Earth and Planetary Science Letters 385 (2014) 89–96 Contents lists available at ScienceDirect Earth and Planetary Science Letters www.elsevier.com/locate/epsl Rupture complexity of the 1994 Bolivia and 2013 Sea of Okhotsk deep earthquakes Zhongwen Zhan ∗ , Hiroo Kanamori, Victor C. Tsai, Donald V. Helmberger, Shengji Wei Seismological Laboratory, California Institute of Technology, 1200 E. California Blvd., Pasadena, CA 91125, USA article info abstract Article history: Received 14 October 2013 Accepted 19 October 2013 Available online 8 November 2013 Editor: Y. Ricard Keywords: deep earthquake earthquake rupture body wave The physical mechanism of deep earthquakes (depth >300 km) remains enigmatic, partly because their rupture dimensions are difficult to estimate due to their low aftershock productivity and absence of geodetic or surface rupture observations. The two largest deep earthquakes, the recent Great 2013 Sea of Okhotsk earthquake (M 8.3, depth 607 km) and the Great 1994 Bolivia earthquake (M 8.3, depth 637 km), together provide a unique opportunity to compare their rupture patterns in detail. Here we extend a travel-time sub-event location method to perform full teleseismic P-waveform inversion. This new method allows us to explain the observed broadband records with a set of sub-events whose model parameters are robustly constrained without smoothing. We find that while the Okhotsk event is mostly unilateral, rupturing 90 km along strike with a velocity over 4 km/s, the Bolivia earthquake ruptured about half this distance at a slow velocity (about 1.5 km/s) and displayed a major change in rupture direction. We explain the observed differences between the two earthquakes as resulting from two fundamentally different faulting mechanisms in slabs with different thermal states. Phase transformational faulting is inferred to occur inside the metastable olivine wedge within cold slab cores whereas shear melting occurs inside warm slabs once triggered.  2013 Elsevier B.V. All rights reserved. 1. Introduction Several mechanisms for deep earthquakes have been proposed, including thermal shear instability (Ogawa, 1987; Kanamori et al., 1998; Karato et al., 2001), dehydration embrittlement of pre- existing faults (Meade and Jeanloz, 1991; Silver et al., 1995), and transformational faulting associated with a metastable olivine wedge in cold subducting slabs (Green and Burnley, 1989; Kirby et al., 1991, 1996; Green and Houston, 1995). These mecha- nisms have been previously evaluated using deep earthquake rup- ture properties (e.g., duration, rupture dimension, rupture speed, stress drop, and radiation efficiency; Wiens, 2001; Tibi et al., 2003; Suzuki and Yagi, 2011), aftershock statistics (Wiens, 2001; Houston, 2007), and their depth dependence (Persh and Houston, 2004a, 2004b; Tocheport et al., 2007). Among these mechanisms, phase transformational faulting and thermal shear instability, pos- sibly involving melting, have so far garnered the most evidence (Wiens, 2001; Houston, 2007). It has also been suggested that deep earthquake mechanisms may depend on the thermal state of the * Corresponding author now at: Institute of Geophysics and Planetary Physics, Scripps Institution of Oceanography, University of California, San Diego, 9500 Gilman Dr., La Jolla, CA 92093-0225, USA. Tel.: +1 858 534 1543. E-mail addresses: zwzhan@ucsd.edu, zwzhan@gmail.com (Z. Zhan). subducting slab (Wiens and McGuire, 1995; Wiens and Gilbert, 1996; Wiens, 2001; Tibi et al., 2003; Houston, 2007), but evidence has been inconclusive (Suzuki and Yagi, 2011). The 1994 Bolivia earthquake was the largest deep earthquake until the recent 2013 Okhotsk earthquake of similar magnitude (Fig. 1), and has provided critical information about deep earth- quake mechanisms. The earthquake was previously characterized by low rupture speed (∼1.5 km/s), high static stress drop, and low radiation efficiency (e.g., Kikuchi and Kanamori, 1994; Silver et al., 1995; Ihmlé, 1998). The earthquake’s rupture dimension (∼30 km × 40 km) is small for its size, yet significantly larger than the predicted width of the metastable olivine wedge (Tibi et al., 2003), unless significant thickening of the slab occurs due to plate bending (Kirby et al., 1995). Kanamori et al. (1998) suggest instead that shear melting could have promoted extensive slid- ing with high energy dissipation which resulted in large slip, high stress drop and slow rupture speed. The 2013 Okhotsk deep earthquake was of similar size as the Bolivia earthquake, but occurred in a different tectonic setting. The subducted Pacific plate in which the Okhotsk earthquake occurred is significantly older and hence colder than the subducted Nazca plate in which the Bolivia earthquake occurred (Wiens and Gilbert, 1996). A thorough comparison of these two earthquakes’ rupture properties thus provides important constraints on the faulting 0012-821X/$ – see front matter  2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.epsl.2013.10.028