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Cores and downhole measurements recovered during International Ocean Discovery Program (IODP) Expedition 376 to Brothers volcano in the Kermadec arc provided unprecedented in situ data in an active submarine arc caldera with extensive hydrothermal alteration. Pressure (P)-wave velocities were measured on the R/V JOIDES Resolution at atmospheric pressures and saturated with seawater. To complement these shipboard measurements, seven new samples were selected representing various primary lithologic and alteration mineralogic compositions in the three deepest holes (U1527C, U1528D, and U1530A) for further shore-based laboratory testing. P- and shear (S)-wave velocities and porosity were measured on seven samples at atmospheric pressure and dry conditions. In addition, P- and S-wave velocities of two of these samples were measured under effective pressure dry and brine saturated. Such data aids in situ porosity, saturation, and pressure sensitivity elastic data interpretation from downhole measurements acquired in Hole U1530A. The clear waveforms obtained and overall similarities to measurements from the nearest shipboard samples ensure that the results are reliable at atmospheric pressures. All shore-based samples have higher porosity than shipboard samples. This difference could be explained by gas- versus water-connected porosity. The two saturated samples measured at effective pressures of the borehole sample depths show that P-wave speeds are 13%–20% higher than the ship atmospheric pressure measurements. The pressure dependence of wave speeds also enables the qualitative interpretation of pore shapes for these submarine, hydrothermally altered rocks.