SCIENCE12 - Earthquakes

The February 6, 2023 major earthquake in southern Turkey and northwestern Syria, with its horrible destruction and loss of life, reminded me that I don’t know much about earthquakes - how, why, and where they happen.  So, researching this article will give me a chance to learn about the subject.

After a short introduction, I will cover historical views of earthquakes, the science of earthquakes, earthquake magnitude and intensity, measuring and locating earthquakes, the damage from earthquakes, major earthquakes in the past, predicting and forecasting earthquakes, and a word about preparedness for earthquakes.  I will close with a short update on the recent Turkey-Syria earthquake.

My principal sources include “Earthquake,” “List of historical earthquakes,” and “Richter magnitude scale,” Wikipedia.com; “The Science of Earthquakes” and “Earthquake Magnitude, Energy Release, and Shaking Intensity,” usgs.gov; “Highest Magnitude & Biggest Earthquakes,” sma-tsunami-warning.com; “Plate Tectonics - Introduction,” nps.gov; “What is Tectonic Shift?,” oceanservice.noaa.gov; “Earthquake Timeline,” sciencelearn.org; “Earthquake Measurements: Magnitude vs Intensity,” earthquakeauthority.com; “GSN - Global Seismographic Network,” usgs.gov; and numerous other online sources.

Introduction

An earthquake is the shaking of part of the surface of the Earth from a sudden release of energy when two blocks of the Earth’s crust suddenly slip past one another.  Earthquakes can occur anywhere between the Earth's surface and about 450 miles below the surface.

Earthquakes can range in intensity, from those that are so weak that they cannot be felt, to those violent enough to propel objects and people into the air, damage critical infrastructure, and wreak destruction across large areas and entire cities, like the recent Turkey-Syria earthquake.

Sometimes the main earthquake is preceded by smaller earthquakes, but the main earthquake is always followed by smaller aftershocks that can continue for weeks, months, and even years.

It is estimated that there are 500,000 detectable naturally-occurring earthquakes in the world each year. 100,000 of those can be felt, and 100 of them cause damage.  Large earthquakes occur about once a year.

Historical Views

In ancient times, earthquakes were attributed to everything from punishing actions by gods, to tensions between Earth and water, to air rushing out of caverns deep in the Earth's interior, to underground thunderstorms.

The true nature and cause of earthquakes began to be understood in the early 18^th century.  Here are a few of the important milestones.  An explanation of the science of earthquakes, based on these milestones, follows this list.

1705 - English scientist Robert Hooke realized that earthquakes are connected to land movements.

1755 - Modern studies begin.  A huge earthquake and tsunami in Portugal killed over 70,000 people.  People began to collect data to help understand the events.

1840 - Italian physicist and meteorologist Luigi Palmieri invented the first accurate electromagnetic seismograph, which could detect earthquakes not felt by humans.

1850 - Irish geophysicist Robert Mallet realized that most earthquake damage is due to moving waves, named seismic waves, caused by a sudden land movement.

1855 - British mathematicians John Pratt and George Airy suggested that surface rocks float on a layer of denser rock.

1872 - American geologist Grove Gilbert figured out that earthquakes are centered around fault lines.

1889 - For the first time, a seismograph (in Germany) detected an earthquake on the other side of the Earth (in Japan).

1897 - British geologist Richard Oldham realized that there were at least two types of seismic waves that travelled at different speeds. We know these now as P-waves and S-waves.

1906 - After the most destructive earthquake in American history in San Francisco, California, American geophysicist Harry Reid suggested that earthquakes are the result of stresses built up along land faults.

1912 - German meteorologist and geophysicist Alfre Wegner proposed that Earth’s continents drift on the surface of the Earth.

1935 - American seismologist and physicist Charles Richter and German-American seismologist Beno Gutenberg developed a magnitude scale for earthquakes, now known as the Richter Scale. 

1961 - A worldwide earthquake monitoring system was set up.  Several systems now exist, including the Global Seismographic Network.  They contribute to the understanding of earthquakes.

