AGU 2015 - Nanometrics Event Schedule - Booth # 1023

 

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Paper Presentations Abstracts

 

S43B-2799: "Discrimination of Earthquake and Blast Seismicity in Western Alberta"   

Thursday, December 17th  Moscone South - Poster Hall I  Session: Seismology Contributions: Earthquake Ground Motions and Engineering Seismology II Posters I  13:40-18:00 hrs

Recorded seismicity in western Alberta is caused by natural and induced earthquakes or blast events from mining and quarry operations. Accurate discrimination of earthquakes from blast events is crucial for evaluating recent seismicity with respect to the historical catalog and for assessing seismic hazards associated with naturally occurring or induced seismicity. In general, blast events are discriminated from earthquakes based on their proximity to active mines and quarries in addition to day-of-week and time-of-day timing patterns. In some parts of western Alberta, however, seismicity originates in regions with active mines, historical earthquake seismicity, and hydraulic fracturing operations. Based on timing patterns or event locations alone, natural or induced seismicity may be misidentified as mining activity. Several studies report that relative differences in Fourier or response spectra can be used to discriminate blast and earthquake events. Other studies report that the relative timing and amplitude of seismic phases may provide useful metrics for classifying blast events.

Here we propose an alternative method that accounts for both differences in phase spectra and phase timing and amplitude. In particular, we evaluate the normalized time integral for characteristic functions of particle motion from confirmed blast and earthquake events recorded by regional Alberta seismic networks. We then use these time-integral profiles to re-classify events that are initially categorized as suspected blasts based on timing pattern and event location indicators.

 

S43B-2812: "Development of an Empirical Local Magnitude Formula for Northern Oklahoma"  

Thursday, December 17th  Moscone South - Poster Hall I  Session: Seismology Contributions: Earthquake Ground Motions and Engineering Seismology II Posters I  13:40-18:00 hrs

In this paper we focus on determining a local magnitude formula for northern Oklahoma that is unbiased with distance by empirically constraining the attenuation properties within the region of interest based on the amplitude of observed seismograms. For regional networks detecting events over several hundred kilometres, distance correction terms play an important role in determining the magnitude of an event. Standard distance correction terms such as Hutton and Boore (1987) may have a significant bias with distance if applied in a region with different attenuation properties, resulting in an incorrect magnitude. We have presented data from a regional network of broadband seismometers installed in bedrock in northern Oklahoma. The events with magnitude in the range of 2.0 and 4.5, distributed evenly across this network are considered. We find that existing models show a bias with respect to hypocentral distance. Observed amplitude measurements demonstrate that there is a significant Moho bounce effect that mandates the use of a trilinear attenuation model in order to avoid bias in the distance correction terms. We present two different approaches of local magnitude calibration. The first maintains the classic definition of local magnitude as proposed by Richter. The second method calibrates local magnitude so that it agrees with moment magnitude where a regional moment tensor can be computed. To this end, regional moment tensor solutions and moment magnitudes are computed for events with magnitude larger than 3.5 to allow calibration of local magnitude to moment magnitude. For both methods the new formula results in magnitudes systematically lower than previous values computed with Eaton’s (1992) model. We compare the resulting magnitudes and discuss the benefits and drawbacks of each method. Our results highlight the importance of correct calibration of the distance correction terms for accurate local magnitude assessment in regional networks.

 

S41A-2703: "Seetwater, Texas Large N Experiment"  

Thursday, December 17th  Moscone South - Poster Hall I  Session: Progress in Ambient Seismic Field Studies Driven by Coplete Wavefields Initiatives II Posters I  8:00-12:20 hrs

From 7 March to 30 April 2014, NodalSeismic, Nanometrics, and IRIS PASSCAL conducted a collaborative, spatially-dense seismic survey with several thousand nodal short-period geophones complemented by a backbone array of broadband sensors near Sweetwater, Texas. This pilot project demonstrates the efficacy of industry and academic partnerships, and leveraged a larger, commercial 3D survey to collect passive source seismic recordings to image the subsurface. This innovative deployment of a large-N mixed-mode array allows industry to explore array geometries and investigate the value of broadband recordings, while affording academics a dense wavefield imaging capability and an operational model for high volume instrument deployment.

