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

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


  STK=     307.15
  DIP=      55.61
 RAKE=      96.94
  STK=     114.99
  DIP=      35.00
 RAKE=      79.99
DEPTH = 6.0 km
Mw = 6.54 km

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

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)   P-first motion
CMB	   14	  266
DAC	   78	  322
HOPS	  335	  404
TPH	   52	  434
TPNV	   71	  457
PFO	  117	  485
WDC	  347	  555
BMN	   32	  625
MOD	    6	  691
WVOR	   15	  776
MVU	   66	  848
WUAZ	   89	  881
CTU	   53	  987
HVU	   44	  988
COR	  350	 1003
TUC	  109	 1024
HLID	   31	 1043
HWUT	   49	 1056
AHID	   45	 1164
BW06	   48	 1266
YMR	   38	 1315
ANMO	   89	 1332
LKWY	   40	 1339
BOZ	   33	 1361
OCWA	  350	 1361
MSO	   24	 1371
SDCO	   76	 1410
NEW	   12	 1432
RSSD	   52	 1727
LTX	  108	 1781
WMOK	   86	 2030
JCT	  100	 2066
HKT	   98	 2443
ULM	   44	 2594
UALR	   84	 2610
CCM	   76	 2663
FVM	   76	 2735
SLM	   74	 2747
JFWS	   64	 2761
GNAR	   80	 2796
PVMO	   79	 2818
MPH	   82	 2820
SIUC	   76	 2844
OXF	   83	 2880
UTMT	   79	 2893
USIN	   75	 2977
WVT	   79	 2988
PLAL	   82	 2989
BLO	   72	 3065
WCI	   74	 3094
LRAL	   86	 3135
DIV	  336	 3321
ACSO	   70	 3360
GOGA	   83	 3442
PMR	  334	 3485
BLA	   75	 3622
MCWV	   70	 3633
MCK	  337	 3643
COLA	  340	 3700
DWPF	   91	 3818
CBN	   73	 3868
BINY	   66	 3930
UNV	  314	 4026
NCB	   62	 4059
PAL	   67	 4116
HNH	   63	 4216
LBNH	   62	 4240
HRV	   64	 4289
BRYW	   65	 4299
WES	   65	 4310
RES	   10	 4558
SCHQ	   45	 4611
PAYG	  135	 5141

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 0.01 3
lp c 0.025 3


The focal mechanism nodal planes are well developed. However since the P-wave first motion data are of poor quality and the local velocity model is poorly known, we cannot resolve whther this is a thrust or normal faulting event.

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.


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 CUS 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

Last Changed Fri Mar 19 08:04:51 CST 2004