The Subsurface Structure of the Kīlauea Caldera Before Its 2018 Collapse Inferred From Ground Magnetic, SP, and Temperatures Anomalies.

The 2018 crisis of Kīlauea volcano stands as a major event in its evolution with a large down‐rift effusive eruption that drained a shallow magma reservoir at the summit. The characterization of such active magmatic systems and associated hazardous events remains a necessity and a challenge. The summit area is hydrothermally active and strongly altered as indicated by geological mapping. A unique data set of geophysical measurements was collected around Halemaʻumaʻu crater before its collapse. Magnetic data are interpreted here in combination with geological information, temperature anomalies at the surface, self‐potential measurements, and a model of electrical conductivity. 3D forward modeling shows that the main magnetic dipole‐like anomaly observed around the crater is not only caused by the crater topography but suggests the presence of an important volume of weakly magnetic material beneath the crater, which may be caused by higher temperature and/or hydrothermal alteration. 3D inversion of the data allows us to explore the first order geometry of the magnetic structures. We complement this inversion with 2D forward modeling in order to refine the geometry of major structures. This study shows the presence of major geological structures in the 2018 collapsed area that may have been associated with mechanical weaknesses and could have played a role in the geometry of the collapse. Therefore, mapping magnetic anomalies and monitoring their temporal evolution are of great interest for constraining the nature and mechanical properties of the underlying formations and their temporal evolution in order to help predict future behavior. Plain Language Summary: Kīlauea ranks among the world's most active volcanoes alternating between effusive and explosive events, implying for example, large caldera collapses. The 2018 eruption is the most recent major volcano‐tectonic event that has significantly affected the Kīlauea summit. Assessment of associated hazards is a necessity and a challenge and requires the imaging of hidden weakness zones within the volcano and the monitoring of their evolution through time. We use here a survey of magnetic measurements collected in 2015 at the Kīlauea summit in order to image the subsurface distribution of rock magnetization. These data allow us to demonstrate the presence below the Halemaʻumaʻu crater of a large volume of material characterized by weak magnetization and to delineate its geometry. This weak magnetization can be caused by high subsurface temperatures and/or rock modifications caused by interaction with hot fluid circulations. We also identify major structures that may have been weakness zones and could have played an important role in the geometry of 2018 collapse. This study suggests that the repetition of magnetic surveys over volcanoes in order to monitor the temporal evolution of their subsurface magnetization could help predict the future evolution of the volcanic system. Key Points: We present a new ground magnetic data set collected at the Kīlauea summit around the Halemaʻumaʻu crater before its collapse in 20182D and 3D modeling of these data indicate the presence of weakly‐magnetized material below the crater and along a major fissure zoneThis data set shows the presence of major structures that may have been pathways to fluids and may have played a role in the 2018 collapse [ABSTRACT FROM AUTHOR]