Research led by Aisling O’Kane, a PhD student in our Department, is helping scientists understand why some sedimentary basins -- low lying regions on Earth’s surface that accumulate sediments -- are particularly prone to hazardous ground shaking following earthquakes, one of the primary causes of building damage.
Seismologists think these sedimentary basins, which are often home to some of the most concentrated urban populations, may focus and trap earthquake waves, causing more intense and longer-lasting tremors.
But they still don’t know exactly what controls the severity of shaking in basins -- whether it’s the strength and depth of the earthquake, or the arrangement and types of the underlying rocks.
In a new study, published in Geophysical Journal International, O’Kane shows that earthquake depth is the main factor controlling ground motion at the surface, but also finds that certain basin shapes can amplify and prolong earthquake tremors more than others.
O’Kane and co-author Dr Alex Copley, also at the Department of Earth Sciences, found the most severe ground shaking happens when the dominant wavelength of the seismic waves -- which increases with the size of the earthquake -- matches the depth of the basin.
Their new model is the first to estimate earthquake ground shaking on a regional scale using a geologically common range of rock configurations – and it can now be applied across the globe. Some of the world’s most densely populated and rapidly developing areas are located in sedimentary basins, including the megacity New Delhi.
“It was clear to me that we needed to identify the main source and structural controls of earthquake shaking in foreland basins on a regional scale. Rather than model a specific basin, we examined the ground motion in a generic geometry found on the edges of mountain belts, to deduce the underlying principles that can be applied to a range of specific locations” said O’Kane.
O’Kane used the freeware model SW4 to simulate how earthquake waves travel through a range of sedimentary basins of different shapes and sizes, upgrading the oft used idealized basin shapes, for the more realistic wedge-shaped basins that form adjacent to mountain ranges.
Taking a representative earthquake as an example, O’Kane estimates that for each earthquake location and size, there is a depth of basin that will result in the most severe ground shaking - shallower or deeper basins wouldn’t produce such amplified ground motions.
“The depth of the earthquake makes sense. It’s like an explosion going off 10km below the surface – you would feel it a lot less than one at 5km depth” said O’Kane, “what was unexpected was how important the relationship between the seismic wavelength and basin depth is in controlling the amplitude and duration of ground shaking”.
Basins magnify ground motion due to their soft sediments, whereas in crystalline rock waves move faster but with lower amplitudes. Earthquake waves move slower through the sediments in the basin as opposed to the crystalline rocks beneath, and they bounce off of the harder rocks underneath and get trapped in the basin. Every time the wave gets to the sediment base more waves are generated and deflected as their speed changes.
O’Kane now plans to apply the model in northwest India, where she has already collected field data on the structural geology and geomorphology. “Northwest India is interesting as it is very different to the rest of the Himalayas. People don’t know a lot about the basin geometry here as the fault bounding the basin is buried at depth. Our new findings tells us about the shape of the fault and how far down it goes.”
Back in 2005 a magnitude 7.6 earthquake struck Muzaffarabad in Pakistan which neighbours Jammu and Kashmir, the northernmost region of the Indian subcontinent. It flattened entire villages killing over 80,000 people and left over 4 million people homeless.
By inputting the field data into models of the Himalayan foreland basin, O’Kane aims to estimate likely ground motions for a range of earthquake magnitudes across the region, which is home to millions of people. “Ultimately we’d like to be able to gain a better understanding of the seismic hazard across the region and produce estimated ground motion maps for this area – a resource that local authorities can use to inform their decision when constructing new infrastructure in the area.”
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Accompanying simulation to O'Kane and Copley (2020) work illustrating the lateral propagation of low-amplitude body waves, followed by higher-amplitude, lower-frequency Rayleigh waves through a model foreland basin setting.
O’Kane, A., & Copley, A. The controls on earthquake ground motion in foreland-basin settings: The effects of basin and source geometry. Geophysical Journal International (2020). DOI: https://doi.org/10.1093/gji/ggaa599