Two earthquakes hit Venezuela: How scientists detect aftershocks and measure intensity

The epicenter of the first earthquake was located near the coastal city of Moron in Yaracuy state, about 160 km west of Caracas, at a depth of 22 km. The second was logged at a depth of 10 km.

Earthquake Caracas’ main airport was closed, the capital’s metro and gas services were suspended, and it was felt as far as the Brazilian Amazon and Bogotá, Colombia.

There was a possibility that the death toll would increase. Venezuela’s acting President Delcy Rodríguez said the death toll currently does not include data from La Guaira state, described as a “disaster zone” and one of the worst-hit areas in the South American country. Forecast modeling by the US Geological Survey (USGS) also estimated the death toll to be in the thousands.

The USGS initially measured the earthquake’s magnitude at 7.1 and revised it to 7.2 within hours. That revision shows how complex it is to collect data and calculate the size of an earthquake.

This motion or collision releases energy which travels outward from the Earth in the form of a wave. Seismic stations located around the world capture that wave as it passes, each recording the movement of the ground at its location.

The instrument that does this is a seismograph. A unit consists of a seismometer, the sensor usually a mass mounted on a pendulum or spring.

At seismological stations, a seismograph is tied to the ground, so when the Earth shakes, the entire instrument shakes with it – except the mass on the spring, which has inertia and stays where it is. A recording device mounted on that mass detects the relative motion between the mass and the rest of the device as the ground moves beneath it. That relative motion becomes a seismogram, a wave chart showing an earthquake.

A single seismograph can confirm where an earthquake occurred nearby and how strong the tremors were felt at that point. This in itself cannot say where the earthquake came from. That process requires a network of stations in an area.

Earthquakes send out two main types of waves at different speeds: a faster “P wave” and a slower “S wave.” The difference between their arrival times at a specific station tells scientists how far that station is from the epicenter of the earthquake. With three or more such distance estimates from different stations, the source can be triangulated.

The USGS describes the underlying calculation as an iterative process: a computer assumes a location, depth, and origin time, compares the wave-arrival times actually recorded by each station, and adjusts its estimate with each pass until the predicted and observed times line up as closely as possible. This is why earthquake-prone countries maintain a dense network of stations.

In India, this network is run by the National Center for Seismology (NCS) under the Ministry of Earth Sciences. As listed on the public monitoring portal of the NCS, the network comprises 172 seismic observatories in every state and union territory – from Campbell Bay in the Andaman and Nicobar Islands to Henley in Ladakh. Some stations are more than a century old: Mumbai’s station has been in operation since 1899 and Kolkata’s since 1898.

Also read: Venezuela earthquake scenes: screams, panic as tremors ‘get stronger’

The first development was made in 1935 by Charles Richter, who was working on earthquakes in Southern California. The Richter scale – formally called local magnitude or ML – measured the amplitude of the largest shock recorded on a seismograph. Richter borrowed the idea of ​​a logarithmic scale from astronomy, with each unit representing a tenfold increase in the amplitude of the recorded wave. In simple terms, this means that the seismogram of a magnitude 7 earthquake shows ten times the wave amplitude of a magnitude 6 earthquake.

But the Richter scale has its limitations. USGS scientists say it becomes “saturated” for very large earthquakes, and that means it may no longer be able to reliably distinguish a large earthquake from an even larger one. Because the scale was calibrated to the geology of Southern California, it also required adjustments for use elsewhere, because rock formations in different areas absorb seismic energy differently.

In 1979, seismologists Thomas Hanks and Hiroo Kanamori introduced moment magnitude (MW), the scale that is now more commonly used.

According to the USGS, rather than measuring a single shock on a seismogram, Mw is calculated from the physical mechanics of the rupture: the stiffness of the rock that ruptures, the area of ​​the fault that slipped, and the average distance it moved. Based on a fault rather than a type of wave on an instrument, the moment magnitude scale is not saturated and can take readings of all earthquake amplitudes.

But calculating Mw takes more time, as it requires modeling the entire waveform rather than reading a single peak. It is also difficult to apply it to small earthquakes. The Richter scale is still used for them.

Then, the Greek philosopher Aristotle came up with a theory that said the Earth moved because of winds trapped within it.

The earliest known seismic instrument was built by the Chinese scholar Zhang Heng in 132 AD. It was a large bronze urn surrounded by eight dragon heads, each facing one of the directions of the compass. Below each dragon’s head was a frog sitting with its mouth open. A blow causes a dragon to release a ball from its mouth into the toad below it, so the direction of the dropped ball will indicate where the blow came from. Historians don’t completely know how it works, but they have speculated that it may have some kind of internal pendulum.

But the study of detecting earthquakes properly began after 1755, when a devastating earthquake and tsunami killed an estimated 70,000 people in Lisbon, Portugal. The disaster pushed scholars to comment, listing when and where Earthquake hit, and what physical effects they left behind.

Three 19th-century scientists, working independently in different countries, laid much of the groundwork for mechanized seismology. In England, engineer Robert Mallet used gunpowder explosions to measure the speed of seismic waves through rock – a method still used in oil exploration today – and was one of the first to estimate how deep underground an earthquake occurred. In France, Alexis Peri combed earthquake catalogs for patterns associated with the seasons and moon phases. And in Italy, Luigi Palmieri built an electromagnetic seismometer installed near Mount Vesuvius, one of the first instruments capable of routinely detecting tremors too small for people to feel.

The modern seismograph arrived soon after. In the 1880s, three English professors – John Milne, James Ewing, and Thomas Gray, who taught at the Imperial College of Tokyo – built the first instrument sensitive enough for true scientific study of earthquakes, UCSB-ERI Account They say.

The USGS separately dates the first seismograph in the modern sense to 1890. In the US, geologist Grove Carl Gilbert, investigating faults left by the 1872 earthquake in Owens Valley, California, established that faults caused earthquakes, not the reverse.

Based on Gilbert’s work, Harry Fielding Reed studied the fault that caused the 1906 San Francisco earthquake and concluded that stress builds up slowly within the Earth over years before being released violently.

It took until 1935 for Richter to add a number to the measurement, and another 44 years for Hanks and Kanamori to refine that number into the moment magnitude scale used today.

The year with the most ‘major’ earthquakes on record was 2010, when 23 earthquakes were detected. The largest earthquake ever recorded by modern instruments occurred in Chile on May 22, 1960, when a magnitude of 9.5 was recorded on a fault spanning approximately 1,600 km.

According to news agency AP report, Venezuela straddles the boundary between the South American and Caribbean tectonic plates, making strong earthquakes very rare there.

Wednesday’s earthquake was one of the most powerful recorded in the country in more than a century. The country’s deadliest earthquakes were a magnitude 6.3 earthquake that killed 236 people in Caracas in 1967, a second earthquake in 1997 that killed 73, and an earthquake in Caracas and Mérida in 1812 that killed an estimated 30,000 people.