Applications: Earthquakes and Tectonics (1)

The 2014 South Napa earthquake occurred in and around the city of Napa, California on August 24 at 3:20 a.m. local time, measuring at 6.0 on the moment magnitude scale. The tremor's epicenter was located south of Napa, approximately 6.0 km northwest of American Canyon near the West Napa Fault. The earthquake was the largest in the San Francisco Bay Area since the 1989 Loma Prieta earthquake. Significant damage and several fires were reported in the southern Napa Valley area.

In this work, we investigate the co-seismic surface deformation related to this earthquake. To this aim, we exploit both the phase and the amplitude information of SAR data acquired in Stripmap mode by the Italian COSMO-SkyMed (CSK), the Canadian RADARSAT-2 (RS2), and the recently launched European Sentinel-1 satellites, to evaluate and analyze the induced surface displacements through the Differential SAR Interferometry (DInSAR) and Pixel-Offset (PO) techniques. In particular, the SAR images, acquired from descending orbits on 26 July and 27 August  2014 by CSK, and on 07 August and 31 August 2014 by Sentinel-1, as well as the ones acquired on 24 July and 10 September by RS2 from ascending passes were used to generate differential SAR interferograms encompassing the main seismic events. The related deformation map, obtained by performing a complex multi-look operation resulting in a pixel size of about 30 m by 30 m, reveals two main lobes of LOS displacement with a range change decrease of about 11 cm to the NE sector and about 7 cm  of range change increase to the SE sector. Moreover, by benefiting from the sensor spatial resolutions (down to 3 meters for both CSK and Sentinel-1 satellites), the Pixel-Offset maps of the same data pairs have been also computed, thus permitting us to retrieve displacement information along the azimuth direction and better describing the deformation field.

In order to retrieve the earthquake source location and its geometrical characteristics, the displacement maps were modeled by finite dislocation faults in an elastic and homogeneous half-space [Okada, 1985]. In particular, we searched for all the free parameters of the fault by using a nonlinear inversion based on the Levenberg-Marquardt least-squares approach. The best fit solution, consists of a right –lateral NNW-SSE oriented fault. The comparison between the model results and the measured InSAR data show a good fit, with residue values smaller than 2 cm. However, small zones far from the epicenter area, with higher residues are individuated.

We also show first results of post-seismic effect by exploiting the high temporal sampling of CSK SAR data, which is currently acquiring new data every 4 days. In particular, we applied the SBAS technique [Berardino et al., 2002] to generate displacement time series and mean velocity maps of the area.

References

Berardino, P., Fornaro, G., Lanari, R., Sansosti, E., 2002, A new Algorithm for Surface Deformation Monitoring based on Small Baseline Differential SAR Interferograms, IEEE Trans. Geo. Rem. Sens., 40, 11, 2375-2383.

Okada, Y. (1992). Internal deformation due to shear and tensile faults in a half-space. Bulletin of the Seismological Society of America, 82, 1018–1040.

The neotectonic regime of South America is dominated by a 6000 km-long subduction zone to the active western margin of the continent. Volcanic processes and crustal deformation in the Andean orogen are driven by the eastward subduction of the oceanic Nazca plate underneath the South American plate with a current convergence rate of about 65 mm/yr. This plate boundary is characterized by frequent megathrust events with a short recurrence time, such as the M_w=9.5 Valdivia earthquake in 1960 or the recent M_w=8.8 Maule earthquake (2010). Almost all segments along this highly active plate boundary broke within the last decades, generating large subduction earthquakes. The only segment that has not ruptured since 1877, but is capable of generating a M_w=9+ earthquake is known as Northern Chile-Southern Peru seismic gap. With an estimated recurrence interval of about 111 ± 30 years and recent earthquakes in the two adjoining segments to the South (Antofagasta 1995, M_w=8.1) and to the North (Arequipa 2001, M_w=8.3), this gap between 18°-22°S latitude was probably the most mature seismic gap along the South American plate boundary.However, on 1 April 2014, a magnitude 8.1 earthquake ocurred in the central-northern part of this gap close to the city of Iquique. An aftershock with a magnitude 7.6 followed two days later. The relatively low magnitude of the main event indicates that the seismic gap was only partially closed. A detailed study of these earthquakes will therefore allow to better understand the regional rheology and to assess the seimic hazard remaining in this region.