Late 1960s - The realization that motion of ocean floors is different from continents led to the theory of plate tectonics, the idea that the earth is covered with tectonic plates, constantly in (slow) motion.

1970s - Japanese seismologist Hiroo Kanamori and American seismologist Thomas C. Hanks developed the Moment Magnitude scale to measure the total energy of earthquakes. 

Science of Earthquakes

The Earth has four major layers: the inner core, outer core, mantle, and crust.  Earth's crust varies in thickness from 22 to 44 miles in the continents and from 3 to 6 miles in the ocean basins. 

Layered structure of the Earth.

 

The surface of our planet looks very different from the way it did 250 million years ago, when there was only one continent, called Pangaea, and one ocean, called Panthalassa.  As Earth’s mantle heated and cooled over many millennia, the huge continent eventually broke apart, creating new and ever-changing land masses and oceans.  That plate motion continues today.

Earth’s land masses move toward and away from each other at an average rate of about 0.6 inch a year.  Some regions, such as coastal California, move quite fast in geological terms - almost two inches a year - relative to the more stable interior of the continental United States. 

Earth’s crust today consists of roughly 20 tectonic plates that are in a constant state of motion.  The plates can be thought of like pieces of a cracked shell that rest on the hot, molten rock of Earth’s mantle and fit snugly against one another. 

The heat from radioactive processes within the planet’s interior causes the plates to move, sometimes toward, and sometimes away from each other.  This movement is called plate motion, or tectonic shift.  At the “seams” where tectonic plates come in contact, the crustal rocks may grind violently against each other, causing earthquakes.

Simplified map of the Earth’s major tectonic plates.  The heavy black lines define the individual tectonic plates. Each continent rides on one or more plates.  (Red arrows indicate direction of movement at plate boundaries).

 

The particular spot on the edge of the plates where they grind or slip is called the fault or fault plane.  The location below the Earth’s surface where the earthquake starts is called the focus or hypocenter, and the location directly above it, on the surface of the Earth, is called the epicenter.  A fault scarp is a small step or offset on the ground surface where one side of a fault has moved vertically with respect to the other.

The plate boundaries are made up of many faults, and most of the earthquakes around the world occur on these faults.  Since the edges of the plates are rough, they get stuck while the rest of the plate keeps moving. 

While the edges of faults are stuck together, and the rest of the block is moving, the energy that would normally cause the blocks to slide past one another is being stored up. When the force of the moving blocks finally overcomes the friction of the jagged edges of the fault and it unsticks, all that stored up energy is released from the earthquake focus.  The energy radiates outward from the fault in all directions in the form of seismic waves like ripples on a pond.  The seismic waves shake the earth as they move through it, and when the waves reach the earth’s surface, they shake the ground and anything on it.

Seismic waves radiate from the focus of an earthquake.

 

Earthquakes can also cause volcanic eruptions.  Such earthquakes can serve as an early warning of volcanic eruptions, as during the 1980 eruption of Mount St. Helens.  The Ring of Fire is a string of volcanoes and sites of seismic activity, or earthquakes, around the edges of the Pacific Ocean.  The Ring of Fire is largely a result of plate tectonics, where the massive Pacific Plate interacts with the less-dense plates surrounding it.  Roughly 90% of all earthquakes occur along the Ring of Fire, and the ring is dotted with 75% of all active volcanoes on Earth.

 

Most earthquakes and volcanic eruptions occur where the moving tectonic plates interact at faults along their boundaries.

 

Magnitude and Intensity of Earthquakes

In this section, I will define magnitude and intensity.  The following section discusses how to measure these earthquake descriptors.

Magnitude.  The magnitude of an earthquake is a single value that describes the size of the earthquake at its source. 

Magnitude is expressed in whole numbers and decimal fractions.  See the table below.  For example, the magnitude of the recent Turkey-Syria earthquake was determined to be 7.8, on the high end of the major earthquake classification.  The magnitude scale is logarithmic, i.e., each whole number increase in magnitude represents a tenfold increase in the size of the earthquake.

This scale classifies the size of an earthquake.

