Abstract

Recording earthquakes with borehole seismometers has become increasingly popular in recent years as the advantages in noise reduction and also the distorting effects of near-surface rocks, especially sediments, have become well known. Borehole recording can be extremely complex, involving active sensors, special cables, and downhole electronics. Such installations, however, are often not very reliable at the high temperatures reached in active tectonic areas at depths of 1 km and greater. Here we describe a simple and reliable system for 3-component recording of local earthquakes at single and multiple depths greater than 1.5 km and temperatures up to 120°C. Our system was designed and developed for experiments in the Cajon Pass Scientific Drillhole of Southern California. The borehole packages are made of titanium with spring-loaded clamping, allowing easy retrieval. Standard seven-conductor oil-well logging cables are used together with a specially designed cablehead. The data recorded have been used for investigating earthquake source scaling, attenuation in the mid-crust, and also near-surface site effects.

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