Your interest in keeping abreast of volcanic and seismic events and in being informed of geologic hazards and ways to mitigate those hazards has sustained our efforts in producing this column. As 1995 draws to a close, we can look back and reflect on the busy year that HVO has experienced. They are seismology, geodesy, gas geochemistry, geophysics, and geology. Of these, the seismic program at HVO is the primary method used to monitor the volcanoes of Hawai'i. It is important because earthquakes occur as magma enters a volcano, and the detection of these tremors provide an early warning of possible activity.
Volcanic earthquakes are usually low in magnitude but can be felt because of their shallow depth.
Earthquakes in Hawai'i are also caused by the weight of the islands, commonly called tectonic adjustment, and are only indirectly related to volcanic activity. Seismologists doing analyses of earthquake locations and waveforms can usually differentiate between volcanic and tectonic origins of an earthquake. The two largest earthquakes this year were of tectonic origin. Last year 67,145 earthquakes were detected, and 36 were felt. The largest earthquakes in 1995 occurred on March 19 and on May 11.