Major Subevents Research Articles

Strong Ground-Motion Characteristics Observed in the November 12, 2017, Mw7.3 Sarpol-e Zahab, Iran Earthquake

ABSTRACT The November 12, 2017, moment magnitude (Mw) 7.3 Sarpol-e Zahab earthquake, which occurred after a seismic quiescence, close to the Iran–Iraq border, is one of the largest events recorded in the Zagros fold and thrust belt. In this study, we analyzed strong-motion data of this event from 38 stations within an epicentral distance of 36 to 200 km. Spectral analysis of strong-motion data resulted in an Mw estimate of 7.34, an average rupture velocity of 3.03 km/s, and a stress drop of 154 bars. We estimated that shear dislocation propagated over a rectangular fault area with a length of ~97 km and width of ~17 km. Comparison of the horizontal peak ground-motions from this earthquake with the ground-motion model for active crustal regions indicated overall good agreement; however, the model underestimates ground-motions at stations within distances of approximately 30 km or less. The model appears to underestimate the observed strong-motion recordings from the Mw7.4 September 16, 1978, Tabas and the Mw7.4 June 20, 1990, Manjil earthquakes as well. Investigation of the observed accelerograms showed that the Sarpol-e Zahab earthquake was nucleated by a sub-event with a local magnitude (ML) of 4.9 which contributed about 0.02% to the total seismic moment. The empirical data indicated multiple arrivals due to the development of the rupture as two major sub-events. The spatial distribution of stations along with the time difference between the sub-events on the recorded time-series implies their occurrence from the hypocenter toward the S-SE direction which is coincident with the principal directions of the majority of damaged buildings. The estimated Mw of these sub-events is 6.79 and 7.08, respectively. The distinctive characteristics of near-field recordings from Sarpol-e Zahab earthquake (e.g., multiple phase arrivals due to complex source process, forward directivity effect, and blind thrust faulting) make these data a valuable supplement to the recorded ground-motion database. The improved strong-motion database plays a fundamental role in calibrating seismological simulation models, developing ground-motion models that capture near-source effects, and performing nonlinear dynamic structural analyses.

Real-Time Unspecified Major Sub-Events Detection in the Twitter Data Stream That Cause the Change in the Sentiment Score of the Targeted Event

Twitter behaves as a social sensor of the world. The tweets provided by the Twitter Firehose reveal the properties of big data (i.e. volume, variety, and velocity). With millions of users on Twitter, the Twitter's virtual communities are now replicating the real-world communities. Consequently, the discussions of real world events are also very often on Twitter. This work has performed the real-time analysis of the tweets related to a targeted event (e.g. election) to identify those potential sub-events that occurred in the real world, discussed over Twitter and cause the significant change in the aggregated sentiment score of the targeted event with time. Such type of analysis can enrich the real-time decision-making ability of the event bearer. The proposed approach utilizes a three-step process: (1) Real-time sentiment analysis of tweets (2) Application of Bayesian Change Points Detection to determine the sentiment change points (3) Major sub-events detection that have influenced the sentiment of targeted event. This work has experimented on Twitter data of Delhi Election 2015.

Rupture Process of the 2012 Mw 7.8 Haida Gwaii Earthquake from an Empirical Green's Function Method

Abstract This article examines rupture processes of the 28 October 2012 M w 7.8 Haida Gwaii earthquake off the coast of British Columbia, Canada, using an empirical Green’s function (EGF) technique. The Haida Gwaii earthquake was the largest event along the Canadian portion of the Pacific–North American plate boundary since the M s 8.1 Queen Charlotte earthquake of 1949. It occurred along a potentially blind thrust fault dipping gently to the northeast rather than the main, subvertical Queen Charlotte fault. Surface waveforms from a 2001 M w 6.3 event, located only 15 km from the 2012 epicenter and with similar mechanism, are used as an EGF and deconvolved from those of the 2012 mainshock. The resulting source time functions contain minimal path effects, focal mechanism effects, and instrument response, so the waveforms display only properties of the 2012 mainshock rupture itself. By examining azimuthal variations in these source time functions, we constrain parameters such as average rupture velocity, extent, and directivity. In addition, information is obtained about the possible existence of major subevents and their relative locations. Results indicate two subevents within this rupture, the first 12 km south and updip of the epicenter and the second approximately 28 km from the first along a heading parallel to the Queen Charlotte terrace (∼323°). Overall, the rupture front propagated roughly 50 km at an azimuth of 308.5°. This evidence for directivity to the northwest is important, given that earthquakes with strong directivity, such as the 2002 M w 7.9 Denali earthquake, have been shown to be capable of triggering earthquakes thousands of kilometers away. In this case, we suggest that northwest directivity of this earthquake is responsible for amplification of surface waves observed at seismic stations in Alaska (Gomberg, 2013) and may provide a potential link between this 2012 event and the 2013 Craig, Alaska, earthquake. Online Material: Figure of all relative source time functions used in directivity analysis.

