The ANSS event ID is ak0201kay1xy and the event page is at https://earthquake.usgs.gov/earthquakes/eventpage/ak0201kay1xy/executive.
2020/02/03 04:49:09 60.580 -152.362 93.5 4.3 Alaska
USGS/SLU Moment Tensor Solution
ENS 2020/02/03 04:49:09:0 60.58 -152.36 93.5 4.3 Alaska
Stations used:
AK.BRLK AK.CNP AK.GHO AK.HOM AK.L22K AK.M20K AK.RC01 AK.SKN
AK.SSN AT.PMR
Filtering commands used:
cut o DIST/3.5 -40 o DIST/3.5 +50
rtr
taper w 0.1
hp c 0.03 n 3
lp c 0.10 n 3
Best Fitting Double Couple
Mo = 6.84e+22 dyne-cm
Mw = 4.49
Z = 92 km
Plane Strike Dip Rake
NP1 70 60 55
NP2 304 45 135
Principal Axes:
Axis Value Plunge Azimuth
T 6.84e+22 59 288
N 0.00e+00 30 89
P -6.84e+22 9 184
Moment Tensor: (dyne-cm)
Component Value
Mxx -6.47e+22
Mxy -1.04e+22
Mxz 1.96e+22
Myy 1.62e+22
Myz -2.80e+22
Mzz 4.85e+22
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########### ##################------##
############ T ###################----####
############ ####################-######
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-------- -----------
---- P -------
Global CMT Convention Moment Tensor:
R T P
4.85e+22 1.96e+22 2.80e+22
1.96e+22 -6.47e+22 1.04e+22
2.80e+22 1.04e+22 1.62e+22
Details of the solution is found at
http://www.eas.slu.edu/eqc/eqc_mt/MECH.NA/20200203044909/index.html
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STK = 70
DIP = 60
RAKE = 55
MW = 4.49
HS = 92.0
The NDK file is 20200203044909.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/3.5 -40 o DIST/3.5 +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 95 40 -85 3.69 0.2047
WVFGRD96 4.0 335 35 -10 3.74 0.2487
WVFGRD96 6.0 335 40 -10 3.78 0.2926
WVFGRD96 8.0 335 40 -10 3.86 0.3142
WVFGRD96 10.0 260 70 45 3.91 0.3211
WVFGRD96 12.0 255 75 40 3.94 0.3331
WVFGRD96 14.0 255 75 40 3.98 0.3360
WVFGRD96 16.0 235 80 40 3.99 0.3371
WVFGRD96 18.0 235 80 40 4.02 0.3378
WVFGRD96 20.0 235 60 30 4.07 0.3503
WVFGRD96 22.0 240 55 35 4.10 0.3674
WVFGRD96 24.0 240 55 30 4.12 0.3781
WVFGRD96 26.0 240 55 30 4.13 0.3804
WVFGRD96 28.0 235 85 35 4.14 0.3815
WVFGRD96 30.0 240 80 35 4.15 0.3852
WVFGRD96 32.0 240 55 25 4.17 0.3995
WVFGRD96 34.0 240 60 30 4.19 0.4151
WVFGRD96 36.0 240 60 30 4.21 0.4256
WVFGRD96 38.0 240 60 30 4.24 0.4269
WVFGRD96 40.0 240 60 35 4.31 0.4230
WVFGRD96 42.0 240 55 35 4.35 0.4172
WVFGRD96 44.0 240 60 35 4.36 0.4097
WVFGRD96 46.0 240 60 35 4.38 0.4029
WVFGRD96 48.0 240 60 35 4.39 0.3969
WVFGRD96 50.0 240 60 30 4.40 0.3928
WVFGRD96 52.0 10 45 -85 4.51 0.3948
WVFGRD96 54.0 10 45 -85 4.52 0.4019
WVFGRD96 56.0 55 70 30 4.40 0.4176
WVFGRD96 58.0 55 70 35 4.41 0.4385
WVFGRD96 60.0 60 70 40 4.43 0.4576
WVFGRD96 62.0 65 65 45 4.44 0.4745
WVFGRD96 64.0 60 70 45 4.44 0.4884
WVFGRD96 66.0 65 65 50 4.46 0.5009
WVFGRD96 68.0 65 65 50 4.46 0.5130
WVFGRD96 70.0 65 65 50 4.46 0.5221
WVFGRD96 72.0 65 65 50 4.47 0.5289
WVFGRD96 74.0 65 65 50 4.47 0.5322
WVFGRD96 76.0 65 65 50 4.47 0.5385
WVFGRD96 78.0 65 65 50 4.47 0.5425
WVFGRD96 80.0 65 65 50 4.47 0.5436
WVFGRD96 82.0 65 65 50 4.47 0.5445
WVFGRD96 84.0 70 60 55 4.48 0.5439
WVFGRD96 86.0 70 60 55 4.48 0.5449
WVFGRD96 88.0 70 60 55 4.48 0.5454
WVFGRD96 90.0 70 60 55 4.48 0.5469
WVFGRD96 92.0 70 60 55 4.49 0.5472
WVFGRD96 94.0 70 60 55 4.49 0.5465
WVFGRD96 96.0 70 60 55 4.49 0.5465
WVFGRD96 98.0 70 60 55 4.49 0.5463
WVFGRD96 100.0 70 60 55 4.49 0.5464
WVFGRD96 102.0 70 60 55 4.49 0.5457
WVFGRD96 104.0 70 60 55 4.49 0.5448
WVFGRD96 106.0 70 60 55 4.49 0.5442
WVFGRD96 108.0 70 60 55 4.49 0.5438
WVFGRD96 110.0 70 60 55 4.50 0.5444
WVFGRD96 112.0 65 60 50 4.48 0.5449
WVFGRD96 114.0 65 60 50 4.48 0.5449
WVFGRD96 116.0 65 60 50 4.49 0.5444
WVFGRD96 118.0 65 60 50 4.49 0.5431
WVFGRD96 120.0 65 60 50 4.49 0.5407
WVFGRD96 122.0 65 60 50 4.49 0.5406
WVFGRD96 124.0 70 55 50 4.49 0.5410
WVFGRD96 126.0 70 55 50 4.50 0.5403
WVFGRD96 128.0 70 55 50 4.50 0.5388
WVFGRD96 130.0 70 55 50 4.50 0.5366
WVFGRD96 132.0 70 55 50 4.50 0.5375
WVFGRD96 134.0 65 55 50 4.50 0.5371
WVFGRD96 136.0 65 55 50 4.50 0.5351
WVFGRD96 138.0 65 55 50 4.50 0.5342
WVFGRD96 140.0 65 55 50 4.50 0.5349
WVFGRD96 142.0 65 55 50 4.51 0.5332
WVFGRD96 144.0 70 50 50 4.51 0.5302
WVFGRD96 146.0 70 50 50 4.51 0.5312
WVFGRD96 148.0 70 50 50 4.51 0.5314
The best solution is
WVFGRD96 92.0 70 60 55 4.49 0.5472
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/3.5 -40 o DIST/3.5 +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