The ANSS event ID is ak019g2w296x and the event page is at https://earthquake.usgs.gov/earthquakes/eventpage/ak019g2w296x/executive.
2019/12/16 03:30:27 61.411 -150.439 62.0 4.1 Alaska
USGS/SLU Moment Tensor Solution
ENS 2019/12/16 03:30:27:0 61.41 -150.44 62.0 4.1 Alaska
Stations used:
AK.GHO AK.KNK AK.L22K AK.PPLA AK.RC01 AK.SAW AK.SKN AK.SLK
AK.SSN AT.PMR AV.STLK TA.M22K TA.O22K
Filtering commands used:
cut o DIST/3.3 -40 o DIST/3.3 +50
rtr
taper w 0.1
hp c 0.03 n 3
lp c 0.10 n 3
Best Fitting Double Couple
Mo = 3.09e+22 dyne-cm
Mw = 4.26
Z = 70 km
Plane Strike Dip Rake
NP1 173 84 -125
NP2 75 35 -10
Principal Axes:
Axis Value Plunge Azimuth
T 3.09e+22 31 291
N 0.00e+00 34 177
P -3.09e+22 41 51
Moment Tensor: (dyne-cm)
Component Value
Mxx -4.02e+21
Mxy -1.64e+22
Mxz -4.68e+21
Myy 9.07e+21
Myz -2.46e+22
Mzz -5.04e+21
####----------
########--------------
###########-----------------
############------------------
###############-------------------
################---------- -------
#################---------- P --------
##### ##########---------- ---------
##### T ##########----------------------
###### ##########----------------------#
###################---------------------##
####################-------------------###
####################------------------####
###################-----------------####
-##################----------------#####
-##################-------------######
---###############----------########
-----############-------##########
----------####--##############
---------------#############
-------------#########
---------#####
Global CMT Convention Moment Tensor:
R T P
-5.04e+21 -4.68e+21 2.46e+22
-4.68e+21 -4.02e+21 1.64e+22
2.46e+22 1.64e+22 9.07e+21
Details of the solution is found at
http://www.eas.slu.edu/eqc/eqc_mt/MECH.NA/20191216033027/index.html
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STK = 75
DIP = 35
RAKE = -10
MW = 4.26
HS = 70.0
The NDK file is 20191216033027.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/3.3 -40 o DIST/3.3 +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 2.0 275 40 -90 3.59 0.2738
WVFGRD96 4.0 310 40 -35 3.61 0.3324
WVFGRD96 6.0 315 50 -25 3.64 0.3704
WVFGRD96 8.0 285 30 -85 3.83 0.3943
WVFGRD96 10.0 345 25 -40 3.87 0.3938
WVFGRD96 12.0 355 30 -35 3.90 0.3794
WVFGRD96 14.0 335 60 25 3.81 0.3690
WVFGRD96 16.0 65 70 35 3.86 0.3781
WVFGRD96 18.0 65 70 35 3.89 0.3891
WVFGRD96 20.0 75 40 25 3.96 0.4122
WVFGRD96 22.0 65 50 -5 3.93 0.4425
WVFGRD96 24.0 65 50 -5 3.95 0.4783
WVFGRD96 26.0 65 50 -5 3.97 0.5039
WVFGRD96 28.0 60 55 -15 3.97 0.5258
WVFGRD96 30.0 60 50 -15 4.00 0.5404
WVFGRD96 32.0 60 50 -15 4.00 0.5468
WVFGRD96 34.0 60 50 -15 4.01 0.5483
WVFGRD96 36.0 75 40 15 4.06 0.5490
WVFGRD96 38.0 75 45 15 4.07 0.5658
WVFGRD96 40.0 75 30 10 4.18 0.5752
WVFGRD96 42.0 75 30 5 4.20 0.5841
WVFGRD96 44.0 75 30 5 4.21 0.5929
WVFGRD96 46.0 75 35 5 4.20 0.5992
WVFGRD96 48.0 75 30 0 4.23 0.6103
WVFGRD96 50.0 75 30 0 4.24 0.6180
WVFGRD96 52.0 70 30 -10 4.24 0.6276
WVFGRD96 54.0 70 30 -10 4.25 0.6354
WVFGRD96 56.0 70 30 -15 4.25 0.6421
WVFGRD96 58.0 70 30 -15 4.26 0.6471
WVFGRD96 60.0 70 30 -15 4.26 0.6512
WVFGRD96 62.0 70 30 -15 4.26 0.6531
WVFGRD96 64.0 70 30 -15 4.27 0.6559
WVFGRD96 66.0 70 30 -15 4.27 0.6559
WVFGRD96 68.0 70 30 -15 4.27 0.6560
WVFGRD96 70.0 75 35 -10 4.26 0.6579
WVFGRD96 72.0 70 35 -15 4.27 0.6575
WVFGRD96 74.0 70 35 -15 4.27 0.6544
WVFGRD96 76.0 75 35 -15 4.27 0.6542
WVFGRD96 78.0 75 35 -15 4.28 0.6532
WVFGRD96 80.0 75 35 -15 4.28 0.6511
WVFGRD96 82.0 70 35 -20 4.28 0.6482
WVFGRD96 84.0 70 35 -20 4.28 0.6461
WVFGRD96 86.0 70 35 -20 4.29 0.6421
WVFGRD96 88.0 70 35 -20 4.29 0.6411
WVFGRD96 90.0 70 35 -20 4.29 0.6369
WVFGRD96 92.0 70 35 -20 4.30 0.6339
WVFGRD96 94.0 75 40 -15 4.29 0.6299
WVFGRD96 96.0 75 40 -15 4.30 0.6269
WVFGRD96 98.0 75 40 -15 4.30 0.6231
WVFGRD96 100.0 75 40 -20 4.31 0.6194
WVFGRD96 102.0 75 40 -20 4.31 0.6162
WVFGRD96 104.0 75 40 -20 4.31 0.6117
WVFGRD96 106.0 75 40 -20 4.31 0.6083
WVFGRD96 108.0 75 40 -20 4.32 0.6044
The best solution is
WVFGRD96 70.0 75 35 -10 4.26 0.6579
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/3.3 -40 o DIST/3.3 +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