The ANSS event ID is uw10753338 and the event page is at https://earthquake.usgs.gov/earthquakes/eventpage/uw10753338/executive.
2008/07/31 05:02:43 48.152 -122.748 55.9 3.87 Washington
USGS/SLU Moment Tensor Solution
ENS 2008/07/31 05:02:43:0 48.15 -122.75 55.9 3.9 Washington
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
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STK = 350
DIP = 85
RAKE = -70
MW = 3.84
HS = 62.0
The NDK file is 20080731050243.ndk The waveform inversion is preferred.
Given the availability of digital waveforms for determination of the moment tensor, this section documents the added processing leading to mLg, if appropriate to the region, and ML by application of the respective IASPEI formulae. As a research study, the linear distance term of the IASPEI formula for ML is adjusted to remove a linear distance trend in residuals to give a regionally defined ML. The defined ML uses horizontal component recordings, but the same procedure is applied to the vertical components since there may be some interest in vertical component ground motions. Residual plots versus distance may indicate interesting features of ground motion scaling in some distance ranges. A residual plot of the regionalized magnitude is given as a function of distance and azimuth, since data sets may transcend different wave propagation provinces.
Left: ML computed using the IASPEI formula for Horizontal components. Center: ML residuals computed using a modified IASPEI formula that accounts for path specific attenuation; the values used for the trimmed mean are indicated. The ML relation used for each figure is given at the bottom of each plot.
Right: Residuals from new relation as a function of distance and azimuth.
Left: ML computed using the IASPEI formula for Vertical components (research). Center: ML residuals computed using a modified IASPEI formula that accounts for path specific attenuation; the values used for the trimmed mean are indicated. The ML relation used for each figure is given at the bottom of each plot.
Right: Residuals from new relation as a function of distance and azimuth.
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The focal mechanism was determined using broadband seismic waveforms. The location of the event (star) and the stations used for (red) 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's 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 3The results of this grid search 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 corresponding 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 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, the velocity model used in the predictions may not be perfect and the epicentral parameters may be be off. 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
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| 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. The time scale is relative to the first trace sample. |
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| Focal mechanism sensitivity at the preferred depth. The red color indicates a very good fit to the waveforms. 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:
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.
The WUS.model used for the waveform synthetic seismograms and for the surface wave eigenfunctions and dispersion is as follows (The format is in the model96 format of Computer Programs in Seismology).
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