E ARTHQUAKES “Whole lotta shakin’ goin’ on” – Jerry Lee Lewis

W HAT IS AN EARTHQUAKE ? An earthquake is a vibration of the Earth produced by the rapid release of energy. This energy release is the result of slippage along a fault.

T YPES OF FAULTS

S EISMIC WAVES With the energy release of an earthquake, waves of seismic energy radiate omnidirectionally and are recorded as lines on a seismograph. Waves that travel along the surface of the Earth are called surface waves. Waves that travel through the Earth’s interior are called body waves. Body waves are divided into primary (P) waves, which occur as “push-pull”, and secondary (S) waves, which occur in a shaking manner.

T HE “R ING OF F IRE ”

S AN A NDREAS F AULT Runs the length of California and occasionally slips, generating frequent earthquakes. People often mistake the motion of the fault, as if California will slip into the Pacific Ocean.

F INDING THE E PICENTER The epicenter is the location on the Earth’s surface directly above the earthquake focus. The difference in the velocity of the P and S waves is used to calculate the location. The key is the time differences between the waves and several locations are used to triangulate the epicenter.

M EASURING THE MAGNITUDE The Mercalli Intensity Scale was developed in 1902 to measure the intensity of an earthquake. Earthquake destructiveness can vary widely based on the depth of the focus, the nature of the soil and sub-soil, and distance from the epicenter. A second scale, the Richter Scale, was adopted in 1935 to better assess the magnitude. This scale uses the amplitude of the largest wave on a seismograph. The largest earthquakes ever recorded measured approximately 8.6 on the Richter scale.

T HE R ICHTER S CALE

M AJOR EARTHQUAKES THROUGH HISTORY San Francisco, CA – 1906 – estimated 7.0, $9.5 billion in damage (2012 dollars) Messina, Italy – 1928 – estimated 7.1, 123,000 dead, shortly after the earthquake, a 40 foot tsunami decimated the coastline. Anchorage, AK – 1964 – 9.2, 143 deaths. Tangshan, China – 1976 – 7.0, nearly 250,000 dead. San Francisco, CA – 1989 – 7.1, 62 dead, $6 billion in damage. Northridge, CA – 1994 – 6.7, 61 dead, $15 billion in damage. Kobe, Japan – 1995 – 6.9, 5,000 dead, damage exceeds $100 billion. Sumatra, Indonesia – 2004 – 9.2, nearly 300,000 dead from ensuing tsunami. Tohoku, Japan – 2011 – 9.0, 15,000 dead, tsunami caused nuclear disaster, over $50 billion in damage.

U SING EARTHQUAKES TO STUDY THE INNER E ARTH Scientists noted that P waves were detected nearly all over the Earth after an earthquake. The time difference between P waves in different locations lead scientists to infer that the Earth’s interior contained layers of differing densities. S waves were less commonly detected. Since S waves were known to be unable to be propagated through a liquid, it was surmised (correctly) that a portion of the Earth’s interior is liquid.

W AVES THROUGH THE E ARTH

T HE A STHENOSPHERE Located just beneath the Lithosphere, in the Earth’s crust, the Asthenosphere consists of hot, weak rock, that exhibits plasticity. It is this characteristic that allows the monstrous tectonic plates to “float” and move about.

C AN WE PREDICT EARTHQUAKES ? Some changes in the land (uplift or subsidence) has been observed prior to some earthquakes. Animals have been observed to behave in an erratic fashion prior to an earthquake. To date, no one has been able to predict the onset of an earthquake with any degree of accuracy. Actuarial scientists have determined the likelihood of future earthquakes along known faults, but this is expressed as a percent.