2008/04/07 09:51:13 28.921 -98.088 5.0 3.7 Texas
USGS Felt map for this earthquake
SLU Moment Tensor Solution
2008/04/07 09:51:13 28.921 -98.088 5.0 3.7 Texas
Best Fitting Double Couple
Mo = 7.76e+21 dyne-cm
Mw = 3.86
Z = 4 km
Plane Strike Dip Rake
NP1 239 46 -80
NP2 45 45 -100
Principal Axes:
Axis Value Plunge Azimuth
T 7.76e+21 0 322
N 0.00e+00 7 52
P -7.76e+21 83 229
Moment Tensor: (dyne-cm)
Component Value
Mxx 4.78e+21
Mxy -3.82e+21
Mxz 6.74e+20
Myy 2.87e+21
Myz 6.74e+20
Mzz -7.64e+21
##############
######################
T ##########################
###########################
#################-----------------
##############--------------------##
############-----------------------###
##########-------------------------#####
########---------------------------#####
########----------------------------######
######------------ --------------#######
#####------------- P -------------########
#####------------- ------------#########
###----------------------------#########
##---------------------------###########
#--------------------------###########
-----------------------#############
--------------------##############
--------------################
############################
######################
##############
Harvard Convention
Moment Tensor:
R T F
-7.64e+21 6.74e+20 -6.74e+20
6.74e+20 4.78e+21 3.82e+21
-6.74e+20 3.82e+21 2.87e+21
Details of the solution is found at
http://www.eas.slu.edu/eqc/eqc_mt/MECH.NA/20080407095113/index.html
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STK = 45
DIP = 45
RAKE = -100
MW = 3.86
HS = 4.0
This was a hard data set to work with because of the wave propagation effects of the Gult Coastal Plain. Stations to the east, e.g., HKT, KTVX and most eastern US stations did not record this event well at the longer periods because of the 2-D nature of the wave propagation and the very low surface velocities. The Rayleigh waves were well recorded to 25 degrees to the W-NW. The surface-wave radiation pattern fits show a log of scatter. The waveform inversion is preferred.
The following compares this source inversion to others
SLU Moment Tensor Solution
2008/04/07 09:51:13 28.921 -98.088 5.0 3.7 Texas
Best Fitting Double Couple
Mo = 7.76e+21 dyne-cm
Mw = 3.86
Z = 4 km
Plane Strike Dip Rake
NP1 239 46 -80
NP2 45 45 -100
Principal Axes:
Axis Value Plunge Azimuth
T 7.76e+21 0 322
N 0.00e+00 7 52
P -7.76e+21 83 229
Moment Tensor: (dyne-cm)
Component Value
Mxx 4.78e+21
Mxy -3.82e+21
Mxz 6.74e+20
Myy 2.87e+21
Myz 6.74e+20
Mzz -7.64e+21
##############
######################
T ##########################
###########################
#################-----------------
##############--------------------##
############-----------------------###
##########-------------------------#####
########---------------------------#####
########----------------------------######
######------------ --------------#######
#####------------- P -------------########
#####------------- ------------#########
###----------------------------#########
##---------------------------###########
#--------------------------###########
-----------------------#############
--------------------##############
--------------################
############################
######################
##############
Harvard Convention
Moment Tensor:
R T F
-7.64e+21 6.74e+20 -6.74e+20
6.74e+20 4.78e+21 3.82e+21
-6.74e+20 3.82e+21 2.87e+21
Details of the solution is found at
http://www.eas.slu.edu/eqc/eqc_mt/MECH.NA/20080407095113/index.html
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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.
