NEW ANIMATION! Foreshock, Mainshock, or Aftershock? (THREAD) #earthquake @NSF_GEO

There is often confusion surrounding the terminology of earthquakes. Large earthquakes are followed by 100s or even 1000s of smaller earthquakes, called aftershocks. In some #earthquake sequences, a smaller earthquake called a “foreshock”, precedes the mainshock.

Although seismologists have carefully analyzed foreshock/mainshock pairs of earthquakes, and earthquake triggering in general, there are no special characteristics of a foreshock that let us know it is a foreshock until the mainshock occurs.

The major redistribution of stress during the mainshock triggers an aftershock sequence of smaller earthquakes along the mainshock rupture zone.

Aftershocks are smaller earthquakes on patches of the rupture zone and adjacent smaller faults that occur as rocks along the rupture adjust to the new state of stress.

In general, larger aftershocks are concentrated in the hours to days following the mainshock.

The magnitude scale is logarithmic & elastic energy released by an #earthquake increases by a factor of 33 with every unit increase in magnitude. So, a M5.6, earthquake will release 1/1000th of the energy of a M7.9 earthquake.

Omori found that the decay of the aftershock rate was best analyzed using a logarithmic graph of aftershocks per day versus days after the mainshock. He showed that the rate of aftershocks decreased w/ the reciprocal of time since the mainshock. This is called Omori’s Law.

In a typical mainshock – aftershock sequence, 97% of energy is released by the mainshock while the hundreds to thousands of aftershocks release the remaining 3%.

Although only a small portion of the energy of an earthquake sequence is released by aftershocks, the *largest* aftershocks and the *highest rate* of aftershocks occur during the critical first hours and days immediately following the major #earthquake.

This is exactly the time when emergency workers are locating and rescuing injured people from collapsed or damaged buildings. Because those buildings can shift or even collapse during aftershocks, rescuers are at greatest risk when their expertise and service is most important.