The ANSS event ID is ak018bt6misc and the event page is at https://earthquake.usgs.gov/earthquakes/eventpage/ak018bt6misc/executive.
2018/09/14 09:25:32 62.331 -149.598 54.5 3.9 Alaska
USGS/SLU Moment Tensor Solution ENS 2018/09/14 09:25:32:0 62.33 -149.60 54.5 3.9 Alaska Stations used: AK.CUT AK.DHY AK.GHO AK.KNK AK.KTH AK.MCK AK.PWL AK.RC01 AK.RND AK.SAW AK.SCM AK.SKN AK.SSN AT.PMR AV.SPU 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 Best Fitting Double Couple Mo = 1.50e+22 dyne-cm Mw = 4.05 Z = 58 km Plane Strike Dip Rake NP1 14 66 -97 NP2 210 25 -75 Principal Axes: Axis Value Plunge Azimuth T 1.50e+22 21 109 N 0.00e+00 6 16 P -1.50e+22 68 270 Moment Tensor: (dyne-cm) Component Value Mxx 1.35e+21 Mxy -3.98e+21 Mxz -1.61e+21 Myy 9.72e+21 Myz 9.80e+21 Mzz -1.11e+22 ############-- #########--------##### ########-------------####### ######----------------######## ######------------------########## ######-------------------########### ######--------------------############ ######---------------------############# #####----------------------############# #####--------- -----------############## #####--------- P ----------############### ####---------- ----------############### ####-----------------------############### ###----------------------######### ### ####--------------------########## T ### ###--------------------########## ## ###------------------############### ##-----------------############### #---------------############## ##------------############## ---------############# ---########### Global CMT Convention Moment Tensor: R T P -1.11e+22 -1.61e+21 -9.80e+21 -1.61e+21 1.35e+21 3.98e+21 -9.80e+21 3.98e+21 9.72e+21 Details of the solution is found at http://www.eas.slu.edu/eqc/eqc_mt/MECH.NA/20180914092532/index.html |
STK = 210 DIP = 25 RAKE = -75 MW = 4.05 HS = 58.0
The NDK file is 20180914092532.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 2.0 45 40 90 3.27 0.2505 WVFGRD96 4.0 170 15 40 3.33 0.2906 WVFGRD96 6.0 175 15 45 3.34 0.3494 WVFGRD96 8.0 170 15 40 3.43 0.3598 WVFGRD96 10.0 155 20 30 3.45 0.3533 WVFGRD96 12.0 220 80 -70 3.47 0.3458 WVFGRD96 14.0 60 25 -60 3.52 0.3438 WVFGRD96 16.0 65 25 -55 3.54 0.3374 WVFGRD96 18.0 205 65 -70 3.57 0.3284 WVFGRD96 20.0 235 30 -60 3.61 0.3170 WVFGRD96 22.0 10 45 -95 3.63 0.3075 WVFGRD96 24.0 190 40 -90 3.65 0.2971 WVFGRD96 26.0 200 40 -80 3.66 0.2908 WVFGRD96 28.0 205 45 -70 3.67 0.2897 WVFGRD96 30.0 210 50 -65 3.69 0.2999 WVFGRD96 32.0 210 50 -65 3.71 0.3109 WVFGRD96 34.0 35 65 -65 3.72 0.3378 WVFGRD96 36.0 25 65 -75 3.75 0.3658 WVFGRD96 38.0 20 60 -85 3.78 0.3952 WVFGRD96 40.0 200 25 -95 3.91 0.4343 WVFGRD96 42.0 25 70 -85 3.94 0.4649 WVFGRD96 44.0 200 20 -95 3.97 0.4952 WVFGRD96 46.0 195 25 -95 3.98 0.5275 WVFGRD96 48.0 20 65 -90 4.00 0.5512 WVFGRD96 50.0 15 65 -95 4.02 0.5715 WVFGRD96 52.0 205 25 -80 4.03 0.5902 WVFGRD96 54.0 210 25 -75 4.04 0.5980 WVFGRD96 56.0 210 25 -75 4.05 0.6032 WVFGRD96 58.0 210 25 -75 4.05 0.6067 WVFGRD96 60.0 210 30 -75 4.06 0.6059 WVFGRD96 62.0 215 30 -70 4.06 0.6035 WVFGRD96 64.0 215 30 -70 4.06 0.6007 WVFGRD96 66.0 215 30 -70 4.06 0.5955 WVFGRD96 68.0 215 30 -70 4.06 0.5897 WVFGRD96 70.0 215 30 -70 4.07 0.5845 WVFGRD96 72.0 215 35 -65 4.08 0.5801 WVFGRD96 74.0 215 35 -65 4.08 0.5766 WVFGRD96 76.0 215 35 -65 4.08 0.5721 WVFGRD96 78.0 215 35 -65 4.08 0.5667 WVFGRD96 80.0 215 35 -65 4.08 0.5611 WVFGRD96 82.0 215 35 -65 4.08 0.5549 WVFGRD96 84.0 215 35 -65 4.08 0.5489 WVFGRD96 86.0 220 40 -60 4.09 0.5438 WVFGRD96 88.0 225 40 -55 4.09 0.5391 WVFGRD96 90.0 225 40 -55 4.09 0.5351 WVFGRD96 92.0 225 40 -55 4.09 0.5312 WVFGRD96 94.0 225 40 -55 4.09 0.5263 WVFGRD96 96.0 225 40 -55 4.09 0.5212 WVFGRD96 98.0 225 40 -55 4.09 0.5171
The best solution is
WVFGRD96 58.0 210 25 -75 4.05 0.6067
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