Challenges in seismic hazard assessment: Analyses of ground motion modelling and seismotectonic sources

Seismic hazard assessment has an important societal impact in describing levels of ground motions to be expected in a given region in the future. Challenges in seismic hazard assessment are closely associated with the fact that different regions, due to their differences in seismotectonics setting (and hence in earthquake occurrence) as well as socioeconomic conditions, require different and innovative approaches. One of the most important aspects in this regard is the seismicity level and the pre-existing knowledge about seismotectonics and fault behaviour in the region. The present thesis focuses on seismic hazard in three regions of very different tectonics in which different approaches for seismic hazard assessment were needed. In seismically active regions, standard probabilistic and deterministic approaches can be followed in assessing the hazard provided that the seismotectonic and geological information is available. In regions of low seismicity, this information is often incomplete and it may be necessary to start by studying in more detail the seismotectonic processes giving rise to the seismic hazard. The Marmara Sea and Sumatra regions are the main geographical areas where challenges in high seismicity areas are addressed. For addressing the seismic hazard assessment in low seismicity areas, the approach was to focus on the seismotectonic source characterization in various locations in Norway and adjacent areas. The Marmara Sea region is under a significant seismic hazard due to the short distance to the North Anatolian Fault which is believed to be close to rupture. This region is well studied in terms of tectonics and fault properties. However, the attenuation properties of the crust in the region have been uncertain. A new attenuation relation is established for the region, based on regressions performed on the background seismicity (paper 1). The obtained relation shows good agreement with previously used relations. Due to the increased level of knowledge about the active faults in the Marmara Sea, scenario based ground motion modelling provides a reliable estimate of the seismic hazard due to a future large earthquake. The predictive nature of such computations leads to uncertainties in the input parameters, the effect of which has not been well known previously. A study of the effect of varying input source and attenuation parameters (paper 2) shows that rise time, rupture velocity, stress drop and rupture initiation point are the most significant parameters in terms of ground motion level. The effect of parameters and the variability of ground motion are strongly frequency dependent. Another factor leading to uncertainties in simulated ground motion is that most simulations are performed at bedrock level without taking possible site amplifications into account. This latter problem is addressed in a separate study in the Ataköy area, SW Istanbul (paper 3), which shows that site amplification is significant over the whole area with amplification up to a factor of 2. The December 26, 2004 Sumatra-Andaman earthquake left many unanswered questions regarding the importance of ground shaking in the observed damage and, more generally, the nature of ground shaking caused by very large earthquakes. To address these issues, the event is modelled in terms of ground motion to see the effect of ground shaking in the regions near the fault rupture (paper 4). Results show that ground shaking was significant in northern Sumatra and the neighbouring islands and set bounds on the ground motion to be expected from such large events. The low seismicity in Norway and the surrounding areas makes it difficult to understand the relationship between the tectonics (active faults) and the earthquake activity. In order to improve this, three regions of significant seismic activity have been chosen for further seismotectonic investigations. The Jan Mayen region is, with its location on the mid-Atlantic ridge, the seismically most active region in Norway. Despite this fact, very little was previously known with respect to active fault structures. Locations of a M=6.0 earthquake and its aftershocks, combined with a detailed bathymetry, have provided new evidence about active tectonic structures in the region (paper 5). It is shown that major strike-slip earthquakes occur along the Koksneset fault, which seems to be the dominant structure in the Jan Mayen Fracture Zone. In addition, NE-SW oriented normal or oblique normal faults are being reactivated in the Jan Mayen Platform as a result of the deformation along the Koksneset fault. Deformation along the plate boundaries is significantly different from intraplate deformation. In this sense, the tectonic setting of Skagerrak situated in a basin within the Eurasian Plate is very different from Jan Mayen. This is reflected in the seismicity, which is much lower than for Jan Mayen but still high in comparison to other regions in Norway. Most earthquakes here have magnitudes less than 3, which in combination with the offshore location makes earthquake location challenging. Increased station coverage during the recent years has improved the location capabilities and the combination of relocated seismicity with reinterpreted seismic profiles and gravity and magnetic anomaly data has provided new clues about the origin of the Skagerrak seismicity (paper 6). A previously unknown graben structure, the Langust fault zone, is found at a location coinciding with the location of the local seismicity. This structure is believed to be the source of a large part of the Skagerrak earthquakes. In addition, activity seems to be present along the Sorgenfri-Tornquist Zone, as it is also the case further southeast in Kattegat. The Rana region in northern Norway is unique in the sense that several earthquake swarms have been registered here earlier. The installation of two temporary stations in this active region has provided high-quality recordings of events down to magnitude less than 0.5. In addition to providing new information about the seismotectonics in the region, these events have been used as ground truth in calibrating event detection based on waveform correlation (paper 7). In combination, the presented studies address some of the challenges associated with seismic hazard assessment, and can hopefully serve as a basis for further investigations in the future

