The science of earthquake detection—in miniature

Technology is zeroing in on earthquake detection to help turn off gas lines and warn people. (Tumisu/Pixabay)

Two powerful earthquakes that hit near Ridgecrest, California, on July 4-5 jolted residents and civil engineers who monitor buildings and bridges.

People weren’t seriously injured, but building and roadway damage from the 6.4 magnitude quake on July 4 and the bigger 7.1 magnitude quake on July 5 caused economic losses estimated at $1 billion. Some residents slept in tents outside their homes for several days after the quakes fearing their homes might collapse.

A new earthquake early warning system for California called ShakeAlert was active during the quakes but the shaking wasn’t serious enough to notify Los Angeles residents 100 miles from the epicenter.

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The system is about 70% complete, Cal OES Earthquake officials told the AP. It should be complete by 2021 and has cost California $25 million to put sensors in the ground.

While the system doesn’t actually predict quakes, it detects when a quake is occurring, calculates its intensity and sends out alerts to give warnings to people further away from the epicenter. The warnings could occur several seconds to a minute in advance. That amount of time could help a surgeon stop surgery or give teachers time to tell students to take cover.

The alerts will eventually be delivered to cell phones like Amber Alerts. Pilot programs have been underway for years and the US Geological Survey said last fall that the system was ready for businesses, utilities and schools. 

However, the only mass notification for the Ridgecrest quakes was through a mobile app for the city of Los Angeles and functional only within LA County, according to the AP.

In fact, ShakeAlertLA did not send alerts for last week’s quakes because the level of shaking in LA was below a trigger threshold. LA was more than 100 miles from the epicenters.

The threshold is set to prevent annoying alerts for very small quakes. Every year there are about 10,000 earthquakes in California, including those that people can’t feel.

For engineers, the situation is a bit different. They might want to use earthquake sensors to shut down sensitive machinery when a quake hits, for example. A sensor might be used to help shut down power near gas line that could explode or to stop an unmanned transport vehicle. Automatic doors, vending machines, household appliances like fan heaters and power distribution boards might also benefit from having an earthquake sensor to shut them off.

To help in such situations, Rohm Semiconductor announced a small sensor detection module on June 25 that will become generally available in August. Pricing was not announced.

Rohm claims the new BP3901 sensor is superior to existing seismic sensors, either mechanical or electronic, because it determines the seismic level, prevents erroneous detection of earthquakes, and has very low standby current and internal data storage.

Accurately detecting vibration from an earthquake can lead to stopping equipment controlling oil or gas flows in order to prevent a secondary disaster like a fire after a quake, Rohm noted.

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The sensor uses a three-axis accelerometer in a compact 11.8 x 8.6 x 2.5 mm module.

Rohm created an original earthquake detection algorithm for greater accuracy. The algorithm is designed to correlate detected seismic activity with historical data for earthquake damage. The device measures spectrum intensity of a quake showing the acceleration of vibration over time and decides if the pattern matches a typical seismic waveform instead of vibration from an external factor like a passing car or train. An angle correction function can correct for unintentional tilt of the module.

The sensor can remain in operation for five years on two AA batteries when used on a device that could be exposed to 10 average earthquakes in a month.   

Also, the last 16 quakes can be stored in non-volatile memory on board the module. Analysis of such data at various locations could make it possible to predict future quakes, Rohm said.

Rohm said it can work with customers to help them use wireless networks such as Wi-SUN for remote control of smart meters that run in combination with its earthquake detection sensor.

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