Location

2005/04/27 00:33:00 30.263 114.125W 2 5.1 BAJA

Arrival Times (from USGS)

Arrival time list

Felt Map

USGS Felt map for this earthquake

USGS Felt reports page for Outside 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

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 0.01 3
lp c 0.02 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   230    40   -50   4.73 0.2570
WVFGRD96    1.0   280    75   -25   4.68 0.2242
WVFGRD96    2.0    55    70   -70   4.81 0.2560
WVFGRD96    3.0    55    70   -70   4.84 0.2777
WVFGRD96    4.0    60    75   -75   4.93 0.3163
WVFGRD96    5.0    55    70   -75   4.92 0.3301
WVFGRD96    6.0    55    65   -75   4.91 0.3382
WVFGRD96    7.0    55    65   -75   4.92 0.3396
WVFGRD96    8.0    50    65   -80   4.95 0.3389
WVFGRD96    9.0   280    40   -15   4.96 0.3387
WVFGRD96   10.0   280    45   -15   4.95 0.3358
WVFGRD96   11.0   285    50    -5   4.94 0.3307
WVFGRD96   12.0   285    55    -5   4.94 0.3267
WVFGRD96   13.0   285    60     0   4.94 0.3235
WVFGRD96   14.0   285    60     0   4.94 0.3186
WVFGRD96   15.0   285    65     5   4.94 0.3143
WVFGRD96   16.0   285    70    10   4.94 0.3094
WVFGRD96   17.0   285    70    10   4.94 0.3047
WVFGRD96   18.0   285    70    10   4.94 0.2998
WVFGRD96   19.0   285    70    10   4.94 0.2939

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 0.01 3
lp c 0.02 3
Figure 3. Waveform comparison for depth of 8 km
NODAL PLANES STK= 39.72 DIP= 59.99 RAKE= -54.74 OR STK= 164.99 DIP= 45.00 RAKE= -134.99 DEPTH = 2.0 km Mw = 5.02 Best Fit 0.8336 - 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
GLA	  348	  316
BAR	  319	  361
TUC	   54	  391
MWC	  321	  574
GSC	  336	  612
ISA	  327	  724
DAC	  335	  741
MNTX	   77	  850
TPH	  343	  912
MVU	   10	  931
MNV	  339	  979
SAO	  319	  990
LTX	   93	 1016
CMB	  328	 1037
SDCO	   42	 1149
BMN	  347	 1162
ELK	  355	 1167
HOPS	  322	 1268
HWUT	   10	 1280
AMTX	   65	 1285
WDC	  329	 1375
JCT	   85	 1376
MOD	  338	 1405
WVOR	  345	 1409
AHID	   10	 1413
BW06	   15	 1446
HLID	  359	 1476
YBH	  331	 1488
HUMO	  332	 1580
LKWY	   11	 1621
CBKS	   50	 1623
BOZ	    7	 1722
COR	  336	 1783
MSO	    0	 1839
HAWA	  347	 1851
KSU1	   54	 1876
LAO	   18	 1947
MIAR	   71	 1986
NEW	  354	 2015
UALR	   70	 2102
LTL	   83	 2240
CCM	   62	 2272
MPH	   70	 2325
FVM	   62	 2340
OXF	   72	 2366
PVMO	   67	 2366
SLM	   61	 2374
SIUC	   64	 2436
UTMT	   67	 2439
PLAL	   71	 2492
WVT	   68	 2528
LRAL	   76	 2584
WCI	   63	 2699
BLO	   61	 2701
GOGA	   75	 2912
ACSO	   60	 3023
DWPF	   86	 3177
NHSC	   76	 3220
MCWV	   62	 3277
ERPA	   57	 3312
SSPA	   61	 3457
CBN	   65	 3474
SDMD	   63	 3530
BINY	   58	 3639
PAL	   61	 3795
NCB	   55	 3813
HRV	   58	 4006

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)   
GLA	  348	  316
BAR	  319	  361
TUC	   54	  391
MWC	  321	  574
GSC	  336	  612
ISA	  327	  724
DAC	  335	  741
MNTX	   77	  850
TPH	  343	  912
MVU	   10	  931
MNV	  339	  979
SAO	  319	  990
LTX	   93	 1016
CMB	  328	 1037
SDCO	   42	 1149
BMN	  347	 1162
ELK	  355	 1167
HOPS	  322	 1268
HWUT	   10	 1280
AMTX	   65	 1285
WDC	  329	 1375
JCT	   85	 1376
MOD	  338	 1405
WVOR	  345	 1409
AHID	   10	 1413
BW06	   15	 1446
HLID	  359	 1476
YBH	  331	 1488
HUMO	  332	 1580
LKWY	   11	 1621
CBKS	   50	 1623
BOZ	    7	 1722
COR	  336	 1783
MSO	    0	 1839
HAWA	  347	 1851
KSU1	   54	 1876
LAO	   18	 1947
MIAR	   71	 1986
NEW	  354	 2015
UALR	   70	 2102
LTL	   83	 2240
CCM	   62	 2272
MPH	   70	 2325
FVM	   62	 2340
OXF	   72	 2366
PVMO	   67	 2366
SLM	   61	 2374
SIUC	   64	 2436
UTMT	   67	 2439
PLAL	   71	 2492
WVT	   68	 2528
LRAL	   76	 2584
WCI	   63	 2699
BLO	   61	 2701
GOGA	   75	 2912
ACSO	   60	 3023
DWPF	   86	 3177
NHSC	   76	 3220
MCWV	   62	 3277
ERPA	   57	 3312
SSPA	   61	 3457
CBN	   65	 3474
SDMD	   63	 3530
BINY	   58	 3639
PAL	   61	 3795
NCB	   55	 3813
HRV	   58	 4006

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 0.01 3
lp c 0.03 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

DUG, HAWA had gains too low. MNV had data dropouts, HAWA seemed to have gains slightly too low.

The following stations did not have a valid response files:

Last Changed Wed Apr 27 11:24:51 CDT 2005