The ANSS event ID is ak018fcpk9xi and the event page is at https://earthquake.usgs.gov/earthquakes/eventpage/ak018fcpk9xi/executive.
2018/11/30 20:26:55 61.384 -150.079 37.7 5 Alaska
USGS/SLU Moment Tensor Solution
ENS 2018/11/30 20:26:55:0 61.38 -150.08 37.7 5.0 Alaska
Stations used:
AK.CAST AK.FID AK.HIN AK.KLU AK.PWL AK.RC01 AK.SAW AK.SCM
AK.SKN AK.SWD AT.PMR AV.ILSW TA.M22K TA.M24K TA.O22K
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 = 2.92e+23 dyne-cm
Mw = 4.91
Z = 46 km
Plane Strike Dip Rake
NP1 195 60 -75
NP2 347 33 -114
Principal Axes:
Axis Value Plunge Azimuth
T 2.92e+23 14 274
N 0.00e+00 13 7
P -2.92e+23 71 139
Moment Tensor: (dyne-cm)
Component Value
Mxx -1.63e+22
Mxy -4.38e+21
Mxz 7.29e+22
Myy 2.60e+23
Myz -1.26e+23
Mzz -2.44e+23
######-----###
#############-########
##############------########
#############----------#######
##############------------########
##############---------------#######
##############-----------------#######
##############-------------------#######
#############--------------------#######
# ##########---------------------#######
# T #########----------------------#######
# #########--------- ----------#######
#############--------- P ----------#######
###########---------- ----------######
###########-----------------------######
##########----------------------######
#########----------------------#####
#########--------------------#####
#######-------------------####
#######-----------------####
####---------------###
#------------#
Global CMT Convention Moment Tensor:
R T P
-2.44e+23 7.29e+22 1.26e+23
7.29e+22 -1.63e+22 4.38e+21
1.26e+23 4.38e+21 2.60e+23
Details of the solution is found at
http://www.eas.slu.edu/eqc/eqc_mt/MECH.NA/20181130202655/index.html
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STK = 195
DIP = 60
RAKE = -75
MW = 4.91
HS = 46.0
The NDK file is 20181130202655.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 1.0 195 50 90 4.02 0.1699
WVFGRD96 2.0 10 45 85 4.19 0.2467
WVFGRD96 3.0 360 45 75 4.24 0.2401
WVFGRD96 4.0 340 40 40 4.23 0.2493
WVFGRD96 5.0 330 40 25 4.25 0.2746
WVFGRD96 6.0 330 40 25 4.27 0.2975
WVFGRD96 7.0 250 60 45 4.30 0.3170
WVFGRD96 8.0 250 60 50 4.38 0.3428
WVFGRD96 9.0 250 60 45 4.40 0.3625
WVFGRD96 10.0 245 60 40 4.42 0.3759
WVFGRD96 11.0 245 60 40 4.44 0.3876
WVFGRD96 12.0 245 60 35 4.45 0.3971
WVFGRD96 13.0 245 60 35 4.47 0.4047
WVFGRD96 14.0 245 60 35 4.49 0.4099
WVFGRD96 15.0 245 60 30 4.50 0.4130
WVFGRD96 16.0 245 60 30 4.52 0.4153
WVFGRD96 17.0 245 60 30 4.53 0.4162
WVFGRD96 18.0 240 60 25 4.55 0.4188
WVFGRD96 19.0 240 60 25 4.56 0.4220
WVFGRD96 20.0 240 55 25 4.58 0.4262
WVFGRD96 21.0 240 55 25 4.59 0.4312
WVFGRD96 22.0 240 55 25 4.60 0.4363
WVFGRD96 23.0 230 70 -30 4.59 0.4365
WVFGRD96 24.0 230 70 -30 4.60 0.4416
WVFGRD96 25.0 230 65 -30 4.61 0.4478
WVFGRD96 26.0 230 65 -30 4.62 0.4542
WVFGRD96 27.0 225 65 -35 4.63 0.4628
WVFGRD96 28.0 225 60 -35 4.64 0.4703
WVFGRD96 29.0 225 60 -35 4.65 0.4823
WVFGRD96 30.0 220 60 -45 4.67 0.4966
WVFGRD96 31.0 220 65 -45 4.68 0.5122
WVFGRD96 32.0 220 60 -50 4.69 0.5295
WVFGRD96 33.0 220 60 -55 4.70 0.5476
WVFGRD96 34.0 215 60 -60 4.71 0.5636
WVFGRD96 35.0 215 60 -60 4.72 0.5752
WVFGRD96 36.0 215 60 -60 4.73 0.5829
WVFGRD96 37.0 215 60 -60 4.73 0.5865
WVFGRD96 38.0 205 60 -65 4.75 0.5914
WVFGRD96 39.0 200 60 -65 4.77 0.5977
WVFGRD96 40.0 205 65 -70 4.86 0.5953
WVFGRD96 41.0 205 65 -70 4.87 0.6035
WVFGRD96 42.0 200 60 -70 4.88 0.6115
WVFGRD96 43.0 200 60 -70 4.89 0.6170
WVFGRD96 44.0 200 60 -70 4.89 0.6203
WVFGRD96 45.0 200 60 -70 4.90 0.6192
WVFGRD96 46.0 195 60 -75 4.91 0.6207
WVFGRD96 47.0 195 60 -75 4.91 0.6190
WVFGRD96 48.0 195 60 -75 4.92 0.6178
WVFGRD96 49.0 195 60 -75 4.92 0.6146
WVFGRD96 50.0 195 60 -75 4.93 0.6102
WVFGRD96 51.0 200 65 -75 4.93 0.6059
WVFGRD96 52.0 200 65 -75 4.93 0.6009
WVFGRD96 53.0 200 65 -75 4.93 0.5955
WVFGRD96 54.0 200 65 -75 4.93 0.5900
WVFGRD96 55.0 200 65 -75 4.93 0.5843
WVFGRD96 56.0 200 65 -75 4.94 0.5779
WVFGRD96 57.0 200 65 -75 4.94 0.5727
WVFGRD96 58.0 200 65 -75 4.94 0.5644
WVFGRD96 59.0 200 65 -75 4.94 0.5579
The best solution is
WVFGRD96 46.0 195 60 -75 4.91 0.6207
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