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30 May 2008 -- The great Sichuan earthquake caused the ground in Colorado to shake up and down by 1mm!

Six gPhone Gravity Meters Simultaneously Capture Sichuan, China Earthquake

2008 Sichuan earthquake
Date May 12, 2008
Magnitude 8.0 Ms / 7.9 Mw
Depth: 19 kilometres (12 mi)
Epicenter location: 31°01?16?N 103°22?01?E? (Sichuan earthquake)
(Wenchuan County in Sichuan province)
The 2008 Sichuan earthquake, which measured at 8.0 Ms and 8.3 Mw according to the China Seismological Bureau, and 7.9 Mw according to USGS, occurred at 14:28:01.42 CST (06:28:01.42 UTC) on 12 May 2008 in Sichuan province of China. The epicenter was 80 kilometres (50 mi) west-northwest of Chengdu, the capital of Sichuan, with a depth of 19 kilometres (12 mi). The earthquake was felt as far away as Beijing (1,500 km away) and Shanghai (1,700 km away), where office buildings swayed with the tremor. The earthquake was also felt in nearby countries.

The earthquake signal was detected on six different gPhone gravity meters that were under test at Micro-g LaCoste, Inc. (Lafayette, CO). Each gPhone recorded gravity data at 1 second intervals. The first graph shows 14 hours of data plotted at 1 second intervals starting at 00:00:00 GMT on 12 May 2008. There are actually 8 different lines plotted using different colors in the graph but the agreement between meters is so good that only one line is visible.

Gravity from Six gPhones

The next graph shows 5 minute time slices of the gravity data. There are actually 8 different lines plotted using different colors in each graph but in most cases the agreement is so good that only one line is visible. You may select different time slices by clicking on the right/left arrows. You can also enter the starting time of the time slice into the textbox.



The velocity was calcuated for each instrument by integrating the position. This signal is similar to what would be measured by a seismometer. Notice that the velocity more clearly shows another signal at around 750 minutes due to the surface waves traveling around the globe. There are actually 8 different lines plotted using different colors in the graph but the agreement between meters is so good that only one line is visible.
Velocity from 6 gPhones

The next graph shows 5 minute time slices of the gravity data. There are actually 8 different lines plotted using different colors in each graph but in most cases the agreement is so good that only one line is visible. You may select different time slices by clicking on the right/left arrows. You can also enter the starting time of the time slice into the textbox.



The vertical displacement of the ground was calcuated for each instrument by integrating the vertical velocity. This signal shows that the earthquake caused the ground in Colorado to move up and down about 1mm!. The displacement even more clearly shows a second signal caused by the surface waves traveling around the globe. There are actually 8 different lines plotted using different colors in the graph but the agreement between meters is so good that only one line is visible.
Displacement from Six gPhones

The next graph shows 5 minute time slices of the gravity data. There are actually 8 different lines plotted using different colors in each graph but in most cases the agreement is so good that only one line is visible. You may select different time slices by clicking on the right/left arrows. You can also enter the starting time of the time slice into the textbox.



Conclusions

The gPhones are capable of recording very precise vertical acceleration, velocity, and displacement during quiet periods as well as during periods of high seismic activity (eg. during earthquake recordings). The gPhones recorded a vertical displacement of about 1 micron at periods of 3-8s during normal quiet periods and up to 1mm at periods of around 20s during the earthquake recording from the great Sichuan earthquake. All gPhone had nearly identical readings throughout the entire recording.
One of our customers has also recorded an earthquake in the Kuril Islands north of Japan simultaneously with a gPhone, superconducting gravity meter, and and STS-2 seismometer. Excellent agreement was observed between the gPhone and superconducting gravity meter during quiet periods and excellent agreement was observed between the gPhone and STS-2 seismometer during the earthquake recording. More earthquake comparisons are given in Earthquake monitoring with gravity meters, Niebauer et. al