There are many seismically active fault zones across the United States just as there are all over the world. Many have yet to even be discovered, and many are too small or too irrelevant to worry about. But there are some that have the potential to do more than crack some dishes and knock grandma's picture off the wall. Most importantly, these are the faults that are known to have caused terrible damage in the past either in human recorded history or the geologic record.
The list won't exhaustively cover ALL the fault zones in the US, just the ones have the potential to cause catastrophic damage.
Hayward Fault Zone
Length: 74 miles
Location: Runs the east side of San Francisco Bay
Capable Magnitude: 7-8
Fault Type: Transform (strike-slip)
Last recorded event: 1868 in Hayward City
Next event probability: 31 percent chance of a magnitude-6.7 earthquake or greater along the Rogers Creek-Hayward Fault in the next 30 years and a 4% of an 8 or greater.
This very unstable fault in California has been threatening the San Francisco Bay Area for generations. It’s capable of producing quakes ranging from 7.0 to 8.0 in magnitude. The last major movement along the Hayward Fault occurred on October 21, 1868, virtually destroying downtown Hayward. In fact, it was considered the “great earthquake” until the San Andreas Fault tore San Francisco apart 38 years later in 1906.
What seismologists consider a 'ticking time bomb' about this particular fault line is that it did not slip during the major 1906 quake. It has been locked since 1868 30km from the surface. Below that point, there is movement -- only increasing the amount of pressure ready to snap like an overstressed rubber band.
The amount of devastation this quake could cause would massively overshadow the Loma Prieta quake of '89 in terms of life lost and structural damage.
Cascadia Fault Zone
Length: 620 miles
Location: Off the coast of Washington and British Columbia
Capable Magnitude: Megathrust quake exceeding 9.0
Fault Type: Subducting
Last large event: 1700
Next event probability: 37% chance of a 8.0+ in the next 50 years
Newport - Inglewood - Rose Canyon Fault Zone
Length: 47 miles
Location: The heart of Los Angeles, running north-south through the center of West LA and into South LA
Potential Magnitude: 6.0 - 7.4
Fault Type: right-lateral strike-slip
Last event: 1933 in Long Beach, killing 120 people
Next event: Unknown
This fault is a bit of a sleeping giant mainly because, like many of the other faults on this list, it runs beneath such a heavily populated area. Los Angeles is better off than most cities in the US in terms of being able to handle a large quake. Due to regulating building codes for seismic, the majority of older buildings have been retrofitted and all newer buildings are as safe as they can be. This does not mean that structures will survive a large quake, instead it means that they have been designed to prevent loss of life.
Newport/Inglewood was first identified after a 4.9 magnitude quake struck near Inglewood, California in1920. Due to the lack of earthquake-resistant construction in southern California at this time, this quake caused considerable damage in the Inglewood area and was a preview of what was to come almost 13 years later when the Long Beach quake hit in 1933. That event was a 6.3, and killed 115 people. Just recently, a new study has uncovered evidence of quakes off this fault so violent that they caused a section of Seal Beach to drop 3 feet in seconds. They are roughly estimated to have happened in 50 BC, 200 AD and 1450 and each were more powerful than the Long Beach event. This tells us that this fault is capable of a lot more than previously thought.
San Andreas Fault Zone - Salton Sea/San Gorgonio
Length: 100-300 miles from Salton towards Cajon Pass (total length of San Andreas 800 miles)
Location: running from the Salton Sea the south up to north of LA ( but the entire fault stretches to San Francisco)
Capable Magnitude: 8+
Fault Type: (transform) strike-slip
Last event: not in recorded human history, but in the geologic body of evidence
Next event probability: ?
Recent data has shown that the entire San Andreas could potentially rupture along its entire 800 mile length, starting with a quake at either end. While still not probable, it is now thought to be possible. Scientists estimate that such a quake could be as high as 8.3 on the Richter scale. However, as the odds of this are not particular likely, this entry is going to focus on the southern end of the San Andreas.
For those who have never been to Southern California, practically the entire basin from beach to San Bernardino is covered in development. This area is bounded in the north and east by mountains, with the little pass of San Gorgonio where the fault slices through into the valley that contains Palm Springs and further on, the Salton Sea. A rupture at the Salton Sea, were it to be large enough, would travel up the fault like like a freight train. It would sever freeways and water mains, electrical grids and emergency services and deliver an enormous amount of directed force into the LA basin. A basin, it should be noted, is built on the soft alluvial runoff of the mountains surrounding it. Such loose material would carry the waves of shock very well, turning into an almost quicksand in a process called liquefaction.
A further catastrophic scenario of a San Andreas rupture could result in aftershocks on the Newport-Inglewood fault ( #3 on this list), or the Sierra Madre fault in the San Gabriel Valley. Hayward (#2 on the list) in the San Francisco Bay Area could also potentially be set off by an earthquake on the southern San Andreas fault.
This has happened before because of the connective nature of the San Andreas. The 1906 earthquake in San Fran sent a 5.5 aftershock to Santa Monica and a 6.0 earthquake almost to Mexico.