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SOSA Tutorial | |
| Synthetic and Observed Seismogram Analysis | ||
| Version 1.1 | ||
| Joanna Muench | ||
This tutorial is designed to provide a basic introduction to the capabilities of SOSA. The tutorial will walk you through retrieving seismograms from a local file and through a DHI, viewing the seismograms, applying some simple analyses and then saving your results. Further information on how to use SOSA can be found in the SOSA User's guide.
If you haven't already installed SOSA, you can download it from the IRIS website. InstallAnywhere will automatically install the correct software on your machine. To run SOSA, double click on the icon. On Unix-based machines it is possible to run SOSA from a command line. Just go to the installation directory and type "sh sosa".
SOSA is configured by default to access the IRIS Data Center. For this tutorial we are going to instead use the Southern California Earthquake Data Center to access event, network and waveform information. To change from the default DHI configuration, open the Configuration window by selecting Edit/Configure from the menu bar. Remove the IRIS servers by either selecting each one and clicking on "Remove" or clicking "Remove All" from each type of server.
To add the SCEDC Event server, click on the "Add" button below "Event Servers" and select "SCEDC_EventDC_new (edu/caltech/gps/k2)" from the list in the popup windows and click "Add". That server should now appear in the main Event Server window. From the Network Servers and Waveform Servers select "SCEDC_NetworkDC (edu/caltech/gps/k2)" and "SCEDC_DataCenter (edu/caltech/gps/k2)" respectively. The window should now look like the below figure:
Click on the "Save" button and proceed to the next section. For further information, see the DHI section of SOSA User's guide.
In this section we'll walk through the steps necessary to read in a local seismogram stored in SAC format and retrieve an observed seismogram remotely from the IRIS Data Center through a DHI. Both seismograms are from the Yorba Linda event, Sept. 3, 2002.
To load a seismogram from a local file, select the File/Load Local option off the menu bar. Using the resulting finder window, navigate to the data directory of your SOSA installation. This directory contains several example seismograms, including "Synth_PAS__Z_20020903T070851_000Z.sac", a simulation of the Yorba Linda event at the Pasadena seismic station. Select this file. The finder window should look like this image:
Clicking the "Open" button should bring up the seismogram in the Display Panel. See loading a seismogram from file for further information.
To retrieve an observed seismogram from a data center, return to the "Request" tab. DHI should be selected in the "FROM:" list. In the Event Search panel, enter a start date of 9/1/2002 and an end date of 9/30/2002, and search for earthquakes of magnitude 4.0 to 10.0. Press the "Search DHI" button. SOSA will send your request to the SCEDC Event Data Center. A popup window will tell you about how the search is proceeding, and if there are any problems. After a few seconds you should end up with something similar to the figure below:
The event of interest, Yorba Linda on 9/3/2002, is already selected. Press the "Get Stations" button to retrieve stations associated with this event. This opens up the lower part of the display. The location panel has already been centered around the location of the selected event.
We want to search for stations within a 3 degree radius of the event. To get data from within a certain radius, select the "Radius Area" radio button. The location panel has already been centered around the location of the event. Leave the Inner radius at 0 degrees, make the outer radius 3 degrees.
In addition, we only want to look for high frequency vertical channels. In many data centers it is possible to constrain the search based on orientation, however the SCEDC does not have that option configured yet. Select "All" in the orientation drop down and leave the sampling range and time interval to the defaults. Under "SPECIFICATION", enter "BHZ" in the Channel window. This will restrict searches to channels that exactly match this string.
Clicking the "Search" button will start the search, first querying the SCEDC Network Data Center for channels that match the search criteria, and then to the SCEDC Waveform Data Center to find channels that have data within the time period of interest. After scrolling down 2/3 of the list, the resulting search should look like the following image.
Selecting the seismogram with station PAS, and clicking "Display Seismograms" will cause SOSA to retrieve that seismogram along with its associated instrument response. The seismogram will be added to the display panel with the previously loaded sythetic seismogram.
At this point you should be viewing the "Display" panel containing both a synthetic and an observed seismogram, both for the Yorba Linda event.
The SCEDC provides a full hour of data, while the synthetic is less than a minute. You can show more detail in the observed seismogram by dragging around the signal of interest. This will zoom the display, as shown below:
Use the slider to bring the section of interest into the window; note that at high zoom factors this display can be very slow.
Once you've selected the interesting portion of the observed seismogram, press the "Process All" button in the upper right corner of the Display panel. This will take both the seismograms and copy them into the Process panel.
The process panel provides many tools for analyzing seismograms. In this example, we will begin by applying the same time window with the same sampling frequency over both seismograms. Select the "Resample" radio button and leave the parameters at the defaults. The panel should now look like this image:
Click the "Apply Process" button, leaving "Display results in new tab" selected. This will cause another Process tab, "Process1", to open. The new tab will contain the results of the previous processing step. That tab, with a new process step selected, is shown below.
In the next process we will convolve the instrument response from the second seismogram with the signal in the first. Selecting the "Convolve" radio button, leave "Use Instrument Response" as true, and click on the "Apply Process" button to bring up the following panel:
In this panel we've already entered in the paramters for the next process, a Butterworth filter. Enter the parameters as shown above and click "Apply Process". In the next panel, choose the "Overlay" process and again click on "Apply Process". A new panel (Process4) should appear looking similar to the below image.
For more information on using SOSA, please refer to the associated User's Guide.