MLOC S-P Data

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.

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