Lassen Swarm
el...@no.spam
el...@no.spam
Thursday, March 19, 2009 19:29 PDT (Friday, March 20, 2009 02:29 UTC)
LASSEN VOLCANIC CENTER VOLCANO (CAVW#1203-08-)
40.49°N 121.51°W, Summit Elevation 10456 ft (3187 m)
Volcano Alert Level: NORMAL
Aviation Color Code: GREEN
Information Statement for the recent March 17-19, 2009, Lassen Peak swarm activity
A series of small earthquake swarms have occurred beneath the
south flank of Lassen Peak in Lassen Volcanic National Park over
the last three days. This activity began at 3:00 AM (PDT) on
March 17 and persisted through 8:16 AM on the 19th. Episodic
swarm activity on the 17th through 4:10 AM on the18th included
some 48 earthquakes large enough to be located, the three largest
of which had magnitudes of M=2.1, 2.0, and 2.2. Activity resumed
at 6:57 AM on the 19th producing 26 earthquakes by 8:19 AM, the
largest of which was a M=1.9. These earthquakes are centered at a
depth of roughly 5 km beneath the surface and, in map view, form
an elongated pattern extending from the vicinity of Eagle Peak on
southwest flank of Lassen Peak to the vicinity of Diamond Peak 3
km further to the south-southwest.
Earthquake swarms are not unusual in this area. This most recent
activity was preceded by a swarm of 12 earthquakes on February
13-14 that included a M=1.8 event. Earlier activity in this same
area over the last five years includes swarms on February 15-16,
2005 (14 events including a M=2.3 earthquake), February 21 2006
(13 events with a M=2.2 earthquake), January 18-19, 2007 (29
events with a M=1.9 earthquake), and September 7-8, 2008 (41
events with a M=2.3 earthquake). This swarm activity over the
past 3 days appears to be within the norm of earthquake activity
detected in the Lassen area since the modern seismic network
was installed in 1980. The USGS will continue to monitor the
situation closely.
All other volcanoes in the Cascade Range are at normal levels of
background seismicity. These include Mount Baker, Glacier Peak,
Mount Rainier, Mount St. Helens, and Mount Adams in Washington
State; Mount Hood, Mount Jefferson, Three Sisters, Newberry
Volcano, and Crater Lake, in Oregon; and Medicine Lake volcano
and Mount Shasta in northern California.
The U.S. Geological Survey and University of Washington continue
to monitor these volcanoes closely and will issue additional
updates and changes in alert level as warranted.