Location

Location ANSS

The ANSS event ID is se610610 and the event page is at https://earthquake.usgs.gov/earthquakes/eventpage/se610610/executive.

2014/02/15 03:23:38 33.817 -82.092 5.2 4.1 Georgia

Focal Mechanism

USGS/SLU Moment Tensor Solution ENS 2014/02/15 03:23:38:0 33.82 -82.09 5.2 4.1 Georgia Stations used: CO.BIRD CO.CASEE CO.HAW CO.PAULI ET.CPCT ET.FPAL IM.TKL N4.154A N4.255A N4.257A N4.456A N4.V48A N4.X48A N4.Y49A NM.CLTN TA.KMSC TA.Q54A TA.R53A TA.R55A TA.R58B TA.S51A TA.S54A TA.S57A TA.S58A TA.T49A TA.T52A TA.T57A TA.T59A TA.U54A TA.U56A TA.U59A TA.V51A TA.V52A TA.V53A TA.V55A TA.V58A TA.V60A TA.W50A TA.W52A TA.W57A TA.X51A TA.Y52A TA.Y57A TA.Y58A TA.Y60A TA.Z56A US.BLA US.GOGA US.NHSC US.TZTN Filtering commands used: cut a -20 a 160 rtr taper w 0.1 hp c 0.02 n 3 lp c 0.10 n 3 Best Fitting Double Couple Mo = 1.84e+22 dyne-cm Mw = 4.11 Z = 5 km Plane Strike Dip Rake NP1 202 66 141 NP2 310 55 30 Principal Axes: Axis Value Plunge Azimuth T 1.84e+22 44 162 N 0.00e+00 45 355 P -1.84e+22 7 258 Moment Tensor: (dyne-cm) Component Value Mxx 7.78e+21 Mxy -6.53e+21 Mxz -8.29e+21 Myy -1.64e+22 Myz 4.98e+21 Mzz 8.65e+21 ############## ################------ #################----------- ################-------------- --------------###----------------- ----------------###----------------- ----------------#######--------------- ----------------##########-------------- ---------------#############------------ ---------------################----------- ---------------#################---------- --------------###################--------- -----------####################-------- P ----------######################------ ----------#######################----- ----------########### ###########--- ---------########### T ###########-- --------########### ###########- ------######################## -----####################### ---################### ############## Global CMT Convention Moment Tensor: R T P 8.65e+21 -8.29e+21 -4.98e+21 -8.29e+21 7.78e+21 6.53e+21 -4.98e+21 6.53e+21 -1.64e+22 Details of the solution is found at http://www.eas.slu.edu/eqc/eqc_mt/MECH.NA/20140215032338/index.html

Preferred Solution

      STK = 310
      DIP = 55
     RAKE = 30
       MW = 4.11
       HS = 5.0

The NDK file is 20140215032338.ndk The waveform inversion is preferred.

Moment Tensor Comparison

The following compares this source inversion to those provided by others. The purpose is to look for major differences and also to note slight differences that might be inherent to the processing procedure. For completeness the USGS/SLU solution is repeated from above.

