The ANSS event ID is ak0232apqri0 and the event page is at https://earthquake.usgs.gov/earthquakes/eventpage/ak0232apqri0/executive.
2023/02/19 01:46:57 59.392 -153.021 82.0 4.2 Alaska
USGS/SLU Moment Tensor Solution
ENS 2023/02/19 01:46:57:0 59.39 -153.02 82.0 4.2 Alaska
Stations used:
AK.BRLK AK.CNP AK.HOM AK.N19K AK.O18K AK.O19K AK.Q19K
AK.SLK AV.ACH AV.P19K AV.RED II.KDAK
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 0.08 n 3
br c 0.12 0.25 n 4 p 2
Best Fitting Double Couple
Mo = 2.11e+22 dyne-cm
Mw = 4.15
Z = 92 km
Plane Strike Dip Rake
NP1 70 70 40
NP2 324 53 155
Principal Axes:
Axis Value Plunge Azimuth
T 2.11e+22 42 293
N 0.00e+00 46 92
P -2.11e+22 11 193
Moment Tensor: (dyne-cm)
Component Value
Mxx -1.75e+22
Mxy -8.85e+21
Mxz 7.89e+21
Myy 8.76e+21
Myz -8.76e+21
Mzz 8.73e+21
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#########-------------------
#############-----------------
##################----------------
#####################---------------
########################--------------
######## ################------------#
######## T #################---------###
######### ##################------######
###############################---########
###############################-##########
############################-----#########
######################----------########
################-----------------#######
--------------------------------######
-------------------------------#####
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-------- ----------------#
----- P --------------
- ----------
Global CMT Convention Moment Tensor:
R T P
8.73e+21 7.89e+21 8.76e+21
7.89e+21 -1.75e+22 8.85e+21
8.76e+21 8.85e+21 8.76e+21
Details of the solution is found at
http://www.eas.slu.edu/eqc/eqc_mt/MECH.NA/20230219014657/index.html
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STK = 70
DIP = 70
RAKE = 40
MW = 4.15
HS = 92.0
The NDK file is 20230219014657.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 0.08 n 3 br c 0.12 0.25 n 4 p 2The results of this grid search are as follow:
DEPTH STK DIP RAKE MW FIT
WVFGRD96 2.0 260 80 15 3.42 0.4434
WVFGRD96 4.0 265 70 20 3.52 0.5189
WVFGRD96 6.0 70 70 -30 3.56 0.5621
WVFGRD96 8.0 65 65 -35 3.61 0.5769
WVFGRD96 10.0 260 75 25 3.62 0.5890
WVFGRD96 12.0 260 75 20 3.64 0.5907
WVFGRD96 14.0 260 75 15 3.66 0.5885
WVFGRD96 16.0 70 75 -20 3.67 0.5939
WVFGRD96 18.0 255 70 15 3.70 0.5956
WVFGRD96 20.0 255 70 15 3.72 0.6001
WVFGRD96 22.0 255 65 15 3.74 0.6042
WVFGRD96 24.0 255 65 10 3.77 0.6082
WVFGRD96 26.0 255 65 10 3.79 0.6126
WVFGRD96 28.0 255 65 10 3.81 0.6181
WVFGRD96 30.0 255 65 10 3.83 0.6252
WVFGRD96 32.0 255 65 10 3.85 0.6352
WVFGRD96 34.0 255 70 10 3.87 0.6467
WVFGRD96 36.0 255 70 10 3.89 0.6562
WVFGRD96 38.0 255 70 10 3.92 0.6592
WVFGRD96 40.0 255 60 20 3.98 0.6702
WVFGRD96 42.0 250 65 20 4.00 0.6686
WVFGRD96 44.0 245 70 -15 4.00 0.6693
WVFGRD96 46.0 245 65 -20 4.02 0.6754
WVFGRD96 48.0 250 70 -20 4.03 0.6798
WVFGRD96 50.0 70 70 30 4.05 0.6867
WVFGRD96 52.0 70 70 30 4.06 0.6928
WVFGRD96 54.0 70 70 30 4.06 0.6977
WVFGRD96 56.0 70 70 30 4.07 0.7033
WVFGRD96 58.0 70 70 35 4.08 0.7117
WVFGRD96 60.0 70 70 35 4.09 0.7193
WVFGRD96 62.0 75 70 40 4.09 0.7271
WVFGRD96 64.0 75 70 40 4.09 0.7325
WVFGRD96 66.0 75 70 40 4.10 0.7385
WVFGRD96 68.0 70 70 40 4.11 0.7438
WVFGRD96 70.0 70 70 35 4.11 0.7478
WVFGRD96 72.0 70 65 35 4.12 0.7532
WVFGRD96 74.0 70 65 35 4.13 0.7563
WVFGRD96 76.0 70 65 35 4.13 0.7599
WVFGRD96 78.0 70 65 35 4.14 0.7627
WVFGRD96 80.0 70 65 35 4.14 0.7656
WVFGRD96 82.0 70 65 35 4.14 0.7673
WVFGRD96 84.0 70 65 35 4.15 0.7687
WVFGRD96 86.0 70 65 35 4.15 0.7696
WVFGRD96 88.0 70 65 35 4.16 0.7706
WVFGRD96 90.0 70 65 35 4.16 0.7709
WVFGRD96 92.0 70 70 40 4.15 0.7713
WVFGRD96 94.0 70 70 40 4.15 0.7708
WVFGRD96 96.0 70 70 40 4.16 0.7696
WVFGRD96 98.0 70 70 40 4.16 0.7692
WVFGRD96 100.0 70 70 40 4.16 0.7689
WVFGRD96 102.0 70 70 40 4.17 0.7686
WVFGRD96 104.0 70 70 40 4.17 0.7670
WVFGRD96 106.0 70 70 40 4.17 0.7647
WVFGRD96 108.0 70 70 40 4.18 0.7640
WVFGRD96 110.0 70 70 40 4.18 0.7622
WVFGRD96 112.0 70 70 40 4.18 0.7600
WVFGRD96 114.0 70 70 40 4.19 0.7571
WVFGRD96 116.0 70 70 40 4.19 0.7557
WVFGRD96 118.0 70 70 40 4.19 0.7537
WVFGRD96 120.0 70 70 40 4.19 0.7507
WVFGRD96 122.0 70 70 45 4.20 0.7492
WVFGRD96 124.0 70 70 45 4.20 0.7463
WVFGRD96 126.0 70 70 40 4.20 0.7441
WVFGRD96 128.0 70 70 40 4.21 0.7429
WVFGRD96 130.0 70 70 40 4.21 0.7394
WVFGRD96 132.0 65 70 40 4.23 0.7385
WVFGRD96 134.0 65 70 40 4.23 0.7365
WVFGRD96 136.0 65 70 40 4.23 0.7291
WVFGRD96 138.0 65 70 35 4.24 0.7189
WVFGRD96 140.0 65 70 35 4.24 0.7047
WVFGRD96 142.0 65 70 30 4.24 0.6892
WVFGRD96 144.0 65 70 30 4.24 0.6670
WVFGRD96 146.0 65 75 25 4.22 0.6185
WVFGRD96 148.0 65 75 20 4.22 0.5463
The best solution is
WVFGRD96 92.0 70 70 40 4.15 0.7713
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 0.08 n 3 br c 0.12 0.25 n 4 p 2
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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