Tidal Waves Can Hit At Four Kilometers Per Second

An earthquake that originated from Japanese seabed was detected in Korea, causing a tsunami warning at one point.

What would happen in Korea if an earthquake hit Japan?

The S wave transmitted to the Korean peninsula-

Earthquake waves caused the people in Korea to experience vibrations and also caused buildings to shake. The seismic waves are due to the movement of waves caused by the energy from an earthquake. Seismic waves consist of P waves, S waves, and surface waves.

P waves arrive the fastest at 8km/s, and S waves arrive after P waves at 4km/s. Surface waves are the slowest to arrive.

The surface wave consists of the largest energy, and causes damage only in the closest regions to the earthquake’s origin.

Among long-ranging earthquake waves, S waves are larger in energy than P waves, causing them to be transmitted further.

Dr. Lee Jeon-hee of the Earthquake Division of the Korea Meteorological Agency said, “The S waves probably caused the Korean peninsula to experience the earthquake in Japan.”

The extent of the Richter scale and seismic intensity-

Though the absolute value of the earthquake is measured in the Richter scale, the degree to which an earthquake is felt is recorded in terms of seismic intensity. Earthquakes farther from the origin record a lower degree of seismic intensity.

Though the earthquake measured 7.0 on the Richter scale, the degree was experienced differently in different regions of Korea. The meteorological agency noted that South Gyeongsang province and Busan recorded scale readings of 4.0 to 5.0, mid-regions including Daejeon measured readings of around 3.0, and that the northern regions recorded scale readings of below 3.0. Earthquakes below 3.0 are hardly detectable in ordinary circumstances.

Reasons why no tsunami was created-

A prime example of oceanic earthquakes may be the tsunami that caused a wave of destruction in South Asia late last year. Measuring 7.0 on the Richter scale, the meteorological agency set up a tsunami warning in case of seismic waves. Fortunately, a tsunami was not detected.

Lee explained that “tidal waves depend not only on the scale of the earthquake, but also the mechanisms in which it has occurred,” and that “if the fault lines are vertical, tidal waves may occur, but there’s less of a chance when the fault line is horizontal.”

A horizontal fault line prevented a seismic wave this time. In addition, the tsunami can be predicted beforehand because the shock waves arrive before the tidal waves.

cosmos@donga.com