Large Inland Earthquakes Research Articles

Analysis of stress changes on the Poso fault caused by large inland earthquakes: Case study of the 2017 Mw 6.6 Poso and the 2018 Mw 7.5 Palu-Donggala earthquakes

The 2017 Mw 6.6 Poso and the 2018 Mw 7.5 Palu-Donggala represent significant seismic events in central Sulawesi Island. These events transferred the stress to the surrounding faults, which can promote or inhibit the critical condition in a fault, thus evaluating the stress transfer from these earthquakes is crucial for a comprehensive understanding of fault behaviour. In this study, we investigate the effect of these large events by resolving the accumulated Coulomb stress change onto a multi-strike segment of the Poso fault. Additionally, we also used GPS measurement (Global Strain Rate Model) to estimate the increase in tectonic stress rate due to coupling of triple-junction plate convergence. The 2017 Poso earthquake induces positive stress changes (7 kPa to 27 kPa) in the central segment, negative changes (–4 kPa to –10 kPa) in the northern segment, and less significant effects in the southern segment. The 2018 Palu-Donggala earthquake imparts a notably high negative Coulomb stress (–16 kPa to –27 kPa) across the entire northern segment. Cumulatively, both earthquakes reduce Coulomb stress predominantly in the northern part of the Poso fault, suggesting a delayed critical point. Furthermore, the analysis of the Coulomb stress changes time function indicates that stress accumulation in the northern Poso fault is more modulated by large earthquakes, while stress accumulation in the southern segments may be more controlled by longterm tectonic stress loading. The stress accumulation model developed in this study can provide a guidance in better understanding further earthquake risk mitigation in central Sulawesi.

Seasonal Modulation of Crustal Seismicity in Northeastern Japan Driven by Snow Load

AbstractNumerous studies have reported that surface hydrological loading can seasonally modulate seismicity rates at crustal depths. For example, substantial winter snow accumulation occurs across the Japanese Islands, and these snowy regions appear to have seasonally modulated the occurrence of previous large inland earthquakes. Therefore, it is important to investigate the impact of seasonal stress changes on crustal seismicity to deepen our understanding of earthquake generation. Here we constrain seasonal changes in the surface load across northeastern Japan using Global Navigation Satellite System surface displacements and evaluate the potential relationship between temporal trends in inland seismicity and estimated seasonal stress changes. The spatial distribution of the seasonal surface load is consistent with snow depth along the Sea of Japan. The inland seismicity beneath northeastern Japan is modestly modulated by the seasonal stress changes that are induced by the annual snow load. However, this seasonal response is weaker than that in other regions. This weak modulation may be due to the small surface‐load‐induced stress perturbation relative to the long‐term‐averaged stressing rate and/or the limited presence of crustal fluids to trigger seismicity in Japan.

Development and Preliminary Analysis of a VLF-Band Electromagnetic-Wave Observation System for Short-Term Earthquake Precursory Monitoring

Preseismic VLF electromagnetic pulses occasionally increase a few days before large earthquakes, especially inland earthquakes. More than two decades ago, the Tokai University group developed a digital recording system for collecting the preseismic electromagnetic pulse data and showed remarkable results. However, due to the limitations of personal computers’ data storage and CPU power during that time, they discontinued the observation. We relaunched this research using current technology. This paper shows the development of the new observation system and presents preliminary results. In addition, we introduce an electromagnetic-wave arrival discrimination algorithm that combines the autoregressive model and the Akaike information criterion, which are commonly used for automatic waveform reading in seismology, to obtain accurate data on the time of arrival (TOA) of electromagnetic waves. Then, source positioning was performed using TOA of electromagnetic waves. Seven electromagnetic pulses near the epicenter were observed 2 days before the largest inland earthquake (M = 5.6) that occurred near the observation network during the observation period (2016–2020). These VLF pulses may be a seismic precursory phenomenon because they were not electromagnetic pulses originating from lightning. These results encourage future observations.

