The ANSS event ID is ak012e0gg0z0 and the event page is at https://earthquake.usgs.gov/earthquakes/eventpage/ak012e0gg0z0/executive.
2012/10/31 02:57:43 62.0497 -146.5462 42.5 4.4 Alaska
USGS/SLU Moment Tensor Solution ENS 2012/10/31 02:57:43:0 62.05 -146.55 42.5 4.4 Alaska Stations used: AK.BAL AK.BMR AK.DHY AK.DIV AK.DOT AK.EYAK AK.FID AK.GLI AK.HDA AK.HIN AK.HMT AK.KLU AK.KNK AK.MCK AK.PAX AK.PWL AK.RAG AK.RC01 AK.RIDG AK.RND AK.SAW AK.SCM AK.WAX AK.WRH XZ.BERG XZ.BGLC XZ.KHIT XZ.VRDI 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 br c 0.12 0.25 n 4 p 2 Best Fitting Double Couple Mo = 2.04e+22 dyne-cm Mw = 4.14 Z = 72 km Plane Strike Dip Rake NP1 240 75 20 NP2 145 71 164 Principal Axes: Axis Value Plunge Azimuth T 2.04e+22 25 103 N 0.00e+00 65 275 P -2.04e+22 3 12 Moment Tensor: (dyne-cm) Component Value Mxx -1.87e+22 Mxy -7.75e+21 Mxz -2.75e+21 Myy 1.52e+22 Myz 7.32e+21 Mzz 3.49e+21 ---------- P - -------------- ----- ##-------------------------- ###--------------------------- ######---------------------------- #######---------------------------## #########-------------------########## ###########-------------################ ###########---------#################### #############-----######################## ########################################## #############---################### #### ##########-------################## T #### #######----------################# ### #####--------------##################### ##-----------------################### --------------------################ ---------------------############# ---------------------######### -----------------------##### ---------------------- -------------- Global CMT Convention Moment Tensor: R T P 3.49e+21 -2.75e+21 -7.32e+21 -2.75e+21 -1.87e+22 7.75e+21 -7.32e+21 7.75e+21 1.52e+22 Details of the solution is found at http://www.eas.slu.edu/eqc/eqc_mt/MECH.NA/20121031025743/index.html |
STK = 240 DIP = 75 RAKE = 20 MW = 4.14 HS = 72.0
The NDK file is 20121031025743.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 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 55 80 -10 3.35 0.3343 WVFGRD96 4.0 60 75 5 3.45 0.4211 WVFGRD96 6.0 60 75 10 3.50 0.4490 WVFGRD96 8.0 60 75 5 3.55 0.4583 WVFGRD96 10.0 65 75 20 3.59 0.4581 WVFGRD96 12.0 60 75 -5 3.61 0.4601 WVFGRD96 14.0 60 80 15 3.64 0.4641 WVFGRD96 16.0 60 75 15 3.67 0.4668 WVFGRD96 18.0 60 75 15 3.69 0.4699 WVFGRD96 20.0 60 75 15 3.71 0.4733 WVFGRD96 22.0 60 75 15 3.74 0.4748 WVFGRD96 24.0 60 75 15 3.75 0.4754 WVFGRD96 26.0 60 75 15 3.77 0.4739 WVFGRD96 28.0 60 75 15 3.79 0.4718 WVFGRD96 30.0 60 75 15 3.80 0.4742 WVFGRD96 32.0 240 80 10 3.81 0.4772 WVFGRD96 34.0 240 80 15 3.83 0.4803 WVFGRD96 36.0 240 75 10 3.86 0.4832 WVFGRD96 38.0 240 75 10 3.89 0.4923 WVFGRD96 40.0 240 70 10 3.95 0.5065 WVFGRD96 42.0 240 70 10 3.97 0.5137 WVFGRD96 44.0 240 70 15 4.00 0.5191 WVFGRD96 46.0 240 70 15 4.01 0.5245 WVFGRD96 48.0 240 70 15 4.03 0.5290 WVFGRD96 50.0 240 70 15 4.05 0.5324 WVFGRD96 52.0 240 70 15 4.06 0.5361 WVFGRD96 54.0 240 70 15 4.07 0.5389 WVFGRD96 56.0 240 70 15 4.08 0.5404 WVFGRD96 58.0 240 70 20 4.09 0.5420 WVFGRD96 60.0 240 70 20 4.10 0.5440 WVFGRD96 62.0 240 70 20 4.11 0.5453 WVFGRD96 64.0 240 70 20 4.12 0.5455 WVFGRD96 66.0 240 70 20 4.13 0.5455 WVFGRD96 68.0 240 70 20 4.13 0.5439 WVFGRD96 70.0 240 75 20 4.13 0.5455 WVFGRD96 72.0 240 75 20 4.14 0.5455 WVFGRD96 74.0 240 75 20 4.14 0.5449 WVFGRD96 76.0 240 75 20 4.15 0.5448 WVFGRD96 78.0 240 75 20 4.15 0.5439 WVFGRD96 80.0 240 75 20 4.15 0.5431 WVFGRD96 82.0 240 75 20 4.16 0.5433 WVFGRD96 84.0 240 75 20 4.16 0.5417 WVFGRD96 86.0 240 80 15 4.16 0.5417 WVFGRD96 88.0 240 80 15 4.16 0.5418 WVFGRD96 90.0 240 80 15 4.17 0.5412 WVFGRD96 92.0 240 80 15 4.17 0.5412 WVFGRD96 94.0 240 80 15 4.17 0.5400 WVFGRD96 96.0 240 80 15 4.18 0.5398 WVFGRD96 98.0 240 80 15 4.18 0.5386
The best solution is
WVFGRD96 72.0 240 75 20 4.14 0.5455
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 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