The ANSS event ID is ak0205k9e7lg and the event page is at https://earthquake.usgs.gov/earthquakes/eventpage/ak0205k9e7lg/executive.
2020/04/30 09:01:47 61.528 -146.342 20.9 3.8 Alaska
USGS/SLU Moment Tensor Solution
ENS 2020/04/30 09:01:47:0 61.53 -146.34 20.9 3.8 Alaska
Stations used:
AK.BMR AK.DHY AK.DIV AK.DOT AK.EYAK AK.GLB AK.HIN AK.KLU
AK.KNK AK.PAX AK.RC01 AK.SAW AK.SCM AK.TGL AT.PMR TA.M22K
TA.M24K TA.N25K
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 = 1.06e+22 dyne-cm
Mw = 3.95
Z = 42 km
Plane Strike Dip Rake
NP1 250 70 -65
NP2 16 32 -139
Principal Axes:
Axis Value Plunge Azimuth
T 1.06e+22 21 321
N 0.00e+00 23 61
P -1.06e+22 58 194
Moment Tensor: (dyne-cm)
Component Value
Mxx 2.75e+21
Mxy -5.21e+21
Mxz 7.43e+21
Myy 3.43e+21
Myz -1.08e+21
Mzz -6.17e+21
##############
####################--
########################----
## ######################---
#### T #######################----
##### #######################-----
#################################-----
########################----------######
##################-----------------#####
##############----------------------######
###########-------------------------######
########---------------------------#######
######-----------------------------#######
###-------------------------------######
#---------------- -------------#######
---------------- P ------------#######
--------------- -----------#######
---------------------------#######
-----------------------#######
--------------------########
--------------########
-----#########
Global CMT Convention Moment Tensor:
R T P
-6.17e+21 7.43e+21 1.08e+21
7.43e+21 2.75e+21 5.21e+21
1.08e+21 5.21e+21 3.43e+21
Details of the solution is found at
http://www.eas.slu.edu/eqc/eqc_mt/MECH.NA/20200430090147/index.html
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STK = 250
DIP = 70
RAKE = -65
MW = 3.95
HS = 42.0
The NDK file is 20200430090147.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 1.0 220 45 85 3.06 0.1764
WVFGRD96 2.0 220 45 85 3.21 0.2402
WVFGRD96 3.0 25 30 60 3.28 0.2436
WVFGRD96 4.0 15 30 40 3.29 0.2620
WVFGRD96 5.0 335 40 -45 3.32 0.2887
WVFGRD96 6.0 330 50 -65 3.38 0.3196
WVFGRD96 7.0 330 50 -65 3.42 0.3426
WVFGRD96 8.0 330 50 -65 3.47 0.3520
WVFGRD96 9.0 330 50 -65 3.50 0.3715
WVFGRD96 10.0 330 50 -65 3.52 0.3858
WVFGRD96 11.0 330 50 -65 3.55 0.3958
WVFGRD96 12.0 325 45 -70 3.57 0.4017
WVFGRD96 13.0 325 45 -75 3.59 0.4044
WVFGRD96 14.0 250 50 -45 3.54 0.4103
WVFGRD96 15.0 255 50 -45 3.56 0.4184
WVFGRD96 16.0 255 50 -45 3.57 0.4244
WVFGRD96 17.0 105 65 55 3.61 0.4282
WVFGRD96 18.0 100 65 50 3.62 0.4330
WVFGRD96 19.0 100 70 50 3.63 0.4384
WVFGRD96 20.0 100 70 45 3.63 0.4435
WVFGRD96 21.0 100 70 50 3.66 0.4488
WVFGRD96 22.0 95 75 45 3.65 0.4531
WVFGRD96 23.0 95 75 45 3.66 0.4579
WVFGRD96 24.0 95 75 45 3.67 0.4630
WVFGRD96 25.0 85 90 40 3.67 0.4702
WVFGRD96 26.0 265 85 -45 3.68 0.4786
WVFGRD96 27.0 260 80 -45 3.69 0.4864
WVFGRD96 28.0 260 80 -50 3.71 0.4963
WVFGRD96 29.0 255 75 -50 3.72 0.5082
WVFGRD96 30.0 255 75 -55 3.74 0.5184
WVFGRD96 31.0 255 75 -55 3.75 0.5298
WVFGRD96 32.0 250 70 -60 3.77 0.5400
WVFGRD96 33.0 250 70 -60 3.78 0.5492
WVFGRD96 34.0 250 70 -60 3.79 0.5578
WVFGRD96 35.0 250 70 -60 3.80 0.5613
WVFGRD96 36.0 250 70 -60 3.81 0.5651
WVFGRD96 37.0 250 70 -60 3.82 0.5663
WVFGRD96 38.0 250 70 -60 3.83 0.5666
WVFGRD96 39.0 250 70 -55 3.83 0.5665
WVFGRD96 40.0 255 75 -65 3.93 0.5651
WVFGRD96 41.0 255 75 -65 3.94 0.5677
WVFGRD96 42.0 250 70 -65 3.95 0.5701
WVFGRD96 43.0 250 70 -65 3.96 0.5689
WVFGRD96 44.0 255 75 -60 3.95 0.5650
WVFGRD96 45.0 255 75 -60 3.96 0.5625
WVFGRD96 46.0 255 75 -60 3.97 0.5578
WVFGRD96 47.0 255 75 -60 3.97 0.5525
WVFGRD96 48.0 255 75 -60 3.98 0.5471
WVFGRD96 49.0 255 75 -60 3.98 0.5403
WVFGRD96 50.0 255 75 -55 3.97 0.5331
WVFGRD96 51.0 255 75 -55 3.97 0.5269
WVFGRD96 52.0 255 75 -55 3.97 0.5198
WVFGRD96 53.0 260 80 -55 3.98 0.5124
WVFGRD96 54.0 260 80 -55 3.98 0.5069
WVFGRD96 55.0 260 80 -55 3.99 0.4999
WVFGRD96 56.0 260 80 -55 3.99 0.4935
WVFGRD96 57.0 260 80 -55 3.99 0.4858
WVFGRD96 58.0 260 80 -50 3.98 0.4791
WVFGRD96 59.0 260 80 -50 3.98 0.4729
The best solution is
WVFGRD96 42.0 250 70 -65 3.95 0.5701
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