Last Updated 05/04/2020
Jonathan R. Bedford, Marcos Moreno, Zhiguo Deng, Onno Oncken, Bernd Schurr, Timm John, Juan Carlos Báez & Michael Bevis
Published online 29 April 2020
Megathrust earthquakes are responsible for some of the most devastating natural disasters. To better understand the physical mechanisms of earthquake generation, subduction zones worldwide are continuously monitored with geophysical instrumentation. One key strategy is to install stations that record signals from Global Navigation Satellite Systems (GNSS), enabling us to track the non-steady surface motion of the subducting and overriding plates before, during and after the largest events. Here we use a recently developed trajectory modelling approach that is designed to isolate secular tectonic motions from the daily GNSS time series to show that the 2010 Maule, Chile (moment magnitude 8.8) and 2011 Tohoku-oki, Japan (moment magnitude 9.0) earthquakes were preceded by reversals of 4–8 millimetres in surface displacement that lasted several months and spanned thousands of kilometres. Modelling of the surface displacement reversal that occurred before the Tohoku-oki earthquake suggests an initial slow slip followed by a sudden pulldown of the Philippine Sea slab so rapid that it caused a viscoelastic rebound across the whole of Japan. Therefore, to understand better when large earthquakes are imminent, we must consider not only the evolution of plate interface frictional processes but also the dynamic boundary conditions from deeper subduction processes, such as sudden densification of metastable slab.
Jamie I. Farquharson & Falk Amelung
Published online 22 April 2020
The May 2018 rift intrusion and eruption of Kīlauea Volcano, Hawai‘i, represented one of its most extraordinary eruptive sequences in at least 200 years, yet the trigger mechanism remains elusive. The event was preceded by several months of anomalously high precipitation. It has been proposed that rainfall can modulate shallow volcanic activity, but it remains unknown whether it can have impacts at the greater depths associated with magma transport. Here we show that immediately before and during the eruption, infiltration of rainfall into Kīlauea Volcano’s subsurface increased pore pressure at depths of 1 to 3 kilometres by 0.1 to 1 kilopascals, to its highest pressure in almost 50 years. We propose that weakening and mechanical failure of the edifice was driven by changes in pore pressure within the rift zone, prompting opportunistic dyke intrusion and ultimately facilitating the eruption. A precipitation-induced eruption trigger is consistent with the lack of precursory summit inflation, showing that this intrusion—unlike others—was not caused by the forceful intrusion of new magma into the rift zone. Moreover, statistical analysis of historic eruption occurrence suggests that rainfall patterns contribute substantially to the timing and frequency of Kīlauea’s eruptions and intrusions. Thus, volcanic activity can be modulated by extreme rainfall triggering edifice rock failure—a factor that should be considered when assessing volcanic hazards. Notably, the increasingly extreme weather patterns associated with ongoing anthropogenic climate change could increase the potential for rainfall-triggered volcanic phenomena worldwide.
Valentí Sallarès & César R. Ranero
Published online 27 November 2019
Seismological data provide evidence of a depth-dependent rupture behaviour of earthquakes occurring at the megathrust fault of subduction zones, also known as megathrust earthquakes1. Relative to deeper events of similar magnitude, shallow earthquake ruptures have larger slip and longer duration, radiate energy that is depleted in high frequencies and have a larger discrepancy between their surface-wave and moment magnitudes. These source properties make them prone to generating devastating tsunamis without clear warning signs. The depth-dependent rupture behaviour is usually attributed to variations in fault mechanics. Conceptual models, however, have so far failed to identify the fundamental physical causes of the contrasting observations and do not provide a quantitative framework with which to predict and link them. Here we demonstrate that the observed differences do not require changes in fault mechanics. We use compressional-wave velocity models from worldwide subduction zones to show that their common underlying cause is a systematic depth variation of the rigidity at the lower part of the upper plate — the rock body overriding the megathrust fault, which deforms by dynamic stress transfer during co-seismic slip. Combining realistic elastic properties with accurate estimates of earthquake focal depth enables us to predict the amount of co-seismic slip (the fault motion at the instant of the earthquake), provides unambiguous estimations of magnitude and offers the potential for early tsunami warnings.
