Malappuram: Persistent underground rumbling sound triggered panic at Pothukkallu and Aanakkal here on Tuesday night. The residents who were alarmed evacuated their homes and alerted government officials. People said that they started hearing the sound around 9 pm. Panchayat officials arrived at the site and evacuated the area. The village officer conducted inspections in places where rumbling sounds was heard.

Officials with the Kerala State Disaster Management Authority (KSDMA) said that this was not the first time such sounds were reported from this place. "There had been previous instances. On October 18, there was a similar rumbling around 4.45 am in the morning. These are acoustic emissions which happen due to minor rupture of rocks at shallow levels and energy comes out as sound waves.

"They may also be called minor earthquakes or shallow earthquakes, which are usually not detected by sensor networks. The area is known for fracture zones, and many a time, this kind of rupturing sound or rupturing itself happens because of groundwater extraction or certain minor shifting of fracture zones," a senior official with the KSDMA said.

The official said that people have a certain fear and there was no need for panic. "We are also looking at commissioning a geophysical study in this region. We are in touch with NIT Surathkal to do a study on subsurface profile and what is happening below ground," the official said.