Location

2006/01/11 10:02:36 43.55N 127.19W 30. 4.9 Offshore Oregon

Arrival Times (from USGS)

Arrival time list

Felt Map

USGS Felt map for this earthquake

USGS Felt reports page for Pacific Northwest US

Focal Mechanism

 SLU Moment Tensor Solution
 2006/01/11 10:02:36 43.55N 127.19W  30. 4.9 Offshore Oregon
 
 Best Fitting Double Couple
    Mo = 1.00e+24 dyne-cm
    Mw = 5.30 
    Z  = 29 km
     Plane   Strike  Dip  Rake
      NP1      117    77   149
      NP2      215    60    15
 Principal Axes:
   Axis    Value   Plunge  Azimuth
     T   1.00e+24     31      72
     N   0.00e+00     57     277
     P  -1.00e+24     11     169



 Moment Tensor: (dyne-cm)
    Component  Value
       Mxx    -8.60e+23
       Mxy     3.91e+23
       Mxz     3.21e+23
       Myy     6.36e+23
       Myz     3.83e+23
       Mzz     2.24e+23
                                                     
                                                     
                                                     
                                                     
                     --------------                  
                 ----------------------              
              ---------------------#######           
             ------------------############          
           ------------------################        
          -----------------###################       
         ##--------------######################      
        ####-----------#################   #####     
        #######-------################## T #####     
       ##########----###################   ######    
       ##########################################    
       ############----##########################    
       ###########--------#######################    
        #########-------------##################     
        #########-----------------##############     
         #######------------------------#######      
          ######------------------------------       
           #####-----------------------------        
             ###---------------------------          
              ##--------------   ---------           
                 ------------- P ------              
                     ---------   --                  
                                                     
                                                     
                                                     

 Harvard Convention
 Moment Tensor:
      R          T          F
  2.24e+23   3.21e+23  -3.83e+23 
  3.21e+23  -8.60e+23  -3.91e+23 
 -3.83e+23  -3.91e+23   6.36e+23 


Details of the solution is found at

http://www.eas.slu.edu/eqc/eqc_mt/NEW/20060111100236/index.html
        
USGS Fast Moment Tensor Solution
06/01/11 10:02:36.42
 OFF COAST OF OREGON             
 Epicenter:  43.548 -127.191
 MW 5.6

 USGS MOMENT TENSOR SOLUTION
 Depth  14         No. of sta: 10
 Moment Tensor;   Scale 10**17 Nm
   Mrr=-0.01       Mtt=-2.31
   Mff= 2.32       Mrt=-0.13
   Mrf=-0.46       Mtf=-1.62
  Principal axes:
   T  Val=  2.89  Plg= 8  Azm= 73
   N       -0.04      80      218
   P       -2.85       5      342

 Best Double Couple:Mo=2.9*10**17
  NP1:Strike=117 Dip=81 Slip= 178
  NP2:       208     88         9
                                      
               -------                
          -- P -----------#           
        ----   ----------####         
      ------------------#######       
    -------------------##########     
   #------------------############    
   ###---------------###########      
  #######-----------############ T    
  ##########-------#############      
  #############---#################   
  ###############-#################   
  ##############------#############   
   ############-----------########    
   ###########------------------##    
    #########--------------------     
      #######------------------       
        ####-----------------         
          #----------------           
               -------                

        
	CENTROID, MOMENT TENSOR SOLUTION
HARVARD EVENT-FILE NAME C011106B
DATA USED: GSN
L.P. BODY WAVES: 57S,112C, T= 40
SURFACE WAVES:   69S,152C, T= 50
CENTROID LOCATION:
ORIGIN TIME       10:02:35.4 0.2
LAT 43.32N 0.01;LON 127.41W 0.01
DEP  18.2 1.0;HALF-DURATION  1.4
MOMENT TENSOR; SCALE 10**24 D-CM
  MRR=-0.17 0.04; MTT=-1.46 0.04
  MPP= 1.62 0.04; MRT=-0.14 0.08
  MRP=-0.24 0.08; MTP=-1.39 0.03
 PRINCIPAL AXES:
 1.(T) VAL=  2.17;PLG= 4;AZM= 69
 2.(N)      -0.15;    82;    191
 3.(P)      -2.02;     7;    339
BEST DOUBLE COUPLE:M0=2.1*10**24
 NP1:STRIKE=114;DIP=82;SLIP=-178
 NP2:STRIKE= 24;DIP=88;SLIP=  -8

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

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 = 215
      DIP = 60
     RAKE = 15
       MW = 5.30
       HS = 29

This solution may be acceptable. The major problem is that it an offshore event and that the wave propagation model should include the effects of a source in an oceanic crust and then wave propagation into continental crust. The waveform solution should be OK since the event was large enough so that low frequencies could be used in the waveform inversion. The use of these low frequencies should mean that the solution is relatively insensitive to source structure and to the ocean-continent transition. Note that the waveform comparisons could not be made at greater depths because such Greens functions were not computed. Finally, the depth control is not that good, although the preferred deeper depth gives better waveform fits.

