The ANSS event ID is ak024ihs2a8 and the event page is at https://earthquake.usgs.gov/earthquakes/eventpage/ak024ihs2a8/executive.
2024/01/11 18:25:40 61.587 -151.120 65.0 3.7 Alaska
USGS/SLU Moment Tensor Solution
ENS 2024/01/11 18:25:40:0 61.59 -151.12 65.0 3.7 Alaska
Stations used:
AK.CUT AK.FIRE AK.GHO AK.KNK AK.L22K AK.N19K AK.O19K AK.SAW
AK.SKN AK.WAT6 AT.PMR 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
br c 0.12 0.25 n 4 p 2
Best Fitting Double Couple
Mo = 1.72e+22 dyne-cm
Mw = 4.09
Z = 82 km
Plane Strike Dip Rake
NP1 152 74 -110
NP2 25 25 -40
Principal Axes:
Axis Value Plunge Azimuth
T 1.72e+22 27 258
N 0.00e+00 19 158
P -1.72e+22 56 37
Moment Tensor: (dyne-cm)
Component Value
Mxx -2.75e+21
Mxy 3.35e+20
Mxz -7.81e+21
Myy 1.12e+22
Myz -1.15e+22
Mzz -8.46e+21
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#--------------------#
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######----------------------##
########-----------------------###
##########-----------------------###
###########------------ ---------###
#############----------- P ---------####
#############----------- ---------####
###############----------------------#####
################---------------------#####
#### ##########--------------------#####
#### T ###########-------------------#####
### ############-----------------#####
###################---------------######
###################-------------######
####################----------######
####################--------######
####################----######
####################-#######
##############------##
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Global CMT Convention Moment Tensor:
R T P
-8.46e+21 -7.81e+21 1.15e+22
-7.81e+21 -2.75e+21 -3.35e+20
1.15e+22 -3.35e+20 1.12e+22
Details of the solution is found at
http://www.eas.slu.edu/eqc/eqc_mt/MECH.NA/20240111182540/index.html
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STK = 25
DIP = 25
RAKE = -40
MW = 4.09
HS = 82.0
The NDK file is 20240111182540.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 3 br c 0.12 0.25 n 4 p 2The results of this grid search are as follow:
DEPTH STK DIP RAKE MW FIT
WVFGRD96 2.0 40 70 15 3.39 0.2362
WVFGRD96 4.0 40 80 25 3.46 0.2656
WVFGRD96 6.0 40 75 25 3.51 0.2820
WVFGRD96 8.0 40 75 25 3.55 0.2875
WVFGRD96 10.0 40 70 20 3.57 0.2859
WVFGRD96 12.0 40 75 15 3.58 0.2813
WVFGRD96 14.0 40 75 15 3.59 0.2779
WVFGRD96 16.0 40 75 10 3.60 0.2771
WVFGRD96 18.0 40 75 10 3.62 0.2776
WVFGRD96 20.0 40 75 10 3.64 0.2786
WVFGRD96 22.0 40 75 10 3.65 0.2805
WVFGRD96 24.0 40 75 10 3.67 0.2827
WVFGRD96 26.0 40 75 10 3.69 0.2861
WVFGRD96 28.0 40 75 10 3.70 0.2905
WVFGRD96 30.0 40 70 15 3.73 0.2954
WVFGRD96 32.0 40 75 10 3.75 0.2993
WVFGRD96 34.0 40 75 10 3.77 0.3011
WVFGRD96 36.0 40 65 15 3.80 0.3065
WVFGRD96 38.0 40 70 15 3.82 0.3114
WVFGRD96 40.0 45 60 20 3.89 0.3107
WVFGRD96 42.0 45 60 20 3.91 0.3116
WVFGRD96 44.0 40 65 15 3.91 0.3126
WVFGRD96 46.0 40 65 15 3.92 0.3141
WVFGRD96 48.0 30 35 0 3.97 0.3174
WVFGRD96 50.0 30 35 0 3.98 0.3239
WVFGRD96 52.0 30 35 0 3.99 0.3291
WVFGRD96 54.0 30 35 -5 4.00 0.3347
WVFGRD96 56.0 30 35 -5 4.01 0.3391
WVFGRD96 58.0 30 35 -5 4.01 0.3428
WVFGRD96 60.0 30 35 -5 4.02 0.3462
WVFGRD96 62.0 30 35 -5 4.03 0.3481
WVFGRD96 64.0 25 30 -30 4.03 0.3505
WVFGRD96 66.0 20 25 -40 4.04 0.3598
WVFGRD96 68.0 20 25 -40 4.04 0.3656
WVFGRD96 70.0 20 25 -40 4.05 0.3703
WVFGRD96 72.0 25 25 -40 4.06 0.3740
WVFGRD96 74.0 25 25 -40 4.07 0.3771
WVFGRD96 76.0 25 25 -40 4.07 0.3795
WVFGRD96 78.0 25 25 -40 4.08 0.3816
WVFGRD96 80.0 25 25 -40 4.08 0.3823
WVFGRD96 82.0 25 25 -40 4.09 0.3824
WVFGRD96 84.0 25 25 -40 4.09 0.3820
WVFGRD96 86.0 25 25 -45 4.10 0.3805
WVFGRD96 88.0 30 25 -40 4.10 0.3792
WVFGRD96 90.0 30 25 -40 4.10 0.3777
WVFGRD96 92.0 30 25 -40 4.11 0.3757
WVFGRD96 94.0 55 30 -30 4.15 0.3740
WVFGRD96 96.0 55 30 -30 4.15 0.3733
WVFGRD96 98.0 55 30 -30 4.15 0.3726
The best solution is
WVFGRD96 82.0 25 25 -40 4.09 0.3824
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 br c 0.12 0.25 n 4 p 2
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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