Location

2006/06/30 16:55:01 42.432 -111.501 1. 4.3 Idaho

Arrival Times (from USGS)

Arrival time list

Felt Map

USGS Felt map for this earthquake

USGS Felt reports page for Intermountain Western US

Focal Mechanism

 SLU Moment Tensor Solution
 2006/06/30 16:55:01  42.432 -111.501   1. 4.3 Idaho
 
 Best Fitting Double Couple
    Mo = 2.11e+22 dyne-cm
    Mw = 4.15 
    Z  = 10 km
     Plane   Strike  Dip  Rake
      NP1       10    65   -70
      NP2      149    32   -126
 Principal Axes:
   Axis    Value   Plunge  Azimuth
     T   2.11e+22     18      85
     N   0.00e+00     18     181
     P  -2.11e+22     64     314



 Moment Tensor: (dyne-cm)
    Component  Value
       Mxx    -1.78e+21
       Mxy     3.55e+21
       Mxz    -5.23e+21
       Myy     1.70e+22
       Myz     1.20e+22
       Mzz    -1.52e+22
                                                     
                                                     
                                                     
                                                     
                     -----------###                  
                 ----------------######              
              ##------------------########           
             ##-------------------#########          
           ###---------------------##########        
          ###----------------------###########       
         ####----------------------############      
        ####----------   ----------#############     
        ####---------- P ----------#############     
       #####----------   ----------#########   ##    
       ######----------------------######### T ##    
       ######---------------------##########   ##    
       #######--------------------###############    
        ######-------------------###############     
        #######------------------###############     
         #######----------------###############      
          ########--------------##############       
           ########------------##############        
             ########---------#############          
              ##########-----#############           
                 ##########--##########              
                     #####---------                  
                                                     
                                                     
                                                     

 Harvard Convention
 Moment Tensor:
      R          T          F
 -1.52e+22  -5.23e+21  -1.20e+22 
 -5.23e+21  -1.78e+21  -3.55e+21 
 -1.20e+22  -3.55e+21   1.70e+22 


Details of the solution is found at

http://www.eas.slu.edu/eqc/eqc_mt/MECH.NA/20060630165501/index.html
        

The focal mechanism was determined using broadband seismic waveforms. The location of the event and the station distribution are given in Figure 1.
Figure 1. Location of broadband stations used to obtain focal mechanism from waveform inversion

Preferred Solution

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

      STK = 10
      DIP = 65
     RAKE = -70
       MW = 4.15
       HS = 10

The waveform inversion is preferred. The surface-wave inversion agrees with it.

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
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   350    45    85   3.76 0.3712
WVFGRD96    1.0    50    85     0   3.72 0.3573
WVFGRD96    2.0    50    75    -5   3.82 0.3898
WVFGRD96    3.0    45    50    10   3.88 0.4397
WVFGRD96    4.0    45    50    15   3.91 0.4940
WVFGRD96    5.0    45    50    20   3.94 0.5261
WVFGRD96    6.0   220    55    10   3.96 0.5486
WVFGRD96    7.0    10    70   -75   4.07 0.5836
WVFGRD96    8.0   360    65   -80   4.10 0.6082
WVFGRD96    9.0   360    65   -75   4.12 0.6221
WVFGRD96   10.0    10    65   -70   4.15 0.6265
WVFGRD96   11.0    10    65   -70   4.16 0.6205
WVFGRD96   12.0    10    65   -65   4.16 0.6083
WVFGRD96   13.0    10    65   -65   4.17 0.5884
WVFGRD96   14.0    10    65   -60   4.17 0.5638
WVFGRD96   15.0    10    65   -60   4.17 0.5362
WVFGRD96   16.0    10    65   -55   4.17 0.5071
WVFGRD96   17.0    10    65   -55   4.17 0.4772
WVFGRD96   18.0   195    70   -50   4.20 0.4494
WVFGRD96   19.0   195    70   -50   4.20 0.4232
WVFGRD96   20.0   205    75    75   4.22 0.4100
WVFGRD96   21.0   205    75    80   4.29 0.3955
WVFGRD96   22.0   200    75    80   4.29 0.3795
WVFGRD96   23.0   200    75    80   4.30 0.3632
WVFGRD96   24.0   200    75    80   4.30 0.3464
WVFGRD96   25.0   120    25    20   4.25 0.3309
WVFGRD96   26.0   125    25    25   4.26 0.3261
WVFGRD96   27.0   125    25    25   4.26 0.3213
WVFGRD96   28.0   125    25    25   4.27 0.3157
WVFGRD96   29.0   125    25    25   4.28 0.3109

