The ANSS event ID is us70008p7a and the event page is at https://earthquake.usgs.gov/earthquakes/eventpage/us70008p7a/executive.
2020/04/05 11:26:57 44.353 -115.197 9.2 3.8 Idaho
USGS/SLU Moment Tensor Solution
ENS 2020/04/05 11:26:57:0 44.35 -115.20 9.2 3.8 Idaho
Stations used:
IE.BCYI IW.DLMT IW.FLWY IW.FXWY IW.LOHW IW.MFID IW.MOOW
IW.PLID IW.SNOW UO.PINE US.AHID US.BMO US.BOZ US.BW06
US.ELK US.HAWA US.HLID US.HWUT US.MSO US.WVOR UU.BGU UU.HVU
UU.SPU UW.BRAN UW.CCRK UW.DAVN UW.DDRF UW.EPH2 UW.KENT
UW.LNO UW.LON UW.LTY UW.MOX UW.PHIN UW.UMAT UW.WA2 UW.WOLL
UW.YPT WY.YDD WY.YFT WY.YHL WY.YMR WY.YNE WY.YPP
Filtering commands used:
cut o DIST/3.3 -30 o DIST/3.3 +50
rtr
taper w 0.1
hp c 0.03 n 3
lp c 0.08 n 3
Best Fitting Double Couple
Mo = 7.76e+21 dyne-cm
Mw = 3.86
Z = 13 km
Plane Strike Dip Rake
NP1 125 90 10
NP2 35 80 180
Principal Axes:
Axis Value Plunge Azimuth
T 7.76e+21 7 350
N 0.00e+00 80 125
P -7.76e+21 7 260
Moment Tensor: (dyne-cm)
Component Value
Mxx 7.18e+21
Mxy -2.61e+21
Mxz 1.10e+21
Myy -7.18e+21
Myz 7.73e+20
Mzz -1.18e+14
## T #########
###### #############
##########################--
##########################----
###########################-------
---########################---------
-------####################-----------
-----------################-------------
-------------#############--------------
-----------------#########----------------
--------------------#####-----------------
-------------------#-------------------
P -------------------###-----------------
------------------#######-------------
------------------###########-----------
----------------###############-------
-------------####################---
-----------#######################
-------#######################
----########################
######################
##############
Global CMT Convention Moment Tensor:
R T P
-1.18e+14 1.10e+21 -7.73e+20
1.10e+21 7.18e+21 2.61e+21
-7.73e+20 2.61e+21 -7.18e+21
Details of the solution is found at
http://www.eas.slu.edu/eqc/eqc_mt/MECH.NA/20200405112657/index.html
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STK = 125
DIP = 90
RAKE = 10
MW = 3.86
HS = 13.0
The NDK file is 20200405112657.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 +50 rtr taper w 0.1 hp c 0.03 n 3 lp c 0.08 n 3The results of this grid search are as follow:
DEPTH STK DIP RAKE MW FIT
WVFGRD96 1.0 305 85 5 3.41 0.3894
WVFGRD96 2.0 305 85 0 3.53 0.5260
WVFGRD96 3.0 305 85 5 3.58 0.5843
WVFGRD96 4.0 305 85 5 3.62 0.6304
WVFGRD96 5.0 125 85 15 3.66 0.6688
WVFGRD96 6.0 125 85 15 3.69 0.7076
WVFGRD96 7.0 125 85 10 3.72 0.7423
WVFGRD96 8.0 125 85 15 3.76 0.7742
WVFGRD96 9.0 125 90 15 3.79 0.7942
WVFGRD96 10.0 305 85 -15 3.81 0.8116
WVFGRD96 11.0 305 85 -15 3.83 0.8218
WVFGRD96 12.0 305 85 -15 3.84 0.8270
WVFGRD96 13.0 125 90 10 3.86 0.8285
WVFGRD96 14.0 305 90 -10 3.87 0.8275
WVFGRD96 15.0 125 90 10 3.88 0.8230
WVFGRD96 16.0 305 90 -10 3.89 0.8158
WVFGRD96 17.0 305 85 -10 3.91 0.8070
WVFGRD96 18.0 125 90 10 3.92 0.7959
WVFGRD96 19.0 305 90 -10 3.93 0.7832
WVFGRD96 20.0 305 90 -5 3.93 0.7684
WVFGRD96 21.0 305 90 -10 3.94 0.7522
WVFGRD96 22.0 125 90 10 3.95 0.7345
WVFGRD96 23.0 125 90 10 3.95 0.7156
WVFGRD96 24.0 125 90 10 3.96 0.6953
WVFGRD96 25.0 125 90 10 3.96 0.6742
WVFGRD96 26.0 305 85 -10 3.96 0.6524
WVFGRD96 27.0 305 85 -10 3.97 0.6296
WVFGRD96 28.0 125 90 15 3.97 0.6057
WVFGRD96 29.0 305 85 -15 3.98 0.5841
The best solution is
WVFGRD96 13.0 125 90 10 3.86 0.8285
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 +50 rtr taper w 0.1 hp c 0.03 n 3 lp c 0.08 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