Location

2005/07/26 04:08:36 45.40N 112.55W 14 5.44 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

05/07/26 04:08:36.09
 WESTERN MONTANA                 
 Epicenter:  45.399 -112.550
 MW 5.5

 USGS MOMENT TENSOR SOLUTION
 Depth   9         No. of sta: 17
 Moment Tensor;   Scale 10**17 Nm
   Mrr=-1.66       Mtt= 0.91
   Mff= 0.75       Mrt=-0.61
   Mrf=-0.39       Mtf=-1.17
  Principal axes:
   T  Val=  2.01  Plg= 3  Azm=222
   N       -0.04      23      131
   P       -1.97      67      319

 Best Double Couple:Mo=2.0*10**17
  NP1:Strike=335 Dip=47 Slip= -57
  NP2:       112     52      -120
                                      
               #######                
          ----#############           
        ----------###########         
      --------------###########       
    ------------------###########     
   ---------------------##########    
   -----------   --------#########    
  ##---------- P ---------#########   
  ###---------   ----------########   
  ####---------------------########   
  ######--------------------#######   
  ########------------------#######   
   ###########--------------######    
   ###############----------####--    
    ########################-----     
         ##################----       
       T ##################--         
          ################-           
               #######              
	
July 26, 2005, WESTERN MONTANA, MW=5.6

Natasha Maternovskaya

CENTROID, MOMENT TENSOR SOLUTION
HARVARD EVENT-FILE NAME C072605A
DATA USED: GSN
L.P. BODY WAVES: 50S, 98C, T= 40
SURFACE WAVES:   69S,155C, T= 50
CENTROID LOCATION:
ORIGIN TIME       04:08:39.4 0.1
LAT 45.34N 0.01;LON 112.50W 0.01
DEP  13.3 0.5;HALF-DURATION  1.5
MOMENT TENSOR; SCALE 10**24 D-CM
  MRR=-1.95 0.04; MTT=-0.04 0.03
  MPP= 1.99 0.04; MRT=-1.13 0.09
  MRP= 0.13 0.08; MTP=-1.46 0.03
 PRINCIPAL AXES:
 1.(T) VAL=  2.84;PLG= 8;AZM=240
 2.(N)      -0.31;    28;    146
 3.(P)      -2.54;    61;    345
BEST DOUBLE COUPLE:M0=2.7*10**24
 NP1:STRIKE=359;DIP=44;SLIP= -48
 NP2:STRIKE=127;DIP=59;SLIP=-123

            -------####
        -------------######
      -----------------######
    --------------------#######
   ##---------   --------#######
  ###--------- P ---------#######
  ####--------   ---------#######
 ######-------------------########
 #######-------------------#######
 ########------------------#######
 ##########---------------########
  ###########-------------#######
  #   ##########----------#######
    T #############------#######
      ##################-#####-
      #################------
        ##############-----
            #######----

	
 SLU Moment Tensor Solution
 2005/07/26 04:08:36 45.40N 112.55W 14 5.44 Montana
 
 Best Fitting Double Couple
    Mo = 1.80e+24 dyne-cm
    Mw = 5.47 
    Z  = 10 km
     Plane   Strike  Dip  Rake
      NP1      359    58   -42
      NP2      115    55   -140
 Principal Axes:
   Axis    Value   Plunge  Azimuth
     T   1.80e+24      2      58
     N   0.00e+00     39     149
     P  -1.80e+24     51     325



 Moment Tensor: (dyne-cm)
    Component  Value
       Mxx     2.78e+22
       Mxy     1.14e+24
       Mxz    -6.92e+23
       Myy     1.06e+24
       Myz     5.49e+23
       Mzz    -1.09e+24
                                                     
                                                     
                                                     
                                                     
                     --------######                  
                 --------------########              
              ------------------##########           
             --------------------##########          
           -----------------------##########         
          -----------   ----------########## T       
         ------------ P -----------#########         
        ##-----------   -----------#############     
        ###-------------------------############     
       #####------------------------#############    
       ######-----------------------#############    
       #######----------------------#############    
       #########--------------------#############    
        ###########-----------------############     
        #############---------------############     
         ################----------############      
          ####################-----#########--       
           #######################-----------        
             ####################----------          
              ##################----------           
                 ##############--------              
                     #########-----                  
                                                     
                                                     
                                                     

 Harvard Convention
 Moment Tensor:
      R          T          F
 -1.09e+24  -6.92e+23  -5.49e+23 
 -6.92e+23   2.78e+22  -1.14e+24 
 -5.49e+23  -1.14e+24   1.06e+24 


Details of the solution is found at

http://www.eas.slu.edu/eqc/eqc_mt/NEW/20050726040836/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 = 115
      DIP = 55
     RAKE = -140
       MW = 5.47
       HS = 10

The preferred solution is based on the surface-wave amplitude spectrum fit because sprectral holes int he Rayleigh mechanism are a direct indicator of depth and mechanism.

