Left: mLg computed using the IASPEI formula. Center: mLg residuals versus epicentral distance ; the values used for the trimmed mean magnitude estimate are indicated. Right: residuals as a function of distance and azimuth.
The ANSS event ID is ak019g675trf and the event page is at https://earthquake.usgs.gov/earthquakes/eventpage/ak019g675trf/executive.
2019/12/18 03:34:35 66.328 -157.270 9.7 4.1 Alaska
USGS/SLU Moment Tensor Solution
ENS 2019/12/18 03:34:35:0 66.33 -157.27 9.7 4.1 Alaska
Stations used:
AK.ANM AK.COLD AK.H21K AK.H22K AK.J17K AK.J19K AK.J20K
AK.RDOG TA.C16K TA.C18K TA.C19K TA.D19K TA.D20K TA.D22K
TA.D23K TA.E18K TA.E19K TA.E21K TA.E22K TA.E23K TA.E24K
TA.F15K TA.F20K TA.F21K TA.F24K TA.G18K TA.G19K TA.G21K
TA.G23K TA.G24K TA.H17K TA.H18K TA.H19K TA.I17K TA.I20K
TA.J18K TA.K17K TA.TOLK
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 = 8.32e+21 dyne-cm
Mw = 3.88
Z = 10 km
Plane Strike Dip Rake
NP1 140 60 -40
NP2 253 56 -143
Principal Axes:
Axis Value Plunge Azimuth
T 8.32e+21 2 197
N 0.00e+00 42 289
P -8.32e+21 48 105
Moment Tensor: (dyne-cm)
Component Value
Mxx 7.35e+21
Mxy 3.24e+21
Mxz 7.22e+20
Myy -2.72e+21
Myz -4.10e+21
Mzz -4.63e+21
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--------#####--------------------------#
--------##------------------------------
--------##--------------------------------
------#####-------------------------------
----########------------------ ---------
---###########---------------- P ---------
-#############--------------- --------
################------------------------
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T ############
Global CMT Convention Moment Tensor:
R T P
-4.63e+21 7.22e+20 4.10e+21
7.22e+20 7.35e+21 -3.24e+21
4.10e+21 -3.24e+21 -2.72e+21
Details of the solution is found at
http://www.eas.slu.edu/eqc/eqc_mt/MECH.NA/20191218033435/index.html
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STK = 140
DIP = 60
RAKE = -40
MW = 3.88
HS = 10.0
The NDK file is 20191218033435.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: mLg computed using the IASPEI formula. Center: mLg residuals versus epicentral distance ; the values used for the trimmed mean magnitude estimate are indicated.
Right: residuals as a function of distance and azimuth.
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 1.0 145 70 -35 3.48 0.3016
WVFGRD96 2.0 145 75 -40 3.62 0.3880
WVFGRD96 3.0 165 60 40 3.69 0.4175
WVFGRD96 4.0 165 55 45 3.74 0.4746
WVFGRD96 5.0 165 55 45 3.77 0.5171
WVFGRD96 6.0 165 55 45 3.78 0.5411
WVFGRD96 7.0 140 60 -40 3.80 0.5571
WVFGRD96 8.0 135 60 -50 3.86 0.5898
WVFGRD96 9.0 135 55 -45 3.87 0.6019
WVFGRD96 10.0 140 60 -40 3.88 0.6029
WVFGRD96 11.0 140 65 -35 3.88 0.5970
WVFGRD96 12.0 140 65 -30 3.89 0.5871
WVFGRD96 13.0 145 70 -25 3.89 0.5764
WVFGRD96 14.0 145 75 -25 3.90 0.5642
WVFGRD96 15.0 145 75 -25 3.91 0.5516
WVFGRD96 16.0 145 75 -20 3.92 0.5396
WVFGRD96 17.0 145 75 -20 3.92 0.5286
WVFGRD96 18.0 145 75 -20 3.93 0.5166
WVFGRD96 19.0 150 70 20 3.94 0.4981
WVFGRD96 20.0 150 70 20 3.94 0.4876
WVFGRD96 21.0 150 70 25 3.95 0.4771
WVFGRD96 22.0 150 75 20 3.96 0.4662
WVFGRD96 23.0 150 70 20 3.97 0.4570
WVFGRD96 24.0 150 70 25 3.97 0.4479
WVFGRD96 25.0 150 70 25 3.98 0.4391
WVFGRD96 26.0 150 70 25 3.98 0.4302
WVFGRD96 27.0 150 70 25 3.99 0.4210
WVFGRD96 28.0 150 70 25 4.00 0.4121
WVFGRD96 29.0 145 75 25 4.00 0.4034
The best solution is
WVFGRD96 10.0 140 60 -40 3.88 0.6029
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