Number of earthquakes in Samoa normal: Seismologist

It is normal for Samoa to experience two to three earthquakes per month.

So says Seismologist and the Samoa Meteorology Division Acting Assistant Chief Executive Officer, Fuimaono Lameko Talia.

Responding to questions from the Samoa Observer, Fuimaono said that based on a presentation he did recently focusing on seismic activities of the Samoa-Tonga Trench, the geographical positioning of the Samoan islands on the edge of a subducting Pacific plate is the main reason for recent seismic, tsunamis and volcanic activities in Samoa.

He said his presentation emphasised that the devastating 29 September 2009 tsunami was also evidence that the Samoa Islands region remains an active earthquake region.

“On average Samoa receives 2 to 3 felt earthquakes per month and 200 recorded earthquakes per month including non-felt earthquakes from the Samoa-Tonga Region.”

According to Earthquake Tracker, Samoa Islands has had one earthquake in the past 7 days, 5 earthquakes in the past 30 days, and 75 earthquakes in the past 365 days.

Professor Emeritus Richard Sibson of the University of Otago's Geology Department also told the Samoa Observer in an email response that the region around the top of the New Zealand–Kermadec–Tonga Trench – where the Pacific plate is subjecting at ~10 cm/yr below Tonga and subduction is 'transforming' into a left-lateral transform fault trending NE-SW and passing south of Fiji – is also one of the most intense areas of earthquake activity on earth.

"The other thing to appreciate is something called the 'frequency-magnitude relationship'.  Basically, for any seismically active region, the frequency of earthquakes increases by about a factor of 10 for every unit decrease in magnitude," added Professor Sibson.

"Thus for each M6 earthquake in a seismically active region, you can generally expect about 10 M5 earthquakes, 100 M4 earthquakes, etc in the same area over the same time period so that earthquakes get much more numerous as their size (magnitude) diminishes. However, EQs with M<3 won't be detected for much of the offshore region."

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