Seismic Hazards Investigations in Puget Sound (SHIPS): I - fig. 2
Seismic Hazards Investigations in Puget Sound (SHIPS): I - fig. 2
Credit:
Tom Brocher • U.S. Geological Survey, Seattle; The SHIPS Working Group/IRIS Consortium
Description
In 1998, 1999, 2000, and 2002, the U.S. Geological Survey, the Geological Survey of Canada, and academic collaborators conducted a series of seismic reflection and refraction studies of crustal and upper mantle structure, as well as site response studies in western Washington and southwestern British Columbia. These experiments have been conducted under the umbrella of Seismic Hazards Investigations in Puget Sound (SHIPS), and each has involved the use of a large (generally between 200 and 670) number of IRIS/PASSCAL short period recording systems and sensors. A variety of controlled sources, including airguns, explosions, and even the implosion of the Kingdome, were used in these studies and a number of local earthquakes and teleseisms, including the 1999 Chi-Chi earthquake, were recorded.
SHIPS provided an exceptionally detailed view of the forearc of the Cascadia subduction zone. Perhaps the most important new result is the mapping in three dimensions of a low velocity zone at the base of the crust (Figure 1) that is electrically conductive, seismically reflective, and aseismic, and which appears to connect updip with the Olympic Core Complex of accretionary sedimentary rocks (Ramachandran et al., 2005). This layer lies at the same depth as preliminary determinations of the depths of non-volcanic tremor events that occur during the Episodic Tremor and Slip (ETS) events every 14 months or so. The strong indication is that these ETS events are occurring within the partially subducted accretionary sediments beneath the forearc.
Another important result of SHIPS is the mapping of a large region of hydrated forearc upper mantle along the Cascadia margin (Brocher et al., 2003). Metamorphic reactions in subducting slabs release water to the overlying forearc upper mantle: this water serpentinizes the upper mantle and reduces the P- and S-wave velocities of the upper mantle. We have mapped locations along the Cascadia margin where PmP arrivals are either very faint or entirely absent. We find that these regions correlate closely with regions of the upper mantle that appear to be strongly magnetic (Figure 2). Our interpretation is that when the mantle is serpentinized, magnetite is formed: thermal calculations indicate that this magnetic wedge of serpentinite lies below the Curie temperature. Thus, low-frequency magnetic anomalies in subduction zones appear to define regions where the forearc upper mantle has been serpentinized as a result of metamorphic reactions in the subducting slab (Blakely et al., 2005).
Blakely, R.J., T.M. Brocher, and R.E. Wells, 2005, Subduction zone magnetic anomalies and implications for hydrated forearc mantle, Geology, in press.
Brocher, T.M., T. Parsons, A.M. Tréhu, C.M. Snelson, and M.A. Fisher, 2003, Seismic evidence for widespread serpentinized forearc upper mantle along the Cascadia margin, Geology, 31, 3, 267-270.
Ramachandran, K., R.D. Hyndman, and T.M. Brocher, 2005a, Northern Cascadia subduction zone: Tomographic 3D P-wave velocity structure, Geophys. J. Int., submitted.
Photographer / Contributor: Tom Brocher • U.S. Geological Survey, Seattle; The SHIPS Working Group
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