bias our views of seismic hazard and earthquake recurrence by focusing attention on presently active features. Comparisons of earthquakes from different areas illustrate cases where earthquakes occur in similar tectonic environments, increasing the data available. Integration of geodetic, seismological, historical, paleoseismic, and other geologic data provides insight into earthquake recurrence and the diffi cult question of why the earthquakes are where they are. Although most earthquakes can be related to structural features, this explanation alone has little predictive value because continents contain many such features, of which a few are the most active. It appears that continental intra- plate earthquakes are episodic, clustered, and migrate. Thus on short time scales seis- micity continues on structures that are active at present, perhaps in part because many events are aftershocks of larger past events. However after periods of activity these structures may become inactive for a long time, so the locus of at least some of the seismicity migrates to other structures. Analysis of the thermo-mechanical struc- ture of the seismic zones gives insight into their mechanics: whether there is some- thing special about them that results in long-lived weak zones on which intraplate strain release concentrates, or as seems more likely, that they are not that unusual, so seismicity migrates. Accepting our lack of understanding of the underlying causes of †E-mail: [email protected]. Stein, S., 2007, Approaches to continental intraplate earthquake issues, in Stein, S., and Mazzotti, S., ed., Continental Intraplate Earthquakes: Science, Hazard, and Policy Issues: Geological Society of America Special Paper 425, p. 1–16, doi: 10.1130/2007.2425(01). For permission to copy, contact [email protected]. ©2007 The Geological Society of America. All rights reserved. 1 spe425-01 page 2 2 Stein the earthquakes, the limitations of the short instrumental record, and the possibility of migrating seismicity helps us to recognize the uncertainties in estimates of seismic hazards. Fortunately, even our limited knowledge can help society develop strategies to mitigate earthquake hazards while balancing resources applied to this goal with those applied to other needs. Keywords: intraplate earthquakes, continental deformation, seismic hazards. INTRODUCTION is thought to be comparable to that from California earth- quakes one magnitude unit larger because rock in the stable The papers in this book represent a range of ongoing conti nental interior transmits seismic energy more effi ciently. research addressing the related topics of the science, hazard, and Because earthquakes of a given magnitude are ~10 times more policy issues of large continental intraplate earthquakes. As sum- frequent than those one-magnitude-unit larger, the shaking dif- marized in the preface, addressing these issues is more diffi cult ference reduces the effect of the difference in earthquake rates than for the far more common earthquakes on plate boundaries, by about a factor of 10. The precise net effect of these differ- for two reasons. First, we lack a model like plate tectonics that ences depends on the recurrence rate of large earthquakes and gives insight into the causes, nature, and rate of the earthquakes. the resulting ground motion, neither of which are well known. Second, because intraplate earthquakes are much rarer owing to Even so, the comparison indicates that different approaches to the slow deformation rate, we know much less about these earth- mitigating the seismic hazard are likely to make sense. quakes and their effects. The hazard posed by large continental intraplate earthquakes As a result, probably none of the authors would claim to be is a small, but still signifi cant, fraction of the threat posed by all an “expert” on intraplate earthquakes. After all, an expert should earthquakes. Earthquakes, in turn, are just one of many challenges know why, where, and when such earthquakes occur, what their societies face. In the United States, on average, fewer than ten effects will be, and how society should address them. Because people per year are killed by earthquakes (Fig. 2), and intraplate none of these issues is well understood at present, the authors are events make up less than 10% of the total. Hence earthquakes are simply researchers exploring these messy issues. at the level of in-line skating or football, but far less than bicycles, These issues involve both fundamental science and societal for risk of loss of life (Stein and Wysession, 2003). Similarly, the implications. The challenge is to understand the nature and causes approximately $5 billion average annual earthquake losses for of these relatively rare but sometimes very destructive earthquakes the United States, though large, is ~2% of that due to automo- and use what we learn to assess the hazard they pose and help bile accidents. Nonetheless, large earthquakes occasionally cause society formulate sensible policies to address the resulting risk. many fatalities and major damage. Similarly, on a global basis, In doing so, it is useful to distinguish between hazards and risks. earthquakes cause an average of ~10,000 deaths per year, signifi - The hazard is the intrinsic natural occurrence of earthquakes and cant but relatively minor compared to other causes. For example, the resulting ground motion and other effects. Although we can malaria causes about a million deaths per year. The challenge to defi ne it in various ways for different purposes, and our estimates societies is to thus to develop strategies that balance resources of it have large uncertainties, the hazard is a natural feature. In allocated to earthquake hazard mitigation with other needs. contrast, the risk is the danger the hazard poses to life and prop- Papers in this volume explore many of the issues in these erty, and can be reduced by human actions. Hence, we seek to examples. Although written by different authors addressing vari- estimate the hazard and choose policies consistent with societal ous geographic areas, and hence often taking different views, goals to reduce the resulting risk. they illustrate approaches that are becoming increasingly com- An underlying theme is that many of the scientifi c and mon and offer the prospect of making signifi cant advances. The societal issues differ signifi cantly from those posed by the far goal of this introduction is to highlight some of these approaches, more common earthquakes at plate boundaries. Figure 1 illus- using North America and New Madrid as examples for compari- trates this point by comparing a type example of a conti nental son with some of the results and ideas presented in this volume. intraplate seismic zone, the New Madrid seismic zone in the central United States, with southern California, part of the DEFINING PLATE INTERIORS boundary zone between the Pacifi c and North American plates. New Madrid seismic zone earthquakes of a given magnitude Although the discovery of plate tectonics explained why are ~30–100 times less frequent because southern California the overwhelming majority of earthquakes and seismic moment earthquakes result from the ~46 mm/yr motion within the plate release occurs on plate boundaries, it remained unclear for some boundary zone, whereas New Madrid is within the interior of the time how to defi ne plate boundaries and distinguish them from North American plate, which is stable to better than 2 mm/yr. plate interiors. Although early papers defi ned narrow plate However, shaking from New Madrid seismic zone earthquakes boundaries between idealized rigid plates, for example, treat- spe425-01 page 3 Approaches to continental intraplate earthquake issues 3 NORTH AMERICAN SEISMICITY 60°N 40°N NMSZ 46 mm/yr < 2 mm/yr Earthquakes M = 5 M = 6 M = 7 20°N 140°W 100°W 60°W SEISMICITY COMPARISON GROUND MOTION COMPARISON 10 1.5 Southern CA Eastern M = 7 ) 1 g Eastern M = 6 1.0 0.1 Western M = 7 Western M = 6 0.01 NMSZ 1811-1812 0.5 0.001 EARTHQUAKES PER YEAR PER EARTHQUAKES PEAK ACCELERATION ( PEAK ACCELERATION 0 567 040608020 MAGNITUDE DISTANCE FROM SOURCE (km) Figure 1. Top: Seismicity (M 5 or greater since 1900) of the continental portion of the North Ameri- can plate and adjacent areas. Seismicity and deformation are concentrated along the Pacifi c–North America plate boundary zone, refl ecting the relative plate motion. The stable eastern portion of the continent, approximately east of 260°, is much less active, with seismicity deformation concentrated in several zones, notably the New Madrid seismic zone.
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