Coronado’s Strongest Quake Since 1952 Produced 60 Aftershocks and One False Alarm

The two quakes that woke the Central Valley before dawn today were the strongest recorded in the Coronado area in more than seven decades, according to Ovsicori seismologist Marino Protti. By mid-morning the sequence had produced roughly 60 aftershocks, and seismologists had answered the question that hung over the first hours of coverage — why the two seismic networks published different magnitudes for the same event. Protti said the last comparable event in the area came in 1952, on the western flank of Irazú volcano, and caused damage across the Coronado region. The magnitude confusion has a straightforward explanation. Lepolt Linkimer of the Red Sismológica Nacional said the two networks calculate magnitude differently. The RSN draws on records from the stations closest to the epicenter and averages them, and the number of stations included can shift the result slightly. An international network measuring the same quake could well arrive at a third figure. Linkimer was clear that this is not a reason to distrust either network. Both are describing the same event, and small variations are a normal feature of how the calculation works. The figures themselves still differ. Ovsicori puts the main quake at magnitude 5.0 and the second at 4.5, both at 11 kilometers (6.8 miles) depth. The RSN revised its readings to 5.4 and 4.8, placing the first 11 kilometers (6.8 miles) north of Cascajal at a depth of 7 kilometers (4.3 miles). The U.S. Geological Survey recorded the main event at 4.9. Aftershock counts diverge for related reasons. The RSN logged 23 by mid-morning, while Ovsicori counted more than 60, ranging from magnitude 2.0 to 4.8. Protti’s tally moved from about 30 at 3:30 a.m. to roughly 60 by 7:30 a.m. The separate matter of the false alarm has also been resolved. In the first minutes after the shaking, an automated Ovsicori report placed a magnitude 5.8 quake in Buenos Aires de Puntarenas, hundreds of kilometers from Coronado. That reading circulated widely before being withdrawn. Protti said a small magnitude 2.0 quake occurred in the southern zone about 15 seconds before the Coronado event. The automatic system located that first, weaker quake, then took the much larger wave amplitudes arriving seconds later from Coronado and attributed them to the southern epicenter. The result was a quake that was too big, in the wrong place, and that never happened. He described it plainly as a system error, corrected once analysts reviewed the records by hand. What caused the sequence is still an open question. Protti attributed it to a local fault — a small fracture in the crust where a minor slip can generate quakes of this size. Local faults are poorly characterized in Costa Rica. Some are drawn on maps, but there is often no evidence establishing which ones are active, and what seismologists mostly have to work with is the record of past quakes in a given area. Ovsicori has said it is still studying which fault produced this sequence, that it cannot rule out further quakes, and that there is no evidence of associated volcanic activity. On the aftershocks, Protti expects them to keep tapering. A magnitude 5.0 sits at the moderate-to-low end of the scale, and a quake that size would not normally produce a sequence lasting several days. Damage stayed limited. The Comisión Nacional de Emergencias reported a single 911 call about structural damage to a perimeter wall at a home in Dulce Nombre de Jesús, in Vázquez de Coronado, along with a downed power pole in San Vicente de Moravia. No deaths or injuries were reported. The most consequential effect was on Route 32. Falling rock, soil and branches in the Braulio Carrillo National Park stretch, concentrated around kilometer 30, forced MOPT to close the San José–Limón highway at about 6:28 a.m. Crews cleared the material through the morning and the road reopened at 10:13 a.m. Residents who felt the shaking can report it through the RSN’s “¿Lo sentiste?” tool, which helps map intensity across Costa Rica.