Location

Location ANSS

The ANSS event ID is nm606657 and the event page is at https://earthquake.usgs.gov/earthquakes/eventpage/nm606657/executive.

2008/04/18 09:37:00 38.4515 -87.8862 14.3 5.2 Illinois

Focal Mechanism

USGS/SLU Moment Tensor Solution ENS 2008/04/18 09:37:00:0 38.45 -87.89 14.3 5.2 Illinois Best Fitting Double Couple Mo = 9.12e+23 dyne-cm Mw = 5.24 Z = 15 km Plane Strike Dip Rake NP1 25 90 -175 NP2 295 85 0 Principal Axes: Axis Value Plunge Azimuth T 9.12e+23 4 160 N 0.00e+00 85 25 P -9.12e+23 4 250 Moment Tensor: (dyne-cm) Component Value Mxx 6.96e+23 Mxy -5.84e+23 Mxz -3.36e+22 Myy -6.96e+23 Myz 7.20e+22 Mzz 0.00e+00 ############## ###################--- #####################------- #####################--------- #######################----------- #######################------------- --#####################--------------- --------###############----------------- -------------#########------------------ ------------------####-------------------- ---------------------#-------------------- ---------------------#####---------------- --------------------##########------------ ---------------##############-------- P --------------##################----- -------------######################- -------------####################### -----------####################### ---------##################### -------##################### ---############# ### ############ T Global CMT Convention Moment Tensor: R T P 0.00e+00 -3.36e+22 -7.20e+22 -3.36e+22 6.96e+23 5.84e+23 -7.20e+22 5.84e+23 -6.96e+23 Details of the solution is found at http://www.eas.slu.edu/eqc/eqc_mt/MECH.NA/20080418093700/index.html

Preferred Solution

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

      STK = 295
      DIP = 85
     RAKE = 0
       MW = 5.24
       HS = 15.0

The NDK file is 20080418093700.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 GCMT

USGS/SLU Moment Tensor Solution ENS 2008/04/18 09:37:00:0 38.45 -87.89 14.3 5.2 Illinois Best Fitting Double Couple Mo = 9.12e+23 dyne-cm Mw = 5.24 Z = 15 km Plane Strike Dip Rake NP1 25 90 -175 NP2 295 85 0 Principal Axes: Axis Value Plunge Azimuth T 9.12e+23 4 160 N 0.00e+00 85 25 P -9.12e+23 4 250 Moment Tensor: (dyne-cm) Component Value Mxx 6.96e+23 Mxy -5.84e+23 Mxz -3.36e+22 Myy -6.96e+23 Myz 7.20e+22 Mzz 0.00e+00 ############## ###################--- #####################------- #####################--------- #######################----------- #######################------------- --#####################--------------- --------###############----------------- -------------#########------------------ ------------------####-------------------- ---------------------#-------------------- ---------------------#####---------------- --------------------##########------------ ---------------##############-------- P --------------##################----- -------------######################- -------------####################### -----------####################### ---------##################### -------##################### ---############# ### ############ T Global CMT Convention Moment Tensor: R T P 0.00e+00 -3.36e+22 -7.20e+22 -3.36e+22 6.96e+23 5.84e+23 -7.20e+22 5.84e+23 -6.96e+23 Details of the solution is found at http://www.eas.slu.edu/eqc/eqc_mt/MECH.NA/20080418093700/index.html

USGS Body-Wave Moment Tensor Solution 08/04/18 09:36:57.31 ILLINOIS Epicenter: 38.523 -87.884 MW 5.2 USGS MOMENT TENSOR SOLUTION Depth 18 No. of sta: 7 Moment Tensor; Scale 10**16 Nm Mrr=-0.41 Mtt= 7.31 Mpp=-6.91 Mrt=-1.11 Mrp=-0.54 Mtp= 5.83 Principal axes: T Val= 9.55 Plg= 7 Azm=160 N -0.56 82 347 P -9.00 0 250 Best Double Couple:Mo=9.3*10**16 NP1:Strike=296 Dip=84 Slip= 4 NP2: 205 86 174 ####### ###############-- ################----- #################-------- ##################----------- ##################------------- -----############-------------- -----------######---------------- ---------------#----------------- ----------------###-------------- ---------------########---------- ------------############------- P -----------################--- ----------#################### ----------################### -------################## ----########## #### --########## T ## #######

April 18, 2008, ILLINOIS, MW=5.3 Goran Ekstrom CENTROID-MOMENT-TENSOR SOLUTION GCMT EVENT: C200804180936A DATA: II IU MN CU IC G GE L.P.BODY WAVES: 74S, 119C, T= 40 SURFACE WAVES: 103S, 186C, T= 50 TIMESTAMP: Q-20080418094937 CENTROID LOCATION: ORIGIN TIME: 09:37:02.2 0.1 LAT:38.55N 0.01;LON: 87.91W 0.01 DEP: 15.0 FIX;TRIANG HDUR: 1.2 MOMENT TENSOR: SCALE 10**24 D-CM RR=-0.011 0.021; TT= 0.983 0.019 PP=-0.972 0.021; RT= 0.067 0.057 RP=-0.154 0.062; TP= 0.736 0.017 PRINCIPAL AXES: 1.(T) VAL= 1.229;PLG= 1;AZM=342 2.(N) 0.012; 82; 77 3.(P) -1.241; 8; 251 BEST DBLE.COUPLE:M0= 1.24*10**24 NP1: STRIKE= 27;DIP=84;SLIP=-175 NP2: STRIKE=296;DIP=85;SLIP= -6 T ######### ### ###########-- ##################----- ###################-------- ####################--------- ---#################----------- --------###########------------ -------------######-------------- -----------------#--------------- -----------------####------------ -------------########--------- P ------------############----- -----------################-- -----------################## ---------################## ------################# ---################ ###########

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

The left panel of the next figure presents the focal mechanism for this earthquake (red) in the context of other nearby events (blue) in the SLU Moment Tensor Catalog. The right panel shows the inferred direction of maximum compressive stress and the type of faulting (green is strike-slip, red is normal, blue is thrust; oblique is shown by a combination of colors). Thus context plot is useful for assessing the appropriateness of the moment tensor of this event.

Surface-Wave Focal Mechanism

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.

Location of broadband stations used to obtain focal mechanism from surface-wave spectral amplitudes

The surface-wave determined focal mechanism is shown here.


  NODAL PLANES 

  
  STK=     199.11
  DIP=      85.08
 RAKE=     169.97
  
             OR
  
  STK=     289.98
  DIP=      80.00
 RAKE=       5.00
 
 
DEPTH = 15.0 km
 
Mw = 5.32
Best Fit 0.8952 - P-T axis plot gives solutions with FIT greater than FIT90

Surface-wave analysis

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.

Data preparation

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.

Best mechanism fit as a function of depth. The preferred depth is given above. Lower hemisphere projection

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.

Love-wave radiation patterns

Rayleigh-wave radiation patterns

Rayleigh-wave radiation patterns.
Rayleigh-wave radiation patterns.
Rayleigh-wave radiation patterns.
Rayleigh-wave radiation patterns.
Rayleigh-wave radiation patterns.
Rayleigh-wave radiation patterns.
Rayleigh-wave radiation patterns.
Rayleigh-wave radiation patterns.
Rayleigh-wave radiation patterns.

Rayleigh-wave radiation patterns.

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

Last Changed Sun Apr 28 01:01:43 PM CDT 2024