Seismotectonic context

Much of the motivation for establishing a borehole observatory at Site U1364 derives from a need to understand the nature of deformation across the full width of a subduction zone like Cascadia that hosts infrequent but very large (Mw ~9) earthquakes generated along the subduction thrust interface. Recent events include Kamchatka (1952), southern Chile (1960), eastern Aleutians (1964), and Sumatra (2004). The most recent "megathrust" earthquake at Cascadia is known from tsunami records in Japan to have taken place in 1700 (Satake et al., 1996), and the interval between large events estimated from sequences of tsunami deposits in coastal and offshore areas ranges from 200 to 800 y (Goldfinger et al., 2003; Leonard et al., 2010). Very large subduction earthquakes are known to be generated by rupture propagating along segments ~1000 km long, but the precise width and position of the seismogenic rupture surface is only modestly well constrained. Updip and downdip limits of rupture are controlled by factors including rock mechanical properties (influenced by temperature and composition), fluid pressure, and interface roughness; observationally, the limits are typically estimated on the basis of seismic radiation patterns of great earthquakes and vertical and horizontal coseismic and postseismic deformation where it can be measured in adjacent subaerial areas. Slow interseismic deformation also provides information about seismogenic deformation, as they are very approximate mirror images of one another. The zone of seismic coupling is offshore in most subduction zones, so constraints from land-based observations provide an incomplete and biased view; observations like those to be made at Site U1364 are bound to add invaluable insight into both interseismic and seismogenic behavior of subduction zones. We anticipate that slow interseismic strain accumulation may be resolved, and it is possible that aseismic deformation events may be present, as have been observed along the subduction thrusts at Costa Rica (Davis and Villinger, 2006; Davis et al., submitted) and the Nankai Trough (Davis et al., 2009) (e.g., Fig. F8).

In addition to the Cascadia subduction zone being host to occasional giant thrust earthquakes, local plate motions generate frequent earthquakes with smaller magnitudes (Fig. F9). To the northwest of Site U1364, strike-slip events are concentrated along the Nootka fault, the strike-slip boundary between the Juan de Fuca and Explorer oceanic plates. Along the continental margin, intraplate events occur within the overriding continental crust and within the oceanic crust of the subducting Juan de Fuca and Explorer plates (Fig. F9A). Further landward, seismic tremor occurs episodically along and above the top of the subducting plate, downdip of the thrust seismogenic zone (Kao et al., 2009) (Fig. F9B). Strain associated with local seismic events is known to be detectable as pressure transients (e.g., Fig. F10), and the comparison of the strain estimated from pressure with the strain estimated from seismic moments will provide invaluable information about seismic rupture processes in the oceanic crust and upper mantle, the continental crust, and the more consolidated parts of the accretionary prism. The possible frequency of occurrence of events that can be resolved in the CORK pressure records at Site U1364 is illustrated in Figure F11, which shows the seismic recurrence relationship for the region around the drill site. The known sensitivity to strain produced by earthquakes of magnitudes as low as Mw = 5 at a range of up to 100 km (Fig. F10) suggests that strain transients of one or more events per year should be resolved at Site U1364.

Beyond these scientific considerations, a number of technical factors added to the motivation to establish a geophysical observatory at this site. High reliability of CORK instrumentation has been demonstrated through successful long-term operations at many sites. Instruments deployed during Leg 196 (Nankai Trough) have been operating for >8 y, and those deployed during ODP Legs 168 and 169 have been in operation for >13 y. Improvements in power consumption, memory capacity, and resolution now permit detection of much subtler signals than were previously possible. And in this instance, connection to the NEPTUNE-Canada cabled observatory infrastructure will open new opportunities. Much higher sampling frequency will be achieved, allowing observations to reach into the seismic frequency band (Fig. F12) and to be placed in the context of collocated seismic and hydrologic records that are currently being collected with a broadband seismometer and a variety of seafloor vent monitoring instruments roughly 3.5 km from the Site U1364 ACORK borehole observatory, as well as those that will be collected with additional instrumentation planned for Hole U1364A.