Texas Earthquakes - UTIG

EARTHQUAKE FAQ

WORLD WIDE EARTHQUAKES

Q: What was the biggest earthquake ever?

A: The biggest earthquake known in historical times occurred on 22 May 1960 in Chile, with a magnitude Mw of 9.5. It occurred because of a rupture in the Earth's crust that extended horizontally about 1000 km along the coast of South America; in some places this quake caused vertical uplifts as large as six meters. The second biggest earthquake known, with a magnitude of 9.2, occurred in Alaska on 27 March 1964; it had a rupture length of 800 km, and caused as much as 11 meters of uplift.

Theoretical calculations suggest that these earthquakes are just about as large as possible for planet Earth. Suppose a hypothetical earthquake ruptures the entire 100 km thickness of the rigid tectonic plates, and suppose this rupture extends along a fault with length 1000 km with an average slip of 10 meters. Calculations show that this hypothetical earthquake would have a magnitude of about 9.3. Larger earthquakes would have to rupture an even longer fault with an even greater average slip; apparently this just doesn't happen.

Q: What was the most deadly earthquake ever?

A: The 27 July 1976 earthquake in Tangshan, China (MS = 7.9) caused the greatest number of deaths of any recent quake known; the most reliable estimates are that about 242,000 lives were lost. Tangshan had a pre-quake population of about a million people; the main industry is mining. One reason the death toll was so high is because most of the structures in Tangshan were multiple-story, unreinforced brick and concrete buildings.

Estimates of loss of life from the worst earthquake disasters are very unreliable; understandably, when tens of thousands of people are dead and most buildings are in ruins, there are more important things to do than to count bodies. However, there have been about five quakes in the twentieth century where reports of lives lost exceeded 100,000. Most experts accept that the most deadly earthquake in historical times occurred in Shanxi Province, China in 1556; about 830,000 lives were lost.

Twentieth Century Earthquakes Where
Estimated Deaths Exceeded 100,000

Date	Location	Estimated Deaths
1908 December 28	Italy, Messina	58,000-110,000
1920 December 16	China, Gansu and Shanxi Provinces	100,000-200,000
1923 September 1	Japan, Tokyo, Yokohama	99,000-143,000
1927 May 22	China, Qinghai Province	200,000
1976 July 27	China, Tangshan	242,000

Source: Catalog of Significant Earthquakes 2150 B. C. - 1991 A. D., National Geophysical Data Center A, U. S. Dept. of Commerce, Boulder, Colorado, 1992.

Q: Why do some big earthquakes cause very little damage, while other relatively small earthquakes are very destructive?

A: The two principal factors affecting damage are: degree of isolation (how far is the epicenter from major cities?), and type of construction (are the buildings easily damaged?). For example, on 23 May 1989 a quake with magnitude Mw of 8.1 occurred near Macquarie Island, about 500 km south of New Zealand. Since the nearest city was about 500 km distant, it caused no damage whatsoever.

In contrast, relatively small but deadly earthquakes occur in regions of dense population where buildings are of unreinforced adobe, stone, or concrete; these structures fail easily and crush inhabitants. An example is the Managua, Nicaragua earthquake of 23 December 1972; it had a magnitude MS of only 6.2 but it killed about 6,000 people. An old seismology saying is, "Earthquakes don't kill people, buildings kill people." A large earthquake in the tropics might shake down every grass hut within 500 km. But, afterwards the people simply rethatch their homes and go on with their lives.

Q: How commonly do "great" earthquakes happen around the world?

A: Over the entire Earth, the number of earthquakes with magnitudes of 8 and greater is less than one each year. However, each year there are about 10 earthquakes of magnitude 7 or greater and 100 earthquakes of magnitude 6 or greater. No matter where they occur, these earthquakes are all powerful enough so that they are recorded by all or mostly all of the world's sensitive seismograph stations, such as UTIG's station HKT.

Each year there are probably also about 1,000 earthquakes of magnitude 5 or more, 10,000 of magnitude 4 or more, etc. However, these smaller earthquakes are only recorded by seismograph stations if they are situated sufficiently near the epicenter; thus, many such earthquakes which occur in remote areas on Earth are never recorded, located, or catalogued.

Q: Can earthquakes occur anywhere?

A: Earthquakes have occurred in all 50 of the United States. Even in so-called 'inactive' regions like the Gulf Coast of Texas most locations will experience small (M ~ 3.0) earthquakes every 100 years or so. For example, an earthquake with magnitude 3.8 occurred on 24 March 1997 near Alice, Texas, about 60 km west of Corpus Christi. This was the first earthquake known in this region in historic times.

Q: Can a volcano cause earthquakes?

A: Most earthquakes have nothing to do with volcanoes; however, numerous small earthquakes do occur in association with most volcanoes. In the Mount Saint Helens eruption of 1980, the main eruption began with an earthquake and an accompanying landslide. Presumably the movement of magma produced stresses responsible for both the earthquake and the main eruption.

Q: Can an earthquake cause a volcano?

A: Most earthquakes have nothing to do with volcanoes; however, small earthquakes often accompany volcanic activity. Scientists who try to forecast volcanic activity rely heavily on interpreting changes in the rate and character of small earthquakes. So, while earthquakes and volcanic eruptions are sometimes caused by the same natural phenomona, the earthquakes do not 'cause' the volcano.

Q: Why are earthquakes common around the Pacific rim?

A: Even though small earthquakes can occur almost anywhere, most earthquakes--large or small--take place at the edges of tectonic plates, which move past one another at rates of a few cm per year. Altogether the earth is covered by approximately 20 such plates; the largest of these plates lies beneath most of the Pacific Ocean; while two others share a boundary with western South America. Thus, almost the entire Pacific rim happens to lie along a plate boundary.

Q: Has human activity triggered any deadly earthquakes around the world?