Location

Location ANSS

The ANSS event ID is nn00242164 and the event page is at https://earthquake.usgs.gov/earthquakes/eventpage/nn00242164/executive.

2008/04/24 22:55:48 39.530 -119.933 2.9 4.1 Nevada

Focal Mechanism

USGS/SLU Moment Tensor Solution ENS 2008/04/24 22:55:48:0 39.53 -119.93 2.9 4.1 Nevada Stations used: BK.CMB BK.HUMO BK.MOD CI.ISA TA.G06A TA.G07A TA.G08A TA.G10A TA.H06A TA.H08A TA.H09A TA.H11A TA.I07A TA.I09A TA.I10A TA.I11A TA.I12A TA.J08A TA.J09A TA.J10A TA.J11A TA.J13A TA.K10A TA.K11A TA.K12A TA.L10A TA.L11A TA.L12A TA.L14A TA.M10A TA.M11A TA.M12A TA.M13A TA.M15A TA.N10A TA.N11A TA.N12A TA.N13A TA.N15A TA.O11A TA.O12A TA.O13A TA.O15A TA.P10A TA.P11A TA.P12A TA.P13A TA.P14A TA.P15A TA.Q10A TA.Q11A TA.Q12A TA.Q13A TA.Q14A TA.Q15A TA.R10A TA.R11A TA.R12A TA.R13A TA.R14A TA.R15A TA.S10A TA.S11A TA.S12A TA.S13A TA.T11A TA.T12A TA.T13A TA.T14A TA.U10A TA.U11A TA.U12A TA.U13A TA.U14A TA.V11A TA.V12A TA.V13A TA.W12A US.WVOR Filtering commands used: cut o DIST/3.3 -40 o DIST/3.3 +50 rtr taper w 0.1 hp c 0.03 n 3 lp c 0.07 n 3 Best Fitting Double Couple Mo = 3.31e+22 dyne-cm Mw = 4.28 Z = 10 km Plane Strike Dip Rake NP1 245 85 25 NP2 153 65 174 Principal Axes: Axis Value Plunge Azimuth T 3.31e+22 21 112 N 0.00e+00 65 256 P -3.31e+22 14 16 Moment Tensor: (dyne-cm) Component Value Mxx -2.49e+22 Mxy -1.83e+22 Mxz -1.14e+22 Myy 2.25e+22 Myz 8.19e+21 Mzz 2.43e+21 ---------- - #------------- P ----- ####------------- -------- #####------------------------- #######--------------------------- ########---------------------------- #########---------------------------## ###########---------------------######## ###########----------------############# #############-----------################## ##############------###################### ##############--########################## ############---########################### ########-------################### ### #####-----------################## T ### #----------------################ ## -----------------################### ------------------################ -----------------############# -------------------######### -------------------### -------------- Global CMT Convention Moment Tensor: R T P 2.43e+21 -1.14e+22 -8.19e+21 -1.14e+22 -2.49e+22 1.83e+22 -8.19e+21 1.83e+22 2.25e+22 Details of the solution is found at http://www.eas.slu.edu/eqc/eqc_mt/MECH.NA/20080424225548/index.html

Preferred Solution

The preferred solution from an analysis of the surface-wave spectral amplitude radiation pattern, waveform inversion or first motion observations is

      STK = 245
      DIP = 85
     RAKE = 25
       MW = 4.28
       HS = 10.0

The NDK file is 20080424225548.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 UCB

