In the past 24 hours, there have been 25 earthquakes around the world. Many of them are of low magnitude, but at least nine were a magnitude of 4.5 or stronger. Earthquakes can potentially occur anywhere (the small earthquake that shook the US East Coast in 2011, an area considered not at risk, is proof of that); however, the likelihood that an area be hit by an earthquake and its strength vary.
The map above, based on data from the Global Seismic Hazard Assessment Program (GSHAP), highlights the areas where there is an increased risk of seismic activity. The GSHAP ran from 1992 to 1999 (as part of the United Nations International Decade for Natural Disaster Reduction), and the data shown in the map have not been updated since. While still accurate overall, the data have a few issues. USGS’s National Regional Coordinator Mark Petersen told Quartz that, for instance, the seismic hazard in Haiti, which was hit by a devastating earthquake in 2010, is underestimated in the map. However, Petersen confirmed this is the most accurate global hazard representation available and, while there is detailed information available regionally, updates to the global representation will be made in the next several years.
On the map, Central and South Asia emerge as the areas of the world where earthquakes are more likely to strike, and with greater intensity. A closer look at the map shows Nepal, as well as parts of northern India, as one of the areas with a higher risk of strong earthquakes, which routinely hit the area. The strongest earthquake recorded in Nepal in the past century had an intensity of 8.2, and killed 10,000 people. In the past 15 years, the area has been hit by several strong earthquakes, including in 2001 in Gujarat, India, when 20,000 died, and in 2005 in Pakistan and Kashmir, where 130,000 died.
The Indian plate’s sliding movement under the Eurasian plate is what causes the intense activity around Nepal; it’s also what causes the Himalayas to continue growing (about 1 cm every year). There is no halting that, and strong earthquakes strike Nepal in a 75-year-cycle, which is why scientists were expecting this disaster.
Earthquakes recur in a certain areas periodically—where the activity is more frequent, it’s easier to identify the pattern. That’s the case in the US state of California, where a strong earthquake of 8.0 intensity is expected to hit every 500 years. The last tremor of such magnitude to hit the area was a 7.9 in 1906. The US Geological Survey says tension has been accumulating and there is a 7% chance of a similar event occurring in the next three decades. The likelihood of strong earthquakes hitting the area—albeit not as devastating—in the next 30 years is higher still, with 19% chance of a 6.7 or larger event.