Keck Expedition 2004
August 3, 2004 Day 5
Action on Nootka - by Taimi Mulder (additional log below)
Update - by Debra StakesToday was an extremely long, challenging day at the Nootka Transform Zone installing a new broadband seismometer station. After steaming during the night the Western Flyer arrived on station at daybreak. The installation will take a lot of equipment and half is sent down first on a benthic elevator, including the PVC caisson, the hopper with glass beads, and the RIN with the datalogger and battery spheres. The broadband sensor package is carried down in the drawer of the ROV, attached to the 60m of coiled underwater cable that ends in the yellow and gray underwater mateable connector that will be used to plug into the RIN module. In front on the sensor package is a large chef’s knife that will be used to cut the caisson free from its lines. The ROV is also equipped with a hydraulically activated water lifter that will be used to excavate a hole for the caisson. Engineer Paul McGill has carefully charted a 23-step procedure for the full installation process. By dinnertime we are only on step 4. The first three steps are easy—find the elevator, set down the sensor package, select a spot for the caisson, etc. However the next step is to actually sink the caisson. This process involves alternatively pushing down on the PVC caisson with the front of the ROV and then vacuuming the sediment from the middle of the caisson.
The result is to create a hole in the seafloor to emplace the sensor package. The science party spends many hours watching the pilots repeat this process until the hole is at least 22 inches deep and somewhat level on the bottom. The pilots improve their technique continuously, getting more efficient with each watch. They discuss how to improve the system with the Western Flyer crew-members at dinner. The level of enthusiasm is phenomenal, especially given the tedium of the task. Finally, at dusk, the broadband sensor in its spherical molded titanium housing is placed into the caisson. The sensor is rotated until the axis is pointed to the north.
The next step is to carry the sensor’s connector back to the electronics module or RIN (remote instrument node) in preparation for connection. The ROV has a similar underwater connector to provide a serial communications connection to the datalogger. The ROV connector needs to be put into place first to be sure that the timing and communications link is in place for the installation. The pilots drive the ROV to the RIN and push the connector into place, but it fails to lock and pops back off. Hmmm. Must have been the wrong angle. They drive closer and use a different camera view. It pops off again. Hmmm. Perhaps the cable is too tight. They drive closer and make sure the cable is slack. The connector pops off again. Hmmm. What the heck is going on?? No one has ever seen a connector behave in this manner before. They were all tested before the equipment was deployed. We look for some explanation—is there a rock in the way? We decide that the connector locks must be stuck. Tony thinks to bring another connector pair into the control room so that everyone can figure out what the problem might be. Part of the locking mechanism requires the connector wire bundle and pins to move backwards on an O-ring. The one on the seafloor is stuck in the open position where the pins cannot move forward and lock. We can even duplicate the problem in the control room, even though none of us have seen it before. Every effort is make to unstick the connector (use your imagination) but nothing works. Finally the pilots decide to use one manipulator to hold onto the RIN frame while the second manipulator will hold the connector in place. The arms are put into position and then securely locked, enabling the necessary communications. Only the minimum communications are established so that we can complete the installation (we are also a bit nervous about holding the connection while powering the pins. One slip could ruin the connector creating even more problems). I don’t thing that anyone in the control room breathes for a few minutes until it becomes obvious that, once the manipulators are locked, this is a very stable configuration.
The next step requires the ROV to carry the bead hopper from the elevator to the caisson and to prepare to bury the sensor in glass beads. The bead hopper has its own small syntactic flotation package to make it light enough for the ROV to move. However, this flotation must be removed BEFORE the beads are released to prevent the hopper from floating away without the weight of the beads to hold them down. The package is released at last daylight and sent zinging to the surface where the small boat snares it and brings it back to the Western Flyer. The “beading-in” process is remarkably efficient and an exotic dragon juvenile sea cucumber provides us with a floor show during the process. The ROV then re-establishes its two-manipulator connection and initiates the sensor’s autoleveling and autocentering routines. Unfortunately one of the axes has trouble centering, so we will try and check on the system before this leg is over. The ROV finally leaves the bottom near midnight with the elevator following. The science party all go to bed hoping that the crew will be able to find our elevator in the middle of the night. Without it we will not be able to install the broadband at Explorer later in the cruise.
