CHARACTERISTICS OF HYPOTHETICAL SUBDUCTION EARTHQUAKES IN THE NORTHWESTERN UNITED STATES

Historic earthquake sequences on subduction zones that are similar to the Cascadia subduction zone are used to hypothesize the nature of shallow subduction earthquakes that might occur in the northwestern United States. Based on systematic comparisons of several physical characteristics, including physiography and seismicity, subduction zones that are deemed most similar to the Cascadia subduction zone are those in southern Chile, southwestern Japan, and Colombia. These zones have all experienced very large earthquake sequences, and if the Cascadia subduction zone is also capable of storing elastic strain energy along its greater than 1000 km length, then earthquakes of very large size (Mw > 81⁄2) must be considered. Circumstantial evidence is presented that suggests (but does not prove) that large subduction earthquakes along the Cascadia subduction zone may have an average repeat time of 400 to 500 yr. INTRODUCTION This is the third in a series of four papers that lead to an estimation of the seismic hazard associated with the subduction of the Juan de Fuca and Gorda plates beneath North America. In the first paper, Heaton and Kanamori (1984) compared physical characteristics of the Cascadia subduction zone (also referred to as the Juan de Fuca subduction zone) with those of other subduction zones and concluded that the Cascadia subduction zone is similar to other subduction zones with strong seismic coupling, and thus may be capable of producing great shallow subduction earthquakes. In the second paper, Hartzell and Heaton (1985) compared the nature of the time history of seismic energy release for 60 of the largest subduction earthquakes to occur in the past 50 yr. In this paper, we extend the results of the previous studies to estimate the nature of shallow subduction earthquakes that could be postulated if the Cascadia subduction zone is assumed to have strong seismic coupling. In the fourth paper, Heaton and Hartzell (1986) estimate the nature of strong ground motions that may result from earthquakes hypothesized in this study. Key issues addressed here are: (1) the dimensions and geometry of hypothetical Cascadia subduction earthquakes; (2) plausible repeat time; (3) identification of analogous historic earthquakes; and (4) estimation of possible tsunami amplitudes. Subduction of the Juan de Fuca and Gorda plates has presented earth scientists with a dilemma. Despite compelling evidence of active plate convergence (Riddihough, 1977; Snavely et al., 1980; Hyndman and Wiechert, 1983; Adams, 1984a; Nishimura et al., 1985), subduction on the Cascadia zone has often been viewed as a relatively benign tectonic process (Riddihough, 1978; Ando and Balazs, 1979; Acharya, 1981, 1985). There is no deep oceanic trench off the coast; there is no extensive Benioff-Wadati seismicity zone; and most puzzling of all, there have not been any historic low-angle thrust earthquakes between the continental and subducted plates. The two simplest interpretations of these observations are: (1) the Cascadia subduction zone is completely decoupled and subduction is occurring aseismically, or (2) the Cascadia subduction zone is uniformly locked and storing elastic energy to be released in future great earthquakes. Full resolution of this issue 675 676 THOMAS H. HEATON AND STEPHEN H. HARTZELL may prove elusive. Although it is somewhat surprising that no shallow subduction earthquakes have been documented in this region, the duration of written history is relatively short. It seems certain that great shallow subduction earthquakes have not occurred in this region since the 1850's and highly probable that they have not occurred since the 1790's. If large shallow subduction earthquakes do occur on the Cascadia subduction zone, we can infer their characteristics only by studying the nature of subduction earthquakes that have occurred on other subduction zones that can be considered as analogous. Unfortunately, no subduction zone is exactly the same as the Cascadia subduction zone, and thus, the search for analogs will always lead to fundamental ambiguities. Systematic variations in the nature of seismic energy release with physical characteristics of subduction zones have been discussed by Kelleher et al. (1974), Kanamori (1977), Uyeda and Kanamori (1979), Ruff and Kanamori (1980), Lay et al. (1982), Peterson and Seno (1984), and Uyeda (1984). Many types of correlations have been suggested, but since quantification of the physical characteristics of subduction zones and their earthquakes is often rather subjective, not all of these studies seem to reach compatible conclusions. Perhaps the most consistent observation is that subduction of young lithosphere is associated with strong seismic coupling whereas subduction of old lithosphere is associated with weak seismic coupling. Ruff and Kanamori {1980) demonstrate that the seismic coupling is wellcorrelated with a simple linear function of subducted plate age and convergence velocity in which decreasing age and increasing convergence velocity imply stronger seismic coupling. Heaton and Kanamori (1984) show that the Cascadia subduction zone is clearly different from the class of aseismic subduction zones that is characterized by the subduction of old lithosphere. They further show that it shares many characteristics with subduction zones that have experienced very large shallow subduction earthquakes. In this report, we do not restate all of the arguments given by Heaton and Kanamori (1984), but instead we extend that study by systematically comparing trench bathymetry and shallow seismicity for a world-wide sampling of subduction zones. TRENCH BATHYMETRY AND GRAVITY Uyeda and Kanamori (1979) suggest that strongly coupled subduction zones are accompanied by relatively shallow trenches, whereas weakly coupled subduction zones are accompanied by deep oceanic trenches. Similarly, they conclude that freeair gravity anomalies tend to be larger for those trenches with weak seismic coupling. The Cascadia subduction zone is somewhat unusual in that it has virtually no bathymetric trench. In order to assess just how anomalous this is, we have constructed profiles of bathymetry and free-air gravity for many circum-Pacific convergent boundaries. Figure 1 shows shiptrack segments that were used to construct the profiles shown in Figure 2. The shiptrack data were obtained in digital form from NOAA (1981). All data are plotted on a common scale, and the subducted plate is always on the right-hand side of the profile. Bathymetry and free-air gravity are plotted together. Unfortunately, gravity data were not available for all the shiptracks selected, and thus, only bathymetry is plotted for some of the profiles. Comparing the profiles from differing subduction zones, we see that the Cascadia subduction zone (profiles 38-43) is indeed remarkable for its lack of a bathymetric trench, very shallow ocean floor, and very small gravity anomalies. However, there are several factors that may help to explain these characteristics. The shallow 3 km depth of the Juan de Fuca plate before subduction is near the average depth of very SUBDUCTION EARTHQUAKES IN THE NORTHWESTERN U.S. 677