Location

Location ANSS

The ANSS event ID is nn00333406 and the event page is at https://earthquake.usgs.gov/earthquakes/eventpage/nn00333406/executive.

2011/04/17 00:55:46 38.367 -118.750 6.1 4 Nevada

Focal Mechanism

USGS/SLU Moment Tensor Solution ENS 2011/04/17 00:55:46:0 38.37 -118.75 6.1 4.0 Nevada Stations used: BK.CMB BK.HOPS BK.HUMO BK.JCC BK.MCCM BK.MOD BK.ORV BK.SAO BK.WDC BK.YBH CI.PASC II.PFO LB.BMN LB.DAC NC.AFD NC.KHMB Filtering commands used: hp c 0.02 n 3 lp c 0.06 n 3 br c 0.12 0.25 n 4 p 2 Best Fitting Double Couple Mo = 1.26e+22 dyne-cm Mw = 4.00 Z = 2 km Plane Strike Dip Rake NP1 354 47 -105 NP2 195 45 -75 Principal Axes: Axis Value Plunge Azimuth T 1.26e+22 1 94 N 0.00e+00 11 4 P -1.26e+22 79 190 Moment Tensor: (dyne-cm) Component Value Mxx -3.37e+20 Mxy -1.04e+21 Mxz 2.23e+21 Myy 1.25e+22 Myz 5.96e+20 Mzz -1.22e+22 #######---#### ##########----######## ###########--------######### ##########-----------######### ##########--------------########## ##########----------------########## ##########------------------########## ##########--------------------########## ##########--------------------########## ##########----------------------########## ##########----------------------######## #########---------- ----------######## T #########---------- P ----------######## ########---------- ----------######### ########----------------------########## ########---------------------######### #######---------------------######## ######--------------------######## #####------------------####### #####----------------####### ###--------------##### #----------### Global CMT Convention Moment Tensor: R T P -1.22e+22 2.23e+21 -5.96e+20 2.23e+21 -3.37e+20 1.04e+21 -5.96e+20 1.04e+21 1.25e+22 Details of the solution is found at http://www.eas.slu.edu/eqc/eqc_mt/MECH.NA/20110417005546/index.html

Preferred Solution

      STK = 195
      DIP = 45
     RAKE = -75
       MW = 4.00
       HS = 2.0

The NDK file is 20110417005546.ndk The waveform inversion is preferred.

Moment Tensor Comparison

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.

SLU	UNR
USGS/SLU Moment Tensor Solution ENS 2011/04/17 00:55:46:0 38.37 -118.75 6.1 4.0 Nevada Stations used: BK.CMB BK.HOPS BK.HUMO BK.JCC BK.MCCM BK.MOD BK.ORV BK.SAO BK.WDC BK.YBH CI.PASC II.PFO LB.BMN LB.DAC NC.AFD NC.KHMB Filtering commands used: hp c 0.02 n 3 lp c 0.06 n 3 br c 0.12 0.25 n 4 p 2 Best Fitting Double Couple Mo = 1.26e+22 dyne-cm Mw = 4.00 Z = 2 km Plane Strike Dip Rake NP1 354 47 -105 NP2 195 45 -75 Principal Axes: Axis Value Plunge Azimuth T 1.26e+22 1 94 N 0.00e+00 11 4 P -1.26e+22 79 190 Moment Tensor: (dyne-cm) Component Value Mxx -3.37e+20 Mxy -1.04e+21 Mxz 2.23e+21 Myy 1.25e+22 Myz 5.96e+20 Mzz -1.22e+22 #######---#### ##########----######## ###########--------######### ##########-----------######### ##########--------------########## ##########----------------########## ##########------------------########## ##########--------------------########## ##########--------------------########## ##########----------------------########## ##########----------------------######## #########---------- ----------######## T #########---------- P ----------######## ########---------- ----------######### ########----------------------########## ########---------------------######### #######---------------------######## ######--------------------######## #####------------------####### #####----------------####### ###--------------##### #----------### Global CMT Convention Moment Tensor: R T P -1.22e+22 2.23e+21 -5.96e+20 2.23e+21 -3.37e+20 1.04e+21 -5.96e+20 1.04e+21 1.25e+22 Details of the solution is found at http://www.eas.slu.edu/eqc/eqc_mt/MECH.NA/20110417005546/index.html	REVIEWED BY NSL STAFF Event ID:333406 Origin ID:789359 Algorithm: Ichinose (2003) Long Period, Regional-Distance Waves Seismic Moment Tensor Solution 2011/04/17 (107) 00:55:48.00 38.3690 -118.7278 789359 Depth = 2.0 (km) Mw = 4.04 Mo = 1.44x10^22 (dyne x cm) Percent Double Couple = 85 % Percent CLVD = 15 % no ISO calculated Epsilon=0.07 Percent Variance Reduction = 67.94 % Total Fit = 4.38 Major Double Couple strike dip rake Nodal Plane 1: 194 41 -79 Nodal Plane 2: 359 50 -100 DEVIATORIC MOMENT TENSOR Moment Tensor Elements: Spherical Coordinates Mrr= -1.45 Mtt= 0.10 Mff= 1.35 Mrt= 0.18 Mrf= -0.23 Mtf= 0.16 EXP=22 Moment Tensor Elements: Cartesian Coordinates 0.10 -0.16 0.18 -0.16 1.35 0.23 0.18 0.23 -1.45 Eigenvalues: T-axis eigenvalue= 1.39 N-axis eigenvalue= 0.11 P-axis eigenvalue= -1.49 Eigenvalues and eigenvectors of the Major Double Couple: T-axis ev= 1.39 trend=96 plunge=4 N-axis ev= 0.00 trend=6 plunge=8 P-axis ev=-1.39 trend=215 plunge=81 Maximum Azmuithal Gap=112 Distance to Nearest Station= 82.7 (km) Number of Stations (D=Displacement/V=Velocity) Used=6 (defining only) MLAC.CI.D CMB.BK.D DAC.LB.D BMN.LB.D FUR.CI.D ORV.BK.D ################## ##############-########## #############------########## #############---------########### ############------------############ ###########---------------############ -###########-----------------############ ###########------------------############ ###########--------------------############ ##########---------------------############ ##########---------------------############# ##########----------------------############ #########------- -------------####### ## #########------- P -------------####### T ## #########------- ------------######## # #########----------------------############ ########----------------------########### ########--------------------############ ########-------------------############ #######-----------------############ #######---------------########### #######------------########## ######---------########## ####----########## All Stations defining and nondefining: Station.Net Def Distance Azi Bazi lo-f hi-f vmodel (km) (deg) (deg) (Hz) (Hz) MLAC.CI (D) Y 82.7 187 7 0.020 0.080 MLAC.CI.wus.glib CMB.BK (D) Y 149.9 256 75 0.020 0.080 CMB.BK.wus.glib DAC.LB (D) Y 252.9 156 337 0.020 0.080 DAC.LB.wus.glib BMN.LB (D) Y 263.0 29 210 0.020 0.080 BMN.LB.wus.glib FUR.CI (D) Y 267.7 141 323 0.020 0.080 FUR.CI.wus.glib ORV.BK (D) Y 273.1 300 118 0.020 0.080 ORV.BK.wus.glib (V)-velocity (D)-Displacement Author: www-data Date: 2011/04/17 01:46:53 mtinv Version 2.1_DEVEL OCT2008