Nucleation process of the 2011 northern Nagano earthquake from nearby seismic observations

The 2011 magnitude (M) 9.0 Tohoku-oki earthquake was followed by seismicity activation in inland areas throughout Japan. An outstanding case is the M6.2 Northern Nagano earthquake, central Japan, occurred 13-h after the megathrust event, approximately 400 km away from its epicenter. The physical processes relating the occurrence of megathrust earthquakes and subsequent activation of relatively large inland earthquakes are not well understood. Here we use waveform data of a dense local seismic network to reveal with an unprecedented resolution the complex mechanisms leading to the occurrence of the M6.2 earthquake. We show that previously undetected small earthquakes initiated along the Nagano earthquake source fault at relatively short times after the Tohoku-oki megathrust earthquake, and the local seismicity continued intermittently until the occurrence of the M6.2 event, being likely ‘modulated’ by the arrival of surface waves from large, remote aftershocks off-shore Tohoku. About 1-h before the Nagano earthquake, there was an acceleration of micro-seismicity migrating towards its hypocenter. Migration speeds indicate potential localized slow-slip, culminating with the occurrence of the large inland earthquake, with fluids playing a seismicity-activation role at a regional scale.

Application of B and Li isotope systematics for detecting chemical disturbance in groundwater associated with large shallow inland earthquakes in Kumamoto, Japan

Application of B and Li isotope systematics for detecting chemical disturbance in groundwater associated with large shallow inland earthquakes in Kumamoto, Japan

The Advancement of Research on Inland Earthquake Generation 2014–2018

The 2011 Tohoku-Oki Earthquake (M9.0) significantly affected inland areas of Japan. The crust and mantle response to the magathrust earthquake induced changes in the mechanical conditions of the seismogenic zone. Here we present important progress in the research into the seismogenesis of inland earthquakes. Stress, strain, strength, and structures are key parameters affecting the occurrence of earthquakes. In particular, both the spatial and temporal changes in these parameters around the focal areas of the large inland earthquakes have been detected and modeled. These results have provided spatial potential evaluation in terms of future inland earthquake occurrence. However, we clearly recognize that, in order to understand and predict the inland earthquake generation process, it will inevitably be necessary to unify the research on various spatial and temporal scales, from problems related to long-term stress loading from plate-relative motion to instant fault response.

Characteristics of relocated hypocenters of the 2018 M6.7 Hokkaido Eastern Iburi earthquake and its aftershocks with a three-dimensional seismic velocity structure

I relocated the hypocenters of the 2018 M6.7 Hokkaido Eastern Iburi earthquake and its surrounding area, using a three-dimensional seismic structure, the double-difference relocation method, and the JMA earthquake catalog. After relocation, the focal depth of the mainshock became 35.4 km. As previous studies show, in south-central Hokkaido, the Hidaka collision zone is formed, and anomalous deep and thickened forearc crust material is subducting at depths of less than 70 km. The mainshock and its aftershocks are located at depths of approximately 10 to 40 km within the lower crust of the anomalous deep and thickened curst near the uppermost mantle material intrusions in the northwestern edge of this Hidaka collision zone. Like the two previous large events, the aftershocks of this event incline steeply eastward and appear to be distributed in the deeper extension of the Ishikari-teichi-toen fault zone. The highly inclined fault in the present study is consistent with a fault model by a geodetic analysis with InSAR. The aftershocks at depths of 10 to 20 km are located at the western edge of the high-attenuation (low-Qp) zone. These kinds of relationships between hypocenters and materials are the same as the 1970 and 1982 events in the Hidaka collision zone. The anomalous large focal depths of these large events compared with the average depth limit of inland earthquakes in Japan could be caused by the locally lower temperature in south-central Hokkaido. This event is one of the approximately M7 large inland earthquakes that occurred repeatedly at a recurrence interval of approximately 40 years and is important in the collision process in the Hidaka collision zone.

