18 years ago today - the Kobe Earthquake - Japan

Eighteen years ago to the day the Kobe earthquake shook Japan.  The Great Hanshin earthquake occurred on Tuesday, January 17, 1995, at 05:46 JST in the southern part of Japan. It measured 6.8 on the moment magnitude scale (USGS), and Mj7.3 (adjusted from 7.2) on JMA magnitude scale.  The tremors lasted for approximately 20 seconds. The focus of the earthquake was located 16 km beneath its epicentre, on the northern end of Awaji Island, 20 km away from the city of Kobe.

An earthquake is a quick release of energy in the Earth’s crust, creating seismic waves. The Earth’s crust is made up of tectonic plates. The tectonic plate edges move against each other all the time, but sometimes they get stuck. When this happens energy builds up to a point of rupture and the seismic energy is released.

The depth of an earthquake is very important.  If it is shallow it will cause much more structural damage to buildings than a deep one. On the Earth’s surface an earthquake will manifest itself by shaking buildings and the ground moving. If an earthquake occurs at sea it can move and displace the seabed. The water on the seabed of an ocean or large lake is normally very still, with waves only occurring near the surface, but if the seabed moves it can cause a deep and harmful that can result in a tsunami as happened in Japan in 2011.

In the Kobe earthquake, approximately 6,434 people lost their lives (final estimate as of December 22, 2005); about 4,600 of them were from Kobe.

A section of the Nojima Fault

The Great Hanshin earthquake belonged to a third type, called an "inland shallow earthquake". Earthquakes of this type occur along active faults. Even at lower magnitudes, they can be very destructive because they often occur near populated areas and because their hypocenters are located less than 20 km below the surface. The Great Hanshin earthquake began north of the island of Awaji, which lies just south of Kobe. It spread toward the southwest along the Nojima Fault on Awaji and toward the northeast along the Suma and Suwayama faults, which run through the center of Kobe. Observations of deformations in these faults suggest that the area was subjected to east-west compression, which is consistent with previously known crustal movements.

The earthquake proved to be a major wake-up call for Japanese disaster prevention authorities. Japan installed rubber blocks under bridges to absorb the shock and rebuilt buildings further apart to prevent them from falling like dominoes. The national government changed its disaster response policies in the wake of the earthquake, and its response to the 2004 Chūetsu earthquake was significantly faster and more effective.  The earthquake and tsunami of 2011 though was much larger than anything that had been seen before causing almost 16,000 deaths.  There was little that could be done to stop such a force of nature.

A large amount of data was collected after the tsunami of 2011 that provides "the possibility to model in great detail what happened during the rupture of an earthquake." The effect of this data is expected to be felt across other disciplines as well, and this disaster will "provide unprecedented information about how buildings hold up under long periods of shaking – and thus how to build them better.

Earthquakes can strike at any time and are unpredictable in their nature.  Fortunately these types of earthquake are not that common.  Early warning systems are in place for many countries around earthquake hot spots so as to try and give people time to get to safety.

See also:-
http://en.wikipedia.org/wiki/Japan_tsunami
http://en.wikipedia.org/wiki/Kobe_earthquake
http://www.bbc.co.uk/learningzone/clips/a-geological-explanation-of-earthquakes/6736.html

New GPS solution provides three-minute tsunami alerts

The invention of GPS Systems has benefitted many people but it is being put to new uses.

Researchers have shown that, by using global positioning systems (GPS) to measure ground deformation caused by a large underwater earthquake, they can provide accurate warning of the resulting tsunami in just a few minutes after the earthquake onset. For the devastating Japan 2011 event, the team reveals that the analysis of the GPS data and issue of a detailed tsunami alert would have taken no more than three minutes.

Most tsunamis, including those in offshore Sumatra, Indonesia in 2004 and Japan in 2011, occur following underwater ground motion in subduction zones, locations where a tectonic plate slips under another causing a large earthquake. To a lesser extent, the resulting uplift of the sea floor also affects coastal regions. There, researchers can measure the small ground deformation along the coast with GPS and use this to determine tsunami information.
“High-precision real-time processing and inversion of these data enable reconstruction of the earthquake source, described as slip at the subduction interface. This can be used to calculate the uplift of the sea floor, which in turn is used as initial condition for a tsunami model to predict arrival times and maximum wave heights at the coast,” says lead-author Andreas Hoechner from the German Research Centre for Geosciences (GFZ).
In the new Natural Hazards and Earth System Sciences paper, the researchers use the Japan 2011 tsunami, which hit the country’s northeast coast in less than half an hour and caused significant damage, as a case study. They show that their method could have provided detailed tsunami alert as soon as three minutes after the beginning of the earthquake that generated it.
“Japan has a very dense network of GPS stations, but these were not being used for tsunami early warning as of 2011. Certainly this is going to change soon,” states Hoechner.

The scientists used raw data from the Japanese GPS Earth Observation Network (GEONET) recorded a day before to a day after the 2011 earthquake. To shorten the time needed to provide a tsunami alert, they only used data from 50 GPS stations on the northeast coast of Japan, out of about 1200 GEONET stations available in the country.

At present, tsunami warning is based on seismological methods. However, within the time limit of 5 to 10 minutes, these traditional techniques tend to underestimate the earthquake magnitude of large events. Furthermore, they provide only limited information on the geometry of the tsunami source (see note). Both factors can lead to underprediction of wave heights and tsunami coastal impact. Hoechner and his team say their method does not suffer from the same problems and can provide fast, detailed and accurate tsunami alerts.
The next step is to see how the GPS solution works in practice in Japan or other areas prone to devastating tsunamis. As part of the GFZ-lead German Indonesian Tsunami Early Warning System project, several GPS stations were installed in Indonesia after the 2004 earthquake and tsunami near Sumatra, and are already providing valuable information for the warning system.

“The station density is not yet high enough for an independent tsunami early warning in Indonesia, since it is a requirement for this method that the stations be placed densely close to the area of possible earthquake sources, but more stations are being added,” says Hoechner.

Note
Traditional tsunami early warning methods use hypocentre (the point directly beneath the epicentre where the seismic fault begins to rupture) and magnitude only, meaning the source of the earthquake and tsunami is regarded as a point source. However, especially in the case of subduction earthquakes, it can have a large extension: in Japan in 2011 the connection between the tectonic plates broke on a length of about 400km and the Sumatra event in 2004 had a length of some 1500km. To get a good tsunami prediction, it is important to consider this extension and the spatial slip distribution.

For more information visit:-
http://www.nat-hazards-earth-syst-sci.net/13/1285/2013/nhess-13-1285-2013.html
http://www.egu.eu/news/64/gps-solution-provides-three-minute-tsunami-alerts/