Location

2007/05/23 19:09:15 22.02N 96.27W 10 5.6 Gulf of Mexico

Arrival Times (from USGS)

Arrival time list

Felt Map

USGS Felt map for this earthquake

USGS Felt reports page for Gulf of Mexico

Focal Mechanism

 SLU Moment Tensor Solution
 2007/05/23 19:09:15 22.02N 96.27W 10 5.6 Gulf of Mexico
 
 Best Fitting Double Couple
    Mo = 3.16e+24 dyne-cm
    Mw = 5.60 
    Z  = 11 km
     Plane   Strike  Dip  Rake
      NP1      190    75   -160
      NP2       95    71   -16
 Principal Axes:
   Axis    Value   Plunge  Azimuth
     T   3.16e+24      3     322
     N   0.00e+00     65     225
     P  -3.16e+24     25      53



 Moment Tensor: (dyne-cm)
    Component  Value
       Mxx     9.98e+23
       Mxy    -2.79e+24
       Mxz    -5.95e+23
       Myy    -4.57e+23
       Myz    -1.06e+24
       Mzz    -5.41e+23
                                                     
                                                     
                                                     
                                                     
                     ##########----                  
                 #############---------              
               T ############-------------           
             #   ###########---------------          
           ################------------   ---        
          #################------------ P ----       
         #################-------------   -----      
        #################-----------------------     
        #################-----------------------     
       ##################------------------------    
       -################-------------------------    
       ----#############-------------------------    
       ---------########----------------------###    
        ---------------##--------------#########     
        ---------------#########################     
         --------------########################      
          -------------#######################       
           ------------######################        
             ----------####################          
              ---------###################           
                 -------###############              
                     ---###########                  
                                                     
                                                     
                                                     

 Harvard Convention
 Moment Tensor:
      R          T          F
 -5.41e+23  -5.95e+23   1.06e+24 
 -5.95e+23   9.98e+23   2.79e+24 
  1.06e+24   2.79e+24  -4.57e+23 


Details of the solution is found at

http://www.eas.slu.edu/eqc/eqc_mt/MECH.NA/20070523190916/index.html
	
May 23, 2007, GULF OF MEXICO, MW=5.6

Meredith Nettles

CENTROID-MOMENT-TENSOR  SOLUTION
GCMT EVENT:     C200705231909A  
DATA: IU II CU IC 
L.P.BODY WAVES: 63S, 132C, T= 40
MANTLE WAVES:   35S,  36C, T=125
SURFACE WAVES:  73S, 159C, T= 50
TIMESTAMP:      Q-20070523231809
CENTROID LOCATION:
ORIGIN TIME:      19:09:20.3 0.1
LAT:22.03N 0.01;LON: 96.31W 0.01
DEP: 17.6  0.7;TRIANG HDUR:  1.5
MOMENT TENSOR: SCALE 10**24 D-CM
RR= 0.313 0.036; TT= 1.150 0.034
PP=-1.460 0.041; RT= 0.111 0.084
RP= 0.646 0.098; TP= 2.660 0.034
PRINCIPAL AXES:
1.(T) VAL=  2.883;PLG=10;AZM=327
2.(N)       0.307;    77;    187
3.(P)      -3.187;     8;     59
BEST DBLE.COUPLE:M0= 3.03*10**24
NP1: STRIKE=103;DIP=77;SLIP=   1
NP2: STRIKE= 13;DIP=89;SLIP= 167

            #########--           
           ##########------       
      ## T #########---------     
    ####   #########---------     
   #################--------- P   
  #################----------   - 
  #################-------------- 
 --###############----------------
 -----############----------------
 ----------#######----------------
 ----------------#----------------
  ---------------################ 
  --------------################# 
   -------------################  
    -----------################   
      ---------##############     
        ------#############       
            --#########           

        

Preferred Solution

The preferred solution from an analysis of the surface-wave spectral amplitude radiation pattern, waveform inversion and first motion observations is

      STK = 190
      DIP = 75
     RAKE = -160
       MW = 5.60
       HS = 11

Only the surface-wave technique was used. The surface-wave grid search used periods greater than 30 seconds only. The synthetics displayed are approximate because the CUS model does not have a realistic upper mantle for describing the waveforms at long periods. Surprisingly the solution is very comparable to that of the Global CMT

Surface-Wave Focal Mechanism

The following figure shows the stations used in the grid search for the best focal mechanism to fit the surface-wave spectral amplitudes of the Love and Rayleigh waves.
Location of broadband stations used to obtain focal mechanism from surface-wave spectral amplitudes

The surface-wave determined focal mechanism is shown here.


