Seismotectonic map for the broader Aegean Sea.Earthquakes occur seemingly at random, yet anyone who follows the news carefully will be familiar with the aftershocks that can follow major crumbles. A paper in the journal Chaos, Solitons & Fractals has found fresh patterns in the way these phenomena occur that might just save lives.
All seismic activity arises from the same processes, which take place along faults in the earth’s crust. Improved understanding of these processes should help improve accuracy in predicting earthquakes and reduce the damage they cause.
Earthquakes have been recorded in Greece, on the boundary between the Eurasian tectonic plate in the north and the African plate in the south. The first documented quake in the region devastated the city-state of Sparta in 464 BC. While quakes as devastating as this are extremely rare, minor shocks are common.
Charikleia Gkarlaouni, of Aristotle University of Thessaloniki in Greece, and her colleagues have published a statistical analysis of historical and recent earthquakes in two very active areas of Greece. Both the Corinth rift and the Mygdonia graben are located on the fault line between the Eurasian and African plates.
The researchers modelled the magnitudes and locations of earthquakes in three series: a historical catalogue covering large quakes in the Corinth rift from 1700-2014; a catalogue of moderate quakes in both regions from 1981-2014; and one of minor shocks in both regions, covering 2008-2015.
The method they chose, Hurst analysis, is often used to study the interdependence of physical events over time. It was first applied to the periodical flooding of the Nile.
Hurst analysis comes up with a value, H, for a series of events. An H-value of 0.5 means that the events – in this case, earthquakes or minor seismic shocks – are completely independent of each other. If they are above 0.5, there is a positive memory effect and events cluster together in time. If they are below 0.5, there is a negative effect and they are not correlated.
All H-values for the historical series of large earthquakes were close to 0.5, indicating that they essentially occurred at random, but values for the series of moderate and minor events were significantly above 0.5, suggesting they were clustered together in a memory effect.
“Our results highlight the important role of minor earthquakes in a fault region,” says Gkarlaouni. “These carry much of the inter-relationship of the system, and in their absence, large earthquakes will occur at random.”
Studies of large quakes in other regions have also found little or no correlation between them, but others show memory-holding properties that differ from one fault system to another. The memory effect seems to be strongest in regions where tectonic faults interact.
“Understanding the interdependence of earthquakes in an area in this way can help in assessing the risk of future shocks, and any improvements in hazard risk assessment can save lives,” adds Gkarlaouni.
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Article details:
Gkarlaouni, C., Lasocki, S., Papadimitriou, E. and George, T.: "Hurst analysis of seismicity in Corinth rift and Mygdonia graben (Greece)", Chaos, Solitons & Fractals (2017)