A small seismic array was deployed at Axial during the summer of 1994. The deployment followed an SOSUS-detected earthquake swarm near the ASHES vent site on the eastern caldera wall. 2 months of continuous seismic recording detected and located 402 events. The tight clustering of events is due in part to the limited extent of the array. Even so, the aperture is wide enough to detect two locusts of activity and the relative seismic quiessence east of the array. Several spatial and temporal patterns are observed in the hypocenter data. details...
The hypocenter data and the velocity structure show significant correlation. The most striking pattern is that much of the seismicity clusters under the eastern wall of the caldera precisely on top of a high velocity "finger" reaching nearly to the surface. In 3-D this finger is actually part of a cylindrical curtain of high velocities which encircles the upper portion of the caldera magma body.
Hypocenters near the low velocity magma body are likely the result of magma movement out of the storage area and small collapses to accomodate the event. While the focused activity under the vent site is not terribly surprising, it corresponds remarkably well with the high velocity curtain and the edge of the caldera. Any interpretation must explain both the seismic activity and the high velocities. A few posibilities include:
1) Vertical diking associated with the event opened fluid pathways. During numerous such events under the caldera walls, seawater has cooled and harded the rock and filled the resulting pore spaces through alteration. There are several flaws with a hydrothermal explanation however. It fails to explain the deep er portions of the high velocity "finger" and ignores the fact that repeated eruption through these pathways would likely heat the column at least as fast as it could be cooled.2) Seismicity around the ring is easy explained by appealing to caldera collapse events. In essence, this region would be the fault system responsible for lowering the caldera. While is explains the seismicity, this also fails to address the high velocities.
3) A more plausible explanation combines elements of these two. The caldera rim fault systems provided preferential pathways to the surface. Diking and strain accomodation are evidenced by the hypocenters. These processes create a region of intrusive magmatism high in the crust where surface processes usually work to keep rock porous and inhomogeneous. Repeated injections have created a region of high velocity material despite the active magmatism.
Post script files
SW-NE transect: cross-section,
locator map
NW-SE transect: cross-section,
locator map