USGS/SLU Moment Tensor Solution ENS 2008/04/24 22:55:48:0 39.53 -119.93 2.9 4.1 Nevada Stations used: BK.CMB BK.HUMO BK.MOD CI.ISA TA.G06A TA.G07A TA.G08A TA.G10A TA.H06A TA.H08A TA.H09A TA.H11A TA.I07A TA.I09A TA.I10A TA.I11A TA.I12A TA.J08A TA.J09A TA.J10A TA.J11A TA.J13A TA.K10A TA.K11A TA.K12A TA.L10A TA.L11A TA.L12A TA.L14A TA.M10A TA.M11A TA.M12A TA.M13A TA.M15A TA.N10A TA.N11A TA.N12A TA.N13A TA.N15A TA.O11A TA.O12A TA.O13A TA.O15A TA.P10A TA.P11A TA.P12A TA.P13A TA.P14A TA.P15A TA.Q10A TA.Q11A TA.Q12A TA.Q13A TA.Q14A TA.Q15A TA.R10A TA.R11A TA.R12A TA.R13A TA.R14A TA.R15A TA.S10A TA.S11A TA.S12A TA.S13A TA.T11A TA.T12A TA.T13A TA.T14A TA.U10A TA.U11A TA.U12A TA.U13A TA.U14A TA.V11A TA.V12A TA.V13A TA.W12A US.WVOR Filtering commands used: cut o DIST/3.3 -40 o DIST/3.3 +50 rtr taper w 0.1 hp c 0.03 n 3 lp c 0.07 n 3 Best Fitting Double Couple Mo = 3.31e+22 dyne-cm Mw = 4.28 Z = 10 km Plane Strike Dip Rake NP1 245 85 25 NP2 153 65 174 Principal Axes: Axis Value Plunge Azimuth T 3.31e+22 21 112 N 0.00e+00 65 256 P -3.31e+22 14 16 Moment Tensor: (dyne-cm) Component Value Mxx -2.49e+22 Mxy -1.83e+22 Mxz -1.14e+22 Myy 2.25e+22 Myz 8.19e+21 Mzz 2.43e+21 ---------- - #------------- P ----- ####------------- -------- #####------------------------- #######--------------------------- ########---------------------------- #########---------------------------## ###########---------------------######## ###########----------------############# #############-----------################## ##############------###################### ##############--########################## ############---########################### ########-------################### ### #####-----------################## T ### #----------------################ ## -----------------################### ------------------################ -----------------############# -------------------######### -------------------### -------------- Global CMT Convention Moment Tensor: R T P 2.43e+21 -1.14e+22 -8.19e+21 -1.14e+22 -2.49e+22 1.83e+22 -8.19e+21 1.83e+22 2.25e+22 Details of the solution is found at http://www.eas.slu.edu/eqc/eqc_mt/MECH.NA/20080424225548/index.html

UCB Seismological Laboratory Inversion method: complete waveform Stations used: BK.ORV BK.CMB BK.MNRC BK.PKD Berkeley Moment Tensor Solution Best Fitting Double-Couple: Mo = 3.80E+22 Dyne-cm Mw = 4.32 Z = 5 km Plane Strike Rake Dip NP1 150 -168 83 NP2 59 -7 78 Event Date/Time: 04/24/2008 22:55:49:290 Event ID: 40215981 ----------- ----------------------- #------------------- -------- ####------------------- P ----------- #######------------------ ------------- ##########----------------------------------- ###########------------------------------------ #############------------------------------------ ################-----------------------------------## ##################---------------------------------#### ###################------------------------------###### ###################---------------------------######### T ####################------------------------############ #####################--------------------############### #########################-----------------################# ##########################-------------#################### ############################----------####################### ############################------######################### #############################--############################ ############################--############################# #########################------############################ ####################-----------########################## ###############-----------------####################### ###########----------------------###################### ####-----------------------------#################### --------------------------------################# ---------------------------------############## ---------------------------------############ --------------------------------######### --------------------------------##### ------------------------------# ----------------------- ----------- Lower Hemisphere Equiangle Projection Deviatoric Solution: Principal Axes: Axis Value Plunge Azimuth T 3.899 4 284 N -0.169 76 179 P -3.730 13 15 Source Composition: Type Percent DC 91.3 CLVD 8.7 Iso 0.0 Moment Tensor: Scale = 10**22 Dyne-cm Component Value Mxx -3.091 Mxy -1.766 Mxz -0.715 Myy 3.435 Myz -0.456 Mzz -0.344 ----------- ----------------------- #------------------- -------- #####------------------ P ----------- #######------------------ ------------- ##########----------------------------------- ############----------------------------------- ##############----------------------------------- ################-----------------------------------## ##################--------------------------------##### ###################-----------------------------####### ###################---------------------------######### T ####################------------------------############ ####################---------------------############### ########################------------------################# #########################---------------################### ###########################------------###################### ##########################----------####################### ##########################--------######################### #########################--------########################## #######################----------########################## ###################--------------######################## ###############------------------###################### ###########-----------------------##################### #####-----------------------------################### ---------------------------------################ ---------------------------------############## ---------------------------------############ --------------------------------######### --------------------------------##### ------------------------------# ----------------------- ----------- Lower Hemisphere Equiangle Projection

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

The left panel of the next figure presents the focal mechanism for this earthquake (red) in the context of other nearby events (blue) in the SLU Moment Tensor Catalog. The right panel shows the inferred direction of maximum compressive stress and the type of faulting (green is strike-slip, red is normal, blue is thrust; oblique is shown by a combination of colors). Thus context plot is useful for assessing the appropriateness of the moment tensor of this event.

