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Mexico earthquakes: What we know so far

Mexico earthquakes: What we know so far

On Friday 8 September 2017 at 04:49 am UTC, a magnitude 8.1 earthquake hit off the coast of Mexico, 87 km SW of Pijijiapan. According to the U.S. Geological Survey, the epicentre was at 15.07 N, 93.72 W at a depth of about 69.7 km. Yesterday, another strong (magnitude 7.1) earthquake hit central Mexico, 55 km SSW of the city of Puebla and 120 km south of Mexico City.

Despite the lower magnitude, yesterday’s earthquake, which struck at a depth of 51 km, has caused widespread destruction. At the time of writing, official estimates put the death toll at 217 (according to Mexico’s National Coordinator for Civil Protection, Luis Felipe Puente), with shaking causing damage to and the collapse of hundreds of buildings in Mexico City and surrounding areas.

“The M 7.1 earthquake was much closer to Mexico City, a city build on a dried lake bed; this caused presumably (needs to be confirmed by data) much higher shaking in the densely populated capital then the larger, but farther M 8.1 event,” explains Martin Mai, President of the EGU’s Seismology Division.

Based on current information, the U.S. Geological Survey estimates that significant causalities are likely in the region. Given the mix of vulnerable and earthquake resistant structures, the economic loss is also expected to be high. For more information visit impact pages of the event on the USGS website.

It is too early to say whether a link exists between the two September earthquakes.

“It could be that stress changes caused by the M 8.1 event brought the fault (system) on which the M 7.1 earthquake happened closer to failure; but this requires detailed quantitative analysis,” clarifies Mai.

Editor’s note: This post will be update as more information about the earthquake becomes available.

Further reading and resources:

U.S. Geological Survey overview of 19.09.2017 M 7.1 earthquake (includes interactive, shake and regional information maps)

U.S. Geological Survey overview of 08/09.2017 M 8.1 earthquake (includes interactive, shake and regional information maps, as well as finite fault results and moment tensor information)

Temblor blog post on M 7.1 earthquake

Temblor blog post on M 8.1 earthquake

Did Mexico dodge a bullet in last week’s M=8.1 earthquake? (Temblor blog post on dynamics of 8th September quake)

European-Mediterranean Seismological Centre information about yesterday’s earthquake

SSN (Mexico) page about yesterday’s earthquake (in Spanish)

GFZ GEOFON Global Seismic Network event page for yesterday’s earthquake

Is it an earthquake, a nuclear test or a hurricane? How seismometers help us understand the world we live in

Is it an earthquake, a nuclear test or a hurricane? How seismometers help us understand the world we live in

Although traditionally used to study earthquakes, like today’s M 8.1 in Mexico,  seismometers have now become so sophisticated they are able to detect the slightest ground movements; whether they come from deep within the bowels of the planet or are triggered by events at the surface. But how, exactly, do earthquake scientists decipher the signals picked up by seismometers across the world? And more importantly, how do they know whether they are caused by an earthquake, nuclear test or a hurricane?  

To find out we asked Neil Wilkins (a PhD student at the University of Bristol) and Stephen Hicks (a seismologist at the University of Southampton) to share some insights with our readers.


Seismometers are highly sensitive and they are able to detect a magnitude 5 earthquake occurring on the other side of the planet. Also, most seismic monitoring stations have sensors located within a couple of meters of the ground surface, so they can be fairly susceptible to vibrations at the surface. Seismologists can “spy” on any noise source, from cows moving in a nearby field to passing trucks and trains.

A nuclear test

On Sunday the 3rd of September, North Korea issued a statement announcing it had successfully tested an underground hydrogen bomb. The blast was confirmed by seismometers across the globe. The U.S.  Geological Survey registered a 6.3 magnitude tremor, located at the Punggye-ri underground test site, in the northwest of the country. South Korea’s Meteorological Administration’s earthquake and volcano center also detected what is thought to be North Korea’s strongest test to date.

However they occur, explosions produce ground vibrations capable of being detected by seismic sensors. Mining and quarry blasts appear frequently at nearby seismic monitoring stations. In the case of nuclear explosions, the vibrations can be so large that the seismic waves they produce can be picked up all over the world, as in the case of this latest test.

It was realised quite early in the development of nuclear weapons that seismology could be used to detect such tests. In fact, the need to have reliable seismic data for monitoring underground nuclear explosions led in part to the development of the Worldwide Standardized Seismograph Network in the 1960s, the first of its kind.

