The ANSS event ID is ak020bpy4mtd and the event page is at https://earthquake.usgs.gov/earthquakes/eventpage/ak020bpy4mtd/executive.
2020/09/11 13:58:32 61.630 -152.179 121.7 4.3 Alaska
USGS/SLU Moment Tensor Solution
ENS 2020/09/11 13:58:32:0 61.63 -152.18 121.7 4.3 Alaska
Stations used:
AK.BRLK AK.BWN AK.CAPN AK.CAST AK.CNP AK.CUT AK.DHY AK.DIV
AK.FID AK.GHO AK.GLI AK.HIN AK.J19K AK.J20K AK.K20K AK.KTH
AK.L18K AK.L19K AK.L20K AK.M20K AK.MCK AK.N18K AK.N19K
AK.NEA2 AK.O18K AK.O19K AK.PPLA AK.PWL AK.RC01 AK.RND
AK.SAW AK.SCM AK.SKN AK.SLK AK.TRF AK.WRH AT.PMR AV.ILSW
AV.STLK TA.J18K TA.K17K TA.M22K TA.N17K TA.O22K
Filtering commands used:
cut o DIST/3.4 -40 o DIST/3.4 +50
rtr
taper w 0.1
hp c 0.03 n 3
lp c 0.10 n 3
Best Fitting Double Couple
Mo = 4.52e+22 dyne-cm
Mw = 4.37
Z = 128 km
Plane Strike Dip Rake
NP1 309 63 127
NP2 70 45 40
Principal Axes:
Axis Value Plunge Azimuth
T 4.52e+22 55 268
N 0.00e+00 33 110
P -4.52e+22 10 13
Moment Tensor: (dyne-cm)
Component Value
Mxx -4.14e+22
Mxy -9.41e+21
Mxz -8.37e+21
Myy 1.23e+22
Myz -2.30e+22
Mzz 2.90e+22
---------- -
-------------- P -----
----------------- --------
------------------------------
##########------------------------
###############---------------------
###################-------------------
#######################----------------#
#########################--------------#
############################-----------###
########### ################---------###
########### T ##################------####
########### ###################---######
########################################
###############################---######
-###########################------####
---####################-----------##
--------#######------------------#
------------------------------
----------------------------
----------------------
--------------
Global CMT Convention Moment Tensor:
R T P
2.90e+22 -8.37e+21 2.30e+22
-8.37e+21 -4.14e+22 9.41e+21
2.30e+22 9.41e+21 1.23e+22
Details of the solution is found at
http://www.eas.slu.edu/eqc/eqc_mt/MECH.NA/20200911135832/index.html
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STK = 70
DIP = 45
RAKE = 40
MW = 4.37
HS = 128.0
The NDK file is 20200911135832.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/3.4 -40 o DIST/3.4 +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 2.0 100 45 -100 3.44 0.1097
WVFGRD96 4.0 325 60 -35 3.45 0.1158
WVFGRD96 6.0 330 80 -40 3.49 0.1254
WVFGRD96 8.0 330 80 -40 3.57 0.1324
WVFGRD96 10.0 165 65 40 3.62 0.1380
WVFGRD96 12.0 340 50 30 3.65 0.1408
WVFGRD96 14.0 340 50 30 3.68 0.1410
