The ANSS event ID is ak0216xu2rod and the event page is at https://earthquake.usgs.gov/earthquakes/eventpage/ak0216xu2rod/executive.
2021/05/31 06:59:54 62.449 -148.251 44.0 6.1 Idaho
USGS/SLU Moment Tensor Solution
ENS 2021/05/31 06:59:54:0 62.45 -148.25 44.0 6.1 Idaho
Stations used:
AK.BARN AK.BMR AK.BPAW AK.CAST AK.CCB AK.DHY AK.DIV AK.DOT
AK.FID AK.GLB AK.GLI AK.GRNC AK.H23K AK.H24K AK.HDA AK.I21K
AK.I23K AK.J19K AK.J20K AK.J25K AK.K20K AK.KAI AK.KLU
AK.L19K AK.L20K AK.LOGN AK.M26K AK.MCK AK.MLY AK.NEA2
AK.O19K AK.PAX AK.POKR AK.PPD AK.PPLA AK.RIDG AK.RND
AK.SCRK AK.SLK AK.TGL AK.TRF AK.VRDI AK.WRH AT.PMR AV.STLK
IU.COLA
Filtering commands used:
cut o DIST/3.3 -40 o DIST/3.3 +50
rtr
taper w 0.1
hp c 0.02 n 3
lp c 0.05 n 3
Best Fitting Double Couple
Mo = 1.60e+25 dyne-cm
Mw = 6.07
Z = 58 km
Plane Strike Dip Rake
NP1 210 59 -106
NP2 60 35 -65
Principal Axes:
Axis Value Plunge Azimuth
T 1.60e+25 12 312
N 0.00e+00 14 219
P -1.60e+25 71 82
Moment Tensor: (dyne-cm)
Component Value
Mxx 6.87e+24
Mxy -7.86e+24
Mxz 1.53e+24
Myy 6.78e+24
Myz -7.29e+24
Mzz -1.37e+25
##############
####################--
##################----------
##############-------------
## T ############-----------------
### ##########--------------------
###############----------------------#
###############-----------------------##
##############------------------------##
#############----------- ------------###
#############----------- P -----------####
############------------ -----------####
###########--------------------------#####
#########--------------------------#####
#########------------------------#######
########----------------------########
#######--------------------#########
-####------------------###########
---#-------------#############
---#########################
######################
##############
Global CMT Convention Moment Tensor:
R T P
-1.37e+25 1.53e+24 7.29e+24
1.53e+24 6.87e+24 7.86e+24
7.29e+24 7.86e+24 6.78e+24
Details of the solution is found at
http://www.eas.slu.edu/eqc/eqc_mt/MECH.NA/20210531065954/index.html
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STK = 60
DIP = 35
RAKE = -65
MW = 6.07
HS = 58.0
The NDK file is 20210531065954.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 -40 o DIST/3.3 +50 rtr taper w 0.1 hp c 0.02 n 3 lp c 0.05 n 3The results of this grid search are as follow:
DEPTH STK DIP RAKE MW FIT
WVFGRD96 2.0 60 50 -65 5.41 0.3128
WVFGRD96 4.0 45 50 -90 5.52 0.3591
WVFGRD96 6.0 55 50 -75 5.53 0.3079
WVFGRD96 8.0 220 40 -95 5.60 0.3199
WVFGRD96 10.0 270 75 30 5.52 0.3282
WVFGRD96 12.0 100 60 35 5.56 0.3561
WVFGRD96 14.0 95 65 30 5.58 0.3808
WVFGRD96 16.0 95 65 30 5.60 0.4037
WVFGRD96 18.0 95 65 30 5.62 0.4237
WVFGRD96 20.0 90 75 30 5.63 0.4434
WVFGRD96 22.0 90 80 30 5.65 0.4618
WVFGRD96 24.0 90 80 30 5.68 0.4811
WVFGRD96 26.0 90 80 30 5.70 0.4993
WVFGRD96 28.0 90 80 30 5.72 0.5161
WVFGRD96 30.0 90 80 25 5.74 0.5330
WVFGRD96 32.0 90 80 25 5.76 0.5497
WVFGRD96 34.0 90 80 25 5.79 0.5666
WVFGRD96 36.0 90 75 20 5.82 0.5846
WVFGRD96 38.0 90 75 20 5.85 0.6101
WVFGRD96 40.0 260 75 -40 5.91 0.6145
WVFGRD96 42.0 260 70 -40 5.93 0.6345
WVFGRD96 44.0 260 70 -40 5.95 0.6533
WVFGRD96 46.0 255 65 -45 5.97 0.6694
WVFGRD96 48.0 70 35 -50 6.00 0.6927
WVFGRD96 50.0 65 35 -55 6.02 0.7098
WVFGRD96 52.0 65 35 -60 6.03 0.7239
WVFGRD96 54.0 60 35 -65 6.04 0.7342
WVFGRD96 56.0 60 35 -65 6.06 0.7405
WVFGRD96 58.0 60 35 -65 6.07 0.7425
WVFGRD96 60.0 70 40 -50 6.08 0.7411
WVFGRD96 62.0 70 40 -50 6.08 0.7380
WVFGRD96 64.0 70 40 -50 6.09 0.7321
WVFGRD96 66.0 70 40 -50 6.10 0.7231
WVFGRD96 68.0 70 40 -50 6.10 0.7117
WVFGRD96 70.0 70 40 -50 6.11 0.6977
WVFGRD96 72.0 70 40 -50 6.11 0.6822
WVFGRD96 74.0 80 45 -35 6.11 0.6677
WVFGRD96 76.0 80 50 -30 6.11 0.6539
WVFGRD96 78.0 80 50 -30 6.12 0.6416
WVFGRD96 80.0 85 55 -25 6.11 0.6288
WVFGRD96 82.0 85 55 -25 6.12 0.6173
WVFGRD96 84.0 85 55 -20 6.12 0.6059
WVFGRD96 86.0 85 55 -20 6.12 0.5941
WVFGRD96 88.0 85 60 -20 6.12 0.5844
WVFGRD96 90.0 85 60 -20 6.13 0.5761
WVFGRD96 92.0 85 60 -20 6.13 0.5678
WVFGRD96 94.0 85 65 -20 6.13 0.5606
WVFGRD96 96.0 85 65 -20 6.13 0.5561
WVFGRD96 98.0 85 65 -20 6.13 0.5521
The best solution is
WVFGRD96 58.0 60 35 -65 6.07 0.7425
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 -40 o DIST/3.3 +50 rtr taper w 0.1 hp c 0.02 n 3 lp c 0.05 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