How Earthquakes Are Studied: Seismographs and the Science of Seismology

An earthquake is the shaking of the ground caused by a sudden release of energy within the Earth's crust, most often where masses of rock slip past one another along a fault. Scientists study earthquakes to understand why the ground moves, to map where shaking is likely, and to reduce the destruction and loss of life that earthquakes have inflicted throughout human history (read more: Which Earthquakes Changed the Face of the Earth). Catastrophic earthquake

What is an earthquake: definition and causes

An earthquake is a sudden movement of the ground produced when accumulated stress in the Earth's crust is released along a fracture called a fault. The rock on either side of the fault locks together under friction until the strain exceeds the rock's strength; the two blocks then snap into a new position, and the energy radiates outward as seismic waves. This mechanism, known as the elastic rebound theory, was formulated by Harry Fielding Reid after studying the 1906 San Francisco earthquake along the San Andreas Fault.

Most earthquakes occur at the boundaries of tectonic plates, the large rigid slabs that make up the Earth's outer shell. Where plates pull apart, collide, or slide past one another, stress builds along fault planes until it is released as a quake. A smaller share of earthquakes are triggered by volcanic activity, the collapse of underground cavities, or human activities such as mining and fluid injection.

A typical earthquake sequence has three phases: smaller foreshocks that may precede the main rupture, the mainshock that releases the bulk of the energy, and aftershocks that follow as the crust readjusts. Aftershocks can continue for days, months, or even years and sometimes cause additional damage to already weakened structures.

How earthquakes are studied: the basics of seismology

Seismology is the science of earthquakes and of the seismic waves that travel through the Earth. Seismologists record ground motion, locate earthquake sources, measure their size, and use the waves themselves as a probe of the planet's interior. The work demands instruments of absolute precision installed where conditions stay stable.

Definition and scope of seismology

Seismology studies the generation, propagation, and recording of seismic waves, along with the structure and dynamics of the Earth they reveal. The discipline ranges from monitoring hazardous faults and issuing early warnings to imaging the deep interior and supporting resource exploration. Geophysicists who specialize in seismology interpret recordings to determine an earthquake's location, depth, and magnitude, and to model how shaking will affect people and buildings.

Work in seismology requires not only highly accurate instruments but also their installation in a room where the temperature stays constant throughout the year and where stray vibrations cannot intrude. For this reason seismic stations are built in deep cellars, with the instruments themselves set on special, deeply anchored foundations isolated from local noise.

Earthquakes of greater or lesser strength happen constantly, but news confirming them does not always reach us, because crustal movements can occur in sparsely populated regions and on the ocean floor, which covers roughly three-quarters of the globe's surface (71%). By specialists' estimates, on average up to ten thousand earthquakes occur every year — in other words, the crust trembles every hour.

If one also counts the signals from the deep sea, the manifestation of the Earth's life — the beat of its "pulse," so to speak — would be even more frequent. Based on many years of observation at seismic stations, whose number in various countries runs into several hundred, scientists now conclude that the Earth's crust is never in a state of seismic rest. Faint movements of remarkable constancy — known as microseisms — are registered by seismic stations even when no earthquake is taking place. Aftermath of an earthquake

Key definitions in seismology

The disturbance of equilibrium in the crustal layers during an earthquake is accompanied by vibrations of varying strength. The region inside the crust where these changes originate is called the earthquake focus, or hypocenter (from the Greek "hypo" — beneath, hence "beneath the center," i.e. beneath the center of the earthquake).

The depth of the hypocenter varies, and the strength of the earthquake varies with it: the closer the hypocenter lies to the surface, the more violent the shaking and the more limited its area of effect. With a deep hypocenter the earthquake's intensity is lower, but its area of spread is enormous. By most definitions, hypocenters form in the crust at depths of up to 50 kilometers from the surface, less often down to 100 kilometers (the Carpathian earthquakes of 1802 and 1940 are examples), and only a few earthquakes occur at 300 kilometers or deeper.

