Five out of six functioning creepmeters on southern California faults recorded slip triggered at the time of some or all of the three largest events of the 1992 Landers earthquake sequence. Digital creep data indicate that dextral slip was triggered within 1 min of each mainshock and that maximum slip velocities occurred 2 to 3 min later. The duration of triggered slip events ranged from a few hours to several weeks. We note that triggered slip occurs commonly on faults that exhibit fault creep. To account for the observation that slip can be triggered repeatedly on a fault, we propose that the amplitude of triggered slip may be proportional to the depth of slip in the creep event and to the available near-surface tectonic strain that would otherwise eventually be released as fault creep. We advance the notion that seismic surface waves, perhaps amplified by sediments, generate transient local conditions that favor the release of tectonic strain to varying depths. Synthetic strain seismograms are presented that suggest increased pore pressure during periods of fault-normal contraction may be responsible for triggered slip, since maximum dextral shear strain transients correspond to times of maximum fault-normal contraction.
The Gulf of California is an excellent laboratory for studying sedimentary processes on time scales that are not resolvable in the open ocean. The high biological productivity and the unique physical character of the gulf combine to produce sedimentological processes that preserve annual phenomena. This volume is organized into six sections. Part 1 covers historical exploration of the area. Part 2 includes 5 chapters detailing information contained on the 5 fold-out maps that accompany the volume. Part 3 consists of chapters on regional geophysics and geology. Part 4 covers satellite geodesy. Part 5's seven chapters discuss physical oceanograpy, primary productivity, and sedimentology. Part 6 covers hydrothermal processes.
Research Article| November 15, 2017 Low‐Frequency Tilt Seismology with a Precision Ground‐Rotation Sensor M. P. Ross; M. P. Ross aCenter for Experimental Nuclear Physics and Astrophysics (CENPA), University of Washington, 4311 Mason Pl NE, Seattle, Washington 98105 U.S.A., mpross2@uw.edu Search for other works by this author on: GSW Google Scholar K. Venkateswara; K. Venkateswara aCenter for Experimental Nuclear Physics and Astrophysics (CENPA), University of Washington, 4311 Mason Pl NE, Seattle, Washington 98105 U.S.A., mpross2@uw.edu Search for other works by this author on: GSW Google Scholar C. A. Hagedorn; C. A. Hagedorn aCenter for Experimental Nuclear Physics and Astrophysics (CENPA), University of Washington, 4311 Mason Pl NE, Seattle, Washington 98105 U.S.A., mpross2@uw.edu Search for other works by this author on: GSW Google Scholar J. H. Gundlach; J. H. Gundlach aCenter for Experimental Nuclear Physics and Astrophysics (CENPA), University of Washington, 4311 Mason Pl NE, Seattle, Washington 98105 U.S.A., mpross2@uw.edu Search for other works by this author on: GSW Google Scholar J. S. Kissel; J. S. Kissel bLIGO Hanford Observatory, 127124 N Rt. 10, Richland, Washington 99352‐0159 U.S.A. Search for other works by this author on: GSW Google Scholar J. Warner; J. Warner bLIGO Hanford Observatory, 127124 N Rt. 10, Richland, Washington 99352‐0159 U.S.A. Search for other works by this author on: GSW Google Scholar H. Radkins; H. Radkins bLIGO Hanford Observatory, 127124 N Rt. 10, Richland, Washington 99352‐0159 U.S.A. Search for other works by this author on: GSW Google Scholar T. J. Shaffer; T. J. Shaffer bLIGO Hanford Observatory, 127124 N Rt. 10, Richland, Washington 99352‐0159 U.S.A. Search for other works by this author on: GSW Google Scholar M. W. Coughlin; M. W. Coughlin cLIGO Laboratory, California Institute of Technology, MC 100‐36, Pasadena, California 91125 U.S.A. Search for other works by this author on: GSW Google Scholar P. Bodin P. Bodin dDepartment of Earth and Space Sciences, University of Washington, 4000 15th Avenue NE, Seattle, Washington 98195‐1310 U.S.A. Search for other works by this author on: GSW Google Scholar Seismological Research Letters (2018) 89 (1): 67–76. https://doi.org/10.1785/0220170148 Article history first online: 15 Nov 2017 Cite View This Citation Add to Citation Manager Share Icon Share Facebook Twitter LinkedIn MailTo Tools Icon Tools Get Permissions Search Site Citation M. P. Ross, K. Venkateswara, C. A. Hagedorn, J. H. Gundlach, J. S. Kissel, J. Warner, H. Radkins, T. J. Shaffer, M. W. Coughlin, P. Bodin; Low‐Frequency Tilt Seismology with a Precision Ground‐Rotation Sensor. Seismological Research Letters 2017;; 89 (1): 67–76. doi: https://doi.org/10.1785/0220170148 Download citation file: Ris (Zotero) Refmanager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search Dropdown Menu toolbar search search input Search input auto suggest filter your search All ContentBy SocietySeismological Research Letters Search Advanced Search ABSTRACT We describe measurements of the rotational component of teleseismic surface waves using an inertial high‐precision ground‐rotation sensor installed at the Laser Interferometer Gravitational‐Wave Observatory (LIGO) Hanford Observatory (LHO). The sensor has a noise floor of 0.4 nrad/Hz at 50 mHz and a translational coupling of less than 1 μrad/m enabling translation‐free measurement of small rotations. We present observations of the rotational motion from Rayleigh waves of six teleseismic events from varied locations and with magnitudes ranging from M 6.7 to 7.9. These events were used to estimate phase dispersion curves that show agreement with a similar analysis done with an array of three STS‐2 seismometers also located at LHO. You do not have access to this content, please speak to your institutional administrator if you feel you should have access.
