JP2013007728A - Real time estimation method of hypocentral region of giant earthquake - Google Patents

Abstract

PROBLEM TO BE SOLVED: To solve the problems that magnitude of the Meteorological Agency does not become larger than around 8.0 due to a problem of saturation of the magnitude when a giant earthquake of around magnitude 9.0 occurs, thus, tidal wave prediction becomes underestimation, which becomes a cause of expansion of tidal wave damages, in addition, also in earthquake early warning, seismic intensity becomes underestimation since the seismic intensity is predicted by a point source model, and a method for estimating expansion of a hypocentral region of the giant earthquake in real time is not developed in a conventional technology.SOLUTION: Temporal and spatial distribution of a hypocentral region is calculated by calculating fault shortest distance from real time seismic intensity by using an empirical formula regarding relation with real time seismic intensity fault shortest distance, and projecting the fault shortest distance on a map. A problem of saturation of magnitude can be checked from expansion of the hypocentral region, and exact tidal wave forecast is expected. In addition, exact prediction of the seismic intensity of earthquake early warning becomes possible from spatial distribution of the hypocentral region.

Description

  The present invention relates to a method for estimating in real time the spatial distribution of an epicenter when a large earthquake occurs.

  Magnitude by the Japan Meteorological Agency definition and Richter, etc., excluding moment magnitude, is known to have a problem of magnitude saturation when the magnitude of the earthquake is larger than 8, even if the magnitude of the earthquake is larger than 8 ing. For this reason, even if a huge earthquake occurs, it is difficult to know how much the earthquake has occurred from the magnitude of the Japan Meteorological Agency. For example, in the Great East Japan Earthquake of March 11, 2011, the magnitude determined immediately after the occurrence of the earthquake was 7.9, but two days later, the magnitude was changed to 9.0 from the result of the moment magnitude. Due to the underestimation of magnitude, the Japan Meteorological Agency's tsunami wave height prediction was underestimated by an order of magnitude compared to the actual tsunami, resulting in delayed evacuation and the loss of many precious lives. The present invention improves the accuracy of a tsunami prediction system, an earthquake early warning distribution system, an earthquake early warning receiving terminal device, etc. by providing a method for obtaining the spatial extent of the source area of a huge earthquake in real time. .

と定義されている。Δは震央距離、ｈは深さで、Ｋは振幅の距離減衰を表す関数である。マグニチュードと断層の長さ（Ｌ）に関する関係式は、多くの著者により示されており、［非特許文献１］では、

となっている。In tsunami wave height prediction, it is necessary to accurately determine the magnitude of the earthquake. The current tsunami prediction wave height is calculated using the JMA magnitude. As described in Non-Patent Document 1, the Japan Meteorological Agency magnitude (Mj) uses the amplitude (A) of a Wiechert seismometer with a natural period of 5 seconds,

It is defined as Δ is the epicenter distance, h is the depth, and K is a function representing the distance attenuation of the amplitude. The relational expression regarding the magnitude and the fault length (L) has been shown by many authors. [Non-Patent Document 1]

It has become.

モーメントマグニチュードは、巨大地震の場合にも飽和することはない。Non-Patent Document 1 also shows that, in a large earthquake, a phenomenon in which the magnitude value does not become large even if the scale is large, so-called magnitude saturation occurs. The seismic moment (MO) is an amount corresponding to the magnitude of the fault motion, and is defined by the amplitude at the limit on the long period side of the seismic wave spectrum. As shown in Non-Patent Document 1, the moment magnitude (Mw) is a magnitude defined from the seismic moment and has the following definition.

Moment magnitude does not saturate even in the event of a huge earthquake.

