WinQuake can calculate the magnitude of an event using several different methods:
The program calculates the ML, or the Richter magnitude, using the distance between the station and the event, the min/max data points, a correction factor for the seismometer, and a lookup table. The lookup table, an ASCII file called MAGTBL.DAT that must be located in the same directory as the WinQuake exe file, is used to correct the magnitude number based on the distance. I have extended this table out to the maximum P and S wave distance calculation of ~12,000km. The table above 600km is purely experimental. I used several teleseismic events received on my long period Lehman sensor and plotted out the correction factor needed to extend the table. I have received many teleseismic events since extending the table, and so far, most have been within +-0.3 ML. The table from 0 to 600km was copied from C. F. Richter's "Elementary Seismology" book.
Ms, or surface wave magnitude, is similar to the ML calculation except a formula is used in place of the look up table. The following formula is used:
Ms = log_10(amp/T)+1.66*log_10(dist)+3.3
Where T is the period of the wave used in seconds (default = 20 seconds). Amplitude is in micrometers (this is derived from the min/max A/D counts and the magnitude correction factor) and distance in degrees (1 degree = ~111.1km).
A magnitude marker can be used to override the min/max number used for ML or Ms calculation. When this marker is turn on, using the Magnitude Marker (ML & MS) item under the Calculate menu, the program will use the height of the marker to calculate the magnitude.
WinQuake can also calculate the magnitude using the Md, sometimes called coda or duration, method. This method uses the length of the event rather then the maximum amplitude. This type of magnitude calculation only works for local events.
To calculate the Md of a local event first pick the P and S waves, using the P and S markers, and then press the Md toolbar icon or select the Md Marker item under the Calculate menu item. The program will then place a "D" marker on the screen at the coda location. This is where the waves from the event go back into the background noise. You may need to fine-tune the coda location by moving the "D" marker.
The magnitude correction factor number used by the ML, Ms and Mb calculation is used to compensate for different seismometer sensitivities. The correction number is located in the header information of PSN event files, or located in a file called MAGCORR.DAT. For SAC, PEPP or SEED volume files the stations ID can be used to supply a correction factor by using any text editor and adding the information to the MAGCORR.DAT file.
For EMON (one of two data logging programs used by PSN stations) users, this number can be set in the EMON.OPT file for EMON Version 6 and up. WinSDR or SDR (the other programs used for data logging by PSN stations) users can set this number in the channels settings dialog box or screen.
The magnitude correction factor for a seismometer is determined by using an event with a known magnitude. After picking the P and S waves, use the Sensor Information dialog box to enter a correction number so that the calculated magnitude matches the reported event magnitude.
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