Attempts to predict seismic events

Overview

Earthquake forecasting and earthquake prediction, seismic gap theory and record of attempted earthquake predictions.

What you need to know

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Questions from this subtopic

Each one with the model answer underneath.

1. 1

   What would a genuine earthquake prediction have to state?

   Model answer

   It would have to specify the place, the time and the magnitude, each within useful limits, and with enough confidence to justify acting on it. Saying that a region will experience a large earthquake at some point in the next century is a forecast of hazard, not a prediction, and no authority can evacuate a city on it. A prediction useful for management would need to narrow the timing to days or weeks and the location to a specific fault segment, and that has never been achieved reliably.

2. 2

   Distinguish between earthquake forecasting and earthquake prediction.

   Model answer

   Forecasting states the probability that an earthquake of a given size will occur in a given area over a long period, for example a stated percentage chance of a major event on a particular fault within thirty years. It is based on the history of past earthquakes and on measured rates of strain accumulation, and it is genuinely useful because it informs building codes, insurance and land-use planning. Prediction claims to specify a particular event at a particular time, and it is this that has proved unattainable. Forecasting works; prediction does not.

3. 3

   What is seismic gap theory?

   Model answer

   Seismic gap theory proposes that along an active fault or subduction zone, segments that have not ruptured for an unusually long time while neighbouring segments have are the segments most likely to rupture next. The reasoning is that plate movement loads every part of the boundary at a similar rate, so a segment that has not released its accumulated strain must be storing it, and the longer the gap since its last earthquake, the closer it must be to failure. Mapping the rupture history of each segment therefore identifies the gaps as the places of highest risk.

4. 4

   How is seismic gap theory used in practice?

   Model answer

   The rupture history of a plate boundary is reconstructed from instrumental records, historical accounts and geological evidence of past ruptures, and the boundary is divided into segments. Segments that have not slipped in a long time are identified as gaps and prioritised for dense monitoring, for strengthening of buildings and infrastructure, for public education and for higher insurance loading. It is used, in other words, as a tool for targeting long-term preparation towards the places most likely to be hit, not as a short-term warning.

5. 5

   What are the weaknesses of seismic gap theory?

   Model answer

   It says nothing about timing, so a gap may not rupture for another century. It assumes each segment repeatedly produces earthquakes of a similar size at roughly regular intervals, which the record does not support. It assumes strain is stored rather than released, but some segments creep steadily and release their strain aseismically, so they are quiet without being dangerous. It assumes ruptures respect segment boundaries, whereas the largest earthquakes are precisely those that break through several segments at once. Large earthquakes have repeatedly occurred where no gap had been identified, and some identified gaps have remained quiet.

6. 6

   What is dilation, and what is the reasoning behind using it to predict earthquakes?

   Model answer

   Dilation, or dilatancy, is the small increase in the volume of rock that occurs as it is placed under increasing stress before it fails. Countless microscopic cracks open within the rock, so its total pore space grows and its physical properties change measurably. Since this occurs during the loading of a fault, before rupture, it should in principle generate detectable precursors in the weeks or months beforehand, and the reasoning is that monitoring those changes would allow the approach of failure to be recognised.

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