The ANSS event ID is uu60422367 and the event page is at https://earthquake.usgs.gov/earthquakes/eventpage/uu60422367/executive.
2020/12/20 02:11:06 38.273 -108.929 0.0 4.32 Colorado
USGS/SLU Moment Tensor Solution ENS 2020/12/20 02:11:06:0 38.27 -108.93 0.0 4.3 Colorado Stations used: AE.BABIT AE.BARN AE.U15A AE.W18A AE.X18A C0.T25A IU.ANMO N4.O20A US.HWUT US.ISCO US.MVCO US.SDCO UU.BRPU UU.BSUT UU.CCUT UU.CTU UU.CVRU UU.ECUT UU.FOR1 UU.HMU UU.KNB UU.LCMT UU.LIUT UU.MPU UU.NLU UU.PKCU UU.RDMU UU.SRU UU.SVWY UU.SZCU UU.TCRU UU.TCU YX.UNM5 Filtering commands used: cut o DIST/3.3 -30 o DIST/3.3 +40 rtr taper w 0.1 hp c 0.03 n 3 lp c 0.07 n 3 Best Fitting Double Couple Mo = 3.76e+21 dyne-cm Mw = 3.65 Z = 9 km Plane Strike Dip Rake NP1 255 76 -159 NP2 160 70 -15 Principal Axes: Axis Value Plunge Azimuth T 3.76e+21 4 27 N 0.00e+00 65 288 P -3.76e+21 24 119 Moment Tensor: (dyne-cm) Component Value Mxx 2.27e+21 Mxy 2.81e+21 Mxz 9.12e+20 Myy -1.64e+21 Myz -1.12e+21 Mzz -6.25e+20 ############## ---################ T ------################ ### -------####################### ---------######################### ----------########################## ------------########################## -------------########################### -------------###########---------------- ---------------###------------------------ ------------###--------------------------- --------########-------------------------- ----#############------------------------- -###############------------------------ #################--------------- ----- #################-------------- P ---- #################------------- --- #################----------------- #################------------- #################----------- ################------ ############## Global CMT Convention Moment Tensor: R T P -6.25e+20 9.12e+20 1.12e+21 9.12e+20 2.27e+21 -2.81e+21 1.12e+21 -2.81e+21 -1.64e+21 Details of the solution is found at http://www.eas.slu.edu/eqc/eqc_mt/MECH.NA/20201220021106/index.html |
STK = 160 DIP = 70 RAKE = -15 MW = 3.65 HS = 9.0
The NDK file is 20201220021106.ndk The waveform inversion is preferred.
The following compares this source inversion to those provided by others. The purpose is to look for major differences and also to note slight differences that might be inherent to the processing procedure. For completeness the USGS/SLU solution is repeated from above.
USGS/SLU Moment Tensor Solution ENS 2020/12/20 02:11:06:0 38.27 -108.93 0.0 4.3 Colorado Stations used: AE.BABIT AE.BARN AE.U15A AE.W18A AE.X18A C0.T25A IU.ANMO N4.O20A US.HWUT US.ISCO US.MVCO US.SDCO UU.BRPU UU.BSUT UU.CCUT UU.CTU UU.CVRU UU.ECUT UU.FOR1 UU.HMU UU.KNB UU.LCMT UU.LIUT UU.MPU UU.NLU UU.PKCU UU.RDMU UU.SRU UU.SVWY UU.SZCU UU.TCRU UU.TCU YX.UNM5 Filtering commands used: cut o DIST/3.3 -30 o DIST/3.3 +40 rtr taper w 0.1 hp c 0.03 n 3 lp c 0.07 n 3 Best Fitting Double Couple Mo = 3.76e+21 dyne-cm Mw = 3.65 Z = 9 km Plane Strike Dip Rake NP1 255 76 -159 NP2 160 70 -15 Principal Axes: Axis Value Plunge Azimuth T 3.76e+21 4 27 N 0.00e+00 65 288 P -3.76e+21 24 119 Moment Tensor: (dyne-cm) Component Value Mxx 2.27e+21 Mxy 2.81e+21 Mxz 9.12e+20 Myy -1.64e+21 Myz -1.12e+21 Mzz -6.25e+20 ############## ---################ T ------################ ### -------####################### ---------######################### ----------########################## ------------########################## -------------########################### -------------###########---------------- ---------------###------------------------ ------------###--------------------------- --------########-------------------------- ----#############------------------------- -###############------------------------ #################--------------- ----- #################-------------- P ---- #################------------- --- #################----------------- #################------------- #################----------- ################------ ############## Global CMT Convention Moment Tensor: R T P -6.25e+20 9.12e+20 1.12e+21 9.12e+20 2.27e+21 -2.81e+21 1.12e+21 -2.81e+21 -1.64e+21 Details of the solution is found at http://www.eas.slu.edu/eqc/eqc_mt/MECH.NA/20201220021106/index.html |
Moment Tensor (TDMT) Moment 3.825e+14 N-m Magnitude 3.66 Depth 5.0 km Percent DC 92% Half Duration - Catalog UU Data Source UU 3 Contributor UU 3 Nodal Planes Plane Strike Dip Rake NP1 166 80 8 NP2 75 82 170 Principal Axes Axis Value Plunge Azimuth T 3.898e+14 N-m 13 31 N -0.150e+14 N-m 77 215 P -3.748e+14 N-m 1 121 |
Given the availability of digital waveforms for determination of the moment tensor, this section documents the added processing leading to mLg, if appropriate to the region, and ML by application of the respective IASPEI formulae. As a research study, the linear distance term of the IASPEI formula for ML is adjusted to remove a linear distance trend in residuals to give a regionally defined ML. The defined ML uses horizontal component recordings, but the same procedure is applied to the vertical components since there may be some interest in vertical component ground motions. Residual plots versus distance may indicate interesting features of ground motion scaling in some distance ranges. A residual plot of the regionalized magnitude is given as a function of distance and azimuth, since data sets may transcend different wave propagation provinces.
Left: ML computed using the IASPEI formula for Horizontal components. Center: ML residuals computed using a modified IASPEI formula that accounts for path specific attenuation; the values used for the trimmed mean are indicated. The ML relation used for each figure is given at the bottom of each plot.
Right: Residuals from new relation as a function of distance and azimuth.
Left: ML computed using the IASPEI formula for Vertical components (research). Center: ML residuals computed using a modified IASPEI formula that accounts for path specific attenuation; the values used for the trimmed mean are indicated. The ML relation used for each figure is given at the bottom of each plot.
Right: Residuals from new relation as a function of distance and azimuth.
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The focal mechanism was determined using broadband seismic waveforms. The location of the event (star) and the stations used for (red) the waveform inversion are shown in the next figure.
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The program wvfgrd96 was used with good traces observed at short distance to determine the focal mechanism, depth and seismic moment. This technique requires a high quality signal and well determined velocity model for the Green's functions. To the extent that these are the quality data, this type of mechanism should be preferred over the radiation pattern technique which requires the separate step of defining the pressure and tension quadrants and the correct strike.
