South Africa / World’s top seismologists to lead conference at Cape Town
PRETORIA, South Africa, December 29, 2008/African Press Organization (APO)/ — The Council for Geoscience will host the IASPEI 2009 General Assembly which will be held at the CTICC from 10 – 16 January 2009. This is the first General Assembly to be held in Africa and is endorsed by the Department of Minerals and Energy (DME), Geological Society of South Africa (GSSA), and the South Africa Geophysical Association (SAGA). The CGS recognizes that this meeting is an ideal opportunity for the South African geoscience community to share their expertise within the international arena.
”We are so proud to have been selected as the venue for the 2009 Assembly of the International Association of Seismology and Physics of the Earth’s Interior (IASPEI),” said Michelle Grobbelaar, head of Seismology at South Africa’s Council for Geoscience and main co-ordinator of the event. “We hope that the delegates will come away from IASPEI remembering not just the conference itself, but the wonderful time they have had here in South Africa.”
An extensive programme will address topics ranging from Mining Seismology to Earthquake Prediction, with Tsunami Early-Warning high on the agenda. Since the tsunami destruction in Indonesia and other countries in December 2004, seismological sensors have been installed — and national early warning systems implemented — in countries around the Indian Ocean. South Africa has played a leading role in this process. The further extension and refinement of this Indian Ocean Tsunami Early Warning System is a top priority of the conference. Many countries around the Indian Ocean live with the threat of devastating tsunamis on a daily basis and every advance is a chance to save more lives when the next tsunami strikes.
More than 400 scientists from 60 countries are expected at the conference in January, with Japan, China and Russia joining South Africa in leading the way. With its significant seismological history, its rich geological heritage, and superb Cape summer weather, Cape Town is the perfect place to host the event.
Cape Town bears the distant scars of South Africa’s largest earthquake. In 1969, a tremor of 6.3 on the Richter scale, centred on the Ceres/Tulbagh area, shook residents of the Western Cape, resulting in nine deaths and extensive damage to infrastructure. Cape Town is also the location of South Africa’s first recorded earthquake in 1620 when the captain of a ship anchored in Table Bay recorded shaking of the earth on Robben Island. In addition, South Africa’s well-known and extensive gold mining provokes a multitude of earthquakes every year, carefully monitored by the Council for Geoscience.
The objectives of the International Association of Seismology and Physics of the Earth’s Interior (IASPEI) are:
To promote the study of problems relating to earthquakes, to the propagation of seismic waves, and to the internal structure, properties and processes of the Earth;
To initiate and co-ordinate the conduct of researches which depend on co- operation between different countries, and to provide for their scientific discussion;
To facilitate particular researches on observational, engineering and applied seismology, such as the comparison of instruments used in different countries, researches on seismicity, man-made seismic disturbances and generally all matters to which seismology is related.
The following are potential benefits for hosting the conference:
Since this meeting attracts the top seismologists throughout the world, it will stimulate interest in South Africa’s seismological and geophysical research programmes. The country will also have an opportunity to showcase its research capabilities in geophysics and seismology.
South African geophysicists, especially young scientists, will be given an opportunity to interact with their counterparts from other countries.
The conference will also provide opportunities for South African scientists from a number of organisations to explore collaborative projects in the fields of earthquake and mine seismology and geophysics.
The Council for Geoscience is a government entity established in terms of the Geoscience Act, Act No. 100 of 1993, and reports to the Department of Minerals and Energy. The CGS is a market leader in the delivery of expert earth science solutions, and has an acclaimed track record of over 100 years both locally and globally. The CGS prides itself on having a winning team of specialist scientists within the various fields of geology.
The Seismology Unit studies the regional seismicity of Southern Africa and provides information and assessments on the seismic hazard in the subcontinent to engineering houses, insurance companies and state organisations. It operates the South African National Seismological Network (SANSN) and the installation and operation of seismic networks to provide these assessments.
The unit, the hub for southern Africa, operates a network of stations throughout South Africa as well as networks in Malawi, Mozambique, Namibia, and Rwanda. In addition, it operates seismic and infrasound stations for the Comprehensive Test Ban Treaty Organisation (CTBTO). Additional seismograph stations provide near real time information for the Indian Ocean Tsunami Warning System (IOTWS).
The unit’s studies go beyond the measurement and recording of seismic events. It advises engineering firms on designs – specifically, the impact of seismic events upon them – and evaluates building codes and regulations in light of seismic activity. It monitors and evaluates explosions and their impact. Members of the unit also conduct studies of the crust and upper mantle of the South Africa lithosphere, and of the African Superplume.
To accomplish all of this, the unit is equipped with the latest in seismological equipment. It has also developed its own in-house, cost-effective data recorder: the Event Acquisition Recorder System (EARS).
The unit encourages the study of seismology and science in general by giving presentations at schools, allowing children hands-on simulations and offering tours of stations and facilities to school groups. It holds on workshops throughout the year to promote training of its staff and that of seismologists, analysts and technicians.
