Submitted by **cpassmore79** t3_10wecl8
in **askscience**

So my understanding is that Earthquakes are a release of pressure when fault lines get "stuck" and the plates can't move.

I live in the PNW, and we're always talking about "the big one" on the Cascadia fault and how we're overdue. But are we? We have a few small quakes every year... doesn't that relieve the pressure?

CrustalTrudgert1_j7nrv48 wroteIt's important to remember that the scales we use for earthquakes (which in the US, is typically the moment magnitude scale, i.e. Mw) are logarithmic. Thus, let's say we define a big earthquake as an Mw 8.0 and a little earthquake as an Mw 2.0, the Mw 8.0 is 1,000,000 times larger than the Mw 2.0 (or alternatively if we say a Mw 3.0 is small, the Mw 8.0 is 100,000 larger, and so on).

Now, this is just thinking about the magnitude as represented on a seismogram, if we want to say how many earthquakes of a given small magnitude equal a given single large magnitude earthquake, we need to consider this through the lens of radiated energy. For this purpose we can use the equation on the linked wiki page that relates Mw and radiated energy Es, specifically,

Mw = 2/3 log(Es) - 3.2

So, we can use this to calculate the amount of energy released by a single Mw 2.0 or Mw 3.0 and a Mw 8.0 earthquake and thus just how many Mw 2.0 or 3.0 events we'd need to equal the energy of a single Mw 8.0. If you go through the math, you'll find that to equal the released energy of a single Mw 8, you would need ~31 million Mw 3.0 or ~1 billion Mw 2.0 events. Let's be more generous and consider something of a more moderate event, like a Mw 5.0, but even then you'd need around 32,000 Mw 5.0 events to release the same energy as a single Mw 8.0.

With this, you could play other games, like lets say the fault system in question has stored enough energy to generate a Mw 8.0, but you have 25 Mw 5.0 earthquakes over a given period, how much energy is left? Again, doing the math, enough to generate a Mw 7.9997 earthquake.

Suffice to say, no, a few small quakes every year are a literal drop in the bucket toward the total strain budget of a system capable of generating a large magnitude earthquake so these do not really do much in terms of preventing an eventual large magnitude event.

EDIT: Writing this answer as I was falling asleep led to me not addressing the "overdue" aspect of the original question. If you would like a deeper dive on why the concept of earthquakes being "overdue" is incredibly problematic, I'll refer to you this FAQ.