An official website of the United States government
Abstract A great earthquake struck the Semidi segment of the plate boundary along the Alaska Peninsula on 29 July 2021, re‐rupturing part of the 1938 rupture zone. The 2021MW8.2 Chignik earthquake occurred just northeast of the 22 July 2020MW7.8 Simeonof earthquake, with little slip overlap. Analysis of teleseismicPandSHwaves, regional Global Navigation Satellite System (GNSS) displacements, and near‐field and far‐field tsunami observations provides a good resolution of the 2021 rupture process. During ∼60‐s long faulting, the slip was nonuniformly distributed along the megathrust over depths from 32 to 40 km, with up to ∼12.9‐m slip in an ∼170‐km‐long patch. The 40–45 km down‐dip limit of slip is well constrained by GNSS observations along the Alaska Peninsula. Tsunami observations preclude significant slip from extending to depths <25 km, confining all coseismic slip to beneath the shallow continental shelf. Most aftershocks locate seaward of the large‐slip zones, with a concentration of activity up‐dip of the deeper southwestern slip zone. Some localized aftershock patches locate beneath the continental slope. The surface‐wave magnitudeMSof 8.1 for the 2021 earthquake is smaller thanMS = 8.3–8.4 for the 1938 event. Seismic and tsunami data indicate that slip in 1938 was concentrated in the eastern region of its aftershock zone, extending beyond the Semidi Islands, where the 2021 event did not rupture.
more »
« less
The 29 July 2021 M W 8.2 Chignik, Alaska Peninsula Earthquake Rupture Inferred From Seismic and Geodetic Observations: Re‐Rupture of the Western 2/3 of the 1938 Rupture Zone
Abstract On 29 July 2021, anMW8.2 thrust‐faulting earthquake ruptured offshore of the Alaska Peninsula within the rupture zone of the 1938MW8.2 earthquake. The spatiotemporal distribution of megathrust slip is resolved by jointly inverting regional and teleseismic broadband waveforms along with co‐seismic static and high‐rate GNSS displacements. The primarily unilateral rupture expanded northeastward, away from the rupture zone of the 22 July 2020MW7.8 Shumagin earthquake. Large slip extends along approximately 175 km, spanning about two third of the estimated 1938 aftershock zone, with well‐bounded depth from 20 to 40 km, and up to 8.6 m slip near the hypocenter. The rupture terminated in the eastern portion of the 1938 aftershock zone in a region of very large geodetic slip deficit where peak slip appears to have occurred in the 1938 rupture. The 2021 and 1938 events do not have similar slip distributions and do not indicate persistent asperities.
more »
« less
- Award ID(s):
- 1802364
- PAR ID:
- 10369655
- Publisher / Repository:
- DOI PREFIX: 10.1029
- Date Published:
- Journal Name:
- Geophysical Research Letters
- Volume:
- 49
- Issue:
- 4
- ISSN:
- 0094-8276
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
More Like this
-
-
Abstract The 1938MS8.3 and 2021MW8.2 earthquakes both ruptured within the Semidi segment of the Aleutian‐Alaska subduction zone. The large‐slip distribution of the 2021 event is well constrained within the depth range 25–45 km, with seaward tsunami observations excluding significant shallower coseismic slip. The 1938 event slip distribution is more uncertain. Regional and far‐field tide gauge observations for the 1938 event are modeled to constrain the location of large coseismic slip. The largest slip (2.0 m) is located below the continental shelf on a 180‐km‐long portion of the rupture extending further northeast than the 2021 rupture, to near Sitkinak Island. Minor slip (1.0 m) extends seaward under the continental slope to 8 km deep, where large slip may have occurred in 1788. The megathrust shallower than 25 km depth to the southwest experienced many small aftershocks and aseismic slip following the 2021 event, and has limited slip deficit.more » « less
-
