What are earthquake aftershocks?

CBC

22-02-2025

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Aftershocks are possible following a 4.7 magnitude earthquake that shook British Columbia's South Coast on Friday afternoon (the quake was initially estimated at a magnitude between 5.1 and 5.4).
In fact, one was already recorded shortly after the initial shake — Earthquakes Canada recorded a 1.9 quake at 2:40 p.m. PST. It was quickly determined there was no threat of a tsunami following the first earthquake.
Aftershocks are smaller earthquakes that happen after a larger earthquake in the same area, and are not an indication that a larger earthquake is on the way.
B.C. Minister of Emergency Management and Climate Readiness Kelly Green said anyone near the epicentre of the earthquake, 24 kilometres north northeast of Sechelt, B.C., should be prepared for aftershocks.
CBC seismologist and meteorologist Johanna Wagstaffe said aftershocks in this particular case will likely be minimal — and definitely lesser in magnitude compared to the first quake.
"We may see a 2 or 3 [magnitude earthquake] in the hours or days to come," she said.
What is happening during aftershocks?
Aftershocks are normal, according to Earthquakes Canada. The agency says what you're feeling during an aftershock is the earth's crust readjusting after it broke during an earthquake.
"We would expect the earth to sort of settle into that new position," Wagstaffe said.
How many will there be?
It's impossible to know how many aftershocks might happen after an initial quake, or how strong they may be. According to the United States Geological Survey (USGS), the rate of aftershocks happening decreases with time.
The province says anytime you feel a quake or aftershock to make sure to drop, cover and hold on.
WATCH | Earthquake preparation drill:
Annual drill urges British Columbians to prepare for earthquakes
4 months ago
Duration 2:12
Thousands of British Columbians practised how to 'drop, cover, and hold on' today during the Great B.C. Shakeout, a provincewide earthquake drill at 10:17 a.m. on Thursday. Michelle Morton has more on how to prepare for the big one.
How strong are they?
Aftershock vibrations are, in most cases, weaker than the initial shock, according to Earthquakes Canada. However, it is possible they're strong enough that you will feel them.
How long could aftershocks happen?
Earthquakes Canada says in most cases, aftershocks could happen several hours or even days after an earthquake.
How can I be prepared?
The province's Prepared B.C. website has over a dozen emergency plans in several languages to help people get ready for future quakes. Plans include things like grab-and-go bags, evacuation routes, understanding what certain warnings and alerts mean and emergency contacts.