The ANSS event ID is ak018fe45vii and the event page is at https://earthquake.usgs.gov/earthquakes/eventpage/ak018fe45vii/executive.
2018/12/01 05:25:40 61.463 -149.885 31.1 4 Alaska
USGS/SLU Moment Tensor Solution
ENS 2018/12/01 05:25:40:0 61.46 -149.88 31.1 4.0 Alaska
Stations used:
AK.GHO AK.KNK AK.PWL AK.RC01 AK.SAW AK.SKN AK.SSN AV.STLK
TA.M22K
Filtering commands used:
cut o DIST/3.3 -30 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 = 1.30e+22 dyne-cm
Mw = 4.01
Z = 46 km
Plane Strike Dip Rake
NP1 175 75 -80
NP2 321 18 -123
Principal Axes:
Axis Value Plunge Azimuth
T 1.30e+22 29 257
N 0.00e+00 10 352
P -1.30e+22 59 99
Moment Tensor: (dyne-cm)
Component Value
Mxx 4.28e+20
Mxy 2.71e+21
Mxz -3.85e+20
Myy 5.99e+21
Myz -1.11e+22
Mzz -6.42e+21
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-#######-------#######
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#############----------------#####
##############------------------####
###############-------------------####
################--------------------####
################---------------------###
#################----------------------###
#################---------- ---------###
##### #########---------- P ---------###
##### T #########---------- ---------###
#### ##########---------------------##
#################---------------------##
################--------------------##
################-------------------#
###############------------------#
##############----------------
#############--------------#
###########-----------
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Global CMT Convention Moment Tensor:
R T P
-6.42e+21 -3.85e+20 1.11e+22
-3.85e+20 4.28e+20 -2.71e+21
1.11e+22 -2.71e+21 5.99e+21
Details of the solution is found at
http://www.eas.slu.edu/eqc/eqc_mt/MECH.NA/20181201052540/index.html
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STK = 175
DIP = 75
RAKE = -80
MW = 4.01
HS = 46.0
The NDK file is 20181201052540.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 -30 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 1.0 10 45 85 3.12 0.1675
WVFGRD96 2.0 15 45 90 3.29 0.2349
WVFGRD96 3.0 295 30 50 3.34 0.2182
WVFGRD96 4.0 280 30 25 3.34 0.2376
WVFGRD96 5.0 260 30 -15 3.36 0.2672
WVFGRD96 6.0 335 75 -55 3.38 0.2932
WVFGRD96 7.0 150 70 -40 3.40 0.3216
WVFGRD96 8.0 0 75 60 3.48 0.3362
WVFGRD96 9.0 150 70 -45 3.49 0.3527
WVFGRD96 10.0 150 70 -45 3.51 0.3668
WVFGRD96 11.0 145 65 -45 3.54 0.3788
WVFGRD96 12.0 145 65 -45 3.55 0.3884
WVFGRD96 13.0 145 65 -45 3.57 0.3952
WVFGRD96 14.0 -5 75 50 3.57 0.4026
WVFGRD96 15.0 355 75 50 3.59 0.4113
WVFGRD96 16.0 355 80 50 3.60 0.4203
WVFGRD96 17.0 355 80 50 3.62 0.4293
WVFGRD96 18.0 355 80 50 3.63 0.4377
WVFGRD96 19.0 -5 80 55 3.64 0.4481
WVFGRD96 20.0 -5 80 55 3.66 0.4589
WVFGRD96 21.0 0 80 60 3.68 0.4693
WVFGRD96 22.0 0 80 60 3.69 0.4794
WVFGRD96 23.0 0 80 60 3.71 0.4905
WVFGRD96 24.0 -5 85 60 3.72 0.4980
WVFGRD96 25.0 0 85 60 3.73 0.5077
WVFGRD96 26.0 175 90 -55 3.73 0.5136
WVFGRD96 27.0 -5 90 60 3.75 0.5228
WVFGRD96 28.0 -5 90 60 3.76 0.5277
WVFGRD96 29.0 175 90 -60 3.77 0.5331
WVFGRD96 30.0 5 90 80 3.80 0.5497
WVFGRD96 31.0 10 90 75 3.81 0.5652
WVFGRD96 32.0 185 85 -75 3.82 0.5903
WVFGRD96 33.0 190 85 -75 3.83 0.6081
WVFGRD96 34.0 190 85 -70 3.84 0.6208
WVFGRD96 35.0 180 80 -75 3.84 0.6358
WVFGRD96 36.0 180 80 -75 3.84 0.6476
WVFGRD96 37.0 175 75 -75 3.85 0.6584
WVFGRD96 38.0 175 75 -75 3.85 0.6672
WVFGRD96 39.0 175 75 -75 3.85 0.6739
WVFGRD96 40.0 175 75 -80 3.99 0.6774
WVFGRD96 41.0 175 75 -80 3.99 0.6785
WVFGRD96 42.0 175 75 -80 3.99 0.6795
WVFGRD96 43.0 175 75 -80 4.00 0.6802
WVFGRD96 44.0 175 75 -80 4.00 0.6806
WVFGRD96 45.0 175 75 -80 4.01 0.6804
WVFGRD96 46.0 175 75 -80 4.01 0.6808
WVFGRD96 47.0 175 75 -80 4.01 0.6797
WVFGRD96 48.0 170 70 -80 4.02 0.6805
WVFGRD96 49.0 170 70 -80 4.03 0.6789
WVFGRD96 50.0 170 70 -80 4.03 0.6796
WVFGRD96 51.0 170 70 -80 4.04 0.6795
WVFGRD96 52.0 170 70 -80 4.04 0.6772
WVFGRD96 53.0 170 70 -80 4.04 0.6780
WVFGRD96 54.0 170 70 -80 4.05 0.6756
WVFGRD96 55.0 170 70 -80 4.05 0.6733
WVFGRD96 56.0 170 70 -80 4.06 0.6710
WVFGRD96 57.0 170 70 -80 4.06 0.6688
WVFGRD96 58.0 165 70 -85 4.06 0.6633
WVFGRD96 59.0 170 70 -80 4.06 0.6626
The best solution is
WVFGRD96 46.0 175 75 -80 4.01 0.6808
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 -30 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