Erin's Abstract:

The dynamic motion of an earthquake begins abruptly, but is often initiated by a small interval of weak motion called the seismic nucleation phase (SNP), first named by Ellsworth and Beroza (1995). In their study, Ellsworth and Beroza (1995, 1996) concluded that the SNP was detectable in near-source records of all of the 41 M 1 to M 8 earthquakes they investigated, with the SNP accounting for ~0.5% of the total moment and lasting ~1/6 of the total duration. Concentrating on large earthquakes, Ji et al (2010) investigated the SNP of 19 M 8.0 earthquakes since 1994 using a new approach applied to teleseismic broadband data. They found that ~50% of the earthquakes had a detectable SNP, and that the relationship between the durations of the SNPs and the final seismic moments were consistent with the relationship that Ellsworth and Beroza (1995) found. Burkhart and Ji (2011) found that, in 68 M 7.5 to M 8.0 since 1994, the SNP is clearly detectable in 31 events, with 27 events showing no nucleation phase and 10 having too much noise or not enough stations to tell.After continuing to investigate these events, modifying the code, and adding events >8.0 from Ji’sstudy, this study finds further evidence that large shallow, continental, and strike-slip earthquakes show a clear SNP. We find that 13 of the 19 strike-slip earthquakes investigated show a clear SNP, with 3 having none. Further correlations are being examined in thrust events between existence of the SNP and characteristics of the subducting plates, including seafloor age, sediment thickness and angle of the down-going slab.

David's Abstract:

Uplift rates along the Gaviota Coast, California have been subject to controversy resulting from incompatible studies over the past 20 years, but recent dates indicate uplift rates must vary by at least five fold along this coastline. Dozens of proposed development projects in the eastern Gaviota Coast highlight the importance of constraining uplift rates (and thus seismic hazard). In order to identify shifts in uplift, the geomorphology of the Gaviota Coast was analyzed using normalized stream steepness indices, mountain crest elevation, synthesis of past research, and pending optically-stimulated luminescence (OSL) dating. Results from these analyses correlate significant geomorphic changes across the Southern Branch of the Santa Ynez Fault (SBSYF). Normalized stream steepness indices vary by approximately two fold across the SBSYF after accounting for rock strength. Average mountain crest elevation jumps ~350 meters across the SBSYF; more than any other location in the mountain range. The only modern river channel to cut through the Santa Ynez Mountains is also located at the SBSYF. Additionally, maps from previous studies of emergent marine terraces along the coastline suggest uplift rates do not vary significantly anywhere else in the study area. These results strongly support the hypothesis that uplift rates are controlled by the SBSYF, and vertical offset across the fault is greater than previously recognized. OSL sampling sites have been identified and partially sampled, and will provide the first set of absolute ages along the entire Gaviota Coast.

Eric's Abstract:

Time series of ?18O analyzed in calcareous cave deposits from China suggest that on orbital-scales the timing and magnitude of changes in Asian monsoon precipitation tightly co-vary with northern high latitude climate.  These studies assume that changes in the ?18O records largely reflect the amount of monsoon precipitation.  This assumption is challenged here through a multi-proxy analysis of an ocean sediment archive from the northern Bay of Bengal spanning Termination II and Marine Isotope Stage 5 (145-70 Kyrs BP).  Due to the proximity to the output of large river systems this location provides a spatially integrated record of Indian monsoon change.  Monsoon variations are determined through analysis of Ba/Ca (a proxy of runoff induced sea-surface salinity) and ?18O in planktonic foraminifera, in addition to a Mg/Ca-based record of sea-surface temperature. Through comparison with other global monsoon records, Ba/Ca proxies of monsoon strength show a general decrease in intensity during MIS 5, as expected from northern hemisphere summer insolation values, while the ?18O records do not.  Using the independent proxies of runoff induced sea-surface salinity and temperature the ?18Oforam record is corrected to primarily reflect the ?18O of precipitation.  The ?18O of precipitation record indicates that the enrichment during MIS 6 and depletions during MIS 5c and 5a are likely related to large-scale changes in atmospheric circulation and/or changes in the seasonality of precipitation and not exclusively due to changes in precipitation amount.