The 1667 Dubrovnik earthquake took place on April 6th with the modified Mercalli intensity scale rating it the most intense degree of X.
6 April 1667 (1667-04-06) X MCS (Disastrous) Also, some of the original criteria of the most intense degrees (X and above), such as bent rails, ground fissures, landslides, etc., are "related less to the level of ground shaking than to the presence of ground conditions susceptible to spectacular failure". 
