The ANSS event ID is ak020sg2qhx and the event page is at https://earthquake.usgs.gov/earthquakes/eventpage/ak020sg2qhx/executive.
2020/01/17 20:05:43 62.868 -148.165 71.0 4 Alaska
USGS/SLU Moment Tensor Solution
ENS 2020/01/17 20:05:43:0 62.87 -148.16 71.0 4.0 Alaska
Stations used:
AK.BPAW AK.CCB AK.DHY AK.DIV AK.GHO AK.J25K AK.K24K AK.KLU
AK.KNK AK.KTH AK.L26K AK.MCK AK.PAX AK.RC01 AK.RND AK.SAW
AK.SCM AK.SKN AK.SSN AK.TRF AK.WRH AT.MENT AT.PMR IM.IL31
TA.M22K TA.M24K TA.N25K
Filtering commands used:
cut o DIST/3.4 -40 o DIST/3.4 +50
rtr
taper w 0.1
hp c 0.03 n 3
lp c 0.10 n 3
br c 0.12 0.25 n 4 p 2
Best Fitting Double Couple
Mo = 2.19e+22 dyne-cm
Mw = 4.16
Z = 80 km
Plane Strike Dip Rake
NP1 38 83 135
NP2 135 45 10
Principal Axes:
Axis Value Plunge Azimuth
T 2.19e+22 36 346
N 0.00e+00 44 211
P -2.19e+22 24 95
Moment Tensor: (dyne-cm)
Component Value
Mxx 1.33e+22
Mxy -1.90e+21
Mxz 1.08e+22
Myy -1.71e+22
Myz -1.08e+22
Mzz 3.80e+21
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########## T ##############---
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-------#################----------- ----
--------###############------------ P ----
---------############-------------- ----
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Global CMT Convention Moment Tensor:
R T P
3.80e+21 1.08e+22 1.08e+22
1.08e+22 1.33e+22 1.90e+21
1.08e+22 1.90e+21 -1.71e+22
Details of the solution is found at
http://www.eas.slu.edu/eqc/eqc_mt/MECH.NA/20200117200543/index.html
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STK = 135
DIP = 45
RAKE = 10
MW = 4.16
HS = 80.0
The NDK file is 20200117200543.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 +50 rtr taper w 0.1 hp c 0.03 n 3 lp c 0.10 n 3 br c 0.12 0.25 n 4 p 2The results of this grid search are as follow:
DEPTH STK DIP RAKE MW FIT
WVFGRD96 2.0 305 80 -5 3.30 0.3518
WVFGRD96 4.0 305 60 -10 3.41 0.4023
WVFGRD96 6.0 40 85 40 3.47 0.4313
WVFGRD96 8.0 40 85 40 3.52 0.4445
WVFGRD96 10.0 45 75 40 3.55 0.4445
WVFGRD96 12.0 45 75 35 3.56 0.4403
WVFGRD96 14.0 40 80 30 3.57 0.4312
WVFGRD96 16.0 45 75 30 3.59 0.4187
WVFGRD96 18.0 305 65 -20 3.61 0.4119
WVFGRD96 20.0 305 65 -20 3.63 0.4044
WVFGRD96 22.0 315 55 20 3.67 0.3985
WVFGRD96 24.0 310 65 20 3.69 0.3981
WVFGRD96 26.0 310 65 20 3.71 0.4025
WVFGRD96 28.0 125 70 5 3.75 0.4136
WVFGRD96 30.0 125 75 0 3.77 0.4252
WVFGRD96 32.0 125 75 0 3.79 0.4345
WVFGRD96 34.0 125 75 0 3.81 0.4402
WVFGRD96 36.0 130 65 15 3.85 0.4472
WVFGRD96 38.0 130 70 10 3.87 0.4588
WVFGRD96 40.0 130 55 10 3.95 0.4766
WVFGRD96 42.0 130 55 10 3.98 0.4848
WVFGRD96 44.0 130 60 10 3.99 0.4958
WVFGRD96 46.0 135 55 20 4.02 0.5118
WVFGRD96 48.0 135 55 20 4.04 0.5270
WVFGRD96 50.0 135 55 20 4.06 0.5425
WVFGRD96 52.0 135 55 15 4.06 0.5579
WVFGRD96 54.0 135 55 15 4.08 0.5745
WVFGRD96 56.0 135 55 15 4.09 0.5883
WVFGRD96 58.0 130 45 10 4.11 0.5995
WVFGRD96 60.0 130 45 10 4.11 0.6140
WVFGRD96 62.0 130 45 10 4.12 0.6267
WVFGRD96 64.0 135 45 15 4.13 0.6388
WVFGRD96 66.0 135 45 15 4.14 0.6489
WVFGRD96 68.0 135 45 15 4.14 0.6579
WVFGRD96 70.0 135 45 15 4.15 0.6645
WVFGRD96 72.0 135 45 15 4.15 0.6694
WVFGRD96 74.0 135 45 15 4.15 0.6732
WVFGRD96 76.0 135 45 15 4.16 0.6752
WVFGRD96 78.0 135 45 15 4.16 0.6760
WVFGRD96 80.0 135 45 10 4.16 0.6764
WVFGRD96 82.0 135 45 10 4.16 0.6763
WVFGRD96 84.0 140 45 15 4.17 0.6758
WVFGRD96 86.0 140 45 15 4.18 0.6757
WVFGRD96 88.0 140 45 15 4.18 0.6737
WVFGRD96 90.0 140 45 15 4.18 0.6721
WVFGRD96 92.0 140 45 15 4.18 0.6685
WVFGRD96 94.0 140 45 15 4.18 0.6645
WVFGRD96 96.0 140 45 15 4.18 0.6618
WVFGRD96 98.0 140 45 15 4.19 0.6579
The best solution is
WVFGRD96 80.0 135 45 10 4.16 0.6764
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 +50 rtr taper w 0.1 hp c 0.03 n 3 lp c 0.10 n 3 br c 0.12 0.25 n 4 p 2
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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