University of Wisconsin-Madison, USA

A geophysicist wonders how and why faults behave in so many different ways.

I’m involved, with colleagues, in a project of the Integrated Ocean Drilling Program (IODP) to drill deep into the Nankai Trough subduction zone off southwestern Japan — a site of numerous great earthquakes and tsunamis.

Major unknowns in the generation of tsunamis include how far earthquake fault slip can propagate up towards the sea bed and what factors control how that slip stops in accretionary wedges — the submarine mountain ranges created as sediment and rock are scraped off the sinking plate.

My research focus is on faults in such wedges, which are generally thought of as aseismic, or incapable of earthquakes. By drilling into the wedge faults at Nankai Trough, we hope to learn how aseismic faults give way with depth to the seismic faulting associated with tsunamis.

Recently, a new kind of slow-motion earthquake was observed in this wedge (Y. Ito & K. Obara Geophys. Res. Lett. 33, L02311; 2006). Suddenly, wedges don’t seem so aseismic after all.

These ‘very-low-frequency’ earthquakes, some as large as magnitude 4.4, have previously gone unrecognized because their seismic waves don’t show up in the frequency range in which earthquakes are normally detected.

By chance, the quakes were detected exactly where my IODP team plans to start drilling later this year. We hope to install sensors deep in the subsurface to record the earthquakes up close and to measure pore fluid pressure and strain in the rock. We’ll also collect samples for laboratory studies of the frictional properties of the rock.

Taken together, the in situ and sample data should yield insight into the processes responsible for these slow-motion quakes. This might help us to understand the aseismic–seismic transition.

University of East Anglia, UK

An oceanographer describes a missing piece of the climate puzzle.

Most school students know that increasing atmospheric carbon dioxide raises global temperatures. But I’ve always been fascinated by the other half of the climate–CO2 connection: why, in the past, have increasing temperatures driven up atmospheric CO2?

That CO2 and temperature are locked in a powerful, positive-feedback embrace is obvious from ice-core evidence. But if we add up all the mechanisms that we know about, we fall short of explaining the rise in CO2 levels seen at the end of glaciations.

Upwelling in the Southern Ocean may be the missing piece. Today, this process brings deep CO2-rich water rapidly to the surface, where it vents carbon to the atmosphere. If upwelling was shut down during glaciations, we could fit the data better.

It has been suggested that sea ice might have blocked the air–sea transfer of CO2 during times of glacial maxima (B. B Stephens & R. F. Keeling Nature 404, 171–174; 2000).

There is good evidence that sea ice was extensive in the region, but any upwelling would have melted that ice, because the rising water has a temperature above freezing point. So the sea ice is evidence that the upwelling itself was absent. What stopped it?

One recent paper (J. R. Toggweiler et al. Paleoceanography 21, PA2005; 2006) argues that the critical factor was a shift to the north of the westerly wind belts that drive the upwelling. I and a colleague propose a subtler connection, a change in the balance of surface heat flux, that would also reduce the upwelling to near-zero (A. J. Watson & A. C. N. Garabato Tellus B 58, 73–87; 2006).

The theories are convergent in many respects, but make distinct predictions that we can test against new proxy evidence. This problem will be solved pretty soon, I think.