Mt. Fitzherbert
USGS Weekly Update 2008-May-30 10:36
RECENT OBSERVATIONS:
Today (May 30): Field crews are at the volcano to make visual observations, to repair time-lapse cameras and retrieve images, and to service other monitoring instruments (seismometers, GPS stations, tiltmeters, acoustic flow monitors). Visual inspection confirms the earlier inference from telemetered monitoring data that the pause in lava extrusion since late January 2008 is continuing. Observers report that the east and west arms of Crater Glacier have come together north of the 1980-1986 lava dome.
In the past week: Critical monitoring tools (time-lapse camera images, tiltmeters, seismometers) indicate no lava extrusion. Time-lapse camera images show no growth and only minor sagging of the lava dome through May 29. The weather rebuffed efforts at field work earlier this week, although needed field repairs are being undertaken today (Friday, May 30).
In the past month: The pause continues, judging from the lack of evidence for renewed lava dome extrusion. Time-lapse camera images, precisely registered to monitor topographic changes, show a static dome that sags ever so slightly as it cools, a condition that includes the time period through May 29. Seismicity from local sources has been nil, aside from sparse microearthquakes (magnitudes less than 2) and sporadically heightened glacier-related earthquakes that spark up whenever a few days of warm weather enhances glacial flow. The glaciogenic earthquakes are tracked mostly on station VALT, near the east and west snouts of Crater Glacier, although a few appear on stations SEP and SUG. As evidence of nearly quiescent seismicity, the Pacific Northwest Seismic Network catalog for May shows only 19 earthquakes larger than magnitude 0. The largest were M 1.0 and two of M 1.1, which occurred May 24, 26, and 28, respectively. Borehole tiltmeters near the dome and in the crater breach shared the same broad patterns of ground response, another sign that no untoward deformation is affecting sites near the recently active lava dome. The GPS station at Johnston Ridge showed a small acceleration in April, now thought a response to snow loading the antenna.
In the four months since the pause began in late January 2008: Instrumentally, the lava dome has been moribund. A GPS spider on the recently active spine has moved south 0.5 m and down about 0.15 m since February 24. We interpret this motion as gentle relaxation as the spine cools. A gas monitoring flight in mid-March reported sulfur dioxide in the range 10-20 tons per day, nearly the lower limit of what can be measured confidently. For comparison, the longest pause in lava extrusion during the previous dome building episode (December 1980 to October 1986) was nearly a year long, from May 1985 to May 1986. That pause was followed by small extrusions in May 1986 and October 1986, when the 1980-1986 dome-building episode ended.