What Causes Earthquakes: Information about Faults, Plate Tectonics and Earth Structure
Q: What is an earthquake and what causes them to happen?
Ans: An earthquake is caused by a sudden slip on a fault. Stresses in the earth's outer layer push the sides of the fault together. Stress builds up and the rocks slips suddenly, releasing energy in waves that travel through the earth's crust and cause the shaking that we feel during an earthquake. An EQ occurs when plates grind and scrape against each other. In California there are two plates the Pacific Plate and the North American Plate. The Pacific Plate consists of most of the Pacific Ocean floor and the California Coast line. The North American Plate comprises most the North American Continent and parts of the Atlantic Ocean floor. These primary boundary between these two plates is the San Andreas Fault. The San Andreas Fault is more than 650 miles long and extends to depths of at least 10 miles. Many other smaller faults like the Hayward (Northern California) and the San Jacinto (Southern California) branch from and join the San Andreas Fault Zone. The Pacific Plate grinds northwestward past the North American Plate at a rate of about two inches per year. Parts of the San Andreas Fault system adapt to this movement by constant "creep" resulting in many tiny shocks and a few moderate earth tremors. In other areas where creep is NOT constant, strain can build up for hundreds of years, producing great EQs when it finally releases.
Q: Can we cause earthquakes? Is there any way to prevent earthquakes?
Ans: Earthquakes induced by human activity have been documented in a few locations in the United States, Japan, and Canada. The cause was injection of fluids into deep wells for waste disposal and secondary recovery of oil, and the use of reservoirs for water supplies. Most of these earthquakes were minor. The largest and most widely known resulted from fluid injection at the Rocky Mountain Arsenal near Denver, Colorado. In 1967, an earthquake of magnitude 5.5 followed a series of smaller earthquakes. Injection had been discontinued at the site in the previous year once the link between the fluid injection and the earlier series of earthquakes was established. (Nicholson, Craig and Wesson, R.L., 1990, Earthquake Hazard Associated with Deep Well Injection--A Report to the U.S. Environmental Protection Agency: U.S. Geological Survey Bulletin 1951, 74 p.) Other human activities, even nuclear detonations, have not been linked to earthquake activity. Energy from nuclear blasts dissipates quickly along the Earth's surface. Earthquakes are part of a global tectonic process that generally occurs well beyond the influence or control of humans. The focus (point of origin) of earthquakes is typically tens to hundreds of miles underground. The scale and force necessary to produce earthquakes are well beyond our daily lives. We cannot prevent earthquakes; however, we can significantly mitigate their effects by identifying hazards, building safer structures, and providing education on earthquake safety.
Q: What do we know about the interior of the Earth?
Five billion years ago the Earth was formed by a massive conglomeration of space materials. The heat energy released by this event melted the entire planet, and it is still cooling off today. Denser materials like iron (Fe) sank into the core of the Earth, while lighter silicates (Si), other oxygen (O) compounds, and water rose near the surface. The earth is divided into four main layers: the inner core, outer core, mantle, and crust. The core is composed mostly of iron (Fe) and is so hot that the outer core is molten, with about 10% sulfur (S). The inner core is under such extreme pressure that it remains solid. Most of the Earth's mass is in the mantle, which is composed of iron (Fe), magnesium (Mg), aluminum (Al), silicon (Si), and oxygen (O) silicate compounds. At over 1000 degrees C, the mantle is solid but can deform slowly in a plastic manner. The crust is much thinner than any of the other layers, and is composed of the least dense calcium (Ca) and sodium (Na) aluminum-silicate minerals. Being relatively cold, the crust is rocky and brittle, so it can fracture in earthquakes. (Univ. of Nevada).
Q: What are plate tectonics?
Ans: Plate tectonics is the continual slow movement of the tectonic plates, the outermost part of the earth. This motion is what causes earthquakes and volcanoes and has created most of the spectacular scenery around the world.
Q: What is a fault and what are the different types?
Ans: A fault is a fracture or zone of fractures between two blocks of rock. Faults allow the blocks to move relative to each other. This movement may occur rapidly, in the form of an earthquake - or may occur slowly, in the form of creep. Faults may range in length from a few millimeters to thousands of kilometers. Most faults produce repeated displacements over geologic time. During an earthquake, the rock on one side of the fault suddenly slips with respect to the other. The fault surface can be horizontal or vertical or some arbitrary angle in between.
