TECTONIC AND SEISMIC SETTINGTectonic Setting
In multichannel seismic profiles, the reflective sequence above a major diffracting horizon represents a seaward transgressive sequence across an extensive angular unconformity (Fig. 4). Landward-dipping reflectors below the unconformity may represent formerly accreted sediments or folded and tilted older sedimentary rocks; they are dated by drilling as Upper Cretaceous at Site 439. The Neogene sequence is cut by landward-dipping normal faults spaced ~10 to 15 km apart (Nasu et al., 1980). Seismic refraction measurements indicate a continental crustal velocity structure beneath the deep-sea terrace (Murauchi and Ludwig, 1980; Suyehiro and Nishizawa, 1994).
Tectonic history in the convergent margin near the Japan Trench is characterized by tectonic subsidence and erosion. Regional subsidence during the latest period of plate convergence was established during DSDP drilling along the Japan Trench margin (von Huene, Nasu, et al., 1978) by coring through a subaerial erosion surface many kilometers below sea level (Fig. 5). That erosion surface corresponds to an angular unconformity that cuts across tilted beds and is buried beneath subhorizontal strata of the outer shelf and slope. The unconformity extends throughout a 150-km-long area (Nasu et al., 1980; von Huene et al., 1982) and shows no sign of ending beyond the published seismic coverage. Across the unconformity, seismic velocity increases abruptly from ~1.9 to 4.2 km/s (Murauchi and Ludwig, 1980; Suyehiro and Nishizawa, 1994), consistent with the contact between unconsolidated Oligocene to Quaternary strata and well-consolidated Cretaceous rock as drilled at DSDP Site 439. The sedimentary strata above the unconformity consist of a 48-m-thick breccia and conglomerate of dacite and rhyolite boulders, covered by 50 m of medium-grained sand containing abundant little-transported macrofossils, which was in turn buried by silt and sand turbidites (Scientific Party, 1980) with a probable seaward source (von Huene et al., 1982). The upper 800 m of the section consists of Miocene diatomaceous mud. The regional extent of rock types and erosion was explained by subsidence of a landmass during the past 22 m.y. (Scientific Party, 1980; von Huene et al., 1982). Benthic microfossils from the sediments indicate a succession of water depth consistent with such a history (Arthur et al., 1980; Keller, 1980).
In the Japan Trench area, seven large (magnitude [M] = >7) interplate events have occurred in the last 30 yr between 38°N and 41°N. Recent large events are the 1968 Tokachi-Oki earthquake (~41°N; moment magnitude [Mw] = 7.9) and the 28 December 1994 Far-off Sanriku earthquake (~40°N; Mw = 7.7) (Fig. 6). These events, however, are not sufficient to account for the subducting rate of ~8-10 cm/yr. Thus, the seismic coupling seems much smaller along the Japan Trench (35°N-41°N) as compared with the Kurile Trench or Nankai Trough regions, which have a higher seismic energy release rate. Subduction at the Japan Trench may be proceeding by stable sliding either only with relatively small (surface-wave magnitude [Ms] = <8) events or with occasional large events.
There is a third important category whereby the subduction rate is accommodated by episodic aseismic events of time constants on the order of 10 min to several days (slow earthquakes). Such events, if they exist, are presently extremely difficult to detect. Kawasaki et al. (1995) reported that an ultra-slow earthquake estimated to be Mw 7.3-7.7 accompanied the 1992 Off-Sanriku (39.42°N, 143.33°E; Mw = 6.9) earthquake, based on strain records observed ~120 170 km away from the source. A postseismic strain change of 10-7 to 10 8 with a time constant of about a day were observed by quartz-tube extensometers (devices that measure absolute strain). Historically, in the same area, the 1896 Sanriku tsunami earthquake (Mw = ~8.5 but body-wave magnitude [Mb] = ~7) killed ~22,000 people. Tsunami earthquakes rupture in a much longer time constant of minutes compared to normal type (Tanioka and Satake, 1996).
More recently, the Japanese global positioning system network revealed a postseismic motion of northern Japan after the 1995 Far-off Sanriku earthquake (M = 7.2) that can be explained by a stress diffusion model assuming slow slip on the earthquake fault (Heki et al., 1997). A different, but previously more prevalent interpretation is that the postseismic deformation is a result of aseismic slip at a depth extending below the seismogenic zone.
The above-mentioned large episodic events are responsible for the plate motion. There are numerous microearthquakes, which contribute little to tectonics, but delineate the characteristics of seismogenic zone (Fig. 7). Microearthquake activity in this area shows spatially clustered occurrence over at least a few tens of years. The plate geometry from microearthquakes suggests that a large bend occurs at ~20 km depth. The seismic activity seems to have a gap above and below this depth along the plate boundary. The shallower activity is conspicuously high between 39°N-41°N.
The seismic velocity structure of this area is characterized by a small accretionary wedge (Fig. 3). High velocity (> 6 km/s) material reaches beneath the inner trench slope. Either an extension of the island arc lower crust or the mantle wedge seems to meet the plate boundary where it plunges. Large thrust earthquakes in this area often initiate from the updip end of the seismogenic zone and reach beneath the coastline. It can be seen that from detailed velocity structure models that there is no obvious correlation with the updip and downdip ends. This suggests that the key factors in controlling the seismic activity are not in bulk properties, but rather in the localized properties at the well-developed décollement.
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