Why did some people not receive an alert after the B.C. earthquake?

Many British Columbians received an alert on their phone after a 4.7-magnitude earthquake struck B.C.'s Sunshine Coast on Friday afternoon.

The alert notified people that an earthquake had been detected and urged them to "drop, cover and hold on."

Natural Resources Canada said the quake occurred at 1:26 p.m. PT at a depth of about 1 kilometre, and there were no immediate reports of damage.

The federal department told CBC News an alert was sent at 1:27 p.m. PT.

While many received an alert shortly after the quake, others reported not receiving anything. 

"It does seem to be the case that some people did not receive alerts while people they were with did," said a statement from Natural Resources Canada, which is responsible for the Earthquake Early Warning (EEW) system. 

"This will need to be investigated." 

The statement said the EEW system only alerts areas where strong and potentially harmful shaking is anticipated. 

"For people further from the epicentre, feeling lighter shaking, an EEW should not be expected," it said. 

The statement also said that people may receive an alert before, during or after shaking arrives at their location depending on their distance from the earthquake and the method by which they receive the alert.

For an alert to be issued, the quake has to be of a magnitude greater than 5.0 with a Modified Mercalli (MM) Intensity of IV or higher.

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Earthquakes Canada initially recorded the magnitude as high as 5.4, but revised the measurement to a 4.7 around 5 p.m. PT — which means it is classified as a minor event. 

It said an MM IV quake "can be felt indoors by many, outdoors by few" and can lead to the rattling of dishes, and the swinging of hanging objects.

Natural Resources Canada said warnings are sent automatically via the National Public Alerting System to cell towers, TVs, and radios within the area where strong shaking is expected. There is no need nor is there a way to sign up for alerts and even visitors to the area should receive an alert.

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Last August, provincial and federal government officials announced that the earthquake early detection system had been activated in B.C.

"We cannot stress this enough, during an earthquake, every second counts," Federal Energy and Natural Resources Minister Jonathan Wilkinson said at the time.

Tony Yang, a professor with the University of British Columbia's Dept. of Civil Engineering, said the system can allow critical infrastructure to take immediate action, such as halting traffic from driving onto bridges or into tunnels.

He said the technology around earthquake alerts is improving, but "we are not real time yet."

"But I think technology, as of now [in] 2025, it should be readily available and we should not have such a discrepancy between different users."