A powerful earthquake shook Istanbul on Wednesday, sending residents of Turkey’s most populous city pouring into the streets as buildings shook and onlookers screamed.

There were no immediate reports of casualties from the magnitude-6.2 tremor. But it rattled nerves in a country still traumatized by the devastating 2023 quake that struck the southeast. Turkey straddles two major fault lines and is regularly hit by earthquakes.

The country’s emergency services said they were still scanning for possible destruction in the city of 16 million, which has long been bracing for a major tremor.

An initial assessment by the U.S. Geological Survey showed the quake had a depth of about six miles, with its epicenter some 25 miles southwest of Istanbul in the Sea of Marmara, the body of water that connects the Mediterranean and the Black Sea. It was felt from the city of Bursa, around 40 miles from Istanbul, all the way to Izmir, a coastal city nearly 300 miles away.

Note: Map shows the area with a shake intensity of 4 or greater, which U.S.G.S. defines as “light,” though the earthquake may be felt outside the areas shown. Source: U.S.G.S. By William B. Davis and John Keefe

In Silivri, the suburb of Istanbul closest to the epicenter, television footage showed patients at the state hospital being evacuated outdoors in beds and stretchers.

On television, news programs were debating whether the temblor may be a precursor to the long-expected major earthquake. Turkish seismologists have been warning authorities for years that an earthquake with a magnitude of seven or higher should be expected in Istanbul, with a risk of major destruction and loss of life

Historical records suggest the ancient city has averaged an earthquake of that magnitude about every 250 years, with the last major Istanbul earthquake in 1766.

The 2023 earthquake in southeastern Turkey forced the country to reassess its preparedness and building standards after residents watched entire city blocks collapse. The 7.8-magnitude quake and its aftershocks killed more than 50,000 people in Turkey and about 6,000 more across the border in neighboring Syria.

The earthquake on Wednesday struck as many in Istanbul were outside enjoying sunny weather on a national holiday that commemorates the 1920 opening of the Grand National Assembly — the Turkish parliament.

Some began to panic as they realized the shaking they felt was an earthquake. Others screamed, took cover, or rushed to make phone calls to check in on loved ones.

Shortly after the quake, Turkish officials warned people to stay away from buildings that looked unstable and to use the internet to communicate with one another.

Telecommunications were choked by phone calls as people frantically sought to check in on one another, but the internet has so far functioned normally.