The point on the surface directly above the hypocenter is called the epicenter (from the Greek "epi" — on, i.e. the area of the earthquake at the Earth's surface). Here the earthquake is felt most sharply. During strong earthquakes the epicenter experiences vertical jolts that throw objects upward. As distance from the epicenter increases, the shocks become more oblique and lateral, gradually losing their destructive force.

The shocks produced by an earthquake travel in every direction, both through the depths of the crust and along its surface. Their spread and force can be roughly compared with the ripples that appear on a smooth water surface when a stone is thrown in. A large stone sets the water particles into greater motion and produces larger waves; a small one, smaller waves. In the same way the strength of earthquakes depends on the power of the underground shocks. The motion of solid particles, of course, differs from that of liquid particles — their movement is constrained by neighboring particles, so they can only compress and change in volume.

Seismic waves and their characteristics

Seismic waves are the vibrations released by an earthquake, and they fall into two broad families: body waves, which travel through the Earth's interior, and surface waves, which travel along the surface. Both kinds arise at the same instant in the hypocenter and spread outward along radii in every direction. The way each wave moves, how fast it travels, and the materials it can pass through let seismologists reconstruct both the earthquake and the planet inside.

Longitudinal and transverse waves (P and S waves)

Body waves come in two types — longitudinal (P waves) and transverse (S waves) — distinguished by how the rock particles move relative to the direction of travel:

1. the first travels from below upward, that is, in the direction of the shock; this is the longitudinal path, and the waves are accordingly called longitudinal, or P waves (pressure waves), because the ground is alternately compressed and stretched;
2. the second travels in the transverse direction, perpendicular to the first, which is why these are called transverse, or S waves (shear waves), shaking the ground from side to side.

Longitudinal P waves spread fastest — from 8 to 10 kilometers per second — and are the first to be recorded by seismographs, which is the meaning of the "P" in primary. Transverse S waves are roughly half as fast — about 4 kilometers per second — and arrive second. A key property is that P waves pass through both solids and liquids, while S waves cannot travel through liquids at all; this difference is precisely what later revealed that the Earth has a liquid outer core.

Surface waves: Rayleigh and Love waves

Surface waves travel along the Earth's outer layer and, although slower, usually cause the most destruction. Their speed is lower still — about 3.5 kilometers per second. According to data from the Pulkovo seismic station, such waves circled the entire globe during the Messina earthquake (1908) in 3 hours, 8 minutes, and 51 seconds.

Two kinds of surface wave are recognized. Rayleigh waves roll the ground in an elliptical, up-and-down motion much like ocean swells; Love waves shake the surface horizontally, side to side. During especially strong earthquakes, gravity waves resembling ripples on water form in the epicentral area, particularly in loose rock. These travel at a very low speed — about 4 meters per second — yet their effect is especially destructive.

Neither the soil nor buildings can resist these waves. It is they that make trees bend and factory chimneys, cathedrals, and tall buildings sway. Often such waves are imprinted on the ground surface itself. Decoding — that is, reading — the seismograms of an earthquake demands great effort from the investigator to make sense of all the records together and to answer the question of how earthquakes are studied.

Instruments for studying earthquakes

Earthquakes are studied with special instruments called seismographs. These devices continuously make a precise record of every vibration of the Earth's crust on a special trace called a seismogram ("gramma" in Greek means "record"). The seismographs designed by Academician B. B. Golitsyn and other scientists once enjoyed worldwide fame. The first seismograph and its trace

History of seismoscope development

The earliest earthquake instruments were seismoscopes, devices that simply signaled that a shock had occurred and, ideally, the direction from which it came. The word comes from the Greek "skopeo" — I look. Over the centuries these simple indicators evolved into the recording seismographs and electronic seismometers used today, but the basic problem they all solve is the same: to detect ground motion too faint or too sudden for human senses.

Zhang Heng's seismograph and ancient instruments

The first known seismoscope was the "earthquake weathercock" built in 132 CE by the Chinese scholar Zhang Heng (also rendered Chang Heng). His bronze vessel held a heavy pendulum connected to eight dragon heads arranged around the rim, each holding a ball above the open mouth of a bronze toad. When the ground shook, the pendulum tripped a lever, and a ball dropped from the dragon facing the direction of the distant earthquake, revealing where the shock had come from long before any report could arrive on foot.

