Data obtained by the U.S. satellite DE-2 are used to investigate possible precursor features in the ionosphere associated with a large earthquake (latitude -33.13 degrees, longitude 73.07 degrees, M = 7.5), which occurred during a moderate geomagnetic disturbance. Atomic oxygen ion and molecular ion distributions show characteristic latitudinal features similar to the well-known equatorial ionization anomaly (EIA) feature but centered around the earthquake epicenter. We name this the precursor ionization anomaly (PIA). The density minima of both the atomic oxygen and molecular ions are in two latitude zones, depending on the distance from the epicenter. One of the PIA minima aligns with the geomagnetic latitude crossing the epicenter. Another minimum is found along the geographic latitude of the epicenter. These minima are located in an area spanning about 40 degrees in latitude and about 140 degrees in longitude. It is noted that the molecular ion minimum is more clearly defined even when the atomic ion density minimum is not indicated clearly. The ion density reduction seems to be caused by a superposition of natural/quiet time ionospheric eastward electric field and an electric field associated with the earthquake. Although we studied one single event, our careful examination of results suggests that the location and day of occurrence of the PIA can be predicted for some large earthquakes even during moderate geomagnetic disturbance if the satellite orbit is properly chosen.