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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 n 3 lp c 0.06 n 3The 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 50 -25 3.54 0.2887
WVFGRD96 1.0 245 25 -75 3.64 0.3335
WVFGRD96 2.0 235 35 -90 3.71 0.4640
WVFGRD96 3.0 240 45 -80 3.79 0.5664
WVFGRD96 4.0 45 45 -100 3.86 0.6178
WVFGRD96 5.0 250 50 -60 3.90 0.5811
WVFGRD96 6.0 90 80 -10 3.91 0.5268
WVFGRD96 7.0 270 90 0 3.94 0.4946
WVFGRD96 8.0 270 85 -5 3.95 0.4493
WVFGRD96 9.0 315 10 10 3.81 0.4161
WVFGRD96 10.0 320 10 20 3.81 0.4359
WVFGRD96 11.0 325 10 25 3.81 0.4530
WVFGRD96 12.0 325 10 25 3.81 0.4685
WVFGRD96 13.0 325 10 25 3.81 0.4820
WVFGRD96 14.0 335 15 35 3.82 0.4942
WVFGRD96 15.0 290 50 40 3.96 0.5102
WVFGRD96 16.0 290 50 40 3.97 0.5224
WVFGRD96 17.0 290 50 40 3.97 0.5338
WVFGRD96 18.0 290 50 40 3.98 0.5418
WVFGRD96 19.0 290 50 40 3.99 0.5479
WVFGRD96 20.0 295 50 45 3.99 0.5536
WVFGRD96 21.0 225 75 80 3.88 0.5592
WVFGRD96 22.0 225 70 80 3.90 0.5670
WVFGRD96 23.0 225 70 80 3.91 0.5733
WVFGRD96 24.0 225 70 80 3.92 0.5778
WVFGRD96 25.0 225 70 80 3.92 0.5822
WVFGRD96 26.0 225 65 80 3.94 0.5854
WVFGRD96 27.0 225 65 80 3.94 0.5886
WVFGRD96 28.0 225 65 80 3.95 0.5907
WVFGRD96 29.0 225 65 80 3.96 0.5910
The best solution is
WVFGRD96 4.0 45 45 -100 3.86 0.6178
The mechanism correspond to the best fit is
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The best fit as a function of depth is given in the following figure:
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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 n 3 lp c 0.06 n 3
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| 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. |
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.
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The surface-wave determined focal mechanism is shown here.
NODAL PLANES
STK= 40.00
DIP= 64.99
RAKE= -105.00
OR
STK= 252.37
DIP= 28.91
RAKE= -60.97
DEPTH = 5.0 km
Mw = 4.08
Best Fit 0.8755 - P-T axis plot gives solutions with FIT greater than FIT90
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The P-wave first motion data for focal mechanism studies are as follow:
Sta Az(deg) Dist(km) First motion
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.
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.
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| 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. |
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| 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. |
The distribution of broadband stations with azimuth and distance is
