The ANSS event ID is ak023f99alme and the event page is at https://earthquake.usgs.gov/earthquakes/eventpage/ak023f99alme/executive.
2023/11/28 11:54:17 61.368 -146.769 31.8 3.5 Alaska
USGS/SLU Moment Tensor Solution ENS 2023/11/28 11:54:17:0 61.37 -146.77 31.8 3.5 Alaska Stations used: AK.BAE AK.DHY AK.DIV AK.EYAK AK.FID AK.GLB AK.GLI AK.HIN AK.KLU AK.KNK AK.RC01 AK.SCM AK.WAT6 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 0.07 n 3 br c 0.12 0.25 n 4 p 2 Best Fitting Double Couple Mo = 4.32e+21 dyne-cm Mw = 3.69 Z = 48 km Plane Strike Dip Rake NP1 27 51 -98 NP2 220 40 -80 Principal Axes: Axis Value Plunge Azimuth T 4.32e+21 5 123 N 0.00e+00 6 32 P -4.32e+21 82 253 Moment Tensor: (dyne-cm) Component Value Mxx 1.25e+21 Mxy -1.98e+21 Mxz -3.46e+19 Myy 2.93e+21 Myz 9.34e+20 Mzz -4.19e+21 ############## #####################- ################---------### #############-------------#### ############-----------------##### ###########-------------------###### ##########---------------------####### #########-----------------------######## ########------------------------######## ########------------------------########## #######---------- ------------########## #######---------- P -----------########### ######----------- -----------########### #####------------------------########### #####-----------------------############ ####---------------------######### # ###--------------------########## T ##------------------############ #---------------############## #------------############### ------################ ############## Global CMT Convention Moment Tensor: R T P -4.19e+21 -3.46e+19 -9.34e+20 -3.46e+19 1.25e+21 1.98e+21 -9.34e+20 1.98e+21 2.93e+21 Details of the solution is found at http://www.eas.slu.edu/eqc/eqc_mt/MECH.NA/20231128115417/index.html |
STK = 220 DIP = 40 RAKE = -80 MW = 3.69 HS = 48.0
The NDK file is 20231128115417.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 0.07 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 35 45 -85 3.11 0.3886 WVFGRD96 4.0 35 45 -85 3.20 0.4153 WVFGRD96 6.0 45 40 -70 3.24 0.4122 WVFGRD96 8.0 285 25 70 3.31 0.4169 WVFGRD96 10.0 270 40 35 3.24 0.4320 WVFGRD96 12.0 270 45 35 3.25 0.4515 WVFGRD96 14.0 270 45 35 3.27 0.4645 WVFGRD96 16.0 270 45 35 3.29 0.4718 WVFGRD96 18.0 265 50 30 3.30 0.4762 WVFGRD96 20.0 230 65 -70 3.36 0.4811 WVFGRD96 22.0 230 65 -70 3.38 0.4948 WVFGRD96 24.0 230 65 -70 3.40 0.5060 WVFGRD96 26.0 235 60 -65 3.41 0.5167 WVFGRD96 28.0 235 55 -65 3.42 0.5272 WVFGRD96 30.0 230 50 -65 3.44 0.5391 WVFGRD96 32.0 230 50 -65 3.46 0.5519 WVFGRD96 34.0 225 45 -75 3.49 0.5637 WVFGRD96 36.0 225 45 -75 3.51 0.5734 WVFGRD96 38.0 225 45 -75 3.53 0.5786 WVFGRD96 40.0 220 40 -80 3.62 0.5794 WVFGRD96 42.0 220 40 -80 3.64 0.5883 WVFGRD96 44.0 220 40 -80 3.66 0.5948 WVFGRD96 46.0 220 40 -80 3.67 0.5987 WVFGRD96 48.0 220 40 -80 3.69 0.6011 WVFGRD96 50.0 220 40 -80 3.70 0.6007 WVFGRD96 52.0 220 40 -80 3.71 0.5987 WVFGRD96 54.0 220 40 -80 3.72 0.5957 WVFGRD96 56.0 220 40 -80 3.72 0.5903 WVFGRD96 58.0 225 40 -75 3.73 0.5843 WVFGRD96 60.0 225 40 -75 3.74 0.5774 WVFGRD96 62.0 225 40 -75 3.75 0.5698 WVFGRD96 64.0 225 40 -75 3.75 0.5610 WVFGRD96 66.0 225 40 -75 3.76 0.5516 WVFGRD96 68.0 220 40 -80 3.76 0.5416 WVFGRD96 70.0 220 40 -80 3.76 0.5346 WVFGRD96 72.0 225 45 -75 3.76 0.5275 WVFGRD96 74.0 225 45 -75 3.76 0.5212 WVFGRD96 76.0 225 45 -80 3.77 0.5150 WVFGRD96 78.0 225 45 -80 3.77 0.5093 WVFGRD96 80.0 225 45 -80 3.78 0.5035 WVFGRD96 82.0 220 45 -85 3.78 0.4981 WVFGRD96 84.0 220 45 -85 3.78 0.4947 WVFGRD96 86.0 35 45 -95 3.79 0.4902 WVFGRD96 88.0 35 45 -95 3.79 0.4869 WVFGRD96 90.0 35 45 -95 3.80 0.4835 WVFGRD96 92.0 220 45 -85 3.80 0.4806 WVFGRD96 94.0 220 45 -85 3.81 0.4772 WVFGRD96 96.0 220 45 -85 3.81 0.4745 WVFGRD96 98.0 220 45 -85 3.82 0.4722
The best solution is
WVFGRD96 48.0 220 40 -80 3.69 0.6011
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 0.07 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