Because of the location of this event in a region of induced seismicity and because of the presence of nearby stations, first arrival times and polarities were picked from the observed waveforms and the program elocate was used for location. The WUS velocity model was used because surface wave tomography for the source region indicated that the WUS model dispersion was closer to observed than that of the CUS model ( http://www.eas.slu.edu/eqc/eqc_research/CEUSTOMO/CEUSTOMO2/NADSP/37.-105.png. The difference is that the SLU location is about 4 km west of the ANSS location. This makes a difference in the azimuth to the nearby stations. The waveforms fits are better.
The results of the relocation are given in the link elocate.txt.
USGS/SLU Moment Tensor Solution ENS 2017/10/10 23:31:45:0 37.03 -104.90 3.7 3.4 Colorado Stations used: TA.KSCO TA.T25A US.SDCO YX.UNM2 YX.UNM3 YX.UNM5 YX.UNM6 Filtering commands used: cut o DIST/3.3 -30 o DIST/3.3 +40 rtr taper w 0.1 hp c 0.04 n 3 lp c 0.08 n 3 br c 0.12 0.25 n 4 p 2 Best Fitting Double Couple Mo = 1.43e+21 dyne-cm Mw = 3.37 Z = 5 km Plane Strike Dip Rake NP1 150 65 -70 NP2 289 32 -126 Principal Axes: Axis Value Plunge Azimuth T 1.43e+21 18 225 N 0.00e+00 18 321 P -1.43e+21 64 94 Moment Tensor: (dyne-cm) Component Value Mxx 6.41e+20 Mxy 6.67e+20 Mxz -2.53e+20 Myy 3.88e+20 Myz -8.51e+20 Mzz -1.03e+21 ############## -##################### ---######################### ----#--------------########### ---###-------------------######### --######---------------------####### -########-----------------------###### ##########-------------------------##### ###########-------------------------#### #############-------------------------#### #############------------- ----------### ##############------------ P -----------## ###############----------- -----------## ###############------------------------- #################----------------------- #################--------------------- #### ##########------------------- ### T ############---------------- # ##############------------ ####################-------- ####################-- ############## Global CMT Convention Moment Tensor: R T P -1.03e+21 -2.53e+20 8.51e+20 -2.53e+20 6.41e+20 -6.67e+20 8.51e+20 -6.67e+20 3.88e+20 Details of the solution is found at http://www.eas.slu.edu/eqc/eqc_mt/MECH.NA/20171010233145/index.html |
STK = 150 DIP = 65 RAKE = -70 MW = 3.37 HS = 5.0
The NDK file is 20171010233145.ndk The waveform inversion is preferred.
The following compares this source inversion to others
USGS/SLU Moment Tensor Solution ENS 2017/10/10 23:31:45:0 37.03 -104.90 3.7 3.4 Colorado Stations used: TA.KSCO TA.T25A US.SDCO YX.UNM2 YX.UNM3 YX.UNM5 YX.UNM6 Filtering commands used: cut o DIST/3.3 -30 o DIST/3.3 +40 rtr taper w 0.1 hp c 0.04 n 3 lp c 0.08 n 3 br c 0.12 0.25 n 4 p 2 Best Fitting Double Couple Mo = 1.43e+21 dyne-cm Mw = 3.37 Z = 5 km Plane Strike Dip Rake NP1 150 65 -70 NP2 289 32 -126 Principal Axes: Axis Value Plunge Azimuth T 1.43e+21 18 225 N 0.00e+00 18 321 P -1.43e+21 64 94 Moment Tensor: (dyne-cm) Component Value Mxx 6.41e+20 Mxy 6.67e+20 Mxz -2.53e+20 Myy 3.88e+20 Myz -8.51e+20 Mzz -1.03e+21 ############## -##################### ---######################### ----#--------------########### ---###-------------------######### --######---------------------####### -########-----------------------###### ##########-------------------------##### ###########-------------------------#### #############-------------------------#### #############------------- ----------### ##############------------ P -----------## ###############----------- -----------## ###############------------------------- #################----------------------- #################--------------------- #### ##########------------------- ### T ############---------------- # ##############------------ ####################-------- ####################-- ############## Global CMT Convention Moment Tensor: R T P -1.03e+21 -2.53e+20 8.51e+20 -2.53e+20 6.41e+20 -6.67e+20 8.51e+20 -6.67e+20 3.88e+20 Details of the solution is found at http://www.eas.slu.edu/eqc/eqc_mt/MECH.NA/20171010233145/index.html |
|
(a) mLg computed using the IASPEI formula; (b) mLg residuals ; the values used for the trimmed mean are indicated.