  NODAL PLANES 

  
  STK=     189.99
  DIP=      75.00
 RAKE=    -160.00
  
             OR
  
  STK=      94.60
  DIP=      70.71
 RAKE=     -15.92
 
 
DEPTH = 11.0 km
 
Mw = 5.60
Best Fit 0.9580 - P-T axis plot gives solutions with FIT greater than FIT90

First motion data

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

Sta Az(deg)    Dist(km)   First motion
KVTX      345  618 -12345
TEIG      103  858 -12345
HKT         3  864 -12345
JCT       340  986 -12345
NATX        8 1076 -12345
VBMS       26 1252 -12345
TGUH      132 1308 -12345
BRAL       41 1343 -12345
MNTX      321 1389 -12345
MIAR       10 1398 -12345
WMOK      351 1415 -12345
AMTX      341 1505 -12345
OXF        25 1525 -12345
DWPF       63 1634 -12345
ANMO      327 1730 -12345
GOGA       43 1772 -12345
TUC       311 1821 -12345
CCM        14 1827 -12345
CBKS      351 1876 -12345
KSU1      359 1878 -12345
NHSC       49 1995 -12345

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. 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)   
KVTX	  345	  618
TEIG	  103	  858
HKT	    3	  864
JCT	  340	  986
NATX	    8	 1076
VBMS	   26	 1252
TGUH	  132	 1308
BRAL	   41	 1343
MNTX	  321	 1389
MIAR	   10	 1398
WMOK	  351	 1415
AMTX	  341	 1505
OXF	   25	 1525
DWPF	   63	 1634
319A	  311	 1645
Y22C	  324	 1682
219A	  313	 1684
318A	  310	 1705
ANMO	  327	 1730
119A	  315	 1740
218A	  312	 1743
GOGA	   42	 1772
Z19A	  317	 1775
118A	  314	 1781
217A	  310	 1791
Z18A	  315	 1816
TUC	  311	 1821
Y19A	  319	 1823
CCM	   14	 1827
117A	  312	 1834
216A	  309	 1855
X19A	  320	 1861
Z17A	  314	 1862
Y18A	  317	 1864
CBKS	  351	 1876
KSU1	  359	 1878
116A	  310	 1909
X18A	  319	 1913
Y17A	  315	 1917
W19A	  321	 1921
Z16A	  313	 1936
W18A	  321	 1944
115A	  310	 1958
Y16A	  314	 1976
Z15A	  312	 1989
NHSC	   49	 1995
W17A	  319	 2006
X16A	  316	 2010
TZTN	   35	 2013
MVCO	  328	 2041
Y15A	  313	 2044
Z14A	  311	 2054
W16A	  317	 2064
X15A	  315	 2072
WUAZ	  319	 2083
113A	  308	 2085
Y14A	  312	 2094
X14A	  314	 2119
W15A	  316	 2121
112A	  306	 2140
HDIL	   16	 2143
SMCO	  334	 2143
ISCO	  338	 2146
Y13A	  311	 2150
W14A	  315	 2182
GLA	  307	 2187
X13A	  313	 2198
Y12C	  310	 2201
T16A	  322	 2202
V14A	  316	 2212
W13A	  314	 2234
BLA	   39	 2260
T15A	  320	 2262
BCIP	  127	 2271
U14A	  318	 2272
V13A	  315	 2290
CNNC	   47	 2302
T14A	  319	 2312
SRU	  327	 2316
W12A	  313	 2321
V12A	  314	 2346
PFO	  307	 2350
ACSO	   29	 2365
MVU	  324	 2365
T13A	  318	 2365
109C	  305	 2369
JFWS	   12	 2369
S14A	  321	 2369
ECSD	  359	 2392
RWWY	  337	 2395
V11A	  314	 2396
R14A	  322	 2400
S13A	  319	 2401
T12A	  316	 2401
