Kazuo Oike

Estimation of the magnitudes and epicenters of Philippine historical earthquakes

Abstract The magnitudes and epicenters of Philippine earthquakes from 1589 to 1895 are estimated based on the review, evaluation and interpretation of historical accounts and descriptions. The first step involves the determination of magnitude–felt area relations for the Philippines for use in the magnitude estimation. Data used were the earthquake reports of 86, recent, shallow events with well-described effects and known magnitude values. Intensities are assigned according to the modified Mercalli intensity scale of I to XII. The areas enclosed by Intensities III to IX [A(III) to A(IX)] are measured and related to magnitude values. The most robust relations are found for magnitudes relating to A(VI), A(VII), A(VIII) and A(IX). Historical earthquake data are obtained from primary sources in libraries in the Philippines and Spain. Most of these accounts were made by Spanish priests and officials stationed in the Philippines during the 15th to 19th centuries. More than 3000 events are catalogued, interpreted and their intensities determined by considering the possible effects of local site conditions, type of construction and the number and locations of existing towns to assess completeness of reporting. Of these events, 485 earthquakes with the largest number of accounts or with at least a minimum report of damage are selected. The historical epicenters are estimated based on the resulting generalized isoseismal maps augmented by information on recent seismicity and location of known tectonic structures. Their magnitudes are estimated by using the previously determined magnitude–felt area equations for recent events. Although historical epicenters are mostly found to lie on known tectonic structures, a few, however, are found to lie along structures that show not much activity during the instrumented period. A comparison of the magnitude distributions of historical and recent events showed that only the period 1850 to 1900 may be considered well-reported in terms of magnitude distribution. Each earthquake is evaluated for its ‘quality’ of determination based on the number of intensity reports. Earlier than 1850, the data collected are few and most earthquakes had fewer than ten reports. Good quality reports began to be collected from 1850, partly correlative to an increase in the number of towns and partly to the start of a systematized collection of earthquake accounts by the Manila Observatory. Parameters of these well-reported earthquakes may be used for conducting various seismological studies. Examples of how the parameters of poorly reported events were arrived at are also discussed.

Relation between characteristics of seismic activity and neotectonics in Honshu, Japan

Abstract The nature of the occurrence of earthquakes in and around the Honshu region of Japan has been studied from the viewpoint of the relation with its neotectonics. Honshu Island (Japan) consists of two large crustal blocks; one is Northeast Japan and the other is Southwest Japan. The junction of these two blocks is the northern part of the Fossa Magna, which crosses the central part of Honshu. Tectonic forces in the crust, by which shallow earthquakes occur in the Inner Zone of Southwest Japan, are considered to be transmitted from Northeast Japan through this junction. Due to the transmission of these tectonic forces, the earthquake activity in the Inner Zone of Southwest Japan shows some significant characteristics that are closely related to major events in Northeast Japan. In particular, occurrences of large earthquakes along the eastern edge of the Japan Sea provide clear evidence of this. In some regions in Southwest Japan, seismic activity increases after the occurrence of large earthquakes along the eastern edge of the Japan Sea. In the northern part of the Fossa Magna region the seismic activity increases over a period of a few years after a large event along the eastern edge of the Japan Sea. Such evidence of seismic activity indicates that force transmission exists from the east to the west crustal block of Honshu Island (Japan).

Error evaluation in acoustic positioning of a single transponder for seafloor crustal deformation measurements

The observation of seafloor crustal deformation is very important to understand plate motions, nucleation processes and mechanisms of great interplate earthquakes as well as the activities of submarine volcanoes. We have been developing an observation system for seafloor crustal deformation. This system consists of two main components; (1) kinematic GPS positioning of an observation vessel and (2) accurate acoustic measurements of distances between a transponder attached on the side of the vessel (onboard station) and one located on the ocean bottom (seafloor station). In this study, we performed numerical simulations to estimate measurement errors with acoustic positioning assuming acoustic velocities in the sea water and the distribution of observation points around the single seafloor station. We found that the position of the seafloor station which we can obtain by analyzing travel-time data might have around 18-cm discrepancy with respect to its “true” position. Colombo et al. (2001) reported that the position of the vessel can be determined with about 10-cm error by kinematic GPS positioning. These results indicate that the system should be able to detect seafloor crustal deformation much larger than 28 cm, including pre-, co-, and post-seismic slips due to the large earthquakes at subduction zones, slow and silent earthquakes, etc. Therefore, we emphasize the importance of continuous observations with a nationwide geodetic observational network for seafloor crustal deformation.

