Location

2007/05/25 13:40:18 25.96N 110.05W 10 4.3 Gulf of California

Arrival Times (from USGS)

Arrival time list

Felt Map

USGS Felt map for this earthquake

USGS Felt reports page for Outside US

Focal Mechanism

 SLU Moment Tensor Solution
 2007/05/25 13:40:18 25.96N 110.05W 10 4.3 Gulf of California
 
 Best Fitting Double Couple
    Mo = 8.41e+22 dyne-cm
    Mw = 4.55 
    Z  = 6 km
     Plane   Strike  Dip  Rake
      NP1      315    80   -160
      NP2      221    70   -11
 Principal Axes:
   Axis    Value   Plunge  Azimuth
     T   8.41e+22      7      87
     N   0.00e+00     68     340
     P  -8.41e+22     21     180



 Moment Tensor: (dyne-cm)
    Component  Value
       Mxx    -7.29e+22
       Mxy     4.92e+21
       Mxz     2.88e+22
       Myy     8.28e+22
       Myz     9.41e+21
       Mzz    -9.84e+21
                                                     
                                                     
                                                     
                                                     
                     --------------                  
                 ----------------------              
              ----------------------------           
             ##----------------------######          
           #######----------------###########        
          ##########-----------###############       
         ##############------##################      
        #################--#####################     
        #################--#####################     
       #################-----#################       
       ###############---------############### T     
       ##############-----------##############       
       #############--------------###############    
        ##########------------------############     
        #########--------------------###########     
         #######-----------------------########      
          ######------------------------######       
           ####--------------------------####        
             ##------------   ------------#          
              ------------- P ------------           
                 ----------   ---------              
                     --------------                  
                                                     
                                                     
                                                     

 Harvard Convention
 Moment Tensor:
      R          T          F
 -9.84e+21   2.88e+22  -9.41e+21 
  2.88e+22  -7.29e+22  -4.92e+21 
 -9.41e+21  -4.92e+21   8.28e+22 


Details of the solution is found at

http://www.eas.slu.edu/eqc/eqc_mt/MECH.NA/20070525134018/index.html
        

      STK = 315
      DIP = 80
     RAKE = -160
       MW = 4.55
       HS = 6

The surface-wave is preferred. Rake is least well determined. This event was studied primarily for surface wave dispersion and not for the source.

Waveform Inversion

The focal mechanism was determined using broadband seismic waveforms. The location of the event and the and stations used for the waveform inversion are shown in the next figure.
Location of broadband stations used for 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 n 3
lp c 0.02 n 3
br c 0.12 0.25 n 4 p 2
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   190    70   -80   4.69 0.3302
WVFGRD96    1.0   185    60   -80   4.62 0.3382
WVFGRD96    2.0   180    65   -85   4.65 0.3328
WVFGRD96    3.0     0    25   -80   4.68 0.3510
WVFGRD96    4.0     5    30   -75   4.70 0.3511
WVFGRD96    5.0    40    50    45   4.36 0.3580
WVFGRD96    6.0    50    50    65   4.39 0.3825
WVFGRD96    7.0    50    50    60   4.42 0.4078
WVFGRD96    8.0    55    50    65   4.45 0.4257
WVFGRD96    9.0    55    50    60   4.48 0.4497
WVFGRD96   10.0    60    50    65   4.51 0.4695
WVFGRD96   11.0    60    50    65   4.53 0.4843
WVFGRD96   12.0    55    55    50   4.55 0.4932
WVFGRD96   13.0    50    60    35   4.55 0.4954
WVFGRD96   14.0    50    65    30   4.57 0.4940
WVFGRD96   15.0    50    65    30   4.58 0.4898
WVFGRD96   16.0    45    75    15   4.58 0.4862
WVFGRD96   17.0    45    75    15   4.58 0.4826
WVFGRD96   18.0    45    80    10   4.60 0.4776
WVFGRD96   19.0    45    80    10   4.60 0.4726
WVFGRD96   20.0    45    80    10   4.61 0.4670
WVFGRD96   21.0    45    80    15   4.62 0.4632
WVFGRD96   22.0    45    80    15   4.62 0.4518
WVFGRD96   23.0    45    80    15   4.62 0.4391
WVFGRD96   24.0    45    85    10   4.63 0.4262
WVFGRD96   25.0   225    90   -10   4.63 0.4044
WVFGRD96   26.0    45    85    10   4.63 0.4008
WVFGRD96   27.0   330    85    65   4.54 0.3782
WVFGRD96   28.0   330    85    65   4.54 0.3820
WVFGRD96   29.0   330    85    65   4.54 0.3831

The best solution is

WVFGRD96   13.0    50    60    35   4.55 0.4954

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 n 3
lp c 0.02 n 3
br c 0.12 0.25 n 4 p 2
Figure 3. Waveform comparison for depth of 8 km
Focal mechanism sensitivity at the preferred depth. The red color indicates a very good fit to thewavefroms. 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.