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:

cut o DIST/3.3 -40 o DIST/3.3 +50
rtr
taper w 0.1
hp c 0.03 n 3 
lp c 0.07 n 3

The results of this grid search are as follow:

           DEPTH  STK   DIP  RAKE   MW    FIT
WVFGRD96    0.5    65    85    20   3.97 0.5052
WVFGRD96    1.0    65    80    10   3.99 0.5341
WVFGRD96    2.0    65    80    10   4.06 0.5987
WVFGRD96    3.0    60    80   -15   4.11 0.6426
WVFGRD96    4.0    60    85   -15   4.14 0.6628
WVFGRD96    5.0   245    80    15   4.17 0.6720
WVFGRD96    6.0    60    90   -25   4.20 0.6778
WVFGRD96    7.0   245    85    20   4.21 0.6905
WVFGRD96    8.0    60    85   -30   4.25 0.7024
WVFGRD96    9.0    60    90   -25   4.27 0.7066
WVFGRD96   10.0   245    85    25   4.28 0.7126
WVFGRD96   11.0   245    85    25   4.29 0.7121
WVFGRD96   12.0    65    90   -25   4.30 0.7053
WVFGRD96   13.0   245    85    20   4.31 0.7022
WVFGRD96   14.0   245    85    20   4.32 0.6940
WVFGRD96   15.0    65    90   -20   4.32 0.6802
WVFGRD96   16.0    65    85    20   4.33 0.6740
WVFGRD96   17.0   245    90   -20   4.34 0.6634
WVFGRD96   18.0    65    85    20   4.35 0.6604
WVFGRD96   19.0   245    90   -20   4.35 0.6471
WVFGRD96   20.0    65    85    20   4.36 0.6402
WVFGRD96   21.0    65    85    20   4.37 0.6280
WVFGRD96   22.0   245    90   -20   4.37 0.6116
WVFGRD96   23.0    65    85    20   4.38 0.6006
WVFGRD96   24.0   245    90   -20   4.38 0.5828
WVFGRD96   25.0    65    85    20   4.39 0.5701
WVFGRD96   26.0   245    90   -25   4.39 0.5513
WVFGRD96   27.0    65    85    20   4.40 0.5377
WVFGRD96   28.0   245    90   -25   4.40 0.5195
WVFGRD96   29.0    65    85    25   4.41 0.5050

The best solution is

WVFGRD96   10.0   245    85    25   4.28 0.7126

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

cut o DIST/3.3 -40 o DIST/3.3 +50
rtr
taper w 0.1
hp c 0.03 n 3 
lp c 0.07 n 3

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)

Assuming only a mislocation, the time shifts are fit to a functional form:

 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.

Surface-Wave Focal Mechanism

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.

Location of broadband stations used to obtain focal mechanism from surface-wave spectral amplitudes

The surface-wave determined focal mechanism is shown here.


  NODAL PLANES 

  
  STK=      58.53
  DIP=      76.43
 RAKE=     -25.77
  
             OR
  
  STK=     154.99
  DIP=      65.00
 RAKE=    -164.99
 
 
DEPTH = 7.0 km
 
Mw = 4.30
Best Fit 0.88945 - P-T axis plot gives solutions with FIT greater than FIT90

Surface-wave analysis

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.

Data preparation

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.

Best mechanism fit as a function of depth. The preferred depth is given above. Lower hemisphere projection

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.

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.

Love-wave radiation patterns

Rayleigh-wave radiation patterns

Rayleigh-wave radiation patterns.
Rayleigh-wave radiation patterns.
Rayleigh-wave radiation patterns.
Rayleigh-wave radiation patterns.
Rayleigh-wave radiation patterns.
Rayleigh-wave radiation patterns.
Rayleigh-wave radiation patterns.

Rayleigh-wave radiation patterns.

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

Last Changed Sun Apr 28 01:01:50 PM CDT 2024