Significance test for seismicity rate changes before the 1987 Chiba-toho-oki earthquake (M 6.7) Japan

A precursory seismic quiescence lasting 1.5 ± 0.5 years was observed prior to the 1987 M 6.7 Chiba-toho-oki earthquake, Central Japan. This event was the largest mainshock to occur in the region in 60 years. A quantitative analysis of the seismicity rates, using two independent catalogs provided by the NIED and JMA networks, shows that the precursory seismic quiescence is centered in the shallower part of the rupture zone of the subsequent mainshock, at a depth of 20-40 km. At the hypocenter of the 1987 Chiba-toho-oki mainshock, a 50% increase in the seismicity rate was detected in the NIED data, coinciding in time with the onset of quiescence (1986.4 ± 0.5). The simultaneous appearance of both quiescence in the shallow part of the rupture zone observed in two catalogs, and a rate increase in the immediate hypocenter region, suggest that these phenomena are causally linked to the subsequent mainshock. However, a quantitative analysis of both catalogs reveals that the precursory quiescence and rate increase are not unique, since rate changes of this duration and significance often occur in the data. A rate change of this significance rating could probably not be detected as a precursor in a real time approach. For the aid of real time monitoring of seismicity rate changes, we introduce the method to calculate the 95-percentile of confidence level for the significant rate changes.

Download Full-text

Seismic quiescence, stress drops, and asperities in the New Hebrides arc

Bulletin of the Seismological Society of America ◽

10.1785/bssa0730010219 ◽

1983 ◽

Vol 73 (1) ◽

pp. 219-236

Author(s):

M. Wyss ◽

R. E. Habermann ◽

Ch. Heiniger

Keyword(s):

High Stress ◽

Plate Boundary ◽

Seismic Quiescence ◽

Data Set ◽

Seismicity Rate ◽

Seismic Gaps ◽

Main Shocks ◽

Seismicity Rates ◽

New Hebrides ◽

Stress Drops

abstract The rate of occurrence of earthquakes shallower than 100 km during the years 1963 to 1980 was studied as a function of time and space along the New Hebrides island arc. Systematic examination of the seismicity rates for different magnitude bands showed that events with mb < 4.8 were not reported consistently over time. The seismicity rate as defined by mb ≧ 4.8 events was examined quantitatively and systematically in the source volumes of three recent main shocks and within two seismic gaps. A clear case of seismic quiescence could be shown to have existed before one of the large main shocks if a major asperity was excluded from the volume studied. The 1980 Ms = 8 rupture in the northern New Hebrides was preceded by a pattern of 9 to 12 yr of quiescence followed by 5 yr of normal rate. This pattern does not conform to the hypothesis that quiescence lasts up to the mainshock which it precedes. The 1980 rupture also did not fully conform to the gap hypothesis: half of its aftershock area covered part of a great rupture which occurred in 1966. A major asperity seemed to play a critical role in the 1966 and 1980 great ruptures: it stopped the 1966 rupture, and both parts of the 1980 double rupture initiated from it. In addition, this major asperity made itself known by a seismicity rate and stress drops higher than in the surrounding areas. Stress drops of 272 earthquakes were estimated by the MS/mb method. Time dependence of stress drops could not be studied because of changes in the world data set of Ms and mb values. Areas of high stress drops did not correlate in general with areas of high seismicity rate. Instead, outstandingly high average stress drops were observed in two plate boundary segments with average seismicity rate where ocean floor ridges are being subducted. The seismic gaps of the central and northern New Hebrides each contain seismically quiet regions. In the central New Hebrides, the 50 to 100 km of the plate boundary near 18.5°S showed an extremely low seismicity rate during the entire observation period. Low seismicity could be a permanent property of this location. In the northern New Hebrides gap, seismic quiescence started in mid-1972, except in a central volume where high stress drops are observed. This volume is interpreted as an asperity, and the quiescence may be interpreted as part of the preparation process to a future large main shock near 13.5°S.

Download Full-text

Stress and pore fluid pressure control of seismicity rate changes following the 2016 Kumamoto earthquake, Japan Version 2021125

Earth Planets and Space ◽

10.1186/s40623-020-01329-5 ◽

2021 ◽

Vol 73 (1) ◽

Author(s):

Kodai Nakagomi ◽

Toshiko Terakawa ◽

Satoshi Matsumoto ◽

Shinichiro Horikawa

Keyword(s):

Focal Mechanism ◽

Fluid Pressure ◽

Pore Fluid ◽

2016 Kumamoto Earthquake ◽

Pore Fluid Pressure ◽

Seismicity Rate ◽

Kumamoto Earthquake ◽

The 2016 Kumamoto Earthquake ◽

Seismicity Rates ◽

Seismicity Rate Changes

AbstractWe quantitatively examined the influence of pore fluid pressure and coseismic stress changes on the seismicity rate changes that followed the 2016 Kumamoto earthquake, on the basis of two approaches. One is a numerical calculation of the classic stress metric of ∆CFS, and the other is an inversion analysis of pore fluid pressure fields with earthquake focal mechanism data. The former calculation demonstrated that seismicity rate changes were consistent with the expectation from ∆CFS in 65% of the target region, whereas they were not in the remaining 35% of the region. The latter analysis indicates that seismicity rates increased in the regions where pore fluid pressure before the Kumamoto earthquake sequence was remarkably enhanced above hydrostatic, regardless of values of ΔCFS. This suggests that the increase in pore fluid pressure is one of the important physical mechanisms triggering aftershock generation. We obtained evidence that pore fluid pressure increased around the southern part of the main rupture zone after the mainshock, examining temporal changes in types of focal mechanism data. The average increases in pore fluid pressure were estimated to be 17, 20, and 17 MPa at depths of 5, 10, and 15 km, respectively. These large increases in pore fluid pressure cannot be explained under the undrained condition. The spatial derivative of the pore fluid pressure field in the depth direction implies that fluid supply from greater depths may have controlled increases in seismicity rates that followed the large earthquake.

