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