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=     134.99
  DIP=      80.00
 RAKE=      19.99
  
             OR
  
  STK=      41.37
  DIP=      70.32
 RAKE=     169.37
 
 
DEPTH = 6.0 km
 
Mw = 4.53
Best Fit 0.8542 - 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
319A        7  605 -12345
TUC       354  707 eP_+
ANMO       18 1054 eP_-

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. 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 distribution

Sta Az(deg)    Dist(km)   
319A	    7	  605
318A	    0	  607
217A	  354	  649
218A	    0	  667
219A	    6	  674
216A	  349	  684
TUC	  354	  707
117A	  355	  736
118A	    1	  740
116A	  348	  749
119A	    5	  758
115A	  345	  776
MNTX	   34	  782
Z18A	    0	  790
Z17A	  357	  814
Z19A	    5	  816
Z16A	  351	  829
113A	  335	  836
Z15A	  346	  838
112A	  330	  851
Y17A	  355	  861
Y18A	    0	  867
Z14A	  342	  867
Y16A	  351	  889
Y19A	    5	  890
Y15A	  347	  913
Y14A	  343	  929
X19A	    4	  942
Y13A	  338	  944
X16A	  352	  947
Y22C	   18	  949
X18A	    1	  950
Y12C	  334	  965
X15A	  348	  968
X14A	  344	  982
W17A	  357	 1013
W18A	    2	 1016
W19A	    3	 1017
W16A	  352	 1023
X13A	  340	 1024
109C	  320	 1028
W15A	  349	 1044
ANMO	   18	 1054
W14A	  345	 1066
WUAZ	  354	 1067
W13A	  341	 1078
V18A	    1	 1081
V19A	    5	 1086
V14A	  346	 1111
V15A	  350	 1112
JCT	   61	 1123
U16A	  355	 1134
W12A	  337	 1134
V13A	  342	 1159
U18A	    1	 1160
V12A	  338	 1176
U15A	  350	 1180
U14A	  346	 1197
V11A	  336	 1209
KVTX	   79	 1221
T16A	  354	 1230
U12A	  341	 1237
T15A	  350	 1246
MVCO	    6	 1256
T14A	  348	 1264
U11A	  338	 1266
T12A	  341	 1273
AMTX	   37	 1274
T13A	  344	 1280
LRL	  327	 1283
U10A	  334	 1304
S15A	  351	 1317
MPM	  330	 1325
S13A	  345	 1338
S14A	  348	 1341
ISA	  325	 1343
T11A	  340	 1343
TPNV	  336	 1353
S12A	  342	 1369
CWC	  329	 1390
R14A	  349	 1396
S11A	  339	 1403
GRA	  332	 1406
R12A	  344	 1437
V05C	  322	 1445
TIN	  330	 1454
WMOK	   45	 1454
SRU	  358	 1459
HELL	  326	 1462
Q15A	  352	 1462
S10A	  337	 1463
HKT	   69	 1466
R11A	  341	 1469
S09A	  334	 1469
Q13A	  346	 1488
R10A	  338	 1488
V04C	  319	 1489
S08C	  331	 1492
SMCO	   10	 1494
U05C	  323	 1496
R09A	  336	 1513
Q12A	  344	 1518
Q11A	  341	 1522
P15A	  353	 1524
T06C	  326	 1529
MLAC	  330	 1537
U04C	  321	 1540
P13A	  347	 1542
V03C	  319	 1542
Q10A	  339	 1546
P12A	  344	 1566
R08A	  333	 1568
Q09A	  337	 1576
T05C	  323	 1576
ISCO	   14	 1589
R07C	  330	 1589
S05C	  325	 1593
P11A	  342	 1598
DUG	  352	 1599
Q08A	  335	 1610
S06C	  328	 1612
O13A	  348	 1614
P10A	  340	 1629
NATX	   63	 1631
O12A	  346	 1646
P09A	  338	 1646
R06C	  330	 1649
CMB	  326	 1655
O11A	  343	 1656
Q07A	  332	 1656
S04C	  323	 1665
N15A	  353	 1672
N14A	  351	 1677
BNLO	  320	 1688
N13A	  348	 1696
P08A	  336	 1697
R05C	  329	 1697
O10A	  341	 1698
R04C	  326	 1705
O09A	  339	 1710
N12A	  346	 1715
P07A	  334	 1717
CBKS	   32	 1722
N11A	  344	 1730
M15A	  353	 1734
LAVA	  327	 1737
HWUT	  356	 1741
N10A	  342	 1742
M13A	  348	 1750
