ALBERT

Description: Lying below Vatnajökull ice cap in Iceland, Bárðarbunga stratovolcano began experiencing wholesale caldera collapse in 2014 August 16, one of the largest such events recorded in the modern instrumental era. Simultaneous with this collapse is the initiation of a plate boundary rifting episode north of the caldera. Observations using the international constellation of radar satellites indicate rapid 50 cm d –1 subsidence of the glacier surface overlying the collapsing caldera and metre-scale crustal deformation in the active rift zone. Anomalous earthquakes around the rim of the caldera with highly nondouble-couple focal mechanisms provide a mechanical link to the dynamics of the collapsing magma chamber. A model of the collapse consistent with available geodetic and seismic observations suggests that the majority of the observed subsidence occurs aseismically via a deflating sill-like magma chamber.

Description: We investigate the relationship between seismic moment M 0 and source duration t w of microearthquakes by using high-quality seismic data recorded with a vertical borehole array installed in central Taiwan. We apply a waveform cross-correlation method to the three-component records and identify several event clusters with high waveform similarity, with event magnitudes ranging from 0.3 to 2.0. Three clusters—Clusters A, B and C—contain 11, 8 and 6 events with similar waveforms, respectively. To determine how M 0 scales with t w , we remove path effects by using a path-averaged Q . The results indicate a nearly constant t w for events within each cluster, regardless of M 0 , with mean values of t w being 0.058, 0.056 and 0.034 s for Clusters A, B and C, respectively. Constant t w , independent of M 0 , violates the commonly used scaling relation ${t_w} \propto M_0^{1/3}$ . This constant duration may arise either because all events in a cluster are hosted on the same isolated seismogenic patch, or because the events are driven by external factors of constant duration, such as fluid injections into the fault zone. It may also be related to the earthquake nucleation size.

Description: 〈span〉〈div〉SUMMARY〈/div〉The strainmeter record observed at Isabella (ISA), California, for the 1960 Chilean earthquake (〈span〉M〈/span〉〈sub〉w〈/sub〉 = 9.5) is one of the most important historical records in seismology because it was one of the three records that provided the opportunity for the first definitive observations of free oscillations of the Earth. Because of the orientation of the strainmeter rod with respect to the back azimuth to Chile, the ISA strainmeter is relatively insensitive to G (Love) waves and higher order (order ≥ 6) toroidal modes, yet long-period G waves and toroidal modes were recorded with large amplitude on this record. This observation cannot be explained with the conventional low-angle thrust mechanism typical of great subduction-zone earthquakes and requires an oblique mechanism with half strike-slip and half thrust. The strain record at Ogdenburg, New Jersey, the Press–Ewing seismograms at Berkeley, California, and the ultra-long period displacement record at Pasadena, California, also support the oblique mechanism. We tested the performance of the ISA strainmeter using other events including the 1964 Alaskan earthquake and found no instrumental problems. Thus, the ISA observation of large G/R and toroidal/spheroidal ratios most likely reflects the real characteristics of the 1960 Chilean earthquake, rather than an observational artefact. The interpretation of the large strike-slip component is not unique, but it may represent release of the strike-slip strain that has accumulated along the plate boundary as a result of oblique convergence at the Nazca–South American plate boundary. The slip direction of the 2010 Chilean (Maule) earthquake ( 〈span〉M〈/span〉〈sub〉w〈/sub〉 = 8.8) is rotated by about 10° clockwise from the plate convergence direction suggesting that right-lateral strain comparable to that of an 〈span〉M〈/span〉〈sub〉w〈/sub〉 = 8.3 earthquake remained unreleased and accumulates near the plate boundary. One possible scenario is that the strike-slip strain accumulated over several great earthquakes like the 2010 Maule earthquake was released during the 1960 Chilean earthquake. If this is the case, we cannot always expect a similar behaviour for all the great earthquakes occurring in the same subduction zone and such variability needs to be considered in long-term hazard assessment of subduction-zone earthquakes.〈/span〉

