Strong quake off Japan's Fukushima causes limited damage due to focal depth, seismic-resistant buildings

TOKYO - Over 150 people were injured but no death was reported as of Sunday after a 7.3-magnitude earthquake hit northeastern Japan Saturday, as experts attributed the limited casualties and damage to the quake's depth of epicenter and the seismic standards of Japanese buildings.

The temblor struck off Fukushima Prefecture at around 11:07 p.m. local time (1407 GMT) Saturday, with its epicenter at a latitude of 37.7 degrees north and a longitude of 141.8 degrees east, according to the Japan Meteorological Agency (JMA).

The quake was preliminarily measured at a magnitude of 7.1 and was later revised up to 7.3, while its depth was placed at 55 km, a revise from the initial 60 km, said the JMA.

While the magnitude of an earthquake is measured in one certain figure, the seismic intensity scale varies from place to place, depending on the hypocentral distance and focal depth.

The JMA puts the seismic intensity scale into 10 degrees, namely 0, 1, 2, 3, 4, Lower 5, Upper 5, Lower 6, Upper 6 and 7. Saturday's quake was measured Upper 6 in some parts of Fukushima and Miyagi prefectures.

On Sunday morning, some shops and metro stations were closed in Soma, where the earthquake was strongly felt. Some sections of the Joban Expressway were closed due to landslides. Other than that, not much visible damage was seen in the downtown area, with the city mainly operating in an orderly manner.

Wang Yuchen, a researcher at Earthquake Research Institute of the University of Tokyo, said that although Saturday's quake fell into the category of a shallow-focus one, its focal depth was relatively deep, which was "close to the range of medium-focus earthquake." Therefore, though the magnitude of the quake was high, it caused few casualties.

In comparison, the focal depth of the devastating earthquake in 2011 that killed more than 15,000 people in the same area was only about 24 km, while about 90 percent of the victims were killed by the following tsunami, a secondary disaster of the powerful 9.0-magnitude quake.

According to Wang, shallow-focus earthquakes occur at depths less than 60 km, medium-focus ones at depths between 60 km and 300 km, while deep-focus earthquakes happen at depths of above 300 km.

As a country with frequent earthquakes, Japan attaches great importance to the seismic resistance of buildings. The 7.3-magnitude quake did not cause a large number of buildings to collapse in places close to the epicenter, including Fukushima Prefecture.

The country has laws that require buildings to be earthquake-resistant, so that buildings that meet seismic standards are less likely to collapse during an earthquake. Meanwhile, most old and run-down houses had already been damaged in previous earthquakes.

In Japan, houses are mostly made of wood to make them anti-seismic, but such houses are poor in terms of fire proof and sound insulation. In the 2011 earthquake, while some 600 people died due to collapsed houses, many more houses were swept away by the ensuing tsunami.

By contrast, Saturday's quake did not trigger a tsunami, so there was little damage to buildings. According to local media, only about 240 people left their homes for shelters after the quake.

The JMA believed the latest quake was an aftershock of the devastating 2011 quake and urged the public to stay alert over the coming week or so, citing the likelihood of more tremors of a scale similar to that of Saturday's quake.