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Waveform complexity caused by near trench structure and its impact on earthquake source study: application to the 2015 Illapel earthquake sequence

Event Type: 
Seminar
Event Date: 
25 Aug 2017
Venue: 
EOS Seminar Room - N2-01B-28
Speaker: 
Yunyi Qian
About the speaker: 

Yunyi Qian is a Ph.D. candidate with particular interests in earthquake physics, seismo-tectonics and structure of the earth's interior. Prior to enrolling in University of Science and Technology of China (USTC), she holds a bachelor's degree in geophysics from China University of Geosciences (CUG). She has focused on the seismic waveform study, which contains valuable information for both the source process and the structure of the earth's interior. She has studied the effects of the reflected body waves along core-mantle boundary (CMB) on earthquake source parameter determinations, and improved a simulation technique to efficiently generate the teleseismic body-waves with an addition of PcP and ScS phases. She is a visiting student at EOS since Sept. 2016, working with Shengji on waveform seismology.

About the event: 

Among various types of 3D heterogeneity in the Earth, trench region might have the most complexities, including rapidly varying bathymetry and usually thick sediments along with water layer. The complex structures can cause substantial waveform complexities, but such impact on seismograms and earthquake source studies has not yet been well understood. These issues place additional difficulties to understand the earthquakes, especially for tsunamigenic earthquakes, and the associated tectonic problems. Here we explore these effects via studies of two moderate aftershocks (a Mw6.8 event near the coast and a Mw6.6 event close to the trench) in the 2015 Illapel earthquake sequence. We found that the seismic waveforms display distinct differences between these two events, in which the trench event has much stronger and longer persisting coda. The coda waves are coherent across stations at different distances and azimuths, indicating an origin of scattering waves due to 3D heterogeneity near trench. To quantitatively model those 3D effects, we adopted a hybrid waveform simulation approach that can precisely generate the 3D wavefield in the source region by the Spectral Element Method (SEM) and then propagates to other parts of the earth through the Direct Solution Method (DSM). In this study, we used the GEBCO bathymetry and water layer in the source region. Compared with 1D synthetics, the 3D synthetics dramatically improved the waveform fittings across a series of frequency bands. Our study suggests that the complex trench structure must be taken into account for a reliable analysis of shallow earthquake near trench, in particular for the shallowest tsunamigenic earthquakes.