Most arrival time datasets contain readings that cannot be usefully employed in a relocation analysis, for a variety of reasons:
- The pick was made on a noise signal rather than a true phase arrival.
- The pick was made on a phase arrival from a different event.
- The station coordinates are grossly wrong.
- The phase was mis-identified.
- The phase name is not one for which mloc can calculate a theoretical arrival time.
- Coordinates cannot be obtained for the reported station code.
The first four cases result in a reading having a large residual against the theoretical arrival time (the others are dealt with elsewhere). All earthquake location codes make some provision to deal with gross outliers. This can be extremely simple, e.g., any reading with a residual greater than N seconds is dropped. mloc employs a more complex algorithm to deal with gross outliers, driven by the wind command. Readings that fail the windowing criteria are dropped from the current run of mloc (listed in the “BAD DATA” section of the ~.phase_data output file with “PRES” in the column “Why Bad”), but under some circumstances they can fall back into a subsequent run as gross outliers unless they are flagged. Those readings are also listed in the ~.xdat output file.
The user seldom needs to look at the ~.xdat file. The only question is whether or not it is appropriate to run xdat after a given run of mloc. It is usually desirable to run xdat at least once early in the analysis, as there will normally be a large number of entries in the ~.xdat file, but in later runs there will be few entries, and often the file will be empty.
The format of the ~.xdat file is kept simple, as it is only meant as input for xdat, i.e., it really only needs to carry the file name and line number that will be flagged. The format is documented in the code module mlocout_phase_data.f90. Here is an example:
7 COP ISC XS 71 x 19670517.0428.50.mnf 15 PYA ISC Sn 15 x 19761128.1113.23.mnf 39 KER ISC Pg 28 x 19990219.1800.10.mnf 41 MLTT ISC Sn 35 x 20020228.0046.31.mnf 41 ZALF ISC Sn 50 x 20020228.0046.31.mnf 41 SALA ISC Sn 64 x 20020228.0046.31.mnf 43 ASAO IIEES Sn 11 x 20030811.2012.06.mnf 55 VRNZ ISC Pn 36 x 20060729.0151.11.mnf 60 BHD ISC Sg 89 x 20090310.1313.02.mnf 61 IBDR ISC Sn 179 x 20090705.2348.25.mnf 66 HOPEN ISC IVmB_BB 1977 x 20111023.1041.23.mnf 66 MTDJ ISC Pdif 3943 x 20111023.1041.23.mnf 74 COAL ISC P 312 x 20111029.2224.24.mnf 75 RAYN ISC Sn 436 x 20111106.0243.14.mnf 103 KIEV ISC 87 x 20200318.2307.22.mnf
The phase name listed here is the one that was read in from the event file, not the result of any phase re-identification done by mloc. In the example the reading from station HOPEN from event 66 should have been caught by mloc’s algorithm to identify readings that are not legitimate phase arrivals (and flagged with “p”), but in this case an amplitude measurement slipped through.