In earthquake science, the epicenter (pronounced /ˈɛpɪˌsɛntər/), also called the epicentre or epicentrum, is the location on Earth's surface directly above the hypocenter or focus. The hypocenter or focus is the point underground where an earthquake or an underground explosion begins.

Determination

The main job of a seismometer is to find the starting point of an earthquake, called the epicenter. Another job is to measure the earthquake's size, or magnitude, but this can only be done after the exact location is known.

The first seismographs were made to show the direction of the first earthquake wave. A Chinese seismograph, called the "frog" model, would drop a ball in the compass direction of the earthquake if a strong wave was detected. Today, scientists know that the first wave can come from almost any direction, depending on how the earthquake starts (called the focal mechanism).

A major improvement in finding earthquake locations was the use of a time scale. Instead of just showing the movement of a pendulum, early seismographs recorded the pendulum's movement on a moving paper, controlled by a clock. This created the first seismogram, which allowed scientists to measure the exact time the first ground movement happened and track how the ground moved afterward.

From the first seismograms, scientists noticed the trace had two main parts. The first wave to arrive was the P wave, followed quickly by the S wave. By knowing how fast these waves travel, scientists could easily calculate how far away the earthquake was.

One seismograph could show the distance to the earthquake, but this would only give a circle of possible locations. Two seismographs would create two circles, showing two possible locations. Only with a third seismograph could scientists find the exact location.

Today, finding an earthquake's location still needs at least three seismometers. Often, many are used together in a group called a seismic array. Scientists focus on accuracy because knowing the location of small earthquakes within a few kilometers helps study how faults work and predict seismic risks. Computer programs use a process that repeats steps to improve accuracy, like trying different answers and adjusting them. A detailed model of how fast waves move through the ground, based on local geology, is also needed. For P waves, the relationship between their speed and the density of the ground has been measured and recorded in a rule called Gardner's relation.

Surface damage

Before modern tools were used to study earthquakes, people believed the epicenter was the place where the most damage happened. However, the underground fault break can be long, causing damage across the entire area where the break occurred. For example, during the magnitude 7.9 Denali earthquake in Alaska in 2002, the epicenter was at the western end of the break, but the most severe damage happened about 330 km (210 mi) away at the eastern end. Earthquakes that occur in continental crust usually happen at depths between 2 and 20 kilometers (1.2 to 12.4 mi). Earthquakes deeper than 20 km (12 mi) in continental areas are uncommon, but earthquakes in subduction zones can occur at depths greater than 600 km (370 mi).

Epicentral distance

During an earthquake, seismic waves travel in all directions from the earthquake's starting point, called the hypocenter. Seismic shadowing happens on the side of Earth opposite the earthquake's epicenter because Earth's liquid outer core bends the longitudinal or compressional waves (P waves) but soaks up the transverse or shear waves (S waves). Outside the seismic shadow zone, both types of waves can be detected. However, because P waves and S waves move at different speeds and take different paths through Earth, they arrive at different times. By measuring the time difference between the waves on a seismograph and using a travel-time graph that shows how long the waves take to travel, scientists can calculate the distance to the earthquake's epicenter. This distance is called the epicentral distance and is usually measured in degrees (°) and written as Δ (delta) in seismology. Láska's empirical rule helps estimate the epicentral distance for distances between 2,000 and 10,000 kilometers.

Once scientists calculate the distance from the epicenter using data from at least three seismographic stations, they can find the earthquake's location by using a method called trilateration.

Epicentral distance is also used to calculate seismic magnitudes, as developed by Richter and Gutenberg.

Fault rupture

The place where the fault starts to slip is called the focus of the earthquake. The break starts at the focus and spreads along the fault. The break stops when the pressure isn't enough to keep breaking the fault (because the rocks are stronger) or when it reaches softer materials. The size of an earthquake is connected to how much of the fault breaks. Most earthquakes are small, with the break not reaching the surface. However, large, damaging earthquakes often cause breaks on the surface. In large earthquakes, the break can be more than 100 km (62 mi) long. If the break starts at one end of the fault (with the epicenter near that end), the shaking is stronger in one direction along the fault.

Macroseismic epicenter

The macroseismic epicenter is the most accurate guess of where an earthquake's epicenter is located when no instruments are used. This estimate can be made by looking at how strong the shaking was, information about earlier or later quakes, understanding nearby fault lines, or comparing data from similar past earthquakes. For earthquakes that happened long ago and were not recorded by instruments, only a macroseismic epicenter can be determined.

Etymology

The word "epicenter" comes from the Neo-Latin noun "epicentrum," which is based on the ancient Greek adjective "epikentros," meaning "occupying a cardinal point, situated on a center." This adjective is made from "epi," meaning "on, upon, at," and "kentron," meaning "center." The term was created by Irish seismologist Robert Mallet.

The word is also used to describe "center of activity," such as in the phrase "Travel is restricted in the Chinese province thought to be the epicenter of the SARS outbreak." According to Garner's Modern American Usage, "epicenter" is sometimes used to mean "center." Garner also mentions a William Safire article in which Safire quotes a geophysicist who says the term's misuse may result from "spurious erudition on the part of writers combined with scientific illiteracy on the part of copy editors." Garner suggests these misuses might be "metaphorical descriptions of focal points of unstable and potentially destructive environments."