The ANSS event ID is ak0236sy9ooj and the event page is at https://earthquake.usgs.gov/earthquakes/eventpage/ak0236sy9ooj/executive.
2023/05/28 11:30:58 63.012 -149.774 77.0 3.7 Alaska
USGS/SLU Moment Tensor Solution
ENS 2023/05/28 11:30:58:0 63.01 -149.77 77.0 3.7 Alaska
Stations used:
AK.BPAW AK.CAST AK.CUT AK.DHY AK.GHO AK.L22K AK.MCK AK.MLY
AK.NEA2 AK.RND AK.SAW AK.SCM AK.WAT6 IU.COLA
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 = 8.32e+21 dyne-cm
Mw = 3.88
Z = 86 km
Plane Strike Dip Rake
NP1 233 78 -112
NP2 115 25 -30
Principal Axes:
Axis Value Plunge Azimuth
T 8.32e+21 29 340
N 0.00e+00 21 237
P -8.32e+21 52 117
Moment Tensor: (dyne-cm)
Component Value
Mxx 4.95e+21
Mxy -7.37e+20
Mxz 5.18e+21
Myy -1.76e+21
Myz -4.79e+21
Mzz -3.19e+21
##############
######################
####### ##################
######## T ###################
########## ###################--
############################--------
##########################------------
-#######################----------------
-####################-------------------
--#################-----------------------
--###############-------------------------
---############---------------------------
---##########--------------- -----------
---#######----------------- P ----------
----####------------------- ----------
-----#--------------------------------
---##-------------------------------
-#####---------------------------#
#######---------------------##
###########-----------######
######################
##############
Global CMT Convention Moment Tensor:
R T P
-3.19e+21 5.18e+21 4.79e+21
5.18e+21 4.95e+21 7.37e+20
4.79e+21 7.37e+20 -1.76e+21
Details of the solution is found at
http://www.eas.slu.edu/eqc/eqc_mt/MECH.NA/20230528113058/index.html
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STK = 115
DIP = 25
RAKE = -30
MW = 3.88
HS = 86.0
The NDK file is 20230528113058.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 2.0 235 50 55 3.12 0.2317
WVFGRD96 4.0 225 55 30 3.17 0.2658
WVFGRD96 6.0 220 65 20 3.21 0.2927
WVFGRD96 8.0 220 70 30 3.29 0.3105
WVFGRD96 10.0 30 70 -25 3.31 0.3161
WVFGRD96 12.0 300 60 -20 3.36 0.3250
WVFGRD96 14.0 300 60 -20 3.39 0.3431
WVFGRD96 16.0 300 65 -20 3.41 0.3574
WVFGRD96 18.0 120 60 -20 3.44 0.3700
WVFGRD96 20.0 120 60 -15 3.46 0.3848
WVFGRD96 22.0 120 55 -15 3.49 0.3979
WVFGRD96 24.0 125 55 -20 3.52 0.4104
WVFGRD96 26.0 125 55 -20 3.53 0.4218
WVFGRD96 28.0 120 50 -20 3.55 0.4314
WVFGRD96 30.0 125 50 -20 3.57 0.4384
WVFGRD96 32.0 120 50 -15 3.58 0.4444
WVFGRD96 34.0 135 45 20 3.61 0.4535
WVFGRD96 36.0 135 45 20 3.63 0.4606
WVFGRD96 38.0 130 50 15 3.63 0.4678
WVFGRD96 40.0 140 35 25 3.75 0.4735
WVFGRD96 42.0 135 35 15 3.76 0.4727
WVFGRD96 44.0 140 35 25 3.78 0.4782
WVFGRD96 46.0 135 35 15 3.78 0.4881
WVFGRD96 48.0 135 35 15 3.79 0.4980
WVFGRD96 50.0 130 40 10 3.78 0.5063
WVFGRD96 52.0 130 40 10 3.79 0.5150
WVFGRD96 54.0 125 40 -20 3.81 0.5234
WVFGRD96 56.0 125 40 -20 3.82 0.5320
WVFGRD96 58.0 125 40 -20 3.82 0.5398
WVFGRD96 60.0 105 20 -35 3.85 0.5495
WVFGRD96 62.0 105 20 -35 3.86 0.5611
WVFGRD96 64.0 105 20 -35 3.86 0.5703
WVFGRD96 66.0 105 20 -35 3.86 0.5778
WVFGRD96 68.0 105 20 -35 3.86 0.5837
WVFGRD96 70.0 115 25 -30 3.86 0.5886
WVFGRD96 72.0 115 25 -30 3.87 0.5935
WVFGRD96 74.0 115 25 -30 3.87 0.5967
WVFGRD96 76.0 115 25 -30 3.87 0.5994
WVFGRD96 78.0 115 25 -30 3.87 0.6012
WVFGRD96 80.0 115 25 -30 3.87 0.6015
WVFGRD96 82.0 115 25 -30 3.88 0.6005
WVFGRD96 84.0 115 25 -30 3.88 0.6015
WVFGRD96 86.0 115 25 -30 3.88 0.6022
WVFGRD96 88.0 120 25 -30 3.89 0.6013
WVFGRD96 90.0 120 25 -30 3.89 0.5988
WVFGRD96 92.0 120 25 -30 3.89 0.5955
WVFGRD96 94.0 120 25 -30 3.89 0.5954
WVFGRD96 96.0 125 25 -25 3.90 0.5927
WVFGRD96 98.0 125 30 -25 3.90 0.5882
The best solution is
WVFGRD96 86.0 115 25 -30 3.88 0.6022
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