Sta Az(deg) Dist(km) JCT 317 239 NATX 46 456 WMOK 354 648 AMTX 334 743 MIAR 33 756 MNTX 296 766 324A 293 792 VBMS 61 813 222A 294 934 122A 298 950 221A 293 991 121A 296 1011 Y22D 306 1015 OXF 51 1029 MPH 46 1032 ANMO 312 1035 320A 288 1040 X22A 308 1047 220A 291 1055 W22A 312 1073 120A 294 1086 BRAL 74 1093 Z20A 298 1105 CBKS 352 1108 319A 287 1111 219A 290 1125 BMO 321 1128 PBMO 37 1128 W21A 310 1134 KSU1 6 1137 PVMO 41 1143 119A 294 1154 PLAL 52 1160 Z19A 297 1170 318A 287 1179 CCM 300 1195 218A 289 1197 SDCO 327 1197 Y19A 301 1198 W20A 308 1199 118A 293 1209 T22A 320 1217 X19A 303 1224 Z18A 295 1230 FVM 33 1231 WVT 48 1251 Y18A 298 1257 217A 288 1263 W19A 306 1268 117A 292 1276 V19A 309 1276 Z17A 295 1276 SIUC 38 1277 X18A 302 1283 W18A 305 1296 SLM 32 1299 R22A 324 1308 Y17A 297 1323 S21A 320 1324 216A 288 1327 MVCO 316 1336 V18A 307 1344 T19A 314 1346 X17A 300 1346 Z16A 294 1362 116A 291 1364 R21A 323 1365 Q22A 326 1370 W17A 303 1373 OGNE 346 1381 U18A 310 1381 Y16A 297 1385 ISCO 332 1390 USIN 41 1393 SMCO 327 1401 R20A 320 1402 X16A 299 1404 Q21A 324 1405 115A 291 1415 Z15A 293 1425 OLIL 38 1426 T18A 313 1427 W16A 302 1441 WUAZ 304 1448 214A 287 1452 U17A 309 1452 P21A 326 1457 Q20A 322 1457 U16A 307 1461 Y15A 296 1463 R19A 318 1466 S18A 314 1473 X15A 298 1474 114A 290 1476 T17A 310 1486 Z14A 293 1498 WCI 44 1502 W15A 301 1506 P20A 324 1507 R18A 317 1514 Q19A 320 1516 O21A 328 1518 HDIL 30 1519 Y14A 295 1521 S17A 313 1526 PHWY 336 1528 V15A 304 1528 X14A 297 1530 T16A 309 1538 O20A 326 1552 P19A 323 1552 113A 290 1557 BLO 40 1557 Z13A 292 1558 U15A 306 1568 N21A 329 1569 R17A 315 1572 W14A 300 1576 Q18A 319 1582 M22A 333 1585 Y13A 294 1591 V14A 302 1596 TZTN 54 1601 T15A 308 1607 SRU 318 1608 N20A 328 1613 O19A 324 1615 X13A 297 1616 R16A 313 1617 112A 288 1628 P18A 320 1629 L22A 335 1638 Q16A 316 1638 M21A 332 1639 RWWY 332 1639 U14A 304 1639 W13A 299 1640 S15A 310 1643 ECSD 4 1649 P17A 318 1649 Y12C 293 1651 GLA 290 1661 O18A 322 1664 T14A 307 1664 N19A 326 1668 R15A 312 1669 L21A 332 1672 V13A 301 1679 N18A 325 1702 U13A 303 1702 O17A 321 1703 JFWS 22 1704 S14A 309 1709 CCUT 308 1712 M19A 327 1719 P16A 317 1719 L20A 330 1721 Q15A 314 1727 T13A 305 1727 R14A 311 1729 W12A 298 1732 V12A 300 1746 U12A 303 1750 LDF 298 1752 O16A 319 1752 S13A 307 1752 P15A 316 1758 M18A 325 1761 N17A 322 1766 RSSD 344 1768 T12A 303 1777 K20A 332 1781 L19A 328 1781 Q14A 313 1789 R13A 309 1793 M17A 324 1799 N16A 321 1799 V11A 300 1800 L18A 326 1801 P14A 315 1813 TCU 322 1815 U11A 302 1819 K19A 331 1820 O15A 318 1820 BAR 288 1824 S12A 306 1831 DUG 317 1837 Q13A 311 1841 T11A 304 1842 BW06 329 1849 R12A 309 1850 HWUT 323 1862 K18A 328 1862 L17A 325 1862 ACSO 44 1865 109C 288 1867 N15A 319 1867 P13A 313 1871 BLA 56 1884 L16A 324 1886 SPU 320 1893 U10A 301 1898 GSC 297 1899 BGU 318 1902 M15A 321 1904 S11A 305 1904 Q12A 310 1906 N14A 318 1907 J18A 329 1911 O13A 315 1912 K17A 326 1918 R11A 307 1926 AHID 326 1929 P12A 312 1937 L15A 322 1942 I18A 331 1945 Q11A 309 1957 J17A 328 1959 K16A 326 1961 M14A 319 1964 REDW 328 1967 SNOW 328 1970 AAM 37 1972 O12A 314 1974 LOHW 329 1975 N13A 316 1975 R10A 306 1979 MWC 292 1981 TPAW 328 1983 S10A 305 1984 RRI2 327 1990 L14A 321 1996 J16A 326 1999 I17A 330 2001 K15A 324 2003 DCID1 328 2004 Q10A 308 2013 IMW 329 2017 K14A 322 2030 O11A 312 2030 N12A 315 2031 ELK 315 2038 H17A 330 2043 I16A 328 2043 L13A 320 2044 LKWY 331 2050 OSI 293 2050 RLMT 334 2050 J15A 325 2054 ISA 296 2056 M12A 