Abstract The eastern portion of the Shumagin gap along the Alaska Peninsula ruptured in anMW7.8 thrust earthquake on 22 July 2020. The megathrust fault space‐time slip history is determined by joint inversion of regional and teleseismic waveform data along with co‐seismic static Global Navigation Satellite System (GNSS) displacements. The rupture expanded westward and along‐dip from the hypocenter, located adjacent to the 1938MW8.2 Alaska earthquake, with slip and aftershocks extending into the gap about 180 to 205 km, respectively, at depths from 15 to 40 km. The deeper half of ~75% of the Shumagin gap experienced faulting. However, the patchy slip is significantly less than possible accumulated slip since the region's last major rupture in 1917, compatible with geodetic seismic‐coupling estimates of 10‐40% beneath the Shumagin Islands. The rupture terminated in the western region of very low seismic coupling. There was a regional decade‐scale decrease in b‐value prior to the 2020 event.more » « less
-
Abstract The 2021MW6.0 Yangbi, Yunnan strike‐slip earthquake occurred on an unmapped crustal fault near the Weixi‐Qiaoho‐Weishan Fault along the southeast margin of the Tibetan Plateau. Using near‐source broadband seismic data from ChinArray, we investigate the spatial and temporal rupture evolution of the mainshock using apparent moment‐rate functions (AMRFs) determined by the empirical Green's function (EGF) method. Assuming a 1D line source on the fault plane, the rupture propagated unilaterally southeastward (∼144°) over a rupture length of ∼8.0 km with an estimated rupture speed of 2.1 km/s to 2.4 km/s. A 2D coseismic slip distribution for an assumed maximum rupture propagation speed of 2.2 km/s indicates that the rupture propagated to the southeast ∼8.0 km along strike and ∼5.0 km downdip with a peak slip of ∼2.1 m before stopping near the largest foreshock, where three bifurcating subfaults intersect. Using the AMRFs, the radiated energy of the mainshock is estimated as ∼. The relatively low moment scaled radiated energyof 1.5 × 10−5and intense foreshock and aftershock activity might indicate reactivation of an immature fault. The earthquake sequence is mainly distributed along a northwest‐southeast trend, and aftershocks and foreshocks are distributed near the periphery of the mainshock large‐slip area, suggesting that the stress in the mainshock slip zone is significantly reduced to below the level for more than a few overlapping aftershock to occur.more » « less
-
Abstract Most great earthquakes on subduction zone plate boundaries have large coseismic slip concentrated along the contact between the subducting slab and the upper plate crust. On 4 March 2021, a magnitude 7.4 foreshock struck 1 hr 47 min before a magnitude 8.1 earthquake along the northern Kermadec island arc. The mainshock is the largest well‐documented underthrusting event along the ∼2,500‐km long Tonga‐Kermadec subduction zone. Using teleseismic, geodetic, and tsunami data, we find that all substantial coseismic slip in the mainshock is located along the mantle/slab interface at depths from 20 to 55 km, with the large foreshock nucleating near the down‐dip edge. Smaller foreshocks and most aftershocks are located up‐dip of the mainshock, where substantial prior moderate thrust earthquake activity had occurred. The upper plate crust is ∼17 km thick in northern Kermadec with only moderate‐size events along the crust/slab interface. A 1976 sequence withMWvalues of 7.9, 7.8, 7.3, 7.0, and 7.0 that spanned the 2021 rupture zone also involved deep megathrust rupture along the mantle/slab contact, but distinct waveforms exclude repeating ruptures. Variable waveforms for eight deep M6.9+ thrusting earthquakes since 1990 suggest discrete slip patches distributed throughout the region. The ∼300‐km long plate boundary in northern Kermadec is the only documented subduction zone region where the largest modeled interplate earthquakes have ruptured along the mantle/slab interface, suggesting that local frictional properties of the putatively hydrated mantle wedge may involve a dense distribution of Antigorite‐rich patches with high slip rate velocity weakening behavior in this locale.more » « less
