The ANSS event ID is ak0094gr40v3 and the event page is at https://earthquake.usgs.gov/earthquakes/eventpage/ak0094gr40v3/executive.
2009/04/07 20:12:55 61.454 -149.743 33.0 4.6 Alaska
USGS/SLU Moment Tensor Solution ENS 2009/04/07 20:12:55:0 61.45 -149.74 33.0 4.6 Alaska Stations used: AK.BMR AK.DIV AK.EYAK AK.PAX AK.SAW AK.SWD AK.TRF AT.PMR Filtering commands used: hp c 0.02 n 3 lp c 0.05 n 3 Best Fitting Double Couple Mo = 8.41e+22 dyne-cm Mw = 4.55 Z = 49 km Plane Strike Dip Rake NP1 216 55 -87 NP2 30 35 -95 Principal Axes: Axis Value Plunge Azimuth T 8.41e+22 10 304 N 0.00e+00 3 34 P -8.41e+22 80 140 Moment Tensor: (dyne-cm) Component Value Mxx 2.33e+22 Mxy -3.62e+22 Mxz 1.95e+22 Myy 5.54e+22 Myz -2.18e+22 Mzz -7.88e+22 ############## ###################### ####################------## #################-----------## ##############--------------### # T ###########-----------------#### ## #########--------------------#### ##############---------------------##### ############-----------------------##### ############------------------------###### ###########-------------------------###### ##########------------ ----------####### ##########------------ P ----------####### ########------------- ---------####### #######-------------------------######## ######------------------------######## #####----------------------######### ####---------------------######### ##-------------------######### #----------------########### ---------############# ############## Global CMT Convention Moment Tensor: R T P -7.88e+22 1.95e+22 2.18e+22 1.95e+22 2.33e+22 3.62e+22 2.18e+22 3.62e+22 5.54e+22 Details of the solution is found at http://www.eas.slu.edu/eqc/eqc_mt/MECH.NA/20090407201255/index.html |
STK = 30 DIP = 35 RAKE = -95 MW = 4.55 HS = 49.0
The NDK file is 20090407201255.ndk The waveform inversion is preferred.
The following compares this source inversion to those provided by others. The purpose is to look for major differences and also to note slight differences that might be inherent to the processing procedure. For completeness the USGS/SLU solution is repeated from above.
USGS/SLU Moment Tensor Solution ENS 2009/04/07 20:12:55:0 61.45 -149.74 33.0 4.6 Alaska Stations used: AK.BMR AK.DIV AK.EYAK AK.PAX AK.SAW AK.SWD AK.TRF AT.PMR Filtering commands used: hp c 0.02 n 3 lp c 0.05 n 3 Best Fitting Double Couple Mo = 8.41e+22 dyne-cm Mw = 4.55 Z = 49 km Plane Strike Dip Rake NP1 216 55 -87 NP2 30 35 -95 Principal Axes: Axis Value Plunge Azimuth T 8.41e+22 10 304 N 0.00e+00 3 34 P -8.41e+22 80 140 Moment Tensor: (dyne-cm) Component Value Mxx 2.33e+22 Mxy -3.62e+22 Mxz 1.95e+22 Myy 5.54e+22 Myz -2.18e+22 Mzz -7.88e+22 ############## ###################### ####################------## #################-----------## ##############--------------### # T ###########-----------------#### ## #########--------------------#### ##############---------------------##### ############-----------------------##### ############------------------------###### ###########-------------------------###### ##########------------ ----------####### ##########------------ P ----------####### ########------------- ---------####### #######-------------------------######## ######------------------------######## #####----------------------######### ####---------------------######### ##-------------------######### #----------------########### ---------############# ############## Global CMT Convention Moment Tensor: R T P -7.88e+22 1.95e+22 2.18e+22 1.95e+22 2.33e+22 3.62e+22 2.18e+22 3.62e+22 5.54e+22 Details of the solution is found at http://www.eas.slu.edu/eqc/eqc_mt/MECH.NA/20090407201255/index.html |
Date: 2009/04/07 Time: 20:12 (UTC) Region: Cook Inlet Region of Alaska Mw=4.6 Location: Lat. 61.4686; Lon. -149.7585; Depth 45 km (Best-fitting depth from moment tensor inversion) Solution quality: good; Number of stations = 8 Best Double Couple: strike dip rake Plane 1: 7.8 48.2 -106.5 Plane 2: 211.7 44.3 -72.4 Moment Tensor Parameters: Mo = 8.83406e+22 dyn-cm Mxx = 0.51; Mxy = -3.01; Mxz = 1.47 Myy = 7.83; Myz = 1.15; Mzz = -8.34 Principal Axes: value azimuth plunge T: 8.93 109.35 1.98 N: -0.18 18.92 12.19 P: -8.74 208.44 77.64 |
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:
hp c 0.02 n 3 lp c 0.05 n 3The results of this grid search are as follow:
DEPTH STK DIP RAKE MW FIT WVFGRD96 30.0 235 55 -65 4.27 0.6456 WVFGRD96 31.0 235 55 -70 4.28 0.6538 WVFGRD96 32.0 230 55 -70 4.29 0.6635 WVFGRD96 33.0 230 55 -70 4.30 0.6723 WVFGRD96 34.0 230 55 -70 4.31 0.6799 WVFGRD96 35.0 230 55 -75 4.32 0.6874 WVFGRD96 36.0 230 55 -75 4.33 0.6936 WVFGRD96 37.0 225 55 -75 4.35 0.7003 WVFGRD96 38.0 225 55 -75 4.36 0.7064 WVFGRD96 39.0 225 55 -75 4.37 0.7112 WVFGRD96 40.0 220 50 -80 4.47 0.7049 WVFGRD96 41.0 215 50 -85 4.49 0.7136 WVFGRD96 42.0 215 50 -85 4.50 0.7223 WVFGRD96 43.0 215 50 -85 4.50 0.7283 WVFGRD96 44.0 30 40 -95 4.51 0.7329 WVFGRD96 45.0 215 50 -85 4.52 0.7362 WVFGRD96 46.0 25 40 -95 4.52 0.7388 WVFGRD96 47.0 35 35 -95 4.54 0.7414 WVFGRD96 48.0 35 35 -95 4.55 0.7424 WVFGRD96 49.0 30 35 -95 4.55 0.7440 WVFGRD96 50.0 215 55 -85 4.55 0.7432 WVFGRD96 51.0 215 55 -90 4.57 0.7432 WVFGRD96 52.0 215 55 -90 4.57 0.7418 WVFGRD96 53.0 215 55 -90 4.57 0.7390 WVFGRD96 54.0 35 35 -90 4.58 0.7359 WVFGRD96 55.0 35 35 -90 4.58 0.7308 WVFGRD96 56.0 40 35 -85 4.59 0.7258 WVFGRD96 57.0 215 55 -90 4.59 0.7193 WVFGRD96 58.0 40 35 -85 4.60 0.7130 WVFGRD96 59.0 215 55 -95 4.60 0.7044
The best solution is
WVFGRD96 49.0 30 35 -95 4.55 0.7440
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
hp c 0.02 n 3 lp c 0.05 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