特別セミナーのお知らせ/Special seminar announcement
日時/Time：6 月 1 日(木) 13:00〜/ June 1 (Thursday), 13:00 ~場所/Place：理学部 1 号館 462 号室 (Science Building No. 1, 4th floor, Room 462)
Title：Full-waveform methods in seismic monitoring operations

Abstract: Microseismic monitoring became a common operation in many seismological and industrial

applications, and in the last decade a significant effort has been spent to develop or improvemethodologies able to perform seismological analysis for weak events at a local scale. This effort wasaccompanied by the improvement of monitoring systems, resulting in an increasing number of large
microseismicity catalogues. The interest in microseismic monitoring operations, involving a synergy
among different scientific communities, is in part due to their occurrence both as a consequence of
natural processes in active regions, swarm areas, hydrothermal and volcanic environments, but also
in relation to human activities, e.g., in the proximity of mining areas, geothermal systems, oil and gas
fields, and water reservoirs. The analysis of microseismicity is challenging because of the large
number of recorded events, often characterized by a low signal-to-noise ratio. A significant limitationof the traditional location approaches is that automated picking procedures are often done on eachseismogram individually, making little or no use of the coherency information between stations.
In order to improve the performance of the traditional location methods, different full waveform
location methods have been recently proposed. These methods exploit the coherence of thewaveforms recorded at different stations and do not require any automated picking procedure. The
main advantage of these methods relies on their robustness even when the recorded waveforms arevery noisy. On the other hand, like any other location method, the location performance strongly
depends on the accuracy of the available velocity model. When dealing with inaccurate velocity
models, location results can be affected by large errors.
In this work we present a location method which combines some features of relative location
techniques (such as the source specific station correction term [Richards-Dinger and Shearer 2000])with a waveform based location method [Grigoli et al., 2016]. This location approach inherits all theadvantages of the full waveform location methods without the main drawback which characterizes
all the absolute location procedures. In fact, this method is less dependent on the knowledge of thevelocity model and presents several benefits, which improve the location accuracy: 1) it accounts forphase delays due to local site effects, e.g. surface topography or variable sediment thickness 2)
theoretical velocity models are only used to estimate travel time within the source volume, and notalong the entire source-sensor path. After successfully testing the method with synthetic data, weapplied it to a real dataset related to fluid induced microseismicity associated with magmatic fluid
migrations.
References:Richards-Dinger, K. & Shearer, P., Earthquake locations in southern California obtained using
source-specific station terms. Journal of Geophysical Research: Solid Earth (1978-2012) 105, 10939–
10960 (2000).
Grigoli, F. et al. Automated microseismic event location using Master-Event Waveform Stacking.
Sci. Rep. 6, 25744; doi: 10.1038/srep25744 (2016).