The earthquake occurred within the Cocos-Caribbean subduction zone. El Salvador sits atop the western part of the Carribbean plate, where it is overriding (subducting) the Cocos plate. Subduction zones such as this are geologically very complex and produce numerous earthquakes from multiple sources. Shallow intraplate (crustal) earthquakes occur within the crust of the overriding Caribbean plate. Deeper intraplate earthquakes occur within the subducting Cocos plate. The earthquake sequence underway in the El Salvador region has involved intraplate faulting in both the Cocos and Caribbean plates, with the largest earthquake in the sequence (January 13) occurring in the lower (Cocos) plate. Today's earthquake was a strong, shallow intraplate earthquake, occurring within the crust of the overlying Caribbean plate. This earthquake was a strike-slip faulting earthquake, which likely occurred in response to the complicated stresses in the Caribbean plate as it overrides the Cocos plate. It is about 85 km away from the 13 January magnitude 7.7 earthquake and about 30 km shallower. These two earthquakes occurred in two different plates. The occurrence of any large earthquake changes the stresses throughout the surrounding region. Aftershocks occur in response to these changes. Occasionally, other earthquakes will occur in response to the altered regional stresses. While not technically aftershocks, these earthquakes are related, becoming part of a regional earthquake sequence. Another example of a regional earthquake sequence is the 1992 Landers-Big Bear sequence in southern California. The magnitude 7.3 Landers earthquake was followed by the magnitude 6.4 Big Bear earthquake, which occurred on a different fault approximately 36 km away.