The research team focuses on two historical seismic shocks that hit a region in Kazakhstan. A major difference between the tremblors is that the first one struck in June while the second one in January. A layer of frozen soil that had been created in the winter affecting the soil's response during the second shock.
Scientists state that both earthquakes (the first with a magnitude of 7.3 and the second with a magnitude of 7.8) resulted in similar ground shaking in the aforementioned areas, nevertheless, ground failure was far more intense during the second shock. The frozen ground led to obstruction of water drainage and, therefore, pore-water pressures significantly increased resulting in liquefaction of deeper ground layers.
The phenomenon caught the attention of researchers as ground failures were very different. In fact, scientists were investigating the impact of the two shocks as part of a larger project handled by the GFZ German Research Centre for Geosciences. The idea of potential ground freezing came from the study of the Great Alaska earthquake that occurred in 1964.
The research team utilized strong motion and temperature data to assess the seismic wave velocity propagating through the ground's profile. In particular, they used strong motion data from the 1999 M 7.6 Chi-Chi earthquake that occurred in Taiwan which was similar to the aforementioned quakes. “Of course there were no digital recordings of these two earthquakes. We had to choose the most reasonable and robust studies for input of strong motion,” Rami Alshembari, co-author of the study and formerly of the International Centre for Theoretical Physics in Italy and currently part of the Camborne School of Mines, at the University of Exeter, UK, stated.
After conducting computer simulations, the team concluded that the observed ground failure during the 1991 earthquake corresponds to a 1-meter frozen layer. Scientists mentioned that those conclusions could not be derived if the damage caused by the seismic shock was not so well documented by the Mining Department of Russia and the Russian Mining Society.
Researchers suggest that such differences should be included in the assessment of the ground's behavior and the design of infrastructure.
Source: Seismological Society of America