The seismograph of Tatev Monastery (Gavazan)

The shocks of earthquakes were detected by other means as well. Of great interest in this respect is the Gavazan — a swaying octagonal stone pillar about 8 meters high in the Tatev Monastery in Zangezur (in Armenia). This ancient monastery was founded in the 9th century. For a long time it was a center of Armenian culture and served as the seat of the head of the clergy.

The Gavazan stands in the monastery courtyard. In 1931 the monastery suffered greatly from an earthquake, and all of its structures were destroyed except the Gavazan itself. Tatev Monastery

For a long time people supposed that the swaying Gavazan pillar had a rounded base that fitted into a cup-shaped hollow. It was suggested that mercury was poured into this hollow to give greater sensitivity to this peculiar seismoscope — an instrument that allowed earthquake shocks to be noted.

The Gavazan is topped by a relief ornament in the form of an openwork cross set in a square stone frame. Hence its second name, khachkar — a memorial stone. By the middle of the 20th century the pillar stood motionless, evidently jammed by rubble that had fallen into its base, although not long before it would tilt easily at the touch of a hand.

Architects dismantled the Gavazan, which helped establish that it was mounted on a hinge. A precise calculation of its center of gravity kept it upright, while the hinge allowed it to sway and return to the vertical. The amplitude of its swing could reach 20 centimeters. At present, in order to preserve the ancient seismograph, scientists have locked the swaying mechanism, drawn the pillar together with metal hoops, and fastened it with bolts, so that it now stands still.

Modern seismographs

Modern seismographs are sophisticated electronic devices. A modern seismograph

A modern seismometer typically uses three components — two horizontal sensors and one vertical — so it can capture ground motion in all directions at once. The signal is digitized and stored as a seismogram that can be analyzed by computer, transmitted in real time, and compared with records from distant stations. Dense arrays of such instruments form the seismic networks that monitor the planet continuously.

How seismic stations work

A seismic station records ground motion at one fixed point and feeds that data into wider monitoring networks. Because the instruments must catch motions far smaller than a footstep, stations are placed away from traffic and on solid foundations, and their clocks are synchronized so the exact arrival time of each wave can be measured. Reading these records — interpreting the seismogram — is what allows seismologists to reconstruct an event they never witnessed directly.

To determine the area affected by an earthquake, observers mark on a map the time each shock was felt at each station and its strength. Joining points with the same arrival time gives lines called homoseists (from the Greek "homos" — the same), while points of equal shaking strength give isoseists (from "isos" — equal in measure or force). These isoseists are drawn into macroseismic maps showing how intensity falls off with distance from the source.

The area of greatest destruction is called the pleistoseismal region (which in Greek means "the most shaken"). Its outline takes various forms — a more or less regular circle, an oval, or a band. During the Verny earthquake (1887) the pleistoseismal area covered 5,476 sq. km, while the total area over which the earthquake was felt reached 1,478,570 sq. km; the area of the second Crimean earthquake exceeded 1 million sq. km.

The duration of any single shock is very brief, in rare cases lasting a minute. At the start of an earthquake the shocks are usually frequent, up to three per second, then become rarer and rarer. The span over which shocks recur in a given area is called the earthquake's period, and it varies greatly — from a few minutes to several years; in the Verny earthquake it lasted about three years. The Phocis earthquake of 1870 in Greece, unmatched in force, continued for three years and produced up to 750,000 shocks (300 of them accompanied by terrible destruction).

