The crack in Mesut Muttaliboglu's bedroom wall is so wide, he can fit a car key into it.

He turns it sideways, and with a flick of his wrist, a large chunk of plaster flies off the wall and crashes to the ground.

It's why he and his family are moving out of the flat they've lived in for the last 15 years. The whole building has been condemned after failing an earthquake safety test. There's a very high chance that a tremor would bring this whole block crashing to the ground.

Here in Istanbul, fear is growing.

The two powerful earthquakes in Turkey's south that claimed almost 50,000 lives have brought a new urgency to its biggest city. Home to 15 million people, it sits on the North Anatolian fault line, and experts predict it's due its own major earthquake before 2030.

Around 70% of the city's buildings were built before rule changes that enforced stricter construction standards in 1999, and so are considered potentially unsafe. Just three months ago, a study said a quake here could kill up to 90,000 people. Now, the race is on to get the city ready.

Mesut knows all too well the devastation a quake can cause. He's just returned from the epicentre in the southern city of Kahramanmaras, where he lost relatives. As we talked in his now-empty flat, he described the moment he found out.

"It happened at 04:17, a relative called and we all woke up screaming." Mesut's face crumples into tears and he turns away to compose himself. "It's a horrible situation. We couldn't get [to Kahramanmaras] for three days because of snow, and when we reached the rubble it was so hard. I can't describe it. I hope God doesn't make anyone else experience this."

When Mesut returned to Istanbul, the authorities had shut off power and water to his flat. "I asked for them back again just so we could move. They gave me two more days."

"The municipality had sent us written warning about it, but the situation wasn't resolved due to rejections from the neighbours. We knew that our utilities were going to be shut off, and we were ready to leave here, but then the quake happened and it all became a shambles."

Since the earthquakes in the south, there've been more than 100,000 new applications to the Istanbul municipality for building safety checks. The waiting list for one shot up to three months, then four, and it keeps rising.

Tenants as well as landlords can now apply, but some still don't because of the financial implications. The compensation to help those who need to move out of condemned buildings is low. There are no official numbers showing how many fail the test.

The city's mayor, Ekrem Imamoglu, has promised more training for rescue teams, and preparation of temporary shelters that could house up to 4.5 million people in the aftermath of a quake. But many fear it still isn't enough.

Engineers like Dr Kurtulus Atasever warn many of Istanbul's buildings have architectural features that weaken their structure

A walk down an average Istanbul street tells you why. Many of the buildings have particular design features that can make them collapse if they're put under pressure during a quake.

Dr Kurtulus Atasever, a structural and earthquake engineer, met me to point some of them out. We stood on an empty patch of ground, strewn with rocks, that used to be the foundations of a building. When a magnitude 5.8 quake hit Istanbul in 2019, it was so badly damaged it had to be knocked down. Up and down the street, its neighbours have many of the same flaws.

Good quality concrete is vital, he tells me. And the architecture is crucial. "We have some overhangs here. In this type of building we have some weak or soft storeys. There are short columns too, they are actually all typical building problems."

In very simple terms, each of them weakens a building at ground level, meaning it struggles to hold up the floors above if there's an earthquake. An overhang makes the rest of the building wider than the footprint. Soft storeys are where the ground floor is taller than the floors above it. Short columns don't have enough length compared to their diameter.

They can be done safely, Dr Atasever points out, but only if proper thought and planning has gone into the design. In old buildings especially, that's rare.

We're standing in the shadow of Yasemin Suleymanoglu's house, and I ask if she's concerned about the apartment block it's in. She holds her daughter's hand and looks up at the face of the building. "I don't feel safe here," she says.

"Our building shook a lot during the 2019 earthquake, and the columns of the one across the street cracked. I've been feeling restless since that sound, and with this latest quake we are really scared. We're losing our sleep because it may hit us any time. And I think we're at risk because our building is old."

The next step is the development of a 50km (31-mile) long fibre-optic based early warning system. But for a city so huge, it's hard to know where people would go to seek shelter, even if they had notice that a quake was coming.

As pictures of the devastation in the south continue to fill Turkish TV screens, these concerns are now front-of-mind for a huge slice of Istanbul's population. And just two months away from important presidential and parliamentary elections, that really matters.

Overnight, the earthquake and its aftermath have joined Turkey's economic crisis as a key issue for voters. Many aren't happy with the government's handling of either. The aftershocks here aren't just physical, they're political too. (BBC)

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