Comparing 3 earthquakes

It has always fascinated me the difference between the signatures of earthquakes of similar sizes as seen from different parts of the world.

On the 22nd August there were 3 Mag 5 quakes (amongst others)

Date/Time UTC,Latitude,Longitude,Magnitude,Depth(Km),Location
2011-08-22 06:04:57, -17.470, 167.244, 5.8, 34.9, Vanuatu
2011-08-22 09:38:37, -28.914, -176.565, 5.5, 6.5, Kermadec Isls. Region
2011-08-22 11:23:38, 36.107, 141.698, 5.9, 34.5, Nr. E. Coast Of Honshu. Japan

CTAO Charter Towers, Northern Australia

Japan 6000Km, Vanuatu 2,240, Kermedecs 3,800

It is not too easy to tell but it certainly seems that the Kermedec quake (the 5.5), which was very shallow, shows better on the plot than the Vanuatu (5.8) even though that one was closer (only just over half the distance)

SNZO South Karori, New Zealand.

Japan 8,400 Km, Vanuatu 2,700 Km, Kermedecs 1,468 Km

Naturally the Kermedec (5.5) shows best. The Japan signal is only a little smaller than Vanuatu, this depite being 3 times the distance. This would be accounted by the fact that in terms of energy released the Japan quake was 4 times stronger. The Vanuatu quake was 2.8 times stronger than the Kermedec quake but shows very much smaller being twice the distance and deeper.

HNR Solomon Islands

Japan 5,200, Vanuatu 1,200, Kermedecs 3,200

This one is quite interesting. Despite the Japan and Vanuatu quakes being only 0.1 off each other in magnitude, and Vaunatu being ¼ the distance to HNR, the Japan quake is still showing stronger. I have measured the peak of the Vanuatu and Japan quakes, 138 pixels and 148 pixels respectively. ( – Seriously recommended tool)


MIDW Midway, Pacific (East of Hawaii)

Japan 3,900 Km, Vanuatu 5,200 Km, Kermedecs 6,000 Km

No great surprise here that Japan is showing the strongest since it is closest. The Kermedecs however are definitely stronger @40 screen pixels to 15 screen pixels for Vanuatu despite there not being proportionately so much different in distance (5200 Vanuatu / 6000 Kermedec) and even though the Vanuatu quakes is 2.8 times stronger.

INCN Inchon, Republic Of Korea

Japan 1,370, Vanuatu 7,000 Km, Kermedec 8,600

No prizes for guessing that Japan will show the strongest! Since Vanuatu and Kermedec are almost in a straight line from INCN the amplitudes are proportional.

WVT Waverly, Tennessee, USA

Japan 9,300 Km, Vanuatu 10,500 Km, Kermedecs 10.100 Km

Once again the shallow Kermedec quake shows stronger than the Vanuatu despite being the same distance virtually, yet much smaller (2.8x). In addition, even though there is not much in it for distance and strength, the Japan quake shows much stronger than the Vanuatu quake.

KEV, Kevo Finland.

Japan 6,800, Vanuatu 11,100, Kermedec 11,800

There is a good signal from Japan; there always is on this instrument despite the distance. The Kermedecs are still showing stronger than Vanuatu again.


So, having looked at the signals on the seismograms, what if any, effect has the P wave and S wave shadow zones had on these?

The P wave shadow zone is between 103 deg and 140 deg, and the S wave shadow zone is beyond the 104 deg mark. This is for direct P and S waves


None of the stations were affected for the Japan quake.


KEV and WVT fall in the P/S wave shadow zones.


KEV and WVT fall into the P/S shadow zones again.

Of course this does not mean no seismic energy arrived at these stations as you can obviously see from the seismograms, but the energy for those in the shadow zones is indirect, reflected or refracted energy.

See the third video on this page for more information.


About PuterMan

A retired programmer.
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