The ANSS event ID is ak0236978fur and the event page is at https://earthquake.usgs.gov/earthquakes/eventpage/ak0236978fur/executive.
2023/05/16 17:04:07 62.336 -149.598 44.3 3.8 Alaska
USGS/SLU Moment Tensor Solution
ENS 2023/05/16 17:04:07:0 62.34 -149.60 44.3 3.8 Alaska
Stations used:
AK.CAST AK.CUT AK.DHY AK.DIV AK.GHO AK.KTH AK.L22K AK.PWL
AK.RC01 AK.RND AK.SAW AK.SCM AK.SKN AK.SLK AK.SSN AK.WAT6
AT.PMR
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 = 9.89e+21 dyne-cm
Mw = 3.93
Z = 62 km
Plane Strike Dip Rake
NP1 245 60 -75
NP2 37 33 -114
Principal Axes:
Axis Value Plunge Azimuth
T 9.89e+21 14 324
N 0.00e+00 13 57
P -9.89e+21 71 189
Moment Tensor: (dyne-cm)
Component Value
Mxx 5.10e+21
Mxy -4.59e+21
Mxz 4.87e+21
Myy 3.17e+21
Myz -8.58e+20
Mzz -8.27e+21
##############
######################
# #######################-
## T ########################-
#### #########################--
###########################----##---
###################----------------###
################--------------------####
#############-----------------------####
###########--------------------------#####
#########---------------------------######
#######-----------------------------######
#####-------------- -------------#######
###--------------- P ------------#######
##---------------- -----------########
------------------------------########
----------------------------########
-------------------------#########
--------------------##########
----------------############
####--################
##############
Global CMT Convention Moment Tensor:
R T P
-8.27e+21 4.87e+21 8.58e+20
4.87e+21 5.10e+21 4.59e+21
8.58e+20 4.59e+21 3.17e+21
Details of the solution is found at
http://www.eas.slu.edu/eqc/eqc_mt/MECH.NA/20230516170407/index.html
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STK = 245
DIP = 60
RAKE = -75
MW = 3.93
HS = 62.0
The NDK file is 20230516170407.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 2.0 35 40 90 3.16 0.2658
WVFGRD96 4.0 205 60 75 3.23 0.2210
WVFGRD96 6.0 130 25 0 3.24 0.2881
WVFGRD96 8.0 140 20 15 3.32 0.3291
WVFGRD96 10.0 150 20 30 3.33 0.3668
WVFGRD96 12.0 20 75 75 3.37 0.3971
WVFGRD96 14.0 20 75 75 3.39 0.4134
WVFGRD96 16.0 10 80 65 3.41 0.4213
WVFGRD96 18.0 10 80 65 3.43 0.4220
WVFGRD96 20.0 90 30 -10 3.45 0.4260
WVFGRD96 22.0 85 30 -15 3.47 0.4302
WVFGRD96 24.0 90 30 -10 3.49 0.4337
WVFGRD96 26.0 85 30 -15 3.51 0.4376
WVFGRD96 28.0 85 30 -20 3.52 0.4419
WVFGRD96 30.0 80 30 -30 3.54 0.4499
WVFGRD96 32.0 80 30 -35 3.55 0.4572
WVFGRD96 34.0 60 25 -60 3.56 0.4619
WVFGRD96 36.0 55 25 -70 3.57 0.4659
WVFGRD96 38.0 55 25 -70 3.59 0.4780
WVFGRD96 40.0 230 70 -85 3.73 0.5023
WVFGRD96 42.0 235 70 -80 3.75 0.5151
WVFGRD96 44.0 230 65 -85 3.77 0.5329
WVFGRD96 46.0 230 65 -85 3.80 0.5596
WVFGRD96 48.0 235 65 -85 3.82 0.5866
WVFGRD96 50.0 235 60 -85 3.85 0.6117
WVFGRD96 52.0 245 60 -75 3.88 0.6388
WVFGRD96 54.0 245 60 -75 3.90 0.6628
WVFGRD96 56.0 245 60 -75 3.91 0.6818
WVFGRD96 58.0 245 60 -75 3.92 0.6940
WVFGRD96 60.0 245 60 -75 3.93 0.7010
WVFGRD96 62.0 245 60 -75 3.93 0.7026
WVFGRD96 64.0 245 60 -75 3.94 0.7006
WVFGRD96 66.0 245 65 -75 3.94 0.6977
WVFGRD96 68.0 245 65 -70 3.94 0.6950
WVFGRD96 70.0 245 65 -70 3.94 0.6918
WVFGRD96 72.0 245 65 -70 3.94 0.6857
WVFGRD96 74.0 245 65 -70 3.94 0.6777
WVFGRD96 76.0 250 70 -75 3.96 0.6722
WVFGRD96 78.0 245 70 -80 3.96 0.6649
WVFGRD96 80.0 245 70 -80 3.96 0.6568
WVFGRD96 82.0 245 70 -80 3.96 0.6461
WVFGRD96 84.0 245 70 -80 3.96 0.6340
WVFGRD96 86.0 240 70 -80 3.96 0.6202
WVFGRD96 88.0 240 70 -80 3.96 0.6065
WVFGRD96 90.0 240 70 -80 3.95 0.5913
WVFGRD96 92.0 240 70 -80 3.95 0.5768
WVFGRD96 94.0 240 70 -80 3.95 0.5619
WVFGRD96 96.0 240 70 -80 3.94 0.5473
WVFGRD96 98.0 240 70 -80 3.94 0.5337
The best solution is
WVFGRD96 62.0 245 60 -75 3.93 0.7026
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