Description: On 2010 March 11, a sequence of large, shallow continental crust earthquakes shook central Chile. Two normal faulting events with magnitudes around M w 7.0 and M w 6.9 occurred just 15 min apart, located near the town of Pichilemu. These kinds of large intraplate, inland crustal earthquakes are rare above the Chilean subduction zone, and it is important to better understand their relationship with the 2010 February 27, M w 8.8, Maule earthquake, which ruptured the adjacent megathrust plate boundary. We present a broad seismological analysis of these earthquakes by using both teleseismic and regional data. We compute seismic moment tensors for both events via a W-phase inversion, and test sensitivities to various inversion parameters in order to assess the stability of the solutions. The first event, at 14 hr 39 min GMT, is well constrained, displaying a fault plane with strike of N145°E, and a preferred dip angle of 55°SW, consistent with the trend of aftershock locations and other published results. Teleseismic finite-fault inversions for this event show a large slip zone along the southern part of the fault, correlating well with the reported spatial density of aftershocks. The second earthquake (14 hr 55 min GMT) appears to have ruptured a fault branching southward from the previous ruptured fault, within the hanging wall of the first event. Modelling seismograms at regional to teleseismic distances ( 〉 10°) is quite challenging because the observed seismic wave fields of both events overlap, increasing apparent complexity for the second earthquake. We perform both point- and extended-source inversions at regional and teleseismic distances, assessing model sensitivities resulting from variations in fault orientation, dimension, and hypocentre location. Results show that the focal mechanism for the second event features a steeper dip angle and a strike rotated slightly clockwise with respect to the previous event. This kind of geological fault configuration, with secondary rupture in the hanging wall of a large normal fault, is commonly observed in extensional geological regimes. We propose that both earthquakes form part of a typical normal fault diverging splay, where the secondary fault connects to the main fault at depth. To ascertain more information on the spatial and temporal details of slip for both events, we gathered near-fault seismological and geodetic data. Through forward modelling of near-fault synthetic seismograms we build a kinematic k –2 earthquake source model with spatially distributed slip on the fault that, to first-order, explains both coseismic static displacement GPS vectors and short-period seismometer observations at the closest sites. As expected, the results for the first event agree with the focal mechanism derived from teleseismic modelling, with a magnitude M w 6.97. Similarly, near-fault modelling for the second event suggests rupture along a normal fault, M w 6.90, characterized by a steeper dip angle (dip = 74°) and a strike clockwise rotated (strike = 155°) with respect to the previous event.

Description: 〈span〉〈div〉SUMMARY〈/div〉The strainmeter record observed at Isabella (ISA), California, for the 1960 Chilean earthquake (〈span〉Mw 〈/span〉= 9.5) is one of the most important historical records in seismology because it was one of the three records that provided the opportunity for the first definitive observations of free oscillations of the earth. Because of the orientation of the strainmeter rod with respect to the back azimuth to Chile, the ISA strainmeter is relatively insensitive to G (Love) waves and higher order (order ≥ 6) toroidal modes, yet long-period G waves and toroidal modes were recorded with large amplitude on this record. This observation cannot be explained with the conventional low-angle thrust mechanism typical of great subduction-zone earthquakes, and requires an oblique mechanism with half strike-slip and half thrust. The strain record at Ogdenburg (OGD), New Jersey, the Press-Ewing seismograms at Berkeley (BRK), California, and the ultra-long period displacement record at Pasadena (PAS), California, also support the oblique mechanism. We tested the performance of the ISA strainmeter using other events including the 1964 Alaskan earthquake and found no instrumental problems. Thus, the ISA observation of large G/R and toroidal/spheroidal ratios most likely reflects the real characteristics of the 1960 Chilean earthquake, rather than an observational artifact. The interpretation of the large strike-slip component is not unique, but it may represent release of strike slip strain that has accumulated along the plate boundary as a result of oblique convergence at the Nazca-South American plate boundary. The slip direction of the 2010 Chilean (Maule) earthquake (〈span〉Mw 〈/span〉= 8.8) is rotated by about 10° clockwise from the plate convergence direction suggesting that right-lateral strain comparable to that of an 〈span〉Mw 〈/span〉= 8.3 earthquake remained unreleased and accumulates near the plate boundary. One possible scenario is that the strike-slip strain accumulated over several great earthquakes like the 2010 Maule earthquake was released during the 1960 Chilean earthquake. If this is the case, we cannot always expect a similar behavior for all the great earthquakes occurring in the same subduction zone and such variability needs to be considered in long-term hazard assessment of subduction-zone earthquakes.〈/span〉

