Simultaneous rupture on conjugate faults during the 2018 Anchorage, Alaska, intraslab earthquake (MW 7.1) inverted from strong-motion waveforms

An MW 7.1 ~ 50-km-deep intraslab earthquake within the Pacific/Yakutat slab underlying the North American Plate struck Anchorage, southern Alaska, on November 30, 2018. The ground-motion records very close to the source region of the Anchorage earthquake provide an important opportunity to better understand the source characteristics of intraslab earthquakes in this subduction zone. We estimated the kinematic rupture process during this earthquake using a series of strong-motion waveform (0.05–0.4 Hz) inversions. Our inversions clearly indicate that the Anchorage earthquake was a rare intraslab event with simultaneous rupture on two conjugate faults, which are recognized sometimes for shallow crustal earthquakes but rarely for deep intraslab earthquakes. Interestingly, one of the conjugate faults had low aftershock productivity. This fault extends to great depth and may reflect a deep oceanic Moho or a local low-velocity and high-VP/VS zone within the oceanic mantle. Even though the Anchorage earthquake was a rare event due to the conjugate faults, we found that its kinematic source parameters such as the slip amplitude and large slip area nearly equal the global averages derived from source scaling relationships for intraslab earthquakes. Because the source parameters comparable to the global averages were also found for another large intraslab earthquake in the subducting Pacific/Yakutat slab, these source parameters are likely an important source characteristic common to this subduction zone.

Simultaneous rupture on conjugate faults during the 2018 Anchorage, Alaska, intraslab earthquake (MW 7.1) inverted from strong-motion waveforms

An MW 7.1 ~50-km-deep intraslab earthquake within the Pacific/Yakutat slab underlying the North American Plate struck Anchorage, southern Alaska, on November 30, 2018. The ground-motion records very close to the source region of the Anchorage earthquake provide an important opportunity to better understand the source characteristics of intraslab earthquakes in this subduction zone. We estimated the kinematic rupture process during this earthquake using a series of strong-motion waveform (0.05–0.4 Hz) inversions. Our inversions clearly indicate that the Anchorage earthquake was a rare intraslab event with simultaneous rupture on two conjugate faults, which are recognized sometimes for shallow crustal earthquakes but rarely for deep intraslab earthquakes. Interestingly, one of the conjugate faults had low aftershock productivity. This fault extends to great depth and may reflect a deep oceanic Moho or a local low-velocity and high-VP/VS zone within the oceanic mantle. Even though the Anchorage earthquake was a rare event due to the conjugate faults, we found that its kinematic source parameters such as the slip amplitude and large-slip area nearly equal the global averages derived from source scaling relationships for intraslab earthquakes. Because the source parameters comparable to the global averages were also found for another large intraslab earthquake in the subducting Pacific/Yakutat slab, these source parameters are likely an important source characteristic common to this subduction zone.

Simultaneous rupture on conjugate faults during the 2018 Anchorage, Alaska, intraslab earthquake (MW 7.1) inverted from strong-motion waveforms

An MW 7.1 ~50-km-deep intraslab earthquake within the Pacific/Yakutat slab underlying the North American Plate struck Anchorage, southern Alaska, on November 30, 2018. The ground-motion records very close to the source region of the Anchorage earthquake provide an important opportunity to better understand the source characteristics of intraslab earthquakes in this subduction zone. We estimated the kinematic rupture process during this earthquake using a series of strong-motion waveform (0.05–0.4 Hz) inversions. Our inversions clearly indicate that the Anchorage earthquake was a rare intraslab event with simultaneous rupture on two conjugate faults, which are recognized sometimes for shallow crustal earthquakes but rarely for deep intraslab earthquakes. Interestingly, one of the conjugate faults had low aftershock productivity. This fault extends to great depth and may reflect a deep oceanic Moho or a local low-velocity and high-VP/VS zone within the oceanic mantle. Even though the Anchorage earthquake was a rare event due to the conjugate faults, we found that its kinematic source parameters such as the slip amplitude and large-slip area nearly equal the global averages derived from source scaling relationships for intraslab earthquakes. Because the source parameters comparable to the global averages were also found for another large intraslab earthquake in the subducting Pacific/Yakutat slab, these source parameters are likely an important source characteristic common to this subduction zone.

