Location

2008/07/31 05:02:43 48.154 -122.747 55.9 4 PNW

Arrival Times (from USGS)

Felt Map

Focal Mechanism

USGS/SLU Moment Tensor Solution ENS 2008/07/31 05:02:43:1 48.15 -122.75 55.9 4.0 PNW Stations used: TA.B06A XU.BS11 XU.C04A XU.E010 XU.N060 XU.PL11 XU.W020 XU.W030 XU.W040 YW.FACA Filtering commands used: cut o DIST/3.3 -30 o DIST/3.3 +70 rtr taper w 0.1 hp c 0.03 n 3 lp c 0.06 n 3 Best Fitting Double Couple Mo = 7.24e+21 dyne-cm Mw = 3.84 Z = 62 km Plane Strike Dip Rake NP1 350 85 -70 NP2 93 21 -166 Principal Axes: Axis Value Plunge Azimuth T 7.24e+21 37 62 N 0.00e+00 20 168 P -7.24e+21 46 280 Moment Tensor: (dyne-cm) Component Value Mxx 8.80e+20 Mxy 2.52e+21 Mxz 9.51e+20 Myy 3.02e+20 Myz 6.64e+21 Mzz -1.18e+21 ----########## --------############## ------------################ -------------################# ----------------################## -----------------################### -------------------########## ###### --------------------########## T ####### -------- ---------########## ####### --------- P ----------#################### --------- ----------#################### #---------------------#################### #----------------------##################- #---------------------################## ##--------------------#################- ##--------------------##############-- ###------------------#############-- ####----------------###########--- #####-------------########---- ########---------####------- ###############------- ###########--- Global CMT Convention Moment Tensor: R T P -1.18e+21 9.51e+20 -6.64e+21 9.51e+20 8.80e+20 -2.52e+21 -6.64e+21 -2.52e+21 3.02e+20 Details of the solution is found at http://www.eas.slu.edu/eqc/eqc_mt/MECH.NA/20080731050243/index.html

Preferred Solution

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

      STK = 350
      DIP = 85
     RAKE = -70
       MW = 3.84
       HS = 62.0

The NDK file is 20080731050243.ndk The waveform inversion is preferred.

Moment Tensor Comparison

The following compares this source inversion to others

SLU

USGS/SLU Moment Tensor Solution ENS 2008/07/31 05:02:43:1 48.15 -122.75 55.9 4.0 PNW Stations used: TA.B06A XU.BS11 XU.C04A XU.E010 XU.N060 XU.PL11 XU.W020 XU.W030 XU.W040 YW.FACA Filtering commands used: cut o DIST/3.3 -30 o DIST/3.3 +70 rtr taper w 0.1 hp c 0.03 n 3 lp c 0.06 n 3 Best Fitting Double Couple Mo = 7.24e+21 dyne-cm Mw = 3.84 Z = 62 km Plane Strike Dip Rake NP1 350 85 -70 NP2 93 21 -166 Principal Axes: Axis Value Plunge Azimuth T 7.24e+21 37 62 N 0.00e+00 20 168 P -7.24e+21 46 280 Moment Tensor: (dyne-cm) Component Value Mxx 8.80e+20 Mxy 2.52e+21 Mxz 9.51e+20 Myy 3.02e+20 Myz 6.64e+21 Mzz -1.18e+21 ----########## --------############## ------------################ -------------################# ----------------################## -----------------################### -------------------########## ###### --------------------########## T ####### -------- ---------########## ####### --------- P ----------#################### --------- ----------#################### #---------------------#################### #----------------------##################- #---------------------################## ##--------------------#################- ##--------------------##############-- ###------------------#############-- ####----------------###########--- #####-------------########---- ########---------####------- ###############------- ###########--- Global CMT Convention Moment Tensor: R T P -1.18e+21 9.51e+20 -6.64e+21 9.51e+20 8.80e+20 -2.52e+21 -6.64e+21 -2.52e+21 3.02e+20 Details of the solution is found at http://www.eas.slu.edu/eqc/eqc_mt/MECH.NA/20080731050243/index.html

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:

cut o DIST/3.3 -30 o DIST/3.3 +70
rtr
taper w 0.1
hp c 0.03 n 3 
lp c 0.06 n 3

The results of this grid search from 0.5 to 19 km depth are as follow:

