The 2013 Bohol earthquake occurred on October 15 at 8:12:31 PST in Bohol, an island province located in Central Visayas, Philippines. The magnitude of the earthquake was recorded at Mw 7.2, with epicenter 6 kilometres (3.7 mi) S 24 W of Sagbayan, and its depth of focus was 12 kilometres (7.5 mi). It affected the whole Central Visayas region, particularly Bohol and Cebu. The earthquake was felt in the whole Visayas area and as far as Masbate island in the north and Cotabato provinces in southern Mindanao.

According to official reports by the National Disaster Risk Reduction and Management Council (NDRRMC), 222 were reported dead, 8 were missing, and 976 people were injured. In all, more than 73,000 structures were damaged, of which more than 14,500 were destroyed.It was the deadliest earthquake in the Philippines in 23 years since the 1990 Luzon earthquake. The energy released by the quake was equivalent to 32 of the bombs dropped in Hiroshima. Previously, Bohol was also hit by an earthquake on February 8, 1990 that damaged several buildings and caused a tsunami.On November 7, just three weeks after the earthquake, Super Typhoon Haiyan (Yolanda) struck the region. Although the storm's eye missed the area affected by the earthquake, it sent some 40,000 Boholanos still living in temporary shelters back to evacuation centers and disrupted relief efforts in the province.

The surface wave magnitude (









M



s









{\displaystyle M_{s}}

) 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 (GB 17740-1999) 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,









M



s









{\displaystyle M_{s}}

, and one on body waves,









M



b









{\displaystyle M_{b}}

.

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









M



s









{\displaystyle M_{s}}

, using an equation similar to that used for









M



L









{\displaystyle M_{L}}

.

Recorded magnitudes of earthquakes through the mid 20th century, commonly attributed to Richter, could be either









M



s









{\displaystyle M_{s}}

or









M



L









{\displaystyle M_{L}}

.