LATE QUATERNARY FAULTING SURROUNDS ALBUQUERQUE, DEFINES THE EAST HEIGHTS SEISMIC GAP

Within 40 km of downtown Albuquerque there are 27 Quaternary faults. Since 1996, the more prominent faults have been studied under NEHRP funding to determine the recency of faulting and other paleoearthquake parameters (displacement per event, recurrence). So far, all four faults investigated in detail on the western side of the Rio Grande display late Pleistocene or Holocene displacement. The County Dump fault has experienced about 14 earthquakes since the abandonment of the Llano de Albuquerque (LdA), the latest three of which occurred ca. 30, 45, and 80 ka. with vertical displacements averaging 1.2 +/- 0.6 m. The southern end of the Calabacillas fault experienced 4 displacements (10 cm, 30 cm, 55 cm, and 20 cm) since 219 ka, with the youngest event at ca. 14 ka. The Zia fault has evidence for mid-Holocene displacement at a trench site at the northern rim of the LdA. The East Paradise fault experienced 3 displacements since 208 ka, the youngest of which was latest Pleistocene. On the eastern side of the rift, all investigated faults also display late Pleistocene movement. The Rincon fault displaces latest Pleistocene-Holocene alluvial fans by 2-3 m. The Tijeras fault has experienced two displacements in the past 130 ka SW of Golden, the latest of which is late Pleistocene. The Hubbell Springs fault shows evidence of three late Quaternary surface faulting events averaging 1.6 m each. Thus, metropolitan Albuquerque is virtually surrounded by faults that have experienced late Pleistocene or Holocene movement. However, the main rift-bounding faults within suburban Albuquerque itself, the East Heights (EHFZ) and Sandia (SFZ) fault zones, have not been studied in detail. Previous reconnaissance studies suggest neither fault zone has moved in the late Pleistocene and Holocene. If this is true, these faults constitute a spatial seismic gap, the East Heights Seismic Gap, that may be the location of the next surface-rupturing earthquake in the Albuquerque region.