3. Seismogram Interpretation

With the addition of the GPS receiver to provide more accurate timing, I have been able to start to try to interpret some of the seismograms I am recording. In 2004 I recorded an earthquake in Chile which had a huge single spike at the S phase arrival. Now, in 2006, there was a similar signal from Argentina. A month earlier, I recorded an earthquake of similar magnitude in Peru that had a markedly different seismogram. This seemed like a good pair to look at.

The signal received at any given seismograph is a signature of the complex mixing of source mechanics, travel path through the earth, mechanical response and physical orientation of the seismograph, and the response of the signal processing electronics and amplifier and is characterized by the amplitudes and arrival times of various phases of vibration. The P and S phases are solid body phases which travel through the body of the earth. The Love and Raleigh phases travel on the surface and arrive later. The arrival times for the various phases were calculated for the Argentina earthquake using the USGS Arrival Time Calculator at the link on the main Seismology page. I was too late to catch the Peruvian arrival times from the USGS calculator (it only goes back two weeks) so I calculated the arrival times for it using data from the Jeffries-Bullens tables obtained from the Public Seismic Net.

Comparison of two spectrograms from South American earthquakes.

The Argentina signal is dominated by a large S phase but is other wise unremarkable. The Peruvian signal shows a respectable S phase arrival but also shows more surface wave activity. Both signals show a significant arrival after the S phase but before the surface wave arrivals.

The Argentina earthquake was deep, 573 km while the Peruvian earthquake was relatively shallow at 26 km. This explains why the surface waves are lacking in the Argentina signal.

Since both events are south of Ohio and my single instrument is more sensitive to East-West motion there is almost no P-phase arrival. The P-phase is a longitudinal compressional vibration so its motion would be mainly in a North-South and vertical direction.

The S-phase is a transverse vibration that is polarized by the fault mechanism and which shows horizontal and vertical components. Depending on the orientation of the fault the E-W sensitivity of my seismograph is ideal for picking up the horizontal component of the S-phase from the south. One would expect that the slip on the Agentina fault was better aligned with my seismograph to transfer that horizontal component but I have not been able to show that. See the “Comparison of Fault Mechanisms” below.

Perhaps the most interesting thing I found on both signals was the significant arrival that followed the S-phase that I alluded to earlier. These peaks are marked ScS on the seismogram and are S-phase reflections off the earth’s core. The time interval between the S-phase and the ScS-phase on the Argentina signal is shorter than it is on the Peru signal because the depth of the Argentina fault makes the length of the direct and reflected paths more similar in length than for the Peruvian case.

Comparison of Fault Mechanisms

The following fault mechanisms were taken in part from the USGS NEIC Fast Moment Tensor page for each of the respective earthquakes. The azimuth calculation for the Argentina epicenter was obtained from the USGS Arrival Time Calculator output page while the azimuth for the Peruvian epicenter was calculated using software I wrote.

06/11/13 01:26:34.67
 SANTIAGO DEL ESTERO, ARGENTINA
 Epicenter: -26.094  -63.339
 MW 6.8 

 USGS MOMENT TENSOR SOLUTION
 Depth 573         No. of sta: 32
 Moment Tensor;   Scale 10**19 Nm
   Mrr=-0.55       Mtt= 0.00
   Mpp= 0.55       Mrt=-0.56
   Mrp= 1.53       Mtp=-0.42
  Principal axes:
   T  Val=  1.86  Plg=34  Azm=246
   N       -0.21       5      339
   P       -1.66      56       76 

 Best Double Couple:Mo=1.8*10**19
  NP1:Strike=316 Dip=12 Slip=-113
  NP2:       160     79       -85 

               #######
          --#---------#####
        #####-------------###
      #######---------------###
    ##########----------------###
   ###########-----------------###
   ############-----------------##
  #############--------   -------##
  ##############------- P --------#
  ##############-------   --------#
  #####   #######-----------------#
  ##### T ########----------------#
   ####   #########--------------#
   ################--------------#
    ################------------#
      ###############----------
        ##############-------
          ##############---
               #######

The double couple information for this earthquake describes a normal fault with a strike of 316 degs, a dip of 12 degs, and a slip of -113 degs. The azimuth to my seismograph is 344.8 degs.

06/10/20 10:48:57.51
 NEAR COAST OF CENTRAL PERU
 Epicenter: -13.427  -76.572
 MW 6.6 

 USGS MOMENT TENSOR SOLUTION
 Depth  26         No. of sta: 77
 Moment Tensor;   Scale 10**19 Nm
   Mrr= 0.53       Mtt= 0.02
   Mpp=-0.55       Mrt= 0.04
   Mrp=-0.85       Mtp= 0.03
  Principal axes:
   T  Val=  1.00  Plg=61  Azm= 87
   N        0.02       0      357
   P       -1.02      29      267 

 Best Double Couple:Mo=1.0*10**19
  NP1:Strike=356 Dip=16 Slip=  89
  NP2:       177     74        90 

               ---#---
          -------#######---
        --------##########---
      ----------############---
    ------------##############---
   -------------###############---
   ------------################---
  -------------#################---
  ----   ------#######   #######---
  ---- P ------####### T #######---
  ----   -------######   #######---
  --------------################---
   -------------################--
   -------------###############---
    -------------#############---
      -----------############--
        ----------#########--
          --------#######--
               ----##-

The double couple information for this earthquake describes a reverse fault with a strike of 356 degs, a dip of 16 degs, and a slip of 89 degs. The azimuth to my seismograph is 355 degs.

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