Wellington: A strong 6.1-magnitude earthquake struck off the west coast of New Zealand's South Island on Thursday, geologists said, but there were no immediate reports of damage and no tsunami warning was issued.

The quake struck at a relatively shallow depth of 18 kilometers, 204 kilometers west of Invercargill, New Zealand's southernmost city, at 5:48 pm (1218 IST), the US Geological Survey said.
It was followed about two hours later by a 5.2-magnitude aftershock with its epicenter in the same area.
Christchurch, New Zealand's second most populous city, which is also on the South Island, was battered by a powerful 6.3-magnitude earthquake in February last year that left 181 people dead and destroyed much of its center.
New Zealand sits on the so-called "Ring of Fire", the boundary of the Australian and Pacific tectonic plates, and experiences up to 15,000 tremors a year.