Three-dimensional numerical simulation of the subduction dynamics in the Sunda arc region, Southeast Asia

Abstract The Sunda arc has been modelled as a test subduction zone with geometrical complexities and varying dynamics. We simulated spatially the structural heterogeneities of the subduction zone and carried out our computations by a three-dimensional finite element approach. The aim is to estimate the lateral variation in the effective horizontal compressive stress in the interplate region, which can produce the present-day observed seismogenic stress field. In our simplified approach, the stress field has been modelled by varying the traction (ridge push), while keeping the age-dependent gravitational body force (slab pull) constant. Our result suggests that there is a considerable amount of stress dissipation in the decoupled segment of Java. At Sumatra, the required traction is much greater than that expected from the age-dependency relations. In our modelling, the transition from the dominance of the horizontal compressive stress to that of the gravitational pull in the regional stress field occurs around a depth range of 150–200 km. Lateral variation in the subducted slab volume emerges crucial to the seismogenic stress field and the derived pattern of displacement. Subduction of the lighter oceanic rise material should enhance the stress level. The effect of abrupt lateral variation in structure e.g., sudden changes in the slab dip or slab depth, may also be significant on the regional stress field. Viscoelastic modelling attempted for estimating the temporal variation in subduction dynamics suggests that the thrust level will increase southward of the present day thrust zone in Java, while strike-slip fault-type stress field will be developed south of Sumatra.

A PHYSICAL MECHANISM FOR TEMPORAL VARIATION IN SEISMICITY IN SOUTHWEST JAPAN RELATED TO THE GREAT INTERPLATE EARTHQUAKES ALONG THE NANKAI TROUGH

Abstract A significant correlation has been reported between the intraplate (inland) and the great interplate earthquakes in Southwest Japan, based on historical earthquake data since 868. The frequency of intraplate earthquakes is high for several decades before and up to a decade after great interplate earthquakes. To explain the correlation, we treated two types of interaction between the inland faults and the plate boundary: (a) in which the stress changes caused by great interplate earthquakes affect the occurrence of intraplate earthquakes; and (b) in which the stress changes caused by intraplate earthquakes trigger other intraplate earthquakes and great interplate earthquakes. To simulate the sequences of these earthquakes, we calculated the static stress changes resulting from large intraplate and great interplate earthquakes and also estimate the rate of increase in tectonic stress on the faults and the plate boundary. The estimated rates of increase here are smaller than those used in previous studies. Thus, in our models, stress changes could more strongly affect the seismicity than previously considered. We simulated the earthquake sequences for our models in four cases: (i) with no interaction among the inland faults in Southwest Japan and the plate boundary along the Nankai trough; (ii) with stress changes on the inland faults caused only by great interplate earthquakes; (iii) with stress changes on the plate boundary caused only by large intraplate earthquakes; and (iv) with stress changes both on the inland faults and on the plate boundary caused by large intraplate earthquakes. The simulated time series in case (i), of course, shows no correlation between intraplate and great interplate earthquakes. Those in cases (iii) and (iv) also show no significant correlation. Only the time series of intraplate earthquakes in case (ii) shows a temporal variation similar to that of the real intraplate earthquakes before and after the great interplate earthquakes. We examined statistically the similarity between the simulated time series and the real earthquake data. About 40% of the time series in case (ii) are significantly similar, although in the other three cases less than 5% of the time series are. Understanding of the effects of stress changes on the inland faults caused by the great interplate earthquakes along the Nankai trough is essential to explain the temporal variation in seismicity in Southwest Japan before and after the great interplate earthquakes.

Stress field in the continental part of China derived from temporal variations of seismic activity

Abstract The seismic activity in East Asia during the last 3000 years and more has systematically been analyzed. A quantitative estimate has been attempted for the seismicity during the last 500 years, employing a statistical approach. The earthquake-generating stresses in and around China have been investigated by analyzing the relationships of the temporal variations of seismicity in various regions of East Asia. Seismicities in the region from the northern part of the North-South Seismic Belt of China (NSB) (using events with M ≧ 7 ) to the Japan trench (using events with M ≧ 7.5 ) through North China (using events with M ≧ 7 ), South China (using events with M ≧ 6 ) and the Korean peninsula (using events with M ≧ 6.5 ) were investigated. They all show high activity from about the 16th century to the beginning of the 18th century. The activity then subsided until the beginning of the 20th century. From the beginning of the 20th century, the activity of each region has risen again and remains high now. The short term variations of seismicity since 1900 in North China, the Korean peninsula and Japan also exhibit the same pattern. Synchronous variations of seismicity in these regions imply that regional earthquake-generating stress fields between the Japan trench and the northern part of the NSB of China originate under common tectonic conditions and that there is a transmission of the tectonic force from the subduction of the Pacific Ocean plate along the Japan trench to the northern part of the NSB through the Korean peninsula and North China. The seismic activities since 1900 in South China (using events with M ≧ 5), the Taiwan (using events M ≧ 7.3 ) and the Ryukyu regions (using events with M ≧ 6.7 ) were found to be high between 1900 and 1940. This high activity was followed by a quiet period starting from around 1945. Since 1960, the active period has continued up to the present time. These results suggest that the earthquake-generating stress fields between Taiwan and South China seem to be formed under common tectonic conditions i.e., due to the transmission of the tectonic force resulting from the subduction of the Philippine Sea plate along the Ryukyu trench and the collision between the Philippine Sea plate and the Eurasian plate in the Taiwan region. During this century, the Tibetan plateau (using events with M ≧ 6.8 ) and the Xinjiang region (using events with M ≧ 7 ) were seismically active from 1900 to 1916, then from around 1931 to 1955, and lately from around 1970 to the present. These active periods correspond to those (using events with M ≧ 7.7 ) at the boundary regions between the Indo-Australian and the Eurasian plate. This implies that the earthquake-generating stress fields in and around West China result from the transmission of the tectonic forces originating from the collision of the Indo-Australian plate and the Eurasian plate along the Himalayan mountain range.