  In the 2011 Great East Japan Earthquake, Mj was 7.9. The Japan Meteorological Agency decided Mw using foreign data two days after the earthquake and changed the magnitude to 9.0. Mw is a parameter that represents the exact magnitude of an earthquake, but the drawback is that it takes tens of minutes to make a decision and is not in time for a tsunami warning or the like. Similarly, the magnitude immediately after the occurrence of the 2004 Sumatra Earthquake was 8.0, but it was changed to 9.1. If the magnitude is different by one, the energy of the seismic wave is different by 32 times, so the magnitude of the earthquake estimated immediately after the occurrence of the earthquake is significantly smaller than the actual magnitude. Because the magnitude 7.9 required immediately after the earthquake occurred was used for the tsunami warning and the emergency earthquake warning for the Great East Japan Earthquake, the tsunami warning was underestimated by an order of magnitude, resulting in delays in the evacuation of many residents and tsunami damage. Expanded.

  As a method for highly accurate prediction of tsunami, there is a method of measuring and transmitting the tsunami wave height offshore using a submarine tsunami meter or GPS. In the 2011 Great East Japan Earthquake, the Japan Meteorological Agency corrected the warning using data from the tsunami meter, but it took time to measure and the timing for issuing the warning was delayed. In addition, a method (for example, Patent Document 1) for measuring a tsunami using SAR technology in an aircraft has been proposed.

Patent Publication 2009-229424, Mitsubishi Electric Corporation March 8, 2008 (2009.10.8) Tsunami monitoring device.

Seismology 2nd edition, written by Tokuharu Utsu, Kyoritsu Shuppan, 1984. Hirotoshi Tsukasa and Saburo Sasakawa (1999): Distance attenuation formula of maximum acceleration and maximum velocity considering fault type and ground conditions, Architectural Institute of Japan, 523, 63-70. Shinichi Matsuzaki, Yoshiaki Hisada, Mimitsu Fukushima (2006): Development of distance attenuation formula of seismic intensity applicable to the vicinity of the fault, Journal of Architectural Institute of Japan, 604, 201-208.

  In the Great East Japan Earthquake with a magnitude of 9.0, the north-south extent of the epicenter was 500 km. In the prior art, the position of the point (seismic source) where the fault motion is started is determined and the seismic intensity is predicted. In the case of a huge earthquake with a large epicenter, there is a problem that the predicted seismic intensity is significantly reduced at locations far from the epicenter. Tsunami prediction is performed in consideration of the relationship between magnitude and fault length shown in Equation (2). However, since the JMA magnitude has a problem of saturation, the magnitude will not increase even if the magnitude of the earthquake increases. As a result, there is a problem that the magnitude of the earthquake is evaluated to be small and the tsunami wave height is underestimated.

  The present invention aims at reducing earthquake damage by contributing to the advancement of tsunami prediction and emergency earthquake warning by providing a method for estimating the extent of the epicenter of a huge earthquake in real time.

  Suppose that an earthquake occurred and the magnitude required in real time increased with time and increased to about 8.0. In this case, even if a larger earthquake occurs due to the above-described saturation problem, it cannot be known from the magnitude.

  In seismic intensity prediction using emergency earthquake bulletin, the use of seismic intensity prediction formulas by Tsukasa Yodogawa (Non-Patent Document 3) is recommended. This prediction formula is empirical about the relationship between the seismic intensity of earthquakes that occurred in the past, the shortest fault distance, and the JMA magnitude. In the earthquake early warning, since the fault spatial distribution information cannot be obtained, the fault equivalent seismic source distance is used instead of the fault shortest distance. Matsuzaki et al. (Non-patent Document 3), for earthquakes that have occurred in the past and whose spatial distribution of the fault is required, the seismic intensity applicable to the fault is calculated based on the relationship between the shortest fault distance, seismic intensity, and JMA magnitude. The distance attenuation formula is obtained. Previous studies have shown that seismic intensity is expressed as a function of fault shortest distance.

