The ANSS event ID is ak023cq3wvg8 and the event page is at https://earthquake.usgs.gov/earthquakes/eventpage/ak023cq3wvg8/executive.
2023/10/04 00:08:11 61.949 -149.303 43.4 3.9 Alaska
USGS/SLU Moment Tensor Solution
ENS 2023/10/04 00:08:11:0 61.95 -149.30 43.4 3.9 Alaska
Stations used:
AK.FID AK.GHO AK.GLI AK.KLU AK.KNK AK.L22K AK.PWL AK.RND
AK.SAW AK.SCM AK.SKN AT.PMR
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 0.08 n 3
Best Fitting Double Couple
Mo = 9.23e+21 dyne-cm
Mw = 3.91
Z = 51 km
Plane Strike Dip Rake
NP1 48 45 -95
NP2 235 45 -85
Principal Axes:
Axis Value Plunge Azimuth
T 9.23e+21 0 141
N 0.00e+00 4 51
P -9.23e+21 86 233
Moment Tensor: (dyne-cm)
Component Value
Mxx 5.63e+21
Mxy -4.51e+21
Mxz 3.26e+20
Myy 3.56e+21
Myz 4.66e+20
Mzz -9.19e+21
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#######----------- --------------#######
######------------ P -------------########
#####------------- ------------#########
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########################## T
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Global CMT Convention Moment Tensor:
R T P
-9.19e+21 3.26e+20 -4.66e+20
3.26e+20 5.63e+21 4.51e+21
-4.66e+20 4.51e+21 3.56e+21
Details of the solution is found at
http://www.eas.slu.edu/eqc/eqc_mt/MECH.NA/20231004000811/index.html
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STK = 235
DIP = 45
RAKE = -85
MW = 3.91
HS = 51.0
The NDK file is 20231004000811.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 0.08 n 3The results of this grid search are as follow:
DEPTH STK DIP RAKE MW FIT
WVFGRD96 1.0 235 45 90 3.10 0.2775
WVFGRD96 2.0 235 45 90 3.23 0.3459
WVFGRD96 3.0 50 40 -85 3.29 0.3314
WVFGRD96 4.0 215 80 75 3.32 0.3754
WVFGRD96 5.0 220 80 75 3.32 0.4270
WVFGRD96 6.0 225 80 75 3.32 0.4649
WVFGRD96 7.0 225 80 75 3.31 0.4896
WVFGRD96 8.0 225 80 75 3.39 0.5068
WVFGRD96 9.0 225 80 75 3.39 0.5205
WVFGRD96 10.0 225 80 75 3.39 0.5290
WVFGRD96 11.0 340 15 5 3.41 0.5343
WVFGRD96 12.0 335 20 0 3.42 0.5364
WVFGRD96 13.0 335 20 0 3.42 0.5358
WVFGRD96 14.0 345 20 15 3.42 0.5331
WVFGRD96 15.0 335 20 0 3.44 0.5299
WVFGRD96 16.0 310 20 -10 3.44 0.5257
WVFGRD96 17.0 305 20 -15 3.45 0.5218
WVFGRD96 18.0 310 20 -10 3.46 0.5178
WVFGRD96 19.0 330 15 5 3.46 0.5135
WVFGRD96 20.0 310 20 -5 3.47 0.5094
WVFGRD96 21.0 305 20 -10 3.49 0.5062
WVFGRD96 22.0 300 20 -10 3.50 0.5025
WVFGRD96 23.0 280 20 -30 3.52 0.5002
WVFGRD96 24.0 275 20 -35 3.53 0.5015
WVFGRD96 25.0 270 20 -40 3.54 0.5029
WVFGRD96 26.0 270 20 -40 3.55 0.5034
WVFGRD96 27.0 275 25 -35 3.57 0.5047
WVFGRD96 28.0 270 25 -40 3.58 0.5077
WVFGRD96 29.0 265 25 -45 3.59 0.5100
WVFGRD96 30.0 265 30 -45 3.61 0.5130
WVFGRD96 31.0 250 30 -65 3.62 0.5218
WVFGRD96 32.0 245 35 -75 3.63 0.5379
WVFGRD96 33.0 55 55 -90 3.65 0.5575
WVFGRD96 34.0 240 35 -80 3.66 0.5844
WVFGRD96 35.0 240 35 -80 3.67 0.6094
WVFGRD96 36.0 245 40 -75 3.69 0.6356
WVFGRD96 37.0 240 40 -80 3.70 0.6582
WVFGRD96 38.0 240 40 -80 3.71 0.6764
WVFGRD96 39.0 240 40 -80 3.73 0.6929
WVFGRD96 40.0 240 40 -80 3.82 0.7132
WVFGRD96 41.0 240 40 -80 3.83 0.7235
WVFGRD96 42.0 240 40 -80 3.84 0.7315
WVFGRD96 43.0 240 40 -80 3.85 0.7370
WVFGRD96 44.0 240 40 -80 3.86 0.7417
WVFGRD96 45.0 240 45 -80 3.87 0.7419
WVFGRD96 46.0 240 45 -80 3.88 0.7494
WVFGRD96 47.0 240 45 -80 3.89 0.7548
WVFGRD96 48.0 240 45 -80 3.89 0.7582
WVFGRD96 49.0 50 45 -95 3.90 0.7621
WVFGRD96 50.0 235 45 -85 3.91 0.7614
WVFGRD96 51.0 235 45 -85 3.91 0.7622
WVFGRD96 52.0 235 45 -85 3.91 0.7600
WVFGRD96 53.0 235 45 -85 3.92 0.7582
WVFGRD96 54.0 235 45 -90 3.92 0.7523
WVFGRD96 55.0 235 45 -90 3.92 0.7506
WVFGRD96 56.0 235 45 -90 3.93 0.7444
WVFGRD96 57.0 235 50 -85 3.93 0.7409
WVFGRD96 58.0 235 50 -90 3.93 0.7366
WVFGRD96 59.0 235 50 -90 3.94 0.7317
The best solution is
WVFGRD96 51.0 235 45 -85 3.91 0.7622
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 0.08 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