Emerging scientific challenges at the interface of surface and deep Earth processes:
Part 1- Surface processes and hydrology needs over next 5-10 years
Submitted by IRIS Grand Challenge Committee on Change and Interactions among Climate, Hydrology, Surface
Processes and Tectonics
Sridhar Anandakrishnan, Julie Elliott, Sean Gulick*, Eric Kirby*, Victor Tsai, Kelin Whipple (*Committee Co-chairs)
Overview
One of the most exciting developments of the past two decades in the Earth Sciences is the
understanding that solid earth deformation is intimately coupled to, and influenced by, changes in
surface boundary conditions. Increasingly, the community recognizes that the feedbacks among
topography, climate and erosion govern the evolution of mountain belts over millennia. At much
shorter timescales, the interactions among warming oceans, wave fields, glacial dynamics, and ice
shelves threatens the stability of large portions of the polar ice caps. Seismological approaches hold
great potential to address these, and other, aspects of how processes in the atmosphere, cryosphere
and at the Earth’s surface are dynamically coupled to the solid Earth. Here, we briefly highlight a few of
key scientific challenges facing the community in the next 5-10 years, offer suggestions for how GAGE
and SAGE facilities may be poised to address these challenges, and highlight the need for partnership
with other disciplines, agencies and organizations to craft integrated experiments that will allow us to
successfully approach questions at the interface between the solid and fluid Earth.
Scientific issues
Surface processes and hydrology
Characterizing the rates of sediment transport by near-surface processes such as landslides and
as bedload in rivers is crucial for both our understanding of long-term erosion and for prevention of
associated natural hazards. While other techniques exist for such monitoring, seismic monitoring is one
of the few techniques that allow us to make measurements during the most extreme events, which are
typically the most important both in terms of mass transport and hazards. We identify four specific
research questions where high-resolution characterization of both seismic wavefields and acoustic noise
in near surface environments will be critical in future studies of both local-scale process mechanics and
their large-scale implications for the interactions among climate, topography, erosion, and deformation
in the evolution of orogenic systems.
1. Quantifying the role of rock strength – the heterogeneity of material properties at and near
the Earth’s surface fundamentally controls the detachment, weathering and transport of sediment. Yet,
it is precisely this heterogeneity that foils most attempts to predict, with any certainty, rock strength
from first principles. Fracture growth and propagation arising from climatic, topographic and biologic
processes largely sets the rate at which rock is converted to regolith, determines the erodibility of
bedrock channel floors, dictates the resultant caliber and size of sediment delivered to channel systems,
and controls the stability of avalanche-prone mountain slopes. A combination of high-resolution studies
focused at marrying field and lab characterization of seismic and acoustic properties of near-surface
materials holds promise for quantitative analysis of rock mass quality at the scale of geomorphic
processes. Likewise, although it is a challenge to fully instrument regions prone to mass wasting,
targeting these areas with long-term seismic deployments will be necessary to accurately monitor
fracture growth, landslide propensity, failure and runout.
2. Quantifying thresholds for transport and erosion in large floods – the acoustic signals
generated by the mobile carpet of boulders, cobbles and gravels moving along the beds of river
channels is a rapidly developing field. This work is poised to tackle one of the outstanding questions in
sediment transport – what proportion of long-term sediment transport is accomplished in extreme flood
events. Measuring flux of bedload sediment is challenging even in the smallest streams, and we have
almost no data on large floods in big rivers. Yet, theory and limited data set an expectation that the rate
of transport should depend non-linearly on discharge in rivers. Although most seismic deployments
avoid the “noise” of rivers, this property may hold the key to better understanding the role of transport
thresholds during extreme events.
3. Measuring the variability of alluvial cover in rivers – the temporal and spatial variability of
bedload serves a dual role modulating the efficiency of river incision. On one hand, the impact of
sediment on the channel bed and banks that generates acoustic noise acts to enhance erosion. On the
other, sufficiently thick sediment can “armor” channel beds and isolate them from impact-related
damage and erosion. What little information we have on the spatial distribution of sediment in
channels has come from shallow exploration geophysics; development of methods in combination with
acoustic measures of transport rate holds future potential.
4. Understanding the controls on flood frequency – why are flood frequency curves so different
in different environments? We now know that much of the non-linearity between uplift rate and
topography (and thus key to all links between climate and tectonics, see orogenic systems below) is
dictated by the variability of flooding. Runoff variability is strongly modulated by the filtering of
precipitation input accomplished by soils, vegetation, and evapotranspiriation. Keys to understanding
how the water cycle operates in the shallow subsurface requires imaging tools that resolve a wide range
of length scales and measure a range of physical properties in order to uniquely map this complex
region. Much current effort in this regard, including refraction, resistivity, and ground-penetrating
radar, has been in association with the Critical Zone Observatories, and future connections to this NSF
program are warranted.
What is needed?
•Enhanced facility for shallow subsurface geophysics
The challenges associated with instrumental monitoring of floods and mass wasting events requires
a large pool of inexpensive instruments. As suggested by other IRIS committees, the advent of lowpower, autonomous geophysical instruments appear to hold promise to open up the possibility of large
arrays aimed at capturing these processes. NSF should invest in developing an instrument pool
specifically tailored to imaging surface processes, perhaps coordinated with a modest RAMP-type
package reserved for response to specific events. There appears to be a growing need for development
of capabilities in exploration geophysical techniques as applied in the shallow Earth. Shallow seismic
reflection/refraction remains a centerpiece of these needs, and the existing PASSCAL equipment serve
this need. However, seismic techniques are increasingly coordinated with other geophysical techniques
(GPR, self-potential, EM, LIDAR, ground-based SAR, etc.). NSF SAGE/GAGE facility is well-positioned to
be a leader in developing and coordinating a national center for the broader range of geophysical
techniques. In addition, a center that provides equipment, user training, and support can play a key role
in the education and training of the next generation of geoscientists. Such geophysical techniques
provide a vehicle to engage students in quantitative, physics-based analytical science and could form the
core of a scientifically literate workforce. Finally, efforts to quantify rock properties would benefit from
a national center(s) in rock physical properties that would facilitate integration of experimental data
with field-based geophysics.