
While the main tremors of an earthquake typically last no more than a few minutes, a seismic event can be accompanied by a series of aftershocks of varying intensity and duration. The overall seismic activity, especially with powerful earthquakes of magnitude 8-9, can continue for several days, even months. For example, the Great Chilean Earthquake of magnitude 9.0, which occurred on May 22, 1960, at 15:11, was preceded by a series of foreshocks:
- May 21 at 6:02, an 8.3 magnitude earthquake struck near Concepción;
- May 22 at 14:55, another earthquake of magnitude 7.8 hit Concepción;
- May 22 at 15:11, the earth in the Corral Bay area moved for 10 minutes.
Strong aftershocks continued along the entire Chilean coast for 5 days following the main shock. Since earthquakes result from the dynamics of tectonic plates, how long can their slippage continue, causing seismic vibrations on the surface? How long did the longest earthquake last, and how was it detected?
Slow Slip Earthquakes
The term “slow slip earthquake” has been used by seismologists since the late 1990s, when GPS technology became sophisticated enough to detect subtle seismic shifts in the Earth’s crust. Traces of slow slip earthquakes were first detected in the Pacific Northwest of North America and in the Nankai Trough region off the coast of Japan. Since then, scientists have discovered numerous occurrences of minor seismic shifts virtually everywhere – from the coasts of New Zealand to Costa Rica and Alaska.
These subtle yet prolonged earthquakes release energy that accumulates as tectonic plates move. Instead of causing an abrupt shift, they gradually relieve stress, posing no immediate danger in themselves. However, these small underground movements can increase stress in adjacent areas along the fault, increasing the risk of more powerful earthquakes nearby. The interest in slow slip events stems from their potential to act as precursors to more significant seismic events. Therefore, continuous monitoring of ground shifts near active faults in subduction zones has been ongoing for two decades.
The Longest Earthquake in Earth’s History
In February 1861, an earthquake measuring approximately 8.5 on the Richter scale struck the Indonesian island of Sumatra, triggering a powerful tsunami that claimed thousands of lives. This tragic event was not isolated but rather the final phase of the longest earthquake in recorded history, which unfolded underground for 32 years. These seismic phenomena, known as “slow slip earthquakes,” develop over days, months, or even years.
More than 150 years later, Indonesia remains a zone of potential seismic hazard. A series of slow slip events continues. For example, small tremors accompany the submergence of the southern part of Enggano Island, suggesting that a powerful earthquake could occur in the island’s vicinity. Similar activity is observed on the west coast of Sumatra, reflecting underground events arising from the interaction between tectonic plates in the Sunda Trench.
The Importance of Understanding Slow Slip Earthquakes
Understanding slow slip events is crucial for identifying the potential risks they pose in terms of triggering more powerful earthquakes. Events of this type preceded many of the strongest earthquakes in history, including:
- The catastrophic 2004 Sumatra-Andaman earthquake of magnitude 9.1 that struck Indonesia;
- The devastating 2011 Tohoku earthquake of magnitude 9.1 in Japan;
- The powerful 2014 Iquique earthquake of magnitude 8.2 in Chile.
However, definitively proving that slow slip events can actually cause major earthquakes remains challenging. Slow seismic activity does not always lead to powerful earthquakes. To learn more, scientists require long-term monitoring of slow slip zones and the development of new technologies to facilitate this monitoring.