S-P Data
Development of the facility for handling S-P data in mloc was motivated by the availability of picks from a widely distributed network of strong motion instruments in Iran, the ISMN. Because there are so many stations, distributed in the most earthquake-prone regions, ISMN stations are often the closest station to an event of interest, and therefore play a major role in depth constraint. Unfortunately most of the ISMN stations do not have calibrated timing so the absolute arrival times of P and S arrivals (which are often clearly recorded) cannot be used in the location analysis. The differential phase S-P still retains information on the distance of the event from the station, however, so readings at very close stations do provide some constraint on focal depth.
S-P data are carried in the standard event file used by mloc. Because there are normally few such readings they are entered by hand. Here is an example, the first few records for an event from the Bojnurd cluster in Iran:
F MNF v1.3 E Iran-Turkmenistan border region 7391378 H = 2004/08/21 03 32 40.98 37.8979 57.6360 10.0 ISC 7439524 M 4.7 mb ISC 7439524 M 4.1 MS ISC 7439524 P QUV Pg 2004 8 21 3 33 2.57 -2 0.4 Pg . .QUV . .B Z RGhods P NJV Pn 2004 8 21 3 33 21.37 -2 1.9 Pn . .NJV . .B Z RGhods P x KHV Pn 2004 8 21 3 33 42.82 -2 1.5 Pn . .KHV . .B Z RGhods P BJV Pg 2004 8 21 3 32 48.44 -2 -0.4 Pg . .BJV . .B Z RGhods P BJV Sg 2004 8 21 3 32 54.86 -2 0.0 Sg . .BJV . .B E RGhods P BRV Pn 2004 8 21 3 34 1.91 -2 1.4 Pn . .BRV . .B Z RGhods P A683 S-P 2004 8 21 0 0 3.38 -2 -0.1 S-P . .A683 . .G N RGhods P A675 S-P 2004 8 21 0 0 4.99 -2 0.3 S-P . .A675 . .G N RGhods P A681 S-P 2004 8 21 0 0 5.55 -2 0.0 S-P . .A681 . .G N RGhods P A682 S-P 2004 8 21 0 0 6.19 -2 0.0 S-P . .A682 . .G N RGhods P A668 S-P 2004 8 21 0 0 8.08 -2 0.3 S-P . .A668 . .G Z RGhods
The first six phase records are from standard seismograph stations with calibrated timing. The next five readings are S-P readings from ISMN stations, all read by Reza Ghods. Notice that the “arrival time” for the S-P readings does carry the year, month, and day, but not hour or minute. The S-P time itself is entered in the seconds field of the arrival time.
If only the S-P time is available the format used above is fine. If the “parent” P and S times are available, however, I recommend a more complete record, in which the P and S arrivals are logged but given the “t” flag to indicate a timing problem:
P B523 S-P 2006 6 3 0 0 3.16 -2 S-P BHRC .ISMN .B523 . . RGhods P t B523 Pg 2006 6 3 7 15 53.38 -2 Pg . .B523 . .G Z ARGhods P t B523 Sg 2006 6 3 7 15 56.54 -2 Sg . .B523 . .G Z ARGhods
Output
If any S-P data are used in direct calibration, mloc keeps a record of the associated stations in the ~.stn file.
One of the options of the pltt command (plot index 9) creates a plot of S-P times as a function of distance.