U.S.S. San Francisco preparing to dock at Apra Harbor
posted Jan 10, 2005.
We have this:
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To: BigSkyFreeper
The crazy thing about the topography of the ocean floor in that area is that the seamounts of the Caroline Ridge (which the sub supposedly hit) are very near to the Marianas Trench -- the deepest spot on Earth...
My guess is that our undersea topographic mapping in this area is not too good.
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Introduction
Fig.1: Location map of the Yap Trench. The contour interval is 2,000 m. The box shows the survey area as shown in Fig.3. The distribution of earthquake hypocenters are represented by solid black symbols: Circles = source depth shallower than 50 km, Diamonds = 50-100 km, Crosses = deeper than 100 km. Global hypocentral data from ISC since 1980. |
Fig.2: Bathymetric map of the circumambient regions of the Yap Trench. The contour interval is 500 m. The bathymetric data are based on the global bathymetry of ETOPO5 (NGDC, 1988). The star symbol shows the pole position of Philippine Sea - Caroline plates estimated by Seno et al. (1993). The solid black symbols show hypocentral distribution. See Figure 1 caption. |
The length of the Yap Trench is about 700 km (Fig. 2). The trench axis elongates in a convex shape toward the southeast. An arc involving the Yap Island on the Philippine Sea Plate side forms a trench-arc system. This arc consists primarily of metamorphic rocks and lacks active volcanism (Shiraki, 1971; Hawkins and Batiza, 1977). The arc-type rocks found on the islands are no younger than Late Oligocene or Miocene. The distance between the island-arc and the trench axis is about 50 km, which is much less than that of other trench-arc systems. The Caroline Ridge, oriented in an ESE-WNW direction, intersects the trench from the east. This ridge consists of a chain of seamounts thought to be of hotspot origin (Keating et al., 1984) of less than 40 m.y. (Hegarty and Weissel, 1988). Seismicity along the trench is low. Earthquakes occur at a depth of less than 50 km, and no deep-focus earthquakes are apparent along the trench. A Wadati-Benioff zone, demonstrated by a plane of deep-focus earthquakes reflecting dynamic interaction between a subducting and an overriding plates, is thus not defined. Considering the above geological and geophysical features, the Yap Trench is controversial regarding the activity of plate subduction at present. Hawkins and Batiza (1977) and McCabe and Uyeda (1983) suggested that subduction at the Yap Trench may have suspended by collision of the Caroline Ridge. McCabe and Uyeda (1983) suggested that the Caroline Ridge collided with the Yap Trench in early Miocene, and that this collision made the volcanic activity in the Yap Arc stop, and narrowing the distance between arc and trench. In contrast, some petrological and geophysical studies suggested that subduction at the trench may still be active. Fresh volcanic rock fragments and hydrothermally affected rocks, dredged in the back-arc region, may suggest in-situ or nearby hydrothermal activity in relation to Quaternary volcanic activity (Fujioka et al., 1986). High heat flow values were observed in the back-arc region and appreciably large values of negative free-air gravity anomalies were observed along the trench axis (Nagihara et al., 1989). Large negative gravity anomaly is indicative of dynamic force exerted on crust under the trench associated with ongoing subduction of the oceanic lithosphere. Observation of micro-seismic activity in the trench area suggested that tectonic force, found in active subduction zones, acted on the crust of the trench (Sato et al., 1997). |
Prior to our study, there were insufficient data for understanding the tectonics and geodynamics of the Yap Trench. We conducted swath bathymetry and gravity surveys of the Yap Trench aboard the R/V Yokosuka in 1993, 1994 and 1996, in combination with dive surveys of the research submersible Shinkai 6500 (Fujioka et al., 1994, 1996). We hope that our study stimulates further geological and geophysical studies in this region, focused on morphotectonic characteristics, by providing a high quality base map, and that it will provide insight into the tectonics of the Yap Trench, the Philippine Sea Plate and the circum-plates. Diagrams : |
Bathymetric map of the circumambient regions of the Yap Trench. The contour interval is 500 m. The bathymetric data are based on the global bathymetry of ETOPO5 (NGDC, 1988). The star symbol shows the pole position of Philippine Sea - Caroline plates estimated by Seno et al. (1993). The solid black symbols show hypocentral distribution. See Figure 1 caption.
Survey ship tracks in the region of the Yap Trench. The solid, gray and dashed lines show the tracks from the Y95-06, Y96-12, and Y93-03 cruises of the R/V Yokosuka, respectively. The dotted lines show the tracks of the KH92-1 cruise of the R/V Hakuho-maru. The area surveyed by Iwabuchi et al. (1990) aboard the S/V Takuyo is stippled.
Looks like this survey was concerned aout mapping the deep trench areas..
No interest in the Seamount areas.
Like we postulated: seamount and inadequate mapping. I'd surely hate to have to navigate that area submerged with any of the charts I've seen.
Don't have them handy right now, but I bet that National Geographic has better relief maps of the seafloor than these... :-(
I'm not a submariner, but I am a Navy navigator. The chart shown at the beginning of the thread was not the chart SAN FRANSISCO would have been using. That was from a small scale general@planning chartDSubmarines and even Navy surface ships in the open ocean use bathymetric charts that show bottom the bottom topography in great detail. Most of the them are classified due to that detail. The problem is they still relay quite a bit on interpolation between actual survey tracks. In the open ocean, 5 or more miles between tracks is not uncommon at all. @