Five questions

Terry Tullis

“Earthquakes don’t kill people; buildings do.”

Powerful earthquakes: Are they linked?

Since January, a series of major earthquakes has struck around the world: Haiti, Chile, Taiwan, Turkey. Geologist Terry Tullis explains what is happening, and why.

By Richard C. Lewis  |  March 10, 2010  |  Email to a friend

Professor Emeritus of Geological Sciences Terry Tullis, an expert on earthquakes, has been busy of late answering questions for the news media in the wake of major tectonic events in Haiti (January 12), Chile (February 27), Taiwan (March 4), and Turkey. Although he formally retired from teaching in 2005, Tullis continues to pursue his research at Brown on the mechanics of earthquakes, using both laboratory experiments and computer modeling. He hopes to identify signals that may help predict the occurrence of major quakes.

Tullis recently spoke with Today at Brown about earthquakes and tsunamis.

The Chilean earthquake was one of the most powerful in recent history. Why was it so powerful, and why was the resulting tsunami in the Pacific less powerful than predicted?

A very large area of a major fault slipped about 30 feet where the Nazca Plate plunges under and rubs against the western edge of the South American plate. The portion of the plate boundary that slipped suddenly during this earthquake was about 400 miles long parallel to the coast and 125 miles wide perpendicular to the coast.

During the earthquake, the ocean floor was moved down in some areas and up in others. When the level of ocean water is disrupted over a large area, a tsunami occurs. This tsunami was about seven feet high in various locations in Chile, where it swept away entire towns. As the tsunami moved across the Pacific Ocean, the magnitude of the plate offset gradually decreased.

The local magnitude of a tsunami in a location such as Hawaii is bigger in places where the gradual slope of the ocean floor and the focusing effect of bays can enhance the effect. For example, during the 1960 magnitude-9.5 Chile earthquake, the largest ever recorded by seismology, the tsunami heights in Hilo Bay were 35 feet, compared to only three to 17 feet elsewhere on the Big Island. These high levels from the 1960 earthquake were partly what caused concern immediately following the latest quake. We can be thankful that the water heights were less than predicted.

Can we expect more aftershocks or even more earthquakes?

There will certainly be more aftershocks in the vicinity of the large part of the fault that slipped in South America. In the first day after the Chile earthquake there was an aftershock of magnitude 6.9, nearly as large as the Haiti earthquake. The aftershocks will go on for months.

Slip from this earthquake should cause an increase in stress on parts of the fault to the north and south of the original quake center. While there is a remote chance that another very large earthquake could be triggered, such “domino” cascades are rare. The magnitude 9.1 Sumatra earthquake of 2004 did trigger large earthquakes to the south, but they were smaller than the 2004 event. Whether they are termed aftershocks or separate earthquakes is mostly a matter of definition.

Is the region where the Chilean quake occurred a major fault area?

The Nazca plate “dives” under the South American plate in the direction of the blue teeth. In red: Feb. 27 fault slip. (USGS): The Nazca plate “dives” under the South American plate in the direction of the blue teeth. In red: Feb. 27 fault slip. (USGS) The area where the Nazca plate is being thrust, or subducted, under the South America plate is part of the “ring of fire” that surrounds much of the Pacific Ocean. It gets its name from the volcanoes that form when the subducted material melts and then rises to the surface.

The 10 largest known earthquakes, including the recent Chile quake, are all in such subduction zones. Of those 10, five were along the west coast of South America, two were
in the Alaska subduction zone, one was along the Russian Kamchatka peninsula, one was in Sumatra, and one was off the Washington and Oregon coast.

The last is the only one known from historic records rather than from direct measurements with seismometers. It occurred on January 26, 1700, and was originally recognized by dating the rings of earthquake-downed trees off the coast of the U.S. Pacific Northwest. Later it was dated precisely by studying historic records in Japan that mentioned the resulting tsunami.

Why has there been less destruction and fewer lives lost from the Chilean earthquake than from the quake that struck Haiti?

This difference is due to several factors. The geological ones are that the Chile earthquake was deeper – about 20 miles compared to 6 miles deep in Haiti – and farther from population centers. Although the total amount of energy was about 500 times larger for the Chile earthquake, the energy was spread over a much larger area, so the intensity of shaking, especially in populated areas, was actually less for the Chile earthquake.

The human factors included the close proximity of Port-au-Prince to the slip in the Haiti earthquake and the much higher quality of building construction in Chile. It is often stated that “earthquakes don’t kill people, buildings do.” Because of the relatively frequent occurrence of large earthquakes in Chile, the standards for building codes are quite good. In Haiti, the poor standard of living and the relative infrequency of significant earthquakes means that building codes range from poor to nonexistent.

As the world’s population grows, more and more people are affected by natural hazards. The impact of earthquakes on the built environment and on human loss of life will probably increase. We are learning how to make more earthquake-resistant structures, but a large part of the world’s population live in zones with low standards of living and construction quality.

Is the timing of three major earthquakes in roughly eight weeks coincidence?

There are many magnitude 7 earthquakes around the world every year. The Haiti earthquake did not cause those in Chile, Taiwan, or Turkey, nor are they evidence for an increasing level of seismic activity from some other cause. Any given 10-year period over the past 100 years would show a generally similar pattern of earthquakes in terms of their number, size, and location