Earthquake safety is a responsibility shared by billions worldwide. The Quake-Catcher Network (QCN) provides software so that individuals can join together to improve earthquake monitoring, earthquake awareness, and the science of earthquakes. The Quake-Catcher Network (QCN) links existing networked laptops and desktops in hopes to form the worlds largest strong-motion seismic network.
The Quake-Catcher Network is a distributed computing network that links volunteer hosted computers into a real-time motion sensing network. QCN is one of many scientific computing projects that runs on the world-renowned distributed computing platform Berkeley Open Infrastructure for Network Computing (BOINC). The volunteer computers monitor vibrational sensors called MEMS accelerometers, and digitally transmit “triggers” to QCN’s servers whenever strong new motions are observed. QCN’s servers sift through these signals, and determine which ones represent earthquakes, and which ones represent cultural noise (like doors slamming, or trucks driving by).
Two Types of Sensors
There are two categories of sensors used by QCN: 1) internal mobile device sensors, and 2) external USB sensors.
Mobile Devices: MEMS sensors are often included in laptops, games, cell phones, and other electronic devices for hardware protection, navigation, and game control. When these devices are still and connected to QCN, QCN software monitors the internal accelerometer for strong new shaking. Unfortunately, these devices are rarely secured to the floor, so they may bounce around when a large earthquake occurs. While this is less than ideal for characterizing the regional ground shaking, many such sensors can still provide useful information about earthquake locations and magnitudes.
USB Sensors: MEMS sensors can be mounted to the floor and connected to a desktop computer via a USB cable. These sensors have several advantages over mobile device sensors. 1) By mounting them to the floor, they measure more reliable shaking than mobile devices. 2) These sensors typically have lower noise and better resolution of 3D motion. 3) Desktops are often left on and do not move. 4) The USB sensor is physically removed from the game, phone, or laptop, so human interaction with the device doesn’t reduce the sensors’ performance. 5) USB sensors can be aligned to North, so we know what direction the horizontal “X” and “Y” axes correspond to.
Apply for a USB Sensor
If you are a science teacher at a K-12 school, please apply for a free USB sensor and accompanying QCN software. QCN has been able to purchase sensors to donate to schools in need. If you are interested in donating to the program or requesting a sensor, click here.
About MEMS Accelerometers
MEMS accelerometers are simply microchips with a very small set of force-balance cantilever beams inside of them. The balance mass weight is compensated by low-level electrical voltage. QCN uses a series of sensor types, some embedded in devices such as laptops, and some are connected to computers via USB connections. The QCN sensors measure accelerations between -2g and +2g (where g is Earth’s gravity – 9.81m/s/s) on three axes. QCN software samples the sensor 50 times per second (50Hz).
(left) This scanning electron microscope (SEM) image illustrates the inner workings of a MEMS accelerometer. The linear features are fixed plates, mobile plates, and the balance beam of the force-balance cantilevers.
Not all sensors are created equal. Some sensors have lower electronic noise levels. In some cases, the analog acceleration is converted to digital signal with greater resolution. In some cases, the analog signal is not fully resolved by the analog-to-digital converter. In other cases, the bottom range of the resolution is entirely filled with electronic noise. The plot to the left roughly shows the magnitude of earthquake you might expect to record at what distances with which type of sensor. If the earthquake acceleration is above the sensor resolution/noise level, the sensor can record and detect the earthquake. Note: Cultural and mechanical noise is often greater than the low-noise level of the better sensors, so you shouldn’t expect to record M2 or M1 earthquakes with these sensors.
Communications and Data Transfer
Laptops connect to the Quake-Catcher Network over the Internet. Typically, when the QCN software is running, there isn’t much need to transfer the data to our headquarters. Instead, the laptop monitors the data locally for new high-energy signals and only sends a single time and a single significance measurement for strong new signals. If our server receives a bunch of these times and significance measurements all at once, then it is likely that an earthquake is happening. If the server receives only a time and significance measurement from one laptop, then the server knows the laptop was shaken by something smaller and more local (like your sister running by, or the door slamming).
Knowing the locations of all QCN’s sensor is very important for tracking earthquake waves and triangulating their source. When you configure the QCN preferences on a desktop, you can set a permanent location through a Google Maps API (right). Once BOINC is installed and attached to the QCN project, you may check and update your sensor location here (here). The Google Maps interface allows you to enter your address or select your computer’s location by clicking on the map. The more precisely you enter your location, the more QCN scientists can understand about earthquakes recorded by your sensor.
Your computer’s location information, sensor type, seismic records, and timing information are shared online. If you do not wish to share your computer’s location, we kindly ask that you opt not to participate. By participating in the Quake-Catcher network, you agree to share information that is critical for earthquake science. For more information on QCN’s policies, please view our legal disclosures and privacy policies.
(left) The sensors can measure acceleration in three directions. The easiest way to think of these directions is as the Z=up/down, Y=front/back and X=side-to-side motions. With these three components of direction, it is possible to find the direction of the 3D acceleration.
(right)There are several kinds of seismic waves, “Primary” or “P” waves (compressional waves), “Secondary” or “S” waves (shear waves), and “Surface waves” (interference waves that bounce along the surface of the Earth). The P waves and S waves vibrate in perpendicular directions to each other. So, measuring acceleration in three directions helps us determine which wave is which.
Orient to North
The USB sensors come with a mini compass so you can align the sensor so the “Y” direction points toward the magnetic North Pole. Note that magnetic North Pole is not always the same as True North – the direction of the Earth’s spin axis. However, we can digitally rotate the signals to True North if we know your location. The better we know the directions of motion, the more precisely we can measure the earthquake.
Timing is Everything
Every 15 minutes the Quake-Catcher software measures the timing difference between your computer’s clock and the Quake-Catcher Network server’s clock. The clock here at the Quake-Catcher Network is synchronized with an atomic clock. Computer time can drift because the speed of a processor depends on voltage, and temperature. Over long times, even small changes in CPU speed can lead to large differences in time. By measuring the time offset between host computers and QCN’s server, we know the timing of any strong new motions measured on the network.
QCN uses Network Timing Protocol (NTP) to measure the timing offset between your host computers and QCN’s server. NTP can usually maintain time to within tens of milliseconds over the public Internet. Repeated NTP checks can typically decrease the timing offset to several milliseconds. NTP works on a tiered system, where stratum 1 NTP servers are connected to atomic clocks, GPS clocks or radio clocks, stratum 2 servers connect to stratum 1 servers and stratum 2 servers, and other computers connect to stratum 3 servers.