Location

2007/09/01 18:32:02 41.64N 112.33W 1.0 3.7 Utah

Arrival Times (from USGS)

Arrival time list

Felt Map

USGS Felt map for this earthquake

USGS Felt reports page for

Focal Mechanism

 SLU Moment Tensor Solution
 2007/09/01 18:32:02 41.64N 112.33W 1.0 3.7 Utah
 
 Best Fitting Double Couple
    Mo = 3.89e+21 dyne-cm
    Mw = 3.66 
    Z  = 9 km
     Plane   Strike  Dip  Rake
      NP1      155    85   175
      NP2      245    85     5
 Principal Axes:
   Axis    Value   Plunge  Azimuth
     T   3.89e+21      7     110
     N   0.00e+00     83     290
     P  -3.89e+21      0     200



 Moment Tensor: (dyne-cm)
    Component  Value
       Mxx    -3.01e+21
       Mxy    -2.46e+21
       Mxz    -1.60e+20
       Myy     2.95e+21
       Myz     4.47e+20
       Mzz     5.89e+19
                                                     
                                                     
                                                     
                                                     
                     --------------                  
                 ##--------------------              
              ######----------------------           
             ########----------------------          
           ##########------------------------        
          ############------------------------       
         ##############----------------------##      
        ################----------------########     
        ################-----------#############     
       ##################------##################    
       ###################-######################    
       ################----######################    
       ############---------#####################    
        ########-------------################        
        #####-----------------############### T      
         #---------------------##############        
          ----------------------##############       
           ----------------------############        
             ---------------------#########          
              ---------------------#######           
                 ---   -------------###              
                     P ------------                  
                                                     
                                                     
                                                     

 Harvard Convention
 Moment Tensor:
      R          T          F
  5.89e+19  -1.60e+20  -4.47e+20 
 -1.60e+20  -3.01e+21   2.46e+21 
 -4.47e+20   2.46e+21   2.95e+21 


Details of the solution is found at

http://www.eas.slu.edu/eqc/eqc_mt/MECH.NA/20070901183202/index.html
        

Preferred Solution

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

      STK = 245
      DIP = 85
     RAKE = 5
       MW = 3.66
       HS = 9

The two solutions are equivalent. The surface-wave solution is essentially the same.

Waveform Inversion

The focal mechanism was determined using broadband seismic waveforms. The location of the event and the and stations used for the waveform inversion are shown in the next figure.
Location of broadband stations used for 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    65    85     0   3.21 0.2165
WVFGRD96    1.0    65    90     5   3.25 0.2401
WVFGRD96    2.0    65    90     5   3.40 0.3365
WVFGRD96    3.0    65    90    10   3.46 0.3826
WVFGRD96    4.0    65    85    10   3.51 0.4217
WVFGRD96    5.0   245    80    -5   3.56 0.4578
WVFGRD96    6.0    65    85   -10   3.60 0.4924
WVFGRD96    7.0    65    85   -10   3.64 0.5238
WVFGRD96    8.0    65    85   -10   3.69 0.5519
WVFGRD96    9.0    65    85   -10   3.71 0.5701
WVFGRD96   10.0    65    85   -10   3.74 0.5802
WVFGRD96   11.0    65    90    -5   3.76 0.5842
WVFGRD96   12.0    65    90    -5   3.78 0.5834
WVFGRD96   13.0    65    90    -5   3.79 0.5784
WVFGRD96   14.0    65    90    -5   3.81 0.5696
WVFGRD96   15.0    65    90    -5   3.82 0.5578
WVFGRD96   16.0    65    90    -5   3.83 0.5439
WVFGRD96   17.0    65    90    -5   3.84 0.5281
WVFGRD96   18.0   245    90     5   3.85 0.5118
WVFGRD96   19.0    65    90    -5   3.85 0.4952
WVFGRD96   20.0    65    90    -5   3.86 0.4777
WVFGRD96   21.0   245    90     0   3.87 0.4609
WVFGRD96   22.0    65    90     0   3.87 0.4442
WVFGRD96   23.0   245    90     0   3.87 0.4280
WVFGRD96   24.0    65    90     5   3.87 0.4119
WVFGRD96   25.0   245    90     0   3.88 0.3971
WVFGRD96   26.0    65    90     0   3.88 0.3838
WVFGRD96   27.0   245    90     0   3.88 0.3703
WVFGRD96   28.0   245    80    10   3.89 0.3582
WVFGRD96   29.0   245    80    10   3.89 0.3467

The best solution is

WVFGRD96   11.0    65    90    -5   3.76 0.5842

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 following figure shows the stations used in the grid search for the best focal mechanism to fit the surface-wave spectral amplitudes of the Love and Rayleigh waves.
Location of broadband stations used to obtain focal mechanism from surface-wave spectral amplitudes

The surface-wave determined focal mechanism is shown here.


