LA PALMA, Spain—A surge in seismic activity on the Spanish Canary Island of La Palma has slowed in the past day but could return at any moment, authorities said on Friday, as they kept almost half the island’s population on yellow alert for an eruption. “The decrease in seismic activity may be transient and does not necessarily imply a halt to the reactivation,” the regional emergency services said in a statement after a meeting with politicians, volcano experts, and civil defense authorities. Scientists have recorded more than 4,000 tremors in the Cumbre Vieja national park in the south of the island, prompting authorities to declare a yellow alert for eruption on Tuesday, the second of a four-level alert system. The yellow alert affects some 35,000 people in the regions of Fuencaliente, Los Llanos de Aridane, El Paso y Mazo. Authorities on Friday advised people in those areas to prepare an …