I’m aware that a small quake with a shallow epicenter can actually be more damaging than a larger quake with a deeper epicenter. But how deep is deep? Seems like all quakes I’ve looked at the data for are centered 3 to 10 miles deep or so. How should I interpret those depth figures? (Also, what’s the frequency distribution look like – are deep or shallow quakes more common? Does it depend on the fault?)
First, the focus or hypocenter is the actual origin point of the earthquake. The epicenter is defined as the point on the Earth’s surface directly above the focus (picture – note that the epicenter may not be on the actual fault).
Almost all earthquakes are shallow , in that seismologists define shallow as within 100 km of the surface. This top 100 km, the lithosphere, is the only part of the earth rigid enough to store strain energy. Deep EQ can occur but only at subduction zones, where two tectonic plates collide and one is deflected down into the mantle. Before this plate reaches thermal equilibrium it remains rigid enough to generate intermediate or deep focus earthquakes.
Most interplate earthquakes, and the vast majority of intraplate earthquakes, occur on very shallow faults, less than a few tens of km in depth. Ignoring rock density and geography, earthquake energy follows the distance squared law, and depth is as important factor in energy dissipation as horizontal distance, so a magnitude 6.5 quake with a focus 10 km (such as the 1994 Northridge earthquake) will produce much more severe shaking at the surface than the same magnitude quake (say, the 1998 Nisqually earthquake in Seattle) with a focus at 30 km (~9X).
Thanks! Also for the “epicenter” clarification – I knew there was a difference between the surface point and actual underground point, but I was thinking center and epicenter — obviously the terminology of a rank amateur. Live and learn.
Now I’ll just have to start thinking in km (surely the USGS site I frequent uses that measurement, though I could swear they speak of miles …)