Location

Location ANSS

The ANSS event ID is nn00242157 and the event page is at https://earthquake.usgs.gov/earthquakes/eventpage/nn00242157/executive.

2008/04/24 22:47:04 39.528 -119.928 2.2 3.6 Nevada

Focal Mechanism

USGS/SLU Moment Tensor Solution ENS 2008/04/24 22:47:04:0 39.53 -119.93 2.2 3.6 Nevada Stations used: BK.HUMO BK.SAO CI.ISA TA.J09A TA.K05A TA.K10A TA.K11A TA.L10A TA.L12A TA.M10A TA.M11A TA.N10A TA.N11A TA.N12A TA.N13A TA.N15A TA.O10A TA.O11A TA.O12A TA.O15A TA.P10A TA.P11A TA.P12A TA.P13A TA.P14A TA.Q10A TA.Q11A TA.Q12A TA.Q14A TA.R10A TA.R11A TA.S10A TA.V11A TA.V12A TA.W12A US.ELK US.WVOR Filtering commands used: cut o DIST/3.3 -30 o DIST/3.3 +60 rtr taper w 0.1 hp c 0.03 n 3 lp c 0.10 n 3 Best Fitting Double Couple Mo = 5.89e+21 dyne-cm Mw = 3.78 Z = 12 km Plane Strike Dip Rake NP1 240 80 15 NP2 147 75 170 Principal Axes: Axis Value Plunge Azimuth T 5.89e+21 18 104 N 0.00e+00 72 273 P -5.89e+21 3 13 Moment Tensor: (dyne-cm) Component Value Mxx -5.24e+21 Mxy -2.57e+21 Mxz -7.46e+20 Myy 4.72e+21 Myz 1.57e+21 Mzz 5.21e+20 ---------- P - -------------- ----- ###------------------------- ####-------------------------- #######--------------------------- #########--------------------------# ##########--------------------######## ############---------------############# #############----------################# ###############------##################### ################--######################## ###############--######################### ############------################## ### ########-----------################ T ## ######--------------############### ## ###-----------------################## ---------------------############### ----------------------############ ---------------------######### -----------------------##### ---------------------- -------------- Global CMT Convention Moment Tensor: R T P 5.21e+20 -7.46e+20 -1.57e+21 -7.46e+20 -5.24e+21 2.57e+21 -1.57e+21 2.57e+21 4.72e+21 Details of the solution is found at http://www.eas.slu.edu/eqc/eqc_mt/MECH.NA/20080424224704/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 = 240
      DIP = 80
     RAKE = 15
       MW = 3.78
       HS = 12.0

The NDK file is 20080424224704.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 22:47:04:0 39.53 -119.93 2.2 3.6 Nevada Stations used: BK.HUMO BK.SAO CI.ISA TA.J09A TA.K05A TA.K10A TA.K11A TA.L10A TA.L12A TA.M10A TA.M11A TA.N10A TA.N11A TA.N12A TA.N13A TA.N15A TA.O10A TA.O11A TA.O12A TA.O15A TA.P10A TA.P11A TA.P12A TA.P13A TA.P14A TA.Q10A TA.Q11A TA.Q12A TA.Q14A TA.R10A TA.R11A TA.S10A TA.V11A TA.V12A TA.W12A US.ELK US.WVOR Filtering commands used: cut o DIST/3.3 -30 o DIST/3.3 +60 rtr taper w 0.1 hp c 0.03 n 3 lp c 0.10 n 3 Best Fitting Double Couple Mo = 5.89e+21 dyne-cm Mw = 3.78 Z = 12 km Plane Strike Dip Rake NP1 240 80 15 NP2 147 75 170 Principal Axes: Axis Value Plunge Azimuth T 5.89e+21 18 104 N 0.00e+00 72 273 P -5.89e+21 3 13 Moment Tensor: (dyne-cm) Component Value Mxx -5.24e+21 Mxy -2.57e+21 Mxz -7.46e+20 Myy 4.72e+21 Myz 1.57e+21 Mzz 5.21e+20 ---------- P - -------------- ----- ###------------------------- ####-------------------------- #######--------------------------- #########--------------------------# ##########--------------------######## ############---------------############# #############----------################# ###############------##################### ################--######################## ###############--######################### ############------################## ### ########-----------################ T ## ######--------------############### ## ###-----------------################## ---------------------############### ----------------------############ ---------------------######### -----------------------##### ---------------------- -------------- Global CMT Convention Moment Tensor: R T P 5.21e+20 -7.46e+20 -1.57e+21 -7.46e+20 -5.24e+21 2.57e+21 -1.57e+21 2.57e+21 4.72e+21 Details of the solution is found at http://www.eas.slu.edu/eqc/eqc_mt/MECH.NA/20080424224704/index.html

