Left: mLg computed using the IASPEI formula. Center: mLg residuals versus epicentral distance ; the values used for the trimmed mean magnitude estimate are indicated. Right: residuals as a function of distance and azimuth.
The ANSS event ID is se609215 and the event page is at https://earthquake.usgs.gov/earthquakes/eventpage/se609215/executive.
2011/08/24 00:04:36 37.915 -77.954 0.0 4.2 Virginia
USGS/SLU Moment Tensor Solution
ENS 2011/08/24 00:04:36:0 37.92 -77.95 0.0 4.2 Virginia
Stations used:
CO.HODGE CO.JSC IU.SSPA LD.BRNJ LD.LUPA LD.MVL LD.PAL
LD.SDMD LD.WVNY PE.PSUB TA.KMSC US.ACSO US.BINY US.BLA
US.CBN US.CNNC US.ERPA US.MCWV US.NHSC US.TZTN
Filtering commands used:
cut o DIST/3.3 -40 o DIST/3.3 +50
rtr
taper w 0.1
hp c 0.03 n 3
lp c 0.07 n 3
Best Fitting Double Couple
Mo = 6.10e+21 dyne-cm
Mw = 3.79
Z = 5 km
Plane Strike Dip Rake
NP1 39 64 106
NP2 185 30 60
Principal Axes:
Axis Value Plunge Azimuth
T 6.10e+21 67 338
N 0.00e+00 14 212
P -6.10e+21 18 117
Moment Tensor: (dyne-cm)
Component Value
Mxx -2.99e+20
Mxy 1.90e+21
Mxz 2.86e+21
Myy -4.27e+21
Myz -2.40e+21
Mzz 4.57e+21
-----#########
------################
------#####################-
-----#######################--
------########################----
------########################------
------######### #############-------
------########## T ############---------
------########## ############---------
-------########################-----------
------########################------------
------######################--------------
------#####################---------------
------###################---------------
------#################----------- ---
------##############------------- P --
-----############--------------- -
-----#########--------------------
----#####---------------------
---##-----------------------
####------------------
###-----------
Global CMT Convention Moment Tensor:
R T P
4.57e+21 2.86e+21 2.40e+21
2.86e+21 -2.99e+20 -1.90e+21
2.40e+21 -1.90e+21 -4.27e+21
Details of the solution is found at
http://www.eas.slu.edu/eqc/eqc_mt/MECH.NA/20110824000436/index.html
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STK = 185
DIP = 30
RAKE = 60
MW = 3.79
HS = 5.0
The NDK file is 20110824000436.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: mLg computed using the IASPEI formula. Center: mLg residuals versus epicentral distance ; the values used for the trimmed mean magnitude estimate are indicated.
Right: residuals as a function of distance and azimuth.
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 -40 o DIST/3.3 +50 rtr taper w 0.1 hp c 0.03 n 3 lp c 0.07 n 3The results of this grid search are as follow:
DEPTH STK DIP RAKE MW FIT
WVFGRD96 0.5 190 15 50 3.89 0.4763
WVFGRD96 1.0 190 15 50 3.91 0.4871
WVFGRD96 2.0 175 20 40 3.84 0.5107
WVFGRD96 3.0 185 25 55 3.81 0.5449
WVFGRD96 4.0 185 30 55 3.79 0.5743
WVFGRD96 5.0 185 30 60 3.79 0.5899
WVFGRD96 6.0 185 35 60 3.77 0.5896
WVFGRD96 7.0 185 35 60 3.77 0.5800
WVFGRD96 8.0 180 35 55 3.75 0.5678
WVFGRD96 9.0 155 45 -15 3.70 0.5607
WVFGRD96 10.0 150 40 -20 3.72 0.5647
WVFGRD96 11.0 155 45 -15 3.72 0.5677
WVFGRD96 12.0 155 45 -15 3.73 0.5684
WVFGRD96 13.0 150 45 -20 3.73 0.5667
WVFGRD96 14.0 150 45 -20 3.73 0.5630
WVFGRD96 15.0 150 45 -20 3.74 0.5583
WVFGRD96 16.0 150 45 -15 3.74 0.5528
WVFGRD96 17.0 155 50 -10 3.74 0.5459
WVFGRD96 18.0 155 50 -10 3.75 0.5384
WVFGRD96 19.0 155 50 -10 3.76 0.5312
WVFGRD96 20.0 155 45 -10 3.78 0.5233
WVFGRD96 21.0 155 45 -10 3.79 0.5146
WVFGRD96 22.0 155 45 -10 3.80 0.5057
WVFGRD96 23.0 155 50 -10 3.79 0.4966
WVFGRD96 24.0 155 50 -10 3.80 0.4875
WVFGRD96 25.0 155 50 -10 3.81 0.4785
WVFGRD96 26.0 155 50 -10 3.81 0.4693
WVFGRD96 27.0 155 50 -10 3.82 0.4598
WVFGRD96 28.0 155 50 -10 3.83 0.4514
WVFGRD96 29.0 150 50 -15 3.83 0.4440
The best solution is
WVFGRD96 5.0 185 30 60 3.79 0.5899
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 -40 o DIST/3.3 +50 rtr taper w 0.1 hp c 0.03 n 3 lp c 0.07 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 CUS.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 CUS Model with Q from simple gamma values 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 1.0000 5.0000 2.8900 2.5000 0.172E-02 0.387E-02 0.00 0.00 1.00 1.00 9.0000 6.1000 3.5200 2.7300 0.160E-02 0.363E-02 0.00 0.00 1.00 1.00 10.0000 6.4000 3.7000 2.8200 0.149E-02 0.336E-02 0.00 0.00 1.00 1.00 20.0000 6.7000 3.8700 2.9020 0.000E-04 0.000E-04 0.00 0.00 1.00 1.00 0.0000 8.1500 4.7000 3.3640 0.194E-02 0.431E-02 0.00 0.00 1.00 1.00