A cascading failure during the 24 May 2013 great Okhotsk deep earthquake

AbstractOn 24 May 2013, the largest ever‐recorded deep earthquake occurred beneath Sea of Okhotsk. A multiple point source inversion procedure is applied to constrain source process of this earthquake, based on waveform modeling of both direct P and SH waves and near‐surface reflected pP and sSH waves. Our results indicate that the earthquake consists of six major subevents separated in space and time, encompassing a horizontal dimension of 64 ± 4 km along ~ N160°E and a downward depth extension of 35 ± 4 km. The geographic distribution and focal mechanisms of the inferred subevents and foreshock/aftershock locations do not fit into plane rupture. We suggest that the earthquake can be best explained by a cascading failure of shear instability within preexisting weak zones in the region, with the perturbation of stress generated by a shear instability triggering another.

The Chichi earthquake sequence, Taiwan: results from source parameter and stress tensor inversions

Abstract The main shock of the Chichi earthquake took place along the north–south-trending Chelungpu fault between the coastal plain and western foothills of Taiwan. It consisted of five major sub-events with the rupture propagating from south to north. The largest seismic moment is located 30–35 km north of the initial rupture. Waveform inversion results indicate a low-angle thrust fault dipping to the east. The centroid depth is 11–12 km. The seismogenic patterns of aftershocks indicate that the Chichi earthquake sequence can be explained by westward block thrusting on the Chelungpu fault, with strike-slip motions at the northern and southern tips. A deeper seismogenic zone lies 15 km directly beneath the Chichi main fault plane, indicating that the deformation associated with the Chichi event involves deep crust and thus cannot be entirely explained by detachment at shallow crust levels. The result of stress tensor inversion is remarkably consistent with that derived from regional background seismic data before the Chichi sequence, and with the palaeostress pattern reconstructed from geological evidences. It is inferred that the stress regime corresponding to the regional collision in Taiwan is the same as that responsible for the occurrence of the Chichi earthquake sequence. Consequently, we propose to consider the Chichi earthquake sequence as a typical manifestation of the mountain-building process in Taiwan.

次の南海トラフ巨大地震に備えて １９４６年南海地震震源過程から推定された南海トラフ巨大地震に対するフィリピン海プレート形状の影響

By carefully analyzing the source process of the 1946 Nankai earthquake and its correlation with plate boundary structure, we attempt to explain the occurrence pattern of historical earthquakes in the Nankai Trough, in which great earthquakes tend to rupture separately either the western or eastern portions of the Nankai Trough. The source process of the 1946 earthquake consists of two major subevents, each corresponding to segments A and B, defined by Ando (1975), which have long been thought to correspond to units of earthquake rupture in the western Nankai Trough. Furthermore, rupture in each subevent begins near the eastern edge of the respective segment, where there are pronounced contortions of the plate boundary : a subducting seamount chain off Cape Muroto and a rapid change in subduction angle beneath the Kii Peninsula. We suggest that these seismotectonic features of the plate boundary shape control to some extent the pattern of great earthquake occurrence in the Nankai Trough.

