Location

2005/10/31 00:23:30 44.90N 113.45W 5. 4.5 Montana

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
 2005/10/31 00:23:30 44.90N 113.45W   5. 4.5 Montana
 
 Best Fitting Double Couple
    Mo = 5.50e+22 dyne-cm
    Mw = 4.46 
    Z  = 14 km
     Plane   Strike  Dip  Rake
      NP1      303    80   -113
      NP2      190    25   -25
 Principal Axes:
   Axis    Value   Plunge  Azimuth
     T   5.50e+22     31      52
     N   0.00e+00     23     307
     P  -5.50e+22     50     187



 Moment Tensor: (dyne-cm)
    Component  Value
       Mxx    -6.66e+21
       Mxy     1.67e+22
       Mxz     4.19e+22
       Myy     2.45e+22
       Myz     2.25e+22
       Mzz    -1.78e+22
                                                     
                                                     
                                                     
                                                     
                     -----#########                  
                 -----#################              
              ------######################           
             -----#########################          
           -----#############################        
          ####-#######################   #####       
         #####-----################### T ######      
        #####---------################   #######     
        #####-------------######################     
       #####-----------------####################    
       #####-------------------##################    
       #####----------------------###############    
       #####------------------------#############    
        ####---------------------------#########     
        #####----------------------------#######     
         ####------------   ---------------####      
          ####----------- P -----------------#       
           ####----------   -----------------        
             ###---------------------------          
              ####------------------------           
                 ###-------------------              
                     #-------------                  
                                                     
                                                     
                                                     

 Harvard Convention
 Moment Tensor:
      R          T          F
 -1.78e+22   4.19e+22  -2.25e+22 
  4.19e+22  -6.66e+21  -1.67e+22 
 -2.25e+22  -1.67e+22   2.45e+22 


Details of the solution is found at

http://www.eas.slu.edu/eqc/eqc_mt/NEW/20051031002330/index.html
        
Harvard CMT Event name: 103105B

Region name: WESTERN MONTANA
Date (y/m/d): 2005/10/31

Information on data used in inversion

Wave    nsta  nrec  cutoff
Body       0     0   0
Mantle    26    36   40

Timing and location information

         hr  min   sec       lat     lon    depth   mb   Ms
PDE       0   23  30.00     44.90  -113.45    5.0  4.5  4.5
CMT       0   23  34.90     44.83  -113.30   13.4
Error              0.50      0.03     0.05    3.0
Assumed half duration:  0.4


Mechanism information

Exponent for moment tensor:  22    units: dyne-cm
         Mrr     Mtt     Mpp     Mrt     Mrp     Mtp
CMT    -4.030   0.340   3.690   5.250  -3.300  -3.050
Error   0.790   0.400   0.530   1.510   1.100   0.320

Mw = 4.5   Scalar Moment = 7.91e+22
Fault plane:  strike=176    dip=25   slip=-44
Fault plane:  strike=307    dip=73   slip=-108
Eigenvector:  eigenvalue:  8.17   plunge: 26   azimuth:  51
Eigenvector:  eigenvalue: -0.53   plunge: 18   azimuth: 312
Eigenvector:  eigenvalue: -7.65   plunge: 58   azimuth: 192

	

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 = 190
      DIP = 25
     RAKE = -25
       MW = 4.46
       HS = 14

Both the surface-wave spectral amplitude and the waveform inversion demand a near vertical dip-slip solution. The depth and moment estimates are considtent. The waveform inversion result is used to characterized this event.

