The 1897 Mindanao earthquakes occurred on September 20, 1897, at 19:06 UTC and September 21, 1897, at 05:12 UTC. The estimated epicentres of the two earthquakes are identical, lying just off the southwestern coast of Mindanao in the Philippines, south of Basilan island. The earthquakes were originally given estimated magnitudes of 8.6 and 8.7 on the surface wave magnitude scale by Charles Richter, but these were revised downwards by Katsuyuki Abe to 7.4 and 7.5 Ms, respectively. Contemporary reports noted that with few exceptions, all the masonry buildings in Zamboanga and Basilan were left in ruins.