How WinQuake Calculates Distance



WinQuake maintains three sets of P and S wave travel-time tables. Two are Jeffreys-Bullen (JB) tables, one set for teleseismic (distant events) and the other for regional or local events. The third set of tables use the IASP91 model of the earth. These tables are used to calculate the distance and the time of origin of the event. They are also used to calculate the location of the P and S markers if the event time and location are known.

Travel-Times Tables:

To change the travel-time table information, use the Tables menu item to open the Travel-Time Tables dialog box. This dialog box is used to select which set of tables (JB regional, JB teleseismic or IASP91), the depth, and what JB travel-time files to use for regional or teleseismic calculation.

When changing JB travel-time table files you must select files that are compatible with each other. The teleseismic or regional P and S wave travel-time table files must have the same distance and depth sequence. You will receive an error when you close the dialog box if one of the files is incompatible with the other. Normally you would leave the P and S teleseismic table files at P.jb and S.jb.

The regional JB travel-time tables shipped with WinQuake are Pn, Sn (P_n.jb and S_n.jb files), Pg, Sg and Pb, Sb. I have found that the Pb and Sb (P_b.jb and S_b.jb) files work best for events here in the San Francisco Bay Area.

When you see the "Table Out Of Range" message on the line that shows distance and magnitude in the event window, you are either viewing an event that is to far or to close for the travel-time table type and depth currently selected. An event can be to close for a table if you have a very deep depth selected and you are trying to calculate the distance of a shallow event. If you do not know the depth of an event, you should use the depth at 33km for teleseismic tables and 5km for regional tables (if the normal events for the region are shallow like in the San Francisco Bay Area).

Calculating Distance:

WinQuake calculates the distance between the station and the event using P and S markers. The user places these markers at the arrival times of the two waves. The distance is then calculated using the time difference between the two markers and the currently selected travel-time table and depth.

Learning to pick the P and S wave arrival times will take some practice. One way of learning how to pick the P and S waves is using the P-S Location feature of WinQuake. To activate this feature, us the "LOC" toolbar icon or the Calculate menu item. To use this feature you will need an event file with a known event location and time of origin. When this feature is activated, WinQuake will places the P and S markers at the calculated arrival times using the currently selected travel-time tables and depth.

Additional Information:

The JB travel-time files used by WinQuake are installed by the setup program in a sub-directory called TABLES. This sub-directory is located in the directory where WinQuake is installed in. You can change the location of the tables directory by using the Tables Directory item under the File menu item. The travel-time table files must have a file extension of *.JB. The IASP91 tables are in one large file named IASP91.DAT. This file must be in the same directory as the WinQuake exe file (winqk32.exe) and not located the in the directory used for the JB tables.

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