The ANSS event ID is ak023a8xq5yn and the event page is at https://earthquake.usgs.gov/earthquakes/eventpage/ak023a8xq5yn/executive.
2023/08/11 08:20:08 62.886 -150.543 90.9 3.7 Alaska
USGS/SLU Moment Tensor Solution
ENS 2023/08/11 08:20:08:0 62.89 -150.54 90.9 3.7 Alaska
Stations used:
AK.BPAW AK.CAST AK.CUT AK.DHY AK.GHO AK.J20K AK.K24K AK.KNK
AK.L20K AK.L22K AK.M20K AK.MCK AK.MLY AK.PPLA AK.SAW AK.SCM
AK.SKN AK.WAT6 AK.WRH
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 0.10 n 3
Best Fitting Double Couple
Mo = 6.76e+21 dyne-cm
Mw = 3.82
Z = 94 km
Plane Strike Dip Rake
NP1 242 83 -135
NP2 145 45 -10
Principal Axes:
Axis Value Plunge Azimuth
T 6.76e+21 24 5
N 0.00e+00 44 249
P -6.76e+21 36 114
Moment Tensor: (dyne-cm)
Component Value
Mxx 4.81e+21
Mxy 2.16e+21
Mxz 3.86e+21
Myy -3.64e+21
Myz -2.70e+21
Mzz -1.17e+21
##############
########### ########
-############# T ###########
--############# ############
---###############################
----################################
-----############################-----
------#######################-----------
------###################---------------
--------##############--------------------
--------###########-----------------------
---------#######--------------------------
----------##-------------------- -------
---------#--------------------- P ------
------#####-------------------- ------
--#########---------------------------
############------------------------
#############---------------------
##############----------------
################------------
######################
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Global CMT Convention Moment Tensor:
R T P
-1.17e+21 3.86e+21 2.70e+21
3.86e+21 4.81e+21 -2.16e+21
2.70e+21 -2.16e+21 -3.64e+21
Details of the solution is found at
http://www.eas.slu.edu/eqc/eqc_mt/MECH.NA/20230811082008/index.html
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STK = 145
DIP = 45
RAKE = -10
MW = 3.82
HS = 94.0
The NDK file is 20230811082008.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 0.10 n 3The results of this grid search are as follow:
DEPTH STK DIP RAKE MW FIT
WVFGRD96 2.0 235 35 70 2.96 0.1974
WVFGRD96 4.0 55 80 -50 3.02 0.2394
WVFGRD96 6.0 55 80 -45 3.06 0.2800
WVFGRD96 8.0 55 80 -45 3.15 0.3060
WVFGRD96 10.0 55 80 -40 3.18 0.3118
WVFGRD96 12.0 240 90 35 3.20 0.3044
WVFGRD96 14.0 70 70 25 3.27 0.3021
WVFGRD96 16.0 70 70 25 3.29 0.2947
WVFGRD96 18.0 70 70 25 3.31 0.2822
WVFGRD96 20.0 40 65 45 3.31 0.2686
WVFGRD96 22.0 350 50 35 3.38 0.2745
WVFGRD96 24.0 350 50 35 3.41 0.2888
WVFGRD96 26.0 350 55 35 3.44 0.3038
WVFGRD96 28.0 350 55 35 3.47 0.3181
WVFGRD96 30.0 345 60 25 3.49 0.3288
WVFGRD96 32.0 345 60 25 3.51 0.3404
WVFGRD96 34.0 330 85 30 3.51 0.3677
WVFGRD96 36.0 330 85 30 3.54 0.3927
WVFGRD96 38.0 330 90 25 3.56 0.4136
WVFGRD96 40.0 330 90 30 3.63 0.4392
WVFGRD96 42.0 145 75 -35 3.65 0.4453
WVFGRD96 44.0 145 70 -30 3.66 0.4539
WVFGRD96 46.0 145 70 -30 3.68 0.4635
WVFGRD96 48.0 145 70 -30 3.69 0.4749
WVFGRD96 50.0 145 65 -30 3.71 0.4876
WVFGRD96 52.0 145 65 -30 3.72 0.5015
WVFGRD96 54.0 145 60 -25 3.73 0.5165
WVFGRD96 56.0 145 60 -25 3.74 0.5308
WVFGRD96 58.0 145 55 -25 3.75 0.5469
WVFGRD96 60.0 145 55 -20 3.76 0.5624
WVFGRD96 62.0 145 55 -20 3.76 0.5780
WVFGRD96 64.0 145 50 -20 3.77 0.5910
WVFGRD96 66.0 145 50 -20 3.78 0.6059
WVFGRD96 68.0 145 50 -20 3.78 0.6167
WVFGRD96 70.0 145 50 -20 3.79 0.6254
WVFGRD96 72.0 145 45 -15 3.80 0.6369
WVFGRD96 74.0 145 45 -15 3.80 0.6440
WVFGRD96 76.0 145 45 -15 3.80 0.6526
WVFGRD96 78.0 145 45 -15 3.81 0.6575
WVFGRD96 80.0 145 45 -15 3.81 0.6619
WVFGRD96 82.0 145 45 -15 3.81 0.6650
WVFGRD96 84.0 145 45 -10 3.81 0.6669
WVFGRD96 86.0 145 45 -10 3.81 0.6694
WVFGRD96 88.0 145 45 -10 3.82 0.6703
WVFGRD96 90.0 145 45 -10 3.82 0.6709
WVFGRD96 92.0 145 45 -10 3.82 0.6720
WVFGRD96 94.0 145 45 -10 3.82 0.6720
WVFGRD96 96.0 145 45 -10 3.82 0.6719
WVFGRD96 98.0 145 45 -5 3.83 0.6709
WVFGRD96 100.0 145 45 -5 3.83 0.6697
WVFGRD96 102.0 145 45 -5 3.83 0.6665
WVFGRD96 104.0 145 45 -5 3.83 0.6667
WVFGRD96 106.0 145 45 -5 3.83 0.6640
WVFGRD96 108.0 145 45 -5 3.83 0.6620
WVFGRD96 110.0 145 45 -5 3.83 0.6578
WVFGRD96 112.0 145 45 -5 3.84 0.6557
WVFGRD96 114.0 145 45 -5 3.84 0.6520
WVFGRD96 116.0 145 45 -5 3.84 0.6478
WVFGRD96 118.0 145 40 -5 3.84 0.6463
WVFGRD96 120.0 145 40 -5 3.84 0.6425
WVFGRD96 122.0 145 40 -5 3.84 0.6374
WVFGRD96 124.0 145 40 -5 3.84 0.6362
WVFGRD96 126.0 145 40 -5 3.84 0.6303
WVFGRD96 128.0 145 40 -5 3.85 0.6288
WVFGRD96 130.0 145 45 -5 3.85 0.6253
WVFGRD96 132.0 150 45 5 3.86 0.6210
WVFGRD96 134.0 150 45 5 3.86 0.6197
WVFGRD96 136.0 150 45 5 3.86 0.6153
WVFGRD96 138.0 150 45 5 3.86 0.6146
WVFGRD96 140.0 150 45 5 3.86 0.6111
WVFGRD96 142.0 150 45 5 3.86 0.6057
WVFGRD96 144.0 150 45 5 3.86 0.6041
WVFGRD96 146.0 150 45 5 3.86 0.5994
WVFGRD96 148.0 150 45 5 3.87 0.5966
The best solution is
WVFGRD96 94.0 145 45 -10 3.82 0.6720
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 0.10 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