The ANSS event ID is ak022auilau5 and the event page is at https://earthquake.usgs.gov/earthquakes/eventpage/ak022auilau5/executive.
2022/08/24 13:03:56 63.834 -148.839 111.5 4.1 Alaska
USGS/SLU Moment Tensor Solution
ENS 2022/08/24 13:03:56:0 63.83 -148.84 111.5 4.1 Alaska
Stations used:
AK.BPAW AK.CAST AK.CCB AK.DHY AK.G23K AK.G24K AK.GHO
AK.H22K AK.H23K AK.H24K AK.HARP AK.I23K AK.J25K AK.J26L
AK.K20K AK.KNK AK.L20K AK.M19K AK.MCK AK.MLY AK.NEA2 AK.PAX
AK.POKR AK.RND AK.SAW AK.SCM AK.WRH AT.MENT AT.PMR IU.COLA
US.EGAK
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 = 9.55e+21 dyne-cm
Mw = 3.92
Z = 110 km
Plane Strike Dip Rake
NP1 72 86 140
NP2 165 50 5
Principal Axes:
Axis Value Plunge Azimuth
T 9.55e+21 30 20
N 0.00e+00 50 247
P -9.55e+21 24 125
Moment Tensor: (dyne-cm)
Component Value
Mxx 3.59e+21
Mxy 6.11e+21
Mxz 5.94e+21
Myy -4.41e+21
Myz -1.44e+21
Mzz 8.20e+20
-#############
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------############ #######
------############# T ########
-------############## ##########
--------############################
--------##############################
---------###############################
---------###########################----
----------######################----------
-----------################---------------
-----------##########---------------------
-----------#####--------------------------
--------###-----------------------------
############----------------------------
###########------------------- -----
###########------------------ P ----
############---------------- ---
###########-------------------
############----------------
###########-----------
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Global CMT Convention Moment Tensor:
R T P
8.20e+20 5.94e+21 1.44e+21
5.94e+21 3.59e+21 -6.11e+21
1.44e+21 -6.11e+21 -4.41e+21
Details of the solution is found at
http://www.eas.slu.edu/eqc/eqc_mt/MECH.NA/20220824130356/index.html
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STK = 165
DIP = 50
RAKE = 5
MW = 3.92
HS = 110.0
The NDK file is 20220824130356.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 2.0 120 60 80 2.97 0.1456
WVFGRD96 4.0 255 75 -40 3.00 0.1756
WVFGRD96 6.0 255 65 -30 3.07 0.2077
WVFGRD96 8.0 250 50 -40 3.19 0.2316
WVFGRD96 10.0 250 50 -40 3.24 0.2489
WVFGRD96 12.0 250 50 -40 3.28 0.2572
WVFGRD96 14.0 255 50 -35 3.31 0.2566
WVFGRD96 16.0 315 65 -30 3.33 0.2587
WVFGRD96 18.0 315 70 -30 3.35 0.2624
WVFGRD96 20.0 5 65 30 3.37 0.2686
WVFGRD96 22.0 5 65 30 3.40 0.2838
WVFGRD96 24.0 5 65 30 3.42 0.2983
WVFGRD96 26.0 5 65 30 3.45 0.3089
WVFGRD96 28.0 5 65 30 3.46 0.3145
WVFGRD96 30.0 5 65 30 3.48 0.3161
WVFGRD96 32.0 0 65 25 3.49 0.3168
WVFGRD96 34.0 5 60 30 3.51 0.3191
WVFGRD96 36.0 0 65 25 3.52 0.3223
WVFGRD96 38.0 0 65 20 3.54 0.3269
WVFGRD96 40.0 5 60 30 3.62 0.3327
WVFGRD96 42.0 5 60 30 3.64 0.3303
WVFGRD96 44.0 5 65 35 3.67 0.3263
WVFGRD96 46.0 -5 70 15 3.66 0.3264
WVFGRD96 48.0 165 75 -40 3.70 0.3301
WVFGRD96 50.0 165 75 -40 3.72 0.3396
WVFGRD96 52.0 165 75 -35 3.72 0.3486
WVFGRD96 54.0 165 70 -30 3.72 0.3618
WVFGRD96 56.0 165 65 -25 3.73 0.3821
WVFGRD96 58.0 165 65 -20 3.74 0.4076
WVFGRD96 60.0 165 60 -15 3.75 0.4332
WVFGRD96 62.0 170 50 5 3.78 0.4623
WVFGRD96 64.0 170 45 10 3.81 0.4854
WVFGRD96 66.0 175 45 20 3.83 0.5042
WVFGRD96 68.0 175 45 20 3.84 0.5206
WVFGRD96 70.0 175 45 20 3.85 0.5332
WVFGRD96 72.0 165 50 0 3.83 0.5445
WVFGRD96 74.0 165 50 0 3.83 0.5595
WVFGRD96 76.0 165 50 0 3.84 0.5730
WVFGRD96 78.0 165 45 5 3.86 0.5856
WVFGRD96 80.0 165 45 5 3.87 0.5967
WVFGRD96 82.0 165 45 5 3.87 0.6077
WVFGRD96 84.0 165 45 5 3.88 0.6165
WVFGRD96 86.0 165 45 5 3.88 0.6247
WVFGRD96 88.0 165 45 5 3.89 0.6315
WVFGRD96 90.0 165 45 5 3.89 0.6386
WVFGRD96 92.0 165 45 5 3.89 0.6443
WVFGRD96 94.0 165 45 5 3.90 0.6481
WVFGRD96 96.0 165 45 5 3.90 0.6521
WVFGRD96 98.0 165 45 5 3.90 0.6548
WVFGRD96 100.0 165 50 5 3.91 0.6579
WVFGRD96 102.0 165 50 5 3.91 0.6596
WVFGRD96 104.0 165 50 5 3.91 0.6617
WVFGRD96 106.0 165 50 5 3.92 0.6618
WVFGRD96 108.0 165 50 5 3.92 0.6630
WVFGRD96 110.0 165 50 5 3.92 0.6643
WVFGRD96 112.0 165 50 5 3.93 0.6642
WVFGRD96 114.0 165 50 5 3.93 0.6638
WVFGRD96 116.0 165 50 5 3.93 0.6629
WVFGRD96 118.0 165 50 5 3.93 0.6622
WVFGRD96 120.0 165 50 5 3.94 0.6604
WVFGRD96 122.0 160 50 0 3.94 0.6593
WVFGRD96 124.0 160 50 0 3.94 0.6581
WVFGRD96 126.0 160 50 0 3.94 0.6567
WVFGRD96 128.0 160 50 0 3.94 0.6558
WVFGRD96 130.0 160 50 0 3.95 0.6535
WVFGRD96 132.0 160 50 0 3.95 0.6514
WVFGRD96 134.0 160 50 0 3.95 0.6482
WVFGRD96 136.0 160 50 0 3.95 0.6454
WVFGRD96 138.0 160 50 0 3.95 0.6424
WVFGRD96 140.0 160 50 0 3.96 0.6391
WVFGRD96 142.0 160 50 0 3.96 0.6356
WVFGRD96 144.0 160 50 0 3.96 0.6320
WVFGRD96 146.0 160 55 0 3.96 0.6286
WVFGRD96 148.0 160 55 0 3.96 0.6255
The best solution is
WVFGRD96 110.0 165 50 5 3.92 0.6643
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