Eric Bergman

Reactivated faulting near Cushing, Oklahoma: Increased potential for a triggered earthquake in an area of United States strategic infrastructure

In October 2014 two moderate-sized earthquakes (Mw 4.0 and 4.3) struck south of Cushing, Oklahoma, below the largest crude oil storage facility in the world. Combined analysis of the spatial distribution of earthquakes and regional moment tensor focal mechanisms indicate reactivation of a subsurface unnamed and unmapped left-lateral strike-slip fault. Coulomb failure stress change calculations using the relocated seismicity and slip distribution determined from regional moment tensors, allow for the possibility that the Wilzetta-Whitetail fault zone south of Cushing, Oklahoma, could produce a large, damaging earthquake comparable to the 2011 Prague event. Resultant very strong shaking levels (MMI VII) in the epicentral region present the possibility of this potential earthquake causing moderate to heavy damage to national strategic infrastructure and local communities.

Collection of a Reference Event Set for Regional and Teleseismic Location Calibration

A three-year consortium project, with members of Science Applications International Corp., University of Colorado at Boulder, Harvard University, Multimax Inc., Geophysical Institute of Israel, Western Services, and University of California at San Diego, was initiated in 2000 to improve locations and reduce uncertainties in the Middle East, North Africa, Europe, and Western Eurasia. The consortium developed high-resolution three-dimensional models of the Earths mantle to generate accurate travel-time predictions for regional and teleseismic P phases. Since the approach was purely model-based, a large set of high-quality reference events was needed to validate the model predictions. The consortium has spent considerable effort to collect, vet, and validate reference events located with 5-km accuracy or better by local networks and “promoted” reference events located with an accuracy of 7 km or better by application of multiple-event location techniques. Consortium members built an extensive network of contacts to solicit candidate reference events from local, regional, and national network operators. Strict methodologies were developed to identify candidate reference events in earthquake bulletins, and to validate and quality control the selected candidate reference events. The outcome of the consortium effort was a quality-controlled reference event list with nearly 2000 events and over 200,000 arrivals. The Reference Event List is provided as an electronic supplement to this article. Online material : Reference event database. Manuscript received 23 June 2003.

The 2013 Mw 6.2 Khaki‐Shonbe (Iran) Earthquake: Insights into seismic and aseismic shortening of the Zagros sedimentary cover

Determining the relationship between folding and faulting in fold and thrust belts is important for understanding the growth of geological structures, the depth extent of seismic slip, and consequently, the potential earthquake hazard. The 2013 Mw 6.2 Khaki-Shonbe earthquake occurred in the Simply Folded Belt of the Zagros Mountains, Iran. We combine seismological solutions, aftershock relocations, satellite interferometry, and field observations to determine fault geometry and its relationship with the structure, stratigraphy, and tectonics of the central Zagros. We find reverse slip on two along-strike, southwest dipping fault segments. The main shock rupture initiated at the lower northern end of the larger northwest segment. Based upon the hypocenter and rupture duration, slip on the smaller southern segment is likely aseismic. Both faults verge away from the foreland, toward the high-range interior, contrary to the fault geometries depicted in many structural cross sections of the Zagros. The modeled slip occurred over two mutually exclusive depth ranges above 10 km, resulting in long (∼16 km), narrow rupture segments (∼7 km). Aftershocks cluster in the depth range 3–14 km. This indicates reverse slip and coseismic shortening occurred mostly or exclusively in the sedimentary cover, with slip distributions likely to be lithologically controlled in depth by the Hormuz salt at the base of the sedimentary cover (∼10–12 km), and the Kazhdumi Formation mudrocks at upper levels (∼4–5 km). Our findings suggest lithology plays a significant role in the depth extent of slip found in reverse faults in folded belts, providing an important control on the potential size of earthquakes.

Toward a Comprehensive Catalog of Global Historical Seismicity

The United States Geological Survey (USGS) and the Colorado School of Mines (CSM) have initiated a project to locate more accurately all earthquakes recorded by instruments during the period 1900 to 1963. Seismicity for this period (hereafter referred to as historical seismicity, following Lee et al. [1988]) is still poorly understood, even for basic parameters such as earthquake locations (see Figure 1). In some cases this is the result of inherent limitations in the distribution, response characteristics, and timing of the instruments. However, locations for most of the pre-1964 earthquakes are poorly determined simply because modern data analysis techniques have yet to be applied to the available arrival-time observations, which are mainly preserved as printed bulletins and not in a computer-ready digital format. The arduous task of hand-entering these data has prevented the systematic analysis and relocation of historical seismicity.

2017 Valparaíso earthquake sequence and the megathrust patchwork of central Chile

In April 2017, a sequence of earthquakes offshore Valparaiso, Chile, raised concerns of a potential megathrust earthquake in the near future. The largest event in the 2017 sequence was a M6.9 on April 24th, seemingly co-located with the last great-sized earthquake in the region - a M8.0 in March 1985. The history of large earthquakes in this region shows significant variation in rupture size and extent, typically highlighted by a juxtaposition of large ruptures interspersed with smaller magnitude sequences. We show that the 2017 sequence ruptured an area between the two main slip patches during the 1985 earthquake, re-rupturing a patch that had previously slipped during the October 1973 M6.5 earthquake sequence. A significant gap in historic ruptures exists directly to the south of the 2017 sequence, with large enough moment deficit to host a great-sized earthquake in the near future, if it is locked.