The ANSS event ID is ak018fhjd3wg and the event page is at https://earthquake.usgs.gov/earthquakes/eventpage/ak018fhjd3wg/executive.
2018/12/03 12:18:58 61.246 -149.972 44.5 2.9 Alaska
USGS/SLU Moment Tensor Solution
ENS 2018/12/03 12:18:58:0 61.25 -149.97 44.5 2.9 Alaska
Stations used:
AK.GHO AK.KNK AK.RC01 AK.SAW AK.SKN AK.SSN TA.M20K TA.M22K
Filtering commands used:
cut o DIST/3.3 -40 o DIST/3.3 +40
rtr
taper w 0.1
hp c 0.03 n 3
lp c 0.10 n 3
Best Fitting Double Couple
Mo = 1.10e+22 dyne-cm
Mw = 3.96
Z = 50 km
Plane Strike Dip Rake
NP1 175 50 -85
NP2 347 40 -96
Principal Axes:
Axis Value Plunge Azimuth
T 1.10e+22 5 261
N 0.00e+00 4 352
P -1.10e+22 84 120
Moment Tensor: (dyne-cm)
Component Value
Mxx 2.09e+20
Mxy 1.65e+21
Mxz 4.47e+20
Myy 1.05e+22
Myz -1.94e+21
Mzz -1.08e+22
####-#########
######-------#########
########-----------#########
########-------------#########
#########----------------#########
#########------------------#########
##########-------------------#########
##########---------------------#########
##########----------------------########
###########----------------------#########
###########---------- ---------#########
########---------- P ----------########
T ########---------- ----------########
#########----------------------#######
###########---------------------########
###########--------------------#######
##########--------------------######
##########------------------######
#########----------------#####
##########-------------#####
########-----------###
#######------#
Global CMT Convention Moment Tensor:
R T P
-1.08e+22 4.47e+20 1.94e+21
4.47e+20 2.09e+20 -1.65e+21
1.94e+21 -1.65e+21 1.05e+22
Details of the solution is found at
http://www.eas.slu.edu/eqc/eqc_mt/MECH.NA/20181203121858/index.html
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STK = 175
DIP = 50
RAKE = -85
MW = 3.96
HS = 50.0
The NDK file is 20181203121858.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 +40 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 1.0 185 45 95 3.10 0.1741
WVFGRD96 2.0 10 45 90 3.27 0.2306
WVFGRD96 3.0 340 25 65 3.33 0.2025
WVFGRD96 4.0 5 85 65 3.37 0.2583
WVFGRD96 5.0 15 80 75 3.37 0.3020
WVFGRD96 6.0 160 15 50 3.37 0.3348
WVFGRD96 7.0 145 20 35 3.38 0.3589
WVFGRD96 8.0 170 15 60 3.47 0.3762
WVFGRD96 9.0 155 20 45 3.48 0.3909
WVFGRD96 10.0 165 20 55 3.49 0.4001
WVFGRD96 11.0 165 25 55 3.50 0.4067
WVFGRD96 12.0 175 25 65 3.52 0.4122
WVFGRD96 13.0 175 25 65 3.53 0.4144
WVFGRD96 14.0 170 25 60 3.54 0.4139
WVFGRD96 15.0 170 25 60 3.55 0.4110
WVFGRD96 16.0 170 25 60 3.56 0.4063
WVFGRD96 17.0 170 25 60 3.57 0.4002
WVFGRD96 18.0 170 20 60 3.57 0.3934
WVFGRD96 19.0 170 20 60 3.58 0.3861
WVFGRD96 20.0 165 20 55 3.59 0.3772
WVFGRD96 21.0 10 80 45 3.68 0.3718
WVFGRD96 22.0 10 80 45 3.69 0.3670
WVFGRD96 23.0 25 85 65 3.65 0.3615
WVFGRD96 24.0 200 90 -65 3.65 0.3575
WVFGRD96 25.0 25 85 65 3.67 0.3564
WVFGRD96 26.0 200 90 -65 3.68 0.3534
WVFGRD96 27.0 25 85 60 3.69 0.3521
WVFGRD96 28.0 200 75 -65 3.69 0.3560
WVFGRD96 29.0 205 75 -65 3.69 0.3625
WVFGRD96 30.0 200 75 -65 3.71 0.3685
WVFGRD96 31.0 200 70 -65 3.72 0.3788
WVFGRD96 32.0 5 30 -85 3.74 0.3927
WVFGRD96 33.0 180 60 -95 3.75 0.4129
WVFGRD96 34.0 5 30 -85 3.76 0.4329
WVFGRD96 35.0 5 30 -85 3.76 0.4473
WVFGRD96 36.0 185 50 -80 3.77 0.4671
WVFGRD96 37.0 185 50 -80 3.77 0.4831
WVFGRD96 38.0 185 50 -80 3.78 0.4987
WVFGRD96 39.0 185 50 -80 3.80 0.5109
WVFGRD96 40.0 180 50 -80 3.88 0.5142
WVFGRD96 41.0 180 50 -80 3.90 0.5225
WVFGRD96 42.0 180 50 -80 3.91 0.5303
WVFGRD96 43.0 180 50 -80 3.92 0.5387
WVFGRD96 44.0 180 50 -80 3.93 0.5455
WVFGRD96 45.0 180 50 -80 3.93 0.5528
WVFGRD96 46.0 180 50 -80 3.94 0.5563
WVFGRD96 47.0 180 50 -80 3.94 0.5612
WVFGRD96 48.0 175 50 -80 3.96 0.5632
WVFGRD96 49.0 175 50 -85 3.96 0.5657
WVFGRD96 50.0 175 50 -85 3.96 0.5665
WVFGRD96 51.0 175 50 -85 3.96 0.5657
WVFGRD96 52.0 345 40 -100 3.97 0.5663
WVFGRD96 53.0 175 50 -85 3.97 0.5645
WVFGRD96 54.0 175 50 -85 3.97 0.5658
WVFGRD96 55.0 350 40 -95 3.97 0.5646
WVFGRD96 56.0 170 50 -95 3.98 0.5642
WVFGRD96 57.0 0 40 -80 3.99 0.5639
WVFGRD96 58.0 0 40 -80 3.99 0.5627
WVFGRD96 59.0 5 40 -75 4.00 0.5634
The best solution is
WVFGRD96 50.0 175 50 -85 3.96 0.5665
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 +40 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