The ANSS event ID is ak0249lwd5zr and the event page is at https://earthquake.usgs.gov/earthquakes/eventpage/ak0249lwd5zr/executive.
2024/07/27 15:19:27 67.895 -160.842 5.5 3.7 Alaska
USGS/SLU Moment Tensor Solution ENS 2024/07/27 15:19:27:0 67.89 -160.84 5.5 3.7 Alaska Stations used: AK.ANM AK.E18K AK.F20K AK.F21K AK.G19K AK.H16K AK.H17K AK.H21K AK.RDOG AK.TNA 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 Best Fitting Double Couple Mo = 2.48e+21 dyne-cm Mw = 3.53 Z = 12 km Plane Strike Dip Rake NP1 150 60 -55 NP2 276 45 -135 Principal Axes: Axis Value Plunge Azimuth T 2.48e+21 9 216 N 0.00e+00 30 311 P -2.48e+21 59 111 Moment Tensor: (dyne-cm) Component Value Mxx 1.51e+21 Mxy 1.38e+21 Mxz 1.08e+20 Myy 2.53e+20 Myz -1.24e+21 Mzz -1.76e+21 ############## ###################### ---######################### ----########################## ------############################ -------#----------------############ -----###----------------------######## ---######-------------------------###### -#########--------------------------#### -##########----------------------------### ############----------------------------## #############-------------- -----------# ##############------------- P ------------ ##############------------ ----------- ###############------------------------- ###############----------------------- ################-------------------- #################----------------- ## ############------------- # T ###############--------- ###################-- ############## Global CMT Convention Moment Tensor: R T P -1.76e+21 1.08e+20 1.24e+21 1.08e+20 1.51e+21 -1.38e+21 1.24e+21 -1.38e+21 2.53e+20 Details of the solution is found at http://www.eas.slu.edu/eqc/eqc_mt/MECH.NA/20240727151927/index.html |
STK = 150 DIP = 60 RAKE = -55 MW = 3.53 HS = 12.0
The NDK file is 20240727151927.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 3The results of this grid search are as follow:
DEPTH STK DIP RAKE MW FIT WVFGRD96 1.0 320 80 5 3.07 0.3299 WVFGRD96 2.0 205 45 80 3.21 0.4063 WVFGRD96 3.0 345 45 10 3.23 0.3885 WVFGRD96 4.0 160 65 -45 3.26 0.4426 WVFGRD96 5.0 155 60 -50 3.32 0.5261 WVFGRD96 6.0 155 60 -50 3.35 0.5960 WVFGRD96 7.0 155 60 -50 3.38 0.6506 WVFGRD96 8.0 150 60 -55 3.46 0.6918 WVFGRD96 9.0 150 60 -60 3.49 0.7284 WVFGRD96 10.0 150 60 -50 3.50 0.7518 WVFGRD96 11.0 150 60 -55 3.52 0.7644 WVFGRD96 12.0 150 60 -55 3.53 0.7685 WVFGRD96 13.0 155 60 -50 3.54 0.7677 WVFGRD96 14.0 155 65 -45 3.55 0.7624 WVFGRD96 15.0 155 65 -45 3.57 0.7540 WVFGRD96 16.0 155 65 -45 3.58 0.7430 WVFGRD96 17.0 155 65 -45 3.59 0.7305 WVFGRD96 18.0 160 65 -45 3.59 0.7167 WVFGRD96 19.0 160 65 -45 3.60 0.7013 WVFGRD96 20.0 160 65 -45 3.61 0.6867 WVFGRD96 21.0 155 65 -50 3.64 0.6751 WVFGRD96 22.0 -5 65 30 3.61 0.6616 WVFGRD96 23.0 -5 65 30 3.62 0.6521 WVFGRD96 24.0 -5 60 30 3.62 0.6411 WVFGRD96 25.0 -5 60 30 3.63 0.6326 WVFGRD96 26.0 -5 60 30 3.63 0.6257 WVFGRD96 27.0 355 60 30 3.64 0.6190 WVFGRD96 28.0 355 60 30 3.65 0.6119 WVFGRD96 29.0 355 60 30 3.65 0.6058
The best solution is
WVFGRD96 12.0 150 60 -55 3.53 0.7685
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
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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