6.9-magnitude earthquake strikes Philippines

• A magnitude 6.9 earthquake struck northern Cebu in the Philippines, killing at least 70 people and displacing more than 20,000.

• The tremor struck off the coast of Bogo City September 30, with the Philippine Institute of Volcanology and Seismology (PHIVOLCS) reporting a shallow depth of around 10 kilometers.

• Initial reports indicate widespread damage to homes, churches, schools, public buildings and transport infrastructure. 

• At least two seaports remain non-operational and several roads are partially blocked, hampering aid delivery.

• More than 340 aftershocks have been recorded since the quake, ranging up to magnitude 4.8. 

• The Philippines sits on the Pacific’s so-called “Ring of Fire” and is highly prone to earthquakes, volcanic eruptions and typhoons.

Some terms related to earthquake

Earthquake: An earthquake is a sudden, rapid shaking of the ground caused by the shifting of rocks deep underneath the Earth’s surface. Earthquakes can cause fires, tsunamis, landslides or avalanches. Earthquakes are classified as, Slight (M<5.0), Moderate (5.0<M<6.9) and Great (M>7.0) depending upon the magnitude on the Richter scale. An earthquake having a magnitude, M<2.0 is termed as a microearthquake. 

Seismograph: A seismograph, or seismometer, is an instrument used to detect and record earthquakes. Generally, it consists of a mass attached to a fixed base. During an earthquake, the base moves and the mass does not. The motion of the base with respect to the mass is commonly transformed into an electrical voltage. The electrical voltage is recorded on paper, magnetic tape, or another recording medium. This record is proportional to the motion of the seismometer mass relative to the earth, but it can be mathematically converted to a record of the absolute motion of the ground. Seismograph generally refers to the seismometer and its recording device as a single unit.

Richter scale: The Richter magnitude scale was developed in 1935 by Charles F. Richter of the California Institute of Technology as a mathematical device to compare the size of earthquakes. The magnitude of an earthquake is determined from the logarithm of the amplitude of waves recorded by seismographs. Adjustments are included for the variation in the distance between the various seismographs and the epicenter of the earthquakes. On the Richter Scale, magnitude is expressed in whole numbers and decimal fractions. 

Epicenter: It is the point on the surface of the Earth, vertically above the place of origin (hypocenter or focus) of an earthquake.

Aftershock: An earthquake that follows a large magnitude earthquake called, “main shock” and originates in or around the rupture zone of the main shock. Generally, major earthquakes are followed by a number of aftershocks, which show a decreasing trend in magnitude and frequency with time.