Scientists at the CSIR-North East Institute of Science and Technology have developed a novel model detailing how earthquake waves travel through the highly active crust of Northeast India. The research measures seismic attenuation, or how much energy an earthquake loses as it ripples through the ground. Over the past century, Northeast India has experienced around twenty major earthquakes, including two catastrophic events with magnitudes greater than 8.0, making it an urgent scientific priority to understand the composition of the ground beneath the surface.
The researchers focused on a specific type of tremor known as an Lg wave. These are energetic, high-frequency seismic waves that form by the superposition of multiple waves trapped within the Earth. They’re also known to cause the most significant surface damage during a quake. As these waves travel, their energy is either absorbed or scattered by the rocks they pass through, a physical process called attenuation. A highly fractured and tectonically active crust will scatter the waves rapidly, resulting in what seismologists term a low crustal quality factor.
To investigate this phenomenon, the team analysed data from 30 moderate earthquakes that occurred between 2007 and 2012, utilising a regional network of sensitive broadband seismic stations. To ensure clarity of the seismic signals and avoid background noise, they restricted their analysis to relatively moderate earthquakes with magnitudes between 3.1 and 4.6. By observing 120 unique underground pathways between the earthquake sources and the recording stations, they mathematically tracked the decay of Lg wave amplitudes over distances up to 600 kilometres. The study capped the observation distance at 600 kilometres to prevent the vital Lg wave readings from blending into the fading echoes of other overlapping seismic phases.
The results confirmed that Northeast India possesses a remarkably shattered and complex crust. The team calculated a low average crustal quality factor for the region alongside a very high frequency dependence, a combination of numbers that aligns with other incredibly earthquake-prone areas globally, such as Colombia and Mexico. They discovered that the heavily fractured nature of the local crust strongly scatters seismic energy. This high degree of scattering acts as a geographical fingerprint, revealing the presence of underlying low-velocity zones, trapped fluids, and the immense stress of ongoing tectonic collisions. By breaking the region into three distinct geological zones—the Arunachal Himalayas, the Indo-Burman Ranges, and the Shillong Plateau—they identified that the Shillong Plateau exhibits the highest level of attenuation, highlighting its intensely deformed subterranean structure.
The Lg wave model provides a high-resolution, region-specific picture of the shallow continental crust of Northeast India, in a region with human infrastructure. It helps us understand the precise mechanics of how the ground shakes and absorbs energy during a tremor. The model provides civil engineers and city planners with the mathematical equations needed to accurately predict future ground motion. By applying these regional calculations to local building codes, society can construct resilient homes, bridges, and essential infrastructure capable of withstanding the inevitable future earthquakes in Northeast India, significantly minimising disaster risks and protecting countless lives.
