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Active Tectonics: Studies in Geophysics
FIGURE 3.7 Recent large earthquakes along the Alaska-Aleutian seismic zone. Areas along this zone that have not ruptured are considered seismic gaps and may be the locations of future large earthquakes. (From McCann et al., 1979.)
earthquakes (Ando and Balazs, 1979; Weaver and Smith, 1983; Adams, 1984).
Recent studies have been undertaken to assess the seismic potential associated with subduction in this region (Heaton and Kanamori, 1984; Adams, 1984). Heaton and Kanamori examined the seismic coupling process and compared the Juan de Fuca subduction zone with subduction of other young oceanic plates. In particular, they compared convergence rates, ages of lithosphere, presence of active back-arc basins, depth of oceanic trench, dip of the Benioff-Wadati zone, topography of the subducted slab, and seismic quiescence. Present-day convergence across the Juan de Fuca subduction zone has been estimated as 3 to 4 cm/yr, a moderate rate for subduction zones. The age of the subducted slab has been estimated at 10 to 15 m.y., a relatively young lithospheric plate (Heaton and Kanamori, 1984). They associated the young buoyant crust with strong coupling with the overriding plate; and when rates of plate movement and the age of ocean floor are used (Ruff and Kanamori, 1980), they estimated a maximum moment magnitude of Mw=8.3±0.5. They also proposed that other parameters such as the dip of the Benioff-Wadati zone (10 to 15° beneath Puget Sound), topography of the subducted slab, absence of a back-arc basin, and depth of trench suggest strong coupling of the plates. In their worst-case model of strong coupling, the Juan de Fuca subduction zone could rupture in one event (approximately 600 km by 200 km; convergence rate=4 cm/yr), with an estimated maximum moment magnitude of Mw=9.0.
Heaton and Kanamori (1984) commented, “this 500-km gap in seismic activity is one of the most remarkable to be found anywhere in the Circum-Pacific belt,” and “if slip is occurring aseismically on the shallow part of the subduction zone, then this particular example would have to be considered unique.” They concluded, “that there is sufficient evidence to warrant further study of the possibility of a great subduction zone earthquake in the Pacific Northwest.”
Adams (1984) suggested types of geologic investigations of paleoseismic activity that could help resolve the seismic hazard. A possible example is Sims’s (1975) study of disturbances of glacio-lacustrine deposits in the western Puget Sound area, which considered 14 disturbed zones in the 40,000-yr-old sediments to be caused by earthquakes. Such evidence needs to be verified as truly seismogenic, synchronous with earthquakes, and distributed over a larger part of the Pacific Northwest.
Definitive studies have yet to resolve the issue of whether this Benioff-Wadati zone is seismogenic or aseismic. Resolution is important to future building, siting, design, and zoning within Washington and Oregon. Many existing engineered structures may have inadequate design for earthquakes of magnitude 7.5, 8.0, or 8.5.
Implications of the Meers Fault, Oklahoma
The Meers Fault (Figure 3.8) is a relatively short fault of about 75-km length in the Frontal Fault zone between the Amarillo-Wichita Uplift and the Anadarko Basin. It was previously recognized to be a major ancient fault with about 3 km of total vertical component of offset in a zone of faults that has a total, mainly Paleozoic, offset of over 10 km. Gilbert (1983) recognized that