Maomao Wang

The 2013 Lushan earthquake: Implications for seismic hazards posed by the Range Front blind thrust in the Sichuan Basin, China

Thrust and reverse faults pose significant earthquake hazards in convergent plate margins around the world, but have proven difficult to study given the complex nature of their ruptures, which often involve multiple along-strike and vertically stacked fault segments. The 2013 Mw 6.6 Lushan earthquake exemplified this complexity, rupturing a blind thrust fault in the southern Longmen Shan, which border the western Sichuan Basin in China. This event occurred 80 km south of the epicenter of the destructive 2008 Mw 7.9 Wenchuan earthquake. The Wenchuan earthquake produced surface ruptures on two parallel fault splays, the Pengguan and Beichuan faults. In contrast, the Lushan earthquake was generated by a ramp in the Range Front blind thrust (RFBT), which is in the footwall of the Wenchuan rupture. We use seismic reflection profiles, petroleum wells, and relocated seismicity to construct a three-dimensional model of this imbricated fault system. Our model illustrates that the 2013 Lushan earthquake ruptured <10% of the RFBT, which extends for 250 km along the Longmen Shan range front and into the western Sichuan Basin. Analysis of growth strata in structures above the RFBT fault along strike shows clear evidence of Quaternary activity and constrains the middle Pleistocene to current slip rate at two locations on the fault. Single segment and multisegment fault rupture scenarios involving the RFBT suggest the potential for large earthquakes (M7.8) that would affect the densely populated western Sichuan Basin. Assessing the hazards posed by such complex thrust systems, which occur in convergent margins worldwide, requires subsurface characterization of fault segments that can be independently associated with geologic and seismologic evidence of fault activity.

Coseismic rupturing stopped by Aso volcano during the 2016 Mw 7.1 Kumamoto earthquake, Japan

A volcanic end to an earthquake The dangerous and active Aso volcanic cluster appears to have put an early end to the damaging magnitude 7.1 Kumamoto earthquake that struck Japan in April 2016. Lin et al. found that the fault rupture stopped underneath the Aso caldera. The unzipping of the fault ended where the rocks went from cold and brittle to a more liquid-like magmatic mush. This distinctive example shows how abrupt changes in rock properties can terminate fault rupture and cap the size of an earthquake. Science, this issue p. 869 The Aso volcanic cluster stopped the fault rupture responsible for the 2015 magnitude 7.1 Kumamoto earthquake. Field investigations and seismic data show that the 16 April 2016 moment magnitude (Mw) 7.1 Kumamoto earthquake produced a ~40-kilometer-long surface rupture zone along the northeast-southwest–striking Hinagu-Futagawa strike-slip fault zone and newly identified faults on the western side of Aso caldera, Kyushu Island, Japan. The coseismic surface ruptures cut Aso caldera, including two volcanic cones inside it, but terminate therein. The data show that northeastward propagation of coseismic rupturing terminated in Aso caldera because of the presence of magma beneath the Aso volcanic cluster. The seismogenic faults of the 2016 Kumamoto earthquake may require reassessment of the volcanic hazard in the vicinity of Aso volcano.

Active thrust sheet deformation over multiple rupture cycles: A quantitative basis for relating terrace folds to fault slip rates

Many recent thrust fault earthquakes have involved coseismic surface faulting and folding, revealing the multifaceted nature of active thrust sheet deformation. We integrate records of surface deformation, subsurface structure and geochronology to investigate active surface deformation over multiple rupture cycles across the Southern Junggar Thrust (SJT) in the southern Junggar basin, NW China. Fluvial terrace geometries – extracted from a 1-m digital elevation model – reveal records of surface faulting across a prominent fault scarp. In addition, terraces exhibit progressive folding across fold scarps. Fault and fold scarps are spatially coincident with a surface-emergent SJT splay and subsurface fault bends along the SJT, respectively, constrained by seismic reflection data. We quantify the magnitude of fault slip at depth implied by fold scarps along Holocene-aged terraces. Our method yields results consistent with independent estimates of slip implied by fault scarp relief for the same terraces. Four late Quaternary terrace records are less continuous, preserved only as fold scarps that suggest folding kinematics involving a component of limb rotation. We develop a new method for quantifying fault slip at depth from terrace folds using a mechanical forward modeling approach. Our analysis yields quantitative relations between fold dip and fault slip, allowing us to quantify SJT fault slip from terrace folds from ~250 ka- present. SJT fault slip rate has decelerated from ~7.0 mm/yr in the Late Quaternary to ~1.3 mm/yr throughout the Holocene. These results provide new insight into the kinematics of fault-bend folding for natural structures and define new methods to accurately estimate fault slip and slip rates from terrace folds in active thrust sheets.

Three‐dimensional seismic velocity structure in the Sichuan basin, China

We present a new three-dimensional velocity model of the crust in the eastern margin of the Tibetan Plateau. The model describes the velocity structure of the Sichuan basin and surrounding thrust belts. The model consists of 3-D surfaces representing major geologic unit contacts and faults and is parameterized with Vp velocity-depth functions calibrated using sonic logs. The model incorporates data from 1166 oil wells, industry isopach maps, geological maps, and a digital elevation model. The geological surfaces were modeled based on structure contour maps for various units from oil wells and seismic reflection profiles. These surfaces include base Quaternary, Mesozoic, Paleozoic, and Proterozoic horizons. The horizons locally exhibit major offsets that are compatible with the locations and displacements of important faults systems. This layered, upper crustal 3-D model extends down to 10–15 km depth and illustrates lateral and vertical variations of velocity that reflect the complex evolution of tectonics and sedimentation in the basin. The model also incorporates 3-D descriptions of Vs and density for sediments that are obtained from empirical relationships with Vp using direct measurements of these properties in borehole logs. To illustrate the impact of our basin model on earthquake hazards assessment, we use it to calculate ground motions and compare these with observations for the 2013 Lushan earthquake. The result demonstrates the effects of basin amplification in the western Sichuan basin. The Sichuan CVM model is intended to facilitate fault systems analysis, strong ground motion prediction, and earthquake hazards assessment for the densely populated Sichuan region.

Active thrusting of the Longquan Fault in the central Sichuan basin, China, and the seismotectonic behavior in the Longmen Shan fold‐and‐thrust belt

Present-day convergence within the Longmen Shan fold-and-thrust belt (LSFTB) was released by the 2008 Mw 7.9 Wenchuan earthquake, which ruptured multiple thrust ramps beneath the range front structures. However, it is still unclear whether fault slip propagated eastward into the foreland, closer to densely populated areas around Chengdu. In this study, we provide constraints on the 3-D subsurface structure, fault activity and seismic hazards of the Longquan fault located in the central Sichuan basin, ~100 km east of the range front structures of the LSFTB. We develop a seismotectonic model and assess the activity of the Longquan fault by a combination of methods, including interpretation of seismic reflection profiles, analysis of satellite images, field mapping, trench logging and radiocarbon dating. Our survey reveals that at least two surface rupturing events occurred on the Longquan fault in the Holocene. Our 3-D structural model and evidence for late Holocene faulting events on the Longquan fault reveals that upper crustal shortening in the Sichuan basin is accommodated by a frontal thrust system that is linked to the recently active range front blind structures by a shallow detachment. We suggest that dynamic weakening following fault activity at the Longmen Shan range front might unlock the up-dip portion of this shallow detachment, sending slip eastward into the foreland and to the surface along the Longquan thrust ramps. Our results have important implications for seismic hazards assessment of active frontal thrusts linked by upper crustal detachments in active fold-and-thrust belts around the world.