The ANSS event ID is ak020bta3ut8 and the event page is at https://earthquake.usgs.gov/earthquakes/eventpage/ak020bta3ut8/executive.
2020/09/13 15:10:09 61.500 -146.392 30.5 4.1 Alaska
USGS/SLU Moment Tensor Solution
ENS 2020/09/13 15:10:09:0 61.50 -146.39 30.5 4.1 Alaska
Stations used:
AK.BMR AK.BRLK AK.CAST AK.CCB AK.DHY AK.DIV AK.EYAK AK.FID
AK.GHO AK.GLB AK.GLI AK.HIN AK.KLU AK.KNK AK.M26K AK.PWL
AK.RC01 AK.SAW AK.SCM AK.SKN AK.VRDI AT.PMR IM.IL31
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 = 1.72e+22 dyne-cm
Mw = 4.09
Z = 45 km
Plane Strike Dip Rake
NP1 215 50 -85
NP2 27 40 -96
Principal Axes:
Axis Value Plunge Azimuth
T 1.72e+22 5 301
N 0.00e+00 4 32
P -1.72e+22 84 160
Moment Tensor: (dyne-cm)
Component Value
Mxx 4.47e+21
Mxy -7.53e+21
Mxz 2.49e+21
Myy 1.24e+22
Myz -1.88e+21
Mzz -1.69e+22
##############
#####################-
##################-------###
###############------------###
#############---------------####
T ###########-----------------#####
# #########--------------------#####
############----------------------######
###########-----------------------######
###########------------------------#######
##########---------- -----------########
#########----------- P -----------########
#########----------- ----------#########
#######------------------------#########
#######------------------------#########
######----------------------##########
#####---------------------##########
####-------------------###########
##-----------------###########
##--------------############
---------#############
##############
Global CMT Convention Moment Tensor:
R T P
-1.69e+22 2.49e+21 1.88e+21
2.49e+21 4.47e+21 7.53e+21
1.88e+21 7.53e+21 1.24e+22
Details of the solution is found at
http://www.eas.slu.edu/eqc/eqc_mt/MECH.NA/20200913151009/index.html
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STK = 215
DIP = 50
RAKE = -85
MW = 4.09
HS = 45.0
The NDK file is 20200913151009.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 15 50 65 3.20 0.2113
WVFGRD96 2.0 25 50 80 3.36 0.2799
WVFGRD96 3.0 10 55 60 3.40 0.2844
WVFGRD96 4.0 5 65 55 3.41 0.2758
WVFGRD96 5.0 0 70 45 3.41 0.2798
WVFGRD96 6.0 340 70 -35 3.43 0.2969
WVFGRD96 7.0 340 70 -35 3.46 0.3155
WVFGRD96 8.0 340 70 -40 3.52 0.3218
WVFGRD96 9.0 345 70 -40 3.54 0.3316
WVFGRD96 10.0 350 75 -40 3.55 0.3383
WVFGRD96 11.0 345 70 -40 3.58 0.3424
WVFGRD96 12.0 245 55 -35 3.60 0.3452
WVFGRD96 13.0 245 55 -35 3.62 0.3636
WVFGRD96 14.0 245 55 -35 3.64 0.3798
WVFGRD96 15.0 245 55 -35 3.66 0.3937
WVFGRD96 16.0 245 55 -35 3.67 0.4053
WVFGRD96 17.0 245 55 -35 3.69 0.4146
WVFGRD96 18.0 245 55 -35 3.70 0.4229
WVFGRD96 19.0 250 60 -30 3.72 0.4292
WVFGRD96 20.0 250 60 -30 3.73 0.4342
WVFGRD96 21.0 250 60 -30 3.75 0.4401
WVFGRD96 22.0 250 60 -30 3.76 0.4439
WVFGRD96 23.0 250 60 -30 3.77 0.4447
WVFGRD96 24.0 250 60 -30 3.78 0.4435
WVFGRD96 25.0 250 50 -20 3.79 0.4465
WVFGRD96 26.0 255 50 -15 3.80 0.4475
WVFGRD96 27.0 250 50 -20 3.81 0.4462
WVFGRD96 28.0 250 75 -40 3.83 0.4569
WVFGRD96 29.0 245 75 -45 3.85 0.4716
WVFGRD96 30.0 245 70 -45 3.86 0.4898
WVFGRD96 31.0 245 70 -45 3.87 0.5076
WVFGRD96 32.0 65 50 -45 3.87 0.5282
WVFGRD96 33.0 60 45 -55 3.88 0.5490
WVFGRD96 34.0 60 45 -55 3.89 0.5648
WVFGRD96 35.0 60 45 -55 3.89 0.5747
WVFGRD96 36.0 60 45 -55 3.90 0.5805
WVFGRD96 37.0 60 45 -55 3.91 0.5819
WVFGRD96 38.0 60 45 -55 3.92 0.5833
WVFGRD96 39.0 50 40 -70 3.93 0.5854
WVFGRD96 40.0 50 40 -65 4.02 0.6234
WVFGRD96 41.0 50 40 -70 4.04 0.6313
WVFGRD96 42.0 25 40 -100 4.06 0.6359
WVFGRD96 43.0 25 40 -100 4.07 0.6412
WVFGRD96 44.0 215 50 -85 4.08 0.6415
WVFGRD96 45.0 215 50 -85 4.09 0.6432
WVFGRD96 46.0 215 50 -85 4.09 0.6424
WVFGRD96 47.0 220 50 -80 4.10 0.6418
WVFGRD96 48.0 220 50 -80 4.11 0.6396
WVFGRD96 49.0 220 50 -80 4.11 0.6361
WVFGRD96 50.0 220 50 -80 4.11 0.6304
WVFGRD96 51.0 230 55 -65 4.12 0.6251
WVFGRD96 52.0 220 50 -80 4.12 0.6200
WVFGRD96 53.0 220 50 -80 4.12 0.6125
WVFGRD96 54.0 220 50 -80 4.12 0.6070
WVFGRD96 55.0 220 50 -80 4.12 0.5988
WVFGRD96 56.0 220 50 -80 4.12 0.5936
WVFGRD96 57.0 220 50 -80 4.12 0.5855
WVFGRD96 58.0 220 50 -80 4.12 0.5792
WVFGRD96 59.0 220 50 -80 4.12 0.5722
The best solution is
WVFGRD96 45.0 215 50 -85 4.09 0.6432
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