Martin Vallée

Rupture process of the Mw = 7.9 2015 Gorkha earthquake (Nepal): Insights into Himalayan megathrust segmentation

We investigate the rupture process of the 25 April 2015 Gorkha earthquake (Mw = 7.9) using a kinematic joint inversion of teleseismic waves, strong motion data, high-rate GPS, static GPS, and synthetic aperture radar (SAR) data. The rupture is found to be simple in terms of coseismic slip and even more in terms of rupture velocity, as both inversion results and a complementing back projection analysis show that the main slip patch broke unilaterally at a steady velocity of 3.1–3.3 km/s. This feature likely contributes to the moderate peak ground acceleration (0.2 g) observed in Kathmandu. The ~15 km deep rupture occurs along the base of the coupled portion of the Main Himalayan Thrust and does not break the area ranging from Kathmandu to the front. The limitation in length and width of the rupture cannot be identified in the preearthquake interseismic coupling distribution and is therefore discussed in light of the structural architecture of the megathrust.

A detailed source model for the Mw9.0 Tohoku‐Oki earthquake reconciling geodesy, seismology, and tsunami records

The 11 March 2011 Mw9.0 Tohoku-Oki earthquake was recorded by an exceptionally large amount of diverse data offering a unique opportunity to investigate the details of this major megathrust rupture. Many studies have taken advantage of the very dense Japanese onland strong motion, broadband, and continuous GPS networks in this sense. But resolution tests and the variability in the proposed solutions have highlighted the difficulty to uniquely resolve the slip distribution from these networks, relatively distant from the source region, and with limited azimuthal coverage. In this context, we present a finite fault slip joint inversion including an extended amount of complementary data (teleseismic, strong motion, high-rate GPS, static GPS, seafloor geodesy, and tsunami records) in an attempt to reconcile them into a single better resolved model. The inversion reveals a patchy slip distribution with large slip (up to 64 m) mostly located updip of the hypocenter and near the trench. We observe that most slip is imaged in a region where almost no earthquake was recorded before the main shock and around which intense interplate seismicity is observed afterward. At a smaller scale, the largest slip pattern is imaged just updip of an important normal fault coseismically activated. This normal fault has been shown to be the mark of very low dynamic friction allowing extremely large slip to propagate up to the free surface. The spatial relationship between this normal fault and our slip distribution strengthens its key role in the rupture process of the Tohoku-Oki earthquake.

The 2010 Haiti earthquake: A complex fault pattern constrained by seismologic and tectonic observations

After the January 12, 2010, Haiti earthquake, we deployed a mainly offshore temporary network of seismologic stations around the damaged area. The distribution of the recorded aftershocks, together with morphotectonic observations and mainshock analysis, allow us to constrain a complex fault pattern in the area. Almost all of the aftershocks have a N‐S compressive mechanism, and not the expected left‐lateral strike‐slip mechanism. A first‐order slip model of the mainshock shows a N264°E north‐dipping plane, with a major left‐lateral component and a strong reverse component. As the aftershock distribution is sub‐parallel and close to the Enriquillo fault, we assume that although the cause of the catastrophe was not a rupture along the Enriquillo fault, this fault had an important role as a mechanical boundary. The azimuth of the focal planes of the aftershocks are parallel to the north‐dipping faults of the Transhaitian Belt, which suggests a triggering of failure on these discontinuities. In the western part, the aftershock distribution reflects the triggering of slip on similar faults, and/or, alternatively, of the south‐dipping faults, such the Trois‐Baies submarine fault. These observations are in agreement with a model of an oblique collision of an indenter of the oceanic crust of the Southern Peninsula and the sedimentary wedge of the Transhaitian Belt: the rupture occurred on a wrench fault at the rheologic boundary on top of the under‐thrusting rigid oceanic block, whereas the aftershocks were the result of the relaxation on the hanging wall along pre‐existing discontinuities in the frontal part of the Transhaitian Belt.