 

Intensity.  Intensity is the measure of shaking at different locations around the earthquake.  Intensity values vary from place to place, depending on distance from the earthquake and underlying rock or soil makeup.

The intensity scale that we use in the United States to classify the amount of shaking at a particular location is called the Modified Mercalli Intensity Scale, but other countries use other scales.  The Modified Mercalli Intensity Scale is composed of increasing levels of intensity that range from observable quake impacts from light shaking to catastrophic destruction.  Intensity is designated by Roman numerals, for example, VI, X, etc.

                

Measuring and Locating Earthquakes

Seismic wave vibrations from earthquakes are recorded by instruments called seismographs, and the recording they make is called a seismogram.  A seismograph has a base that sets firmly in the ground, and a heavy weight that hangs free.  When an earthquake causes the ground to shake, the base of the seismograph shakes too, but the hanging weight does not.  Instead, the spring or string that it is hanging from absorbs all the movement.  The difference in position between the shaking part of the seismograph and the motionless part is what is recorded.  The concept of a seismograph is shown below.

This cartoon sketch of a seismograph shows how the instrument shakes with the earth below it, but the recording device remains stationary.

 

The Global Seismographic Network, a permanent digital network - over 150 seismic stations connected by a telecommunications network - monitors earthquake activity around the world.

Worldwide distribution of Global Seismograph Network stations.

 

There are two broad classes of seismic waves: body waves and surface waves.  Body waves travel within the body of Earth.  They include P, or primary, waves and S, or secondary, waves.  P-waves cause the ground to compress and expand, that is, to move back and forth, in the direction of travel.  They are called primary waves because they are the first type of wave to arrive at seismic recording stations.  S-waves shake the ground in a shearing, or crosswise, motion that is perpendicular to the direction of travel.  These are the shake waves that move the ground up and down or from side to side.  S-waves are called secondary waves because they always arrive after P-waves at seismic recording stations.  After both P and S waves have moved through the body of Earth, they are followed by surface waves, which travel along Earth’s surface.  They are slower-moving than body waves, but are much larger and therefore more destructive.

The differences in travel time of P-waves and S-waves from the earthquake epicenter to the observatory are a measure of the distance and can be used to determine both sources of earthquakes and structures within the Earth. 

Magnitude.  Although modern seismographs have been around since the 19^th century, it wasn’t until 1935 that Charles F. Richter, a seismologist at the California Institute of Technology, invented a mathematical formula - now known as the Richter Scale - to compare earthquake magnitudes.  But the Richter Scale worked largely for earthquakes in Southern California, and only those occurring within about 370 miles of seismographs.  In addition, the scale was calculated for only one type of earthquake wave. 

By 1970, Scientists had developed far-more sensitive seismographs that, with faster computers, enabled them to record and interpret a broader spectrum of seismic signals than was possible in the 1930s, when the Richter magnitude was developed. 

So, in 1970, the Richter Scale was replaced with the Moment Magnitude Scale, which records all the different seismic waves from an earthquake.  The Moment Magnitude Scale also measures the movement of rock along the fault.  “Moment” is a product of the distance a fault moved and the force required to move it.  That information is plugged into the Moment Magnitude Scale to give us the amount of energy that is released during an earthquake.

The Moment Magnitude is derived from modeling recordings of the earthquake at multiple stations.  It accurately measures larger earthquakes, which can last for minutes, affect a much larger area, and cause more damage.

Intensity.  Traditionally the intensity of an earthquake at a particular location is a subjective measure derived from human observations and reports of felt shaking and damage.  In the past, the data was gathered from postal questionnaires, but with the advent of the internet, is now collected using a web-based form.

Location.  Scientists use a method called triangulation to determine exactly where the earthquake was located on the surface of the Earth (see image below).  It is called triangulation because a triangle has three sides, and it takes three seismographs to locate an earthquake.   If you draw a circle on a map around three different seismographs, where the radius of each is the distance from that station to the earthquake (determined from the time difference in the arrival of the different seismic waves), the intersection of those three circles is the earthquake’s epicenter.

