Doublet earthquake

In seismology, doublet earthquakes—also known as multiple or twin earthquakes—were first noticed as groups of earthquakes that had nearly the same waveforms and happened in the same location. Today, they are described as sequences with two or more main shocks that are very similar in strength, sometimes happening just seconds apart or even years later. These earthquakes are special because their strengths are almost the same, unlike regular aftershocks, which are smaller and happen after the main quake.

Doublet earthquakes share almost identical seismic waveforms because they come from the same rupture zone. They happen when parts of a fault, called asperities, stop the main earthquake from moving smoothly. This means only part of the pressure built up by tectonic stress is released, and the rest can cause another quake soon after. For example, in the 1997 Harnai earthquake, a big quake was followed by another big one just 19 seconds later, making the shaking last longer and affect more areas.

Scientists did not fully understand doublet earthquakes until the 1970s and 1980s, when they studied seismograms and realized some big earthquakes were not just large foreshocks or aftershocks. They found that about 20% of very large earthquakes (with a magnitude above 7.5) are doublets. These earthquakes challenge older ideas about how to predict seismic hazard, because they can involve more than one part of a fault breaking at the same time, changing how we think about earthquake risks.