Emmy-Noether-Project "Bridging Geodesy and Seismology"

Publications

Repeated large-magnitude earthquakes in a tectonically active, low-strain continental interior: the northern Tien Shan, Kyrgyzstan

Landgraf A., Dzhumabaeva A., Abdrakhmatov K.E., Strecker M. R., Macaulay E. A., Arrowsmith J. R., Sudhaus H., Preusser F., Rugel G. and Merchel S., (2016).

Journal of geophysical research, DOI: 10.1002/2015JB012714

wiley online library

 

Abstract

The northern Tien Shan of Kyrgyzstan and Kazakhstan has been affected by a series of major earthquakes in the late 19th and early 20th centuries.

To assess the significance of such a pulse of strain release in a continental interior, it is important to analyze and quantify strain release over multiple time scales. We have undertaken paleoseismological investigations at two geomorphically distinct sites (Panfilovkoe and Rot Front) near the Kyrgyz capital Bishkek.

Although located near the historic epicenters, both sites were not affected by these earthquakes. Trenching was accompanied by dating stratigraphy and offset surfaces using luminescence, radiocarbon, and 10Be terrestrial cosmogenic nuclide methods.

At Rot Front, trenching of a small scarp did not reveal evidence for surface rupture during the last 5000 years. The scarp rather resembles an extensive debris-flow lobe.

At Panfilovkoe, we estimate a Late Pleistocene minimum slip rate of 0.2 ± 0.1 mm/a, averaged over at least two, probably three earthquake cycles. Dip-slip reverse motion along segmented, moderately steep faults resulted in hanging wall collapse scarps during different events.

The most recent earthquake occurred around 3.6 ± 1.3 kyr ago (1σ), with dip-slip offsets between 1.2 and 1.4 m. We calculate a probabilistic paleomagnitude to be between 6.7 and 7.2, which is in agreement with regional data from the Kyrgyz range.

The morphotectonic signals in the northern Tien Shan are a prime example of deformation in a tectonically active intracontinental mountain belt and as such can help understand the longer-term coevolution of topography and seismogenic processes in similar structural settings worldwide.

InSAR Sensitivity Analysis of Tandem-L Mission for Modeling Volcanic and Seismic Deformation Sources

Ansari, H., Goel K., Parizzi A., Sudhaus, H., Adam, N., and Eineder, M., (2015)

Proceedings of ESA (SP-371), 1-8,

electronic library

Abstract

We assess the reliability of space-borne InSAR-derived deformation source parameters for volcanic and seismic events, with special focus on the future L-band data of the proposed Tandem-L mission [1-3].

Using representative simulation cases, the influence of certain characteristics of the InSAR measurements on the source model parameter precision is quantified. The performance drivers are assessed from two aspects: the data acquisition geometry as well as the measurement noise; in particular governed by signal coherence and superposed atmospheric signal.

The significance of each these governing noise components is shown to be dependent on the spatial scale of the geophysical signal of interest as well as the deformation source mechanism in question.

Here, we estimate the error bounds for the inferred source parameters as a function of the signal coherence and atmospheric signal parameters.


 

Satellite radar data reveal short-term pre-explosive displacements and a complex conduit system at Volcán de Colima, Mexico

Salzer, J. T., Nikkhoo, M., Walter,T. R., Sudhaus, H., G. Reyes-Dávila, H. G., Bretón, M. and R. Arambula-Mendoza (2014).

Frontiers in Earth Sciences, doi: 10.3389/feart.2014.00012

Frontiers journal


Abstract

The geometry of the volcanic conduit is a main parameter controlling the dynamics and the style of volcanic eruptions and their precursors, but also one of the main unknowns. Pre-eruptive signals that originate in the upper conduit region include seismicity and deformation of different types and scales.

However, the locality of the source of these signals and thus the conduit geometry often remain unconstrained at steep sloped and explosive volcanoes due to the sparse instrumental coverage in the summit region and difficult access.Here we infer the shallow conduit system geometry of Volcán de Colima, Mexico, based on ground displacements detected in high resolution satellite radar data up to 7 h prior to an explosion in January 2013.

We use Boundary Element Method modeling to reproduce the data synthetically and constrain the parameters of the deformation source, in combination with an analysis of photographs of the summit.