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 0.01 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   200    60    20   5.07 0.3415
WVFGRD96    1.0   200    65    25   5.10 0.3552
WVFGRD96    2.0   195    70    15   5.17 0.3942
WVFGRD96    3.0   195    70    10   5.22 0.4003
WVFGRD96    4.0   190    65    -5   5.27 0.3968
WVFGRD96    5.0    10    75   -40   5.32 0.4063
WVFGRD96    6.0   360    60   -45   5.37 0.4345
WVFGRD96    7.0   360    60   -45   5.38 0.4591
WVFGRD96    8.0   360    60   -50   5.42 0.4839
WVFGRD96    9.0   360    55   -45   5.43 0.5033
WVFGRD96   10.0   360    55   -45   5.44 0.5175
WVFGRD96   11.0   360    55   -45   5.44 0.5252
WVFGRD96   12.0    10    65   -35   5.42 0.5316
WVFGRD96   13.0    10    65   -35   5.43 0.5361
WVFGRD96   14.0    10    65   -30   5.44 0.5379
WVFGRD96   15.0    10    65   -30   5.45 0.5377
WVFGRD96   16.0    10    65   -30   5.45 0.5358
WVFGRD96   17.0    10    65   -25   5.46 0.5309
WVFGRD96   18.0    10    65   -25   5.47 0.5258
WVFGRD96   19.0    15    75   -25   5.47 0.5200

The best solution is

WVFGRD96   14.0    10    65   -30   5.44 0.5379

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 0.01 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=     114.99
  DIP=      59.99
 RAKE=    -135.00
  
             OR
  
  STK=     358.42
  DIP=      52.24
 RAKE=     -39.24
 
 
DEPTH = 10.0 km
 
Mw = 5.47
Best Fit 0.8617 - 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
MSO       326  192 eP_X
LKWY      118  193 iP_C
REDW      149  264 eP_-
AHID      158  315 eP_+
BW06      140  378 iP_C
WALA      346  420 iP_-
HWUT      169  429 eP_+
WVOR      238  590 iP_C

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)   
MSO	  326	  192
LKWY	  118	  193
REDW	  148	  264
AHID	  158	  315
BW06	  140	  378
WALA	  346	  420
HWUT	  169	  429
LAO	   71	  510
WVOR	  238	  590
PNT	  312	  688
TTW	  293	  746
SLEB	  329	  763
GNW	  291	  825
EDM	  356	  872
TPH	  207	  901
PGC	  298	  904
LLLB	  313	  905
SHB	  303	  972
WDC	  240	  974
SDCO	  143	 1033
OZB	  297	 1057
CBB	  303	 1090
DAC	  204	 1098
GSC	  199	 1178
ISA	  207	 1191
EDB	  300	 1201
SAO	  221	 1214
PHC	  303	 1254
MWC	  203	 1327
ULM	   61	 1356
BBB	  309	 1367
GLA	  189	 1385
BAR	  196	 1456
TUC	  173	 1461
KSU1	  112	 1487
MNTX	  155	 1643
MOBC	  309	 1646
FNBB	  338	 1660
DLBC	  327	 1874
YKW3	  357	 1917
LTX	  153	 1946
SLM	  104	 1987
FVM	  106	 2010
KNDN	    5	 2019
SNPN	    2	 2022
MGTN	    4	 2046
BOXN	    4	 2064
LON	  285	 2076
MLON	    4	 2076
UALR	  118	 2079
MCKN	    3	 2100
ILKN	  356	 2104
GALN	  354	 2107
LGSN	    3	 2115
SIUC	  106	 2116
CTLN	  355	 2132
DVKN	    3	 2134
LDGN	    3	 2141
EKTN	    2	 2154
PVMO	  110	 2158
GLWN	    4	 2163
NATX	  128	 2164
YMBN	    1	 2171
ACKN	    2	 2186
COWN	    2	 2216
USIN	  103	 2220
UTMT	  109	 2225
MPH	  113	 2226
WHY	  327	 2244
LUPN	    2	 2268
KAPO	   68	 2297
OXF	  114	 2306
WCI	  101	 2311
YRTN	   27	 2334
PLAL	  111	 2370
LRAL	  113	 2582
SADO	   80	 2614
ALLY	   88	 2636
MCWV	   92	 2743
QILN	   24	 2832
SDMD	   90	 2989
NHSC	  105	 3082
PAL	   85	 3149
HRV	   81	 3272
DWPF	  114	 3343

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 0.01 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 Thu Aug 11 12:20:29 CDT 2005