Paper summaries 2015

Abstract: Detailed analysis of continuously cored boreholes and cone penetrometer tests (CPTs), high-resolution seismic-reflection data, and luminescence and ¹⁴C dates from Holocene strata folded above the tip of the Ventura blind thrust fault constrain the ages and displacements of the two (or more) most recent earthquakes. These two earthquakes, which are identified by a prominent surface fold scarp and a stratigraphic sequence that thickens across an older buried fold scarp, occurred before the 235-yr-long historic era and after 805 ± 75 yr ago (most recent folding event[s]) and between 4065 and 4665 yr ago (previous folding event[s]). Minimum uplift in these two scarp-forming events was ∼6 m for the most recent earthquake(s) and ∼5.2 m for the previous event(s). Large uplifts such as these typically occur in large-magnitude earthquakes in the range of M_w 7.5–8.0. Any such events along the Ventura fault would likely involve rupture of other Transverse Ranges faults to the east and west and/or rupture downward onto the deep, low-angle décollements that underlie these faults. The proximity of this large reverse-fault system to major population centers, including the greater Los Angeles region, and the potential for tsunami generation during ruptures extending offshore along the western parts of the system highlight the importance of understanding the complex behavior of these faults for probabilistic seismic hazard assessment.

Abstract: We improve constraints on the slip distribution and geometry of faults involved in the complex, multisegment, M_w 8.6 April 2012 Wharton Basin earthquake sequence by joint inversion of high-rate GPS data from the Sumatran GPS Array (SuGAr), teleseismic observations, source time functions from broadband surface waves, and far-field static GPS displacements. This sequence occurred under the Indian Ocean, ∼400 km offshore Sumatra. The events are extraordinary for their unprecedented rupture of multiple cross faults, deep slip, large strike-slip magnitude, and potential role in the formation of a discrete plate boundary between the Indian and Australian plates. The SuGAr recorded static displacements of up to ∼22 cm, along with time-varying arrivals from the complex faulting, which indicate that the majority of moment release was on young, WNW trending, right-lateral faults, counter to initial expectations that an old, lithospheric, NNE trending fracture zone played the primary role. The new faults are optimally oriented to accommodate the present-day stress field. Not only was the greatest moment released on the younger faults, but it was these that sustained very deep slip and high stress drop (>20 MPa). The rupture may have extended to depths of up to 60 km, suggesting that the oceanic lithosphere in the northern Wharton Basin may be cold and strong enough to sustain brittle failure at such depths. Alternatively, the rupture may have occurred with an alternative weakening mechanism, such as thermal runaway.

Abstract: For years, many studies of subduction zones and on-land fold-and-thrust belts have assumed that the frontal portions of accretionary prisms are too weak to rupture coseismically and must therefore be fully creeping. We present a series of examples, both on-land and offshore, demonstrating that in many cases, shallow décollements are capable of large, coseismic slip events that rupture to the toes of the fault systems. Some of these events are associated with ruptures that initiate down-dip, while others appear to be limited to the frontal, shallow portion of the wedge.

We suggest that this behavior is not limited to the examples described here, but rather is common to many (perhaps most) accretionary wedges and fold-and-thrust belts around the world. Indeed, there may be many other examples of similar earthquakes, where existing data cannot constrain slip at the toe. We do not characterize the regions and events described here as unusual, as they encompass a wide range of settings. This study indicates that there is an urgent need to reevaluate seismic and tsunami hazard in fold-and-thrust belts and subduction zones around the world, allowing for the possibility of shallow décollement rupture.