Edwin Nissen

Slip in the 2010–2011 Canterbury earthquakes, New Zealand

The 3rd September 2010 Mw 7.1 Darfield and 21st February 2011 Mw 6.3 Christchurch (New Zealand) earthquakes occurred on previously unknown faults. We use InSAR ground displacements, SAR amplitude offsets, field mapping, aerial photographs, satellite optical imagery, a LiDAR DEM and teleseismic body-wave modeling to constrain the pattern of faulting in these earthquakes. The InSAR measurements reveal slip on multiple strike-slip segments and secondary reverse faults associated with the Darfield main shock. Fault orientations are consistent with those expected from the GPS-derived strain field. The InSAR line-of-sight displacement field indicates the main fault rupture is about 45 km long, and is confined largely to the upper 10 km of the crust. Slip on the individual fault segments of up to 8 m at 4 km depth indicate stress drops of 6–10 MPa. In each event, rupture initiated on a reverse fault segment, before continuing onto a strike-slip segment. The non-double couple seismological moment tensors for each event are matched well by the sum of double couple equivalent moment tensors for fault slip determined by InSAR. The slip distributions derived from InSAR observations of both the Darfield and Christchurch events show a 15-km-long gap in fault slip south-west of Christchurch, which may present a continuing seismic hazard if a further unknown fault structure of significant size should exist there.

Rapid mapping of ultrafine fault zone topography with structure from motion

Structure from Motion (SfM) generates high-resolution topography and coregistered texture (color) from an unstructured set of overlapping photographs taken from varying viewpoints, overcoming many of the cost, time, and logistical limitations of Light Detection and Ranging (LiDAR) and other topographic surveying methods. This paper provides the first investigation of SfM as a tool for mapping fault zone topography in areas of sparse or low-lying vegetation. First, we present a simple, affordable SfM workflow, based on an unmanned helium balloon or motorized glider, an inexpensive camera, and semiautomated software. Second, we illustrate the system at two sites on southern California faults covered by existing airborne or terrestrial LiDAR, enabling a comparative assessment of SfM topography resolution and precision. At the first site, an ∼0.1 km 2 alluvial fan on the San Andreas fault, a colored point cloud of density mostly >700 points/m 2 and a 3 cm digital elevation model (DEM) and orthophoto were produced from 233 photos collected ∼50 m above ground level. When a few global positioning system ground control points are incorporated, closest point vertical distances to the much sparser (∼4 points/m 2 ) airborne LiDAR point cloud are mostly 530 points/m 2 and a 2 cm DEM and orthophoto were produced from 450 photos taken from ∼60 m above ground level. Closest point vertical distances to existing terrestrial LiDAR data of comparable density are mostly

Reinterpretation of the active faulting in central Mongolia

We present remote-sensing and fi eld observations of an ~350-km-long east-west left-lateral strike-slip fault (the South Hangay fault) in the Hangay Mountains of central Mongolia, an area previously believed to be deforming solely by slip on scattered, and randomly oriented, normal faults. The known dip-slip faults are shown to be short segments introduced at bends in the much longer strike-slip fault. Our observations show that the active faulting in the Hangay Mountains is consistent with the regional strain fi eld of Mongolia and does not require, as suggested in other studies, that the faults result from stresses introduced by the locally elevated topography. Our observations help to defi ne the active tectonics of this important part of the India-Eurasia collision. The South Hangay strike-slip fault is a potential source of large-magnitude earthquakes and constitutes a previously unrecognized hazard in this part of Mongolia.

Zagros “phantom earthquakes” reassessed—The interplay of seismicity and deep salt flow in the Simply Folded Belt?

Unraveling the contributions of main shock slip, aftershocks, aseismic afterslip, and postseismic relaxation to the deformation observed in earthquake sequences heightens our understanding of crustal rheology, triggering phenomena, and seismic hazard. Here, we revisit two recent earthquakes in the Zagros mountains (Iran) which exhibited unusual and contentious aftereffects. The Mw ~6 earthquakes at Qeshm (2005) and Fin (2006) are both associated with large interferometric synthetic aperture radar (InSAR) signals, consistent with slip on steep reverse faults in carbonate rocks of the middle sedimentary cover, but small aftershocks detected with local seismic networks were concentrated at significantly greater depths. This discrepancy can be interpreted in one of two ways: either (1) there is a genuine vertical separation between main shock and aftershocks, reflecting a complex stress state near the basement-cover interface, or (2) the aftershocks delimit the main shock slip and the InSAR signals were caused by shallow, updip afterslip (phantom earthquakes) with very similar magnitudes, mechanisms, and geographical positions as the original earthquakes. Here, we show that main shock centroid depths obtained from body waveform modeling—which in this instance is the only method that can reveal for certain the depth at which seismic slip was centered—strongly support the first interpretation. At Qeshm, microseismic aftershock depths are centered at the level of the Hormuz Formation, an Infracambrian sequence of intercalated evaporitic and nonevaporitic sediments. These aftershocks may reflect the breaking up of harder Hormuz sediments and adjacent strata as the salt flows in response to main shock strain at the base of the cover. This work bolsters recent suggestions that most large earthquakes in the Zagros are contained within carbonate rocks in the midlower sedimentary cover and that the crystalline basement shortens mostly aseismically.

Optimization of legacy lidar data sets for measuring near-field earthquake displacements

Airborne lidar (light detection and ranging) topography, acquired before and after an earthquake, can provide an estimate of the coseismic surface displacement field by differencing the preevent and postevent lidar point clouds. However, estimated displacements can be contaminated by the presence of large systematic errors in either of the point clouds. We present three-dimensional displacements obtained by differencing airborne lidar point clouds collected before and after the El Mayor–Cucapah earthquake, a Mw 7.2 earthquake that occurred in 2010. The original surface displacement estimates contained large, periodic artifacts caused by systematic errors in the preevent lidar data. Reprocessing the preevent data, detailed herein, removed a majority of these systematic errors that were largely due to misalignment between the scanning mirror and the outgoing laser beam. The methodology presented can be applied to other legacy airborne laser scanning data sets in order to improve change estimates from temporally spaced lidar acquisitions.

