St Helens' loud quiescence

[igorsboss]

Watching the washington state webicorders, I've noticed something that looks a little strange happening recently.<br /><br />When I compare webicorder records for Oct 4th to those from Oct 8, and I pay special attention to the times in-between quakes, the webicorder line looks a lot more wavy now than it did then.<br /><br />On Oct 4, when a webicorder was quiescent, that is, when it was recording minimal activity, the trace looked like a straight line.<br /><br />On Oct 8, when a webicorder was quiescent, the trace appeared to have a consistent low-amplitude and ultra low-frequency background wave.<br /><br />What's going on here???? Have the webicorders changed their recording resolution somehow? Is this a real rumble?<br /><br />I observed this on the following webicorders: <br />LON EHZ UW : Longmire, Mount Rainier, <br />http://www.pnsn.org/WEBICORDER/GREEN/LON_EHZ_UW.2004100400.html versus<br />http://www.pnsn.org/WEBICORDER/GREEN/LON_EHZ_UW.2004100812.html<br /><br />JCW EHZ UW : Jim Creek, West Cascades, comparing<br />http://www.pnsn.org/WEBICORDER/GREEN/JCW_EHZ_UW.2004100400.html versus<br />http://www.pnsn.org/WEBICORDER/GREEN/JCW_EHZ_UW.2004100812.html<br /><br />and also <br />STW EHZ UW : Striped Peak, Olympics <br />HTW EHZ UW : Haystack Peak, West Cascades <br />GNW EHZ UW : Green Mt, Kitsap<br /><br />See also: http://www.geophys.washington.edu/SEIS/PNSN/WEBICORDER/GREEN/welcome.html

 

[silylene old]

Igor: good observation. I dno't know why.<br /><br />Mt. St. Helens has picked up in actvitiy a lot in the last few hours. It seems the quiescent phase may be ending again. <div class="Discussion_UserSignature"> <div class="Discussion_UserSignature" align="center"><em><font color="#0000ff">- - - - - - - - - - - - - - - - - - - - - -</font></em> </div><div class="Discussion_UserSignature" align="center"><font color="#0000ff"><em>I really, really, really miss the "first unread post" function.</em></font> </div> </div>

 

[nexium]

I presume the webcorders were mounted on well anchored tripods. Perhaps the recent 4 days of minor shocks has damaged the anchoring, so now the webcams wobble even in a gentle breeze. Neil

 

[nexium]

My guess is bed rock is rarely available on the rim of Mount St. Helens, so siezmonitors are anchored to the largest peices of solid rock available so as to minimise vibration that occur within 100 feet of the seizmonitor but not generally at other locations. It is possible that something important is happening within 100 feet of the web cam which it is recording. Neil

 

[igorsboss]

Longmire is about 40 miles away from St Helens.<br /><br />Jim Creek West is over 100 miles away from St. Helens.<br /><br />That's too far away for the seismometers to be damaged from St. Helens.<br /><br />That also means that this low frequency low amplitude rumble, if it is real, is being detected all over Washington's Cascades.<br /><br />Hence my concern...

 

[igorsboss]

The waves I mentioned have diminished greatly today.<br />I think I understand what was going on. IMO:<br /><br />The waves I was seeing were the 'S' waves from the St. Helens earthquakes superimposing themselves on one another.<br /><br />Here's why I think this:<br /><br />The LON (Longmire) webicorder record for Oct 9, 2004, between 21:10 and 22:10, records the P waves and the S waves from the 6.9 earthquake in central america. The S waves from that quake has a waveform that looks similar to the wavy quiescent line I observed earlier. Also, the S waves tend to last for a longer duration than the P-wave burst.<br /><br />Now, if the S waves from each of the St. Helens earthquakes typically have a duration of 'S'-wave shaking that is significantly longer than the period between the quakes, the S waves from one quake will superimpose on the S-waves on the next few quakes.<br /><br />Given the frequent and so evenly spaced quakes, the S-waves from each quake acted as a kind of forcing function, keeping the whole northwest ringing with S waves.<br /><br />An analogy... It is like a drummer doing a light drum roll on a cymbal. After a while, you just get a constant amplitude ringing. When the drummer stops, it takes a while for the cymbal to stop ringing.

 

[silylene old]

<i>Eruptions from lava domes generally are preceded by a change from high-frequency quakes -- the ones St. Helens is now experiencing that usually connote rock breaking beneath the surface -- to low-frequency events, such as undulating tremors.<br /><br />Moran said such changes haven't been seen yet.<br /></i><br /><br />Interesting discussion in the Seattle Post today:<br /><br /><b>St. Helens hot under the collar<br />Spreading bulge signals that molten rock is nearing surface</b><br /><br />By LEWIS KAMB<br />SEATTLE POST-INTELLIGENCER REPORTER<br /><br />VANCOUVER, Wash. -- A massive loaf of rock bulging from Mount St. Helens' crater is spreading rapidly and growing piping hot -- signs that upchurning molten rock is nearing the volcano's surface, scientists say.<br /><br />Thermal imaging of the ever-growing bulge on the south side of the volcano's lava dome for the first time has shown "significantly heated areas" ranging from 200 to 300 degrees Celsius -- about 400 to 570 degrees Fahrenheit, U.S. Geological Survey scientists said yesterday.<br /><br />The rising temperatures, along with the bulge's persistent growth since St. Helens' reawakening late last month -- it now stands up to 400 feet high and measures 1,300 feet by 1,600 feet -- may indicate "magma is getting very close to the surface," said Willie Scott, a USGS geologist.<br /><br />"It just seems to be a matter of time before we see some sort of extrusion on the surface, or some sort of explosion," he added.<br /><br />Magma likely has risen to a point within half a mile of the crater's floor, perhaps even as close as 100 meters, Scott said.<br /><br />And recent readings taken during flyovers of the crater detected high concentrations of carbon dioxide and hydrogen sulfide -- volcanic gases contained within magma that, depending on amounts, can cause volatile eruptions.<br /><br />Still, Scott said, scientists can't predict exactly how explosive a potential eruption would be, or when it might occur. Scientists have said v <div class="Discussion_UserSignature"> <div class="Discussion_UserSignature" align="center"><em><font color="#0000ff">- - - - - - - - - - - - - - - - - - - - - -</font></em> </div><div class="Discussion_UserSignature" align="center"><font color="#0000ff"><em>I really, really, really miss the "first unread post" function.</em></font> </div> </div>