The ANSS event ID is ak0195f5rhn0 and the event page is at https://earthquake.usgs.gov/earthquakes/eventpage/ak0195f5rhn0/executive.
2019/04/28 00:58:50 61.382 -149.908 44.5 4 Alaska
USGS/SLU Moment Tensor Solution
ENS 2019/04/28 00:58:50:0 61.38 -149.91 44.5 4.0 Alaska
Stations used:
AK.FIRE AK.GHO AK.GLI AK.HIN AK.KLU AK.KNK AK.PWL AK.RC01
AK.RND AK.SAW AK.SKN AK.SLK AK.SSN AK.SWD AK.TRF AT.PMR
AV.STLK TA.M22K TA.M23K TA.M24K TA.O22K
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 = 1.72e+22 dyne-cm
Mw = 4.09
Z = 54 km
Plane Strike Dip Rake
NP1 34 65 -95
NP2 225 25 -80
Principal Axes:
Axis Value Plunge Azimuth
T 1.72e+22 20 127
N 0.00e+00 4 36
P -1.72e+22 69 295
Moment Tensor: (dyne-cm)
Component Value
Mxx 5.22e+21
Mxy -6.48e+21
Mxz -5.78e+21
Myy 7.74e+21
Myz 9.60e+21
Mzz -1.30e+22
##############
##############---#####
#########-----------------#-
#######--------------------###
#######----------------------#####
######-----------------------#######
#####-------------------------########
#####-------------------------##########
####--------- --------------##########
#####--------- P -------------############
####---------- ------------#############
####------------------------##############
###------------------------###############
##----------------------################
##---------------------#################
##------------------########### ####
#-----------------############ T ###
#--------------############## ##
----------####################
------######################
######################
##############
Global CMT Convention Moment Tensor:
R T P
-1.30e+22 -5.78e+21 -9.60e+21
-5.78e+21 5.22e+21 6.48e+21
-9.60e+21 6.48e+21 7.74e+21
Details of the solution is found at
http://www.eas.slu.edu/eqc/eqc_mt/MECH.NA/20190428005850/index.html
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STK = 225
DIP = 25
RAKE = -80
MW = 4.09
HS = 54.0
The NDK file is 20190428005850.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 55 40 90 3.24 0.1899
WVFGRD96 2.0 55 40 90 3.38 0.2562
WVFGRD96 3.0 55 40 90 3.43 0.2336
WVFGRD96 4.0 185 60 -55 3.41 0.2231
WVFGRD96 5.0 25 80 60 3.42 0.2423
WVFGRD96 6.0 25 85 55 3.43 0.2603
WVFGRD96 7.0 25 85 50 3.44 0.2724
WVFGRD96 8.0 25 85 55 3.51 0.2813
WVFGRD96 9.0 110 45 40 3.54 0.2981
WVFGRD96 10.0 110 55 45 3.56 0.3133
WVFGRD96 11.0 110 55 45 3.58 0.3265
WVFGRD96 12.0 110 55 45 3.59 0.3358
WVFGRD96 13.0 290 45 25 3.58 0.3453
WVFGRD96 14.0 290 45 20 3.59 0.3532
WVFGRD96 15.0 290 45 25 3.60 0.3605
WVFGRD96 16.0 290 45 20 3.61 0.3670
WVFGRD96 17.0 270 60 -35 3.64 0.3729
WVFGRD96 18.0 270 60 -35 3.65 0.3788
WVFGRD96 19.0 270 60 -35 3.66 0.3835
WVFGRD96 20.0 270 60 -35 3.68 0.3865
WVFGRD96 21.0 285 45 -5 3.67 0.3884
WVFGRD96 22.0 285 45 -5 3.68 0.3928
WVFGRD96 23.0 280 45 -10 3.69 0.3973
WVFGRD96 24.0 280 45 -15 3.71 0.4021
WVFGRD96 25.0 280 45 -20 3.72 0.4077
WVFGRD96 26.0 280 45 -20 3.73 0.4144
WVFGRD96 27.0 275 45 -25 3.74 0.4211
WVFGRD96 28.0 270 25 -25 3.75 0.4297
WVFGRD96 29.0 270 25 -25 3.77 0.4382
WVFGRD96 30.0 265 25 -35 3.78 0.4482
WVFGRD96 31.0 260 25 -40 3.79 0.4601
WVFGRD96 32.0 255 25 -45 3.81 0.4734
WVFGRD96 33.0 250 25 -50 3.82 0.4862
WVFGRD96 34.0 250 25 -55 3.83 0.5030
WVFGRD96 35.0 250 25 -55 3.84 0.5175
WVFGRD96 36.0 245 25 -60 3.85 0.5311
WVFGRD96 37.0 245 25 -60 3.86 0.5426
WVFGRD96 38.0 240 25 -70 3.88 0.5524
WVFGRD96 39.0 235 25 -75 3.89 0.5621
WVFGRD96 40.0 235 25 -70 4.00 0.5580
WVFGRD96 41.0 235 25 -70 4.01 0.5747
WVFGRD96 42.0 235 25 -70 4.02 0.5876
WVFGRD96 43.0 235 25 -70 4.03 0.5990
WVFGRD96 44.0 230 25 -75 4.04 0.6089
WVFGRD96 45.0 230 25 -75 4.05 0.6164
WVFGRD96 46.0 230 25 -75 4.05 0.6241
WVFGRD96 47.0 230 25 -75 4.06 0.6295
WVFGRD96 48.0 230 25 -75 4.06 0.6352
WVFGRD96 49.0 230 25 -75 4.07 0.6383
WVFGRD96 50.0 230 25 -75 4.08 0.6426
WVFGRD96 51.0 225 25 -80 4.08 0.6441
WVFGRD96 52.0 225 25 -80 4.09 0.6466
WVFGRD96 53.0 225 25 -80 4.09 0.6463
WVFGRD96 54.0 225 25 -80 4.09 0.6471
WVFGRD96 55.0 225 25 -80 4.10 0.6457
WVFGRD96 56.0 225 25 -80 4.10 0.6445
WVFGRD96 57.0 225 25 -80 4.10 0.6430
WVFGRD96 58.0 225 25 -80 4.10 0.6383
WVFGRD96 59.0 225 25 -80 4.10 0.6363
The best solution is
WVFGRD96 54.0 225 25 -80 4.09 0.6471
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