The ANSS event ID is ak02183gai9z and the event page is at https://earthquake.usgs.gov/earthquakes/eventpage/ak02183gai9z/executive.
2021/06/25 23:02:12 60.914 -149.493 37.4 3.9 Alaska
USGS/SLU Moment Tensor Solution
ENS 2021/06/25 23:02:12:0 60.91 -149.49 37.4 3.9 Alaska
Stations used:
AK.CUT AK.FIRE AK.GHO AK.GLI AK.KNK AK.L26K AK.PWL AK.RC01
AK.RIDG AK.SAW AK.SCM AK.SKN AK.SLK AK.SWD AT.PMR
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
Best Fitting Double Couple
Mo = 1.35e+22 dyne-cm
Mw = 4.02
Z = 49 km
Plane Strike Dip Rake
NP1 325 85 175
NP2 55 85 5
Principal Axes:
Axis Value Plunge Azimuth
T 1.35e+22 7 280
N 0.00e+00 83 100
P -1.35e+22 0 10
Moment Tensor: (dyne-cm)
Component Value
Mxx -1.27e+22
Mxy -4.48e+21
Mxz 2.77e+20
Myy 1.25e+22
Myz -1.62e+21
Mzz 2.04e+20
--------- P --
------------- ------
###-------------------------
######------------------------
#########-------------------------
############---------------------###
##############------------------######
#################--------------#########
################----------############
T ##################------###############
###################---#################
######################-###################
###################-----##################
###############----------###############
#############-------------##############
#########-----------------############
####----------------------##########
--------------------------########
-------------------------#####
-------------------------###
----------------------
--------------
Global CMT Convention Moment Tensor:
R T P
2.04e+20 2.77e+20 1.62e+21
2.77e+20 -1.27e+22 4.48e+21
1.62e+21 4.48e+21 1.25e+22
Details of the solution is found at
http://www.eas.slu.edu/eqc/eqc_mt/MECH.NA/20210625230212/index.html
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STK = 55
DIP = 85
RAKE = 5
MW = 4.02
HS = 49.0
The NDK file is 20210625230212.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 3The results of this grid search are as follow:
DEPTH STK DIP RAKE MW FIT
WVFGRD96 1.0 235 85 0 3.07 0.1554
WVFGRD96 2.0 235 90 0 3.21 0.2131
WVFGRD96 3.0 235 90 0 3.27 0.2375
WVFGRD96 4.0 50 85 -35 3.36 0.2612
WVFGRD96 5.0 50 85 -30 3.39 0.2834
WVFGRD96 6.0 50 85 -30 3.42 0.3000
WVFGRD96 7.0 50 85 -25 3.44 0.3148
WVFGRD96 8.0 50 85 -30 3.49 0.3298
WVFGRD96 9.0 50 85 -25 3.51 0.3381
WVFGRD96 10.0 230 90 25 3.53 0.3417
WVFGRD96 11.0 50 85 -25 3.55 0.3469
WVFGRD96 12.0 230 90 25 3.56 0.3475
WVFGRD96 13.0 50 85 -20 3.58 0.3494
WVFGRD96 14.0 230 90 20 3.59 0.3507
WVFGRD96 15.0 50 90 -20 3.60 0.3536
WVFGRD96 16.0 230 90 20 3.62 0.3565
WVFGRD96 17.0 50 85 -20 3.63 0.3610
WVFGRD96 18.0 230 90 20 3.64 0.3656
WVFGRD96 19.0 50 85 -15 3.66 0.3751
WVFGRD96 20.0 50 85 -15 3.68 0.3856
WVFGRD96 21.0 50 85 -15 3.69 0.3959
WVFGRD96 22.0 50 80 -10 3.71 0.4070
WVFGRD96 23.0 50 80 -10 3.72 0.4181
WVFGRD96 24.0 55 80 5 3.73 0.4286
WVFGRD96 25.0 55 80 5 3.74 0.4394
WVFGRD96 26.0 55 80 5 3.76 0.4512
WVFGRD96 27.0 55 80 5 3.77 0.4609
WVFGRD96 28.0 55 80 5 3.78 0.4697
WVFGRD96 29.0 55 80 5 3.79 0.4778
WVFGRD96 30.0 55 80 5 3.80 0.4852
WVFGRD96 31.0 55 75 0 3.82 0.4919
WVFGRD96 32.0 55 80 0 3.82 0.4985
WVFGRD96 33.0 55 80 0 3.83 0.5042
WVFGRD96 34.0 55 80 0 3.84 0.5071
WVFGRD96 35.0 55 80 0 3.85 0.5097
WVFGRD96 36.0 55 80 5 3.87 0.5117
WVFGRD96 37.0 55 80 5 3.88 0.5132
WVFGRD96 38.0 55 80 5 3.90 0.5168
WVFGRD96 39.0 55 85 5 3.91 0.5196
WVFGRD96 40.0 55 80 5 3.94 0.5241
WVFGRD96 41.0 55 80 5 3.96 0.5261
WVFGRD96 42.0 55 80 5 3.97 0.5292
WVFGRD96 43.0 55 80 5 3.98 0.5294
WVFGRD96 44.0 55 80 5 3.99 0.5300
WVFGRD96 45.0 55 80 5 4.00 0.5300
WVFGRD96 46.0 55 85 5 4.00 0.5297
WVFGRD96 47.0 55 85 5 4.01 0.5305
WVFGRD96 48.0 55 85 5 4.01 0.5300
WVFGRD96 49.0 55 85 5 4.02 0.5308
WVFGRD96 50.0 55 85 5 4.03 0.5302
WVFGRD96 51.0 55 85 5 4.03 0.5305
WVFGRD96 52.0 235 90 -5 4.03 0.5265
WVFGRD96 53.0 55 85 5 4.05 0.5292
WVFGRD96 54.0 55 85 5 4.05 0.5289
WVFGRD96 55.0 235 90 -5 4.05 0.5279
WVFGRD96 56.0 235 90 -5 4.05 0.5287
WVFGRD96 57.0 55 90 5 4.06 0.5275
WVFGRD96 58.0 55 90 5 4.06 0.5288
WVFGRD96 59.0 235 90 -5 4.07 0.5275
The best solution is
WVFGRD96 49.0 55 85 5 4.02 0.5308
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
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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