Laura Gulia & Stefan Wiemer
Published online 09 October 2019
Immediately after a large earthquake, the main question asked by the public and decision-makers is whether it was the mainshock or a foreshock to an even stronger event yet to come. So far, scientists can only offer empirical evidence from statistical compilations of past sequences, arguing that normally the aftershock sequence will decay gradually whereas the occurrence of a forthcoming larger event has a probability of a few per cent. Here we analyse the average size distribution of aftershocks of the recent Amatrice–Norcia and Kumamoto earthquake sequences, and we suggest that in many cases it may be possible to discriminate whether an ongoing sequence represents a decaying aftershock sequence or foreshocks to an upcoming large event. We propose a simple traffic light classification to assess in real time the level of concern about a subsequent larger event and test it against 58 sequences, achieving a classification accuracy of 95 per cent.
Satoshi Ide
Published online 04 September 2019
Every gigantic earthquake begins as a tiny rock failure at almost a point, followed by successive slip of the complex fault system, before radiating strong shaking from a vast rupture area extending over hundreds of kilometres. Whether the growth process of the rupture of a large earthquake is predictable and whether it produces observable signatures different from that of smaller events1–5 are fundamental questions related to the potential for earthquake early warning and probabilistic forecasting. Inspired by a recent discovery that large earthquakes might have seismic waves, and probably rupture processes, that are almost identical to those of smaller events6–8, we show that such similarity characterized by large cross-correlation is a common feature of earthquakes in the Tohoku– Hokkaido subduction zone, Japan. A systematic comparison of 15 years of high-sensitivity seismograph records for approximately 100,000 events reveals 80 extremely similar and 390 very similar pairs of large (moment magnitude M > 4.5) and small (M < 4.0) earthquakes, co-located within about 100 metres. An extremely high similarity is observed for pairs of subduction-type earthquakes (170 of 899 large events) separated by a long period of up to 15 years, whereas for pairs of other types of large earthquakes only the foreshocks and aftershocks are similar. This frequently occurring similarity between different-sized subduction-type earthquakes suggests repeated cascading rupture processes in a widespread hierarchical structure9–12 along the plate interface and indicates a specific but probabilistically limited predictability of the final size of the earthquake (that is, the location and a set of possible sizes of an earthquake are well predicted, but its final size is not at all well constrained).
Phoebe M. R. DeVries, Fernanda Viégas, Martin Wattenberg & Brendan J. Meade
Published online 29 August 2018
Aftershocks are a response to changes in stress generated by large earthquakes and represent the most common observations of the triggering of earthquakes. The maximum magnitude of aftershocks and their temporal decay are well described by empirical laws (such as Bath’s law1 and Omori’s law2), but explaining and forecasting the spatial distribution of aftershocks is more difficult. Coulomb failure stress change3 is perhaps the most widely used criterion to explain the spatial distributions of aftershocks4,5,6,7,8, but its applicability has been disputed9,10,11. Here we use a deep-learning approach to identify a static-stress-based criterion that forecasts aftershock locations without prior assumptions about fault orientation. We show that a neural network trained on more than 131,000 mainshock–aftershock pairs can predict the locations of aftershocks in an independent test dataset of more than 30,000 mainshock–aftershock pairs more accurately (area under curve of 0.849) than can classic Coulomb failure stress change (area under curve of 0.583). We find that the learned aftershock pattern is physically interpretable: the maximum change in shear stress, the von Mises yield criterion (a scaled version of the second invariant of the deviatoric stress-change tensor) and the sum of the absolute values of the independent components of the stress-change tensor each explain more than 98 per cent of the variance in the neural-network prediction. This machine-learning-driven insight provides improved forecasts of aftershock locations and identifies physical quantities that may control earthquake triggering during the most active part of the seismic cycle.
Bjørn Jamtveit, Yehuda Ben-Zion, François Renard & Håkon Austrheim
Published online 25 April 2018
The structural and metamorphic evolution of the lower crust has direct effects on the lithospheric response to plate tectonic processes involved in orogeny, including subsidence of sedimentary basins, stability of deep mountain roots and extension of high-topography regions. Recent research shows that before orogeny most of the lower crust is dry, impermeable and mechanically strong1. During an orogenic event, the evolution of the lower crust is controlled by infiltration of fluids along localized shear or fracture zones. In the Bergen Arcs of Western Norway, shear zones initiate as faults generated by lower-crustal earthquakes. Seismic slip in the dry lower crust requires stresses at a level that can only be sustained over short timescales or local weakening mechanisms. However, normal earthquake activity in the seismogenic zone produces stress pulses that drive aftershocks in the lower crust2. Here we show that the volume of lower crust affected by such aftershocks is substantial and that fluid-driven associated metamorphic and structural transformations of the lower crust follow these earthquakes. This provides a ‘top-down’ effect on crustal geodynamics and connects processes operating at very different timescales.
