The wech-o-meter takes over all of Cascadia
March 30, 2012
by Steve Malone
Developed by Aaron Wech as part of his PhD dissertation the Interactive Tremor Map is the web interface to a rather complicated tremor detection and location process that runs once a day on waveforms archived at the PNSN over the previous day. The Cascadia margin is now divided into 9 overlapping sub regions. In each an algorithm is run generating waveform envelope traces and then cross-correlating these to determine time lags to establish relative arrivals for inverting for common sources and then combining all sources to look for continuity......blah blah blah. If really interested in the details see the paper published by Wech in Seismological Research Letters in 2010. A couple of months ago John wrote a new kind of earthquake blog that is quite interesting with references on more and better explanations of this strange stuff.
What is of current interest is, with the significant help from other networks in the region, this system now is monitoring all of the Cascadia subduction zone. Besides data from the PNSN (41 stations) we now are receiving a selection of stations on and near Vancouver Island from the Canadian Geological Survey (15 stations) and from the Northern California Seismic Network run by the U.S. Geological Survey (19 stations) and the University of California at Berkeley ( 4 stations). We add in data obtained from EarthScope's Transportable USArray (13 stations) and Plate Boundary Observatory (24 stations) provided by the IRIS Data Management Center for a total of 116 stations. The web interface to this plots up tremor epicenters by day but has a nice exploratory interface allowing one to look at past tremor, tremor rates in different regions, envelope functions and station participation maps. WOW! This is enough to keep the wildest tremor geek happy for weeks. One additional change in the current tremor catalog is a recent improvement in the tremor detection algorithm that has been re-run on all old data resulting in about 10% more tremor locations than the previous catalog.
While we don't really understand the fundamental source for this deep tremor the wech-o-meter has already allowed us to establish many interesting patterns including the close association with slow slip, migration patterns, variable sizes and timing of events and much more. These data when combined with other studies such as the array processing done on the Array of Arrays data may help unlock additional secrets of this strange phenomenon.
By the way, we are cleaning up and migrating our previous tremor monitoring logs to the new web pages and preparing ourselves to do an even better job of tracking the next ETS when it hits our area; probably in fall, 2012.