The Shocks of Earthquake

But first, what is an Earthquake?

Earthquakes are usually caused when rock underground suddenly breaks along a fault. This sudden release of energy causes the seismic waves that make the ground shake. When two blocks of rock or two plates are rubbing against each other, they stick a little.

The rocks are still pushing against each other, but not moving. After a while, the rocks break because of all the pressure that’s built up. When the rocks break, the earthquake occurs.

photo by:

The Shocks (categories) of an Earthquake

We all know that tectonics, faulting and volcanic eruption are the major causes of earthquake, but earthquake have categories which may classified according to their relation to other earthquakes, with respect to location, magnitude, and time.


In a series of earthquake which occur every relatively short period of time in a particular area, the mainshock is the quake of the greatest magnitude. Some earthquake sequences have obvious mainshocks, but instead have several quakes of similar magnitude that constitute the largest events in the series.


Foreshocks are those earthquakes which occur immediately preceeding a mainshock in the exact same area in which the mainshock occurs. The time between the last foreshock and the mainshock varies somewhat, but is typically less than a day.

Foreshocks cannot be positively identified as foreshocks until after the mainshock has occurred. Even then, the “mainshock” sometimes proves to be a large foreshock of an even bigger earthquake, which will then assume the position as the “mainshock”. Also, not all mainshocks have foreshocks. This means that while foreshock identification (before the mainshock strikes) would be useful, it can never be a cure-all solution for earthquake forecasting.


Aftershocks are tougher to define than both foreshocks and mainshocks. To be considered an aftershock, an earthquake must meet two criteria:

  1. It must occur within one rupture length of the mainshock rupture surface, or alternatively, within an “aftershock zone” based upon early aftershock activity and defined by seismologists.
  2. It must occur within that designated area before the seismicity rate in that area returns to its “background”, meaning pre-mainshock, level.

Of course, mainshocks are of the greatest interest to the public, simply because they do the most damage. Mainshocks also represent great opportunities for learning about the processes behind earthquakes, and about the structure of the Earth’s interior.

Studying the aftershock sequences of mainshocks provides another way to compare and contrast large earthquakes, and may ultimately lead to a better understanding of the processes controlling the scale of rupture, the timing and location, and the long-term repercussions of earthquakes.

Foreshocks. These earthquakes are followed by larger earthquakes — mainshocks — in a manner that is roughly the opposite of the relation between aftershocks and mainshocks. Because of this, earthquakes can only be classified as foreshocks once an obvious mainshock strikes.

photo by: steven panambo

Categorizing earthquakes into foreshocks, mainshocks, and aftershocks gives us a way to break down seismicity into smaller groups of events with at least one common characteristic. This allows us to study those groups more closely, and look for more subtle similarities common to most or all members of that group.


Michigan Technological University (2007) Why Do Earthquakes Happen? Retrieved from

Foreshocks, Mainshocks, and Aftershocks. Retrieved from


Leave a Reply

Fill in your details below or click an icon to log in: Logo

You are commenting using your account. Log Out /  Change )

Google+ photo

You are commenting using your Google+ account. Log Out /  Change )

Twitter picture

You are commenting using your Twitter account. Log Out /  Change )

Facebook photo

You are commenting using your Facebook account. Log Out /  Change )


Connecting to %s