Description: 〈span〉〈div〉SUMMARY〈/div〉We determine 〈span〉m〈/span〉〈sub〉B〈/sub〉, the original body wave magnitude developed by Gutenberg and Richter over the period 1942–1956, for about 3300 〈span〉M〈/span〉〈sub〉w〈/sub〉 ≥ 6 earthquakes for the period 1988–present using modern broad-band seismograms. The main objective is to extend the database of energy-related parameters by combining 〈span〉m〈/span〉〈sub〉B〈/sub〉 databases for recent and old events. The radiated energy 〈span〉E〈/span〉〈sub〉R_B〈/sub〉 (in erg) computed from 〈span〉m〈/span〉〈sub〉B〈/sub〉 using the Gutenberg & Richter relation $\log {E_{\mathrm{ R}\_\mathrm{ B}}} = 2.4{m_\mathrm{ B}} + 5.8$ agrees very well with 〈span〉E〈/span〉〈sub〉R〈/sub〉 estimated with modern techniques, especially for large deep earthquakes. Thus, 〈span〉E〈/span〉〈sub〉R_B〈/sub〉 is useful as a proxy for 〈span〉E〈/span〉〈sub〉R〈/sub〉 to investigate the global diversity of earthquake characteristics and physics over an extended period of time.〈/span〉

Description: 〈span〉〈div〉Summary〈/div〉We determine 〈span〉mB〈/span〉, the original body-wave magnitude developed by Gutenberg and Richter over the period 1942–1956, for about 3,300 〈span〉Mw 〈/span〉≥ 6 earthquakes for the period 1988-present using modern broad-band seismograms. The main objective is to extend the database of energy-related parameters by combining 〈span〉mB〈/span〉 databases for recent and old events. The radiated energy 〈span〉ER_B〈/span〉 (in erg) computed from 〈span〉mB〈/span〉 using the Gutenberg & Richter relation log 〈span〉ER_B 〈/span〉=〈span〉 〈/span〉2.4〈span〉mB 〈/span〉+〈span〉 〈/span〉5.8 agrees very well with 〈span〉ER〈/span〉 estimated with modern techniques, especially for large deep earthquakes. Thus, 〈span〉ER_B〈/span〉 is useful as a proxy for 〈span〉ER〈/span〉 to investigate the global diversity of earthquake characteristics and physics over an extended period of time.〈/span〉

Description: Stress drop, a measure of static stress change in earthquakes, is the subject of numerous investigations. Stress drop in an earthquake is likely to be spatially varying over the fault, creating a stress drop distribution. Representing this spatial distribution by a single number, as commonly done, implies averaging in space. In this study, we investigate similarities and differences between three different averages of the stress drop distribution used in earthquake studies. The first one, $\overline{\Delta \sigma }_M$ , is the commonly estimated stress drop based on the seismic moment and fault geometry/dimensions. It is known that $\overline{\Delta \sigma }_M$ corresponds to averaging the stress drop distribution with the slip distribution due to uniform stress drop as the weighting function. The second one, $\overline{\Delta \sigma }_A$ , is the simplest (unweighted) average of the stress drop distribution over the fault, equal to the difference between the average stress levels on the fault before and after an earthquake. The third one, $\overline{\Delta \sigma }_E$ , enters discussions of energy partitioning and radiation efficiency; we show that it corresponds to averaging the stress drop distribution with the actual final slip at each point as the weighting function. The three averages, $\overline{\Delta \sigma }_M$ , $\overline{\Delta \sigma }_A$ , and $\overline{\Delta \sigma }_E$ , are often used interchangeably in earthquake studies and simply called ‘stress drop’. Yet they are equal to each other only for ruptures with spatially uniform stress drop, which results in an elliptical slip distribution for a circular rupture. Indeed, we find that other relatively simple slip shapes—such as triangular, trapezoidal or sinusoidal—already result in stress drop distributions with notable differences between $\overline{\Delta \sigma }_M$ , $\overline{\Delta \sigma }_A$ , and $\overline{\Delta \sigma }_E$ . Introduction of spatial slip heterogeneity results in further systematic differences between them, with $\overline{\Delta \sigma }_E$ always being larger than $\overline{\Delta \sigma }_M$ , a fact that we have proven theoretically, and $\overline{\Delta \sigma }_A$ almost always being the smallest. In particular, the value of the energy-related $\overline{\Delta \sigma }_E$ significantly increases in comparison to the moment-based $\overline{\Delta \sigma }_M$ with increasing roughness of the slip distribution over the fault. Previous studies used $\overline{\Delta \sigma }_M$ in place of $\overline{\Delta \sigma }_E$ in computing the radiation ratio R that compares the radiated energy in earthquakes to a characteristic part of their strain energy change. Typical values of R for large earthquakes were found to be from 0.25 to 1. Our finding that $\overline{\Delta \sigma }_E \ge \overline{\Delta \sigma }_M$ allows us to interpret the values of R as the upper bound. We determine the restrictions placed by such estimates on the evolution of stress with slip at the earthquake source. We also find that $\overline{\Delta \sigma }_E$ can be approximated by $\overline{\Delta \sigma }_M$ if the latter is computed based on a reduced rupture area.