Download Full-text

Modeling seismicity rate changes along the Hellenic subduction Zone (Greece).

Bulletin of the Geological Society of Greece ◽

10.12681/bgsg.10971 ◽

2016 ◽

Vol 47 (3) ◽

pp. 1157

Author(s):

M. K. Leptokaropoulos ◽

E. E. Papadimitriou ◽

B. Orlecka–Sikora ◽

G. V. Karakostas ◽

F. Vallianatos

Keyword(s):

Subduction Zone ◽

Parameter Determination ◽

Physical Parameters ◽

Coulomb Stress Changes ◽

Seismicity Rate ◽

Hellenic Subduction Zone ◽

Seismicity Rates ◽

Stress Changes ◽

Seismicity Rate Changes ◽

Constitutive Parameters

The Dieterich (1994) Rate/State formulation was applied for the seismicity rate changes in the western part of the Hellenic arc to be investigated. The completeness magnitude of the shallow seismicity (h<60km) was firstly evaluated for different time windows. The spatio-temporal changes of these seismicity rates (reference rates) were studied then for the interevent periods between successive strong(M≥6.0) earthquakes. These changes were correlated with the Coulomb stress changes (ΔCFF) produced by the stronger events, through a Rate/State model which incorporates physical parameters associated with fault dynamics such as the tectonic stressing rate, fault constitutive parameters and frictional response of the rupture zones. The influence of the former parameters in the model performance wastested by evaluating the linear correlation coefficient between modeled and real earthquake production rates along with their confidence limits. Application of different parameter values was attempted for the sensitivity of the calculated seismicity rates and their fit to the real data to be tested. Given the geographical peculiarity of the Hellenic Subduction zone, that yields to high uncertainties in the earthquake focal parameter determination, the results demonstrate that the present formulation and the available data sets are sufficient enough to contribute to a robust seismic hazard assessment.

Download Full-text

Seismicity rate change before the Irpinia (M = 6.9) 1980 earthquake

Bulletin of the Seismological Society of America ◽

10.1785/bssa0870020318 ◽

1997 ◽

Vol 87 (2) ◽

pp. 318-326 ◽

Cited By ~ 1

Author(s):

M. Wyss ◽

R. Console ◽

M. Murru

Keyword(s):

Southern Italy ◽

Seismic Quiescence ◽

Magnitude Scale ◽

Data Sets ◽

Data Set ◽

Background Rate ◽

Seismicity Rate ◽

Rupture Area ◽

The Stability ◽

Precursory Seismic Quiescence

Abstract The stability of seismicity rate in central and southern Italy was examined in two data sets: from 1975.0 to 1995.0 with M ≧ 3.4 and from 1987.5 to 1996.0 with M ≧ 2.5. These are the approximate minimum magnitudes of complete reporting for the respective periods. The first set was used to evaluate the possibility that the 23 November 1980 Irpinia (M 6.9) earthquake was preceded by precursory seismic quiescence; the second was used to evaluate the conditions under which a current seismic quiescence could be detected in central or southern Italy. During the years before the Irpinia earthquake, the seismicity rate in the northern half of the rupture area and north of it was low. Whether this was a case of precursory quiescence or not is subject to interpretation because the background rate cannot be established for the years before 1975. If we accept the relatively constant seismicity rate in the Irpinia volume during the decade after the mainshock as representative for the background rate, we have a clear case of precursory quiescence lasting at least 1.3 yr up to the mainshock. Alternatively, it can be postulated that the seismicity rate during the decade following this shock was elevated regionally because of the stress redistribution and that the low rates seen before it represent the normal background rate. Even if this reasoning is accepted, the fact remains that a volume including the northern part of the rupture produced no M ≧ 3.4 earthquakes during 1.3 yr before the Irpinia earthquake but produced 10 earthquakes during the 4.7 previous yr. Given these facts, we favor the interpretation that the Irpinia 1980 earthquake was preceded by precursory seismic quiescence. In the entire data set, there are three other cases of quiescence of higher significance without a mainshock following. Since no other mainshock with M > 6 exists in the data set, no missed event exists. We propose that in Italy precursory seismic quiescence may precede mainshocks and that it may be detected in the future by the improved catalog of the modern data set beginning in 1987.5. Major magnitude scale changes give the mistaken appearance that fewer large-magnitude earthquakes occur in Italy now than in years before 1987. We postulate that the rate of earthquakes has not changed and that the magnitude scale should be adjusted.

Download Full-text