The ANSS event ID is ak023be9qxdf and the event page is at https://earthquake.usgs.gov/earthquakes/eventpage/ak023be9qxdf/executive.
2023/09/05 07:50:50 60.680 -146.876 29.3 3.6 Alaska
USGS/SLU Moment Tensor Solution
ENS 2023/09/05 07:50:50:0 60.68 -146.88 29.3 3.6 Alaska
Stations used:
AK.BMR AK.DIV AK.EYAK AK.FID AK.GHO AK.GLI AK.HIN AK.KLU
AK.KNK AK.PWL AK.Q23K AK.SAW AK.SCM AK.SLK AK.SWD AK.WAT6
AT.PMR
Filtering commands used:
cut o DIST/3.3 -30 o DIST/3.3 +40
rtr
taper w 0.1
hp c 0.03 n 3
lp c 0.06 n 3
Best Fitting Double Couple
Mo = 6.31e+21 dyne-cm
Mw = 3.80
Z = 47 km
Plane Strike Dip Rake
NP1 70 80 -15
NP2 163 75 -170
Principal Axes:
Axis Value Plunge Azimuth
T 6.31e+21 3 117
N 0.00e+00 72 217
P -6.31e+21 18 26
Moment Tensor: (dyne-cm)
Component Value
Mxx -3.36e+21
Mxy -4.78e+21
Mxz -1.80e+21
Myy 3.92e+21
Myz -4.70e+20
Mzz -5.59e+20
#-------------
#####------------ --
#######------------- P -----
#########------------ ------
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#############-------------------------
##############-------------------------#
###############---------------------####
################------------------########
#################-------------############
#################---------################
##################---#####################
##############----######################
######------------###################
------------------################## T
------------------#################
------------------################
-----------------#############
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Global CMT Convention Moment Tensor:
R T P
-5.59e+20 -1.80e+21 4.70e+20
-1.80e+21 -3.36e+21 4.78e+21
4.70e+20 4.78e+21 3.92e+21
Details of the solution is found at
http://www.eas.slu.edu/eqc/eqc_mt/MECH.NA/20230905075050/index.html
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STK = 70
DIP = 80
RAKE = -15
MW = 3.80
HS = 47.0
The NDK file is 20230905075050.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 -30 o DIST/3.3 +40 rtr taper w 0.1 hp c 0.03 n 3 lp c 0.06 n 3The results of this grid search are as follow:
DEPTH STK DIP RAKE MW FIT
WVFGRD96 1.0 40 50 -60 3.06 0.2303
WVFGRD96 2.0 40 50 -60 3.20 0.3220
WVFGRD96 3.0 45 45 -50 3.25 0.3474
WVFGRD96 4.0 65 55 -20 3.22 0.3622
WVFGRD96 5.0 70 60 -5 3.24 0.3790
WVFGRD96 6.0 255 60 30 3.29 0.4060
WVFGRD96 7.0 255 60 30 3.31 0.4329
WVFGRD96 8.0 260 55 40 3.38 0.4550
WVFGRD96 9.0 260 55 40 3.40 0.4738
WVFGRD96 10.0 250 70 25 3.37 0.4892
WVFGRD96 11.0 250 65 20 3.39 0.5044
WVFGRD96 12.0 250 65 20 3.41 0.5202
WVFGRD96 13.0 250 65 20 3.42 0.5345
WVFGRD96 14.0 250 70 10 3.43 0.5486
WVFGRD96 15.0 250 70 10 3.44 0.5612
WVFGRD96 16.0 250 70 10 3.46 0.5722
WVFGRD96 17.0 250 75 10 3.47 0.5818
WVFGRD96 18.0 250 75 10 3.48 0.5912
WVFGRD96 19.0 250 75 5 3.49 0.5992
WVFGRD96 20.0 250 75 5 3.50 0.6063
WVFGRD96 21.0 250 80 10 3.51 0.6126
WVFGRD96 22.0 250 75 5 3.52 0.6178
WVFGRD96 23.0 250 80 5 3.53 0.6224
WVFGRD96 24.0 250 80 5 3.54 0.6267
WVFGRD96 25.0 250 80 5 3.55 0.6297
WVFGRD96 26.0 70 90 -10 3.56 0.6316
WVFGRD96 27.0 250 80 5 3.57 0.6341
WVFGRD96 28.0 250 85 5 3.57 0.6354
WVFGRD96 29.0 70 85 -10 3.58 0.6372
WVFGRD96 30.0 70 85 -10 3.59 0.6380
WVFGRD96 31.0 70 85 -10 3.60 0.6379
WVFGRD96 32.0 250 90 15 3.61 0.6376
WVFGRD96 33.0 70 85 -15 3.62 0.6376
WVFGRD96 34.0 70 85 -15 3.64 0.6366
WVFGRD96 35.0 250 90 10 3.64 0.6335
WVFGRD96 36.0 70 85 -15 3.66 0.6361
WVFGRD96 37.0 250 90 10 3.67 0.6359
WVFGRD96 38.0 250 90 10 3.69 0.6385
WVFGRD96 39.0 250 90 10 3.71 0.6412
WVFGRD96 40.0 70 85 -15 3.74 0.6486
WVFGRD96 41.0 70 80 -15 3.75 0.6517
WVFGRD96 42.0 70 80 -15 3.76 0.6541
WVFGRD96 43.0 70 80 -15 3.77 0.6555
WVFGRD96 44.0 70 80 -15 3.78 0.6561
WVFGRD96 45.0 70 80 -15 3.79 0.6568
WVFGRD96 46.0 70 80 -15 3.79 0.6565
WVFGRD96 47.0 70 80 -15 3.80 0.6568
WVFGRD96 48.0 70 80 -15 3.81 0.6557
WVFGRD96 49.0 70 80 -15 3.82 0.6554
WVFGRD96 50.0 70 80 -15 3.82 0.6543
WVFGRD96 51.0 70 85 -15 3.83 0.6533
WVFGRD96 52.0 70 85 -15 3.84 0.6522
WVFGRD96 53.0 70 85 -15 3.84 0.6506
WVFGRD96 54.0 70 85 -15 3.85 0.6499
WVFGRD96 55.0 70 85 -15 3.85 0.6485
WVFGRD96 56.0 250 90 10 3.85 0.6430
WVFGRD96 57.0 70 85 -15 3.86 0.6454
WVFGRD96 58.0 70 85 -10 3.86 0.6433
WVFGRD96 59.0 70 85 -10 3.87 0.6414
The best solution is
WVFGRD96 47.0 70 80 -15 3.80 0.6568
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 -30 o DIST/3.3 +40 rtr taper w 0.1 hp c 0.03 n 3 lp c 0.06 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