An extensive seismic refraction experiment with the use of explosive sources was conducted in the Kitakami region, northern Honshu, Japan, on November 1, 1990. The experiment area is divided into two geological units by the Hayachine Tectonic Belt (HTB). The southern terrane consists of pre-Silurian basements and Silurian-lower Cretaceous marine sediments, while the northern one is characterized by a Jurassic accretionary complex. Both of the units were intruded by Cretaceous granitic rocks.An almost N-S seismic refraction profile of 194-km length was extended from Kuji City, Iwate Prefecture to Ishinomaki City, Miyagi Prefecture, on which 4 shots with a charge size of 450-700 kg were fired. The generated seismic signals were recorded at 179 stations, from which a precise crustal structure was determined. The uppermost crust is covered with a very thin (0.5-1 km) surface layer with a velocity of 3.1-5.4 km/s. Our results show a remarkable structural difference between the northern and southern Kitakami terranes. A velocity of the uppermost crust is 5.90 km/s in the northern part of the profile while 6.05-6.15 km/s in the southern part. This lateral velocity change occurs just beneath the HTB. A mid-crustal interface corresponding to well-recorded wide-angle reflections (PiP phase) shows an abrupt southward depth decrease from 25 to 20 km beneath the HTB. The reflections from the Moho boundary (PmP phase) were well observed in all of the record sections. The Moho depth determined from the PmP phase also decreases southward from 34 to 32 km. The velocity of the lower crust is 6.8-7.0 km/s. The refracted waves from the uppermost mantle (Pn phase) were observed with weak amplitudes at offsets greater than 160 km. These data suggest that the Pn velocity in the experiment area is less than 7.7 km/s.The VP/VS ratio within the crust was determined from S-waves recognized in the vertical component seismograms. The VP/VS ratio in the upper crust also shows a significant lateral change at the HTB, namely 1.72-1.73 in the northern part whereas 1.75-1.76 in the southern part. In the lower crust, the VP/VS ratio is 1.73-1.76. In the present experiment, weak refracted waves from the upper mantle (Sn phase) were observed at offsets greater than 170 km, from which the Sn velocity was estimated to be 4.3 km/s.
Space-time variations of very-shallow (0-20km) earthquakes occurrence in the northern part of Nagano Prefecture are studied from the hypocenter data by the Matsushiro Seismic Array System (MSAS) and JMA. The very-shallow earthquakes are distinctively characterized by two active zones, or the one from Omachi to Nozawaonsen and the other from Sakakita to Suzaka including hypocenters of the Matsushiro Swarm Earthquake. A significant variation of very-shallow earhtquakes occurrence is recently observed along the Omachi-Nozawaonsen zone. After two aftershock activities from Dec. 30, 1986 in Ogawa and from Feb. 12, 1987 in Nozawaonsen, a very low seismic activity or gap between Ogawa and Nozawaonsen is now observed.