Magnitudes

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 M_L by application of the respective IASPEI formulae. As a research study, the linear distance term of the IASPEI formula for M_L is adjusted to remove a linear distance trend in residuals to give a regionally defined M_L. The defined M_L 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.

ML Magnitude

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.

Context

Waveform Inversion using wvfgrd96

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.

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'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 3
br c 0.12 0.25 n 4 p 2

           DEPTH  STK   DIP  RAKE   MW    FIT
WVFGRD96    0.5   200    45   -65   3.91 0.7086
WVFGRD96    1.0   195    45   -75   3.94 0.7014
WVFGRD96    2.0   195    45   -75   4.00 0.7552
WVFGRD96    3.0   195    50   -75   4.06 0.7250
WVFGRD96    4.0   200    70   -75   4.14 0.6773
WVFGRD96    5.0   195    70   -80   4.15 0.6834
WVFGRD96    6.0   195    70   -80   4.13 0.6884
WVFGRD96    7.0   190    70   -80   4.12 0.6873
WVFGRD96    8.0   190    70   -85   4.17 0.7007
WVFGRD96    9.0   195    70   -80   4.15 0.6951
WVFGRD96   10.0   195    70   -80   4.14 0.6868
WVFGRD96   11.0   195    70   -75   4.13 0.6779
WVFGRD96   12.0   195    70   -75   4.12 0.6697
WVFGRD96   13.0   195    70   -75   4.11 0.6615
WVFGRD96   14.0    60    60    45   4.14 0.6618
WVFGRD96   15.0    60    60    45   4.15 0.6639
WVFGRD96   16.0    60    60    40   4.15 0.6643
WVFGRD96   17.0    60    60    40   4.15 0.6627
WVFGRD96   18.0    60    60    40   4.16 0.6599
WVFGRD96   19.0    60    60    40   4.17 0.6557
WVFGRD96   20.0    60    60    40   4.17 0.6505
WVFGRD96   21.0    60    60    40   4.18 0.6397
WVFGRD96   22.0    60    60    40   4.19 0.6324
WVFGRD96   23.0    60    60    40   4.19 0.6242
WVFGRD96   24.0    60    60    40   4.20 0.6153
WVFGRD96   25.0    60    60    40   4.20 0.6060
WVFGRD96   26.0    60    60    40   4.21 0.5963
WVFGRD96   27.0    60    60    40   4.22 0.5861
WVFGRD96   28.0    60    60    40   4.22 0.5758
WVFGRD96   29.0    65    55    45   4.22 0.5656

The best solution is

WVFGRD96    2.0   195    45   -75   4.00 0.7552

The mechanism corresponding 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, 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
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. The time scale is relative to the first trace sample.

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:

• 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.

Velocity Model

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