A 7.8 MS earthquake struck the densely populated Philippine island of Luzon, killing an estimated 1,621 people.

The surface wave magnitude scale is one of the magnitude scales used in seismology to describe the size of an earthquake. It is based on measurements of Rayleigh surface waves that travel along the uppermost layers of the Earth. This magnitude scale is related to the local magnitude scale proposed by Charles Francis Richter in 1935, with modifications from both Richter and Beno Gutenberg throughout the 1940s and 1950s. It is currently used in People's Republic of China as a national standard for categorising earthquakes.

The successful development of the local-magnitude scale encouraged Gutenberg and Richter to develop magnitude scales based on teleseismic observations of earthquakes. Two scales were developed, one based on surface waves, , and one on body waves, . Surface waves with a period near 20 s generally produce the largest amplitudes on a standard long-period seismograph, and so the amplitude of these waves is used to determine , using an equation similar to that used for .

The 1990 Luzon earthquake occurred on July 16 at 4:26 p.m. (PDT) or 3:26 p.m. (PST) on the densely populated island of Luzon in the Philippines. The shock had a surface-wave magnitude of 7.8 and produced a 125 km-long ground rupture that stretched from Dingalan to Kayapa. The event was a result of strike-slip movements along the Philippine Fault and the Digdig Fault within the Philippine fault system. The earthquake's epicenter was near the town of Rizal, Nueva Ecija, northeast of Cabanatuan. An estimated 1,621 people were killed, most of the fatalities located in Central Luzon and the Cordillera region.