 

Triangulation from three seismographs is used to locate earthquakes on the Earth’s surface.

 

Damage from Earthquakes

Shaking and Ground Rupture.  Shaking and ground rupture are the main effects created by earthquakes, principally resulting in more or less severe damage to buildings and other rigid structures.  The severity of the local effects depends on the complex combination of the earthquake magnitude, the distance from the epicenter, and the local geological and geomorphological conditions, which may amplify or reduce wave propagation. 

Ground rupture is a visible breaking and displacement of the Earth's surface along the trace of the fault, which may be of the order of several yards in the case of major earthquakes.  Ground rupture is a major risk for large engineering structures such as dams, bridges, and nuclear power stations, and requires careful mapping of existing faults to identify any that are likely to break the ground surface within the life of the structure.

The San Adreas Fault, northwest of Los Angeles.

 

Soil Liquefaction.  Soil liquefaction occurs when, because of the shaking, water- saturated granular material (such as sand) temporarily loses its strength and transforms from a solid to a liquid.  Soil liquefaction may cause rigid structures, like buildings and bridges, to tilt or sink into the liquefied deposits. For example, in the 1964 Alaska earthquake, soil liquefaction caused many buildings to sink into the ground, eventually collapsing upon themselves.

Human Impacts.  Physical damage from an earthquake will vary depending on the intensity of shaking in a given area.  Impacts may include:

·         Injuries and loss of life

·         Damage to critical infrastructure (short and long term)

o    Roads, bridges and public transportation networks

o    Water, power, sewer and gas interruption

o    Communication systems

·         Loss of critical community services including hospitals, police, and fire

·         General property damage

·         Collapse or destabilization (potentially leading to future collapse) of buildings

 

With these impacts and others, the aftermath may bring disease, lack of basic necessities, mental consequences such as panic attacks, depression to survivors, and higher insurance premiums.  Recovery times will vary based on the level of damage along with the socioeconomic status of the impacted community.

Landslides.  Earthquakes can produce slope instability leading to landslides, a major geological hazard. Landslide danger may persist while emergency personnel are attempting rescue work.

Fires.  Earthquakes can cause fires by damaging electrical power or gas lines.  In the event of water mains rupturing and a loss of pressure, it may also become difficult to stop the spread of a fire once it has started.  For example, more deaths in the 1906 San Francisco earthquake (magnitude 7.9) were caused by fire than by the earthquake itself.

Fires in San Francisco after the 1906 earthquake.

 

Tsunamis.  Tsunamis are sea waves produced by the sudden or abrupt movement of large volumes of water - including when an earthquake occurs at sea.  In the open ocean, the distance between wave crests can surpass 62 miles, and the wave periods can vary from five minutes to one hour.  Such tsunamis travel 373-497 miles per hour, depending on water depth.  Large waves produced by an earthquake, or a submarine landslide, can overrun nearby coastal areas in a matter of minutes.  Tsunamis can also travel thousands of miles across open ocean and wreak destruction on far shores, hours after the earthquake that generated them.

Floods.  Floods may be secondary effects of earthquakes, if dams are damaged.    Earthquakes may cause landslips to dammed rivers, which collapse and cause floods.

Major Earthquakes of the Past

Before the 20^th century, we could not record earthquakes as they happened.  Descriptions of earthquake events that happened prior to the 20^th century, therefore, rely mainly on after-the-fact analysis of written sources.  There is often significant uncertainty in location and magnitude, and sometimes the date, for these earlier earthquakes.  The number of fatalities is also often highly uncertain, particularly for the older events.  Earthquakes that caused the greatest loss of life, while powerful, were deadly because of their proximity to either heavily populated areas or the ocean, where earthquakes often created tsunamis that devastated communities thousands of miles away. 

The deadliest earthquake in history was the AD 1556 Shaanxi earthquake (estimated magnitude 8.2-8.3), which occurred on January 23, 1556 in Shaanxi, China.  More than 830,000 people reportedly died, including those displaced by the earthquake, who died from famine.  Most homes in the area were carved out of hillsides; many victims were killed when these structures collapsed.

