S.O.S. (Stuff on the Seafloor)

As discussed in an earlier post (see “Meet Bill, the multi-beam guy!” from July 15), we are collecting multi-beam data around our OBS sites and while in transit to map out important and interesting features on the seafloor.  It is truly amazing how little of the ocean has been surveyed!  For today’s post, we’ll highlight some of the features we’ve found.

Seamounts (Submarine Volcanoes)

While some of the ocean floor is fairly smooth and covered in sediment, other portions are riddled with seamounts.  Seamounts are mountains that rise off the ocean floor, most of which are volcanic in origin.

The image below show an ~800 m high seamount that we discovered while in transit between sites WD65 and WD67.  Science teacher Shannon Hendricks was the first person to recognize the crater at the top, so we thought naming this feature after her might be appropriate.  Given her bout with seasickness, Mt. Spew was suggested, but she doesn’t seem keen on that idea.

New multi-beam bathymetry data showing discovered seamount near site WD67

So, what is the source of this seamount?  One idea that was suggested is that it is associated with petit-spot volcanism.  Petit volcanoes have been recognized in Japan, for instance, and generally form along fractures caused by flexure and tension in the subducting plate.  However, this seamount is somewhat further from the trench and is significantly larger than those attributed to petit-spot volcanism in Japan, so this idea would need to be further explored.

Cartoon of petit volcanism in Japan from Snow (2016).

Normal Faults Leading Into the Trench

Leading up to the trench, the oceanic crust is deformed and offset by numerous normal faults, also caused by tensional stresses in the subducting plate.  While not typically associated with as large of earthquakes as the thrust interface between the subducting and over-riding plates, earthquakes along these outer rise normal faults can have large magnitudes.  For example, in 1965 a Mw 7.2 outer rise normal event occurred offshore the Rat Islands in southwestern Alaska (Abe, 1972).

Subduction cartoon from USGS Pacific Coastal and Marine Science Center 

Bathymetry data showing oceanic crust deformed by normal faults and the Aleutian Trench

It’s also worth mentioning that there were two earthquakes (magnitudes 5.8 and 6.0, respectively) in the last two days right under recently deployed site WS75.  So the action is already starting!  Stay tuned!

– Samantha Hansen (Associate Professor, The University of Alabama)

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