Tuesday’s early-morning text message that awoke Marin County residents with a warning that a tsunami watch was in effect for the California coast came as no shock to the county’s emergency services coordinator, Ursula Hank. The 7.9-magnitue earthquake in the Gulf of Alaska triggered evacuations in some coastal areas of that state and alerts along the Pacific coast, though officials lifted the watch hours later. Later that day, the Board of Supervisors approved an updated tsunami report that Ms. Hank recently completed as an annex to the county’s emergency operations plan. “The annex primarily provides officials with the information to know when they should evacuate and what portion of a community specifically should be evacuated, based on the analysis given during a warning,” Ms. Hank said. Currently, the Marin County Sheriff’s Office receives tsunami alerts from the National Oceanic and Atmospheric Administration’s two national tsunami warning centers. Depending on the level of threat, the alert could be a statement, a watch, an advisory or a warning, which calls for an evacuation. In turn, the county’s Office of Emergency Services broadcasts messages to local disaster groups—including local fire departments, state and national parks, hospitals and other county agencies—that can warn residents through calls, texts and radio transmissions and, if necessary, launch a localized evacuation effort. (If you’re signed up for the county’s AlertMarin emergency notification system, the sheriff’s office will also send you updates from the get-go, as it did on Tuesday morning.) The annex delineates the roles and responsibilities of the county’s response agencies. Ms. Hank revised the three-year-old document to reflect new data accumulated at the state level. Those include new “playbooks” created in 2016 that go beyond previous maps that only addressed “worst-case” tsunami evacuation zones. The playbooks, which are not available to the public, provide more detailed depictions of the varying impacts of events of different sizes and source locations. Ms. Hank used the new information to update evacuation maps for Dillon Beach, Point Reyes Station, Bolinas, Stinson Beach, Muir Beach and Drakes Bay. There are two types of tsunamis to prepare for in West Marin: those emanating from distant earthquake sources around the Pacific Ocean and those from a local source. Eight tsunamis have reached California shores over the past 70 years. In 1964, a 9.2-magnitude quake from Alaska sent a tsunami that killed one person in Bolinas 13 hours after the first wave arrived. Most recently, in 2011, California coastlines—including Bolinas—felt ripples from a massive tsunami that ravaged Japan’s coastal towns in the wake of an 8.9-magnitude earthquake that brought waves in excess of 100 feet in some places. Marin residents can sign up to receive real-time alerts in the event of a tsunami or other disaster through the county’s AlertMarin system at alertmarin.org. The tsunami annex will soon be available online at marinsheriff.org.