2007/06/09 10:45:44 36.93N 104.79W 1 3.3 New Mexico
USGS Felt map for this earthquake
USGS Felt reports page for Central and Southeastern US
SLU Moment Tensor Solution 2007/06/09 10:45:44 36.93N 104.79W 1 3.3 New Mexico Best Fitting Double Couple Mo = 1.33e+21 dyne-cm Mw = 3.35 Z = 4 km Plane Strike Dip Rake NP1 195 50 -60 NP2 333 48 -121 Principal Axes: Axis Value Plunge Azimuth T 1.33e+21 1 264 N 0.00e+00 23 355 P -1.33e+21 67 172 Moment Tensor: (dyne-cm) Component Value Mxx -1.79e+20 Mxy 1.58e+20 Mxz 4.66e+20 Myy 1.32e+21 Myz -8.28e+19 Mzz -1.14e+21 -----------### ######-----########### ############-############### ###########------############# ############---------############# ###########-------------############ ###########---------------############ ############----------------############ ###########------------------########### ###########--------------------########### ###########---------------------########## #########---------------------########## T ########---------- ----------######### ########---------- P ----------######## #########---------- ----------######## ########-----------------------####### ########----------------------###### #######----------------------##### ######---------------------### ######-------------------### ####-----------------# #------------- Harvard Convention Moment Tensor: R T F -1.14e+21 4.66e+20 8.28e+19 4.66e+20 -1.79e+20 -1.58e+20 8.28e+19 -1.58e+20 1.32e+21 Details of the solution is found at http://www.eas.slu.edu/eqc/eqc_mt/MECH.NA/20070609104544/index.html |
STK = 195 DIP = 50 RAKE = -60 MW = 3.35 HS = 4
This a marginal event which had some good signals. The waveforms inversion is preferred. The moment is OK but hte depth is not well determined.
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.
|
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 0.025 n 3 lp c 0.06 n 3 br c 0.12 0.25 n 4 p 2The results of this grid search from 0.5 to 19 km depth are as follow:
DEPTH STK DIP RAKE MW FIT WVFGRD96 0.5 210 30 -20 3.32 0.2225 WVFGRD96 1.0 210 30 -20 3.34 0.2262 WVFGRD96 2.0 205 50 -40 3.26 0.2330 WVFGRD96 3.0 200 50 -50 3.31 0.2452 WVFGRD96 4.0 195 50 -60 3.35 0.2532 WVFGRD96 5.0 185 45 -75 3.40 0.2488 WVFGRD96 6.0 155 40 -100 3.38 0.2376 WVFGRD96 7.0 35 65 50 3.30 0.2345 WVFGRD96 8.0 30 70 45 3.28 0.2360 WVFGRD96 9.0 35 70 45 3.29 0.2362 WVFGRD96 10.0 35 70 45 3.30 0.2365 WVFGRD96 11.0 30 75 40 3.29 0.2373 WVFGRD96 12.0 30 75 40 3.29 0.2380 WVFGRD96 13.0 30 75 40 3.30 0.2378 WVFGRD96 14.0 30 75 40 3.30 0.2369 WVFGRD96 15.0 30 75 35 3.30 0.2359 WVFGRD96 16.0 30 75 35 3.31 0.2346 WVFGRD96 17.0 25 80 35 3.31 0.2334 WVFGRD96 18.0 25 80 35 3.31 0.2320 WVFGRD96 19.0 25 80 35 3.32 0.2305
The best solution is
WVFGRD96 4.0 195 50 -60 3.35 0.2532
The mechanism correspond to the best fit is
|
The best fit as a function of depth is given in the following figure:
|
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 0.025 n 3 lp c 0.06 n 3 br c 0.12 0.25 n 4 p 2
|
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.
|
The surface-wave determined focal mechanism is shown here.
NODAL PLANES STK= 175.15 DIP= 60.21 RAKE= -70.70 OR STK= 319.97 DIP= 35.00 RAKE= -120.00 DEPTH = 5.0 km Mw = 3.46 Best Fit 0.8945 - P-T axis plot gives solutions with FIT greater than FIT90
The P-wave first motion data for focal mechanism studies are as follow:
Sta Az(deg) Dist(km) First motion ANMO 215 267 eP_- SMCO 323 315 eP_X ISCO 348 327 eP_X MVCO 276 331 eP_X AMTX 128 361 eP_X CBKS 63 491 iP_D
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.
|
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. |
The distribution of broadband stations with azimuth and distance is
Sta Az(deg) Dist(km) ANMO 215 267 ISCO 348 327 MVCO 276 331 AMTX 128 361 Y22C 212 371 V19A 252 405 U18A 264 458 W19A 245 461 V18A 255 481 W18A 247 489 SRU 297 558 U16A 263 574 Z19A 227 574 MNTX 186 583 Y18A 235 591 WMOK 112 595 Q16A 293 603 TUC 229 751 DUG 300 787 ECSD 40 1026 MIAR 101 1048 CCM 80 1203
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 0.025 n 3 lp c 0.06 n 3 br c 0.12 0.25 n 4 p 2
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
Here we tabulate the reasons for not using certain digital data sets
The following stations did not have a valid response files: