Tectonics/Volcanoes 1

Modelling the slip distribution along fault planes is an essential part of earthquake investigations. From a societal perspective, understanding slip distributions is important for constraining seismic hazard, since patches that did not rupture in one earthquake may fail in future events. From a scientific perspective, slip gives insight into stress distribution and frictional properties of a fault.

Earthquake slip can be determined using data from a variety of sources, with different data offering different benefits. Satellite-borne Interferometric Synthetic Aperture Radar (InSAR) provides spatially-dense coverage but poor temporal resolution, which can instead be provided by seismology.

With the launch of ESA’s satellite Sentinel-1A in 2014, the scientific community is now in a position to routinely investigate all large continental earthquakes using InSAR, and inverting for slip is a crucial part of that procedure. However, in order for the slip inversions to be useful we need to ensure that the inversion processes give results that are properly representing the slip distribution.

There is growing evidence that faults show fractal properties, and that slip distribution is well described by a von Karman distribution, which incorporates fractal properties through the Hurst parameter. In such a distribution a geometry term allows for a different correlation length in the along-strike and down-dip directions. This can be incorporated into the slip inversion using a Bayesian approach and has more physical meaning than standard regularisation methods.

Here we present the results of solving for earthquake slip using von Karman regularisation for the M_w 6.0 Napa Valley, California, earthquake of 24^th August 2014, which was the first earthquake captured by Sentinel-1A.

A common simplifying assumption in inversions of geodetic data for the earthquake source is that the geodetic observations were made on a flat surface at zero elevation. In many cases, this approximation is reasonable, however in the case of large thrust faults, where the hangingwall of the fault is often accompanied by significant relief, including steep slopes and high elevations, it is much less secure.  

We can anticipate two potential biases in source models that do not account for topography. First, geodetic observations made at high elevations are correspondingly further away from the faults that generated them, compared with a flat surface at sea level, and will thus require greater amounts of fault slip to provide the same surface displacements; models that do not account for topography may, therefore, underestimate seismic moment. Second, in situations where the topographic profile contains slopes on a length scale similar to that of the fault, the different elevations of the actual observation points may cause a bias in dip in a model that does not take topography into account.

We test these hypotheses against data from the 2015 Gorkha, Nepal earthquake. Using interferometric surface displacements imaged by the ALOS-2 satellite in SCANSAR mode, we first invert for the geometry of the source fault, assuming first a flat elastic half space, and then accounting for the difference of elevation of the different data points, using a Metropolis algorithm to explore the model parameter space. Our preliminary set of solutions shows two principal modes; a steeper fault (dip 26 degrees), and a shallower fault (dip 10–13 degrees). The former group of solutions has a better fit to data in the flat Earth case, and the latter in the topographic model case, although both groups of solutions are obtained in both cases. The shallower fault dips are close to seismic estimates (7-10 degrees), tentatively supporting our hypothesis of a possible bias in the flat Earth case. We will present the preliminary results of our modeling of the same data using finite element models that include a realistic representation of the topography, using both our preferred dip from our flat Earth model, and the shallower preferred dip from the topographic model.

The 25^th of April 2015 M_w 7.8 Gorkha Earthquake in Nepal affected a large area of central Nepal and adjacent parts of India and Tibet. It was followed by a number of large aftershocks, with the largest so far an M_w 7.3 aftershock on the 12^th of May 2015. We integrate geodetic measurements from Global Positioning System (GPS) data and synthetic aperture radar (SAR) satellite images to image the three-dimensional vector field of coseismic surface deformation for these two large events. We analyze SAR data from the Copernicus Sentinel-1A satellite operated by the European Space Agency; the RADARSAT-2 satellite operated by MacDonald, Dettwiler and Associates (MDA); and the Advanced Land Observation Satellite-2 (ALOS-2) satellite operated by the Japanese Aerospace Exploration Agency. We combine less precise analysis of large scale displacements from the SAR images of the three satellites by pixel offset tracking or sub-pixel correlation, including the along-track component of surface motion, with the more precise SAR interferometry (InSAR) measurements in the radar line-of-sight direction to estimate all three components of the surface displacement for the mainshock and large aftershock. A large area of central Nepal was pushed southward, due to thrust slip on the Main Himalayan Thrust (MHT) at depth extending about 170 km along-strike. The InSAR measurements show that there was no detectable slip on the shallower part of the MHT up-dip from the large coseismic slip or on other thrust faults in the Himalayas, except for one area of very shallow triggered slip of up to 5 cm on a thrust to the north of the Himalayan Frontal Thrust, during the two event. We also image postseismic deformation after these earthquakes with ongoing continuous GPS measurements and InSAR analysis of the SAR satellite data. Initial analysis of the GPS measurements indicates the most likely process in the first months is afterslip down-dip from the main coseismic slip. Large atmospheric effects in the InSAR measurements make it challenging to image deep afterslip, but the early interferograms appear to rule out any shallow afterslip up-dip from the mainshock and aftershock ruptures at the time of this writing. Preliminary modeling suggests that viscoelastic relaxation may not be measurable until we have 6-12 months of GPS and InSAR data.