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.

Preliminary estimate of Holocene slip rate on active normal faults bounding the southern coast of the Gulf of Evia, central Greece

We investigate the late Quaternary history of slip on the Kamena Vourla and Arkitsa normal faults, which are segments of a fault system bounding the south coast of the Gulf of Evia in central Greece, and which we refer to as the Coastal Fault System. We examine two river terraces, near the village of Molos, which are found within the uplifted footwall of the Kamena Vourla fault. The upper terrace is ~20 m above the present river level and appears to represent fan deposition into the main river channel from surrounding tributaries. The lower terrace, ~8 m above the present-day river bed, represents an interval of river-bed aggradation and correlates with the surface of a delta on the hanging-wall side of the fault. GPS profiles show a 6 ± 0.1 m vertical offset of the lower terrace surface as it crosses the fault. Preliminary dating of the two terrace levels, using both optical luminescence and radiocarbon methods, provides inconclusive results. The lower terrace, however, grades toward the present-day sea level and correlates with the surface of a delta on the hanging-wall side of the fault; it is, therefore, likely to date from ~6 ka, when sea level stabilized at its present-day highstand. With an age of ~6 ka, the 6 m vertical displacement of the lower terrace yields an estimate of ~1.2-2.0 mm/yr for the Holocene rate of slip across the Kamena Vourla fault. This rate of slip is comparable with an estimated rate of ~0.7-2.0 mm/yr for the central (Arkitsa) segment of the Coastal Fault System, and with a 0.4-1.6 mm/yr slip rate measured on the easternmost (Atalanti) segment. These estimates of Holocene slip rates are consistent with the 1-3 mm/yr of present-day extension across the Gulf of Evia measured by GPS, arguing against large changes in rate of extension through the Holocene. Both the Arkitsa and Kamena Vourla faults are clearly active and despite an absence of historical earthquakes on either fault, they should be considered to be a major hazard to local populations. However, further dating studies and palaeoseismic investigations are required before the slip rate and history can be fully quantified.

Change Detection Using Airborne LiDAR: Applications to Earthquakes

We present a method for determining 3-dimensional, local ground displacements caused by an earthquake. The technique requires pre- and post-earthquake point cloud datasets, such as those collected using airborne Light Detection and Ranging (Lidar). This problem is formulated as a point cloud registration problem in which the full point cloud is divided into smaller windows, for which the local displacement that best restores the post-earthquake point cloud onto its pre-earthquake equivalent must be found. We investigate how to identify the size of window to be considered for registration. We then present an information theoretic approach that classifies whether a region contains an earthquake fault. These methods are first validated on simulated earthquake datasets, for which the input displacement field is known, and then tested on a real earthquake. We show results and error analyses for a variety of different window sizes, as well as results for our fault detection algorithm.

The Egiin Davaa prehistoric rupture, central Mongolia: a large magnitude normal faulting earthquake on a reactivated fault with little cumulative slip located in a slowly deforming intraplate setting

Abstract The prehistoric Egiin Davaa earthquake rupture is well-preserved in late Quaternary deposits within the Hangay Mountains of central Mongolia. The rupture is expressed by a semi-continuous 80 km-long topographic scarp. Geomorphological reconstructions reveal a relatively constant scarp height of 4–4.5 m and a NW-directed slip vector. Previous researchers have suggested that the scarps exceptional geomorphological preservation indicates that it may correspond to an earthquake that occurred in the region c. 500 years ago. However, we constrain the last rupture to have been at least 4 ka ago from morphological dating and <7.4 ka ago based on radiocarbon dating from one of two palaeoseismic trenches. Our study shows that discrete earthquake ruptures, along with details such as the locations of partially infilled fissures, can be preserved for periods well in excess of 1000 years in the interior of Asia, providing an archive of fault movements that can be directly read from the Earths surface over a timescale appropriate for the study of slowly deforming continental interiors. The Egiin Davaa rupture involved c. 8 m of slip which, along with the observations that it is largely unsegmented along its length and that the ratio of cumulative slip (c. 250 m) to fault length (c. 80 km) is small, suggests relatively recent reactivation of a pre-existing geological structure. Supplementary material: All scarp profiles are available at http://www.geolsoc.org.uk/SUP18871

3D change detection using low cost aerial imagery

We present a method to register point clouds obtained from aerial images through Structure from motion (SFM) techniques with data from airborne LiDAR systems. The data was obtained by the United States Geological Survey (USGS) over a 800 sq km stretch in California using airborne LiDAR. The images were obtained by a downward looking camera on an autonomous helicopter along the San Andreas fault [9]. A 3D point cloud is built by fusing GPS information with the aerial images. Our approach to detect changes is to compare the LiDAR data with 3D point cloud derived from aerial images. This comparison necessitates the two point clouds to be in the same co-ordinate frame. We adopt a registration approach to bring the point clouds to the same co-ordinate frame. We highlight the challenges involved in registering aerial point clouds and propose a semi automated way for registration. We also present a simulation of a change detection scenario by introducing displacement fields in the source point cloud and obtaining a target point cloud by additionally simulating the GPS offsets. We recover the displacement vectors in two steps (1) globally registering the source and target point clouds using the method described in this paper (2) using our change detection module [5] for computing the displacement fields. We present results for global registration and change detection.