Thursday, August 22, 2013

Earthquake Monitoring at NHM and in Los Angeles

We are increasing our online presence on the Mineral Science webpage! First step: an Earthquake-dedicated page. It describes Earthquakes in the LA area, and also the Quake Catcher Network, a program dedicated to encourage the public monitoring earthquake from home!

The majority of the text and pictures from the site has been added in the remainder of the post below. The website page was created by me, Austin Baca, a volunteer working with the Mineral Sciences Department. I am currently a fourth year student at the California State Polytechnic University of Pomona, studying Geological Sciences with an emphasis in Geophysics. My interests are in Seismology, which is the study of earthquakes, but I also enjoy learning as much as I can about Geology which is what brought me here to the Mineral Sciences Department at the Natural History Museum. When I first began assisting in the Mineral Sciences Department I was informed about the Quake Catcher Network and tasked with reading about it and creating a webpage on how others can become involved. I seriously hope that people will check out the actual post on the Natural History Museum webpage and follow the links to get involved in this project. I believe that it can open people up to the Geosciences while getting them involved in acquiring data that can help individuals who study Seismology to better understand how earthquakes affect the Los Angeles area.

The Quake-Catcher Network 

Partnering with the Quake Catcher Network (QCN), the public has the unique experience of turning their computer into a mini-seismometer capable of sensing moderate to large-sized earthquakes. The public signs-up to request a sensor from the Quake-Catcher Network website along with the necessary drivers and software. The more people who participate become part of an ever expanding Southern California array of sensors recording ground motion from earthquakes. The ground motion recorded is used to better understand earthquakes and help to mitigate damages in the future. Earthquakes and earthquake hazards are real issues for people living in California, so it is important for people to learn about this incredible force of nature.

What are earthquakes and why should Angelenos care?

The greater Los Angeles area is no stranger to the dangers of earthquakes. But what are earthquakes exactly? Earthquakes are powerful waves of energy that travel through the ground normally created when two blocks of rock slide past one another at a fault boundary. Normally faults are locked in place and it’s the area immediately next to the fault that is moving and deforming. The deformation, called strain to geologists, builds up a force called stress. At some point the stress built up is released in an earthquake and the rock on both sides of the fault move. Plate tectonics best explains the existence of faults in the world as well as in California and is a fundamental concept for Earth scientists. The Earth is made up of eight large crustal plates and about a dozen smaller ones moving slowly due to the movement of hot mantle material. There are continental and oceanic plates. The Earth’s tectonic plates move in different directions and speeds. Figure 1 below shows the outline of every major tectonic plate in yellow. The red arrows point in the overall direction of plate movement and the length of the arrow represents the speed at which the plates move on average. A majority of the world’s earthquakes occur at or near plate boundaries. There are three kinds of plate boundaries, divergent, convergent, and transform.
Fig. 1: Relative motion of major tectonic plates. copyright

Two plates split apart from one another at a divergent plate boundary, also called a spreading center or rift. An example would be the Mid-Atlantic Oceanic Ridge which is slowly pulling apart ocean crust and expanding the Atlantic Ocean. As the plates slowly move apart new lava brought up from the mantle crystallizes, forming new crust. Earthquakes at these ridges are normally small to moderate.

On the other hand two plates can come into contact with one another at a convergent plate boundary. Continental plates can collide, or converge, with one another crumpling and forming huge mountain ranges like the Himalayans. When one continental plate collides with an oceanic plate, the denser oceanic plate is forced down into the mantle creating a subduction zone. Some of the Earth’s largest earthquakes have occurred at subduction zones. Most of the Earth’s volcanoes form parallel to subduction zone boundaries. The chains of volcanoes form because water is carried down into the mantle within the minerals of the ocean’s crust. The water is released as the minerals break down rising into the overlying mantle rock and initiating melting.The figure 2 shows a regional subduction zone off of the Northwest coast of the United States. Here the Juan de Fuca plate is colliding with the North American plate and being pushed down into the hotter mantle where melting occurs. The red ovals within the crust of the North America Plate displays the melted material moving up and eventually breaking the surface where Mt. St. Helen and Mt. Hood are plotted.

