Abstract: The Seismic Research Centre of the University of the West Indies, Trinidad and Tobago, operates a network of about 70 seismic stations in the eastern Caribbean islands from St. Kitts to Trinidad. Since 2008 the Centre has embarked on an initiative to expand and modernize this network by furnishing the stations with various combinations of broadband and/or strong motion sensors, high dynamic range digitizers and networking equipment to link each station to centralized observatories via high speed digital data transmission medium. After accomplishing about a half the task, the centre has come to the realization that it has neither the fiscal nor the technical capability to complete the full upgrade and provide maintenance to a network totally equipped with commercial instrumentation in a sustainable manner. The primary inhibiting factors are 1) insular geographical layout of the network, giving rise to several access and communications related challenges and 2) the high cost of procuring and maintaining commercial instrumentation. The Centre has devised a sustainable solution to complete the upgrade. We have developed a low-cost digitizer that makes heavy use of open-source hardware and software tools. The hardware is based on an Arduino Mega microcontroller that is programmed to capture data from a 24-bit four-channel analogue-digital-converter. A GPS Arduino shield provides time synchronization and data is continuously fed to a Raspberry Pi asynchronously but in a deterministic manner. On the Raspberry Pi, data are inserted into a Ring Buffer structure from which Miniseed files are written to flash memory for long-term storage. Near real-time data can also be streamed out to subscribers via the seedlink protocol and/or also be fed to a local Earthworm or SeisComp installation for manipulation and processing. Our prototype costs a lot less than commercial digitizers do and the data quality is comparable. The component modules are easy to get, build up and replace, thus maintenance is much more feasible than commercial systems.
Slidecast:
https://vimeo.com/277552854
Abstract: The Alaska Earthquake Center’s mission is to advance the state’s earthquake resilience by integrating monitoring, research, and public engagement. Previously, the Center approached this mission primarily by operating a network of seismic sensors and reporting on earthquake activity via traditional avenues such as catalog production and email alerts to emergency managers and journalists. This is no longer enough, though, as the advent of smart buildings, social media, and instant communication has raised expectations for direct engagement. The center has adopted several strategies to meet these expectations. Using social media, we address public demand for immediate post-earthquake information by serving interpreted products tailored to our stakeholders rather than cookie-cutter announcements. Because strong motion provides a link between abstract source parameters and how earthquakes impact society, we have emphasized the installation of strong-motion instrumentation and production of related products designed for news media and the general public. We also emphasize the common ground between different types of hazard monitoring, such as landslides and volcanic eruptions, which rely on seismic monitoring. We place a particularly strong emphasis on the integrated nature of tsunamis and earthquakes because in Alaska these are simply two facets of the same hazard. This approach helps compensate for the awkward bifurcation of these hazards at the federal level. Finally, in an era of political, programmatic, and budgetary volatility, the Alaska Earthquake Center draws purpose from our singular focus on the needs of state-level stakeholders.
Slidecast:
https://vimeo.com/277555873
Abstract: Regional seismic networks (RSNs) in areas like Cascadia, with high seismic hazard but where earthquakes are not often felt, face special challenges maintaining the enthusiasm and involvement of stakeholders over time. Negative consequences of stakeholder disinterest include a society poorly prepared to face the very real hazard, and lack of stakeholder support to maintain regional monitoring excellence. In PNSN we have adopted a strategy of coupling a rather assertive social media and web presence with information products that provide compelling, informative, and relevant content and involve the entire RSN staff in Seattle. Cascadia, while shy on significant earthquakes over the past decade, does have both active volcanoes and abundant non-volcanic tremor to provide focus. Glacier-clad volcanoes are always noisy and tremor is frequent so there is almost always something to show and something to talk about. Research projects have led to the development of A. Hotovec-Ellis’ (USGS) REDPy (Repeating Earthquake Detector) and A. Wech’s (USGS) “Wechometer” tremor page. With help from L. Simmons (USGS) we have used the rumbling noise of football crowds and the nearly insane level of Seattle NFL enthusiasm to test our readiness for seismic crises and to explain to the public how Earthquake Early Warning could save lives and property. We have developed real-time waveform displays (QuickShake) so that stakeholders can look over our shoulder when there are interesting phenomena…swarms, aftershocks, etc. An example is our monitoring of the Rattlesnake Ridge (or Union Gap) landslide with T. Yelin and D. Bowen (USGS). We also aggressively market ANSS products, such as ShakeMap and ShakeCast, to regional stakeholders like the city of Seattle and to the US Department of Energy, for whom we monitor the Hanford nuclear reservation. It has been our experience that regional stakeholders highly value regional expertise and involvement with these 3rd-party information products.
