Location

2004/11/07 11:20:25 32.97N 87.90W 5 4.0 Alabama

Arrival Times (from USGS)

Arrival time list

Felt Map

USGS Felt map for this earthquake

USGS Felt reports page for Central and Southeastern 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


  NODAL PLANES 

  
  STK=     246.32
  DIP=      54.37
 RAKE=     -70.48
  
             OR
  
  STK=      35.00
  DIP=      40.00
 RAKE=    -114.99
 
 
DEPTH = 3.0 km
 
Mw = 4.25
Best Fit 0.7933 - P-T axis plot gives solutions with FIT greater than FIT90

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 0.02 3
lp c 0.06 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   270    30   -45   4.17 0.4932
WVFGRD96    1.0   270    40   -50   4.13 0.4981
WVFGRD96    2.0   265    50   -65   4.14 0.4958
WVFGRD96    3.0    90    40    55   4.08 0.4928
WVFGRD96    4.0    95    40    65   4.09 0.4867
WVFGRD96    5.0   110    35    80   4.08 0.4686
WVFGRD96    6.0   105    90   -60   4.10 0.4584
WVFGRD96    7.0   285    90    55   4.08 0.4562
WVFGRD96    8.0   105    90   -55   4.08 0.4545
WVFGRD96    9.0   105    90   -55   4.08 0.4519
WVFGRD96   10.0   290    85    50   4.08 0.4504
WVFGRD96   11.0   290    85    45   4.08 0.4555
WVFGRD96   12.0   290    85    45   4.09 0.4608
WVFGRD96   13.0   290    85    45   4.09 0.4647
WVFGRD96   14.0   290    85    45   4.10 0.4682
WVFGRD96   15.0   290    85    45   4.10 0.4716
WVFGRD96   16.0   290    85    45   4.11 0.4749
WVFGRD96   17.0   290    85    45   4.12 0.4780
WVFGRD96   18.0   290    80    50   4.12 0.4812
WVFGRD96   19.0   290    80    50   4.12 0.4845

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.
Figure 3. Waveform comparison for depth of 8 km

First motion data

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

Sta Az(deg)    Dist(km)   First motion
LRAL	   85	   85	e+
OXF	  321	  221	X
MPH	  322	  304	X
WVT	    1	  351	X
UTMT	  347	  384	X
PVMO	  337	  416	X
UALR	  297	  457	e+
SIUC	  348	  540	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.

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

The distribution of broadband stations with azimuth and distance is

Sta Az(deg)    Dist(km)   
LRAL	   85	   85
OXF	  321	  221
MPH	  322	  304
WVT	    1	  351
UTMT	  347	  384
PVMO	  337	  416
UALR	  297	  457
SIUC	  348	  540
MIAR	  290	  554
USIN	    2	  555
WCI	   14	  601
FVM	  338	  602
CCM	  333	  641
NATX	  260	  650
SLM	  342	  663
BLO	   10	  699
NHSC	   87	  721
DWPF	  129	  821
ACSO	   27	  918
WMOK	  284	 1025
MCWV	   42	 1035
KSU1	  313	 1038
AAM	   19	 1101
CBN	   56	 1116
JFWS	  350	 1123
ALLY	   33	 1182
SDMD	   51	 1223
ERPA	   32	 1232
SSPA	   44	 1232
CBKS	  305	 1248
BINY	   42	 1466
SADO	   28	 1513
PAL	   50	 1529
KGNO	   35	 1594
SDCO	  293	 1682
NCB	   40	 1703
ISCO	  301	 1757
GAC	   33	 1769
HRV	   49	 1783
ULM	  344	 2029
TUC	  274	 2144
BW06	  306	 2185
DGMT	  326	 2195
MVU	  292	 2276
HWUT	  301	 2297
AHID	  305	 2302
LKWY	  310	 2329
BOZ	  312	 2469
HLID	  305	 2586
TPNV	  288	 2617
MSO	  313	 2687
TPH	  290	 2707
BMN	  296	 2733
DAC	  286	 2737
MNV	  291	 2791
FCC	  353	 2906
NEW	  314	 2972
CMB	  290	 2985
EDM	  326	 3028
SAO	  287	 3080
DRLN	   45	 3096
COR	  303	 3304
OCWA	  309	 3449

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 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 Wed Nov 10 14:16:42 CST 2004