Arrival Times (from USGS)

Arrival time list

Felt Map

USGS Felt map for this earthquake

USGS Felt reports page for California

Focal Mechanism

UC Berkeley Solution

UC Berkeley moment tensor solution

SLU Determination

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


  NODAL PLANES 

  
  STK=     104.99
  DIP=      54.99
 RAKE=      69.99
  
             OR
  
  STK=     317.40
  DIP=      39.68
 RAKE=     116.04
 
 
DEPTH = 5.0 km
 
Mw = 4.62
Best Fit 0.8414 - P-T axis plot gives solutions with FIT greater than FIT90

Focal Mechanism

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, intreument 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. The figure
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 rpeferred 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. The Each solution is plotted as a vector at a given value of strike and dip witht he 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

First motion data

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

Sta Az(deg)    Dist(km)   First motion

Sta	Az	Dist	First motion
CMB	14	267	i+

Broadband station distributiuon

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

Sta Az(deg)    Dist(km)   
CMB	   14	  267
DAC	   78	  323
HOPS	  335	  405
TPH	   52	  435
PFO	  117	  485
WDC	  347	  556
BMN	   32	  626
MOD	    6	  692
WVOR	   15	  777
MVU	   66	  849
WUAZ	   88	  881
HVU	   44	  989
COR	  350	 1004
TUC	  109	 1024
HLID	   31	 1044
HWUT	   49	 1057
AHID	   45	 1165
BW06	   48	 1266
ANMO	   89	 1332
OCWA	  350	 1362
SDCO	   76	 1410
NEW	   12	 1433
RSSD	   52	 1728
WMOK	   86	 2030
HKT	   98	 2443
ULM	   44	 2595
CCM	   76	 2663
JFWS	   64	 2761
SIUC	   76	 2844
WVT	   79	 2988
PLAL	   82	 2989
WCI	   74	 3094
LRAL	   85	 3135
MCWV	   70	 3634
CBN	   73	 3868
BINY	   66	 3930
PAL	   67	 4116
LBNH	   62	 4241
HRV	   64	 4290
SCHQ	   45	 4612

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 osberved 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 3
lp c 0.05 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

We did not ahve response files for the following stations: AAM KSU1 ACCN LON BEKR LUPA BGU MIAR MPU CBKS MVL CPCT NEN DAWY NLU DLBC PGC FCC PVMO REDW GNW SAO HAWA SPUT HUMO SRU INK SSPA ISCO SWET JLU TCUT KBO KEBM WCN KHMM YBH KRMB YKW3

The data from LKWY, GOGA CBN and JCT were not used because of odd spectral amplitudes. This may indicate instrument or response file problems.

Last Changed Fri Mar 19 08:06:32 CST 2004