The ANSS event ID is us10008nsl and the event page is at https://earthquake.usgs.gov/earthquakes/eventpage/us10008nsl/executive.
2017/05/03 11:20:49 59.770 -136.761 1.7 3.5 Alaska
USGS/SLU Moment Tensor Solution ENS 2017/05/03 11:20:49:0 59.77 -136.76 1.7 3.5 Alaska Stations used: AK.BAL AK.BCP CN.YUK5 CN.YUK6 TA.M29M TA.M30M TA.N30M TA.N31M TA.N32M TA.O29M TA.O30N TA.P29M TA.Q32M TA.R31K TA.R33M Filtering commands used: cut o DIST/3.3 -30 o DIST/3.3 +70 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 = 2.48e+21 dyne-cm Mw = 3.53 Z = 11 km Plane Strike Dip Rake NP1 280 75 35 NP2 180 56 162 Principal Axes: Axis Value Plunge Azimuth T 2.48e+21 35 145 N 0.00e+00 52 300 P -2.48e+21 12 46 Moment Tensor: (dyne-cm) Component Value Mxx -1.87e+19 Mxy -1.97e+21 Mxz -1.31e+21 Myy -6.93e+20 Myz 3.04e+20 Mzz 7.12e+20 #####--------- ########-------------- #########------------------- #########------------------ ###########------------------ P -- ###########------------------- --- ###########--------------------------- ############---------------------------- #####-------##-------------------------- -------------#############---------------- -------------####################--------- -------------########################----- -------------###########################-- ------------############################ ------------############################ ------------############# ########## -----------############# T ######### -----------############ ######## ----------#################### ----------################## --------############## ------######## Global CMT Convention Moment Tensor: R T P 7.12e+20 -1.31e+21 -3.04e+20 -1.31e+21 -1.87e+19 1.97e+21 -3.04e+20 1.97e+21 -6.93e+20 Details of the solution is found at http://www.eas.slu.edu/eqc/eqc_mt/MECH.NA/20170503112049/index.html |
STK = 280 DIP = 75 RAKE = 35 MW = 3.53 HS = 11.0
The NDK file is 20170503112049.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 +70 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 90 90 0 3.13 0.2644 WVFGRD96 2.0 95 90 0 3.26 0.4025 WVFGRD96 3.0 95 90 0 3.31 0.4353 WVFGRD96 4.0 95 75 25 3.38 0.4555 WVFGRD96 5.0 100 70 35 3.42 0.4755 WVFGRD96 6.0 280 80 40 3.44 0.4926 WVFGRD96 7.0 280 80 35 3.44 0.5041 WVFGRD96 8.0 285 70 45 3.51 0.5157 WVFGRD96 9.0 285 70 40 3.51 0.5212 WVFGRD96 10.0 280 75 35 3.51 0.5241 WVFGRD96 11.0 280 75 35 3.53 0.5250 WVFGRD96 12.0 280 75 30 3.53 0.5237 WVFGRD96 13.0 280 75 30 3.55 0.5215 WVFGRD96 14.0 280 75 30 3.56 0.5176 WVFGRD96 15.0 280 80 30 3.57 0.5128 WVFGRD96 16.0 280 80 30 3.58 0.5065 WVFGRD96 17.0 280 80 25 3.59 0.4990 WVFGRD96 18.0 280 80 25 3.60 0.4911 WVFGRD96 19.0 280 75 25 3.62 0.4831 WVFGRD96 20.0 280 75 25 3.63 0.4741 WVFGRD96 21.0 280 75 25 3.64 0.4640 WVFGRD96 22.0 280 75 30 3.65 0.4533 WVFGRD96 23.0 280 75 30 3.66 0.4419 WVFGRD96 24.0 280 75 30 3.67 0.4297 WVFGRD96 25.0 275 65 20 3.69 0.4170 WVFGRD96 26.0 275 70 25 3.70 0.4038 WVFGRD96 27.0 275 70 25 3.70 0.3905 WVFGRD96 28.0 275 70 25 3.71 0.3775 WVFGRD96 29.0 275 65 25 3.72 0.3636
The best solution is
WVFGRD96 11.0 280 75 35 3.53 0.5250
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 +70 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 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