Frequency-Dependent Seismic Attenuation in the Eastern United States as Observed from the 2011 Central Virginia Earthquake and Aftershock Sequence
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Research Article| January 14, 2014 Frequency‐Dependent Seismic Attenuation in the Eastern United States as Observed from the 2011 Central Virginia Earthquake and Aftershock Sequence Daniel E. McNamara; Daniel E. McNamara aU.S. Geological Survey, MS966, Box 25046, Denver, Colorado 80225 Search for other works by this author on: GSW Google Scholar Lind Gee; Lind Gee bU.S. Geological Survey, Albuquerque Seismological Laboratory, P.O. Box 82010, Albuquerque,New Mexico 87198‐2010 Search for other works by this author on: GSW Google Scholar Harley M. Benz; Harley M. Benz aU.S. Geological Survey, MS966, Box 25046, Denver, Colorado 80225 Search for other works by this author on: GSW Google Scholar Martin Chapman Martin Chapman cDepartment of Geosciences, 4044 Derring Hall, Virginia Tech, Blacksburg, Virginia 24061 Search for other works by this author on: GSW Google Scholar Bulletin of the Seismological Society of America (2014) 104 (1): 55–72. https://doi.org/10.1785/0120130045 Article history first online: 14 Jul 2017 Cite View This Citation Add to Citation Manager Share Icon Share MailTo Twitter LinkedIn Tools Icon Tools Get Permissions Search Site Citation Daniel E. McNamara, Lind Gee, Harley M. Benz, Martin Chapman; Frequency‐Dependent Seismic Attenuation in the Eastern United States as Observed from the 2011 Central Virginia Earthquake and Aftershock Sequence. Bulletin of the Seismological Society of America 2014;; 104 (1): 55–72. doi: https://doi.org/10.1785/0120130045 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 SocietyBulletin of the Seismological Society of America Search Advanced Search Abstract Ground shaking due to earthquakes in the eastern United States (EUS) is felt at significantly greater distances than in the western United States (WUS) and for some earthquakes it has been shown to display a strong preferential direction. Shaking intensity variation can be due to propagation path effects, source directivity, and/or site amplification. In this paper, we use S and Lg waves recorded from the 2011 central Virginia earthquake and aftershock sequence, in the Central Virginia Seismic Zone, to quantify attenuation as frequency‐dependent Q(f). In support of observations based on shaking intensity, we observe high Q values in the EUS relative to previous studies in the WUS with especially efficient propagation along the structural trend of the Appalachian mountains. Our analysis of Q(f) quantifies the path effects of the northeast‐trending felt distribution previously inferred from the U.S. Geological Survey (USGS) “Did You Feel It” data, historic intensity data, and the asymmetrical distribution of rockfalls and landslides. You do not have access to this content, please speak to your institutional administrator if you feel you should have access.Keywords:
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The northern part of Miyagi Prefecture is one of the most seismically active areas in the northeastern Japan arc. At present, many shallow earthquakes occur in and around the focal area of the 1962 Northern Miyagi Earthquake (M 6.5). The daily number of these earthquakes occurring now coincides with that expected from the lapse time-aftershock frequency relation, the extended Omori's law, of the 1962 event. A temporary seismic network set up in this area has revealed that the present seismicity is distributed on a plane dipping to the west-northwest at an angle of about 50°. Hypocenters of aftershocks within one month of the main shock occurrence are relocated by using S-P time data of the aftershocks. Relocated aftershocks are also distributed on a plane dipping to the west-northwest at approximately the same angle which corresponds to one of the nodal planes of the focal mechanism solution of the main shock. These observations indicate that the 1962 event ruptured along a plane inclined toward the west-northwest at an angle of -50° and that aftershocks of this event are still actively occurring now, more than 30 years after the main shock occurrence, along the fault plane or its northward extension.
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In order to clarify the origin of aftershocks, we precisely analyze the hypocenters and focal mechanisms of the aftershocks following the 2000 Western Tottori Earthquake, which occurred in the western part of Japan, using data from dense seismic observations. We investigate whether aftershocks occur on the mainshock fault plane on which coseismic slip occurred or they represent the rupture of fractures surrounding the mainshock fault plane. Based on the hypocenter distribution of the aftershocks, the subsurface fault structure of the mainshock is estimated using principal component analysis. As a result, we can obtain the detail fault structure composed of 8 best-fit planes. We demonstrate that the aftershocks around the mainshock fault are distributed within zones of 1.0–1.5 km in thicknesses, and their focal mechanisms are significantly diverse. This result suggests that most of the aftershocks represent the rupture of fractures surrounding the mainshock fault rather than the rerupture of the mainshock fault. The aftershocks have a much wider zone compared with the exhumed fault zone in field observations, suggesting that many aftershocks occur outside the fault damage zone. We find that most aftershocks except in and around the large-slip region are well explained by coseismic stress changes. These results suggest that the thickness of the aftershock distribution may be controlled by the stress changes caused by the heterogeneous slip distribution during the mainshock. The aftershock is also distributed within a much wider zone than the hypocenter distribution observed in swarm activity in the geothermal region, which is thought to be caused by the migration of hydrothermal fluid. This result implies a difference in generation processes: Stress changes due to the mainshock contribute primarily to the occurrence of aftershocks, whereas earthquake swarms in the geothermal region are caused by fluid migration within the localized zone.
