The ANSS event ID is ak023dz554ac and the event page is at https://earthquake.usgs.gov/earthquakes/eventpage/ak023dz554ac/executive.
2023/10/31 23:12:35 60.953 -150.662 44.9 4 Alaska
USGS/SLU Moment Tensor Solution
ENS 2023/10/31 23:12:35:0 60.95 -150.66 44.9 4.0 Alaska
Stations used:
AK.BAE AK.BRLK AK.CAPN AK.CNP AK.FID AK.FIRE AK.GHO AK.GLI
AK.HOM AK.KNK AK.M19K AK.PPLA AK.PWL AK.RC01 AK.SAW AK.SKN
AK.SLK AK.SWD AK.WAT6 AT.PMR AV.RED AV.STLK
Filtering commands used:
cut o DIST/3.3 -40 o DIST/3.3 +50
rtr
taper w 0.1
hp c 0.03 n 3
lp c 0.07 n 3
Best Fitting Double Couple
Mo = 1.78e+22 dyne-cm
Mw = 4.10
Z = 56 km
Plane Strike Dip Rake
NP1 170 65 -65
NP2 302 35 -132
Principal Axes:
Axis Value Plunge Azimuth
T 1.78e+22 16 242
N 0.00e+00 23 339
P -1.78e+22 62 119
Moment Tensor: (dyne-cm)
Component Value
Mxx 2.70e+21
Mxy 8.51e+21
Mxz 1.33e+21
Myy 9.64e+21
Myz -1.08e+22
Mzz -1.23e+22
---###########
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--#######--------#############
-#########-------------###########
###########----------------#########
############------------------########
#############--------------------#######
#############---------------------######
##############-----------------------#####
##############-----------------------#####
###############---------- ----------####
###############---------- P -----------###
###############--------- -----------##
### #########-----------------------##
## T ##########----------------------#
# ##########----------------------
##############--------------------
#############-----------------
#############---------------
############----------
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Global CMT Convention Moment Tensor:
R T P
-1.23e+22 1.33e+21 1.08e+22
1.33e+21 2.70e+21 -8.51e+21
1.08e+22 -8.51e+21 9.64e+21
Details of the solution is found at
http://www.eas.slu.edu/eqc/eqc_mt/MECH.NA/20231031231235/index.html
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STK = 170
DIP = 65
RAKE = -65
MW = 4.10
HS = 56.0
The NDK file is 20231031231235.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 -40 o DIST/3.3 +50 rtr taper w 0.1 hp c 0.03 n 3 lp c 0.07 n 3The results of this grid search are as follow:
DEPTH STK DIP RAKE MW FIT
WVFGRD96 1.0 165 45 90 3.25 0.1343
WVFGRD96 2.0 345 45 90 3.37 0.1674
WVFGRD96 3.0 320 65 65 3.42 0.1712
WVFGRD96 4.0 125 90 -65 3.47 0.2067
WVFGRD96 5.0 125 90 -65 3.49 0.2407
WVFGRD96 6.0 125 90 -65 3.50 0.2663
WVFGRD96 7.0 305 85 60 3.50 0.2868
WVFGRD96 8.0 120 90 -65 3.58 0.2986
WVFGRD96 9.0 120 90 -65 3.59 0.3128
WVFGRD96 10.0 300 90 65 3.59 0.3229
WVFGRD96 11.0 305 90 60 3.59 0.3305
WVFGRD96 12.0 115 80 -65 3.61 0.3383
WVFGRD96 13.0 115 80 -65 3.62 0.3432
WVFGRD96 14.0 115 80 -65 3.63 0.3466
WVFGRD96 15.0 115 80 -65 3.64 0.3516
WVFGRD96 16.0 115 80 -65 3.65 0.3557
WVFGRD96 17.0 115 80 -60 3.66 0.3594
WVFGRD96 18.0 115 80 -60 3.67 0.3626
WVFGRD96 19.0 115 80 -60 3.67 0.3649
WVFGRD96 20.0 115 80 -60 3.68 0.3665
WVFGRD96 21.0 115 80 -60 3.70 0.3677
WVFGRD96 22.0 120 85 -60 3.70 0.3682
WVFGRD96 23.0 120 85 -60 3.71 0.3680
WVFGRD96 24.0 120 85 -60 3.72 0.3663
WVFGRD96 25.0 120 85 -60 3.73 0.3638
WVFGRD96 26.0 120 85 -60 3.74 0.3604
WVFGRD96 27.0 105 90 -75 3.77 0.3551
WVFGRD96 28.0 105 90 -75 3.78 0.3542
WVFGRD96 29.0 105 90 -75 3.78 0.3520
WVFGRD96 30.0 105 90 -75 3.79 0.3494
WVFGRD96 31.0 180 85 -80 3.78 0.3501
WVFGRD96 32.0 180 85 -75 3.79 0.3532
WVFGRD96 33.0 180 85 -75 3.80 0.3581
WVFGRD96 34.0 185 85 -75 3.81 0.3646
WVFGRD96 35.0 180 80 -70 3.82 0.3712
WVFGRD96 36.0 180 80 -70 3.83 0.3784
WVFGRD96 37.0 175 70 -60 3.86 0.3905
WVFGRD96 38.0 170 65 -60 3.87 0.4107
WVFGRD96 39.0 175 65 -60 3.89 0.4305
WVFGRD96 40.0 170 65 -65 3.99 0.4318
WVFGRD96 41.0 170 65 -65 4.00 0.4404
WVFGRD96 42.0 170 65 -65 4.01 0.4481
WVFGRD96 43.0 170 65 -65 4.02 0.4549
WVFGRD96 44.0 170 65 -65 4.03 0.4602
WVFGRD96 45.0 170 65 -65 4.04 0.4658
WVFGRD96 46.0 170 65 -65 4.05 0.4712
WVFGRD96 47.0 170 65 -65 4.05 0.4768
WVFGRD96 48.0 170 65 -65 4.06 0.4807
WVFGRD96 49.0 170 65 -65 4.07 0.4843
WVFGRD96 50.0 170 65 -65 4.07 0.4884
WVFGRD96 51.0 170 65 -65 4.08 0.4906
WVFGRD96 52.0 170 65 -65 4.08 0.4924
WVFGRD96 53.0 170 65 -65 4.09 0.4952
WVFGRD96 54.0 170 65 -65 4.09 0.4955
WVFGRD96 55.0 170 65 -65 4.10 0.4962
WVFGRD96 56.0 170 65 -65 4.10 0.4973
WVFGRD96 57.0 170 65 -65 4.11 0.4967
WVFGRD96 58.0 170 65 -65 4.11 0.4968
WVFGRD96 59.0 170 65 -65 4.12 0.4959
The best solution is
WVFGRD96 56.0 170 65 -65 4.10 0.4973
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 -40 o DIST/3.3 +50 rtr taper w 0.1 hp c 0.03 n 3 lp c 0.07 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