Evaluating two critical zone hypotheses using active source seismic refraction in Antelope Valley, California

Introduction. 1 GEOLOGIC BACKGROUND. 3 Field Location. 3 Previous Studies. 5 THE CRITICAL ZONE. 6 Ambiguities and Unknowns in the Critical Zone. 8 Prominent Critical Zone Hypotheses. 9 Hydrology Model. 9 Topographic Stress Model. 10 SEISMIC REFRACTION METHODS. 13 Background. 13 Inversion. 14 Process Overview. 14 Synthetic Tests. 16 Seismic Data Collection. 19 Data Processing. 20 RESULTS. 24 MH3 and MH4. 24 MH7. 25 DISCUSSION. 31 Interpretations of Velocity Structure. 31 Disaggregated Materials. 31 Saprolite. 32 Fractured Bedrock. 33 Unweathered Bedrock. 33 Hypothesis Interpretation. 36 Hydrology Model. 36 Stress Model. 38 CONCLUSIONS. 41 APPENDIX. 42 ACKNOWLEDGEMENTS. 45 REFERENCES CITED.45 Evaluating two critical zone hypotheses using active source seismic refraction in Antelope Valley, California Berit Hudson-Rasmussen Senior Integrative Exercise March 11, 2020 Advisors: Dr. Sarah J. Titus Department of Geology, Carleton College, Northfield, MN Dr. Mary Savina Department of Geology, Carleton College, Northfield, MN Dr. Mong-Han Huang Department of Geology, University of Maryland, College Park ABSTRACT Understanding critical zone structure in different landscapes is necessary to quantify water storage in the shallow subsurface, predict landslide hazards, and monitor plant-soil interactions in a changing climate. Two prominent hypotheses for critical zone formation predict the weathered to unweathered bedrock boundary to be controlled by 1) hydrologic properties, or 2) tectonic compressive stress. Near-surface geophysical techniques such as active-source seismic refraction can reveal changes in material strength that may correspond to different amounts of physical or chemical weathering at depth. In this study, I utilize seismic refraction along three hillslopes in Antelope Valley, east of the California Coastal Ranges, to characterize weathering-defined boundaries within the critical zone in an effort to test critical zone hypotheses. I assess the resolution power of the transdimensional hierarchical Bayesian inversion method by performing synthetic tests, which indicate a limited ability to detect velocity changes at depth. The seismic data, in combination with preliminary borehole analyses, indicate a sharp velocity boundary between disaggregated materials and saprolite, as well as a gradual transition from saprolite to fractured bedrock to unweathered bedrock. Evidence in favor of both hypotheses may be present at Antelope Valley, with a possible effect of hillslope relief on the weathering profile. While seismic refraction is a powerful tool for assessing changes in weathering with depth, further analyses of the stress field and water table elevation are needed to make more definitive conclusions about the dominance of tectonic versus hydrologic processes at Antelope Valley.Understanding critical zone structure in different landscapes is necessary to quantify water storage in the shallow subsurface, predict landslide hazards, and monitor plant-soil interactions in a changing climate. Two prominent hypotheses for critical zone formation predict the weathered to unweathered bedrock boundary to be controlled by 1) hydrologic properties, or 2) tectonic compressive stress. Near-surface geophysical techniques such as active-source seismic refraction can reveal changes in material strength that may correspond to different amounts of physical or chemical weathering at depth. In this study, I utilize seismic refraction along three hillslopes in Antelope Valley, east of the California Coastal Ranges, to characterize weathering-defined boundaries within the critical zone in an effort to test critical zone hypotheses. I assess the resolution power of the transdimensional hierarchical Bayesian inversion method by performing synthetic tests, which indicate a limited ability to detect velocity changes at depth. The seismic data, in combination with preliminary borehole analyses, indicate a sharp velocity boundary between disaggregated materials and saprolite, as well as a gradual transition from saprolite to fractured bedrock to unweathered bedrock. Evidence in favor of both hypotheses may be present at Antelope Valley, with a possible effect of hillslope relief on the weathering profile. While seismic refraction is a powerful tool for assessing changes in weathering with depth, further analyses of the stress field and water table elevation are needed to make more definitive conclusions about the dominance of tectonic versus hydrologic processes at Antelope Valley.