The Making of the NEAM Tsunami Hazard Model 2018 (NEAMTHM18)

The NEAM Tsunami Hazard Model 2018 (NEAMTHM18) is a probabilistic hazard model for tsunamis generated by earthquakes. It covers the coastlines of the North-eastern Atlantic, the Mediterranean, and connected seas (NEAM). NEAMTHM18 was designed as a three-phase project. The first two phases were dedicated to the model development and hazard calculations, following a formalized decision-making process based on a multiple-expert protocol. The third phase was dedicated to documentation and dissemination. The hazard assessment workflow was structured in Steps and Levels. There are four Steps: Step-1) probabilistic earthquake model; Step-2) tsunami generation and modeling in deep water; Step-3) shoaling and inundation; Step-4) hazard aggregation and uncertainty quantification. Each Step includes a different number of Levels. Level-0 always describes the input data; the other Levels describe the intermediate results needed to proceed from one Step to another. Alternative datasets and models were considered in the implementation. The epistemic hazard uncertainty was quantified through an ensemble modeling technique accounting for alternative models’ weights and yielding a distribution of hazard curves represented by the mean and various percentiles. Hazard curves were calculated at 2,343 Points of Interest (POI) distributed at an average spacing of ∼20 km. Precalculated probability maps for five maximum inundation heights (MIH) and hazard intensity maps for five average return periods (ARP) were produced from hazard curves. In the entire NEAM Region, MIHs of several meters are rare but not impossible. Considering a 2% probability of exceedance in 50 years (ARP≈2,475 years), the POIs with MIH >5 m are fewer than 1% and are all in the Mediterranean on Libya, Egypt, Cyprus, and Greece coasts. In the North-East Atlantic, POIs with MIH >3 m are on the coasts of Mauritania and Gulf of Cadiz. Overall, 30% of the POIs have MIH >1 m. NEAMTHM18 results and documentation are available through the TSUMAPS-NEAM project website (http://www.tsumaps-neam.eu/), featuring an interactive web mapper. Although the NEAMTHM18 cannot substitute in-depth analyses at local scales, it represents the first action to start local and more detailed hazard and risk assessments and contributes to designing evacuation maps for tsunami early warning.publishedVersio

Towards the new Thematic Core Service Tsunami within the EPOS Research Infrastructure

Tsunamis constitute a significant hazard for European coastal populations, and the impact of tsunami events worldwide can extend well beyond the coastal regions directly affected. Understanding the complex mechanisms of tsunami generation, propagation, and inundation, as well as managing the tsunami risk, requires multidisciplinary research and infrastructures that cross national boundaries. Recent decades have seen both great advances in tsunami science and consolidation of the European tsunami research community. A recurring theme has been the need for a sustainable platform for coordinated tsunami community activities and a hub for tsunami services. Following about three years of preparation, in July 2021, the European tsunami community attained the status of Candidate Thematic Core Service (cTCS) within the European Plate Observing System (EPOS) Research Infrastructure. Within a transition period of three years, the Tsunami candidate TCS is anticipated to develop into a fully operational EPOS TCS. We here outline the path taken to reach this point, and the envisaged form of the future EPOS TCS Tsunami. Our cTCS is planned to be organised within four thematic pillars: (1) Support to Tsunami Service Providers, (2) Tsunami Data, (3) Numerical Models, and (4) Hazard and Risk Products. We outline how identified needs in tsunami science and tsunami risk mitigation will be addressed within this structure and how participation within EPOS will become an integration point for community development.publishedVersio