U11A	  315	 2429
GSC	  311	 2470
S12A	  318	 2471
T11A	  317	 2471
MCWV	   35	 2487
U10A	  314	 2499
R12A	  320	 2505
Q13A	  322	 2513
AAM	   25	 2524
TPNV	  315	 2526
CBN	   41	 2535
S11A	  317	 2536
DUG	  326	 2537
RSSD	  346	 2538
P13A	  323	 2551
Q12A	  321	 2571
R11A	  319	 2573
N15A	  327	 2577
HWUT	  330	 2587
SNCC	  303	 2592
BW06	  335	 2596
O13A	  324	 2601
P12A	  322	 2608
N14A	  326	 2612
Q11A	  320	 2612
S10A	  316	 2614
R10A	  318	 2618
ISA	  310	 2621
M15A	  328	 2622
SDDR	   93	 2626
S09A	  315	 2646
O12A	  323	 2658
Q10A	  318	 2661
AHID	  332	 2667
P11A	  321	 2669
R09A	  317	 2669
TPH	  316	 2669
N13A	  325	 2671
M14A	  327	 2674
SSPA	   36	 2682
ERPA	   31	 2684
S08C	  314	 2702
O11A	  322	 2705
M13A	  326	 2709
REDW	  334	 2710
SNOW	  334	 2714
Q09A	  318	 2715
HELL	  311	 2719
N12A	  324	 2721
P10A	  320	 2721
LOHW	  334	 2722
TPAW	  334	 2726
GLMI	   20	 2731
V05C	  309	 2746
R08A	  316	 2750
K14A	  329	 2752
L13A	  327	 2756
M12A	  325	 2757
N11A	  323	 2761
P09A	  319	 2761
IMW	  334	 2763
FLWY	  335	 2769
Q08A	  317	 2770
O10A	  321	 2771
U05C	  310	 2787
T06C	  311	 2790
N10A	  322	 2802
R07C	  314	 2802
O09A	  320	 2807
RLMT	  338	 2809
K13A	  328	 2810
M11A	  324	 2811
V04C	  308	 2813
L12A	  326	 2821
BMN	  320	 2828
P08A	  318	 2835
U04C	  309	 2841
S06C	  313	 2850
K12A	  327	 2854
S05C	  312	 2857
R06C	  315	 2858
V03C	  308	 2861
M10A	  323	 2864
LAO	  344	 2865
L11A	  325	 2866
J13A	  329	 2877
N09A	  321	 2877
O08A	  319	 2880
P07A	  317	 2881
EYMN	    7	 2893
CMB	  313	 2899
AGMN	    1	 2900
L10A	  324	 2901
HAST	  308	 2904
HLID	  329	 2904
N08A	  320	 2912
M09A	  322	 2913
SAO	  309	 2914
J12A	  328	 2915
R05C	  314	 2915
BINY	   36	 2917
I13A	  330	 2918
O07A	  318	 2920
K11A	  326	 2929
G15A	  334	 2949
R04C	  313	 2953
BOZ	  336	 2955
P06A	  316	 2961
PAL	   40	 2962
LAVA	  314	 2967
N07B	  319	 2968
J11A	  327	 2969
L09A	  323	 2970
M08A	  321	 2975
K10A	  325	 2979
H13A	  331	 2981
O06A	  317	 2986
P05C	  315	 2989
BNLO	  310	 2990
DGMT	  348	 3003
F15A	  335	 3010
H12A	  330	 3010
I11A	  328	 3016
Q04C	  313	 3019
M07A	  320	 3022
J10A	  326	 3023
K09A	  324	 3024
N06A	  318	 3024
G13A	  332	 3025
L08A	  322	 3025
F14A	  334	 3047
O05C	  316	 3051
Q03C	  312	 3056
WVOR	  322	 3061
E15A	  335	 3070
SUTB	  314	 3072
K08A	  323	 3073
J09A	  325	 3075
I10A	  327	 3077
L07A	  321	 3077
ELFS	  317	 3078
H11A	  329	 3084
O04C	  316	 3085
MCCM	  311	 3097
M06C	  319	 3098
EGMT	  340	 3111
H10A	  328	 3114
J08A	  324	 3118
K07A	  322	 3118
D15A	  336	 3122
I09A	  326	 3122
MOD	  320	 3132
F12A	  331	 3133
O03C	  315	 3134
