Where Should We Look for Clues to Resolve the Physical Mechanism Causing Deep Earthquakes? Consider: Deep Moonquakes, and Isolated Deep Earthquakes

The physical mechanism responsible for deep earthquakes has been a puzzle since their discovery by Wadati 85 years ago. Today the three most widely accepted mechanisms are dehydration embrittlement (for depths above about 300 km), ‘anticracks’ in metastable olivine (for deeper events), and thermal runaway. Yet, the puzzle remains: no single mechanism seems satisfactory, and having three mechanisms seems contrary to Occams Razor. Where might we look for observations to help resolve this problem?

 One of the 20^thcenturies greatest scientific surprises was when seismic data collected during the Apollo projects proved that the Moon was seismically active at depths of 850-1100 km. These deep moonquakes occurred repeatedly within clusters or “nests”, with occurrence times strongly correlated with solid tides. Although they occur under temperature-pressure conditions highly similar to terrestrial earthquakes at depths of ~120 km, it seems unlikely they are caused by dehydration embrittlement, as there is no subduction on the Moon, and possibly little or no water. Must we propose a fourth mechanism for deep quakes? Or are our explanations for terrestrial deep quakes incorrect?

 The development of plate tectonics was one of the 20^thcentury science’s great paradigm shifts; one observation it helped explain was why numerous earthquakes with focal depths exceeding 400 km occurred in the mantle in areas like Tonga where very old, fast-moving plates converged. But some “isolated deep earthquakes” are exceptional; these earthquakes occur separated from neighboring events by hundreds of km. Some, like earthquakes in Spain in 1954 and Colombia in 1970, with Mw exceeding 7.5 are among the largest deep-focus earthquakes known, yet occurred singly without any fore- or aftershocks. What physical mechanism allows such large, isolated earthquakes to occur? The tendency is to focus attention on areas like Tonga where the majority of deep events occur. But understanding rare isolated events is equally important if we hope to understand the physical mechanism.

Figure. Estimated temperature-pressure conditions where seismicity occurs within the Earth (shaded dark grey) and Moon (plotted symbols).