Vahe Gabuchian, Ares J. Rosakis, Harsha S. Bhat, Raúl Madariaga & Hiroo Kanamori
Published online 01 May 2017
Many of Earth’s great earthquakes occur on thrust faults1. These earthquakes predominantly occur within subduction zones, such as the 2011 moment magnitude 9.0 eathquake in Tohoku-Oki, Japan, or along large collision zones, such as the 1999 moment magnitude 7.7 earthquake in Chi-Chi, Taiwan2. Notably, these two earthquakes had a maximum slip that was very close to the surface3, 4. This contributed to the destructive tsunami that occurred during the Tohoku-Oki event and to the large amount of structural damage caused by the Chi-Chi event. The mechanism that results in such large slip near the surface is poorly understood as shallow parts of thrust faults are considered to be frictionally stable5. Here we use earthquake rupture experiments to reveal the existence of a torquing mechanism of thrust fault ruptures near the free surface that causes them to unclamp and slip large distances. Complementary numerical modelling of the experiments confirms that the hanging-wall wedge undergoes pronounced rotation in one direction as the earthquake rupture approaches the free surface, and this torque is released as soon as the rupture breaks the free surface, resulting in the unclamping and violent ‘flapping’ of the hanging-wall wedge. Our results imply that the shallow extent of the seismogenic zone of a subducting interface is not fixed and can extend up to the trench during great earthquakes through a torquing mechanism.
Yusuke Yokota, Tadashi Ishikawa, Shun-ichi Watanabe, Toshiharu Tashiro & Akira Asada
Nature 534, 374–377 (16 June 2016) doi:10.1038/nature17632 Received 04 November 2015 Accepted 25 February 2016 Published online 23 May 2016
Interplate megathrust earthquakes have inflicted catastrophic damage on human society. Such an earthquake is predicted to occur in the near future along the Nankai Trough off southwestern Japan—an economically active and densely populated area in which megathrust earthquakes have already occurred1, 2, 3, 4, 5. Megathrust earthquakes are the result of a plate-subduction mechanism and occur at slip-deficit regions (also known as ‘coupling’ regions)6, 7, where friction prevents plates from slipping against each other and the accumulated energy is eventually released forcefully. Many studies have attempted to capture distributions of slip-deficit rates (SDRs) in order to predict earthquakes8, 9, 10. However, these studies could not obtain a complete view of the earthquake source region, because they had no seafloor geodetic data. The Hydrographic and Oceanographic Department of the Japan Coast Guard (JHOD) has been developing a precise and sustainable seafloor geodetic observation network11 in this subduction zone to obtain information related to offshore SDRs. Here, we present seafloor geodetic observation data and an offshore interplate SDR-distribution model. Our data suggest that most offshore regions in this subduction zone have positive SDRs. Specifically, our observations indicate previously unknown regions of high SDR that will be important for tsunami disaster mitigation, and regions of low SDR that are consistent with distributions of shallow slow earthquakes and subducting seamounts. This is the first direct evidence that coupling conditions might be related to these seismological and geological phenomena. Our findings provide information for inferring megathrust earthquake scenarios and interpreting research on the Nankai Trough subduction zone.
Deepa Mele Veedu & Sylvain Barbot
Nature 532, 361–365 (21 April 2016) doi:10.1038/nature17190 Received 14 May 2015 Accepted 26 January 2016 Published online 04 April 2016
The deep extension of the San Andreas Fault is believed to be creeping, but the recent observations of tectonic tremors from these depths indicate a complex deformation style1. In particular, an isolated tremor source near Parkfield has been producing a sequence of low-frequency earthquakes2 that indicates an uncommon mechanism of stress accumulation and release. The tremor pattern regularly oscillated between three and six days from mid-2003 until it was disrupted by the 2004 magnitude 6.0 Parkfield earthquake. After that event, the tremor source ruptured only about every three days, but over the next two years it gradually returned to its initial alternating recurrence pattern. The mechanism that drives this recurrence pattern is unknown. Here we use physics-based models to show that the same tremor asperity—the region from which the low-frequency earthquakes radiate—can regularly slip in slow and fast ruptures, naturally resulting in recurrence intervals alternating between three and six days. This unusual slip behaviour occurs when the tremor asperity size is close to the critical nucleation size of earthquakes. We also show that changes in pore pressure following the Parkfield earthquake can explain the sudden change and gradual recovery of the recurrence intervals. Our findings suggest a framework for fault deformation in which the same asperity can release tectonic stress through both slow and fast ruptures.
