The ANSS event ID is ak0233339thf and the event page is at https://earthquake.usgs.gov/earthquakes/eventpage/ak0233339thf/executive.
2023/03/08 17:24:25 61.485 -149.449 44.9 3.8 Alaska
USGS/SLU Moment Tensor Solution
ENS 2023/03/08 17:24:25:0 61.49 -149.45 44.9 3.8 Alaska
Stations used:
AK.CUT AK.GHO AK.KNK AK.L22K AK.PWL AK.RC01 AK.SAW AK.SCM
AK.SKN AK.SLK AK.WAT6 AT.PMR
Filtering commands used:
cut o DIST/3.8 -15 o DIST/3.8 +25
rtr
taper w 0.1
hp c 0.03 n 3
lp c 0.25 n 3
Best Fitting Double Couple
Mo = 6.10e+21 dyne-cm
Mw = 3.79
Z = 37 km
Plane Strike Dip Rake
NP1 338 61 -132
NP2 220 50 -40
Principal Axes:
Axis Value Plunge Azimuth
T 6.10e+21 6 97
N 0.00e+00 36 3
P -6.10e+21 53 195
Moment Tensor: (dyne-cm)
Component Value
Mxx -1.93e+21
Mxy -1.28e+21
Mxz 2.74e+21
Myy 5.79e+21
Myz 1.41e+21
Mzz -3.86e+21
--------------
#######---------------
############---------#######
###############--#############
###############----###############
##############-------###############
#############----------###############
############-------------###############
###########---------------##############
###########-----------------##############
##########-------------------#############
#########--------------------##########
########----------------------######### T
######-----------------------#########
######---------- ----------###########
#####---------- P -----------#########
####---------- -----------########
###------------------------#######
#-----------------------######
#----------------------#####
-------------------###
--------------
Global CMT Convention Moment Tensor:
R T P
-3.86e+21 2.74e+21 -1.41e+21
2.74e+21 -1.93e+21 1.28e+21
-1.41e+21 1.28e+21 5.79e+21
Details of the solution is found at
http://www.eas.slu.edu/eqc/eqc_mt/MECH.NA/20230308172425/index.html
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STK = 220
DIP = 50
RAKE = -40
MW = 3.79
HS = 37.0
The NDK file is 20230308172425.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/3.8 -15 o DIST/3.8 +25 rtr taper w 0.1 hp c 0.03 n 3 lp c 0.25 n 3The results of this grid search are as follow:
DEPTH STK DIP RAKE MW FIT
WVFGRD96 1.0 315 30 25 2.87 0.1275
WVFGRD96 2.0 335 30 50 3.10 0.1476
WVFGRD96 3.0 180 15 70 3.18 0.1746
WVFGRD96 4.0 340 65 45 3.19 0.1870
WVFGRD96 5.0 210 55 -50 3.24 0.1931
WVFGRD96 6.0 150 45 35 3.30 0.2001
WVFGRD96 7.0 150 45 35 3.35 0.1989
WVFGRD96 8.0 40 60 -45 3.43 0.1907
WVFGRD96 9.0 30 70 -55 3.49 0.1958
WVFGRD96 10.0 30 70 -55 3.53 0.2027
WVFGRD96 11.0 35 70 -50 3.56 0.2048
WVFGRD96 12.0 35 70 -50 3.60 0.2065
WVFGRD96 13.0 35 70 -45 3.62 0.2090
WVFGRD96 14.0 40 70 -40 3.64 0.2182
WVFGRD96 15.0 35 65 -45 3.67 0.2326
WVFGRD96 16.0 40 65 -40 3.68 0.2436
WVFGRD96 17.0 35 60 -45 3.69 0.2468
WVFGRD96 18.0 40 70 -40 3.72 0.2522
WVFGRD96 19.0 40 70 -40 3.73 0.2556
WVFGRD96 20.0 225 50 -25 3.66 0.2593
WVFGRD96 21.0 230 50 -20 3.69 0.2657
WVFGRD96 22.0 225 50 -25 3.69 0.2700
WVFGRD96 23.0 45 50 -35 3.76 0.2861
WVFGRD96 24.0 45 50 -35 3.76 0.3016
WVFGRD96 25.0 45 45 -35 3.76 0.3175
WVFGRD96 26.0 45 50 -35 3.77 0.3365
WVFGRD96 27.0 45 50 -35 3.77 0.3498
WVFGRD96 28.0 50 45 -35 3.78 0.3603
WVFGRD96 29.0 45 45 -35 3.77 0.3710
WVFGRD96 30.0 225 45 -25 3.77 0.3905
WVFGRD96 31.0 225 45 -25 3.78 0.4047
WVFGRD96 32.0 225 45 -25 3.79 0.4191
WVFGRD96 33.0 225 45 -25 3.79 0.4406
WVFGRD96 34.0 220 45 -35 3.79 0.4534
WVFGRD96 35.0 220 50 -40 3.79 0.4780
WVFGRD96 36.0 220 50 -40 3.79 0.4847
WVFGRD96 37.0 220 50 -40 3.79 0.4855
WVFGRD96 38.0 220 50 -35 3.79 0.4824
WVFGRD96 39.0 220 50 -35 3.80 0.4681
WVFGRD96 40.0 220 45 -35 3.89 0.4666
WVFGRD96 41.0 220 45 -35 3.91 0.4591
WVFGRD96 42.0 220 45 -35 3.92 0.4480
WVFGRD96 43.0 220 50 -40 3.92 0.4386
WVFGRD96 44.0 220 50 -35 3.93 0.4388
WVFGRD96 45.0 220 45 -35 3.94 0.4284
WVFGRD96 46.0 220 45 -35 3.95 0.4340
WVFGRD96 47.0 220 45 -35 3.96 0.4297
WVFGRD96 48.0 220 45 -30 3.96 0.4373
WVFGRD96 49.0 210 45 -55 3.96 0.4336
WVFGRD96 50.0 210 45 -50 3.97 0.4432
WVFGRD96 51.0 210 45 -50 3.97 0.4469
WVFGRD96 52.0 210 45 -50 3.98 0.4489
WVFGRD96 53.0 215 50 -50 3.98 0.4521
WVFGRD96 54.0 215 50 -50 3.98 0.4548
WVFGRD96 55.0 215 50 -50 3.99 0.4600
WVFGRD96 56.0 215 50 -50 3.99 0.4593
WVFGRD96 57.0 210 50 -50 3.99 0.4659
WVFGRD96 58.0 210 50 -50 3.99 0.4625
WVFGRD96 59.0 210 50 -50 3.99 0.4667
The best solution is
WVFGRD96 37.0 220 50 -40 3.79 0.4855
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/3.8 -15 o DIST/3.8 +25 rtr taper w 0.1 hp c 0.03 n 3 lp c 0.25 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