THIN LOW-VELOCITY ZONE WITHIN THE KRAFLA CALDERA, NE-ICELAND ATTRIBUTED TO A SMALL MAGMA CHAMBER

We examine seismograms from microearthquakes recorded at Krafla central.volcano, in the Northern Volcanic Zone of Iceland, during a short inflation period in 1988. We produce record sections for propagation both to the north and to the south of the center of the caldera by combining seismograms from many earthquakes. The northern record section contains clear evidence of a low velocity zone (LVZ), including a low amplitude diffraction of the P wave as it impinges on top of the LVZ and high amplitude reflections from beneath the LVZ. The absence of clear shear waves indicates that the LVZ is at least partially molten and should be interpretated as a magma chamber. The magma chamber is less than 1 km thick, with a top approximately 3 km beneath the surface' We find no evidence for a LVZ in the southern part of the caldera.