Today, more than 150 seismic stations are operating as part of the International Monitoring System (IMS) to detect nuclear tests in breach of the Comprehensive Test-Ban Treaty (CTBT), which opened for signatures in 1996. The IMS also incorporates other technologies, including infrasound, hydroacoustics and radionuclide monitoring.

The key to determining whether a seismic signal is from an explosion or an earthquake lies in the nature of the waves that are present. There are three kinds of seismic wave seismologists can detect. The fastest, called Primary (P) waves, cause ground vibrations in the same direction that they travel, similar to sound waves in the air. Secondary (S) waves cause shaking in a perpendicular direction. Both P and S waves travel deep through the Earth and are known collectively as body waves. In contrast, the third type of seismic waves are known as surface waves, because they are trapped close to the surface of the Earth. In an earthquake, it is normally surface waves that cause the most ground shaking.

In an explosion, most of the seismic energy is released outwards as the explosive material rapidly expands. This means that the largest signal in the seismogram comes as P waves. Explosions therefore have a distinctive shape in the seismic data when compared with an earthquake, where we expect S and surface waves to have higher amplitude.

Forensic seismologists can therefore make measurements of the seismic data to determine whether there was an explosion. An extra indication that a nuclear test occurred can also be revealed by measuring the depth of the source of the waves, as it would not be possible to place a nuclear device deeper than around 10 km below the surface.

Yet while seismic data can tell us that there has been an explosion, there is nothing that can directly identify that explosion as being nuclear. Instead, the IMS relies on the detection of radioactive gases that can leak from the test site for final confirmation of what kind of bomb was used.

The figure shows (at the bottom) the seismic recording of the latest test in North Korea made at NORSAR’s station in Hedmark, Norway. The five upper traces show recordings at the same station for the five preceding tests, conducted by North Korea in 2006, 2009, 2013 and 2016 (two explosions in 2016). The 2017 test, is as can be seen from this figure, clearly the strongest so far. Credit: NORSAR.

When North Korea conducted a nuclear test in 2013, radioactive xenon was detected 55 days later, but this is not always possible. Any detection of such gases depends on whether or not a leak occurs in the first place, and how the gases are transported in the atmosphere.

Additionally, the seismic data cannot indicate the size of the nuclear device or whether it could be attached to a ballistic missile, as the North Korean government claims.

What seismology can give us is an idea of the size of the explosion by measuring the seismic magnitude. This is not straightforward, and depends on knowledge of exactly how deep the bomb was buried and the nature of the rock lying over the test site. However, by comparing the magnitude of this latest test with those from the previous five tests conducted in North Korea, we can see that this is a much larger explosion.

The Norwegian seismic observatory NORSAR has estimated a blast equivalent to 120 kilotons of TNT, six times larger than the atomic bomb dropped on Nagasaki in 1945, and consistent with the expected yield range of a hydrogen bomb.

Hurriquakes?

Nuclear tests are not the only hazard keeping our minds busy in the past few weeks. In the Atlantic, Hurricanes Harvey, Irma and Katia have wreaked havoc in the southern U.S.A, Mexico and the Caribbean.

Hurricanes in the Atlantic can occur at any time between June and November. According to hurricane experts, we are at the peak of the season. It is not uncommon for storms to form in rapid succession between August, September and October.

The National Hurricane Centre (NHC) is the de facto regional authority for producing hurricane forecasts and issuing alerts in the Atlantic and eastern Pacific. For their forecasts, meteorologists use a combination of on the ground weather sensors (e.g. wind, pressure, Doppler radar) and satellite data.

As hurricane Irma tore its way across the Atlantic, gaining strength and approaching the Caribbean island of Guadeloupe, local seismometers detected its signature, sending the global press into a frenzy. It may come as a slight surprise to some people that storms and hurricanes also show on seismometers.

However, a seismometer detecting an approaching hurricane is not actually that astonishing. There is no evidence to suggest that hurricanes directly cause earthquakes, so what signals can we detect from a hurricane? Rather than “signals”, seismologists tend to refer to this kind of seismic energy as “noise” as it thwarts our ability to see what we’re normally looking out for – earthquakes.

The seismic noise from a storm doesn’t look like distinct “pings” that we would see with an earthquake. What we see are fairly low-pitched “hums” that gradually get louder in the days and hours preceding the arrival of a storm. As the storm gets closer to the sensor, these hums turn into slightly higher-pitched “rustling”. This seismic energy then wanes as the hurricane drifts away. We saw this effect clearly for Hurricane Irma with recordings from a seismometer on the island of Guadeloupe.

