R. J. Walters

The 2009 L’Aquila earthquake (central Italy): A source mechanism and implications for seismic hazard

An edited version of this paper was published by AGU. Copyright (2009) American Geophysical Union.

Spatial variations in fault friction related to lithology from rupture and afterslip of the 2014 South Napa, California, earthquake

Following earthquakes, faults are often observed to continue slipping aseismically. It has been proposed that this afterslip occurs on parts of the fault with rate-strengthening friction that are stressed by the main shock, but our understanding has been limited by a lack of immediate, high-resolution observations. Here we show that the behavior of afterslip following the 2014 South Napa earthquake in California varied over distances of only a few kilometers. This variability cannot be explained by coseismic stress changes alone. We present daily positions from continuous and survey GPS sites that we remeasured within 12 h of the main shock and surface displacements from the new Sentinel-1 radar mission. This unique geodetic data set constrains the distribution and evolution of coseismic and postseismic fault slip with exceptional resolution in space and time. We suggest that the observed heterogeneity in behavior is caused by lithological controls on the frictional properties of the fault plane.

Constraining crustal velocity fields with InSAR for Eastern Turkey: Limits to the block‐like behavior of Eastern Anatolia

The Sentinel-1 satellite mission will enable global strain rate mapping from interferometric synthetic aperture radar (InSAR) and GPS data, and methods to combine these data in velocity fields will become increasingly important. Here we use InSAR to measure interseismic deformation in Eastern Turkey, across a ∼250,000 km2 area that spans the Arabia-Eurasia plate boundary zone. From our InSAR data we first estimate slip rates and locking depths for the North and East Anatolian Faults (NAF and EAF) of 20 ± 3 mm/yr and ∼16 km and 11 ± 3 mm/yr and ∼16 km, respectively, but we also combine the InSAR data with existing GPS velocity measurements to construct high-resolution velocity and strain rate fields across the region for the first time. We calculate 2-D and 3-D velocity fields and find that strain is mainly localized across the NAF and EAF and that there is negligible differential vertical motion across the Eastern Anatolian plateau. We also show that high-resolution 2-D strain rate fields can be calculated from InSAR alone, even in the absence of GPS data. We fit a block model to our velocity field and estimate slip rates of ∼21 mm/yr and ∼8 mm/yr for the NAF and EAF, showing that our previous estimates differ from these values because they neglected crustal rotation. Although this rotation is an important component of the velocity field in Eastern Turkey, systematic residuals between our velocity field and the best fitting block for Anatolia suggest that the region is not block-like as proposed by previous authors.

The role of space-based observation in understanding and responding to active tectonics and earthquakes

The quantity and quality of satellite-geodetic measurements of tectonic deformation have increased dramatically over the past two decades improving our ability to observe active tectonic processes. We now routinely respond to earthquakes using satellites, mapping surface ruptures and estimating the distribution of slip on faults at depth for most continental earthquakes. Studies directly link earthquakes to their causative faults allowing us to calculate how resulting changes in crustal stress can influence future seismic hazard. This revolution in space-based observation is driving advances in models that can explain the time-dependent surface deformation and the long-term evolution of fault zones and tectonic landscapes.

Geodetic observations of postseismic creep in the decade after the 1999 Izmit earthquake, Turkey: Implications for a shallow slip deficit

The relationship between aseismic slip and tectonic loading is important for understanding both the pattern of strain accumulation along a fault and its ability to generate large earthquakes. We investigate the spatial distribution and temporal evolution of aseismic creep on the western North Anatolian Fault (NAF) using time series analysis of Envisat interferometric synthetic aperture radar (InSAR) data, covering the full extent of the 1999 Izmit and Duzce earthquake ruptures and spanning 2002–2010. Discontinuities in the line-of-sight velocity across the fault imply that fault creep reaches the Earths surface at an average fault-parallel rate of ∼5 mm/yr along an ∼80 km section of the NAF. By combining InSAR and published GPS velocities, we are able to extract the vertical and east-west components of motion and show that the Adapazari basin is subsiding at a rate of ∼6 mm/yr. Vertical motions have biased previous estimates of creep in this region. The displacement time series close to the fault is consistent with an afterslip model based on rate-and-state friction, which predicts a rapid deceleration in fault creep rate after the Izmit earthquake to a near-steady state ∼5 mm/yr after 5 years. Projecting our model 200 years into the future we find that the cumulative displacement of 1–1.3 m is insufficient to account for the shallow coseismic slip deficit observed in previous studies. Distributed off-fault deformation in the shallow crust or transient episodes of faster slip are likely required to release some of the long-term strain during the earthquake cycle.

