Finding the buried record of past earthquakes with GPR-based palaeoseismology: a case study on the Hope fault, New Zealand

Abstract

International audienceIn places where sedimentation and erosion compete at fast rates, part of the record of past earthquakes on faults may be buried, hence hidden, in the first few metres below the surface. We developed a novel form of palaeoseismology, of geophysical type, based on the use of a dense pseudo-3-D Ground Penetrating Radar (GPR) survey to investigate such possible buried earthquake traces, on a long, fast-slipping strike-slip fault (Hope fault, New Zealand), at a site (Terako) where marked alluvial conditions prevail. We first used LiDAR data to analyse the ground surface morphology of the 2 km2 site at the greatest resolution. Nineteen morphological markers were observed, mainly alluvial terrace risers and small stream channels that are all dextrally offset by the fault by amounts ranging between 3 and 200 m. The measurements document about 10 past earthquake slip events with a mean coseismic slip of 3.3 +/- 1 m, with the most recent earthquake event having a slip of 3 +/- 0.5 m. We then investigated a detailed area of the site (400 x 600 m2) with pseudo-3-D GPR. We measured 56, similar to 400 m long, 510 m spaced GPR profiles (250 MHz), parallel to the fault and evenly distributed on either side. The analysis revealed the existence of a palaeosurface buried at about 3 m depth, corresponding to the top of alluvial terraces of different ages. That buried surface is incised by a dense network of stream channels that are all dextrally offset by the fault. We measured 48 lateral offsets in the buried channel network, more than twice than at the surface. These offsets range between 6 and 108 m, as observed at the surface, yet provide a more continuous record of the fault slip. The similarity of the successive slip increments suggests a slip per event averaging 4.4 +/- 1 m, fairly similar to that estimated from surface data. From the total surface and buried 67 offset collection, we infer that a minimum of 30 large earthquakes have broken the Hope fault at the Terako site in the last about 67 kyr, with an average coseismic slip of 3.2 +/- 1 m, a minimum average recurrence time of about 200 yr, and a magnitude of at least Mw 7.07.4. Our study therefore confirms that part of the record of past earthquakes may indeed reside in the first few metres below the surface, where it may be explored with geophysical, GPR-based palaeoseismology. Developing such a new palaeoseismological tool should provide rich information that may complement surface observations and help to document the past earthquakes on faults

Stratigraphic architecture and fault offsets of alluvial terraces at Te Marua, Wellington fault, New Zealand, revealed by pseudo‐3D GPR investigation

“…Along with our earlier work on the Hope fault [ Beaupretre et al ., ], the present study thus suggests that pseudo‐3D GPR is a powerful paleoseismological tool that could be used in complement to surface observation to recover the past fault slips. In the Te Marua case, the GPR data additionally allow documenting the 3D stratigraphic architecture of the alluvial terraces, an information that was previously unknown.…”

“…Finally, in each horizon, we represent the entire collection of mean offset values as a probability density function (PDF) [e.g., McGill and Sieh , ; Lowell , ; Zielke et al ., ; Beaupretre et al ., ]. Each individual mean offset is represented with a Gaussian distribution.…”

“…We approach this question differently. In an earlier paper [ Beaupretre et al ., ], we suggested, as others before [e.g., McCalpin , ], that part of the past earthquake offsets might be buried in the first few meters below the current ground surface. Yet, to investigate this hypothesis, we developed a novel paleoseismological approach, based on a specific use of pseudo‐3D ground penetrating radar (GPR) surveying.…”

Ground-penetrating radar and surface nuclear magnetic resonance monitoring of an englacial water-filled cavity in the polythermal glacier of Tête Rousse

“…Such an extremely dense acquisition necessarily restricts 3D applications to small areas. Fortunately, in cases in which the investigated subsurface features are expected to exhibit fairly 2D variations, anisotropic acquisition with dense GPR-trace collection in the direction perpendicular to the expected 2D features (here transverse to the glacier flow) and sparser trace collection in the other direction (longitudinal to the glacier flow) may be an appropriate approach (Beauprêtre et al, 2012). In addition to common-offset profiles, CMP surveys were systematically acquired in different parts of the glacier using 100 MHz unshielded antennae.…”

Applying FDEM, ERT and GPR at a site with soil contamination: A case study

“…GPR has proven its ability to provide high-resolution images for strata. Thus, it is useful for investigations in geology, archeology and buried structures and pipelines (Al-Nuaimy et al, 2000;Beauprˆetre et al, 2012;Porsani et al, 2010;Tohge et al, 1998). In environmental pollution investigations, GPR can survey the NAPL leakage distribution (Cassidy, 2007;Greenhouse et al, 1993;Lopes de Castro and Branco, 2003).…”