The Kitakami massif, which is located in the eastern part of Northern Honshu, Japan, is composed of two geological units. The northern Kitakami terrane is characterized as a Jurassic accretionary complex, while the southern Kitakami terrane consists of pre‐Silurian basement and Silurian‐lower Cretaceous marine sediments. The boundary region of these two units, called the Hayachine tectonic belt (HTB), is composed of mafic to ultramafic rocks. The Kitakami massif experienced intense granitic intrusions in the Cretaceous. We present a detailed crustal structure model for the eastern part of the massif derived from an extensive seismic refraction experiment conducted on a 194‐km N‐S line. The uppermost crust is covered with a very thin (0.5–1 km) surface layer with a velocity of 3.1–5.4 km/s. The velocity structure below this layer shows remarkable lateral variation. In the northern Kitakami terrane the P wave velocity and V p / V s at the top of the basement are 5.85–5.95 km/s and 1.68–1.70, respectively. The seismic attenuation in this region is high ( Q p = 150–200 and Q s = 70–100). In contrast, the uppermost crust in the southern Kitakami terrane is characterized by a high P wave velocity (6.05–6.15 km/s) and V p / V s (1.74–1.77). The Q p and Q s also show high values of 300–400 and 150–200, respectively. Such a structural difference persists to 14‐to 16‐km depth, at which the P wave velocity increases to 6.45 km/s. The low velocity and high attenuation in the northern Kitakami terrane represent a highly deformed structure of the accretionary complex. The high P wave velocity and V p / V s in the southern Kitakami terrane indicate the relatively mafic crustal composition, which may result from the fragment of the oceanic crust incorporated by the accretion process or the uplifting in the latest Jurassic‐early Cretaceous. A midcrustal interface determined from wide‐angle reflections shows an abrupt southward depth decrease from 25 to 20 km under the HTB. The P wave velocity and V p / V s between 14‐ and 16‐km depth and the midcrustal interface are 6.45–6.55 km/s and 1.74–1.78, respectively. The Moho depth under the northern Kitakami terrane decreases southward from 34 to 32 km. In the southern Kitakami terrane the Moho dips slightly southward. The P wave velocity and the V p / V s ratio in the lower crust are 6.9–7.0 km/s and 1.75–1.76, respectively. The P wave velocity in the uppermost mantle is not well resolved but is probably less than 7.7 km/s. The S wave velocity derived from relatively clear S n is 4.35–4.40 km/s. Our results show that the HTB is a prominent structural boundary extending to the Moho. The crust of Kitakami massif was not homogenized by the Cretaceous granitic intrusions, and the original structural difference remains in the upper crust.
The 1923 Kanto earthquake (M7.9), the most disastrous earthquake in Japan, is the first earthquake which left abundant seismometrical data of the main shock and aftershocks. It provides us with one of the best opportunities of studying aftershock activity and long term change in seismicity after an interplate earthquake. However, due to an uncertainty of data quality of that day and difficulty in integrating the available data, detailed examination of aftershocks has been remained untouched except several preliminary investigations carried out by the Central Meteorological Observatory and universities immediately after the earthquake in term of the current knowledge of seismology.In this study, we made a complete integration of the data existing separately. The intensive data of half year after the 1923 event ensures statistical treatments such as making of S-P histograms and daily frequency of earthquakes observed at each station which may approximately indicate the extent of aftershock zone and time variation of the aftershock activity. By using S-P times mainly and P and S arrivals independently in some cases, we locate about 400 earthquakes. Although each epicenter may include location error of an order of 20km, relatively large extent of aftershock area makes the aftershock distribution show a good correspondence with the faulting models and the seismicity pattern of today.A high concentration of aftershocks is recognized in the border of Yamanashi and Kanagawa Prefectures, the western boundary of faulting area where the main shock originated. Another concentration of aftershocks suggests that eastern boundary of the faulting zone exists in the middle of the Boso Peninsula. While northern boundary of aftershock region is defined rather clearly near the border of Kanagawa and Tokyo Prefectures, southern limit of aftershock zone is ambiguous because poor location accuracy.Compared to the periphery of the aftershock area, two inactive areas of aftershock activity are recognized at the middle of faulting region which corresponds to low seismicity areas found with recent microearthquake studies and the nucleation zone of the main shock where large dislocation of the faulting is estimated by the dynamic source process studies. This result suggests that basic seismicity pattern of present day in the southern Kanto region including the 1923 focal region has been created from its aftershock activity. Seismicity pattern before 1923 might be almost similar to those of today except middle of the Boso Peninsula. Relatively high seismic activity is recognized there immediately after the 1923 earthquake and it has taken ten to twenty years the seismicity to recover the normal level.Magnitude frequency relation of the aftershocks was compared with those of four interplate earthquakes of similar large size in Japan. It shows that the Kanto earthquake is accompanied with larger number of aftershock of M>6. Prominence of aftershock activity may be attributed to the complexity of tectonic setting of the focal region which is located near the collision boundary of the Philippine Sea plate and the Eurasian plate.