The broadband array consists of 25 continuously-recording stations from IRIS PASSCAL and Nanometrics, with an array design that maximized recording of horizontal-traveling seismic energy for surface wave analysis over the primary target area with sufficient offset for imaging objectives at depth. In addition, 2639 FairfieldNodal Zland nodes from NodalSeismic were deployed in three sub-arrays: the outlier, backbone, and active source arrays. The backbone array consisted of 292 nodes that covered the entire survey area, while the outlier array consisted of 25 continuously-recording nodes distributed at a ~3 km distance away from the survey perimeter. Both the backbone and outlier array provide valuable constraints for the passive source portion of the analysis.

This project serves as a learning platform to develop best practices in the support of large-N arrays with joint industry and academic expertise. Here we investigate lessons learned from a facility perspective, and present examples of data from the various sensors and array geometries. We will explore first-order results from local and teleseismic earthquakes, and show visualizations of the data across the array. Data are archived at the IRIS DMC under stations codes XB and 1B.

 

S33D-2803: "Quantifying the Benefits of Shallow Posthole Installation for the Future French Permanent Broadband Stations"  

Wednesday, December 16th  Moscone South - Poster Hall I  Session: Seismology Contributions: Advances in Instrumentation and Installation Posters I  13:40-18:00 hrs

In the framework of the RESIF (réseau sismologique et géodésique français) infrastructure, more than one hundred new permanent broadband stations have to be deployed in metropolitan France within the forthcoming years. This requires a standardized installation method able to provide good noise level performance at a reasonable cost, especially for the 60 percent of stations that we expect to be settled in open environments.

During the last two years we tested various types of sensor’s hosting infrastructures with a strong focus on recently released posthole sensors that can be deployed at the bottom of shallow boreholes. Tests were performed at 3 different sites (two GEOSCOPE stations and a dedicated open-field prototype site) with geological conditions spanning from hard rocks to very soft soils. On each site, posthole sensors were deployed at different depths, from the surface to a maximum of 20m deep, and in different types of casing. Moreover, a reference sensor, either installed in a tunnel, a cellar or a seismic vault, has been operated continuously.

We present a comprehensive comparison of the seismic noise level measured in the different hosting infrastructures and for several frequency bands corresponding to various sources of noise. At high and low frequencies, seismic noise level in some boreholes equals or outperforms the one obtained for the reference sensors. Between 0.005 and 0.05Hz, we observe a strong decrease of seismic noise level on the horizontal components in the deepest boreholes compared to near surface installations. This improvement can reach up to 30dB and is mostly due to a reduction in tilt noise induced by wind or local pressure variations. However, the absolute noise level that can be achieved clearly depends on the local geology.

All these tests, together with estimated installation costs, point toward the deployment of sensors in shallow boreholes at the future French broadband stations located in open environments.

 

S33D-2815: "Field Testing GEOICE: A Next-Generation Polar Seismometer"  

Wednesday, December 16th  Moscone South - Poster Hall I  Session: Seismology Contributions: Advances in Instrumentation and Installation Posters I  13:40-18:00 hrs

We report on the development of a new NSF MRI-community supported seismic observatory designed for studies in ice-covered regions – the Geophysical Earth Observatory for Ice Covered Environs (GEOICE). This project is motivated by the need to densify and optimize the collection of high-quality seismic data relevant to key solid Earth and cryosphere science questions.

The GEOICE instruments and their power and other ancillary systems are being designed to require minimal installation time and logistical load (i.e., size and weight), while maximizing ease-of-use in the field. The system is capable of advanced data handling and telemetry while being able to withstand conditions associated with icy environments, including cold/wet conditions and high-latitude solar limitations.

The instrument capability will include a hybrid seismograph pool of broadband and intermediate elements for observation of both long-period signals (e.g, long-period surface waves and slow sources) and intermediate-to-short-period signals (e.g., teleseismic body waves, local seismicity, and impulsive or extended glaciogenic signals).

Key features will include a design that integrates the seismometer and digitizer into a single, environmentally and mechanically robust housing; very low power requirements (~1 watt) for the intermediate-band systems; and advanced power systems that optimize battery capacity and operational limits. The envisioned ~100 element GEOICE instruments will nearly double the current polar inventory of stations and will be maintained and supported at the IRIS PASSCAL Instrument Center to ensure full and flexible peer-reviewed community use.

Prototype instruments are currently deployed in Antarctica and Alaska, with a larger Antarctic deployment planned for the 2015-2016 season. The results of these field tests will help to refine instrumentation design and lead to the production of robust and capable next-generation seismic sensors.

Dec 9, 2014