1. 1.Earthquake Relocation
   

I have been working on earthquake location improvements by applying the source-specific station term (SSST) method, similar event cluster analysis and waveform cross-correlation. The most recent work on this is the development of a new location catalog (LSH catalog) for southern California. 

Figure 1. Map view of the relocated seismicity from 1981 to 2005 in the LSH catalog. Black dots show similar event clusters relocated using cross-correlation data. About 25% of events do not correlate and are plotted in color by year at their 3-D locations or 1-D locations. For more details, please refer to Lin et al. (2007a).

2. Seismic Velocity Tomography

I have been developing 3-D seismic velocity models for crust and uppermost mantle in California at different scales.

1. (a) Southern California
   

The new crustal seismic velocity model for southern California was derived from P and S arrival times from local earthquakes and explosions by applying a composite event method. For more details, please refer to Lin et al. (2007b).

      

Figure 2. Vp and Vp/Vs perturbations relative to the average value in each layer. The black contours circle the area that we are able to resolve with the resolution above 0.1.

2. (b) Entire State of California
   

Recently, we determined the seismic velocity model of the California crust and uppermost mantle using a regional-scale double-difference tomography algorithm. 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.   

Figure 3. Map views of the P-wave velocity model at 1 and 4 km depth slices. Pink dots represent relocated earthquakes. Black lines denote coast line and lakes, gray lines rivers and surface traces of mapped faults. The white contours enclose the areas where the derivative weight sum is greater than 50.

3. 3.Temporal Variations of Seismic Properties
   

Recently, we use relative arrival times and locations for similar earthquake pairs that are found using a cross-correlation method to analyze the time dependence of P and S station terms in southern California from 1984 to 2002 (Lin et al., 2008).

              

Figure 4. Time-varying station-term estimates for nine stations that have sharp timing offsets. Filled circles show station terms at the stations where the equipment changes are reported. No reported equipment changes correspond to the station offsets shown by open circles at the other three stations.

4. Subduction Earthquakes

We applied a modified joint hypocenter determination (JHD) method to relocate intermediate-depth events in order to revisit a recent global assessment of Double Seismic Zone (DSZ) layer separations. Our results suggest that antigorite dehydration alone is not able to account for the range of double seismic zone separations we find.

Figure 5. The 31 slab segments in our study with the identification codes next to them.

5. Volcano Seismology

We are applying a high-resolution Vp/Vs ratio estimation method described in Lin and Shearer (2007) using differential times from waveform cross-correlation to study spatial and temporal variations of near-source Vp/Vs ratios in different tectonic regions. The results will help to track movement of magma and volatiles within a volcano and to estimate spatial variations in fracturing and fluid content in active fault zones.