The ANSS event ID is ak019b11fo13 and the event page is at https://earthquake.usgs.gov/earthquakes/eventpage/ak019b11fo13/executive.
2019/08/28 07:48:25 61.322 -150.075 40.9 4.1 Alaska
USGS/SLU Moment Tensor Solution
ENS 2019/08/28 07:48:25:0 61.32 -150.07 40.9 4.1 Alaska
Stations used:
AK.BRLK AK.CAPN AK.CNP AK.CUT AK.DHY AK.FID AK.FIRE AK.GHO
AK.GLI AK.HOM AK.KNK AK.KTH AK.MCK AK.PAX AK.PPLA AK.PWL
AK.RC01 AK.RND AK.SCM AK.SKN AK.SLK AK.SSN AK.SWD AK.VRDI
AT.PMR AV.SPU AV.STLK TA.L19K TA.M19K TA.M20K TA.M22K
TA.O22K TA.P19K
Filtering commands used:
cut o DIST/3.4 -40 o DIST/3.4 +50
rtr
taper w 0.1
hp c 0.05 n 3
lp c 0.10 n 3
Best Fitting Double Couple
Mo = 1.91e+22 dyne-cm
Mw = 4.12
Z = 44 km
Plane Strike Dip Rake
NP1 205 85 -60
NP2 304 30 -170
Principal Axes:
Axis Value Plunge Azimuth
T 1.91e+22 33 270
N 0.00e+00 30 22
P -1.91e+22 42 143
Moment Tensor: (dyne-cm)
Component Value
Mxx -6.76e+21
Mxy 5.00e+21
Mxz 7.62e+21
Myy 9.62e+21
Myz -1.44e+22
Mzz -2.87e+21
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------#######--------#######
-###################-#########
######################---#########
######################-------#######
######################----------######
######################------------######
#####################--------------#####
#####################----------------#####
###### ###########------------------####
###### T ##########--------------------###
###### #########---------------------###
#################---------------------##
################----------------------##
##############----------- ---------#
############------------ P ---------
##########------------- --------
########----------------------
######----------------------
###-------------------
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Global CMT Convention Moment Tensor:
R T P
-2.87e+21 7.62e+21 1.44e+22
7.62e+21 -6.76e+21 -5.00e+21
1.44e+22 -5.00e+21 9.62e+21
Details of the solution is found at
http://www.eas.slu.edu/eqc/eqc_mt/MECH.NA/20190828074825/index.html
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STK = 205
DIP = 85
RAKE = -60
MW = 4.12
HS = 44.0
The NDK file is 20190828074825.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 -40 o DIST/3.4 +50 rtr taper w 0.1 hp c 0.05 n 3 lp c 0.10 n 3The results of this grid search are as follow:
DEPTH STK DIP RAKE MW FIT
WVFGRD96 2.0 125 65 15 3.32 0.2519
WVFGRD96 4.0 120 65 0 3.42 0.3037
WVFGRD96 6.0 300 65 0 3.49 0.3308
WVFGRD96 8.0 120 70 -20 3.57 0.3465
WVFGRD96 10.0 120 70 -20 3.62 0.3460
WVFGRD96 12.0 120 70 -20 3.65 0.3389
WVFGRD96 14.0 235 70 20 3.69 0.3382
WVFGRD96 16.0 235 70 20 3.72 0.3395
