Earthquakes and tectonics I

Slow slip events (SSEs), also known as silent earthquakes or episodic tremor and slip (ETS), are essentially earthquakes unfolding in slow motion.  The seismic events we call earthquakes may last from a few seconds to perhaps a minute or two, whereas as an SSE may unfold over hours, days, or even weeks.  The dispersal of released energy over a long time causes seismic waves with such long periods that the motion is imperceptible to all but the most sensitive instruments, and with none of the short-period shaking we usually associate with earthquakes.  Since the ground acceleration resulting from such a slow moving wave is tiny, these events in themselves are much less hazardous to structures than are their more rapid earthquake cousins.  Nonetheless, the total moment released in an SSE may be quite large, and stress transfer from the event may either increase or relax strain stored elastically in a fault or subduction boundary.  Thus it is important from a hazard viewpoint to understand the nature and impact of these often regularly-repeating and predictable events, that is whether they are adding to hazard potential or ameliorating it. 

Here we report on our efforts to help assess the earthquake hazard along the Cascadia region of the Pacific northwest and determine, from spaceborne InSAR data, if SSE events are affecting the earthquake risk.  The challenge of successfully implementing InSAR over Cascadia is that the dense forest cover leads to significant InSAR decorrelation.  We use persistent scatterer (PS) point identification, interpolation of the sparse phase field, inverse solutions optimized for transient detection, and separation of the secular and transient signals, which when brought together with specific improvements for the local Cascadia environment are a new and untested effort that may allow for a superior assessment of the hazard potential of a massive earthquake in the Pacific northwest. This work is, to our knowledge, the first implementation of the type of hazard analysis from crustal deformation that has been shown in Hawaii and Mexico to an environment where nearly all of the surface is hidden beneath a very thick vegetation canopy.  If successful, the new approach could be applied in subduction zones around the world lacking in the kind of GPS coverage we have in this country. 

In our approach we produce spatially dense crustal deformation observations that, when used with the existing GPS and seismic data, can localize and assess the size of slow displacements on the subduction boundary. Our goal is to measure SSE crustal displacements in Cascadia, using InSAR with finer and more comprehensive coverage than the existing GPS network, and solve for a model of the slip at depth that helps us understand the potential for a large and destructive earthquake.  This combination of InSAR measurement and data inversion should help obtain accurate crustal deformation maps with wide ground coverage, separate the SSE signatures from the secular background motions, and interpret the solutions as slip at depth with sufficient localization to assess the increase or decrease to hazard potential from these events.

The Levantine fault system forms a transpressive zone in Lebanon, associating left-lateral faults and thrust faults, responsible for large historical earthquakes. It spreads over more than 1000km, from the Aqaba Gulf by the Red Sea to the East-Anatolian Fault in Turkey. It limits the Arabia tectonic plate eastward from the Sinaï plate westward.

We use Synthetic Aperture Radar Interferometry (InSAR) to quantify the surface displacements associated with active faults in Lebanon. Measuring interseismic deformation rate across the Lebanese fault system by InSAR is very challenging because of several limitations of InSAR measurements in this region. First of all, the slow strike-slip deformation rate on the Yammoûneh fault (about 5 mm/y) accounts for only 0.7 mm/y when projected along the satellite line of sight (LOS), due to the fault azimuthal direction almost perpendicular to the LOS. Moreover, Lebanon exhibits high topography variations (between 0 and 3 km in elevation) close to the Mediterranean Sea inducing very strong tropospheric delay variations on the interferograms. These tropospheric delay variations are in turn partly correlated to topography, as well as the expected vertical deformation. Furthermore, the interferometric coherence is low on a large part of the study zone. Lastly, the measurements interpretation is difficult due to the 3D complex structure of the fault system [Daëron et al., 2005].

We processed the complete Envisat ASAR archive of descending tracks 78 and 307. Each track covers Lebanon and neighboring Syria over 300 km in azimuthal direction and 100 km in range direction. On track 78, 38 images were acquired between 2003 and 2010 and 165 interferograms have been computed. 35 images were acquired between 2002 and 2010 on track 307 and 146 interferograms have been computed. The interferograms have been computed with the interferometric processing chain NSBAS [Doin et al., 2011]. In order to reduce the noise, we also used the MuLSAR (Multi-Link Interferograms) method [Pinel-Puysségur et al., 2012] to process the network of wrapped interferograms. Afterwards, the wrapped interferograms were corrected from stratified tropospheric delays estimated from global atmospheric reanalysis ERA Interim data from ECMWF [Doin et al., 2009, Jolivet et al., 2011]. Residual DEM errors were evaluated and compensated as in Ducret et al., (2014). After filtering and unwrapping, the time series was inverted following the Small Baseline Subsets approach [Berardino, 2002; Lopez-Quiroz et al., 2009; Jolivet et al., 2012].

