The ANSS event ID is ak0236phrb1h and the event page is at https://earthquake.usgs.gov/earthquakes/eventpage/ak0236phrb1h/executive.
2023/05/26 02:17:59 61.704 -150.826 56.6 4.2 Alaska
USGS/SLU Moment Tensor Solution
ENS 2023/05/26 02:17:59:0 61.70 -150.83 56.6 4.2 Alaska
Stations used:
AK.CAST AK.CUT AK.FID AK.GHO AK.GLI AK.KLU AK.KNK AK.KTH
AK.L22K AK.N19K AK.P23K AK.PWL AK.RC01 AK.SCM AK.SKN AK.SLK
AT.PMR AV.P19K AV.RED 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 = 2.43e+22 dyne-cm
Mw = 4.19
Z = 66 km
Plane Strike Dip Rake
NP1 210 65 -70
NP2 349 32 -126
Principal Axes:
Axis Value Plunge Azimuth
T 2.43e+22 18 285
N 0.00e+00 18 21
P -2.43e+22 64 154
Moment Tensor: (dyne-cm)
Component Value
Mxx -2.15e+21
Mxy -3.80e+21
Mxz 1.04e+22
Myy 1.96e+22
Myz -1.09e+22
Mzz -1.75e+22
#######-------
###############-----##
####################-#######
##################------######
##################---------#######
#################------------#######
#################--------------#######
## ############----------------#######
## T ##########-------------------######
### #########--------------------#######
##############---------------------#######
#############----------------------#######
#############----------------------#######
###########----------- ---------######
##########------------ P ---------######
#########------------ ---------#####
########-----------------------#####
######-----------------------#####
####----------------------####
###---------------------####
#------------------###
-------------#
Global CMT Convention Moment Tensor:
R T P
-1.75e+22 1.04e+22 1.09e+22
1.04e+22 -2.15e+21 3.80e+21
1.09e+22 3.80e+21 1.96e+22
Details of the solution is found at
http://www.eas.slu.edu/eqc/eqc_mt/MECH.NA/20230526021759/index.html
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STK = 210
DIP = 65
RAKE = -70
MW = 4.19
HS = 66.0
The NDK file is 20230526021759.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 2.0 260 40 85 3.45 0.2483
WVFGRD96 4.0 45 75 50 3.46 0.2022
WVFGRD96 6.0 45 80 50 3.47 0.2417
WVFGRD96 8.0 50 75 60 3.55 0.2716
WVFGRD96 10.0 180 25 30 3.54 0.2961
WVFGRD96 12.0 140 50 -35 3.62 0.3328
WVFGRD96 14.0 140 50 -35 3.65 0.3649
WVFGRD96 16.0 140 50 -35 3.68 0.3879
WVFGRD96 18.0 140 50 -35 3.70 0.4048
WVFGRD96 20.0 140 50 -35 3.73 0.4147
WVFGRD96 22.0 140 50 -30 3.75 0.4180
WVFGRD96 24.0 140 50 -30 3.77 0.4166
WVFGRD96 26.0 165 30 25 3.75 0.4193
WVFGRD96 28.0 165 30 20 3.78 0.4258
WVFGRD96 30.0 160 30 15 3.80 0.4281
WVFGRD96 32.0 145 25 -15 3.82 0.4324
WVFGRD96 34.0 140 25 -20 3.84 0.4404
WVFGRD96 36.0 130 25 -45 3.86 0.4526
WVFGRD96 38.0 125 25 -50 3.87 0.4639
WVFGRD96 40.0 90 20 -80 4.01 0.4643
WVFGRD96 42.0 85 20 -80 4.03 0.4866
WVFGRD96 44.0 80 20 -85 4.04 0.5072
WVFGRD96 46.0 255 70 -90 4.06 0.5322
WVFGRD96 48.0 65 20 -95 4.07 0.5543
WVFGRD96 50.0 70 20 -90 4.09 0.5748
WVFGRD96 52.0 75 25 -80 4.10 0.5928
WVFGRD96 54.0 75 25 -80 4.11 0.6109
WVFGRD96 56.0 215 65 -70 4.13 0.6213
WVFGRD96 58.0 215 65 -70 4.14 0.6343
WVFGRD96 60.0 210 65 -70 4.16 0.6439
WVFGRD96 62.0 210 65 -70 4.17 0.6508
WVFGRD96 64.0 210 65 -70 4.18 0.6536
WVFGRD96 66.0 210 65 -70 4.19 0.6539
WVFGRD96 68.0 210 65 -70 4.19 0.6500
WVFGRD96 70.0 210 65 -70 4.20 0.6422
WVFGRD96 72.0 210 65 -70 4.20 0.6307
WVFGRD96 74.0 210 65 -70 4.21 0.6175
WVFGRD96 76.0 210 65 -70 4.21 0.6013
WVFGRD96 78.0 205 60 -75 4.19 0.5864
WVFGRD96 80.0 210 65 -85 4.19 0.5727
WVFGRD96 82.0 210 65 -85 4.19 0.5602
WVFGRD96 84.0 85 35 -85 4.15 0.5469
WVFGRD96 86.0 85 35 -85 4.15 0.5384
WVFGRD96 88.0 85 40 -75 4.15 0.5295
WVFGRD96 90.0 85 40 -75 4.15 0.5209
WVFGRD96 92.0 85 40 -75 4.15 0.5119
WVFGRD96 94.0 85 40 -75 4.15 0.5024
WVFGRD96 96.0 85 40 -85 4.14 0.4934
WVFGRD96 98.0 85 40 -85 4.14 0.4857
The best solution is
WVFGRD96 66.0 210 65 -70 4.19 0.6539
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