Earthquakes may recur repeatedly in the same district. Accordingly, seismic regions are distinguished where earthquakes are frequent — for example the Pacific coast (the Japanese, Kuril, Aleutian, and Philippine islands, the Andes, and among other mountain regions the Apennines, Carpathians, Caucasus, Tien Shan, and Himalayas) and the countries of Latin America. Earthquakes are often a consequence of the movement of the crust's tectonic plates. Map of the Earth's seismic regions

Aseismic regions, where earthquakes are rarer or entirely absent, include the Russian Plain, the West Siberian Lowland, and others. The first earthquakes are noted in the chronicles at Kiev in 1107, 1122, 1196, and 1211. An especially strong impression was made on contemporaries by the "shaking of the earth" at Vladimir, Kiev, and Pereyaslavl in 1230. In Moscow, noticeable oscillations were recorded several times: in 1445, as the chronicle says:

At the sixth hour of the night the city of Moscow shook, the Kremlin and the whole posad, and the church trembled; many people who were not asleep heard it and were in great distress, despairing of their lives.

Tremors were also noted in 1802 and 1940. More minor oscillations were local in character and were caused by the collapse of the roofs of limestone cavities. Old whitestone Moscow rests mainly, at a depth of 10–30 meters, on Carboniferous limestones in which significant voids occur, produced by the eroding action of water (read more: The Rocks That Make Up the Earth's Crust). When the metro was built, such voids were indeed discovered in many places.

Determining earthquake distance and magnitude

The distance to an earthquake is found from the time gap between the fast P waves and the slower S waves: the bigger the gap, the farther away the source. Since P waves travel at about 8–10 km/s and S waves at about 4 km/s, a single station can estimate how far away the quake was, but not in which direction. To pin down the epicenter, seismologists use triangulation, combining readings from at least three stations.

Triangulation works by drawing a circle around each station with a radius equal to its calculated distance to the earthquake. The single point where three or more such circles intersect marks the epicenter. This use of seismic wave arrival times from multiple stations is the standard method for locating earthquakes worldwide, and modern networks perform it automatically within seconds.

Earthquake size is described in two distinct ways — intensity and magnitude. Intensity measures how strongly the ground shaking is felt and how much damage it causes at a given place, and it decreases with distance from the source. Magnitude measures the total energy released at the source and has a single value for each earthquake.

The world uses a large number of seismic scales. Intensity is reported on graded scales of observed effects, while magnitude is reported on the Richter scale, developed in 1935 by Charles Richter at Caltech, and, for large events, on the modern moment magnitude scale, which better captures the energy of the greatest earthquakes:

• Originally the strength of earthquakes was judged on the special Rossi–Forel scale. In 1883 these scientists, Rossi and Forel, arranged in order the characteristic signs observed during earthquakes and divided them into ten numbers, or points; this scale is now used in Latin American countries.
• In Russia the most widely used scale in the world is applied, the 12-point MSK-64 earthquake scale (Medvedev–Sponheuer–Karnik), based on the Mercalli–Cancani scale of 1902.
• In Europe the European Macroseismic Scale (EMS-98) is the modern standard for intensity, also using 12 degrees.
• In Japan a 7-point earthquake scale is used.

Let us look at the content of a 12-degree intensity scale, degree by degree:

1. The vibrations are not felt directly by people and are detected only by special instruments.
2. Very weak tremors, noticed only by individual people at rest.
3. Very faint tremors, noticed by most people only inside a building and often confused with the shaking of a building caused by a passing truck.
4. Weak ground motion, felt by people in movement and at work. Rattling of windowpanes and crockery, creaking of doors and floors, cracking of ceilings.
5. A fairly strong earthquake, felt by everyone. Buildings shake. Beds, chairs, and other furniture move. Tree branches sway as in a light wind. Curtains, chandeliers, and pictures swing. Doors and windows open and close. Sleepers wake.
6. A strong earthquake, perceptible shocks. Pictures fall from walls and books from shelves. Furniture moves. Pendulum clocks stop. Plaster begins to crumble. All sleepers awake.
7. A very strong earthquake. Strong shocks. Items of furniture are overturned. Large bells ring. Pieces of plaster and stucco break off. Chimneys are damaged. Landslides appear; well levels change.
8. A destructive earthquake, very strong shocks. Cracks form in the walls of buildings. Chimneys are destroyed. Factory chimneys fall. Trees sway violently and even break. Many feel "seasick."
9. A devastating earthquake. Destruction of individual parts of buildings or of whole structures. Cracks appear in the ground.
10. Extraordinarily strong shocks. General destruction. Cracks form in the earth; faults, collapses, landslides. Railway rails bend. Water, sewer, gas, and electrical mains rupture. Cracks and wavy bulges appear on roads.
11. A catastrophic earthquake. Wide cracks form, with numerous landslides and collapses. Complete destruction of masonry structures and bridge piers; destruction of embankments, dams, and other structures.
12. A great catastrophic earthquake. Fault cracks, shifts, and ruptures form. Enormous changes in the natural setting: waterfalls appear, river currents are diverted, lakes form where rivers are dammed by debris. Final destruction of all structures built by man.

