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Earthquake: the story on HearLore | HearLore
Earthquake
The 1556 Shaanxi earthquake on the 23rd of January 1556 in China claimed more than 100,000 lives, yet the true death toll may have reached 730,000 when accounting for the subsequent emigration, plague, and famine that followed. This disaster was not merely a geological event but a societal collapse triggered by the destruction of yaodongs, the dwellings carved into loess hillsides that housed the region's population. When the ground ruptured, these earthen structures collapsed with terrifying efficiency, burying thousands in the soft soil that formed their homes. The sheer scale of this tragedy highlights how the specific geology of a region can turn a natural phenomenon into a catastrophe of unprecedented proportions. While modern seismology measures energy release in joules, the human cost of that energy remains the most haunting metric of an earthquake's power.
Earthquakes are the sudden release of energy within the lithosphere, creating seismic waves that shake the surface of the Earth. This shaking can range from imperceptible tremors to violent movements that propel objects and people into the air. The initial point where the rock breaks is known as the hypocenter or focus, while the point directly above it on the ground is the epicenter. Most of the world's seismic activity occurs along tectonic plate boundaries, particularly within the Pacific Ring of Fire, a horseshoe-shaped zone that encircles the Pacific Ocean. This area accounts for 90 percent of the world's earthquakes and 81 percent of the largest ones, making it the most dangerous place on the planet for anyone living near a fault line. The energy released during these events is not just a force of destruction but a mechanism that lowers the Earth's available elastic potential energy while raising its temperature, though these changes are negligible compared to the heat flowing from the Earth's deep interior.
The Mechanics of Rupture
The 1960 Chilean earthquake on the 22nd of May 1960 stands as the largest earthquake ever recorded on a seismograph, reaching a magnitude of 9.5. The energy released by this event was approximately twice that of the next most powerful earthquake, the Good Friday earthquake of 1964 in Alaska. This massive release of energy occurred because the fault plane within the brittle crust of the Earth can reach depths of 100 kilometers, allowing for the most powerful earthquakes to occur along converging plate margins. The mechanics of this rupture are governed by the elastic-rebound theory, which describes how stress builds up along a fault until it breaks through asperities, suddenly allowing sliding over the locked portion of the fault. Only 10 percent or less of an earthquake's total energy is radiated as seismic energy, with the rest powering the fracture growth or converting into heat generated by friction.
Faults are categorized into three main types: normal, reverse, and strike-slip, each generating different levels of stress and energy. Normal faults occur where the crust is being extended, such as at divergent boundaries, while reverse faults happen where the crust is being shortened, such as at convergent boundaries. Strike-slip faults are steep structures where the two sides of the fault slip horizontally past each other, and they can produce major earthquakes up to about magnitude 8. The maximum observed lengths of ruptures can be approximately 1,000 kilometers, as seen in the earthquakes in Alaska in 1957, Chile in 1960, and Sumatra in 2004. These massive ruptures are constrained by the brittle-ductile transition zone, which limits the depth extent of the rupture. The width of the faulted area is the most important parameter controlling the maximum earthquake magnitude, as it varies by a factor of 20 along converging plate margins.
Common questions
What was the death toll of the 1556 Shaanxi earthquake on the 23rd of January 1556 in China?
The 1556 Shaanxi earthquake claimed more than 100,000 lives, yet the true death toll may have reached 730,000 when accounting for the subsequent emigration, plague, and famine that followed. This disaster was not merely a geological event but a societal collapse triggered by the destruction of yaodongs, the dwellings carved into loess hillsides that housed the region's population.
Which earthquake holds the record for the largest magnitude ever recorded on a seismograph?
The 1960 Chilean earthquake on the 22nd of May 1960 stands as the largest earthquake ever recorded on a seismograph, reaching a magnitude of 9.5. The energy released by this event was approximately twice that of the next most powerful earthquake, the Good Friday earthquake of 1964 in Alaska.
What was the deadliest earthquake of the 20th century and how many people did it kill?
The 1976 Tangshan earthquake in China killed between 240,000 and 655,000 people, making it the deadliest earthquake of the 20th century. This disaster was not only a result of the magnitude of the quake but also of its proximity to heavily populated areas and the lack of seismic building codes.
Where does most of the world's seismic activity occur and what percentage of earthquakes happen there?
Most of the world's seismic activity occurs along tectonic plate boundaries, particularly within the Pacific Ring Fire, a horseshoe-shaped zone that encircles the Pacific Ocean. This area accounts for 90 percent of the world's earthquakes and 81 percent of the largest ones, making it the most dangerous place on the planet for anyone living near a fault line.
Can scientists predict the specific day or month when an earthquake will occur?
Despite considerable research efforts by seismologists, scientifically reproducible predictions cannot yet be made to a specific day or month. Earthquake forecasting is concerned with the probabilistic assessment of general earthquake hazards, including the frequency and magnitude of damaging earthquakes in a given area over years or decades.
