Seismic Swarm PS20210306.2: Analysis of Activity in the Mariana Islands Region

A seismic swarm designated PS20210306.2 occurred in the Mariana Islands region from 18:00 on 6 March 2021 to 01:03 on 7 March 2021. Over this 7-hour, 2-minute period, eight earthquakes were recorded, all at a focal depth of 10 km. The sequence began with a magnitude 4.4 event at 18:00:38, followed by a magnitude 5.2 event at 18:16:11. Subsequent shocks included magnitudes 5.5 at 18:18:33, 5.7 at 18:51:12, 5.0 at 18:52:52, another 5.0 at 20:28:37, 5.0 at 23:37:14, and a final 5.0 at 01:03:08 on 7 March.

This swarm reflects clustered seismicity typical of subduction-related stress release. The Mariana Islands lie along the Mariana subduction zone, where the Pacific Plate converges with and subducts beneath the Philippine Sea Plate at rates of approximately 6–7 cm per year. The region hosts the Mariana Trench, the deepest oceanic trench on Earth, reaching depths exceeding 10,900 m. Volcanic arc activity, including active volcanoes on islands such as Pagan and Anatahan, is driven by this plate interaction.

Historical records document frequent moderate-to-large earthquakes in the Mariana Islands. Notable events include the 1993 magnitude 7.8 earthquake near Guam and multiple magnitude 6+ shocks associated with the 2014 seismic sequence. Updated monitoring from global networks confirms persistent background seismicity, with swarms often preceding or accompanying volcanic unrest or aseismic slip episodes along the trench.

Swarm events such as PS20210306.2 provide insight into localized stress migration within the shallow crust. The tight temporal clustering and consistent depths suggest fluid migration or fault interaction rather than a single mainshock-aftershock sequence. All events remained below magnitude 6, producing no reported damage but highlighting the dynamic nature of the subduction interface.

References

• USGS Earthquake Hazards Program: Mariana Islands regional seismicity summaries (2021–2024 updates).
• International Seismological Centre: Bulletin of earthquake locations and depths.