ALBERT

Description: [1]  We use high resolution interferometric synthetic aperture radar and GPS measurements of crustal motion across the southern San Andreas Fault system to investigate the effects of elastic heterogeneity and fault geometry on inferred slip rates and locking depths. Geodetically measured strain rates are asymmetric with respect to the mapped traces of both the southern San Andreas and San Jacinto faults. Two possibilities have been proposed to explain this observation: large contrasts in crustal rigidity across the faults, or an alternate fault geometry such as a dipping San Andreas fault or a blind segment of the San Jacinto Fault. We evaluate these possibilities using a two-dimensional elastic model accounting for heterogeneous structure computed from the Southern California Earthquake Center crustal velocity model CVM-H 6.3. The results demonstrate that moderate variations in elastic properties of the crust do not produce a significant strain rate asymmetry and have only a minor effect on the inferred slip rates. However, we find that small changes in the location of faults at depth can strongly impact the results. Our preferred model includes a San Andreas Fault dipping northeast at 60°, and two active branches of the San Jacinto fault zone. In this case, we infer nearly equal slip rates of 18 ± 1 and 19 ± 2 mm/yr for the San Andreas and San Jacinto fault zones, respectively. These values are in good agreement with geologic measurements representing average slip rates over the last 10 4 –10 6  years, implying steady long-term motion on these faults.

Description: Detailed geodetic imaging of earthquake ruptures enhances our understanding of earthquake physics and associated ground shaking. The 25 April 2015 moment magnitude 7.8 earthquake in Gorkha, Nepal was the first large continental megathrust rupture to have occurred beneath a high-rate (5-hertz) Global Positioning System (GPS) network. We used GPS and interferometric synthetic aperture radar data to model the earthquake rupture as a slip pulse ~20 kilometers in width, ~6 seconds in duration, and with a peak sliding velocity of 1.1 meters per second, which propagated toward the Kathmandu basin at ~3.3 kilometers per second over ~140 kilometers. The smooth slip onset, indicating a large (~5-meter) slip-weakening distance, caused moderate ground shaking at high frequencies (〉1 hertz; peak ground acceleration, ~16% of Earth's gravity) and minimized damage to vernacular dwellings. Whole-basin resonance at a period of 4 to 5 seconds caused the collapse of tall structures, including cultural artifacts.〈br /〉〈span class="detail_caption"〉Notes: 〈/span〉Galetzka, J -- Melgar, D -- Genrich, J F -- Geng, J -- Owen, S -- Lindsey, E O -- Xu, X -- Bock, Y -- Avouac, J-P -- Adhikari, L B -- Upreti, B N -- Pratt-Sitaula, B -- Bhattarai, T N -- Sitaula, B P -- Moore, A -- Hudnut, K W -- Szeliga, W -- Normandeau, J -- Fend, M -- Flouzat, M -- Bollinger, L -- Shrestha, P -- Koirala, B -- Gautam, U -- Bhatterai, M -- Gupta, R -- Kandel, T -- Timsina, C -- Sapkota, S N -- Rajaure, S -- Maharjan, N -- New York, N.Y. -- Science. 2015 Sep 4;349(6252):1091-5. doi: 10.1126/science.aac6383. Epub 2015 Aug 6.〈br /〉〈span class="detail_caption"〉Author address: 〈/span〉Department of Geology and Planetary Sciences, California Institute of Technology (Caltech), Pasadena, CA 91125, USA. ; BerkeleySeismological Laboratory, University of California (UC)-Berkeley, Berkeley, CA 94720, USA. ; Cecil H. and Ida M. Green Institute of Geophysics and Planetary Physics, Scripps Institution of Oceanography, UC-San Diego, La Jolla, CA 92037, USA. ; Jet Propulsion Laboratory (JPL), Caltech, Pasadena, CA 91109, USA. ; Department of Earth Sciences, University of Cambridge, Cambridge CB2 3EQ, UK. Department of Geology and Planetary Sciences, California Institute of Technology (Caltech), Pasadena, CA 91125, USA. ; Department of Mines and Geology, Lainchour, Kathmandu, Nepal. ; Nepal Academy of Science and Technology, Khumaltar, Lalitpur, Nepal. ; Department of Geological Sciences, Central Washington University (CWU), Ellensberg, WA 98926, USA. ; Tri-Chandra Campus, Tribhuvan University, Ghantaghar, Kathmandu, Nepal. ; U.S. Geological Survey (USGS), Pasadena, CA 91106, USA. ; Pacific Northwest Geodetic Array and Department of Geological Sciences, CWU, Ellensberg, WA 98926, USA. ; UNAVCO, Boulder, CO 80301, USA. ; Departement Analyse et Sureveillance de l'Environnement (DASE), Commissariat a l'Energie Atomique (CEA), 91297 Bruyeres-le-Chatel, Arpajon, France.〈br /〉〈span class="detail_caption"〉Record origin:〈/span〉 〈a href="http://www.ncbi.nlm.nih.gov/pubmed/26249228" target="_blank"〉PubMed〈/a〉

Description: Assessment of seismic hazard relies on estimates of how large an area of a tectonic fault could potentially rupture in a single earthquake. Vital information for these forecasts includes which areas of a fault are locked and how the fault is segmented. Much research has focused on exploring downdip limits to fault rupture from chemical and thermal boundaries, and along-strike barriers from subducted structural features, yet we regularly see only partial rupture of fully locked fault patches that could have ruptured as a whole in a larger earthquake. Here we draw insight into this conundrum from the 25 April 2015 M w 7.8 Gorkha (Nepal) earthquake. We invert geodetic data with a structural model of the Main Himalayan thrust in the region of Kathmandu, Nepal, showing that this event was generated by rupture of a décollement bounded on all sides by more steeply dipping ramps. The morphological bounds explain why the event ruptured only a small piece of a large fully locked seismic gap. We then use dynamic earthquake cycle modeling on the same fault geometry to reveal that such events are predicted by the physics. Depending on the earthquake history and the details of rupture dynamics, however, great earthquakes that rupture the entire seismogenic zone are also possible. These insights from Nepal should be applicable to understanding bounds on earthquake size on megathrusts worldwide.

Description: Assessment of seismic hazard relies on estimates of how large an area of a tectonic fault could potentially rupture in a single earthquake. Vital information for these forecasts includes which areas of a fault are locked and how the fault is segmented. Much research has focused on exploring downdip limits to fault rupture from chemical and thermal boundaries, and along-strike barriers from subducted structural features, yet we regularly see only partial rupture of fully locked fault patches that could have ruptured as a whole in a larger earthquake. Here we draw insight into this conundrum from the 25 April 2015 M w 7.8 Gorkha (Nepal) earthquake. We invert geodetic data with a structural model of the Main Himalayan thrust in the region of Kathmandu, Nepal, showing that this event was generated by rupture of a décollement bounded on all sides by more steeply dipping ramps. The morphological bounds explain why the event ruptured only a small piece of a large fully locked seismic gap. We then use dynamic earthquake cycle modeling on the same fault geometry to reveal that such events are predicted by the physics. Depending on the earthquake history and the details of rupture dynamics, however, great earthquakes that rupture the entire seismogenic zone are also possible. These insights from Nepal should be applicable to understanding bounds on earthquake size on megathrusts worldwide.