Tag Archives: strain energy

Turkish fault reveals seismic steadiness

Satellite data has shed new light on seismic hazard in one of the world’s most deadly earthquake zones.

Published today in Nature Communications, the study describes how tectonic strain builds up along Turkey’s North Anatolian Fault at a remarkably steady rate.

This means that present-day measurements can not only reflect past and future strain accumulation, but also provide vital information on events still to come.

The strain, which builds up as Turkey is squeezed between three major tectonic plates, has caused almost the entire length of the fault to rupture since 1939 in a series of major earthquakes gradually migrating east to west towards Istanbul.

NAF_strain
Surface velocities along Turkey’s North Anatolian Fault (past ruptures shown in purple/yellow) alongside westward progression of earthquakes since 1939

Led by Ekbal Hussain, the team used satellite images from the European Space Agency’s Envisat mission to identify tiny ground movements at earthquake locations along the fault.

Dr Hussain explained: “Because we know so much about the fault’s recent history, we could look at the strain build up at specific places knowing how much time had passed since the last earthquake.”

The 600-plus satellite images, taken between 2002 and 2010, provided insights into the equivalent of 250 years of the fault’s earthquake repeat cycle.

Remarkably, apart from the ten years immediately after an earthquake, strain rates levelled out at about 0.5 microstrain per year, equivalent to 50mm over a 100km region, regardless of where or when the last earthquake took place.

Dr Hussain added: “This means that the strain rates we measure over the short term can also reflect what’s happening in the longer term, telling us how much energy is being stored on the fault that could eventually be released in an earthquake.”

Until the satellite era, it was difficult to get a clear picture of how strain built up on the fault.  Now, satellites like Envisat, alongside the newer Sentinel-1 mission, can detect ground movements of less than a millimetre, indicating how and where strain is accumulating.

The findings suggest that some existing hazard assessment models, which presume that strain rates vary over time, need to be rethought.  This is especially true for regions where there are long gaps between earthquakes, such as the Himalayas.

Co-author Professor Tim Wright said: “Discovering this consistent strain accumulation will help us to reassess how we model seismic hazards, as well as improving understanding of the earthquake cycle worldwide.”

[1] The full paper is: Hussain et al. (2018) Constant strain accumulation rate between major earthquakes on the North Anatolian Fault, Nature Communications
[2] Lead author Ekbal Hussain is now a Remote Sensing Geoscientist at the British Geological Survey

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How do we measure earthquake strain energy?

I was asked a very interesting question on one of my recent LinkedIn posts that I thought deserved a slightly detailed answer. Here’s the question:

QuestionAnd here’s my answer:

Hi David. Firstly, this is not a basic question at all. In fact, it’s one that is of considerable importance. Your question has two parts and I’ll address them individually.

How do we measure strain energy released in earthquakes?

The main way we do this is by measuring how much the ground moves in an earthquake, this can be done with high precision GPS instruments on the ground or from space based satellite measurements.

Here’s an example from the giant Tohoku earthquake, that struck Japan in 2011, of what we can do with GPS instruments. Each little arrow is a GPS station and it records how much the ground moved during the earthquake.

We can use these kind of measurements to calculate how much the ground has moved all along the fault and at depth during an earthquake. Once we have the full displacement of the fault we can relate that directly to the stress drop and strain release.

How do we measure strain energy being stored on faults?

A similar method is used to determine strain stored on a fault. The ground very slowly warps itself around a locked fault in the decades to centuries before an earthquake.

Here’s an animated model of what I mean (might need a screen refresh to play it). Imagine looking down onto the ground from above with the top half of the earth moving to the right and the bottom moving to the left.

strain_accummulation2
The warping of the ground before an earthquake. Source: Philip England

If we can measure the degree of warping before the earthquake, again by using GPS and satellites, we can relate that to the strain energy getting stored on the fault. Here represented by the different colours, with red being the area of highest strain storage.

I hope that helps to answer your question!

Ekbal