An earthquake of magnitude 7.1 (M7.1) which occurred off the coast of Japan on 25 October UTC (26 October local time) inevitably raised the spectre of the devastating Tohoku-Oki earthquake of March 2011. The 2011 quake, which, along with the resulting tsunami, cost an estimated 15,000 lives and continues to wreak havoc in the region with the on-going problems of the Fukushima nuclear plant. How was this seismic event different?
Honshu Earthquake, 25 October 2013
Tsunamis occur when submarine earthquakes involve vertical movement, displacing large volumes of seawater. At a depth of just 10km, with an offshore location and a significant magnitude (reported elsewhere as being up to M7.5) the October 25 tremor was theoretically capable of generating a tsunami and although a widespread event did not occur, early news reports indicate that a small wave (around 30cm in height ) came onshore in Miyagi Province. It’s also reported that officials evacuated the Fukushima plant as a precaution.
Tectonically, Japan is very vulnerable to earthquakes because of its location on the Pacific margin, where several of the earth’s tectonic plates come together. The October 25 earthquake occurred close to the epicentre of the 2011 quake in Japan, where the Pacific plate is moving westwards and being subducted beneath a southward extension of the North American plate.
The North American plate lies between the Pacific plate and the Eurasian continent. According to the United States Geological Survey (USGS) the cause of the earthquake was normal faulting within the overriding plate. This is in contrast to the 2011 quake, which occurred as a result of a massive rupture, 440km long and 220km wide and with vertical displacement of up to 50m, along the plate margin itself.
Japan’s Earthquakes: After 2011
Earthquakes of M7 or more are by no means unusual along this particular section of plate margin. Although the March 2011 earthquake was exceptionally large (at M9.0) it was preceded by a significant tremor of M7.7 which caused no damage, while a tremor of M7.3 occurred in December 2012. In addition, the area has experienced thousands of aftershocks as a result of the earthquake of 2011. Further, the entire north west Pacific is regularly subject to significant of earthquakes of M7.0 or more.
Today’s Honshu Earthquake vs. Tohoku Quake of 2011
Despite the fact that the Tohoku earthquake of 2011 was preceded (by two days) by a significant -and in itself newsworthy – earthquake, there’s no indication that the most recent M7.1 presages further significant movement. Although, given that earthquakes are unpredictable in their behaviour, we can’t rule out a follow-up quake to today’s tremor. What is certain is that the offshore plate margin to the west of Honshu will continue to experience regular and major seismic occurrences, as the Pacific plate continues its westward progress against Eurasia.
RT. 7.3 magnitude earthquake off Japan prompts Fukushima plant evacuation. (2013). Accessed October 25, 2013.
USGS. M7.1 – Off the east coast of Honshu, Japan. (2013). Accessed October 25, 2013.
Yeats, R. Active Faults of the World. (2012). Cambridge University Press.