Mainshock and aftershock analysis of the June 17, 1996, deep Flores Sea earthquake sequence: Implications for the mechanism of deep earthquakes and the tectonics of the Banda Sea

One of the largest (Mw = 7.8) deep earthquakes (595 km) in recorded history occurred beneath the Flores Sea on June 17, 1996. We analyze broadband body waveforms to determine the seismic source characteristics of this important earthquake. The source time function has a small initial subevent (5 s duration) followed by three major subevents with a total source duration of 29 s. Directivity analysis indicates an unilateral rupture to the east‐southeast with a subhorizontal rupture length of 75 km. A time‐independent moment tensor (TIMT) inversion gives a total seismic moment of 5.4×1020 N m ( Mw = 7.8). A time‐dependent moment tensor (TDMT) inversion indicates rupture on a near‐vertical fault (strike of 130°, dip of 80°SW, rake of −166°) during the first 5 s, releasing 8% of the seismic moment. The major part of the moment was released along a shallower dipping plane (strike of ∼100°, dip of ∼55°, rake of ∼−45°). This change of faulting geometry may imply that the rupture initiated at a structural, thermal, or phase boundary of high stress concentration and then triggered the major moment release along a regional weak zone. The mainshock produced at least 23 (mb>2.9) aftershocks within the first 3 days. Seventeen of the largest aftershocks were located using the Joint Hypocenter Determination algorithm. The aftershocks define a plane dipping antithetically to the dip of the subducting slab with lateral and depth dimensions of 145 km and 76 km, respectively. The aftershock locations correlate with the southeast striking nodal plane of the moment tensor mechanism.

The Rupture Process of the Manjil, Iran Earthquake of 20 June 1990 and Implications for Intraplate Strike-Slip Earthquakes

In terms of seismically radiated energy or moment release, the earthquake of 20 January 1990 in the Manjil Basin-Alborz Mountain region of Iran is the second largest strike-slip earthquake to have occurred in an intracontinental setting in the past decade. It caused enormous loss of life and the virtual destruction of several cities. Despite a very large meizoseismal area, the identification of the causative faults has been hampered by the lack of reliable earthquake locations and conflicting field reports of surface displacement. Using broadband data from global networks of digitally recording seismographs, we analyse broadband seismic waveforms to derive characteristics of the rupture process. Complexities in waveforms generated by the earthquake indicate that the main shock consisted of a tiny precursory subevent followed in the next 20 seconds by a series of four major subevents with depths ranging from 10 to 15 km. The focal mechanisms of the major subevents, which are predominantly strike-slip, have a common nodal plane striking about 285°–295°. Based on the coincidence of this strike with the dominant tectonic fabric of the region we presume that the EW striking planes are the fault planes. The first major subevent nucleated slightly south of the initial precursor. The second subevent occurred northwest of the initial precursor. The last two subevents moved progressively southeastward of the first subevent in a direction collinear with the predominant strike of the fault planes. The offsets in the relative locations and the temporal delays of the rupture subevents indicate heterogeneous distribution of fracture strength and the involvement of multiple faults. The spatial distribution of teleseismic aftershocks, which at first appears uncorrelated with meizoseismal contours, can be decomposed into stages. The initial activity, being within and on the periphery of the rupture zone, correlates in shape and length with meizoseismal lines. In the second stage of activity the aftershock zone expands and appears to cluster about the geomorphic and geologic features several tens of kilometres from the rupture zone. The activity is interpreted as a regional response to quasistatic stress migration along zones of tectonic weakness. The radiated energy of the main shock and the estimate of seismic moment yields an apparent stress of 20 bars. High apparent stress may be typical of strike slip earthquakes occurring in intracontinental environments undergoing continental collision.

Spatio-temporal moment-tensor inversion for multiple shocks: an application to the deep 1972 Banda Sea earthquake

