The ANSS event ID is ak018fe58uyv and the event page is at https://earthquake.usgs.gov/earthquakes/eventpage/ak018fe58uyv/executive.
2018/12/01 07:07:38 61.466 -149.968 32.7 4.3 Alaska
USGS/SLU Moment Tensor Solution
ENS 2018/12/01 07:07:38:0 61.47 -149.97 32.7 4.3 Alaska
Stations used:
AK.BERG AK.BPAW AK.BRLK AK.CAST AK.CNP AK.CUT AK.DHY AK.DIV
AK.EYAK AK.FID AK.GHO AK.GLI AK.HDA AK.HOM AK.KLU AK.KNK
AK.KTH AK.PPLA AK.PWL AK.RC01 AK.SAW AK.SCM AK.SKN AK.SLK
AK.SSN AK.SWD AK.TRF AK.VRDI AK.WRH AT.SVW2 AV.ILSW AV.STLK
TA.L18K TA.M19K TA.M20K TA.M22K TA.M24K TA.N18K TA.N19K
TA.O18K TA.O19K TA.O22K TA.P18K TA.P19K
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 = 2.37e+23 dyne-cm
Mw = 4.85
Z = 52 km
Plane Strike Dip Rake
NP1 205 80 -60
NP2 312 31 -161
Principal Axes:
Axis Value Plunge Azimuth
T 2.37e+23 29 271
N 0.00e+00 29 19
P -2.37e+23 47 146
Moment Tensor: (dyne-cm)
Component Value
Mxx -7.69e+22
Mxy 4.82e+22
Mxz 1.00e+23
Myy 1.47e+23
Myz -1.66e+23
Mzz -7.02e+22
--------------
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---############-----########
##############################
#####################----#########
####################--------########
####################-----------#######
####################-------------#######
###################---------------######
##### ###########-----------------######
##### T ###########------------------#####
##### ##########--------------------####
#################---------------------####
###############----------------------###
##############---------- ----------###
#############---------- P ----------##
###########----------- ----------#
##########-----------------------#
#######-----------------------
######----------------------
##--------------------
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Global CMT Convention Moment Tensor:
R T P
-7.02e+22 1.00e+23 1.66e+23
1.00e+23 -7.69e+22 -4.82e+22
1.66e+23 -4.82e+22 1.47e+23
Details of the solution is found at
http://www.eas.slu.edu/eqc/eqc_mt/MECH.NA/20181201070738/index.html
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STK = 205
DIP = 80
RAKE = -60
MW = 4.85
HS = 52.0
The NDK file is 20181201070738.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 0 45 75 3.93 0.1832
WVFGRD96 2.0 5 45 80 4.09 0.2508
WVFGRD96 3.0 20 40 105 4.15 0.2369
WVFGRD96 4.0 135 70 -40 4.12 0.2429
WVFGRD96 5.0 135 65 -35 4.15 0.2608
WVFGRD96 6.0 140 70 -30 4.17 0.2753
WVFGRD96 7.0 140 70 -30 4.20 0.2882
WVFGRD96 8.0 135 65 -35 4.26 0.2978
WVFGRD96 9.0 135 65 -35 4.28 0.3082
WVFGRD96 10.0 140 70 -35 4.30 0.3178
WVFGRD96 11.0 140 70 -35 4.32 0.3248
WVFGRD96 12.0 140 70 -35 4.33 0.3304
WVFGRD96 13.0 140 70 -35 4.35 0.3354
WVFGRD96 14.0 140 70 -35 4.36 0.3388
WVFGRD96 15.0 140 70 -35 4.38 0.3411
WVFGRD96 16.0 140 70 -35 4.39 0.3426
WVFGRD96 17.0 340 75 45 4.39 0.3503
WVFGRD96 18.0 340 75 45 4.40 0.3550
WVFGRD96 19.0 345 70 50 4.42 0.3589
WVFGRD96 20.0 345 70 50 4.43 0.3624
WVFGRD96 21.0 350 70 55 4.44 0.3650
WVFGRD96 22.0 350 70 55 4.45 0.3670
WVFGRD96 23.0 350 70 60 4.47 0.3664
WVFGRD96 24.0 365 75 60 4.47 0.3676
WVFGRD96 25.0 5 75 60 4.48 0.3689
WVFGRD96 26.0 10 75 65 4.50 0.3707
WVFGRD96 27.0 10 75 65 4.51 0.3720
WVFGRD96 28.0 25 90 65 4.53 0.3760
WVFGRD96 29.0 25 90 65 4.54 0.3954
WVFGRD96 30.0 25 90 65 4.56 0.4145
WVFGRD96 31.0 30 90 60 4.58 0.4349
WVFGRD96 32.0 30 90 60 4.59 0.4556
WVFGRD96 33.0 210 85 -60 4.60 0.4808
WVFGRD96 34.0 210 85 -60 4.61 0.5009
WVFGRD96 35.0 205 80 -65 4.62 0.5196
WVFGRD96 36.0 205 80 -65 4.63 0.5390
WVFGRD96 37.0 205 80 -60 4.64 0.5558
WVFGRD96 38.0 205 80 -60 4.65 0.5714
WVFGRD96 39.0 205 80 -60 4.65 0.5842
WVFGRD96 40.0 205 80 -65 4.78 0.5993
WVFGRD96 41.0 205 80 -65 4.78 0.6088
WVFGRD96 42.0 205 80 -65 4.79 0.6175
WVFGRD96 43.0 205 80 -65 4.80 0.6228
WVFGRD96 44.0 205 80 -65 4.81 0.6291
WVFGRD96 45.0 205 80 -65 4.81 0.6354
WVFGRD96 46.0 205 80 -65 4.82 0.6411
WVFGRD96 47.0 205 80 -65 4.83 0.6454
WVFGRD96 48.0 205 80 -65 4.83 0.6482
WVFGRD96 49.0 205 80 -65 4.84 0.6516
WVFGRD96 50.0 205 80 -60 4.84 0.6536
WVFGRD96 51.0 205 80 -60 4.85 0.6536
WVFGRD96 52.0 205 80 -60 4.85 0.6545
WVFGRD96 53.0 205 80 -60 4.86 0.6541
WVFGRD96 54.0 205 80 -60 4.86 0.6518
WVFGRD96 55.0 205 80 -60 4.86 0.6512
WVFGRD96 56.0 205 80 -65 4.87 0.6488
WVFGRD96 57.0 205 80 -65 4.87 0.6460
WVFGRD96 58.0 205 80 -65 4.87 0.6433
WVFGRD96 59.0 200 75 -65 4.88 0.6404
The best solution is
WVFGRD96 52.0 205 80 -60 4.85 0.6545
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