Dan Bassett, David T. Sandwell, Yuri Fialko & Anthony B. Watts
Nature 531, 92–96 (03 March 2016) doi:10.1038/nature16945 Received 26 June 2015 Accepted 11 December 2015 Published online 02 March 2016
The March 2011 Tohoku-oki earthquake was only the second giant (moment magnitude Mw ≥ 9.0) earthquake to occur in the last 50 years and is the most recent to be recorded using modern geophysical techniques. Available data place high-resolution constraints on the kinematics of earthquake rupture1, which have challenged prior knowledge about how much a fault can slip in a single earthquake and the seismic potential of a partially coupled megathrust interface2. But it is not clear what physical or structural characteristics controlled either the rupture extent or the amplitude of slip in this earthquake. Here we use residual topography and gravity anomalies to constrain the geological structure of the overthrusting (upper) plate offshore northeast Japan. These data reveal an abrupt southwest–northeast-striking boundary in upper-plate structure, across which gravity modelling indicates a south-to-north increase in the density of rocks overlying the megathrust of 150–200 kilograms per cubic metre. We suggest that this boundary represents the offshore continuation of the Median Tectonic Line, which onshore juxtaposes geological terranes composed of granite batholiths (in the north) and accretionary complexes (in the south)3. The megathrust north of the Median Tectonic Line is interseismically locked2, has a history of large earthquakes (18 with Mw > 7 since 1896) and produced peak slip exceeding 40 metres in the Tohoku-oki earthquake1. In contrast, the megathrust south of this boundary has higher rates of interseismic creep2, has not generated an earthquake with MJ > 7 (local magnitude estimated by the Japan Meteorological Agency) since 1923, and experienced relatively minor (if any) co-seismic slip in 20111. We propose that the structure and frictional properties of the overthrusting plate control megathrust coupling and seismogenic behaviour in northeast Japan.
Keishi Okazaki & Greg Hirth
Nature 530, 81–84 (04 February 2016) doi:10.1038/nature16501 Received 17 August 2015 Accepted 26 November 2015 Published online 03 February 2016
Intermediate-depth earthquakes in cold subduction zones are observed within the subducting oceanic crust, as well as the mantle1, 2. In contrast, intermediate-depth earthquakes in hot subduction zones predominantly occur just below the Mohorovičić discontinuity1. These observations have stimulated interest in relationships between blueschist-facies metamorphism and seismicity, particularly through dehydration reactions involving the mineral lawsonite1, 2. Here we conducted deformation experiments on lawsonite, while monitoring acoustic emissions, in a Griggs-type deformation apparatus. The temperature was increased above the thermal stability of lawsonite, while the sample was deforming, to test whether the lawsonite dehydration reaction induces unstable fault slip. In contrast to similar tests on antigorite, unstable fault slip (that is, stick–slip) occurred during dehydration reactions in the lawsonite and acoustic emission signals were continuously observed. Microstructural observations indicate that strain is highly localized along the fault (R1 and B shears), and that the fault surface develops slickensides (very smooth fault surfaces polished by frictional sliding). The unloading slope during the unstable slip follows the stiffness of the apparatus at all experimental conditions, regardless of the strain rate and temperature ramping rate. A thermomechanical scaling factor3 for the experiments is within the range estimated for natural subduction zones, indicating the potential for unstable frictional sliding within natural lawsonite layers.
