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An expedition to Axial Volcano by David Clague (MBARI) collected high-resolution bathymetry using the Simrad EM300 system. This bathymetry data has been compared by Bill Chadwick (OSU/NOAA) to previously collected data using the 16-beam Sea Beam mapping system to see if there has been any detectable changes in the topography. Preliminary results of the comparison:
A first pass of the data through comparing the May 1998 EM300 bathymetry survey at Axial (collected by Dave Clague, MBARI) to NOAA Vents 1981 and 1991 SeaBeam data (roughly 46 05' to 45 27') has been completed.
Two locations where it looked like there were significant depth changes, both are within 1-3 miles south of the caldera on the axis of the upper south rift zone. The first is near the detection limit (5-10 m) and so somewhat less certain than the second which is larger (5-25 m), and fairly certain. The first anomaly is located within a circular area between 45 54.3'-55.0' and -129 59.4'-58.1', although it could extend a little further SW to 45 54.0'/-130 00.2'. The second is between 45 51.7'-52.3' and -130 00.5' to -129 59.1'. No convincing anomalies south of there have been detected.
*If* these two anomalies are indeed new lava flows (and we won't know for sure until we have some imagery of the bottom there), they both appear to have come out at the axis of the south rift zone and flowed to the east. It is *possible* that the two anomalies could be linked along the area between them (making it all one flow) - there were some discontinuous anomalies in there, but the data must be reexamined and looked at more closely. It is also possible that thinner lava flows extend further north and/or south of these two anomalies along the rift zone.
Another conclusion that can be made at this point is that no *major* (>5-10 m) subsidence of the floor of Axial caldera occurred during the earthquake swarm (a broad depth change in the caldera is not seen). However, there still could have been movement along the caldera faults and/or deflation leading to "single-digit" subsidence of the caldera floor.