In this study, we present co- and postseismic ground displacements for the Pisagua Earthquake 2014 observed by InSAR and GPS. Our InSAR observations are derived from the TerraSAR-X and Radarsat-2 satellite missions. For TerraSAR-X, we processed three ascending datasets, one from three years before the earthquake and two from just one day and 79 days after the event. This dataset allows us to separate deformation caused by the main event and the aftershock two days later. The Radarsat-2 data was acquired in descending mode, four days before and 19 and 44 days after the earthquake and in ascending mode, three and 76 days after the event. The resulting interferometric pairs enable us to quantify the co- and post-seismic ground displacement in the line-of-sight (LOS) direction of the satellites. Moreover, the combination of the descending and ascending mode of the Radarsat-2 SAR imagery reveals the horizontal and vertical component of the ground displacement. The co-seismic interferograms show a circular deformation pattern centered at the North Chilean coast near Iquique, with ground displacement up to 45 mm in LOS. In the post-seismic interferograms the displacement is much smaller and the deformation pattern is somewhat less symmetric. Additionally, we present continuous GPS time series provided by the Integrated Plate Boundary Observatory Chile (IPOC) network in Northern Chile covering more than seven years (from 2007 to 2014). The campaign GPS data were measured immediately after the event. However, the pre-event data was acquired between a few days only to a few years before the earthquake. Using a subsampled dataset of the high-resolution InSAR data we jointly invert the co- and postseismic observations acquired by InSAR and GPS data. In our modelling approach we use principle component analysis and the decomposition of displacement vectors. The Green´s functions of dislocations are supposed to be calculated using elastic half-space approach.

Using this modelling approach we present preliminary slip respectively afterslip models of the Pisagua earthquake. Thisstudywill not only help to betterunderstand the co- and postseismic deformation patterns of the event, but also resolve and separate the displacement linked to the main shock and the aftershock. The Northern Chile seismic gap brokeonly partly in April 2014, which leads to a high level of observational attention at this segment. Our analysis contributes to the ongoing research in this region of increased seismic hazard potential.

ScanSAR interferometry offers the unique capability of large-scale coverage and frequent acquisitions. It has proven to be critical for measuring coseismic deformation after great earthquakes. Interseismic strain applications along the San Andreas Fault system need accurate measurement on the order of one millimeter per year.  Achieving this high accuracy will require the satellite to maintain small spatial baselines (<~300 m) and accurate orbits (< 20 cm) and to have multiple look directions.  Using GMTSAR we have successfully produced strip-mode interferograms associated with the M6 South Napa earthquake in California from Sentinel-1 and a forest area in the Amazon from ALOS-2. Based on this limited processing we believe that both systems will meet the performance requirements for our interseismic application.  There are, however, technical challenges associated with ScanSAR interferometry. For example, ScanSAR interferometry requires precise burst alignment for ALOS-2 and precise image alignment for Sentinel-1 (TOPS). Moreover, a consistent geometric model is essential for automated processing and mosaicking of subswaths.  We will discuss these issues at the meeting as more ScanSAR data become available from these two satellites.