A Trial Application of Geodetic Data for Inland Fault Assessment – Coulomb Stress Changes Estimated from GNSS Surface Displacements

We examine a method to calculate changes in Coulomb failure stress (ΔCFS) from observed GNSS displacements. The method assumes no stress changes on a horizontal plane and a linear elastic relation between strain and stress, represented by Hooke’s law. The ΔCFS distributions calculated using this method are applied to the 2003 Tokachi-oki and the 2016 Kumamoto earthquakes and they are compared with those using a standard dislocation model buried in an elastic half-space. The results suggest that the ΔCFS distribution at a depth of 10 km in a region far from a deformation source can give a first-order approximation using observations of surface displacements. However, ΔCFS distributions near the source cannot be reproduced by the examined method and need to be evaluated using the standard method. We apply the examined method and GNSS displacement data to calculate ΔCFS on major active faults as well as source faults of large inland earthquakes in southwest Japan for the period 1996-2017. ΔCFS from five large earthquakes, including the 2016 Kumamoto earthquake are separately calculated using the standard method with published fault models. Calculated ΔCFS increases by an order of 10 KPa at most faults over the past 21 years. ΔCFS on the source faults for the 2000 Western Tottori, the 2016 Kumamoto, and the 2016 Central Tottori earthquakes reached a maximum just before their rupture. Coseismic and postseismic deformation of the 2011 Tohoku-oki earthquake accelerated an increase of ΔCFS at some faults, including the source fault of the 2016 Central Tottori earthquake and the Arima-Takatsuki fault zone. The examined method can provide information on the activity of inland earthquakes using contemporarily observed deformation, and can hopefully improve the preparedness for earthquakes.

Remote triggering of seismicity at Japanese volcanoes following the 2016 M7.3 Kumamoto earthquake

The MJMA7.3 Kumamoto earthquake occurred on April 16, 2016, in the western part of Kyushu, at a depth of 12 km, on an active strike-slip fault. Here, we report on a relatively widespread activation of small remote earthquakes, which occurred as far as Hokkaido, detected by analyzing the continuous waveform data recorded at seismic stations all over Japan. Such relatively widespread remote seismicity activation, following a large inland earthquake, has not been reported before for Japan. Our analysis demonstrates that the remote events were triggered dynamically, by the passage of the surface waves from the Kumamoto earthquake. Most of the remotely triggered events in the Tohoku and Hokkaido regions, as well as close to Izu Peninsula, occur at or close to volcanoes, which suggests that the excitation of crustal fluids, by the passage of Rayleigh waves, played an important triggering role. Nevertheless, remote activation in other regions, like Noto Peninsula, occurred away from volcanoes. The relatively large-amplitude Love waves, enhanced by a source directivity effect during the Kumamoto earthquake, may have triggered seismicity on local active faults. The dynamic stresses in the areas where remote activation has been observed range from several kPa to tens of kPa, the thresholds being lower than in previous dynamic triggering cases for Japan; this might relate to a change in the crustal conditions following the 2011 M9.0 Tohoku-oki earthquake, in particular at volcanoes in NE Japan.

Interseismic deformation and moment deficit along the Manila subduction zone and the Philippine Fault system

AbstractWe examine interseismic coupling of the Manila subduction zone and fault activity in the Luzon area using a block model constrained by GPS data collected from 1998 to 2015. Estimated long‐term slip rates along the Manila subduction zone show a gradual southward decrease from 90–100 mm/yr at the northwest tip of Luzon to 65–80 mm/yr at the southern portion of the Manila Trench. We provide two block models (models A and B) to illustrate possible realizations of coupling along the Manila Trench, which may be used to infer future earthquake rupture scenarios. Model A shows a low coupling ratio of 0.34 offshore western Luzon and continuous creeping on the plate interface at latitudes 18–19°N. Model B includes the North Luzon Trough Fault and shows prevalent coupling on the plate interface with a coupling ratio of 0.48. Both models fit GPS velocities well, although they have significantly different tectonic implications. The accumulated strain along the Manila subduction zone at latitudes 15–19°N could be balanced by earthquakes with composite magnitudes of Mw 8.8–9.2, assuming recurrence intervals of 500–1000 years. GPS observations are consistent with full locking of the majority of active faults in Luzon to a depth of 20 km. Inferred moments of large inland earthquakes in Luzon fall in the range of Mw 6.9–7.6 assuming a recurrence interval of 100 years.