WVFGRD96 18.0 235 65 20 3.75 0.3456
WVFGRD96 20.0 235 65 20 3.78 0.3631
WVFGRD96 22.0 230 65 20 3.81 0.3849
WVFGRD96 24.0 230 65 20 3.83 0.4060
WVFGRD96 26.0 30 85 20 3.85 0.4232
WVFGRD96 28.0 30 80 25 3.87 0.4442
WVFGRD96 30.0 30 80 25 3.89 0.4620
WVFGRD96 32.0 30 80 25 3.91 0.4739
WVFGRD96 34.0 30 90 55 3.95 0.5013
WVFGRD96 36.0 30 90 55 3.97 0.5346
WVFGRD96 38.0 210 90 -55 3.98 0.5589
WVFGRD96 40.0 205 85 -65 4.10 0.5702
WVFGRD96 42.0 30 90 60 4.11 0.5696
WVFGRD96 44.0 205 85 -60 4.12 0.5731
WVFGRD96 46.0 205 85 -60 4.13 0.5713
WVFGRD96 48.0 205 85 -60 4.14 0.5679
WVFGRD96 50.0 200 80 -60 4.15 0.5638
WVFGRD96 52.0 205 85 -55 4.15 0.5586
WVFGRD96 54.0 205 85 -55 4.16 0.5536
WVFGRD96 56.0 205 85 -55 4.17 0.5462
WVFGRD96 58.0 205 85 -55 4.17 0.5407
WVFGRD96 60.0 205 85 -55 4.18 0.5328
WVFGRD96 62.0 30 85 50 4.18 0.5256
WVFGRD96 64.0 25 90 70 4.19 0.5246
WVFGRD96 66.0 205 90 -70 4.20 0.5249
WVFGRD96 68.0 25 90 65 4.20 0.5249
WVFGRD96 70.0 25 90 65 4.20 0.5239
WVFGRD96 72.0 25 90 65 4.21 0.5234
WVFGRD96 74.0 25 90 65 4.21 0.5201
WVFGRD96 76.0 25 90 65 4.21 0.5179
WVFGRD96 78.0 25 90 65 4.21 0.5147
WVFGRD96 80.0 205 90 -65 4.22 0.5094
WVFGRD96 82.0 25 90 65 4.22 0.5059
WVFGRD96 84.0 205 90 -65 4.22 0.5002
WVFGRD96 86.0 25 90 60 4.22 0.4936
WVFGRD96 88.0 205 90 -60 4.22 0.4868
WVFGRD96 90.0 205 90 -60 4.22 0.4763
WVFGRD96 92.0 25 90 60 4.22 0.4650
WVFGRD96 94.0 205 90 -60 4.22 0.4485
WVFGRD96 96.0 25 90 60 4.22 0.4371
WVFGRD96 98.0 40 70 30 4.24 0.4256
WVFGRD96 100.0 205 90 -55 4.22 0.4247
WVFGRD96 102.0 205 90 -55 4.22 0.4017
WVFGRD96 104.0 30 85 50 4.21 0.3609
WVFGRD96 106.0 225 90 15 4.23 0.3419
WVFGRD96 108.0 45 90 -15 4.23 0.3271
WVFGRD96 110.0 225 90 15 4.23 0.3143
WVFGRD96 112.0 45 90 -15 4.23 0.3083
WVFGRD96 114.0 45 90 -15 4.23 0.3006
WVFGRD96 116.0 225 90 15 4.23 0.2916
WVFGRD96 118.0 45 90 -15 4.23 0.2832
WVFGRD96 120.0 45 90 -15 4.23 0.2722
WVFGRD96 122.0 45 90 -15 4.23 0.2624
WVFGRD96 124.0 225 90 20 4.23 0.2563
WVFGRD96 126.0 30 85 -40 4.20 0.2495
WVFGRD96 128.0 30 85 -40 4.20 0.2434
WVFGRD96 130.0 30 85 -40 4.19 0.2317
WVFGRD96 132.0 30 85 -40 4.18 0.2137
WVFGRD96 134.0 30 80 -35 4.18 0.1939
WVFGRD96 136.0 40 80 -25 4.19 0.1744
WVFGRD96 138.0 40 75 -25 4.17 0.1559
WVFGRD96 140.0 40 80 -25 4.16 0.1417
WVFGRD96 142.0 40 80 -25 4.15 0.1249
WVFGRD96 144.0 245 65 70 4.12 0.1042
WVFGRD96 146.0 135 75 5 4.14 0.0994
WVFGRD96 148.0 135 70 0 4.13 0.0946
The best solution is
WVFGRD96 44.0 205 85 -60 4.12 0.5731
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 -40 o DIST/3.4 +50 rtr taper w 0.1 hp c 0.05 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