Read more: 鈥�Instant Expert: Earthquakes鈥�
In the 1970s, during the heyday of earthquake prediction research, Charles Richter remained an ardent and vocal sceptic, a stance that drove a wedge between him and more optimistic colleagues. Overwhelmingly, the lessons of subsequent decades have vindicated Richter鈥檚 views. Yet asked in 1979 if he thought earthquake prediction would ever be possible, he replied: 鈥淣othing is less predictable than the development of an active scientific field.鈥�
Indeed, the 25 years since Richter鈥檚 death have witnessed developments he could not have imagined, including the recent recognition that many subduction zones generate a kind of seismic chatter, dubbed non-volcanic tremor, and that patches along subduction zones can slip slowly without releasing seismic waves. Non-volcanic tremor, which is thought to occur along the deep extension of faults into layers that are too hot to remain fully brittle, has also been identified along a few faults outside subduction zones. Could the processes at play in the deeper layers be the key to understanding the occurrence of large earthquakes?
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Other intriguing but controversial ideas have been proposed, including the theory that electromagnetic precursors are generated before faults rupture. Scientific discourse about such research is couched within polarised debates about proposed prediction methods. Some scientists now wonder if the pessimism about the feasibility of reliable earthquake prediction has led the field to shy away from investigations that could help us understand earthquake processes.
Some fairly basic questions still beg for answers: why does an earthquake start at a particular time and place? Why does an earthquake stop? As a big earthquake starts, does it 鈥渒now鈥� it will be a big earthquake? Or is it merely a small one that gets out of hand?
As seismologists work to develop a more complete understanding of earthquakes, and to refine hazard assessments, one sobering lesson has emerged: expect the unexpected. While hazard maps characterise the expected long-term rates of earthquakes in many regions, an 鈥渙verdue鈥� earthquake might not strike for another 100 years.
Moreover, even in well studied areas, the historical record is too short to understand fully the variability of the earthquake cycle associated with a given plate boundary. Geological investigations of prehistoric earthquakes can start to extend our knowledge to more geologically meaningful timescales, but such results are limited and typically characterised by high uncertainties. Our understanding of both the variability of earthquake repeat times and the largest possible earthquake in a given area is limited at best. Our expectations for the largest possible earthquakes are often too strongly shaped by the events in the historical record only. We should know better.