Magnitude 6.9 quake off Kamchatka triggers false magnitude 9.9 alert in Japan, which is withdrawn after 8 minutes

The earthquake

A magnitude 6.9 earthquake struck near the southern tip of Russia's Kamchatka Peninsula at 3:35 p.m. local time on Friday, Oct. 16 (0335 GMT), about 76 kilometers (47 miles) northeast of Ozernovskiy, according to the U.S. Geological Survey, as reported by Anadolu Agency. The USGS event record, which is marked as reviewed, lists a depth of about 135 km; early reports cited 143.2 km. ^{[3] [1]}

Some early reports, including KITV in Honolulu, gave the magnitude as 7.0. The USGS figure is 6.9. The USGS rated its pager alert green, its lowest level, and its record shows one felt report. ^{[2] [1]}

The U.S. tsunami warning system said no tsunami was expected because of the quake's depth, Anadolu reported, and the National Weather Service said Hawaii was not under a tsunami watch, according to KITV. Anadolu said there were no immediate reports of casualties or damage. Russian authorities' assessment was not available in the reports reviewed. ^{[3] [2]}

A false alarm in Japan

At 12:40 p.m. Japan time, about two minutes after the quake, the Japan Meteorological Agency issued an earthquake early warning describing a magnitude 9.9 quake that would produce shaking up to 7, the top of its scale, from Hokkaido to the Tokai region, the Japan Times reported. Phones across the affected regions sounded alerts, including in Tokyo, according to The Watchers. ^{[4] [5]}

No strong shaking followed. The highest intensity recorded in Japan was 1, in Hakodate, Hokkaido, and the agency withdrew the warning at about 12:48 p.m. The Japan Times described it as the first time the agency had withdrawn an emergency quake warning with a second alarm. The strongest earthquake on record, in Chile in 1960, was magnitude 9.5. ^{[5] [4]}

JR Central's earthquake protection system cut power and suspended the Tokaido Shinkansen in both directions between Shin-Yokohama and Toyohashi for about 15 minutes, The Watchers reported, citing the railway. JR Hokkaido suspended some conventional services. Demand for quake information briefly disrupted access to Yahoo Japan Search, the outlet said. ^[5]

What the agency has said

Ayataka Ebita, who heads the agency's earthquake and tsunami observation division, said the system had wrongly placed the epicenter off the Kuril Islands and overestimated the magnitude. The agency ruled out a cyberattack, the Japan Times reported. Officials later said the overestimate came from applying a magnitude formula designed for domestic earthquakes to one far from Japan, and that different locations were used to calculate magnitude and shaking even though both should have used the same one. ^[4]

The agency apologized at a news conference, The Watchers reported, and said the underlying cause remained under investigation. Chief Cabinet Secretary Minoru Kihara said the government had received no reports of damage from the Kamchatka earthquake and took seriously the burden on people who evacuated or whose travel was disrupted, the Japan Times reported. ^{[5] [4]}

Background and what is unknown

The Japan Times noted that earlier false alarms occurred in July 2020, when the agency warned 15 prefectures including Tokyo after misjudging a quake's location and size, and in August 2013, when seabed sensor noise combined with a small quake's data. Neither produced shaking of intensity 1 or above. ^[4]

The agency has not yet explained exactly how the magnitude reached 9.9, and no aftershock or damage information from Kamchatka was available in the reports reviewed. ^{[4] [5] [3]}