Alert Level 1 has been raised over Mount Pinatubo in Central Luzon on Thursday, March 4, after 25 years due to “persistence of seismic activity.”

In its latest bulletin, the Philippine Institute of Volcanology and Seismology (Phivolcs) said there is low-level unrest in the Pinatubo’s vicinity “that may be related to tectonic processes beneath the volcano and that no imminent eruption is foreseen.”

Here's what you should know about Pinatubo volcano alert levels, according to Phivolcs:

• Alert Level 0: Normal

    Quiescence; no eruption in foreseeable future.

    Continued vigilance and community preparedness.

• Alert Level 1: Low-level unrest

    Local tectonic or hydrothermal activity; no eruption imminent.

    Extreme caution when venturing into the Pinatubo Crater recommended.

• Alert Level 2: Increasing unrest

    Probable deep-seated magmatic intrusion and/or increased hydrothermal activity; could eventually lead to an eruption.

    No entry into Pinatubo Crater; Preparation of communities in case of escalation of unrest.

• Alert Level 3: Intensified unrest

    Certain magmatic intrusion into the shallow magma system or summit region with increased likelihood of an eruption. Precursory eruptive activity as hydrothermal system is disrupted. Lava dome extrusion may occur.

    Evacuation of upland communities up to 10 kilometers from the crater.

• Alert Level 4: Hazardous eruption imminent

    Phreatomagmatic or pre-climactic magmatic eruption; if magma ascent rates increase, highly explosive eruption probable within hours to a few days; if magma ascent rates decrease, prolonged lava dome extrusion may occur.

    Evacuation of communities within pre-determined hazard zones for PDCs, heavy ashfall and syn-eruption lahars.

• Alert Level 5: Hazardous eruption ongoing

    Climactic Subplinian to Plinian eruption; volcanic hazards within a 30-kilometer radius of the crater and ashfall hazards downwind of the eruption plume.

    Evacuation of additional communities, downwind of the eruption plume, along major river systems and in buffer extensions of hazard zones.

Mount Pinatubo last erupted on June 15, 1991.

The eruption, which is considered as “the second largest volcanic eruption of the 20th century,” reportedly killed more than 840 people.

Phivolcs said the volcano, which is part of the Zambales Mountain Range located on the boundaries of Pampanga, Tarlac, and Zambales, has been on Alert Level 0 or “normal” status since January 1996.