The observed signal is principally vertical, due to the uplift of Mount Lebanon and Mount Hermon. The horizontal left component is visible south of Lebanon and north of Lebanon, across Ghab fault. Moreover, important patterns of subsidence due to water-pumping have been identified, from small scale patterns located principally in Lebanon and Israel to large-scale patterns in Syria.

We also evaluate the potential of the present-day database of Sentinel-1 to measure tectonic deformation along the Levantine fault system. A first series of interferograms from Sentinel-1 have been computed in our study zone.

References

Berardino, P; Fornaro, G; Lanari, R; et al., A new algorithm for surface deformation monitoring based on small baseline differential SAR interferograms, IEEE Trans. on Geosc. and Remote Sens., 40:11, p. 2375-2383, 2002

Daëron, M., Klinger, Y., Tapponnier, P., Elias, A., Jacques, E., Sursock, A., Sources of the large AD 1202 and 1759 Near East earthquakes, Geology, 33 (7), 529-532, 2005.

Doin, M.-P., Guillaso, S., Jolivet, R., Lasserre, C., Lodge, F., Ducret, G., Grandin, R., Presentation of the small-baseline NSBAS processing chain on a case example: the Etna deformation monitoring from 2003 to 2010 using ENVISAT data, Proceedings of the European Space Agency Symposium « Fringe », Frascati, Italy, 2011.

Doin, M.-P., C. Lasserre, G. Peltzer, O. Cavalié, C. Doubre, Corrections of stratified tropospheric delays in SAR interferometry : validation with global atmospheric models, J. of Applied Geophysics, 69, p35-50, doi:10.1016/j.jappgeo.2009.03.010, 2009

Ducret, G., M.-P. Doin, R. Grandin, C. Lasserre, S. Guillaso, DEM Corrections before Unwrapping in a Small Baseline Strategy for InSAR Time Series Analysis, IEEE Geoscience and Remote Sensing Letters, doi:10.1109/LGRS.2013.2276040, 2013.

Jolivet, R., C. Lasserre, M.-P. Doin, S. Guillaso, G. Peltzer, R. Dailu, J. Sun, Z.-K. Shen, and X. Xu, Shallow creep on the Haiyuan Fault (Gansu, China) revealed by SAR Interferometry, J. Geophys. Res., 117, B06401, doi:10.1029/2011JB008732, 2012.

Lopez Quiroz, P., M.-P. Doin, F. Tupin, P. Briole, J.-M. Nicolas, Time series analysis of Mexico city subsidence constrained by radar Interferometry, Journal of Applied Geophysics, 69 (1), 1-15, doi : 10.1016/j.jappgeo.2009.02.006, 2009

Pinel-Puysségur, B., R. Michel, J.-P. Avouac, Multi-Link InSAR time series: Enhancement of a wrapped interferometric database, IEEE Journal of Selected Topics in Applied Earth Observations and Remote Sensing, 5 (3):784-794, doi:10.1109/JSTARS.2012.2196758, 2012.

In the last few years impressive achievements have been made in improving inferences about earthquake sources. Several factors aided these developments. The open data basis of earthquake observations has expanded vastly with the two powerful Sentinel-1 sensors up in space. Computer power is continuously increasing enabling us to process enlarged sets of data for better and more detailed source models. Moreover, data inversion approaches for earthquake source inferences are becoming more and more advanced. By now data error propagation is widely implemented and the estimation of model uncertainties is a regular feature of reported optimum earthquake source models. Also, more regularly InSAR-derived surface displacements and seismological waveforms are combined, which requires finite rupture models instead of point-source approximations and layered medium models instead of homogeneous half-spaces. In other words the disciplinary differences in geodetic and seismological earthquake source modelling shrink towards common source-medium descriptions and a source near-field/far-field data point of view. With the work we present we explore and facilitate the combination of InSAR-derived near-field static surface displacement maps and dynamic far-field seismological waveform data for global earthquake source inferences.

We join in the community efforts with the particular goal to improve crustal earthquake source inferences in generally not well instrumented areas, where often only the global backbone observations of earthquakes are available provided by seismological broadband sensor networks and, since recently, by Sentinel-1 SAR acquisitions. In these general cases automated locations of earthquake hypocenters may be inaccurate and information on location and orientation of the causative faults highly uncertain such that fully non-linear and practically unconstrained source inferences are necessary. We present our work on modelling standards for the combination of static and dynamic surface displacements in the source's near-field and far-field, e.g. on data and prediction error estimations as well as model uncertainty estimation. The data combination is driven by estimations of the data error covariances in space and time. Rectangular dislocations and moment-tensor point sources are exchanged by simple planar finite rupture models. 1d-layered medium models are implemented for both near- and far-field data predictions. Non-linear source optimizations and Bayesian sampling of the model parameter space is carried out to provide quantified source model uncertainties estimations. A highlight of our approach is a weak dependence on earthquake bulletin information: hypocenter locations and source origin times are relatively free source model parameters. The near-field data do well constrain the source location and the higher frequencies of the far-field dynamic waveforms potentially constrain the rupture propagation from a variable nucleation point on the rupture plane.

