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

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

UCB Moment Tensor

Inversion method:   complete waveform
Stations used:      HUMO MOD YBH WDC JCC
 
 Berkeley Moment Tensor Solution
 
 Best Fitting Double-Couple:
    Mo = 7.96E+22 Dyne-cm
    Mw = 4.57
    Z  = 24. km
    Plane   Strike   Rake   Dip
     NP1      159       8    87
     NP2       69     177    82
 Principal Axes:
    Axis    Value   Plunge   Azimuth
      T     7.960       8       24
      N     0.000      81      179
      P    -7.960       4      294
 
 
 Event Date/Time: August 19, 2004 at 06:06:03 UTC
 Event ID:        uw08190606
 
 Moment Tensor: Scale = 10**22 Dyne-cm
    Component   Value
       Mxx      5.252
       Mxy      5.811
       Mxz      0.780
       Myy     -5.368
       Myz      0.881
       Mzz      0.116
                                               
                    #######                    
              --##############                 
           ------############# T ###           
         --------#############   #####         
       -----------######################       
      -------------######################      
       ------------#######################     
     P -------------######################-    
       --------------###################---    
   ------------------###############--------   
   -------------------##########------------   
   --------------------#####----------------   
   --------------------#--------------------   
   ----------------#####--------------------   
    ---------############------------------    
    -#####################-----------------    
     #####################----------------     
      #####################--------------      
       #####################------------       
         ####################---------         
           ###################------           
              #################--              
                    #######                    
                                               
NODAL PLANES STK= 154.99 DIP= 64.99 RAKE= 74.99 OR STK= 7.38 DIP= 28.91 RAKE= 119.04 DEPTH = 23.0 km Mw = 4.70 Best Fit 0.8539 - P-T axis plot gives solutions with FIT greater than FIT90

Focal Mechanism

First motion data

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

Sta Az(deg)    Dist(km)   First motion
GNW	   19	  343	i+
OCWA	    2	  344	i+
HAWA	   61	  420	i+

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. A nearly vertical strike-slip fault striking at 75 or 165 degrees is preferred. 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)   
LON	   39	  303
GNW	   19	  343
OCWA	    2	  344
HAWA	   61	  420
PGC	    8	  449
WDC	  162	  476
WVOR	  116	  521
CBB	  353	  603
WSLR	    9	  617
PNT	   33	  629
NEW	   52	  682
LLLB	   14	  686
HLID	   95	  801
CMB	  155	  806
MSO	   70	  842
MNV	  141	  861
BBB	  343	  883
TPH	  139	  941
SPU	  107	 1036
DUG	  114	 1067
YFT	   86	 1070
AHID	   97	 1084
REDW	   93	 1088
HWUT	  104	 1090
DAC	  146	 1091
NOQ	  110	 1093
ISA	  152	 1115
CTU	  109	 1119
TCU	  106	 1124
NLU	  113	 1135
JLU	  109	 1146
MPU	  112	 1164
BW06	   95	 1206
EDM	   36	 1244
SRU	  114	 1298
LAO	   74	 1425
WUAZ	  128	 1496
DGMT	   68	 1595
ISCO	  103	 1631
SDCO	  110	 1749
YKW3	   14	 2090
CBKS	   99	 2136
WMOK	  108	 2437
FCC	   42	 2578
INK	  352	 2690
MIAR	  103	 2847
FVM	   93	 2910
UALR	  101	 2930
PVMO	   95	 3047
MPH	   98	 3099
USIN	   91	 3130
OXF	   99	 3174
LRAL	   99	 3451
ERPA	   79	 3560
NCB	   73	 3935
NHSC	   93	 3984
ACCN	   74	 3999
PAL	   78	 4077

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

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 Fri Aug 20 09:24:20 CDT 2004