Earth scientists use the angle of the fault with respect to the surface (known as the dip) and the direction of slip along the fault to classify faults. Faults which move along the direction of the dip plane are dip-slip faults and described as either normal or reverse, depending on their motion. Faults that move horizontally are known as strike-slip faults and are classified as either right-lateral or left-lateral. Faults, which show both dip-slip and strike-slip motion are known as oblique-slip faults.
The following definitions are adapted from The Earth by Press and Siever.
Normal fault- a dip-slip fault in which the block above the fault has moved downward relative to the block below. This type of faulting occurs in response to extension and is often observed in the Western United States Basin and Range Province and along oceanic ridge systems.
Thrust fault- a dip-slip fault in which the upper block, above the fault plane, moves up and over the lower block. This type of faulting is common in areas of compression, such as regions where one plate is being sub ducted under another as in Japan. When the dip angle is shallow, a reverse fault is often described as a thrust fault.
Strike-slip fault - a fault on which the two blocks slide past one another. The San Andreas Fault is an example of a right lateral fault.
A left-lateral strike-slip fault is one on which the displacement of the far block is to the left when viewed from either side.
A right-lateral strike-slip fault is one on which the displacement of the far block is to the right when viewed from either side.
Q: At what depth do earthquakes occur?
Ans: Earthquakes occur in the crust or upper mantle, which ranges from the earth's surface to about 800 kilometers deep (about 500 miles).
Q: What is "surface rupture" in an earthquake?
Ans: Surface rupture occurs when movement on a fault deep within the earth breaks through to the surface. NOT ALL earthquakes result in surface rupture.
Q: What is the relationship between faults and earthquakes? What happens to a fault when an earthquake occurs?
Ans: Earthquakes occur on faults - strike-slip earthquakes occur on strike-slip faults, normal earthquakes occur on normal faults, and thrust earthquakes occur on thrust or reverse faults. When an earthquake occurs on one of these faults, the rock on one side of the fault slips with respect to the other. The fault surface can be vertical, horizontal, or at some angle to the surface of the earth. The slip direction can also be at any angle.
Q: How do we know a fault exists?
- if the EQ left surface evidence, such as surface ruptures or fault scarps (cliffs made by EQs).
- if a large EQ has broken the fault since we began instrumental recordings in 1932.
- if the faults produces small EQs that we can record with the denser seismographic network established in the 1970s.
Q: Where can I go to see the/a fault?
Ans: The closest fault depends on where you live. Some earthquakes produce spectacular fault scarps, and others are completely buried beneath the surface. Sometimes you may not even know that you are looking at a fault scarp.
Q: What does an earthquake feel like?
Ans: Generally, during an earthquake you first will feel a swaying or small jerking motion, then a slight pause, followed by a more intense rolling or jerking motion. The duration of the shaking you feel depends on the earthquake's magnitude, your distance from the epicenter, and the geology of the ground under your feet. Shaking at a site with soft sediments, for example, can last 3 times as long as shaking at a stable bedrock site such as one composed of granite. If the site is in a building, then the height of the building and type of material it is constructed from are also factors. For minor earthquakes, ground shaking usually lasts only a few seconds. Strong shaking from a major earthquake usually lasts less than one minute. For example, shaking in the 1989 magnitude 7.1 Loma Prieta (San Francisco) earthquake lasted 15 seconds; for the 1906 magnitude 8.3 San Francisco earthquake it lasted about 40 seconds. Shaking for the 1964 magnitude 9.2 Alaska earthquakes, however, lasted three minutes.
Q: Foreshocks, aftershocks - what is the difference?
Ans: "Foreshock" and "aftershock" are relative terms. Foreshocks are earthquakes, which precede larger earthquakes in the same location. Aftershocks are smaller earthquakes, which occur in the same general area during the days to years following a larger event or "mainshock", defined as within 1-2 fault lengths away and during the period of time before the background seismicity level has resumed. As a general rule, aftershocks represent minor readjustments along the portion of a fault that slipped at the time of the main shock. The frequency of these aftershocks decreases with time. Historically, deep earthquakes (>30km) are much less likely to be followed by aftershocks than shallow earthquakes. (Univ. of Washington).
Q: Two earthquakes occurred on the same day. Are they related?
Ans: Often, people wonder if an earthquake in Alaska may have triggered an earthquake in California; or if an earthquake in Chile is related to an earthquake that occurred a week later in Mexico. Over these distances, the answer is no. Even the Earth's rocky crust is not rigid enough to transfer stress fields efficiently over thousands of miles.