Though they seem stable, rock and soil can move very quickly and with devastating force, creaking and rumbling as they fall down a cliff side. For years, seismologists and geologists have tried to understand the complex triggers that set off landslides and mudslides. Earthquakes, heavy rainfall, and changes in the plant community are known to drive the sudden shifting of the Earth, but clear data that combine these factors have been hard to collect. Since landslides and mudslides can be so sudden, it is difficult to study them as they occur.
To get around this problem, researchers from France and Peru teamed up to find a steady and slow-moving slide that they could study. The Maca landslide in Peru is made of clay and silt, sliding at about 4 feet per year. However, with enough rainfall in the wetter months, this rate can jump to around 24 feet per year. To understand exactly what changes the rate of the slide, the researchers set up monitoring equipment at the site of the sliding earth that could measure both the amount of soil that slides as well as listen to the sounds of the slide itself. By measuring the sound waves produced by the rumbles, creaks, and clicks, the researchers could tell how quickly the slide was moving as well as the ways the soil was compacted.
For two years, the researchers also tracked rainfall and earthquake activity along with the amount of sliding soil and sound waves. During this time, there were 165 small to medium sized earthquakes in the area, providing a lot of data on how they affect the slide. Two earthquakes that were recorded in 2016 gave the researchers some useful information. They found that during these earthquakes the slide moved suddenly, but afterwards the slide relaxed into a new position. This is similar to disturbing dirt at the bottom of a pond and watching it slowly settle again. Though the earthquake itself caused a great deal of movement initially, the relaxing after the Earthquake caused 10 times as much movement.
As they compared the two large earthquakes from 2016, the researchers also realized that more intense sliding occurred during the rainy season. This led them to wonder if differences in water levels below the ground could be playing a role in how the slides happen. The researchers built mathematical models of how different amounts of water might support or weaken the soil compactness and found some important results. When combined with the collected sound wave data, the researchers realized that rain prevented the ground from healing after earthquake damage, resulting in worse slides
When all of their findings were considered, the researchers found that earthquakes create cracks and pores that water can get into. When water seeps in, the soil can’t relax and compact, creating more sliding events. So, when earthquakes happen just after a big rain storm, slides can’t stay still and fall in large amounts.
If you live in a community near mountains and cliffs, it can be important to be aware of how much water might be affecting the riskier ground in your community. The next time you go outside and see the scar of a landslide, think back to last week’s weather and wonder how the shaky, breaky earth might be moving today.