Kermadec Quake Sixth Largest

Sunday’s magnitude 7.8 earthquake in the northern Kermadec Islands was the sixth largest quake worldwide so far this year.

Larger events during 2007 have been:

  • The magnitude 8.4 earthquake in Southern Sumatra on September 13th,
  • the magnitude 8.1 events in the Solomon Islands on April 2nd and the Kuril Islands on January 13th,
  • the magnitude 8.0 quake off the coast of Central Peru on the 2nd of April, and
  • the magnitude 7.9 event in the Kepulauan region of Indonesia on September 13th.

The northern Kermadec Islands have been remarkably free of earthquake activity since Sunday’s quake, with only a magnitude 5.1 quake reported nearly 500 km south (105 km south of Raoul) at 6:46 Wednesday morning December 12th New Zealand time, and two smaller events in the Fiji Islands far to the north.

The absence of aftershocks is unusual after such a strong quake. However, the area in which the quake struck is itself unusual, and activity often doesn’t occur as expected. Just to the north of the area where Sunday’s quake struck is an area where the planet’s deepest earthquakes are recorded. These quakes at depths of 500 km and more occur in a large slab of the Pacific Plate that has slid under the Tonga microplate and, because of its density and low temperature, has survived to great depths at which other rock would have been recycled into the planet’s molten mantle.

To the south, in the Kermadec Islands, seismologists have noticed that some of the earthquakes occur in pairs – far more commonly than average. The earthquakes can be at different depths and separated by distance, but are usually similar in magnitude and occur within a short period of each other.

This indicates that nature follows some of the rules, some of the time. What is, in fact, happening is that nature is following the rules all of the time, but the understanding of those rules by mice and men only contains part of the picture. Fortunately, our understanding is improving by leaps and bounds.

Analysis of earthquake data gathered since 1990 indicates that there will be 152 earthquakes of magnitude 6.0 or greater worldwide every year. This is an average that was remarkably accurate during the four-year period 2003 to 2006. The years 2001 and 2002 were below this average, and the year 2000 and this year somewhat above the average.

So far this year the US Geological Survey has reported 176 quakes worldwide with magnitudes of 6.0 or greater. The 13 events between magnitude 7.0 and 7.9 are below the annual average of 17, but the four quakes of mag. 8.0 or above so far this year exceed the annual average of 1.

Improved coverage by earthquake-detecting networks is refining our understanding of earthquake activity below magnitude 8 and dramatically improving knowledge of quakes below magnitude 6. However, because of their great size and dramatic effects, the magnitude 8 or greater quakes have been statistically analysed for some time, and the annual average of 1 such event worldwide each year has been determined from observations made since 1900.

[Compiled from data supplied by the U.S. Geological Survey and its contributing agencies.]

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