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=     310.00
  DIP=      90.00
 RAKE=     150.00
  
             OR
  
  STK=      40.00
  DIP=      60.00
 RAKE=       0.00
 
 
DEPTH = 5.0 km
 
Mw = 4.43
Best Fit 0.8531 - 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

Broadband station distributiuon

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

Sta Az(deg)    Dist(km)   
CMB	   36	  169
HOPS	  331	  278
DAC	   98	  356
TPH	   68	  405
WDC	  348	  428
BMN	   42	  549
MOD	   10	  575
PFO	  126	  582
WVOR	   21	  672
COR	  351	  876
WUAZ	   96	  924
CTU	   60	  952
HLID	   37	  963
HWUT	   55	 1010
TUC	  114	 1104
AHID	   50	 1109
BW06	   53	 1218
OCWA	  351	 1234
YMR	   42	 1246
LKWY	   44	 1273
NEW	   14	 1322
ANMO	   94	 1374
SDCO	   81	 1423
RSSD	   56	 1686
LTX	  111	 1858
WMOK	   90	 2064
HKT	  100	 2499
CCM	   78	 2671
JFWS	   66	 2743
FVM	   78	 2744
SLM	   76	 2752
PVMO	   81	 2834
SIUC	   78	 2853
UTMT	   81	 2909
WVT	   81	 3005
WCI	   76	 3099
LRAL	   87	 3165
MCWV	   72	 3630
CBN	   74	 3869
PAL	   68	 4105

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

The following stations did not have a valid response files: BGU, HUMO, KBO, KHMM, NEN, REDW, TCUT, GNW, JLU, KEBM, KRMB, NLU, SPUT, YKW3

Last Changed Fri Mar 19 08:06:49 CST 2004