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Everything you always wanted to know about earthquakes | Blue Mountain Gazette

Why did you feel the Victorian earthquake at home


The 5.9 earthquake was felt from afar.

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2021-09-22T18: 00: 00 + 10: 00

You probably didn’t even realize, when you got up this morning, how you don’t know nothing about earthquakes.

An earthquake in Victoria, felt as far away as Tasmania and the central coast of New South Wales, was 5.9 on the Richter scale.

Here is the truth about earthquakes from the experts at Geoscience Australia.

What is an earthquake?

Earthquakes are the vibrations caused by rocks breaking under stress. The underground surface along which the rock breaks and moves is called a fault plane.

The focus, or “hypocenter,” of an earthquake is the point where it originated in the Earth. The point on the Earth’s surface directly above the focus is called the epicenter of the earthquake.

The size or magnitude of earthquakes is determined by measuring the amplitude of seismic waves recorded on a seismograph and the distance of the seismograph from the earthquake. These are put into a formula that converts them to a magnitude, which is a measure of the energy released by the earthquake. For every unit of increase in magnitude, there is about thirty times more energy released. For example, a 6.0 magnitude earthquake releases approximately 30 times more energy than a 5.0 magnitude earthquake, while a 7.0 magnitude earthquake releases approximately 900 times (30×30) more energy than ‘an earthquake of magnitude 5.0.

A magnitude 8.6 earthquake releases energy equivalent to about 10,000 atomic bombs of the type developed during World War II. Fortunately, small earthquakes occur much more frequently than large ones, and most cause little or no damage.

The magnitude of earthquakes was traditionally measured on the Richter scale. It is often now calculated from the seismic moment, which is proportional to the area of ​​the fault multiplied by the average displacement over the fault.

Where do earthquakes occur?

No part of the Earth’s surface is immune to earthquakes, but some areas experience them more frequently than others. They are most common and most important at the boundaries of tectonic plates where two plates collide and / or slide over each other. They occur particularly around the margins of the Pacific plate, for example in New Zealand, Vanuatu, Solomon Islands, Papua New Guinea, Japan and the Americas, as well as along the Indonesian island arc. , where the Indo-Australian Plate collides with the Eurasian Plate. Plaque. The depths of focus in these collision zones can range from the surface to a depth of 700 km. Large, shallow earthquakes also occur when two plates separate with the creation of new oceanic crust along mid-ocean ridges and on transforming faults that intersect them.

In the relatively stable interior of continents, far from the boundaries of the plates, earthquakes are less frequent and do not follow easily recognizable patterns. These “intraplate” earthquakes generally originate at shallow depths (ie less than 20 km), but can still be of great magnitude. In 1811-1812, four earthquakes with an estimated magnitude of 7 occurred in the New Madrid area in the eastern United States. More recently, the magnitude 7.7 earthquake of 2001 at Bhuj in Intraplate India killed more than 20,000 people.

Australia’s biggest earthquake

The largest earthquake recorded in Australia occurred in 1988 in Tennant Creek in the Northern Territory, with an estimated magnitude of 6.6. It occurred in a sparsely populated area and damaged a major gas pipeline. A magnitude 6.5 earthquake in Meckering in 1968 caused extensive damage to buildings and was felt across much of southern Western Australia. These earthquakes are two of eleven that are recorded to have produced a surface rupture in historical times, forming fault scarps.

Earthquakes don’t have to be “the biggest” to cause massive damage

In 1989, on December 28, an earthquake measuring 5.6 on the Richter scale struck Newcastle. The epicenter of the earthquake was about 15 kilometers from Newcastle’s central business district.

This earthquake killed 13 people; and 160 people were injured.

More than 350 fault scarps are mapped across Australia, the majority believed to be linked to large prehistoric earthquakes. The study of these characteristics makes it possible to deduce recurrence and magnitude models of the largest Australian earthquakes. While large earthquakes have occurred across Australia, the failing faults and margins of the ancient cratonic building blocks that make up the continent are particularly prone to earthquakes.

