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We are currently embarking on a seismic marine research voyage in the NW Pacific ocean operated jointly between CSUN and Brown University. PI's (Principle Investigators), Dr. Dayanthie Weeraratne and Dr. Donald Forsyth are heading this NSF funded project to study the physical properties of old oceanic seafloor. We propose to measure the azimuthal anisotropy of Rayleigh wave propagation within two ocean-bottom seismometer arrays in the western Pacific (see map) as a means of unambiguously determining the thickness of the old oceanic lithosphere. In addition, the results will provide a test of global mantle convection models. Thermal models of seafloor subsidence indicate that the oceanic plates should be ~ 90 - 125 km thick, with temperatures approaching steady state in very old seafloor. In contrast, seismic surface wave studies indicate that velocities continue to increase as a function of age, with the velocity changes occurring at depths greater than the thickness of the best-fitting cooling slab models. Surprisingly Earth scientists have very few measurements of seafloor plate thickness due to the high cost of seagoing research and new technology only currently being developed to put seismometers on the bottom of the ocean. The most direct and unambiguous way to determine the thickness of the lithosphere and to resolve this issue is to map the transition from static structure frozen in the plate to actively deforming fabric in the deforming or convecting asthenosphere.
This particular location was chosen because of the unique observation of a "magnetic bight" anomaly on this section of old (155 Ma) seafloor. The magnetic lineations in two adjacent regions show magnetic anomalies which are perpendicular to each other (red lines on map). This implied change in seafloor and lithospheric structure should induce a change in anisotropic fabric, which we propose to detect by measuring the variation of azimuthal anisotropy of Rayleigh waves as a function of period. With a deployment of two arrays (large circles) of ocean-bottom seismometers (OBS) on the limbs of a magnetic anomaly bight, it should be possible to clearly distinguish the fossil component of anisotropy from the dynamically maintained component in the asthenosphere. A 12-month deployment of OBS's should be sufficient to provide the needed data and is underway now. CSUN students, Alex Hanna and Cristo Ramirez recently traveled with the science crew to Kaohsiung, Taiwan to board the Research Vessel (R/V) Revelle (below) and are now at sea deploying seismometers. When finished, the ship will dock in Guam and the science party will disembark and return home. The seismometers will rest on the ocean floor for 12 months collecting passive earthquake data from around the world arriving at this location on the seafloor. One year from now, the marine scientists will return to the western Pacific to retrieve the instruments and the seismic data for analysis.
The Research Vessel (R/V) Revelle named after Roger Revelle, was built in 1996, is roughly 273 ft long (nearly the length of a football field!), and can travel at speeds of 10-19 knots. It sports 6 engines, 2 rear propellers, and a bow thruster that allows for dynamic positioning. Dynamic positioning allows the ship to stay still at one location anywhere in the middle of the ocean for hours. The ship can also move sideways, and spin in a circle on axis at 4 knots. The Revelle has about 23 fulltime crew members who maintain the ship and assist scientists in their research. See the live link for the R/V Revelle for our current position, ship track, and live webcam pictures (refreshed every 10 minutes).
An equidistant projection of our study area (red square in the middle of the map) with the world wrapped around it, showing the source locations for earthquakes we expect to record during this deployment. These earthquakes send seismic energy radiating outward through the Earth's interior that will be recorded on our ocean bottom seismeters. Dr. Dayanthie Weeraratne and CSUN students will analyze this seismic data to study the physical properties of the oceanic plate, crust, lithosphere, and asthenosphere in this part of the western Pacific.
Cristo Ramirez (CSUN MS student in Geophysics in the Bridge to Doctorate Program) assisting David Gassier from Lamont Doherty Earth Observatory (LDEO at Columbia University) to load seismometers on board the Revelle from the harbor at Kaohsiung, Taiwan.
Alex Hanna (CSUN undergrad in Geophysics) hard at work in the computer lab making seafloor maps and locating good places to drop our seismometers.
Alex Hanna and Cristo Ramirez assisting Andrew Barclay (LDEO) to hoist 2 ocean bottom seismometers (OBS) on to a rack for a test deployment.
