Question

Differences between seismic focus and epicenter

Answer 1

Answer:

Epicenter is the location on the surface of the Earth directly above where the earthquake starts. Focus (aka Hypocenter) is the location in the Earth where the earthquake starts.

Explanation:

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Answer 2

What's the difference between the focus and the epicenter of an earthquake?

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The focus is the point under ground where the earthquake rupture physically starts.

The epicentre is the location of the focus on a map. Basically, epicentre + depth = focus.

To expand on this, the focus is only where the earthquake starts, not really the ‘location’ of the earthquake, because an earthquake rupture has a physical size; and for large earthquakes (magnitude 7+), the focus and epicentre is not always a good indicator of where the worst damage may be because the strongest shaking could potentially be several hundred kilometres from the epicentre/focus, and possibly even on completely different faults.

Some examples of this are the September 2010 earthquake in Canterbury, NZ, where the epicentre was about 10km north of the main rupture; the April 2015 Nepal earthquake, where the rupture moved primarily east of the epicentre, causing severe damage to Kathmandu, while Pokhara was relatively unscathed and the November 2016 Kaikoura earthquake in NZ, where more than 10 faults ruptured, some of them up to 200km north of the epicentre.

Depth is important too. A magnitude 7 with a focus 5km deep will likely rupture to the surface, and could flatten a city, while a magnitude 7with a focus 200km deep is unlikely to do anything more than scare a few people.

The reason the epicentre is used is because it is fairly easy to find using the time the first earthquake waves reach seismometers; it is usually a good indicator of where the worst damage will be; and people like to see a dot on a map they can point at.