The ANSS event ID is pr2018335008 and the event page is at https://earthquake.usgs.gov/earthquakes/eventpage/pr2018335008/executive.
2018/12/01 12:44:53 19.072 -66.938 44.0 2.89 Alaska
USGS/SLU Moment Tensor Solution
ENS 2018/12/01 12:44:53:0 19.07 -66.94 44.0 2.9 Alaska
Stations used:
AK.CAPN AK.CUT AK.FID AK.GHO AK.GLI AK.KLU AK.KNK AK.KTH
AK.PWL AK.RC01 AK.RND AK.SAW AK.SCM AK.SKN AK.SSN AV.ILSW
AV.STLK TA.M19K TA.M20K TA.M22K TA.O22K
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 = 5.96e+22 dyne-cm
Mw = 4.45
Z = 48 km
Plane Strike Dip Rake
NP1 185 55 -80
NP2 348 36 -104
Principal Axes:
Axis Value Plunge Azimuth
T 5.96e+22 9 268
N 0.00e+00 8 359
P -5.96e+22 77 129
Moment Tensor: (dyne-cm)
Component Value
Mxx -1.05e+21
Mxy 3.56e+21
Mxz 7.66e+21
Myy 5.62e+22
Myz -1.95e+22
Mzz -5.51e+22
######-#######
#########-----########
##########----------########
##########------------########
###########---------------########
###########-----------------########
############------------------########
############--------------------########
############---------------------#######
############----------------------########
# ########----------------------########
# T ########----------- --------########
# ########----------- P ---------#######
###########----------- --------#######
###########----------------------#######
###########---------------------######
##########--------------------######
##########------------------######
########------------------####
########---------------#####
######-------------###
####---------#
Global CMT Convention Moment Tensor:
R T P
-5.51e+22 7.66e+21 1.95e+22
7.66e+21 -1.05e+21 -3.56e+21
1.95e+22 -3.56e+21 5.62e+22
Details of the solution is found at
http://www.eas.slu.edu/eqc/eqc_mt/MECH.NA/20181201124453/index.html
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STK = 185
DIP = 55
RAKE = -80
MW = 4.45
HS = 48.0
The NDK file is 20181201124453.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 -5 45 80 3.59 0.2105
WVFGRD96 2.0 0 40 80 3.75 0.2727
WVFGRD96 3.0 -5 45 75 3.80 0.2533
WVFGRD96 4.0 340 30 55 3.79 0.2438
WVFGRD96 5.0 135 75 -40 3.80 0.2707
WVFGRD96 6.0 135 75 -40 3.83 0.2933
WVFGRD96 7.0 130 70 -40 3.86 0.3120
WVFGRD96 8.0 130 70 -45 3.92 0.3213
WVFGRD96 9.0 130 70 -45 3.94 0.3326
WVFGRD96 10.0 130 70 -45 3.96 0.3411
WVFGRD96 11.0 130 70 -45 3.98 0.3471
WVFGRD96 12.0 130 70 -45 4.00 0.3500
WVFGRD96 13.0 130 70 -45 4.01 0.3508
WVFGRD96 14.0 70 60 50 4.03 0.3497
WVFGRD96 15.0 70 60 50 4.05 0.3545
WVFGRD96 16.0 65 60 45 4.06 0.3576
WVFGRD96 17.0 70 55 45 4.08 0.3605
WVFGRD96 18.0 70 55 45 4.09 0.3628
WVFGRD96 19.0 65 55 40 4.11 0.3661
WVFGRD96 20.0 65 55 40 4.12 0.3694
WVFGRD96 21.0 65 55 45 4.14 0.3703
WVFGRD96 22.0 210 70 -55 4.10 0.3783
WVFGRD96 23.0 210 70 -55 4.11 0.3875
WVFGRD96 24.0 210 70 -55 4.12 0.3959
WVFGRD96 25.0 210 65 -55 4.14 0.4032
WVFGRD96 26.0 210 65 -55 4.15 0.4122
WVFGRD96 27.0 210 70 -55 4.16 0.4195
WVFGRD96 28.0 210 70 -55 4.17 0.4264
WVFGRD96 29.0 210 70 -55 4.18 0.4313
WVFGRD96 30.0 210 75 -65 4.19 0.4479
WVFGRD96 31.0 210 75 -65 4.20 0.4650
WVFGRD96 32.0 205 70 -65 4.21 0.4845
WVFGRD96 33.0 205 65 -65 4.22 0.5031
WVFGRD96 34.0 205 65 -65 4.23 0.5241
WVFGRD96 35.0 200 60 -70 4.24 0.5434
WVFGRD96 36.0 195 60 -70 4.25 0.5631
WVFGRD96 37.0 195 60 -70 4.26 0.5774
WVFGRD96 38.0 195 60 -70 4.27 0.5906
WVFGRD96 39.0 190 55 -75 4.28 0.6022
WVFGRD96 40.0 195 60 -75 4.37 0.6041
WVFGRD96 41.0 195 60 -75 4.38 0.6115
WVFGRD96 42.0 190 55 -75 4.40 0.6196
WVFGRD96 43.0 185 55 -80 4.41 0.6262
WVFGRD96 44.0 185 55 -80 4.42 0.6316
WVFGRD96 45.0 185 55 -80 4.43 0.6362
WVFGRD96 46.0 185 55 -80 4.44 0.6387
WVFGRD96 47.0 185 55 -80 4.44 0.6400
WVFGRD96 48.0 185 55 -80 4.45 0.6404
WVFGRD96 49.0 185 55 -80 4.45 0.6402
WVFGRD96 50.0 185 55 -80 4.46 0.6376
WVFGRD96 51.0 185 55 -80 4.46 0.6354
WVFGRD96 52.0 180 55 -85 4.47 0.6305
WVFGRD96 53.0 -10 35 -100 4.47 0.6275
WVFGRD96 54.0 -5 35 -95 4.47 0.6204
WVFGRD96 55.0 -5 35 -95 4.47 0.6162
WVFGRD96 56.0 -5 35 -95 4.48 0.6093
WVFGRD96 57.0 -5 35 -95 4.48 0.6043
WVFGRD96 58.0 -5 35 -95 4.48 0.5982
WVFGRD96 59.0 5 35 -80 4.49 0.5909
The best solution is
WVFGRD96 48.0 185 55 -80 4.45 0.6404
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