Location

Location ANSS

Focal Mechanism

USGS/SLU Moment Tensor Solution ENS 2013/01/24 04:46:39:0 38.32 -108.99 2.5 3.9 Colorado Stations used: AE.U15A IU.ANMO IW.RWWY IW.SMCO RE.PV02 RE.PV13 TA.N23A TA.O20A TA.Q24A TA.S22A TA.W18A US.DUG US.ISCO US.MVCO US.SDCO US.WUAZ UU.BRPU UU.CCUT UU.CTU UU.CVRU UU.HVU UU.JLU UU.KNB UU.LCMT UU.MTPU UU.NLU UU.PKCU UU.PSUT UU.RDMU UU.SPU UU.SRU UU.SZCU UU.TCRU UU.TCU UU.TMU Filtering commands used: cut o DIST/3.3 -30 o DIST/3.3 +70 rtr taper w 0.1 hp c 0.03 n 3 lp c 0.06 n 3 br c 0.12 0.25 n 4 p 2 Best Fitting Double Couple Mo = 1.06e+22 dyne-cm Mw = 3.95 Z = 5 km Plane Strike Dip Rake NP1 175 80 20 NP2 81 70 169 Principal Axes: Axis Value Plunge Azimuth T 1.06e+22 21 40 N 0.00e+00 68 201 P -1.06e+22 7 307 Moment Tensor: (dyne-cm) Component Value Mxx 1.69e+21 Mxy 9.55e+21 Mxz 2.02e+21 Myy -2.93e+21 Myz 3.24e+21 Mzz 1.24e+21 -----######### ---------############# -----------################# -----------############# ## P -----------############# T #### - -----------############# ##### ----------------###################### -----------------####################### -----------------####################### ------------------#######################- ------------------#####################--- -------------------################------- -------------------############----------- ####--------------#####----------------- ##################---------------------- #################--------------------- #################------------------- ################------------------ ###############--------------- ##############-------------- ############---------- ########------ Global CMT Convention Moment Tensor: R T P 1.24e+21 2.02e+21 -3.24e+21 2.02e+21 1.69e+21 -9.55e+21 -3.24e+21 -9.55e+21 -2.93e+21 Details of the solution is found at http://www.eas.slu.edu/eqc/eqc_mt/MECH.NA/20130124044639/index.html

Preferred Solution

      STK = 175
      DIP = 80
     RAKE = 20
       MW = 3.95
       HS = 5.0

The NDK file is 20130124044639.ndk The waveform inversion is preferred.

Moment Tensor Comparison

The following compares this source inversion to others

SLU	USGSMT	SLUFM
USGS/SLU Moment Tensor Solution ENS 2013/01/24 04:46:39:0 38.32 -108.99 2.5 3.9 Colorado Stations used: AE.U15A IU.ANMO IW.RWWY IW.SMCO RE.PV02 RE.PV13 TA.N23A TA.O20A TA.Q24A TA.S22A TA.W18A US.DUG US.ISCO US.MVCO US.SDCO US.WUAZ UU.BRPU UU.CCUT UU.CTU UU.CVRU UU.HVU UU.JLU UU.KNB UU.LCMT UU.MTPU UU.NLU UU.PKCU UU.PSUT UU.RDMU UU.SPU UU.SRU UU.SZCU UU.TCRU UU.TCU UU.TMU Filtering commands used: cut o DIST/3.3 -30 o DIST/3.3 +70 rtr taper w 0.1 hp c 0.03 n 3 lp c 0.06 n 3 br c 0.12 0.25 n 4 p 2 Best Fitting Double Couple Mo = 1.06e+22 dyne-cm Mw = 3.95 Z = 5 km Plane Strike Dip Rake NP1 175 80 20 NP2 81 70 169 Principal Axes: Axis Value Plunge Azimuth T 1.06e+22 21 40 N 0.00e+00 68 201 P -1.06e+22 7 307 Moment Tensor: (dyne-cm) Component Value Mxx 1.69e+21 Mxy 9.55e+21 Mxz 2.02e+21 Myy -2.93e+21 Myz 3.24e+21 Mzz 1.24e+21 -----######### ---------############# -----------################# -----------############# ## P -----------############# T #### - -----------############# ##### ----------------###################### -----------------####################### -----------------####################### ------------------#######################- ------------------#####################--- -------------------################------- -------------------############----------- ####--------------#####----------------- ##################---------------------- #################--------------------- #################------------------- ################------------------ ###############--------------- ##############-------------- ############---------- ########------ Global CMT Convention Moment Tensor: R T P 1.24e+21 2.02e+21 -3.24e+21 2.02e+21 1.69e+21 -9.55e+21 -3.24e+21 -9.55e+21 -2.93e+21 Details of the solution is found at http://www.eas.slu.edu/eqc/eqc_mt/MECH.NA/20130124044639/index.html	USGS/SLU Regional Moment Solution 13/01/24 04:46:40.22 Epicenter: 38.351 -109.003 MW 3.9 USGS/SLU REGIONAL MOMENT TENSOR Depth 5 No. of sta: 86 Moment Tensor; Scale 10**15 Nm Mrr= 0.01 Mtt= 0.30 Mpp=-0.32 Mrt=-0.05 Mrp= 0.25 Mtp=-0.94 Principal axes: T Val= 1.02 Plg=11 Azm=217 N 0.01 75 355 P -1.03 10 125 Best Double Couple:Mo=1.0*10**15 NP1:Strike=351 Dip=89 Slip= 15 NP2: 261 75 179	First motions and takeoff angles from an elocate run.

