Did you know that Los Angeles has already experienced an earthquake today? In fact, at the time of writing, six earthquakes have occurred in the last week, and 568 in the past year. This should come as no surprise to the citizens of Los Angeles, a city built on the truly massive San Andreas Fault, the slipping, grating boundary between the Pacific and North American tectonic plates. Although these earthquakes – generated by the constantly moving fault – were far from devastating, residents of the City of Angels know that their metropolis has suffered cataclysmic tremors before, and it will one day do so again. A new study by NASA has revealed with 99.9% certainty that a magnitude 5.0 earthquake will hit the city of 3.9 million people before the summer of 2018.

The research, published in the journal Earth and Space Science, is a rare seismology study in that the authors state that they are essentially 100% confident such a quake will hit Los Angeles within a short time period. They also look into the likelihood of more powerful quakes; the probability for an M6.0 earthquake within the same time frame is lower, at 35%. The most powerful earthquake in the 20th century, measuring at M7.8, was centered on the nearby city of San Francisco in 1906, where 3,000 people died in the subsequent destruction.

Earthquakes occur on the San Andreas Fault line when stress builds up between the two tectonic plates, which are sliding against each other in opposite directions. When parts of the tectonic plates get stuck, the plates continue to try and keep moving, which builds up stress. Eventually, the stress is released, and the tectonic plates judder forwards, causing an earthquake. The greater this stress, the larger the eventual earthquake. The more powerful stress releases are far less common, so more energetic quakes happen less often.

A moderate M5.1 quake that impacted the greater Los Angeles area in 2014 appears to have been part of a series of more shallow tremors. Seismologists at NASA’s Jet Propulsion Laboratory (JPL), who have been mapping the deformation under Los Angeles using GPS data and ground-penetrating radar, have concluded that this shallow trend of tremors suggests that there is a deeper part of the fault zone still accumulating stress that has yet to be released. The longer this stress builds up, the more powerful the eventual quake will be.

Earthquakes are measured these days by how much energy they release at the focal point of the fault rupture, known as the epicenter. This energy release (in joules) is placed on the moment magnitude scale (M), which is used by seismologists instead of the older Richter magnitude scale. To put this in perspective, an apple falling from a small tree to the ground releases one joule of energy. An M5.0, which according to NASA will definitely occur in the next three years in Los Angeles, typically releases one hundred billion joules, the equivalent of one million sticks of dynamite.

The certainty of the study’s prediction, however, is already being debated by the United States Geological Survey, who aren’t sure how the 99.9% value was attained.