~.tomo Files
~.tomo files are produced by the tomo command. The files are written in the module mlocout_tomo.f90, in a format intended to provide convenient access to the aspects of a relocation study, especially a calibrated relocation, that are of greatest interest to someone conducting research in seismic tomography. Tomographers normally work with large datasets extracted from more-or-less standard bulletins of (biased) single event locations, so the input data are individual travel times of specific phases. The fact that source hypocenters are biased to variable unknown degrees is conventionally ignored.
For a dataset of (possibly) calibrated events relocated as a cluster by mloc, there are several possibilities for the nature of the data set offered for tomography, basically trading quantity of data against quality of data. This is reflected in the three types of output that may be selected in the tomo command:
- Extract all readings of the specified phase
- Extract only readings which were used for the cluster vectors
- Extract empirical path anomalies
The type 1 option is very similar to the typical tomographic dataset, especially for an uncalibrated cluster, although the noise in the data may be somewhat lessened by improved relative locations of the events. The type 2 option sacrifices numbers of data in the interest of better quality control. Readings that have survived the cleaning process and are still contributing to cluster vectors should be quite reliable. Readings that are not multiply-observed may still be good readings but they cannot be validated so they are not included. So this dataset will be a bit more sparse but of higher quality. This is even more true for the type 3 option, which reduces all the readings for a give station-phase to a single estimate, known as the empirical path anomaly.
Each ~.tomo file includes an identification of the phase it contains and the type of output file (1-3). All such files are stored in a subdirectory of the cluster series directory named ~_tomo.
Types 1 and 2 Format
The only difference between type 1 and type 2 tomo files is the selection criteria, i.e., type 2 is more selective and there will be fewer outliers. Here is an example, the first 10 lines from the file wells6.1_P_2.tomo of the Wells, Nevada cluster, with a head row added for clarity:
Delta Res RdErr N ArrTime Event Origin TTO EvLat EvLon H StaLat StaLon Elev Sta Phase Ev# Cal Run 21.384 1.06 0.15 14 20070228 115228.90 20070228 114740.75 288.15 41.143-114.851 12.3 62.493 245.395 0.2 YKA P 1 1 wells6.1 18.039 1.39 0.33 2 20080221 142014.40 20080221 1416 2.04 252.36 41.141-114.869 11.4 34.545 266.423 0.2 MIAR P 3 1 wells6.1 18.724 3.37 1.14 2 20080221 142023.93 20080221 1416 2.04 261.89 41.141-114.869 11.4 31.760 265.339 0.2 NATX P 3 1 wells6.1 18.841 2.36 1.31 2 20080221 142024.18 20080221 1416 2.04 262.14 41.141-114.869 11.4 34.775 267.657 0.1 UALR P 3 1 wells6.1 19.053 1.98 0.74 2 20080221 142026.10 20080221 1416 2.04 264.07 41.141-114.869 11.4 38.636 269.764 0.2 SLM P 3 1 wells6.1 19.092 2.73 1.39 4 20080221 142027.30 20080221 1416 2.04 265.27 41.141-114.869 11.4 37.984 269.574 0.3 FVM P 3 1 wells6.1 19.456 4.08 4.09 2 20080221 142032.66 20080221 1416 2.04 270.62 41.141-114.869 11.4 27.546 262.107 0.0 KVTX P 3 1 wells6.1 19.804 5.25 1.63 2 20080221 142037.53 20080221 1416 2.04 275.49 41.141-114.869 11.4 58.437 229.973 1.0 DLBC P 3 1 wells6.1 21.129 4.06 1.81 3 20080221 142050.80 20080221 1416 2.04 288.77 41.141-114.869 11.4 34.512 270.591 0.1 OXF P 3 1 wells6.1 21.386 1.43 0.15 14 20080221 142050.70 20080221 1416 2.04 288.66 41.141-114.869 11.4 62.493 245.395 0.2 YKA P 3 1 wells6.1 21.665 3.19 0.41 2 20080221 142055.70 20080221 1416 2.04 293.66 41.141-114.869 11.4 36.130 272.170 0.2 WVT P 3 1 wells6.1
Most of the headers are easily understood. TTO is the observed travel time (arrival time – origin time). For a calibrated cluster this will be an empirical true travel time. Calibration state is indicated by the Cal column. A number less than 99 will be the calibration level. A value of 99 indicates an uncalibrated cluster, in which case the observed travel time is understood to be biased to an unknown degree, as has been the case in virtually every global and regional tomography study ever done.
The file is written by looping over the events in the cluster and extracting readings that meet the criteria. This example was for the P phase. In the Wells cluster most of the events are rather small; many of them have few or no P observations. Event 1 has a single observation (YKA). Event 2 has none. Event 3 is the mainshock (5.8 Mw) on Feb 21, 2008, and it has the largest contribution to the set of P data in this file. Many of the entries in this file have a small value for number of samples (N), so in this case there is perhaps not much advantage to asking for a type 2 output rather than type 1.
Type 3 Format
Here are the first 10 lines from the file wells6.1_Pn_3.tomo, from the Wells, Nevada cluster:
1.309 -0.80 0.20 16 99999999 999999.99 99999999 999999.99 24.53 41.173-114.887 10.5 40.719-116.508 1.4 N10A Pn 0 1 wells6.1 1.298 -0.68 0.11 27 99999999 999999.99 99999999 999999.99 24.49 41.166-114.882 10.5 41.522-116.540 1.7 M10A Pn 0 1 wells6.1 1.427 -1.10 0.25 40 99999999 999999.99 99999999 999999.99 25.90 41.160-114.891 10.4 40.925-113.030 1.6 BGU Pn 0 1 wells6.1 1.503 -0.65 0.18 40 99999999 999999.99 99999999 999999.99 27.40 41.163-114.887 10.7 40.292-116.500 1.5 O10A Pn 0 1 wells6.1 1.462 -1.21 0.18 33 99999999 999999.99 99999999 999999.99 26.28 41.173-114.888 10.5 42.636-114.903 1.1 K12A Pn 0 1 wells6.1 1.496 -0.69 0.28 39 99999999 999999.99 99999999 999999.99 27.29 41.164-114.889 10.5 42.077-116.471 1.5 L10A Pn 0 1 wells6.1 1.603 -0.53 0.28 37 99999999 999999.99 99999999 999999.99 28.95 41.162-114.888 10.6 42.649-114.084 1.2 K13A Pn 0 1 wells6.1 1.677 -0.81 0.12 24 99999999 999999.99 99999999 999999.99 29.73 41.152-114.892 10.5 39.473-114.908 1.9 P12A Pn 0 1 wells6.1 1.702 -0.73 0.21 40 99999999 999999.99 99999999 999999.99 30.18 41.163-114.887 10.5 41.780-112.775 1.6 HVU Pn 0 1 wells6.1 1.720 -0.56 0.15 29 99999999 999999.99 99999999 999999.99 30.59 41.145-114.897 10.4 39.553-115.754 1.8 P11A Pn 0 1 wells6.1
The columns in a type 3 tomo file are formatted the same as the type 1 or type 2 file, but some of the columns are interpreted differently. The full travel time associated with an empirical path anomaly is calculated by adding the empirical path anomaly to the theoretical travel time between the hypocentroid and the appropriate station. In this case, the hypocentroid is not of the entire cluster, but the hypocentroid of those events which contribute to this station-phase. The entries for origin time and reading time are ignored. All the events in this cluster are well observed at far regional distances so the number of samples for these entries are much larger than for the type 2 example above. The event number (Ev#) is always 0 because the reference point for each entry is the average location of the events contributing to each station-phase (the EvLat, EvLon and H columns).