Location

2006/07/14 17:06:01 42.43N 111.54W 5. 4.0 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
 2006/07/14 17:06:01  42.43N 111.54W   5. 4.0 Idaho
 
 Best Fitting Double Couple
    Mo = 9.77e+21 dyne-cm
    Mw = 3.96 
    Z  = 10 km
     Plane   Strike  Dip  Rake
      NP1       10    75   -70
      NP2      135    25   -142
 Principal Axes:
   Axis    Value   Plunge  Azimuth
     T   9.77e+21     27      84
     N   0.00e+00     19     185
     P  -9.77e+21     56     305



 Moment Tensor: (dyne-cm)
    Component  Value
       Mxx    -9.66e+20
       Mxy     2.25e+21
       Mxz    -2.23e+21
       Myy     5.56e+21
       Myz     7.68e+21
       Mzz    -4.59e+21
                                                     
                                                     
                                                     
                                                     
                     -----------###                  
                 ---------------#######              
              -------------------#########           
             --------------------##########          
           #---------------------############        
          #----------------------#############       
         ##----------------------##############      
        ##----------   ----------###############     
        ##---------- P ----------###############     
       ###----------   ---------##########   ####    
       ####---------------------########## T ####    
       ####---------------------##########   ####    
       #####-------------------##################    
        ####-------------------#################     
        #####------------------#################     
         #####----------------#################      
          ######-------------#################       
           #######-----------################        
             #######--------###############          
              #########----##############-           
                 #########----####-----              
                     ####----------                  
                                                     
                                                     
                                                     

 Harvard Convention
 Moment Tensor:
      R          T          F
 -4.59e+21  -2.23e+21  -7.68e+21 
 -2.23e+21  -9.66e+20  -2.25e+21 
 -7.68e+21  -2.25e+21   5.56e+21 


Details of the solution is found at

http://www.eas.slu.edu/eqc/eqc_mt/MECH.NA/20060714170601/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

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 = 75
     RAKE = -70
       MW = 3.96
       HS = 10

The waveform inversion is preferred. The surface-wave spevtral amplitude solution is in agreemnt.

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   175    40    90   3.59 0.2832
WVFGRD96    1.0   185    70   -25   3.48 0.2461
WVFGRD96    2.0    15    60   -45   3.67 0.2731
WVFGRD96    3.0   175    15    75   3.81 0.2648
WVFGRD96    4.0   195    85    70   3.80 0.3178
WVFGRD96    5.0    10    85   -75   3.82 0.3695
WVFGRD96    6.0    10    80   -75   3.85 0.4096
WVFGRD96    7.0    10    80   -70   3.87 0.4391
WVFGRD96    8.0    10    80   -70   3.93 0.4581
WVFGRD96    9.0    10    80   -70   3.94 0.4696
WVFGRD96   10.0    10    75   -70   3.96 0.4739
WVFGRD96   11.0    10    80   -65   3.97 0.4732
WVFGRD96   12.0    10    75   -65   3.98 0.4669
WVFGRD96   13.0    10    75   -65   3.99 0.4547
WVFGRD96   14.0    10    75   -65   4.00 0.4370
WVFGRD96   15.0   200    75    70   3.99 0.4207
WVFGRD96   16.0   200    75    75   4.00 0.4118
WVFGRD96   17.0   200    75    75   4.01 0.4026
WVFGRD96   18.0   200    75    75   4.01 0.3929
WVFGRD96   19.0   200    75    80   4.02 0.3840
WVFGRD96   20.0   200    75    85   4.03 0.3754
WVFGRD96   21.0   200    75    85   4.10 0.3678
WVFGRD96   22.0   200    75    85   4.11 0.3572
WVFGRD96   23.0    90    25   -20   4.10 0.3502
WVFGRD96   24.0    90    25   -20   4.11 0.3433
WVFGRD96   25.0    90    25   -15   4.10 0.3356
WVFGRD96   26.0    90    25   -15   4.11 0.3293
WVFGRD96   27.0    90    25   -15   4.12 0.3219
WVFGRD96   28.0    90    25   -15   4.13 0.3139
WVFGRD96   29.0    90    30   -15   4.13 0.3051

The best solution is

WVFGRD96   10.0    10    75   -70   3.96 0.4739

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


  NODAL PLANES 

  
  STK=     199.99
  DIP=      90.00
 RAKE=      69.99
  
             OR
  
  STK=     109.97
  DIP=      20.01
 RAKE=     179.99
 
 
DEPTH = 9.0 km
 
Mw = 4.03
Best Fit 0.8872 - 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       44   52 ePgc
RRI2       10  103 ePg
HVU       235  125 iP_D
SPU       212  146 iP_D
TCU       176  146 eP_X
MOOW       24  160 ePn
BW06       76  167 eP_+
IMW        16  170 ePn
CTU       185  194 eP_-
BGU       217  209 iP_D
YFT        14  232 eP_X
YMR        10  253 eP_X
LKWY       21  254 eP_X
HLID      299  266 iP_D
MPU       182  268 eP_-
DUG       204  270 iP_D
NLU       189  279 iP_D
TMU       175  349 eP_X
BOZ       359  358 eP_-
ELK       240  361 eP_-
RWWY      102  368 eP
SRU       167  379 eP_X
MVU       188  440 eP
MVU       188  440 eP_X
HMU       172  504 eP_X
PHWY      102  520 eP_+
MSO       340  525 eP_-
ISCO      118  577 eP_X
WVOR      272  584 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)   
HVU	  235	  125
SPU	  212	  146
TCU	  176	  146
MOOW	   24	  160
BW06	   76	  167
IMW	   16	  170
CTU	  185	  194
BGU	  217	  209
YFT	   14	  232
YMR	   10	  253
HLID	  299	  266
MPU	  182	  268
DUG	  204	  270
NLU	  190	  279
BOZ	  359	  358
ELK	  240	  361
RWWY	  102	  368
SRU	  167	  379
MVU	  188	  440
HMU	  172	  504
PHWY	  102	  520
MSO	  340	  525
CCUT	  197	  564
ISCO	  118	  577
WVOR	  272	  584
LAO	   40	  634
TPH	  227	  684
MNV	  234	  716
SDCO	  133	  732
WUAZ	  179	  768
HAWA	  307	  774
DAC	  219	  860
GSC	  212	  914
HUMO	  275	  938
WDC	  261	  941
LON	  304	  946
ISA	  221	  961
GNW	  307	 1056
SAO	  237	 1057
MWC	  214	 1074
GLA	  197	 1080
TUC	  176	 1125
AMTX	  132	 1244
WMOK	  123	 1399
LTX	  152	 1616
EYMN	   62	 1685
JCT	  138	 1689
MIAR	  113	 1790
SLM	   96	 1848
FVM	   98	 1859
UALR	  111	 1870
SIUC	   98	 1969
PVMO	  102	 1989
MPH	  106	 2038
USIN	   96	 2085
WVT	  101	 2154
WCI	   94	 2187
PLAL	  104	 2193
LTL	  119	 2272
AAM	   81	 2287
ACSO	   86	 2392
ERPA	   80	 2589
SSPA	   83	 2798
CBN	   88	 2922
SDMD	   85	 2922
NCB	   74	 3013
PAL	   80	 3111
DWPF	  111	 3142
HRV	   76	 3258

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

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 Mon Jul 17 09:21:10 CDT 2006