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Saunders, JK, Haase JS.  2018.  Augmenting onshore GNSS displacements with offshore observations to improve slip characterization for Cascadia Subduction Zone earthquakes. Geophysical Research Letters. 45:6008-6017.   10.1029/2018gl078233   AbstractWebsite

For the Cascadia subduction zone, M-w similar to 8 megathrust earthquake hazard is of particular interest because uncertainties in the predicted tsunami size affect evacuation alerts. To reduce these uncertainties, we examine how augmenting the current Global Navigation Satellite Systems (GNSS) network in Cascadia with offshore stations improves static slip inversions for M-w similar to 8 megathrust earthquakes at different rupture depths. We test two offshore coseismic data types: vertical-only bottom pressure sensors and pressure sensors combined with GNSS-Acoustic aided horizontal positions. We find that amphibious networks best constrain slip for a shallow earthquake compared to onshore-only networks when offshore stations are located above the rupture. However, inversions using vertical-only offshore data underestimate shallow slip and tsunami impact. Including offshore horizontal observations improves slip estimates, particularly maximum slip. This suggests that while real-time GNSS-Acoustic sensors may have a long development timeline, they will have more impact for static inversion-based tsunami early warning systems than bottom pressure sensors. Plain Language Summary The Cascadia subduction zone is the region of highest tsunami hazard within the contiguous United States. This region has experienced many tsunamis over the last 10,000years that were generated by earthquakes of magnitude 8 to 9. Magnitude 8 earthquakes in the subduction zone can be tricky for tsunami early warning systems because it is difficult to determine the depth of the earthquake rupture, which strongly affects the anticipated tsunami size. This can make the difference between an evacuation order being issued or not. This study tests how estimating total slip on the earthquake fault during rupture and the resulting tsunami wave height for magnitude 8 earthquakes can be improved when combining the current land-based Global Navigation Satellite Systems monitoring network in the Pacific Northwest with offshore seafloor networks. We test hypothetical arrangements of offshore stations that measure the vertical seafloor motion with ocean bottom pressure sensors. We also test networks that measure motion in all three directions by including Global Navigation Satellite Systems measurements at the sea surface linked by acoustic communication to measurement points on the seafloor. This work can help plan where best to put new offshore instruments as they are developed for future tsunami early warning systems.

Melgar, D, Geng JH, Crowell BW, Haase JS, Bock Y, Hammond WC, Allen RM.  2015.  Seismogeodesy of the 2014 M(w)6.1 Napa earthquake, California: Rapid response and modeling of fast rupture on a dipping strike-slip fault. Journal of Geophysical Research-Solid Earth. 120:5013-5033.   10.1002/2015jb011921   AbstractWebsite

Real-time high-rate geodetic data have been shown to be useful for rapid earthquake response systems during medium to large events. The 2014 M(w)6.1 Napa, California earthquake is important because it provides an opportunity to study an event at the lower threshold of what can be detected with GPS. We show the results of GPS-only earthquake source products such as peak ground displacement magnitude scaling, centroid moment tensor (CMT) solution, and static slip inversion. We also highlight the retrospective real-time combination of GPS and strong motion data to produce seismogeodetic waveforms that have higher precision and longer period information than GPS-only or seismic-only measurements of ground motion. We show their utility for rapid kinematic slip inversion and conclude that it would have been possible, with current real-time infrastructure, to determine the basic features of the earthquake source. We supplement the analysis with strong motion data collected close to the source to obtain an improved postevent image of the source process. The model reveals unilateral fast propagation of slip to the north of the hypocenter with a delayed onset of shallow slip. The source model suggests that the multiple strands of observed surface rupture are controlled by the shallow soft sediments of Napa Valley and do not necessarily represent the intersection of the main faulting surface and the free surface. We conclude that the main dislocation plane is westward dipping and should intersect the surface to the east, either where the easternmost strand of surface rupture is observed or at the location where the West Napa fault has been mapped in the past.