Location - USGS NEIC

2006/06/18 00:05:33 45.74N 111.80W 5. 4.3 Montana

Location - Montana Bureau of Mines and Geology

  DATE    ORIGIN   LAT N LONG W  DEPTH    MAG NO DM GAP M  RMS  ERH ERZ Q
2060618  000532.40 45.60 111.90  11.53   4.03 51 21  44 1 0.14  0.3 0.4 B

Michael Stickney, Director
Earthquake Studies Office
Montana Bureau of Mines and Geology
Montana Tech of the University of Montana
1300 W Park St
Butte, MT 59701

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
 2006/06/18 00:05:32 45.60N 111.90W 11 4.0 Montana
 
 Best Fitting Double Couple
    Mo = 2.02e+22 dyne-cm
    Mw = 4.17 
    Z  = 10 km
     Plane   Strike  Dip  Rake
      NP1      305    60   -120
      NP2      174    41   -49
 Principal Axes:
   Axis    Value   Plunge  Azimuth
     T   2.02e+22     10      56
     N   0.00e+00     26     321
     P  -2.02e+22     62     166



 Moment Tensor: (dyne-cm)
    Component  Value
       Mxx     1.94e+21
       Mxy     1.01e+22
       Mxz     1.01e+22
       Myy     1.32e+22
       Myz     8.80e+20
       Mzz    -1.51e+22
                                                     
                                                     
                                                     
                                                     
                     ---###########                  
                 -----#################              
              ------######################           
             ------########################          
           #######-#######################           
          #######--------################# T #       
         ########------------#############   ##      
        ########----------------################     
        ########------------------##############     
       #########---------------------############    
       #########----------------------###########    
       #########------------------------#########    
       #########-------------------------########    
        #########-----------   ------------#####     
        #########----------- P ------------#####     
         #########----------   -------------###      
          #########--------------------------#       
           #########-------------------------        
             ########----------------------          
              ########--------------------           
                 #######---------------              
                     ######--------                  
                                                     
                                                     
                                                     

 Harvard Convention
 Moment Tensor:
      R          T          F
 -1.51e+22   1.01e+22  -8.80e+20 
  1.01e+22   1.94e+21  -1.01e+22 
 -8.80e+20  -1.01e+22   1.32e+22 


Details of the solution is found at

http://www.eas.slu.edu/eqc/eqc_mt/NEW/20060618000533/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 = 305
      DIP = 60
     RAKE = -120
       MW = 4.17
       HS = 10

Both techniques give the same depth and moment. The waveforms inversionis more strike-slip like. However, To use the data at the closest station, BOZ, we had to use the Montana Bureau of Mines and Geology location. The surface-wave spectral amplitude solution is preferred.

First motion data

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

Sta Az(deg)    Dist(km)   First motion
BOZ       129   17 eP_X
BOZ        77   22 iP_C
LKWY      139  171 eP_+
MSO       307  205 eP_X
IMW       161  216 iP_D
MOOW      159  237 iP_D
REDW      164  275 eP_D
EGMT       31  298 iP_C
HLID      222  319 eP_-
AHID      170  336 eP_X
BW06      151  376 ePn
LAO        74  443 ePn
HWUT      178  460 ePn

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   105    75   -20   3.63 0.3196
WVFGRD96    1.0   105    80   -10   3.66 0.3310
WVFGRD96    2.0   105    75   -15   3.77 0.3634
WVFGRD96    3.0   290    85    35   3.84 0.3395
WVFGRD96    4.0   290    80    45   3.89 0.3591
WVFGRD96    5.0   290    80    45   3.92 0.3796
WVFGRD96    6.0   360    50   -35   3.93 0.3997
WVFGRD96    7.0   360    50   -35   3.95 0.4174
WVFGRD96    8.0   360    50   -35   4.00 0.4397
WVFGRD96    9.0    10    65   -25   4.02 0.4484
WVFGRD96   10.0   190    60   -20   4.04 0.4628
WVFGRD96   11.0   190    60   -15   4.06 0.4754
WVFGRD96   12.0   190    60   -15   4.08 0.4840
WVFGRD96   13.0   190    65   -15   4.10 0.4887
WVFGRD96   14.0   190    65   -15   4.11 0.4897
WVFGRD96   15.0   190    65   -15   4.12 0.4858
WVFGRD96   16.0   190    70   -15   4.14 0.4800
WVFGRD96   17.0   190    70   -15   4.15 0.4708
WVFGRD96   18.0   195    75    -5   4.18 0.4596
WVFGRD96   19.0   190    75   -10   4.18 0.4463
WVFGRD96   20.0   190    75   -10   4.19 0.4348
WVFGRD96   21.0   190    70   -15   4.22 0.4248
WVFGRD96   22.0   190    70   -10   4.24 0.4164
WVFGRD96   23.0   195    70     0   4.26 0.4085
WVFGRD96   24.0   195    70     0   4.27 0.3998
WVFGRD96   25.0   195    75     0   4.28 0.3922
WVFGRD96   26.0   195    75     0   4.28 0.3846
WVFGRD96   27.0   195    75     0   4.29 0.3762
WVFGRD96   28.0   195    75     0   4.30 0.3666
WVFGRD96   29.0   195    55   -15   4.29 0.3585

The best solution is

WVFGRD96   14.0   190    65   -15   4.11 0.4897

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=     304.98
  DIP=      59.99
 RAKE=    -120.00
  
             OR
  
  STK=     174.08
  DIP=      41.41
 RAKE=     -49.12
 
 
DEPTH = 10.0 km
 
Mw = 4.17
Best Fit 0.8864 - 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.

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.
Figure 2. Location of broadband stations for surface wave study

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)   
MSO	  307	  205
IMW	  161	  216
MOOW	  159	  237
REDW	  164	  275
EGMT	   31	  298
HLID	  222	  319
AHID	  170	  336
BW06	  151	  376
LAO	   74	  443
HWUT	  178	  460
RWWY	  139	  583
HAWA	  280	  603
WVOR	  238	  660
PHWY	  132	  711
LON	  282	  780
MVU	  183	  805
ISCO	  140	  832
HUMO	  253	  958
TPH	  210	  963
MNV	  215	  967
SDCO	  147	 1031
WDC	  240	 1044
DAC	  207	 1158
GSC	  202	 1234
ISA	  209	 1253
SAO	  222	 1282
MWC	  205	 1385
GLA	  192	 1433
TUC	  176	 1494
BAR	  198	 1510
AMTX	  141	 1525
EYMN	   74	 1564
MNTX	  158	 1655
CCM	  109	 1900
SLM	  106	 1941
LTX	  156	 1956
FVM	  108	 1965
JCT	  144	 1990
UALR	  120	 2046
SIUC	  108	 2071
PVMO	  112	 2117
NATX	  130	 2143
UTMT	  110	 2182
MPH	  115	 2188
WVT	  110	 2272
PLAL	  112	 2330
LTL	  126	 2486
ERPA	   88	 2570
MCWV	   93	 2685
SSPA	   90	 2794
CBN	   94	 2949
NCB	   80	 2951
NHSC	  106	 3036
PAL	   86	 3087
HRV	   82	 3208
DWPF	  116	 3306

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 3
lp c 0.10 3
br c 0.12 0.2 n 4 p 2

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 Fri Jun 23 13:11:41 CDT 2006