Measurements of Ground-Based Magnetics and Vertical Deformation From a Leveling Line Across the San Andreas Fault at Durmid Hill

Karen Alvarez, & Jascha Polet

Published August 15, 2017, SCEC Contribution #7713, 2017 SCEC Annual Meeting Poster #163

The Durmid Hill region is located near the termination of the San Andreas Fault (SAF) at Bombay Beach. This section of the fault has not experienced any major earthquakes in the past three centuries. Fuis et al. (2017) showed that the SAF is dipping to the northeast at ~50˚-60˚. The 100 m elevation difference between the Salton Sea and Bat Cave Buttes (located at the top of Durmid Hill) is a result of transpression along the SAF (Billham and Williams, 1985). The portion of the SAF within the Coachella Valley region experiences mild seismicity and an average dextral shear strain rate of 25 mm/yr. During a six year study, Sylvester et al. (1993) collected differential height measurements at Durmid Hill from monuments they installed along a 2.37 km leveling line normal to the SAF. They concluded that interseismic processes account for most of the growth at Durmid Hill and estimated more than 9 mm of uplift within the leveling line during those six years, with uniform tilt at distances greater than 500 m from the fault. Langenheim et al. (2014) created a model based on ground-based magnetic data that they collected in the same area and found a complex magnetic structure with a broad band magnetic anomaly present on the northeast side of SAF and a prominent magnetic high along the main mapped trace of the SAF.

A primary objective of our study is to reoccupy the leveling line from Sylvester et al. (1993), across the SAF at Durmid Hill. Additionally, we will utilize subsurface geophysical techniques to enhance our understanding of the fault geometry and its relationship to the aseismic deformation. Elevation profiles are measured using Nikon Nivo 5C total stations and magnetic field intensity measurements are made by a GSM-19TGW v7.0 walking magnetometer. We will present preliminary results from data sets gathered in our initial surveys. The maps produced from the results of these first magnetic surveys show two significant and distinct magnetic anomalies consistent with earlier studies. Initial monument elevation comparisons could only be made for monuments located at the north-eastern end of the leveling line, at a distance of about 1.5 km. There appear to be sections of both positive and negative tilt within this segment, which would be in contrast to the observations made in the early 1990’s. We plan to collect additional data in more extensive surveys later this year, and include the use of RTK GPS and Very Low Frequency equipment.

Citation
Alvarez, K., & Polet, J. (2017, 08). Measurements of Ground-Based Magnetics and Vertical Deformation From a Leveling Line Across the San Andreas Fault at Durmid Hill. Poster Presentation at 2017 SCEC Annual Meeting.

Related Projects & Working Groups
San Andreas Fault System (SAFS)