| Ground Motion Waveforms of 9:55AM, May 25th, 2009 |
Launched:
May 25, 20009
Updated:
May 28, 2009
|
According to USGS, an event occurred at 9:54:43AM, May 25th 2009 (Japan time), in the vicinity
of North Korea. The observed ground motions from this event show features
different from a natural earthquake. We will update information on this
event occasionally (Outreach Office).
[Observed Waveform]
The ground motions recorded on May 25th, around 9:55AM (Japan time), were
observed with a high-sensitivity seismograph network of the National Research
Institute for Earth Science and Disaster Prevention (NIED) .
According to USGS, the source of this event is located at lat=41.331 N，lon=129.011
E，depth=0km. The vertical axis indicates the distance from the source,
and the horizontal axis shows the lapse time from 9:56:00AM.
 （Click to enlarge）
[Comparison with natural earthquakes - No.1]
The following compares the observed waveforms from this event (Left) with those from the natural earthquake, which occurred in the vicinity of Vladivostok (Right: 12:56PM, April 18th, 2009, lat=42.844N， lon=130.496E， depth=564km, data by IRIS).
The red arrow indicates the P-wave, the blue indicates the S-wave. Although
the figures show vertical ground motions and the S-wave is less visible,
it would still be observed in the figure for the natural earthquake, however,
no S-wave can be seen in the figure for this event.
Earthquake Location：（click to enlarge）
[Comparison with natural earthquakes - No.2 ]
After the waveforms were low-pass filtered with a corner period of 1s,
the S-wave for the natural earthquake becomes distinct, but no S-wave can
be seen for this event.
Left: Low-pass filtered waveforms of the event on May 25th, 2009
Right:Low-pass filtered waveforms of the natural earthquake on April 18th,
2009
 
On the other hand, the ground motion on May 25th, 2009 becomes more distinct
after the waveforms are bandpass filtered with a frequency band of 1Hz
to 5Hz.
Left: Bandpass filtered waveforms of the event on May 25th, 2009
Right:Bandpass filtered waveforms of the natural earthquake on April 18th,
2009
 
It is unlikely for a distant M5-class natural earthquake that long-period
components are barely observed. (The magnitude of this event was determined
by USGS and Japan Meteorological Agency)
[Comparison with the event in Oct. 2006]
Earthquake Observation Center
USGS reported that there occurred a similar event fairly close to the source
ofthis event, at 10:35AM, October 9th, 2006 (Japan time). Following is
a comparison of waveforms of events both observed at ERI.
The red waveforms indicate the 2006 event, and the blue indicates the 2009
event. Seen that the time difference of both waveforms are the same, it
can be said that both events occurred in the same place, and from the similarity
of the waveforms, it can be presumed that they hold the same source mechanism.
The numbers at the right end are the maximum amplitudes (x10^-6m/s). It
is clear that there is a difference of 3-4times . This makes a difference
of 9-16 times if converted into energy.
↓Click to enlarge
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