Seismometer Orientation of iMUSH Array Using Rayleigh Wave Polarization

Abstract:

Imaging Magma Under St. Helens (iMUSH) is a collaborative project with the intention of obtaining more detailed information about the internal structure of Mount St. Helens, the most active volcano in the Cascade Arc. Combining data from a magnetotelluric survey, an active source experiment, a passive source experiment, and geochemical/petrological studies at their highest resolution will illuminate the magmatic structure beneath the volcano from surface to slab. The detail from this specific section of the arc will be used to answer broader questions about subduction zone magmatism. In June and July, 2014 we installed and serviced 70 broadband seismometers in a 50 km radius around the volcano. Having an accurate seismometer orientation is important for many applications including Ambient Noise Tomography and Receiver Functions. Since it is difficult to get an accurate orientation when installing the seismometers, the initial data were used to determine this orientation following Stachnik et. al. [Seismol. Res. Lett. 2012]. Theoretically, Rayleigh waves have an elliptical polarization and should only appear in the radial and vertical components. The horizontal components are rotated by steps through 360 degrees to find the angle that maximizes their correlation with the Hilbert transformed (90 degree phase shifted) vertical component. The difference between this angle and the back azimuth is an estimate of the sensor orientation. This is applied to earthquakes with magnitudes greater than 6 and focal depths less than 50 km. For the limited data from the first service run we found orientations to be generally within 5 degrees of expected orientations. To increase statistical significance, this will be done again in October, 2014 when more data are available.