ALBERT

Description: Geophysical observations from the 2011 moment magnitude (M(w)) 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 M(w) 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.〈br /〉〈span class="detail_caption"〉Notes: 〈/span〉Simons, Mark -- Minson, Sarah E -- Sladen, Anthony -- Ortega, Francisco -- Jiang, Junle -- Owen, Susan E -- Meng, Lingsen -- Ampuero, Jean-Paul -- Wei, Shengji -- Chu, Risheng -- Helmberger, Donald V -- Kanamori, Hiroo -- Hetland, Eric -- Moore, Angelyn W -- Webb, Frank H -- New York, N.Y. -- Science. 2011 Jun 17;332(6036):1421-5. doi: 10.1126/science.1206731. Epub 2011 May 19.〈br /〉〈span class="detail_caption"〉Author address: 〈/span〉Seismological Laboratory, Division of Geological and Planetary Sciences, California Institute of Technology, Pasadena, CA 91125, USA. simons@caltech.edu〈br /〉〈span class="detail_caption"〉Record origin:〈/span〉 〈a href="http://www.ncbi.nlm.nih.gov/pubmed/21596953" target="_blank"〉PubMed〈/a〉

Description: Earth's deepest earthquakes occur in subducting oceanic lithosphere, where temperatures are lower than in ambient mantle. On 24 May 2013, a magnitude 8.3 earthquake ruptured a 180-kilometer-long fault within the subducting Pacific plate about 609 kilometers below the Sea of Okhotsk. Global seismic P wave recordings indicate a radiated seismic energy of ~1.5 x 10(17) joules. A rupture velocity of ~4.0 to 4.5 kilometers/second is determined by back-projection of short-period P waves, and the fault width is constrained to give static stress drop estimates (~12 to 15 megapascals) compatible with theoretical radiation efficiency for crack models. A nearby aftershock had a stress drop one to two orders of magnitude higher, indicating large stress heterogeneity in the deep slab, and plausibly within the rupture process of the great event.〈br /〉〈span class="detail_caption"〉Notes: 〈/span〉Ye, Lingling -- Lay, Thorne -- Kanamori, Hiroo -- Koper, Keith D -- New York, N.Y. -- Science. 2013 Sep 20;341(6152):1380-4. doi: 10.1126/science.1242032.〈br /〉〈span class="detail_caption"〉Author address: 〈/span〉Department of Earth and Planetary Sciences, University of California Santa Cruz, Santa Cruz, CA 95064, USA.〈br /〉〈span class="detail_caption"〉Record origin:〈/span〉 〈a href="http://www.ncbi.nlm.nih.gov/pubmed/24052306" target="_blank"〉PubMed〈/a〉

Description: Earthquake rupture speeds exceeding the shear-wave velocity have been reported for several shallow strike-slip events. Whether supershear rupture also can occur in deep earthquakes is unclear, because of their enigmatic faulting mechanism. Using empirical Green's functions in both regional and teleseismic waveforms, we observed supershear rupture during the 2013 moment magnitude (M(w)) 6.7 deep earthquake beneath the Sea of Okhotsk, an aftershock of the large deep earthquake (M(w) 8.3). The M(w) 6.7 event ruptured downward along a steeply dipping fault plane at an average speed of 8 kilometers per second, suggesting efficient seismic energy generation. Comparing it to the highly dissipative 1994 M(w) 8.3 Bolivia earthquake, the two events represent end members of deep earthquakes in terms of energy partitioning and imply that there is more than one rupture mechanism for deep earthquakes.〈br /〉〈span class="detail_caption"〉Notes: 〈/span〉Zhan, Zhongwen -- Helmberger, Donald V -- Kanamori, Hiroo -- Shearer, Peter M -- New York, N.Y. -- Science. 2014 Jul 11;345(6193):204-7. doi: 10.1126/science.1252717.〈br /〉〈span class="detail_caption"〉Author address: 〈/span〉Institute of Geophysics and Planetary Physics, Scripps Institution of Oceanography, University of California, San Diego, La Jolla, CA 92093-0225, USA. Seismological Laboratory, California Institute of Technology, 1200 East California Boulevard, Pasadena, CA 91125, USA. zwzhan@ucsd.edu zwzhan@gmail.com. ; Seismological Laboratory, California Institute of Technology, 1200 East California Boulevard, Pasadena, CA 91125, USA. ; Institute of Geophysics and Planetary Physics, Scripps Institution of Oceanography, University of California, San Diego, La Jolla, CA 92093-0225, USA.〈br /〉〈span class="detail_caption"〉Record origin:〈/span〉 〈a href="http://www.ncbi.nlm.nih.gov/pubmed/25013073" target="_blank"〉PubMed〈/a〉

