PLAIN AND CIVIL: WHAT DO SEISMIC ZONES MEAN?

A seismic zone is a region in which there is a fairly consistent seismic activity. This means that the frequency and magnitude of seismic activities may be rare or may be extremely common. The term "seismic zone" is used to talk about an area with increase in seismic activities, whereby other people refer it as "seismic hazard zone" when discussing areas with more frequent seismic activities.

According to United States Geological Survey, USGS, seismic zoning numbered 0, 1, 2, 3, 4 are practically outdated. This designation had been used for the last time in 1969. The Uniform Building Code of 1997 is the only building code that still uses such zone. For the last past two decades, building codes have replaced maps with contours of design ground motion.

The Medvedev-Sponheuer-Karnik (MSK or MSK-64) is a macroseismic intensity scale for the evaluation of the severity of ground excitation using observed effects in the area of occurence. MSK intensity broadly associates the different seismic zones 2, 3, 4, 5 respective with seismic zones VI (or less), VII, VIII, and IX (and above) corresponding to Mazimum Considered Earthquake (MCE). Each of these zones indicates the effects of an earthquake at a particular place from observations on the affected areas. This can also be described using scales like Modified Mercalli or Medvedev-Sponheue-Karnik scale.

ZONES	DEFINITION
5	VERY HIGH RISK ZONE
	Covers the areas with the highest risk zones that suffer earthquakes of intensity MSK IX or greater.
	Structural designers use this factor for earthquake resistant design of structures.
	The areas having trap rock or basaltic rock are prone to earthquakes of this zone.
4	HIGH RISK ZONE / HIGH DAMAGE RISK ZONE
	Covers areas liable to MSK VIII.
3	MODERATE RISK ZONE / MODERATE DAMAGE RISK ZONE
	Liable to MSK VII
2	LOW RISK ZONE / LOW DAMAGE RISK ZONE
	This region is liable to MSK VI (or less)