Location

2008/04/24 22:55:48 39.550 -119.887 0.0 4.0 Nevada

Arrival Times (from USGS)

Felt Map

Focal Mechanism

SLU Moment Tensor Solution 2008/04/24 22:55:48 39.550 -119.887 0.0 4.0 Nevada Best Fitting Double Couple Mo = 3.20e+22 dyne-cm Mw = 4.27 Z = 8 km Plane Strike Dip Rake NP1 245 85 30 NP2 152 60 174 Principal Axes: Axis Value Plunge Azimuth T 3.20e+22 24 113 N 0.00e+00 60 254 P -3.20e+22 17 15 Moment Tensor: (dyne-cm) Component Value Mxx -2.34e+22 Mxy -1.67e+22 Mxz -1.33e+22 Myy 2.06e+22 Myz 8.85e+21 Mzz 2.78e+21 ---------- - #------------- P ----- ####------------- -------- ####-------------------------- ######---------------------------- #######----------------------------- ########----------------------------## ##########----------------------######## ##########-----------------############# ############------------################## ############--------###################### #############---########################## ############--############################ ########------################### #### #####----------################## T #### #--------------################# ### ----------------#################### ----------------################## ----------------############## ------------------########## ------------------#### -------------- Harvard Convention Moment Tensor: R T F 2.78e+21 -1.33e+22 -8.85e+21 -1.33e+22 -2.34e+22 1.67e+22 -8.85e+21 1.67e+22 2.06e+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 and first motion observations is

      STK = 245
      DIP = 85
     RAKE = 30
       MW = 4.27
       HS = 8.0

The waveform inversion is preferred.

Moment Tensor Comparison

The following compares this source inversion to others

SLU UCB

SLU Moment Tensor Solution 2008/04/24 22:55:48 39.550 -119.887 0.0 4.0 Nevada Best Fitting Double Couple Mo = 3.20e+22 dyne-cm Mw = 4.27 Z = 8 km Plane Strike Dip Rake NP1 245 85 30 NP2 152 60 174 Principal Axes: Axis Value Plunge Azimuth T 3.20e+22 24 113 N 0.00e+00 60 254 P -3.20e+22 17 15 Moment Tensor: (dyne-cm) Component Value Mxx -2.34e+22 Mxy -1.67e+22 Mxz -1.33e+22 Myy 2.06e+22 Myz 8.85e+21 Mzz 2.78e+21 ---------- - #------------- P ----- ####------------- -------- ####-------------------------- ######---------------------------- #######----------------------------- ########----------------------------## ##########----------------------######## ##########-----------------############# ############------------################## ############--------###################### #############---########################## ############--############################ ########------################### #### #####----------################## T #### #--------------################# ### ----------------#################### ----------------################## ----------------############## ------------------########## ------------------#### -------------- Harvard Convention Moment Tensor: R T F 2.78e+21 -1.33e+22 -8.85e+21 -1.33e+22 -2.34e+22 1.67e+22 -8.85e+21 1.67e+22 2.06e+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

Waveform Inversion

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:

hp c 0.02 n 3
lp c 0.10 n 3
br c 0.12 0.25 n 4 p 2

The results of this grid search from 0.5 to 19 km depth are as follow:

