The ANSS event ID is ak01843dldfm and the event page is at https://earthquake.usgs.gov/earthquakes/eventpage/ak01843dldfm/executive.
2018/03/30 11:36:00 61.548 -149.943 48.1 4.1 Alaska
USGS/SLU Moment Tensor Solution ENS 2018/03/30 11:36:00:0 61.55 -149.94 48.1 4.1 Alaska Stations used: AK.CUT AK.FIRE AK.GHO AK.HDA AK.KNK AK.KTH AK.RC01 AK.SAW AK.SCM AK.SKN AK.SSN AK.TRF AK.WRH AT.PMR TA.O22K 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 0.10 n 3 Best Fitting Double Couple Mo = 1.84e+22 dyne-cm Mw = 4.11 Z = 60 km Plane Strike Dip Rake NP1 236 55 -87 NP2 50 35 -95 Principal Axes: Axis Value Plunge Azimuth T 1.84e+22 10 324 N 0.00e+00 3 54 P -1.84e+22 80 160 Moment Tensor: (dyne-cm) Component Value Mxx 1.10e+22 Mxy -8.33e+21 Mxz 5.65e+21 Myy 6.21e+21 Myz -3.02e+21 Mzz -1.72e+22 ############## #################### ## T ####################### ### ######################## #######################----------- ##################-----------------# ################--------------------## ##############-----------------------### ############-------------------------### ###########---------------------------#### #########----------------------------##### ########------------- -------------##### ######--------------- P ------------###### ####---------------- -----------###### ####----------------------------######## ##----------------------------######## #--------------------------######### ------------------------########## ------------------############ #####------################# ###################### ############## Global CMT Convention Moment Tensor: R T P -1.72e+22 5.65e+21 3.02e+21 5.65e+21 1.10e+22 8.33e+21 3.02e+21 8.33e+21 6.21e+21 Details of the solution is found at http://www.eas.slu.edu/eqc/eqc_mt/MECH.NA/20180330113600/index.html |
STK = 50 DIP = 35 RAKE = -95 MW = 4.11 HS = 60.0
The NDK file is 20180330113600.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 0.10 n 3The results of this grid search are as follow:
DEPTH STK DIP RAKE MW FIT WVFGRD96 2.0 55 45 90 3.39 0.2719 WVFGRD96 4.0 310 75 -40 3.44 0.2945 WVFGRD96 6.0 310 70 -35 3.50 0.3457 WVFGRD96 8.0 295 75 55 3.58 0.3842 WVFGRD96 10.0 290 55 35 3.59 0.4218 WVFGRD96 12.0 290 60 35 3.63 0.4389 WVFGRD96 14.0 90 65 -45 3.69 0.4384 WVFGRD96 16.0 90 65 -40 3.71 0.4343 WVFGRD96 18.0 260 55 -50 3.73 0.4309 WVFGRD96 20.0 260 60 -50 3.75 0.4323 WVFGRD96 22.0 260 60 -50 3.77 0.4340 WVFGRD96 24.0 260 60 -50 3.79 0.4327 WVFGRD96 26.0 100 85 -30 3.79 0.4321 WVFGRD96 28.0 105 75 30 3.78 0.4422 WVFGRD96 30.0 105 75 30 3.79 0.4505 WVFGRD96 32.0 90 60 -30 3.82 0.4510 WVFGRD96 34.0 240 50 -80 3.89 0.4610 WVFGRD96 36.0 235 50 -85 3.90 0.4897 WVFGRD96 38.0 240 55 -80 3.92 0.5104 WVFGRD96 40.0 240 55 -85 4.02 0.5370 WVFGRD96 42.0 240 55 -85 4.04 0.5575 WVFGRD96 44.0 240 55 -85 4.06 0.5730 WVFGRD96 46.0 240 55 -85 4.07 0.5832 WVFGRD96 48.0 50 35 -95 4.08 0.5904 WVFGRD96 50.0 240 55 -85 4.09 0.5958 WVFGRD96 52.0 240 55 -85 4.09 0.5987 WVFGRD96 54.0 50 35 -95 4.09 0.6015 WVFGRD96 56.0 240 55 -85 4.10 0.6027 WVFGRD96 58.0 50 35 -95 4.10 0.6027 WVFGRD96 60.0 50 35 -95 4.11 0.6031 WVFGRD96 62.0 50 35 -95 4.11 0.6013 WVFGRD96 64.0 235 55 -90 4.12 0.5984 WVFGRD96 66.0 235 55 -90 4.12 0.5978 WVFGRD96 68.0 55 35 -90 4.12 0.5966 WVFGRD96 70.0 235 55 -90 4.13 0.5937 WVFGRD96 72.0 55 35 -90 4.13 0.5896 WVFGRD96 74.0 55 35 -90 4.13 0.5856 WVFGRD96 76.0 235 55 -90 4.13 0.5811 WVFGRD96 78.0 55 35 -90 4.14 0.5760 WVFGRD96 80.0 235 55 -90 4.14 0.5709 WVFGRD96 82.0 235 55 -90 4.14 0.5650 WVFGRD96 84.0 60 30 -90 4.14 0.5609 WVFGRD96 86.0 60 30 -90 4.15 0.5576 WVFGRD96 88.0 60 30 -90 4.15 0.5531 WVFGRD96 90.0 60 30 -90 4.16 0.5495 WVFGRD96 92.0 240 60 -90 4.16 0.5441 WVFGRD96 94.0 90 30 -65 4.19 0.5417 WVFGRD96 96.0 90 30 -65 4.19 0.5379 WVFGRD96 98.0 90 30 -65 4.20 0.5359
The best solution is
WVFGRD96 60.0 50 35 -95 4.11 0.6031
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 0.10 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