OK so the reason New York was high on the list of seismic cities is actually quite chilling.
The programme that I watched had structural engineers, seismologists, architects along with the various government agencies who deal with the monitoring of seismic events (both earthquake and volcanic since both are connected to each other). They also had people who deal with evacuations and the planning thereof.
For those who doubt how serious the seismic threat is to New York and its surrounding areas......a few days ago epicentered close to West Seneca they experienced the strongest earthquake for over 40 years, a 3.8.
The threat is very real.
If you look at the buildings in and around the city of New York there are alot of tall skinny ones. Look at the bridges...all very ornate. Look at the parks.....alll very pretty.
New York subway trains......several parts of the track are above ground. The most well known is the 125th Street bridge.....it has hinges on each end. Back in the days when the subway was built there was no safe way to burrow a tunnel through a fault line, so the trains were placed above ground over the fault lines and the 125th Street bridge has hinges to allow for movement in an earthquake. One slight flaw in the design is that earthquakes tend to move side to side, not up and down so as well meaning as those hinges are, they will fail in the event of an earthquake. The road surfaces in that part of the city are pockmarked with earth movement scars from previous smaller earthquakes that were enough to open fissues in the tarmac but not enough to be noticed by residents.
Many of the older bridges across the city's rivers were built before rebar. Although work is in progress to upgrade the bridges, they are still vulnerable in the event of a 6.5 or higher earthquake.
Now the buildings.
With seismic building codes not introduced until 1995, much of what you see is vulnerable. The experts feel that there are just two pre 1995 buildings that could maybe survive an earthquake greater than 6.5.....the Chrysler and the Empire State. They were built not only to last but they are firmly sat on bedrock.
The area around Battery Park is extremely vulnerable. All that area is made up of spoil and was reclaimed. The spoil came from the foundation pits, including the very deep cut basements, of the Twin Towers. The spoil is mostly sand/clay mix, typical riverbed material.
The combined effect of the rivers, the high water table, the use of reclaimed land backfilled with spoil from construction makes the ground underneath those gleaming buildings very vulnerable to liquefaction.
To understand liquefaction.....if you stand on a beach where the tide can reach and stamp up and down for 5-10 minutes, the water held deep below the surface rises to the surface and you start stamping in puddles, the sand becomes liquid and unstable.
The majority of Manhattan and surrounding boroughs are built on reclaimed spoil land. It is extremely wet due to the high water table thanks to the rivers. The majority of buildings have what are called friction piles, they do not reach bedrock cos there isn't any. Instead the piles are driven deep into the spoil before basement areas are tanked and the highrise built above.
This makes Manhattan and its surrounding boroughs extremely vulnerable to liquefaction during and after earthquakes. Those buildings that resonate at the right frequency during an earthquake will trigger liquefaction in their piles and they will collapse (along with any bridges that were built without rebar, any new extensions to the subway system that cut through fault lines)
There is a contentious issue within the various studies.
The collapse of the Twin Towers and Building Seven. It has split the experts somewhat. There is a degree of evidence that cannot be ignored. The fact that after the 1993 explosion under the Twin Towers, both buildings and the surrounding buildings were inspected from bottom to top. It was discovered that in relation to the Twin Towers, the standard fire prevention measure when built - namely asbestos coatings on all internal steels - had been damaged and much of it was missing. The coatings were likely have been damaged both during construction (due to steels being out in the elements and waiting to be hoisted, the action of being hoisted and installed...plus the resonation from the explosion releasing already damaged areas). After the 1993 explosion and the introduction of the 1995 seismic building codes, the Twin Towers and its associated buildings were subject to annual inspection and plans had been drawn up to carry out extensive repairs to all steels that showed damage or loss of fire preventative coatings and corrosion. This work included Building Seven where similar issues were discovered even though that building was completed long after the Twin Towers. The remedial work was set to begin in late 2001/early 2002 and was estimated to take around 10 years to complete.
On top of the construction issues, the resonance of the buildings during the 1993 explosion was potentially enough to cause minor liquefaction around the vicinity of the friction piles.....think of a loose tooth, it wiggles alot but doesn't fall out. Come 9/11 and that resonance is far greater still. Coupled with the construction issues, the liquefaction issues plus the event itself and what it consisted of. Add into the mix that days before 9/11 an earthquake epicentered on Central Park with a magnitude 5....and another magnitude 5 hit days later.
Even though many experts shy away from the possibility, the building collapse and the construction issues and the high water table, reclaimed land, liquefaction, explosion and earthquakes does lend itself to being a chain reaction.....and its that chain reaction that has experts extremely concerned about the survivability of a 6.5 or higher earthquake in New York. The consensus is that the majority of buildings, bridges and tunnels will collapse and any tsunami afterwards will essentially wipe the city and its surroundings off the map. A taste of what could well occur in the event of a 6.5 plus earthquake came with both 9/11 and Hurricane Sandy.
