Research

Small to Large

I am interested in earthquake source processes from various points of view, such as the scaling of earthquake rupture growth and frequency-dependent seismic radiation from an earthquake source. In addition to the above, I am currently working on source properties of small and moderate earthquakes, which may give us a clue to understand past and future large earthquakes on the same active fault. 

"Small to Large" well represents my research. The scaling study was on small to large earthquakes. The frequency-dependent radiation study was on the rupture process of single earthquakes at small to large scale. In addition, source properties of small earthquakes were studied for the assessment of the rupture process of large earthquakes. My research was well summarized in the SSA's "At Work" article published online in 2020.

If you are interested in working with me, please see here.

Earthquake Source Physics inferred from Seismic Observations

Scaling of Earthquake Rupture Growth

How are the final sizes of earthquake determined? In order to answer this question, we investigated the initial rupture process of large earthquakes in detail and compared it to the rupture process of small earthquakes. A multiscale slip inversion method we developed (Uchide and Ide, 2007) is a tool for analyzing the initial rupture process of large earthquakes in detail and their whole rupture process at the same time. Using this method, we revealed that the 2004 Parkfield earthquake had a high-speed rupture process with a slip rate > 1 m/s since at least the first 0.2 s (Uchide et al., 2009). We also found that the 2011 Tohoku-oki earthquake had a high-speed rupture process with varying rupture propagation direction (Uchide, 2013). A systematic study for the comparison of earthquake rupture growth processes of small to large earthquakes in Parkfield, California suggested a self-similar growth curve of the moment rate function, but the finite thickness of the seismogenic zone suppresses the growth of the moment rate function (Uchide and Ide, 2010).

Frequency-dependent Seismic Radiation

Seismic waves in different frequency bands reflect different aspects of earthquake rupture process. We revealed the complementary distributions of the fault slip and the high-frequency radiation sources during the 2010 El Mayor-Cucapah, Baja California, Mexico earthquake by comparing the slip inversion and the back-projection analyses (Uchide et al., 2013), which suggests that the resultant large slip area must have a smooth initial stress field in space. It is still unclear why, but a possible underlying mechanisms is the stress diffusion due to the viscoelastic rheology of the fault zone (Sone and Uchide, 2016).

Source Properties of Small Earthquakes

Aiming general understanding of earthquake source physics, I am studying the source properties (such as stress drops, focal mechanisms, and seismic moment (moment magnitude)) of small earthquakes. We studied the spatial variation in the stress drops of small earthquakes in the Tohoku-oki area, northeast Japan (Uchide et al., 2014). We were trying to estimate source spectra more precisely by introducing spectral ratio method with many empirical Green's function (EGF) events, and it turns out more complexity in earthquake source spectra (Uchide and Imanishi, 2016). 

The spectral ratio analyses inferred seismic moment ratios of target and EGF events. This indicated a systematic deviation of moment magnitude and JMA magnitude scales from a 1:1 relationship for microearthquakes (M < 3) (Uchide and Imanishi, 2018), which will be important for earthquake statistical analyses using microearthquakes. 

Implication of Source Properties of Small Earthquakes to Fault Properties

Source properties of small earthquakes are useful as proxies of fault properties such as the stress, frictional properties, which will help us assess the rupture process of large earthquakes.

Stress Estimation

Focal mechanisms have been indicators of the stress orientation. We studied the crustal stress in Kanto region, East Japan (Imanishi et al., 2019) including Yamanashi prefecture (Uchide et al., 2015; Imanishi et al., 2016) and expanded it to the whole Japan (Uchide et al. 2022) thanks to the automatic P-wave first-motion polarity picking by a deep learning model (Uchide, 2020). The estimated stress orientations are visualized on the GSJ Crustal Stress Database.

Rupture Process of Large Earthquakes

I have been studying the rupture process of large earthquakes by the slip inversion technique using strong-motion record: