Seismic Swarm S20140311.1: Earthquake Sequence Offshore Ferndale, California

Seismic swarm S20140311.1 occurred approximately 87 km west of Ferndale, California, in a tectonically complex offshore region. The sequence began at 05:27 on 10 March 2014 and concluded at 07:49 on 12 March 2014, encompassing 31 earthquakes over 50 hours and 21 minutes. This activity reflects the persistent seismicity along the Mendocino Fracture Zone and the southern edge of the Cascadia Subduction Zone, where the Gorda, Pacific, and North American plates interact.

The swarm initiated with a magnitude 4.3 event at a depth of 10 km. Subsequent shocks included additional magnitude 4.3 and 3.7 events within the first hour, followed by a series of smaller events ranging from magnitude 2.3 to 3.4. Depths varied between 4 km and 24 km, with many events clustering around 20–24 km in the later stages. The largest events occurred early, while later activity consisted primarily of magnitude 2.7–3.1 tremors. Such patterns are characteristic of swarms, where energy releases through numerous moderate events rather than a single mainshock-aftershock sequence.

The Ferndale offshore area lies near the Mendocino Triple Junction, one of the most seismically active zones in the contiguous United States. Historical records document frequent moderate earthquakes driven by right-lateral strike-slip faulting along the Mendocino Fracture Zone and convergence at the Cascadia margin. Notable prior events include the 1992 magnitude 7.2 Cape Mendocino earthquake and multiple magnitude 6+ events in the 20th century. Updated monitoring by regional networks confirms ongoing microseismicity in this locale, consistent with long-term plate boundary deformation.

Analysis of the provided event times and magnitudes shows a rapid initial rate of occurrence that gradually declined, with events distributed across a range of depths indicative of distributed faulting rather than a single planar rupture. No events exceeded magnitude 4.3, underscoring the swarm’s moderate character.

References
USGS Earthquake Catalog
California Geological Survey Regional Fault Maps
NOAA National Centers for Environmental Information Tsunami and Earthquake Database