Earthquake Facts & Figures

A guide to earthquakes, from fault lines to seismographs to damage estimations.

Sources: U.S. Geological Survey, National Earthquake Information Center, Federal Emergency Management Agency

What are earthquakes?
An earthquake is caused by a sudden slip on a fault and it occurs when plates grind and scrape against each other. Stresses in the earth's outer layer push the sides of the fault together. Stress builds up and the rocks slips suddenly, releasing energy in waves that travel through the rock to cause the shaking that we feel during an earthquake.

What is a fault?
A fault is a thin zone of crushed rock between two blocks of rock, and can be any length, from centimeters to thousands of kilometers. It is a fracture in the crust of the earth along which rocks on one side have moved relative to those on the other side. Most faults are the result of repeated displacements over a long period of time.

What are the basic types of earthquakes?
The two basic types:
A strike-slip earthquake occurs on an approximately vertical fault plane as the rock on one side of the fault slides horizontally past the other. In a dip-slip earthquake, the fault is at an angle to the surface of the earth and the movement of the rock is up or down.

Are earthquakes really on the increase?
Not necessarily. Earthquakes of magnitude 7.0 or greater have remained fairly constant throughout this century and according to records have actually seemed to decrease in recent years. In the last 20 years more earthquakes are noticed yearly because of the increase in the number of seismograph stations in the world and improved global communications. This increase has helped seismological centers to locate many small earthquakes which were undetected in earlier years.

What is the hypocenter?
The hypocenter is the point where the earthquake rupture begins, usually deep down on the fault.

What is the epicenter?
The epicenter is the point on the surface directly above the hypocenter.

How many earthquakes are reported yearly?
The National Earthquake Information Center now locates about 12,000 to 14,000 earthquakes worldwide each year, or 35 a day on average.

Earthquake frequency worldwide:

Description	Magnitude	Annual Average
> Great	8 or higher	1
> Major	7 - 7.9	18
> Strong	6 - 6.9	120
> Moderate	5 - 5.9	800
> Light	4 - 4.9	6,200
> Minor	3 - 3.9	49,000
> Very Minor	1 - 3	9,000 (daily)

What is the Richter Scale?
The Richter Scale is used to measure the magnitude of earthquakes, as determined by seismograph measurements of the height of ground oscillations during an earthquake.
Because the scale is based on a logarithm, every whole-number step in the scale represents about 31 times more energy than the amount represented by the preceding whole number value. The Richter scale has no upper limit; the largest known earthquakes have magnitudes in the 8.8 to 8.9 range.

What is the Modified Mercalli Scale?
The Modified Mercalli Scale measures the intensity of an earthquake; in other words, the effect of the earthquake on the Earth's surface. This scale is composed of 12 increasing levels of intensity that range from imperceptible shaking to catastrophic destruction, and is designated by Roman numerals. The scale does not have a mathematical basis; instead it is an arbitrary ranking based on observed effects.

How can I prepare for an earthquake?
Fasten shelves securely to walls. Place large or heavy objects on lower shelves. Store breakable items such as bottled foods, glass, and china in low, closed cabinets with latches. Hang heavy items such as pictures and mirrors away from beds, couches, and anywhere people sit. Brace overhead light fixtures. Repair defective electrical wiring and leaky gas connections. These are potential fire risks. Secure a water heater by strapping it to the wall studs and bolting it to the floor. Repair any deep cracks in ceilings or foundations. Get expert advice if there are signs of structural defects. Store weed killers, pesticides, and flammable products securely in closed cabinets with latches and on bottom shelves.

Damage of recent major earthquakes in the U.S.:
Northridge Earthquake, California (20 miles from Los Angeles) January 17, 1994 at 4:31 a.m. Magnitude: 6.7 Deaths: 57 Injuries: 9,000 Property Damage: $15 billion
Loma Prieta Earthquake (south of San Francisco) October 17, 1989 at 5:04 p.m.
Magnitude: 7.1 Deaths: 62 Injuries: 3,757 Property Damage: More than $6 billion