The 2013 Mw 6.2 Khaki‐Shonbe (Iran) Earthquake: Insights into seismic and aseismic shortening of the Zagros sedimentary cover

Determining the relationship between folding and faulting in fold and thrust belts is important for understanding the growth of geological structures, the depth extent of seismic slip, and consequently, the potential earthquake hazard. The 2013 Mw 6.2 Khaki-Shonbe earthquake occurred in the Simply Folded Belt of the Zagros Mountains, Iran. We combine seismological solutions, aftershock relocations, satellite interferometry, and field observations to determine fault geometry and its relationship with the structure, stratigraphy, and tectonics of the central Zagros. We find reverse slip on two along-strike, southwest dipping fault segments. The main shock rupture initiated at the lower northern end of the larger northwest segment. Based upon the hypocenter and rupture duration, slip on the smaller southern segment is likely aseismic. Both faults verge away from the foreland, toward the high-range interior, contrary to the fault geometries depicted in many structural cross sections of the Zagros. The modeled slip occurred over two mutually exclusive depth ranges above 10 km, resulting in long (∼16 km), narrow rupture segments (∼7 km). Aftershocks cluster in the depth range 3–14 km. This indicates reverse slip and coseismic shortening occurred mostly or exclusively in the sedimentary cover, with slip distributions likely to be lithologically controlled in depth by the Hormuz salt at the base of the sedimentary cover (∼10–12 km), and the Kazhdumi Formation mudrocks at upper levels (∼4–5 km). Our findings suggest lithology plays a significant role in the depth extent of slip found in reverse faults in folded belts, providing an important control on the potential size of earthquakes.

Interseismic strain accumulation across the central North Anatolian Fault from iteratively unwrapped InSAR measurements

The North Anatolian Fault (NAF) is a major tectonic feature in the Middle East and is the most active fault in Turkey. The central portion of the NAF is a region of Global Navigation Satellite Systems (GNSS) scarcity. Previous studies of interseismic deformation have focused on the aseismic creep near the town of Ismetpasa using radar data acquired in a single line-of-sight direction, requiring several modeling assumptions. We have measured interseismic deformation across the NAF using both ascending and descending data from the Envisat satellite mission acquired between 2003 and 2010. Rather than rejecting incorrectly unwrapped areas in the interferograms, we develop a new iterative unwrapping procedure for small baseline interferometric synthetic aperture radar (InSAR) processing that expands the spatial coverage. Our method corrects unwrapping errors iteratively and increases the robustness of the unwrapping procedure. We remove long wavelength trends from the InSAR data using GNSS observations and deconvolve the InSAR velocities into fault-parallel motion. Profiles of fault-parallel velocity reveal a systematic eastward decrease in fault slip rate from 30 mm/yr (25–34, 95% confidence interval (CI)) to 21 mm/yr (14–27, 95% CI) over a distance of ∼200 km. Direct offset measurements across the fault reveal fault creep along a ∼130 km section of the central NAF, with an average creep rate of 8 ± 2 mm/yr and a maximum creep rate of 14 ± 2 mm/yr located ∼30 km east of Ismetpasa. As fault creep is releasing only 30–40% of the long-term strain in the shallow crust, the fault is still capable of producing large, damaging earthquakes in this region.

Near-field fault slip of the 2016 Vettore Mw 6.6 earthquake (Central Italy) measured using low-cost GNSS.

