A total of 37 earthquakes occurred within Tonga in the month of September, but none of them were felt by communities, the Tonga Geological Services, reported in its Tonga Seismic Bulletin for September. (Yesterday's felt tremor will be covered in their October bulletin).

“These are earthquakes with magnitudes which are greater than or equal to four. Earthquakes with magnitudes less than 4 are insignificant and are likely to not cause any damages at the Earth’s surface.”

• 18 were shallow-focus earthquakes with an average magnitude of 4.7;
• 11 were intermediate- focus earthquakes with 4.5 average magnitude;
• 8 were deep-focus earthquakes with 4.5 average magnitude.

The highest magnitude for earthquakes in September was 5.6 and the lowest was 4.

“There were no felt events for September.”

Figure 1 shows the average magnitude and number of earthquakes that occurred at each depth. Shallow earthquake (0-70KM), Intermediate (70-300KM) and Deep focus (300-700KM).

“The spatial distribution of the epicentres of the earthquakes for September were random throughout Tongan waters, as shown in Figure 2, with various magnitudes from 4 – 5.6.

Based on the maps, the deep earthquakes are located on the Western side of Tonga while the shallow earthquakes on the East.

“This indicates that the stress experienced by faults of these earthquakes is directly related to the subduction zone”, they stated.

The area covered lies between 13.00°S – 26.00°S latitudes and 170.00°W – 178.00°W longitudes.