The best solution is

WVFGRD96   10.0    10    65   -70   4.15 0.6265

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
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 focal mechanism was determined using broadband seismic waveforms. The location of the event and the station distribution are given in Figure 2.
Figure 2. Location of broadband stations used to obtain focal mechanism from surface-wave spectral amplitude inversion

  NODAL PLANES 

  
  STK=       8.94
  DIP=      65.82
 RAKE=     -71.68
  
             OR
  
  STK=     150.00
  DIP=      30.00
 RAKE=    -124.99
 
 
DEPTH = 10.0 km
 
Mw = 4.16
Best Fit 0.9025 - 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(deg)    Dist(km)   First motion
AHID       41   50 iP_C
HWUT      183   92 eP_+
REDW       27  116 eP_+
SNOW       28  130 iP_C
LOHW       29  150 iP_C
MOOW       23  159 iP_C
BW06       76  164 eP_+
M13A      243  251 iP_D
LKWY       20  253 eP_X
HLID      299  269 iP_D
DUG       204  272 iP_D
M12A      249  305 eP_X
BOZ       358  358 eP_-
ELK       240  364 eP_X
RWWY      102  365 iP_C
SRU       167  378 eP_X
N11A      244  396 eP_X
O11A      235  432 iP_D
MVU       188  440 iP_D
Q12A      218  470 eP_-
P11A      230  480 eP_X
O10A      242  481 eP_X
M09A      259  506 eP_X
L09A      267  511 eP_X
PHWY      102  517 iP_C
Q11A      223  531 iP_D
O09A      244  539 iP_D
TPNV      215  733 eP_X
WUAZ      179  768 eP_X

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.

The velocity model used for the search is a modified Utah model .

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 distributiuon

Sta Az(deg)    Dist(km)   
HWUT	  183	   92
REDW	   27	  116
SNOW	   28	  130
LOHW	   29	  150
MOOW	   23	  159
BW06	   76	  164
M13A	  243	  251
HLID	  299	  269
DUG	  204	  272
M12A	  249	  305
BOZ	  358	  358
ELK	  240	  364
RWWY	  102	  365
SRU	  167	  378
N11A	  244	  396
O11A	  235	  432
MVU	  188	  440
Q12A	  218	  470
P11A	  230	  480
O10A	  242	  481
M09A	  259	  506
L09A	  267	  511
PHWY	  102	  517
Q11A	  223	  531
O09A	  244	  539
ISCO	  119	  574
K08A	  276	  574
J08A	  283	  579
WVOR	  272	  587
O08A	  249	  605
Q09A	  232	  625
P08A	  243	  631
K07A	  275	  637
Q08A	  236	  673
S09A	  225	  715
U12A	  202	  716
K06A	  276	  719
I06A	  286	  728
TPNV	  215	  733
U11A	  208	  746
WUAZ	  179	  768
L05A	  270	  771
HAWA	  307	  776
U10A	  214	  786
V12A	  202	  799
R06C	  240	  801
W14A	  190	  813
H05A	  291	  824
O05C	  254	  836
W12A	  201	  844
S06C	  237	  872
X15A	  184	  884
X14A	  188	  892
H04A	  290	  899
O03C	  256	  923
G04A	  293	  935
S05C	  236	  942
M02C	  267	  949
Y14A	  188	  952
F04A	  298	  955
Y13A	  193	  978
D05A	  306	  983
J02A	  280	  991
L02A	  272	  998
N02C	  264	  999
S04C	  240	 1001
U05C	  230	 1004
Z14A	  188	 1015
D04A	  304	 1034
M01C	  271	 1045
U04C	  233	 1045
P01C	  256	 1049
O01C	  260	 1062
116A	  181	 1096
ECSD	   78	 1221
JCT	  138	 1687
SLM	   96	 1846
FVM	   98	 1856
UALR	  111	 1867
PVMO	  102	 1986
MPH	  106	 2035
USIN	   96	 2082
BLO	   91	 2131
PLAL	  104	 2190
AAM	   81	 2284
ACSO	   86	 2389

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 velocity model used for the waveform fit is a modified Utah model .

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

this solution was made possible by the use of wavefroms from the University of Utah Seismograph Network

Appendix A

The figures below show the observed spectral amplitudes (units of cm-sec) at each station and the theoretical predictions as a function of period for the mechanism given above. The modified Utah model earth model was used to define the Green's functions. For each station, the Love and Rayleigh wave spectrail amplitudes are plotted with the same scaling so that one can get a sense fo the effects of the effects of the focal mechanism and depth on the excitation of each.

Quality Control

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

The following stations did not have a valid response files:

Last Changed Tue Jan 16 14:52:51 CST 2007