The ANSS event ID is ak018feal6jf and the event page is at https://earthquake.usgs.gov/earthquakes/eventpage/ak018feal6jf/executive.
2018/12/01 16:04:59 61.542 -149.852 47.6 3.9 Alaska
USGS/SLU Moment Tensor Solution
ENS 2018/12/01 16:04:59:0 61.54 -149.85 47.6 3.9 Alaska
Stations used:
AK.GHO AK.GLI AK.KNK AK.PWL AK.SAW AK.SKN AK.SSN AV.STLK
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 = 1.60e+22 dyne-cm
Mw = 4.07
Z = 58 km
Plane Strike Dip Rake
NP1 180 60 -65
NP2 317 38 -126
Principal Axes:
Axis Value Plunge Azimuth
T 1.60e+22 12 252
N 0.00e+00 21 347
P -1.60e+22 65 136
Moment Tensor: (dyne-cm)
Component Value
Mxx -1.03e+15
Mxy 5.87e+21
Mxz 3.39e+21
Myy 1.26e+22
Myz -7.27e+21
Mzz -1.26e+22
------########
--------##############
-#########--################
##########--------############
###########------------###########
############--------------##########
############-----------------#########
#############------------------#########
#############-------------------########
#############----------------------#######
#############----------------------#######
#############-----------------------######
#############----------- ---------######
# #########---------- P ----------####
# T #########---------- ----------####
#########-----------------------###
############----------------------##
###########---------------------##
##########--------------------
##########------------------
########--------------
######--------
Global CMT Convention Moment Tensor:
R T P
-1.26e+22 3.39e+21 7.27e+21
3.39e+21 -1.03e+15 -5.87e+21
7.27e+21 -5.87e+21 1.26e+22
Details of the solution is found at
http://www.eas.slu.edu/eqc/eqc_mt/MECH.NA/20181201160459/index.html
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STK = 180
DIP = 60
RAKE = -65
MW = 4.07
HS = 58.0
The NDK file is 20181201160459.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 1.0 170 45 80 3.21 0.2332
WVFGRD96 2.0 170 55 100 3.37 0.3107
WVFGRD96 3.0 320 30 50 3.42 0.3018
WVFGRD96 4.0 300 25 20 3.43 0.3126
WVFGRD96 5.0 290 30 0 3.44 0.3369
WVFGRD96 6.0 130 80 -60 3.47 0.3674
WVFGRD96 7.0 130 80 -60 3.49 0.3923
WVFGRD96 8.0 320 90 60 3.55 0.4042
WVFGRD96 9.0 320 90 60 3.57 0.4164
WVFGRD96 10.0 140 90 -60 3.58 0.4224
WVFGRD96 11.0 290 85 75 3.68 0.4327
WVFGRD96 12.0 290 80 75 3.70 0.4387
WVFGRD96 13.0 290 80 75 3.72 0.4421
WVFGRD96 14.0 290 80 75 3.74 0.4420
WVFGRD96 15.0 290 80 75 3.75 0.4398
WVFGRD96 16.0 290 80 75 3.76 0.4360
WVFGRD96 17.0 290 80 75 3.77 0.4314
WVFGRD96 18.0 290 75 75 3.78 0.4305
WVFGRD96 19.0 290 75 75 3.79 0.4295
WVFGRD96 20.0 110 20 75 3.76 0.4235
WVFGRD96 21.0 100 25 60 3.77 0.4254
WVFGRD96 22.0 105 25 65 3.78 0.4251
WVFGRD96 23.0 120 25 70 3.77 0.4221
WVFGRD96 24.0 125 25 75 3.78 0.4190
WVFGRD96 25.0 120 25 70 3.79 0.4138
WVFGRD96 26.0 115 30 60 3.78 0.4065
WVFGRD96 27.0 170 90 -60 3.75 0.3989
WVFGRD96 28.0 355 85 60 3.76 0.3992
WVFGRD96 29.0 35 75 65 3.83 0.4005
WVFGRD96 30.0 35 75 65 3.84 0.4067
WVFGRD96 31.0 35 75 65 3.85 0.4111
WVFGRD96 32.0 30 80 65 3.85 0.4138
WVFGRD96 33.0 30 80 60 3.85 0.4166
WVFGRD96 34.0 25 90 55 3.84 0.4224
WVFGRD96 35.0 25 90 55 3.84 0.4347
WVFGRD96 36.0 195 75 -50 3.84 0.4444
WVFGRD96 37.0 195 70 -50 3.86 0.4667
WVFGRD96 38.0 190 65 -50 3.87 0.4907
WVFGRD96 39.0 190 65 -50 3.88 0.5139
WVFGRD96 40.0 185 65 -60 3.97 0.5319
WVFGRD96 41.0 185 60 -60 3.98 0.5432
WVFGRD96 42.0 185 60 -60 3.99 0.5514
WVFGRD96 43.0 185 60 -60 4.00 0.5621
WVFGRD96 44.0 185 60 -60 4.01 0.5697
WVFGRD96 45.0 185 60 -60 4.02 0.5776
WVFGRD96 46.0 180 60 -65 4.03 0.5856
WVFGRD96 47.0 185 60 -60 4.03 0.5928
WVFGRD96 48.0 180 60 -65 4.04 0.5989
WVFGRD96 49.0 185 60 -60 4.04 0.6047
WVFGRD96 50.0 180 60 -65 4.05 0.6102
WVFGRD96 51.0 180 60 -65 4.05 0.6160
WVFGRD96 52.0 180 60 -65 4.06 0.6205
WVFGRD96 53.0 180 60 -65 4.06 0.6245
WVFGRD96 54.0 180 60 -65 4.06 0.6289
WVFGRD96 55.0 180 60 -65 4.07 0.6309
WVFGRD96 56.0 180 60 -65 4.07 0.6349
WVFGRD96 57.0 180 60 -65 4.07 0.6358
WVFGRD96 58.0 180 60 -65 4.07 0.6387
WVFGRD96 59.0 180 60 -65 4.08 0.6382
The best solution is
WVFGRD96 58.0 180 60 -65 4.07 0.6387
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