Studying the Earth's internal structure from seismic data

Seismic waves reveal the Earth's internal structure because they change speed, bend, and reflect as they pass through layers of different composition and state. By timing waves from distant earthquakes recorded at stations all over the globe, geophysicists have mapped the crust, mantle, and core without ever drilling to them. The behavior of P and S waves at each boundary is the key evidence.

In 1909 the Croatian seismologist Andrija Mohorovičić found a sharp jump in wave speed that marks the base of the crust — now called the Mohorovičić discontinuity, or "Moho." Richard Dixon Oldham showed that the Earth has a distinct core, and because S waves cannot cross it, the outer core must be liquid. In 1936 Inge Lehmann discovered, from waves bending unexpectedly, that within the liquid outer core lies a solid inner core; later work by Harold Jeffreys refined the travel-time tables that made such conclusions possible.

Wave conversion at the core boundaries — where P waves are bent and partly converted, and where S waves are stopped — creates "shadow zones" on the far side of the Earth that fingerprint the core's size and state. The whole planet also rings like a bell after the largest quakes; these normal modes, or free oscillations, confirm the density of the deep interior, including the composition and solidification of the Earth's inner core.

Plate tectonics and continental boundaries

Plate tectonics explains earthquakes as the product of slow movement of the rigid plates that form the Earth's outer shell. These tectonic plates drift a few centimeters a year, and most seismic activity is concentrated along their boundaries, where they pull apart, collide, or slide past one another. The theory unified earlier observations of earthquakes, volcanoes, and mountain ranges into a single picture.

Evidence for plate movement came from the ocean floor. The Mid-Atlantic Ridge is a place where two plates spread apart, and the symmetrical magnetic stripes recorded on the seafloor on either side of such mid-ocean ridges showed that new crust is created there and pushed outward. Where plates converge instead, crust is consumed or crumpled: the collision of two continental plates raised the Himalayas, which are still rising today.

The three main types of plate boundary each produce a characteristic earthquake style. At divergent boundaries plates separate; at convergent boundaries they collide; and at transform boundaries they grind past one another laterally. The San Andreas Fault in California is the classic transform boundary, where the lateral sliding of plates builds the strain that the elastic rebound theory says is later released as an earthquake. Sudden plate motion offshore can also displace the seabed and generate tsunamis, as in the 2004 Indian Ocean earthquake.

Active methods of generating seismic waves

Seismic waves do not have to come from earthquakes — they can be generated deliberately to image the ground. Sources are divided into passive sources (natural earthquakes and background noise) and active sources (controlled energy releases such as small explosions, vibrating trucks, or air guns at sea). Sensors called geophones on land, or hydrophones at sea, record the waves that bounce back from buried rock layers.

The core technique is seismic reflection: a controlled source sends waves downward, and detectors measure how long they take to return after reflecting off the boundaries between rock layers. From these travel times analysts build a cross-section of the subsurface, identifying rock layers, folds, and faults without excavation. Dense lines of geophones make the resulting images sharp enough to guide drilling and engineering decisions.

Applications of seismology in resource exploration

Active-source seismology is the backbone of petroleum exploration, where reflection surveys map the structures that can trap oil and gas. The same physics has uncovered features unrelated to energy, including the buried Chicxulub Crater off Mexico, the impact scar linked to the mass extinction of the dinosaurs.

