The ANSS event ID is ak019ecbo7vz and the event page is at https://earthquake.usgs.gov/earthquakes/eventpage/ak019ecbo7vz/executive.
2019/11/08 20:15:26 61.300 -149.936 42.5 4 Alaska
USGS/SLU Moment Tensor Solution ENS 2019/11/08 20:15:26:0 61.30 -149.94 42.5 4.0 Alaska Stations used: AK.FIRE AK.GHO AK.GLI AK.PWL AK.RC01 AK.RND AK.SAW AK.SCM AK.SKN AK.SLK AK.SSN AT.PMR AV.ILSW AV.SPU AV.STLK TA.M22K Filtering commands used: cut o DIST/3.3 -30 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 = 1.22e+22 dyne-cm Mw = 3.99 Z = 54 km Plane Strike Dip Rake NP1 175 70 -75 NP2 317 25 -125 Principal Axes: Axis Value Plunge Azimuth T 1.22e+22 24 253 N 0.00e+00 14 350 P -1.22e+22 62 108 Moment Tensor: (dyne-cm) Component Value Mxx 5.71e+20 Mxy 3.57e+21 Mxz 2.88e+20 Myy 6.98e+21 Myz -9.06e+21 Mzz -7.55e+21 -----######### ----####--############ -#########---------######### ###########------------####### ############---------------####### #############-----------------###### ##############-------------------##### ###############--------------------##### ###############---------------------#### ################---------------------##### ################----------------------#### ################---------- ---------#### #### #########---------- P ---------#### ### T #########---------- ----------## ### ##########---------------------### ###############---------------------## ###############--------------------# ##############-------------------# #############----------------- #############--------------- ###########----------- ########------ Global CMT Convention Moment Tensor: R T P -7.55e+21 2.88e+20 9.06e+21 2.88e+20 5.71e+20 -3.57e+21 9.06e+21 -3.57e+21 6.98e+21 Details of the solution is found at http://www.eas.slu.edu/eqc/eqc_mt/MECH.NA/20191108201526/index.html |
STK = 175 DIP = 70 RAKE = -75 MW = 3.99 HS = 54.0
The NDK file is 20191108201526.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 -30 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 2.0 -5 55 85 3.27 0.2723 WVFGRD96 4.0 130 65 -20 3.24 0.2981 WVFGRD96 6.0 125 60 -25 3.34 0.3346 WVFGRD96 8.0 305 60 -35 3.43 0.3608 WVFGRD96 10.0 305 60 -35 3.48 0.3696 WVFGRD96 12.0 130 60 -25 3.50 0.3653 WVFGRD96 14.0 130 60 -30 3.53 0.3565 WVFGRD96 16.0 130 60 -30 3.55 0.3456 WVFGRD96 18.0 40 65 -40 3.57 0.3513 WVFGRD96 20.0 40 65 -35 3.60 0.3652 WVFGRD96 22.0 40 65 -40 3.62 0.3730 WVFGRD96 24.0 40 65 -35 3.64 0.3748 WVFGRD96 26.0 35 60 -40 3.67 0.3787 WVFGRD96 28.0 35 50 -30 3.69 0.3861 WVFGRD96 30.0 15 40 -40 3.76 0.4084 WVFGRD96 32.0 195 85 -70 3.76 0.4308 WVFGRD96 34.0 195 85 -70 3.78 0.4662 WVFGRD96 36.0 190 80 -70 3.79 0.4975 WVFGRD96 38.0 180 75 -70 3.80 0.5169 WVFGRD96 40.0 180 80 -80 3.93 0.5223 WVFGRD96 42.0 180 75 -75 3.94 0.5262 WVFGRD96 44.0 180 75 -75 3.95 0.5286 WVFGRD96 46.0 180 75 -75 3.95 0.5313 WVFGRD96 48.0 175 70 -75 3.97 0.5358 WVFGRD96 50.0 175 70 -75 3.98 0.5376 WVFGRD96 52.0 175 70 -75 3.99 0.5383 WVFGRD96 54.0 175 70 -75 3.99 0.5389 WVFGRD96 56.0 175 70 -75 4.00 0.5371 WVFGRD96 58.0 175 70 -75 4.00 0.5357 WVFGRD96 60.0 175 70 -75 4.00 0.5318 WVFGRD96 62.0 175 75 -80 4.00 0.5297 WVFGRD96 64.0 175 75 -80 4.00 0.5265 WVFGRD96 66.0 175 75 -80 4.00 0.5240 WVFGRD96 68.0 175 75 -80 4.01 0.5204 WVFGRD96 70.0 175 75 -80 4.01 0.5186 WVFGRD96 72.0 175 75 -80 4.01 0.5144 WVFGRD96 74.0 175 75 -80 4.02 0.5097 WVFGRD96 76.0 175 75 -80 4.02 0.5070 WVFGRD96 78.0 175 75 -80 4.02 0.5027 WVFGRD96 80.0 175 75 -80 4.02 0.4968 WVFGRD96 82.0 175 75 -75 4.03 0.4936 WVFGRD96 84.0 175 75 -75 4.03 0.4886 WVFGRD96 86.0 175 75 -75 4.03 0.4828 WVFGRD96 88.0 175 75 -75 4.04 0.4782 WVFGRD96 90.0 175 80 -75 4.04 0.4716 WVFGRD96 92.0 175 80 -75 4.04 0.4672 WVFGRD96 94.0 175 80 -70 4.05 0.4601 WVFGRD96 96.0 170 70 -80 4.06 0.4555 WVFGRD96 98.0 170 70 -80 4.06 0.4514
The best solution is
WVFGRD96 54.0 175 70 -75 3.99 0.5389
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 -30 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