During the 2004 field season we recorded drill noise vertical seismic profile data using both downhole and surface receiver arrays at the site of the San Andreas Fault Observatory at Depth. The site is located ~2 kilometers from the trace of the San Andreas Fault near Parkfield, California. At the end of the field season a ~M6 earthquake occurred on September 28. While processing the drill noise data recorded by the surface array we noticed unusual seismic events in records collected before the ~M6 event. An initial analysis of these seismic records began after noting that some events coincided with the times listed for earthquakes occurring along the San Andreas Fault. After noting times of specific events which were recorded by the surface array and also cataloged by the USGS earthquake monitoring network, time-frequency analysis was applied to some of the cataloged events. While inspecting the timefrequency displays it was noted that the surface array had also recorded many singular seismic events not cataloged by the network. In addition, it was discovered that a series of microseismic tremors were also recorded by the surface array. These tremors are associated with many singular microearthquake events as well as the larger earthquakes cataloged by the network. These acoustic emissions are briefly described and preliminary conclusions regarding microearthquake and tremor cycles are presented.
The San Andreas Fault Observatory at Depth (SAFOD) well is located southwest of the Middle Mountain structure (colored dominantly green in Figure 1). As part of the SAFOD project, the well was designed to penetrate, sample and monitor a section of the San Andreas Fault (SAF) that cyclically generates micro-earthquakes. During drilling of the well ~4 months of drill noise data was recorded for construction of a reverse vertical seismic profiles using both a downhole array and a surface array for recording. Seismic data from the site indicates that there is a complex system of faults present at SAFOD (Catchings et al, 2004; Hole et al, 2006; Taylor, 2006). It is within the vicinity of SAFOD that the San Andreas Fault changes from a locked portion of the SAF, moving only during great earthquakes, to the creeping portion which due to microearthquakes, moves continuously. Due to frequent earthquakes, both the downhole array in the pilot hole at SAFOD and the surface array recorded acoustic emissions from both microearthquakes and larger earthquakes.
Time-frequency analysis was applied to uncorrelated records of drill bit noise. Median filters were tested and one was selected to be applied to the output time-frequency data. The time-frequency plots are organized in the following manner. Panels display the variation of frequency content with time such that the vertical axis is frequency and one vertical column of pixels displays the amplitude values over the specified frequency range. The horizontal axis is time. Increment in time is 100 ms with the panel showing 65 seconds of data. Amplitudes are relative for the entire display with low amplitude indicated by dark blue and high amplitude by dark red.