317 2060 P10A 310 2060 COWI 20 2061 N11A 314 2077 G18A 334 2081 K13A 321 2093 LAO 342 2095 SNCC 289 2097 O10A 312 2100 H16A 330 2103 I15A 326 2106 J14A 323 2110 L12A 318 2116 M11A 316 2120 N10A 313 2126 GLMI 30 2128 G17A 332 2133 F18A 335 2142 I14A 325 2154 J13A 322 2154 H15A 327 2166 L11A 317 2166 G16A 330 2176 M10A 315 2177 HLID 322 2181 F17A 333 2183 EYMN 13 2187 I13A 324 2191 G15A 329 2204 H14A 326 2204 L10A 316 2208 E18A 336 2213 F16A 331 2216 K11A 318 2225 DLMT 329 2226 DGMT 348 2232 I12A 322 2241 E17A 334 2243 H13A 325 2249 J11A 320 2256 G14A 327 2261 F15A 330 2262 D18A 337 2268 SDMD 54 2270 K10A 317 2280 H12A 324 2282 G13A 326 2290 D17A 335 2299 I11A 321 2300 F14A 329 2303 J10A 319 2317 E15A 331 2320 D16A 334 2325 MVL 52 2335 L08A 314 2343 EGMT 338 2346 SAO 298 2346 F13A 327 2351 M07A 312 2356 E14A 330 2361 H11A 323 2361 I10A 320 2364 D15A 332 2369 N06A 309 2372 J09A 318 2375 WVOR 315 2378 B18A 338 2383 K08A 315 2385 E13A 329 2398 F12A 326 2398 L07A 313 2403 I09A 319 2416 D14A 331 2418 MSO 330 2418 J08A 317 2424 G11A 324 2428 K07A 314 2436 C15A 333 2438 H09A 321 2458 B16A 335 2459 F11A 325 2459 D13A 329 2466 E12A 327 2468 A17A 338 2470 G10A 322 2470 BINY 48 2474 B15A 334 2485 E11A 326 2498 A16A 336 2504 D12A 328 2508 G09A 322 2508 MCCM 300 2510 H08A 319 2512 C13A 330 2521 F10A 324 2526 J06A 315 2531 I07A 317 2538 F09A 322 2547 K05A 313 2554 E10A 325 2558 D11A 327 2559 C12B 329 2568 PAL 52 2570 H07A 318 2574 G08A 320 2581 F08A 322 2605 D10A 326 2610 E09A 324 2619 A13A 332 2622 H06A 318 2633 B12A 330 2637 D09A 324 2669 C10A 327 2672 B11A 329 2673 G06A 318 2684 LTH 322 2690 HAWA 322 2692 HUMO 311 2697 NEW 328 2697 D08A 324 2703 B10A 328 2705 LONY 44 2710 A11A 330 2715 C09A 326 2722 E07A 322 2723 H04A 316 2753 B09A 327 2763 C08A 325 2764 D07A 323 2774 A10A 329 2777 E06A 321 2790 B08A 326 2822 COR 315 2824 D06A 322 2826 A09A 328 2829 F04A 318 2843 A08A 327 2864 LBNH 47 2872 B07A 325 2873 C06A 323 2879 D05A 321 2895 A07A 326 2931 B06A 324 2955 A05A 324 3023 PKME 46 3110
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:
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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 n 3 lp c 0.06 n 3
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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. 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.
Thanks also to the many seismic network operators whose dedication make this effort possible: University of Alaska, University of Washington, Oregon State University, University of Utah, Montana Bureas of Mines, UC Berkely, Caltech, UC San Diego, Saint L ouis University, Universityof Memphis, Lamont Doehrty Earth Observatory, Boston College, the Iris stations and the Transportable Array of EarthScope.
The WUS used for the waveform synthetic seismograms and for the surface wave eigenfunctions and dispersion is as follows:
Here we tabulate the reasons for not using certain digital data sets
The following stations did not have a valid response files:
DATE=Mon Apr 7 14:38:13 CDT 2008
