2007/09/24 06:20:54 45.10N 123.03W 22.0 3.6 Oregon
USGS Felt map for this earthquake
SLU Moment Tensor Solution 2007/09/24 06:20:54 45.10N 123.03W 22.0 3.6 Oregon Best Fitting Double Couple Mo = 3.76e+21 dyne-cm Mw = 3.65 Z = 20 km Plane Strike Dip Rake NP1 125 90 -45 NP2 215 45 -180 Principal Axes: Axis Value Plunge Azimuth T 3.76e+21 30 180 N 0.00e+00 45 305 P -3.76e+21 30 70 Moment Tensor: (dyne-cm) Component Value Mxx 2.50e+21 Mxy -9.09e+20 Mxz -2.18e+21 Myy -2.50e+21 Myz -1.52e+21 Mzz -6.47e+13 ############## ###################### ################------------ ##############---------------- #############--------------------- ---#########------------------------ -------####--------------------------- -------------------------------- ----- ----------###------------------- P ----- ----------#######---------------- ------ ---------###########---------------------- --------##############-------------------- --------#################----------------- ------#####################------------- ------#######################----------- -----##########################------- ----#############################--- ---############# ############### --############ T ############# -############ ############ ###################### ############## Harvard Convention Moment Tensor: R T F -6.47e+13 -2.18e+21 1.52e+21 -2.18e+21 2.50e+21 9.09e+20 1.52e+21 9.09e+20 -2.50e+21 Details of the solution is found at http://www.eas.slu.edu/eqc/eqc_mt/MECH.NA/20070924062054/index.html |
STK = 125 DIP = 90 RAKE = -45 MW = 3.65 HS = 20
This is a small event with little Love wave. The two techniques give the same solution
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 0.02 n 3 lp c 0.10 n 3 br c 0.11 0.20 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 0.5 120 45 90 3.18 0.2709 WVFGRD96 1.0 295 45 90 3.23 0.2750 WVFGRD96 2.0 120 45 90 3.35 0.3407 WVFGRD96 3.0 290 90 0 3.42 0.2960 WVFGRD96 4.0 115 70 20 3.47 0.2859 WVFGRD96 5.0 20 35 0 3.44 0.2974 WVFGRD96 6.0 35 15 -10 3.47 0.3583 WVFGRD96 7.0 50 10 10 3.46 0.4032 WVFGRD96 8.0 310 85 90 3.54 0.4284 WVFGRD96 9.0 320 75 75 3.58 0.4530 WVFGRD96 10.0 310 75 80 3.55 0.4690 WVFGRD96 11.0 310 70 80 3.56 0.4808 WVFGRD96 12.0 310 70 85 3.56 0.4884 WVFGRD96 13.0 135 20 95 3.56 0.4937 WVFGRD96 14.0 130 70 -90 3.55 0.4977 WVFGRD96 15.0 130 70 -90 3.56 0.5023 WVFGRD96 16.0 125 70 -90 3.56 0.5058 WVFGRD96 17.0 120 85 -45 3.62 0.5096 WVFGRD96 18.0 125 90 -45 3.63 0.5125 WVFGRD96 19.0 125 90 -45 3.64 0.5138 WVFGRD96 20.0 125 90 -45 3.65 0.5138 WVFGRD96 21.0 125 90 -45 3.67 0.5127 WVFGRD96 22.0 125 90 -40 3.70 0.5101 WVFGRD96 23.0 125 90 -45 3.69 0.5066 WVFGRD96 24.0 305 90 45 3.70 0.5022 WVFGRD96 25.0 305 90 45 3.71 0.4964 WVFGRD96 26.0 305 90 45 3.72 0.4892 WVFGRD96 27.0 305 85 45 3.73 0.4815 WVFGRD96 28.0 305 85 45 3.74 0.4729 WVFGRD96 29.0 305 85 45 3.75 0.4631
The best solution is
WVFGRD96 20.0 125 90 -45 3.65 0.5138
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 0.02 n 3 lp c 0.10 n 3 br c 0.11 0.20 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= 141.72 DIP= 81.69 RAKE= -66.33 OR STK= 249.98 DIP= 25.01 RAKE= -159.99 DEPTH = 16.0 km Mw = 3.65 Best Fit 0.8962 - 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 HEBO 283 58 -12345 J03A 178 154 -12345 LON 27 206 -12345 NLWA 346 263 -12345 GNW 3 275 -12345 HUMO 179 277 -12345 HAWA 61 308 -12345 M04C 165 381 -12345
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) J03A 178 192 LON 27 206 GNW 3 275 HUMO 179 277 HAWA 61 308 YBH 176 375 M04C 165 381 F09A 79 407 MOD 147 418 BMO 92 452 O01C 187 555 NEW 50 572 D12A 67 632 HLID 101 708 MSO 72 730 C14A 64 772 A13A 54 781 CMB 163 815 A15A 58 892 MOOW 94 989 EGMT 68 1067 BW06 99 1111
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 n 3 lp c 0.10 n 3 br c 0.11 0.20 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
The WUS used for the waveform synthetic seismograms and for the surface wave eigenfunctions and dispersion is as follows:
MODEL.01 Model after 8 iterations ISOTROPIC KGS FLAT EARTH 1-D CONSTANT VELOCITY LINE08 LINE09 LINE10 LINE11 H(KM) VP(KM/S) VS(KM/S) RHO(GM/CC) QP QS ETAP ETAS FREFP FREFS 1.9000 3.4065 2.0089 2.2150 0.302E-02 0.679E-02 0.00 0.00 1.00 1.00 6.1000 5.5445 3.2953 2.6089 0.349E-02 0.784E-02 0.00 0.00 1.00 1.00 13.0000 6.2708 3.7396 2.7812 0.212E-02 0.476E-02 0.00 0.00 1.00 1.00 19.0000 6.4075 3.7680 2.8223 0.111E-02 0.249E-02 0.00 0.00 1.00 1.00 0.0000 7.9000 4.6200 3.2760 0.164E-10 0.370E-10 0.00 0.00 1.00 1.00
Here we tabulate the reasons for not using certain digital data sets
The following stations did not have a valid response files:
DATE=Sat Sep 29 15:46:55 CDT 2007