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Pdf/11/5/750/4830085/750.Pdf by Guest on 02 October 2021 JACOBI and EBEL | Berne Earthquake Swarms RESEARCH RESEARCH Seismotectonic implications of the Berne earthquake swarms west-southwest of Albany, New York Robert D. Jacobi1,* and John E. Ebel2,* 1DEPARTMENT OF GEOLOGY, UNIVERSITY AT BUFFALO, 126 COOKE HALL, BUFFALO, NEW YORK 14260, USA 2WESTON OBSERVATORY, DEPARTMENT OF EARTH & ENVIRONMENTAL SCIENCES, BOSTON COLLEGE, 381 CONCORD ROAD, WESTON, MASSACHUSETTS 02493, USA ABSTRACT Five earthquake swarms occurred from 2007 to 2011 near Berne, New York. Each swarm consisted of four to twenty-four earthquakes ranging from M 1.0 to M 3.1. The network determinations of the focal depths ranged from 6 km to 24 km, 77% of which were ≥14 km. High-precision, relative location analysis showed that the events in the 2009 and 2011 swarms delineate NNE-SSW orientations, collinear with NNE trends established by the distribution of the spatially distinct swarms; the events in the 2010 swarm aligned WNW-ESE. Focal mechanisms determined from the largest event in the swarms include one nodal plane that strikes NNE, collinear with the distribution of the swarms and relative events within the swarms. Two, possibly related explanations exist for the Berne earthquake swarms. (1) The swarms were caused by reactivations of proposed blind NE- and NW-striking rift structures associated with the NE-trending Scranton gravity high. These rift structures, of uncertain age (Protero zoic or Neoproterozoic/Iapetan opening), have been modeled at depths appropriate for the seismicity. (2) The NNE-trending swarms were caused by reactivations of NNE-striking faults mapped at the surface north-northeast of the earthquake swarms. Both mod- els involve reactivation of rift-related faults, and the development of the NNE-striking surficial faults in the second model probably was guided by the blind rift faults in the first model. The Berne swarms may be evidence that these faults are seismically capable and, if so, could sustain a maximum event on the order of Mw 5.7–6.6, based on fault segment length. LITHOSPHERE; v. 11; no. 5; p. 750–764 | Published online 6 September 2019 https:// doi .org /10 .1130 /L1066 .1 INTRODUCTION on seismically active faults is generally lacking, with modern regional seismic networks so far the hazard computations are based primarily on have been generally insufficient in number to The most accurate seismic hazard assess- the locations of the small and moderate earth- delineate the local geologic structures that are ments that can be made for a region utilize quakes in the region (Petersen et al., 2014). seismically active (e.g., Ebel and Kafka, 1991; knowledge about the potentially active faults For these areas, inferences about the possible Coppersmith et al., 2012). For this reason, earth- in the region, particularly, the locations of indi- locations of future strong earthquakes and the quake swarms in eastern North America provide vidual active faults, the maximum magnitudes magnitudes of those earthquakes are made from a special opportunity to look for alignments of that can occur on those faults, and the recurrence the seismicity data set alone, because direct geo- epicenters that might illuminate the local struc- rates of potentially damaging earthquakes on logical information about active faults is lacking, ture or structures on which the earthquakes took those faults. For active plate-boundary regions except at a few localities, such as Mineral, Vir- place. One such locality in the eastern United like California, active faults are revealed by ginia (e.g., McNamara et al., 2014; Horton et States is southwest of Albany, New York, in the (1) surface faulting in areas of recent strong al., 2015a; Shah et al., 2015), and Charlevoix, Mohawk Valley region near the town of Berne, earthquakes, (2) geomorphic signatures of Quebec, Canada (for a review of this and other where five earthquake swarms were recorded geologically recent earthquake activity, and sites, see Coppersmith et al., 2012; Horton et by local and regional seismic network stations (3) alignments of seismicity on suspected active al., 2015a). Thus, any new information about from 2007 to 2011 (Figs. 1 and 2; Jacobi et structures. For example, Petersen et al. (2014) possible seismically active geologic structures al., 2012; Ebel et al., 2014; Kim et al., 2016). included information on over 2000 active faults in an intraplate region like the central and east- Each swarm consisted of four to twenty-four in the western United States in their 2014 ver- ern United States is important to improve future earthquakes ranging in magnitude from 1.0 to sion of the U.S. National Seismic Hazard Maps. seismic hazard assessment. 