Gareth Davies

Enhanced methane desorption characteristics from South Wales anthracites affected by tectonically induced fracture sets

Abstract It has generally been assumed that anthracite, despite containing potentially high levels of sorbed methane, is unsuitable for coalbed methane (CBM) production because of low levels of fracture permeability. South Wales anthracites are commonly strongly affected by tectonically developed fracture systems formed during Late Carboniferous Variscan compressional deformation. The fracture systems occur as thrust-related slip, slickenside and feather fractures, and are superimposed on previously formed extensional fracture systems that include cleat, conchoidal and bed-parallel fractures. When these fracture systems are intensely developed, the anthracite becomes incompetent and friable as a result of extremely close fracture spacing. According to current models for gas desorption and migration, methane initially diffuses through the micropore system of the coal matrix before flowing freely through larger pores and fracture systems. The models suggest that the more closely spaced the fractures, the more rapidly will the coal desorb methane. The tectonically fractured South Wales anthracites were investigated to determine the relationship between the rate of desorption of methane and tectonically induced fracture spacing. The former was established using gravimetric desorption isotherm experiments and in situ quantification of the latter was achieved by using a hand-drill penetrometer. The results demonstrate a clear relationship in anthracite between both the rate of methane desorbed and the amount of structural deformation. It is concluded that regions of deformed anthracite, previously considered unsuitable for CMB production, should be re-investigated.

Tsunami inundation from heterogeneous earthquake slip distributions: Evaluation of synthetic source models

This study investigates whether eight different synthetic finite fault models (SFFM) can simulate stochastic earthquake-tsunami with similar statistical properties to “real” earthquake-tsunami events, where the latter are represented using heterogeneous slip distributions from 66 Finite Fault Inversions (FFI) for oceanic subduction interface earthquakes. A new method is derived to estimate SFFM parameters from FFI, and predictive relations between the earthquake moment magnitude and the SFFM corner wave numbers are developed to support model applications. SFFM with more capacity to spatially localize slip are better able to simulate higher slip regions on the FFI, and this strongly influences their associated tsunami inundation, which is computed in two dimensions over idealized topography. The best performing SFFM generates tsunami inundation which envelopes the FFI inundation in 81% of cases using 10 synthetic events (close to the ideal value of 82%), while the other SFFM show greater tendencies to underestimate inundation. These differences are related to the capacity of each SFFM to produce spatially localized slip distributions. None of the SFFM showed a tendency to overpredict inundation. The results highlight that SFFM cannot be assumed to reliably quantify uncertainties in the tsunami inundation of real earthquakes, and the use of untested SFFM could create nonconservative bias in tsunami hazard assessments. However, the most successful model used here performs quite well, although it may still underestimate inundation more often than an optimal model.

A global probabilistic tsunami hazard assessment from earthquake sources

Abstract Large tsunamis occur infrequently but have the capacity to cause enormous numbers of casualties, damage to the built environment and critical infrastructure, and economic losses. A sound understanding of tsunami hazard is required to underpin management of these risks, and while tsunami hazard assessments are typically conducted at regional or local scales, globally consistent assessments are required to support international disaster risk reduction efforts, and can serve as a reference for local and regional studies. This study presents a global-scale probabilistic tsunami hazard assessment (PTHA), extending previous global-scale assessments based largely on scenario analysis. Only earthquake sources are considered, as they represent about 80% of the recorded damaging tsunami events. Globally extensive estimates of tsunami run-up height are derived at various exceedance rates, and the associated uncertainties are quantified. Epistemic uncertainties in the exceedance rates of large earthquakes often lead to large uncertainties in tsunami run-up. Deviations between modelled tsunami run-up and event observations are quantified, and found to be larger than suggested in previous studies. Accounting for these deviations in PTHA is important, as it leads to a pronounced increase in predicted tsunami run-up for a given exceedance rate.

Assessing tsunami hazard using heterogeneous slip models in the Mentawai Islands, Indonesia

Abstract Tsunami hazard maps are generated for the coastline of the Mentawai Islands, West Sumatra, Indonesia, to support evacuation and disaster response planning. A random heterogeneous slip generator is used to forward model a suite of earthquake rupture scenarios on the Mentawai Segment of the Sunda Subduction Zone. Up to 1000 rupture models that fit constraints provided by coral and geodetic records of coseismic vertical deformation from major earthquakes in 1797, 1833 and 2007 are used to model inundation and to define a maximum inundation zone that envelopes all of these scenarios. Comparison with single-scenario hazard assessments developed by experts and agreed through scientific consensus shows that there is value in modelling a suite of scenarios in order to obtain a more robust and conservative estimate of potential inundated areas. Although both the model presented here and the single-scenario models are based on assumptions about the characteristics of future events using knowledge of past events, by sampling a range of plausible outcomes we gain a more robust estimate of which areas may be inundated during a tsunami within the bounds of the assumptions applied.