This article presents evidence for the channeling of strain energy released by the Ms = 7.4 Landers, California, earthquake within the eastern California shear zone (ECSZ). We document an increase in seismicity levels during the 22-hr period starting with the Landers earthquake and culminating 22 hr later with the Ms = 5.4 Little Skull Mountain (LSM), Nevada, earthquake. We evaluate the completeness of regional seismicity catalogs during this period and find that the continuity of post-Landers strain release within the ECSZ is even more pronounced than is evident from the catalog data. We hypothesize that regional-scale connectivity of faults within the ECSZ and LSM region is a critical ingredient in the unprecedented scale and distribution of remotely triggered earthquakes and geodetically manifest strain changes that followed the Landers earthquake. The viability of static strain changes as triggering agents is tested using numerical models. Modeling results illustrate that regional-scale fault connectivity can increase the static strain changes by approximately an order of magnitude at distances of at least 280 km, the distance between the Landers and LSM epicenters. This is possible for models that include both a network of connected faults that slip “sympathetically” and realistic levels of tectonic prestrain. Alternatively, if dynamic strains are a more significant triggering agent than static strains, ECSZ structure may still be important in determining the distribution of triggered seismic and aseismic deformation.
We identified seven locations on or near the transform plate boundary in California where nonvolcanic tremor was triggered by the 2002 Denali earthquake. This result implies that the conditions essential for nonvolcanic tremor exist in a range of tectonic environments. Models explaining tremor typically require conditions endemic to subduction zones, that is, high temperatures and fluid pressures, because previously tremor was nearly exclusively documented in subduction zones. The absence of tremor in geothermal areas is inconsistent with such models. Additionally, we found no correlation between creeping or locked faults and tremor, contrary to predictions of frictional models of tremor.
Abstract We develop simple relations to estimate dynamic displacement gradients (and hence the strains and rotations) during earthquakes in the lake-bed zone of the Valley of Mexico, where the presence of low-velocity, high-water content clays in the uppermost layers cause dramatic amplification of seismic waves and large strains. The study uses results from a companion article (Bodin et al., 1997) in which the data from an array at Roma, a lake-bed site, were analyzed to obtain displacement gradients. In this article, we find that the deformations at other lake-bed sites may differ from those at Roma by a factor of 2 to 3. More accurate estimates of the dominant components of the deformation at an individual instrumented lake-bed site may be obtained from the maximum horizontal velocity and displacement, νmax and umax, at the surface. The maximum surface strain ɛmax is related to νmax by ɛmax = νmax/C, with C ∼ 0.6 km/sec. From the analysis of data from sites equipped with surface and borehole sensors, we find that the vertical gradient of peak horizontal displacement (Δumax/Δz) computed from sensors at 0 and 30 m equals (umax)z=0/Δz, Δz = 30 m, within a factor of 1.5. This is the largest gradient component, and the latter simple relation permits its estimation from surface records alone. The observed profiles of umax versus depth suggest a larger gradient in some depth range of 10 to 20 m, in agreement with synthetic calculations presented in Bodin et al. (1997). From the free-field recordings of the 19 September 1985 Michoacan earthquake, we estimate a maximum surface strain, ɛmax, between 0.05% and 0.11%, and a lower bound for the peak vertical gradient (Δumax/Δz) between 0.3% and 1.3%. This implies that (1) the extensive failure of water pipe joints during the Michoacan earthquake in the valley occurred at axial strains of about 0.1%, not 0.38% as previously reported, and (2) the clays of the valley behave almost linearly even at shear strain of about 1%, in agreement with laboratory tests. The available data in the valley can be used to predict deformations during future earthquakes using self-similar earthquake scaling.
