The ANSS event ID is ak0249guafvl and the event page is at https://earthquake.usgs.gov/earthquakes/eventpage/ak0249guafvl/executive.
2024/07/24 09:37:54 62.935 -150.495 89.6 3.9 Alaska
USGS/SLU Moment Tensor Solution
ENS 2024/07/24 09:37:54:0 62.94 -150.49 89.6 3.9 Alaska
Stations used:
AK.BPAW AK.GHO AK.J19K AK.J20K AK.KNK AK.L22K AK.MCK AK.MLY
AK.NEA2 AK.PAX AK.RC01 AK.RND AK.SAW AK.SCM AK.WRH AT.PMR
AT.TTA IM.IL31
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 = 8.91e+21 dyne-cm
Mw = 3.90
Z = 92 km
Plane Strike Dip Rake
NP1 181 82 -140
NP2 85 50 -10
Principal Axes:
Axis Value Plunge Azimuth
T 8.91e+21 21 307
N 0.00e+00 49 190
P -8.91e+21 33 51
Moment Tensor: (dyne-cm)
Component Value
Mxx 3.45e+20
Mxy -6.75e+21
Mxz -7.59e+20
Myy 1.18e+21
Myz -5.60e+21
Mzz -1.52e+21
#######-------
###########-----------
#############---------------
##############----------------
## ###########------------------
### T ###########---------- ------
#### ##########----------- P -------
##################----------- --------
##################----------------------
###################-----------------------
###################----------------------#
###################---------------------##
-##################-------------------####
--################-----------------#####
----##############---------------#######
-------##########-----------##########
----------------####################
---------------###################
-------------#################
-------------###############
----------############
------########
Global CMT Convention Moment Tensor:
R T P
-1.52e+21 -7.59e+20 5.60e+21
-7.59e+20 3.45e+20 6.75e+21
5.60e+21 6.75e+21 1.18e+21
Details of the solution is found at
http://www.eas.slu.edu/eqc/eqc_mt/MECH.NA/20240724093754/index.html
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STK = 85
DIP = 50
RAKE = -10
MW = 3.90
HS = 92.0
The NDK file is 20240724093754.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 0 75 -15 2.93 0.2088
WVFGRD96 4.0 185 70 30 3.06 0.2341
WVFGRD96 6.0 185 70 25 3.11 0.2565
WVFGRD96 8.0 185 70 25 3.19 0.2672
WVFGRD96 10.0 120 70 -25 3.21 0.2631
WVFGRD96 12.0 115 70 -20 3.26 0.2670
WVFGRD96 14.0 115 70 -15 3.29 0.2663
WVFGRD96 16.0 115 70 -15 3.32 0.2639
WVFGRD96 18.0 115 70 -10 3.34 0.2635
WVFGRD96 20.0 270 90 -15 3.38 0.2807
WVFGRD96 22.0 270 80 -10 3.42 0.3045
WVFGRD96 24.0 270 85 -10 3.44 0.3300
WVFGRD96 26.0 265 80 -15 3.47 0.3586
WVFGRD96 28.0 85 90 15 3.49 0.3828
WVFGRD96 30.0 265 85 -15 3.52 0.4073
WVFGRD96 32.0 90 90 10 3.53 0.4289
WVFGRD96 34.0 90 90 10 3.55 0.4420
WVFGRD96 36.0 90 90 10 3.57 0.4505
WVFGRD96 38.0 90 90 5 3.60 0.4545
WVFGRD96 40.0 90 90 10 3.65 0.4617
WVFGRD96 42.0 270 90 -10 3.68 0.4570
WVFGRD96 44.0 270 85 -10 3.71 0.4564
WVFGRD96 46.0 90 70 5 3.70 0.4618
WVFGRD96 48.0 90 70 5 3.72 0.4687
WVFGRD96 50.0 85 60 -5 3.74 0.4783
WVFGRD96 52.0 85 55 -10 3.75 0.4908
WVFGRD96 54.0 85 50 -10 3.76 0.5029
WVFGRD96 56.0 85 55 -5 3.77 0.5196
WVFGRD96 58.0 80 50 -10 3.79 0.5369
WVFGRD96 60.0 80 50 -10 3.80 0.5529
WVFGRD96 62.0 80 50 -10 3.81 0.5726
WVFGRD96 64.0 80 45 -15 3.83 0.5896
WVFGRD96 66.0 80 45 -15 3.83 0.6054
WVFGRD96 68.0 80 45 -15 3.84 0.6174
WVFGRD96 70.0 80 45 -15 3.85 0.6311
WVFGRD96 72.0 80 45 -15 3.86 0.6442
WVFGRD96 74.0 80 45 -15 3.86 0.6524
WVFGRD96 76.0 80 45 -15 3.87 0.6628
WVFGRD96 78.0 80 45 -15 3.87 0.6702
WVFGRD96 80.0 80 45 -15 3.88 0.6739
WVFGRD96 82.0 85 50 -10 3.88 0.6810
WVFGRD96 84.0 85 50 -10 3.88 0.6855
WVFGRD96 86.0 85 50 -10 3.89 0.6887
WVFGRD96 88.0 85 50 -10 3.89 0.6920
WVFGRD96 90.0 85 50 -10 3.89 0.6927
WVFGRD96 92.0 85 50 -10 3.90 0.6953
WVFGRD96 94.0 85 50 -10 3.90 0.6929
WVFGRD96 96.0 85 50 -10 3.90 0.6944
WVFGRD96 98.0 90 50 -10 3.91 0.6909
WVFGRD96 100.0 90 50 -10 3.92 0.6934
WVFGRD96 102.0 90 50 -10 3.92 0.6891
WVFGRD96 104.0 90 50 -10 3.92 0.6891
WVFGRD96 106.0 90 50 -10 3.93 0.6844
WVFGRD96 108.0 90 50 -10 3.93 0.6835
WVFGRD96 110.0 90 50 -10 3.93 0.6798
WVFGRD96 112.0 90 50 -5 3.93 0.6776
WVFGRD96 114.0 90 50 -5 3.93 0.6741
WVFGRD96 116.0 90 50 -5 3.93 0.6691
WVFGRD96 118.0 95 50 -5 3.95 0.6661
WVFGRD96 120.0 95 50 -5 3.95 0.6612
WVFGRD96 122.0 95 50 -5 3.95 0.6607
WVFGRD96 124.0 95 50 -5 3.95 0.6553
WVFGRD96 126.0 95 55 0 3.95 0.6532
WVFGRD96 128.0 95 55 0 3.95 0.6511
WVFGRD96 130.0 95 55 0 3.96 0.6466
WVFGRD96 132.0 95 55 0 3.96 0.6453
WVFGRD96 134.0 95 55 0 3.96 0.6414
WVFGRD96 136.0 95 55 0 3.96 0.6393
WVFGRD96 138.0 95 55 0 3.97 0.6364
WVFGRD96 140.0 95 55 0 3.97 0.6329
WVFGRD96 142.0 95 55 0 3.97 0.6316
WVFGRD96 144.0 95 55 0 3.97 0.6279
WVFGRD96 146.0 95 55 0 3.97 0.6232
WVFGRD96 148.0 95 55 0 3.98 0.6156
The best solution is
WVFGRD96 92.0 85 50 -10 3.90 0.6953
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