Measuring slow slip offshore

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Science  06 May 2016:
Vol. 352, Issue 6286, pp. 654-655
DOI: 10.1126/science.aaf6534

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Tsunami earthquakes are rare events and are distinguished by an anomalously shallow source depth and slow rupture propagation speed (1, 2). They can be particularly dangerous because they generate less high-frequency seismic energy than a normal earthquake; consequently, they are not preceded by the strong shaking that would warn coastal residents that a locally generated tsunami is on the way. A better understanding of the range of slip behaviors in the offshore portion of subduction zones is needed to constrain models and improve earthquake hazard forecasts. On page 701 of this issue, Wallace et al. (3) present the first offshore geodetic data from a slow-slip event on the Hikurangi subduction zone offshore from New Zealand (see the figure). This segment of the Hikurangi subduction zone is unusual in that it experienced two “tsunami” earthquakes in 1947. Continuous global positioning system (GPS) instrumentation onshore indicates that the southern part of the subduction zone is locked above a plate depth of ~40 km, with slow-slip events occurring at greater depth, similar to what is observed in many other subduction zones (4, 5). In contrast, the northern part of the Hikurangi subduction zone appears to accommodate plate motion primarily through geodetically detected slow-slip events at shallow depths. The up-dip extent of slow slip, however, cannot be determined from onshore data only, and offshore locking models depend strongly on model assumptions.