Observations of Rayleigh-Wave Phase Velocity and Coseismic Deformation Using an Optical Fiber, Interferometric Vertical Strainmeter at the SAFOD Borehole, California by John Blum, Heiner Igel, and Mark Zumberge Abstract We present observations from a vertical, optical fiber interferometric strainmeter in the San Andreas Fault Observatory at Depth borehole near Parkfield, California. The sensor detects both teleseismic earthquakes and local events, along with coseismic strain steps consistent with theoretical dislocation models. For tele- seismic events, we investigate the possibility of determining local Rayleigh-wave phase velocities beneath the borehole by comparing the ratio of vertical ground accel- eration from a nearby seismometer to vertical strain. While similar studies have used horizontal components and rotations, this is the first such attempt utilizing vertical measurements. We show that at periods from around 16–40 seconds, we can recover general dispersion characteristics that are within a few percent of models of realistic local structure. Introduction The San Andreas Fault Observatory at Depth (SAFOD) project (Fig. 1) began in 2002 as part of the Earthscope in- itiative (see Data and Resources) to investigate the physical and chemical processes and properties responsible for earth- quake generation and crustal deformation near Parkfield, California (Hickman et al., 2004). The project was designed to study recurring ≈M 6 events (in 1881, 1901, 1922, 1934, 1966; Bakun and McEvilly, 1984) and the overdue event of 28 September 2004 (Langbein et al., 2005), as well as recur- rent microearthquakes for which the hypocenters are mostly distributed in a sparse number of clusters (Nadeau et al., 1994, 1995). The SAFOD borehole was designed to actively penetrate the fault trace near the depth and location of the microearthquakes. As part of this experiment, we developed an optical fiber, interferometric strainmeter to be installed outside the well casing of the upper 1-km vertical portion of the borehole (Blum et al., 2008). Our goals are to observe strain before, during, and after earthquakes in the region to explore the behavior driving and accompanying the earthquake cycle. While the long-awaited 28 September 2004 event occurred shortly prior to our installation, we are able to observe ground motions from local microearthquakes and teleseismic events. In this study, we describe the functionality and ben- efits of our sensor, present results of local deformation, and investigate the possibility of determining local Rayleigh- wave phase velocities by comparing colocated measurements of vertical strain and acceleration. Description of the Instrument Design Optical fiber strain sensors have been developed to mea- sure a variety of geophysical process, including the flow of glacial ice (Zumberge et al., 2002), seafloor deformation (Zumberge, 1997; Blum et al., 2008), and atmospheric pres- sure signals (Zumberge et al., 2003). Blum et al. (2008) describe two ways to interrogate optical fibers: with an elec- tronic distance meter or utilizing interferometry. Optical fiber interferometers can measure picometer displacements of the fiber; Zumberge and Wyatt (1998) have used this tech- nique to monitor displacements of end monuments of longer baseline strainmeters and tiltmeters in shallow boreholes. Interferometric methods have an advantage in that they can be sampled at intervals and dynamic ranges equivalent to con- ventional seismometers. (Using a similar interferometric tech- nique, an optical fiber seismometer can rival the performance of observatory class seismometers [Otero, 2009].) A borehole sensor of this type can therefore be treated similar to a broad- band seismometer at short periods but also can capture long period strain. Installation details of our optical fiber, interferometric strainmeter are in Blum et al. (2008). The instrument consists of a single-mode optical fiber stretched vertically along the outside of several hundred meters of borehole casing cemented into the Earth. The fiber, slightly elongated during 1879 Bulletin of the Seismological Society of America, Vol. 100, No. 5A, pp. 1879–1891, October 2010, doi: 10.1785/0120090333