We favor a two-source model, indicative of distinct regions of pressurization at very shallow levels. The horizontal location of the upper pressurization source coincides with that of post-explosive extrusion. The pattern and degree of deformation reverses again during the eruption; we therefore attribute the displacements to transient (elastic) pre-explosive pressurization of the conduit system.

Our results highlight the geometrical complexity of shallow conduit systems at explosive volcanoes and its effect on the distribution of pre-eruptive deformation signals. An apparent absence of such signals at many explosive volcanoes may relate to its small temporal and spatial extent, partly controlled by upper conduit structures.

Modern satellite radar instruments allow observations at high spatial and temporal resolution that may be the key for detecting and improving our understanding of the generation of precursors at explosive volcanoes.

Strain partitioning at the eastern Pamir-Alai revealed through SAR data analysis of the 2008 Nura earthquake

Teshebaeva, K., Sudhaus, H., Echtler, H., Schurr, B. and S. Roessner (2014)

Geophysical Journal International, 198(2), 760-774 ,  doi: 10.1093/gji/ggu158


Oxford journals


Abstract

On 2008 October 5, a magnitude 6.6 earthquake struck the eastern termination of the intermontane Alai valley between the southern Tien Shan and the northern Pamir of Kyrgyzstan.

The shallow thrust earthquake occurred in the footwall of the Main Pamir thrust, where the Pamir orogen is colliding with the southern Tien Shan mountains. We measure the coseismic surface displacements using SAR (Synthetic Aperture RADAR) data; the results show clear gradients in the vertical and horizontal directions along a complex pattern of surface ruptures and active faults.

To integrate and to interpret these observations in the context of the regional tectonics, we complement the SAR data analysis with seismological data and geological field observations.

While the main moment release of the Nura earthquake appears to be on the Pamir Frontal thrust, the main surface displacements and surface rupture occurred in the footwall along the NE–SW striking Irkeshtam fault.

With InSAR data from ascending and descending tracks along with pixel offset measurements, we model the Nura earthquake source as a segmented rupture. One fault segment corresponds to high-angle brittle faulting at the Pamir Frontal thrust and two more fault segments show moderate-angle and low-friction thrusting at the Irkeshtam fault.

Our integrated analysis of the coseismic deformation argues for rupture segmentation and strain partitioning associated to the earthquake. It possibly activated an orogenic wedge in the easternmost segment of the Pamir-Alai collision zone. Further, the style of the segmentation may be associated with the presence of Palaeogene evaporites.





 

An active ring fault detected at Tendürek volcano by using InSAR

Bathke, H., Sudhaus, H., Holohan, E.P., Walter T.R. and Shirzaei, M., (2013).

Journal of Geophysical Research, 118(8), 4488-4502.  DOI: 10.1002/jgrb.50305

Wiley online library

Abstract

Although ring faults are present at many ancient, deeply eroded volcanoes, they have been detected at only very few modern volcanic centers. At the so far little studied Tendürek volcano in eastern Turkey, we generated an ascending and a descending InSAR time series of its surface displacement field for the period from 2003 to 2010.

We detected a large (~105 km2) region that underwent subsidence at the rate of ~1 cm/yr during this period. Source modeling results show that the observed signal fits best to simulations of a near-horizontal contracting sill located at around 4.5 km below the volcano summit.

Intriguingly, the residual displacement velocity field contains a steep gradient that systematically follows a system of arcuate fractures visible on the volcano's midflanks.

RapidEye satellite optical images show that this fracture system has deflected Holocene lava flows, thus indicating its presence for at least several millennia. We interpret the arcuate fracture system as the surface expression of an inherited ring fault that has been slowly reactivated during the detected recent subsidence.

These results show that volcano ring faults may not only slip rapidly during eruptive or intrusive phases, but also slowly during dormant phases.

Reliability of Coulomb Stress Changes inferred from correlated uncertainties of finite-fault source models

Woessner J., S. Jónsson S., Sudhaus H. and Baumann C., (2012).

Journal of Geophysical Research,  doi:10.1029/2011JB009121.

Wiley online library

Abstract

Static stress transfer is one physical mechanism to explain triggered seismicity. Coseismic stress-change calculations strongly depend on the parameterization of the causative finite-fault source model.

