================================================================================ This file contains an explanation of the "ndk" file format used to store and distribute the Global Centroid-Moment-Tensor (CMT) catalog (formerly the Harvard CMT catalog). The "ndk" format replaces the earlier "dek" format. Last modified: 2006-09-26 ================================================================================ 12345678901234567890123456789012345678901234567890123456789012345678901234567890 The format is ASCII and uses five 80-character lines per earthquake. PDE 2005/01/01 01:20:05.4 13.78 -88.78 193.1 5.0 0.0 EL SALVADOR C200501010120A B: 4 4 40 S: 27 33 50 M: 0 0 0 CMT: 1 TRIHD: 0.6 CENTROID: -0.3 0.9 13.76 0.06 -89.08 0.09 162.8 12.5 FREE S-20050322125201 23 0.838 0.201 -0.005 0.231 -0.833 0.270 1.050 0.121 -0.369 0.161 0.044 0.240 V10 1.581 56 12 -0.537 23 140 -1.044 24 241 1.312 9 29 142 133 72 66 PDE 2005/01/01 01:42:24.9 7.29 93.92 30.0 5.1 0.0 NICOBAR ISLANDS, INDIA R C200501010142A B: 17 27 40 S: 41 58 50 M: 0 0 0 CMT: 1 TRIHD: 0.7 CENTROID: -1.1 0.8 7.24 0.04 93.96 0.04 12.0 0.0 BDY S-20050322125628 23 -1.310 0.212 2.320 0.166 -1.010 0.241 0.013 0.535 -2.570 0.668 1.780 0.151 V10 3.376 16 149 0.611 43 44 -3.987 43 254 3.681 282 48 -23 28 73 -136 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 [57-80] Geographical location (24 characters) Second line: CMT 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: "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 moment-rate function, "BOXHD" indicates a boxcar moment-rate function. The value given is half the duration of the moment-rate function. This value is assumed in the inversion, following a standard scaling relationship (see note (2) below), and is not derived from the analysis. Third line: CMT 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 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 info (3) [1-2] The exponent for all following moment values. For example, if the exponent is given as 24, the moment values that follow, expressed in dyne-cm, should be multiplied by 10**24. [3-80] 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 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] 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] Scalar moment, to be multiplied by 10**(exponent) as given on line four. [58-80] 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. ================================================================================ Notes (additional information): (1) CMT 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. (2) 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 some very shallow earthquakes, the CMT inversion does not well constrain 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. ================================================================================