The observed and predicted traces are filtered using the following gsac commands:
cut o DIST/3.3 -30 o DIST/3.3 +40 rtr taper w 0.1 hp c 0.03 n 3 lp c 0.07 n 3The results of this grid search are as follow:
DEPTH STK DIP RAKE MW FIT WVFGRD96 1.0 160 75 -10 3.38 0.4097 WVFGRD96 2.0 160 75 -20 3.47 0.4840 WVFGRD96 3.0 165 90 -5 3.49 0.5108 WVFGRD96 4.0 165 80 15 3.53 0.5282 WVFGRD96 5.0 345 90 -15 3.55 0.5341 WVFGRD96 6.0 345 90 -20 3.58 0.5395 WVFGRD96 7.0 165 75 5 3.59 0.5428 WVFGRD96 8.0 160 70 -15 3.63 0.5479 WVFGRD96 9.0 160 70 -15 3.65 0.5480 WVFGRD96 10.0 160 70 -15 3.66 0.5472 WVFGRD96 11.0 160 70 -15 3.67 0.5439 WVFGRD96 12.0 160 70 -15 3.69 0.5399 WVFGRD96 13.0 160 70 -15 3.70 0.5344 WVFGRD96 14.0 160 75 -15 3.71 0.5287 WVFGRD96 15.0 160 75 -15 3.72 0.5220 WVFGRD96 16.0 160 75 -15 3.73 0.5148 WVFGRD96 17.0 160 75 -15 3.74 0.5063 WVFGRD96 18.0 160 75 -15 3.75 0.4973 WVFGRD96 19.0 160 75 -15 3.76 0.4872 WVFGRD96 20.0 160 75 -15 3.77 0.4766 WVFGRD96 21.0 160 75 -15 3.77 0.4651 WVFGRD96 22.0 160 75 -15 3.78 0.4528 WVFGRD96 23.0 160 75 -15 3.79 0.4402 WVFGRD96 24.0 160 80 -25 3.80 0.4266 WVFGRD96 25.0 160 85 -25 3.80 0.4149 WVFGRD96 26.0 160 85 -25 3.81 0.4056 WVFGRD96 27.0 160 85 -25 3.81 0.3965 WVFGRD96 28.0 165 90 -25 3.82 0.3874 WVFGRD96 29.0 165 90 -25 3.83 0.3787
The best solution is
WVFGRD96 9.0 160 70 -15 3.65 0.5480
The mechanism corresponding to the best fit is
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The best fit as a function of depth is given in the following figure:
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The comparison of the observed and predicted waveforms is given in the next figure. The red traces are the observed and the blue are the predicted. Each observed-predicted component is plotted to the same scale and peak amplitudes are indicated by the numbers to the left of each trace. A pair of numbers is given in black at the right of each predicted traces. The upper number it the time shift required for maximum correlation between the observed and predicted traces. This time shift is required because the synthetics are not computed at exactly the same distance as the observed, the velocity model used in the predictions may not be perfect and the epicentral parameters may be be off. A positive time shift indicates that the prediction is too fast and should be delayed to match the observed trace (shift to the right in this figure). A negative value indicates that the prediction is too slow. The lower number gives the percentage of variance reduction to characterize the individual goodness of fit (100% indicates a perfect fit).
The bandpass filter used in the processing and for the display was
cut o DIST/3.3 -30 o DIST/3.3 +40 rtr taper w 0.1 hp c 0.03 n 3 lp c 0.07 n 3
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Figure 3. Waveform comparison for selected depth. Red: observed; Blue - predicted. The time shift with respect to the model prediction is indicated. The percent of fit is also indicated. The time scale is relative to the first trace sample. |
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Focal mechanism sensitivity at the preferred depth. The red color indicates a very good fit to the waveforms. Each solution is plotted as a vector at a given value of strike and dip with the angle of the vector representing the rake angle, measured, with respect to the upward vertical (N) in the figure. |
A check on the assumed source location is possible by looking at the time shifts between the observed and predicted traces. The time shifts for waveform matching arise for several reasons:
Time_shift = A + B cos Azimuth + C Sin Azimuth
The time shifts for this inversion lead to the next figure:
The derived shift in origin time and epicentral coordinates are given at the bottom of the figure.
The WUS.model used for the waveform synthetic seismograms and for the surface wave eigenfunctions and dispersion is as follows (The format is in the model96 format of Computer Programs in Seismology).
MODEL.01 Model after 8 iterations ISOTROPIC KGS FLAT EARTH 1-D CONSTANT VELOCITY LINE08 LINE09 LINE10 LINE11 H(KM) VP(KM/S) VS(KM/S) RHO(GM/CC) QP QS ETAP ETAS FREFP FREFS 1.9000 3.4065 2.0089 2.2150 0.302E-02 0.679E-02 0.00 0.00 1.00 1.00 6.1000 5.5445 3.2953 2.6089 0.349E-02 0.784E-02 0.00 0.00 1.00 1.00 13.0000 6.2708 3.7396 2.7812 0.212E-02 0.476E-02 0.00 0.00 1.00 1.00 19.0000 6.4075 3.7680 2.8223 0.111E-02 0.249E-02 0.00 0.00 1.00 1.00 0.0000 7.9000 4.6200 3.2760 0.164E-10 0.370E-10 0.00 0.00 1.00 1.00