The ANSS event ID is ak017brhvmq0 and the event page is at https://earthquake.usgs.gov/earthquakes/eventpage/ak017brhvmq0/executive.
2017/09/13 07:22:18 62.898 -149.921 79.6 4.2 Alaska
USGS/SLU Moment Tensor Solution
ENS 2017/09/13 07:22:18:0 62.90 -149.92 79.6 4.2 Alaska
Stations used:
AK.CAST AK.CUT AK.DHY AK.DIV AK.FID AK.GHO AK.GLI AK.KNK
AK.KTH AK.MLY AK.NEA2 AK.PPLA AK.PWL AK.RC01 AK.RND AK.SAW
AK.SCM AK.SKN AK.SSN AK.TRF AT.PMR TA.I23K TA.J20K TA.K20K
TA.M19K TA.M22K TA.M24K
Filtering commands used:
cut o DIST/3.5 -30 o DIST/3.5 +70
rtr
taper w 0.1
hp c 0.03 n 3
lp c 0.10 n 3
Best Fitting Double Couple
Mo = 2.34e+22 dyne-cm
Mw = 4.18
Z = 78 km
Plane Strike Dip Rake
NP1 21 80 102
NP2 150 15 40
Principal Axes:
Axis Value Plunge Azimuth
T 2.34e+22 53 305
N 0.00e+00 11 199
P -2.34e+22 34 101
Moment Tensor: (dyne-cm)
Component Value
Mxx 2.14e+21
Mxy -9.38e+20
Mxz 8.50e+21
Myy -9.68e+21
Myz -2.00e+22
Mzz 7.53e+21
##############
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--######################--------------
--######### ###########---------------
--######### T ##########----------------
---######### ##########-----------------
---#####################------------------
---####################---------- ------
---####################---------- P ------
---##################----------- -----
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Global CMT Convention Moment Tensor:
R T P
7.53e+21 8.50e+21 2.00e+22
8.50e+21 2.14e+21 9.38e+20
2.00e+22 9.38e+20 -9.68e+21
Details of the solution is found at
http://www.eas.slu.edu/eqc/eqc_mt/MECH.NA/20170913072218/index.html
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STK = 150
DIP = 15
RAKE = 40
MW = 4.18
HS = 78.0
The NDK file is 20170913072218.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 -30 o DIST/3.5 +70 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 10 70 5 3.21 0.1439
WVFGRD96 4.0 10 70 10 3.32 0.1688
WVFGRD96 6.0 10 70 10 3.39 0.1758
WVFGRD96 8.0 275 75 -15 3.48 0.1854
WVFGRD96 10.0 275 70 -15 3.53 0.1885
WVFGRD96 12.0 100 75 10 3.57 0.1952
WVFGRD96 14.0 100 70 10 3.61 0.2023
WVFGRD96 16.0 100 70 10 3.64 0.2077
WVFGRD96 18.0 100 75 5 3.67 0.2144
WVFGRD96 20.0 100 70 0 3.70 0.2254
WVFGRD96 22.0 100 70 0 3.72 0.2361
WVFGRD96 24.0 100 70 0 3.74 0.2450
WVFGRD96 26.0 100 65 5 3.77 0.2547
WVFGRD96 28.0 105 60 20 3.79 0.2645
WVFGRD96 30.0 105 60 15 3.81 0.2742
WVFGRD96 32.0 105 55 20 3.83 0.2841
WVFGRD96 34.0 105 50 15 3.85 0.2918
WVFGRD96 36.0 100 55 0 3.85 0.2988
WVFGRD96 38.0 100 60 0 3.87 0.3072
WVFGRD96 40.0 155 25 55 4.02 0.3137
WVFGRD96 42.0 150 25 50 4.03 0.3205
WVFGRD96 44.0 150 25 45 4.05 0.3452
WVFGRD96 46.0 150 25 45 4.06 0.3697
WVFGRD96 48.0 150 25 45 4.08 0.3908
WVFGRD96 50.0 150 25 45 4.09 0.4126
WVFGRD96 52.0 150 25 45 4.11 0.4297
WVFGRD96 54.0 150 25 45 4.12 0.4462
WVFGRD96 56.0 150 25 45 4.13 0.4587
WVFGRD96 58.0 155 15 50 4.13 0.4706
WVFGRD96 60.0 150 15 45 4.14 0.4846
WVFGRD96 62.0 145 15 40 4.14 0.4983
WVFGRD96 64.0 145 15 35 4.14 0.5115
WVFGRD96 66.0 145 15 35 4.15 0.5235
WVFGRD96 68.0 145 15 35 4.16 0.5324
WVFGRD96 70.0 145 15 35 4.16 0.5408
WVFGRD96 72.0 145 15 35 4.17 0.5478
WVFGRD96 74.0 150 15 40 4.17 0.5512
WVFGRD96 76.0 150 15 40 4.18 0.5524
WVFGRD96 78.0 150 15 40 4.18 0.5536
WVFGRD96 80.0 145 15 30 4.18 0.5531
WVFGRD96 82.0 145 15 30 4.18 0.5503
WVFGRD96 84.0 145 15 30 4.18 0.5520
WVFGRD96 86.0 140 15 25 4.18 0.5512
WVFGRD96 88.0 140 15 25 4.18 0.5472
WVFGRD96 90.0 140 15 25 4.19 0.5478
WVFGRD96 92.0 140 15 25 4.19 0.5439
WVFGRD96 94.0 140 15 25 4.19 0.5399
WVFGRD96 96.0 140 15 20 4.19 0.5357
WVFGRD96 98.0 145 10 30 4.19 0.5302
The best solution is
WVFGRD96 78.0 150 15 40 4.18 0.5536
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 -30 o DIST/3.5 +70 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