The ANSS event ID is ak018fe7wu5e and the event page is at https://earthquake.usgs.gov/earthquakes/eventpage/ak018fe7wu5e/executive.
2018/12/01 11:55:08 61.351 -149.995 40.6 4.1 Alaska
USGS/SLU Moment Tensor Solution
ENS 2018/12/01 11:55:08:0 61.35 -149.99 40.6 4.1 Alaska
Stations used:
AK.FID AK.GHO AK.GLI AK.KNK AK.PWL AK.RC01 AK.SAW AK.SKN
AK.SSN AV.ILSW AV.STLK TA.M19K TA.M20K 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 = 2.19e+22 dyne-cm
Mw = 4.16
Z = 46 km
Plane Strike Dip Rake
NP1 190 60 -70
NP2 334 36 -121
Principal Axes:
Axis Value Plunge Azimuth
T 2.19e+22 13 266
N 0.00e+00 17 360
P -2.19e+22 68 141
Moment Tensor: (dyne-cm)
Component Value
Mxx -1.68e+21
Mxy 3.04e+21
Mxz 5.47e+21
Myy 1.95e+22
Myz -9.47e+21
Mzz -1.78e+22
---------#####
##########-###########
############-----###########
############---------#########
#############------------#########
#############--------------#########
#############-----------------########
##############------------------########
#############--------------------#######
##############--------------------########
# #########----------------------#######
# T #########----------------------#######
# #########---------- ---------#######
############---------- P ----------#####
############---------- ---------######
###########----------------------#####
##########----------------------####
##########--------------------####
########-------------------###
########-----------------###
######---------------#
###-----------
Global CMT Convention Moment Tensor:
R T P
-1.78e+22 5.47e+21 9.47e+21
5.47e+21 -1.68e+21 -3.04e+21
9.47e+21 -3.04e+21 1.95e+22
Details of the solution is found at
http://www.eas.slu.edu/eqc/eqc_mt/MECH.NA/20181201115508/index.html
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STK = 190
DIP = 60
RAKE = -70
MW = 4.16
HS = 46.0
The NDK file is 20181201115508.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 1.0 180 45 85 3.31 0.2003
WVFGRD96 2.0 0 45 85 3.47 0.2690
WVFGRD96 3.0 335 30 55 3.52 0.2618
WVFGRD96 4.0 325 30 40 3.53 0.2814
WVFGRD96 5.0 315 30 20 3.53 0.3066
WVFGRD96 6.0 310 35 10 3.55 0.3287
WVFGRD96 7.0 305 45 -30 3.59 0.3459
WVFGRD96 8.0 305 35 5 3.63 0.3553
WVFGRD96 9.0 330 45 30 3.67 0.3684
WVFGRD96 10.0 330 50 30 3.69 0.3830
WVFGRD96 11.0 330 50 30 3.71 0.3912
WVFGRD96 12.0 330 50 30 3.72 0.3956
WVFGRD96 13.0 330 55 30 3.74 0.3963
WVFGRD96 14.0 330 55 30 3.75 0.3934
WVFGRD96 15.0 330 60 35 3.77 0.3913
WVFGRD96 16.0 240 55 30 3.78 0.3933
WVFGRD96 17.0 240 55 30 3.79 0.3959
WVFGRD96 18.0 35 75 45 3.79 0.3969
WVFGRD96 19.0 35 80 50 3.80 0.4035
WVFGRD96 20.0 35 80 50 3.81 0.4108
WVFGRD96 21.0 35 80 50 3.83 0.4179
WVFGRD96 22.0 35 80 50 3.84 0.4259
WVFGRD96 23.0 35 80 50 3.86 0.4347
WVFGRD96 24.0 35 80 50 3.87 0.4446
WVFGRD96 25.0 30 85 50 3.88 0.4541
WVFGRD96 26.0 30 85 50 3.89 0.4633
WVFGRD96 27.0 210 90 -50 3.90 0.4738
WVFGRD96 28.0 30 90 50 3.91 0.4855
WVFGRD96 29.0 30 90 50 3.92 0.4971
WVFGRD96 30.0 30 90 50 3.93 0.5076
WVFGRD96 31.0 205 80 -55 3.94 0.5314
WVFGRD96 32.0 205 80 -55 3.95 0.5461
WVFGRD96 33.0 205 75 -60 3.97 0.5647
WVFGRD96 34.0 205 75 -60 3.97 0.5816
WVFGRD96 35.0 200 70 -60 3.98 0.5962
WVFGRD96 36.0 200 70 -60 3.99 0.6124
WVFGRD96 37.0 200 70 -60 3.99 0.6243
WVFGRD96 38.0 200 65 -60 4.00 0.6365
WVFGRD96 39.0 200 65 -60 4.02 0.6495
WVFGRD96 40.0 190 65 -70 4.11 0.6467
WVFGRD96 41.0 195 65 -65 4.12 0.6597
WVFGRD96 42.0 195 65 -70 4.13 0.6700
WVFGRD96 43.0 195 65 -65 4.14 0.6767
WVFGRD96 44.0 195 65 -65 4.15 0.6824
WVFGRD96 45.0 190 60 -70 4.16 0.6857
WVFGRD96 46.0 190 60 -70 4.16 0.6885
WVFGRD96 47.0 190 60 -70 4.17 0.6879
WVFGRD96 48.0 190 60 -70 4.18 0.6877
WVFGRD96 49.0 190 60 -70 4.18 0.6853
WVFGRD96 50.0 190 60 -70 4.18 0.6822
WVFGRD96 51.0 190 60 -70 4.19 0.6796
WVFGRD96 52.0 190 60 -70 4.19 0.6749
WVFGRD96 53.0 190 60 -70 4.20 0.6704
WVFGRD96 54.0 190 60 -70 4.20 0.6646
WVFGRD96 55.0 190 60 -70 4.20 0.6605
WVFGRD96 56.0 190 60 -70 4.20 0.6540
WVFGRD96 57.0 190 60 -70 4.20 0.6491
WVFGRD96 58.0 190 60 -70 4.21 0.6438
WVFGRD96 59.0 185 60 -75 4.21 0.6367
The best solution is
WVFGRD96 46.0 190 60 -70 4.16 0.6885
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