Probabilistic Tsunami Hazard Analysis: Multiple Sources and Global Applications

Applying probabilistic methods to infrequent but devastating natural events is intrinsically nchallenging. For tsunami analyses, a suite of geophysical assessments should be in principle evaluated nbecause of the different causes generating tsunamis (earthquakes, landslides, volcanic activity, nmeteorological events, and asteroid impacts) with varying mean recurrence rates. Probabilistic Tsunami nHazard Analyses (PTHAs) are conducted in different areas of the world at global, regional, and local scales nwith the aim of understanding tsunami hazard to inform tsunami risk reduction activities. PTHAs enhance nknowledge of the potential tsunamigenic threat by estimating the probability of exceeding specific nlevels of tsunami intensity metrics (e.g., run-up or maximum inundation heights) within a certain period of ntime (exposure time) at given locations (target sites); these estimates can be summarized in hazard maps nor hazard curves. This discussion presents a broad overview of PTHA, including (i) sources and mechanisms nof tsunami generation, emphasizing the variety and complexity of the tsunami sources and their generation nmechanisms, (ii) developments in modeling the propagation and impact of tsunami waves, and (iii) nstatistical procedures for tsunami hazard estimates that include the associated epistemic and aleatoric nuncertainties. Key elements in understanding the potential tsunami hazard are discussed, in light of the nrapid development of PTHA methods during the last decade and the globally distributed applications, nincluding the importance of considering multiple sources, their relative intensities, probabilities of noccurrence, and uncertainties in an integrated and consistent probabilistic framework.

Modelling of historical tsunami in Eastern Indonesia: 1674 Ambon and 1992 Flores case studies

In order to reliably assess tsunami hazard in eastern Indonesia, we need to understand how historical events were generated. Here we consider two such events: the 1674 Ambon and the 1992 Flores tsunamis. Firstly, Ambon Island suffered a devastating earthquake that generated a tsunami with 100u2005m run-up height on the north coast of the island in 1674. However, there is no known active fault around the island capable of generating such a gigantic wave. Rumphius’ report describes that the initial wave was coming from three villages that collapsed immediately after the earthquake with width as far as a musket shot. Moreover, a very high tsunami was only observed locally. We suspect that a submarine landslide was the main cause of the gigantic tsunami on the north side of Ambon Island. Unfortunately, there is no data available to confirm if landslide have occurred in this region. Secondly, several tsunami source models for the 1992 Flores event have been suggested. However, the fault strike is quite different c...

A Framework for Modelling Shoreline Response to Clustered Storm Events: A Case Study from Southeast Australia

ABSTRACT Nichol, S.L.; McPherson, A., Davies, G., Jiang, W., Howard, F., Baldock, T., Callaghan, D., and Gravois, U., 2016. A framework for modelling shoreline response to clustered storm events: A case study from southeast Australia. In: Vila-Concejo, A.; Bruce, E.; Kennedy, D.M., and McCarroll, R.J. (eds.), Proceedings of the 14th International Coastal Symposium (Sydney, Australia). Journal of Coastal Research, Special Issue, No. 75, pp. 1197 - 1201. Coconut Creek (Florida), ISSN 0749-0208. An overview of a framework for modelling shoreline response to clustered storm events is presented for a case study area on the high energy coast of southeast Australia. We adopt the coastal sediment compartment as the functional management and modelling unit and use sub-surface information (ground-penetrating radar) to assess sediment thickness in the upper beach and foredune. Results for an actively eroding beach face at Old Bar Beach (New South Wales) indicate that sand cover is highly variable at the critical beach-dune interface, ranging from less than 1 m where bedrock occurs in shallow sub-crop to greater than 4 m across a former tidal inlet. The temporal distribution of storm events is examined through statistical modelling. For the duration of the data, modeled wave parameters are in good agreement with wave buoy observations. Event clustering does not appear to be stronger than is expected from events that occur randomly in time. Together, these data provide site-specific information necessary to inform shoreline response modeling to storms by establishing the requisite conditions describing the geomorphic setting and nearshore process regime.