The ANSS event ID is us7000fct3 and the event page is at https://earthquake.usgs.gov/earthquakes/eventpage/us7000fct3/executive.
2021/09/20 04:41:47 44.353 -114.797 14.8 3.8 Idaho
USGS/SLU Moment Tensor Solution
ENS 2021/09/20 04:41:47:0 44.35 -114.80 14.8 3.8 Idaho
Stations used:
IE.BCYI IW.DLMT IW.FLWY IW.FXWY IW.IMW IW.LOHW IW.MFID
IW.MOOW IW.PLID IW.SNOW IW.TPAW MB.JTMT MB.SRMT US.BOZ
US.ELK US.HAWA US.HLID US.HWUT US.LKWY US.MSO US.WVOR
UU.MOUT UU.SPU UW.BRAN UW.DDRF UW.IZEE WY.YDD WY.YFT WY.YHB
WY.YHL WY.YMP WY.YMR WY.YNE WY.YNM WY.YNR
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 0.10 n 3
br c 0.12 0.25 n 4 p 2
Best Fitting Double Couple
Mo = 5.31e+21 dyne-cm
Mw = 3.75
Z = 13 km
Plane Strike Dip Rake
NP1 340 85 15
NP2 249 75 175
Principal Axes:
Axis Value Plunge Azimuth
T 5.31e+21 14 205
N 0.00e+00 74 358
P -5.31e+21 7 113
Moment Tensor: (dyne-cm)
Component Value
Mxx 3.26e+21
Mxy 3.84e+21
Mxz -8.83e+20
Myy -3.49e+21
Myz -1.12e+21
Mzz 2.39e+20
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--######################------------ -
#######################------------ P
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##### T ############--------
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Global CMT Convention Moment Tensor:
R T P
2.39e+20 -8.83e+20 1.12e+21
-8.83e+20 3.26e+21 -3.84e+21
1.12e+21 -3.84e+21 -3.49e+21
Details of the solution is found at
http://www.eas.slu.edu/eqc/eqc_mt/MECH.NA/20210920044147/index.html
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STK = 340
DIP = 85
RAKE = 15
MW = 3.75
HS = 13.0
The NDK file is 20210920044147.ndk The waveform inversion is preferred.
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 0.10 n 3 br c 0.12 0.25 n 4 p 2The results of this grid search are as follow:
DEPTH STK DIP RAKE MW FIT
WVFGRD96 1.0 340 90 0 3.32 0.3734
WVFGRD96 2.0 340 90 5 3.49 0.6152
WVFGRD96 3.0 340 90 5 3.54 0.6925
WVFGRD96 4.0 340 90 15 3.58 0.7330
WVFGRD96 5.0 340 90 20 3.61 0.7581
WVFGRD96 6.0 340 90 20 3.63 0.7777
WVFGRD96 7.0 340 90 20 3.65 0.7924
WVFGRD96 8.0 340 90 25 3.69 0.8043
WVFGRD96 9.0 160 90 -20 3.70 0.8129
WVFGRD96 10.0 340 85 20 3.71 0.8193
WVFGRD96 11.0 340 85 20 3.73 0.8237
WVFGRD96 12.0 340 85 15 3.74 0.8268
WVFGRD96 13.0 340 85 15 3.75 0.8287
WVFGRD96 14.0 340 85 15 3.76 0.8283
WVFGRD96 15.0 340 80 15 3.78 0.8258
WVFGRD96 16.0 340 80 10 3.79 0.8221
WVFGRD96 17.0 340 80 10 3.80 0.8174
WVFGRD96 18.0 340 80 10 3.81 0.8108
WVFGRD96 19.0 340 80 -10 3.82 0.8045
WVFGRD96 20.0 340 85 -10 3.83 0.7972
WVFGRD96 21.0 340 85 -15 3.84 0.7888
WVFGRD96 22.0 340 85 -15 3.84 0.7800
WVFGRD96 23.0 340 85 -15 3.85 0.7700
WVFGRD96 24.0 340 85 -15 3.86 0.7587
WVFGRD96 25.0 340 85 -15 3.87 0.7468
WVFGRD96 26.0 340 85 10 3.87 0.7330
WVFGRD96 27.0 340 85 10 3.88 0.7197
WVFGRD96 28.0 340 80 10 3.89 0.7056
WVFGRD96 29.0 340 85 10 3.89 0.6913
The best solution is
WVFGRD96 13.0 340 85 15 3.75 0.8287
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 0.10 n 3 br c 0.12 0.25 n 4 p 2
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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