Publications

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2015
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.

2013
Crowell, BW, Melgar D, Bock Y, Haase JS, Geng JH.  2013.  Earthquake magnitude scaling using seismogeodetic data. Geophysical Research Letters. 40:6089-6094.   10.1002/2013gl058391   AbstractWebsite

The combination of GPS and strong-motion data to estimate seismogeodetic waveforms creates a data set that is sensitive to the entire spectrum of ground displacement and the full extent of coseismic slip. In this study we derive earthquake magnitude scaling relationships using seismogeodetic observations of either P wave amplitude or peak ground displacements from five earthquakes in Japan and California ranging in magnitude from 5.3 to 9.0. The addition of the low-frequency component allows rapid distinction of earthquake size for large magnitude events with high precision, unlike accelerometer data that saturate for earthquakes greater than M 7 to 8, and is available well before the coseismic displacements are emplaced. These results, though based on a limited seismogeodetic data set, support earlier studies that propose it may be possible to estimate the final magnitude of an earthquake well before the rupture is complete.

1997
Hauksson, E, Haase JS.  1997.  Three-dimensional V-P and V-P/V-S velocity models of the Los Angeles basin and central Transverse Ranges, California. Journal of Geophysical Research-Solid Earth. 102:5423-5453. AbstractWebsite

We use P and S arrival times from 5225 earthquakes and 53 explosions, recorded by the Southern California Seismographic Network, to invert for the three-dimensional P- velocity (V-p) and the P and S-velocity ratio (V-p/V-S) in the central Transverse Ranges and the Los Angeles basin. To model long-wavelength features of the velocity structure, we invert for the models by interpolating models determined successively from a sparse, medium, to a dense grid, with 40, 20, and 10 km spacing of horizontal grid nodes. Layers of grid nodes are placed at depths of 1, 4, 6, 8, 12, 16, and 20 km. The data variance decreased about 80% in the gradational inversion. Ample data from the 1994 Northridge and other earthquake sequences, the rich background seismicity, and the dense station distribution along with controlled sources made the model well resolved, except along the edges, to the southwest in the offshore region, and at depths greater than 20 km. The Vp model images the shape of the Los Angeles and east Ventura basins down to depths of 8 and 12 km. Three low-velocity areas at 1 km depth in the Los Angeles basin that coincide with recent sediment depositional areas are also imaged. The north edge of the Peninsular Ranges, the Santa Monica, and the San Gabriel Mountains, form discontinuous high-velocity ridges extending to depths df 20 km. The high V-P/V-S ratios in the near surface are consistent with high pore fluid pressures in the basin sediments. At depth beneath the east Ventura basin the high V-p/V-S and high. Vp suggest the presence of Ophiolitic assemblages or mid-Miocene volcanics. In contrast, a body with normal Vp and low V-P/V-S that is identified in the upper crust beneath Santa Monica Bay may be a fragment of quartz-rich continental crust. The Vp model of both the Ventura and the Los Angeles basins shows features that suggest deformation of the hanging wall or basin closure. At seismogenic depths of 16 km the hypocenters of moderate-sized and large earthquakes are located within or adjacent to high- Vp-velocity bodies. In most cases these high-velocity bodies form the upper block, consistent with shortening of the lower crust as described in thick-skinned tectonic interpretations.