Behavior of Coastal Amagmatic Geothermal Systems: Thermal–Hydraulic Modeling Insights From La Jolla Beach, Baja California, Mexico

Abstract Amagmatic geothermal systems in coastal locations are promising energy resources, yet the processes that localize and sustain their hot‐spring discharge remain unclear. We investigate La Jolla Beach (NW Baja California, Mexico)—one of the hottest known examples worldwide (∼100°C)—using large‐scale 3D coupled thermal–hydraulic simulations. The models are calibrated against observed temperature, salinity, surface area and location of the springs, and verified using meteoric‐water residence times. The results imply that a highly permeable coastal segment of the Agua Blanca Fault (ABF) transfers meteoric water from a hinterland recharge zone to the coast via deep (>5 km) circulation. Within the coastal fault, dense seawater forms a hydraulic barrier to the meteoric water, while thermal buoyancy steepens the meteoric–seawater interface and creates a near‐vertical upflow plume that focuses hot, mixed fluids to the shoreline, resulting in high discharge temperatures. Tracer simulations indicate that deep fault flow is supplied approximately equally by infiltration through the exposed fault trace and by lateral inflow from the surrounding fractured country rocks. This underscores the system's inherently 3D nature and the capacity of regional faults to collect recharge from broad catchments, even where overall infiltration rates are low. Permeable sediments draped over a basement high and bounded by less‐permeable sediments focus the hot upwelling water at La Jolla Beach. Our findings explain the thermal‐hydraulic coupling that controls amagmatic coastal fault–controlled geothermal systems, providing a basis to assess the geothermal potential of analogous systems worldwide.