           DEPTH  STK   DIP  RAKE   MW    FIT
WVFGRD96    2.0    10    45   -60   3.25 0.3335
WVFGRD96    4.0   220    75   -30   3.31 0.3921
WVFGRD96    6.0   210    65   -30   3.35 0.4410
WVFGRD96    8.0   210    65   -35   3.42 0.4763
WVFGRD96   10.0   215    60    40   3.42 0.4935
WVFGRD96   12.0   215    65    40   3.45 0.5181
WVFGRD96   14.0   215    65    40   3.47 0.5350
WVFGRD96   16.0   210    70    35   3.47 0.5453
WVFGRD96   18.0   210    70    40   3.49 0.5535
WVFGRD96   20.0   215    70    40   3.51 0.5598
WVFGRD96   22.0   215    70    45   3.53 0.5667
WVFGRD96   24.0   215    70    45   3.54 0.5730
WVFGRD96   26.0   215    70    45   3.54 0.5785
WVFGRD96   28.0     5    80   -35   3.55 0.5807
WVFGRD96   30.0     0    80   -35   3.57 0.5843
WVFGRD96   32.0     0    80   -40   3.58 0.5883
WVFGRD96   34.0     0    80   -40   3.59 0.5969
WVFGRD96   36.0    -5    80   -40   3.61 0.6044
WVFGRD96   38.0    -5    80   -40   3.62 0.6144
WVFGRD96   40.0    -5    80   -55   3.72 0.6267
WVFGRD96   42.0   355    80   -55   3.73 0.6316
WVFGRD96   44.0   355    80   -55   3.74 0.6355
WVFGRD96   46.0   355    80   -55   3.75 0.6384
WVFGRD96   48.0   350    80   -55   3.77 0.6416
WVFGRD96   50.0   350    80   -60   3.78 0.6443
WVFGRD96   52.0   350    80   -60   3.79 0.6456
WVFGRD96   54.0   355    85   -60   3.79 0.6468
WVFGRD96   56.0   355    85   -60   3.80 0.6483
WVFGRD96   58.0   350    85   -65   3.82 0.6484
WVFGRD96   60.0   350    85   -70   3.83 0.6489
WVFGRD96   62.0   350    85   -70   3.84 0.6492
WVFGRD96   64.0   350    85   -75   3.85 0.6488
WVFGRD96   66.0   350    85   -80   3.86 0.6471
WVFGRD96   68.0   200     5   -60   3.89 0.6417
WVFGRD96   70.0   200     5   -60   3.89 0.6411
WVFGRD96   72.0   210     5   -50   3.90 0.6394
WVFGRD96   74.0   210     5   -50   3.90 0.6363
WVFGRD96   76.0   220    10   -35   3.92 0.6333
WVFGRD96   78.0   225    10   -30   3.92 0.6297
WVFGRD96   80.0   235    15   -20   3.94 0.6266
WVFGRD96   82.0   235    15   -20   3.94 0.6211
WVFGRD96   84.0   240    20   -15   3.96 0.6182
WVFGRD96   86.0   240    20   -15   3.97 0.6126
WVFGRD96   88.0   240    25   -15   3.98 0.6080
WVFGRD96   90.0   240    25   -15   3.99 0.6029
WVFGRD96   92.0   245    30   -10   4.01 0.5972
WVFGRD96   94.0   240    30   -15   4.01 0.5921
WVFGRD96   96.0   245    35   -10   4.04 0.5874
WVFGRD96   98.0   245    35   -10   4.04 0.5816

The best solution is

WVFGRD96   62.0   350    85   -70   3.84 0.6492

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 component is plotted to the same scale and peak amplitudes are indicated by the numbers to the left of each trace. A pair of numbers is given in black at the right of each predicted traces. The upper number 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 lower number gives the percentage of variance reduction to characterize the individual goodness of fit (100% indicates a perfect fit).

The bandpass filter used in the processing and for the display was

cut o DIST/3.3 -30 o DIST/3.3 +70
rtr
taper w 0.1
hp c 0.03 n 3 
lp c 0.06 n 3

Figure 3. Waveform comparison for selected depth. Red: observed; Blue - predicted. The time shift with respect to the model prediction is indicated. The percent of fit is also indicated.

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.

A check on the assumed source location is possible by looking at the time shifts between the observed and predicted traces. The time shifts for waveform matching arise for several reasons:

The origin time and epicentral distance are incorrect
The velocity model used for the inversion is incorrect
The velocity model used to define the P-arrival time is not the same as the velocity model used for the waveform inversion (assuming that the initial trace alignment is based on the P arrival time)

Assuming only a mislocation, the time shifts are fit to a functional form:

 Time_shift = A + B cos Azimuth + C Sin Azimuth

The time shifts for this inversion lead to the next figure:

The derived shift in origin time and epicentral coordinates are given at the bottom of the figure.

Discussion

Acknowledgements

Thanks also to the many seismic network operators whose dedication make this effort possible: University of Nevada Reno, University of Alaska, University of Washington, Oregon State University, University of Utah, Montana Bureas of Mines, UC Berkely, Caltech, UC San Diego, Saint Louis University, University of Memphis, Lamont Doherty Earth Observatory, the Iris stations and the Transportable Array of EarthScope.

Velocity Model

The WUS.model 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

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 Thu Apr 30 15:42:18 CDT 2015