ALBERT

Description: Between 2010 and 2013, the Pollino Mountains region (south Italy), already proposed as a seismic gap, was affected by a seismic crisis of more than 5000 small-to-moderate earthquakes (maximum magnitude M L  5.0). Preliminary analyses performed in a previous work highlighted that this activity can be ascribed to normal faulting on north-northwest-trending west-dipping dislocation surfaces consistent with the general seismotectonic frame of the southern Apennines. This work contributes additional data and a more sophisticated analyses that highlight new features of the seismic swarm and support a new interpretation for the study area. We obtained high-precision locations and focal mechanisms using the double-difference method and the cut-and-paste waveform inversion method, respectively. The 3D patterns of hypocenters and focal mechanisms consistently image an ~10-km-long north-northwest-striking and west-dipping fault zone between 5 and 10 km depth, with predominantly extensional kinematics. The high-resolution data show that this zone broadens from north to south as a result of secondary faulting. The depicted geometry, with preliminary geological observation, leads to the hypothesis of multiple seismogenic normal faults rooted into more regional shallow-dipping detachments inherited from the pre-existing Apennine thrust tectonics. Online Material: Table of estimated focal mechanism parameters.

Description: A persistent earthquake sequence in northeast Ohio includes many distinct fore-main-aftershock subsequences, illuminates two faults, and was triggered by fluid injection. The first known earthquake from within 30 km of Ashtabula was an M (sub blg) 3.8 mainshock that shook the downtown area in 1987. Seismicity has continued at an average of about one felt event per year. The largest magnitude so far, M (sub blg) 4.3, caused slight damage (modified Mercalli intensity VI) on 26 January 2001. The latest subsequence started in July 2003 with an M (sub blg) 2.6 event. Accurate hypocenters and focal mechanisms are available from three local seismograph deployments in 1987, 2001, 2003 and from regional broadband seismograms. These hypocenters are in the Precambrian basement, 0-2 km below the 1.8-km-deep Paleozoic unconformity, and illuminate two distinct planar east-west-striking sources zones 4 km apart, one in 1987 about 1.5 km long, the other in 2001 and 2003 about 5 km long. We interpret them as steep subparallel faults slipping left laterally in the current regime. Like many of the faults that ruptured in hazardous stable continental region (SCR) earthquakes, these faults were previously unknown and probably have small post-Precambrian displacements. The 1987 source was active a year after onset of class 1 fluid injection only 0.7 km north of the fault. The second fault, 5 km south of the injection well, became active in 2000, while the 1987 source was inactive. The well injected about 164 m (super 3) /day of waste fluid into the 1.8-km-deep basal sand-stone with about 100 bars of wellhead pressure from May 1986 to June 1994. An annular high pore-pressure anomaly is expected to expand along this hydraulically confined horizon at the top of the basement, even after injection ends and pressure drops near the well. Over 16 years, seismicity has shifted southward from 〈 or =1 to 5-8 km from the point of injection. It seems to initiate when and where a significant pore-pressure rise intersects pre-existing faults close to failure and to be turned off when pressure starts dropping back. The largest earthquakes postdated the end of injection at both Ashtabula and at the Rocky Mountain Arsenal near Denver, Colorado. Anthropogenic earthquake hazard may thus persist after the causative activity has ceased but can generally be closely monitored. High-stress and low-strain rates in SCRs can account for a larger proportion of triggered earthquakes in the eastern United States and other SCRs than in active regions.

Description: A catalog of 383 earthquakes in southeastern New York, southwestern Connecticut, northern New Jersey, and eastern Pennsylvania, including metropolitan New York City and Philadelphia, is compiled from historical and instrumental data from 1677 through 2006. A magnitude-felt area relationship is used to calculate the equivalent magnitude m (sub b) Lg prior to the advent of abundant instrumental data in 1974. Revised locations are computed for a number of historic earthquakes. Most hypocenters are concentrated in older terranes bordering the Mesozoic Newark basin in the Reading, Manhattan, and Trenton prongs and in similar rocks found at a shallow depth beneath the coastal plain from south of New York City across central New Jersey. Historic shocks of m (sub b) Lg 3 and larger were most numerous in the latter zone. The largest known event, m (sub b) Lg 5.25, occurred just offshore of New York City in 1884. Many earthquakes have occurred beneath the 12-km wide Ramapo seismic zone (RSZ) in the eastern part of the Reading prong, where station coverage was the most extensive since 1974. The southeastern boundary of the RSZ, which is nearly vertical, extends from near the surface trace of the Mesozoic Ramapo fault to depths of 12-15 km. Because the Mesozoic border fault dips about 50 degrees -60 degrees southeast, earthquakes of the RSZ are occurring within middle Proterozoic through early Paleozoic rocks. Which faults within the RSZ are active is unclear. Well-located activity in the Manhattan prong since 1974 extends to similar depths but cuts off abruptly at all depths along a northwest-striking boundary extending from Peekskill, New York, to Stamford, Connecticut. That boundary, which is subparallel to brittle faults farther south, is inferred to be a similar fault or fault zone. Those brittle features may have formed between the Newark, Hartford, and New York bight basins to accommodate Mesozoic extension. The Great Valley in the northwestern part of the study region is nearly devoid of known earthquakes. While few focal mechanism solutions and in situ stress measurements of high quality are available, the maximum compressive stress is nearly horizontal and is oriented about N64 degrees E, similar to that in adjacent areas. The catalog is likely complete for events of m (sub b) Lg〉5 since 1737, 〉 or =3.5 since 1840, and 〉 or =3.0 since 1928. Extrapolation of the frequency-magnitude relationship indicates that an event of m (sub b) Lg〉 or =6.0 is expected about once per 670 yr.

Description: Between 2010 and 2013, the Pollino Mountains region (south Italy), already proposed as a seismic gap, was affected by a seismic crisis of more than 5000 small-to-moderate earthquakes (maximum magnitude ML 5.0). Preliminary analyses performed in a previous work highlighted that this activity can be ascribed to normal faulting on north-northwest-trending west-dipping dislocation surfaces consistent with the general seismotectonic frame of the southern Apennines. This work contributes additional data and a more sophisticated analyses that highlight new features of the seismic swarm and support a new interpretation for the study area. We obtained high-precision locations and focal mechanisms using the double-difference method and the cut-and-paste waveform inversion method, respectively. The 3D patterns of hypocenters and focal mechanisms consistently image an ∼10-km-long north-northwest-striking and west-dipping fault zone between 5 and 10 km depth, with predominantly extensional kinematics. The high-resolution data show that this zone broadens from north to south as a result of secondary faulting. The depicted geometry, with preliminary geological observation, leads to the hypothesis of multiple seismogenic normal faults rooted into more regional shallow-dipping detachments inherited from the pre-existing Apennine thrust tectonics.Online Material: Table of estimated focal mechanism parameters.