Hayward Fault & Bridges Research

T. V. McEvilly, R. Uhrhammer, R. W. Clymer, R. Nadeau, W.Johnson, L. Hutchings, P. Hipley

Overview

Using advanced borehole-based technology it is possible to monitor Hayward fault seismicity with far greater precision than we can accomplish with the existing surface-based networks whose data effectively mask meters-scale details of the fault zone geometry because of lack of detection at low magnitudes, lack of signal bandwidth, and the generally low seismicity rate on the northern Hayward fault (NHF). The evolving borehole network is a joint UCB/USGS/Caltrans project designed to provide the seismological community with state-of-art high- frequency and wide dynamic range data on the Hayward fault zone. In addition, it provides needed bedrock ground motion recordings at the toll bridges on San Francisco Bay. The low- noise borehole network is allowing the threshold of detection for microearthquakes on the fault zone to be reduced to a magnitude range near -1.0 on the NHF - almost two orders of magnitude better than that attained with NCSN data alone. The resulting large increase in visible seismicity on the northern Hayward fault zone (roughly tenfold, an event every three days or so) is providing enhanced detail in the patterns of earthquakes in space and time. Recurrence times of many months, rather than many years, are detectable so that slip-rate changes or clustering anomalies on the fault can be monitored on much finer space and time scales than with the surface data alone and the M>1 limit of NCSN in the East Bay. At low detection levels and location resolutions attained at Parkfield using borehole networks, the image of the fault zone heterogeneity and microearthquake process develops a sharpness that reveals systematics not visible with surface observations alone. It has been suggested that the base of seismicity along the Hayward defines a major detachment surface that accommodates the plate boundary motion (Jones et al., 1994, Burgmann, 1997). Precise definition of the nature of events at the bottom of the Hayward fault seismogenic zone will provide new evidence on this hypothesis, and an improved 3-D velocity structure will help understand the nature of the mid-crustal reflector observed in the region (Brocher et al., 1994). Finally, the complete detection of repeating sequences at the very small magnitudes offers the opportunity for slip-rate estimation on the fault surface at depth (Nadeau and McEvilly, 1999).

Hayward Fault Events Archive

We have begun to go back and build a NHF-specific data archive from the existing waveform data that have been collected by the heterogeneous set of recording systems in operation along the Hayward fault. Working with NHFN, SHFN, NCSN, and BDSN waveforms, both continuous and triggered data sets, we have undertaken a massive association of event and trigger times for the test years of 1997 and 1998. The process will reduce more than 300,000 individual time segments to real events along the Hayward fault during the period. This approach then will be run backwards and forwards in time to finally collect all the small events along the HF since the network began (originally with only RefTek triggered recorders). As the remaining few event recorders are replaced with the new Quanterra hardware with telemetry, and the central triggering is fully implemented, this archive will grow without such special handling.

Repeating Events and Slip Rate Estimation

One of the most important contributions to our understanding of the processes underway on the fault has come from the unique capability of the high-resolution borehole data in defining details of hypocenter clustering and geometric characteristics at a scale of meters (Nadeau and McEvilly, 1997, 1999). This is not possible using surface-based stations, even though such networks (NCSN) can certainly be used to find similar event sequences at some detection level and within the frequency range available. The potential value of slip-rate estimation from repeating sequences is illustrated for NHF in Figure 12.1.

  

Figure 12.1: Surface (solid circles) and subsurface (open squares) creep rates along the northern 50 km of the Hayward fault. Surface rates are averages over the last several decades ( Lienkaemper et al., 1997) and subsurface rates are computed from the individual repeating-earthquake sequences for the NHF. Solid squares are spatial average creep rates for the three clusters of sequences (i.e. El Cerrito, Berkeley and San Leandro resp.).

Burgmann et al. (2000) combined slip rates at depth from repeating sequences evident from NCSN data with surface deformation observations in concluding that the NHF likely slips freely throughout its entire seismogenic depth. The borehole data offer the chance to increase by an order of magnitude or more the density of such slip rate estimates over the slipping fault surface. Equally promising is the ability to observe the mapped slip rate varying in space and time as the fault responds to changes in loading or fault-zone properties. At Parkfield, the NCSN can detect few of the smaller magnitude sequence members (those with the shorter recurrence times needed for high-resolution fault slip monitoring), and it cannot separate adjacent sequences in clusters that collectively do not exhibit the characteristic regularity seen only when such clusters can be broken into individual repeating sequences. Figure 12.2 is an example of a Hayward fault sequence of similar events readily captured from NCSN waveforms that we fully expect with borehole-observed waveforms to further separate into individual highly similar (correlation coefficients > 0.95 in a 100 Hz bandwidth) repeating sequences.

  

Figure 12.2: Suite of similar events ( 1.0 < M < 1.5) on the Hayward fault available in NCSN catalog. High-resolution borehole data will separate such clusters into individual sequences.

Ground Motion at the Bay Bridges

The borehole instruments beneath the bridges will provide multiple use data that is important to geotechnical, structural engineering, and seismological studies. The holes, as deep as 300 m, were drilled by Caltrans. There are twenty-one sensor packages at fifteen sites, capable of recording a micro-g from a local M =1.0 earthquake or 0.5 g strong ground motion from large Bay Area earthquakes. Hutchings et al. (2000) list earthquakes and stations where recordings were obtained during the period December 1998 to December 1999, along with preliminary results on phasing across the Bay Bridge and wave amplification at Yerba Buena Island.

Hutchings et al. (1999) provide a computation of linear strong ground motion along the San Francisco/Oakland Bay bridges (western and eastern spans) at the base of pier supports on rock or at the basement, rock/soil interface at other pier locations. They synthesized a M=7.25 Hayward fault earthquake to use as input into soils models of the Bay sediments, or directly as input into non-linear finite element modeling of the bridge for sites with no sedimentary cover. The proximity of the bridges to the Hayward fault requires full wavetrain ground motion modeling that includes frequencies from D.C. to 25 Hz at several points along the bridges and accounts for the effects of finite rupture and directivity, fling, wave passage, and the loss of motion spatial coherency at high frequencies. This is achieved by providing a numerical solution of finite rupture along the faults, using a three-dimensional finite element method for frequencies below 0.2 Hz, and empirical Green's functions for frequencies from 0.2 to 25.0 Hz. The ground motion was computed at seven points along the structure. The empirical Green's functions, obtained from actual recordings at the borehole sites along the structure, explicitly account for high frequency incoherency due to variations in the geology.

References

Brocher, T. M., J. McCarthy, P. E. Hart, W. S. Holbrook, K. P. Furlong, T. V. McEvilly, J. A. Hole and BASIX Working Group, Seismic evidence for a possible lowercrustal detachment beneath San Francisco Bay, Science, 265, 1436-1439, 1994.

Burgmann, R., Active detachment faulting in the San Francisco Bay area?, Geology , 25, 1135-1138, 1997.

Burgmann, R., D. Schmidt, R.M. Nadeau, M. d'Alessio, E. Fielding, D. Manaker, T. V. McEvilly, and M.H. Murray, Earthquake Potential along the Northern Hayward Fault, California, Science, 289, 1178-1182, 2000.

Hutchings, Lawrence, William Foxall, Shawn Larsen, and Paul Kasameyer, Synthetic Strong Ground Motion at the Oakland/San Francisco Bay Bridge from a M=7.25 Earthquake on the Hayward Fault, Lawrence Livermore National Laboratory, UCRL-ID-183645, 1999.

Hutchings, L., P. Kasameyer, C. Turpin, L. Long, W. Foxall, J. Hollfelder, T. McEvilly, R. Clymer, and R. Uhrhammer, Deep Borehole Instrumentation Along San Francisco Bay Bridges - 2000,. Lawrence Livermore National Laboratory, UCRL 132137-00, 2000.

Jones, D.L., R. Graymer, C. Wang, T.V. McEvilly and A. Lomax, Neogene transpressive evolution of the California coast ranges, Tectonics, 13, 561-574, 1994.

Lienkaemper, J.J., J.S. Galehouse and R. W. Simpson, Creep response of the Hayward fault to stress changes caused by the Loma Prieta earthquake, Science 276, 2014-2016, 1997.

Nadeau, R. M. and T. V. McEvilly, Seismological Studies at Parkfield V: Characteristic microearthquake sequences as fault-zone drilling targets, Bull. Seis m. Soc. Am., 87, 1463-1472, 1997.

Nadeau, R.M. and T.V. McEvilly , Fault slip rates at depth from recurrence inter vals of repeating microearthquakes, Science, 285, 718-721, 1999.

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