A shallow magnitude 4.0 earthquake struck 9.5 kilometers southwest of Pinotepa Nacional, Oaxaca, on Sunday at 11:56 a.m. local time — 4.3 kilometers deep, with moderate shaking reported near the epicenter. [1] Saturday night, a magnitude 4.1 shook Baja California 45 kilometers south of Mexicali at 10:09 p.m. local time, also at very shallow depth (8.3 km). [2] Both events carried into Monday's seismic logs; neither produced damage or casualties.

The weekend cluster traces the arc the paper has circled before. The Tacana stratovolcano — Guatemala's second-highest peak, 4,060 meters on the Chiapas-San Marcos border — has been classified as active but quiescent since the mid-1980s monitoring protocols developed after El Chichon's catastrophic 1982 eruption. [3] El Chichon itself sits north of the Sunday epicenter in Chiapas. Isla Tortuga, a youthful shield volcano in the Gulf of California, lies forty kilometers off the Baja coast and within the broader plate-boundary envelope of the Mexicali epicenter. [4]

The cluster is ordinary for a country that straddles the Pacific, Rivera, Cocos, Caribbean, and North American plates — six magnitude-four-or-above quakes in a typical 24-hour window, per Mexico's Servicio Sismologico Nacional averages. What makes the weekend notable is proximity: the Sunday shallow shaker and the Saturday-night Baja tremor crossed a single long arc in thirty-eight hours. Mexico's SASMEX early-warning system triggers only for M5-plus. The weekend stayed under that threshold. The arc did not.

-- LUCIA VEGA, São Paulo