SAN JOSE, Costa Rica -- A powerful earthquake with a magnitude of 7.6 struck the northwest coast of Costa Rica Wednesday, collapsing homes, blocking highways and causing widespread panic.

The quake struck Costa Rica's Nicoya Peninsula, on the Pacific coast about 87 miles west of the capital, San Jose.

It was centered more than 28 miles deep.

The Geological Survey originally measured the quake at magnitude 7.9, but revised it downward.

The earthquake did not generate any aftershocks, said Julie Dutton of the USGS. She said the agency has received reports of some damage.

Two people also reportedly died of heart attacks following the quake.

The epicenter was just six miles away from the city of Hojancha.

"It started out pretty mild, but then it really got going," said Bill Root, owner of a hotel in Samara, also near the epicenter. "It was a very strong earthquake. Everything was falling off the shelves and the ground was rolling."

Authorities issued tsunami warnings for much of the Pacific coastlines of Costa Rica, Panama and Nicaragua, according to the Pacific Tsunami Warning Center. The warning was canceled for other locations.

An earlier tsunami watch for the Caribbean was sent out inadvertently and subsequently retracted.

CNN affiliate Teletica showed people streaming out of buildings after the quake.

Telephone service was knocked out in the cities near the epicenter, Teletica reported. The station showed photos submitted by viewers of a partially collapsed bridge over the Rio Sucio and a landslide on a highway. Other photos showed floors littered with merchandise inside of stores and homes with minor damages.

"It's unnerving when solid concrete beneath you is shaking from side to side. It was pretty scary," said David Boddinger, editor of the English-language Tico Times in San Jose.

There is some damage, including debris that has fallen on the roads, and damage to the roads themselves at a spot on the Nicoya Peninsula about 14 kilometers (8 miles) from the epicenter, Boddinger said.

Seismologists have been predicting a strong quake for that area for years now, and Costa Ricans were wondering if this was the big one, he said.