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 0.033 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   210    90   -10   4.95 0.3471
WVFGRD96    1.0   210    90    -5   4.96 0.3582
WVFGRD96    2.0   210    85   -10   5.00 0.3884
WVFGRD96    3.0    30    75    -5   5.03 0.4065
WVFGRD96    4.0    25    65   -20   5.07 0.4217
WVFGRD96    5.0    25    60   -20   5.09 0.4343
WVFGRD96    6.0    25    60   -20   5.11 0.4430
WVFGRD96    7.0    30    70   -10   5.10 0.4499
WVFGRD96    8.0    25    55   -20   5.15 0.4567
WVFGRD96    9.0    30    65    -5   5.14 0.4581
WVFGRD96   10.0    30    65    -5   5.14 0.4586
WVFGRD96   11.0    30    75     5   5.14 0.4590
WVFGRD96   12.0   215    60    25   5.18 0.4660
WVFGRD96   13.0   215    60    25   5.18 0.4727
WVFGRD96   14.0   215    60    25   5.19 0.4789
WVFGRD96   15.0   215    60    25   5.19 0.4845
WVFGRD96   16.0   215    60    20   5.20 0.4900
WVFGRD96   17.0   215    60    20   5.21 0.4954
WVFGRD96   18.0   215    60    20   5.21 0.5000
WVFGRD96   19.0   215    60    20   5.22 0.5040
WVFGRD96   20.0   215    60    15   5.23 0.5077
WVFGRD96   21.0   215    60    20   5.25 0.5133
WVFGRD96   22.0   215    60    20   5.25 0.5167
WVFGRD96   23.0   215    60    20   5.26 0.5195
WVFGRD96   24.0   215    60    20   5.27 0.5217
WVFGRD96   25.0   215    60    15   5.27 0.5237
WVFGRD96   26.0   215    60    15   5.28 0.5260
WVFGRD96   27.0   215    60    15   5.29 0.5281
WVFGRD96   28.0   215    60    15   5.29 0.5296
WVFGRD96   29.0   215    60    15   5.30 0.5309

The best solution is

WVFGRD96   29.0   215    60    15   5.30 0.5309

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 0.033 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=     204.99
  DIP=      75.00
 RAKE=      34.99
  
             OR
  
  STK=     104.72
  DIP=      56.36
 RAKE=     161.88
 
 
DEPTH = 28.0 km
 
Mw = 5.37
Best Fit 0.8253 - 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
HUMO      105  360 eP_X
WDC       129  507 ePn
HAWA       60  682 eP_+
WVOR       97  708 eP
SAO       145  898 ePn
MNV       124  949 eP
HLID       86 1032 eP
MSO        66 1102 eP_+
HWUT       94 1299 eP
MWC       140 1302 eP
AHID       88 1310 eP
REDW       85 1321 eP_+
MOOW       83 1325 eP
SNOW       85 1328 eP
LOHW       84 1338 eP
LKWY       79 1348 eP
BW06       87 1435 eP_+

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)   
HUMO	  105	  360
WDC	  129	  507
LON	   48	  553
GNW	   36	  562
HAWA	   60	  682
WVOR	   97	  708
SAO	  145	  898
MNV	  124	  949
HLID	   86	 1032
MSO	   66	 1102
ISA	  137	 1151
GSC	  132	 1278
HWUT	   94	 1299
MWC	  140	 1302
AHID	   88	 1310
REDW	   85	 1321
MOOW	   83	 1325
SNOW	   85	 1328
LOHW	   84	 1338
BW06	   87	 1435
BAR	  139	 1516
GLA	  133	 1586
SIT	  342	 1609
WUAZ	  118	 1622
RWWY	   90	 1649
LAO	   71	 1682
PHWY	   90	 1801
ISCO	   96	 1839
TUC	  126	 1901
SDCO	  102	 1938
MNTX	  117	 2320
CBKS	   94	 2352
PMR	  332	 2475
KSU1	   90	 2593
LTX	  119	 2618
WMOK	  102	 2628
EYMN	   67	 2796
JCT	  112	 2817
MIAR	   98	 3053
MPH	   94	 3316
OXF	   94	 3390
WVT	   90	 3435
WCI	   86	 3462
LTL	  102	 3512
AAM	   77	 3518
ACSO	   80	 3646
LRAL	   95	 3665
ERPA	   76	 3812
MCWV	   80	 3919
GOGA	   92	 3929
CBN	   81	 4179
MVL	   78	 4181
NCB	   70	 4193
NHSC	   90	 4212
SMY	  305	 4362
DWPF	   97	 4409

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 0.033 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 Wed Jan 11 14:42:01 CST 2006