What we know about the earthquake off the Japanese coast of Fukushima

(CNN) — A deadly 7.4-magnitude earthquake struck off the coast of Japan’s Fukushima prefecture on Wednesday, injuring dozens of people. For some, the incident brought back painful memories of 2011, when an earthquake triggered a tsunami that triggered a nuclear meltdown at the Fukushima power plant, a disaster still felt to this day. What to do during a tremor or earthquake? Although the earthquake hit a similar area, Wednesday’s earthquake did not trigger a national emergency, for several reasons. This is what you should know. What, where and when? The quake struck around 12:30 a.m. local time (11:30 a.m. ET) off the coast of Fukushima, north of Japan’s capital Tokyo. It was originally designated a 7.3 magnitude quake, but was upgraded to 7.4 on Thursday. As of Thursday, all tsunami warnings issued after the earthquake have been lifted. The epicenter of Wednesday’s quake was about 89 kilometers (55 miles) from the center of the devastating 2011 quake. Robert Geller, a seismologist and professor emeritus at the University of Tokyo, suggested Wednesday’s quake could have been an aftershock from 2011. “In geological terms, aftershocks will persist for 50 to 100 years, but as time goes on, the frequency of aftershocks and their size will decrease,” he said. Four people were confirmed dead, including a man in his 60s, and at least 160 others were injured, Japanese authorities said on Thursday. Images from the capital Tokyo show street lights and apartments shaking. Tens of thousands of homes were without power across the city, but service was restored within a few hours. A worker performs electrical repairs after a power outage in Soma, Fukushima Prefecture, Japan, on March 17. No “anomalies” have been detected at the country’s nuclear plants, Japan’s Prime Minister Fumio Kishida said. A bullet train traveling through Miyagi prefecture derailed during the earthquake, trapping 78 people on board for four hours. All eventually escaped unharmed through an emergency exit, according to public broadcaster NHK. Photos from Fukushima and Miyagi show buildings damaged by the earthquake, with broken windows, raised tiles and floors, and collapsed roofs. Merchandise and debris littered the floors of stores and supermarkets. Wednesday’s quake occurred off the coast, 60 kilometers (37 miles) deep, which could have limited the damage. The most damaging earthquakes occur near the earth’s surface rather than deep in the earth’s crust, Geller said. How does it compare to the 2011 earthquake? The difference between the magnitude 7.4 earthquake on Wednesday and the magnitude 9.1 earthquake of 2011 is staggering. The 2011 earthquake was about 63 times stronger than Wednesday’s and released about 500 times more energy, the most powerful earthquake ever to hit Japan. And it was only 15.2 miles (26 kilometers) deep, meaning its impact was much larger. While Wednesday’s quake caused tsunami waves only 8 inches (20 centimeters) high, the 2011 quake triggered 30-foot (9.1-meter) waves that damaged several nuclear reactors in the area. The 2011 disaster left more than 22,000 dead or missing, from the initial earthquake, tsunami and post-disaster sanitary conditions. As of last year, more than 35,000 people were still displaced, 10 years after the collapse. A bullet train derailed while traveling through Japan’s Miyagi prefecture on March 17. The cleanup is expected to last decades and cost billions of dollars. Since 2011, Japan has stepped up its response systems to better deal with such disasters, including improving earthquake early warning systems and seismic observation technology. Could more earthquakes or tsunamis be coming? The Japan Meteorological Agency warned the public to remain alert for aftershocks and the possible risk of landslides or mudslides. He also urged people in the affected areas to stay away from the coast and not go into the sea. In a tweet, the Prime Minister’s office said the government had set up a countermeasures office that will work with local governments to carry out emergency measures, including search and rescue of potential victims. A supermarket full of fallen goods in Shiroishi, Miyagi prefecture, Japan, on March 17. Geller, the seismologist, said Japan can expect more aftershocks in the coming week that will gradually subside. Although he said there is also a possibility that Wednesday’s quake was a “pre-quake” before a larger quake, he cautioned that the chances of this happening are very low but “not zero.” “Yesterday’s earthquake is a good reminder for people in Japan to remember that Japan is prone to earthquakes and earthquakes can strike at any time,” he said. “So people should be prepared.”