The ANSS event ID is ak0191piyf73 and the event page is at https://earthquake.usgs.gov/earthquakes/eventpage/ak0191piyf73/executive.
2019/02/06 20:27:59 61.376 -150.025 43.0 3.6 Alaska
USGS/SLU Moment Tensor Solution
ENS 2019/02/06 20:27:59:0 61.38 -150.02 43.0 3.6 Alaska
Stations used:
AK.GHO AK.KNK AK.PWL AK.RC01 AK.SAW AK.SSN AT.PMR AV.STLK
GM.AD09 TA.M22K TA.O22K
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
br c 0.12 0.25 n 4 p 2
Best Fitting Double Couple
Mo = 3.51e+21 dyne-cm
Mw = 3.63
Z = 39 km
Plane Strike Dip Rake
NP1 230 75 -55
NP2 340 38 -155
Principal Axes:
Axis Value Plunge Azimuth
T 3.51e+21 22 294
N 0.00e+00 34 40
P -3.51e+21 48 177
Moment Tensor: (dyne-cm)
Component Value
Mxx -1.07e+21
Mxy -1.04e+21
Mxz 2.24e+21
Myy 2.51e+21
Myz -1.20e+21
Mzz -1.44e+21
##------------
############----------
##################----------
#####################---------
#########################----#####
##########################-#########
### ##################-----#########
#### T ###############---------#########
#### #############------------########
##################----------------########
################------------------########
##############--------------------########
############-----------------------#######
##########------------------------######
########--------------------------######
######------------ -----------######
####------------- P -----------#####
##-------------- ----------#####
--------------------------####
------------------------####
--------------------##
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Global CMT Convention Moment Tensor:
R T P
-1.44e+21 2.24e+21 1.20e+21
2.24e+21 -1.07e+21 1.04e+21
1.20e+21 1.04e+21 2.51e+21
Details of the solution is found at
http://www.eas.slu.edu/eqc/eqc_mt/MECH.NA/20190206202759/index.html
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STK = 230
DIP = 75
RAKE = -55
MW = 3.63
HS = 39.0
The NDK file is 20190206202759.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 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 1.0 320 75 15 2.90 0.1887
WVFGRD96 2.0 320 75 15 3.12 0.3323
WVFGRD96 3.0 140 75 10 3.19 0.3819
WVFGRD96 4.0 140 70 10 3.24 0.4152
WVFGRD96 5.0 320 90 40 3.31 0.4453
WVFGRD96 6.0 130 55 -25 3.34 0.4766
WVFGRD96 7.0 135 45 -10 3.35 0.4970
WVFGRD96 8.0 135 40 -10 3.40 0.5065
WVFGRD96 9.0 130 45 -20 3.41 0.5145
WVFGRD96 10.0 130 45 -25 3.42 0.5193
WVFGRD96 11.0 130 45 -25 3.43 0.5220
WVFGRD96 12.0 130 50 -25 3.43 0.5245
WVFGRD96 13.0 130 50 -25 3.44 0.5262
WVFGRD96 14.0 130 50 -25 3.45 0.5269
WVFGRD96 15.0 130 50 -25 3.46 0.5275
WVFGRD96 16.0 130 50 -25 3.46 0.5279
WVFGRD96 17.0 130 50 -25 3.47 0.5279
WVFGRD96 18.0 130 50 -25 3.48 0.5276
WVFGRD96 19.0 130 50 -25 3.49 0.5276
WVFGRD96 20.0 130 50 -25 3.50 0.5272
WVFGRD96 21.0 135 50 -10 3.50 0.5279
WVFGRD96 22.0 135 50 -10 3.51 0.5291
WVFGRD96 23.0 135 50 -10 3.52 0.5301
WVFGRD96 24.0 160 40 15 3.52 0.5321
WVFGRD96 25.0 160 40 15 3.53 0.5356
WVFGRD96 26.0 160 40 15 3.54 0.5383
WVFGRD96 27.0 160 40 15 3.55 0.5416
WVFGRD96 28.0 35 85 50 3.59 0.5479
WVFGRD96 29.0 35 85 50 3.60 0.5549
WVFGRD96 30.0 210 90 -45 3.61 0.5553
WVFGRD96 31.0 35 85 50 3.62 0.5635
WVFGRD96 32.0 35 85 50 3.62 0.5663
WVFGRD96 33.0 35 85 50 3.62 0.5671
WVFGRD96 34.0 35 85 50 3.63 0.5680
WVFGRD96 35.0 35 85 45 3.63 0.5680
WVFGRD96 36.0 235 80 -60 3.63 0.5698
WVFGRD96 37.0 230 75 -60 3.63 0.5742
WVFGRD96 38.0 230 75 -55 3.63 0.5793
WVFGRD96 39.0 230 75 -55 3.63 0.5825
WVFGRD96 40.0 220 80 -65 3.74 0.5804
WVFGRD96 41.0 220 80 -65 3.74 0.5795
WVFGRD96 42.0 215 75 -65 3.74 0.5783
WVFGRD96 43.0 215 75 -65 3.75 0.5780
WVFGRD96 44.0 215 75 -65 3.76 0.5769
WVFGRD96 45.0 215 75 -65 3.76 0.5761
WVFGRD96 46.0 215 75 -65 3.76 0.5740
WVFGRD96 47.0 210 70 -65 3.77 0.5741
WVFGRD96 48.0 210 70 -65 3.77 0.5727
WVFGRD96 49.0 210 70 -65 3.78 0.5729
WVFGRD96 50.0 210 70 -65 3.78 0.5713
WVFGRD96 51.0 210 70 -65 3.79 0.5707
WVFGRD96 52.0 210 70 -65 3.79 0.5694
WVFGRD96 53.0 210 70 -65 3.79 0.5690
WVFGRD96 54.0 210 70 -65 3.80 0.5686
WVFGRD96 55.0 210 70 -65 3.80 0.5670
WVFGRD96 56.0 210 70 -65 3.80 0.5682
WVFGRD96 57.0 210 70 -65 3.81 0.5668
WVFGRD96 58.0 210 70 -65 3.81 0.5663
WVFGRD96 59.0 210 70 -65 3.81 0.5647
The best solution is
WVFGRD96 39.0 230 75 -55 3.63 0.5825
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 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 CUS.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 CUS Model with Q from simple gamma values 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 1.0000 5.0000 2.8900 2.5000 0.172E-02 0.387E-02 0.00 0.00 1.00 1.00 9.0000 6.1000 3.5200 2.7300 0.160E-02 0.363E-02 0.00 0.00 1.00 1.00 10.0000 6.4000 3.7000 2.8200 0.149E-02 0.336E-02 0.00 0.00 1.00 1.00 20.0000 6.7000 3.8700 2.9020 0.000E-04 0.000E-04 0.00 0.00 1.00 1.00 0.0000 8.1500 4.7000 3.3640 0.194E-02 0.431E-02 0.00 0.00 1.00 1.00