Erratic Aftershocks Near Tonga

Aftershocks following Thursday’s magnitude 7.9 earthquake centred 160 km north-east of Tonga’s capital Nuku’alofa have been erratic over the past four days.

Normally, a large earthquake is followed by a series of earthquakes which tend to decrease in frequency and strength over time, with occasional members of the sequence being of higher magnitude. This illustrates the crust surrounding the area of the rupture reacting to the change in stress caused by the large earthquake.

Tonga’s aftershock series has not followed this trend, but I hasten to add that not all earthquake sequences follow this trend of decreasing activity. Even in New Zealand, we see earthquake and aftershock sequences that don’t “follow the rules.”

Data published by the US Geological Survey for the four days (New Zealand time) since the initial magnitude 7.9 earthquake show erratic earthquake activity in the area.

A summary of earthquake magnitudes is as follows:
Thursday May 4 – M7 (1), M6 (1), M5 (6), M4 (4)
Friday May 5 – M7 (0), M6 (0), M5 (3), M4 (8)
Saturday May 6 – M7 (0), M6 (0), M5 (1), M4 (0)
Sunday May 7 – M7 (0), M6 (0), M5 (2), M4 (0)

The list includes the main earthquake event. Since Wild Land published data for the 4th and 5th late on the 6th of May, information on three magnitude 4 events has been added to the USGS’s activity list for those days.

The summary shows that there were eleven aftershocks on the 4th, eleven on the 5th, one on the 6th and two on the 7th of May. The aftershocks are spread across several kilometres of seabed, and range between 2 and 58 km deep.

So far today, two magnitude 5 events have been reported for the 8th of May.

Published data for the region near Tonga is limited, and near magnitude 8 events have not been covered. It is difficult to know whether this aftershock trend is normal for that location.

One Response to “Erratic Aftershocks Near Tonga”

  1. Chris says:

    The erratic behaviour does make you wonder of the possibility of a foreshock-mainshock scenario.

    In saying this, a foreshock with a magnitude 7.9 is pretty rare…and I’m sure the data has already been checked for this type of scenario

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