California had been settled by various Atlantean tribes until the Europeans arrived.
This happened in 1581, when a Castillian expedition crossed the Arizona desert, until they reached the Pacific and claimed the area west of the Rockies. 1590, the Triple Monarchy of England-Castille-Portugal founded the sixth capitanate, of California.
When the Triple Monarchy fell apart, California stayed a colony of Spain. In the second half of the 17th century, the Russians arrived in northern California; 1669, at OTL San Francisco Bay, Novorussians clashed with Spain.
1794, after Spain had been conquered by republican France, California declared independence, as did the other Spanish colonies. In 1840, Gold Rush in Northern California happened. People from all parts of Northern and Central Atlantis tried their luck - Spaniards, Russians, Germans, Italians, even English- and French-speaking Canadians; and Mexica, other native Atlanteans, and run-away Arab and African slaves. The population of the area exploded from 50,000 (mostly Russians) to more than a quarter of a million. In fact, many Russian-settled lands in Atlantis were depopulated by the rush.
1841, formerly Spanish California was added to New Rome.
In 1857, an earthquake in Southern California (OTL Fort Tejon) happened, maybe the strongest earthquake of history in the region.