K. Maeda and F. Hirose (MRI) Probability Estimation of Large Earthquakes by Possible Foreshocks along the Japan Trench K. Maeda and F. Hirose (MRI) Foreshocks has been thought one of the most promising phenomena to predict large earthquakes. However, foreshocks are mostly found after a large earthquake occurred and it is very difficult to distinguish them deterministically from background seismicity before a mainshock occurs. Therefore, probabilistic approach is realistic way to use foreshock activity as a precursor of a mainshock. So, we investigate a probabilistic features of foreshocks and search for effective parameters to define foreshocks which present relatively good performance to predict large earthquakes.

Contents Intoduction of the foreshock definition proposed in 1996 (Data period: 1980-1993) Results for case that the method is applied to the recent data ( Data period:1994-2009 ) Combination of long-term prediction and foreshock information (Off-Ibaraki region) I have once proposed a foreshock definition which gives relatively high performance to predict large earthquakes. In this talk, we firstly introduce that method and parameters for foreshock definition, and then show the results for case that method was applied to the recent data after the method was proposed. Finally, I will talk about the effect of combination of ling-term prediction and foreshock information for the Off-Ibaraki region where characteristic earthquakes are known to occur recurrently with the period about 21 years.

Procedure for Selecting Foreshock Candidates (Maeda, 1996) (1) To Eliminate Small Aftershocks 　　・ Condition for Distance： 　　・ Condition for Time： 　　・ Condition for Magnitude： (2) To Segment Investigated Area 　　・ Size of Segmentation　　D°(Latitude)×　D°(Longitude) (D=0.5°) (3) To Define Time Window and Number of Earthquakes for Foreshock Candidates 　　・ Time Window：Tf (=10 days) 　　・ Number of Eqs. to Define Foreshock Candidates：Nf (= 3) (4) To Define Alarm Period for Mainshocks 　　・ Ta ( = 5 days) 　　 (Mainshocks: Mm ≥ 6.0, All aftershocks are removed.) This slide shows the procedure for selecting foreshock candidates I adopted in the paper of 1996.The procedure is as follows: Firstly, we eliminate small aftershocks defined like this. Secondary, we divided the investigated region into square segments with the size of D by D degree in latitude and longitude. Next, we define the time window and number of earthquakes for foreshock candidates. In this study we fixed the time window to select foreshocks as 10 days. The number of earthquakes for foreshock candidates is a variable parameter. Lastly, the we define the alarm period for mainshock to occur. The parameters of D, Tf, and Ta are variable parameters, and we search for values of them which give good performance for forecasting large earthquakes.

Schematic Diagram for Foreshock Candidates and Alarm Period (Maeda, 1996) Tf=10days Md=1.0 Ta=5days [ false foreshock ] [ true foreshock ]

Indices for Prediction Performance ・ Alarm Rate（AR）＝（Alarmed Mainshocks） 　　　　　　　　　　 /（Total Number of Mainshocks） ・ Truth Rate（TR）＝（Number of True Foreshocks） 　　　　　　　　　　 /（Number of Proposed Foreshocks） ・ Probability Gain（PG）＝ （Occurrence Rate of M.S. in Predicted Space-Time） /（Background Occurrence Rate of M.S.） ・ dAIC＝AIC Difference of the Foreshock Model from the Stationary Poisson Model

Data for Optimizing Foreshock Parameters JMA Data ・ Period: 1980-1993 ・ Dep ≤ 100km Foreshock Candedates ・ Mf ≥ 5.0 ・ Small aftershocks are removed. Target Mainshocks ・Mm ≥ 6.0 ・All aftershocks are removed.

Optimizing Parameter Nf Basing on dAIC (Maeda, 1996) Optimized Parameters for Old JMA Data ・ D = 0.5 ° ・ Ta = 5 days ・ Mf ≥ 5.0 ・ Nf = 3 Target Mainshocks ・ Mm ≥ 6.0 Note JMA magnitude was revised at Sep/25/2003

Prediction Results for the Testing Period (1994-2009) Prediction Performance ・ AR = 4% (=2/52), ・TR = 8% (=2/24) ・ PGa = 217, ・ PGf = 87 ・ dAIC = 16

Prediction Results for the Modeling Period (1980-1993) Prediction Performance ・ AR = 13% (=7/55), ・TR = 19% (=9/47) ・ PGa = 365, ・ PGf = 145 ・ dAIC = 68

Data for Testing Foreshock Performance JMA Data ・ Period: 1994-2009 ・ Dep ≤ 100km Foreshock Candedates ・ Mf ≥ 5.0 ・ Small aftershocks are removed. Target Mainshocks ・Mm ≥ 6.0 ・All aftershocks are removed.

Prediction Results for the Total Period (1980-2009) Prediction Performance ・ AR = 8% (=9/107), ・TR = 15% (=11/71) ・ PGa = 327, ・ PGf = 172, ・ dAIC =85

Performance in the Off-Ibaraki Region (1980-2009) Prediction Performance ・ AR = 50% (=2/4) ・TR = 43% (=3/7)

Long-term Probability for Off-Ibaraki Eqs. (ERC, 2009) (ERC, 2009) ―slip for 1982 (Mochizuki et al.,2008) ―slip for 2008 (Nagoya Univ,2008)

Combination of Long-term Probability and Probability Gain by Foreshocks for Off-Ibaraki Eqs.

Summary Modeling Period:1980-1993 The parameter values for selecting foreshocks which bring high prediction performance for mainshocks with M≥6 are 　　　Mf ≥ 5, Nf=3, D=0.5°,Ta=5 days 　（AR=13％, TR=19％, PG=145, dAIC=68） Testing Period:1994-2009 The prediction performance using foreshock candidates are AR=4％, TR=8％, PG=87, dAIC=16. All the Period:1980-2009 The performance are AR=8％, TR=15％, PG=172, dAIC=85. Off-Ibaraki Region Combination of long-term prediction and potential foreshocks may be effective to predict the characteristic earthquake. (Maeda, 1996)