The 16^th of September 2015 M_w 8.3 Illapel Earthquake ruptured in the Chilean subduction zone and triggered a substantial tsunami. We integrated InSAR measurements from Sentinel-1 images with GPS, tsunami buoy, and seismic data to image the slip distribution of the quake. Most of the slip was far off the coast near the trench.

Large thrust faults accommodate crustal shortening caused by the collision of tectonic plates, contributing to the growth of topography over geological timescales. The Himalayan belt, which results from the collision of India into Asia, has been the locus of some of the largest earthquakes to strike the continents, including the recent 2015 magnitude 7.8 Gorkha earthquake. Competing hypotheses have been proposed to explain how topography is sustained and how the current convergence across the Himalaya is accommodated – whether this is predominately along a single thrust or is more distributed, involving out-of-sequence additional faulting. Here we use geodetically-derived surface displacements from Sentinel-1 data to show that whilst the Gorkha earthquake was blind, it ruptured the Main Himalayan Thrust (MHT), highlighting its ramp-and-flat geometry. Reconciling independent geological, geomorphological, geophysical and geodetic observations, we quantify the geometry of the MHT in the Kathmandu area. Present-day convergence across the Himalaya is mostly accommodated along the MHT, and no out-of-sequence thrusting is required to explain the higher uplift and incision rates at the front of the high range. In addition to the region west of the Gorkha rupture, a large portion of the MHT remains unbroken south of Kathmandu presenting a continuing seismic hazard. Constraining the geometry of the structure accommodating most of the convergence is a landmark for further studies on the development of the Himalayan range and on the seismic behaviour of the broader region of Nepal.

Sentinel-1A has now been in orbit since April 2014, and collecting data routinely for more than a year. Here we review progress within COMET(*) towards our ultimate goal of building a fully-automated processing system that provides deformation results and derived products to the community for all tectonic and volcanic areas.

The Sentinel-1 constellation (the 1B satellite will be launched in early 2016) has several advantages over previous radar missions for InSAR applications: (1) Data are being acquired systematically for tectonic and volcanic areas, (2) Images cover a wide footprint, 250 km from near to far range in Interferometric Wide Swath (TOPS) mode, (3) Small perpendicular and temporal baselines greatly improve interferometric coherence at C-band, (4) The mission is planned to be operational for 20 years, with 1C and 1D planned for future launches, (5) Data are freely available to all users.

Since reaching its operational orbit in August 2014, Sentinel-1A has provided valuable data for a number of geological events. These include earthquakes in Napa (August 2014), Nepal (April 2015), and Chile (September 2015) and eruptions at Fogo (November 2014) and Calbuco (April 2015). We will show results from these events, as well as the ongoing monitoring of postseismic deformation following the earthquakes.

Many tectonic faults and volcanoes are deforming very slowly. To provide results with this accuracy comparable to GPS (~1 mm/yr) on tectonic length scales (~100 km) requires time series analysis of 3-5 years of data acquired every 6-12 days, and atmospheric corrections. With 1 year of data, we can only expect to resolve slow deformation in areas where the deformation occurring in 1 year exceeds ~40% of the uncorrected atmospheric noise. This condition should be met along the North Anatolian Fault, and for some volcanic systems in South America. We will show preliminary regional analyses of these areas.

We will also show results of a systematic analysis of interferometric coherence in tectonic and volcanic areas, and discuss the future goals and timeline for our processing system.

* COMET is the UK Natural Environment Research Council’s Centre for the Observation and Modelling of Earthquakes, Volcanoes, and Tectonics.