  As described above, the seismic intensity can be expressed as a function of the fault shortest distance. In the present invention, the shortest fault distance is obtained for each observation point from the real-time seismic intensity using the conventional research, and it is projected onto the position shown in FIG. When the length from the epicenter to the end of the epicenter is obtained for many observation points, the orientation distribution of the distance to the end of the epicenter is obtained, and the spatial extent of the epicenter is required. In addition, it is estimated temporally and spatially how the epicenter of a huge earthquake expands from the time change. The detailed method is described below.

と置く。Suppose that the seismic intensity greater than the magnitude estimated from the magnitude was observed at the jth observation point, and using the relational expressions such as Non-Patent Document 2 and Non-Patent Document 3, the distance from the observation point to the fault was reversed. It is possible to determine the time variation of the shortest distance. Therefore, the shortest fault distance at time t, Rj (t), by j observation points,

Put it.

  Here, M (t) is the magnitude of the emergency earthquake bulletin at time t, Δj is the epicenter distance, h is the depth of the epicenter, and Sj (t) is the real-time seismic intensity at observation point j at time t. . When the shortest tomographic distance is obtained using maximum value data such as speed or acceleration, Sj (t) is the maximum value of speed or acceleration.

と置かれる。As shown in FIG. 1, the position of the end of the epicenter at time t is assumed to be a position away from observation point j by Rj (t) on the straight line connecting the epicenter and the observation point, and its horizontal coordinates Is (Xj (t), Yj (t)). Similarly, at the observation point i, the fault shortest distance Ri (t) is obtained, and the coordinates of the position of the end of the epicenter area (Xi (t), Yi (t)) are obtained. For simplicity, assuming that the fault is horizontal, the distance from the epicenter to the end of the epicenter, ie, the length of the fault from the epicenter, Dj is

And put.

  From the distribution of straight lines connecting the epicenter and the end of the epicenter at many observation points, the interior of the region indicated by the dotted line in FIG. By examining this temporally, it is possible to estimate the spatial and temporal distribution of the epicenter region.

  Seismic intensity prediction before the arrival of a large shake can be performed by determining the shortest fault distance for each observation point, assuming that the end of the fault has expanded to (Xj (t), Yj (t)). Since the shortest fault distance is an amount obtained using the S-wave amplitude, a large shake has already arrived at the observation point used for the analysis, and prediction before the shake cannot be performed. However, it is possible to transmit a warning before a large shake to an observation point located far from the observation point used for the analysis.

  It is difficult to predict how far the fault will expand without completing the fault movement. There is a possibility that the epicenter area will continue to expand by 50 km or 100 km in a certain direction with respect to the hypocenter area estimated at a certain time. In the case of a huge earthquake, the seismic intensity is predicted using a model in which a fault obtained at a certain time is further expanded by a specific distance.

  FIG. 2 shows an example of real-time estimation results of the epicenter area of the Great East Japan Earthquake using the method of the present invention. The figure shows the result 171 seconds after the earthquake early warning was distributed. It is in good agreement with the hypocenter distribution estimated from the aftershock distribution. FIG. 3 is a diagram showing a seismic intensity estimation method according to the present invention. By calculating the shortest fault distance and predicting seismic intensity for each direction, accurate seismic intensity prediction in real time becomes possible. This result shows that according to the present invention, the spread of the epicenter region can be accurately determined in real time, and high-precision seismic intensity prediction in real time becomes possible.

It is the figure which showed the estimation method of the breadth of a fault by this invention. It is the figure which showed the hypocenter area calculated | required from the distribution of the fault of the Great East Japan Earthquake calculated | required in real time by this invention, and the hypocenter distribution by the Meteorological Agency. It is the figure which showed the breadth of the fault plane of the Great East Japan Earthquake for every direction calculated | required by applying the method by this invention.

  Disseminate emergency earthquake warning receiving terminals with built-in seismometers. When an earthquake occurs, the receiving terminal transmits information such as real-time seismic intensity to the center server about once every second. The center server enters the giant earthquake occurrence mode, assuming that if the magnitude by the Japan Meteorological Agency is greater than a threshold value of around 8.0, Mk, the magnitude is saturated and an earthquake that may be a giant earthquake is occurring.

