The ANSS event ID is ak02049tx7pg and the event page is at https://earthquake.usgs.gov/earthquakes/eventpage/ak02049tx7pg/executive.
2020/04/02 01:58:34 62.874 -149.576 77.1 3.9 Idaho
USGS/SLU Moment Tensor Solution
ENS 2020/04/02 01:58:34:0 62.87 -149.58 77.1 3.9 Idaho
Stations used:
AK.BPAW AK.CCB AK.CUT AK.DHY AK.J20K AK.K20K AK.KLU AK.KTH
AK.L22K AK.M20K AK.MCK AK.PAX AK.PPLA AK.RC01 AK.RIDG
AK.RND AK.SAW AK.SCM AK.SKN AK.TRF AV.STLK TA.M22K TA.M24K
Filtering commands used:
cut o DIST/3.3 -40 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 = 1.06e+22 dyne-cm
Mw = 3.95
Z = 96 km
Plane Strike Dip Rake
NP1 30 76 159
NP2 125 70 15
Principal Axes:
Axis Value Plunge Azimuth
T 1.06e+22 24 346
N 0.00e+00 65 177
P -1.06e+22 4 78
Moment Tensor: (dyne-cm)
Component Value
Mxx 7.85e+21
Mxy -4.12e+21
Mxz 3.73e+21
Myy -9.61e+21
Myz -1.66e+21
Mzz 1.76e+21
##############
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######### T ############----
########## ############-----
##########################--------
--#########################---------
----#######################-----------
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-------###################------------ P
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------------##############----------------
--------------###########-----------------
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Global CMT Convention Moment Tensor:
R T P
1.76e+21 3.73e+21 1.66e+21
3.73e+21 7.85e+21 4.12e+21
1.66e+21 4.12e+21 -9.61e+21
Details of the solution is found at
http://www.eas.slu.edu/eqc/eqc_mt/MECH.NA/20200402015834/index.html
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STK = 125
DIP = 70
RAKE = 15
MW = 3.95
HS = 96.0
The NDK file is 20200402015834.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 0.03 n 3 lp c 0.10 n 3The results of this grid search are as follow:
DEPTH STK DIP RAKE MW FIT
WVFGRD96 2.0 20 55 -50 3.08 0.2359
WVFGRD96 4.0 220 65 15 3.10 0.2632
WVFGRD96 6.0 225 70 30 3.18 0.2945
WVFGRD96 8.0 225 65 30 3.27 0.3126
WVFGRD96 10.0 225 65 25 3.31 0.3158
WVFGRD96 12.0 220 70 20 3.34 0.3109
WVFGRD96 14.0 215 65 0 3.37 0.3014
WVFGRD96 16.0 315 65 25 3.40 0.2924
WVFGRD96 18.0 315 65 25 3.43 0.3010
WVFGRD96 20.0 315 65 25 3.46 0.3105
WVFGRD96 22.0 315 65 20 3.49 0.3225
WVFGRD96 24.0 310 75 20 3.50 0.3374
WVFGRD96 26.0 305 75 -30 3.54 0.3567
WVFGRD96 28.0 305 75 -25 3.55 0.3729
WVFGRD96 30.0 130 85 25 3.56 0.3846
WVFGRD96 32.0 130 85 25 3.58 0.3994
WVFGRD96 34.0 130 85 25 3.59 0.4070
WVFGRD96 36.0 135 75 30 3.62 0.4080
WVFGRD96 38.0 130 80 20 3.63 0.4076
WVFGRD96 40.0 135 70 30 3.70 0.4081
WVFGRD96 42.0 135 70 30 3.72 0.4092
WVFGRD96 44.0 135 70 30 3.74 0.4101
WVFGRD96 46.0 135 70 30 3.76 0.4131
WVFGRD96 48.0 135 65 30 3.79 0.4207
WVFGRD96 50.0 135 65 30 3.80 0.4353
WVFGRD96 52.0 135 60 30 3.83 0.4477
WVFGRD96 54.0 135 60 30 3.84 0.4620
WVFGRD96 56.0 130 60 25 3.85 0.4749
WVFGRD96 58.0 130 60 25 3.86 0.4891
WVFGRD96 60.0 130 60 25 3.87 0.5037
WVFGRD96 62.0 130 60 25 3.88 0.5173
WVFGRD96 64.0 130 60 25 3.89 0.5300
WVFGRD96 66.0 130 60 25 3.90 0.5405
WVFGRD96 68.0 130 60 25 3.91 0.5492
WVFGRD96 70.0 130 60 25 3.91 0.5575
WVFGRD96 72.0 130 60 25 3.92 0.5641
WVFGRD96 74.0 130 60 25 3.92 0.5697
WVFGRD96 76.0 125 60 20 3.93 0.5747
WVFGRD96 78.0 125 65 20 3.93 0.5784
WVFGRD96 80.0 125 65 20 3.93 0.5829
WVFGRD96 82.0 125 65 20 3.94 0.5854
WVFGRD96 84.0 125 65 20 3.94 0.5871
WVFGRD96 86.0 125 65 15 3.94 0.5896
WVFGRD96 88.0 125 65 15 3.94 0.5919
WVFGRD96 90.0 125 65 15 3.95 0.5929
WVFGRD96 92.0 125 65 15 3.95 0.5933
WVFGRD96 94.0 125 65 15 3.95 0.5930
WVFGRD96 96.0 125 70 15 3.95 0.5940
WVFGRD96 98.0 125 70 15 3.96 0.5935
The best solution is
WVFGRD96 96.0 125 70 15 3.95 0.5940
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 0.03 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