Shallow dynamic overshoot and energetic deep rupture in the 2011 Mw 9.0 Tohoku-Oki earthquake

Strong spatial variation of rupture characteristics in the moment magnitude (M(w)) 9.0 Tohoku-Oki megathrust earthquake controlled both the strength of shaking and the size of the tsunami that followed. Finite-source imaging reveals that the rupture consisted of a small initial phase, deep rupture for up to 40 seconds, extensive shallow rupture at 60 to 70 seconds, and continuing deep rupture lasting more than 100 seconds. A combination of a shallow dipping fault and a compliant hanging wall may have enabled large shallow slip near the trench. Normal faulting aftershocks in the area of high slip suggest dynamic overshoot on the fault. Despite prodigious total slip, shallower parts of the rupture weakly radiated at high frequencies, whereas deeper parts of the rupture radiated strongly at high frequencies.     

The 2011 Tohoku-Oki earthquake: displacement reaching the trench axis

We detected and measured coseismic displacement caused by the 11 March 2011 Tohoku-Oki earthquake [moment magnitude (M(W)) 9.0] by using multibeam bathymetric surveys. The difference between bathymetric data acquired before and after the earthquake revealed that the displacement extended out to the axis of the Japan Trench, suggesting that the fault rupture reached the trench axis. The sea floor on the outermost landward area moved about 50 meters horizontally east-southeast and ~10 meters upward. The large horizontal displacement lifted the sea floor by up to 16 meters on the landward slope in addition to the vertical displacement.     

A high-frequency secondary event during the 2004 Parkfield earthquake

By using seismic records of the 2004 magnitude 6.0 Parkfield earthquake, we identified a burst of high-frequency seismic radiation that occurred about 13 kilometers northwest of the hypocenter and 5 seconds after rupture initiation. We imaged this event in three dimensions by using a waveform back-projection method, as well as by timing distinct arrivals visible on many of the seismograms. The high-frequency event is located near the south edge of a large slip patch seen in most seismic and geodetic inversions, indicating that slip may have grown abruptly at this point. The time history obtained from full-waveform back projection suggests a rupture velocity of 2.5 kilometers per second. Energy estimates for the subevent, together with long-period slip inversions, indicate a lower average stress drop for the northern part of the Parkfield earthquake compared with that for the region near its hypocenter, which is in agreement with stress-drop estimates obtained from small-magnitude aftershocks.     

Coseismic and Postseismic Fault Slip for the 17 August 1999, M = 7.5, Izmit, Turkey Earthquake

We use Global Positioning System (GPS) observations and elastic half-space models to estimate the distribution of coseismic and postseismic slip along the Izmit earthquake rupture. Our results indicate that large coseismic slip (reaching 5.7 meters) is confined to the upper 10 kilometers of the crust, correlates with structurally distinct fault segments, and is relatively low near the hypocenter. Continued surface deformation during the first 75 days after the earthquake indicates an aseismic fault slip of as much as 0.43 meters on and below the coseismic rupture. These observations are consistent with a transition from unstable (episodic large earthquakes) to stable (fault creep) sliding at the base of the seismogenic zone.     

Extended nucleation of the 1999 Mw 7.6 Izmit earthquake

Laboratory and theoretical studies suggest that earthquakes are preceded by a phase of developing slip instability in which the fault slips slowly before accelerating to dynamic rupture. We report here that one of the best-recorded large earthquakes to date, the 1999 moment magnitude (M(w)) 7.6 Izmit (Turkey) earthquake, was preceded by a seismic signal of long duration that originated from the hypocenter. The signal consisted of a succession of repetitive seismic bursts, accelerating with time, and increased low-frequency seismic noise. These observations show that the earthquake was preceded for 44 minutes by a phase of slow slip occurring at the base of the brittle crust. This slip accelerated slowly initially, and then rapidly accelerated in the 2 minutes preceding the earthquake.     

Under the hood of the earthquake machine: toward predictive modeling of the seismic cycle

Advances in observational, laboratory, and modeling techniques open the way to the development of physical models of the seismic cycle with potentially predictive power. To explore that possibility, we developed an integrative and fully dynamic model of the Parkfield segment of the San Andreas Fault. The model succeeds in reproducing a realistic earthquake sequence of irregular moment magnitude (M(w)) 6.0 main shocks--including events similar to the ones in 1966 and 2004--and provides an excellent match for the detailed interseismic, coseismic, and postseismic observations collected along this fault during the most recent earthquake cycle. Such calibrated physical models provide new ways to assess seismic hazards and forecast seismicity response to perturbations of natural or anthropogenic origins.     