These models are uncertain due to uncertainties in input data, model assumptions, and modeling procedures. However, fault model uncertainties have usually been ignored in stress-triggering studies and have not been propagated to assess the reliability of Coulomb failure stress change (ΔCFS) calculations.

We show how these uncertainties can be used to provide confidence intervals for co-seismic ΔCFS-values. We demonstrate this for the MW = 5.9 June 2000 Kleifarvatn earthquake in southwest Iceland and systematically map these uncertainties. A set of 2500 candidate source models from the full posterior fault-parameter distribution was used to compute 2500 ΔCFS maps.

We assess the reliability of the ΔCFS-values from the coefficient of variation (CV) and deem ΔCFS-values to be reliable where they are at least twice as large as the standard deviation (CV ≤ 0.5). Unreliable ΔCFS-values are found near the causative fault and between lobes of positive and negative stress change, where a small change in fault strike causes ΔCFS-values to change sign.

The most reliable ΔCFS-values are found away from the source fault in the middle of positive and negative ΔCFS-lobes, a likely general pattern. Using the reliability criterion, our results support the static stress-triggering hypothesis. Nevertheless, our analysis also suggests that results from previous stress-triggering studies not considering source model uncertainties may have lead to a biased interpretation of the importance of static stress-triggering.

Source model for the 1997 Zirkuh earthquake (MW=7.2) in Iran derived from JERS and ERS InSAR observations

Sudhaus, H. and S. Jónsson (2011).

Geophysical Journal International, 185(2), 676-692.

DOI: 10.1111/j.1365-246X.2011.04973.x

Wiley online library

Abstract

We present the first detailed source model of the 1997 M7.2 Zirkuh earthquake that ruptured the entire Abiz fault in East Iran producing a 125 km long, bended and segmented fault trace.

Using SAR data from the ERS and JERS-1 satellites we first determined a multisegment fault model for this predominately strike-slip earthquake by estimating fault-segment dip, slip, and rake values using an evolutionary optimization algorithm.

We then inverted the InSAR data for variable slip and rake in more detail along the multisegment fault plane. We complement our optimization with importance sampling of the model parameter space to ensure that the derived optimum model has a high likelihood, to detect correlations or trade-offs between model parameters, and to image the model resolution.

Our results are in an agreement with field observations showing that this predominantly strike-slip earthquake had a clear change in style of faulting along its rupture. In the north we find that thrust faulting on a westerly dipping fault is accompanied with the strike-slip that changes to thrust faulting on an eastward dipping fault plane in the south. The centre part of the fault is vertical and has almost pure dextral strike-slip.

The heterogeneous fault slip distribution shows two regions of low slip near significant fault step-overs of the Abiz fault and therefore these fault complexities appear to reduce the fault slip. Furthermore, shallow fault slip is generally reduced with respect to slip at depth.

This shallow slip deficit varies along the Zirkuh fault from a small deficit in the North to a much larger deficit along the central part of the fault, a variation that is possibly related to different interseismic repose times.

Improved source modeling through combined use of InSAR and GPS under consideration of correlated data errors: Application to the June 2000 Kleifarvatn earthquake Iceland

Sudhaus, H. and S. Jónsson (2009)

Geophysical Journal International, 176(2), 389-404.

DOI: 10.1111/j.1365-246X.2008.03989.x

Oxford journals

Abstract

Simultaneous use of multiple independent data sets can improve constraints on earthquake source-model parameters. However, the ways in which data sets have been combined in the past are manifold and usually qualitative.

In this paper we present a method to combine geodetic data in source model estimations, which includes characterizing the data errors and estimating realistic model-parameter uncertainties caused by these errors.

We demonstrate this method in a case study of the June 2000 Kleifarvatn earthquake, which occurred on Reykjanes Peninsula in Iceland. We begin by showing to what extent additional data can positively influence the source modelling results, by combining both GPS and descending-orbit InSAR data, which were used in two earlier studies of that event, with InSAR data from an ascending orbit.

We estimate the data error covariances of the InSAR observations and base the data weights in our model-parameter optimization on the corresponding data variance–covariance matrix. We also derive multiple sets of synthetic data errors from the estimated data covariances that we use to modify the original data to generate numerous data realizations.

