Pacific Northwest Geodetic Array
Central Washington University

Slow Earthquakes in the Pacific Northwest


Tiltmeter Installation

We completed installation of the first biaixal tiltmeter near Olympia at the Shelton airport. Since it is difficult to excavate trenches deeper than 2.5 m, the long baseline tiltmeter must follow topogrpahic contours. Ideally, the ends of the tiltmeter will be in a region that is well drained although we are constructing sensor systems that can operate submerged for extended periods.

Shelton airport employees
Shelton airport employees have provided much assistance and support.

The tiltmeter consists of two arms approximaely at right angles, in an L-shape plan view. The sensors are placed ~2 m underground and operate in three vaults accessed via 2' diameter concrete manhole lids. A 10' pole near each vault supports a radio modem through which data are transmitted to a data-logging computer operated in a shed, or in an electrical enclosure, up to a mile from the tiltmeter.

Tiltmeter Installation
Pipe in the ground
Tiltmeter Installation
Laser leveling in the trench
Helicapiles and covers
Culverts for manhole
covers and helical piles
for stable monumnents.
SE vault
The southeast corner vault will house two separate reservoirs and floats.



Underground view of tiltmeter

The tiltmeter consists of two arms approximaely at right angles, in an L-shape plan view. The sensors are placed ~2 m underground and operate in three vaults accessed via 2' diameter concrete manhole lids. A 10' pole near each vault supports a radio modem through which data are transmitted to a data-logging computer operated in a shed, or in an electrical enclosure, up to a mile from the tiltmeter.

Data stored in the computer are accessed via cell-phone through the internet. Power to the tiltmeter comes through an AC hookup (120V at 0.2 Amp is equivalent to a 25W light bulb).

Data are transmitted to a nearby computer via radio links every few seconds; the computer forwards the data to analysis centers in Ellensburg, Washington and Boulder, Colorado. Five of these new tiltmeters will be installed initially in the Puget sound region monitoring earthquake activity tens of miles underground. Data from the tiltmeters will help understand how increments in stress eventually lead to large earthquakes.