WVFGRD96 16.0 250 55 35 3.70 0.1411
WVFGRD96 18.0 250 50 35 3.72 0.1393
WVFGRD96 20.0 250 50 35 3.75 0.1370
WVFGRD96 22.0 250 50 35 3.77 0.1367
WVFGRD96 24.0 250 50 35 3.79 0.1365
WVFGRD96 26.0 250 55 30 3.82 0.1375
WVFGRD96 28.0 250 60 30 3.84 0.1400
WVFGRD96 30.0 250 60 30 3.86 0.1436
WVFGRD96 32.0 250 65 25 3.88 0.1451
WVFGRD96 34.0 250 65 25 3.89 0.1446
WVFGRD96 36.0 250 65 25 3.91 0.1423
WVFGRD96 38.0 250 70 20 3.94 0.1410
WVFGRD96 40.0 250 70 25 4.01 0.1408
WVFGRD96 42.0 250 70 25 4.03 0.1401
WVFGRD96 44.0 250 70 25 4.06 0.1407
WVFGRD96 46.0 250 70 25 4.08 0.1415
WVFGRD96 48.0 250 70 25 4.09 0.1431
WVFGRD96 50.0 245 75 25 4.10 0.1458
WVFGRD96 52.0 245 75 25 4.12 0.1512
WVFGRD96 54.0 245 80 25 4.14 0.1611
WVFGRD96 56.0 245 80 20 4.15 0.1734
WVFGRD96 58.0 60 90 -25 4.17 0.1883
WVFGRD96 60.0 60 85 -20 4.19 0.2079
WVFGRD96 62.0 60 85 -20 4.21 0.2283
WVFGRD96 64.0 60 85 -20 4.22 0.2480
WVFGRD96 66.0 55 75 -15 4.23 0.2670
WVFGRD96 68.0 70 45 40 4.22 0.2799
WVFGRD96 70.0 70 45 40 4.23 0.2944
WVFGRD96 72.0 65 45 40 4.24 0.3095
WVFGRD96 74.0 65 45 40 4.25 0.3220
WVFGRD96 76.0 65 45 35 4.26 0.3315
WVFGRD96 78.0 65 45 35 4.27 0.3409
WVFGRD96 80.0 65 45 35 4.27 0.3501
WVFGRD96 82.0 65 45 35 4.28 0.3592
WVFGRD96 84.0 65 45 35 4.29 0.3670
WVFGRD96 86.0 65 45 35 4.29 0.3742
WVFGRD96 88.0 65 45 35 4.30 0.3810
WVFGRD96 90.0 65 45 35 4.31 0.3872
WVFGRD96 92.0 65 45 35 4.31 0.3930
WVFGRD96 94.0 65 45 35 4.32 0.3994
WVFGRD96 96.0 65 45 35 4.32 0.4046
WVFGRD96 98.0 65 45 35 4.33 0.4101
WVFGRD96 100.0 65 45 35 4.33 0.4146
WVFGRD96 102.0 65 45 35 4.33 0.4195
WVFGRD96 104.0 70 45 40 4.34 0.4235
WVFGRD96 106.0 70 45 40 4.34 0.4276
WVFGRD96 108.0 70 45 40 4.34 0.4309
WVFGRD96 110.0 70 45 40 4.35 0.4341
WVFGRD96 112.0 70 45 40 4.35 0.4364
WVFGRD96 114.0 70 45 40 4.35 0.4383
WVFGRD96 116.0 70 45 40 4.36 0.4405
WVFGRD96 118.0 70 45 40 4.36 0.4426
WVFGRD96 120.0 70 45 40 4.36 0.4442
WVFGRD96 122.0 70 45 40 4.37 0.4449
WVFGRD96 124.0 70 45 40 4.37 0.4449
WVFGRD96 126.0 70 45 40 4.37 0.4447
WVFGRD96 128.0 70 45 40 4.37 0.4457
WVFGRD96 130.0 70 45 40 4.38 0.4453
WVFGRD96 132.0 70 45 40 4.38 0.4445
WVFGRD96 134.0 70 45 40 4.38 0.4431
WVFGRD96 136.0 70 45 40 4.38 0.4428
WVFGRD96 138.0 70 45 40 4.38 0.4413
WVFGRD96 140.0 70 45 40 4.38 0.4384
WVFGRD96 142.0 70 45 40 4.39 0.4374
WVFGRD96 144.0 70 45 40 4.39 0.4355
WVFGRD96 146.0 70 50 40 4.39 0.4335
WVFGRD96 148.0 70 50 40 4.39 0.4320
The best solution is
WVFGRD96 128.0 70 45 40 4.37 0.4457
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/3.4 -40 o DIST/3.4 +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