Taiwan’s Plate Tectonics and Seismic Activity

30-Second Overview

Taiwan lies at the convergent boundary between the Eurasian Plate and the Philippine Sea Plate. Intense plate collision has produced Taiwan’s distinctive geological structures and frequent seismic activity. Across the island, 36 active faults have been identified. In the western seismic belt, earthquake hypocenters are shallower, but the area is densely populated; in the eastern seismic belt, hypocenters are deeper, but seismic activity is more frequent.

The 1999 921 earthquake, with a Richter magnitude of 7.3, was the most severe earthquake disaster in modern Taiwan. The fault map updated by the Geological Survey and Mining Management Agency in 2021 is currently a core basis for disaster-prevention planning. Taiwan ranks among the world’s most seismically dense regions and is also an important field site for the study of plate tectonics.

Keywords: plate convergence, active faults, seismic belts, Chelungpu Fault, orogeny

Why It Matters

Taiwan’s Geological Origins

Taiwan’s very existence is a geological marvel. Around 5 million years ago, the area that is now Taiwan was still open ocean. Only after the Philippine Sea Plate pushed northwestward and collided with the Eurasian Plate did the land gradually rise above sea level. This intense geological process continues today, making Taiwan one of the best field sites for studying plate tectonics.

Plate collision proceeds at a rate of 7-8 centimeters per year, continuously uplifting Taiwan’s surface and making earthquakes part of the island’s fate.

Living on a Dynamic Earth

For people in Taiwan, earthquakes are part of the background of everyday life. From seismic design in building codes to the widespread teaching of disaster-prevention education, Taiwanese society has developed a culture of coexistence with earthquakes. Understanding Taiwan’s geological characteristics helps with disaster prevention and mitigation, and deepens our knowledge of this land.

After the 1999 921 earthquake, Taiwan comprehensively revised its seismic building codes, and its earthquake early warning system was gradually upgraded into one of the most advanced systems in Asia.

Plate Tectonics and Geological Background

A Geographic Position Caught Between Two Plates

Taiwan happens to sit on one of the most active plate boundaries on Earth. To Taiwan’s west is the stable Eurasian Plate; to its east is the active Philippine Sea Plate. The two plates push against each other at about 7-8 centimeters per year, roughly the speed at which fingernails grow. The movement appears slow, but its force is enormous.

Off northeastern Taiwan, the Philippine Sea Plate subducts beneath the Eurasian Plate, forming the Ryukyu Trench. Off southeastern Taiwan, the situation is reversed: the Eurasian Plate subducts beneath the Philippine Sea Plate, forming the Manila Trench. Taiwan lies precisely within the transition zone between these two opposite directions of subduction, producing extremely complex geological structures.

Orogeny in Progress

The formation of Taiwan’s mountains is an ongoing process. The Central Mountain Range is the most direct product of plate collision, and most of Taiwan’s mountains above 3,000 meters are concentrated there. Taiwan’s orogeny is not uniform: eastern Taiwan is uplifted by about 3-4 millimeters per year, while western Taiwan is relatively stable, and some coastal areas are even subsiding.

This “asymmetric uplift” explains why eastern Taiwan has many high, rugged mountains, while western Taiwan has more plains and hills. Taiwan’s geographic landscape is still evolving. The Taiwan we see now is completely different from the Taiwan of a million years ago, and from the Taiwan that will exist a million years from now.

Distribution of Taiwan’s Seismic Belts

Western Seismic Belt: A Shallow Threat in a Densely Populated Region

The western seismic belt covers all of western Taiwan, extending from the Taipei Basin to the Pingtung Plain. This seismic belt has several characteristics:

Shallow hypocenters: Most earthquake hypocenters are between 10 and 20 kilometers deep, reflecting faulting within the crust. Although shallow earthquakes are not necessarily very large in magnitude, their proximity to the surface often produces stronger shaking and more serious damage.

Complex fault systems: Multiple active faults are distributed across the western seismic belt, including the Chelungpu Fault, Changhua Fault, and Hsincheng Fault. Most of these are reverse faults, reflecting an east-west compressional tectonic environment.

Major population risk: Because about 80% of Taiwan’s population lives in western Taiwan, any large-scale seismic activity in the western seismic belt could cause severe casualties and economic losses. The 1999 921 earthquake is the clearest example.

Eastern Seismic Belt: Frequent Shaking from Deep-Sea Subduction

The eastern seismic belt is mainly located from the offshore waters east of Taiwan to eastern Taiwan’s land area. Its characteristics differ sharply from those of the west:

Large variation in hypocenter depth: Earthquakes range from shallow events only a few kilometers deep to deep earthquakes at depths of 300 kilometers. Together they form a westward-dipping seismic zone, which is evidence of the subduction of the Philippine Sea Plate.

Extremely high earthquake frequency: Earthquakes occur far more frequently in eastern Taiwan than in western Taiwan, with events of varying size happening almost every day. However, because population density is relatively low, disaster impacts are usually smaller.

Tsunamigenic earthquake risk: Large earthquakes off eastern Taiwan may trigger tsunamis, threatening eastern coastal areas. The 2022 Chishang earthquake on September 18 is a typical example of activity in the eastern seismic belt.

Northeastern Seismic Belt: A Symphony of Volcanoes and Earthquakes

Northeastern Taiwan, including the Greater Taipei area, is influenced by the expansion of the Okinawa Trough and volcanic activity, creating a distinctive seismic environment. Although the Tatun Volcano Group is currently dormant, weak seismic activity continues there, reminding us of the potential volcanic threat in northern Taiwan.

