Seismic Velocity Tomography
Seismic velocity models are necessary for determining accurate earthquake locations and focal mechanisms, developing ground motion models, and relating seismicity to geologic structures. In volcanic systems, tomographic images can be used to improve earthquake location and to resolve conduits and magma chambers.
Body Wave Tomography
(a) Southern California
We present a new crustal seismic velocity model for southern California derived from P and S arrival times from local earthquakes and explosions (Lin et al., 2007b). To constrain absolute event locations and shallow velocity structure, we also use times from controlled sources, including both refraction shots and quarries. Our new velocity model in general agrees with previous studies, resolving low velocity features at shallow depths in the basins and some high velocity features in the mid-crust.
(b) State of California
We determined the seismic velocity model of the California crust and uppermost mantle using a regional-scale double-difference tomography algorithm (Lin et al., 2010). Our model is the first 3D seismic velocity model for the entire state of California based on local and regional arrival time data that has ever been developed. It has improved areal coverage compared to the previous northern and southern California models, and extends to greater depth due to the inclusion of substantial data at large epicentral distances.
(c) Salton Trough
The Vp model in this study (Lin, 2013b) generally agrees with previous studies and is more consistent with the refraction data than the starting regional model. The most significant features in the Vp model are the velocity contrasts across the southernmost San Andreas Fault and the low velocity anomalies within the sea level contour at shallow depths, which are more consistent with the most recent active-source model by the Salton Seismic Imaging Project (SSIP) than the starting 3-D model.
We present a new three-dimensional seismic velocity model of the crustal and upper mantle structure for Mauna Loa and Kilauea volcanoes in Hawaii (Lin et al., 2014b). An anomalous body with low Vp, low Vs, and high Vp/Vs anomalies is observed at 8–11 km depth beneath the upper east rift zone of Kilauea volcano. We interpret this body to be a crustal magma reservoir beneath the volcanic pile, similar to those widely recognized beneath mid-ocean ridge volcanoes. Combined seismic velocity and petrophysical models suggest the presence of 10% melt in a cumulate magma mush (Lin et al., 2014a).
Ambient Noise Tomography
Traditional ambient noise tomography methods using regular grid nodes are often ill posed because the inversion grids do not always represent the distribution of ray paths. Large grid spacing is usually used to reduce the number of inversion parameters, which may not be able to solve for small-scale velocity structure. We present a new adaptive tomography method with irregular grids that provides a few advantages over the traditional methods (Li and Lin, 2014). First, irregular grids with different sizes and shapes can fit the ray distribution better and the traditionally ill-posed problem can become more stable owing to the different parameterizations. Second, the data in the area with dense ray sampling will be sufficiently utilized so that the model resolution can be greatly improved. Both synthetic and real data are used to test the newly developed tomography algorithm. In synthetic data tests, we compare the resolution and stability of the traditional and adaptive methods. The results show that adaptive tomography is more stable and performs better in improving the resolution in the area with dense ray sampling.
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