Why do we have earthquakes in Australia?

The Australian Plate is the fastest moving landmass on Earth and collides with the Pacific Plate to the north and east of Australia, and the Eurasian Plate to the northwest. This primarily generates a compressive stress within the Australian mainland, which slowly builds up across the plate as it moves northeast by about 7cm per year. Earthquakes in Australia are caused by the sudden release of this stress when deep rocks shatter and move along a fault line. While some areas of the country are more likely to experience earthquakes than others, large earthquakes can occur anywhere on the continent, and without warning.

On average, 100 earthquakes of magnitude 3 or more are recorded in Australia each year. Earthquakes greater than magnitude 5.0, such as the destructive Newcastle earthquake in 1989, occur on average every one to two years. About every ten years, Australia experiences a potentially damaging earthquake of magnitude 6.0 or greater, such as the magnitude 6.5 Meckering earthquake in October 1968.

Why are earthquakes felt in different ways?

Magnitude is only one indicator of the expected impacts that an earthquake can have in a given region. The severity of the resulting tremors during an earthquake is affected by other specific and unique factors such as the depth of the earthquake, local geology and soil conditions.

Damage to buildings also depends on the quality and type of construction. Obviously, the distance from the epicenter will also have an effect on the vibrations felt in a place.

The epicenter

The two-dimensional location of an earthquake is known as its epicenter. This is the point plotted on a map. Sometimes the location of the earthquake is also referred to as the hypocenter. The hypocenter is the three-dimensional location of the earthquake, which along with its location includes its depth in the Earth. Because a seismogram clearly represents the start of an earthquake, the measured location defines the point where the earthquake begins to rupture along the fault plane. For large earthquakes, the epicenter is not necessarily the point along the fault plane where the most tremors or damage will occur.

Monitoring earthquakes and tsunamis

Geoscience Australia monitors, analyzes and reports significant earthquakes to alert emergency managers, government and the public to earthquakes in Australia and abroad. These alerts then allow emergency managers to inform the community of earthquakes in their local area and the appropriate level of emergency response and assistance to send.

No warning

Earthquakes cannot be predicted. An earthquake can only be alerted for its time, location and magnitude after it has been recorded and analyzed by the Geoscience Australia Duty seismologist. First, the seismic energy must be large enough to achieve a minimum of three seismometers. It may take about 3 to 4 minutes.

Once the location of the earthquake has been calculated, the magnitude can be determined. It may take another 3-4 minutes. Thus, earthquakes are usually notified in about 10 minutes, but are always alerted as soon as possible.

Sometimes alerts can be as fast as 6-7 minutes depending on the distribution and the number of seismometers around the epicenter. However, small earthquakes with magnitude less than 2.5 can take longer because seismic waves can sometimes be difficult to analyze.

Inform the community in the event of an earthquake

Earthquake alerts are important to emergency managers and the general public. For small earthquakes that occur nearby, the quake may sound like an explosion or a thud, as if something has crashed in the house.

The affected population wants to know what it was and what is likely to happen. If Geoscience Australia can locate a seismic event, then it can determine its magnitude and inform the public through the Geoscience Australia website and @EarthquakesGA Twitter feed.

For large, potentially damaging earthquakes that can shake an entire state, the need for an immediate emergency response to save lives becomes vital. Although a large area or region may have been affected by the tremors of the earthquake, the earthquake alert provides emergency responders with the location of areas most likely to suffer the greatest damage and where to initially focus their efforts. safety.

What to do in the event of an earthquake

There are many ways to remember what to do in the event of an earthquake. People experiencing an earthquake should stay indoors and seek refuge under a table, bench, or door frame – you can remember to “Drop, Cover, Hold” when you feel an earthquake. Next, staying alert is essential – expect aftershocks, listen to local radio for updates, avoid using phones or driving, stay calm, and help others if possible.

You can help

This information has been reproduced with permission from Geoscience Australia.

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