Why earthquake depth matters
Depth is how far below the surface the rock broke. It is the number people skip past, and it does more to decide whether an earthquake is felt than almost anything except the magnitude itself.
A shallow M4 can shake a town harder than a deep M6 four hundred kilometres down. The energy has less rock to travel through and arrives less spread out.
The rough bands
| Depth | Called | What it means for shaking |
|---|---|---|
| 0–70 km | Shallow | Strongest shaking at the surface for a given magnitude. Most damaging earthquakes are here, and most earthquakes full stop. |
| 70–300 km | Intermediate | Felt over a wider area but less intensely near the epicentre. |
| 300–700 km | Deep | Can be felt hundreds of kilometres away and rarely causes damage. Occurs in subduction zones. |
Nothing is recorded below about 700 km: deeper than that, rock deforms rather than breaking suddenly.
Why California earthquakes are nearly all shallow
Depth depends on the kind of boundary. Along a strike-slip system like the San Andreas the crust slides past itself in the top ten to fifteen kilometres, so events there are almost all shallow — which is why a modest magnitude in California can be felt sharply and is often reported by thousands of people.
In a subduction zone — Alaska, the Aleutians, Cascadia, and much of the Pacific — one plate descends beneath another, and earthquakes occur along that descending slab to great depths. That is where intermediate and deep events come from.
Depth gets revised, sometimes a lot
Depth is the hardest of the basic parameters to determine quickly. It is constrained by how the arrival times at different stations compare, and when the nearest station is far away that constraint is weak. A first automatic solution can put an event at a default depth and move it substantially on review.
So a depth that changes from 11 km to 24 km in the first hour is normal, not a mistake. QuakeIndex lists depth revisions on the event page alongside magnitude revisions, with the time each was observed.
Reading an event page with depth in mind
If you are trying to work out why something was or was not felt, three numbers together tell you most of it: the magnitude, the depth, and the distance from you to the epicentre. A M3.2 at 5 km depth eight kilometres away is a sharp jolt. The same magnitude at 60 km depth and eighty kilometres away is usually nothing at all.
Check what was recorded near you →
Sources
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