Anthony Sladen

The 2011 Magnitude 9.0 Tohoku-Oki Earthquake: Mosaicking the Megathrust from Seconds to Centuries

Detailed geophysical measurements reveal features of the 2011 Tohoku-Oki megathrust earthquake. Geophysical observations from the 2011 moment magnitude (Mw) 9.0 Tohoku-Oki, Japan earthquake allow exploration of a rare large event along a subduction megathrust. Models for this event indicate that the distribution of coseismic fault slip exceeded 50 meters in places. Sources of high-frequency seismic waves delineate the edges of the deepest portions of coseismic slip and do not simply correlate with the locations of peak slip. Relative to the Mw 8.8 2010 Maule, Chile earthquake, the Tohoku-Oki earthquake was deficient in high-frequency seismic radiation—a difference that we attribute to its relatively shallow depth. Estimates of total fault slip and surface secular strain accumulation on millennial time scales suggest the need to consider the potential for a future large earthquake just south of this event.

Partial rupture of a locked patch of the Sumatra megathrust during the 2007 earthquake sequence

The great Sumatra–Andaman earthquake and tsunami of 2004 was a dramatic reminder of the importance of understanding the seismic and tsunami hazards of subduction zones. In March 2005, the Sunda megathrust ruptured again, producing an event of moment magnitude (Mw) 8.6 south of the 2004 rupture area, which was the site of a similar event in 1861 (ref. 6). Concern was then focused on the Mentawai area, where large earthquakes had occurred in 1797 (Mw = 8.8) and 1833 (Mw = 9.0). Two earthquakes, one of Mw = 8.4 and, twelve hours later, one of Mw = 7.9, indeed occurred there on 12 September 2007. Here we show that these earthquakes ruptured only a fraction of the area ruptured in 1833 and consist of distinct asperities within a patch of the megathrust that had remained locked in the interseismic period. This indicates that the same portion of a megathrust can rupture in different patterns depending on whether asperities break as isolated seismic events or cooperate to produce a larger rupture. This variability probably arises from the influence of non-permanent barriers, zones with locally lower pre-stress due to the past earthquakes. The stress state of the portion of the Sunda megathrust that had ruptured in 1833 and 1797 was probably not adequate for the development of a single large rupture in 2007. The moment released in 2007 amounts to only a fraction both of that released in 1833 and of the deficit of moment that had accumulated as a result of interseismic strain since 1833. The potential for a large megathrust event in the Mentawai area thus remains large.

Seismic and aseismic slip on the Central Peru megathrust

Slip on a subduction megathrust can be seismic or aseismic, with the two modes of slip complementing each other in time and space to accommodate the long-term plate motions. Although slip is almost purely aseismic at depths greater than about 40 km, heterogeneous surface strain suggests that both modes of slip occur at shallower depths, with aseismic slip resulting from steady or transient creep in the interseismic and postseismic periods. Thus, active faults seem to comprise areas that slip mostly during earthquakes, and areas that mostly slip aseismically. The size, location and frequency of earthquakes that a megathrust can generate thus depend on where and when aseismic creep is taking place, and what fraction of the long-term slip rate it accounts for. Here we address this issue by focusing on the central Peru megathrust. We show that the Pisco earthquake, with moment magnitude Mw = 8.0, ruptured two asperities within a patch that had remained locked in the interseismic period, and triggered aseismic frictional afterslip on two adjacent patches. The most prominent patch of afterslip coincides with the subducting Nazca ridge, an area also characterized by low interseismic coupling, which seems to have repeatedly acted as a barrier to seismic rupture propagation in the past. The seismogenic portion of the megathrust thus appears to be composed of interfingering rate-weakening and rate-strengthening patches. The rate-strengthening patches contribute to a high proportion of aseismic slip, and determine the extent and frequency of large interplate earthquakes. Aseismic slip accounts for as much as 50–70% of the slip budget on the seismogenic portion of the megathrust in central Peru, and the return period of earthquakes with Mw = 8.0 in the Pisco area is estimated to be 250 years.

A detailed source model for the Mw9.0 Tohoku‐Oki earthquake reconciling geodesy, seismology, and tsunami records

The 11 March 2011 Mw9.0 Tohoku-Oki earthquake was recorded by an exceptionally large amount of diverse data offering a unique opportunity to investigate the details of this major megathrust rupture. Many studies have taken advantage of the very dense Japanese onland strong motion, broadband, and continuous GPS networks in this sense. But resolution tests and the variability in the proposed solutions have highlighted the difficulty to uniquely resolve the slip distribution from these networks, relatively distant from the source region, and with limited azimuthal coverage. In this context, we present a finite fault slip joint inversion including an extended amount of complementary data (teleseismic, strong motion, high-rate GPS, static GPS, seafloor geodesy, and tsunami records) in an attempt to reconcile them into a single better resolved model. The inversion reveals a patchy slip distribution with large slip (up to 64 m) mostly located updip of the hypocenter and near the trench. We observe that most slip is imaged in a region where almost no earthquake was recorded before the main shock and around which intense interplate seismicity is observed afterward. At a smaller scale, the largest slip pattern is imaged just updip of an important normal fault coseismically activated. This normal fault has been shown to be the mark of very low dynamic friction allowing extremely large slip to propagate up to the free surface. The spatial relationship between this normal fault and our slip distribution strengthens its key role in the rupture process of the Tohoku-Oki earthquake.