  NODAL PLANES 

  
  STK=     244.99
  DIP=      85.00
 RAKE=      14.99
  
             OR
  
  STK=     153.65
  DIP=      75.06
 RAKE=     174.82
 
 
DEPTH = 9.0 km
 
Mw = 3.79
Best Fit 0.8670 - 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
HWUT       93   64 -12345
M16A      121   69 -12345
L16A       61   85 -12345
M14A      260   86 -12345
L14A      300   87 -12345
N16A      138  112 -12345
K14A      325  123 -12345
N17A      121  147 -12345
O15A      184  151 -12345
M13A      259  156 -12345
AHID       39  161 -12345
DUG       194  166 -12345
N13A      242  180 -12345
RRI2       23  207 -12345
J14A      333  211 -12345
O13A      220  218 -12345
P15A      179  230 -12345
TPAW       28  235 -12345
K12A      298  240 -12345
SNOW       32  240 -12345
N12A      250  243 -12345
BW06       60  261 -12345
ELK       249  263 -12345
I14A      341  270 -12345
HLID      322  274 -12345
I13A      331  292 -12345
Q15A      181  293 -12345
P12A      223  325 -12345
G13A      339  413 -12345
BOZ         7  449 -12345
F13A      341  488 -12345

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.

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 distribution

The distribution of broadband stations with azimuth and distance is

Sta Az(deg)    Dist(km)   
K14A	  325	  123
N17A	  121	  147
O15A	  184	  151
M13A	  259	  156
AHID	   39	  161
DUG	  194	  166
O16A	  156	  174
N13A	  242	  180
K13A	  308	  183
J16A	   18	  191
J15A	  358	  196
O17A	  140	  209
J14A	  333	  211
M12A	  264	  217
O13A	  220	  218
REDW	   32	  227
P15A	  179	  230
P16A	  166	  233
TPAW	   28	  235
P14A	  196	  236
K12A	  298	  240
N12A	  250	  243
J13A	  323	  247
O18A	  127	  248
O12A	  234	  254
I16A	   15	  258
BW06	   60	  261
LOHW	   32	  261
I15A	  357	  262
ELK	  249	  263
I14A	  340	  270
HLID	  322	  274
IMW	   24	  275
P17A	  150	  276
N19A	  106	  277
P13A	  211	  282
P18A	  141	  285
J12A	  309	  290
L11A	  283	  290
M11A	  267	  290
I13A	  330	  292
Q15A	  181	  293
N11A	  253	  300
Q14A	  196	  305
Q16A	  162	  318
P12A	  223	  325
O11A	  240	  327
K11A	  294	  331
Q13A	  206	  331
H15A	  356	  332
Q18A	  146	  338
H14A	  346	  341
L10A	  279	  347
J11A	  306	  348
M10A	  269	  351
H16A	   14	  352
Q12A	  217	  358
P19A	  127	  360
H13A	  335	  361
N10A	  255	  365
P11A	  232	  371
R14A	  189	  376
R16A	  169	  380
R15A	  179	  381
O10A	  248	  382
H12A	  328	  383
R17A	  158	  383
I11A	  312	  390
G15A	  358	  392
K10A	  290	  396
Q19A	  138	  396
G14A	  348	  411
G13A	  339	  413
J10A	  300	  415
P10A	  239	  415
R12A	  209	  416
R18A	  149	  417
Q11A	  223	  420
RWWY	   88	  426
M09A	  268	  428
S14A	  190	  437
S16A	  172	  440
O09A	  250	  441
N09A	  260	  444
L09A	  277	  445
BOZ	    7	  449
H11A	  320	  453
I10A	  308	  455
R19A	  144	  455
R11A	  218	  459
K09A	  287	  461
S17A	  163	  464
Q10A	  229	  466
F15A	  358	  467
P09A	  242	  468
S18A	  155	  482
H10A	  314	  486
F13A	  341	  488
L08A	  279	  503
R10A	  224	  504
M08A	  270	  505
S19A	  147	  512
T14A	  188	  512
I09A	  302	  513
O08A	  255	  513
T15A	  180	  513
F12A	  334	  514
Q09A	  234	  517
T16A	  172	  522
K08A	  285	  523
G11A	  324	  525
WVOR	  282	  529
T13A	  195	  531
E15A	  357	  532
P08A	  248	  532
S11A	  215	  532
T17A	  165	  532
BMO	  313	  539
E14A	  350	  539
T18A	  156	  543
T11A	  208	  548
H09A	  310	  549
S10A	  223	  551
E13A	  345	  554
R09A	  229	  554
G10A	  318	  561
F11A	  328	  563
N07B	  262	  565
Q08A	  239	  568
M07A	  270	  572