Futoshi Yamashita, Eiichi Fukuyama, Kazuo Mizoguchi, Shigeru Takizawa, Shiqing Xu & Hironori Kawakata
Nature 528, 254–257 (10 December 2015) doi:10.1038/nature16138 Received 23 April 2015 Accepted 06 October 2015 Published online 09 December 2015
Determination of the frictional properties of rocks is crucial for an understanding of earthquake mechanics, because most earthquakes are caused by frictional sliding along faults. Prior studies using rotary shear apparatus1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13 revealed a marked decrease in frictional strength, which can cause a large stress drop and strong shaking, with increasing slip rate and increasing work rate. (The mechanical work rate per unit area equals the product of the shear stress and the slip rate.) However, those important findings were obtained in experiments using rock specimens with dimensions of only several centimetres, which are much smaller than the dimensions of a natural fault (of the order of 1,000 metres). Here we use a large-scale biaxial friction apparatus with metre-sized rock specimens to investigate scale-dependent rock friction. The experiments show that rock friction in metre-sized rock specimens starts to decrease at a work rate that is one order of magnitude smaller than that in centimetre-sized rock specimens. Mechanical, visual and material observations suggest that slip-evolved stress heterogeneity on the fault accounts for the difference. On the basis of these observations, we propose that stress-concentrated areas exist in which frictional slip produces more wear materials (gouge) than in areas outside, resulting in further stress concentrations at these areas. Shear stress on the fault is primarily sustained by stress-concentrated areas that undergo a high work rate, so those areas should weaken rapidly and cause the macroscopic frictional strength to decrease abruptly. To verify this idea, we conducted numerical simulations assuming that local friction follows the frictional properties observed on centimetre-sized rock specimens. The simulations reproduced the macroscopic frictional properties observed on the metre-sized rock specimens. Given that localized stress concentrations commonly occur naturally, our results suggest that a natural fault may lose its strength faster than would be expected from the properties estimated from centimetre-sized rock samples.
Thorsten W. Becker, Anthony R. Lowry, Claudio Faccenna, Brandon Schmandt, Adrian Borsa & Chunquan Yu
Nature 524, 458–461 (27 August 2015) doi:10.1038/nature14867 Received 01 March 2015 Accepted 22 June 2015 Published online 26 August 2015
Understanding the causes of intraplate earthquakes is challenging, as it requires extending plate tectonic theory to the dynamics of continental deformation. Seismicity in the western United States away from the plate boundary is clustered along a meandering, north–south trending ‘intermountain’ belt1. This zone coincides with a transition from thin, actively deforming to thicker, less tectonically active crust and lithosphere. Although such structural gradients have been invoked to explain seismicity localization2, 3, the underlying cause of seismicity remains unclear. Here we show results from improved mantle flow models that reveal a relationship between seismicity and the rate change of ‘dynamic topography’ (that is, vertical normal stress from mantle flow). The associated predictive skill is greater than that of any of the other forcings we examined. We suggest that active mantle flow is a major contributor to seismogenic intraplate deformation, while gravitational potential energy variations have a minor role. Seismicity localization should occur where convective changes in vertical normal stress are modulated by lithospheric strength heterogeneities. Our results on deformation processes appear consistent with findings from other mobile belts4, and imply that mantle flow plays a significant and quantifiable part in shaping topography, tectonics, and seismic hazard within intraplate settings.
Nature (2014) doi:10.1038/nature13778
Volume 512 Number 7514 pp231-342
Volume 510 Number 7505 pp312-436
Volume 509 Number 7501 pp399-526
Volume 509 Number 7499 pp134-254
Volume 493 Number 7433 pp451-570 24 January 2013
An earthquake source model in which stable, rate-strengthening behaviour at low slip rates is combined with coseismic weakening due to rapid shear heating of pore fluids, allowing unstable slip to occur in segments that can creep between events, explains a number of both long-term and coseismic observations of faults that hosted the 2011 Tohoku-Oki earthquake and the 1999 Chi-Chi earthquake.
Volume 491 Number 7422 pp7-154 1 November 2012
Laboratory experiments and seismological observations show that increased fault healing causes a disproportionately large amount of high-frequency seismic radiation to be produced during fault rupture, which may help to explain recent observations of large megathrust earthquakes.
The two earthquakes of respective magnitudes 8.6 and 8.2 that occurred off the coast of the Sumatra subduction zone on 11 April 2012 are shown to be part of a continuing boost of the intraplate deformation between India and Australia that followed the Aceh 2004 and Nias 2005 megathrust earthquakes.
See also
The magnitude 8.7 earthquake that occurred off the coast of the Sumatra subduction zone on 11 April 2012 is shown to have had an extraordinarily complex four-fault rupture; these great ruptures represent large lithospheric deformation that may eventually lead to a localized boundary between the Indian and Australian plates.
