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Earthquake Facts

1. There is no such thing as "earthquake weather" . Statistically, there is an equal distribution of earthquakes in cold weather, hot weather, rainy weather, etc. Furthermore, there is no physical way that the weather could affect the forces several miles beneath the surface of the earth. The changes in barometric pressure in the atmosphere are very small compared to the forces in the crust, and the effect of the barometric pressure does not reach beneath the soil.

2. From 1975-1995 there were only four states that did not have any earthquakes . They were: Florida, Iowa, North Dakota, and Wisconsin.

3. The core of the earth was the first internal structural element to be identified. In 1906 R.D. Oldham discovered it from his studies of earthquake records. The inner core is solid, and the outer core is liquid and so does not transmit the shear wave energy released during an earthquake.

4. The swimming pool at the University of Arizona in Tucson lost water from sloshing (seiche) caused by the 1985 M8.1 Michoacan, Mexico earthquake 2000 km (1240 miles) away.

5. Earthquakes occur in the central portion of the United States too! Some very powerful earthquakes occurred along the New Madrid fault in the Mississippi Valley in 1811-1812 . The effects of shaking from these magnitude 8+ earthquakes caused church bells to ring in Boston, Massachusetts, nearly 1600 km (1000 miles) away.

6. Most earthquakes occur at depths of less than 80 km (50 miles) from the Earth's surface.

7. The San Andreas fault is NOT a single, continuous fault , but rather is actually a fault zone made up of many segments. Movement may occur along any of the many fault segments along the zone at any time. The San Andreas fault system is more that 1300 km (800 miles) long, and in some spots is as much as 16 km (10 miles) deep.

8. The world's deadliest recorded earthquake occurred in 1556 in central China. It struck a region where most people lived in caves carved from soft rock. These dwellings collapsed during the earthquake, killing an estimated 830,000 people. In 1976 another deadly earthquake struck in Tangshan, China, where more than 250,000 people were killed.

9. Florida and North Dakota have the smallest number of earthquakes in the United States.

10. The deepest earthquakes typically occur at plate boundaries where the Earth's crust is being subducted into the Earth's mantle. These occur as deep as 750 km (400 miles) below the surface.

11. Alaska is the most earthquake-prone state and one of the most seismically active regions in the world. Alaska experiences a magnitude 7 earthquake almost every year, and a magnitude 8 or greater earthquake on average every 14 years.

12. The majority of the earthquakes and volcanic eruptions occur along plate boundaries such as the boundary between the Pacific Plate and the North American plate. One of the most active plate boundaries where earthquakes and eruptions are frequent, for example, is around the massive Pacific Plate commonly referred to as the Pacific Ring of Fire.

13. The earliest recorded evidence of an earthquake has been traced back to 1831 BC in the Shandong province of China, but there is a fairly complete record starting in 780 BC during the Zhou Dynasty in China.

14. It was recognized as early as 350 BC by the Greek scientist Aristotle that soft ground shakes more than hard rock in an earthquake.

15. The cause of earthquakes was stated correctly in 1760 by British engineer John Michell, one of the first fathers of seismology, in a memoir where he wrote that earthquakes and the waves of energy that they make are caused by "shifting masses of rock miles below the surface".

16. In 1663 the European settlers experienced their first earthquake in America .

17. Human beings can detect sounds in the frequency range 20-10,000 Hertz. If a P wave refracts out of the rock surface into the air, and it has a frequency in the audible range, it will be heard as a rumble. Most earthquake waves have a frequency of less than 20 Hz , so the waves themselves are usually not heard. Most of the rumbling noise heard during an earthquake is the building and its contents moving.

18. When the Chilean earthquake occurred in 1960, seismographs recorded seismic waves that traveled all around the Earth . These seismic waves shook the entire earth for many days! This phenomenon is called the free oscillation of the Earth.

19. The San Andreas Fault was named in 1895 by geologist A.C. Lawson. He named it after the San Andreas Lake, a sag pond through which the fault passes about 20 miles south of San Francisco. He likely did not realize at the time that the fault ran almost the entire length of California!

Source: USGS