The 2015–2017 Pamir Earthquake Sequence: Fore-, Main-, and Aftershocks, Seismotectonics, Fault Interaction, and Fluid Processes
Wasja BlochSabrina MetzgerBernd SchurrXiaohui YuanLothar RatschbacherSanaa ReuterQiang XuJunmeng ZhaoShohrukh MurodkulovIlhomjon Oimuhammadzoda
0
Citation
119
Reference
10
Related Paper
Keywords:
Seismotectonics
Sequence (biology)
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.
Microearthquake
Focal mechanism
Fault plane
Cite
Citations (7)
On June 9, 1994 the M w 8.3 Bolivia earthquake (636 km depth) occurred in a region which had not experienced significant, deep seismicity for at least 30 years. The mainshock and aftershocks were recorded in Bolivia on the BANJO and SEDA broadband seismic arrays and on the San Calixto Network. We used the joint hypocenter determination method to determine the relative location of the aftershocks. We have identified no foreshocks and 89 aftershocks ( m > 2.2) for the 20‐day period following the mainshock. The frequency of aftershock occurrence decreased rapidly, with only one or two aftershocks per day occuring after day two. The temporal decay of aftershock activity is similar to shallow aftershock sequences, but the number of aftershocks is two orders of magnitude less. Additionally, a m b ∼6, apparently triggered earthquake occurred just 10 minutes after the mainshock about 330 km east‐southeast of the mainshock at a depth of 671 km. The aftershock sequence occurred north and east of the mainshock and extends to a depth of 665 km. The aftershocks define a slab striking N68°W and dipping 45°NE. The strike, dip, and location of the aftershock zone are consistent with this seismicity being confined within the downward extension of the subducted Nazca plate. The location and orientation of the aftershock sequence indicate that the subducted Nazca plate bends between the NNW striking zone of deep seismicity in western Brazil and the N‐S striking zone of seismicity in central Bolivia. A tear in the deep slab is not necessitated by the data. A subset of the aftershock hypocenters cluster along a subhorizontal plane near the depth of the mainshock, favoring a horizontal fault plane. The horizontal dimensions of the mainshock [ Beck et al., this issue; Silver et al., 1995] and slab defined by the aftershocks are approximately equal, indicating that the mainshock ruptured through the slab.
Hypocenter
Cite
Citations (40)
Hypocenter
Earthquake rupture
Cite
Citations (12)
Fault plane
Cite
Citations (3)
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.
Focal mechanism
Thrust fault
Accretionary wedge
Cite
Citations (5)
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.
Fault plane
Focal mechanism
Slab
Deep-focus earthquake
Cite
Citations (8)
Abstract The Pasadena earthquake (ML = 4.9) of 3 December 1988 occurred at a depth of 16 km, probably on the Santa Monica - Raymond fault, which is recognized as one of the most important faults in the Los Angeles basin for its potential seismic hazard. Prior to this event, no earthquake larger than magnitude 4 had been recorded since 1930 in this area. High-quality seismograms were recorded with the Pasadena very broadband (VBB) system (IRIS-TERRAscope station) not only for the mainshock but also for the aftershocks at epicentral distances of 3 to 4 km. We determined the focal mechanisms of 9 aftershocks using these data; for most of the aftershocks, the first-motion data are too sparse to determine the mechanism. We combined the first-motion data and the waveform data of P, SV, and SH waves recorded with the VBB instrument to determine the mechanism and seismic moment of nine aftershocks. The average orientations of the P and T axes of the aftershocks are consistent with the strike of the Raymond fault. The ratio of the logarithm of cumulative seismic moment of aftershocks to that of the seismic moment of the mainshock is significantly smaller than commonly observed.
Seismogram
Seismic moment
Focal mechanism
Asperity (geotechnical engineering)
Cite
Citations (12)
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.
Hypocenter
Focal mechanism
Cite
Citations (0)
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.
Fault plane
Fault trace
Hypocenter
Cite
Citations (11)
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.
Epicenter
Focal mechanism
Microseism
Shoal
Earthquake rupture
Cite
Citations (0)