IT鈥橲 a kind of geological butterfly effect. of Rice University in Houston, Texas, and colleagues believe they have found two clear cases where remote events weakened the San Andreas fault near Parkfield, California. The finding suggests powerful earthquakes 鈥� like the one that has just hit Sumatra 鈥� may trigger further quakes worldwide.
The first changes to the San Andreas occurred in 1992 after a 7.3-magnitude earthquake several hundred kilometres to the south. The second took place in 2004 after a quake of magnitude 9.1, also in Sumatra, 8000 kilometres away. In both cases, there were distinct changes in the movement of fluids and an increase in the frequency of micro-earthquakes deep within the fault below Parkfield (Nature, ).
Niu and colleagues believe these changes are linked to a weakening of the fault, and that monitoring them could lead to more accurate earthquake forecasts. They suggest that very large quakes might push faults all round the world closer to the point of failure, and so lead to a temporary increase in global seismicity.
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Rumble relation?
So does Niu鈥檚 finding suggest the two quakes that struck the Pacific on 30 September were linked?
The magnitude-8.0 rumble that sent a tsunami towards the Samoan islands on Wednesday was followed within a matter of hours by the magnitude-7.6 quake on the island of Sumatra in Indonesia. However, other scientists have been quick to rule out a link between the two, pointing out they occurred on separate fault systems.
Niu says this may have been too hasty. 鈥淚t鈥檚 possible that the Samoan earthquake did trigger Indonesia鈥檚 earthquake,鈥� says Niu, not least because the epicentres are geographically close.
However, he concedes that the Samoan tremor may not have been big enough to produce the same kind of fault weakening his team observed.
