DOE OSTI2019
The location of an earthquake is a classic inverse problem. Arrival times of various seismic phases are combined with the location of the instrument to form the data vector. The inverse operator is applied in an iterative manor until the solution converges and the resultant solution is the hypocentral location of the earthquake in space and time. In this presentation, I will present how seismologists obtain local earthquake hypocenters. I will present how the generalized inverse operator is built using the simple distance = rate x time equation and how the inverse operator is used in an iterative manor to converge on the hypocenter of the earthquake. Once an earthquake location is determined, seismologists can then use the location to infer other information about the medium through which the waves traveled. One possible application of the earthquake locations is a branch of seismology known as tomography. In tomography, source and station locations are used to compute the travel times of seismic phases and these phases are then inverted to obtain a velocity model of the medium. Seismic tomography can be applied in both 2-D and 3-D settings and in this presentation, I will present examples from both. The first example comes from a technique known as ambient noise tomography. Rayleigh waves are obtained by cross correlating the background noise between two stations. Because we know the interstation distance between the two stations and the arrival times of the Rayleigh waves, we can use tomography to build a group velocity map of the study region. I will present results from a study of the Eastern Tennessee Seismic Zone using the EarthScope’s Transportable Array together with different seismic networks in the region, figure 1. The second example uses P and S-waves from 412 earthquakes which occurred during the 1994 Draney Peak swarm to calculate a 3-D velocity model of the area. Preliminary results show a strong velocity contrast at a depth of about 4 km where the majority of the earthquakes occur, figure 2.