The ANSS event ID is ak018by72yh3 and the event page is at https://earthquake.usgs.gov/earthquakes/eventpage/ak018by72yh3/executive.
2018/09/17 12:35:08 59.708 -153.214 104.8 3.9 Alaska
USGS/SLU Moment Tensor Solution
ENS 2018/09/17 12:35:08:0 59.71 -153.21 104.8 3.9 Alaska
Stations used:
AT.OHAK AV.ILSW AV.SPU II.KDAK TA.L19K TA.M19K TA.O18K
TA.P18K TA.Q19K TA.Q20K
Filtering commands used:
cut o DIST/3.4 -30 o DIST/3.4 +70
rtr
taper w 0.1
hp c 0.03 n 3
lp c 0.10 n 3
Best Fitting Double Couple
Mo = 9.55e+21 dyne-cm
Mw = 3.92
Z = 118 km
Plane Strike Dip Rake
NP1 319 63 127
NP2 80 45 40
Principal Axes:
Axis Value Plunge Azimuth
T 9.55e+21 55 278
N 0.00e+00 33 120
P -9.55e+21 10 23
Moment Tensor: (dyne-cm)
Component Value
Mxx -7.72e+21
Mxy -3.81e+21
Mxz -8.98e+20
Myy 1.58e+21
Myz -5.09e+21
Mzz 6.14e+21
-------------
----------------- P --
#------------------- -----
########----------------------
##############--------------------
#################-------------------
####################------------------
#######################-----------------
#########################---------------
########### ##############-------------#
########### T ###############-----------##
########### ################---------###
###############################-------####
###############################----#####
--##############################-#######
----#########################---######
-------#################-------#####
------------------------------####
-----------------------------#
----------------------------
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Global CMT Convention Moment Tensor:
R T P
6.14e+21 -8.98e+20 5.09e+21
-8.98e+20 -7.72e+21 3.81e+21
5.09e+21 3.81e+21 1.58e+21
Details of the solution is found at
http://www.eas.slu.edu/eqc/eqc_mt/MECH.NA/20180917123508/index.html
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STK = 80
DIP = 45
RAKE = 40
MW = 3.92
HS = 118.0
The NDK file is 20180917123508.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 -30 o DIST/3.4 +70 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 2.0 330 65 -35 2.96 0.2171
WVFGRD96 4.0 160 85 40 3.08 0.2533
WVFGRD96 6.0 160 85 40 3.16 0.2911
WVFGRD96 8.0 340 90 -50 3.27 0.3149
WVFGRD96 10.0 -5 75 65 3.37 0.3289
WVFGRD96 12.0 185 75 75 3.49 0.3345
WVFGRD96 14.0 185 75 75 3.53 0.3336
WVFGRD96 16.0 230 70 15 3.45 0.3367
WVFGRD96 18.0 230 70 10 3.48 0.3503
WVFGRD96 20.0 230 70 10 3.51 0.3578
WVFGRD96 22.0 230 70 10 3.53 0.3585
WVFGRD96 24.0 70 85 15 3.48 0.3662
WVFGRD96 26.0 75 75 20 3.49 0.3871
WVFGRD96 28.0 75 75 15 3.50 0.4027
WVFGRD96 30.0 65 55 -25 3.60 0.4132
WVFGRD96 32.0 70 65 -15 3.57 0.4210
WVFGRD96 34.0 70 65 -15 3.58 0.4242
WVFGRD96 36.0 70 70 -10 3.59 0.4275
WVFGRD96 38.0 70 75 -5 3.62 0.4316
WVFGRD96 40.0 250 80 -35 3.66 0.4397
WVFGRD96 42.0 250 80 -35 3.68 0.4473
WVFGRD96 44.0 255 90 -35 3.70 0.4540
WVFGRD96 46.0 255 90 -35 3.72 0.4597
WVFGRD96 48.0 255 90 -35 3.74 0.4660
WVFGRD96 50.0 255 90 -35 3.76 0.4731
WVFGRD96 52.0 255 90 -35 3.77 0.4808
WVFGRD96 54.0 75 75 40 3.82 0.4974
WVFGRD96 56.0 75 70 35 3.83 0.5190
WVFGRD96 58.0 75 70 40 3.85 0.5407
WVFGRD96 60.0 75 65 35 3.85 0.5539
WVFGRD96 62.0 75 65 35 3.86 0.5658
WVFGRD96 64.0 75 65 35 3.86 0.5780
WVFGRD96 66.0 75 65 35 3.86 0.5842
WVFGRD96 68.0 75 65 35 3.87 0.5918
WVFGRD96 70.0 75 65 35 3.87 0.5979
WVFGRD96 72.0 75 65 35 3.87 0.6039
WVFGRD96 74.0 75 65 35 3.88 0.6085
WVFGRD96 76.0 75 65 35 3.88 0.6141
WVFGRD96 78.0 75 65 35 3.88 0.6151
WVFGRD96 80.0 75 60 35 3.88 0.6192
WVFGRD96 82.0 75 60 35 3.89 0.6226
WVFGRD96 84.0 80 50 40 3.89 0.6264
WVFGRD96 86.0 80 50 40 3.89 0.6294
WVFGRD96 88.0 80 50 40 3.89 0.6345
WVFGRD96 90.0 80 50 40 3.89 0.6376
WVFGRD96 92.0 80 50 40 3.90 0.6396
WVFGRD96 94.0 80 50 40 3.90 0.6410
WVFGRD96 96.0 80 50 40 3.90 0.6409
WVFGRD96 98.0 80 50 40 3.90 0.6439
WVFGRD96 100.0 80 50 40 3.90 0.6460
WVFGRD96 102.0 80 50 40 3.91 0.6468
WVFGRD96 104.0 80 50 40 3.91 0.6471
WVFGRD96 106.0 80 45 40 3.91 0.6473
WVFGRD96 108.0 80 45 40 3.91 0.6487
WVFGRD96 110.0 80 45 40 3.91 0.6504
WVFGRD96 112.0 80 45 40 3.91 0.6503
WVFGRD96 114.0 80 45 40 3.92 0.6503
WVFGRD96 116.0 80 45 40 3.92 0.6519
WVFGRD96 118.0 80 45 40 3.92 0.6519
WVFGRD96 120.0 80 45 40 3.92 0.6510
WVFGRD96 122.0 80 45 40 3.92 0.6497
WVFGRD96 124.0 80 45 40 3.93 0.6493
WVFGRD96 126.0 80 45 40 3.93 0.6478
WVFGRD96 128.0 75 45 35 3.93 0.6475
WVFGRD96 130.0 75 45 35 3.93 0.6480
WVFGRD96 132.0 75 45 30 3.92 0.6464
WVFGRD96 134.0 75 45 30 3.92 0.6470
WVFGRD96 136.0 75 45 30 3.92 0.6458
WVFGRD96 138.0 75 45 30 3.93 0.6427
WVFGRD96 140.0 75 45 30 3.93 0.6421
WVFGRD96 142.0 75 45 30 3.93 0.6411
WVFGRD96 144.0 75 45 30 3.93 0.6416
WVFGRD96 146.0 75 45 30 3.93 0.6395
WVFGRD96 148.0 75 45 30 3.94 0.6360
The best solution is
WVFGRD96 118.0 80 45 40 3.92 0.6519
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 -30 o DIST/3.4 +70 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