The ANSS event ID is ak024am4pt2a and the event page is at https://earthquake.usgs.gov/earthquakes/eventpage/ak024am4pt2a/executive.
2024/08/18 06:00:17 60.030 -143.176 26.4 3.7 Alaska
USGS/SLU Moment Tensor Solution ENS 2024/08/18 06:00:17:0 60.03 -143.18 26.4 3.7 Alaska Stations used: AK.BARN AK.BERG AK.BMR AK.DHY AK.DOT AK.GLB AK.GLI AK.HIN AK.ISLE AK.KAI AK.KLU AK.KNK AK.LOGN AK.MCAR AK.MESA AK.PAX AK.PIN AK.PWL AK.SAMH AK.SAW AK.SCM AK.SUCK AK.TGL AK.VRDI AK.WAX AT.MENT CN.HYT 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 0.08 n 3 br c 0.12 0.25 n 4 p 2 Best Fitting Double Couple Mo = 6.10e+21 dyne-cm Mw = 3.79 Z = 25 km Plane Strike Dip Rake NP1 353 80 -165 NP2 260 75 -10 Principal Axes: Axis Value Plunge Azimuth T 6.10e+21 4 126 N 0.00e+00 72 24 P -6.10e+21 18 217 Moment Tensor: (dyne-cm) Component Value Mxx -1.47e+21 Mxy -5.54e+21 Mxz 1.17e+21 Myy 2.00e+21 Myz 1.37e+21 Mzz -5.29e+20 #####--------- ##########------------ #############--------------- ###############--------------- #################----------------- ###################----------------- ####################------------------ #####################------------------- ######################----#########----- ################-------################### #########--------------################### #####-------------------################## ##----------------------################## -----------------------################# -----------------------################# ----------------------################ ---------------------############ ----- ------------############ T --- P ------------############ -- ------------########### --------------######## ----------#### Global CMT Convention Moment Tensor: R T P -5.29e+20 1.17e+21 -1.37e+21 1.17e+21 -1.47e+21 5.54e+21 -1.37e+21 5.54e+21 2.00e+21 Details of the solution is found at http://www.eas.slu.edu/eqc/eqc_mt/MECH.NA/20240818060017/index.html |
STK = 260 DIP = 75 RAKE = -10 MW = 3.79 HS = 25.0
The NDK file is 20240818060017.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 0.08 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 1.0 255 85 0 3.26 0.1882 WVFGRD96 2.0 260 80 -5 3.40 0.2737 WVFGRD96 3.0 255 65 -15 3.47 0.3004 WVFGRD96 4.0 255 65 -20 3.51 0.3191 WVFGRD96 5.0 250 60 -40 3.58 0.3364 WVFGRD96 6.0 250 60 -40 3.59 0.3535 WVFGRD96 7.0 255 65 -30 3.59 0.3629 WVFGRD96 8.0 250 60 -40 3.64 0.3770 WVFGRD96 9.0 255 65 -30 3.63 0.3844 WVFGRD96 10.0 255 65 -25 3.64 0.3913 WVFGRD96 11.0 260 70 -20 3.64 0.3982 WVFGRD96 12.0 260 70 -20 3.65 0.4058 WVFGRD96 13.0 260 75 -15 3.66 0.4130 WVFGRD96 14.0 260 75 -15 3.67 0.4200 WVFGRD96 15.0 260 75 -15 3.69 0.4257 WVFGRD96 16.0 260 75 -15 3.70 0.4310 WVFGRD96 17.0 260 75 -10 3.71 0.4356 WVFGRD96 18.0 260 75 -10 3.72 0.4399 WVFGRD96 19.0 260 75 -10 3.73 0.4437 WVFGRD96 20.0 260 75 -10 3.74 0.4467 WVFGRD96 21.0 260 75 -10 3.75 0.4496 WVFGRD96 22.0 260 75 -10 3.76 0.4514 WVFGRD96 23.0 260 75 -10 3.77 0.4526 WVFGRD96 24.0 260 75 -10 3.78 0.4529 WVFGRD96 25.0 260 75 -10 3.79 0.4529 WVFGRD96 26.0 260 75 -5 3.79 0.4518 WVFGRD96 27.0 260 75 -5 3.80 0.4511 WVFGRD96 28.0 260 75 10 3.80 0.4500 WVFGRD96 29.0 260 75 10 3.81 0.4494 WVFGRD96 30.0 260 75 10 3.82 0.4481 WVFGRD96 31.0 260 75 10 3.83 0.4465 WVFGRD96 32.0 260 75 10 3.84 0.4443 WVFGRD96 33.0 260 70 10 3.85 0.4418 WVFGRD96 34.0 260 70 10 3.86 0.4387 WVFGRD96 35.0 260 70 10 3.87 0.4347 WVFGRD96 36.0 260 70 10 3.88 0.4301 WVFGRD96 37.0 260 70 5 3.89 0.4250 WVFGRD96 38.0 260 70 5 3.90 0.4206 WVFGRD96 39.0 260 70 5 3.91 0.4170 WVFGRD96 40.0 260 65 5 3.96 0.4153 WVFGRD96 41.0 260 65 -5 3.97 0.4156 WVFGRD96 42.0 260 65 -5 3.98 0.4147 WVFGRD96 43.0 260 65 -5 3.99 0.4135 WVFGRD96 44.0 260 65 -5 4.00 0.4115 WVFGRD96 45.0 260 65 -5 4.00 0.4099 WVFGRD96 46.0 260 65 -5 4.01 0.4075 WVFGRD96 47.0 260 65 -5 4.02 0.4048 WVFGRD96 48.0 260 65 -5 4.02 0.4019 WVFGRD96 49.0 260 65 -5 4.03 0.3994 WVFGRD96 50.0 260 65 -5 4.03 0.3964 WVFGRD96 51.0 260 70 -5 4.04 0.3930 WVFGRD96 52.0 260 70 -5 4.04 0.3902 WVFGRD96 53.0 260 70 -5 4.05 0.3871 WVFGRD96 54.0 260 70 -5 4.05 0.3840 WVFGRD96 55.0 260 65 -5 4.06 0.3814 WVFGRD96 56.0 260 65 -5 4.06 0.3791 WVFGRD96 57.0 260 65 0 4.06 0.3763 WVFGRD96 58.0 260 70 10 4.06 0.3745 WVFGRD96 59.0 260 70 10 4.07 0.3724
The best solution is
WVFGRD96 25.0 260 75 -10 3.79 0.4529
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 0.08 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