The ANSS event ID is ak020246zot2 and the event page is at https://earthquake.usgs.gov/earthquakes/eventpage/ak020246zot2/executive.
2020/02/15 07:17:21 62.831 -149.579 83.1 4.2 Alaska
USGS/SLU Moment Tensor Solution
ENS 2020/02/15 07:17:21:0 62.83 -149.58 83.1 4.2 Alaska
Stations used:
AK.BPAW AK.CCB AK.CUT AK.DHY AK.GHO AK.K20K AK.K24K AK.KLU
AK.KNK AK.KTH AK.L22K AK.M20K AK.MCK AK.NEA2 AK.RND AK.SAW
AK.SCM AK.SKN AK.SSN AK.TRF AK.WRH TA.M22K
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 = 4.07e+22 dyne-cm
Mw = 4.34
Z = 106 km
Plane Strike Dip Rake
NP1 140 70 30
NP2 39 62 157
Principal Axes:
Axis Value Plunge Azimuth
T 4.07e+22 35 2
N 0.00e+00 54 171
P -4.07e+22 5 268
Moment Tensor: (dyne-cm)
Component Value
Mxx 2.72e+22
Mxy -6.90e+20
Mxz 1.93e+22
Myy -4.03e+22
Myz 4.20e+21
Mzz 1.31e+22
##############
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-############ ###########-
--############ T ###########--
----############ ###########----
------#########################-----
--------#######################-------
----------######################--------
-----------####################---------
-------------##################-----------
-----------################------------
P ------------##############-------------
--------------###########--------------
-----------------########---------------
-------------------#####----------------
--------------------#-----------------
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----#######################-
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Global CMT Convention Moment Tensor:
R T P
1.31e+22 1.93e+22 -4.20e+21
1.93e+22 2.72e+22 6.90e+20
-4.20e+21 6.90e+20 -4.03e+22
Details of the solution is found at
http://www.eas.slu.edu/eqc/eqc_mt/MECH.NA/20200215071721/index.html
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STK = 140
DIP = 70
RAKE = 30
MW = 4.34
HS = 106.0
The NDK file is 20200215071721.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 25 55 -55 3.45 0.2088
WVFGRD96 4.0 235 70 45 3.51 0.2466
WVFGRD96 6.0 235 75 50 3.60 0.2868
WVFGRD96 8.0 235 75 55 3.71 0.3100
WVFGRD96 10.0 235 75 50 3.74 0.3164
WVFGRD96 12.0 235 75 50 3.78 0.3105
WVFGRD96 14.0 235 75 50 3.81 0.2957
WVFGRD96 16.0 325 45 15 3.83 0.3015
WVFGRD96 18.0 325 60 25 3.83 0.3110
WVFGRD96 20.0 325 60 25 3.86 0.3267
WVFGRD96 22.0 325 60 20 3.89 0.3457
WVFGRD96 24.0 325 60 20 3.91 0.3669
WVFGRD96 26.0 320 70 20 3.92 0.3880
WVFGRD96 28.0 320 70 20 3.94 0.4075
WVFGRD96 30.0 320 75 20 3.95 0.4236
WVFGRD96 32.0 320 75 20 3.96 0.4321
WVFGRD96 34.0 320 75 20 3.97 0.4330
WVFGRD96 36.0 320 75 20 3.99 0.4276
WVFGRD96 38.0 135 80 -15 4.00 0.4213
WVFGRD96 40.0 130 65 -20 4.07 0.4175
WVFGRD96 42.0 130 55 -15 4.11 0.4129
WVFGRD96 44.0 140 55 20 4.14 0.4128
WVFGRD96 46.0 140 60 20 4.14 0.4145
WVFGRD96 48.0 140 60 25 4.16 0.4214
WVFGRD96 50.0 140 65 25 4.17 0.4296
WVFGRD96 52.0 140 65 25 4.18 0.4404
WVFGRD96 54.0 140 65 25 4.19 0.4499
WVFGRD96 56.0 140 65 25 4.20 0.4611
WVFGRD96 58.0 140 65 30 4.22 0.4730
WVFGRD96 60.0 140 65 30 4.23 0.4836
WVFGRD96 62.0 140 65 30 4.24 0.4931
WVFGRD96 64.0 140 70 30 4.24 0.5026
WVFGRD96 66.0 140 70 30 4.25 0.5106
WVFGRD96 68.0 140 70 30 4.25 0.5183
WVFGRD96 70.0 140 70 30 4.26 0.5239
WVFGRD96 72.0 140 70 30 4.26 0.5315
WVFGRD96 74.0 140 70 30 4.27 0.5357
WVFGRD96 76.0 140 70 30 4.28 0.5429
WVFGRD96 78.0 140 70 30 4.28 0.5469
WVFGRD96 80.0 140 70 30 4.29 0.5512
WVFGRD96 82.0 140 70 30 4.29 0.5556
WVFGRD96 84.0 140 70 30 4.30 0.5592
WVFGRD96 86.0 140 70 30 4.30 0.5628
WVFGRD96 88.0 140 70 30 4.31 0.5664
WVFGRD96 90.0 140 70 30 4.31 0.5679
WVFGRD96 92.0 140 70 30 4.31 0.5692
WVFGRD96 94.0 140 70 30 4.32 0.5726
WVFGRD96 96.0 140 70 30 4.32 0.5735
WVFGRD96 98.0 140 70 30 4.33 0.5735
WVFGRD96 100.0 140 70 30 4.33 0.5754
WVFGRD96 102.0 140 70 30 4.33 0.5754
WVFGRD96 104.0 140 70 30 4.34 0.5740
WVFGRD96 106.0 140 70 30 4.34 0.5754
WVFGRD96 108.0 135 70 25 4.34 0.5741
WVFGRD96 110.0 135 70 25 4.35 0.5736
WVFGRD96 112.0 135 70 25 4.35 0.5742
WVFGRD96 114.0 135 70 25 4.35 0.5722
WVFGRD96 116.0 135 70 25 4.36 0.5730
WVFGRD96 118.0 135 70 25 4.36 0.5708
The best solution is
WVFGRD96 106.0 140 70 30 4.34 0.5754
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