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Abstract Regional distance surface waves are used to study the source parameters for moderate-size aftershocks of the 25 April 1992 Petrolia earthquake sequence. The Cascadia subduction zone had been relatively seismically inactive until the onset of the mainshock (Ms = 7.1). This underthrusting event establishes that the southern end of the North America-Gorda plate boundary is seismogenic. It was followed by two separate and distinct large aftershocks (Ms = 6.6 for both) occurring at 07:41 and 11:41 on 26 April, as well as thousands of other small aftershocks. Many of the aftershocks following the second large aftershock had magnitudes in the range of 4.0 to 5.5. Using intermediate-period surface-wave spectra, we estimate focal mechanisms and depths for one foreshock and six of the larger aftershocks (Md = 4.0 to 5.5). These seven events can be separated into two groups based on temporal, spatial, and principal stress orientation characteristics. Within two days of the mainshock, four aftershocks (Md = 4 to 5) occurred within 4 hr of each other that were located offshore and along the Mendocino fault. These four aftershocks comprise one group. They are shallow, thrust events with northeast-trending P axes. We interpret these aftershocks to represent internal compression within the North American accretionary prism as a result of Gorda plate subduction. The other three events compose the second group. The shallow, strike-slip mechanism determined for the 8 March foreshock (Md = 5.3) may reflect the right-lateral strike-slip motion associated with the interaction between the northern terminus of the San Andreas fault system and the eastern terminus of the Mendocino fault. The 10 May aftershock (Md = 4.1), located on the coast and north of the Mendocino triple junction, has a thrust fault focal mechanism. This event is shallow and probably occurred within the accretionary wedge on an imbricate thrust. A normal fault focal mechanism is obtained for the 5 June aftershock (Md = 4.8), located offshore and just north of the Mendocino fault. This event exhibits a large component of normal motion, representing internal failure within a rebounding accretionary wedge. These two aftershocks and the foreshock have dissimilar locations in space and time, but they do share a north-northwest oriented P axis.
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The M w 8.3 Bolivia earthquake occurred on June 9, 1994, at a depth of 636 km. This is the largest deep event in recorded history and ruptured a portion of the down‐going Nazca slab unknown to have ruptured previously. We recorded the main shock and aftershocks on the BANJO and SEDA portable, broadband seismic arrays deployed in Bolivia during this event. Myers et al. (this issue) identified and located 36 aftershocks (M>2) for the 10‐day period following the main shock. We use a grid search technique to determine focal mechanisms for 12 of these aftershocks ranging in magnitude from 2.7 to 5.3. We compare the observed P to SV and SH ratios to a series of synthetics that represent different fault plane orientations. We find consistent focal mechanisms with the T‐axis roughly horizontal and oriented approximately east‐west, and the P‐axis predominantly vertical. The aftershock focal mechanisms indicate a rotation of the P‐axis within the slab from down‐dip compression prior to the main shock to a near‐vertical direction afterwards. This observation is consistent with the release of shear stress on the near‐horizontal rupture plane and the subsequent rotation of the maximum compressive stress to a fault ‐normal orientation.
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We conducted a temporary seismic observation just after the occurrence of July 26, 2003, M6.4 northern Miyagi earthquake, in order to precisely locate aftershock hypocenters. Thirteen portable data-logger stations and one communication satellite telemetry station were installed in and around the focal area of the earthquake. Hypocenters of aftershocks were located by using data observed at those temporary stations and nearby stationary stations of Tohoku University, Hi-net and Japan Meteorological Agency. Obtained aftershock distribution delineates the fault planes of this M6.4 event in the depth range of 3-12km, dipping to the west at an angle of -50 degree in the northern part of the aftershock area and to the northwest again at -40 degree in the southern part. Temporary observation data also allowed us to determine focal mechanisms of many aftershocks. The results show that focal mechanism of reverse fault type is predominant in this earthquake sequence including foreshock (M5.6), main shock (M6.4) and most aftershocks. Directions of P axes, however, are classified into three groups. P axes of M5.6 foreshock and the main shock estimated from P-wave poralities have NW-SE directions. On the other hand, moment tensor solution of the main shock has a P axis of east-west direction. Moreover, the largest aftershock (M5.5), that occurred in northernmost part of the aftershock area, has a P axis of NE-SW direction. Aftershocks with P axis of NW-SE direction occurred mainly in the southern part of the aftershock area where M5.6 foreshock and the main shock ruptures initiated. Many aftershocks with P axes of east-west direction took place in the central part of the aftershock area where large amount of fault slips by the main shock were estimated by wave form inversions. Many aftershocks in the northernmost part of the aftershock area have the same focal mechanisms as that of the largest aftershock.
