(Alex) Yanjun Hao

Intraplate Earthquakes

ME The plate tectonics theory provides a framework to understand the dynamic processes of our planet. The theory offers an explanation for why earthquakes happen at plate boundaries, but fails to explain earthquakes occurring inside the tectonic plates and within continental interiors, specified as intraplate earthquakes. In spite of the fact that intraplate earthquakes are very rare, they are rather catastrophic, because the continental interiors are usually densely populated. Typical examples are the 2012 Wenchuan Mw7.9 earthquake and 1811-1812 New Madrid earthquakes (>M7.0).

Despite the fact that intraplate earthquakes are a real challenge for modern seismology, in the last two decades new observations have shed light on the enigma of intraplate seismicity. These observations can be summarized by the following points: 1) continental intraplate earthquakes appear to be episodic, clustered, and migrating; 2) the current seismicity in mid-continental interiors usually does not reflect the long-term behavior of the seismogenic faults; 3) the spatial distribution of large continental intraplate earthquakes coincides with weak zones of lithoshperic strength that appear to be controlled by paleotectonic structures. My research broadly concern seismogenesis and powering tectonic processes in intraplate regions. I employ active-source seismology to explore the subsurface targets.

Seismic exploration

MSF Seismic exploration (aka active-source seismology, exploration seismology) is an exploration technique using mechanical perturbations (seismic waves) to image the underground structures in the Earth. It contains seismic reflection, seismic refraction, and vertical seismic profile (VSP) methods according to the type of waves studied. Seismic waves are generated by man-made sources, for instance dynamite, air gun, and seismic vibrator. In seismic reflection, the waves propagate into the underground and reflected back by layer boundaries, then are detected by receivers at the surface (i.e. geophones on land or hydrophones in water). Knowing the times of the seismic waves traveling underground and the velocity field, a geological model/image can be recovered to represent the Earth structure, after a series of seismic data processing procedures.

Primarily the data I use are acquired using seismic reflection methods, both on land and in marien settings. The source, receiver, velocity field, and the property of the layer boundaries are inherent factors that affect the quality of the final image. The effectiveness of seismic wave being reflected is expressed as a reflection coefficient, and is controlled by the “abruptness” of media property (acoustic impedance) change at the layer boundary. The normal reflection coefficient is defined as R=(Z2-Z1)/(Z2+Z1), where Z1, Z1 are the acoustic impedances of the layers at both sides of the boundary and are defined as the products of the wave velocities and the media densities, Z = Vρ.

I am interested in and working on various aspects of seismic exploration, including seismic survey design, data acquisition, processing, and interpretation.