(a) ML computed using the IASPEI formula for Horizontal components; (b) 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.
(a) ML computed using the IASPEI formula for Vertical components (research); (b) 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.
The focal mechanism was determined using broadband seismic waveforms. The location of the event and the and stations used for the waveform inversion are shown in the next figure.
|
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 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 0.04 n 3 lp c 0.08 n 3 br c 0.12 0.25 n 4 p 2The results of this grid search from 0.5 to 19 km depth are as follow:
DEPTH STK DIP RAKE MW FIT WVFGRD96 1.0 160 60 -60 3.08 0.4025 WVFGRD96 2.0 340 40 -50 3.26 0.6039 WVFGRD96 3.0 335 30 -55 3.34 0.6983 WVFGRD96 4.0 150 70 -70 3.37 0.7958 WVFGRD96 5.0 150 65 -70 3.37 0.8289 WVFGRD96 6.0 145 60 -75 3.39 0.8202 WVFGRD96 7.0 155 65 -60 3.37 0.7928 WVFGRD96 8.0 155 65 -60 3.42 0.7849 WVFGRD96 9.0 165 75 -50 3.40 0.7564 WVFGRD96 10.0 175 90 -40 3.38 0.7376 WVFGRD96 11.0 10 55 30 3.41 0.7298 WVFGRD96 12.0 10 55 30 3.41 0.7207 WVFGRD96 13.0 10 55 30 3.41 0.7117 WVFGRD96 14.0 5 60 30 3.42 0.7007 WVFGRD96 15.0 5 60 30 3.43 0.6890 WVFGRD96 16.0 10 55 35 3.44 0.6784 WVFGRD96 17.0 10 55 35 3.44 0.6662 WVFGRD96 18.0 10 55 35 3.45 0.6558 WVFGRD96 19.0 10 50 35 3.46 0.6443 WVFGRD96 20.0 10 50 40 3.47 0.6342 WVFGRD96 21.0 15 45 40 3.49 0.6212 WVFGRD96 22.0 15 45 40 3.50 0.6100 WVFGRD96 23.0 20 45 65 3.53 0.6037 WVFGRD96 24.0 20 45 65 3.54 0.5989 WVFGRD96 25.0 20 45 65 3.54 0.5927 WVFGRD96 26.0 20 45 65 3.54 0.5886 WVFGRD96 27.0 20 45 65 3.55 0.5849 WVFGRD96 28.0 15 40 65 3.56 0.5805 WVFGRD96 29.0 15 40 65 3.57 0.5782
The best solution is
WVFGRD96 5.0 150 65 -70 3.37 0.8289
The mechanism correspond to the best fit is
|
The best fit as a function of depth is given in the following figure:
|
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 and because the velocity model used in the predictions may not be perfect. 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 0.04 n 3 lp c 0.08 n 3 br c 0.12 0.25 n 4 p 2
|
Focal mechanism sensitivity at the preferred depth. The red color indicates a very good fit to thewavefroms. 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.
Thanks also to the many seismic network operators whose dedication make this effort possible: University of Nevada Reno, University of Alaska, University of Washington, Oregon State University, University of Utah, Montana Bureas of Mines, UC Berkely, Caltech, UC San Diego, Saint Louis University, University of Memphis, Lamont Doherty Earth Observatory, the Iris stations and the Transportable Array of EarthScope.
The WUS.model used for the waveform synthetic seismograms and for the surface wave eigenfunctions and dispersion is as follows:
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
Here we tabulate the reasons for not using certain digital data sets
The following stations did not have a valid response files: