C. Kyriakopoulos

The May 12, 2008, (Mw 7.9) Sichuan Earthquake (China): Multiframe ALOS-PALSAR DInSAR Analysis of Coseismic Deformation

A destructive (Mw 7.9) earthquake affected the Sichuan province (China) on May 12, 2008. The seismic event ruptured approximately 270 km of the Yingxiu-Beichuan fault and about 70 km of the Guanxian-Anxian fault. Surface effects were suffered over a wide epicentral area (about 300 km E-W and 250 km N-S). We apply the differential synthetic aperture radar interferometry (DInSAR) technique to detect and measure the surface displacement field, using a set of ALOS-PALSAR L-band SAR images. We combine an unprecedented high number of data (25 frames from six adjacent tracks) to encompass the entire area which has coseismically displaced. The resulting mosaic of differential interferograms covers an overall area of about 340 km E-W and 240 km N-S. We investigate the source of the Sichuan earthquake by modeling the DInSAR data. The geometry and position of the fault parameters are inferred by a nonlinear inversion, followed by a linear inversion to retrieve the relative slip distribution. Our results show two different source mechanisms for the 145-long Yingxiu-Beichuan fault and for the 105-long Beichuan-Qingchuan fault. Both faults are characterized by slip concentrations of up to 8 m.

A new seismically constrained subduction interface model for Central America

We provide a detailed, seismically defined three-dimensional model for the subducting plate interface along the Middle America Trench between Northern Nicaragua through to Southern Costa Rica. The model uses data from a weighted catalog of about 30,000 earthquake hypocenters compiled from nine catalogs to constrain the interface through a process we term the “Maximum Seismicity Method”. The method determines the average position of the largest cluster of microseismicity beneath an a priori functional surface above the interface. This technique is applied to all seismicity above 40 km depth, the approximate intersection of the hanging-wall Mohorovicic discontinuity, where seismicity likely lies along the plate interface. Below this depth, an envelope above 90% of seismicity approximates the slab surface. Because of station proximity to the interface, this model provides highest precision along the interface beneath the Nicoya Peninsula of Costa Rica, an area where marked geometric changes coincide with crustal transitions and topography observed seaward of the trench. The new interface is useful for a number of geophysical studies that aim to understand subduction zone earthquake behavior, geodynamic and tectonic development of convergent plate boundaries.

Did the September 2010 (Darfield) earthquake trigger the February 2011 (Christchurch) event

We have investigated the possible cause-and-effect relationship due to stress transfer between two earthquakes that occurred near Christchurch, New Zealand, in September 2010 and in February 2011. The Mw 7.1 Darfield (Canterbury) event took place along a previously unrecognized fault. The Mw 6.3 Christchurch earthquake, generated by a thrust fault, occurred approximately five months later, 6 km south-east of Christchurchs city center. We have first measured the surface displacement field to retrieve the geometries of the two seismic sources and the slip distribution. In order to assess whether the first earthquake increased the likelihood of occurrence of a second earthquake, we compute the Coulomb Failure Function (CFF). We find that the maximum CFF increase over the second fault plane is reached exactly around the hypocenter of the second earthquake. In this respect, we may conclude that the Darfield earthquake contributed to promote the rupture of the Christchurch fault.

Structural Asperity focusing locking and earthquake slip along the Nicoya megathrust, Costa Rica

On 5 September 2012, a moment magnitude 7.6 earthquake occurred along the locked megathrust interface directly beneath the Nicoya Peninsula in Costa Rica. The event was anticipated given the history of frequent large earthquakes and recent GPS results. Strain accumulation observed before and coseismic slip observed during the earthquake create a novel and important data set, offering a unique look at the strain and locking behavior around a large megathrust environment. Given this geodetic data, and a newly developed regional 3-D subduction interface, we develop a regionally appropriate finite element model to evaluate the role of interface topography in generating interseismic locking and ultimately coseismic slip following the 2012 earthquake. We find that the interface is most strongly coupled in a patch immediately beneath the central portion of Nicoya in an area where a topographic high in the downgoing Cocos Plate is resisting further subduction. This zone is the dominant slip environment in the 2012 earthquake. Reevaluation of the interseismic locking over 62 years before the most recent event finds that the total accumulated moment potential (M0 = 3.48 × 1020 N m) well matches the total moment release in the 2012 rupture found here (M0 = 3.73 × 1020 N m, Mw 7.68), with rupture extending to the SE outside of the strongly locked region, possibly as early afterslip. In contrast to a prior study, little locking is required immediately offshore Nicoya—a conclusion important for understanding the rupture area of the 2012 event, regional tsunami potential, and relation with regionally observed shallow slow-slip events.

