2018.11.16
Challenging Conventional Wisdom: Discovery and Understanding of a Fault Being Re-activated with and Extreme Short Interval of Six Years
Discovery of an exception to rare movements of active faults
Disaster Science Division Associate Professor Yo FukushimaActive faults are faults that have experienced repeated ruptures in the past and are likely to cause earthquakes in the future. The cause of intraplate earthquakes shallow in the Earth’s crust is an abrupt rupture of such active faults. The number of active faults identified on the islands of Japan is approximately 2,000. Naturally, large intraplate earthquakes occur once per a few years somewhere in Japan. The frequency of earthquake occurrence along a single active fault, however, is rare – once per a thousand to a few tens of thousands of years on average 1), reflecting the slow strain accumulation in the crust.
A new study led by Dr. Yo Fukushima, Associate Professor of IRIDeS, discovered that an active fault in northern Ibaraki Prefecture, approximately 130 km north of Tokyo, had ruptured twice on March 19, 2011 and December 28, 2016, with a time interval of only five years and nine months. The research team proposes that the 2011 Great East Japan Earthquake is the cause of these movements. Discovery of this exception to the widely accepted conventional wisdom of infrequent ruptures of active faults leads to updating the way we see earthquake generation on active faults and changing the way we assess future earthquake occurrence.
Evidence and cause of reactivation of an identical fault within six years
[Figure 1] The region in northern Ibaraki Prefecture (shown by the red square)where the two earthquakes occurred. Orange: The main segment of the plate interface that slipped during the 2011 Great East Japan Earthquake. Black dots: Epicenters of earthquakes of depths less than 30 km that occurred between March 11, 2011, and December 31, 2011.
Fukushima’s expertise has been on the satellite geodesy. There are a number of earth-observation satellites flying around the Earth that are equipped with a specific type of radar called synthetic aperture radar (SAR). A SAR antenna on a satellite emits and receives microwaves to know about the change over the ground surface. More specifically, by detecting the difference in the return time of the microwave signals at two different timings, we can measure the amount of ground displacements that occurred during the acquisition interval on a wide area in the looking direction of the satellite radar. Just after the 2011 Great East Japan Earthquake, Fukushima processed the data of Advanced Land Observing Satellite (ALOS) operated by the Japan Aerospace Exploration Agency (JAXA) to map the ground deformation on northeastern Honshu Island, which included the area of the magnitude 6.1, March 19, 2011 earthquake. When the magnitude 6.3 earthquake on December 28, 2016 struck, Fukushima analyzed the data of ALOS-2, successor of ALOS, caused by the event upon request from another researcher. He did not, however, intend to look further in detail about this event at first. Approximately 5,000 earthquakes on a yearly average are recorded by the Japanese seismological network, and many of them fall into the same region2). The northern Ibaraki area in particular had experienced a swarm of earthquakes after the 2011 megaquake, including a few earthquakes larger than magnitude six. “The magnitude 6.3 event was one of them”, Fukushima states. Fukushima was much more intrigued by the earthquake when he heard from his room-mate colleague Professor Shinji Toda that surface ruptures had been observed at the same location as the ones caused by the March 19, 2011 earthquake, and that an identical fault may have been reactivated. Could that really happen? Shouldn’t faults be tranquil once they rupture? Maybe the surface ruptures were caused by some superficial process or landslides triggered by the earthquake? Fukushima, who had already computed the deformation caused by the two earthquakes, decided to investigate into further detail [Figure 1].
[Figure 2] Crustal deformation maps associated with the March 19, 2011, and December 28, 2016 earthquakes, obtained from satellite radar data analysis. Black lines indicate surface rupture of the active fault. One color cycle change corresponds to about 12 cm of displacement in the looking direction of the satellite radar.
[Figure 3] Fault slip distributions estimated for the March 19, 2011, and December 28, 2016 earthquakes.
[Figure 4] Figure on the left shows the displacements in blue arrows that occurred between March 20, 2011 (one day after the fist earthquake) and December 27, 2016 (one day before the second earthquake). Figure on the right shows the strain map calculated from the displacement data. Areas with a darker blue color indicate larger amounts of strain. The red line shows the location of the active fault.
