Is That an Accelerometer in Your Pocket?

Thanks to relatively recent advances in technology, many portable digital devices now have accelerometers embedded in them. As can be guessed by its name, an accelerometer measures an object’s acceleration, which provides an accurate assessment of any force being applied to the object. An example of such a force is gravity, the force that keeps us firmly planted on Mother Earth. Acceleration is typically measured relative to gravitational acceleration and this ratio is popularly known as g-force, where 1 g is equivalent to the pull of gravity at the Earth’s surface.

Any time an object changes speed or direction, the force required to make the change produces a measurable acceleration. But how do accelerometers work?

Early accelerometers were relatively large and consisted of a large mass on a spring. When a force was applied to the device, the mass would move separately from the rest of the accelerometer. Smaller accelerometers were developed using piezoelectric crystals (such as crystals utilized in quartz watches) that produce an electric current when squeezed.

Modern accelerometers can be even smaller and are typically engraved on a single computer chip. The design is not so different from the original mass-on-a-spring design, only now the mass and spring consist of a micro-machined silicon cantilever beam and mass.

Accelerometers have proven to be useful in a wide range of environments and applications. Some applications include measuring vehicle loads on bridges, measuring wind load on buildings, and measuring ground and building motions during earthquakes.

Because accelerometers can accurately measure changes in speed and direction, they are used along with gyroscopes in internal guidance systems. In automobiles, they are used to determine when airbags should be deployed and to measure the force of impact in a collision.

Due to their small size, modern accelerometers are used in a myriad of devices: in video game controllers to detect motion, in video cameras to stabilize images, in sports watches to count the steps of a runner, and in computer laptops to detect whether they have been dropped and are about to hit the floor.

Acceleration is always measured relative to the constant pull of gravity, so accelerometers can also detect when they are being tipped. This is the feature used to automatically align landscape versus portrait photos when a cell phone is rotated.

The recent explosion in cell phone “apps” has resulted in several applications that use the internal accelerometers to display real-time acceleration graphs. There are also laptop applications that can produce a similar data display using internal accelerometers. These applications can also send recorded data to research projects, such as the quake-catcher network, which utilizes thousands of laptops world-wide for analyzing ground motion from earthquakes (

Scientists are interested in real-time recording of this type of data because, by measuring the peak ground acceleration resulting from an earthquake in real time, they can rapidly forecast how much damage might have been caused by the earthquake. Rapid distribution of this information to critical services such as police, fire, and rescue officials can quickly get help to the areas that were hardest hit by the earthquake.

The U.S. Geological Survey has produced software, called ShakeCast, to do this with data from a network of accelerometers. Through this effort, critical facilities can receive notification of potentially damaging shaking levels within minutes of an earthquake.

Although some cell phone apps that use accelerometers, such as electronic bobble-head dolls or one that measures how high you can throw your cell phone, might seem silly or a waste of time (or, as some might argue, just misunderstood), you never know when creative applications like these might lead to a new, life-saving use for accelerometers.

Kilauea activity update

Over the past week, activity on the east rift zone flow field continued to build low lava shields above the pali and to shed lava flows southward toward the top of the Royal Gardens subdivision. No surface flows have been reported for several days along the recently active Quarry flow from the top of the pali to the coast near the end of Highway 130. In addition to the surface flows, lava also continues to erupt within Pu`u `O`o crater, though this activity has waned over the past several days.
At Kilauea’s summit, a circulating lava pond deep in the collapse pit within the floor of Halema`uma`u Crater was visible via the Webcam throughout the past week. The baseline lava level appears to have leveled off after a slow rise that started in late May, but was punctuated a few times by short-lived lava-level increases that brought the lava surface to its highest levels yet recorded. Volcanic gas emissions remain elevated, resulting in high concentrations of sulfur dioxide downwind.

No earthquakes beneath Hawai`i Island were reported felt during the past week.

Visit the HVO Web site for detailed Kilauea and Mauna Loa activity updates, recent volcano photos, recent earthquakes, and more; call (808) 967-8862 for a Kilauea summary; email questions to

Volcano Watch is a weekly article and activity update written by scientists at the U.S. Geological Survey’s Hawaiian Volcano Observatory.

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