We've been monitoring seismic activity since July 24 with an AS1 seismometer loaned to us by IRIS and the USGS Albuquerque Seismic Laboratory .
The Exploratorium is located in a poor location for seismic monitoring, as it is far from bedrock. This location, combined with the noise of city traffic, building ventillation system, exhibits, and foot traffic, create a very high level of background vibrations. At least one earthquake, which occurred at 5:09 PM local time, was large enough to ve visible above this backgound noise.
I've been watching the
USGS
listing of global earthquakes
and noted that a magnitude 6.8
event occurred at 01:09 UT on August 3, 2000.
00/08/03 01:09:38 12.09S 166.38E 33.0 6.8Mw B SANTA CRUZ ISLANDS
I then used the USGS travel time calculator page to find when the waves from this event arrived at the Exploratorium. On this page I entered the latitude and longitude of the Exploratorium that I had earlier determined from the Microsoft Terraserver web site . Instructions for using the site to find your coordinates are located here .
From the calculator page I found that this earthquake was located 82.9 degrees from the Exploratorium and that the P wave arrived here at 1:21:59 UT. I used the AmaSeis program that has been saving data on PC disk from the AS1 seismometer since July 24 to select about 45 minutes of data starting at 01:15:00 UT. I saved this interval of time in a file on a floppy disk, using SAC binary format. SAC binary format is one of the formats that the program WinQuake can read. WinQuake is a PC program for displaying and analyzing seismograms. It is available from Larry Cochrane's Public Seismic Network site . This program may be used free for one month and full registration costs $30.
Here's what the seismogram looks like in WinQuake. I've band-pass filtered the data between 0.01 Hz and 0.4 Hz to eliminate some of the background noise at frequencies lower than 0.01 Hz and higher than 0.4 Hz. Note that the Rayleigh waves (LR) start at about 47:30. Although the S phase is marked, I don't really see a recognizable phase at this time.