DOI: 10.7302/krah-yx31
title: Profiles of near-surface rock mass strength across gradients in erosion, burial, and time [Data set]

description: These datasets support the findings of Townsend et al. (2020). In this article, we project profiles of rock mass shear strength into the shallow subsurface (~30 m depth) using the Hoek and Brown criterion with Geological Strength Index (GSI) observations of outcrop structure and surface conditions, and Schmidt hammer rebound values of intact (unfractured) rock hardness. We compare these projected rock mass shear strength profiles to shear-wave velocity profiles collected using shallow geophysical arrays. We evaluate our methods in the Western Transverse Ranges of southern California, which exhibit strong gradients in the depth of latest-Mesozoic through Cenozoic sedimentary rocks exposed at the surface today, and in erosion rates quantified from catchment-average cosmogenic radionuclide concentrations and low-temperature apatite and zircon (U-Th)/He thermochronometry. We find that stratigraphic age and burial depth exerts the strongest apparent control on rock strength and S-wave velocities, likely due to diagenetic changes associated with burial. For rocks of the same age and inferred burial history, we observe that shear strength and S-wave velocities are positively correlated with erosion rate. We suggest that increasing erosion rates cause decreased residence time of rock masses within the critical zone, resulting in less weathered rocks.
description: The data are Schmidt hammer rebound values for intact rock hardness, Geological Strength Index (GSI) observations of outcrop structure and surface conditions, raw seismic velocities collected in the field, and analyzed dispersion curves and S-wave velocity profiles.

Seismic data was recorded using a 16-channel Geometrics ES-3000 portable seismometer, and a 24-channel Geometrics Geode portable seismometer, using 4.5 Hz geophones spaced at 1.5 to 3 m intervals. Impulsive sources were produced by striking a 25-cm square, 5-cm thick plastic plate with a 7.2 kg sledge hammer, and shots were stacked 8-10 times to improve the signal-to-noise ratio. Shots were produced from the end of the array at an offset of 15-20% of the total array length to avoid near-field effects. Total line lengths varied from 53 to 78 m. At 10 sites, a second survey was recorded with a smaller geophone spacing of 0.7 to 1.0 m in order to increase the resolution of the S-wave velocity profile near the surface. These shorter surveys were centered over the midpoint of the longer array.

All active seismic surveys were collected with a source interval of 0.125 milliseconds, a record length of 1.0 seconds, and 8 stacks. All passive seismic data was collected with a sample interval of 2.0 milliseconds, a record length of 30 seconds, and a single stack. Twenty 30-second recordings were made for each passive seismic survey.

We generated S-wave velocity profiles using Geometrics SeisImager/SW software (Pickwin Version 5.2.1.3, WaveEq Version 4.0.1.0, 2016). Initial velocity structures with 30 layers were assumed for each site, and the maximum depth of each profile was set to half the longest measured wavelength.

We assessed outcrop fractures and surface conditions in the field using the Geological Strength Index (GSI) of Hoek & Marinos (2000). Schmidt hammer measurements of intact rock hardness were recorded from the same outcrops as the GSI. We used an "Original Schmidt" (type N, manufactured by Proceq). Reported values are the mean of 20 measurements recorded from a horizontal position.

We used the Hoek & Brown (2002) criterion to calculate shear strength depth profiles, assuming that the minimum principal effective stress is the lateral earth pressure. More details on this methodology are presented in the journal article associated with this data set.