Fig. 2: Block diagram of Juan de Fuca Plate subducting beneath the North America Plate and the movement of magma up to the surface. copyright

A transform fault boundary is one where plates slide past one another in opposite directions. This is the type of faulting common in California, New Zealand, and Turkey. Transform motion is completely horizontal and crust is neither created nor subducted. Transform plate boundaries can produce small, moderate, and large earthquakes. Figure 3 shows all three types of plate boundary can be seen in our state alone. Divergence ends in the Salton Sea area (ending in the Gulf of California in the figure) transitioning to transform motion on the San Andreas Fault with subduction taking over off the coast of Northern California.

Fig. 3: Regional tectonic picture of California and the west coast. copyright

A majority of the world’s earthquakes occur on these plate boundaries. One plate boundary all Californians should be aware of is the San Andreas Fault, which divides the Pacific from the North American plate. The San Andreas is a transform boundary. Movement on both sides of the fault is on the order of a few centimeters per year. However the San Andreas isn’t the only fault in California. Due to the movement of the two larger plates many smaller, but still significant faults are found throughout California. Figure 4 below is an example of the number and complexity of faulting in Los Angeles and the rest of Southern California resembling cracks in a pane of glass. Many faults are parallel to the San Andreas and exhibit the similar transform motion. Added complexity is due to the San Andreas Fault bending towards the Pacific Ocean. This “Big Bend” acts like a wrench jammed in a series of gears impeding the transform motion on both sides of the San Andreas Fault and creating numerous thrust faults. The movement on thrust faults is vertical, with one block of rock being pushed over another, the same as in subduction. This vertical movement is called uplift and is the reason for the height of the San Gabriel and San Bernardino mountain ranges. Thrust faults are numerous in the Los Angeles area, many of these faults can’t be seen on the surface.

Fig. 4: Known faults in Southern California (plotted red) and the larger San Andreas Fault (plotted yellow). Map created through Google Earth and downloadable KML files provided freely through the USGS.

Earthquake scientists, called seismologists, work to better understand how earthquake waves move through the ground using seismometers and how they can effect major populations, like the one in Los Angeles and neighboring communities. The seismometers pick up ground motion through sensors that measure displacement due to surface disturbances, a much more advanced version of the old spring and pencil system. Some devices are so sensitive that they can easily pick up the sound of the wind hitting the ground. The final recording from a seismometer is called a seismogram; it records displacement with respect to time. The seismogram can be used to determine where the earthquake occurred, at what depth it occurred at, and at precisely what time it occurred.

Becoming part of a network

The Quake-Catcher Network (QCN) is a collaborative effort sponsored by the National Science Foundation and the Southern California Earthquake Center. The goal is to create a large-scale seismic network in hopes to attain better earthquake data, to assist in early warning response, and to be used as a teaching tool about earthquakes and earthquake hazards. To achieve these goals, the public is encouraged to sign up for a sensor plus software that turns any internet connected computer into a seismometer.

The USB sensors can be secured to the floor or attached via USB cable. The floor-mounted approach is far better for picking up seismic signals while reducing any background levels of ground shaking, like typing on your keyboard or people stomping about. A simple configuration of the sensor is required in order to get an accurate position and activation time of the sensor. The configuration is important in aiding earthquake scientists observing ground motion data by giving them the location and time of the earthquake signal. All of this aids in a final determination of when, where, and even how powerful the recorded earthquake was. A software package is also available to read the data that the sensors pick up. It is even possible to view the ground motion on your computer screen!

Picture of USB sensor one can receive. On the left-hand corner of device is a small compass that is used to align sensor to North. For all sensors the y-direction aligns with North.

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