Hidden depths

A staggering lack of undersea data hampers our understanding of earthquakes and tsunamis. Geophysicists must put more instruments offshore.

The magnitude-9.0 Tohoku-Oki earthquake and tsunami that struck Japan on 11 March were devastating not just to the Japanese people, but also to the scientific community. Seismologists had underestimated the earthquake size by a factor of 4 or more, and the tsunami hazard by so much that the protective walls around the Fukushima nuclear power plant were overtopped.

The past 20 years have seen great strides in understanding earthquake faults and volcanoes, largely thanks to technological advances allowing a huge increase in ground-deformation measurements. Precise measurements of ground movement can provide nearly direct information about the strain energy accumulating along a fault, which can be released by a giant quake. The number of researchers interested in this field has skyrocketed.
But underwater monitoring lags behind. The tools for monitoring subtle ground movements rely largely on the Global Positioning System (GPS), and so only work when they have a clear view of the satellites they rely on. Their radio waves cannot penetrate water. This makes underwater monitoring expensive and time-consuming, so few opt for it. Instead, most of the community has been drawn to rely on cheaper, land-based tools for understanding earthquake behaviour, including that around underwater faults — where the real action happens offshore. Seismometers, which detect shaking during earthquakes, are routinely deployed on land and at sea. But ground-deformation monitors have been restricted almost entirely to land. Geophysicists are relying on the data they have rather than the data they need.

This needs to change. We must improve undersea monitoring and make it cheaper, increasing measurements of the sea floor 100-fold. By so doing, we will vastly improve both our understanding of plate-boundary dynamics and volcanic processes under-
water, and our assessment of earthquake and tsunami hazards.