The ANSS event ID is ak019cqb6c9o and the event page is at https://earthquake.usgs.gov/earthquakes/eventpage/ak019cqb6c9o/executive.
2019/10/04 12:28:02 62.496 -151.569 92.8 5 Alaska
USGS/SLU Moment Tensor Solution
ENS 2019/10/04 12:28:02:0 62.50 -151.57 92.8 5.0 Alaska
Stations used:
AK.BPAW AK.BWN AK.CUT AK.DHY AK.FIRE AK.GHO AK.GLI AK.KLU
AK.KNK AK.KTH AK.MCK AK.MLY AK.NEA2 AK.PPLA AK.PWL AK.RC01
AK.RND AK.SAW AK.SCM AK.SKN AK.SLK AK.SSN AK.TRF AK.WRH
AT.PMR AV.ILSW AV.RED AV.SPU AV.STLK
Filtering commands used:
cut o DIST/3.3 -50 o DIST/3.3 +30
rtr
taper w 0.1
hp c 0.03 n 3
lp c 0.10 n 3
Best Fitting Double Couple
Mo = 3.59e+23 dyne-cm
Mw = 4.97
Z = 100 km
Plane Strike Dip Rake
NP1 312 51 124
NP2 85 50 55
Principal Axes:
Axis Value Plunge Azimuth
T 3.59e+23 64 288
N 0.00e+00 26 109
P -3.59e+23 1 19
Moment Tensor: (dyne-cm)
Component Value
Mxx -3.15e+23
Mxy -1.30e+23
Mxz 3.93e+22
Myy 2.52e+22
Myz -1.36e+23
Mzz 2.90e+23
------------ P
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#################-----------------
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###########################-------------
############ ##############-----------
############# T ################----------
############# #################--------#
##################################------##
-##################################---####
--######################################
----#############################--#####
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Global CMT Convention Moment Tensor:
R T P
2.90e+23 3.93e+22 1.36e+23
3.93e+22 -3.15e+23 1.30e+23
1.36e+23 1.30e+23 2.52e+22
Details of the solution is found at
http://www.eas.slu.edu/eqc/eqc_mt/MECH.NA/20191004122802/index.html
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STK = 85
DIP = 50
RAKE = 55
MW = 4.97
HS = 100.0
The NDK file is 20191004122802.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 -50 o DIST/3.3 +30 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 2.0 120 40 -80 4.18 0.2480
WVFGRD96 4.0 340 45 -10 4.20 0.2509
WVFGRD96 6.0 345 50 5 4.24 0.2840
WVFGRD96 8.0 345 45 10 4.32 0.3044
WVFGRD96 10.0 345 45 10 4.35 0.3192
WVFGRD96 12.0 265 70 40 4.40 0.3347
WVFGRD96 14.0 265 70 40 4.43 0.3400
WVFGRD96 16.0 265 70 40 4.45 0.3365
WVFGRD96 18.0 265 75 35 4.47 0.3279
WVFGRD96 20.0 270 70 35 4.50 0.3181
WVFGRD96 22.0 270 70 40 4.51 0.3082
WVFGRD96 24.0 260 65 -40 4.54 0.3025
WVFGRD96 26.0 260 65 -40 4.56 0.3012
WVFGRD96 28.0 255 60 -45 4.58 0.3011
WVFGRD96 30.0 255 60 -40 4.59 0.2940
WVFGRD96 32.0 270 70 35 4.60 0.2843
WVFGRD96 34.0 270 70 35 4.61 0.2817
WVFGRD96 36.0 70 60 -25 4.65 0.2866
WVFGRD96 38.0 75 65 -20 4.66 0.2909
WVFGRD96 40.0 120 45 85 4.76 0.3068
WVFGRD96 42.0 125 45 90 4.80 0.3249
WVFGRD96 44.0 305 45 90 4.82 0.3366
WVFGRD96 46.0 305 45 90 4.84 0.3432
WVFGRD96 48.0 130 45 95 4.85 0.3473
WVFGRD96 50.0 100 40 70 4.86 0.3549
WVFGRD96 52.0 95 45 60 4.86 0.3705
WVFGRD96 54.0 90 45 60 4.87 0.3880
WVFGRD96 56.0 90 45 55 4.88 0.4070
WVFGRD96 58.0 90 45 55 4.89 0.4262
WVFGRD96 60.0 90 45 55 4.90 0.4456
WVFGRD96 62.0 80 45 50 4.91 0.4654
WVFGRD96 64.0 80 45 50 4.92 0.4864
WVFGRD96 66.0 80 45 50 4.92 0.5054
WVFGRD96 68.0 80 45 50 4.93 0.5238
WVFGRD96 70.0 80 45 50 4.93 0.5386
WVFGRD96 72.0 80 50 50 4.94 0.5535
WVFGRD96 74.0 80 50 50 4.94 0.5655
WVFGRD96 76.0 80 50 50 4.94 0.5779
WVFGRD96 78.0 80 50 50 4.95 0.5889
WVFGRD96 80.0 80 50 50 4.95 0.5958
WVFGRD96 82.0 80 50 50 4.95 0.6030
WVFGRD96 84.0 80 50 50 4.96 0.6099
WVFGRD96 86.0 85 50 55 4.96 0.6148
WVFGRD96 88.0 85 50 55 4.96 0.6189
WVFGRD96 90.0 85 50 55 4.96 0.6230
WVFGRD96 92.0 85 50 55 4.97 0.6237
WVFGRD96 94.0 85 50 55 4.97 0.6260
WVFGRD96 96.0 85 50 55 4.97 0.6264
WVFGRD96 98.0 85 50 55 4.97 0.6264
WVFGRD96 100.0 85 50 55 4.97 0.6265
WVFGRD96 102.0 85 50 55 4.97 0.6245
WVFGRD96 104.0 85 50 55 4.98 0.6246
WVFGRD96 106.0 85 50 55 4.98 0.6220
WVFGRD96 108.0 85 50 55 4.98 0.6212
WVFGRD96 110.0 80 55 55 4.98 0.6185
WVFGRD96 112.0 80 55 55 4.99 0.6169
WVFGRD96 114.0 80 55 55 4.99 0.6148
WVFGRD96 116.0 80 55 55 4.99 0.6130
WVFGRD96 118.0 80 55 55 4.99 0.6101
WVFGRD96 120.0 80 55 55 4.99 0.6072
WVFGRD96 122.0 80 55 55 4.99 0.6036
WVFGRD96 124.0 80 55 55 4.99 0.6002
WVFGRD96 126.0 80 55 55 5.00 0.5963
WVFGRD96 128.0 80 55 55 5.00 0.5928
The best solution is
WVFGRD96 100.0 85 50 55 4.97 0.6265
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 -50 o DIST/3.3 +30 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