From these data realizations we estimate the model-parameter uncertainties. We first model the Kleifarvatn earthquake as a simple uniform-slip fault and subsequently as a fault with variable slip and rake.

Our fault model matches well with the field observations of coseismic surface ruptures and its near-vertical dip (83°) agrees with the regional faulting style as well as with aftershock locations. The two published source models of the event, on the other hand, both differ from our model as well as differing for one another.

These studies, which were based on the descending InSAR data alone (the first study) and on that same data and GPS data (the second study), both neglect correlations in the InSAR data and do not report model-parameter uncertainties. Therefore, to compare these results with our model, we simulate the earlier model estimation set-ups and provide realistic estimates of the model-parameters uncertainties for these cases.

We then discuss the significance of the difference between the existing fault models and demonstrate that both the inclusion of additional independent data as well as the covariance-based data weights improve the model-parameter estimation.

Broadband frequency-dependent amplification of seismic waves across Bucharest, Romania

Sudhaus, H. and J. R. R. Ritter (2009)

 Journal of Seismology, 13(4), 479 – 497.   DOI: 10.1007/s10950-008-9140-0


Springer Link

Abstract

The determination of seismic amplitude amplification is a fundamental contribution to seismic hazard assessment.

While often only high-frequency amplitude variations (>1 Hz) are taken into account, we analyse broadband waveforms from 0.14 to 8.6 Hz using a temporary network of 32 stations in and around the earthquake-prone city of Bucharest.

Spectral amplitudes are calculated with an adaptive multiple-taper approach. Across our network (aperture 25 km × 25 km), we find a systematic northwest/southeast-oriented structural influence on teleseismic P-wave amplitudes from 0.14 to 0.86 Hz that can be explained by constructive interference in the dipping Cenozoic sedimentary layers.

For higher frequencies (1.4–8.75 Hz), more local site effects prevail and can be correlated partly among neighbouring stations. The transition between systematic and localised amplitude variations occurs at about 1 Hz.




 

Characterization of small local noise sources with array seismology

Ritter, J. R. R. and H. Sudhaus (2007)

Near Surface Geophysics 5(4): 253-261.

DOI: 10.3997/1873-0604.2007007

earthdoc.org
 

Abstract:

Seismic noise is often regarded as a stochastic, incoherent and unwanted signal. However, at a closer look, noise also contains coherent signals which can be used to characterize its sources and/ or the subsurface.

Here, we propose the use of array seismological methods for engineering purposes to locate and identify small, local noise sources. A detailed knowledge of noise sources enables appropriate measures to be taken to mitigate unwanted noise effects.

As a case study, we report results from a temporary array of ten seismic broadband stations which were installed at the building site of a high-precision physics laboratory.

Standard analyses of power spectra and amplitude measurements indicate that the highest noise amplitudes are caused by heavy trucks passing by. Burst-like coherent waves, excited by vehicles, appear to have a recurring pattern. Using array techniques, we were able to measure the slowness and backazimuth of the coherent arrivals.

These could be identified as Rayleigh waves generated at small pot-holes and bumps of a near-by road and were found to propagate with a speed of 130–200 m/s.

High-resolution measurement of the seismic attenuation across the Vrancea region, Romania

Sudhaus, H. and J. R. R. Ritter (2005)

Geophysical Research Letters 32(10). DOI: 10.1029/2004GL022148

Wiley online library

Abstract

We use teleseismic waveforms from a seismic refraction experiment along a N-S profile across the Vrancea region, Romania, to study seismic attenuation.

This unique data set with an average station spacing of about 2–3 km provides spatial high-resolution estimates of the variability in seismic attenuation. Time windows of the analyzed seismic phases are extracted with a multiple-taper technique and spectral ratios (δt*) are determined.

Our results from 4 teleseismic phases (P, pP, PKPdf, and PP) of two events underneath Hokkaido, Japan and New Guinea find a significant variation of the seismic attenuation along the line.

A major influence of local site effects on the values of δt* can be excluded as well as a correlation with the Vrancea slab. The variations of the δt* operators from different phases at individual stations seem to be caused by the projection of three-dimensional attenuating structures at deep lithospheric levels.

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