By Dave Tucker
October 22, 2010
The seismometer on the east flank of Mount Baker is still not able to send seismic data. The instrument, near the ski area and code-named SHUK (for Shuksan Arm) was apparently damaged by lightning in early September. The instrument was replaced a couple of weeks ago by Pacific Northwest Seismic Network personnel. According to PNSN seismologist Karl Hagel, the ongoing problem is now the GPS antenna. He tells us that neither Cascade Volcano Observatory nor PNSN have a replacement antenna due to routine failures of the type of GPS antenna being used. A GPS receiver is necessary to provide a satellite time signature to seismic signals received, and then relayed to the PNSN, by the seismometer. The time that a seismometer registers an earthquake is critical for determining the distance from the earthquake when combined with data received by other seismometers. Cascade Volcano Observatory is working on a replacement antenna system. The MBVRC subscription service will post a news item as soon as we know that the unit is back on-line.
SHUK was installed less than a year ago, made possible by an infusion of federal stimulus money for volcano monitoring in 2009 (remember Louisiana governor Bobby Jindal’s outrage at spending federal money on something as frivolous as volcano monitoring?). SHUK is the second seismometer at Baker, joining long-running old reliable MBW over on the west side. Two seismometers are better than one, and hopefully a third will at some time be installed to more accurately pinpoint the location of tiny earthquakes under the volcano caused by renewed intrusion of magma into the edifice.
Data are available to the public by going here for web postings (“webicorders”) of seismic signals from all volcano seismometers in the Cascade arc. Choose the station you are interested in to see the data for a particular 12 hour period. Enter the code MBW (or any seismometer code identifier) in the search box on this page to see data from the instrument on Baker’s west flank (no data is posted for SHUK until the station is back online). There are now 21 seismometers on the US side of the Cascade volcanic arc. In addition to the 2 on Baker, Mount Saint Helens (8) and Three Sisters (6) are densely covered with seismometers; there are 2 at Mount Rainier, but only one on Glacier Peak, Adams, and Hood, and none at Crater Lake.
For information on volcano monitoring methods in general, the Geological Survey of Canada has a nice synopsis here. However, there are no seismometers on the flanks of any of the several potentially active volcanoes in Canada.
Understanding data in the ‘latest earthquakes’ table for Mount Baker
Some of the following is reprinted from an update posted to the MBVRC main website last winter, repeated for the benefit of new subscribers to our update service:
Several visitors to the MBVRC website have asked about data on the Baker seismology data table to which we have a link from the links page. In particular, people wanted to know the meaning of the code in the column labeled ‘QUAL’. Wes Thelen, at PNSN, explained that The QUAL value stands for “quality” and is assigned to each event depending on how well the event has been located. Wes continues: “The first is a measure of the earthquake signal residuals. These are the errors between the actual arrival time of an earthquake wave and the expected arrival time, based on a model of the Earth’s structure in the area. The second letter is a measure of the station distribution around the earthquake. Of course neither quality measure is purely independent of the other, but it provides a quick check of the overall quality of the location. For research purposes, we usually only consider AA, BB, or BA events to be good. We don’t like to publish things worse than a CC. Unfortunately at Baker, the station configuration means that most events will have a C or D in the second quality metric. Hopefully, the addition of SHUK will improve that.”
Jackie Caplan-Auerbach, the seismologist on the faculty at the geology department at Western Washington University, further explains residuals. “Earthquake residuals are also known as ‘travel time residuals’. This is the difference between the amount of time that a seismic wave is expected to take to reach a given seismic station and the amount of time it was observed to take. To locate an earthquake you take the times at which seismic waves arrived at a number of stations and find the earthquake location and origin time that best fit these data. To do this you have to tell the computer how fast seismic waves move in the area. This is called a velocity model, and is generally a rough estimate of the true wave speeds in the region. Heterogeneities in velocity structure are typically not built into the program. So if a seismic wave moves through a region with a velocity different from that of the model the expected travel time will be different from the observed time and voila, a ‘travel time residual’ is produced. Residuals may also result from other issues with earthquake location. Not all location algorithms account for topography, for example, so this introduces uncertainty.
Of the 20 most recent earthquakes recorded at Baker (as of Tuesday, October 12, 2010, the most latest update and the date of the most recent microquake), a number of locations have ‘C’ quality or better in the second metric; this is in part likely due to the improved monitoring capacity since SHUK was established. Only two, however, are rated at ‘B’ and none at ‘A’. However, the ‘network’ at Baker has not done so well locating very small earthquakes that do not occur inside the area defined by the 2-station ‘net’. See the mapped locations here (scroll down); note that there is no black triangle locating SHUK on that map. It would be about the same distance from the summit as MBW, but to the NE. With time, we should see improvement in locations when SHUK comes back on board, and certainly if a third seismometer can be installed. As a contrast, visit the Mount Saint Helens earthquake location map, (note that there are also really cool cross section interpretations available there, too).