Magnitude 7.1 – IZU ISLANDS, JAPAN REGION

2009Aug09Japan

A Magnitude 7.1 Earthquake occurred this morning (10:55:56 UTC) off the coast of Japan.  This is the extracted earthquake from my seismograph in Holmes County, Ohio.

This seismogram has several interesting features.  The two largest peaks appear to be related to the S body waves as indicated by the arrival time table generated for my location by the USGS Arrival Time Calculator linked at the right and copied below.  The first large peak is the direct S wave phase but due to the exact location of the hypocenter relative to my location and to its depth of 303 km, the ScS phase arrives at exactly the same time.   The ScS phase is the reflection of the S phase off the core!  The other tall peak is the SS body phase which is due to one reflection of the S phase off the earth’s surface!

So why do the S phase arrivals show so strongly in this seismogram?  Notice that the station-to-eq azimuth in the table below is 327.1°.  My seismograph is oriented to be most sensitive to east-west motion and the S phase is a transverse vibration.  The first arrival was out of the northwest with the vibration transverse to that direction, ie roughly east-west.

The hypocenter was located at a depth of 303 km so there is a noticeable absence of surface waves when compared with earthquakes at shallower depths.

Here is the arrival time table:

  DATE-(UTC)-TIME    LAT    LON     DEPTH MAG   Q   COMMENTS
  2009/08/09 10:55:56  33.14N 138.04E 303.1 7.1      US: IZU ISLANDS, JAPAN REGIO
   Expected 1s period body wave amplitude     [  1.02E+00 µm]  [  6.40E+00 µm/s]
 
 delta   azimuth (degrees clockwise from north)
 (deg)      eq-to-station     station-to-eq
 97.92           29.6             327.1
 
                 travel   arrival time
    #  code      time(s)  dy hr mn sec
    1  Pdiff      781.30   0 11  8 57
    2  pP         853.74   0 11 10  9
    3  sP         885.48   0 11 10 41
    4  PP        1024.18   0 11 13  0
    5  PKiKP     1053.11   0 11 13 29
    6  pPKiKP    1129.12   0 11 14 45
    7  sPKiKP    1160.03   0 11 15 16
    8  SKiKP     1238.36   0 11 16 34
    9  SKSac     1388.76   0 11 19  4
   10  SKKSac    1417.53   0 11 19 33
   11  S         1440.19   0 11 19 56
   12  ScS       1440.19   0 11 19 56
   13  pSKSac    1491.85   0 11 20 47
   14  sSKSac    1523.95   0 11 21 19
   15  SP        1526.97   0 11 21 22
   16  pS        1534.98   0 11 21 30
   17  PS        1565.69   0 11 22  1
   18  sS        1570.00   0 11 22  6
   19  PKKPbc    1772.87   0 11 25 28
   20  PKKPdf    1776.94   0 11 25 32
   21  SS        1851.15   0 11 26 47
   22  SKKPdf    1962.24   0 11 28 38
   23  SKKPbc    1963.43   0 11 28 39
   24  PKKSdf    1993.16   0 11 29  9
   25  PKKSbc    1994.50   0 11 29 10
   26  SKKSdf    2178.30   0 11 32 14
   27  SKKSac    2183.63   0 11 32 19
   28  P'P'df    2269.45   0 11 33 45
   29  S'S'ac    3100.03   0 11 47 36
   30  S'S'df    3104.71   0 11 47 40
   31  LQ        2485.30   0 11 37 21
   32  LR        2758.33   0 11 41 54
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