Simultaneous Rupture on Conjugate Faults During the 2018 Anchorage, Alaska, Intraslab Earthquake (MW 7.1) Inverted from Strong-Motion Waveforms

An MW 7.1 intraslab earthquake within the Pacific/Yakutat slab underlying the North American Plate struck Anchorage, southern Alaska, on November 30, 2018. The ground-motion records very close to the source region of the Anchorage earthquake provide an important opportunity to better understand the source characteristics of intraslab earthquakes in this subduction zone. We estimated the kinematic rupture process during this earthquake using a series of strong-motion waveform (0.05–0.4 Hz) inversions. Our inversions clearly indicate that the Anchorage earthquake was a rare intraslab event with simultaneous rupture on two conjugate faults, which are worldwide recognized sometimes for shallow crustal earthquakes but rarely for deep intraslab earthquakes. Interestingly, one of the conjugate faults is located in a zone that had low aftershock activity. This fault extends to great depth and may reflect a deep oceanic Moho or a local low-velocity and high-VP/VS zone within the oceanic mantle. Even though the Anchorage earthquake was a rare event due to the conjugate faults, we found that its kinematic source parameters were not abnormal compared to the average parameters of global intraslab earthquakes. The normal source parameters suggest that the larger low-frequency (0.33-Hz) ground-motion amplitudes than predicted by the ground-motion prediction equation observed in downtown Anchorage were primarily due to site amplification effects associated with a sedimentary basin, not source effects. Because such normality of the source parameters was also found for another large intraslab earthquake in the subducting Pacific/Yakutat slab, this normality is likely an important source characteristic common to this subduction zone.

Simultaneous Rupture of an Aortomitral Intervalvular Fibrosa Pseudoaneurysm to the Aorta and the Left Atrium

A 55-year-old man was admitted to our surgical ward with a diagnosis of an aorto-left atrial fistula. The diagnosis of infectious endocarditis had been ruled out in another hospital before the patient was referred to our hospital through multiple blood cultures, examinations of inflammatory markers, and the absence of signs and symptoms of infection. He had a history of aortic and mitral valve replacement with mechanical bileaflet valves 3 years earlier and had been on hemodialysis for several years. Physical examinations revealed a continuous murmur at the left parasternal border. Transthoracic and transesophageal echocardiographic examinations demonstrated mild left ventricular enlargement with a normal systolic function, a normal right ventricular size with systolic dysfunction, and normally functioning aortic and mitral prosthetic valves without any leakage or thrombosis. There was an echo-free space between the aortic and mitral valves that expanded in systole and decompressed in diastole with a connection to the left ventricular outflow tract, suggestive of the pseudoaneurysm of the aortomitral intervalvular fibrosa (AMIVF). This pseudoaneurysm was connected to the aorta on one side and to the left atrium on the other side, constituting a cavity between the ascending aorta and the left atrium that conducted a continuous flow from the aorta to the left atrium (Figure 1A and B; Video 1 and Video 2). Computed tomography angiography documented this space and its connection to the ascending aorta and the left atrium (Figure 2A, B, and C). It appears that surgical trauma was the most probable etiology of the pseudoaneurysm of the AMIVF in our patient. The patient refused surgical repair of this pseudoaneurysm. The AMIVF is a zone between the anterior mitral leaflet and the non-coronary and left coronary aortic cusp that is fibrotic and vascular. These properties predispose it to injury due to infection and surgical trauma as the most common etiologies for the pseudoaneurysm of the AMIVF. The pseudoaneurysm of the AMIVF can be complicated by fistulation to the adjacent structures such as the left atrium the and aorta, which has been reported in about 20% of patients with the pseudoaneurysm of the AMIVF.1 On the follow-up of patients with aortic or mitral valve replacement or both, the presence of a pseudoaneurysm in the AMIVF as an uncommon complication of such valve replacements should be considered at the time of physical examinations and echocardiography. J Teh Univ Heart Ctr 2019;14(4):195-197 This paper should be cited as: Mohseni-Badalabadi R, Hosseinsabet A, Mohseni-Badalabadi M, Forozannia K. Simultaneous Rupture of an Aortomitral Intervalvular Fibrosa Pseudoaneurysm to the Aorta and the Left Atrium. J Teh Univ Heart Ctr 2019;14(4):195-197.