The analysis proved that between 1982 and 1993, earthquake frequency reduced drastically, the foci of earthquakes being located at the depth of less than 70 kilometers (known as shallow-focus earthquakes). Less powerful earthquakes (magnitude 7-7.5) took place 3-4 times less often during that period, and more powerful earthquakes (magnitude 8 and higher) – 10 times less often as compared to the previous and subsequent periods.
The most powerful shallow-focus earthquakes (magnitude exceeding 8.5 scores) arouse special interest. Between 1965 and 2001 there were none at all, and after 2001 occurred almost every year. These include the strongest shocks near the Sumatran coast on December 26, 2004 and on March 28, 2005, which were followed by gigantic tsunami.
After the eleven-year calm, deep-focus earthquakes also occurred. Up to mid-1993, there were no deep-focus earthquakes over 7 in magnitude, but between the end of 1993 and 2005, researchers recorded 12 earthquakes with magnitudes of 7.5, four with magnitudes of 8, and one with magnitude of 8.8. This most violent earthquake took place in the Atlantic Ocean near the eastern coast of South America at the depth of about 90 kilometers. As per relatively weak deep-focus events with magnitudes of about seven, they happened about five times a year since mid-1993 and later (which is several times more often than during the calm period).
Deep-focus earthquakes are connected with submersion of lithospheric plates into the upper earth mantle. It is probable that plates used to submerge slowly and smoothly during the calm period, thus loosening the tension in the earth lithosphere and reducing the number of seismic events in it. It is also possible that slow submersion of lithospheric plates contributes to the accumulation of geodynamic tension, followed by relaxation – elastic stress falloff in the form of earthquakes.
To confirm these findings, V.I. Ulomov listed several recent events that signify violent seismic activity of last decades:
- April 21, 2006 – an earthquake took place in Kamchatka (magnitude 7.6-7.8);
- November 15, 2006 – the most violent earthquake in the world in terms of magnitude took place in the central part of the Kurile Islands (magnitude 8.3 and 10-11-score seismic effect in the epicentre).
- January 13, 2007 – one more earthquake happened (magnitude 8.2) in the same focal area of the Kurile Islands, where the largest in magnitude earthquake had occurred two month before that.
- Quite recently, on September 12, 2007 – a recurrent violent earthquake (magnitude 8.4) occurred near Sumatra coast.
Nadezda Markina | alfa
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