A waveform inversion scheme is presented for multiple shocks. An earthquake source is modeled as a sequence of point sources with different focal mechanisms. Each point source is defined by an onset time, relative spatial coordinates, and a moment tensor. All the point sources are allowed to occur at only grid points with the same interval on a single flat plane and to have the same trapezoidal time history. Unknown source parameters are obtained by an iterative procedure. Each iteration step consists of two substeps; first, onset times and relative spatial coordinates of subevents with respect to a reference point are determined. Second, seismic moment-tensors of the subevents are obtained by a conventional linear moment-tensor inversion. A statistical test of a change in the focal mechanism during the source process is made using confidence ellipses of principal stress axes for each subevent. We apply the inversion to WWSSN teleseismic long-period P waveforms from a deep focus earthquake of the 1972 Banda Sea ( m b = 6.6). The result suggests that the earthquake consisted of two major subevents with a time interval of 7.5 s, and that the main rupture, connecting the two subevents, propagated westward by approximately 20 km. The best double-couple seismic moment is 5.7 × 10 18 Nm and 2.0 × 10 18 Nm for the first and the second subevents, respectively. Differences between directions of the P- and T-axes of the two major subevents may suggest that the main rupture of the earthquake propagaged along a northward convex fault surface.

Teleseismic interpretation of the 1968 Dasht‐e Bayaz, NE Iran, Earthquake

The 1968 Dasht‐e Bayaz earthquake is the oldest destructive earthquake in north‐east Iran recorded by the WWSSN network at teleseismic distances. These records are well distributed in azimuth and allow a detailed study of the rupture process of the main shock. We invert the WWSSN long‐period body waves of the Dasht‐e Bayaz earthquake and its principal aftershock, which occurred about 20 hours later, to their sources to study the complexity of the sources. The results of our analysis suggest that the main shock consists of three major subevents while the principal aftershock is a single event. Mechanisms of the subevents of the main shock are mainly strike‐slip and consistent with the observed fault movement. The principal aftershock has a dip‐slip mechanism. The sum of the seismic moments of the main shock is ∑M0 = 4.5×1026 dyn cm. The moment of the largest subevent of the main shock is M0 = 2.4×1026 dyn cm and its stress drop is about 200 bar. This shows that the main shock has a high stress drop nature. The largest subevent with shallow depth and large lateral displacement mostly caused the great destruction during the source process of the main shock. The moment of the principal aftershock is M0 = 1.9×1025 dyn cm and its stress drop is about 40 bar. Mechanism of the earthquakes suggest that there is a shortening in north‐east Iran mostly due to the north‐east compressional stress in the Iranian plateau.

Source characteristics of the Loma Prieta, California, Earthquake of October 18, 1989 from global digital seismic data

Displacement, velocity and acceleration records of P and SH body waves recorded at teleseismic distances are analysed to determine the static and dynamic source parameters of the Loma Prieta, California earthquake of October 18, 1989 (mb 6.6, Ms 7.1). Three distinct bursts of energy corresponding to three subevents can be recognized in most records. The displacement waveforms indicate that the first subevent contributes negligible moment while the largest releases of moment and energy are controlled by the second and third subevents. The second and third subevents are located north and south of the initial nucleation. A fourth small subevent needed to model later features of the P waveforms suggests that slow slip continued after the major releases of energy occurred. The waveforms are fit with a fault plane solution having strike 130°, dip 65° and slip 140° for all the subevents. The focal depths of the two major subevents are 16 and 12 km, and their asperity radii are 4.0 and 6.0 km, respectively. The seismic moment, M0, is 2.2×1026 dyne‐cm. From spectral analysis of teleseismic velocity, the radiated energy, Es, is estimated to be 1.1×1022 dyne‐cm, implying an apparent stress of 15 bars. From the high‐frequency level of the teleseismic acceleration spectrum and a rupture area of 440 km2, we derive a dynamic stress drop of 51 bars.

A comparison of the spatio‐temporal distribution of moment release for the 1986 Andreanof Islands Earthquake with relocated seismicity