SLU	USGSMT
USGS/SLU Moment Tensor Solution ENS 2014/02/15 03:23:38:0 33.82 -82.09 5.2 4.1 Georgia Stations used: CO.BIRD CO.CASEE CO.HAW CO.PAULI ET.CPCT ET.FPAL IM.TKL N4.154A N4.255A N4.257A N4.456A N4.V48A N4.X48A N4.Y49A NM.CLTN TA.KMSC TA.Q54A TA.R53A TA.R55A TA.R58B TA.S51A TA.S54A TA.S57A TA.S58A TA.T49A TA.T52A TA.T57A TA.T59A TA.U54A TA.U56A TA.U59A TA.V51A TA.V52A TA.V53A TA.V55A TA.V58A TA.V60A TA.W50A TA.W52A TA.W57A TA.X51A TA.Y52A TA.Y57A TA.Y58A TA.Y60A TA.Z56A US.BLA US.GOGA US.NHSC US.TZTN Filtering commands used: cut a -20 a 160 rtr taper w 0.1 hp c 0.02 n 3 lp c 0.10 n 3 Best Fitting Double Couple Mo = 1.84e+22 dyne-cm Mw = 4.11 Z = 5 km Plane Strike Dip Rake NP1 202 66 141 NP2 310 55 30 Principal Axes: Axis Value Plunge Azimuth T 1.84e+22 44 162 N 0.00e+00 45 355 P -1.84e+22 7 258 Moment Tensor: (dyne-cm) Component Value Mxx 7.78e+21 Mxy -6.53e+21 Mxz -8.29e+21 Myy -1.64e+22 Myz 4.98e+21 Mzz 8.65e+21 ############## ################------ #################----------- ################-------------- --------------###----------------- ----------------###----------------- ----------------#######--------------- ----------------##########-------------- ---------------#############------------ ---------------################----------- ---------------#################---------- --------------###################--------- -----------####################-------- P ----------######################------ ----------#######################----- ----------########### ###########--- ---------########### T ###########-- --------########### ###########- ------######################## -----####################### ---################### ############## Global CMT Convention Moment Tensor: R T P 8.65e+21 -8.29e+21 -4.98e+21 -8.29e+21 7.78e+21 6.53e+21 -4.98e+21 6.53e+21 -1.64e+22 Details of the solution is found at http://www.eas.slu.edu/eqc/eqc_mt/MECH.NA/20140215032338/index.html	Moment 1.63e+15 N-m Magnitude 4.1 Percent DC 52% Depth 6.0 km Updated 2014-02-15 03:48:13 UTC Author us Catalog us Contributor us Code us_c000mr27_mwr Principal Axes Axis Value Plunge Azimuth T 1.436 64 352 N 0.338 26 161 P -1.774 5 253 Nodal Planes Plane Strike Dip Rake NP1 141 55 58 NP2 9 46 127

Magnitudes

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 M_L by application of the respective IASPEI formulae. As a research study, the linear distance term of the IASPEI formula for M_L is adjusted to remove a linear distance trend in residuals to give a regionally defined M_L. The defined M_L 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.

mLg Magnitude

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.

ML Magnitude

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.

Context

Waveform Inversion using wvfgrd96

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.

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'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 a -20 a 160
rtr
taper w 0.1
hp c 0.02 n 3 
lp c 0.10 n 3 

           DEPTH  STK   DIP  RAKE   MW    FIT
WVFGRD96    0.5   300    20   -10   4.22 0.3517
WVFGRD96    1.0   300    25   -10   4.19 0.3611
WVFGRD96    2.0   305    40     0   4.10 0.3835
WVFGRD96    3.0   310    50    20   4.09 0.4051
WVFGRD96    4.0   315    50    35   4.12 0.4265
WVFGRD96    5.0   310    55    30   4.11 0.4324
WVFGRD96    6.0   310    55    30   4.11 0.4293
WVFGRD96    7.0   310    60    25   4.11 0.4226
WVFGRD96    8.0   310    60    25   4.11 0.4137
WVFGRD96    9.0   305    65    20   4.12 0.4036
WVFGRD96   10.0   310    60    25   4.13 0.3929
WVFGRD96   11.0   310    60    25   4.14 0.3797
WVFGRD96   12.0   305    65    20   4.15 0.3671
WVFGRD96   13.0   305    65    20   4.15 0.3559
WVFGRD96   14.0   305    65    20   4.16 0.3454
WVFGRD96   15.0   305    65    20   4.16 0.3353
WVFGRD96   16.0   305    65    20   4.17 0.3258
WVFGRD96   17.0   305    65    20   4.18 0.3164
WVFGRD96   18.0   305    65    20   4.18 0.3073
WVFGRD96   19.0   305    65    20   4.19 0.2988
WVFGRD96   20.0   310    60    25   4.20 0.2903
WVFGRD96   21.0   310    60    25   4.21 0.2809
WVFGRD96   22.0   310    60    25   4.21 0.2721
WVFGRD96   23.0   310    60    25   4.22 0.2638
WVFGRD96   24.0   310    60    25   4.22 0.2559
WVFGRD96   25.0   310    60    25   4.23 0.2493
WVFGRD96   26.0   310    60    25   4.23 0.2439
WVFGRD96   27.0   310    60    25   4.24 0.2401
WVFGRD96   28.0   310    60    25   4.24 0.2377
WVFGRD96   29.0   310    60    20   4.25 0.2365

The best solution is

WVFGRD96    5.0   310    55    30   4.11 0.4324

The mechanism corresponding 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, 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 a -20 a 160
rtr
taper w 0.1
hp c 0.02 n 3 
lp c 0.10 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. The time scale is relative to the first trace sample.

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:

• 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)

 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.

Velocity Model

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