The 2014 Mw 6.1 South Napa Earthquake: A Unilateral Rupture with Shallow Asperity and Rapid Afterslip

The Mw 6.1 South Napa earthquake occurred near Napa, California, on 24 August 2014 at 10:20:44.03 (UTC) and was the largest inland earthquake in northern California since the 1989 Mw 6.9 Loma Prieta earthquake. The first report of the earthquake from the Northern California Earthquake Data Center (NCEDC) indicates a hypocentral depth of 11.0 km with longitude and latitude of (122.3105° W, 38.217° N). Surface rupture was documented by field observations and Light Detection and Ranging (LiDAR) imaging (Brooks et al., 2014; Hudnut et al., 2014; Brocher et al., 2015), with about 12 km of continuous rupture starting near the epicenter and extending to the northwest. The southern part of the rupture is relatively straight, but the strike changes by about 15° at the northern end over a 6 km segment. The peak dextral offset was observed near the Buhman residence with right‐lateral motion of 46 cm, near the location where the strike of fault begins to rotate clockwise (Hudnut et al., 2014). The earthquake was well recorded by the strong‐motion network operated by the NCEDC, the California Geological Survey and the U.S. Geological Survey (USGS). There are about 12 sites within an epicentral distance of 15 km that had relatively good azimuthal coverage (Fig. 1). The largest peak ground velocity (PGV) of nearly 100 cm/s was observed on station 1765, which is the closest station to the rupture and lies about 3 km east of the northern segment (Fig. 1). The ground deformation associated with the earthquake was also well recorded by the high resolution COSMO–SkyMed (CSK) satellite and Sentinel-1A satellite, providing independent static observations.

A statistical feature of anomalous seismic activity prior to large shallow earthquakes in Japan revealed by the pattern informatics method

Abstract. To reveal the preparatory processes of large inland earthquakes, we systematically applied the pattern informatics (PI) method to earthquake data of Japan. We focused on 12 large earthquakes with magnitudes greater than M = 6.4 (based on the magnitude scale of the Japan Meteorological Agency) that occurred at depths shallower than 30 km between 2000 and 2010. We examined the relationship between the spatiotemporal locations of these large shallow earthquakes and the locations of PI hotspots, which correspond to grid cells of anomalous seismic activity during a designated time span. Based on a statistical test conducted using Molchan's error diagram, we investigated whether precursory anomalous seismic activity occurred in association with these large earthquakes and, if so, studied the characteristic time spans of such activity. Our results indicate that Japanese inland earthquakes with M ≥ 6.4 are typically preceded by anomalous seismic activity in timescales of 8–10 years.

An Evaluation of the 11<sup>th</sup> September, 2009 Earthquake and Its Implication for Understanding the Seismotectonics of South Western Nigeria

