Severity of earthquake impact may change with the seasons, study shows

In this study, the team which includes an expert from the Camborne School of Mines, Rami Alshembari, investigated two historical earthquakes that took place in close proximity near the city of Almaty more than 100 years ago, but in different seasons.

The research team found that, although of similar earthquake shaking, the quake that took place in the winter caused greater ground failure and hence damage – a phenomenon that means the ground loses its strength and can lead to cracks.

They believe this was due to a shallow frozen ground layer present during the winter months, which led to a build-up of excessive pore-pressure that couldn’t be released.

Lead author Rami Alshembari, who is formerly of the International Centre for Theoretical Physics in Italy and currently part of the Camborne School of Mines, based at the University of Exeter's Penryn Campus in Cornwall said: "We show that the presence of a shallow frozen ground layer can generate more severe ground fracturing during earthquakes in the winter".  

The earthquakes in Verny in June 1887 and the nearby Kemin in January 1911 produced very similar ground shaking at the surface, when considering the estimated epicenter positions of the two events and their corresponding magnitudes.

However, the Kemin earthquake caused significant ground failure – which the team believe was due to a shallow frozen ground layer present at the time, which inhibited the release of pore-pressure through the surface during the earthquake.

Rami said: “The daily temperature and historical data show that the uppermost layer at the time of Kemin quake was totally frozen.”

For the study, the team of researchers reviewed historical records of the two earthquakes, produced by the Mining Department of Russians and the Russian Mining Society at the time.

Having discovered the difference in ground failures between the two earthquakes, the team then set about determining whether the time of the year could play a role.

The research team created computer simulations of the earthquakes, which used different models of soil profiles alongside existing temperature data to determine which parameters control the internal redistribution of pore pressure with the soil profiles in both the winter and summer.

Rami said: “One of the problems was finding a way to include appropriate strong motion data recordings in the simulations, since there were no digital recordings of these two earthquakes.  

“We had to choose the most reasonable and robust studies for input of strong motion before finally settling on data taken from the 1999 7.6 magnitude Chi-Chi earthquake in Taiwan, appropriately modified, as being most similar to the Almaty earthquakes.”

The results suggested that models with a one-meter deep frozen layer of ground, that acts like a seal against pore pressure drainage, were the best fit for generating the ground failure seen in the Kemin earthquake. 

Rami explained: “Similar behaviour has been noticed in several seismically active permafrost regions, such as the recent 30 November 2018 Alaska earthquake. This dramatically provided new evidence of such an effect and a significantly greater data coverage.”

Seasonality in site response: An example from two historical earthquakes in Kazakhstan, is published in Seismological Research Letters.