Frequently Asked Questions

What is an accelerometer? What is an accelerograph? And what is an accelerogram?

An accelerometer is a sensor that measures acceleration, like a speedometer measures speed. An accelerometer is often part of an accelerograph, which is an instrument that contains accelerometers and records the acceleration record. The acceleration record is called an accelerogram. Note the similarity to something much more familiar: The telegraph, is an instrument, and it produces a record that is called the telegram

What is acceleration?  What does “g” stand for?

Acceleration, in physics, corresponds to the force applied to something that causes it to change its position or speed.  It is the force you feel when a car accelerates from a stop sign, pushing you back into the car seat (it’s a horizontal force).  Similarly, when an elevator starts moving, you feel more weight on your legs (it’s a vertical force).  When a roller coaster car makes a hairpin turn, the acceleration may push you to the side, or up or down.

Acceleration is measured in “g”, where 1 g corresponds to the vertical acceleration force due to gravity.  Roller coasters experience accelerations of 2 or more g, and fighter pilots may have to handle accelerations of 8g or more without passing out.

During an earthquake, the forces vary a lot and keep changing, back and forth and side to side.  These forces, if they’re strong enough, can damage structures unless the structures have been specially designed.  The largest earthquake forces that have been measured are about 1 to 2 g; most earthquakes have much lower forces, but those forces can still damage many structures.

What does strong motion mean?

The motion of a point on the ground during a small or distant earthquake can be so small that only specialized, precision instruments can record it.  When the earthquake is larger (or closer), that motion will be larger.  When the motion reaches the level where humans can feel it, typically a 1-2 %g, it is often called strong motion.  This is an actually an arbitrary level, meant to communicate the level qualitatively, as when one says “heavy rain” vs. “light rain”.   

What other sensors besides accelerometers are used to measure earthquake motion?

The most common sensor is not the accelerometer, which measures acceleration, but the seismometer, which measures the velocity or speed of a point on the ground as it moves during earthquake shaking. Most velocity sensors are high precision, sensitive instruments designed to record motions from distant earthquakes rather than the strong shaking that occurs near to earthquakes. Another instrument is the displacement sensor, which can be used in certain applications; in strong motion, they are most useful in measuring relative displacement (the distance between two points). Finally, GPS position sensing has become available and can be used to track the position of a point. Earthquake forces changes so rapidly during an earthquake that they must be measured many times each second (as many as 200). GPS doesn’t measure changes that rapid, but is ideal to get final locations after an earthquake.

Why do you measure acceleration, instead of velocity, or displacement?

Acceleration gives the forces directly, so it can be used to establish the forces that a structure experiences during an earthquake. Also, acceleration sensors are generally the most hardy of all seismic sensors. In addition, they are usually small, only a few inches on a side, so they are easy to place at key locations in a structure. The acceleration record can be computer processed and integrated to obtain the velocity and displacement records.

What does near-real-time vs. real-time mean?

A real-time signal is continuously being sent and received, with little or no delay, much like a newscast or program on TV. Near-real-time is delayed slightly because a communication link is established to send the data once an event occurs, much as one makes phone call to someone in order to give them a message. Near-real-time communication is generally more economical than a continuous data link, which makes it attractive for communication of infrequent events like the occurrence of earthquake shaking. Real-time communication is thought to be more reliable, but a big earthquake can interrupt many conventional communication pathways. To achieve the most robustness, all types of communication paths should be used achieve maximum redundancy (as is being done in CISN).

What is the difference between Richter magnitude and acceleration?

The Richter magnitude indicates the size or strength of an earthquake. For illustration, a parallel can be drawn with how the strength of an explosion will often be reported in terms of tons of TNT (or sticks of dynamite, in old movies). In contrast, the acceleration, or “g force” usually refers to the shaking experienced at a specific point due to the earthquake. This shaking is generally lower at greater distances from an earthquake, just as the sound level experienced from the dynamite blast is lower at points farther away from the explosion.

What does TriNet stand for?

What does CISN stand for?TriNet is a combined seismic network involving the networks of three agencies, Caltech, the US Geological Survey in Pasadena, and the California Strong Motion Program (in the Department of Conservation’s California Geological Survey). The TriNet effort was primarily supported by FEMA, through OES, as a result of the 1994 Northridge earthquake, and the funding ended for that in late 2000.CISN is the California Integrated Seismic Network, and similar to TriNet but with the addition of two northern California networks, the US Geological Survey in Menlo Park and UC Berkeley. With State support through OES, and federal support though the ANSS effort of the USGS, it makes the effort statewide.

Where can I find lists of strong-motion stations in California?

The lists of strong motion stations in California of the CSMIP and NSMP can be found on the web site of each program.  In addition, the Northern and Southern California Seismic Networks maintain lists of seismic stations of their networks for both strong and weak motion stations.

Where can I find strong-motion data?

There are several strong-motion data collections for earthquakes of California and elsewhere.  These include the CISN Engineering Data Center, of which the primary data sources are the CSMIP and NSMP networks, in CISN.  For data immediately after an earthquake, see the “Internet Quick Report” at the Engineering Data Center.  Additional data collections are the PEER database and COSMOS Virtual Data Center.  For earthquakes outside California, the NSMP and COSMOS Virtual Data Center also host datasets for the US and selected regions in the world.  An important European data collection can be accessed at the European Strong Motion Database, and Japanese records can be obtained at Kik-Net.

What types of buildings/structures does CSMIP instrument?

The CSMIP installs earthquake-monitoring devices in structures such as buildings, hospitals, bridges, dams, utilities and industrial facilities.  The program has installed more than 1000 stations, including 700 ground-response stations, 170 buildings, 20 dams and 60 bridges.  Sites are selected according to long-term strategies developed in consultation with the Strong Motion Instrumentation Advisory Committee, a committee of the Seismic Safety Commission.

What are the products of strong-motion data?

Strong motion data are used both for earthquake emergency response and for engineering and scientific research.  For earthquake emergency response, applications such as the ShakeMaps use strong-motion data as input and generate near-real time information on the levels of ground shaking and loss assessment for emergency responders.  Research products in engineering and scientific applications include evaluation of building/structure performance, near surface wave propagation and attenuation, local site characterization, and strong motion prediction, etc.

How do I find out about recent earthquakes in California?

Near-real-time earthquake maps and lists are available at the USGS Pasadena and in Menlo Park recent-earthquake sites and at the Southern California Earthquake Center.

Maps of shaking, called ShakeMaps, which show the areas of ground shaking for significant recent earthquakes are available for both Northern California and Southern California.