This is a preliminary NCSS moment tensor solution for the event located 3 km ENE of Verdi-Mogul, NV; 39.5252N 119.9228W; Z=2.0km; ML=3.81; (USGS/UCB Joint Notification System) on 04/24/2008 22:47:04:390 UTC. Other information about this event can be viewed at: http://earthquake.usgs.gov/recenteqsus/Quakes/nc40215976.php Reviewed by: Rick UCB Seismological Laboratory Inversion method: complete waveform Stations used: NN.BEK BK.ORV BK.CMB Berkeley Moment Tensor Solution Best Fitting Double-Couple: Mo = 5.10E+21 Dyne-cm Mw = 3.74 Z = 5 km Plane Strike Rake Dip NP1 236 -11 81 NP2 328 -171 79 Event Date/Time: 04/24/2008 22:47:04:390 Event ID: 40215976 ----------- ----------------------- ------------------------------- ####--------------------------------- ########--------------------------------- ############--------------------------------- ##############--------------------------------- #################-------------------------------- ####################--------------------------####### #######################---------------------########### ########################----------------############### ###########################-----------################### ###########################------######################## T ############################--########################### ############################-############################ ###########################------########################## #########################----------########################## ######################-------------######################## ###################-----------------####################### #################--------------------###################### ###############-----------------------##################### ###########---------------------------################### ########------------------------------################# ######---------------------------------################ ####-----------------------------------############## -------------------------------------############ -------------------------------------########## -------------------------------------######## -------------- -------------------##### ------------ P -------------------### --------- ------------------- ----------------------- ----------- Lower Hemisphere Equiangle Projection Deviatoric Solution: Principal Axes: Axis Value Plunge Azimuth T 5.028 2 282 N 0.089 76 19 P -5.117 14 191 Source Composition: Type Percent DC 96.5 CLVD 3.5 Iso 0.0 Moment Tensor: Scale = 10**21 Dyne-cm Component Value Mxx -4.413 Mxy -1.918 Mxz 1.248 Myy 4.634 Myz 0.090 Mzz -0.220 ----------- ----------------------- ------------------------------- ####--------------------------------- ########--------------------------------- ############--------------------------------- ##############--------------------------------- #################-------------------------------- #####################-------------------------####### #######################--------------------############ #########################---------------############### ###########################----------#################### #############################---######################### T ######################################################### ######################################################### #############################--############################ ##########################--------########################### ######################-------------######################## ####################----------------####################### #################--------------------###################### ###############-----------------------##################### ############--------------------------################### ########------------------------------################# ######---------------------------------################ ####-----------------------------------############## --------------------------------------########### -------------------------------------########## -------------------------------------######## -------------- -------------------##### ------------ P -------------------### --------- ------------------- ----------------------- ----------- 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 -30 o DIST/3.3 +60
rtr
taper w 0.1
hp c 0.03 n 3 
lp c 0.10 n 3

The results of this grid search are as follow:

           DEPTH  STK   DIP  RAKE   MW    FIT
WVFGRD96    0.5   145    50   -10   3.45 0.3766
WVFGRD96    1.0   145    65   -15   3.43 0.3941
WVFGRD96    2.0   145    75   -20   3.50 0.4460
WVFGRD96    3.0   150    75    15   3.54 0.4790
WVFGRD96    4.0   150    75    20   3.58 0.4966
WVFGRD96    5.0   150    80    15   3.60 0.5053
WVFGRD96    6.0    60    80    20   3.63 0.5144
WVFGRD96    7.0    60    75    10   3.65 0.5298
WVFGRD96    8.0   240    70    10   3.70 0.5469
WVFGRD96    9.0    60    70    10   3.72 0.5565
WVFGRD96   10.0   240    80    20   3.74 0.5654
WVFGRD96   11.0   240    80    15   3.76 0.5708
WVFGRD96   12.0   240    80    15   3.78 0.5728
WVFGRD96   13.0   240    80    15   3.79 0.5708
WVFGRD96   14.0   240    80    15   3.81 0.5655
WVFGRD96   15.0   240    80    15   3.82 0.5566
WVFGRD96   16.0   240    80    15   3.83 0.5448
WVFGRD96   17.0    60    75    10   3.84 0.5307
WVFGRD96   18.0    60    75    10   3.85 0.5148
WVFGRD96   19.0    60    75    10   3.85 0.4970
WVFGRD96   20.0    60    75    10   3.86 0.4775
WVFGRD96   21.0    60    75    10   3.86 0.4562
WVFGRD96   22.0    60    70     5   3.87 0.4348
WVFGRD96   23.0    60    70    10   3.87 0.4127
WVFGRD96   24.0    60    70    10   3.87 0.3903
WVFGRD96   25.0    60    65     5   3.87 0.3678
WVFGRD96   26.0    60    65     5   3.87 0.3461
WVFGRD96   27.0    60    65     5   3.87 0.3248
WVFGRD96   28.0    55    65   -15   3.87 0.3047
WVFGRD96   29.0    55    60   -15   3.88 0.2861

The best solution is

WVFGRD96   12.0   240    80    15   3.78 0.5728

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 -30 o DIST/3.3 +60
rtr
taper w 0.1
hp c 0.03 n 3 
lp c 0.10 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=     239.99
  DIP=      75.00
 RAKE=      24.99
  
             OR
  
  STK=     143.11
  DIP=      65.91
 RAKE=     163.53
 
 
DEPTH = 17.0 km
 
Mw = 3.92
Best Fit 0.8279 - 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.

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 01:01:49 PM CDT 2024