Slidecast:
https://vimeo.com/277555132
Abstract: A moment tensor is a symmetric matrix that expresses the source for a seismic event. The fundamental lune of eigenvalues is a certain subset of the unit sphere whose points represent the source types for all moment tensors. Familiar source types such as double couple or pure isotropic have natural locations on the lune. Although the lune consists only of source types, it gives a useful, if incomplete, picture of moment tensor space as a whole. For each subset B of the lune we therefore consider the associated set BU of unit moment tensors that have their source types in B. We then wish to get a sense of BU from looking at B. We succeed in calculating both the volume and the angular diameter of BU. We also calculate the angular diameter and the volume of the set LambdaU of unit moment tensors that have source type Lambda, and we plot the results as contours on the lune. We show that great circle arc lengths on the lune are closely related to angles between moment tensors, and that arc length on the lune gives a natural measure of difference in source type. We show how to calculate volume elements for a variety of moment tensor coordinates. Volumes are relevant in part because we equate fractional volume with the probability that expresses randomness. We thus can find the probability that a random moment tensor have its source type in a given subset of the lune. These insights could be useful for characterizing uncertainties for individual events and for distinguishing source types among a set of events. Additional challenges of uncertainty characterization are expected if additional source parameters are considered, such as magnitude, hypocenter, origin time and source-time function.
Slidecast:
https://vimeo.com/277549361
Abstract: We report the results of a probabilistic seismic hazard analysis for Iquique, northern Chile located near the Nazca-South America plate boundary subduction zone. We constructed a seismic source model to characterize the geometry and earthquake activity at the plate interface (0 to 40 km depth) and for the deep subduction zone intraslab sources (40 to >150 km depth). Because of the high uncertainties associated with intraslab earthquake locations, the intraslab source has two equally weighted models—a 3D surface model and a progressively deeper stepped area source model. The source zone model also includes potential earthquakes generated by crustal faults and shallow crustal area sources to account for background earthquakes and unmapped faults. Ground motion prediction equations (GMPE) were selected and weighted using the method proposed by Scherbaum et al. (2009) and the Chile strong motion database developed by Bastías and Montalva (2016). The final GMM applied both global and local GMPEs selected and weighted based on their goodness-of-fit to the Chile strong motion dataset. Epistemic uncertainties in the seismic source and ground motion models were incorporated using a logic tree approach. Tornado plots indicate that the estimated seismic hazard at Iquique at short structural periods is most sensitive to the GMPE selection and recurrence parameters for the intraslab sources, followed by sensitivity to the alternative intraslab source models.The estimated seismic hazard at 1-second structural period is most sensitive to the selection and weighting of the subduction GMPE for both the interface and intraslab subduction sources. The 95th fractile PGA, 0.2- and 1.0-second spectral accelerations for the 475-year return period are about 1.3, 1.4 and 1.5 times of the mean spectral accelerations, respectively. Local fault sources(e.g., Atacama fault) make little contribution to the 475-year return period hazard, probably because of their low average slip rates of ≤ 0.5 mm/yr.
Slidecast:
https://vimeo.com/277552599
Abstract: We have compiled a thorough database of earthquake source parameters for seismic events occurring in Mexico, obtained by means of two different methods: finite-fault slip inversions and spectral analysis. From this compilation we employed stress drop and apparent stress values calculated for three (subduction, strike-slip, and deep intra-plate) tectonic environments and compared the results of employing a formulation, which allows investigating the difference between average final stress and frictional stress (a quantity which has been referred to as “shoot stress”). By analyzing these results we want to discern if Orowan´s model is obeyed by events occurring in a particular tectonic environment. Comparing all three types we observed that subduction events are the ones, which more closely follow Orowan´s model. Strike-slip events are not too far apart from this condition. Deep intra-plate events, on the other hand, depart further from this model. We also calculated the partial stress drop ratio (e), which is the ratio between the static stress drop and the average dynamic stress drop (Brune, 1976), since Orowan’s model would predict an e ratio equal to one. We found that in the case of subduction events, e ratios mostly indicate a partial stress drop process while intraplate and strike-slip events show a mixture of both partial and overshoot stress processes. We also notice that the e ratios for intraplate events have an apparent dependence on magnitude, which is not seen for the other two types of events. If the e ratio is close to 2 (maximum overshoot), it would indicate that the average frictional stress is equal to the “average” stress. In our study, some events are close to this condition, for the three types analyzed.It is important to emphasize that all these estimates are highly compromised by uncertainties in the estimation of energy, moment and static stress drop, but it is useful to attempt to make inferences as those above which can help differentiate between probable variations in source processes acting at different tectonic environments, depth, etc.
Slidecast:
https://vimeo.com/277551871
