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Long-term Effects

Long-term Effects

Rising and Receding Land

How thrust-fault tsunamis occur.

Large, mega-thrust earthquakes at subduction zones can produce long-term changes in the land. The overriding plate was bent like a bow before the quake, and now the land near the fault springs up while the land behind it drops down. This effect can be large, and can make coasts even more vulnerable to tsunamis when land subsides.

Plate uplift and downdrop around the epicenter of the Indian Ocean Tsunami

In this image from the Indian Ocean Tsunami, you can see how the plate rose near the boundary and sank further behind it, close to shore.

Aerial view of Valdez following the 1964 tsunami.

The 1964 Alaska earthquake produced similar effects. This earthquake struck the coast of Alaska near Valdez and forced the entire city to relocate.

Alaska Earthquake March 27, 1964. Hinchinbrook Coast Guard dock, raised above all but the highest tides by regional uplift in Prince William Sound. Land in this area rose about 8 feet during the earthquake.

In some places land permanently rose up to 15 m (49 ft), and in other places it sank up to two (seven ft). Shipping lanes were altered. Nautical charts of Prince William Sound became inaccurate as depth soundings no longer matched the maps.

In addition to the effects of uplift and subsidence, sandbars in harbors can also shift—blocking the entrance—or disappear entirely.

Coastal environments

Alaska Earthquake March 27, 1964. Uplifted sea floor at Cape Cleare on Montague Island in Prince William Sound in the area of the greatest recorded tectonic uplift on land (33 feet). The very gently slopping flat rocky surface with the white coating which lies between the cliffs and the water is about a quarter of a mile wide. The white coating consists of the remains of calcareous marine organisms that were killed by desiccation when the wave-cut surface was lifted above high tide during the earthquake.

People aren’t the only ones affected by uplift or subsidence. Where this happens, coastlines can advance or retreat by several hundred meters, greatly altering coastal environments. In the great Alaska quake, a wetland near Cordova was lifted up, draining it. The clam bed went dry. Migratory birds could no longer stop there, and the plant communities changed.

Alaska Earthquake March 27, 1964. These spruce trees on a gravel spill on Resurrection Bay on the Kenai Peninsula are in an area which tectonically subsided 3 feet during the earthquake. The subsidence dropped the shallow roots of these trees below high tide, where they were killed by repeated inundation in salt water.

In other places, the land dropped and plunged conifers into saltwater, killing them and converting the land into wetland and ghost forest.

Alaska Earthquake March 27, 1964. The stumps in the foreground are part of an ancient forest on Latouche Island in Prince William Sound that was submerged below sea level and buried in prehistoric times. Tectonic uplift of 9 feet during the earthquake raised these stumps above sea level once again, demonstrating that the area is tectonically restless.

Soil in these places often reveals a centuries-long alternating pattern of sand and dirt—and sometimes ancient buried tree stumps—that tells a story of repeated earthquake-induced elevation change.

In other parts of the world, different ecosystems suffer similar effects.

Exposure of the reef around Ranongga Island in the Solomon Islands after a 2007 earthquake lifted the land.

In the tropics, salt water-dwelling coastal mangrove forests and coral are often the organisms that suffer from subsidence and uplift. You can see that here on the coast of Ranongga Island in the Solomon Islands after a magnitude 8.1 earthquake raised the coast in 2007. In these cases new land forms on the old reef. At one place in Banda Aceh, Indonesia, tangible evidence of coastal subsidence was experienced by villagers when the holes of mangrove mud lobsters suddenly appeared in their vegetable gardens.

The landslide scar caused by the rockfall that started the Lituya Bay tsunami in Alaska.

Massive waves may also shear off trees, creating a distinctive “trimline” that takes years to reforest, like this one in Lituya Bay, or this one in Sumatra after the Indian Ocean tsunami.

A trimline -- or deforested area -- around the coast of Sumatra created by the 2004 Indian Ocean Tsunami.
Indian Ocean Tsunami Damage around the perimeter of the Andaman Islands.

On low-lying islands or flat coasts, vast swaths of forest may be wiped out. In the North Andaman Islands, the 2004 Indian Ocean tsunami destroyed 3,292.5 hectares (8,100 acres) of coastal forest.


Farm fields inundated by a tsunami and land subsidence after the 1960 Chilean earthquake

In cases where seawater sweeps over farm fields, the soil grows saltier.

Farm fields inundated by a tsunami and land subsidence after the 1960 Chilean earthquake

The damage to soil is twofold: direct salt water infiltration of the topsoil, and deposits of salty sand or clay from the sea. The longer stranded tsunami waters sit on the land, the worse the damage will be.

Tsunami deposits of sand over soil in Papua New Guinea after the 1998 tsunami.

After the Indian Ocean tsunami, 47,000 hectares (116,000 acres) of agricultural land was damaged. Plants were killed by sand or salt and soil fertility plunged. In most cases, abundant rain and irrigation quickly flushed away the salt. Of the 47,000 hectares of farmland damaged, 38,000 hectares were deemed recoverable by the U.N. Food and Agriculture Organization. The other 9,000 hectares were lost for agriculture.

 the Banda Aceh coast remains flooded six weeks after the tsunami of 2004

Farmland may also be permanently lost by land recession.

Groundwater Intrusion

Destroyed houses, a well, and laundry in Sri Lanka after the 2004 Indian Ocean Tsunami.

Tsunamis can also harm local aquifers by flushing them with saltwater. This happened widely in Asia after the 2004 Indian Ocean tsunami. Tsunami waves penetrated the aquifer at dug domestic wells or by infiltrating the permeable sands that usually make up coastal aquifers. In Sri Lanka, over 40,000 drinking water wells were either destroyed or contaminated in these ways.

Alaska Earthquake March 27, 1964. Large ground crack with vertical displacement on the Kenai Lowland. Thick deposits of sand border the left side of the crack. Photo by H.L. Foster, 1964.

Earthquakes that generate tsunamis may also crack the bedrock of the aquifer, making it even easier for seawater propelled by tsunamis to infiltrate them, as happened at Neill Island in the Indian Ocean after the 2004 tsunami. Groundwater recharge from monsoons or other rains may help dilute these problems with time, but freshening of wells can take months or years.

Summary Questions

Question 1

What are the chief long-term effects of tsunamigenic earthquakes? (Choose all that apply.)

The answers are A, B, C, and D. All of these changes can be experienced after major tsunamigenic earthquakes.

Question 2

Which of the following are long-term environmental effects of tsunamis? (Choose all that apply.)

The answers are A, B, and D. Subsidence does not alter the ocean temperatures of nearby currents.

Question 3

What are the ways tsunamis can be destructive to agriculture? (Choose all that apply.)

The answers are A, C, and D. Tsunamis do not strip soils of nutrients. Erosion can, however, carry soil away. Deposition of sediments and infiltration of saltwater by tsunamis are much more common.

Question 4

What are the ways tsunamis can damage aquifers? (Choose all that apply.)

The answers are B and C. Tsunamis cannot drain or pressurize aquifers.

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