EIC seismological note: No. 105
EIC seismological note: No. 105             June 24, 2001
                                             (rev. July 12, 2001)
                                         by M.Kikuchi & Y.Yamanaka
                             (EIC: Eearthquake Information Center)

   Near Coast of Peru earthquake (Mw 8.2) 
           on June 23, 2001 (REVISED)

  A very large earthquake with Mw 8.2 occurred near coast of Peru
at 3:33 PM (local time) on June 23, 2001. It is reported that at
least 71 people were dead, 61 missing, and 1,178 injured.
QED information given by USGS is as follows:

   date        time        epicenter    depth  M
 2001/06/23 20:33:14 (UT) 16.14S 73.31W  8km  8.4

<<Data processing>>
  We analysed teleseismic broadband P waves retrieved from IRIS-DMC.
(In revised version, P-wave records at 24 stations were used.)
First, assuming a point source located at QED, we determined the focal
mechanism and depth in the least squares method. Next, fixing the fault
plane as inferred from one of the P-wave nodal planes (shallower dip
plane), we determined the spacio-temporal distribution of fault slip.

  Results are shown in Fig.1(Model), Fig.2(Waveform), and Fig.3(Map-view).
Main source parameters are as follows:

 Initial break: epicenter = 16.15S 73.40W
                depth, H0 = 30 km
 (Strike, Dip, Rake) = (309, 21, 61)
 Seismic moment, Mo = 2.2x10**21 Nm  (Mw = 8.2)
 Source duration,  T = 107 s
 Fault area,  S = 200 km x 100 km
 Dislocation, D (maximum) = 4.5 m
              D (mean)  = Mo/mu S = 2.8 m  (mu = 40 GPa is assumed)
 Stress drop  DELTA sigma = 2.5 Mo / S**1.5 = 1.9 MPa

  This is the first M8 class earthquake in 21st century. The
mechanism, the location and the stress drop indicate that this is
an inter-plate thrust event between a subducting oceanic plate (Nazca
plate) and an overriding continental plate (South American plate).
A very large asperity was derived in the southeast of the epicenter.
The center locates about 150 km from the epicenter. Therefore, heavy
damages are estimated to be concentrated rather far south from the

1. Aftershocks within a week
    Aftershock distribution determined by USGS and the focal mechanism
 by Harvard Univ. are shown.

2. The largest aftershock on July 7, 2001 (M7.2)
    Source process

    Main source parameters are:
 (Strike, Dip, Rake) = (315, 16, 64) (Harvard CMT)
  Mo = 2.8x10**20 Nm  (Mw = 7.6)
  Depth of the initial break, H0 = 20 km
  Source duration,  T = 30 s