3.1 (Tables 1 and 2). Because so many of these In contrast, for intraplate regions like the central For an intraplate region like eastern North events were scattered across such a small region, and eastern United States, because information America, the rate of seismicity is generally the relative location method of Ebel et al. (2008) much lower than for plate-boundary regions, can be used to compute very precise spatial dis- and strong earthquakes associated with surface tributions of the events in these swarms. Precise Robert Jacobi http://orcid.org /0000 -0001 -5352 -4084 ruptures are rare. Furthermore, in most places, relative locations of the events in these swarms *[email protected]; [email protected] the routine, small earthquakes that are detected provide an opportunity to delineate the faults Geological© 2019 The SocietyAuthors. of Gold America Open |Access: LITHOSPHERE This paper | Volume is published 11 | underNumber the 5 terms| www.gsapubs.org of the CC-BY-NC license. 750 Downloaded from http://pubs.geoscienceworld.org/gsa/lithosphere/article-pdf/11/5/750/4830085/750.pdf by guest on 02 October 2021 JACOBI AND EBEL | Berne earthquake swarms RESEARCH that may have been active in these earthquake 0 77 W 750 W 0 swarms and to relate the geology to any lin- 73 W eaments or surfaces defined by the earthquake 450 N loci established by the relative location anal- MAGNITUDE ysis. The 2011 swarm began ~1 d before the LC 2011 M 5.8 Mineral, Virginia, earthquake and A > M 5.0 continued for ~110 h after the time of the Min- VT NY eral main shock (e.g., Chapman, 2013; Horton 4.0 - 4.9 ADK et al., 2015b), which occurred ~600 km to the Taconic thrust f. south-southwest along the general strike of the 3.0 - 3.9 Mesozoic Atlantic-opening structures in east- 2.0 - 2.9 ern North America. Although the occurrence G < M 2.0 Taconic of the 2011 Berne swarm at the same time as faults “normal” the Mineral, Virginia, Mw 5.8 main shock may have been coincidental, both the Mineral main NH shock and the 2011 Berne swarm took place on GF Mohawk Valley 0 NE-SW–oriented structures. Saratoga-McGregor F. 43 N The purpose of this study was twofold. The B first objective was to compute highly precise relative locations of the events in each local 100 km MA swarm in order to look for alignments of the epicenters. Relative locations of the events in ALBANY the different swarms near Berne were computed, Mesozoic rifts Fig. 2 and the locations of the different swarms rel- HB Appalachian Orogen ative to each other also were estimated. The CT second purpose of this study was to compare Siluro-Devonian in the locations of the events and the orientations Foreland NY of the hypocentral alignments in the different Cambro-Ordovician in swarms to the local geology in order to assess Foreland the local faults that may have been associated Precambrian NJ 410 N with the seismicity. 2011 SEISMICITY Faults NB The major result of this study is that the AND OUTCROPS NYC hypocenters of the Berne area swarms are con- sistent in depth and alignment with presumed and modeled Proterozoic or Neoproterozoic/ Iapetan-opening rift structures in the region of Figure 1. General geology and earthquake epicenters (yellow squares) for eastern New York the northeastern termination of the Scranton State and western New England. Seismicity is from January 1975 to February 2014, from the gravity high (Benoit et al., 2014). The epicenters Weston Observatory Network. The focal mechanism “A” is for the M 5.0 2002 Au Sable Forks, New York, earthquake (http://www.ldeo.columbia.edu/LCSN/NYQuake_2002/hrv.cmt.txt; See- of the Berne area swarms also align with the ber et al., 2002); the epicenter is indicated by the yellow star. The southern focal mechanisms southern extensions of Taconic normal faults, in the black box labeled “B” show the end members of the range of possible focal mechanisms which themselves may have been guided by the (this study) for the largest event of the Berne area earthquake swarms. The focal mechanism older rift structures. The relationship between “G” is for the 1983 Mw 4.7 Goodnow, New York, earthquake (Seeber and Armbruster, 1986). the Berne seismicity and known geologic The focal mechanism labeled “GF” (this study) is for a MLg 2.6 earthquake on 25 August 2013 structures is inherently speculative due to the that occurred near Glen Falls, New York. ADK—Adirondack dome as outlined by the extent of the Precambrian rocks in New York State, LC—Lake Champlain, HB—Hartford Basin, NB— midcrustal depths at which the Berne-area earth- Newark Basin, NYC—New York City, Saratoga-McGregor F.—Saratoga-McGregor fault, Taconic quake swarms took place. thrust f.—Taconic thrust faults. General geology and faults are after Bradley and Kidd (1991). Selected Taconic “normal” faults were omitted for clarity (more complete suite of faults shown SEISMOTECTONIC AND GEOLOGIC in Fig. 2). The “normal” faults may have varying degrees of oblique and strike-slip motion (see SETTING text); the fault locations in the Precambrian basement are primarily inferred from topographic lineaments.
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