The ANSS event ID is usp000j0xn and the event page is at https://earthquake.usgs.gov/earthquakes/eventpage/usp000j0xn/executive.
2011/05/03 02:58:30 30.665 -105.734 10.0 3.8 Chihuahua, MX
USGS/SLU Moment Tensor Solution ENS 2011/05/03 02:58:30:0 30.67 -105.73 10.0 3.8 Chihuahua, MX Stations used: AR.113A AR.319A AR.U15A AR.X16A AR.X18A AR.Y14A IM.TX31 IU.ANMO IU.TUC MX.HPIG MX.HSIG MX.LNIG TA.133A TA.134A TA.214A TA.233A TA.236A TA.333A TA.334A TA.433A TA.434A TA.435B TA.436A TA.533A TA.534A TA.535A TA.633A TA.634A TA.733A TA.735A TA.832A TA.833A TA.933A TA.ABTX TA.MSTX TA.T25A TA.U32A TA.V32A TA.W18A TA.W32A TA.W33A TA.WHTX TA.X32A TA.X33A TA.X34A TA.Y12C TA.Y22D TA.Y33A TA.Y34A TA.Z33A TA.Z34A TA.Z35A US.JCT US.WMOK US.WUAZ Filtering commands used: hp c 0.02 n 3 lp c 0.06 n 3 Best Fitting Double Couple Mo = 7.00e+21 dyne-cm Mw = 3.83 Z = 13 km Plane Strike Dip Rake NP1 5 60 -65 NP2 142 38 -126 Principal Axes: Axis Value Plunge Azimuth T 7.00e+21 12 77 N 0.00e+00 21 172 P -7.00e+21 65 321 Moment Tensor: (dyne-cm) Component Value Mxx -4.03e+20 Mxy 2.05e+21 Mxz -1.75e+21 Myy 5.90e+21 Myz 3.03e+21 Mzz -5.49e+21 ----------#### ---------------####### #------------------######### #--------------------######### ##----------------------########## ###----------------------########### ####----------------------############ #####---------- ----------######### #####---------- P ----------######### T ######---------- ----------######### # ######-----------------------############# #######----------------------############# ########--------------------############## ########-------------------############# #########------------------############# #########----------------############# ##########-------------############# ###########-----------############ ############-------########### ##############---########### ############---------- #######------- Global CMT Convention Moment Tensor: R T P -5.49e+21 -1.75e+21 -3.03e+21 -1.75e+21 -4.03e+20 -2.05e+21 -3.03e+21 -2.05e+21 5.90e+21 Details of the solution is found at http://www.eas.slu.edu/eqc/eqc_mt/MECH.NA/20110503025830/index.html |
STK = 5 DIP = 60 RAKE = -65 MW = 3.83 HS = 13.0
The NDK file is 20110503025830.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.
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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 0.06 n 3The results of this grid search are as follow:
DEPTH STK DIP RAKE MW FIT WVFGRD96 0.5 210 60 -45 3.52 0.2881 WVFGRD96 1.0 205 55 -50 3.56 0.2972 WVFGRD96 2.0 205 55 -50 3.62 0.3191 WVFGRD96 3.0 200 60 -60 3.69 0.3157 WVFGRD96 4.0 25 85 -60 3.73 0.3026 WVFGRD96 5.0 25 85 -60 3.73 0.3247 WVFGRD96 6.0 25 90 -55 3.72 0.3459 WVFGRD96 7.0 25 90 -50 3.71 0.3627 WVFGRD96 8.0 20 85 -60 3.77 0.3763 WVFGRD96 9.0 5 70 -70 3.80 0.3913 WVFGRD96 10.0 0 65 -70 3.82 0.4071 WVFGRD96 11.0 0 60 -70 3.83 0.4192 WVFGRD96 12.0 0 60 -70 3.83 0.4265 WVFGRD96 13.0 5 60 -65 3.83 0.4281 WVFGRD96 14.0 5 60 -65 3.82 0.4258 WVFGRD96 15.0 10 60 -60 3.82 0.4212 WVFGRD96 16.0 15 65 -55 3.82 0.4151 WVFGRD96 17.0 15 65 -50 3.82 0.4089 WVFGRD96 18.0 20 70 -45 3.82 0.4016 WVFGRD96 19.0 20 75 -45 3.82 0.3944 WVFGRD96 20.0 20 75 -45 3.82 0.3859 WVFGRD96 21.0 25 80 -40 3.83 0.3783 WVFGRD96 22.0 25 80 -40 3.84 0.3702 WVFGRD96 23.0 25 80 -40 3.84 0.3611 WVFGRD96 24.0 25 80 -40 3.84 0.3516 WVFGRD96 25.0 25 85 -40 3.85 0.3421 WVFGRD96 26.0 210 90 35 3.86 0.3322 WVFGRD96 27.0 30 90 -35 3.86 0.3229 WVFGRD96 28.0 30 90 -35 3.87 0.3131 WVFGRD96 29.0 30 90 -35 3.87 0.3034
The best solution is
WVFGRD96 13.0 5 60 -65 3.83 0.4281
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 0.06 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