Temporal variations in seismic and volcanic activity and relationship with stress fields in East Asia

Abstract The temporal characteristics in seismic and volcanic activity and relationship with seismogenic stress in East Asia were analyzed mainly based on the qualitative and statistical analysis of historical and instrumental earthquake data. A long-term synchronous variation in seismicity since 1400 A.D. exists in the intraplate region from northeastern China to the Inner Zone of Southwest Japan through the Korean Peninsula. Short-term synchronous variation in seismicity since 1900 is also found in the region from northeastern China and the Korean Peninsula to the western part of the Inner Zone of Southwest Japan. Historical volcanic activity in and around the Korean Peninsula is closely correlated with active seismicity. The seismogenic stress field in this intraplate region may be formed under the common tectonic conditions due to the regional tectonic forces originating from the collision between the Indo-Australian and the Eurasian plates in the west and the combined effects of subduction of the Pacific and the Philippine Sea plates around the Izu and Tokai area in the east, and the regional vertical force by uplifting of upper mantle. The direction of the regional forces is almost parallel to the great circle connecting the eastern part of the Himalayas to Japan. The intraplate regions from northeastern China and the Korean Peninsula to the western part of the Inner Zone of Southwest Japan may be responding as a unitary block to the regional tectonic force and comprising a province with a common seismogenic stress field in the eastern part of the Eurasian plate.

Regional characteristics of stress field in the southern part of the north-south seismic belt in China and its relation with plate movement

The regional characteristics of stress field in the southern part of the north-south seismic belt (NSB) have been analyzed in detail based on the mechanism solutions of 134 medium and large earthquakes from 1933 to 1991. The results show that the southern part of the NSB is a shallow earthquake zone where most earthquakes are caused by the strike-slip faulting. There is a systematic distribution of the directions ofP- andT-axes in the western and the eastern regions of the southern part of the NSB.P- andT-axes in the western region are in the NE-SW direction and in the NW-SE direction.P- andT-axes in the eastern region are oriented in NW-SE and NE-SW, respectively. The directions ofP-axes in the western and the eastern regions show a pattern of a reversal “V” as a whole. The boundary between the eastern and western regions coincides with that between the Tibetan Plateau and the Yangtze crustal block. Based on a lot of mechanism solutions, the result indicates that the direction ofP-axes roughly shows the consistent distributions from the Himalayan collision zone to the eastern region and from the eastern coast collision zone in Taiwan to the eastern region of southern part of the NSB, respectively. It is suggested that the tectonic force due to relative movement between the Indo-Australian and the Eurasian plates is transmitted from the Himalayan collision zone to the western region of the southern part of NSB, simultaneously, the tectonic force due to the relative movement between the Philippine Sea and the Eurasian plates is transmitted from the eastern region coast in Taiwan to the eastern region of the southern part of NSB, and control the stress field there, respectively.

Spatiotemporal distribution of seismic activity in East Asia and its relation to regional stress field

Tectonic forces from the relative movements between plates are transmitted into the continental crust, and then they create the earthquake generating stress field there. The space-time distribution of the seismic activity including the small earthquakes in a region reflects the variation of the stress field in the region.According to this idea, the characteristics of the stress fields in the various regions of East Asia have been analyzed in detail in this paper based on a lot of solutions of focal mechanisms and data of seismic activity during the last 500 years. The results indicate that the tectonic forces from the subduction of the Pacific Ocean plate underneath the Eurasian plate control the stress field in the region from North China to the northern part of the North-South Seismic Belt.The variation of the regional stress field shown by the variation of seismic activity in some regions of Japan has also been discussed based on characteristics of variation of the seimicity of small earthquakes.Synchronous variations of seismicity in the past 100 years or so in West China and in the boundary region between the Indo-Australian and Eurasian plates implicate that there is the transmission of tectonic forces into West China through the collision between the Indo-Australian and Eurasian plates. The active seismic activity in the boundary region between the Indo-Australian and the Eurasian plates and in West China is continuing consistently.

Earthquake Environments at Mogao Grottoes, Dunhuang, China

This paper describes fundamental settings of the Mogao Grottoes, Dunhuang against earthquake damage. The potential damage caused by an earthquake might be generally estimated by Intensity scale. The intensity is related to human reaction and structural damages. Based upon the characteristics distribution pattern of the relation between magnitudes and epicentral distances or distances from faults, general effects to structures could be estimated as a preliminary base. Earthquake data from PCSeis (1606–2011) show little effects on the Mogao Grottoes. However, “Sanwei Shan fault,” which locates nearby the site, may cause a very big magnitude earthquake of M = 8. The active fault is extended to the very near to the Grottoes, the Sanwei Shan fault is identified as to produce the most dangerous earthquake to Mogao Grottoes in the future.