The search for survivors continues in both Indonesia and western Colombia. A magnitude 7.7 quake killed at least 47 people off Indonesia’s coast on Friday. Earlier this week, a magnitude 7.4 earthquake killed at least 281 people and injured more than 3,900 in Colombia. Another 379 are still reported missing.
“Unfortunately, we are reaching a point where the likelihood of finding survivors is dwindling, but our mission is to keep searching and find all those people who are reported missing,” David Tamayo, head of Colombia’s National Unit for Disaster Risk Management, told Reuters.
The disasters have revived a familiar question after major earthquakes: Can scientists predict when and where the next one will hit?
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The short answer, seismologists say, is no. Neither the U.S. Geological Survey nor any other scientists have ever predicted the date, time, location and magnitude of a major earthquake. Scientists can estimate the probability that a damaging earthquake will occur in a region over years or decades, but they cannot issue a reliable short-term prediction.
Suzan van der Lee, a professor of earth and planetary sciences at Northwestern University, said researchers continue to look for useful warning signs, but none have proven reliable enough to predict quakes in practical terms.
“We will never give up trying,” van der Lee said. “But so far, there is nothing promising that is reliable.”
That is why experts are skeptical of viral claims that one earthquake “loads” a distant fault enough to make another major quake imminent somewhere else. California, for example, remains at risk of large earthquakes because the Pacific and North American plates continue to move past one another along faults such as the San Andreas, Van der Lee said. But the Colombia quake did not suddenly load the San Andreas Fault thousands of miles away, she said.
A magnitude 3.8 earthquake struck near San Leandro in the San Francisco Bay Area on Thursday, followed by smaller aftershocks. The quake was centered north of San Leandro, and no damage was immediately reported, according to the U.S. Geological Survey. The shaking, experts said, was not unexpected for the Bay Area and did not mean a larger earthquake was imminent.
Colombia sits in a complex seismic region where several tectonic plates interact. van der Lee said the quake appears to be an “oddball” compared with the classic shallow subduction-zone earthquakes most associated with the world’s largest and most damaging events. The Colombia quake occurred at a depth of about 110 kilometers, meaning the fault rupture did not reach the surface or displace the seafloor enough to generate a tsunami.
Still, van der Lee said the magnitude was large enough to send strong seismic waves over a broad area, causing widespread shaking and damage above and around the hypocenter, the underground point where the quake began.
Aftershocks are expected after a major quake. They are smaller earthquakes that occur on or near the same fault area as the main shock as the crust adjusts to a changed stress pattern. van der Lee said they typically diminish over days or weeks, depending on the size of the main earthquake.
While the recent cluster of strong quakes in South America and elsewhere can make it feel as if the planet is becoming unusually active, global earthquake activity naturally fluctuates. The USGS estimates around 20,000 earthquakes are detected around the world each year, or roughly 55 a day. Long-term records suggest about 16 major earthquakes annually — about 15 in the magnitude 7 range and one magnitude 8 or greater.
So far this year, the USGS list of magnitude 7 and larger earthquakes includes 10 events. Van der Lee said that number is not unusual; even a year that ended with 17 would remain within natural fluctuation.
“There are lots of earthquakes in South America every day, but most earthquakes all over the world do not cause any damage,” van der Lee said. “It’s the occasional ones that do cause damage that, of course, are frightening and terrible.”
Places such as Japan, Alaska, Greece and parts of South America experience frequent earthquakes because they lie near plate boundaries, where tectonic plates move away from each other, slide past each other or collide. The most powerful quakes often occur in subduction zones, where one plate dives beneath another.
Improved monitoring also contributes to the perception that earthquakes are increasing. More instruments can better detect small earthquakes than in the past, and modern communications spread reports almost instantly. A temporary rise or decline in seismicity, the USGS says, is part of normal variation and does not mean a large earthquake is imminent.
For people in earthquake-prone areas, experts say the practical takeaway is preparation, not prediction: stronger buildings, emergency plans, seismic monitoring and rapid public alerts where earthquake early-warning systems exist.
“It is very important that we keep monitoring earthquakes with seismic stations, that we keep recording them and sharing the data internationally,” van der Lee said, “so that we can learn a lot more from possible bigger quakes that could occur by studying the smaller ones that occur in the same regions.”
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