See also
Although strong remote aftershocks are exceedingly rare, their rate increased fivefold during the six days following the 2012 east Indian Ocean earthquake, perhaps as a result of the strike-slip nature of the 2012 event or a build up of close-to-failure nucleation sites.
See also
Continental collision slowing due to viscous mantle lithosphere rather than topography (29 February 2012)
Marin Kristen Clark
Origin of Columbia River flood basalt controlled by propagating rupture of the Farallon slab (15 February 2012)
Liu and Stegman
Correlation between deep fluids, tremor and creep along the central San Andreas fault (30 November 2011)
Becken et al.
Frictional ageing from interfacial bonding and the origins of rate and state friction (30 November 2011)
Li et al.
Subduction dynamics and the origin of Andean orogeny and the Bolivian orocline (23 November 2011)
Capitanio et al.
East Antarctic rifting triggers uplift of the Gamburtsev Mountains (16 November 2011)
Ferraccioli et al.
Coseismic and postseismic slip of the 2011 magnitude-9 Tohoku-Oki earthquake (15 June 2011)
S. Ozawa et al.
Shaping mobile belts by small-scale convection (3 June 2010)
C. Facenna and T. Becker
Striations, duration, migration and tidal response in deep tremor (15 July 2010)
S. Ide
Triggering of New Madrid seismicity by late-Pleistocene erosion (29 July 2010)
E. Calais et al.
Near-simultaneous great earthquakes at Tonga megathrust and outer rise in September 2009 (19 August 2010)
J. Beavan et al.
The 2009 Samoa-Tonga great earthquake triggered doublet (19 August 2010)
T. Lay et al.
Lithospheric layering in the North American craton (26 August 2010)
H. Yuan and B. Romanowicz
Olivine water contents in the continental lithosphere and the longevity of cratons (2 September 2010)
A. Peslier et al.
Remote triggering of fault-strength changes on the San Andreas fault at Parkfield (1 October 2009)
Taira et al.
Fluid and deformation regime of an advancing subduction system at Marlborough, New Zealand (6 August 2009)
Wannamaker et al.
BSSA Last Updated 3/7/12
Scaling Relationships of Source Parameters for Slow Slip Events (February 2012)
Gao et al.
A New Perspective on the Geometry of the San Andreas Fault in Southern California and Its Relationship to Lithospheric Structure (February 2012)
Fuis et al.
Spontaneous Dynamic Rupture Propagation beyond Fault Discontinuities: Effect of Thermal Pressurization (February 2012)
Urata et al.
Long-Term Creep Rates on the Hayward Fault: Evidence for Controls on the Size and Frequency of Large Earthquakes (February 2012)
Lienkaemper et al.
Nonvolcanic Tremor in the Aleutian Arc (December 2011)
Peterson et al.
Science Last Updated 3/21/12
A Change in the Geodynamics of Continental Growth 3 Billion Years Ago (16 March 2012)
Dhuime et al.
Plate Motions and Stresses from Global Dynamic Models (17 February 2012)
Ghosh and Holt
Near-Field Deformation from the El Mayor–Cucapah Earthquake Revealed by Differential LIDAR (10 February 2012)
Oskin et al.
Propagation of Slow Slip Leading Up to the 2011 Mw 9.0 Tohoku-Oki Earthquake (January 19 2012)
Kato et al.
The 2011 Tohoku-Oki Earthquake: Displacement Reaching the Trench Axis (2 December 2011)
Fujiwara et al.
The 2010 Mw 8.8 Maule Megathrust Earthquake of Central Chile, Monitored by GPS (17 June 2011)
Vigny et al.
Shallow Dynamic Overshoot and Energetic Deep Rupture in the 2011 Mw 9.0 Tohoku-Oki Earthquake (17 June 2011)
Ide et al.
Global surface wave tomography using seismic hum (2 October 2009)
Nishida et al.
Underplating in the Himalaya-Tibet Colision Zone Revealed by the Hi-CLIMB Experiment (11 September 2009)
Nabelek et al.
Adjoint Tomography of the Southern California Crust (21 August 2009)
Tape et al.
Nonvolcanic Tremor Evolution and the San Simeon and Parkfield, California, Earthquakes (10 July 2009)
Nadeau and Guilhem