Bilham's research includes: Application of space geodesy and strain and tilt instrumentation to monitor deformation of the Earth at plate boundaries, mostly in southern Asia and western North America; Design and operation of new geophysical instruments to monitor strain and tilt in the Earth; Archival research into historical earthquakes; Statistics of urban earthquakes and global seismic hazards.
CIRES prepares to monitor slip in the forthcoming Bay Area earthquake
Earthquakes are the result of the rapid release of accumulated strain on a fault. In a few seconds, strain energy that has been developing for hundreds or thousands of years can manifest as meters of violent fault rupture. A special class of creeping faults, however, slip slowly, releasing strain before it amounts to dangerous levels. Few such faults slip entirely by creep, but numerous vertical faults in California do both, releasing strain in the form of viscous creep in the uppermost 5 kilometers of a fault and in the form of seismic rupture at deeper depths.
The Hayward Fault in San Francisco’s East Bay area is one such fault. It last slipped in 1868, and experiences major earthquakes (6.5-7.1 moment-magnitude) every 161 ± 65 years, yet the surface fault slips steadily at 3 to 9 millimeters per year. Much of the surface fault has slipped 1 meter since 1868. For the past two decades, CIRES, with funding from the United States Geological Survey (USGS), has been monitoring surface creep at five locations on the Hayward Fault. Steady slip in Fremont at 6.9 millimeters per year occurs at rates of less than 1 micrometer per hour in the uppermost hundred meters of the fault, interrupted at several-month intervals by approximately 1.5-millimeter amplitude creep-events, each lasting a few hours, signifying slip in the uppermost 3 to 5 kilometers of the fault. Fluctuations in rate in the past two decades have been caused by local earthquakes (Lienkaemper et al, 2013).
Realizing that a magnitude 7 earthquake will soon occur on the fault, we have devised a new sensor designed to capture both slow slip and catastrophic rupture. A tensioned flexible wire fastened obliquely across the fault is wrapped around a 30-centimer circumference wheel. Slip on the fault pulls the cable and rotates the wheel whose angular position is monitored by a Hall effect sensor. The sensor has a sub-millimeter resolution per turn, and 10 turns realizes a range of up to 3 meters—more than adequate to capture the 1 to 2 meters of slip anticipated on the Hayward Fault during the forthcoming earthquake. Cumulative slip is telemetered