Deforming Land and Earthquakes in Japan

The earth’s crust undulates dramatically across the Japanese archipelago and its surrounds, ranging from mountains of more than 3,000 meters to ocean trenches with depths exceeding 10,000 meters. This area is also a global hotspot for seismic activity, with around 20% of the world’s earthquakes occurring in and around Japan. The rugged topography and frequent earthquakes are attributable to the high degree of tectonic movement around the Japanese archipelago. Apart from the sudden and dramatic movement associated with earthquakes, tectonic movement proceeds extremely slowly and can only be detected using precise measurement technologies such as the global navigation satellite system (GNSS). GNSS is a system that allows users to track a precise location by receiving radio waves from satellites in space. A GNSS observation network in Japan was established some 30 years ago, enabling detailed insights into the movements of the four tectonic plates that intersect around the Japanese archipelago. Analysis of these movements has been instrumental to understanding the mechanisms of earthquake generation and forecasting their distribution.

This lecture will explain the relationship between earthquakes and the tectonic movements observed around the Japanese archipelago, using the 2024 M7.6 Noto Peninsula earthquake as an example.

Exploring the Mysteries of Earthquakes

The Japanese archipelago is known as a hotbed of earthquakes, as they occur here with high frequency by global standards. The shaking felt during earthquakes is caused by the bedrock beneath the earth’s surface cracking and abruptly shifting out of place. The shaking grows larger as the area of cracking and shifting expands over a short period of time. Recent observations have shed light on the complexity of this process in megaquakes, as well as drawing attention to the phenomenon of “slow earthquakes” in which the bedrock shifts at a much slower pace than in regular earthquakes. There are some reports of the concurrence of earthquakes with differing time constants—regular and slow—with each influencing how the other unfolds. It is crucial to explicate the relationship between the two types in order to understand the mechanisms by which megaquakes occur.

How does a megaquake begin? Is it preceded by a slow earthquake? This lecture will introduce the basic features of earthquakes, including intensity, magnitude, frequency, and scaling, and use examples to explain the diverse ways in which megaquakes can begin.

Making Sense of Earthquake Early Warnings

Most people know about the earthquake early warnings issued on television and mobile telephones, but do you know how they work, or how accurate they are? Earthquake early warnings are a useful source of information just before the ground begins to shake, but they are also subject to limitations, such as accuracy and warning time. Many researchers at universities and the Japan Meteorological Agency are constantly working to address these limitations. At the time of the Noto Peninsula earthquake of January 1, 2024, 12 earthquake early warnings were issued in the space of one day. Those warnings were highly varied: some of them were issued as intended, others could have been handled better. This lecture offers a deep dive into the largely unknown world of earthquake early warnings, including their current status and prospects for evolution in the future.