The ANSS event ID is ak02381rp2xn and the event page is at https://earthquake.usgs.gov/earthquakes/eventpage/ak02381rp2xn/executive.
2023/06/24 21:24:47 61.277 -150.103 38.7 3.6 Alaska
USGS/SLU Moment Tensor Solution
ENS 2023/06/24 21:24:47:0 61.28 -150.10 38.7 3.6 Alaska
Stations used:
AK.CNP AK.FIRE AK.GHO AK.GLI AK.HOM AK.KNK AK.M20K AK.MCK
AK.PWL AK.RC01 AK.SKN AT.PMR AV.P19K AV.SPCP
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 = 5.50e+21 dyne-cm
Mw = 3.76
Z = 47 km
Plane Strike Dip Rake
NP1 180 60 -60
NP2 311 41 -131
Principal Axes:
Axis Value Plunge Azimuth
T 5.50e+21 10 249
N 0.00e+00 26 344
P -5.50e+21 62 139
Moment Tensor: (dyne-cm)
Component Value
Mxx -4.16e+14
Mxy 2.38e+21
Mxz 1.37e+21
Myy 4.12e+21
Myz -2.38e+21
Mzz -4.12e+21
------########
--------##############
----------##################
##########-----###############
###########----------#############
############-------------###########
############----------------##########
#############------------------#########
#############-------------------########
#############---------------------########
#############----------------------#######
#############-----------------------######
#############----------- ----------#####
# #########---------- P ----------####
# T #########---------- ----------####
#########-----------------------###
############----------------------##
###########----------------------#
##########--------------------
##########------------------
########--------------
######--------
Global CMT Convention Moment Tensor:
R T P
-4.12e+21 1.37e+21 2.38e+21
1.37e+21 -4.16e+14 -2.38e+21
2.38e+21 -2.38e+21 4.12e+21
Details of the solution is found at
http://www.eas.slu.edu/eqc/eqc_mt/MECH.NA/20230624212447/index.html
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STK = 180
DIP = 60
RAKE = -60
MW = 3.76
HS = 47.0
The NDK file is 20230624212447.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 1.0 290 55 -85 2.96 0.1802
WVFGRD96 2.0 290 60 -85 3.10 0.2384
WVFGRD96 3.0 290 60 -80 3.12 0.2343
WVFGRD96 4.0 50 45 -35 3.11 0.2588
WVFGRD96 5.0 50 50 -35 3.12 0.2856
WVFGRD96 6.0 25 45 -30 3.15 0.3048
WVFGRD96 7.0 25 45 -30 3.17 0.3239
WVFGRD96 8.0 100 80 -85 3.28 0.3335
WVFGRD96 9.0 95 75 -85 3.30 0.3455
WVFGRD96 10.0 25 45 -30 3.24 0.3488
WVFGRD96 11.0 30 50 -35 3.25 0.3544
WVFGRD96 12.0 25 45 -35 3.27 0.3596
WVFGRD96 13.0 25 45 -35 3.28 0.3643
WVFGRD96 14.0 30 45 -30 3.28 0.3671
WVFGRD96 15.0 30 45 -30 3.29 0.3701
WVFGRD96 16.0 30 45 -30 3.30 0.3723
WVFGRD96 17.0 35 50 -30 3.31 0.3740
WVFGRD96 18.0 30 45 -30 3.33 0.3748
WVFGRD96 19.0 40 50 -30 3.34 0.3765
WVFGRD96 20.0 40 50 -30 3.35 0.3766
WVFGRD96 21.0 35 45 -25 3.36 0.3762
WVFGRD96 22.0 35 45 -25 3.38 0.3753
WVFGRD96 23.0 40 50 -30 3.39 0.3745
WVFGRD96 24.0 30 45 -30 3.39 0.3735
WVFGRD96 25.0 30 50 -35 3.40 0.3738
WVFGRD96 26.0 35 50 -40 3.42 0.3758
WVFGRD96 27.0 30 50 -45 3.43 0.3795
WVFGRD96 28.0 35 50 -45 3.44 0.3836
WVFGRD96 29.0 20 45 -50 3.45 0.3881
WVFGRD96 30.0 20 45 -50 3.46 0.3923
WVFGRD96 31.0 175 50 -75 3.51 0.4082
WVFGRD96 32.0 175 50 -70 3.53 0.4251
WVFGRD96 33.0 175 50 -70 3.54 0.4407
WVFGRD96 34.0 185 55 -55 3.57 0.4560
WVFGRD96 35.0 180 55 -60 3.57 0.4683
WVFGRD96 36.0 180 55 -60 3.59 0.4763
WVFGRD96 37.0 180 55 -60 3.60 0.4826
WVFGRD96 38.0 180 55 -60 3.61 0.4861
WVFGRD96 39.0 180 55 -55 3.63 0.4878
WVFGRD96 40.0 180 60 -60 3.70 0.4768
WVFGRD96 41.0 180 60 -60 3.71 0.4870
WVFGRD96 42.0 180 60 -60 3.72 0.4947
WVFGRD96 43.0 180 60 -60 3.73 0.5010
WVFGRD96 44.0 180 60 -60 3.74 0.5059
WVFGRD96 45.0 180 60 -60 3.75 0.5093
WVFGRD96 46.0 180 60 -60 3.76 0.5102
WVFGRD96 47.0 180 60 -60 3.76 0.5105
WVFGRD96 48.0 180 60 -60 3.77 0.5101
WVFGRD96 49.0 180 60 -60 3.77 0.5076
WVFGRD96 50.0 175 60 -65 3.78 0.5046
WVFGRD96 51.0 180 60 -60 3.78 0.5009
WVFGRD96 52.0 175 60 -65 3.79 0.4967
WVFGRD96 53.0 175 60 -65 3.79 0.4917
WVFGRD96 54.0 175 60 -65 3.79 0.4860
WVFGRD96 55.0 175 60 -60 3.80 0.4794
WVFGRD96 56.0 175 60 -60 3.80 0.4737
WVFGRD96 57.0 180 65 -60 3.79 0.4671
WVFGRD96 58.0 180 65 -60 3.80 0.4611
WVFGRD96 59.0 180 65 -60 3.80 0.4548
The best solution is
WVFGRD96 47.0 180 60 -60 3.76 0.5105
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