Towards the new Thematic Core Service Tsunami within the EPOS Research Infrastructure

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Reconnaissance report and preliminary ground motion simulation of the 12 May 2008 Wenchuan earthquake

The Mw = 8.0Wenchuan earthquake of May 12, 2008, caused destruction over a wide area. The earthquake cost more than 69,000 lives and the damage is reported to have left more than 5 million people homeless. It is estimated that 5.36 million buildings were destroyed and 21 million buildings were damaged in Sichuan and the nearby provinces. Economic losses due to the event are estimated to be 124 billion USD. From a field reconnaissance trip conducted in October 2008, it is evident that the combination of several factors, including mountainous landscape, strong ground shaking, extensive landslides and rock-falls, has exacerbated the human and economic consequences of this earthquake. Extensive damage occurred over a wide area due to the shear size of the earthquake rupture combined with poor quality building construction. In order to investigate the ground shaking during the earthquake, we have conducted a strong ground motion simulation study, applying a hybrid broadband frequency technique. The preliminary results show large spatial variation in the ground shaking, with the strongest ground motions along the fault plane. The simulation results have been calibrated against the recorded ground motion from several near-field stations in the area, and acceleration values of the order of 1 g are obtained, similar to what was recorded during the event. Comparison with the damage distribution observed in the field confirms that the effect of fault rupture complexity on the resulting ground motio

The seismotectonics of western Skagerrak

The seismically active Skagerrak region in the border area between Denmark and Norway has traditionally been associated with uncertain earthquake locations due to the limited station coverage in the region. A new seismic station in southern Norway and a recent update of the earthquake database of the Danish National Network have led to a much more complete and homogeneous data coverage of the Skagerrak area, giving the possibility of improved earthquake locations in the region. In this study, we relocate earthquakes in the Skagerrak area to obtain a more exact picture of the seismicity and investigate well-recorded events to determine the depth distribution. Hypocenter depths are found to be generally in the range 11–25 km. Furthermore, new composite focal mechanisms are determined for clusters of events with similar waveforms. Results indicate that the Skagerrak seismicity is associated with shallow, crustal faults oriented in the NS direction south of the Sorgenfrei–Tornquist Zone (STZ) as well as with the STZ itself. Mainly reverse faulting mechanisms along NE– SW oriented faults indicate maximum horizontal compression in the NW–SE direction. This is in agreement with World Stress Map generalizations, most likely associated with ridge push forces from the mid-Atlantic ridge, though modified probably by local crustal weaknesses

Improved seismic monitoring with OBS deployment in the Arctic: A pilot study from offshore western Svalbard

The mid‐ocean ridge system is the main source of earthquakes within the Arctic region. The earthquakes are recorded on the permanent land‐based stations in the region, although, smaller earthquakes remain undetected. In this study, we make use of three Ocean Bottom Seismographs (OBSs) that were deployed offshore western Svalbard, along the spreading ridges. The OBS arrival times were used to relocate the regional seismicity, using a Bayesian approach, which resulted in a significant improvement with tighter clustering around the spreading ridge. We also extended the regional magnitude scales for the northern Atlantic region for OBSs, by computing site correction terms. Besides location and magnitude improvement, the OBS network was able to detect hundreds of earthquakes, mostly with magnitude below Mw 3, including a swarm activity at the Molloy Deep. Our offshore observations provide further evidence of a low‐velocity anomaly offshore Svalbard, at the northern tip of Knipovich ridge that was previously seen in full‐waveform inversion. We conclude that even a single permanent OBS near the ridge would make a significant difference to earthquake catalogs and their interpretation

Improved seismic monitoring with obs deployment in the arctic: A pilot study from offshore western svalbard

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