Retrieving
Data from the Global Seismic Network
To
Use in SWAP or WinQuake
The
SPYDER system, part of the
http://www.iris.washington.edu/SPYDER/spyderdata/data/catalog.html.
I
suggest you bookmark this website, or go to the IRIS website at http://www.iris.edu/. At this site,
click onto DMS, and then choose WILBER from the menu on the left side of the
page. WILBER provides a place where seismograms from many sites around the
globe can be viewed. By following the prompts at that site, it is possible
to pick stations to download data from. Unfortunately, at this time SWAP
does not have the capability to read data directly from that site.
There
are several advantages of GSN data: 1) it provides information on all 3 components
of ground motion: Vertical, North-South, and East-West. This can be used to
identify different types of seismic waves and to determine direction from
wave motion, 2) the SPYDER data is generally very high quality; the stations
are well calibrated, well maintained, and seldom have timing or other major
instrumentation problems, 3) the sites are widely distributed around the world,
providing data at a wide variety of distances and directions from any earthquake,
and 4) the data are often pre-filtered into long-period, broad-band, and short-period
components.
From
the SPYDER website, the process for downloading data is somewhat like getting
data from PEPP. The first step is to select the event you want to download.
There is quite an extensive database to select from. Once you decide on an
event, click on the date-time label for that event.
A
map of the world appears, showing the location of the event and the surface
path from the epicenter to the recording stations. There are over 130 possible
stations that are part of this network: (http://www.iris.washington.edu/pub/spyder/station.list). The greatest disadvantage of this system
is that there is no easy way to preview seismograms before downloading. You
can preview some data by clicking on the links 'Broad-band' or 'Long -period'
in the sentence, "Show CORAL record section (Based on SEED volume) of
Broad-band data or Long-period data."
The
stations are listed in order of distance from the event. It is possible to
download the bh (Broadband) channel or the lh
(Long Period) channel. The bh
is a smaller file, much like a triggered file in QUAKES, containing very detailed
information about the initial part of the seismogram. The lh file is like the event file in
QUAKES, but it contains 20 samples/second, like the files in SCREAM. The lh file generally provides data on
the entire earthquake, and is most likely what you will want.
Selecting
"z" will get the Vertical component, "e" the East-West
component, and "n" the North-South component. You may only download
one component at a time.
Once
you have decided on which station(s) to get data from, select a component
to download. (If you plan to get all three components, it is best to download
them in order: vertical first, north-south second, then east-west.) You
will get a 'save dialog' box, where you can rename your file. Simply
press Return, and the file will carry a name that SWAP can identify.
Save the file to a pre selected folder for that event, to avoid confusion.
Repeat this process until you have all the files you want to download on your
computer.
Putting
the files into SWAP
There
are probably two types of file sets you will make. One set is of single seismograms
from an event from different locations, which can be used to show the time
difference in arrival of different seismic wave phases. To do this, it is
best to load the closer events into SWAP first, then the later ones.
When you choose the Display menu in SWAP, choose Absolute Time,
and the seismograms will be ordered on the screen by arrival time. You can
then go through the process of selecting P-wave arrivals and use the Locator
Window to triangulate the quake location as an exercise.
Once
you have loaded the set you want into SWAP, you can save the whole set by
using the "Save File Set As" choice in the File menu of SWAP.
It can then be put on the desktop, floppy, etc