The ANSS event ID is ak019f5wxnfc and the event page is at https://earthquake.usgs.gov/earthquakes/eventpage/ak019f5wxnfc/executive.
2019/11/26 09:38:36 62.922 -150.512 86.6 4 Alaska
USGS/SLU Moment Tensor Solution
ENS 2019/11/26 09:38:36:0 62.92 -150.51 86.6 4.0 Alaska
Stations used:
AK.CCB AK.CUT AK.DHY AK.GHO AK.J20K AK.K24K AK.KLU AK.KNK
AK.L19K AK.L20K AK.MCK AK.PPLA AK.RND AK.SAW AK.SCM AK.SKN
AK.SSN AK.TRF AT.PMR AV.STLK IU.COLA TA.M22K
Filtering commands used:
cut o DIST/3.4 -40 o DIST/3.4 +45
rtr
taper w 0.1
hp c 0.03 n 3
lp c 0.08 n 3
Best Fitting Double Couple
Mo = 2.11e+22 dyne-cm
Mw = 4.15
Z = 98 km
Plane Strike Dip Rake
NP1 43 62 139
NP2 155 55 35
Principal Axes:
Axis Value Plunge Azimuth
T 2.11e+22 48 6
N 0.00e+00 42 194
P -2.11e+22 4 100
Moment Tensor: (dyne-cm)
Component Value
Mxx 8.83e+21
Mxy 4.75e+21
Mxz 1.08e+22
Myy -2.02e+22
Myz -4.39e+20
Mzz 1.14e+22
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------########### ###########---
-------########### T ###########----
--------########### ##########------
---------########################-------
---------#######################--------
-----------#####################----------
-----------####################-----------
------------#################-------------
-------------###############-----------
------------#############------------- P
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--------------#######-----------------
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################------------
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Global CMT Convention Moment Tensor:
R T P
1.14e+22 1.08e+22 4.39e+20
1.08e+22 8.83e+21 -4.75e+21
4.39e+20 -4.75e+21 -2.02e+22
Details of the solution is found at
http://www.eas.slu.edu/eqc/eqc_mt/MECH.NA/20191126093836/index.html
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STK = 155
DIP = 55
RAKE = 35
MW = 4.15
HS = 98.0
The NDK file is 20191126093836.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/3.4 -40 o DIST/3.4 +45 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 2.0 195 45 -90 3.35 0.2441
WVFGRD96 4.0 220 45 -50 3.37 0.2263
WVFGRD96 6.0 245 50 15 3.36 0.2478
WVFGRD96 8.0 250 45 20 3.44 0.2681
WVFGRD96 10.0 250 50 30 3.48 0.2845
WVFGRD96 12.0 250 50 30 3.51 0.2935
WVFGRD96 14.0 250 55 30 3.53 0.2932
WVFGRD96 16.0 345 55 45 3.57 0.2953
WVFGRD96 18.0 345 65 45 3.59 0.3027
WVFGRD96 20.0 345 65 45 3.61 0.3092
WVFGRD96 22.0 340 70 40 3.63 0.3159
WVFGRD96 24.0 340 65 35 3.66 0.3218
WVFGRD96 26.0 340 65 35 3.68 0.3257
WVFGRD96 28.0 335 75 35 3.70 0.3331
WVFGRD96 30.0 335 75 35 3.72 0.3418
WVFGRD96 32.0 335 75 35 3.74 0.3486
WVFGRD96 34.0 335 75 35 3.76 0.3535
WVFGRD96 36.0 335 75 35 3.78 0.3565
WVFGRD96 38.0 335 75 30 3.80 0.3561
WVFGRD96 40.0 340 70 45 3.89 0.3608
WVFGRD96 42.0 340 65 40 3.90 0.3548
WVFGRD96 44.0 340 65 40 3.92 0.3477
WVFGRD96 46.0 150 50 20 3.94 0.3393
WVFGRD96 48.0 150 50 20 3.96 0.3502
WVFGRD96 50.0 150 50 20 3.98 0.3622
WVFGRD96 52.0 155 55 30 3.98 0.3768
WVFGRD96 54.0 155 55 35 4.01 0.3954
WVFGRD96 56.0 155 55 35 4.02 0.4154
WVFGRD96 58.0 155 55 35 4.04 0.4348
WVFGRD96 60.0 155 55 35 4.05 0.4532
WVFGRD96 62.0 155 55 35 4.06 0.4707
WVFGRD96 64.0 160 50 40 4.08 0.4887
WVFGRD96 66.0 160 50 45 4.09 0.5074
WVFGRD96 68.0 160 50 45 4.10 0.5253
WVFGRD96 70.0 160 50 45 4.11 0.5406
WVFGRD96 72.0 160 50 45 4.11 0.5552
WVFGRD96 74.0 160 50 45 4.12 0.5670
WVFGRD96 76.0 160 50 45 4.12 0.5776
WVFGRD96 78.0 160 50 40 4.12 0.5864
WVFGRD96 80.0 160 50 40 4.13 0.5941
WVFGRD96 82.0 160 50 40 4.13 0.6003
WVFGRD96 84.0 160 50 40 4.13 0.6053
WVFGRD96 86.0 160 50 40 4.14 0.6093
WVFGRD96 88.0 160 50 40 4.14 0.6124
WVFGRD96 90.0 160 50 40 4.14 0.6150
WVFGRD96 92.0 155 55 35 4.14 0.6172
WVFGRD96 94.0 155 55 35 4.14 0.6189
WVFGRD96 96.0 155 55 35 4.14 0.6202
WVFGRD96 98.0 155 55 35 4.15 0.6211
WVFGRD96 100.0 155 55 35 4.15 0.6205
WVFGRD96 102.0 155 55 35 4.15 0.6203
WVFGRD96 104.0 155 55 35 4.15 0.6195
WVFGRD96 106.0 155 55 35 4.15 0.6193
WVFGRD96 108.0 155 55 35 4.15 0.6179
WVFGRD96 110.0 155 55 35 4.16 0.6157
WVFGRD96 112.0 155 60 45 4.15 0.6144
WVFGRD96 114.0 155 60 45 4.15 0.6133
WVFGRD96 116.0 155 60 45 4.15 0.6119
WVFGRD96 118.0 155 60 45 4.15 0.6099
The best solution is
WVFGRD96 98.0 155 55 35 4.15 0.6211
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/3.4 -40 o DIST/3.4 +45 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