E13A	  333	 3141
HATC	  317	 3142
G11A	  330	 3154
M05C	  318	 3154
HOPS	  312	 3156
NCB	   35	 3156
I08A	  325	 3165
MSO	  334	 3165
D14A	  335	 3168
H09A	  327	 3171
J07A	  323	 3171
L05A	  320	 3177
SJG	   93	 3177
LONY	   33	 3185
F11A	  330	 3189
K06A	  322	 3189
G10A	  328	 3191
P01C	  313	 3201
D13A	  334	 3212
M03C	  317	 3216
H08A	  326	 3217
G09A	  328	 3226
K05A	  321	 3226
M04C	  318	 3228
E11A	  331	 3231
I07A	  324	 3234
C14A	  335	 3241
D12A	  333	 3249
F10A	  329	 3252
L04A	  319	 3256
F09A	  328	 3267
I06A	  323	 3267
C13A	  334	 3269
O01C	  314	 3273
K04A	  320	 3274
N02C	  315	 3274
M02C	  317	 3275
YBH	  318	 3285
J05A	  321	 3286
E10A	  330	 3288
G08A	  327	 3293
D11A	  332	 3296
BBSR	   63	 3311
LBNH	   37	 3314
F08A	  328	 3322
B13A	  335	 3327
G07A	  326	 3331
H06A	  324	 3331
J04A	  320	 3340
D10A	  331	 3344
E09A	  329	 3344
I05A	  323	 3348
HUMO	  319	 3356
L02A	  318	 3373
A13A	  336	 3376
H05A	  324	 3379
B12A	  334	 3385
G06A	  325	 3386
E08A	  328	 3391
M01C	  317	 3391
F07A	  326	 3394
I04A	  321	 3396
D09A	  330	 3397
K02A	  319	 3402
J03A	  320	 3404
C10A	  332	 3409
HAWA	  328	 3410
B11A	  333	 3417
G05A	  324	 3427
A12A	  335	 3429
D08A	  329	 3429
F06A	  326	 3433
NEW	  332	 3438
E07A	  328	 3440
H04A	  323	 3441
J02A	  320	 3445
B10A	  332	 3446
C09A	  331	 3456
A11A	  334	 3463
I03A	  321	 3464
K01A	  318	 3472
F05A	  325	 3482
C08A	  330	 3494
G04A	  323	 3495
D07A	  328	 3496
B09A	  332	 3501
E06A	  326	 3502
I02A	  320	 3502
COR	  322	 3505
H03A	  322	 3511
A10A	  333	 3520
C07A	  329	 3539
F04A	  324	 3542
D06A	  328	 3544
E05A	  326	 3548
PKME	   37	 3548
G03A	  323	 3552
B08A	  330	 3555
H02A	  321	 3556
A09A	  332	 3568
C06A	  328	 3602
D05A	  326	 3607
E04A	  325	 3607
F03A	  323	 3611
C05A	  328	 3628
FFC	  354	 3646
D04A	  326	 3649
E03A	  324	 3654
A07A	  330	 3664
B06A	  329	 3680
B05A	  328	 3693
C04A	  326	 3702
D03A	  325	 3706
A06A	  330	 3724
NLWA	  325	 3731
A05A	  329	 3748
B04A	  326	 3757
A04A	  328	 3761
C03A	  326	 3810
GRGR	  101	 3837
BBGH	   98	 4011
CRAG	  330	 4786
WRAK	  332	 4809
SIT	  331	 4999
SKAG	  334	 5153

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 0.005 n 3
lp c 0.02 n 3
br c 0.12 0.25 n 4 p 2

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. The digital data used in this study were provided by Natural Resources Canada through their AUTODRM site http://www.seismo.nrcan.gc.ca/nwfa/autodrm/autodrm_req_e.php, and IRIS using their BUD interface

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 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 Tue Jan 8 11:21:38 CST 2008