Rupture characteristics of the A.D. 1912 Mürefte (Ganos) earthquake segment of the North Anatolian fault (western Turkey)

Abstract: The Ganos fault is the westernmost segment of the North Anatolian Fault (NAF) that generated the 9 August 1912 Murefte (Ganos) earthquake (Mw=7.4). We study the 1912 earthquake characteristics using co-seismic fault slip and fault segmentation coupled with an analysis of historical seismic records. Surface ruptures with small releasing and restraining structures and 1.5 to 5.5 m right-lateral offsets have been measured at 45 sites of the onland ~45-km-long fault section. Similar structures are delineated by fresh fault scarps and prominent pull-apart basins in the Sea of Marmara and Saros Bay. A second shock with Mw=6.8 occurred on 13 September 1912 implying 20 to 40-km- long rupture; the damage distribution and analysis of seismic records suggest an epicenter located further west near Saros Bay, which indicates the location of western termination of the 9 August rupture. Our modeling of historical seismic records reveals a relative source time function between the two events and indicates a 40 second rupture duration, in agreement for a 120±30-km-long fault rupture for the 9 August shock. An estimated total rupture length of 150±30 km for the two earthquakes combined with onshore and offshore fault segmentation allow us to better constrain the western limit of the Marmara Sea seismic gap and related potential for producing a large earthquake that was sharply increased by the devastating 1999 Izmit earthquake in the east.

The Mw = 6.3, November 21, 2004, Les Saintes earthquake (Guadeloupe): Tectonic setting, slip model and static stress changes

On November 21, 2004, a magnitude 6.3 earthquake occurred offshore, 10 km south of Les Saintes archipelago in Guadeloupe (French West Indies). There were more than 30000 aftershocks recorded in the following two years, most of them at shallow depth near the islands of the archipelago. The main shock and its main aftershock of February 14, 2005 (Mw = 5.8) ruptured a NE-dipping normal fault (Roseau fault), mapped and identified as active from high-resolution bathymetric data a few years before. This fault belongs to an arc-parallel en echelon fault system that follows the inner edge of the northern part of the Lesser Antilles arc, accommodating the sinistral component of oblique convergence between the North American and Caribbean plates. The distribution of aftershocks and damage (destruction and landslides) are consistent with the main fault plane location and attitude. The slip model of the main shock, obtained by inverting jointly global broadband and local strong motion records, is characterized by two main slip zones located 5 to 10 km to the SE and NW of the hypocenter. The main shock is shown to have increased the Coulomb stress at the tips of the ruptured plane by more than 4 bars where most of the aftershocks occurred, implying that failures on fault system were mainly promoted by static stress changes. The earthquake also had an effect on volcanic activity since the Boiling Lake in Dominica drained twice, probably as a result of the extensional strain induced by the earthquake and its main aftershock.

Ten year recurrence time between two major earthquakes affecting the same fault segment

Earthquake ruptures stop when they encounter barriers impeding further propagation. These barriers can theoretically originate from changes of geometry or nature of the seismic faults or from a strong lowering of the tectonic stresses, typically due to the occurrence of a recent major earthquake. We show here that this latter mechanism can be ineffective at stopping rupture expansion: the 17 November 2013 magnitude 7.8 Scotia Sea earthquake has propagated into a 100 km long zone already ruptured 10 years ago by a magnitude 7.6 earthquake. Given the plate velocities between Scotia and Antarctic plates (8–9 mm/yr), simple recurrence models would have predicted that the segment affected by the 2003 earthquake could not be reruptured by a major earthquake during several hundreds of years. This earthquake pair indicates that the variations of the tectonic stress during the seismic history of the fault are small compared to the stresses dynamically generated by a large earthquake.

Observation of far-field Mach waves generated by the 2001 Kokoxili supershear earthquake

Regional surface wave observations offer a powerful tool for determining source properties of large earthquakes, especially rupture velocity. Supershear ruptures, being faster than surface wave phase velocities, create far-field surface wave Mach cones along which waves from all sections of the fault arrive simultaneously and, over a sufficiently narrow frequency band, in phase. We present the first observation of far-field Mach waves from the major Kokoxili earthquake (Tibet, 2001/11/14,Mw7.9) and confirm that ground motion amplitudes are indeed enhanced on the Mach cone. Theory predicts that on the Mach cone, bandpassed surface wave seismograms from a large supershear rupture will be identical to those from much smaller events with similar focal mechanisms, with an amplitude ratio equal to the ratio of the seismic moments of the two events. Cross-correlation of 15-25 s Love waves from the Kokoxili event with those from a much smaller (Mw5) foreshock indicates a high degree of similarity (correlation coefficients ranging from 0.8 to 0.95) in waveforms recorded at stations near the far-field Mach cone. This similarity vanishes away from the Mach cone. These observations provide further evidence for supershear propagation of the Kokoxili rupture, and demonstrate how this simple waveform correlation procedure can be used to identify supershear ruptures.

