The ANSS event ID is ak0221pb6nv5 and the event page is at https://earthquake.usgs.gov/earthquakes/eventpage/ak0221pb6nv5/executive.
2022/02/06 07:22:33 62.747 -148.705 61.6 5.2 Alaska
USGS/SLU Moment Tensor Solution
ENS 2022/02/06 07:22:33:0 62.75 -148.71 61.6 5.2 Alaska
Stations used:
AK.BMR AK.BPAW AK.CAST AK.CCB AK.CUT AK.DHY AK.DIV AK.FIRE
AK.GHO AK.GLI AK.HARP AK.HDA AK.K20K AK.K24K AK.KLU AK.KNK
AK.KTH AK.L20K AK.L22K AK.MCK AK.NEA2 AK.PAX AK.RC01 AK.SAW
AK.SCM AK.SLK AK.SSN AK.WRH AT.PMR AV.SPCP AV.STLK IU.COLA
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 = 6.46e+23 dyne-cm
Mw = 5.14
Z = 62 km
Plane Strike Dip Rake
NP1 260 60 -80
NP2 61 31 -107
Principal Axes:
Axis Value Plunge Azimuth
T 6.46e+23 14 343
N 0.00e+00 9 75
P -6.46e+23 73 195
Moment Tensor: (dyne-cm)
Component Value
Mxx 5.01e+23
Mxy -1.85e+23
Mxz 3.23e+23
Myy 4.98e+22
Myz 1.92e+16
Mzz -5.51e+23
###########
#### T ###############
####### ##################
##############################
##################################
####################################
#####################################-
##############---------------------####-
#########-----------------------------#-
#######--------------------------------###
#####----------------------------------###
###-----------------------------------####
#----------------- ----------------#####
----------------- P ---------------#####
----------------- --------------######
-------------------------------#######
----------------------------########
#------------------------#########
###----------------###########
############################
######################
##############
Global CMT Convention Moment Tensor:
R T P
-5.51e+23 3.23e+23 -1.92e+16
3.23e+23 5.01e+23 1.85e+23
-1.92e+16 1.85e+23 4.98e+22
Details of the solution is found at
http://www.eas.slu.edu/eqc/eqc_mt/MECH.NA/20220206072233/index.html
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STK = 260
DIP = 60
RAKE = -80
MW = 5.14
HS = 62.0
The NDK file is 20220206072233.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 2.0 260 40 -85 4.26 0.1215
WVFGRD96 4.0 95 45 -70 4.33 0.1155
WVFGRD96 6.0 45 30 40 4.31 0.1293
WVFGRD96 8.0 60 35 65 4.44 0.1552
WVFGRD96 10.0 20 40 45 4.46 0.1779
WVFGRD96 12.0 15 40 35 4.49 0.1913
WVFGRD96 14.0 15 40 35 4.52 0.1975
WVFGRD96 16.0 15 40 30 4.55 0.1988
WVFGRD96 18.0 10 40 20 4.57 0.1977
WVFGRD96 20.0 10 40 20 4.60 0.1963
WVFGRD96 22.0 10 35 20 4.63 0.1944
WVFGRD96 24.0 295 70 -30 4.68 0.1987
WVFGRD96 26.0 295 70 -30 4.70 0.1969
WVFGRD96 28.0 100 75 60 4.69 0.1932
WVFGRD96 30.0 95 80 60 4.71 0.1937
WVFGRD96 32.0 295 65 -30 4.76 0.2172
WVFGRD96 34.0 295 60 -30 4.79 0.2461
WVFGRD96 36.0 290 55 -45 4.81 0.2717
WVFGRD96 38.0 285 55 -50 4.84 0.2954
WVFGRD96 40.0 280 60 -60 4.96 0.3323
WVFGRD96 42.0 280 60 -60 4.98 0.3380
WVFGRD96 44.0 275 60 -65 5.01 0.3539
WVFGRD96 46.0 270 60 -75 5.04 0.3764
WVFGRD96 48.0 270 60 -75 5.06 0.4001
WVFGRD96 50.0 270 60 -75 5.08 0.4210
WVFGRD96 52.0 260 60 -85 5.10 0.4410
WVFGRD96 54.0 260 60 -85 5.11 0.4594
WVFGRD96 56.0 260 60 -85 5.12 0.4726
WVFGRD96 58.0 260 60 -80 5.13 0.4812
WVFGRD96 60.0 260 60 -80 5.14 0.4861
WVFGRD96 62.0 260 60 -80 5.14 0.4868
WVFGRD96 64.0 260 60 -80 5.14 0.4855
WVFGRD96 66.0 260 60 -80 5.14 0.4820
WVFGRD96 68.0 260 60 -80 5.14 0.4776
WVFGRD96 70.0 260 60 -80 5.14 0.4726
WVFGRD96 72.0 260 60 -80 5.14 0.4686
WVFGRD96 74.0 260 60 -80 5.14 0.4662
WVFGRD96 76.0 265 60 -75 5.15 0.4630
WVFGRD96 78.0 265 60 -75 5.15 0.4605
WVFGRD96 80.0 265 60 -75 5.15 0.4573
WVFGRD96 82.0 265 60 -75 5.14 0.4528
WVFGRD96 84.0 270 65 -70 5.16 0.4486
WVFGRD96 86.0 270 65 -70 5.16 0.4461
WVFGRD96 88.0 270 65 -70 5.16 0.4435
WVFGRD96 90.0 270 65 -70 5.16 0.4404
WVFGRD96 92.0 270 65 -70 5.16 0.4370
WVFGRD96 94.0 270 65 -70 5.16 0.4328
WVFGRD96 96.0 270 65 -70 5.16 0.4288
WVFGRD96 98.0 270 65 -70 5.16 0.4247
The best solution is
WVFGRD96 62.0 260 60 -80 5.14 0.4868
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