Extended Data Fig. 7: Comparison of resistive bodies to other geophysical data.

(a) Pseudo-gravity derived from magnetics⁷⁹ with contours (20 and 30 km) for depth to bottom of resistive body from Fig. 3a overlain. There is a good correlation between the 3D geometry of the core of Siletzia inferred from MT, and magnetic anomalies converted to pseudo-gravity. There is no clear correlation with resistive block ‘e’ but this body is under the thick (8 km) sedimentary Seattle basin, and is also outside our area of good data coverage. Deeply extending resistive bodies ‘g’ and ‘f’, also do not exhibit strong magnetic anomalies but these are likely not part of Siletzia per se, and may have different composition. (b) Crustal seismicity (M > 2, 1990–2020) from ANSS catalogue with the same resistivity contours overlain. Resistive blocks in the core of Siletzia are mostly aseismic, while block ‘e’ has little seismicity below the level of the Seattle basin. As is well known, there is almost no crustal seismicity in central-southern Oregon. This seismic gap includes the main thick block of Siletzia, but extends further south to the California border.