Why Choco Recorded 26 Earthquakes in Less Than 24 Hours
The Department of Choco in Colombia, recorded 26 earthquakes in less than 24 hours, including a cluster of stronger tremors in the municipality of Istmina on Sept. 14 that were felt across several departments. The Colombian Geological Survey, or SGC, said the activity belongs to two seismic sequences occurring simultaneously: aftershocks associated with the magnitude 7.4 earthquake that struck the region on Aug. 10 and a separate seismic swarm.
The distinction helps explain why earthquakes have continued to occur in Choco more than a month after the major earthquake. The SGC says the two groups differ in their depth, location and behavior, and that the recent concentration of tremors does not by itself indicate that another major earthquake is imminent.
Stronger earthquakes struck Istmina on Monday
The most severe seismic activity struck Istmina on the afternoon of Sept. 14. The SGC recorded a magnitude 4.9 earthquake at 3:02 p.m., followed by events measuring 4.1 at 3:04 p.m. and 4.4 at 3:08 p.m. A fourth magnitude 4.9 earthquake occurred at 4:13 p.m.
The tremors were felt outside Choco, with reports reaching parts of Valle del Cauca, Risaralda, Antioquia and Tolima. By 4:30 p.m., the SGC had received more than 2,000 reports from residents through its Sismo Sentido system, which collects information about where earthquakes are perceived and how strongly they are felt.
Monday’s tremors followed a series of earthquakes that struck Choco earlier in the day. The concentration of events in Istmina renewed concern in a department still dealing with the effects of the Aug. 10 earthquake.
The region is experiencing two different seismic processes
The SGC says its analysis identified two groups of earthquakes in Choco rather than a single sequence. The first consists of aftershocks from the Aug. 10 earthquake. The SGC’s Red Sismologica Nacional has identified more than 260 earthquakes in this group around San Jose del Palmar, at depths generally between 70 and 105 kilometers. Their location and the gradual decline in their frequency and magnitude are consistent with aftershock activity.
The second group is concentrated mainly around Sipi, Istmina, Medio San Juan, and Litoral del San Juan, at depths of roughly 30 to 60 kilometers. The SGC classifies its behavior as compatible with a seismic swarm. Its previous analysis identified more than 900 earthquakes in this group.
That difference is important because a seismic swarm does not follow the usual pattern of one main earthquake followed by progressively weaker aftershocks. Instead, earthquakes of different magnitudes can occur repeatedly without a single dominant event organizing the sequence.
What the SGC says about the ongoing activity
Freddy Tovar, coordinator of the SGC’s National Seismological Network, said the earthquakes recorded on Sept. 14 belong to the swarm identified in the area, rather than representing a new chain of aftershocks from the Aug. 10 earthquake.
The SGC says a swarm can continue for days, weeks, or months before gradually decreasing. The duration cannot be determined from the number of earthquakes recorded over a short period.
The agency has also stressed that earthquakes cannot be predicted. A concentration of earthquakes, even when several occur in a short period and in the same area, does not establish what will happen next. The SGC continues to monitor the region through the National Seismological Network and said it will report significant changes in the activity.