Location

2005/05/01 12:37:32 35.83N 90.15W 9.7 4.1 Arkansas

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

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   305    65    20   4.09 0.6740
WVFGRD96    1.0   305    80    40   4.14 0.6864
WVFGRD96    2.0   305    80    30   4.13 0.7083
WVFGRD96    3.0   310    70    25   4.14 0.7188
WVFGRD96    4.0   310    65    20   4.16 0.7291
WVFGRD96    5.0   310    65    20   4.16 0.7388
WVFGRD96    6.0   310    65    20   4.17 0.7439
WVFGRD96    7.0   310    65    20   4.17 0.7464
WVFGRD96    8.0   310    65    15   4.18 0.7466
WVFGRD96    9.0   310    65    15   4.18 0.7450
WVFGRD96   10.0   310    65    15   4.20 0.7422
WVFGRD96   11.0   310    65    15   4.20 0.7371
WVFGRD96   12.0   310    70    15   4.20 0.7304
WVFGRD96   13.0   310    70    15   4.20 0.7220
WVFGRD96   14.0   310    70    15   4.21 0.7151
WVFGRD96   15.0   310    70    15   4.21 0.7056
WVFGRD96   16.0   310    70    10   4.22 0.6981
WVFGRD96   17.0   310    70    10   4.22 0.6898
WVFGRD96   18.0   130    70    15   4.23 0.6825
WVFGRD96   19.0   130    70    15   4.23 0.6787

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 0.02 3
lp c 0.06 3
Figure 3. Waveform comparison for depth of 8 km
NODAL PLANES STK= 214.67 DIP= 72.77 RAKE= 148.43 OR STK= 314.98 DIP= 60.00 RAKE= 20.00 DEPTH = 8.0 km Mw = 4.22 Best Fit 0.8468 - 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
PVMO       32   76 iP_D
MPH       166   81 iP_C
UTMT       63  129 iP_D
OXF       155  161 iP_C
PLAL      116  211 iP_C
WVT        80  212 iP_D
UALR      240  231 iP_D
FVM       354  240 eP_-
CCM       339  266 iP_D
SLM       359  312 eP_X
OLIL       29  370 eP_X
LRAL      136  424 eP_+
WCI        51  434 iP_D
BLO        40  490 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)   
PVMO	   32	   76
MPH	  166	   81
UTMT	   64	  129
OXF	  155	  161
PLAL	  116	  211
WVT	   80	  212
UALR	  240	  231
FVM	  354	  240
CCM	  339	  266
SLM	  359	  312
USIN	   42	  324
LRAL	  136	  424
WCI	   51	  434
BLO	   40	  490
LTL	  186	  590
GOGA	  112	  669
KSU1	  304	  677
WMOK	  264	  794
ACSO	   50	  796
CBKS	  294	  912
NHSC	  105	  964
MCWV	   62	 1002
AMTX	  265	 1035
JCT	  239	 1077
ERPA	   48	 1122
CBN	   73	 1166
DWPF	  134	 1187
SSPA	   60	 1197
SDMD	   67	 1239
MVL	   65	 1297
SADO	   40	 1362
LTX	  244	 1457
MNT	  256	 1482
MNTX	  256	 1482
KGNO	   47	 1490
PAL	   63	 1528
NCB	   52	 1631
KAPO	   20	 1636
GAC	   44	 1650
ULM	  346	 1668
VLDQ	   33	 1710
HRV	   60	 1765
LAO	  317	 1802
BW06	  301	 1838
WUAZ	  275	 1918
AHID	  300	 1958
HWUT	  296	 1964
LKWY	  306	 1971
MVU	  285	 1977
DUG	  290	 2042
LMQ	   45	 2092
BOZ	  308	 2107
GGN	   55	 2220
HLID	  300	 2241
ELK	  292	 2254
GLA	  269	 2282
MSO	  309	 2323
LMN	   54	 2396
ICQ	   44	 2399
GSC	  276	 2410
TPH	  284	 2416
BMN	  290	 2418
WALA	  315	 2429
BAR	  270	 2460
MNV	  285	 2496
MWC	  274	 2544
ISA	  278	 2553
WVOR	  295	 2557
FCC	  355	 2567
NEW	  310	 2606
EDM	  324	 2649
MOD	  294	 2688
CMB	  284	 2693
HAWA	  305	 2712
SCHQ	   33	 2768
SAO	  281	 2801
PNT	  311	 2819
WDC	  290	 2869
YBH	  293	 2886
HUMO	  295	 2912
HOPS	  287	 2920
COR	  299	 2962
YRTN	  358	 3006
LLLB	  313	 3023
DRLN	   50	 3036
SHB	  310	 3122
OZB	  308	 3217
CBB	  310	 3239
KNDN	  343	 3336
EDB	  309	 3356
MGTN	  343	 3371
IHLN	  342	 3372
BOXN	  343	 3390
SNPN	  342	 3392
PHC	  311	 3400
MLON	  343	 3406
CAMN	  342	 3411
YKW1	  338	 3412
NODN	  342	 3434
FRB	   19	 3437
QILN	    3	 3441
GLWN	  344	 3463
BBB	  313	 3490
EKTN	  343	 3493
FNBB	  327	 3504
ACKN	  343	 3527
YMBN	  343	 3534
COWN	  344	 3561
LUPN	  344	 3607
CTLN	  339	 3626
GALN	  338	 3642
DLBC	  324	 3841
WHY	  325	 4189
SFJD	   24	 4285
RES	  358	 4338
INK	  337	 4501

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:

BLA

In addition we did not use the following stations because data were not consistent with the solution either due to response problems or data problems: ISCO,JFWS, LTL, MIAR, NCB, MIAR, AHID, EYMN, SNCC, AHID

Last Changed Mon May 2 11:56:07 CDT 2005