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Following the 1995 Hyogo-ken Nanbu earthquake, six seismic recorder sites were deployed in the northern part of the Awaji-shima island during the period between 8 and 29 August 1995. Analyzing continuous seismic recordings, we find an activity of micro-earthquakes with local magnitude below 2. Total number of well-located aftershocks is 448. The epicentral distribution and its NE-SW azimuthal trend are similar to those obtained just after the main shock. The trend is interrupted and changed by the Nojima fanit at the east-shore of the Awaji-shima island. The epicentral locations of aftershocks are generally coincident with surface active fault traces: e. g., the Nojima fault, the Higashiura fault, and the Asano fault. An aftershock cluster just beneath the south margin of the Nojima fault is found at depths between about 2 and 10km by dipping toward inland direction with an angle of 80°. We also find an aftershock cluster beneath the Higashiura fault, which did not slip coseismically at the mainshock. The cluster can be traced down to about 9km depth. Such distributions of aftershocks seem to indicate that these faults are structurally independent of each other down to this depth.
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The (1994) Arthur's Pass earthquake (Mw 6.7, South Island, New Zealand) had a complex aftershock sequence including events aligned with major mapped faults. To determine whether the major NE–SW-trending strike-slip faults in the region were activated during this aftershock sequence, we investigate the largest well-recorded aftershocks. The Arthur's Pass earthquake itself was a reverse-faulting event, but the majority of the aftershocks were strike-slip. We use the empirical Green's function method to obtain source time functions for four aftershocks (ML 4.1–5.1). We then invert for slip on each nodal plane and compare the variance reduction to determine which is the fault plane. The two largest earthquakes (ML 5.1 and ML 4.2) located close to the mapped trace of the Bruce fault both occurred on fault planes striking NNW–SSE, perpendicular to the strike of the Bruce and other regional strike-slip faults. The third earthquake studied (ML 4.1), located on a lineation of aftershocks parallel to the regional mapped trend, had a preferred fault plane with a NE–SW strike. The fourth aftershock (ML 4.1) was located close to the main-shock fault plane and had an oblique reverse mechanism. This earthquake exhibited northward directivity, but the fault plane could not be identified. The earthquake stress drops ranged from 1 to 10 MPa.
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On November 26, 2018, a Mw5.7 earthquake occurred on the northern edge of the Taiwan Shoal. The epicenter was not on the known deep fault, and the direction of the rupture was doubtful due to the lack of near-station control. Based on the broadband station recordings in Fujian, Guangdong and Taiwan, we use microseismic detection technology to obtain a more complete aftershock sequence. The number of detected aftershocks is 4 times that of the Fujian network catalogue. These aftershocks are distributed in a 2*10 km east-west trending strip. In addition, the focal mechanism solutions of the main shock and five strong aftershocks were inverted by the GCAP method. The inversion results showed that the main shock and the strong aftershocks were both strike-slip earthquakes with high dip angles, and the principal compressive stress direction was in the NW-SE direction. The obtained focal depths are slightly different, the focal depth of the main shock is 14 km, and the focal depth of strong aftershocks above Mw3.9 is between 12 and 17 km. There are significant differences in aftershock activities between the east and west of the main shock. The aftershock activities on the east are mainly concentrated within one month after the main shock, while the aftershock activities on the west continued to be active within the six months after the main shock, indicating that the stress on the east side is relatively fully released after the main earthquake. Moreover, the multi-channel seismic profiles passing through the epicenter reveal that the shallow active faults in the epicenter are EW, with significant strike-slip characteristics, and their spatial locations are consistent with the distribution of aftershocks and the focal mechanism solution. Based on the temporal-spatial distribution of aftershocks, focal mechanism solutions and the characteristics of shallow active faults, we inferred that the seismic fault of the Mw5.7 earthquake is a near east-west trending Taiwan Shoal Fault, which may be an extension of the B Fault of Taiwan Island. The strong right-handed shear stress in the upper crust generated by the lateral subduction rate difference is the dynamic cause of the 2018 Taiwan Shoal Mw5.7 earthquake.
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