We present this harmonized source modelling environment based on example earthquake studies, e.g. the 2010 Haiti earthquake, the 2009 L'Aquila earthquake and others. We discuss the benefit of combined-data non-linear modelling on the resolution of first-order rupture parameters, e.g. location, size, orientation, mechanism, moment/slip and rupture propagation.

The presented studies apply our newly developed software tools which build up on the open-source seismological software toolbox pyrocko (www.pyrocko.org) in the form of modules. We aim to facilitate a better exploitation of open global data sets for a wide community studying tectonics, but the tools are applicable also for a large range of regional to local earthquake studies. Our developments therefore ensure a large flexibility in the parametrization of medium models (e.g. 1d to 3d medium models), source models (e.g. explosion sources, full moment tensor sources, heterogeneous slip models, etc) and of the predicted data (e.g. (high-rate) GPS, strong motion, tilt).

This work is conducted within the project “Bridging Geodesy and Seismology” (www.bridges.uni-kiel.de) funded by the German Research Foundation DFG through an Emmy-Noether grant.

The 17 November 2015 onshore Lefkada earthquake (M6.5, Ganas et al., 2016) resulted in tens of centimetres of co-seismic motion in both Lefkada and Cephalonia islands (Ionian Sea, Greece). We present the full picture of the co-seismic deformation as mapped by space geodetic techniques (Sentinel 1A INSAR and permanent GPS stations). We use this data to invert for fault slip distribution model that matches well published slip models from seismology.

We also observed postseismic motions throughout most of southern Lefkada and northern Cephalonia islands using GPS data from six permanent and six non-permanent stations (established after the earthquake). The GPS data were processed with GIPSY software in PPP mode. Postseismic displacements ranged from a few centimetres near the epicentre to a few millimetres far from the fault. We modelled the postseismic displacements as due to uniform slip on the east-dipping fault that ruptured on 17 November 2015, with RMS of residuals near 3mm.

Our model shows a right-lateral afterslip along the seismic fault but with slightly larger dimensions in comparison to the co-seismic slip. Transient strain followed the Lefkada earthquake during a short period (80 days decay time when modelled as an exponential). 

Currently, the postseismic deformation is being investigated by exploiting multitemporal INSAR using Sentinel 1A/B acquisitions. The challenging issue is the minimization of the tropospheric noise that can be achieved by estimating the tropospheric delay from a WRF meteorological model with a spatial resolution of 1X1km.

The earthquakes occurred in the Central Ionian area since 1983, studied both by seismology and space geodesy imply a seismic gap offshore NW Cephalonia that needs to be monitored.

Different data types used in space-based geodesy (e.g., InSAR and GPS) and seismology (e.g., local, regional and teleseismic waveforms) provide complementary information about the earthquake’s source. Thus, simultaneous inversions for source mechanisms using these data sets are highly beneficial for accurate descriptions of source processes.

In routinely estimated earthquake source models, the event is often described by a single fault or point source. This simplifying assumption can be a considerable limitation since not only large earthquakes may rupture on various fault segments, but even small-moderate magnitude events can show complex faulting. In these cases, a single fault model oversimplifies the source process. This can be a serious issue, since reliable seismic hazard assessments, active tectonics and earthquake physics studies depend on accurate and robust earthquake source models.

We present a joint seismo-geodesy inversion method for the simultaneous determination of multiple fault source solutions. Our technique takes 3-D Earth structure effects fully into account when modelling seismic data and uses a Monte Carlo method to explore the model space.

We study the 21st February 2008, Mw 6.0 Wells earthquake in Nevada, USA using local seismic, teleseismic and InSAR data to obtain its source parameters and associated uncertainties. As a more recent example with an excellent data coverage of geodetic and seismic data, we also investigate the 16^th April 2016 Mw 7.0 Kumamoto (Japan) earthquake. The substantial non-double component of the moment tensor solution by the Japan Meteorological Agency indicates a composite rupture of multiple faults. The earthquake happened on a branching fault and is studied jointly with GPS, InSAR and seismological data sets.

A two-fault solution for the Wells event leads to an improvement in the data fit compared to a single fault source inversion and seems to match the geometry of the aftershocks well. The estimated source parameters are highly beneficial to explore earthquake physics, notably to constrain the earthquake's stress drop and energy budget.