Bibek Parajuli

When an earthquake occurs, the intensity of ground shaking can vary significantly from one location to another. This variation is primarily due to the geological properties of the site where the earthquake is taking place. Understanding these local site effects is essential for predicting the potential impact of an earthquake on a specific location and taking appropriate measures to mitigate its effects.

What are Local Site Effects?

Local site effects refer to the amplification or attenuation of ground motion during an earthquake that occurs due to the geological properties of the site. The intensity of ground shaking during an earthquake is affected by a combination of factors such as the earthquake’s magnitude, distance from the epicenter, and the type of soil and rock present at the site. Different types of soil and rock have different levels of stiffness and density, which affect the way they respond to seismic waves.

For instance, soft soil and loose sediment layers can amplify the ground shaking during an earthquake, while hard rock can attenuate it. This amplification effect can be particularly severe in urban areas, where man-made structures can interact with the soil to create resonance, leading to a significant increase in ground motion. In contrast, hilly or mountainous areas with hard rock are less likely to experience strong ground motion during an earthquake.

Why is it important to consider Local Site Effects?

Understanding local site effects is crucial for assessing the potential impact of an earthquake on a specific location. It helps in predicting the intensity of ground shaking and the resulting damage to buildings, infrastructure, and human life. Site-specific assessments can also help in developing effective earthquake-resistant designs and building codes that consider the geological properties of the site.

For example, tall buildings located on soft soil or reclaimed land can be particularly vulnerable to earthquake-induced ground motion. These buildings may need additional reinforcement and foundation design to withstand the amplified shaking. Similarly, bridges, tunnels, and other critical infrastructure located in high-risk areas may require special attention to ensure their safety during an earthquake.

Local Site Effect during 2015 Gorkha Earthquake

The 2015 Gorkha earthquake, also known as the Nepal earthquake, was a devastating earthquake that occurred on April 25, 2015, with a magnitude of 7.8 on the Richter scale. The earthquake caused widespread damage and loss of life in Nepal and neighboring countries, with a death toll of over 8,000 people.

In the case of the 2015 Gorkha earthquake, the local site effect played a significant role in the damage that occurred in Kathmandu, the capital of Nepal. Kathmandu is situated on a sedimentary basin, which is known to amplify the seismic waves generated by earthquakes. The city’s geology, coupled with the haphazard urban development, created an environment that was highly susceptible to seismic waves.

The valley is composed of layers of sediments that have been deposited by the rivers that flow through it over millions of years. The city is built on a mix of soft and hard sediments, which means that the ground shaking was amplified in some parts of the city and attenuated in others. This led to significant variations in the damage that was caused to buildings and infrastructure. In some areas of the city, the ground shaking was amplified by a factor of two or more. In other areas of the city, the ground shaking was attenuated by the presence of hard rock, which meant that buildings were less likely to collapse.

To demonstrate the local site effect, I have conducted a simulation using python to analyze the response of a multi-degree-of-freedom (MDOF) system to the acceleration time histories obtained from three different locations in the Kathmandu Valley: Kritipur, Patan, and Thimi. The locations of the stations are clearly marked on the map below:

The simulation involved modeling the MDOF system using appropriate structural and mechanical properties and inputting the acceleration time histories obtained from the three different locations in the valley. The properties of the MDOF system are as follows:

Lumped mass at each floor = 1000 KN
Stiffness at each floor = 2467 KN/m
Number of floors = 4

The result from the simulation is shown in the video below:

The simulation presented above provides interesting insights into the local site effects that occurred during the 2015 Gorkha earthquake in the Kathmandu Valley. We observed that, although the peak ground acceleration (PGA) values were similar in all three sites (Kritipur, Patan, and Thimi), the response of the MDOF system to the acceleration time histories varied significantly depending on the geological conditions of the site.

In Kritipur, where the soil is underlain by rocky strata, the energy dissipation during the earthquake was high, leading to attenuation of lower frequency wave forms and higher frequency content in the wave. Consequently, the MDOF system responded with low oscillation to the acceleration time history from Kritipur, as the attenuated waveform did not significantly affect the system modeled with the structural properties used in the simulation.

On the other hand, in Thimi, where there is sediment deposit, the wave was amplified and the frequency content decreased. This led to higher oscillation of the MDOF system to the acceleration time history obtained from Thimi. The amplification of the wave was due to the interaction of the seismic waves with the soft sedimentary layers, which tend to amplify the ground motion. The decrease in frequency content was due to the dispersion of the wave, as the different frequencies traveled at different speeds through the heterogeneous soil layers.

Conclusion

Overall, the simulation highlights the importance of understanding the local site effects in earthquake risk assessments and designing more resilient structures. By accounting for the local site effects, it is possible to identify areas of high seismic hazard and design structures that are better able to withstand the effects of earthquakes. This can help to reduce the risk of damage and loss of life during earthquakes, particularly in regions of high seismic hazard such as the Kathmandu Valley.