The ANSS event ID is ak018acfg224 and the event page is at https://earthquake.usgs.gov/earthquakes/eventpage/ak018acfg224/executive.
2018/08/13 19:36:09 69.547 -144.459 17.3 4.9 Alaska
USGS/SLU Moment Tensor Solution ENS 2018/08/13 19:36:09:0 69.55 -144.46 17.3 4.9 Alaska Stations used: AK.CCB AK.COLD AK.HDA AK.MDM AK.MLY AK.NEA2 AK.PPD AK.WRH CN.INK IU.COLA TA.B22K TA.C26K TA.D24K TA.D25K TA.D27M TA.D28M TA.E23K TA.E24K TA.E27K TA.E28M TA.E29M TA.EPYK TA.F20K TA.F24K TA.F25K TA.F26K TA.F28M TA.F31M TA.G23K TA.G24K TA.G27K TA.G29M TA.G30M TA.H21K TA.H23K TA.H24K TA.I23K TA.I28M TA.J25K TA.J26L TA.POKR TA.TOLK US.EGAK Filtering commands used: cut o DIST/3.3 -20 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 = 1.04e+23 dyne-cm Mw = 4.61 Z = 14 km Plane Strike Dip Rake NP1 99 76 154 NP2 195 65 15 Principal Axes: Axis Value Plunge Azimuth T 1.04e+23 28 55 N 0.00e+00 61 253 P -1.04e+23 8 149 Moment Tensor: (dyne-cm) Component Value Mxx -4.67e+22 Mxy 8.36e+22 Mxz 3.64e+22 Myy 2.62e+22 Myz 2.76e+22 Mzz 2.05e+22 -----------### -------------######### ---------------############# --------------################ ---------------################### ---------------############## #### ---------------############### T ##### ---------------################ ###### ---------------######################### #--------------########################### ####-----------########################### #########-----############################ ##############---######################### #############--------------------------- ############---------------------------- ###########--------------------------- ##########-------------------------- #########------------------------- ########---------------- --- #######---------------- P -- #####--------------- #------------- Global CMT Convention Moment Tensor: R T P 2.05e+22 3.64e+22 -2.76e+22 3.64e+22 -4.67e+22 -8.36e+22 -2.76e+22 -8.36e+22 2.62e+22 Details of the solution is found at http://www.eas.slu.edu/eqc/eqc_mt/MECH.NA/20180813193609/index.html |
STK = 195 DIP = 65 RAKE = 15 MW = 4.61 HS = 14.0
The NDK file is 20180813193609.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 -20 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 1.0 105 90 10 4.10 0.2582 WVFGRD96 2.0 245 55 -80 4.33 0.3242 WVFGRD96 3.0 100 60 -15 4.30 0.3454 WVFGRD96 4.0 100 60 -10 4.33 0.3653 WVFGRD96 5.0 285 75 50 4.38 0.3941 WVFGRD96 6.0 25 50 40 4.42 0.4320 WVFGRD96 7.0 195 55 20 4.43 0.4735 WVFGRD96 8.0 35 45 60 4.54 0.5211 WVFGRD96 9.0 200 55 30 4.53 0.5523 WVFGRD96 10.0 195 60 20 4.54 0.5772 WVFGRD96 11.0 195 60 20 4.56 0.5948 WVFGRD96 12.0 195 65 15 4.58 0.6060 WVFGRD96 13.0 195 65 15 4.60 0.6116 WVFGRD96 14.0 195 65 15 4.61 0.6127 WVFGRD96 15.0 190 70 10 4.62 0.6108 WVFGRD96 16.0 190 70 5 4.63 0.6072 WVFGRD96 17.0 190 70 5 4.65 0.6015 WVFGRD96 18.0 190 70 5 4.65 0.5937 WVFGRD96 19.0 190 70 5 4.66 0.5847 WVFGRD96 20.0 190 70 5 4.67 0.5746 WVFGRD96 21.0 190 70 5 4.68 0.5638 WVFGRD96 22.0 190 65 5 4.69 0.5546 WVFGRD96 23.0 190 65 5 4.69 0.5445 WVFGRD96 24.0 190 65 5 4.70 0.5338 WVFGRD96 25.0 190 65 5 4.71 0.5217 WVFGRD96 26.0 190 65 5 4.71 0.5103 WVFGRD96 27.0 190 60 5 4.72 0.4986 WVFGRD96 28.0 190 60 5 4.72 0.4874 WVFGRD96 29.0 185 55 -5 4.72 0.4773
The best solution is
WVFGRD96 14.0 195 65 15 4.61 0.6127
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 -20 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 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