The ANSS event ID is ak018fzs646q and the event page is at https://earthquake.usgs.gov/earthquakes/eventpage/ak018fzs646q/executive.
2018/12/14 15:36:57 61.437 -151.652 84.3 4 Alaska
USGS/SLU Moment Tensor Solution
ENS 2018/12/14 15:36:57:0 61.44 -151.65 84.3 4.0 Alaska
Stations used:
AK.CUT AK.PWL AK.SKN AK.SLK AK.SSN IU.COLA 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 0.08 n 3
Best Fitting Double Couple
Mo = 2.43e+22 dyne-cm
Mw = 4.19
Z = 90 km
Plane Strike Dip Rake
NP1 215 85 -35
NP2 308 55 -174
Principal Axes:
Axis Value Plunge Azimuth
T 2.43e+22 20 267
N 0.00e+00 55 28
P -2.43e+22 28 166
Moment Tensor: (dyne-cm)
Component Value
Mxx -1.78e+22
Mxy 5.64e+21
Mxz 9.28e+21
Myy 2.02e+22
Myz -1.02e+22
Mzz -2.42e+21
--------------
----------------------
-------------------------###
------------------------######
#############-----------##########
###################-----############
#######################-##############
#######################---##############
######################------############
#####################----------###########
### ##############------------##########
### T #############---------------########
### ############-----------------#######
###############--------------------#####
##############---------------------#####
############-----------------------###
##########------------------------##
########----------- -----------#
#####------------ P ----------
###------------- ---------
----------------------
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Global CMT Convention Moment Tensor:
R T P
-2.42e+21 9.28e+21 1.02e+22
9.28e+21 -1.78e+22 -5.64e+21
1.02e+22 -5.64e+21 2.02e+22
Details of the solution is found at
http://www.eas.slu.edu/eqc/eqc_mt/MECH.NA/20181214153657/index.html
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STK = 215
DIP = 85
RAKE = -35
MW = 4.19
HS = 90.0
The NDK file is 20181214153657.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 0.08 n 3The results of this grid search are as follow:
DEPTH STK DIP RAKE MW FIT
WVFGRD96 2.0 125 70 5 3.38 0.2116
WVFGRD96 4.0 295 65 -35 3.51 0.2434
WVFGRD96 6.0 325 75 -40 3.54 0.2621
WVFGRD96 8.0 295 70 -40 3.63 0.2782
WVFGRD96 10.0 130 50 20 3.68 0.2792
WVFGRD96 12.0 130 50 20 3.71 0.2809
WVFGRD96 14.0 215 75 30 3.72 0.2819
WVFGRD96 16.0 215 75 30 3.75 0.2885
WVFGRD96 18.0 215 80 25 3.76 0.2933
WVFGRD96 20.0 215 75 20 3.78 0.3004
WVFGRD96 22.0 215 75 20 3.80 0.3065
WVFGRD96 24.0 215 80 20 3.83 0.3137
WVFGRD96 26.0 215 80 20 3.85 0.3216
WVFGRD96 28.0 215 85 25 3.88 0.3315
WVFGRD96 30.0 215 85 20 3.89 0.3441
WVFGRD96 32.0 215 85 15 3.91 0.3643
WVFGRD96 34.0 35 90 -10 3.93 0.3823
WVFGRD96 36.0 215 85 10 3.96 0.3978
WVFGRD96 38.0 35 90 -5 3.99 0.4128
WVFGRD96 40.0 215 85 10 4.04 0.4313
WVFGRD96 42.0 35 90 -10 4.06 0.4315
WVFGRD96 44.0 215 85 5 4.08 0.4322
WVFGRD96 46.0 215 85 5 4.09 0.4347
WVFGRD96 48.0 215 85 5 4.11 0.4370
WVFGRD96 50.0 215 90 5 4.12 0.4406
WVFGRD96 52.0 215 90 0 4.13 0.4448
WVFGRD96 54.0 215 90 0 4.14 0.4509
WVFGRD96 56.0 215 90 0 4.15 0.4581
WVFGRD96 58.0 215 90 0 4.16 0.4651
WVFGRD96 60.0 215 90 0 4.17 0.4694
WVFGRD96 62.0 215 90 0 4.18 0.4736
WVFGRD96 64.0 35 90 5 4.18 0.4778
WVFGRD96 66.0 215 85 -25 4.16 0.4831
WVFGRD96 68.0 40 90 25 4.16 0.4847
WVFGRD96 70.0 40 90 30 4.16 0.4891
WVFGRD96 72.0 215 85 -30 4.17 0.4995
WVFGRD96 74.0 215 85 -30 4.18 0.5023
WVFGRD96 76.0 215 85 -30 4.18 0.5067
WVFGRD96 78.0 215 85 -30 4.18 0.5075
WVFGRD96 80.0 215 85 -30 4.18 0.5113
WVFGRD96 82.0 215 85 -35 4.18 0.5130
WVFGRD96 84.0 215 85 -35 4.19 0.5145
WVFGRD96 86.0 215 85 -35 4.19 0.5152
WVFGRD96 88.0 215 85 -35 4.19 0.5173
WVFGRD96 90.0 215 85 -35 4.19 0.5181
WVFGRD96 92.0 215 85 -35 4.20 0.5178
WVFGRD96 94.0 215 85 -35 4.20 0.5175
WVFGRD96 96.0 215 85 -35 4.20 0.5169
WVFGRD96 98.0 215 85 -35 4.20 0.5174
WVFGRD96 100.0 215 85 -35 4.20 0.5180
WVFGRD96 102.0 215 85 -40 4.20 0.5165
WVFGRD96 104.0 215 85 -40 4.21 0.5168
WVFGRD96 106.0 215 90 -40 4.21 0.5147
WVFGRD96 108.0 215 85 -40 4.21 0.5152
WVFGRD96 110.0 215 85 -40 4.21 0.5139
WVFGRD96 112.0 215 90 -40 4.21 0.5129
WVFGRD96 114.0 215 90 -40 4.22 0.5113
WVFGRD96 116.0 215 90 -40 4.22 0.5108
WVFGRD96 118.0 215 90 -40 4.22 0.5096
WVFGRD96 120.0 215 90 -40 4.22 0.5077
WVFGRD96 122.0 215 90 -40 4.22 0.5076
WVFGRD96 124.0 210 85 -45 4.23 0.5059
WVFGRD96 126.0 215 90 -40 4.23 0.5049
WVFGRD96 128.0 210 85 -45 4.23 0.5055
WVFGRD96 130.0 210 85 -45 4.24 0.5047
WVFGRD96 132.0 210 85 -45 4.24 0.5050
WVFGRD96 134.0 210 85 -45 4.24 0.5044
WVFGRD96 136.0 210 85 -45 4.24 0.5027
WVFGRD96 138.0 210 85 -45 4.25 0.5023
WVFGRD96 140.0 210 85 -45 4.25 0.5014
WVFGRD96 142.0 210 85 -45 4.25 0.5019
WVFGRD96 144.0 210 85 -45 4.25 0.5011
WVFGRD96 146.0 210 85 -45 4.25 0.4893
WVFGRD96 148.0 210 80 -40 4.26 0.4553
The best solution is
WVFGRD96 90.0 215 85 -35 4.19 0.5181
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 0.08 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