What causes these hums and rustles? If you look at the frequency content of seismic data from any monitoring station around the globe, noise levels light up at frequencies of ~0.2 Hz (5 s period). We call these hums “microseism”. Microseism is caused by persistent seismic waves unrelated to earthquakes, and it occurs over huge areas of the planet.  One of the strongest sources of microseism is caused by ocean waves and swell. During a hurricane, swell increases and ocean waves become more energetic, eventually crashing into coastlines, transferring seismic energy into the ground. This effect is more obvious on islands as they are surrounded by water.

As the hurricane gets closer to the island, wind speeds dramatically increase and may dwarf the noise level of the longer period microseism. Wind rattles trees, telegraph poles, and the surface itself, transferring seismic energy into the ground and moving the sensitive mass inside the seismometer. This effect causes higher-pitched “rustles” as the centre of the storm approaches. Gusts of wind can also generate pressure changes inside the seismometer installation and within the seismometer itself, generating longer period fluctuations.

During Hurricane Irma, a seismic monitoring station located in the Dutch territory of St. Maarten clearly recorded the approach of the storm, leading to an intense crescendo as the eyewall crossed the area. As the centre of the eye passed over, the seismometer seems to have recorded a slightly lower noise level. This observation could be due to the calmer conditions and lower pressure within the eye. The station went down shortly after, probably from a power outage or loss in telemetry which provides the data in real-time.

Seismometers measuring storms is not a new observation. Recently, Hurricane Harvey shook up seismometers located in southern Texas. Even in the UK, the approach of winter storms across the Atlantic causes much higher levels of microseism.

It would be difficult to use seismometer recordings to help forecast a hurricane – the recordings really depend on how close the sensor is to the coast and how exposed the site is to wind. In the event of outside surface wind and pressure sensors being damaged by the storm, protected seismometers below the ground could possibly prove useful in delineating the rough location of the hurricane eye, assuming they maintain power and keep sending real-time data.

At least several seismic monitoring stations in the northern Antilles region were put out of action by the effects of the Hurricane. Given the total devastation on some islands, it is likely that it will take at least several months to bring these stations back online. The Lesser Antilles are a very tectonically active and complex part of Earth; bringing these sensors back into operation will be crucial to earthquake and volcano hazard monitoring in the region.

By Neil Wilkins (PhD student at the University of Bristol) and Steven Hicks (a seismologist at the University of Southampton)

References and further reading

GeoSciences Column: Can seismic signals help understand landslides and rockfalls?

NORSAR Press Release: Large nuclear test in North Korea on 3 September 2017

The Comprehensive Nuclear-Test-Ban Organization Press Release: CTBTO Executive Secretary Lassina Zerbo on the unusual seismic event detected in the Democratic People’s Republic of Korea

First Harvey, Then Irma and Jose. Why? It’s the Season (The New York Times)

NOAA  National Hurricane Center

IRIS education and outreach series: How does a seismometer work?

January GeoRoundup: the best of the Earth sciences from across the web

January Georoundup: the best of the Earth sciences from across the web

The start of the new year sees the launch of a new series here on GeoLog. Drawing inspiration from popular stories on our social media channels, as well as unique and quirky research news, this monthly column aims to bring you the best of the Earth and planetary sciences from around the web.

Major stories

One of the biggest stories of this month was the anticipated release of the average global surface temperatures for 2016. It probably wasn’t a great surprise to discover that newly released National Oceanic and Atmospheric Administration (NOAA), NASA and the UK’s MetOffice data showed 2016 was the hottest year on record. On average, temperatures last year were 0.99℃ higher than the mid-century mean. It marks the third year in a row that Earth has registered record-breaking temperatures and highlights a trend, as climate blogger, Dana Nuccitelli, explains in an article for The Guardian:

“We’re now breaking global temperature records once every three years”

This video, showing NASA global surface temperature record since 1880, illustrates the point clearly. There were no record breaking years between 1945 and 1976, but since 1980 there have been 12.

 

This month also saw the inauguration of the 45th President of the United States. A fierce climate-change denier, Donald Trump’s rise to power has many worried about the future of climate change policy at the White House. To shine a light on the realities of climate change in the face of a largely climate sceptic administration and despite the ever rising global temperatures, The Guardian dedicate 24 hours to reporting on how climate changes is affecting regions across the globe. Among the comprehensive coverage this collection of climate facts stands out.