A Method for Rapid Determination of Moment Magnitude Mw for Moderate to Large Earthquakes from the Near-Field Spectra of Strong-Motion Records (MWSYNTH)

Seismic moment and the corresponding moment magnitude Mw are clas- sically obtained from the spectrum of far-field body waves. Near-field records are generally not used for that purpose, particularly in the case of large earthquakes be- cause different types of wave arrive simultaneously, preventing the definition of a simple relation between the seismic moment and the spectrum. We developed an orig- inal method to determine Mw from the displacement spectra of near-field records. The spectral amplitude at low frequency obtained from the real records is compared to that of synthetic records computed using kinematic rupture models scaled with Mw. Syn- thetic records are computed and averaged for various fault orientations and for epi- central distances ranging from 1 to 100 km. The initial portion of the spectrum affected by baseline shift in the acceleration records is automatically identified and removed by high-pass filtering using a cutoff frequency adapted to each station. The synthetic spectral values as a function of moment magnitude, epicentral distance, and filtering are computed only once and stored in tables. The spectral amplitudes of the real records are simply interpolated in the tables of synthetic data, allowing a fast determination of Mw. The method has been validated using 22 shallow earthquakes (depth < 50 km) with magnitude ranging from 3.9 to 7.7. We show that a window of 80 sec of signal after the earthquake origin time provides robust values of Mw for the whole magnitude range considered here. Shorter time windows may be used but with Mw underestimated for large events. The method is well suited for near real-time fast determination of Mw.

Observations and modeling of the elastogravity signals preceding direct seismic waves

Gravity gets into the earthquake game Earthquakes generate large movements of mass, which slightly change the gravitational field. Unlike the elastic waves that propagate from the earthquake, the gravity perturbations travel at the speed of light. Vallée et al. have finally observed these gravity perturbations in seismometer records from the great Tohoku earthquake in Japan in 2011. The signal would have allowed an accurate magnitude estimation in minutes, rather than hours, for this catastrophic earthquake. Science, this issue p. 1164 The observation of elastogravity signals provides a much faster method for determining the size of great earthquakes. After an earthquake, the earliest deformation signals are not expected to be carried by the fastest (P) elastic waves but by the speed-of-light changes of the gravitational field. However, these perturbations are weak and, so far, their detection has not been accurate enough to fully understand their origins and to use them for a highly valuable rapid estimate of the earthquake magnitude. We show that gravity perturbations are particularly well observed with broadband seismometers at distances between 1000 and 2000 kilometers from the source of the 2011, moment magnitude 9.1, Tohoku earthquake. We can accurately model them by a new formalism, taking into account both the gravity changes and the gravity-induced motion. These prompt elastogravity signals open the window for minute time-scale magnitude determination for great earthquakes.

Areas prone to slow slip events impede earthquake rupture propagation and promote afterslip

Frequent slow slip events and rapid postseismic slip reveal persistent aseismic fault areas delineating future seismic ruptures. At subduction zones, transient aseismic slip occurs either as afterslip following a large earthquake or as episodic slow slip events during the interseismic period. Afterslip and slow slip events are usually considered as distinct processes occurring on separate fault areas governed by different frictional properties. Continuous GPS (Global Positioning System) measurements following the 2016 Mw (moment magnitude) 7.8 Ecuador earthquake reveal that large and rapid afterslip developed at discrete areas of the megathrust that had previously hosted slow slip events. Regardless of whether they were locked or not before the earthquake, these areas appear to persistently release stress by aseismic slip throughout the earthquake cycle and outline the seismic rupture, an observation potentially leading to a better anticipation of future large earthquakes.