38.4 hypoDD

This program Waldhauser and Ellsworth (2000) is meant to improve relative locations in clusters of events. Quoting the manual (hypodd.pdf in INF):

"The DD technique (described in detail in Waldhauser and Ellsworth, 2000) takes advantage of the fact that if the hypocentral separation between two earthquakes is small compared to the event-station distance and the scale length of velocity heterogeneity, then the ray paths between the source region and a common station are similar along almost the entire ray path. In this case, the difference in travel times for two events observed at one station can be attributed to the spatial offset between the events with high accuracy."

Thus arrival time differences between stations is used. Data from catalogs or corss correlation can be used, here we will only illustrate the use of catalog data. For details, see the manual. hypoDD is installed in SEISAN and can also be obtained from http://geopubs.wr.usgs.gov/open-file/of01-113/. For simplicity, the program is called hypodd and not hypoDD. To run the program, the steps are:

1:

Select a cluster of events.

2:

Create an input file to hypodd program ph2dt with SEISAN program NOR2DD. NOR2DD takes a CAT file (like select.out) and makes a file phase.dat (default input file to PH2DT).

3:

Created difference times with PH2DT (supplied with hypodd, included in SEISAN).

4:

Run HYPODD.

5:

Plot the new and original locations with EPIMAP, output is provided in Nordic format. Command is epimap hypodd.map

NOR2DD

The program takes an input CAT file and creates an output file phase.dat (default name) with the arrival times and hypocenter information:

# 2015  4 13 15 32 40.6 66.6120  13.3800  15.1  0.7 5.2 9.4 0.6    1
KONS    3.13  1.0 P
KONS    5.94  1.0 S
STOK    7.02  0.5 P
STOK   11.13  1.0 S
MOR8   19.50  1.0 S
FAUS   18.71  0.5 P
FAUS   33.65  1.0 S
STEI   42.45  1.0 S

The phases P and S are assumed to be first arrivals, whether Pg or Pn. There are 2 options for selecting phases, either use all phases P and S irrespective of given names so e.g. Pn becomes P or only use phases labeled P and S in input file. It is recommend to use all phases since phases labeled Pn and Pg most likely are first arrivals. Any double entry of a phase will be eliminated by giving the latest arrival phase zero weight. In this way a Pg following a Pn for the same station will be weighted out.

Optionally, phases from given stations can be given a different weight than given in the S-file. This can be used to remove station by giving weight 0 or lowering the weight. The weigh -9 can also be given to indicate that this station must be used in all cases (see hypodd manual).

The program also creates the station file for HYPODD using the STATION0.HYP (in local or DAT directory) with the default name station.dat:

 KONS   66.4990   13.1177
 STOK   66.3330   13.0177
 MOR8   66.2852   14.7317
 FAUS   67.3820   15.2877
 STEI   67.9300   15.2420

Once the location is made with HYPODD, the output locations can be converted to Nordic format with NOR2DD. It is assumed that the default file hypoDD.loc is used for the original locations and hypoDD.reloc is used for the relocations. The output file with the relocated events is called hypodd.out and the corresponding original locations is hypodd.org. Hypodd.org will usually contain fewer events than the original input in phase.dat since events are eliminated during the relocation process, see later.

Examples:

nor2dd
  Convert from Nordic to hypodd (1=enter) or hypodd to Nordic (2)

  Give input Nordic file
swarm.out
  Use any P and S-phase (enter) or only P and S (o)

  Stations and weights  0-1 or -9 to force use, enter to terminate
NBB13 0

  The input file had          388  events
  Output phases in file phase.dat
  Output of station list in station.dat



nor2dd
  Convert from Nordic to hypodd (1=enter) or hypodd to Nordic (2)
2
  Give input hypodd file, hypoDD.reloc is default (enter)

  Number of events in original file          39
  Number of events in relocated file           24
  Output file of relocated events is hypodd.out
  Output file of corresponding original events is hypodd.org

PH2DT

PH2DT find the stations pairs to be used. It has a default parameter file ph2dt.inp. The most important parameters are event separation and maximum distances. Since these parameters also can be set on the HYPODD parameter file, hypodd.inp , the simplest is to leave ph2dt with default parameters. That includes the default phase data file phase.dat and station.dat made with NOR2DD:

* ph2dt.inp - input control file for program ph2dt
* Input station file:
station.dat
* Input phase file:
phase.dat
*MINWGHT: min. pick weight allowed [0]
*MAXDIST: max. distance in km between event pair and stations [200]
*MAXSEP: max. hypocentral separation in km [10]
*MAXNGH: max. number of neighbors per event [10]
*MINLNK: min. number of links required to define a neighbor [8]
*MINOBS: min. number of links per pair saved [8]
*MAXOBS: max. number of links per pair saved [20]
*MINWGHT MAXDIST MAXSEP MAXNGH MINLNK MINOBS MAXOBS
   0      200      10      10     8      8     20

To run the program, write

ph2dt ph2dt.inp

HYPODD

The program has a large number of parameters and it can be a bit confusing to adjust for best performance, see manual for details. The most important seem to be the event separation WDCT (see example parameter file below and example runs), weight for P and S-data WTCTP and WTCTS, respectively and cut-off threshold for outliers WRCT. Inversion is normally with conjugate gradients but for a small data set (less than 40-50 events), least squares solution can be used. The parameter file must also be set up for the model used.

Running HYPODD
Hypodd hypodd.inp

For explanation of output, see hypodd manual. The program will, in the SEISAN version, also execute the NOR2DD program to generate, in Nordic format, the files hypodd.out and hypodd.org as describe under NOR2DD. The result can then be plotted immediately.