And it turns out the fears about the newly elected administration may not have been unfounded. Earlier this week the US Department of Agriculture and other federal agencies issued a ban preventing its scientists from communicating with the press and public about their research findings; even on social media. The order has since been rescinded, but US- based scientists remain concerned. In response, the AGU has written a letter to federal agencies in the US defending the protection of scientific integrity and open communications.

Closer to home, central Italy was struck by a sequence of four earthquakes on 18th of January, with the largest registering a magnitude of 5.7. The epicentres were located close to the town of Amatrice – in a region already shaken by several strong, and sometimes devastating, earthquakes in 2016. Later that day, and following a period of very heavy snowfall, a deadly avalanche in the Apennines buried a hotel in the Grand Sasso resort area, which had also been affected by the earthquakes. Although some news reports were quick to suggest the avalanche had been triggered by the earthquakes, researchers will need more data and a more detailed analysis to make this connection.

What you might have missed

While we are on the topic of climate change, a newly published report by the  European Environmental Agency is not to be missed.

“Climate change poses increasingly severe risks for ecosystems, human health and the economy in Europe.”

The document assesses the latest trends and projections on climate change and its impacts across Europe. While the effects of increasing temperatures will be felt across the continent, Southern and south-eastern Europe is projected to be a climate change hotspot with the forecasts showing the region will bear the brunt of the impacts.

A powerful 7.9 magnitude earthquake struck off the coast of Papua New Guinea on 22nd January. While the USGS estimated there was a low risk to property, tsunami warnings were issued across the South Pacific. In the wake of the tremor, the Incorporated Research Institutions for Seismology (IRIS) tweeted this neat ground motion visualisation of the earthquake waves.

A study published in Nature and summarised in Eos, highlights that while overt discrimination of women in the geosciences has not been as prevalent in recent years, many female scientists are still subject to subtle and unconscious bias leading to barriers to success in the geosciences.

Five links we liked

  • NOAA’s new GOES-R satellite for weather monitoring returned its first stunning photos of planet Earth – here are six reasons why the data it will acquire matter.
  • Snow fell in the Sahara for the first time in 37 years! This photogallery shows the usually red sand dunes of the desert covered in a sprinkling of white snow.
  • Scientist at The University of Cambridge have published the first global map of flow within the Earth’s mantle, showing that the surface moves up & down “like a yo-yo”.

The EGU story

At the EGU, the highlight of the month is the number of abstracts we received from researchers wishing to present and discuss their science at the EGU 2017 General Assembly. With over 17,500 abstracts, and an improved set-up to accommodate the high number of expected participants, the conference promises to be the largest and most exciting to date. We look forward to welcoming everyone in Vienna on 23–28 April!

And don’t forget! To stay abreast of all the EGU’s events and activities, from highlighting papers published in our open access journals to providing news relating to EGU’s scientific divisions and meetings, including the General Assembly, subscribe to receive our monthly newsletter.

Shaking on Christmas Day: what we know about the 7.6 M Chile earthquake

Chile, Chiloe earthquake

While the majority of us were midway through our Christmas Day celebrations, a powerful 7.6 M earthquake struck off the western coast of the Chile. Natural hazards are not bound by time, location or festivities; an earthquake can happen at any time in any place, regardless of the significance of the day. As a result, in this earthquake prone region, raising awareness of the risk posed by natural hazards is vitally important.

The Christmas Day quake struck 42 km south west of the port city of Quellón, on the rural island of Chiloé at a depth of 34 km. Despite the powerful shaking, the tremor caused no casualties and damage to infrastructure was limited. For a time, services (such as water and power) to the southern tip of Chiloé were cut. Most affected were roads and bridges, particularly the recently renovated highway 5, which links Quellón with the fishing town of Chonchi.

The earthquake triggered a tsunami warning, leading to the evacuation of 4000 people in the coastal areas of Los Lagos Region, including the towns of Quellón and Chonchi. However, no tsunami waves were reported and the warning was lifted some 90 minutes after the temblor.

Chile’s long history of powerful earthquakes

As recently as September 2015, an 8.3 M tremor hit Illapel, causing 13 casualties, 6 missing and triggering a 4.5 m tsunami wave, with shaking felt as far as Bolivia and Argentina.

