Earthquakes and Tsunamis

Plate tectonics, the theory that describes the movement of the 60-mile (100-kilometer)-thick lithospheric plates that make up the rigid outer skin of Earth, provides a way to understand the causes of many earthquakes.

Faults occur where the lithospheric plates grind against each other. As the plates continue to move—typically at the rate at which a fingernail grows—the rocks on either side of the fault bend to accommodate that movement. Stress builds up over time and, eventually, exceeds the rock strength, at which time the fault ruptures, the rocks snap back to their original shapes, and the stored energy is released as a shock wave that is the earthquake.

Watch the video above to learn how geologists are studying earthquake risk in Bangladesh, Earth’s most densely populated nation.

The San Francisco Earthquake

April 18, 1906

Aftermath of the San Francisco earthquake on April 18, 1906.
© Library of Congress

At 5:12 am, the Earth ruptured beneath the Pacific Ocean just off the young city’s shores. The Sun rose on a scene of total devastation.

What happened? 

• A stretch of the San Andreas Fault almost 500 kilometers (300 miles) long snapped upwards—more than 8 meters (25 feet) in some places.
• Some 28,000 San Francisco buildings were toppled or were ravaged by a fire that burned for three days.
• At least 3,000 people lost their lives.

Scientists in 1906 knew about the great gash in the California ground called the San Andreas Fault. But they understood little about why that ground sometimes shook so violently.

The “Great San Francisco Earthquake” changed all that. It jump-started the modern field of seismology, or the study of earthquakes. Scientific studies in the aftermath of the tragedy led to recognition that the San Andreas Fault was the main source of California’s numerous earthquakes, as well as to our modern understanding of earthquake genesis.

Striking without warning, earthquakes are the deadliest of natural disasters. Over the last 25 years, earthquakes have claimed the lives of more than half a million people and affected more than 120 million.

The vast majority of deaths were due to collapse of buildings. Although earthquake risks can be substantially reduced by strict building codes, requiring earthquake-proof construction, many earthquake-prone regions have not adopted such codes. Proper codes increase construction costs, which may be a particularly heavy burden in poor countries.

Tsunamis

Tsunamis are caused by sudden displacements of the seafloor, usually when a fault on the ocean floor ruptures to produce a large earthquake. As the seafloor is displaced, so is the water above it, creating waves that travel across the open ocean at the speed of a jet plane. As they approach, the waves slow and increase in height.

The Indian Ocean Tsunami

December 26, 2004

People fleeing as a tsunami wave comes crashing ashore on December 26, 2004, at Koh Raya, part of Thailand’s territory in the Andaman Islands. The photographer who took this picture escaped without injury but retreated at the first wave and watched as a second wave tore apart the wooden buildings, with a third and largest wave coming forward and “ripping apart the cement buildings like they were made of balsa wood.”
© John Russell/AFP/Getty Images

One of the most powerful earthquakes ever recorded unleashed a colossal tsunami—and no one saw it coming.

What happened?

• Just before 8 am, the seafloor off the coast of Sumatra snapped upward as much as 50 feet (15 meters), leaving a gash the length of California.
• The magnitude 9.3 earthquake continued for almost 10 minutes and caused the entire planet to vibrate as much as a centimeter (0.4 inch)—and launched a giant tsunami.
• The tsunami inundated coastlines of more than 11 countries around the Indian Ocean. More than 226,000 people perished, and more than 1 million were left homeless.

Tsunami warning systems are used throughout much of the Pacific Ocean, where tsunamis happen most frequently. But the Indian Ocean tragedy had no historic precedent, so warning systems, which could have saved many lives, were not in place.

The Samoan Tsunami

September 29, 2009

Two earthquakes off the coast of Samoa— one of magnitude 8.1 and, a few seconds later, another of magnitude 8.0— triggered a tsunami that arrived with only a few minutes’ warning.

What happened?

• Just after 7 am, a tsunami swept the Samoan island chain in the South Pacific.
• Wave heights reached about 15 feet (4.5 meters).
• The waves claimed 192 lives in Samoa, American Samoa, and neighboring Tonga.

A large wooden bowl like this one would have been used to prepare 'ava by a young Samoan woman, likely the daughter of a high chief.
©AMNH/Division of Anthropology #80.0/4760

Samoans feel that focus on family and community is at the core of their strength. Families have helped each other rebuild homes and lives and to cope with losing loved ones.

Ceremonial events help reinforce the connections and obligations that make Samoan society resilient. Passing a cup of the ceremonial drink called ‘ava around a group of leaders is one of those rituals—and is a living part of Samoan culture.