Japanese expert warns Bangkok over long-period ground motion risks

The Earthquake Research Center of Thailand (EARTH), together with the National Research Council of Thailand (NRCT), the Faculty of Engineering, Chulalongkorn University, and The Engineering Institute of Thailand under H.M. The King’s Patronage, held a major academic conference to mark one year since the major earthquake under the theme, “One year after the March 28, 2025 earthquake: Lessons and the future of safety for Thailand”, to review safety measures and strengthen preparedness for future disasters.

A key highlight of the event was a special lecture by Prof Gaku Shoji, an expert from the University of Tsukuba in Japan, on the topic “Dynamic Response Analysis and Monitoring of Bridges: From Observation to Design Code”.

In the lecture, he presented in-depth findings on the vulnerability of infrastructure in major cities to earthquake events.

Long-period waves and the danger to Bangkok

Professor Shoji began by expressing his condolences to those affected by the March 28, 2025, earthquake.

He pointed out that although the epicentre was more than 1,000 kilometres from Bangkok, the shaking could still have a significant impact on large structures in the capital.

This phenomenon is caused by long-period seismic waves, which are often observed in areas with thick, soft sedimentary soil layers, such as the Bangkok Basin.

From his analysis, Professor Shoji said infrastructure classified as “long-period” structures faces a high risk of resonance with this type of wave.

Examples of structures that require particularly close monitoring include elevated transport systems and urban rail networks, such as the MRT Blue Line, whose natural period ranges from 0.5 to 1.2 seconds; large cable-stayed bridges, such as the new Rama IX Bridge, whose natural swaying period can be as high as 3.0 to 5.0 seconds; and oil storage tanks and wastewater treatment systems, where liquid sloshing may have long periods ranging from 2.0 to 30 seconds, depending on the size and shape of the tank.

A costly lesson from Japan: low acceleration, heavy damage

To illustrate the danger more clearly, Professor Shoji cited the 2011 Tohoku Earthquake in Japan.

Although the Tokyo Bay and Yokohama areas were as far as 400 kilometres from the epicentre and recorded a peak acceleration of only around 2.0 m/s², which is considered relatively low, the shaking lasted for more than five minutes, or 300 seconds, and included long-period wave components exceeding two seconds.

This resulted in minor to moderate structural damage to cable-stayed bridges and high-rise buildings due to severe dynamic response.

“The damage was not caused by a violent immediate impact, but by prolonged shaking that allowed the structure to accumulate energy and resonate, which could lead to widespread disruption of transport systems.”

From observation to a virtual model

To tackle this challenge, Professor Shoji presented a structural monitoring approach using the Tsurumi Tsubasa Bridge in Tokyo as a case study.

The cable-stayed bridge, which is more than 1,020 metres long, has been equipped with a modern seismic sensor system.

The system enables engineers to carry out “system identification” in order to determine the bridge’s actual physical characteristics while it is in service.

The key steps are data collection, which records the structure’s movement in the vertical, horizontal and torsional directions; computer modelling, in which a highly detailed mathematical model is created to simulate the bridge’s behaviour, with more than 773 nodes used in the case study; and validation, in which real measured data is compared with the model to improve the accuracy of damage prediction.

Professor Shoji said data from actual observation is highly important because it helps engineers determine whether protective devices, such as bridge bearings and seismic damping devices, are still functioning as designed.

Simulating future scenarios

The research team also simulated scenarios by using earthquake waves from events actually recorded in Bangkok, together with projected future earthquake waves, to test the cable-stayed bridge model.

The study found that under earthquake levels previously experienced in Bangkok, cable-stayed bridge structures designed to current standards still maintain an adequate level of safety.

However, if a maximum projected earthquake were to occur, such as a magnitude-7.9 Kanto earthquake scenario, the structure could begin to display nonlinear behaviour, meaning permanent damage could start to occur at some points in the structure.

Conclusion and recommendations

At the end of the lecture, Professor Shoji proposed a “Data-driven Infrastructure Resilience Design” approach as a model for Thailand, with emphasis on integrating data from the real world and the virtual world.

Installing sensor systems on bridges and critical infrastructure in Thailand is not merely a matter of collecting research data, he said, but an important tool for preventive maintenance.

It would help relevant agencies assess damage quickly after an event and plan structural reinforcement with precision before the next disaster occurs.

The conference was not only a commemoration of a past event, but also an effort to lay an important engineering foundation so that Thailand can overcome future earthquake challenges and build a society with resilient and safer infrastructure in the long term on a sustainable basis.