Two OBS's going down into the water for a test at 1000 m depth.
Alex Hanna helping to deploy the magnetometer which measures the Earth's magnetic field. This instrument is towed about 400 meters behind the ship (to get away from the metal ship!) as we travel.
Cristo Ramirez watching monitors displaying active data collection of underway geophysical measurements including Seabeam bathymetry (ship board bathymetry is the highest resolution data scientists can obtain of the seafloor), acoustic doppler current profiles, GPS, gravity, and magnetics.
Dr. Weeraratne is shown on the ship's webcam as she answers questions from her Geology 101 class (on the CSUN campus) via an internet chat session. Here is some of the dialog:
Class: Made any discoveries?
Dr. Weeraratne: We steamed 7 days to our study area from Kaohsiung, Taiwan. We dropped our first OBS (ocean bottom seismometer) in the ocean on Oct 20, 2009. Can anyone guess how many earthquakes our instrument has recorded worldwide since then?
Dr. Weeraratnee: Close, about 30. A few 5.3 in the Kuril Islands (near Russia), and 5 of 6 at 4.7 in the Tonga Trench, and lots of smaller ones. Can you all see the camera on the back deck of the ship ? The yellow objects are seismometers. A csun student, Alex Hanna is standing there with me. We pick these up with a crane and drop them overboard (with a lot of technical and electrical work first!)
Class: How man OBS's have been deployed?
Dr. Weeraratnee: We've deployed 4 OBS's so far. We work 24 hours around the clock. It's 2:30am right now (Oct 22). We're steaming at 12 knots to our next site to deploy at 4am!
Class: Can you get real time data from the seismometers?
Dr. Weeraratnee: No, they are too deep and we can't exchange data through the water. If only we were on the surface of Mars where things are easier! We also don't have power at the bottom of the ocean (no solar energy like the Rovers use on Mars with solar cells). The 10 computer screens you see behind me is "command central" where we monitor all the geophysical data collected as we steam. Someone is here 24 hrs a day to watch this (namely me and other students!)
Class: What is the smallest earthquake that you can measure?
Dr. Weeraratnee: We can measure earthquakes as small as 2.0-3.0 but we might need a different kind of instrument for that. With these OBS's which are "broadband" we generally use earthquakes of 4.0 or higher.
Class: What is the most exciting thing that has happened so far?
Dr. Weeraratnee: Dropping the first OBS was pretty exciting. When it reaches the bottom (6000 m) that is 6 km, that is 3 miles! We can send a sonar signal to it and receive a ping back at a specific frequency. To hear it respond is a reassuring feeling! We return in 12 months to ping to it again, then bring it to the surface and get the data on the hard drive.
Dr. Weeraratnee: Did you see the videocam shot with your styrofoam cups? Katie Zuniga wrote on hers "My geology professor went to the seafloor and all I got was this cup!" The other cup was from Ben Sherman. We haven't dropped these to the seafloor yet, but we will soon. Can you see the dark sky behind the ship? There was a "Orionid" meteor shower tonight visible by anyone all over the world did you see any shooting stars in LA? Does anyone know what happens during a meteor shower?
Class:The meteor gets wet?
Dr. Weeraratnee:The Earth travels through the debris left behind from Halley's Comet. The particles burn up in Earth's atmosphere. Any guesses as to how big these particles are?
Dr. Weeraratnee: Yes, they are "pea sized" or the size of a typical sand grain.
Dr. Weeraratne inspects one of the components of an ocean bottom seismometer.
Cristo Ramirez holding a tag line with Martin Rapa (Scripps) to hoist an OBS (ocean bottom seismometer) over the side of the ship. It will sink to ~6000 m depth (3 miles down!) to record earthquake energy traveling through this part of the world.
Dayanthie Weeraratne and CSUN students Alex Hanna and Cristo Ramirez checking an OBS on board the Revelle before deployment.
Alex Hanna, Cristo Ramirez, and Martin Rappa bring in the tag lines after a Scripps OBS deployment.