Hawaiian travel times*

Abstract

Summary In Part I a special study has been made on seismogram phases, showing how to locate Hawaiian local earthquakes without the necessity of accurate time checks on all of the master clocks of the several seismograph stations on Hawaii. This was done by searching old records of the Hawaiian Volcano Research Association network for dates and times when an earthquake occurred very close to one station and was well recorded on the seismographs 37, 70, or 89 kilometers away. Records gave fourteen earthquakes of this kind, of which ten were used. These records were studied and the interval in seconds between the arrivals of the distortional (S) and compressional (P) waves was plotted for the distance of the station away from the origin of the disturbance. On each record there appeared more than one compressional wave and more than one distortional wave, giving rise to various differences, as Sn — Pn, S* — P*, and Sg — Pg. Simple linear relations were found between time-interval and distance for special earthquakes. These linear relations were to be used in locating other earthquakes away from all stations by the geometric method of Isikawa. In Part II a study has been made on travel times; that is, the speeds with which different seismic waves travel through rock constituting the Hawaiian Islands, between the islands of Hawaii and Oahu. This study is based on Part I, for practically no earthquakes have been recorded near any one seismograph station on Hawaii that were large enough to record clearly on Oahu. Oahu is about 300 kilometers away from the seismic origins considered. This study came within five per cent of verifying the speeds of seismic waves through the floor of the Pacific Ocean as found by others. These velocities may prove useful in determining the physical constants of the rocks and indicate what makes up the basement of the Hawaiian Islands. Other waves were found which may indicate a complex structure of four to seven discontinuities in the Hawaiian ridge, it being distinct from the floor of the Pacific and analogous to the structure of continental areas. A travel-time, distance graph has been included which should aid in locating earthquakes in the Hawaiian Islands. These wave velocities, for further corroboration, should lead to geophysical experiments.