Magnitude 4.6 – ILLINOIS

M4.5 Illinois in Ohio

Comparison of Fault Mechanisms

The following fault mechanisms were taken in part from the USGS/SLU Regional Moment Tensor Solution page for each of the respective earthquakes. Both show pure vertical faults running roughly North-South, bearing 25 degrees and 315 degrees. In the first and larger earthquake the slip was -175 degrees which means that the east side of the fault moved south relative to the west side. During the second earthquake the slip was +10 degrees indicating that it “relaxed back” toward its original position. (This analysis has not been reviewed by a seismologist so take it for what it is worth…and leave a comment if I’m not right.)

08/04/18 09:36:57   
 ILLINOIS                        
 Epicenter:  38.481  -87.826
 MW 5.2

 USGS/SLU REGIONAL MOMENT TENSOR
 Depth  15         No. of sta: 15
 Moment Tensor;   Scale 10**16 Nm
   Mrr= 0.00       Mtt= 5.79
   Mpp=-5.79       Mrt=-0.28
   Mrp=-0.60       Mtp= 4.86
  Principal axes:
   T  Val=  7.59  Plg= 3  Azm=159
   N        0.00      85       25
   P       -7.59       3      250

 Best Double Couple:Mo=7.6*10**16
  NP1:Strike= 25 Dip=90 Slip=-175
  NP2:       295     85         0
                                      
               #######                
          ###############--           
        #################----         
      ##################-------       
    ###################----------     
   ###################------------    
   -----#############-------------    
  ----------########---------------   
  ---------------##----------------   
  -----------------##--------------   
  ----------------######-----------   
     ------------###########-------   
   P -----------###############---    
     ----------###################    
    ----------###################     
      -------##################       
        ----###########   ###         
          --########### T #           
               #######                
                                      

The double couple information for this earthquake describes a right-lateral vertical slip fault with a strike of 25 degs, a dip of 90 degs, and a slip of -175 degs.

 08/04/18 15:14:16   
 ILLINOIS                        
 Epicenter:  38.539  -87.865
 MW 4.6

 USGS/SLU REGIONAL MOMENT TENSOR
 Depth  15         No. of sta: 12
 Moment Tensor;   Scale 10**15 Nm
   Mrr= 0.00       Mtt= 9.74
   Mpp=-9.74       Mrt=-1.21
   Mrp= 1.21       Mtp= 0.00
  Principal axes:
   T  Val=  9.89  Plg= 7  Azm=180
   N        0.00      80      314
   P       -9.89       7       89

 Best Double Couple:Mo=9.9*10**15
  NP1:Strike=315 Dip=90 Slip=  10
  NP2:       225     80       180
                                      
               #######                
          #################           
        #####################         
      -######################--       
    -----#################-------     
   --------#############----------    
   ----------#########------------    
  -------------#####---------------   
  ---------------#---------------     
  --------------##--------------- P   
  ------------######-------------     
  -----------#########-------------   
   --------#############----------    
   -------################--------    
    ----####################-----     
      -#######################-       
        #########   #########         
          ####### T #######           
               ##   ##                

The double couple information for this earthquake describes a left-lateral vertical slip fault with a strike of 315 degs, a dip of 90 degs, and a slip of 10 degs.

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