The ANSS event ID is ak0234a0vpig and the event page is at https://earthquake.usgs.gov/earthquakes/eventpage/ak0234a0vpig/executive.
2023/04/03 13:27:05 58.271 -151.228 47.2 3.7 Alaska
USGS/SLU Moment Tensor Solution
ENS 2023/04/03 13:27:05:0 58.27 -151.23 47.2 3.7 Alaska
Stations used:
AK.BRLK AK.CNP AK.HOM AK.N19K AK.O18K AK.O19K AK.P16K
AK.P17K AK.PWL AK.RC01 AK.SLK AK.SWD AV.ACH II.KDAK
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
Best Fitting Double Couple
Mo = 5.31e+21 dyne-cm
Mw = 3.75
Z = 31 km
Plane Strike Dip Rake
NP1 303 80 -165
NP2 210 75 -10
Principal Axes:
Axis Value Plunge Azimuth
T 5.31e+21 4 76
N 0.00e+00 72 334
P -5.31e+21 18 167
Moment Tensor: (dyne-cm)
Component Value
Mxx -4.26e+21
Mxy 2.33e+21
Mxz 1.57e+21
Myy 4.72e+21
Myz -1.48e+19
Mzz -4.61e+20
--------------
---------------------#
---------------------#######
--------------------##########
---------------------#############
####----------------################
#########-----------##################
##############------##################
#################-#################### T
##################---##################
#################-------##################
################-----------###############
###############--------------#############
#############------------------#########
#############--------------------#######
###########-----------------------####
#########--------------------------#
########--------------------------
#####-------------------------
####------------ ---------
#------------ P ------
--------- --
Global CMT Convention Moment Tensor:
R T P
-4.61e+20 1.57e+21 1.48e+19
1.57e+21 -4.26e+21 -2.33e+21
1.48e+19 -2.33e+21 4.72e+21
Details of the solution is found at
http://www.eas.slu.edu/eqc/eqc_mt/MECH.NA/20230403132705/index.html
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STK = 210
DIP = 75
RAKE = -10
MW = 3.75
HS = 31.0
The NDK file is 20230403132705.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 3The results of this grid search are as follow:
DEPTH STK DIP RAKE MW FIT
WVFGRD96 1.0 35 90 0 3.13 0.2842
WVFGRD96 2.0 215 80 5 3.25 0.3715
WVFGRD96 3.0 35 90 0 3.29 0.4051
WVFGRD96 4.0 40 70 20 3.35 0.4328
WVFGRD96 5.0 40 70 15 3.38 0.4508
WVFGRD96 6.0 40 70 15 3.40 0.4662
WVFGRD96 7.0 40 70 10 3.43 0.4808
WVFGRD96 8.0 40 65 10 3.47 0.4963
WVFGRD96 9.0 35 65 0 3.48 0.5041
WVFGRD96 10.0 35 65 0 3.50 0.5152
WVFGRD96 11.0 35 65 0 3.51 0.5258
WVFGRD96 12.0 35 65 0 3.53 0.5356
WVFGRD96 13.0 40 65 15 3.55 0.5448
WVFGRD96 14.0 40 65 20 3.57 0.5548
WVFGRD96 15.0 40 75 30 3.59 0.5659
WVFGRD96 16.0 40 75 30 3.60 0.5778
WVFGRD96 17.0 210 70 -15 3.60 0.5877
WVFGRD96 18.0 210 70 -15 3.61 0.6001
WVFGRD96 19.0 210 75 -15 3.62 0.6124
WVFGRD96 20.0 210 75 -15 3.64 0.6244
WVFGRD96 21.0 210 75 -15 3.65 0.6355
WVFGRD96 22.0 210 75 -15 3.66 0.6472
WVFGRD96 23.0 210 75 -15 3.67 0.6577
WVFGRD96 24.0 210 75 -15 3.69 0.6673
WVFGRD96 25.0 210 75 -10 3.69 0.6761
WVFGRD96 26.0 210 75 -10 3.70 0.6837
WVFGRD96 27.0 210 75 -10 3.71 0.6897
WVFGRD96 28.0 210 75 -10 3.72 0.6945
WVFGRD96 29.0 210 75 -10 3.73 0.6978
WVFGRD96 30.0 210 75 -10 3.74 0.6992
WVFGRD96 31.0 210 75 -10 3.75 0.6993
WVFGRD96 32.0 210 75 -10 3.76 0.6985
WVFGRD96 33.0 210 75 -10 3.77 0.6968
WVFGRD96 34.0 210 75 -10 3.78 0.6947
WVFGRD96 35.0 210 75 -10 3.79 0.6922
WVFGRD96 36.0 210 75 -10 3.80 0.6903
WVFGRD96 37.0 210 75 -5 3.81 0.6893
WVFGRD96 38.0 210 75 -5 3.83 0.6895
WVFGRD96 39.0 210 75 -5 3.85 0.6910
WVFGRD96 40.0 205 65 -20 3.90 0.6926
WVFGRD96 41.0 210 65 -5 3.89 0.6907
WVFGRD96 42.0 210 65 -5 3.90 0.6882
WVFGRD96 43.0 210 70 -5 3.91 0.6850
WVFGRD96 44.0 210 70 -5 3.92 0.6813
WVFGRD96 45.0 210 70 -5 3.93 0.6771
WVFGRD96 46.0 210 70 -5 3.94 0.6721
WVFGRD96 47.0 210 70 -5 3.94 0.6666
WVFGRD96 48.0 210 70 -5 3.95 0.6605
WVFGRD96 49.0 210 70 -5 3.96 0.6541
WVFGRD96 50.0 210 70 0 3.96 0.6481
WVFGRD96 51.0 210 70 0 3.96 0.6427
WVFGRD96 52.0 210 70 0 3.97 0.6370
WVFGRD96 53.0 210 70 0 3.97 0.6307
WVFGRD96 54.0 210 70 0 3.98 0.6239
WVFGRD96 55.0 210 70 0 3.98 0.6166
WVFGRD96 56.0 210 70 0 3.99 0.6102
WVFGRD96 57.0 210 70 0 3.99 0.6044
WVFGRD96 58.0 210 70 0 3.99 0.5980
WVFGRD96 59.0 210 65 0 4.00 0.5910
The best solution is
WVFGRD96 31.0 210 75 -10 3.75 0.6993
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
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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