The ANSS event ID is usp000j0wm and the event page is at https://earthquake.usgs.gov/earthquakes/eventpage/usp000j0wm/executive.
2011/05/02 11:43:31 30.734 -105.724 10.0 4.2 Chihuahua, MX
USGS/SLU Moment Tensor Solution
ENS 2011/05/02 11:43:31:0 30.73 -105.72 10.0 4.2 Chihuahua, MX
Stations used:
AR.113A AR.X16A AR.X18A AR.Y14A IM.TX31 IU.ANMO IU.TUC
MX.HPIG MX.HSIG MX.SRIG TA.034A TA.121A TA.133A TA.134A
TA.214A TA.234A TA.434A TA.435B TA.534A TA.633A TA.634A
TA.635A TA.733A TA.833A TA.ABTX TA.MSTX TA.S22A TA.T25A
TA.W18A TA.W32A TA.WHTX TA.X32A TA.Y22D TA.Y33A TA.Z33A
US.AMTX US.JCT US.MNTX US.MVCO US.SDCO US.WMOK US.WUAZ
Filtering commands used:
hp c 0.02 n 3
lp c 0.05 n 3
Best Fitting Double Couple
Mo = 2.69e+22 dyne-cm
Mw = 4.22
Z = 12 km
Plane Strike Dip Rake
NP1 0 60 -75
NP2 152 33 -114
Principal Axes:
Axis Value Plunge Azimuth
T 2.69e+22 14 79
N 0.00e+00 13 172
P -2.69e+22 71 304
Moment Tensor: (dyne-cm)
Component Value
Mxx 0.00e+00
Mxy 6.03e+21
Mxz -3.48e+21
Myy 2.25e+22
Myz 1.30e+22
Mzz -2.25e+22
--------######
#-------------########
##----------------##########
##------------------##########
###--------------------###########
####--------------------############
####----------------------############
#####----------------------#############
#####--------- ----------######### #
######--------- P -----------######## T ##
######--------- -----------######## ##
#######----------------------#############
#######----------------------#############
#######--------------------#############
########-------------------#############
########------------------############
########----------------############
#########--------------###########
#########-----------##########
##########--------##########
###########---########
########------
Global CMT Convention Moment Tensor:
R T P
-2.25e+22 -3.48e+21 -1.30e+22
-3.48e+21 0.00e+00 -6.03e+21
-1.30e+22 -6.03e+21 2.25e+22
Details of the solution is found at
http://www.eas.slu.edu/eqc/eqc_mt/MECH.NA/20110502114331/index.html
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STK = 0
DIP = 60
RAKE = -75
MW = 4.22
HS = 12.0
The NDK file is 20110502114331.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:
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 0.5 190 50 -70 3.93 0.3282
WVFGRD96 1.0 30 50 -40 3.96 0.3306
WVFGRD96 2.0 25 50 -45 4.03 0.3872
WVFGRD96 3.0 20 45 -50 4.07 0.4027
WVFGRD96 4.0 200 60 -60 4.10 0.3978
WVFGRD96 5.0 20 65 -55 4.09 0.4087
WVFGRD96 6.0 20 75 -60 4.11 0.4258
WVFGRD96 7.0 5 65 -70 4.14 0.4479
WVFGRD96 8.0 5 65 -75 4.21 0.4733
WVFGRD96 9.0 0 65 -75 4.21 0.4975
WVFGRD96 10.0 0 65 -75 4.22 0.5125
WVFGRD96 11.0 0 60 -75 4.22 0.5196
WVFGRD96 12.0 0 60 -75 4.22 0.5216
WVFGRD96 13.0 0 60 -75 4.22 0.5180
WVFGRD96 14.0 5 65 -65 4.20 0.5109
WVFGRD96 15.0 5 65 -65 4.20 0.5037
WVFGRD96 16.0 220 75 40 4.17 0.4976
WVFGRD96 17.0 220 75 40 4.17 0.4967
WVFGRD96 18.0 220 75 40 4.18 0.4943
WVFGRD96 19.0 220 75 40 4.18 0.4908
WVFGRD96 20.0 220 75 35 4.19 0.4869
WVFGRD96 21.0 220 75 35 4.20 0.4814
WVFGRD96 22.0 220 75 35 4.20 0.4760
WVFGRD96 23.0 220 80 35 4.21 0.4701
WVFGRD96 24.0 220 80 35 4.22 0.4642
WVFGRD96 25.0 220 80 35 4.22 0.4576
WVFGRD96 26.0 220 80 35 4.23 0.4506
WVFGRD96 27.0 220 80 30 4.23 0.4437
WVFGRD96 28.0 220 80 30 4.24 0.4365
WVFGRD96 29.0 220 80 30 4.24 0.4291
The best solution is
WVFGRD96 12.0 0 60 -75 4.22 0.5216
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 following figure shows the stations used in the grid search for the best focal mechanism to fit the surface-wave spectral amplitudes of the Love and Rayleigh waves.
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The surface-wave determined focal mechanism is shown here.
NODAL PLANES
STK= 12.73
DIP= 74.24
RAKE= -70.35
OR
STK= 140.00
DIP= 25.00
RAKE= -139.99
DEPTH = 6.0 km
Mw = 4.34
Best Fit 0.8295 - P-T axis plot gives solutions with FIT greater than FIT90
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Surface wave analysis was performed using codes from Computer Programs in Seismology, specifically the multiple filter analysis program do_mft and the surface-wave radiation pattern search program srfgrd96.
Digital data were collected, instrument response removed and traces converted
to Z, R an T components. Multiple filter analysis was applied to the Z and T traces to obtain the Rayleigh- and Love-wave spectral amplitudes, respectively.
These were input to the search program which examined all depths between 1 and 25 km
and all possible mechanisms.
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| Pressure-tension axis trends. Since the surface-wave spectra search does not distinguish between P and T axes and since there is a 180 ambiguity in strike, all possible P and T axes are plotted. First motion data and waveforms will be used to select the preferred mechanism. The purpose of this plot is to provide an idea of the possible range of solutions. The P and T-axes for all mechanisms with goodness of fit greater than 0.9 FITMAX (above) are plotted here. |
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| Focal mechanism sensitivity at the preferred depth. The red color indicates a very good fit to the Love and Rayleigh wave radiation patterns. 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. Because of the symmetry of the spectral amplitude rediation patterns, only strikes from 0-180 degrees are sampled. |
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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