Analysis of the 2014 Rowland Heights Earthquake Swarm

The earthquake swarm designated S20140329.1 occurred 3 km south-southeast of Rowland Heights, California. It began at 03:03 UTC on 29 March 2014 and concluded at 10:59 UTC on 3 April 2014, spanning 127 hours and 56 minutes. During this period, 214 earthquakes were recorded. The sequence was characterized by a rapid onset of activity followed by an extended series of aftershocks.

Rowland Heights lies within the Los Angeles Basin, a tectonically active region influenced by the interaction between the Pacific and North American plates. The area is underlain by sedimentary rocks of the Puente Formation and is proximate to the Whittier Fault and the Puente Hills Blind Thrust Fault system. These structures accommodate regional compression and have produced damaging earthquakes historically. Shallow crustal depths in this setting typically range from 0 to 10 km, consistent with the observed swarm events.

The first 100 events provide key insights into swarm dynamics. Activity initiated with a magnitude 3.5 earthquake at 7 km depth. Within 66 minutes, a magnitude 5.1 mainshock occurred at 5 km depth, marking the largest event. Subsequent events were predominantly small, with magnitudes between 0.7 and 3.6. Depths clustered between 1 and 4 km, indicating shallow slip along minor faults or fractures. Frequency peaked immediately after the mainshock, with events occurring at intervals of minutes before gradually declining over the following days. Only a few events exceeded magnitude 3.0, underscoring the swarm’s low-to-moderate energy release.

Historical records indicate limited prior swarm activity in the immediate vicinity. Since 1 January 2000, just three swarms have been documented: one each in 2002, 2008, and 2012. These earlier episodes each involved a single notable event, contrasting with the higher event count observed in 2014. Such infrequency highlights the episodic nature of seismic swarms in this portion of the basin.

The 2014 sequence aligns with known patterns of induced or natural swarm behavior in southern California, where fluid migration or stress triggering can sustain prolonged low-magnitude activity without a single dominant rupture. No significant damage or injuries were associated with the swarm, though it served as a reminder of the region’s persistent seismic hazard.

SeismoSight internal classification
USGS Earthquake Catalog (historical context)
California Geological Survey regional fault maps