Propagation of slow slip leading up to the 2011 M(w) 9.0 Tohoku-Oki earthquake

Many large earthquakes are preceded by one or more foreshocks, but it is unclear how these foreshocks relate to the nucleation process of the mainshock. On the basis of an earthquake catalog created using a waveform correlation technique, we identified two distinct sequences of foreshocks migrating at rates of 2 to 10 kilometers per day along the trench axis toward the epicenter of the 2011 moment magnitude (M(w)) 9.0 Tohoku-Oki earthquake in Japan. The time history of quasi-static slip along the plate interface, based on small repeating earthquakes that were part of the migrating seismicity, suggests that two sequences involved slow-slip transients propagating toward the initial rupture point. The second sequence, which involved large slip rates, may have caused substantial stress loading, prompting the unstable dynamic rupture of the mainshock.     

The 2010 Mw 8.8 Maule megathrust earthquake of Central Chile, monitored by GPS

Large earthquakes produce crustal deformation that can be quantified by geodetic measurements, allowing for the determination of the slip distribution on the fault. We used data from Global Positioning System (GPS) networks in Central Chile to infer the static deformation and the kinematics of the 2010 moment magnitude (M(w)) 8.8 Maule megathrust earthquake. From elastic modeling, we found a total rupture length of ~500 kilometers where slip (up to 15 meters) concentrated on two main asperities situated on both sides of the epicenter. We found that rupture reached shallow depths, probably extending up to the trench. Resolvable afterslip occurred in regions of low coseismic slip. The low-frequency hypocenter is relocated 40 kilometers southwest of initial estimates. Rupture propagated bilaterally at about 3.1 kilometers per second, with possible but not fully resolved velocity variations.     

Displacement above the hypocenter of the 2011 Tohoku-Oki earthquake

The moment magnitude (M(w)) = 9.0 2011 Tohoku-Oki mega-thrust earthquake occurred off the coast of northeastern Japan. Combining Global Positioning System (GPS) and acoustic data, we detected very large sea-floor movements associated with this event directly above the focal region. An area with more than 20 meters of horizontal displacement, that is, four times larger than those detected on land, stretches several tens of kilometers long along the trench; the largest amount reaches about 24 meters toward east-southeast just above the hypocenter. Furthermore, nearly 3 meters of vertical uplift occurred, contrary to observed terrestrial subsidence.     

Frictional afterslip following the 2005 Nias-Simeulue earthquake, Sumatra

Continuously recording Global Positioning System stations near the 28 March 2005 rupture of the Sunda megathrust [moment magnitude (Mw) 8.7] show that the earthquake triggered aseismic frictional afterslip on the subduction megathrust, with a major fraction of this slip in the up-dip direction from the main rupture. Eleven months after the main shock, afterslip continues at rates several times the average interseismic rate, resulting in deformation equivalent to at least a M(w) 8.2 earthquake. In general, along-strike variations in frictional behavior appear to persist over multiple earthquake cycles. Aftershocks cluster along the boundary between the region of coseismic slip and the up-dip creeping zone. We observe that the cumulative number of aftershocks increases linearly with postseismic displacements; this finding suggests that the temporal evolution of aftershocks is governed by afterslip.     

Prospects for larger or more frequent earthquakes in the los angeles metropolitan region

Far too few moderate earthquakes have occurred within the Los Angeles, California, metropolitan region during the 200-year-long historic period to account for observed strain accumulation, indicating that the historic era represents either a lull between clusters of moderate earthquakes or part of a centuries-long interseismic period between much larger (moment magnitude, M(w), 7.2 to 7.6) events. Geologic slip rates and relations between moment magnitude, average coseismic slip, and rupture area show that either of these hypotheses is possible, but that the latter is the more plausible of the two. The average time between M(w) 7.2 to 7.6 earthquakes from a combination of six fault systems within the metropolitan area was estimated to be about 140 years.     

Major earthquakes occur regularly on an isolated plate boundary fault

The scarcity of long geological records of major earthquakes, on different types of faults, makes testing hypotheses of regular versus random or clustered earthquake recurrence behavior difficult. We provide a fault-proximal major earthquake record spanning 8000 years on the strike-slip Alpine Fault in New Zealand. Cyclic stratigraphy at Hokuri Creek suggests that the fault ruptured to the surface 24 times, and event ages yield a 0.33 coefficient of variation in recurrence interval. We associate this near-regular earthquake recurrence with a geometrically simple strike-slip fault, with high slip rate, accommodating a high proportion of plate boundary motion that works in isolation from other faults. We propose that it is valid to apply time-dependent earthquake recurrence models for seismic hazard estimation to similar faults worldwide.     

