Physical insights into the recnet large earthquakes
Our technical improvements of BP allow me to address the questions of earthquake source dynamics in the case studies of recent large earthquakes.
By comparing the back projection result with the geodetic and teleseismic slip model of the Tohoku-Oki earthquake, we first demonstrate that the peak low frequency slip are up-dip from the hypocenter and the high frequency radiation are generated in the deeper portion of the megathrust (Simons et al, Science, 2011). We relate the deep high frequency burst to the small brittle asperities embedded in the ductile matrix at the root of the seismogenic zone (Meng et al, GRL, 2011). This concept is demonstrated by our physical models of earthquake rupture (Huang et al., EPS, 2012).
In another example, we image the 2010 Haiti earthquake with the Venezuela seismic network. We observe two high frequency subevents at the terminal end of the major geodetic slip regions, which can be interpreted as stopping phases associated with abrupt rupture speed reduction at the edge of the slip area (Meng et al., JGR, 2012).
The back-projection source imaging also indicates that the rupture of the 2012 Off-Sumatra earthquake occurs on distinct planes in an orthogonal conjugate fault system. The rupture branched twice into faults where dynamic stresses were compressional, challenging our conventional view of the dynamic clamping effect (Meng et al, Science, 2012).
Deep earthquakes occur at depths where high pressure strongly inhibits brittle failure and where high temperatures result in increasingly ductile deformation. The plausible mechanisms to promote catastrophic failure below 70 km is subject to intense debates. Meng et al., GRL, 2014 reports the rupture process of the 2013 M8.2 Okhotsk earthquake, the largest deep earthquake ever recorded in modern seismology. The high-resolution imaging allows the comparison of the earthquake rupture with the local geometry of the subducting slab. We find that two different mechanisms: phase transformation of olivine to spinel and shear instability induced by thermal runaway effect control different segments of the rupture. This observation provides new insights into the long-debating mechanisms of the deep earthquakes.
The 2014 M8.2 Iquique earthquake in northern Chile highlights the complexity of interactions between dynamic rupture and geometrical barriers on megathrusts (Meng et al., EPSL, 2015). Our work reveals that rupture expands in bursts along the rim of a semi-elliptical region with episodes of re-ruptures within an area of local high gravity anomaly, indicating the presence of subducting seamounts that promote high-frequency generation.
In the 2015 Mw 7.8 Gorkha earthquake, our refined source imaging reveals a narrow (less than 20 km in width) unilateral eastward rupture unzipping the lower bottom of the locked portion of the Main Himalaya Thrust (Avouac et al., Nature Geoscience, 2015). Such limited rupture extent indicates that the Gorkha earthquake in Nepal is possibly a medium-size event during the inter-seismic period of larger earthquakes (Meng et al., GRL, 2016). Such intermediate events are predicted by earthquake cycle simulations with heterogeneous loading stress.
We have also performed a joint seismic and geodetic investigation of the 2015 Mw 7.2 Tajikistan earthquake (Sangha et al., EPSL, 2017). The coincidence of the locations of high frequency radiators and the fault geometry derived from space-based geodesy is remarkable. This study demonstrates the capability of the BP method, enhanced by aftershock calibrations, to describe details of the rupture kinematics of a moderate size event in a remote part of the world.
The complexity of dynamic rupture can be also appreciated in the 2015 M8.4 Illapel earthquake in central Chile. This earthquake is featured by splitting of rupture fronts around the rim of a large asperity (Meng et al., EPSL, 2018). This encircling pattern is analogous to the double-pincer movement in military tactics. Such degree of complexity is previously only seen in simulations and it is observed for the first time in real earthquakes enabled by enhanced high-resolution BP.