Prospects for Studying Short Time Scale Mantle Dynamics at Oceanic Transforms
Oceanic transform faults present a unique opportunity to utilize the new OBSIP seismometers to learn about several dynamical processes occurring in the uppermost oceanic mantle, including earthquake nucleation, aseismic fault slip, and uppermantle responses to coseismic stress changes. Many of these processes occur in significantly different ways on oceanic faults than on continental ones. Unlike continental regions, the seismogenic zone on oceanic transforms extends below the moho to depths approximately coincident with the 600 C isotherm which can reach ~20 km depth in the middle of large offset transforms. Thus, much of the seismogenic zone is within the uppermost mantle. The extent of the seismogenic zone into the uppermantle may be directly related to the unusual dynamical properties of oceanic transforms that require ocean bottom seismometers to be studied in detail.
Foreshock Sequences: Foreshocks in the 0-1000 second period before magnitude 5 and greater earthquakes are the rule rather than the exception on East Pacific Rise transforms.
Approximately 50% of the M>5 earthquakes (excluding aftershocks) on the
Quebrada/Discovery/Transform fault system (11 out of 23, including 6 of 9 CMT catalog events) between 1996 and 1999 were preceded by a foreshock in the one hour before and 20 km distance window around the mainshock. Almost all of the foreshocks occurred in the 1000 s before the mainshock, particularly the 100-500 s period. While the existence of these astoundingly abundant foreshock sequences can be documented with the current hydrophone arrays, determining their implications for the large earthquake nucleation process will require the increased location accuracy and lower detection thresholds that OBSs can provide.
Aseismic Fault Slip: Brune [1968] first recognized a difference between oceanic and continental transforms in the ratio of their observed seismic slip to that expected from plate-motion. His study indicated that either the seismogenic thickness of oceanic transforms is extremely small or that much of plate motion on oceanic faults is accommodated by aseismic slip. While the exact percentage of aseismic slip is still uncertain, teleseismic earthquake studies have suggested the
600°C isotherm as the lower boundary of the seismogenic zone on oceanic transforms [Engeln et al, 1986; Bergman and Solomon 1988, Abercrombie and Ekstrom 2001]. Assuming this isotherm for the base of the seismogenic zone leads to estimates of seismic deficits that vary from about 50 to 100% on the global transform system [Boettcher and Jordan 2001; Bird et al., 2002]. OBSIP instruments are necessary for defining the exact geometry of the seismogenic zone on oceanic transforms and hence pinning down the exact magnitude of the seismic slip deficit. They can also be used for constraining the material properties of the fault-zones, which may provide an explanation for the unusual abundance of aseismic slip. Continuous GPS arrays have recently revealed that much of the aseismic slip in subduction zones occurs episodically with characteristic time scales ranging from days to years. Given the abundance of aseismic slip on transforms it is quite likely that dynamical processes with similar time-scales occur on these faults and are reflected in the microseismicity.
Postseismic Deformation: It is likely that the oceanic uppermantle responds in a observable way to the stress changes imposed by transform fault earthquakes. While the seafloor geodetic instruments are not yet as routinely operational as OBSs, it appears that they will be able to undertake these types of studies in the near future. Oceanic transforms are a prime target for measuring the viscosity of the uppermantle using post-seismic transients due to their high rate of occurrence of moderate to large earthquakes. For instance, the Discovery transform averages ~5 magnitude 5.0 and larger events per year over a ~50 km long fault. Thus even with the short deployment times required by OBS battery limitations, it is possible to guarantee recording a large event within a relatively modest sized array.