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 0.014 3
lp c 0.06 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   315    50   -90   4.20 0.4428
WVFGRD96    1.0   115    50   -80   4.20 0.4089
WVFGRD96    2.0   135    50   -90   4.30 0.4446
WVFGRD96    3.0   325    30   -75   4.34 0.3774
WVFGRD96    4.0   180    20   -25   4.39 0.4059
WVFGRD96    5.0   180    15   -30   4.41 0.4678
WVFGRD96    6.0   185    15   -25   4.41 0.5185
WVFGRD96    7.0   185    20   -25   4.41 0.5598
WVFGRD96    8.0   185    15   -25   4.45 0.5917
WVFGRD96    9.0   180    20   -35   4.46 0.6183
WVFGRD96   10.0   185    20   -30   4.46 0.6397
WVFGRD96   11.0   185    20   -30   4.46 0.6538
WVFGRD96   12.0   180    20   -35   4.46 0.6638
WVFGRD96   13.0   185    20   -30   4.46 0.6686
WVFGRD96   14.0   190    25   -25   4.46 0.6699
WVFGRD96   15.0   195    25   -20   4.46 0.6694
WVFGRD96   16.0   195    25   -20   4.46 0.6662
WVFGRD96   17.0   195    25   -20   4.46 0.6602
WVFGRD96   18.0   200    25   -15   4.46 0.6532
WVFGRD96   19.0   200    25   -15   4.46 0.6448

The best solution is

WVFGRD96   14.0   190    25   -25   4.46 0.6699

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 0.014 3
lp c 0.06 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=     319.99
  DIP=      69.99
 RAKE=     -74.99
  
             OR
  
  STK=     101.90
  DIP=      24.82
 RAKE=    -125.42
 
 
DEPTH = 13.0 km
 
Mw = 4.54
Best Fit 0.8947 - 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
HLID      208  168 iP_D
MSO       350  218 iP_C
LKWY       98  244 iP_C
REDW      129  269 iP_D
AHID      141  303 iP_D
BW06      126  393 eP_X
HWUT      157  397 iP_D
HAWA      291  503 eP_-
LAO        68  596 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)   
HLID	  208	  168
MSO	  350	  218
REDW	  129	  269
AHID	  141	  303
BW06	  126	  393
HWUT	  157	  397
WVOR	  239	  500
HAWA	  291	  503
LAO	   68	  596
LON	  290	  682
TTW	  299	  707
MVU	  171	  718
HUMO	  255	  807
MNV	  210	  818
TPH	  204	  821
ISCO	  129	  858
WDC	  240	  885
LLLB	  318	  895
EDM	    0	  926
DAC	  202	 1020
SDCO	  137	 1036
WUAZ	  170	 1057
CBB	  307	 1063
GSC	  196	 1104
ISA	  204	 1111
SAO	  219	 1127
EDB	  303	 1169
MWC	  200	 1250
GLA	  186	 1321
CBKS	  116	 1322
BBB	  312	 1349
BMBC	  337	 1383
BAR	  193	 1385
TUC	  170	 1417
KSU1	  109	 1533
MNTX	  152	 1625
WMOK	  127	 1684
FNBB	  341	 1688
EYMN	   71	 1716
DLBC	  329	 1884
LTX	  150	 1931
YKW3	  358	 1969
CCM	  105	 1997
JCT	  139	 1997
FCC	   34	 2026
SIT	  320	 2032
SLM	  102	 2044
MIAR	  117	 2047
FVM	  104	 2063
UALR	  115	 2118
SIUC	  103	 2171
NATX	  125	 2187
PVMO	  107	 2208
MPH	  110	 2271
USIN	  101	 2277
OXF	  111	 2350
WVT	  105	 2367
WCI	   99	 2371
KAPO	   67	 2383
ACSO	   91	 2540
LTL	  121	 2542
LRAL	  111	 2627
DAWY	  332	 2680
ERPA	   85	 2705
GOGA	  106	 2866
KGNO	   78	 2913
GAC	   75	 2953
CBN	   91	 3074
NCB	   78	 3096
NHSC	  103	 3137
PAL	   84	 3225
LSCT	   82	 3252
HRV	   80	 3351
DWPF	  112	 3386

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 0.014 3
lp c 0.06 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.

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 Nov 1 08:02:02 CST 2005