The 1917 San Salvador earthquake occurred on June 7 at 18:55 local time near the Salvadoran capital. The hypocenter of the 6.7 was at a shallow depth of , and occurred along a shallow crustal fault near San Salvador. The earthquake caused significant destruction of the city and left approximately 1,050 dead. It was followed by an eruption on San Salvador that killed another 1,100. It is the second-deadliest earthquake in El Salvador, behind the 1986 San Salvador earthquake.
There were two earthquakes felt in the San Salvador area on 6 June; one at 18:55 and 19:10. The largest earthquake was the first event, recorded at 6.7. The latter event was a 6.3 aftershock. Another 6.0 aftershock was centered around San Jacinto Hill, close to the epicenter of the 1986 shock.
Subduction of the Cocos Plate beneath the Caribbean Plate is associated with a volcanic arc that extends from Costa Rica to Guatemala. Across the Central America Volcanic Arc, there are three major geological structures; the Nicaraguan depression, El Salvador Fault Zone and Jalpatagua Fault. The El Salvador Fault Zone represents an area of geological strain with a dimension of by . Earthquakes in the volcanic arc are concentrated within this region.
The fault zone predominantly comprise east to southeast-striking strike-slip faults. Some southeast to southeast-striking normal faults also occur. These faults are seismically active and was the source of the 13 February 2001 and 1917 earthquakes. Their displacement rates were estimated to be as high as annually and 7.2 earthquakes are possible on these faults. One of these faults, the Guaycume Fault, is a possible source of the earthquake. The fault slips at a rate of per year, and is one of the most dangerous in El Salvador due to its proximity to San Salvador. Damage from the earthquake was mainly concentrated to the south of the fault while no major destruction was reported to its north. Near the fault, districts of Nejapa, Opico and Quezaltepeque experienced the greatest impact. Modelling of peak ground acceleration from an earthquake on the Guaycume Fault also indicated higher values to the south while lower values were predicted in the north.
An area west of San Salvador volcano sustained the greatest damage from the initial earthquake. San Salvador and the region to the north, including around the volcano were devastated by the aftershock. At least 1,050 people died and many were injured. The towns of Santa Tecla, San Julián, Sacacoyo, Tepecoyo, Ateos, Caluco and San Vicente sustained significant damage. Hundreds of people died in San Salvador.
In the towns of Armenia and Quezaltepeque, 40 people were killed and over 100 were injured. Damage in San Salvador was severe; described as "80 out of every 100 homes" levelled. The business district of San Salvador was also incinerated by fires. Out of the 9,000 homes in San Salvador, only 200 remained standing. All government buildings with the exception of the national theater and palace withstood the earthquake. Hospitals, university buildings, schools, and other public buildings were razed. The National Palace, National Press building, President Residence and banks were also devastated. Surviving buildings eventually toppled during the second major earthquake at 19:30. In addition to damaged buildings, the local coffee plantations were ruined. The Colombian poet, Porfirio Barba-Jacob, experienced the earthquake and documented it in a book and the local newspaper Diario del Salvador.
At the San Salvador volcano, eruptive activity occurred after the second shock, at 20:11. A fracture formed on the northwest flank of the Boquerón cone in the San Salvador volcano crater. The fissure later evolved into smaller craters. Lava erupted from the vent and flowed outwards in the direction of Sitio del Niño. The eruption produced an ash cloud and lava flow extended some along the northern slopes of San Salvador. The eruption was the largest since 1671, and was possibly triggered by the earthquakes disrupting plumbing within the magma chamber. About north of the vent, the lava flows buried a section of railroad.
The eruption caused the Boquerón crater lake to evaporate and form a cinder cone in the crater named BoqueroncitoâÂÂit ejected material up to high. Several people died when they were caught in the lava flows. The eruption rated a 3 on the Volcanic explosivity index. The lava flows also destroyed homes between Quezaltepeque and Sitio del Niño. By 11 June, the eruption had generally subsided, and the lava flows affected a area, burying it under ûaûÃÂ. A few of the craters had merged while some occasionally produced explosions. Two craters at the southern part of the fracture displayed fumarolic activity. One of these eruptive craters, El Tornador, located at in elevation, continued to produce lava and Strombolian explosions. Localised earthquakes were also felt. Explosions on 28 and 29 June expelled volcanic material into the air. The eruptions continued until November.