Double Difference Earthquake Location(hypoDD) - Geophysics at ...

2 HypoDD
HypoDD is a Fortran77 computer program package for relocating earthquakes with the doubledifference
algorithm of Waldhauser and Ellsworth [2000]. The latest version of HypoDD is version
1.3 and is available from http://www.ldeo.columbia.edu/˜felixw/hypoDD.html.
This tutorial follows instructions on http://geophysics.eas.gatech.edu/people/cwu/teaching/hypoDD/hypoDD.
2.1 Input files
hypoDD reads input parameters and files from the hypoDD.inp, which contains general information
such station, DD time, output file, and etc. It also includes a few important parameters that are
used during inversion.
1. OBSCC and OBSCT
OBSCC and OBSCT define the minimum number of links between event pair for cross correlation
differetial time (CC) and catalog differential time (CT). This parameters determine the
strength of links between event pair, which is crucial for relocation process. Below is quoted
from the hypoDD manual.
“To prevent an ill-conditioned system of DD-equations, hypoDD ensures connectedness between
events by grouping events into clusters, with each cluster having a chain of links from
any event to any other event in the cluster. The strength of this chain is defined by a minimum
number of observations per event pair that build up the chain (OBSCC, OBSCT for
cross-correlation and catalog data, respectively). Typical thresholds are similar to the number
of degrees of freedom for an event pair, which is 8 (3 spatial and 1 time for each event). If P-
and S-wave data is used, the threshold has to be higher to actually reach 8 stations per event
pair. Increasing the threshold might increase the stability of the solution, but it might also
split up a large cluster into subclusters. Lowering the threshold might include more events in
a single cluster, but might decrease the stability of the solution. A high threshold, however,
does not necessarily guarantee stable relocations, as the spread of the partial derivatives is
not taken into account directly.”
2. ISTART
This parameter tells hypoDD whether the intial location of events is from a centroid location,
or is taken from network sources (different initial locations).
3. ISOLV
For a small number of events, it is easy to contol the performance of hypoDD by SVD
(ISOLV=1). However, it is more difficulte to assess the reliability of the locations in the case
of large number of events (thousands or more) that can only be relocated in LSQR mode
(ISOLV=2).
4. NSET and Weighting
NSET is number of iterations during inversion. Specifications during each iteration can be
defined by values of weighting and re-weighting.
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