Complementary to the regional broadband network, the Hayward Fault Network (HFN) (Figure 3.10 and Table 3.5) is a deployment of borehole-installed, wide-dynamic range seismographic stations along the Hayward Fault and throughout the San Francisco Bay toll bridges network. Development of the HFN initiated through a cooperative effort between the BSL and the USGS, with support from the USGS, Caltrans, EPRI, the University of California Campus/Laboratory Collaboration (CLC) program, LLNL, and LBNL. The project's objectives included both an initial characterization period followed by longer-term monitoring effort using a backbone of stations from among the initial characterization set. Subsequent funding from Caltrans, however, has allowed for continued expansion of the backbone station set for additional coverage in critical locations.

The HFN consists of two components. The Northern Hayward Fault Network (NHFN) is operated by the BSL and currently consists of 28 stations with various operational status. These include stations located on Bay Area bridges and now at borehole sites of the Mini-PBO (MPBO) project, which were installed with support from NSF and the member institutions of the MPBO project. The NHFN is considered part of the BDSN and uses the network code BK. The Southern Hayward Fault Network (SHFN) is operated by the USGS and currently consists of 5 stations. This network is considered part of the NCSN and uses the network code NC. The purpose of the HFN is threefold: 1) to increase substantially the sensitivity of seismic data to low amplitude seismic signals, 2) to increase the recorded bandwidth for seismic events along the Hayward fault, and 3) to obtain bedrock ground motion signals at the bridges from more frequent smaller earthquakes.

Data with these attributes contribute significantly to a variety of scientific objectives including: a) the investigation of bridge responses to stronger ground motions from real earthquakes, b) obtaining a significantly lower detection threshold for microearthquakes and nonvolcanic tremor signals, c) increasing the resolution of the fault-zone seismic structure (e.g., in the vicinity of the Rodgers Creek/Hayward Fault step over), d) improving monitoring of spatial and temporal evolution of seismicity (to magnitudes approaching $M \sim >
-1.0$) that may signal behavior indicative of the nucleation of large damaging earthquakes, e) the investigation of earthquake scaling, physics and related fault processes, f) improving working models for the Hayward fault, and g) using these models to make source-specific response calculations for estimating strong ground shaking throughout the Bay Area.

This chapter is primarily focused on the NHFN and activities associated with the BSL operations.

Figure 3.10: Map showing the locations of the HFN stations operated by the BSL (NHFN - squares and diamonds) and the USGS (SHFN - circles). Operational sites are filled blue/black, while sites in progress are yellow/grey. Other instrumented but currently non-operational boreholes are indicated as open symbols.
\epsfig{file=nhfn07_map.eps, width=15cm}\end{center}\end{figure*}

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