High-resolution backprojection at regional distance: Application to the Haiti M7.0 earthquake and comparisons with finite source studies,

A catastrophic M_w7 earthquake ruptured on 12 January 2010 on a complex fault system near Port-au-Prince, Haiti. Offshore rupture is suggested by aftershock locations and marine geophysics studies, but its extent remains difficult to define using geodetic and teleseismic observations. Here we perform the multitaper multiple signal classification (MUSIC) analysis, a high-resolution array technique, at regional distance with recordings from the Venezuela National Seismic Network to resolve high-frequency (about 0.4 Hz) aspects of the earthquake process. Our results indicate westward rupture with two subevents, roughly 35 km apart. In comparison, a lower-frequency finite source inversion with fault geometry based on new geologic and aftershock data shows two slip patches with centroids 21 km apart. Apparent source time functions from USArray further constrain the intersubevent time delay, implying a rupture speed of 3.3 km/s. The tips of the slip zones coincide with subevents imaged by backprojections. The different subevent locations found by backprojection and source inversion suggest spatial complementarity between high- and low-frequency source radiation consistent with high-frequency radiation originating from rupture arrest phases at the edges of main slip areas. The centroid moment tensor (CMT) solution and a geodetic-only inversion have similar moment, indicating most of the moment released is captured by geodetic observations and no additional rupture is required beyond where it is imaged in our preferred model. Our results demonstrate the contribution of backprojections of regional seismic array data for earthquakes down to M ≈ 7, especially when incomplete coverage of seismic and geodetic data implies large uncertainties in source inversions.

Mega-earthquakes rupture flat megathrusts

Mega-earthquakes go the flat way Megathrust faults in subduction zones cause large and damaging earthquakes. Bletery et al. argue that certain geometric features of the subduction zones relate to earthquake size. The key parameter is the curvature of the megathrust. Larger earthquakes occur where the subducting slab is flatter, providing a rough metric for estimating where mega-earthquakes may occur in the future. Science, this issue p. 1027 Large earthquakes in subduction zones are most likely to occur where the subducting slab is relatively flat. The 2004 Sumatra-Andaman and 2011 Tohoku-Oki earthquakes highlighted gaps in our understanding of mega-earthquake rupture processes and the factors controlling their global distribution: A fast convergence rate and young buoyant lithosphere are not required to produce mega-earthquakes. We calculated the curvature along the major subduction zones of the world, showing that mega-earthquakes preferentially rupture flat (low-curvature) interfaces. A simplified analytic model demonstrates that heterogeneity in shear strength increases with curvature. Shear strength on flat megathrusts is more homogeneous, and hence more likely to be exceeded simultaneously over large areas, than on highly curved faults.

Postseismic relocking of the subduction megathrust following the 2007 Pisco, Peru, earthquake

Characterizing the time evolution of slip over different phases of the seismic cycle is crucial to a better understanding of the factors controlling the occurrence of large earthquakes. In this study, we take advantage of interferometric synthetic aperture radar data and 3.5 years of continuous Global Positioning System (GPS) measurements to determine interseismic, coseismic, and postseismic slip distributions in the region of the 2007, Mw 8.0 Pisco, earthquake, Peru, using the same fault geometry and inversion method. Our interseismic model, based on pre-2007 campaign GPS data, suggests that the 2007 Pisco seismic slip occurred in a region strongly coupled before the earthquake while afterslip occurred in low coupled regions. Large afterslip occurred in the peripheral area of coseismic rupture in agreement with the notion that afterslip is mainly induced by coseismic stress changes. The temporal evolution of the region of maximum afterslip, characterized by a relaxation time of about 2.3 years, is located in the region where the Nazca ridge is subducting, consistent with rate-strengthening friction promoting aseismic slip. We estimate a return period for the Pisco earthquake of about 230 years with an estimated aseismic slip that might account for about 50% of the slip budget in this region over the 0–50 km seismogenic depth range. A major result of this study is that the main asperity that ruptured during the 2007 Pisco earthquake relocked soon after this event.

Investigating dynamic triggering of seismicity by regional earthquakes: the case of the Corinth Rift (Greece)

Dynamic triggering has been commonly observed after large teleseismic events, but the physics behind it is still under debate. To broaden observations, we here focus on the dynamic triggering by regional earthquakes, that is, by events with magnitude lower than 6.2 at distances smaller than 600 km. The western part of the Corinth Rift (Greece) is characterized by intense seismic swarms and is therefore adapted to study such responses. The microseismicity rates before and after the transient perturbations are high enough to analyze 30 regional earthquakes out of the 59 occurring in 2013. More than 40% of those 30 events, including earthquakes with magnitude as small as 4.5, are associated with a significant seismicity rate increase. The triggerability primarily depends on the amplitude of the seismic waves. However, triggering is mainly observed when the seismic perturbations are orthogonal to the faults, which suggests that fluid pressurization is likely involved.