Magnitudes

mLg Magnitude

(a) mLg computed using the IASPEI formula; (b) mLg residuals ; the values used for the trimmed mean are indicated.

ML Magnitude

(a) ML computed using the IASPEI formula for Horizontal components; (b) 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.

(a) ML computed using the IASPEI formula for Vertical components (research); (b) 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.

Context

Waveform Inversion using wvfgrd96

The focal mechanism was determined using broadband seismic waveforms. The location of the event and the and stations used for the waveform inversion are shown in the next figure.

Location of broadband stations used for waveform inversion

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 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 0.03 n 3 
lp c 0.06 n 3 
br c 0.12 0.25 n 4 p 2

           DEPTH  STK   DIP  RAKE   MW    FIT
WVFGRD96    1.0   350    90   -15   3.84 0.6641
WVFGRD96    2.0   170    90    20   3.90 0.7380
WVFGRD96    3.0   170    90    20   3.92 0.7713
WVFGRD96    4.0   175    80    20   3.94 0.7868
WVFGRD96    5.0   175    80    20   3.95 0.7920
WVFGRD96    6.0   175    80    20   3.97 0.7883
WVFGRD96    7.0   175    80    15   3.98 0.7819
WVFGRD96    8.0   175    75    20   4.00 0.7771
WVFGRD96    9.0   175    80    25   4.00 0.7679
WVFGRD96   10.0   175    85    25   4.01 0.7635
WVFGRD96   11.0   175    85    25   4.01 0.7609
WVFGRD96   12.0   350    90   -25   4.01 0.7564
WVFGRD96   13.0   175    80    25   4.04 0.7563
WVFGRD96   14.0   175    80    25   4.05 0.7537
WVFGRD96   15.0   175    80    20   4.06 0.7511
WVFGRD96   16.0   350    90   -20   4.06 0.7469
WVFGRD96   17.0   170    85    20   4.08 0.7438
WVFGRD96   18.0   170    85    20   4.09 0.7403
WVFGRD96   19.0   170    90    20   4.09 0.7342
WVFGRD96   20.0   350    90   -20   4.10 0.7291
WVFGRD96   21.0   170    90    20   4.11 0.7220
WVFGRD96   22.0   350    90   -20   4.12 0.7151
WVFGRD96   23.0   170    90    20   4.13 0.7063
WVFGRD96   24.0   170    90    20   4.14 0.6968
WVFGRD96   25.0   170    90    20   4.15 0.6871
WVFGRD96   26.0   170    90    20   4.16 0.6763
WVFGRD96   27.0   350    85   -20   4.16 0.6647
WVFGRD96   28.0   345    85   -25   4.17 0.6543
WVFGRD96   29.0   345    80   -25   4.17 0.6438

The best solution is

WVFGRD96    5.0   175    80    20   3.95 0.7920

The mechanism correspond to the best fit is

Figure 1. Waveform inversion focal mechanism

The best fit as a function of depth is given in the following figure:

Figure 2. Depth sensitivity for waveform mechanism

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 and because the velocity model used in the predictions may not be perfect. 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 0.03 n 3 
lp c 0.06 n 3 
br c 0.12 0.25 n 4 p 2

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.

Focal mechanism sensitivity at the preferred depth. The red color indicates a very good fit to thewavefroms. 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:

• The origin time and epicentral distance are incorrect
• The velocity model used for the inversion is incorrect
• The velocity model used to define the P-arrival time is not the same as the velocity model used for the waveform inversion (assuming that the initial trace alignment is based on the P arrival time)

 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.

Discussion

Acknowledgements

Thanks also to the many seismic network operators whose dedication make this effort possible: University of Nevada Reno, University of Alaska, University of Washington, Oregon State University, University of Utah, Montana Bureas of Mines, UC Berkely, Caltech, UC San Diego, Saint Louis University, University of Memphis, Lamont Doherty Earth Observatory, the Iris stations and the Transportable Array of EarthScope.

Velocity Model

The WUS.model used for the waveform synthetic seismograms and for the surface wave eigenfunctions and dispersion is as follows:

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    

Quality Control

Here we tabulate the reasons for not using certain digital data sets

The following stations did not have a valid response files: