Earthquake locations are fundamental for studies of earthquake physics, fault orientations and Earth’s deformation. Accurate earthquake locations are also essential for characterizing magmatic pathways and identifying magma migration. Improving earthquake location accuracy has been an important and challenging goal in seismology for the past few decades. In our relocation studies, we improve both absolute and relative earthquake location accuracies by applying the three-dimensional ray tracing and differential time relocation methods using waveform cross-correlation data. These approaches have been applied to different tectonically and volcanically active areas, including California, Puerto Rico, and Hawaii.
(a) Southern California
We present high-precision earthquake locations for southern California from 1981 to 2005 computed using waveform cross-correlation with a new robust least squares method (Lin et al., 2007a). We use a new 3-D velocity model (Lin et al., 2007b) to improve absolute location accuracy and apply a new differential time relocation method that is very robust to outliers in the data. These results build on our earlier relocation work and provide additional insight regarding the fine-scale seismicity structure in southern California.
(b) Mammoth Mountain
The relocation (available here) is a by-product of the reinvestigation of the seismic activity during an 11-mo-long seismic swarm between 1989 and 1990 beneath Mammoth Mountain in eastern California (United States) (Lin, 2013a). This swarm, believed to be the result of a shallow intrusion of magma beneath Mammoth Mountain, was followed by the emission of magmatic CO2 gas, resulting in tree die-off in 1990 and posing a significant human health risk around Mammoth Mountain.
(c) Salton Trough
We present a new three-dimensional seismic velocity model and high-precision earthquake relocations between 1981 and 2010 near the Salton Trough and the San Jacinto Fault Zone (Lin, 2013b). The newly resolved model is used to relocate all the seismicity in the study area. We then apply the similar-event cluster analysis and differential-time relocation approach based on waveform cross-correlation data to the 3-D relocated events. A dramatic sharpening of seismicity patterns is obtained after using these methods. The new locations are more tightly clustered than the previous catalogs mainly due to the different absolute locations. The depth distribution along a basal surface may suggest the existence of a detachment fault from the north end of the Salton Trough to US-Mexico border.
(d) Puerto Rico
The island of Puerto Rico, sandwiched between the Caribbean and North American plates, experiences frequent earthquakes of small magnitudes, especially in the southwestern portion. Accurate velocity model and earthquake relocations are essential for investigating the tectonics and possible reasons that cause the excess seismic activity. We invert for minimum one-dimensional compressional-wave velocity models for the southwestern, the eastern and the entire island of Puerto Rico. With the velocity model for the island, we relocate the seismicity on the island between 1986 and 2009 using the source-specific station term location method. Our relocations show clustered seismicity around the main fault zones and depict sharper and clearer lineament in depth compared to the PRSN catalog locations.
We use a new 3-D velocity model and waveform cross-correlation data to relocate all the background seismicity (53,000 events with phase picks) between 1992 and 2009 on the Island of Hawaii.
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