85.7 – The HYPOELLIPSE Earthquake Location Program

This chapter focuses on the HYPOELLIPSE Earthquake Location Program. The earthquake location program HYPOELLIPSE, first published in 1979 and subsequently updated with added features, was initially developed to locate crustal and subcrustal earthquakes of southern Alaska using arrival times recorded by a sparse regional seismograph network. This is a more difficult problem than locating shallow earthquakes within a dense network, which is the problem generally faced within California, so HYPOELLIPSE includes many more user-adjustable parameters than does HYPO71, the earthquake location program developed for California that is the ancestor of HYPOELLIPSE. Travel times may be computed from velocity models or travel-time tables. The program HYPOTABLE, which is included with this distribution, can create spherical-earth travel-time tables for use with HYPOELLIPSE. This allows the program to be used with stations beyond the distance at which significant travel-time errors would be introduced by a fiat-layer travel-time model. A relatively new feature of HYPOELLIPSE is the ability to work in areas with large topographic relief. Previous versions assumed that all of the stations were located at the same elevation and that small elevation differences could be accounted for by station elevation corrections. HYPOELLIPSE currently allows stations to be “embedded” within the velocity model, and will correctly compute travel time and take-off angles even for stations that are at a lower elevation than the earthquake.