2013 Itomori Impact Event

The Itomori Impact Event, formally known as "Itomori Disaster", refers to the fall and crash of a piece of Comet Tiamat happened at 8:42 pm on October 4, 2013 in Itomori.

Background
Comet Tiamat has an orbital period of 50,200 years. For some reasons, every time the comet approaches Earth, a piece of itself always mysteriously breaks out and crashes somewhere around the area that would become Itomori. In the last time it crashed, circa the 9th century, its crater formed Lake Itomori.

Because of this, Itomori inhabitants developed a traditional ritual that reminds and warns future generations of the upcoming crashes. Unfortunately, a fire happened 1,000 years after the first impact destroyed all documents of its meaning, leaving its modern inhabitants clueless of the foreseen disaster.

The impact
As it was 120,000 km away from Earth, part of the Comet Tiamat broke away, with a 40-meter piece of its core falling to Earth at 30 km/s, hitting Itomori shrine at 8:42 pm. Its impact created a crater 1 km in diameter, with its shockwave spreading as far as 5 km, causing an earthquake measuring 4.8 on Richter scale.

Original timeline
As people were unaware of the upcoming disaster, they gathered at the autumn festival. Therefore, the impact caused massive casualties, a third of the population, including the death of Mitsuha Miyamizu. Only about 1,000 people in the southern districts were alive after the event.

Alternate timeline
Thanks to the plan from the future relayed by Taki Tachibana and, after being persuaded, the order of the mayor of Itomori Toshiki Miyamizu, all Itomori inhabitants were safely evacuated to the high school grounds, with only 104 people injured.

Aftermath
In both timelines, the town was eventually abandoned, as some of its citizens moved to other places. Although nobody died in the new timeline, the event is still considered a major disaster continued to be commemorated even 8 years later.

Trivia

 * The disaster is a homage to the 2011 Tōhoku earthquake and tsunami, which devastated Japan and killed at least 15,896 people.
 * It also pays tribute to the 2014 Sinking of MV Sewol in South Korea, which killed at least 299 people.