33 FEBRUARY 2023 WorldWide Drilling Resource® Deep Lake Cores Could Help Define History of Southern Andreas Fault Adapted from Information by the Seismological Society of America Sediment cores from the ancient Lake Cahuilla at Coachella in southern California could help seismologists determine whether lake filling events are connected to earthquakes on the Southern San Andreas Fault. Lake Cahuilla was a prehistoric lake in California and northern Mexico covering more than 2000 square miles in the Coachella and Imperial valleys, in the same region as the Southern San Andreas Fault. Today’s Salton Sea occupies part of the original lake footprint. Recent paleoseismology studies along the Southern San Andreas Fault suggest the last seven major ruptures on the fault occurred at intervals between 116 to 221 years. However, the most recent earthquake in the Lake Cahuilla region occurred about 300 years ago, during the last time the lake was filled. “One of our goals is to see whether there is any relationship between fluctuations in the lake and the activation of the Southern San Andreas Fault,” said Sourav Saha of the Kentucky Geological Survey, University of Kentucky. Some geophysicists believe past fault ruptures might be linked to stages of lake filling, he explained. The increase in pore pressure which could result from lake filling “would most likely reactivate smaller normal faults along the Salton Sea stepover zone; and Coulomb stress models indicate that if you reactivate those extensional faults, it is very likely that you don’t need much to reactivate the Southern San Andreas Fault.” Before seismologists can evaluate this possible triggering mechanism, they need a more complete and detailed history of the lake itself. The [114- to 131-footlong] cores dated and analyzed by Saha and his colleagues provided valuable data about this history. Sediment layers in two cores, dated with a technique called single-grain luminescence, extended lake level history back to about 7000 years, from the previous record of about 2000 years. The technique calculated the last time a mineral grain such as feldspar was exposed to sunlight, to carefully trace the history of sediment deposition in the lake. This allowed researchers to evaluate some assumptions made by paleoseismologists in tracing past earthquakes. “Usually, paleoseismologists assume that places with high sedimentation rates would likely preserve older rupture events in much more detail than areas with lower sedimentation rates, but that hypothesis has never been tested,” Saha explained. Single-grain luminescence dating offered additional details about events between earthquakes, such as “pulses” of sedimentation which might be related to changes in climate or tectonic activity. Although lake cores of the ancient Lake Cahuilla offered an extended history of lake fluctuations, they were insufficient to reconstruct past earthquake rupture history. Subtle changes in grain size of sediments suggested the lake had a more complicated sedimentation history than previously thought, making it a little more difficult to distinguish lake from nonlake layers. Saha said the research team is planning to extract more cores and use cone penetration testing (CPT) to reveal more vertical layers and the horizontal extent of those layers to better distinguish lake and nonlake layers and look for complete evidence of past earthquakes. Coring fieldwork at Lake Cahuilla. ENV New Location Location Expanding Mfg., Scree Atlan Inc. n & tic E-mail: atlantic@ce Atlantic-Screen.c Manufactu 302-684-3197 Manholes j Pipe Clear PVC j Inline Chemical Mixers j ell Rehabilitation Produ jW .net om rers cts DE 19968 Milton, 142 Broadkill Rd ½” - 24 ranging f Perforated of Slotted Fax: 302-384-0643 more! MUCH And j Bailers Sampling j Locking Caps j Filter Sock j Bentonite j ” rom Pipe and
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