O07A	  256	  575
U17A	  165	  578
U15A	  180	  579
K07A	  284	  583
T12A	  201	  583
L07A	  276	  584
U14A	  188	  584
E12A	  335	  591
MSO	  348	  591
U13A	  194	  597
G09A	  314	  598
D15A	  359	  600
P07A	  249	  602
S09A	  226	  604
T19A	  151	  605
H08A	  304	  607
E11A	  331	  609
D14A	  352	  612
R08A	  235	  614
U16A	  170	  619
TPNV	  214	  621
F10A	  322	  622
Q07A	  244	  627
D13A	  345	  629
F09A	  317	  638
V15A	  179	  646
D12A	  339	  648
O06A	  258	  652
E10A	  326	  660
MOD	  275	  663
V14A	  186	  670
G08A	  309	  672
H07A	  302	  674
D11A	  333	  680
S08C	  229	  680
WCN	  250	  681
M06C	  269	  683
WUAZ	  173	  685
C14A	  351	  690
C13A	  346	  694
F08A	  314	  695
R07C	  238	  697
R06C	  243	  699
L05A	  276	  707
E09A	  321	  714
ELFS	  264	  714
W14A	  186	  716
V19A	  156	  717
D10A	  328	  719
W16A	  174	  730
R05C	  246	  736
W17A	  168	  742
LAO	   39	  743
O05C	  258	  748
P05C	  252	  748
C11A	  337	  756
W18A	  162	  759
E08A	  317	  762
B13A	  348	  767
S06C	  240	  767
D09A	  324	  768
W19A	  160	  769
HATC	  266	  771
HAWA	  315	  782
K04A	  281	  784
LAVA	  249	  784
C10A	  332	  791
M04C	  274	  791
X15A	  179	  794
L04A	  278	  795
CMB	  242	  797
D08A	  321	  798
B12A	  342	  801
GSC	  210	  804
X16A	  174	  805
E07A	  315	  813
X18A	  164	  816
T06C	  233	  818
B11A	  339	  819
M03C	  270	  819
X17A	  170	  822
R04C	  246	  825
A13A	  349	  827
NEW	  334	  827
C09A	  328	  830
G05A	  302	  830
Q04C	  251	  830
B10A	  334	  834
S05C	  238	  837
X19A	  161	  844
SUTB	  255	  845
A12A	  343	  851
ISA	  221	  852
YBH	  274	  864
OGNE	   92	  865
Y16A	  175	  865
D07A	  318	  866
A11A	  340	  872
M02C	  272	  878
Y13A	  189	  879
B09A	  331	  882
Q03C	  251	  890
Y17A	  171	  891
O02C	  263	  895
Y18A	  166	  896
S04C	  242	  898
C07A	  320	  909
A10A	  335	  913
D06A	  315	  916
N02C	  268	  924
Z16A	  175	  925
Z15A	  179	  927
B08A	  326	  928
K02A	  281	  929
A09A	  331	  951
SAO	  238	  955
HOPS	  256	  958
OSI	  218	  960
GLA	  194	  978
MCCM	  250	  981
117A	  172	 1016
119A	  164	 1021
A07A	  325	 1036
B06A	  320	 1049
BAR	  202	 1066
A06A	  322	 1094
CBKS	  102	 1116
NLWA	  309	 1116
AMTX	  125	 1195
ECSD	   74	 1307
KSU1	   97	 1362
JCT	  134	 1672
FVM	   95	 1913

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 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


Spectra fit plots to each station

Velocity Model

The WUS used for the waveform synthetic seismograms and for the surface wave eigenfunctions and dispersion is as follows:

MODEL.01
Model after     8 iterations
ISOTROPIC
KGS
FLAT EARTH
1-D
CONSTANT VELOCITY
LINE08
LINE09
LINE10
LINE11
      H(KM)   VP(KM/S)   VS(KM/S) RHO(GM/CC)         QP         QS       ETAP       ETAS      FREFP      FREFS
     1.9000     3.4065     2.0089     2.2150  0.302E-02  0.679E-02   0.00       0.00       1.00       1.00    
     6.1000     5.5445     3.2953     2.6089  0.349E-02  0.784E-02   0.00       0.00       1.00       1.00    
    13.0000     6.2708     3.7396     2.7812  0.212E-02  0.476E-02   0.00       0.00       1.00       1.00    
    19.0000     6.4075     3.7680     2.8223  0.111E-02  0.249E-02   0.00       0.00       1.00       1.00    
     0.0000     7.9000     4.6200     3.2760  0.164E-10  0.370E-10   0.00       0.00       1.00       1.00    

Quality Control

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

The following stations did not have a valid response files:

DATE=Fri Sep 7 15:47:34 CDT 2007

Last Changed 2007/09/01