The ANSS event ID is ak024bpxi37y and the event page is at https://earthquake.usgs.gov/earthquakes/eventpage/ak024bpxi37y/executive.
2024/09/11 12:53:14 59.825 -153.323 126.3 3.9 Alaska
USGS/SLU Moment Tensor Solution
ENS 2024/09/11 12:53:14:0 59.83 -153.32 126.3 3.9 Alaska
Stations used:
AK.HOM AK.L19K AK.M20K AK.N18K AK.O18K AK.O19K AK.P17K
AK.PWL AK.SWD AV.ACH AV.PLBL AV.RED AV.STLK II.KDAK
Filtering commands used:
cut o DIST/3.5 -40 o DIST/3.5 +50
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 = 136 km
Plane Strike Dip Rake
NP1 286 84 114
NP2 30 25 15
Principal Axes:
Axis Value Plunge Azimuth
T 9.55e+21 46 221
N 0.00e+00 24 104
P -9.55e+21 34 356
Moment Tensor: (dyne-cm)
Component Value
Mxx -3.85e+21
Mxy 2.77e+21
Mxz -8.03e+21
Myy 1.96e+21
Myz -2.80e+21
Mzz 1.89e+21
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------------ -----------##
------------- P -------------#
--------------- --------------##
---------------------------------###
-----------------------------------###
------------------------------------####
########-----------------------------###
#################---------------------####
########################--------------####
#############################--------#####
##################################---#####
###################################---##
########### ####################------
########## T ###################------
######### ##################------
############################------
########################------
####################--------
#############---------
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Global CMT Convention Moment Tensor:
R T P
1.89e+21 -8.03e+21 2.80e+21
-8.03e+21 -3.85e+21 -2.77e+21
2.80e+21 -2.77e+21 1.96e+21
Details of the solution is found at
http://www.eas.slu.edu/eqc/eqc_mt/MECH.NA/20240911125314/index.html
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STK = 30
DIP = 25
RAKE = 15
MW = 3.92
HS = 136.0
The NDK file is 20240911125314.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/3.5 -40 o DIST/3.5 +50 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 90 65 -15 2.90 0.1918
WVFGRD96 4.0 10 70 -5 2.97 0.2361
WVFGRD96 6.0 10 75 0 3.05 0.2737
WVFGRD96 8.0 10 75 5 3.14 0.3011
WVFGRD96 10.0 10 75 10 3.20 0.3110
WVFGRD96 12.0 10 85 10 3.23 0.3113
WVFGRD96 14.0 190 90 -10 3.26 0.3058
WVFGRD96 16.0 190 75 -5 3.29 0.2988
WVFGRD96 18.0 185 70 -10 3.33 0.2894
WVFGRD96 20.0 185 65 -5 3.35 0.2823
WVFGRD96 22.0 185 65 -5 3.37 0.2778
WVFGRD96 24.0 185 60 -5 3.39 0.2764
WVFGRD96 26.0 185 60 0 3.41 0.2770
WVFGRD96 28.0 185 65 15 3.43 0.2782
WVFGRD96 30.0 190 65 30 3.46 0.2795
WVFGRD96 32.0 190 65 35 3.49 0.2814
WVFGRD96 34.0 190 65 40 3.51 0.2785
WVFGRD96 36.0 190 70 45 3.54 0.2776
WVFGRD96 38.0 165 85 -25 3.58 0.2819
WVFGRD96 40.0 160 80 -45 3.69 0.3008
WVFGRD96 42.0 340 50 -15 3.71 0.3004
WVFGRD96 44.0 340 45 -15 3.74 0.3112
WVFGRD96 46.0 340 45 -15 3.77 0.3265
WVFGRD96 48.0 340 40 -20 3.79 0.3465
WVFGRD96 50.0 340 35 -20 3.82 0.3702
WVFGRD96 52.0 340 35 -20 3.84 0.3955
WVFGRD96 54.0 340 30 -25 3.86 0.4179
WVFGRD96 56.0 340 30 -25 3.87 0.4363
WVFGRD96 58.0 345 30 -20 3.87 0.4545
WVFGRD96 60.0 345 30 -15 3.87 0.4644
WVFGRD96 62.0 350 30 -15 3.87 0.4781
WVFGRD96 64.0 350 30 -15 3.87 0.4844
WVFGRD96 66.0 355 30 -5 3.87 0.4921
WVFGRD96 68.0 360 15 -15 3.88 0.5107
WVFGRD96 70.0 5 15 -10 3.88 0.5262
WVFGRD96 72.0 10 15 -5 3.88 0.5377
WVFGRD96 74.0 15 15 0 3.88 0.5527
WVFGRD96 76.0 20 15 5 3.89 0.5662
WVFGRD96 78.0 20 15 5 3.89 0.5769
WVFGRD96 80.0 25 15 10 3.89 0.5864
WVFGRD96 82.0 25 15 10 3.89 0.5940
WVFGRD96 84.0 30 15 15 3.89 0.6002
WVFGRD96 86.0 30 15 15 3.89 0.6056
WVFGRD96 88.0 30 15 15 3.89 0.6110
WVFGRD96 90.0 35 15 20 3.89 0.6172
WVFGRD96 92.0 35 15 20 3.89 0.6218
WVFGRD96 94.0 35 15 20 3.89 0.6248
WVFGRD96 96.0 35 15 20 3.89 0.6291
WVFGRD96 98.0 40 15 25 3.89 0.6321
WVFGRD96 100.0 40 15 25 3.89 0.6329
WVFGRD96 102.0 35 20 20 3.89 0.6345
WVFGRD96 104.0 35 20 20 3.89 0.6382
WVFGRD96 106.0 35 20 20 3.90 0.6419
WVFGRD96 108.0 40 20 25 3.90 0.6463
WVFGRD96 110.0 40 20 25 3.90 0.6474
WVFGRD96 112.0 35 20 20 3.90 0.6465
WVFGRD96 114.0 35 20 20 3.90 0.6488
WVFGRD96 116.0 35 20 20 3.90 0.6516
WVFGRD96 118.0 35 20 20 3.90 0.6541
WVFGRD96 120.0 35 20 20 3.90 0.6514
WVFGRD96 122.0 35 20 20 3.90 0.6537
WVFGRD96 124.0 35 20 20 3.90 0.6559
WVFGRD96 126.0 35 20 20 3.90 0.6551
WVFGRD96 128.0 35 20 20 3.91 0.6545
WVFGRD96 130.0 35 20 20 3.91 0.6561
WVFGRD96 132.0 35 20 20 3.91 0.6551
WVFGRD96 134.0 30 25 15 3.91 0.6545
WVFGRD96 136.0 30 25 15 3.92 0.6566
WVFGRD96 138.0 30 25 15 3.92 0.6537
WVFGRD96 140.0 30 25 15 3.92 0.6551
WVFGRD96 142.0 30 25 15 3.92 0.6551
WVFGRD96 144.0 30 25 15 3.92 0.6506
WVFGRD96 146.0 30 25 15 3.92 0.6544
WVFGRD96 148.0 30 25 15 3.92 0.6510
The best solution is
WVFGRD96 136.0 30 25 15 3.92 0.6566
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/3.5 -40 o DIST/3.5 +50 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