The Science of Earthquakes, Tsunamis, and Seismic Waves
By Joan Whetzel
Violent natural events occurring beneath the Earth's crust (earthquakes), below the water's surface (tsunamis), or along the surface of the earth (volcanoes), often trigger shockwaves that spread outward from the epicenter of the event. These shockwaves occur either as tsunamis or seismic waves and, like the original event, can leave behind a overwhelming amount of destruction. The forces of earthquakes that are strong enough to collapse buildings and buckle roads and the massive flooding from tsunamis that can wipe out entire buildings and devastate the coastal communities of an entire nation cannot be reliably predicted. People living in the communities affected by these natural disasters must remain aware of the dangers, the indications of an impending event, and must be prepared for such events at all times.
What is an earthquake?
Earthquakes occur when two sections of the Earth's tectonic plates rub against each other or when one rides up over the other. This area where the two plates strike is called a fault or fault plane. The site where an earthquake begins (just below the earth's surface) is called the hypocenter and the epicenter is the spot just above the earth's surface where the earthquake begins. Occasionally foreshocks (small shockwaves) forewarn of an impending earthquake. Of course, scientists can't always determine if this shockwave is a foreshock or an actual until a larger earthquake (called the mainshock) occurs - or doesn't. Aftershocks will always follow a mainshock and, depending on the size of the mainshock, can continue for weeks or months, sometimes even years, after the main event.
What Causes Earthquakes?
The earth is constructed in 4 layers known as the inner and outer cores, the mantle and the crust. The Earth's surface, or skin, is made up of the top layer of the mantle and the crust. This skin is fractured into many pieces (tectonic plates) that sometimes shift and either rub each other the wrong way or strike each other. Since the edges are rough hewn, the sometimes get caught up on the edges of other tectonic plates, while the plates continue moving. At some point the edges that have gotten caught up suddenly release, causing the earthquake.
How are earthquakes recorded?
Specialized scientific equipment called seismographs make records (seismographs) of the earthquakes. The seismograph machine has a flat base, allowing it sit securely on the ground, and a suspended weight. During an earthquake, the seismograph shakes along with the ground while the hanging weight remains stationary because the spring or cord that the weight is attached to absorbs all of the motion. The seismograph then records the difference in position between the moving base and the stationary suspended weight.
Seismic Waves and Tsunamis
Seismic waves are the vibrations (shockwaves) brought on by an earthquake or an explosion. These shock waves produce a temporary deformation, known as an elastic change, in the Earth's rocky structures as the rumble past them.
Seismic waves move extremely fast (kilometers per second) depending on a variety factors, such as the composition of rock in the terrain that they move through. The earthquake's shock waves move outward from the epicenter like the rings in a pond when a rock has been tossed in. The first set of waves are called P-waves (primary waves which are powerful) which are followed by S-waves (the secondary waves, which are superficial). The seismograph records tracings of the P-waves and the S-waves. Scientists then measure the time between the two waves to determine how far the seismograph is from the epicenter of the quake and how quickly the seismic waves are moving.
Tsunamis (erroneously referred to as tidal waves) are a form of seismic wave that transfers its energy into the sea's wave motion. Disturbances underwater (earthquakes or volcanoes) spawn tsunamis. These huge waves can move along the bottom of the ocean at speeds of 100s of miles per hour in the open sea, that is until they arrive in the shallower depths near a shoreline. While far out at sea, the ocean floor is deep enough that the tsunami may not even break the surface. However, the closer the tsunami gets to shore, the higher the wave rises up above the surface, sometimes reaching heights of 100 feet (30.48 meters) or more. Tsunamis are sometimes composed of multiple wave, and the waves that follow the initial wave can be even more powerful than the initial wave.
The Risk for Earthquake and Tsunami Damage
Houses that are not attached to a foundation, mobile homes, and any structures built on top of landfills or on unstable and unpredictable ground (e.g. prone to rock slides or mud slides) are in danger of receiving the most damage. Even doorways are not safe in these buildings.
Safety and Disaster Preparedness
The American Red Cross recommends the making following preparations for those who live in earthquake prone areas:
- · A Disaster Kits with a 3-day supply of nonperishable food and water for everyone in your family.
- · A first aid kit.
- · Medications, both prescriptions and basic medications like aspirin or ibuprofen, over the counter allergy medicines, antacids, and vitamins.
- · Flashlights and batteries.
- · Clothing, at least a 3-day supply for everyone.
- · Blankets, pillows and moveable bedding or sleeping bags.
- · Cash and Identification.
- · Emergency contact information.
- · Medical records and immunization records.
Everyone should also have the following in place in case they are separated from their families or friends:
- · a communication plan.
- · a meeting place.
- · several escape routes.
- · plans for pets.
If stuck at home or in a building when an earthquake hits, know where the strongest, most solid parts of the building are located. Also it's a good idea to make sure that large furniture (bookshelves) are bolted to the wall and cabinets have latches that prevent the contents from falling out during a quake. The Red Cross also recommends people drop to the floor, seek cover, and hold on to something solid when caught indoors during a quake, and people caught outside during a quake should move clear of buildings, electrical wires, and anything else that might topple onto them (i.e. billboards, trees, freeway overpasses).
USGS. The Science of Earthquakes. and Tsunamis and Seismic Sea Waves.
Penn State University. Earthquake Seismology: Seismic Waves and Earth's Interior.
American Red Cross, Bay Area Chapter. Earthquake Safety. http://www.redcrossbayarea.org/general_calltoaction.asp?CTA=2&SN=5011&OP=7973&SUOP=8230&IDCapitulo=VA24T92924
NASA Science, Science News. Anticipating Earthquakes.