Description: On 2010 March 11, a sequence of large, shallow continental crust earthquakes shook central Chile. Two normal faulting events with magnitudes around M w 7.0 and M w 6.9 occurred just 15 min apart, located near the town of Pichilemu. These kinds of large intraplate, inland crustal earthquakes are rare above the Chilean subduction zone, and it is important to better understand their relationship with the 2010 February 27, M w 8.8, Maule earthquake, which ruptured the adjacent megathrust plate boundary. We present a broad seismological analysis of these earthquakes by using both teleseismic and regional data. We compute seismic moment tensors for both events via a W-phase inversion, and test sensitivities to various inversion parameters in order to assess the stability of the solutions. The first event, at 14 hr 39 min GMT, is well constrained, displaying a fault plane with strike of N145°E, and a preferred dip angle of 55°SW, consistent with the trend of aftershock locations and other published results. Teleseismic finite-fault inversions for this event show a large slip zone along the southern part of the fault, correlating well with the reported spatial density of aftershocks. The second earthquake (14 hr 55 min GMT) appears to have ruptured a fault branching southward from the previous ruptured fault, within the hanging wall of the first event. Modelling seismograms at regional to teleseismic distances ( 〉 10°) is quite challenging because the observed seismic wave fields of both events overlap, increasing apparent complexity for the second earthquake. We perform both point- and extended-source inversions at regional and teleseismic distances, assessing model sensitivities resulting from variations in fault orientation, dimension, and hypocentre location. Results show that the focal mechanism for the second event features a steeper dip angle and a strike rotated slightly clockwise with respect to the previous event. This kind of geological fault configuration, with secondary rupture in the hanging wall of a large normal fault, is commonly observed in extensional geological regimes. We propose that both earthquakes form part of a typical normal fault diverging splay, where the secondary fault connects to the main fault at depth. To ascertain more information on the spatial and temporal details of slip for both events, we gathered near-fault seismological and geodetic data. Through forward modelling of near-fault synthetic seismograms we build a kinematic k –2 earthquake source model with spatially distributed slip on the fault that, to first-order, explains both coseismic static displacement GPS vectors and short-period seismometer observations at the closest sites. As expected, the results for the first event agree with the focal mechanism derived from teleseismic modelling, with a magnitude M w 6.97. Similarly, near-fault modelling for the second event suggests rupture along a normal fault, M w 6.90, characterized by a steeper dip angle (dip = 74°) and a strike clockwise rotated (strike = 155°) with respect to the previous event.

Description: Author(s): Y. Kanamori, H. Matsueda, and S. Ishihara [Phys. Rev. Lett. 107, 167403] Published Tue Oct 11, 2011

