Publication Date:
2017-04-04
Description:
We estimate the static stress drop on small exhumed strike-slip faults in the Lake
Edison granodiorite of the central Sierra Nevada (California). The subvertical strike-slip
faults were exhumed from 4 to 15 km depth and were chosen because they are exposed in
outcrop along their entire tip-to-tip lengths of 8–12 m. Slip nucleated on joints and
accumulated by crystal-plastic shearing (forming quartz mylonites from early quartz vein
filling in joints) and successive brittle faulting (forming epidote-bearing cataclasites).
The occurrence of thin, 300 mm wide, pseudotachylytes along some small faults
throughout the study area suggests that some, if not all, of the brittle slip on the study area
faults may have been seismic. We suggest that the contribution of brittle, cataclastic slip to
the total slip along the studied cataclasite-bearing small faults may be estimated by the
length of epidote-filled, rhombohedral dilatational jogs (rhombochasms) distributed
quasi-periodically along the length of the faults. The interpretation that slip recorded by
rhombochasms occurred in single events is based on evidence that (1) epidote crystals
are randomly oriented and undeformed within the rhombochasm; (2) cataclasite in
principal slip zones does not include clasts of previous cataclasite, and (3)
rhombochasm lengths vary systematically along the length of the faults with slip
maximum occurring near the fault center, tapering to the fault tips. We thereby constrain
both the rupture length and slip. On the basis of these measurements, we calculate stress
drops ranging over 90–250 MPa, i.e., one to two orders of magnitude larger than
typical seismological estimates for earthquakes, but similar in magnitude to
seismological estimates of small (〈M2) earthquakes from the San Andreas Fault
Observatory at Depth (SAFOD). The slip events described in the present study occurred
along small, deep-seated faults, and, given the calculated stress drops and observations
that brittle faults exploited joints sealed by quartz-bearing mylonite, we conclude
that these were ‘‘strong’’ faults.
Description:
Published
Description:
B02402
Description:
3.1. Fisica dei terremoti
Description:
JCR Journal
Description:
reserved
Keywords:
earthquakes
;
stress drops
;
04. Solid Earth::04.04. Geology::04.04.99. General or miscellaneous
Repository Name:
Istituto Nazionale di Geofisica e Vulcanologia (INGV)
Type:
article
