2005/02/10 14:04:53 35.748N 90.233 10 4.2 Arkansas

ANSS-MA Location

Arrival Times (from USGS)

Arrival time list

Felt Map

USGS Felt map for this earthquake

USGS Felt reports page for Central and Southeastern US

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:

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   335    55    45   4.01 0.4790
WVFGRD96    1.0   335    55    45   4.02 0.4822
WVFGRD96    2.0   140    80     5   3.92 0.5007
WVFGRD96    3.0   140    75     0   3.95 0.5203
WVFGRD96    4.0   140    75     0   3.96 0.5317
WVFGRD96    5.0   140    75     0   3.98 0.5391
WVFGRD96    6.0   140    80     5   3.99 0.5464
WVFGRD96    7.0   140    80     5   4.00 0.5566
WVFGRD96    8.0   140    80     5   4.01 0.5669
WVFGRD96    9.0   140    85     5   4.03 0.5756
WVFGRD96   10.0   140    80     5   4.04 0.5837
WVFGRD96   11.0   140    80     5   4.05 0.5893
WVFGRD96   12.0   140    85     5   4.07 0.5936
WVFGRD96   13.0   140    85    10   4.08 0.5961
WVFGRD96   14.0   140    85    10   4.08 0.5968
WVFGRD96   15.0   140    85    10   4.09 0.5958
WVFGRD96   16.0   140    85    10   4.10 0.5935
WVFGRD96   17.0   140    80    10   4.10 0.5908
WVFGRD96   18.0   140    80    15   4.11 0.5871
WVFGRD96   19.0   320    75     5   4.12 0.5836

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

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


  STK=      54.99
  DIP=      79.99
 RAKE=    -165.00
  STK=     322.33
  DIP=      75.23
 RAKE=     -10.35
DEPTH = 15.0 km
Mw = 4.11
Best Fit 0.6359 - 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
MPH       158   75 eP_+
PVMO       33   88 eP_+
UTMT       61  140 eP_-
OXF       151  156 iP_+
UALR      241  220 eP_X
SIUC       22  236 eP_X
FVM       356  249 eP_X
CCM       341  272 iP_+
SLM       360  321 P
MIAR      247  333 eP_+
USIN       42  336 eP_X
LRAL      134  423 eP_X
WCI        51  446 eP_X

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)   
MPH	  158	   75
PVMO	   33	   88
UTMT	   62	  140
OXF	  151	  156
PLAL	  113	  214
UALR	  241	  220
SIUC	   22	  236
FVM	  356	  249
CCM	  341	  272
SLM	  360	  321
MIAR	  247	  333
USIN	   42	  336
LRAL	  134	  423
WCI	   51	  446
GOGA	  111	  673
WMOK	  264	  786
JFWS	  360	  796
HKT	  221	  829
CBKS	  295	  908
AAM	   36	  923
NHSC	  105	  969
MCWV	   62	 1013
AMTX	  266	 1027
ERPA	   48	 1134
DWPF	  133	 1186
SSPA	   60	 1209
EYMN	  356	 1359
SDCO	  284	 1380
ISCO	  293	 1426
MNTX	  257	 1472
PAL	   63	 1538
HRV	   59	 1776
LAO	  317	 1804
DGMT	  325	 1820
BW06	  301	 1836
TUC	  264	 1932
HWUT	  296	 1961
LKWY	  306	 1970
HLID	  300	 2240
MSO	  309	 2323
NEW	  311	 2606
MOD	  294	 2685
SAO	  282	 2795

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 0.04 3
lp c 0.15 3


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 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 Sat Feb 12 10:34:36 CST 2005