The evaluation of an intra-plate earthquake that occurred on Friday, 11th September, 2009 and felt in parts of Abeokuta, Ago-Iwoye, Ajambata, Ajegunle, Imeko, Ijebu-Ode, Ilaro and Ibadan, all in south western Nigeria is presented. This event has been the largest inland earthquake recorded since the inception of digital seismological recording in Nigeria in 2008 was incepted. The event was recorded by three seismological stations operated by Centre for Geodesy and Geodynamics (CGG), Toro. Data obtained from the CGG stations and others distributed around the world were analysed to determine precise earthquake locations and focal mechanism and to assess the regional tectonic stress. The data recorded in MiniSEED format at a sampling rate of 40 samples per second (sps) were analyzed using the SEISAN earthquake analysis software. The resultshowed an epicentral location situated about 108 km west of Lagos, a focal depth of 10.0 km and an origin time of 03:10:21.60 GMT. The local and moment magnitudes were 4.5 and 4.2 respectively. The P-wave to S-wave velocity ratio was 1.72. The fault plane solutions obtained for the rupture process indicated that a normal dip-slip fault with median solution of strike 325°, dip 40° and rake -90° was the probable trigger mechanism for this earthquake. It suggested that the event was a reactivation of a buried high-angle fault in the Precambrian basement represented by the contemporary northeast-southwest trending regional horizontal compressive stress. Generally, the seismotectonics of the region was linked to the fracture zones in the Atlantic Ocean.

Spatial distribution of similar aftershocks of a large inland earthquake, the 2000 Western Tottori earthquake, in Japan

We detected similar aftershocks of the 2000 Western Tottori earthquake and we have examined their spatial distribution on the source fault. Many similar aftershocks are distributed in the northern part of the source fault, but few in the southern part. Specifically, similar aftershocks on the source fault are located outside of the asperity. The cumulative slip estimated from the similar aftershocks is 0.4–2.4 cm. The largest one is found at the edge of the major asperity. We observe that shallower events show a larger cumulative slip than deeper events. Large cumulative slip in the shallow depth in the northern part is coincident with the geodetic observation of afterslip. These facts suggest that similar aftershocks can provide information related to afterslip. However, the slip velocity estimated from both the slip and the recurrence interval of the similar aftershocks shows no distinct distribution.

Spatial variations in 3He/4He ratios along a high strain rate zone, central Japan

A linear zone with high strain rates along the Japan Sea coast, the Niigata-Kobe Tectonic Zone (NKTZ), is considered to be associated with rheological heterogeneities in the lower crust and/or upper mantle. Helium isotope variations along the NKTZ reveal a close association with the geophysical evidence for rheological heterogeneities in the crust and mantle. In the southern NKTZ, the 3He/4He ratios lower than 3.4Ra (Ra denotes the atmospheric 3He/4He ratio of 1.4×10−6) could be interpreted as a two-component mixture of helium stored in aqueous fluids driven off the subducting oceanic crust and radiogenic crustal helium. Higher 3He/4He ratios are observed in the central NKTZ where Quaternary volcanoes and high-temperature hot springs are concentrated, suggesting that the 3He emanation manifest in the central NKTZ results from the effective transfer of mantle helium by intrusion and degassing of mantle-derived magma in the crust. In the northern NKTZ where two large inland earthquakes occurred recently, there appears to be many samples with 3He/4He ratios significantly higher than those observed in the fore-arc side of northeast Japan. A plausible source of mantle helium could be attributed to upward mobilization of aqueous fluids generated by dehydration of the subducting Pacific Plate slab.

Modeling of crustal deformation and fault development in the Ou Backbone range and the midNiigata region: Toward an understanding of the mechanisms of large inland earthquakes

内陸大地震の発生過程を理解するためには，外力としてのプレート間の相互作用による応力蓄積過程と，島弧内で応力を開放する内的な非弾性変形と断層成長過程を解明する必要がある．内的な非弾性変形と断層運動は，島弧地殻・最上部マントルの不均質なレオロジー構造に支配される．本稿では，内陸地震発生解明に向けた，不均質なレオロジー構造を考慮した東北日本および中越地域における変形と断層形成過程のモデル化を紹介する．脊梁山脈周辺では地温勾配が高く，その領域の地殻深部で非線形流動による短縮変形が生じ，それにより上部地殻で断層が形成される．他方，日本海拡大時に背弧で地溝帯形成に伴って生成された正断層が，短縮変形場の中で，逆断層として再活動することが考えられており，中越地域においては，埋没した地溝や基盤内の摩擦強度の小さな領域を考慮することで最近発生した大地震の断層成長過程を再現できる．