           DEPTH  STK   DIP  RAKE   MW    FIT
WVFGRD96    0.5   245    80    15   3.99 0.5427
WVFGRD96    1.0   245    85    10   4.01 0.5779
WVFGRD96    2.0    65    80    10   4.09 0.6616
WVFGRD96    3.0    65    90   -10   4.13 0.7045
WVFGRD96    4.0    60    80   -20   4.18 0.7253
WVFGRD96    5.0    60    85   -30   4.21 0.7352
WVFGRD96    6.0    60    85   -25   4.23 0.7405
WVFGRD96    7.0    65    90   -25   4.24 0.7444
WVFGRD96    8.0   245    85    30   4.27 0.7497
WVFGRD96    9.0    60    85   -30   4.29 0.7463
WVFGRD96   10.0   245    85    25   4.30 0.7488
WVFGRD96   11.0   245    85    25   4.31 0.7444
WVFGRD96   12.0    65    90   -20   4.32 0.7356
WVFGRD96   13.0   245    85    20   4.33 0.7317
WVFGRD96   14.0   245    85    20   4.33 0.7210
WVFGRD96   15.0   245    85    15   4.34 0.7097
WVFGRD96   16.0   245    85    15   4.35 0.6967
WVFGRD96   17.0   245    85    15   4.36 0.6819
WVFGRD96   18.0   245    85    15   4.37 0.6664
WVFGRD96   19.0   245    85    15   4.37 0.6501
WVFGRD96   20.0   245    80    10   4.38 0.6317
WVFGRD96   21.0   245    90   -15   4.39 0.6118
WVFGRD96   22.0   245    90   -15   4.39 0.5963
WVFGRD96   23.0    65    85    15   4.40 0.5834
WVFGRD96   24.0   245    90   -15   4.40 0.5616
WVFGRD96   25.0   245    90   -20   4.41 0.5443
WVFGRD96   26.0   245    90   -20   4.41 0.5266
WVFGRD96   27.0   245    90   -20   4.42 0.5085
WVFGRD96   28.0   245    90   -20   4.42 0.4907
WVFGRD96   29.0   245    90   -25   4.43 0.4733

The best solution is

WVFGRD96    8.0   245    85    30   4.27 0.7497

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 componnet is plotted to the same scale and peak amplitudes are indicated by the numbers to the left of each trace. The number in black at the rightr of each predicted traces 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 bandpass filter used in the processing and for the display was

hp c 0.02 n 3
lp c 0.10 n 3
br c 0.12 0.25 n 4 p 2

Figure 3. Waveform comparison for depth of 8 km

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.

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

First motion data

The P-wave first motion data for focal mechanism studies are as follow:

Sta Az    Dist   First motion

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

Broadband station distribution

The distribution of broadband stations with azimuth and distance is
Listing of broadband stations used

Waveform comparison for this mechanism

Since the analysis of the surface-wave radiation patterns uses only spectral amplitudes and because the surfave-wave radiation patterns have a 180 degree symmetry, each surface-wave solution consists of four possible focal mechanisms corresponding to the interchange of the P- and T-axes and a roation of the mechanism by 180 degrees. To select one mechanism, P-wave first motion can be used. This was not possible in this case because all the P-wave first motions were emergent ( a feature of the P-wave wave takeoff angle, the station location and the mechanism). The other way to select among the mechanisms is to compute forward synthetics and compare the observed and predicted waveforms.

The fits to the waveforms with the given mechanism are show below:

This figure shows the fit to the three components of motion (Z - vertical, R-radial and T - transverse). For each station and component, the observed traces is shown in red and the model predicted trace in blue. The traces represent filtered ground velocity in units of meters/sec (the peak value is printed adjacent to each trace; each pair of traces to plotted to the same scale to emphasize the difference in levels). Both synthetic and observed traces have been filtered using the SAC commands:

hp c 0.02 n 3
lp c 0.10 n 3

Discussion

The Future

Should the national backbone of the USGS Advanced National Seismic System (ANSS) be implemented with an interstation separation of 300 km, it is very likely that an earthquake such as this would have been recorded at distances on the order of 100-200 km. This means that the closest station would have information on source depth and mechanism that was lacking here.

Acknowledgements

Dr. Harley Benz, USGS, provided the USGS USNSN digital data. The digital data used in this study were provided by Natural Resources Canada through their AUTODRM site http://www.seismo.nrcan.gc.ca/nwfa/autodrm/autodrm_req_e.php, and IRIS using their BUD interface.

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

Appendix A

Spectra fit plots to each station

Velocity Model

The WUS 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:

DATE=Sat Apr 26 11:45:18 CDT 2008

Last Changed 2008/04/24