Description:
Geomorphic and Quaternary geologic studies provide data with which to
assess seismic hazard in southeastern Arizona and adjacent New Mexico and
Sonora, Mexico, where one large (M ~ 7 1/4) historic earthquake has occurred
against a background of very low seismicity. Conclusions regarding the
distribution and timing of late Quaternary faulting are based on (1) estimated
ages of soils based on correlation with soils near Las Cruces in southern New
Mexico; (2) use of surface age-fault offset relationships to constrain the age
of most-recent fault movement and to estimate the frequency of movement along
individual faults; and (3) morphologic analyses of fault scarps to estimate
their ages. Individual late Quaternary faults in the region have surface
rupture recurrence intervals on the order of 105 years. However, the major
earthquake that occurred in 1887 in northeastern Sonora is evidently part of a
series of 5 or 6 surface-rupturing earthquakes that have occurred since 20 ka
in a N-S-trending zone straddling the Arizona-New Mexico border. Surface
ruptures during the late Pleistocene (about 20-120 ka) occurred from near
Tucson east to the border area, but the rate of surface rupture occurrence was
evidently 4-25 times lower than during the past 20 kyo The rate of Holocenelatest
Pleistocene surface-rupturing, while much lower than some portions of
the northern Basin and Range province, evidently represents a burst of
activity relative to the average long-term rate of faulting in southern
Arizona.