Postscript: Out of bed at 6 am, my first thought is to check whether the elevator is safely stowed on the back deck. I check on this even before I go in search of coffee. It is! And we are on our way back to finish our work at Endeavour.Left: Taimi Mulder and John Ristau of PGC helping to
prepare the RIN for the Nootka broadband installation. Right: This is a map of the Juan de Fuca plate and Vancouver Island that shows the three areas of Keck seismometer deployments. The Endeavour segment is shown by the red and blue diamonds (several instruments in a small area). The Explorer site (not yet installed) is shown by the red star. This location was selected at the recent Neptune Canada science planning meeting because it is north of most of the regional seismic activity. Earthquakes are shown by the small red dots. The new Nootka Broadband installation KNBB is shown by the large green dot. You can see that the station is just offshore off a large cluster of earthquakes.
Yesterday we installed the Nootka broadband station. It is nestled on the upper right-hand corner of the Juan de Fuca plate just before it descends (subducts) beneath the North America plate. When this seismic station is hooked up to the Neptune cable it will provide us with real-time seismic data from this location. This site will provide important data on the plate tectonic behaviour in this region. The Canadian portion of this important project is already underway and will be installed by 2007-8. http://www.neptunecanada.ca)
West of North America lies the Juan de Fuca plate system. This system extends offshore from Northern Vancouver Island, BC, to northernmost California. Over the last tens of millions of years the Juan de Fuca plate has broken up into a series of smaller plates on the north and south end of the main Juan de Fuca plate. The Nootka transform fault zone marks the boundary between the Juan de Fuca plate and the Explorer plate to the north of it (both part of the Juan de Fuca plate system). The Juan de Fuca plate is actively subducting beneath the North America plate at the rate of fingernail growth (~4cm/yr). The Explorer plate is not actively subducting anymore. The movement of all of these plates is delineated by earthquakes.
The Nootka broadband site will greatly assist in the routine location of these earthquakes for the Canadian Earthquake Catalogue.It will provide important constraint on these locations and their depth. Locating earthquakes is a bit like finding a needle in a haystack. Ideally you want to make your haystack as small as possible. This can be accomplished by having stations on all sides of the earthquake and relatively close to it. If one station happens to also be directly above the earthquake then the distance from this station to the earthquake is the depth of the earthquake.
With offshore earthquakes this is very difficult as all stations providing real-time data feeds are far away on land and on only one side of the earthquakes. We are looking forward to having a station closer and outboard of the plate margin, placing it to the west of some of these events. This will help us do our job better. In fact, if the July M5.8 and M6.4 earthquakes could have just waited until after Aug 4th then we would have some great data from which to determine accurate depths for these events. These earthquakes probably occurred within 10km of the broadband site we installed yesterday. Knowing exactly which plate these earthquakes occurred on and at what depth would provide us with information on the stress regime of these plates and add to our accumulating store of knowledge concerning earthquake hazard in this region.
In the Pacific Northwest earthquakes occur in three main regions: within the North America plate, within the Juan de Fuca plate, and along the boundary between the two. There have been large, damaging earthquakes, which originate in any of these three regions. The largest and most infrequent earthquake is the “Cascadia megathrust earthquake” which occurs on the boundary between the Juan de Fuca and North America plate as the Juan de Fuca plate sticks and slips underneath North America on the timescale of several hundreds of years. The last megathust event was a magnitude 9 earthquake on January 26, 1700, recently putting us into the approximately 500 year time window for another megathrust earthquake occurrence. However there are more frequent, somewhat smaller earthquakes that occur both within the Juan de Fuca and within the North America plates. These damaging events are in the magnitude 6 to 7 range and more than 10 of these have occurred in the past 100 years. Near the Nootka broadband site there are earthquakes on the western margin of the North America plate west of Vancouver Island. There are many more earthquakes along the Nootka Transform Fault as well. Clearly defining the location of these events will lead to greater understanding of the stresses operating in this region and lead us closer to understanding stress cycles which would be forerunners to rupture. For further information go to: http://www.pgc.nrcan.gc.ca/seismo/table.htm