家具の転倒率関数を用いた住宅内の地震被害推定

The functions to estimate overturning ratios of furniture in residential buildings are developed using indoor damage data during the 1995 Hyogo-ken Nambu Earthquake and results of analytical studies for overturning of rigid bodies. The functions are verified to match the indoor damage of tall buildings in Tokyo during the 2011 Off the Pacific Coast of Tohoku Earthquake. By using these functions, indoor damage in tall residential buildings in Tokyo is estimated during Nankai Trough earthquake and a large inland earthquake. The situation of indoor damage might be different depend on characteristic of seismic motions and buildings.

A non‐accelerating foreshock sequence followed by a short period of quiescence for a large inland earthquake

Using continuous waveforms recorded at a seismic station located in close proximity to the epicenter of the 2008 Iwate‐Miyagi inland earthquake, we conducted a detailed investigation of foreshock behavior. Calculation of cross‐correlation coefficients between the continuous waveform record and one of the officially recognized foreshock events revealed 20 micro foreshocks in addition to the two previously recognized foreshocks. Our analysis shows that all of the foreshocks occurred within the same general area relative to the main event. Over the two week period leading up to the Iwate‐Miyagi earthquake, foreshocks only occurred during the last 45 minutes, specifically over a 35 minute period followed by a 10 minute period of quiescence just before the mainshock. We found no evidence of acceleration of the foreshock sequence.

Spatiotemporal Relationship between Geodetic and Seismic Quantities: A Possible Clue to Preparatory Processes of M ≥ 6.0 Inland Earthquakes in Japan

Constructing a statistical model to characterize the physical conditions associated with large earthquake occurrence is crucial for disaster mitigation. With the aim to formulate such a model, we previously developed a statistical evaluation system which assesses the correlation of the spatiotemporal relationship between different kinds of physical quantities with the occurrence times of large inland earthquakes. In this study, we focused on assessing the relationship between geodetic and seismic quantities and attempted to find the pair of related quantities that most likely indicates preparatory processes of large earthquakes in Japan. We assessed the quantities prior to M ≥ 6.0 inland earthquakes for the period of 2001–2007 in terms of probability gains and error diagram. Our system revealed that the pair of absolute value of dilatation rate and seismic energy showed the highest statistical performance. Further validation of this result is required by updating the database of physical quantities.

Lateral variation of the cutoff depth of shallow earthquakes beneath the Japan Islands and its implications for seismogenesis

We estimated the spatial distribution of the cut-off depth of shallow seismicity beneath the Japan Islands based on data from a recently constructed nationwide dense seismic network. In order to avoid the effect of heterogeneous seismic velocity structure, we determined the 3D seismic velocity structure for the whole of Japan, which was then used to relocate the hypocenters. The cut-off depth was determined by calculating D90, the depth above which 90% of the earthquakes occur. A spatial distribution of estimated D90 values shows a considerable lateral variation, ranging from about 5km to about 40km. The D90's are deep along the Pacific coast of eastern Japan. Locally shallow D90's extend along belt-like stretches of volcanic areas. Heat flow is low along the Pacific coast in eastern Japan and high in the volcanic areas. This indicates that the lateral variation of D90's is formed mainly by lateral geotherm variation. Cooling by the subducting old, very cold Pacific plate may cause the deep D90's along the Pacific coast in eastern Japan. Heating by magma supplied from the mantle wedge below is considered to be the main cause of the shallow D90's in the volcanic areas. Magma and/or aqueous fluids derived from solidified magma contribute not only to locally shallow D90's but also to the weakening of ambient rocks in the lower crust, resulting in stress concentrations in the upper crust just above them. Large shallow inland earthquakes tend to occur in such areas, suggesting that a model of shallow earthquake generation applied to Tohoku can be used in other areas in Japan as well.