Distribution and Characteristics of Active Faults

Current Status of Active Faults

Taiwan currently has 36 confirmed active faults, according to the latest 2021 version from the Geological Survey and Mining Management Agency¹. Major examples include the Milun Fault, Chishang Fault, and Chelungpu Fault. The former activity classification system has been uniformly abolished and replaced by management through the designation of geologically sensitive areas. The potential earthquake threat posed by each fault varies, making these faults a core basis for disaster-prevention planning.

Building Restrictions in Fault Zones

To reduce earthquake disasters, the government imposes strict land-use controls on active faults. Within 15 meters on each side of an exposed fault trace, that is, within the “fault zone,” construction is completely prohibited on public land. On private land, buildings are restricted to two stories and no more than 7 meters in height. Although this regulation limits land development, it is important for protecting public safety.

Lessons from the Chelungpu Fault

The Chelungpu Fault was the main rupture fault of the 1999 921 earthquake. It is about 105 kilometers long and produced a maximum vertical displacement of 8 meters during the earthquake, directly causing surface rupture and building collapse. Post-earthquake studies found that building damage was most severe within 200 meters on the hanging-wall side of the fault and 100 meters on the footwall side. This finding directly influenced subsequent revisions to building regulations.

Major Modern Earthquake Events

921 Jiji Earthquake (1999/9/21)

At 1:47 a.m. on September 21, 1999, a Richter magnitude 7.3 earthquake occurred near Jiji Township in Nantou County, with a maximum intensity of 7. The earthquake caused 2,415 deaths and 11,305 injuries. More than 100,000 buildings fully or partially collapsed, and direct economic losses were estimated at more than NT$300 billion. It was Taiwan’s deadliest natural disaster of the 20th century. Surface rupture along the Chelungpu Fault extended for about 105 kilometers, with vertical displacement reaching 8 meters in some locations. The 921 earthquake led Taiwan to comprehensively revise its seismic building standards and greatly strengthened its capacity for earthquake engineering research, laying the foundation for Taiwan’s modern earthquake-resilience system.²

0403 Hualien Earthquake (2024/4/3)

At 7:58 a.m. on April 3, 2024, an earthquake of M_L 7.2, or local magnitude, and M_w 7.4, or moment magnitude, occurred offshore eastern Taiwan near Hualien. It was the largest earthquake in Taiwan since the 1999 921 earthquake. The earthquake caused 13 deaths and more than 1,000 injuries, and several buildings in Hualien City tilted severely. On the Shakadang Trail, the earthquake triggered large-scale rock and debris collapse, burying multiple hikers and extending search-and-rescue operations for several days. The Central Weather Administration’s earthquake early warning system completed nationwide alert issuance within about 10 seconds after the mainshock, becoming an important real-world validation case for the EEW system.³

Earthquake Monitoring and Early Warning Systems

Dense Monitoring Network

Taiwan has established one of the densest earthquake monitoring networks in the world. The Central Weather Administration’s Seismological Center has installed more than 150 real-time seismic monitoring stations across Taiwan, averaging one monitoring point for every 300 square kilometers. This dense network can determine an earthquake’s epicenter and magnitude within 20-30 seconds after it occurs.

Earthquake Early Warning System

Taiwan’s earthquake early warning system, or EEW, is one of the most advanced in Asia. When a strong earthquake occurs, the system can issue alerts from several seconds to several tens of seconds before seismic waves arrive, giving the public valuable time to take shelter. Although warning time is limited, the system has crucial disaster-prevention value for high-speed trains, precision industrial equipment, and other vulnerable systems.

Real-Time Strong-Motion Alert Service

The real-time strong-motion alert service launched in 2016 automatically issues warnings through television, radio, mobile push notifications, and other channels when the estimated seismic intensity reaches level 4 or above. Taiwan was among the early countries to build a nationwide earthquake early warning system and is considered a pioneer in Asia.

Earthquakes and Taiwanese Society

Development of Earthquake-Resistant Building Technologies

Taiwan’s seismic building technologies have accumulated and improved through repeated earthquake disasters. From early static seismic design to today’s base isolation and vibration-control technologies, the earthquake resistance of buildings in Taiwan has reached advanced global standards. The tuned mass damper, or TMD, in Taipei 101 is a well-known application of vibration-control technology.

Formation of an Earthquake Culture

Taiwanese attitudes toward earthquakes are both cautious and composed. Earthquake evacuation drills beginning in elementary school, the preparation of household emergency kits, and the “drop, cover, and hold on” response during earthquakes have all become part of Taiwanese culture.

The saying “Taiwanese people are not afraid of earthquakes” does not mean that people in Taiwan are indifferent to earthquakes. Rather, it means they have learned to coexist with them, reducing disaster risk through science and technology while maintaining respect for nature.

Contributions to International Earthquake Research

Taiwan’s distinctive geological environment and rich seismic data have attracted seismologists from many countries to collaborate on research in Taiwan. Taiwanese research findings in areas such as fault behavior and early warning have been published in international journals and have direct reference value for global disaster-prevention practice.

Challenges for the Future

New Threats from Climate Change

In recent years, scientists have found that climate change may affect seismic activity. Increased extreme rainfall may alter groundwater pressure, thereby affecting fault stability. Taiwan needs to incorporate these new risk factors into earthquake disaster-prevention planning.

The Double Impact of Urbanization

Taiwan’s urbanization has produced a double effect. On one hand, the seismic performance of modern buildings has greatly improved. On the other hand, population and assets are increasingly concentrated, so the potential losses from a major earthquake are also growing. How to balance development and safety is a long-term challenge facing Taiwan.

Opportunities from Technological Innovation

New technologies such as artificial intelligence, big data analytics, and the Internet of Things are creating new possibilities for earthquake research and disaster prevention. Taiwan is exploring how to use these technologies to improve earthquake forecasting and early warning systems, making disaster-prevention work more precise and effective.

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