Teleseismic search for slow precursors to large earthquakes

Some large earthquakes display low-frequency seismic anomalies that are best explained by episodes of slow, smooth deformation immediately before their high-frequency origin times. Analysis of the low-frequency spectra of 107 shallow-focus earthquakes revealed 20 events that had slow precursors (95 percent confidence level); 19 were slow earthquakes associated with the ocean ridge-transform system, and 1 was a slow earthquake on an intracontinental transform fault in the East African Rift system. These anomalous earthquakes appear to be compound events, each comprising one or more ordinary (fast) ruptures in the shallow seismogenic zone initiated by a precursory slow event in the adjacent or subjacent lithosphere.     

Slip deficit on the san andreas fault at parkfield, california, as revealed by inversion of geodetic data

A network of geodetic lines spanning the San Andreas fault near the rupture zone of the 1966 Parkfield, California, earthquake (magnitude M = 6) has been repeatedly surveyed since 1959. In the study reported here the average rates of line-length change since 1966 were inverted to determine the distribution of interseismic slip rate on the fault. These results indicate that the Parkfield rupture surface has not slipped significantly since 1966. Comparison of the geodetically determined seismic moment of the 1966 earthquake with the interseismic slip-deficit rate suggests that the strain released by the latest shock will most likely be restored between 1984 and 1989, although this may not occur until 1995. These results lend independent support to the earlier forecast of an M = 6 earthquake near Parkfield within 5 years of 1988.     

Detection of large prehistoric earthquakes in the pacific northwest by microfossil analysis

Geologic and palynological evidence for rapid sea level change approximately 3400 and approximately 2000 carbon-14 years ago (3600 and 1900 calendar years ago) has been found at sites up to 110 kilometers apart in southwestern British Columbia. Submergence on southern Vancouver Island and slight emergence on the mainland during the older event are consistent with a great (magnitude M >/= 8) earthquake on the Cascadia subduction zone. The younger event is characterized by submergence throughout the region and may also record a plate-boundary earthquake or a very large crustal or intraplate earthquake. Microfossil analysis can detect small amounts of coseismic uplift and subsidence that leave little or no lithostratigraphic signature.     

Surface displacements in the 1906 san francisco and 1989 loma prieta earthquakes

The horizontal displacements accompanying the 1906 San Francisco earthquake and the 1989 Loma Prieta earthquake are computed from geodetic survey measurements. The 1906 earthquake displacement field is entirely consistent with right-lateral strike slip on the San Andreas fault. In contrast, the 1989 Loma Prieta earthquake exhibited subequal components of strike slip and reverse faulting. This result, together with other seismic and geologic data, may indicate that the two earthquakes occurred on two different fault planes.     

Earthquakes, active faults, and geothermal areas in the imperial valley, california

A dense seismograph network in the Imperial Valley recorded a series of earthquake swarms along the Imperial and Brawley faults and a diffuse pattern of earthquakes along the San Jacinto fault. Two known geothermal areas are closely associated with these earthquake swarms. This seismicity pattern demonstrates that seismic slip is occurring along both the Imperial-Brawley and San Jacinto fault systems.     

Active normal faulting in the upper Rhine graben and paleoseismic identification of the 1356 Basel earthquake

We have identified an active normal fault in the epicentral area of the Basel (Switzerland) earthquake of 18 October 1356, the largest historical seismic event in central Europe. The event of 1356 and two prehistoric events have been characterized on the fault with geomorphological analysis, geophysical prospecting, and trenching. Carbon-14 dating indicates that the youngest event occurred in the interval 610 to 1475 A.D. and may correspond to the 1356 Basel earthquake. The occurrence of the three earthquakes induced a total of 1.8 meters of vertical displacement in the past 8500 years for a mean uplift rate of 0.21 millimeters per year. These successive ruptures on the normal fault indicate the potential for strong ground movements in the Basel region and should be taken into account to refine the seismic hazard estimates along the Rhine graben.     

Observation of long supershear rupture during the magnitude 8.1 Kunlunshan earthquake

The 2001 Kunlunshan earthquake was an extraordinary event that produced a 400-km-long surface rupture. Regional broadband recordings of this event provide an opportunity to accurately observe the speed at which a fault ruptures during an earthquake, which has important implications for seismic risk and for understanding earthquake physics. We determined that rupture propagated on the 400-km-long fault at an average speed of 3.7 to 3.9 km/s, which exceeds the shear velocity of the brittle part of the crust. Rupture started at sub-Rayleigh wave velocity and became supershear, probably approaching 5 km/s, after about 100 km of propagation.     

Earthquake source fault beneath Tokyo

Devastating earthquakes occur on a megathrust fault that underlies the Tokyo metropolitan region. We identify this fault with use of deep seismic reflection profiling to be the upper surface of the Philippine Sea plate. The depth to the top of this plate, 4 to 26 kilometers, is much shallower than previous estimates based on the distribution of seismicity. This shallower plate geometry changes the location of maximum finite slip of the 1923 Kanto earthquake and will affect estimations of strong ground motion for seismic hazards analysis within the Tokyo region.