Two damaging earthquakes struck California in 2014. The Los Angeles area was shaken by an Mw 5.1 earthquake on 29 March 2014 at 04:10 UTC with an epicenter in La Habra. Damage was concentrated in a zone south of the epicenter. The Italian Space Agency’s (ASI) was acquiring COSMO-SkyMed™ (CSK) synthetic aperture radar (SAR) images of the La Habra area from June 2013 through October 2014 with one look direction (ascending). The Canadian Space Agency (CSA) and partner MDA have been acquiring RADARSAT-2 (RS2) SAR images over the Los Angeles area for several years, including the La Habra area. TerraSAR-X (TSX) images have also been acquired over the La Habra area by DLR and their commercial partner Airbus Defense and Space. We used individual interferograms from the RS2 and TSX (descending tracks) satellites plus time series analysis of the CSK data stack (ascending) to analyze the coseismic deformation from the La Habra earthquake. We ran SBAS-type time series analysis with GIAnT using a step function at the time of the earthquake to estimate the coseismic deformation separating other effects better than a single interferogram. The combined CSK time series fit plus RS2 and TSX interferograms enabled a finite fault model for the slip distribution of the M 5.1 earthquake. We are also studying the InSAR time series from CSK, TSX, ERS and Envisat data acquired before the La Habra earthquake.

The Mw 6.0 South Napa earthquake shook the San Francisco Bay area at 10:21 UTC on 24 August 2014, causing extensive damage in and around the city of Napa. There was ASI CSK coverage of the Napa area with two different look directions (ascending and descending) from June 2013 through October 2014. The CSA and MDA acquisitions over the San Francisco area mostly cover the area to the south, but partly cover the Napa area. We use GIAnT to extend the time series analysis of the CSK data on the two tracks across the time of the earthquake and calculate a better estimate of the coseismic deformation as a step function in the time interval between the date of the last pre-quake scene and first post-quake scene, plus a postseismic deformation time function. The Napa earthquake has an unusually large amount of shallow afterslip on the main rupture, mostly in the southern part that had less coseismic slip. The CSK time series and GPS data at nearby stations show that this postseismic deformation was largely completed by the end of October 2014. 

We present results that make use of the fortuitous early acquisition of image mode data by Sentinel-1A over northern California in the weeks prior to the 24th August 2014 Mw 6.0 South Napa earthquake. In addition to presenting a coseismic model augmented with other InSAR datasets, we capitalise on the short and regular repeat intervals of Sentinel-1 acquisitions in the months following the earthquake to measure the amount, extent and nature of postseismic motion and near-surface slip. We further constrain the time series of these deformation models with near-field GPS which measured both the coseismic and decay of postseismic offsets resulting from the earthquake. To date, it has been a challenge to observe the spatial pattern and temporal evolution of post-earthquake processes including decaying after-slip and visco-elastic relaxation processes, and the distinction between them. Persistent or induced surface creep has also been observed along many major faults in California (e.g. central San Andreas Fault, Hayward Fault and Rodgers Creek Fault) but no evidence existed of such a phenomenon on the West Napa Fault system, on a branch of which this earthquake occurred. The South Napa earthquake provides the first opportunity to employ the regular 12-day repeat cycle of Sentinel-1 SAR acquisition to address these phenomena. The InSAR data show several unexpected features: (1) unequivocal evidence of a sustained displacement discontinuity across the co-seismic fault plane in the post-earthquake period, indicating after-slip on the fault plane; (2) off-fault surface displacements, suggesting either a complex, multi-plane co-seismic rupture or induced slip on secondary faults; and (3) extension of surface displacements to the north and possible south of the limits of co-seismic rupture. Augmented by near-field daily GPS observations, we model the data using each SAR acquisition interval to study the temporal evolution of after-slip. Models show that co-seismic slip is north and upwards of the reported hypocentre, with a maximum slip of the order 2-3 metres concentrated in an area between 4-10 km depth. Conversely, the post-earthquake interferograms suggest after-slip occurs in two places: (1) to the south of the region of maximum co-seismic displacement at approximately the same depth, where the majority of aftershocks occurred; and (2) in the shallow region from 3 km depth to the surface along the entire length of the plane experiencing co- and post-seismic slip at depth. The earthquake was followed by a weak aftershock sequence, with the largest aftershock being only Mw 3.9, that may be related to the observed after-slip. This has implications for the total moment release of earthquakes in northern California and beyond and may provide insights into the frictional properties and evolution of faults displaying such time-dependent behaviour.

During the round table, seed questions proposed by the chairs will be discussed with the audience.