Other pre-20^th century earthquakes, with estimated deaths exceeding 100,000 people, include: AD 526: Antioch, Turkey, 250,000 deaths; AD 893: Ardabil, Iran, 150,000 deaths; AD 1138: Aleppo, Syria, 230,000 deaths; AD 1139: Ganja, Azerbaijan, 230-300,000 deaths; and AD 1303: Hongdong, China, 270,000 deaths.

These are the 10 largest earthquakes ever recorded:

1. Valdivia, Chile 22 May 1960 (magnitude 9.5)

This earthquake killed 1,655 people, injured 3,000 and displaced two million.  It caused $550 million damage in Chile, while the tsunami that it spawned caused deaths and damage as far away as Hawaii, Japan and the Philippines.  The “rupture zone” of the quake was more than 620 miles long.  Two days after the initial quake, the nearby volcano Puyehue erupted, sending ash and steam up to four miles into the atmosphere over a period of several weeks.

Devastation in Valdivia, Chile after May, 22, 1960 earthquake.

 

2. Prince William Sound, Alaska 28 March 1964 (magnitude 9.2)

Compared to the Chilean earthquake, this earthquake was less damaging: the resulting tsunami took 128 lives and caused overall $311 million in damage.  The earthquake was felt mainly over Alaska, as well as some places in Canada, while the tsunami created by it caused damage as far away as Hawaii. The most damage was sustained by the city of Anchorage, 75 miles north-west of the epicenter.  Shaking from the quake itself is reported to have lasted for three minutes.

3. Sumatra, Indonesia 26 December 2004 (magnitude 9.1)

In terms of damage and loss of life, the scale of the disaster caused by the resulting Boxing Day Tsunami was enormous.  In total, 227,900 people were killed or presumed dead, with around 1.7 million were displaced over 14 countries in South Asia and East Africa.  The epicenter was 155 miles southeast of Band Aceh, Indonesia, at a depth of 20 miles.  Several days later, on 28 December, a mud volcano began erupting near Baratang, Andamar Islands, which is thought to have been associated with the earthquake.

4. Sendai, Japan 11 March 2011 (magnitude 9.1)

The earthquake happened 81 miles off Sendai, Miyagi Prefecture, on the east coast of the Tohoku of Japan.  It was at a depth of 15.2 miles.  It was the most powerful earthquake to hit Japan in recorded history.  The death toll, from the combined effect of the powerful earthquake, aftershocks, and the tsunami was 16,000.  Total damage was $360 billion, with the shutting down of nuclear reactors which many industries rely on for power.

Damage from Sendai, Japan earthquake on March 11, 2011.

 

5. Kamchatka, Russia 4 November 1952 (magnitude 9.0)

This earthquake generated a tsunami that caused widespread damage in the Hawaiian Islands.  Property damage was estimated at around $1,000,000.  Some reports describe waves of over 30 feet high at Kaena Point, Oahu.  No people were reported killed.

6. Bio-bio, Chile 27 February 2010 (magnitude 8.8)

This earthquake and subsequent tsunami killed at least 521 people, with 56 missing and 12,000 injured.  More than 800,000 people were displaced with a total of 1.8 million people affected across Chile, where damage was estimated at $30 billion.  The epicenter was 210 miles southwest of Santiago, at a depth of 22 miles.  A minor tsunami travelled across the Pacific, causing damage to boats as far away as San Diego, California.

7. Off the coast of Ecuador 31 January 1906 (magnitude 8.8)

This earthquake caused a tsunami that is reported to have killed between 500 and 1,500 in Ecuador and Colombia.  The tsunami travelled as far north as San Francisco, on the west coast of the U.S., and west to Hawaii and Japan.  The tsunami took roughly 12 hours to cross the Pacific to Hilo, Hawaii.

8. Rat Islands, Alaska 2 April 1965 (magnitude 8.7)

The worst of the damage attributed to this earthquake was caused by a tsunami, reported to be about 33 feet high on Shemya Island.  The wave caused flooding on Amchitka Island, causing $10,000 in property damage.  No deaths or injuries were reported.

