Location and size of the shallow magma reservoir beneath Kīlauea caldera, constraints from near‐sourceVp/Vsratios

We present high-resolution compressional wave to shear wave velocity ratios (Vp/Vs) beneath Kīlauea’s summit caldera by applying an in situ estimation method using waveform cross-correlation data for three similar earthquake clusters. We observe high Vp/Vs ratios (1.832 and 1.852) for two event clusters surrounded by the low background Vp/Vs value of 1.412 at ∼2.1 km depth below the surface. These high and low Vp/Vs ratios can be explained by melt- and CO2-filled cracks, respectively, based on a theoretical crack model. The event cluster with the highest Vp/Vs ratio consists of long-period events that followed the 1997 East Rift Zone eruption, indicating their association with fluid and magma movement. The depths of the two clusters with high Vp/Vs ratios are consistent with the magma reservoir location inferred from geodetic observations. Their locations east and north of Halema‘uma‘u crater suggest a horizontal extent of a few kilometers for the reservoir.