The ANSS event ID is ak019g813maq and the event page is at https://earthquake.usgs.gov/earthquakes/eventpage/ak019g813maq/executive.
2019/12/19 14:09:53 59.273 -153.554 107.8 3.9 Alaska
USGS/SLU Moment Tensor Solution
ENS 2019/12/19 14:09:53:0 59.27 -153.55 107.8 3.9 Alaska
Stations used:
AK.BRLK AK.CNP AK.HOM AK.N18K AK.O18K AK.O19K AK.Q19K
II.KDAK TA.P18K TA.P19K
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 0.08 n 3
br c 0.12 0.25 n 4 p 2
Best Fitting Double Couple
Mo = 1.97e+22 dyne-cm
Mw = 4.13
Z = 104 km
Plane Strike Dip Rake
NP1 230 80 -65
NP2 340 27 -157
Principal Axes:
Axis Value Plunge Azimuth
T 1.97e+22 31 300
N 0.00e+00 25 45
P -1.97e+22 49 167
Moment Tensor: (dyne-cm)
Component Value
Mxx -4.50e+21
Mxy -4.44e+21
Mxz 1.38e+22
Myy 1.06e+22
Myz -9.69e+21
Mzz -6.11e+21
#####---------
###############-------
#####################-------
########################------
############################--####
############################--######
##### ##################------######
###### T ###############----------######
###### #############-------------#####
####################----------------######
##################-------------------#####
################---------------------#####
##############-----------------------#####
###########-------------------------####
#########--------------------------#####
#######------------- -----------####
####--------------- P ----------####
##---------------- ----------###
----------------------------##
-------------------------###
--------------------##
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Global CMT Convention Moment Tensor:
R T P
-6.11e+21 1.38e+22 9.69e+21
1.38e+22 -4.50e+21 4.44e+21
9.69e+21 4.44e+21 1.06e+22
Details of the solution is found at
http://www.eas.slu.edu/eqc/eqc_mt/MECH.NA/20191219140953/index.html
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STK = 230
DIP = 80
RAKE = -65
MW = 4.13
HS = 104.0
The NDK file is 20191219140953.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 0.08 n 3 br c 0.12 0.25 n 4 p 2The results of this grid search are as follow:
DEPTH STK DIP RAKE MW FIT
WVFGRD96 2.0 350 50 50 3.42 0.2947
WVFGRD96 4.0 180 50 65 3.52 0.3238
WVFGRD96 6.0 40 65 90 3.60 0.3391
WVFGRD96 8.0 225 30 95 3.63 0.3418
WVFGRD96 10.0 315 55 -50 3.59 0.3448
WVFGRD96 12.0 260 35 -25 3.62 0.3517
WVFGRD96 14.0 260 40 -25 3.63 0.3579
WVFGRD96 16.0 265 40 -15 3.64 0.3623
WVFGRD96 18.0 265 40 -15 3.66 0.3659
WVFGRD96 20.0 265 40 -15 3.68 0.3694
WVFGRD96 22.0 260 35 -20 3.72 0.3749
WVFGRD96 24.0 260 35 -20 3.74 0.3809
WVFGRD96 26.0 260 35 -15 3.77 0.3869
WVFGRD96 28.0 255 35 -20 3.79 0.3910
WVFGRD96 30.0 255 35 -20 3.81 0.3954
WVFGRD96 32.0 255 35 -20 3.82 0.3983
WVFGRD96 34.0 250 35 -25 3.85 0.4005
WVFGRD96 36.0 245 35 -30 3.87 0.4007
WVFGRD96 38.0 240 35 -35 3.90 0.4006
WVFGRD96 40.0 250 45 -40 3.93 0.4087
WVFGRD96 42.0 250 45 -40 3.95 0.4180
WVFGRD96 44.0 255 50 -35 3.96 0.4249
WVFGRD96 46.0 255 50 -40 3.98 0.4306
WVFGRD96 48.0 255 50 -40 4.00 0.4332
WVFGRD96 50.0 245 80 -55 4.01 0.4604
WVFGRD96 52.0 245 80 -55 4.02 0.4833
WVFGRD96 54.0 245 85 -50 4.01 0.5075
WVFGRD96 56.0 245 85 -50 4.03 0.5301
WVFGRD96 58.0 245 85 -55 4.05 0.5487
WVFGRD96 60.0 235 90 -55 4.05 0.5687
WVFGRD96 62.0 55 90 55 4.05 0.5832
WVFGRD96 64.0 55 90 55 4.06 0.5956
WVFGRD96 66.0 235 90 -55 4.06 0.6077
WVFGRD96 68.0 240 90 -60 4.08 0.6167
WVFGRD96 70.0 65 85 65 4.10 0.6301
WVFGRD96 72.0 240 90 -65 4.09 0.6362
WVFGRD96 74.0 65 85 65 4.10 0.6464
WVFGRD96 76.0 240 90 -65 4.10 0.6521
WVFGRD96 78.0 240 90 -65 4.10 0.6574
WVFGRD96 80.0 60 90 65 4.11 0.6634
WVFGRD96 82.0 240 90 -65 4.11 0.6680
WVFGRD96 84.0 60 90 65 4.11 0.6721
WVFGRD96 86.0 240 90 -65 4.11 0.6741
WVFGRD96 88.0 240 90 -65 4.11 0.6764
WVFGRD96 90.0 60 90 65 4.12 0.6786
WVFGRD96 92.0 60 90 65 4.12 0.6800
WVFGRD96 94.0 235 85 -65 4.12 0.6831
WVFGRD96 96.0 235 85 -65 4.12 0.6842
WVFGRD96 98.0 230 80 -65 4.13 0.6856
WVFGRD96 100.0 230 80 -65 4.13 0.6866
WVFGRD96 102.0 230 80 -65 4.13 0.6872
WVFGRD96 104.0 230 80 -65 4.13 0.6876
WVFGRD96 106.0 230 80 -65 4.13 0.6869
WVFGRD96 108.0 230 80 -65 4.14 0.6872
WVFGRD96 110.0 225 75 -65 4.15 0.6854
WVFGRD96 112.0 225 75 -65 4.15 0.6856
WVFGRD96 114.0 215 70 -65 4.17 0.6839
WVFGRD96 116.0 215 70 -65 4.17 0.6845
WVFGRD96 118.0 215 70 -65 4.18 0.6838
WVFGRD96 120.0 215 70 -65 4.18 0.6828
WVFGRD96 122.0 215 70 -65 4.18 0.6822
WVFGRD96 124.0 215 70 -65 4.18 0.6796
WVFGRD96 126.0 215 70 -65 4.19 0.6785
WVFGRD96 128.0 215 70 -65 4.19 0.6766
WVFGRD96 130.0 215 70 -65 4.19 0.6750
WVFGRD96 132.0 215 70 -65 4.19 0.6736
WVFGRD96 134.0 215 70 -65 4.19 0.6703
WVFGRD96 136.0 215 70 -65 4.20 0.6682
WVFGRD96 138.0 210 65 -65 4.22 0.6672
The best solution is
WVFGRD96 104.0 230 80 -65 4.13 0.6876
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 0.08 n 3 br c 0.12 0.25 n 4 p 2
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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