9. Sumatra, Indonesia 28 March 2005 (magnitude 8.6)

This earthquake killed 1,313, with over 400 people injured by the tsunami as far away as Sri Lanka. The epicenter was 130 miles northwest of Sibolga, Sumatra, at a depth of 20 miles. This region is particularly geologically active, with three of the 15 largest known earthquakes having happened here.

10. Assam - Tibet 15 August 1950 (magnitude 8.6)

This inland earthquake caused widespread damages to buildings as well as large landslides.  780 people were killed in eastern Tibet, with many villages and towns affected across Assam, China, Tibet and India.  Oscillations to lake levels occurred as far away as Norway.  The total death toll is likely to have been higher, as no definitive total was ever estimated.  While the earthquake itself is known as the Assam Earthquake, it is believed the epicenter may have been in Tibet

Prediction and Forecasting

Earthquake prediction is the branch of the science of seismology concerned with the specification of the time, location, and magnitude of future earthquakes.  Many methods have been developed for predicting the time and place in which earthquakes will occur, but despite considerable research efforts by seismologists, scientifically reproducible predictions cannot yet be made to a specific day or month.

Earthquake forecasting is often differentiated from earthquake prediction, and is concerned with the probabilistic assessment of general earthquake hazard, including the frequency and magnitude of damaging earthquakes in a given area over years or decades.  For well-understood faults, the probability that a segment may rupture during the next few decades can be estimated.

Earthquake warning systems have been developed that can provide regional notification of an earthquake in progress, but before the ground surface has begun to move, potentially allowing people within the system's range to seek shelter before the earthquake's impact is felt.

Preparedness

The objective of earthquake engineering is to foresee the impact of earthquakes on buildings and other structures, and to design such structures to minimize the risk of damage.  For instance, Japanese high-rise construction today commonly uses a grid of steel beams and columns that evenly distributes seismic forces across the structure and diagonal dampers that serve as shock absorbers.  American high-rises are typically built with a concrete core that resists most of the seismic forces of an earthquake.

Existing structures can be modified by seismic retrofitting to improve their resistance to earthquakes. Earthquake insurance can provide building owners with financial protection against losses resulting from earthquakes.  Emergency management strategies can be employed by a government or organization to mitigate risks and prepare for consequences.

Pat took this photo (safe zone in case of earthquake) on the outside wall of a museum in Lima, Peru.

  

Artificial intelligence may help to assess buildings and plan precautionary operations: the Igor expert system is part of a mobile laboratory that supports the seismic assessment of masonry buildings and the planning of retrofitting operations on them.  It has been successfully applied to assess buildings in Lisbon, Rhodes, Naples.

Individuals can also take preparedness steps like securing water heaters and heavy items that could injure someone, locating shutoffs for utilities, and being educated about what to do when the shaking starts. 

Update on Turkey-Syria Earthquake

The 7.8-magnitude earthquake struck at depth of 11 miles in southern Turkey in the early hours of the morning on February 6, 2023, with the large Turkish city (two million residents) of Gaziantep near its epicenter.  This was followed, less than 10 hours later by a 7.5-magnitude aftershock, slightly to Gaziantep’s north.

Turkey is a tectonically active area, where three tectonic plates touch and interact with each other.

In addition to its impact on Turkey, the twin quakes hit the heart of the Turkey-Syria border area, home to millions of Syrian refugees at a time of great economic and geostrategic uncertainty in Turkey and across the region.

The devastation was particularly deadly because the shocks were both powerful and shallow, and also struck a region where few buildings were fully compliant with codes designed to make them more resistant to earthquakes.

The reverberations from the earthquakes were felt in Iraq, Israel, and Lebanon. 

As of the day of this blog post, the death toll had risen to over 45,000, making the natural disaster one of the worst of the century.

Damage region of the 7.8-magnitude Turkey-Syria earthquake.

 

Collapsed residential buildings in Kahramanmaras, Turkey, a city of 400,000, located between the epicenters of the magnitude 7.8 and 7.5 earthquakes.