A powerful, and destructive, 8.8 M quake struck Maule in February 2010. On land, there was severe loss to infrastructure and housing, while a tsunami wave caused significant damage to coastal areas. Combined, the earthquake and tsunami resulted in the deaths of more than 500 people.

The most powerful tremor ever recorded, the estimated 9.5 M Valdivia earthquake, struck Chile in May 1960. More than 2,000 people were reported dead, a further 3,000 went missing and over 2,000,000 were left homeless. The damage in Southern Chile alone amounted to over $550 million. Tsunami waves generated by the quake struck Hawaii, Japan, the Philippines and the western USA coast, causing a further $50.5 million in damages and killing 231 people.

Damage to houses after the Valdivia earthquake, Chile

Damage to several houses in Chile after the earthquake. Credit: Pierre St. Amand – NGDC Natural Hazards Slides with Captions Header, Public Domain (distributed by Wikimedia Commons)

What causes earthquakes in Chile and what does the future hold?

Chile lies along the Pacific Ring of Fire, an area known for its high seismic and volcanic activity. Here, tectonic plates slide against each other, pull apart or converge and subduct under one another generating geologically active zones.

To understand why powerful earthquakes occur in Chile, we asked Cindy Mora Stock, a seismologist at the University of Concepción (Chile), to give us a more detailed insight into the tectonics of the region:

Earthquakes along the Chilean coast occur at the interface between the South American plate and the subducted Nazca plate. The rapid velocity between these plates (66 – 90 mm/yr) increases the potential for great earthquakes in the region, presenting on average an event of magnitude 8, or larger, every ten years. As a comparison, the Antarctic plate subducts under South American plate at a much slower rate (16 – 22 mm/yr).

The latest Mw 7.6 earthquake near Quellón on 25th of December [1], falls in the central part of the rupture zone (the portion of the fault which slipped during) of  the Valdivia earthquake – roughly 380 km south from Valdivia.

A study by Lange et al in 2007 showed a cluster of four main 4.0 < Ml < 4.4 events and their afteshocks, occurring at the interface between 12-30 km depth, beneath the western coast of Chiloe Island. Another study by Moreno et al in 2011 shows some patches at the interface that ruptured during the previous 1960 event, which are more stuck than other areas at the same interface.

Especially, computer simulations show the interface at the center part of the 1960’s rupture zone is fully locked, this means that part is “stuck”, not moving, and accumulating energy. Zones that present a high locking rate have shown to be prone areas for the nucleation of a great earthquake in the future. Although in all presented scenarios the Chiloe Island presents a high locking rate, this is not enough to state a range of time when an earthquake will occur at this patch.  Considering this, the previous seismicity, and the present Mw7.6 earthquake in the region it might seem like the interface might have ended its and it is starting to build up stress for a future earthquake.

By Laura Roberts, EGU Communications Officer, and Cindy Mora Stock, postdoctoral researcher at the University of Concepcion, Chile.

 

References and further reading

[1] Intensities of shaking felt after the 25 December earthquake (in Spanish): http://www.sismologia.cl/events/sensibles/2016/12/25-1422-28L.S201612.html

[2] Lange, D., Rietbrock, A., Haberland, E., et al.: Seismicity and geometry of the south Chilean subduction zone (41.5°S–43.5°S): Implications for controlling parameters, Geophysical Research Letters, 34, L06311, doi: 0.1029/2006GL029190, 2007

[3] Moreno, M., Melnick, D., Rosenau, M., et al.: Heterogeneous plate locking in the South–Central Chile subduction zone: Building up the next great earthquake, Earth and Planetary Research Letters, 305, 3-4, 413-424, doi: 10.1016/j.epsl.2011.03.025, 2011 (Paywalled)

USGS overview of M7.6 – 42km SW of Puerto Quellon, Chile (includes shake maps, regional tectonic information and moment tensor details): http://earthquake.usgs.gov/earthquakes/eventpage/us10007mn3#executive

Understanding Tectonic Processes Following Great Earthquakes (Eos: Earth & Space Science News)

25 December earthquake in the news:
·         Chile earthquake tsunami warning lifted (BBC News report)
·         Major quake jolts Chile tourist region on Christmas Day (Reuters in-depth news report)
·         Chile jolted by major 7.6-magnitude earthquake (Guardian News)
·         Imagenes del terremoto al sur de Chile (in Spanish: Images of the earthquake in Southern Chile – Gestión, diario de econimía y negocios de Perú)

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