We compare relocated, teleseismically recorded seismicity before and after the May 7, 1986, Andreanof Islands earthquake (Mw= 8) to the spatial and temporal distribution of seismic moment release during the mainshock. Relocated seismicity from 1964 through April 1987 clusters strongly along the main thrust zone, in similar locations for the pre‐ and post‐mainshock periods.We performed simultaneous iterative inversions of broadband teleseismic GDSN and NARS P‐waveforms using the method of Kikuchi and Fukao [1987]. Fault plane parameters (e.g., strike, dip, depth, and extent of fault plane) consistent with regional tectonics and aftershock locations were specified. The results of the inversions are spatial and temporal maps of moment release on the fault plane. Most of the moment found by the inversion is localized in several regions of the fault plane. Resolution of the location and timing of the major subevents is good. The rupture was bilateral, with a highly variable rupture velocity. A shallow, updip region of the fault plane ruptured toward the end of the mainshock process.The mainshock moment release tended to occur in regions of the fault plane in which no or few aftershocks or preshocks are located. This tendency suggests that regions of high moment release in the 1986 Andreanof Islands earthquake resulted from rupture of mechanically strong regions (i.e., asperities) on the fault plane, which exhibited little seismicity before or after the mainshock that ruptured them. We infer that the asperity regions were locked and quiescent before the mainshock, while surrounding seismically active areas did not store as much strain and slipped mostly aseismically.

Comparison of a complex rupture model with the precursor asperities of the 1975 HawaiiM s-7.2 earthquake

A simplified multiple source model was constructed for the 1975 HawaiiM s=7.2 earthquake by matching synthetic signals with three component accelerograms at two stations located approximately 45 km from the epicenter. Six major subevents were identified and located approximately. The signals of these are larger by factors of 1.4 to 3.2 than that of theM L=5.9 foreshock which occurred 70 minutes before the main rupture and also triggered the SAM-1 recorders at the two stations. Dividing the rupture length (40 km) by the duration of strong ground shaking (∼ 50 sec) an, average rupture velocity of 0.8 km/sec (about 25% of S-velocity) is obtained. Thus it is likely that the rupture stopped between subevents. The approximate epicenters of the 6 major subevents, and of the foreshock, support the hypothesis that they were located in high stress asperities which rupture during the main shock, except for the last events which is interpreted as a stopping phase generated at a barrier. These asperities have been previously defined on the basis of differences in the precursor pattern before the mainshock. Thus, it appears that both the details of the precursors and of the main rupture depended critically on the heterogeneous tress distribution in the source volume. This suggests that main rupture initiation points and locations of high rupture accelerations may be identified before the mainshock occurs, based on precursor anomaly patterns. A satisfactory match of synthetic signals with the observations could be obtained only if the aximuth of the fault plane of subevents was rotated from N60°E to N90°E and back to N30°E. These orientations are approximately parallel to the nearest Kilauea rift segments. Hence the slip directions and greatest principal stresses were oriented perpendicular to the rifts everywhere. From this analysis and other work, it is concluded that this fault surface consisted of three types of segments with different strength: hard asperities (radius ≈ 5 km), soft but ‘brittle’ segments between the asperities (radius ≈ 5 km), and a ‘viscous’ half (10×40 km) which slipped during the mainshock, but where microearthquakes and aftershocks are not common.

Iterative deconvolution of complex body waves from great earthquakes—the Tokachi-Oki earthquake of 1968

A method of body-wave inversion is developed in an attempt to extract the information about asperities or barriers in a fault zone. A sequence of point sources, each being characterized with the seismic moment, the onset time and the location, are iteratively derived from observed records at multi-stations, where the two-dimensional extent of the source location is taken into account. A modification is made of the iterative method of Kikuchi and Kanamori on the formulation of inversion procedure to facilitate the computation. Using this method, we analyse long period P waves of the Tokachi-Oki earthquake of 1968 ( M w = 8.2) and obtain several significant subevents with time durations of ∼ 10 s. Their spatio-temporal distribution shows that the rupture process consists of three characteristic stages: (A) a stage of introductory rupture, (B) a stage of main rupture and (C) a stage of aftershocks. The main rupture takes place in the form of clustering around a few sites of the fault plane. The largest subevent occurs in the northwestern corner. The stress drop associated with this event is estimated to be ∼ 200 bars, one order of magnitude higher than the stress drop averaged over the entire fault plane. The sum of the seismic moments of the individual subevents amounts to 2.3 × 10 28 dyn. cm which approximately coincides with the one estimated from the analysis of long-period surface waves. This implies that the source of the Tokachi-Oki earthquake consists of several major subevents with time durations of ∼ 10 s in addition to other minor subevents.