Description: Stress drop, a measure of static stress change in earthquakes, is the subject of numerous investigations. Stress drop in an earthquake is likely to be spatially varying over the fault, creating a stress drop distribution. Representing this spatial distribution by a single number, as commonly done, implies averaging in space. In this study, we investigate similarities and differences between three different averages of the stress drop distribution used in earthquake studies. The first one, $\overline{\Delta \sigma }_M$ , is the commonly estimated stress drop based on the seismic moment and fault geometry/dimensions. It is known that $\overline{\Delta \sigma }_M$ corresponds to averaging the stress drop distribution with the slip distribution due to uniform stress drop as the weighting function. The second one, $\overline{\Delta \sigma }_A$ , is the simplest (unweighted) average of the stress drop distribution over the fault, equal to the difference between the average stress levels on the fault before and after an earthquake. The third one, $\overline{\Delta \sigma }_E$ , enters discussions of energy partitioning and radiation efficiency; we show that it corresponds to averaging the stress drop distribution with the actual final slip at each point as the weighting function. The three averages, $\overline{\Delta \sigma }_M$ , $\overline{\Delta \sigma }_A$ , and $\overline{\Delta \sigma }_E$ , are often used interchangeably in earthquake studies and simply called ‘stress drop’. Yet they are equal to each other only for ruptures with spatially uniform stress drop, which results in an elliptical slip distribution for a circular rupture. Indeed, we find that other relatively simple slip shapes—such as triangular, trapezoidal or sinusoidal—already result in stress drop distributions with notable differences between $\overline{\Delta \sigma }_M$ , $\overline{\Delta \sigma }_A$ , and $\overline{\Delta \sigma }_E$ . Introduction of spatial slip heterogeneity results in further systematic differences between them, with $\overline{\Delta \sigma }_E$ always being larger than $\overline{\Delta \sigma }_M$ , a fact that we have proven theoretically, and $\overline{\Delta \sigma }_A$ almost always being the smallest. In particular, the value of the energy-related $\overline{\Delta \sigma }_E$ significantly increases in comparison to the moment-based $\overline{\Delta \sigma }_M$ with increasing roughness of the slip distribution over the fault. Previous studies used $\overline{\Delta \sigma }_M$ in place of $\overline{\Delta \sigma }_E$ in computing the radiation ratio R that compares the radiated energy in earthquakes to a characteristic part of their strain energy change. Typical values of R for large earthquakes were found to be from 0.25 to 1. Our finding that $\overline{\Delta \sigma }_E \ge \overline{\Delta \sigma }_M$ allows us to interpret the values of R as the upper bound. We determine the restrictions placed by such estimates on the evolution of stress with slip at the earthquake source. We also find that $\overline{\Delta \sigma }_E$ can be approximated by $\overline{\Delta \sigma }_M$ if the latter is computed based on a reduced rupture area.

Description: Pacific Ocean crust west of southwest North America was formed by Cenozoic seafloor spreading between the large Pacific Plate and smaller microplates. The eastern limit of this seafloor, the continent–ocean boundary, is the fossil trench along which the microplates subducted and were mostly destroyed in Miocene time. The Pacific–North America Plate boundary motion today is concentrated on continental fault systems well to the east, and this region of oceanic crust is generally thought to be within the rigid Pacific Plate. Yet, the 2012 December 14 M w 6.3 earthquake that occurred about 275 km west of Ensenada, Baja California, Mexico, is evidence for continued tectonism in this oceanic part of the Pacific Plate. The preferred main shock centroid depth of 20 km was located close to the bottom of the seismogenic thickness of the young oceanic lithosphere. The focal mechanism, derived from both teleseismic P -wave inversion and W -phase analysis of the main shock waveforms, and the 12 aftershocks of M  ~3–4 are consistent with normal faulting on northeast striking nodal planes, which align with surface mapped extensional tectonic trends such as volcanic features in the region. Previous Global Positioning System (GPS) measurements on offshore islands in the California Continental Borderland had detected some distributed Pacific and North America relative plate motion strain that could extend into the epicentral region. The release of this lithospheric strain along existing zones of weakness is a more likely cause of this seismicity than current thermal contraction of the oceanic lithosphere or volcanism. The main shock caused weak to moderate ground shaking in the coastal zones of southern California, USA, and Baja California, Mexico, but the tsunami was negligible.