Two minor earthquakes occurred within the space of one minute in Mexico City early Wednesday.

A 2.8 magnitude quake struck at 6:42 a.m. and a 1.8 magnitude one followed 49 seconds later at 6:43 a.m., according to the National Seismological Service (SSN).

Both micro-quakes had their epicenter three kilometers north of La Magdalena Contreras, the SSN said. La Magdalena Contreras is a borough in the southwest of Mexico City. It is bordered to the north by the borough of Álvaro Obregón.

Residents of several boroughs felt the quakes, but they didn’t cause any damage, Mexico City authorities said. Some residents said they were awoken by the shaking they caused.

Mayor Martí Batrés said in a television interview that the earthquake alarm didn’t go off in Mexico City because the quakes were only minor and had their epicenters in the capital.

The alarm “is activated in advance when the telluric wave comes from afar,” he said.

The alarm, amplified through loudspeakers situated across the capital, sounds up to a minute before a sufficiently powerful earthquake begins to be felt, but it is dependent on sensors outside the capital. It can give residents a brief window of opportunity to evacuate to the safety of the street and thus avoid the risk of being caught in a building that collapses.

In recent times, there have been numerous minor earthquakes in the capital. A total of 23 were recorded between Dec. 3 and Jan. 10.

Scientists have confirmed the presence of a roughly one-kilometer-long seismic fault beneath the ground of a densely populated area on the western side of Mexico City.

Researchers from the National Autonomous University (UNAM) are investigating its relation to the frequent occurrence of micro-quakes in Mexico City.

With reports from Milenio, El Economista, Reforma and El Financiero