The ANSS event ID is ak0201zcjff3 and the event page is at https://earthquake.usgs.gov/earthquakes/eventpage/ak0201zcjff3/executive.
2020/02/12 14:16:04 58.816 -154.489 117.7 4.5 Alaska
USGS/SLU Moment Tensor Solution
ENS 2020/02/12 14:16:04:0 58.82 -154.49 117.7 4.5 Alaska
Stations used:
AK.BRLK AK.CNP AK.HOM AK.N18K AK.O18K AK.O19K AK.P16K
AK.P17K AK.Q19K AV.ACH AV.ILSW AV.RED II.KDAK TA.N17K
TA.O15K TA.P18K TA.P19K
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 = 6.38e+22 dyne-cm
Mw = 4.47
Z = 134 km
Plane Strike Dip Rake
NP1 319 76 154
NP2 55 65 15
Principal Axes:
Axis Value Plunge Azimuth
T 6.38e+22 28 275
N 0.00e+00 61 113
P -6.38e+22 8 9
Moment Tensor: (dyne-cm)
Component Value
Mxx -6.10e+22
Mxy -1.32e+22
Mxz -6.25e+21
Myy 4.83e+22
Myz -2.74e+22
Mzz 1.27e+22
-------- P ---
------------ -------
----------------------------
####--------------------------
#########-------------------------
#############----------------------#
################-------------------###
###################----------------#####
####################--------------######
#### ################----------#########
#### T ##################-------##########
#### ###################----############
##########################################
########################----############
######################-------###########
##################-----------#########
############------------------######
-##--------------------------#####
----------------------------##
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Global CMT Convention Moment Tensor:
R T P
1.27e+22 -6.25e+21 2.74e+22
-6.25e+21 -6.10e+22 1.32e+22
2.74e+22 1.32e+22 4.83e+22
Details of the solution is found at
http://www.eas.slu.edu/eqc/eqc_mt/MECH.NA/20200212141604/index.html
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STK = 55
DIP = 65
RAKE = 15
MW = 4.47
HS = 134.0
The NDK file is 20200212141604.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 2.0 270 40 -85 3.57 0.1897
WVFGRD96 4.0 310 85 30 3.55 0.2121
WVFGRD96 6.0 135 70 30 3.63 0.2529
WVFGRD96 8.0 135 70 30 3.72 0.2839
WVFGRD96 10.0 135 70 30 3.77 0.3031
WVFGRD96 12.0 135 70 30 3.81 0.3063
WVFGRD96 14.0 135 70 30 3.85 0.2998
WVFGRD96 16.0 135 70 30 3.87 0.2868
WVFGRD96 18.0 135 75 35 3.89 0.2701
WVFGRD96 20.0 130 80 35 3.91 0.2484
WVFGRD96 22.0 220 60 25 3.93 0.2384
WVFGRD96 24.0 230 60 35 3.94 0.2406
WVFGRD96 26.0 230 60 35 3.95 0.2405
WVFGRD96 28.0 230 60 35 3.97 0.2389
WVFGRD96 30.0 230 70 40 3.98 0.2370
WVFGRD96 32.0 230 65 40 4.00 0.2354
WVFGRD96 34.0 230 65 40 4.00 0.2295
WVFGRD96 36.0 230 65 40 4.01 0.2236
WVFGRD96 38.0 230 65 40 4.03 0.2171
WVFGRD96 40.0 20 55 -45 4.13 0.2177
WVFGRD96 42.0 20 55 -45 4.15 0.2176
WVFGRD96 44.0 20 55 -45 4.17 0.2138
WVFGRD96 46.0 45 60 30 4.19 0.2235
WVFGRD96 48.0 45 60 30 4.21 0.2337
WVFGRD96 50.0 45 60 30 4.23 0.2442
WVFGRD96 52.0 45 60 30 4.25 0.2554
WVFGRD96 54.0 50 60 35 4.26 0.2692
WVFGRD96 56.0 50 60 35 4.28 0.2895
WVFGRD96 58.0 50 60 30 4.29 0.3143
WVFGRD96 60.0 55 55 30 4.31 0.3393
WVFGRD96 62.0 55 55 25 4.32 0.3638
WVFGRD96 64.0 55 55 25 4.34 0.3875
WVFGRD96 66.0 60 50 20 4.36 0.4073
WVFGRD96 68.0 60 50 20 4.37 0.4254
WVFGRD96 70.0 60 50 15 4.39 0.4403
WVFGRD96 72.0 60 50 15 4.39 0.4544
WVFGRD96 74.0 60 50 15 4.40 0.4650
WVFGRD96 76.0 60 55 25 4.39 0.4757
WVFGRD96 78.0 60 55 25 4.40 0.4865
WVFGRD96 80.0 60 55 25 4.40 0.4957
WVFGRD96 82.0 60 55 25 4.41 0.5041
WVFGRD96 84.0 60 55 25 4.41 0.5123
WVFGRD96 86.0 60 55 25 4.42 0.5182
WVFGRD96 88.0 60 55 20 4.42 0.5246
WVFGRD96 90.0 60 55 20 4.43 0.5292
WVFGRD96 92.0 60 55 20 4.43 0.5352
WVFGRD96 94.0 60 55 20 4.43 0.5401
WVFGRD96 96.0 60 55 20 4.44 0.5447
WVFGRD96 98.0 60 55 20 4.44 0.5486
WVFGRD96 100.0 60 55 15 4.45 0.5520
WVFGRD96 102.0 60 60 20 4.44 0.5556
WVFGRD96 104.0 60 60 20 4.44 0.5592
WVFGRD96 106.0 60 60 20 4.44 0.5626
WVFGRD96 108.0 60 60 20 4.44 0.5661
WVFGRD96 110.0 60 60 20 4.45 0.5680
WVFGRD96 112.0 55 65 20 4.44 0.5702
WVFGRD96 114.0 55 65 20 4.44 0.5717
WVFGRD96 116.0 55 65 20 4.45 0.5734
WVFGRD96 118.0 55 65 20 4.45 0.5754
WVFGRD96 120.0 55 65 20 4.45 0.5766
WVFGRD96 122.0 55 65 15 4.46 0.5781
WVFGRD96 124.0 55 65 15 4.46 0.5799
WVFGRD96 126.0 55 65 15 4.46 0.5807
WVFGRD96 128.0 55 65 15 4.47 0.5810
WVFGRD96 130.0 55 65 15 4.47 0.5812
WVFGRD96 132.0 55 65 15 4.47 0.5819
WVFGRD96 134.0 55 65 15 4.47 0.5823
WVFGRD96 136.0 55 65 15 4.48 0.5817
WVFGRD96 138.0 55 65 15 4.48 0.5806
WVFGRD96 140.0 55 70 15 4.48 0.5810
WVFGRD96 142.0 55 70 15 4.48 0.5809
WVFGRD96 144.0 55 70 15 4.48 0.5800
WVFGRD96 146.0 55 70 15 4.48 0.5788
WVFGRD96 148.0 55 70 15 4.49 0.5791
The best solution is
WVFGRD96 134.0 55 65 15 4.47 0.5823
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