The temporal evolution of slip on surface ruptures during an earthquake is important for assessing fault displacement, defining seismic hazard and for predicting ground motion. However, measurements of near-field surface displacement at high temporal resolution are elusive. We present a novel record of near-field co-seismic displacement, measured with 1-second temporal resolution during the 30th October 2016 Mw 6.6 Vettore earthquake (Central Italy), using low-cost Global Navigation Satellite System (GNSS) receivers located in the footwall and hangingwall of the Mt. Vettore - Mt. Bove fault system, close to new surface ruptures. We observe a clear temporal and spatial link between our near-field record and InSAR, far-field GPS data, regional measurements from the Italian Strong Motion and National Seismic networks, and field measurements of surface ruptures. Comparison of these datasets illustrates that the observed surface ruptures are the propagation of slip from depth on a surface rupturing (i.e. capable) fault array, as a direct and immediate response to the 30th October earthquake. Large near-field displacement ceased within 6–8 seconds of the origin time, implying that shaking induced gravitational processes were not the primary driving mechanism. We demonstrate that low-cost GNSS is an accurate monitoring tool when installed as custom-made, short-baseline networks.

Partitioning of oblique convergence coupled to the fault locking behavior of fold-and-thrust belts : evidence from the Qilian Shan, northeastern Tibetan Plateau.

Oblique plate convergence is common, but it is not clear how the obliquity is achieved by continental fold-and-thrust belts. We address this problem in the Qilian Shan, northeastern Tibetan Plateau, using fieldwork observations, geomorphic analysis and elastic dislocation modeling of published geodetic data. A thrust dips SSW from the northern range front, and underlies steeper thrusts in the interior. Cenozoic thrust-related shortening across the Qilian Shan is ~155-175 km, based on two transects. Elastic dislocation modeling indicates that horizontal strain in the interseismic period is consistent with oblique slip on a single low angle detachment thrust below ~26 km depth, dipping SSW at ~17o. We suggest this detachment is located above North China Block crust, originally underthrust during Paleozoic orogeny. Horizontal shear strain is localized directly above the up-dip limit of creep on the detachment, and is coincident with the left-lateral Haiyuan Fault. This configuration implies oblique slip on the detachment below seismogenic depths is partitioned in the shallow crust onto separate strike-slip and thrust faults. This is consistent with strain partitioning in oceanic subduction zones, but has not previously been found by dislocation models of continental interiors. The marginal, strike-slip, Altyn Tagh Fault influences thrusting within the Qilian Shan for 100-200 km from the fault, but does not control the regional structure, where Paleozoic basement faults have been reactivated. The Qilian Shan resembles the main Tibetan Plateau in nascent form: active thrusts are marginal to an interior that is developing plateau characteristics, involving low relief, and low seismicity.

Normal faulting in the Simav graben of western Turkey reassessed with calibrated earthquake relocations

Western Turkey has a long history of large earthquakes, but the responsible faults are poorly characterized. Here we reassess the past half century of instrumental earthquakes in the Simav-Gediz region, starting with the 19 May 2011 Simav earthquake (Mw 5.9), which we image using interferometric synthetic aperture radar and regional and teleseismic waveforms. This event ruptured a steep, planar normal fault centered at 7–9 km depth but failed to break the surface. However, relocated main shock and aftershock hypocenters occurred beneath the main slip plane at 10–22 km depth, implying rupture initiation in areas of low coseismic slip. These calibrated modern earthquakes provide the impetus to relocate and reassess older instrumental events in the region. Aftershocks of the 1970 Gediz earthquake (Mw 7.1) form a narrow band, inconsistent with source models that invoke low-angle detachment faulting, and may include events triggered dynamically by the unilateral main shock rupture. Epicenters of the 1969 Demirci earthquakes (Mw 5.9, 6.0) are more consistent with slip on the south dipping Akdag fault than the larger, north dipping Simav fault. A counterintuitive aspect of recent seismicity across our study area is that the largest event (Mw 7.1) occurred in an area of slower extension and indistinct surface faulting, yet ruptured the surface, while recent earthquakes in the well-defined and more rapidly extending Simav graben are smaller (Mw <6.0) and failed to produce surface breaks. Though our study area bounds a major metamorphic core complex, there is no evidence for involvement of low-angle normal faulting in any of the recent large earthquakes.