Lead Author

Roger Bilham is on a mission to improve earthquake forecasts in the Himalayan region. The geophysicist and his student Nicole Feldl, from the University of Colorado, Boulder, braved high altitudes and Maoist guerillas to gather GPS points in the Himalaya. Using these data and a subsurface model of the northern Indian region helped clarify the seismic significance of the squishy Tibetan plateau that has confounded modelling efforts up to now. On page 165, the model represents how time lag between earthquakes, the length of ruptures and their magnitude are linked.

How soon could the Himalaya experience a massive earthquake?

Our calculations show that two-thirds of the Himalaya are ready today for an earthquake of magnitude 8 or greater. Some parts of the Himalaya are apparently long overdue.

Describe your model.

By assuming that most of the Tibetan plateau is elastic, or able to return to its original shape following temporary stress, we calculated how earthquake magnitude grows with rupture length. Using data from past earthquakes, we calibrated our results by adjusting the time interval in our models that's necessary to obtain the observed geologic movement. Unfortunately, it's not useful for accurately predicting earthquakes.

How does this work change assumptions about plate activity in the region?

It brings a new perspective, with a couple of unexpected consequences. For example, if you have a devastating magnitude 7.6 earthquake (like in Kashmir, 2005), we find that it doesn't drain the system's energy, and the same area can have another, bigger earthquake soon afterwards. This contradicts seismic gap theory, which says if you've recently had a big earthquake you shouldn't expect another for a long time.

If pressure is released by smaller earthquakes, how can conditions for a megaquake still exist?

Short ruptures are unable to tap deep into the Tibetan plateau's cumulative reservoir of strain energy. It requires megaquakes to do this. We've had four fairly substantial earthquakes in the past 200 years, but they haven't been big enough to release much of this cumulative strain.

How serious is the threat of earthquake in the region?

Quite frankly, the countries bordering the Himalaya are in terrible trouble. We estimate that one million people could die in a single event unless city structures are made earthquake-resistant.