The ANSS event ID is ak0183b4m226 and the event page is at https://earthquake.usgs.gov/earthquakes/eventpage/ak0183b4m226/executive.
2018/03/13 03:22:25 59.233 -151.984 50.8 4.3 Alaska
USGS/SLU Moment Tensor Solution
ENS 2018/03/13 03:22:25:0 59.23 -151.98 50.8 4.3 Alaska
Stations used:
AK.BRLK AK.CNP AK.HOM AK.SKN AV.ILSW TA.O18K TA.O19K
TA.P18K TA.P19K TA.Q19K TA.Q20K
Filtering commands used:
cut o DIST/3.4 -20 o DIST/3.4 +50
rtr
taper w 0.1
hp c 0.03 n 3
lp c 0.10 n 3
Best Fitting Double Couple
Mo = 4.22e+22 dyne-cm
Mw = 4.35
Z = 58 km
Plane Strike Dip Rake
NP1 195 90 -70
NP2 285 20 -180
Principal Axes:
Axis Value Plunge Azimuth
T 4.22e+22 42 266
N 0.00e+00 20 15
P -4.22e+22 42 124
Moment Tensor: (dyne-cm)
Component Value
Mxx -7.21e+21
Mxy 1.25e+22
Mxz 1.03e+22
Myy 7.21e+21
Myz -3.83e+22
Mzz 3.46e+15
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-----#############--########
---################------#####
--##################----------####
-####################------------###
-####################--------------###
-#####################---------------###
#####################-----------------##
######################------------------##
####### ###########--------------------#
####### T ###########--------------------#
####### ##########---------------------#
###################--------- ---------
##################---------- P ---------
#################---------- --------
###############---------------------
##############--------------------
###########-------------------
##########------------------
######----------------
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Global CMT Convention Moment Tensor:
R T P
3.46e+15 1.03e+22 3.83e+22
1.03e+22 -7.21e+21 -1.25e+22
3.83e+22 -1.25e+22 7.21e+21
Details of the solution is found at
http://www.eas.slu.edu/eqc/eqc_mt/MECH.NA/20180313032225/index.html
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STK = 195
DIP = 90
RAKE = -70
MW = 4.35
HS = 58.0
The NDK file is 20180313032225.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/3.4 -20 o DIST/3.4 +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 2.0 0 65 25 3.60 0.2813
WVFGRD96 4.0 355 75 -10 3.68 0.3043
WVFGRD96 6.0 300 30 0 3.68 0.3292
WVFGRD96 8.0 205 85 50 3.76 0.3606
WVFGRD96 10.0 20 85 -45 3.80 0.3825
WVFGRD96 12.0 200 90 40 3.84 0.3934
WVFGRD96 14.0 200 90 40 3.88 0.3957
WVFGRD96 16.0 205 90 40 3.90 0.3923
WVFGRD96 18.0 205 90 40 3.92 0.3828
WVFGRD96 20.0 200 75 -45 3.93 0.3972
WVFGRD96 22.0 200 75 -50 3.96 0.4206
WVFGRD96 24.0 205 80 -50 3.99 0.4426
WVFGRD96 26.0 45 65 60 4.04 0.4740
WVFGRD96 28.0 35 75 60 4.06 0.5079
WVFGRD96 30.0 35 75 60 4.08 0.5467
WVFGRD96 32.0 30 80 60 4.10 0.5802
WVFGRD96 34.0 30 80 60 4.11 0.6066
WVFGRD96 36.0 25 85 60 4.12 0.6290
WVFGRD96 38.0 25 85 60 4.13 0.6475
WVFGRD96 40.0 25 85 70 4.27 0.6578
WVFGRD96 42.0 25 85 70 4.28 0.6811
WVFGRD96 44.0 25 85 70 4.29 0.6975
WVFGRD96 46.0 15 90 65 4.29 0.7123
WVFGRD96 48.0 15 90 65 4.31 0.7276
WVFGRD96 50.0 15 90 65 4.32 0.7385
WVFGRD96 52.0 15 90 65 4.32 0.7442
WVFGRD96 54.0 15 90 65 4.33 0.7488
WVFGRD96 56.0 195 90 -70 4.34 0.7517
WVFGRD96 58.0 195 90 -70 4.35 0.7550
WVFGRD96 60.0 15 90 70 4.36 0.7545
WVFGRD96 62.0 15 90 70 4.36 0.7499
WVFGRD96 64.0 15 90 70 4.36 0.7464
WVFGRD96 66.0 195 90 -70 4.37 0.7414
WVFGRD96 68.0 195 90 -70 4.37 0.7317
WVFGRD96 70.0 15 90 70 4.37 0.7204
WVFGRD96 72.0 15 90 70 4.37 0.7084
WVFGRD96 74.0 190 85 -65 4.37 0.6997
WVFGRD96 76.0 190 85 -65 4.37 0.6894
WVFGRD96 78.0 190 85 -65 4.37 0.6781
WVFGRD96 80.0 190 85 -65 4.37 0.6699
WVFGRD96 82.0 10 90 60 4.39 0.6496
WVFGRD96 84.0 15 90 60 4.38 0.6440
WVFGRD96 86.0 25 70 60 4.43 0.6368
WVFGRD96 88.0 25 70 60 4.43 0.6290
WVFGRD96 90.0 20 75 55 4.43 0.6282
WVFGRD96 92.0 20 75 55 4.43 0.6189
WVFGRD96 94.0 15 80 50 4.43 0.6185
WVFGRD96 96.0 15 80 50 4.43 0.6111
WVFGRD96 98.0 20 80 50 4.42 0.6114
The best solution is
WVFGRD96 58.0 195 90 -70 4.35 0.7550
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/3.4 -20 o DIST/3.4 +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