The October 23, 2011, Van (Turkey) earthquake and its relationship with neighbouring structures

The present work reports the analysis of a possible relationship due to stress transfer between the two earthquakes that hit the province of Van, Eastern Turkey, on October 23, 2011 (Mw = 7.2) and on November 9, 2011 (Mw = 5.6). The surface displacement field of the mainshock has been obtained through a combined data set made up of differential interferograms from COSMO-SkyMed and ENVISAT satellites, integrated with continuous GPS recordings from the Turkish TUSAGA-AKTIF network. This allowed us to retrieve the geometry and the slip distribution of the seismic source and to compute the Coulomb Failure Function (CFF) variation on the aftershock plane, in order to assess a possible causal relationship between the two events. Our results show that the November 9 earthquake could have been triggered by the October 23 shock, with transferred stress values largely exceeding 1 bar.

Large and primarily updip afterslip following the 2012 Mw 7.6 Nicoya, Costa Rica, earthquake

We present detailed surface measurements of the first 3.5 years of postseismic deformation following the 5 September 2012 moment magnitude (Mw) 7.6 Nicoya, Costa Rica earthquake. The dominant signal in the first 2.5 years is uniform horizontal trenchward motion totaling 7-26 cm across 40 stations. Trenchward velocity is strongly diminished by mid 2014 and appears by 2016 to have begun reversing. We invert the first 2.5 years to determine the corresponding afterslip on a detailed 3D interface. Results show significant afterslip both up and down dip of the main coseismic rupture zone, with as much as 1.7 m of offset in two patches at 15-20 km depth and immediately up-dip of the maximum coseismic slip. This updip slip represents an important mechanism to address unrelieved interseismic locking, although sufficient strain energy remains to generate up to a Mw 7.1 event near the coastline. The afterslip patches are anticorrelated with strongly-clustered aftershocks at the same depth, which is indicative of varying frictional behavior along strike. An additional patch of slip is colocated with reoccurring slow slip events beneath the Gulf of Nicoya. The magnitude of the observed slip, however, cannot be sufficiently explained by the known slow-slip events. Ongoing measurements will be crucial to understanding the relocking process in Nicoya

The SIGRIS Project: A Remote Sensing System for Seismic Risk Management

SIGRIS (SIstema di osservazione spaziale per la Gestione del RIschio Sismico) is a pilot project aiming to the realization of a system, based on satellite remote sensing data, for the seismic risk management. The project is funded by the Italian Space Agency (ASI). ASI is deeply interested on the development of new applications, using satellite data, dedicated to the monitoring and management of the natural hazards. SIGRIS is focused on providing the information services for mapping, monitoring, forecasting and awareness of seismic risk. The Earth Observation products are generated by using GPS data, optical and SAR (Synthetic Aperture Radar) images. This project deals with the data exploitation of the new Italian Earth Observation mission: COSMO-SkyMed, a constellation of four satellites equipped with an X-band high resolution SAR.

Joint inversion of the 2011 Tohoku (Japan) earthquake from dinsar and GPS data

On March 11, 2011 a Mw 9.0 earthquake hit Honsu island in Japan. The so called “Tohoku-Oki” giant earthquake occurred near the northeast coast resulting from thrust faulting on or near the subduction zone plate boundary between the Pacific and North America plates. The rupture zone is roughly centered on the earthquake epicenter alongstrike, while peak slips were up-dip of the hypocenter, towards the Japan Trench axis. The rupture was also responsible of a big tsunami that struck a large portion of the coastal area of Tohoku-Oki region. Soon after the earthquake numerous space remote sensing sensors were tasked to acquire as much as possible information about the situation on ground. In particular, a large numbers of SAR images were collected from the ENVISAT and ERS-2 satellite. For this event, a group of researchers from the Istituto Nazionale di Geofisica e Vulcanologia (INGV), named Tohoku-Oki INGV Team, decided to apply a multidisciplinary approach to carry on some analysis addressed to achieve added value outcomes. In this work we present part of the work done by the Team concerning DInSAR data analysis and the methodology to retrieve the seismic source of this extraordinary and disastrous event.

DInSAR techniques for studying the October 23, 2011, Van earthquake (Turkey), and its relationship with neighboring structures

In October 2011 a strong earthquake hit the Van province, Eastern Turkey. Few days later (November 9th) an aftershock occurred few km southward. Finally in November 1976 another mainshock took place north of Van along the Caldiran fault. We have investigated the possible relations between 2011 mainshock and aftershock and the link with the 1976 earthquake. In order to complete the work SAR interferometry has been applied to measure surface displacements, while the fault geometries of the mainshock have been retrieved using a Simulated Annealing approach. Moreover the CFF has been calculated to evaluate the role of 1976 earthquake in promoting the 2011 mainshock and, later on, the role of this latter respect to the aftershock in November 9th, 2011.