NEW INSIGHTS INTO THE VELOCITY AND FAULT STRUCTURE OF THE NEW MADRID SEISMIC ZONE

New detailed P- and S-wave velocity (Vp and Vs) models are determined for the active New Madrid seismic zone (NMSZ) based on local earthquake arrival time data in the Cooperative New Madrid seismic network catalog for the period January 1, 2020 through July 17, 2024. The dataset consists of 5,181 earthquakes, 82,645 P-wave phases and 58,946 S-wave phases recorded at 59 stations. The velocity models provide much greater resolution along the major arms of seismicity than previous models and indicate the strong influence that the Reelfoot axial intrusion has on NMSZ seismicity. Northern Reelfoot fault earthquakes in the depth range 4.65 – 6.65 km occur along the steep northeastern edge of the intrusion and deeper earthquakes occur within the intrusion in a high Vp/Vs ratio region indicative of high pore pressure. The southern Reelfoot fault is cut by the northeast trending Axial and Ridgely faults. The most active portion of the Axial fault is in very low Vp and high Vs granitic crust; low Vp/Vs values reaching 1.58 occur along portions of the fault. The Bootheel lineament corresponds to a linear, negative Vp anomaly in the depth range 6.65 to 8.65 km. Three-dimensional views of the seismicity patterns indicate that the southern and northern Reelfoot fault contain numerous northeast trending faults, in agreement with a recent focal mechanism study. We suggest that both strike-slip and reverse motion occur on the Reelfoot fault during large earthquakes and that topography on the Lake County uplift cannot be used to estimate the age of the seismic zone.