Presentation
Earthquake Forecasting and Prediction: Progress in Model Development and Evaluation
Thomas H. Jordan
Director, Southern California Earthquake Center (SCEC), University of Southern California,
Los Angeles, 90089-0742 USA,
Forecasting and prediction are the central problems of earthquake system science. This presentation will summarize recent research, which has produced a new generation of time-dependent forecasting models and a new global infrastructure for evaluating prediction experiments. Earthquake forecasts estimate the probability that N fault ruptures with magnitudes ≥ M will occur in a geographic region R during the time interval of length T beginning at t0 ≥ tnpw. Earthquake predictions are alarms derived from forecasts or other information, such as precursory signals, and therefore involve tradeoffs between false alarms (type-I errors) and failures-to-predict (type-II errors).
Long-term earthquake rupture forecasts (T ~ decades to centuries) are the basis for probabilistic seismic hazard analysis. The simplest is a time-independent ERF, a Poissonian model independent of t0 consistent with the long-term occurrence rates in R. The Working Group on California Earthquake Probabilities (2007), in cooperation with the U.S. National Seismic Hazard Mapping Project, has recently developed a new time-independent model for California, and built on top of it a time-dependent Uniform California Earthquake Rupture Forecast. In UCERF, the event probabilities are conditioned on the dates of previous earthquakes using stress-renewal models and calibrated for variations in the earthquake cycle using historical and paleoseismic observations. A second type of time-dependent ERF, appropriate for short-term forecasting (T ~ hours to weeks), conditions the probabilities using seismic-triggering models calibrated to account for observed aftershock activity, such as epidemic-type aftershock sequence models. In California, the Short-Term Earthquake Probability (STEP) model of Gerstenberger et al. (2005) has been turned into an operational forecast that is updated hourly. Unifying the two types of time-dependent models requires a better understanding of medium-term predictability (T ~ weeks to years). This unification is not straightforward, because long-term models based on stress renewal are less clustered than simple Poissonian models, whereas short-term models based on seismic triggering are more clustered. Current research is thus focused on gaining insights into the physical processes that control stress changes and evolution on intermediate time scales.
The SCEC-USGS Working Group on Regional Earthquake Likelihood Models (Field et al., 2007; Schorlemmer et al., 2007) has been prospectively testing of a variety of medium-term forecasts (T = 5 yr, M = 5) in California. Based on this experience, an international partnership has been formed that is extending earthquake prediction experiments to other fault systems in a variety of tectonic environments through a global infrastructure for comparative testing, the Collaboratory for the Study of Earthquake Predictability. CSEP testing regions have been established in California, New Zealand, Japan, Italy, and the western Pacific. An open-source software system has been developed to automate the blind-testing of forecasting and prediction experiments using both likelihood and alarm-based scoring methods. Experiments currently being evaluated at the CSEP testing centers include seismicity-based forecasts updated at 1-day, 3-mo, and 1-yr intervals, as well as time-independent 5-yr forecasts of the RELM type. I will describe how the CSEP infrastructure is promoting rigorous research on earthquake predictability and the challenges of extending this infrastructure to include a wider range of prediction hypotheses.
Presentation
Earthquake dynamics: from source to radiation
Raul Madariaga (1), Sara Di Carli(1) and Sophie Peyrat(2)
(1) Laboratoire de Geologie CNRS-Ecole Normale Supérieure,
(2) Institut de Physique du Globe, Paris, France
The most successful earthquake radiation model, Brune’s circular crack, was developed on the basis of a combination of simple geometrical arguments and the amplitude of seismic waves generated by a prescribed instantaneous stress drop. Later this model was shown to model also the radiation from a circular crack running at finite speed and stopping abruptly. Under closer scrutiny, Brune’s model actually specifies a particular partition of available strain energy into fracture energy and radiated energy. Seismic efficiency, defined as the ratio between radiated energy and the part of strain energy that can be used for radiation, can be computed for different models of radiation from circular cracks. Efficiency is close to 50 % for Brune’s model independently of the size of the earthquake. In order to study energy balance from near field data, we have recently developed a non-linear dynamic inversion method for low frequency weak and strong motion records using the Neighbourhood algorithm of Sambridge and colleagues. We look for source models that have a simple elliptical geometry (one or more ellipses can be used). The forward problem is solved using a finite difference numerical simulation of the seismic rupture process for given distributions of initial stress and fracture resistance (Gc) . We applied the method to a couple of well-studied earthquakes in Japan and Chile showing that seismic waveforms are dominated by a combination of stress drop, energy release rate and the overall earthquake geometry. Only average values of these parameters can be derived from dynamic inversion as suggested by several previous studies. We demonstrate that the general properties of the models retrieved from inversion can be encapsulated by the kappa parameter that controls numerical seismic ruptures. This number derives from the ratio between the strain energy released by the earthquake that is available for radiation and the amount of energy that is required to make the rupture propagate. At low rupture speeds, this number converges to the Griffith criterion of fracture initiation. It is surprising that such a simple number may encapsulate much of the low frequency properties of earthquake radiation. We argue that the reason is that radiation from these simple models are controlled by stopping phases, that is most of the elastic energy radiated by these events comes from the border of the rupture zone, where the ratio of radiated to available strain energy (seismic efficiency) is at a maximum. A consequence of this observation is that energy release rate necessarily grows with earthquake size, so that it is not a property of the fault interface, but that it evolves with the growth of the rupture as suggested by Aki, Ohnaka and others.
SOURCE : The Council for Geoscience