  In the huge earthquake occurrence mode, the magnitude of the meteorological agency is used to determine the radius of the fault from the relationship between the fault length and the magnitude in equation (2), and a circle is drawn around the epicenter, assuming that it is a fault area.

  Next, for each observation point, the seismic intensity predicted from the magnitude is compared with the observed seismic intensity. If the measured seismic intensity is larger than the seismic intensity predicted from the magnitude, the fault length is calculated from the equation (5). To plot a line segment of length Dj (t) in the direction of the azimuth of the observation point. The distribution of the epicenter is estimated from the distribution of this plot. Tsunami predictions are made based on the size of the epicenter and whether or not a magnitude saturation problem has occurred.

  In the seismic intensity prediction, first, the azimuth of the observation point viewed from the epicenter is obtained, the azimuth interval of 10 degrees to 20 degrees is set, and which azimuth block is entered for each observation point. Next, the average value of the fault length is obtained for each orientation block. At stations near the epicenter, large tremors arrive early, but at distant stations it takes time to reach the tremors. When determining the average value for each azimuth block, it is necessary not to include data of observation points far away. Therefore, the average value is the average of a plurality of observation points in descending order of seismic intensity or in order of increasing distance from the epicenter.

  In general, there are large variations in predicted seismic intensity and observed seismic intensity due to inhomogeneities within the earth and the influence of ground amplification characteristics. In the method of the present invention, there is a possibility that it is determined that the earthquake is a huge earthquake because it is not a huge earthquake due to the variation.

となる。ここに、ＴＯ，Ｔｓは発震時刻、震源から観測点迄のＳ波走時であり、Ｖｒ，Ｖｓは、断層破壊伝播速度、Ｓ波速度である。From the conventional research, it is known that the speed at which the failure of the fault propagates is about 2.8 km / sec at the maximum. It is assumed that an earthquake that has an epicenter area larger than the size of the epicenter area estimated from the magnitude, that is, a huge earthquake, has occurred due to the magnitude saturation problem. The fault radius assumed from the magnitude is De. In this case, the fault motion corresponding to the huge earthquake starts after the fault has traveled by the distance De, that is, after De / Vr seconds. Here, Vr is the fracture propagation speed. The time Tf when the S wave corresponding to the huge earthquake arrives at the observation point is

It becomes. Here, TO and Ts are the time of earthquake occurrence, the S wave travel time from the epicenter to the observation point, and Vr and Vs are the fault destruction propagation speed and the S wave speed.

  Since the real-time seismic intensity measured by the time Tf is due to the shaking before a huge earthquake, even if this value increases, it is necessary not to assume that the fault has expanded in a specific direction. In determining the shortest fault distance, the seismic intensity in the section up to the time Tf is regarded as a correction term due to inhomogeneity in the earth and ground amplification characteristics, and the seismic intensity after the time Tf is corrected, so that a more accurate fault It seems that the spread of can be estimated.

  It can be used as an emergency earthquake warning device and tsunami warning device corresponding to a huge earthquake.

Claims (4)

  Using the time change of the maximum value of any of the real-time seismic intensity, acceleration, and velocity at the time of the earthquake, the shortest fault distance is obtained, and that value is on the straight line connecting the epicenter and the observation point, or an estimated fault A real-time method for determining the location of the epicenter area, which is obtained by projecting onto a surface.   A real-time ground motion prediction system, wherein the ground motion prediction is performed using the position of the hypocenter region obtained by using the determination method according to claim 1.   A tsunami prediction system for performing tsunami prediction using the position of the epicenter area obtained by using the determination method according to claim 1   An earthquake information reception / warning terminal device using earthquake information from the earthquake motion prediction system according to claim 2.

Images