The ANSS event ID is ak014eubm38t and the event page is at https://earthquake.usgs.gov/earthquakes/eventpage/ak014eubm38t/executive.
2014/11/19 08:44:44 61.771 -149.139 34.7 3.7 Alaska
USGS/SLU Moment Tensor Solution ENS 2014/11/19 08:44:44:0 61.77 -149.14 34.7 3.7 Alaska Stations used: AK.BPAW AK.CAST AK.EYAK AK.GLI AK.KTH AK.PPD AK.RND AK.SAW AK.SCM AK.SSN AK.SWD AK.TRF AT.PMR IM.IL31 IU.COLA TA.POKR 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 0.07 n 3 br c 0.12 0.25 n 4 p 2 Best Fitting Double Couple Mo = 3.89e+21 dyne-cm Mw = 3.66 Z = 38 km Plane Strike Dip Rake NP1 270 80 -50 NP2 12 41 -165 Principal Axes: Axis Value Plunge Azimuth T 3.89e+21 24 330 N 0.00e+00 39 82 P -3.89e+21 41 217 Moment Tensor: (dyne-cm) Component Value Mxx 1.02e+21 Mxy -2.46e+21 Mxz 2.80e+21 Myy -5.87e+13 Myz 4.34e+20 Mzz -1.02e+21 ############-- ##################---- ##### ###############----- ###### T ################----- ######## #################------ #############################------- ###############################------- ################################-------- ################################-------- #################################--------- ###------------------------------######--- ---------------------------------######### ---------------------------------######### -------------------------------######### -------------------------------######### ---------- ----------------######### --------- P ---------------######### -------- --------------######### ---------------------######### -------------------######### -------------######### ------######## Global CMT Convention Moment Tensor: R T P -1.02e+21 2.80e+21 -4.34e+20 2.80e+21 1.02e+21 2.46e+21 -4.34e+20 2.46e+21 -5.87e+13 Details of the solution is found at http://www.eas.slu.edu/eqc/eqc_mt/MECH.NA/20141119084444/index.html |
STK = 270 DIP = 80 RAKE = -50 MW = 3.66 HS = 38.0
The NDK file is 20141119084444.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 0.07 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 2.0 230 45 -85 3.28 0.3485 WVFGRD96 4.0 240 75 -35 3.37 0.3937 WVFGRD96 6.0 235 65 -30 3.45 0.4350 WVFGRD96 8.0 85 70 55 3.47 0.4562 WVFGRD96 10.0 85 75 50 3.45 0.4702 WVFGRD96 12.0 90 70 50 3.46 0.4945 WVFGRD96 14.0 90 70 45 3.47 0.5144 WVFGRD96 16.0 85 70 45 3.50 0.5315 WVFGRD96 18.0 80 75 40 3.52 0.5450 WVFGRD96 20.0 80 75 40 3.53 0.5549 WVFGRD96 22.0 80 75 40 3.55 0.5643 WVFGRD96 24.0 270 70 -40 3.57 0.5805 WVFGRD96 26.0 270 75 -40 3.59 0.6004 WVFGRD96 28.0 270 75 -45 3.61 0.6179 WVFGRD96 30.0 270 80 -50 3.61 0.6289 WVFGRD96 32.0 270 80 -50 3.63 0.6404 WVFGRD96 34.0 270 80 -50 3.64 0.6487 WVFGRD96 36.0 270 80 -50 3.65 0.6539 WVFGRD96 38.0 270 80 -50 3.66 0.6568 WVFGRD96 40.0 270 85 -65 3.77 0.6514 WVFGRD96 42.0 275 85 -65 3.79 0.6515 WVFGRD96 44.0 95 90 65 3.80 0.6485 WVFGRD96 46.0 95 90 65 3.81 0.6456 WVFGRD96 48.0 275 90 -65 3.82 0.6410 WVFGRD96 50.0 95 90 65 3.83 0.6348 WVFGRD96 52.0 95 90 70 3.84 0.6275 WVFGRD96 54.0 100 85 70 3.84 0.6200 WVFGRD96 56.0 100 85 70 3.85 0.6133 WVFGRD96 58.0 100 85 70 3.86 0.6054 WVFGRD96 60.0 100 85 70 3.86 0.5979 WVFGRD96 62.0 100 85 75 3.87 0.5896 WVFGRD96 64.0 100 80 75 3.87 0.5834 WVFGRD96 66.0 100 80 75 3.88 0.5772 WVFGRD96 68.0 105 80 80 3.89 0.5700 WVFGRD96 70.0 105 80 85 3.90 0.5639 WVFGRD96 72.0 105 80 85 3.90 0.5587 WVFGRD96 74.0 105 80 85 3.91 0.5518 WVFGRD96 76.0 105 80 85 3.91 0.5456 WVFGRD96 78.0 105 80 85 3.91 0.5393 WVFGRD96 80.0 105 80 85 3.91 0.5329 WVFGRD96 82.0 105 75 90 3.93 0.5285 WVFGRD96 84.0 285 15 90 3.94 0.5230 WVFGRD96 86.0 280 15 85 3.94 0.5174 WVFGRD96 88.0 280 15 85 3.94 0.5088 WVFGRD96 90.0 280 20 85 3.96 0.5008 WVFGRD96 92.0 105 70 90 3.96 0.4910 WVFGRD96 94.0 280 20 85 3.96 0.4793 WVFGRD96 96.0 105 70 90 3.96 0.4643 WVFGRD96 98.0 110 70 90 3.95 0.4383
The best solution is
WVFGRD96 38.0 270 80 -50 3.66 0.6568
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 0.07 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