The ANSS event ID is ak0152xyw58h and the event page is at https://earthquake.usgs.gov/earthquakes/eventpage/ak0152xyw58h/executive.
2015/03/05 07:55:52 62.635 -149.461 65.8 4.5 Alaska
USGS/SLU Moment Tensor Solution ENS 2015/03/05 07:55:52:0 62.63 -149.46 65.8 4.5 Alaska Stations used: AK.BPAW AK.BRLK AK.CCB AK.CNP AK.DOT AK.EYAK AK.FID AK.FYU AK.GHO AK.GLI AK.HDA AK.HIN AK.KLU AK.KNK AK.KTH AK.MDM AK.PAX AK.PIN AK.PPLA AK.RND AK.SAW AK.SCM AK.SKN AK.SSN AK.SWD AK.TGL AK.TRF AK.WRH AT.PMR II.KDAK IU.COLA TA.I23K TA.M24K TA.O22K TA.POKR Filtering commands used: cut o DIST/3.3 -30 o DIST/3.3 +70 rtr taper w 0.1 hp c 0.02 n 3 lp c 0.07 n 3 Best Fitting Double Couple Mo = 3.55e+22 dyne-cm Mw = 4.30 Z = 66 km Plane Strike Dip Rake NP1 85 80 80 NP2 310 14 135 Principal Axes: Axis Value Plunge Azimuth T 3.55e+22 54 343 N 0.00e+00 10 87 P -3.55e+22 34 184 Moment Tensor: (dyne-cm) Component Value Mxx -1.29e+22 Mxy -4.94e+21 Mxz 3.26e+22 Myy 9.63e+20 Myz -3.93e+21 Mzz 1.20e+22 -------------- --##############------ -######################----- ###########################--- ###############################--- ############## ################--- ############### T #################--- ################ ##################--- ######################################-- ########################################-- #######################################-## #################################-------## -------###########----------------------## ---------------------------------------# ---------------------------------------# -------------------------------------# ------------------------------------ --------------- ---------------- ------------- P -------------- ------------ ------------- ---------------------- -------------- Global CMT Convention Moment Tensor: R T P 1.20e+22 3.26e+22 3.93e+21 3.26e+22 -1.29e+22 4.94e+21 3.93e+21 4.94e+21 9.63e+20 Details of the solution is found at http://www.eas.slu.edu/eqc/eqc_mt/MECH.NA/20150305075552/index.html |
STK = 85 DIP = 80 RAKE = 80 MW = 4.30 HS = 66.0
The NDK file is 20150305075552.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 +70 rtr taper w 0.1 hp c 0.02 n 3 lp c 0.07 n 3The results of this grid search are as follow:
DEPTH STK DIP RAKE MW FIT WVFGRD96 2.0 80 45 -90 3.52 0.2106 WVFGRD96 4.0 80 40 -90 3.61 0.1595 WVFGRD96 6.0 265 90 65 3.58 0.1892 WVFGRD96 8.0 345 20 0 3.65 0.2137 WVFGRD96 10.0 355 25 10 3.66 0.2412 WVFGRD96 12.0 5 25 25 3.68 0.2630 WVFGRD96 14.0 5 25 25 3.70 0.2802 WVFGRD96 16.0 5 25 25 3.72 0.2937 WVFGRD96 18.0 5 25 25 3.74 0.3045 WVFGRD96 20.0 265 75 60 3.79 0.3164 WVFGRD96 22.0 265 75 60 3.82 0.3269 WVFGRD96 24.0 265 75 60 3.84 0.3354 WVFGRD96 26.0 265 75 60 3.86 0.3411 WVFGRD96 28.0 255 85 75 3.86 0.3449 WVFGRD96 30.0 255 85 70 3.88 0.3470 WVFGRD96 32.0 260 85 -60 3.91 0.3540 WVFGRD96 34.0 260 85 -65 3.93 0.3745 WVFGRD96 36.0 80 90 70 3.94 0.3941 WVFGRD96 38.0 75 90 70 3.95 0.4180 WVFGRD96 40.0 75 85 80 4.11 0.4343 WVFGRD96 42.0 255 90 -75 4.13 0.4757 WVFGRD96 44.0 75 85 80 4.15 0.5147 WVFGRD96 46.0 75 85 80 4.17 0.5481 WVFGRD96 48.0 75 85 75 4.19 0.5769 WVFGRD96 50.0 80 80 80 4.20 0.6037 WVFGRD96 52.0 80 80 80 4.22 0.6276 WVFGRD96 54.0 80 80 80 4.23 0.6498 WVFGRD96 56.0 80 80 80 4.25 0.6704 WVFGRD96 58.0 80 80 80 4.26 0.6871 WVFGRD96 60.0 80 80 80 4.27 0.6993 WVFGRD96 62.0 80 80 80 4.28 0.7088 WVFGRD96 64.0 80 80 80 4.29 0.7134 WVFGRD96 66.0 85 80 80 4.30 0.7165 WVFGRD96 68.0 85 80 80 4.31 0.7161 WVFGRD96 70.0 85 80 80 4.32 0.7144 WVFGRD96 72.0 85 80 80 4.33 0.7102 WVFGRD96 74.0 85 80 80 4.33 0.7032 WVFGRD96 76.0 85 80 80 4.34 0.6964 WVFGRD96 78.0 85 80 80 4.35 0.6881 WVFGRD96 80.0 80 85 80 4.35 0.6787 WVFGRD96 82.0 80 85 80 4.36 0.6707 WVFGRD96 84.0 80 85 80 4.37 0.6618 WVFGRD96 86.0 85 85 85 4.37 0.6532 WVFGRD96 88.0 225 5 50 4.38 0.6400 WVFGRD96 90.0 225 5 50 4.39 0.6303 WVFGRD96 92.0 215 5 40 4.39 0.6177 WVFGRD96 94.0 215 5 40 4.40 0.6057 WVFGRD96 96.0 215 5 40 4.40 0.5926 WVFGRD96 98.0 215 5 40 4.41 0.5785
The best solution is
WVFGRD96 66.0 85 80 80 4.30 0.7165
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 +70 rtr taper w 0.1 hp c 0.02 n 3 lp c 0.07 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