TODAY | April 05, 2014
>>> one week ago this morning, southern california residents woke up with rattled nerves. a 5.1 earthquake had struck and led to over 100 aftershocks. now in the past week, we're learning more about a newly discovered fault line , one that could turn much of los angeles to rubble if a bigger quake were to strike. more now from miguel almaguer.
>> reporter: for 25 miles in southern california , it snakes beneath densely populated urban centers , below bridges and right underneath downtown l.a. skyscrapers. this is where the puente hills fault line lies, what's been dubbed the most terrifying fault line in the nation.
>> we have buildings that will kill people. we know which ones they are. we need to find a way to get them retrofitted.
>> reporter: last week, a 5.1 here rattled shelves, splintered homes and even triggered a rockslide. but experts say this was just a warning. a 7.5 on this fault would be catastrophic.
>> if a big one hits, we estimate that the damage to the economy and to our structures would be somewhere in the $200 to $300 billion range. it would be the largest disaster in u.s. history .
>> reporter: the problem is l.a.'s aging infrastructure. more than 1,000 buildings aren't up to modern earthquake codes. some of the most critical are built of vulnerable concrete. l.a.'s iconic general hospital had to be shut down in '94 after the north ridge earthquake. now it's just medical offices. but within five miles of here, six other major hospitals could face catastrophic damage in a big quake, including children's hospital los angeles . at the hall of justice, they've spent $234 million to retrofit one building alone, and it's taken three years to complete. the city is also scrambling to shore up 11 bridges. the cost, $400 million.
>> i think we're as well prepared as any big city in america, which is to say we're woefully unprepared. there is no city that gets this right.
>> reporter: today, city leaders in los angeles say they're doing what they can to prepare for the big one, but it's impossible to predict when it will strike, though experts agree it's certain to hit. for "today," miguel almaguer, nbc news,