Proposal Summary

This proposal seeks funds to conduct a piggy-backed active source tomography experiment on Axial Seamount on the Juan de Fuca Ridge. The instruments to be used are 16 Ocean Bottom Seismometers (OBSs) and 4 Ocean Bottom Hydrophones (OBHs) owned by Spahr Webb (Scripps Institution of Oceanography), which are funded to be deployed on Axial Seamount from March to November of 1998, as part of a passive seismic experiment. We propose to use the 20-gun 8300 cubic inch array of the R/V Ewing to shoot to these instruments for 7 days during the 8 month deployment. The goal of this experiment is to understand the relationship between the Axial Volcano magma chamber, volcanism and crustal genesis on the surrounding Juan de Fuca Ridge. This will be done using 3D seismic imaging based on P, PmP and Pn wave traveltime from the shot locations to OBS/OBH receivers. The advantage of using an airgun source is that it provides far superior spatial coverage than relying on earthquakes alone (as well as having a precisely known source location and origin time). This will allow us to map out crustal structure and thickness all around Axial Volcano, as well as the internal structure of the volcano. The tomographic model from the passive experiment would otherwise be limited to areas with good rays coverage from earthquakes. Our experiment will therefore be very complementary to the passive experiment in that it will provide an accurate 3D velocity model which will greatly improve earthquake hypocenter accuracy. The tomographic image of the Axial Volcano should indicate the location and size of the magma chamber, if it exists, as well as constraining whether it sits atop a mush zone, as seen on the East Pacific Rise, or a solid, cool dome of gabbroic cumulates, as seen on Krafla Volcano in Iceland. The regional crustal models surrounding Axial Volcano will put the magma chamber and the volcano in a structural context. We have a unique opportunity to conduct this experiment at unusually low cost, because the engineering and instrument costs are already covered by a funded experiment. The passive experiment was only funded in the last few months, and since it is to be conducted next year, this is the only funding window in which this active source experiment can be conducted at such low cost.