M14A	  351	 1750
M12A	  347	 1772
O07A	  335	 1775
P06A	  331	 1776
P05C	  329	 1778
Q04C	  326	 1779
N09A	  339	 1790
M11A	  344	 1796
Q03C	  324	 1798
N08A	  338	 1806
L13A	  350	 1824
M10A	  342	 1826
SUTB	  326	 1835
N07B	  336	 1839
M09A	  340	 1845
MIAR	   55	 1845
L12A	  347	 1852
K14A	  352	 1862
BW06	    1	 1865
AHID	  357	 1867
L11A	  345	 1872
N06A	  334	 1873
M08A	  338	 1881
L10A	  343	 1882
K13A	  350	 1888
O04C	  330	 1893
ELFS	  331	 1902
K12A	  348	 1902
M07A	  336	 1906
L09A	  340	 1913
KSU1	   38	 1924
REDW	  358	 1932
K11A	  345	 1943
SNOW	  358	 1943
P01C	  324	 1944
M06C	  333	 1945
TPAW	  358	 1946
L08A	  339	 1954
HATC	  330	 1956
LOHW	  359	 1959
K10A	  343	 1966
J13A	  350	 1970
O02C	  326	 1970
J12A	  348	 1972
WVOR	  339	 1989
HLID	  350	 1992
J11A	  346	 2006
K08A	  340	 2013
VBMS	   65	 2020
I13A	  351	 2026
O01C	  324	 2026
M03C	  330	 2030
J09A	  342	 2052
N02C	  327	 2053
I11A	  346	 2062
M04C	  331	 2062
LKWY	  359	 2064
J08A	  340	 2076
RSSD	   14	 2087
H13A	  351	 2098
L04A	  332	 2100
I10A	  345	 2101
K05A	  335	 2106
H12A	  349	 2107
J07A	  339	 2107
I09A	  343	 2116
RLMT	    2	 2127
I08A	  341	 2135
G15A	  355	 2142
H11A	  347	 2148
H10A	  345	 2153
G13A	  351	 2156
I07A	  339	 2184
M01C	  327	 2184
H09A	  344	 2185
BOZ	  357	 2189
OXF	   60	 2196
H08A	  342	 2200
F15A	  355	 2217
CCM	   48	 2219
F14A	  353	 2223
G11A	  347	 2227
F13A	  351	 2233
G10A	  345	 2236
H07A	  340	 2237
F12A	  349	 2245
G09A	  344	 2252
J03A	  332	 2259
I04A	  334	 2273
F11A	  348	 2277
E15A	  355	 2282
G08A	  342	 2288
J02A	  331	 2288
E14A	  353	 2290
F09A	  344	 2301
E13A	  352	 2302
G07A	  341	 2309
F10A	  346	 2311
TEIG	  101	 2316
ECSD	   28	 2317
LAO	    7	 2325
E11A	  348	 2328
I03A	  332	 2328
F08A	  343	 2334
MSO	  352	 2341
SCIA	   37	 2347
D15A	  355	 2350
I02A	  332	 2357
D14A	  354	 2364
D13A	  352	 2377
F07A	  341	 2380
D12A	  350	 2387
E09A	  345	 2391
H03A	  334	 2393
D11A	  348	 2405
E08A	  343	 2414
HAWA	  342	 2418
H02A	  332	 2425
C14A	  353	 2442
C13A	  352	 2443
EGMT	    0	 2449
D09A	  345	 2452
D08A	  344	 2468
F04A	  337	 2474
C10A	  347	 2508
E05A	  339	 2510
HDIL	   45	 2510
D07A	  342	 2513
B13A	  352	 2517
C09A	  346	 2529
B12A	  350	 2544
C08A	  344	 2547
DGMT	   10	 2550
NEW	  348	 2551
B11A	  349	 2555
B10A	  348	 2557
C07A	  343	 2566
A13A	  353	 2578
B09A	  346	 2591
A12A	  351	 2596
D04A	  338	 2602
JFWS	   39	 2604
A11A	  350	 2611
B08A	  344	 2614
A10A	  348	 2637
A09A	  346	 2660
C04A	  338	 2667
NLWA	  336	 2671
GOGA	   66	 2693
B06A	  341	 2694
AGMN	   23	 2771
DWPF	   79	 2843
COWI	   34	 2905
EYMN	   29	 2930
ACSO	   51	 2963
NHSC	   68	 2991
BLA	   59	 3060
GLMI	   41	 3093
MCWV	   54	 3189
CNNC	   64	 3235
ERPA	   49	 3277
SSPA	   53	 3382
NCB	   49	 3790
WRAK	  338	 3827
LBNH	   49	 3973

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 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


Spectra fit plots to each station

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 Jun 26 12:36:39 CDT 2007