The ANSS event ID is ak0179f3lhxg and the event page is at https://earthquake.usgs.gov/earthquakes/eventpage/ak0179f3lhxg/executive.
2017/07/24 04:07:56 62.105 -150.247 16.8 3.8 Alaska
USGS/SLU Moment Tensor Solution ENS 2017/07/24 04:07:56:0 62.10 -150.25 16.8 3.8 Alaska Stations used: AK.BPAW AK.BWN AK.CAST AK.CCB AK.DHY AK.DIV AK.EYAK AK.FIRE AK.GHO AK.GLI AK.HDA AK.KLU AK.KNK AK.KTH AK.MCK AK.MDM AK.MLY AK.NEA2 AK.PAX AK.PPLA AK.PWL AK.RC01 AK.RND AK.SAW AK.SCM AK.SCRK AK.SSN AK.SWD AK.TRF AK.WAX AK.WRH AT.PMR AT.SVW2 AT.TTA IM.IL31 IU.COLA TA.H21K TA.I21K TA.I23K TA.J20K TA.J25K TA.K20K TA.L19K TA.L26K TA.M19K TA.M20K TA.M22K TA.M24K TA.N19K TA.N25K TA.O18K TA.O22K TA.P19K TA.POKR TA.TCOL Filtering commands used: cut o DIST/3.3 -30 o DIST/3.3 +50 rtr taper w 0.1 hp c 0.03 n 3 lp c 0.08 n 3 Best Fitting Double Couple Mo = 6.10e+21 dyne-cm Mw = 3.79 Z = 22 km Plane Strike Dip Rake NP1 5 50 95 NP2 177 40 84 Principal Axes: Axis Value Plunge Azimuth T 6.10e+21 84 310 N 0.00e+00 4 182 P -6.10e+21 5 91 Moment Tensor: (dyne-cm) Component Value Mxx 2.52e+19 Mxy 1.18e+20 Mxz 4.32e+20 Myy -6.01e+21 Myz -1.02e+21 Mzz 5.98e+21 ----######---- -----###########------ -------#############-------- ------################-------- -------##################--------- -------####################--------- --------####################---------- --------######################---------- --------######################---------- ---------######### ##########----------- ---------######### T ##########-------- ---------######### ##########-------- P ---------######################-------- --------#####################----------- ---------####################----------- --------###################----------- --------##################---------- --------################---------- -------##############--------- --------##########---------- -------#######-------- -----##------- Global CMT Convention Moment Tensor: R T P 5.98e+21 4.32e+20 1.02e+21 4.32e+20 2.52e+19 -1.18e+20 1.02e+21 -1.18e+20 -6.01e+21 Details of the solution is found at http://www.eas.slu.edu/eqc/eqc_mt/MECH.NA/20170724040756/index.html |
STK = 5 DIP = 50 RAKE = 95 MW = 3.79 HS = 22.0
The NDK file is 20170724040756.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 +50 rtr taper w 0.1 hp c 0.03 n 3 lp c 0.08 n 3The results of this grid search are as follow:
DEPTH STK DIP RAKE MW FIT WVFGRD96 1.0 200 45 -90 3.32 0.3207 WVFGRD96 2.0 15 45 -90 3.45 0.4086 WVFGRD96 3.0 200 45 -95 3.48 0.3098 WVFGRD96 4.0 285 30 -30 3.44 0.2322 WVFGRD96 5.0 290 25 -20 3.44 0.2724 WVFGRD96 6.0 210 10 -40 3.50 0.3185 WVFGRD96 7.0 205 10 -45 3.51 0.3564 WVFGRD96 8.0 200 10 -50 3.60 0.3848 WVFGRD96 9.0 200 10 -50 3.61 0.4218 WVFGRD96 10.0 205 10 -45 3.62 0.4517 WVFGRD96 11.0 210 10 -40 3.63 0.4759 WVFGRD96 12.0 155 30 70 3.65 0.5087 WVFGRD96 13.0 155 35 70 3.68 0.5535 WVFGRD96 14.0 170 35 75 3.69 0.5943 WVFGRD96 15.0 170 40 75 3.71 0.6312 WVFGRD96 16.0 170 40 75 3.72 0.6617 WVFGRD96 17.0 175 40 80 3.73 0.6863 WVFGRD96 18.0 175 40 80 3.74 0.7057 WVFGRD96 19.0 175 40 80 3.75 0.7202 WVFGRD96 20.0 180 40 85 3.76 0.7303 WVFGRD96 21.0 180 40 85 3.78 0.7343 WVFGRD96 22.0 5 50 95 3.79 0.7376 WVFGRD96 23.0 5 45 95 3.80 0.7368 WVFGRD96 24.0 5 45 95 3.81 0.7339 WVFGRD96 25.0 175 45 85 3.82 0.7283 WVFGRD96 26.0 180 45 90 3.82 0.7203 WVFGRD96 27.0 0 45 90 3.83 0.7102 WVFGRD96 28.0 0 45 90 3.83 0.6975 WVFGRD96 29.0 0 45 85 3.84 0.6826
The best solution is
WVFGRD96 22.0 5 50 95 3.79 0.7376
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 +50 rtr taper w 0.1 hp c 0.03 n 3 lp c 0.08 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