Engineering and hazard surveys apply these methods at shallow depths. In Kansas, the Kansas Geological Survey (KGS) has used seismic and related geophysical methods to map coal beds and abandoned mines, to assess salt dissolution and subsurface voids that lead to sinkholes near Hutchinson, and to evaluate subsidence risk for infrastructure. Comparable surveys assess bedrock fracturing and voids beneath wind turbine foundations and probe permafrost and other geological hazards in the Arctic. Newer fiber-sensing technology, which turns ordinary optical cables into long strings of vibration sensors, is being used to track volcanic eruptions and dense seismic activity in real time.

The history of ideas about the causes of earthquakes

Ideas about why the ground shakes evolved from myth toward mechanics over many centuries. In the 17th century, scholars proposed that earthquakes were driven by underground fires, exploding gases, or the collapse of caverns — physical explanations that, while wrong, replaced purely supernatural ones. Folklore long held that "earthquake weather" or unusual animal behavior could foretell a quake, beliefs that modern seismology has found no reliable basis for.

The 1755 Lisbon earthquake spurred the first systematic attempts to study seismic effects scientifically. In the 19th century the Irish engineer Robert Mallet conducted experiments on how waves travel through the ground and is often called a founder of instrumental seismology. The decisive shift to recording instruments came when Ernst von Rebeur-Paschwitz noticed that a delicate instrument near Potsdam had registered a distant Japanese earthquake in 1889 — the first recognized teleseismic recording. Together with Reid's elastic rebound theory, these milestones turned earthquake science into a quantitative discipline; later research into friction laws on faults continues to refine how and when ruptures occur.

Earthquake-resistant construction and protection from earthquakes

Reducing earthquake casualties depends as much on engineering as on science: buildings designed to flex and absorb shaking save far more lives than any forecast. Because no method can reliably predict the exact time, place, and size of a future earthquake, modern protection rests on building resilience, early warning, and preparedness rather than prediction.

Earthquake-resistant design uses flexible frames, reinforced connections, and base isolation so that structures sway without collapsing under surface waves. Earthquake early warning systems detect the fast P waves at stations near the source and send an alert ahead of the slower, more damaging S and surface waves, giving seconds to minutes for trains to brake, utilities to shut off, and people to take cover. These warnings are paired with community preparedness and response plans so that the time gained is actually used.

Citizens contribute directly to monitoring through felt reports, in which people describe the shaking they experienced; agencies like the USGS combine thousands of such reports into rapid intensity maps. Paleoseismology — reading the geological record of ancient ruptures preserved in fault trenches and sediment layers — extends the catalogue of earthquakes far beyond written history, helping engineers estimate how often and how strongly a fault is likely to move.

Where to access seismic data

Seismic data is freely available from national agencies and international research networks, making it possible for anyone to view recent earthquakes and download recordings. These organizations run the dense monitoring networks that locate quakes and publish the results within minutes, and several offer developer tools and APIs for automated access.

• US Geological Survey (USGS) — through its Earthquake Hazards Program, the U.S. Geological Survey publishes real-time global earthquake locations, magnitudes, felt reports, and hazard maps.
• IRIS — the research consortium that archives and distributes seismic waveform data from stations worldwide, and which runs the Seismographs in Schools program for educators.
• EMSC — the European-Mediterranean Seismological Centre, a fast source of felt reports and rapid earthquake information across Europe.
• BGS — the British Geological Survey monitors and reports earthquakes in and around the United Kingdom.
• Caltech Seismological Laboratory — together with the USGS it operates the Southern California Seismic Network (SCSN) and carries a long legacy of earthquake science, from Charles Richter onward.
• American Geophysical Union — the professional body that publishes much of the peer-reviewed research underlying these data sets.

The science is also kept alive by educators and communicators. Figures such as Lisa Wald of the USGS have built public earthquake education resources, while researchers including Emile Okal of Northwestern University, Kathy Whaler, and specialists at the French Atomic Energy Commission such as Yves Cansi have advanced the analysis of historic events from the 1996 Flores Sea earthquake to the 2001 Seattle earthquake. For broader reading on the Earth sciences, explore our sections on Astronomy, Speleology, and more articles about nature and science.