The 1976 Tangshan earthquake in China killed between 240,000 and 655,000 people, making it the deadliest earthquake of the 20th century. This disaster was not only a result of the magnitude of the quake but also of its proximity to heavily populated areas and the lack of seismic building codes. Regions most at risk for great loss of life are those where earthquakes are relatively rare but powerful, and poor regions with lax, unenforced, or nonexistent seismic building codes. The 2004 Indian Ocean earthquake, while not the largest in magnitude, was one of the deadliest in history because it generated a tsunami that devastated communities thousands of kilometers away. The shaking of the earth can cause soil liquefaction, where water-saturated granular material temporarily loses its strength and transforms from a solid to a liquid, causing buildings to sink into the ground.
The aftermath of an earthquake often brings disease, a lack of basic necessities, and mental consequences such as panic attacks and depression to survivors. Recovery times vary based on the level of damage and the socioeconomic status of the impacted community. In the 1906 San Francisco earthquake, more deaths were caused by fire than by the earthquake itself, as damaged electrical power or gas lines ignited fires that spread uncontrollably due to the loss of water pressure. The terrain below the Sarez Lake in Tajikistan remains in danger of catastrophic flooding if the landslide dam formed by an earthquake, known as the Usoi Dam, were to fail during a future earthquake. Impact projections suggest that such a flood could affect roughly five million people, highlighting the long-term risks that persist long after the initial shaking has ceased.
The Science of Prediction
The 2011 Tohoku-Oki earthquake, also known as the Fukushima earthquake, confirmed that gravitational measurement could provide instantaneous detection of earthquakes, a breakthrough realized by scientists in 2016. Despite considerable research efforts by seismologists, scientifically reproducible predictions cannot yet be made to a specific day or month. Earthquake forecasting is concerned with the probabilistic assessment of general earthquake hazards, 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.
The instrumental scales used to describe the size of an earthquake began with the Richter scale in the 1930s, but standard practice by most seismological authorities is to express an earthquake's strength on the moment magnitude scale. This scale takes into account the seismic moment, which is the total rupture area, average slip of the fault, and rigidity of the rock. The number of seismic stations has increased from about 350 in 1931 to many thousands today, allowing for more accurate reporting of earthquakes. The United States Geological Survey estimates that, since 1900, there have been an average of 18 major earthquakes and one great earthquake per year, and that this average has been relatively stable. However, accurate recordings of earthquakes only began in the early 1900s, so it is too early to categorically state that the number of major earthquakes is increasing.
Myths and Modern Myths
From the lifetime of the Greek philosopher Anaxagoras in the 5th century BCE to the 14th century CE, earthquakes were usually attributed to air vapors in the cavities of the Earth. Pliny the Elder called earthquakes underground thunderstorms, while Thales of Miletus was the only documented person who believed that earthquakes were caused by tension between the earth and water. In Norse mythology, earthquakes were explained as the violent struggle of the god Loki being punished for the murder of Baldr, god of beauty and light. In Greek mythology, Poseidon was the cause and god of earthquakes, and in Japanese mythology, Namazu is a giant catfish who causes earthquakes. In Taiwanese folklore, the Tē-gû is a giant earth buffalo who causes earthquakes. These myths reflect the profound influence of earthquakes on human societies, from ancient times to the present day.
In modern popular culture, the portrayal of earthquakes is shaped by the memory of great cities laid waste, such as Kobe in 1995 or San Francisco in 1906. Fictional earthquakes tend to strike suddenly and without warning, as seen in novels like Richter 10, Goodbye California, 2012, and San Andreas. The most popular single earthquake in fiction is the hypothetical Big One expected of California's San Andreas Fault someday. The cultural impact of earthquakes spans myths, religious beliefs, and modern media, reflecting their profound influence on human societies. The New Testament refers to earthquakes occurring both after the death of Jesus and at his resurrection, showing how these events have been woven into religious narratives for centuries.
Shaking Beyond Earth
Phenomena similar to earthquakes have been observed on other planets, such as marsquakes on Mars and moonquakes on the Moon, indicating the universality of such events beyond Earth. These seismic activities are caused by the movement of tectonic plates or other geological processes, similar to those on Earth. The study of these phenomena helps scientists understand the internal structure and dynamics of other celestial bodies. The 2001 Kunlun earthquake, which ruptured along segments of the East Anatolian Fault at supershear speeds, caused an unusually wide zone of damage attributed to the effects of the sonic boom developed in such earthquakes. This phenomenon is one of the few exceptions where the rupture speed exceeds the shear wave velocity of the surrounding rock.
The study of earthquakes has also led to the development of earthquake engineering, which aims to design structures that withstand shaking. Seismic retrofitting can modify existing structures to improve their resistance to earthquakes, and earthquake insurance can provide building owners with financial protection against losses resulting from earthquakes. Artificial intelligence may help to assess buildings and plan precautionary operations, as seen in the Igor expert system, which has been applied to assess buildings in Lisbon, Rhodes, and Naples. The future of earthquake management lies in the combination of scientific research, technological innovation, and global cooperation to mitigate the risks posed by these powerful natural events.