Mathilde Vergnolle

Slow slip events and strain accumulation in the Guerrero gap, Mexico

Note: Best student presentation award Reference EPFL-TALK-183540 Record created on 2013-02-01, modified on 2016-08-09

Present-day kinematics and fault slip rates in eastern Iran, derived from 11 years of GPS data

We analyze new GPS data spanning 11 years at 92 stations in eastern Iran. We use these data to analyze the present-day kinematics and the slip rates on most seismogenic faults in eastern Iran. The east Lut, west Lut, Kuhbanan, Anar, Dehshir, and Doruneh faults are confirmed as the major faults and are found to currently slip laterally at 5.6 ± 0.6, 4.4 ± 0.4, 3.6 ± 1.3, 2.0 ± 0.7, 1.4 ± 0.9, and 1.3 ± 0.8 mm/yr, respectively. Slip is right-lateral on the ~NS striking east Lut, west Lut, Kuhbanan, Anar, and Dehshir faults and left-lateral on the ~EW Doruneh fault. The ~NS faults slice the eastern Iranian crust into five blocks that are moving northward at 6–13 mm/yr with respect to the stable Afghan crust at the eastern edge of the collision zone. The collective behavior of the ~NS faults might thus allow the Arabian promontory to impinge northward into the Eurasian crust. The ~NS faults achieve additional NS shortening by rotating counterclockwise in the horizontal plane, at current rates up to 0.8°/Ma. Modeling the GPS and available geological data with a block rotation model suggests that the rotations have been going on at a similar rate (1 ± 0.4°/Ma) over the last 12 Ma. We identify large strains at the tips of the rotating east Lut, west Lut, and Kuhbanan faults, which we suspect to be responsible for the important historical and instrumental seismicity in those zones.

Anticipating the Next Large Silent Earthquake in Mexico

Silent earthquakes, or slow slip events (SSEs), in subduction zones [Schwartz and Rokosky, 2007] release accumulated strain energy within tens of minutes to a few months, as opposed to a few seconds or minutes for “regular” earthquakes [Kostoglodov et al., 2003]. This phenomenon has important implications for the seismic cycle because SSEs significantly modify the loading-unloading budget of faults; their existence suggests that the buildup and relaxing mechanisms of the earthquake cycle are much more complex than previously thought. Numerous important questions have to be answered concerning SSEs, in particular, their specific location on the fault, the amount of slip at depth, and their recurrence. Depending on whether they occur on the seismogenic or creeping section of the fault, they may release some accumulated elastic strain or further load the brittle part of the fault, effectively lengthening or shortening the time before the next large regular earthquake. In that framework, assessing the repartition of the displacement on the subduction interface and the frequency of SSEs is of particular importance, because these parameters govern the extent to which SSEs may slow or accelerate the regular earthquake clock.

Two successive slow slip events evidenced in 2009–2010 by a dense GPS network in Guerrero, Mexico

A large slow slip event (SSE) had been expected for the Guerrero gap for 2010. It was actually observed with an onset in July 2009. Comparison with the preceding large SSEs, which occurred in 2002 and 2006, highlights both persistent characteristics of the Guerrero SSEs (e.g. the localization of slip in the seismogenic part of the subduction interface), and also particularities of the 2009/2010 event (namely two distinct slip patches on the fault interface moving consecutively). The long GPS time series and the density of the GPS network provide evidence that the Guerrero SSEs, like classical earthquakes, have complex features. Despite having very short and relatively regular repeat times (∼4 yr), Guerrero SSEs appear aperiodic. A shorter loading time before the 2009/2010 event than before the 2006 SSE seems to produce consistently reduced surface displacements for a group of stations in a core zone.

Fault Geometry and Slip Distribution at Depth of the 1997 Mw 7.2 Zirkuh Earthquake: Contribution of Near‐Field Displacement Data

In this study, we reestimate the source model of the 1997 Mw 7.2 Zirkuh earthquake (northeastern Iran) by jointly optimizing intermediate‐field Interferometry Synthetic Aperture Radar data and near‐field optical correlation data using a two‐step fault modeling procedure. First, we estimate the geometry of the multisegmented Abiz fault using a genetic algorithm. Then, we discretize the fault segments into subfaults and invert the data to image the slip distribution on the fault. Our joint‐data model, although similar to the Interferometry Synthetic Aperture Radar‐based model to the first order, highlights differences in the fault dip and slip distribution. Our preferred model is ∼80° west dipping in the northern part of the fault, ∼75° east dipping in the southern part and shows three disconnected high slip zones separated by low slip zones. The low slip zones are located where the Abiz fault shows geometric complexities and where the aftershocks are located. We interpret this rough slip distribution as three asperities separated by geometrical barriers that impede the rupture propagation. Finally, no shallow slip deficit is found for the overall rupture except on the central segment where it could be due to off‐fault deformation in quaternary deposits.

Ocean Loading in Brittany, Northwest France: Impact of the GPS Analysis Strategy

In this contribution, we analyze the impact of different GPS processing strategies on ocean tide loading estimation. We use continuous GPS data acquired during a 4-month campaign performed in 2004 in Brittany, Northwest France. Since the expected geodynamical signal in the estimated positions is exceeding the typical GPS data analysis noise, this data set can be used to compare the results obtained with different analysis software packages. Moreover, in this specific case we need short sub-daily solutions to study short-period signals instead of classical 24 h-solutions. The GPS capability for measuring 3D ocean tide loading deformation has already been assessed, but since we are looking for the finest signal as the one induced by the shallow water constituents, it is essential to be sure that the position time series represent a geodynamical signal and are not biased by the data processing strategy used. To analyze the possible effect of the methodology used on the geodynamical results, we compare different solutions computed with different strategies (Double Differencing and Precise Point Positioning) with various GPS analysis software packages (Bernese, GAMIT, GINS, and GIPSY/OASIS). We show that the different solution consistency is at the level of 1–3 mm. We also show that the data processing strategy has a mean effect of about 10–20% of the ocean tide loading signal amplitude.