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This file contains an explanation of the "ndk" file format used to store
and distribute Centroid-Single-Force (CSF) parameters determined by the
GCMT project. The format is identical to the ndk format used to store
and distribute earthquake parameters for the Global Centroid-Moment-
Tensor catalog, but some fields have a different meaning.
Last update to CMT description: 2006-09-26
This file last modified: 2011-12-09, to describe CSF option (-MN)
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12345678901234567890123456789012345678901234567890123456789012345678901234567890
The format is ASCII and uses five 80-character lines per earthquake.
SWEC 2008/07/13 04:59:44.0 69.50 -49.50 10.0 0.0 4.8 WESTERN GREENLAND
S200807130459X B: 0 0 0 S: 49 74 50 M: 0 0 0 CSF:11 BOXHD: 20.0
CENTROID: 25.5 0.7 69.24 0.04 -49.53 0.08 12.0 0.0 FIX Q-20111018102547
18 -0.352 0.112 1.170 0.143 -1.460 0.127 0.000 0.000 0.000 0.000 0.000 0.000
V20 1.904 11 231 0.000 0 0 0.000 0 0 1.904 0 0 0 0 0 0
First line: Hypocenter line
[1-4] Hypocenter reference catalog (e.g., PDE for USGS location, ISC for
ISC catalog, SWE for surface-wave location, [Ekstrom, BSSA, 2006])
[6-15] Date of reference event
[17-26] Time of reference event
[28-33] Latitude
[35-41] Longitude
[43-47] Depth
[49-55] Reported magnitudes, usually mb and MS, or MSW
[57-80] Geographical location (24 characters)
Second line: CMT/CSF info (1)
[1-16] CMT event name. This string is a unique CMT-event identifier. Older
events have 8-character names, current ones have 14-character names.
See note (1) below for the naming conventions used.
[18-61] Data used in the CMT inversion. Three data types may be used:
Long-period body waves (B), Intermediate-period surface waves (S),
and long-period mantle waves (M). For each data type, three values
are given: the number of stations used, the number of components
used, and the shortest period used.
[63-68] Type of source inverted for:
"CSF:11" - centroid single force.
For the CMT catalog, allowable values are
"CMT: 0" - general moment tensor;
"CMT: 1" - moment tensor with constraint of zero trace (standard);
"CMT: 2" - double-couple source.
[70-80] Type and duration of moment-rate function assumed in the inversion.
"TRIHD" indicates a triangular force-time function, "BOXHD" indicates
a boxcar force-time function. The value given is half the duration
of the force-time function. This value is assumed in the inversion,
and is not derived from the analysis. For CMT inversions, the half
duration follows a standard scaling relationship (see note (2) below).
Third line: CMT/CSF info (2)
[1-58] Centroid parameters determined in the inversion. Centroid time, given
with respect to the reference time, centroid latitude, centroid
longitude, and centroid depth. The value of each variable is followed
by its estimated standard error. See note (3) below for cases in
which the hypocentral coordinates are held fixed.
[60-63] Type of depth. "FREE" indicates that the depth was a result of the
inversion; "FIX " that the depth was fixed and not inverted for;
"BDY " that the depth was fixed based on modeling of broad-band
P waveforms.
[65-80] Timestamp. This 16-character string identifies the type of analysis that
led to the given CMT/CSF results and, for recent events, the date and
time of the analysis. This is useful to distinguish Quick CMTs ("Q-"),
calculated within hours of an event, from Standard CMTs ("S-"), which
are calculated later. The format for this string should not be
considered fixed.
Fourth line: CMT/CSF info (3)
[1-2] The exponent for all following CSF/moment values. For example, if the
exponent is given as 18, the CSF values that follow, expressed in
g-cm, should be multiplied by 10**18. Moment values are expressed in
dyne-cm.
[3-80] The three CSF vector elements: Vr, Vt, Vp, where r is up, t is south,
and p is east. The value of each vector element is followed by its
estimated standard error. These three elements (plus errors) are
followed by six fields filled with zeros.
For CMT results, the fields [3-80] contain:
The six moment-tensor elements: Mrr, Mtt, Mpp, Mrt, Mrp, Mtp, where r
is up, t is south, and p is east. See Aki and Richards for conversions
to other coordinate systems. The value of each moment-tensor
element is followed by its estimated standard error. See note (4)
below for cases in which some elements are constrained in the inversion.
Fifth line: CMT/CSF info (4)
[1-3] Version code. This three-character string is used to track the version
of the program that generates the "ndk" file.
[4-48] CSF vector expressed as: amplitude, plunge, azimuth. The amplitude
should be multiplied by 10**(exponent) as given on line four. The
plunge is positive downward; azimuth is counter-clockwise from North.
These three fields are followed by six fields filled with zeros.
For CMT values, [4-48] contain:
Moment tensor expressed in its principal-axis system: eigenvalue,
plunge, and azimuth of the three eigenvectors. The eigenvalue should be
multiplied by 10**(exponent) as given on line four.
[50-56] CSF amplitude ("mass-distance product"), to be multiplied by
10**(exponent) as given on line four.
For CMT results, [50-56] contain:
Scalar moment, to be multiplied by 10**(exponent) as given on line four.
[58-80] For CSF results, these fields are zero.
For CMT results, [58-80] contain:
Strike, dip, and rake for first nodal plane of the best-double-couple
mechanism, repeated for the second nodal plane. The angles are defined
as in Aki and Richards.
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Notes (additional information):
(1) CMT/CSF event names follow two conventions. Older events use an 8-character
name with the structure XMMDDYYZ, where MMDDYY represents the date of
the event, Z is a letter (A-Z followed by a-z) distinguishing different
events on the same day, and X is a letter (B,M,Z,C,...) used to identify
the types of data used in the inversion. Newer events use 14-character event
names with the structure XYYYYMMDDhhmmZ, in which the time is given to greater
precision, and the initial letter is limited to four possibilities: B - body
waves only, S - surface waves only, M - mantle waves only, C - a combination
of data types. All CSF events use the newer naming convention.
(2) For CMT events:
The source duration is generally estimated using an empirically determined
relationship such that the duration increases as the cube root of the scalar
moment. Specifically, we currently use a relationship where the half duration
for an event with moment 10**24 is 1.05 seconds, and for an event with moment
10**27 is 10.5 seconds.
(3) For some small earthquakes for which the azimuthal distribution of stations
with useful seismograms is poor, we constrain the epicenter of the event to
the reference location. This is reflected in the catalog by standard
errors of 0.0 for both the centroid latitude and the centroid longitude.
(4) For CMT events:
For some very shallow earthquakes, the CMT inversion does not constrain
well the vertical-dip-slip components of the moment tensor (Mrt and Mrp),
and we constrain these components to zero in the inversion. The standard
errors for Mrt and Mrp are set to zero in this case.
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