Northridge Earthquake Epicenter EarthCache

So why was this earthquake so special? One of the reasons was that is occurred on a blind thrust fault. Another reason is that it happened literally in the middle of a major metropolitan area. But before we go into any more details, you must understand what a fault is and how they work.

Let's first discuss the basic fundamentals of how fault lines are created and now they work. The answer can be found in your kitchen, literally. When boiling pasta on the stove, the hotter pasta floats to the surface then when it cools it sinks. This effect has a name. Convection currents in your pot at home and in the earth are nearly identical. There are a few difference though. One is with water, rather than boiling rocks and the other is that one is less a few inches while one is miles deep. Convection currents are continues cycles in the earth's mantel that make hotter rock rise, and cooler rock fall and the cycle begins over and over again. When this happens, it causes fractures in the earth's crust. When fractures occur, a fault line is created. When they move in a circular pattern in one direction, they can push rock with them. When tension is released you get an earthquake in an that moves in waves on the X, Y, and Z axises. X is up or down, Y is right or left, and Z is pivoting side to side.

There are four types of main earthquake waves. Each wave moves at about 700 miles an hour and some move through different mediums and depths. There are P, S, Rayleigh, and Love waves in that order. P or primary waves are the first to arrive. These are compressional waves that have push and pull the land horizontally. Next are S or secondary waves. These waves move up and down along the vertical access. P and S waves are used to determine the epicenter of an earthquake. By looking at a seismograph, we can see these waves individually and measure the time in between them. By using three seismographs, we can draw circles around the locations of the devises and the intersection of these circles are the epicenter.

The other two types are earthquake waves are the most destructive. Rayleigh waves roll the ground on the vertical like a perfect surfing wave in the ocean. The last major wave, although there are more that come after, are love waves. Love waves roll the ground on the horizontal, perpendicular to the epicenter. The diagram below will show you the differences in these four main types of waves.

Depending on the pressure, it can create a magnitude 1 earthquake on the Richter Scale or a 10. A 10 has never happened in recorded history, but is believed to have only occur when large meteors hit the earth. The largest earthquake ever recorded happened in Chile on May 22, 1960. It happened in open waters, just off the coast in the Pacific where it created a Tsunami. You can see my earthcache about that Tsunami here. A Richter Scale works like this. Say there was a 5.0 earthquake, which are common, and a 7.0, a little less. A 7.0 on the Richter scale is 100 times greater of that of a 5.0 on the same scale. Each decimal point in-between is logtmartihicly worse as the preceding one, not twice as bad as most people would believe. The amount of energy an earthquake produces is logtmartihic not linear.

The 1994 Northridge earthquake occurred about 11 and a half miles below your feet. So why were houses built all around this very active fault? Well, this the fault that the north ridge earthquake occurred on is called a blind thrust fault. It's exactly what it sounds like. It's blind because its near impossible to detect. Unlike the San Andreas Fault with is very easy to detect, the blind thrust faults are hidden. We only know when and where one exists if we have recorded an earthquake of happening. So, as far as the planners and developers knew there were no active fault lines in the area. Unfortunately, they were wrong.

Blind thrust faults can have periods of inactivity and periods of activity, similar to how a volcano can be dormant and explosively active. We now know that there is a fault zone in the middle of the San Fernando Valley, that wasn't known until January 17, 1994. The reason why they are so hard to detect is because of erosion and deposition. Because earthquakes are infrequent, but violent on this fault means that in the long waiting periods, sediment can build up along the fault and become hidden under thousands of feet of new sediment before a new earthquake strikes. When faults do move, they move the land and shape the land. If the land is eroded faster than it is being shaped by a fault, then it become hard for geologists to pinpoint faults since there is little to no evidence of there being a fault.

On January 17, 1994 at about 4:30 in the morning the 6.7 magnitude earthquake rattled all of Southern and Central California as well as parts of Arizona and Nevada. The earthquake caused billions in damaged and took the lives of 57 people. More than likely, there are more blind thrust faults that we don't know of yet. Some can be even more powerful than the one that cause the Northridge earthquake. We won't know exactly how many faults there are until we have a better way of knowing where they are before an earthquake strikes along them.

One of the reasons why Northridge was so powerful is because it sits in the middle of a valley. This is an earthquake's paradise. Valleys form a natural barrier for seismic waves to bounce off of and reverberate against each other, making the shaking more severe inside the valley, than outside. The safest place to be during an earthquake if you are inside is under a table. A doorway would be the safest place in older homes, but not ones build after the 80s and 90s. If you are outside, the best place would be on top of a mountain and away from buildings. Being on a solid piece of rock such as a mountain will make the seismic waves move through the rock very easily. Since solid rock doesn't bend well, the earthquake waves travel through it very easily. The worst place to be is in a valley or sandy areas. Sandy areas are prone to liquefaction.

Liquefaction is what cause most of the damage inside the San Fernando Valley during the earthquake. Liquefaction is literally turning the ground into quick sand. Buildings are not being supported by anything so they ultimately fall and sink in this water. To understand liquefaction, think of it like a water bottle with water at the bottom and dry sand at the top. When you shake it, water travels up to the surface as the sand settles. What really happens is that when you shake and bottle full of sand, the elevation gets lower due to small particles of sand go under the larger ones. In reality, this squeezes up groundwater to the surface which makes puddles and can deform the land up to a few feet.

https://en.wikipedia.org/wiki/1994_Northridge_earthquake

http://www.britannica.com/science/soil-liquefaction

http://quakeinfo.ucsd.edu/~gabi/sio15/lectures/Lecture05.html

*http://scedc.caltech.edu/significant/northridge1994.html*

TO LOG A FIND ON THIS CACHE YOU MUST GIVE ME THE CORECT ANSWERS BEFORE YOU LOG A FIND. YOU CAN CONTACT ME THROUGH MY EMAIL OR THE GEOCACHING MESSAGE CENTER. ANY INCORRECT ANSWERS WILL RESULT IN A DELETED LOG

1. Northridge Earthquake Epicenter on the first line of your email.

2. Look at the buildings around GZ. Do you see any damage from the 1994 Northridge earthquake? Which buildings look like they were built after 1994? Which buildings looked like they were built before 1994?

3. Using the first diagram on the cache page, would you expect to see similar damage if another 6.7 earthquake occurred here today? If the photos are too small, go to the works cited and click the link that as these symbols *website*. The damaged photos are explained in much greater detail on that website.

4. Which seismic wave(s) would you feel first here on on January 17th, 1994 at 4:30:55 am PST? Which wave(s) would you feel first in Las Vegas on January 17th, 1994 at 4:30:55 am PST? Which seismic wave(s) would you feel first in Los Angeles on January 17th, 1994 at 4:30:55 am PST? Which seismic wave(s) would you feel first in San Francisco on January 17th, 1994 at 4:30:55 am PST?

5. If another 6.7 earthquake occurred today here, would you expect liquefaction occur where you are standing? Explain.

* NOTE - You will likely not receive an email back from me, unless I need clarification on your answers. Please, don't wait for me to tell you you have it correct or not. I do review your emails within 12 hours of getting them in my inbox and with other of my earthcaches, it has become too hard to do. As soon as you send the email, you can log this earthcache.