Legend of Wiang Nong Lom and Earthquake Engineering Study

The northern region of Thailand has large cities with significant populations, which is a result of Thailand's economic growth. To manage cities for sustainable development, it is necessary to consider the impact of natural disasters to reduce potential future damage. Although earthquakes are unpredictable natural disasters, lessons from various incidents in many countries show that preparation through studying earthquake risk areas from the past can significantly reduce the impact of earthquakes.

The Mae Chan fault is a large active fault that runs through several provinces in northern Thailand. With a length of more than 100 kilometers, it passes through Mae Ai District in Chiang Mai Province, through Mae Chan and Chiang Saen Districts in Chiang Rai Province, before entering Laos. It can cause earthquakes of magnitude 7.0 Richter or greater. According to a study by Panya Jarusiri (2000), the Mae Chan fault has caused at least 5 major earthquakes (>7.0 Richter) between 940,000 and 1,600 years ago.

An event that emphasizes the importance of earthquakes in the Mae Chan fault area in history is the story from the Singhanawati legend, which mentions the establishment of a city called Wiang Yonok, ruled by the Singhanawati dynasty for a long time. According to the legend, there were forty-five consecutive rulers, and finally, the legend tells of the end of Wiang Yonok by saying that it sank due to an earthquake. Scholars have speculated that Yonok City was likely located in one of two areas: either at the border between Mae Chan and Chiang Saen Districts in Chiang Rai Province, or in the Nong Luang area, Wiang Chai District, Chiang Rai Province. From geological studies and evidence of ancient monuments and artifacts, it was found that the Wiang Nong Lom basin area (Figure 1) is aligned with the Mae Chan fault line. Therefore, it is more likely that Yonok City was located in the Wiang Nong Lom area, because this area is situated near a location where earthquakes could occur.

However, there is another perspective in earthquake engineering that has never been mentioned in explaining the legend of the end of Yonok City. It is possible that instead of the entire city sinking and becoming a swamp, as understood from the legend, from an engineering perspective, a phenomenon called liquefaction may have occurred. This is a phenomenon that happens when there is an earthquake of a certain severity, combined with suitable soil properties in the affected area. When liquefaction occurs, groundwater may rise to the surface, reducing the load-bearing capacity of the soil, causing structures or buildings on the ground to subside. In some areas, water may surge up from the ground, which could be the origin of the legend about flooding and the city sinking.