The ANSS event ID is mb80151884 and the event page is at https://earthquake.usgs.gov/earthquakes/eventpage/mb80151884/executive.
2016/06/09 03:31:06 44.733 -111.763 9.8 3.66 Montana
USGS/SLU Moment Tensor Solution ENS 2016/06/09 03:31:06:0 44.73 -111.76 9.8 3.7 Montana Stations used: IW.DLMT IW.FLWY IW.LOHW IW.MFID IW.MOOW IW.REDW IW.SNOW IW.TPAW US.BOZ US.HLID US.HWUT US.LKWY US.RLMT WY.YHL WY.YMR WY.YNR WY.YPP Filtering commands used: cut o DIST/3.3 -20 o DIST/3.3 +50 rtr taper w 0.1 hp c 0.03 n 3 lp c 0.10 n 3 Best Fitting Double Couple Mo = 1.64e+21 dyne-cm Mw = 3.41 Z = 11 km Plane Strike Dip Rake NP1 340 80 -25 NP2 75 65 -169 Principal Axes: Axis Value Plunge Azimuth T 1.64e+21 10 29 N 0.00e+00 63 140 P -1.64e+21 25 295 Moment Tensor: (dyne-cm) Component Value Mxx 9.69e+20 Mxy 1.20e+21 Mxz -1.98e+19 Myy -7.32e+20 Myz 7.01e+20 Mzz -2.37e+20 ############## -----############### ----------############# T ## ------------############ ### ---------------################### -----------------################### --- ------------#################### ---- P -------------#################### ---- --------------##################- -----------------------################--- -----------------------##############----- ------------------------##########-------- -------------------------######----------- ------------------------##-------------- ##------------------#####--------------- ########################-------------- ########################------------ #######################----------- ######################-------- #####################------- ##################---- ############## Global CMT Convention Moment Tensor: R T P -2.37e+20 -1.98e+19 -7.01e+20 -1.98e+19 9.69e+20 -1.20e+21 -7.01e+20 -1.20e+21 -7.32e+20 Details of the solution is found at http://www.eas.slu.edu/eqc/eqc_mt/MECH.NA/20160609033106/index.html |
STK = 340 DIP = 80 RAKE = -25 MW = 3.41 HS = 11.0
The NDK file is 20160609033106.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 2016/06/09 03:31:06:0 44.73 -111.76 9.8 3.7 Montana Stations used: IW.DLMT IW.FLWY IW.LOHW IW.MFID IW.MOOW IW.REDW IW.SNOW IW.TPAW US.BOZ US.HLID US.HWUT US.LKWY US.RLMT WY.YHL WY.YMR WY.YNR WY.YPP Filtering commands used: cut o DIST/3.3 -20 o DIST/3.3 +50 rtr taper w 0.1 hp c 0.03 n 3 lp c 0.10 n 3 Best Fitting Double Couple Mo = 1.64e+21 dyne-cm Mw = 3.41 Z = 11 km Plane Strike Dip Rake NP1 340 80 -25 NP2 75 65 -169 Principal Axes: Axis Value Plunge Azimuth T 1.64e+21 10 29 N 0.00e+00 63 140 P -1.64e+21 25 295 Moment Tensor: (dyne-cm) Component Value Mxx 9.69e+20 Mxy 1.20e+21 Mxz -1.98e+19 Myy -7.32e+20 Myz 7.01e+20 Mzz -2.37e+20 ############## -----############### ----------############# T ## ------------############ ### ---------------################### -----------------################### --- ------------#################### ---- P -------------#################### ---- --------------##################- -----------------------################--- -----------------------##############----- ------------------------##########-------- -------------------------######----------- ------------------------##-------------- ##------------------#####--------------- ########################-------------- ########################------------ #######################----------- ######################-------- #####################------- ##################---- ############## Global CMT Convention Moment Tensor: R T P -2.37e+20 -1.98e+19 -7.01e+20 -1.98e+19 9.69e+20 -1.20e+21 -7.01e+20 -1.20e+21 -7.32e+20 Details of the solution is found at http://www.eas.slu.edu/eqc/eqc_mt/MECH.NA/20160609033106/index.html |
Regional Moment Tensor (Mwr) Moment 2.246e+14 N-m Magnitude 3.5 Mwr Depth 14.0 km Percent DC 44 % Half Duration – Catalog MB Data Source US2 Contributor US2 Nodal Planes Plane Strike Dip Rake NP1 116 49 -99 NP2 310 42 -79 Principal Axes Axis Value Plunge Azimuth T 2.515e+14 N-m 4 212 N -0.708e+14 N-m 7 122 P -1.807e+14 N-m 82 329 |
Montana Bureau of Mines and Geology First motion mechanism |
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 -20 o DIST/3.3 +50 rtr taper w 0.1 hp c 0.03 n 3 lp c 0.10 n 3The results of this grid search are as follow:
DEPTH STK DIP RAKE MW FIT WVFGRD96 1.0 345 60 30 2.95 0.3094 WVFGRD96 2.0 165 65 30 3.10 0.4254 WVFGRD96 3.0 340 90 -25 3.14 0.4578 WVFGRD96 4.0 340 85 -30 3.19 0.4964 WVFGRD96 5.0 335 75 -30 3.24 0.5351 WVFGRD96 6.0 335 70 -30 3.27 0.5659 WVFGRD96 7.0 335 70 -25 3.30 0.5889 WVFGRD96 8.0 335 70 -30 3.36 0.6071 WVFGRD96 9.0 335 70 -30 3.38 0.6167 WVFGRD96 10.0 335 70 -30 3.40 0.6200 WVFGRD96 11.0 340 80 -25 3.41 0.6201 WVFGRD96 12.0 340 80 -25 3.43 0.6171 WVFGRD96 13.0 340 80 -25 3.44 0.6111 WVFGRD96 14.0 340 80 -25 3.46 0.6020 WVFGRD96 15.0 340 85 -25 3.47 0.5909 WVFGRD96 16.0 165 80 25 3.48 0.5835 WVFGRD96 17.0 165 75 25 3.50 0.5711 WVFGRD96 18.0 165 75 25 3.51 0.5588 WVFGRD96 19.0 165 75 25 3.51 0.5463 WVFGRD96 20.0 165 75 25 3.52 0.5342 WVFGRD96 21.0 165 75 25 3.53 0.5216 WVFGRD96 22.0 165 75 25 3.54 0.5093 WVFGRD96 23.0 165 75 25 3.54 0.4980 WVFGRD96 24.0 170 75 30 3.55 0.4869 WVFGRD96 25.0 180 70 35 3.55 0.4768 WVFGRD96 26.0 180 70 35 3.56 0.4678 WVFGRD96 27.0 180 70 35 3.57 0.4592 WVFGRD96 28.0 180 70 35 3.57 0.4491 WVFGRD96 29.0 180 70 35 3.57 0.4390
The best solution is
WVFGRD96 11.0 340 80 -25 3.41 0.6201
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 -20 o DIST/3.3 +50 rtr taper w 0.1 hp c 0.03 n 3 lp c 0.10 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