RECENT GROUND MOVEMENT AND SEISMIC ACTIVITY IN CAMPI FLEGREI, SOUTHERN ITALY - EPISODIC GROWTH OF A RESURGENT DOME

Leveling surveys in Campi Flegrei, an active volcanic caldera in southern Italy, show that this caldera has undergone both subsidence and uplift of a few meters during the past 85 years. A single source located at a depth of about 3 km beneath the center of the caldera can account for almost all elevation changes recorded in Campi Flegrei since the first leveling survey conducted in 1905/1907. This includes leveling surveys that span periods of subsidence (such as between 1905/1907 and 1919 and between January 1985 and October 1986) and periods of rapid uplift (between March 1970 and October 1971 and between January 1981 and December 1984). A possible geometry for the source of uplift and subsidence beneath Campi Flegrei is a rectangular horizontal sheet located at a depth of about 3 km and centered a few hundred meters east of the city of Pozzuoli. The horizontal dimensions of this sheet are about 1.5 km by 3.0 km; the long axis is oriented N70-degrees-W. This sheet lies adjacent to the most intense area of seismicity that occurred from mid-1983 to December 1984. During an uplift episode this sheet may represent the top of a magma body undergoing internal pressure changes, possibly by intrusion of additional magma. Subsidence is caused by groundwater removal from the water-saturated deposits that fill this caldera to a depth of a few kilometers. A temporary increase in subsidence rate after an uplift episode is caused by increased rate of groundwater removal from newly fractured regions that lie immediately above the magma body. The close proximity of this water-saturated region and the magma body makes it impossible to separate the depth to these two sources from the results of leveling surveys. Two observations indicate that uplift has occurred in the center of Campi Flegrei for at least the past several thousand years. First, marine deposits in the center of Campi Flegrei were uplifted at least 40 m between 4000 and 8000 years ago. Second, continuous seismic profiles of the undersea portion of Campi Flegrei have identified only one region where rocks are fractured and faulted: adjacent to the uplifted marine deposits that surround the point of maximum uplift indicated by recent leveling surveys. On the basis of these observations and the historical record of activity we suggest that Campi Flegrei caldera is in a stage of growing a resurgent dome. The present diameter of the resurgent dome is about 4 km, and the maximum amount of vertical uplift during the past 10,000 years has probably not exceeded 100 m. The episodes of rapid uplift and shallow earthquake swarms during the past 20 years are probably related to the growth of this resurgent dome.