Seismicity - Axial Seamount Volcanic Event, January, 1998

The initial epicenters locate on the summit of Axial Seamount on the central Juan de Fuca Ridge near 45 55'N and 130 00'W. After about 10 hours of activity, migration down the southern rift zone was observed and continues. A gap in epicenter locations between 45 48'N and 45 52'N is not readily explained.

Three individual earthquakes from the summit caldera were of sufficiently large magnitudes (4.5-4.7) to be recorded by land-based seismometers in the Pacific Northwest. Their mechanisms indicate high-angle normal faulting and a northwest strike consistent with movement along the faults bounding the summit caldera.

The onset of activity and subsequent migration of epicenters is characteristic of shield volcano eruptions and earlier seafloor spreading episodes detected by SOSUS. The initiation of high levels of seismic activity without the presence of a large seismic main shock is typical of volcanic activity. Also, the relatively slow migration of epicenters down rift is very characteristic of lateral magma dike injection.

Land Seismometer Details

Magnitude 4.5 Earthquake - Source Time: 01:07:33Z, 98/1/26
Magnitude 4.5 Earthquake - Source Time: 19:28:01Z, 98/1/26
Magnitude 4.7 Earthquake - Source Time: 23:40:34Z, 98/1/27

The high-angle, normal faulting focal mechanisms with northwest strike are consistent with movement along the faults bounding the summit caldera. This faulting is likely associated with readjustment of the caldera in response to the removal of magma from beneath the summit. All these events occur after activity begins in the southern flank, which also tends to support this interpretation. The hypocentral depths are considerably deeper than would be expected for caldera collapse, however.

Histogram. Time histogram showing number of events per hour. A gap is left between each Julian Day. Major activity begins at 1200Z on 1/25/98 (JD 25), although there are some events at 0300-0400Z. Intensity of the initial activity exceeds that of either the 1993 event at CoAxial Segment or the 1996 event at the northern Gorda Ridge.

Latest Seismicity.

Time-dependent epicenters. Epicenter map showing the time dependence of the activity. Activity begins at the summit, propagates down the flanks and concentrates at 45 30'N, 130 06'W. Activity midway down the flank stops after the initial propagation.

Time vs. latitude. Time versus latitude plot showing the migration of epicenters into the southern flanks of Axial Seamount. Migration rate is similar that observed at both CoAxial and Gorda Ridge sites. Activity appears to be concentrating at 45.5+ degrees with continued activity at the summit. Focussed activity at 45.725 on the first half of 1/26 and at 45.64 late on 1/26 may be small eruptive centers.

Spectrogram. Example spectrogram derived from SOSUS of Axial Seamount volcanic seismicity. Image represents about 15 minutes of data from the early stages of the activity. Rapidly repeating, low-level earthquakes without a large main shock is consistent with earlier observations of seafloor magmatic activity.