Description: Pacific Ocean crust west of southwest North America was formed by Cenozoic seafloor spreading between the large Pacific Plate and smaller microplates. The eastern limit of this seafloor, the continent–ocean boundary, is the fossil trench along which the microplates subducted and were mostly destroyed in Miocene time. The Pacific–North America Plate boundary motion today is concentrated on continental fault systems well to the east, and this region of oceanic crust is generally thought to be within the rigid Pacific Plate. Yet, the 2012 December 14 M w 6.3 earthquake that occurred about 275 km west of Ensenada, Baja California, Mexico, is evidence for continued tectonism in this oceanic part of the Pacific Plate. The preferred main shock centroid depth of 20 km was located close to the bottom of the seismogenic thickness of the young oceanic lithosphere. The focal mechanism, derived from both teleseismic P -wave inversion and W -phase analysis of the main shock waveforms, and the 12 aftershocks of M  ~3–4 are consistent with normal faulting on northeast striking nodal planes, which align with surface mapped extensional tectonic trends such as volcanic features in the region. Previous Global Positioning System (GPS) measurements on offshore islands in the California Continental Borderland had detected some distributed Pacific and North America relative plate motion strain that could extend into the epicentral region. The release of this lithospheric strain along existing zones of weakness is a more likely cause of this seismicity than current thermal contraction of the oceanic lithosphere or volcanism. The main shock caused weak to moderate ground shaking in the coastal zones of southern California, USA, and Baja California, Mexico, but the tsunami was negligible.

Description: 〈span〉〈div〉Summary〈/div〉We recently found the original Omori seismograms recorded at Hongo, Tokyo, of the 1922 Atacama, Chile, earthquake (〈span〉MS 〈/span〉= 8.3) in the historical seismogram archive of the Earthquake Research Institute (ERI) of the University of Tokyo. These recordings enable a quantitative investigation of long-period seismic radiation from the 1922 earthquake. We document and provide interpretation of these seismograms together with a few other seismograms from Mizusawa, Japan, Uppsala, Sweden, Strasbourg, France, Zi-ka-wei, China, and De Bilt, Netherlands. The 1922 event is of significant historical interest concerning the cause of tsunami, discovery of G wave, and study of various seismic phase and first-motion data. Also, because of its spatial proximity to the 1943, 1995, and 2015 great earthquakes in Chile, the 1922 event provides useful information on similarity and variability of great earthquakes on a subduction-zone boundary. The 1922 source region, having previously ruptured in 1796 and 1819, is considered to have significant seismic hazard. The focus of this paper is to document the 1922 seismograms so that they can be used for further seismological studies on global subduction zones. Since the instrument constants of the Omori seismographs were only incompletely documented, we estimate them using the waveforms of the observed records, a calibration pulse recorded on the seismogram and the waveforms of better calibrated Uppsala Wiechert seismograms. Comparison of the Hongo Omori seismograms with those of the 1995 Antofagasta, Chile, earthquake (〈span〉M〈/span〉〈sub〉w 〈/sub〉= 8.0) and the 2015 Illapel, Chile, earthquake (〈span〉M〈/span〉〈sub〉w 〈/sub〉= 8.3) suggests that the 1922 event is similar to the 1995 and 2015 events in mechanism (i.e. on the plate boundary megathrust) and rupture characteristics (i.e. not a tsunami earthquake) with 〈span〉M〈/span〉〈sub〉w〈/sub〉 = 8.6 ± 0.25. However, the initial fine scale rupture process varies significantly from event to event. The G1 and G2, and R1 and R2 of the 1922 event are comparable in amplitude, suggesting a bilateral rupture, which is rare for large megathrust earthquakes.〈/span〉