The ANSS event ID is ak018fcr8hkj and the event page is at https://earthquake.usgs.gov/earthquakes/eventpage/ak018fcr8hkj/executive.
2018/11/30 23:07:46 61.533 -149.947 39.7 4.1 Alaska
USGS/SLU Moment Tensor Solution
ENS 2018/11/30 23:07:46:0 61.53 -149.95 39.7 4.1 Alaska
Stations used:
AK.CUT AK.GHO AK.KNK AK.PWL AK.RC01 AK.SAW AK.SCM AK.SKN
AK.SSN AT.PMR 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 = 1.60e+22 dyne-cm
Mw = 4.07
Z = 47 km
Plane Strike Dip Rake
NP1 185 65 -75
NP2 333 29 -119
Principal Axes:
Axis Value Plunge Azimuth
T 1.60e+22 19 264
N 0.00e+00 14 359
P -1.60e+22 67 122
Moment Tensor: (dyne-cm)
Component Value
Mxx -5.63e+20
Mxy 2.67e+21
Mxz 2.61e+21
Myy 1.24e+22
Myz -9.76e+21
Mzz -1.19e+22
--------######
##########--##########
############-------#########
############-----------#######
#############--------------#######
##############---------------#######
##############-----------------#######
###############------------------#######
##############--------------------######
###############---------------------######
## ##########---------------------######
## T ##########--------- ----------#####
## #########---------- P ----------#####
##############--------- ----------####
##############----------------------####
#############---------------------####
############---------------------###
###########--------------------###
##########------------------##
#########-----------------##
#######--------------#
####----------
Global CMT Convention Moment Tensor:
R T P
-1.19e+22 2.61e+21 9.76e+21
2.61e+21 -5.63e+20 -2.67e+21
9.76e+21 -2.67e+21 1.24e+22
Details of the solution is found at
http://www.eas.slu.edu/eqc/eqc_mt/MECH.NA/20181130230746/index.html
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STK = 185
DIP = 65
RAKE = -75
MW = 4.07
HS = 47.0
The NDK file is 20181130230746.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 1.0 175 50 75 3.20 0.2064
WVFGRD96 2.0 15 40 95 3.36 0.2814
WVFGRD96 3.0 180 60 75 3.40 0.2429
WVFGRD96 4.0 145 85 -55 3.39 0.2654
WVFGRD96 5.0 145 80 -50 3.41 0.2985
WVFGRD96 6.0 140 75 -50 3.44 0.3228
WVFGRD96 7.0 140 70 -45 3.47 0.3419
WVFGRD96 8.0 145 80 -50 3.52 0.3531
WVFGRD96 9.0 145 80 -50 3.54 0.3681
WVFGRD96 10.0 145 80 -50 3.56 0.3804
WVFGRD96 11.0 145 75 -50 3.58 0.3913
WVFGRD96 12.0 145 75 -50 3.60 0.3999
WVFGRD96 13.0 340 75 50 3.61 0.4096
WVFGRD96 14.0 340 75 50 3.62 0.4170
WVFGRD96 15.0 340 75 50 3.64 0.4219
WVFGRD96 16.0 345 75 50 3.65 0.4250
WVFGRD96 17.0 345 75 50 3.66 0.4266
WVFGRD96 18.0 345 75 50 3.67 0.4262
WVFGRD96 19.0 345 80 55 3.68 0.4250
WVFGRD96 20.0 345 80 55 3.69 0.4257
WVFGRD96 21.0 345 80 55 3.71 0.4274
WVFGRD96 22.0 340 85 55 3.72 0.4275
WVFGRD96 23.0 345 85 55 3.73 0.4276
WVFGRD96 24.0 345 85 60 3.74 0.4277
WVFGRD96 25.0 170 90 -60 3.75 0.4281
WVFGRD96 26.0 170 85 -60 3.76 0.4313
WVFGRD96 27.0 165 80 -60 3.77 0.4349
WVFGRD96 28.0 170 80 -60 3.79 0.4382
WVFGRD96 29.0 200 80 -65 3.83 0.4580
WVFGRD96 30.0 200 80 -65 3.84 0.4748
WVFGRD96 31.0 200 80 -65 3.85 0.4912
WVFGRD96 32.0 195 75 -70 3.86 0.5093
WVFGRD96 33.0 200 75 -65 3.87 0.5260
WVFGRD96 34.0 200 75 -65 3.88 0.5407
WVFGRD96 35.0 200 75 -65 3.88 0.5512
WVFGRD96 36.0 195 70 -65 3.89 0.5649
WVFGRD96 37.0 195 70 -65 3.89 0.5764
WVFGRD96 38.0 195 70 -65 3.90 0.5866
WVFGRD96 39.0 190 65 -65 3.91 0.5948
WVFGRD96 40.0 190 70 -70 4.01 0.6083
WVFGRD96 41.0 190 70 -70 4.02 0.6137
WVFGRD96 42.0 185 65 -75 4.03 0.6222
WVFGRD96 43.0 185 65 -75 4.04 0.6295
WVFGRD96 44.0 185 65 -75 4.05 0.6338
WVFGRD96 45.0 185 65 -75 4.06 0.6399
WVFGRD96 46.0 185 65 -75 4.06 0.6407
WVFGRD96 47.0 185 65 -75 4.07 0.6441
WVFGRD96 48.0 185 65 -75 4.08 0.6430
WVFGRD96 49.0 185 65 -70 4.08 0.6430
WVFGRD96 50.0 185 65 -70 4.09 0.6430
WVFGRD96 51.0 185 65 -70 4.09 0.6401
WVFGRD96 52.0 185 65 -70 4.10 0.6389
WVFGRD96 53.0 185 65 -70 4.10 0.6365
WVFGRD96 54.0 185 65 -70 4.10 0.6329
WVFGRD96 55.0 185 65 -70 4.11 0.6297
WVFGRD96 56.0 185 65 -70 4.11 0.6249
WVFGRD96 57.0 185 65 -70 4.11 0.6209
WVFGRD96 58.0 185 65 -70 4.12 0.6157
WVFGRD96 59.0 185 65 -70 4.12 0.6104
The best solution is
WVFGRD96 47.0 185 65 -75 4.07 0.6441
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