11:45 AM - 12:00 PM
[J06-4-06] 1972 Slow Mega Slip Event in Mexico Recoded with Tide Gauges
The aseismic slow slip events (SSEs) are essential elements of the seismic cycle of large thrust earthquakes in subduction zones. GPS networks so far are the main tools for the SSE detection. Tide gauges data may also provide useful information about large subduction thrust SSEs in the pre-GPS epoch (PGE).
Since 1997 there were five large Mw~7.5, long-term slow slip events recorded by GPS in the the subduction zone of Mexico. Older SSEs are not detectable using campaign GPS measurements. Research quality tide gauge data in the Pacific coast of Mexico exists back to the year 1953 that gives a chance to recover the SSE in the PGE. Before a search for the PGE SSEs the monthly averaged tide gauge (TG) records had been processed to get rid of the long-term tide harmonics, partially reduced for the el Ninio effect, long-period solar harmonics were removed and the residual TG records smoothed.
There are two unconformities observed in the final reduced TG record at the Acapulco station in 1962 and 1972. The first step-like signal corresponds to the coseismic coastal uplift of ~22 cm produced by a doublet of earthquakes with Mw 7.1 and 7.0 in May 1962 below Acapulco. The second distinct coastal rise of 10-12 cm lasted almost one year in 1972 without any noticeable seismic activity in Guerrero. The amplitude and time span of this event closely reminds recent long-term SSEs observed at the same area. Analysis of differential residuals (with respect to the Manzanillo) of TG records reveals similar concurrent but smaller amplitude coastal rise at Puerto Angel and Salina Cruz TG stations in the Oaxaca state. The records at TG stations located NW from the Acapulco do not show any anomalous signals corresponding to the apparent 1972 slow slip in Guerrero-Oaxaca.
Our result suggest that the along coast extent of the 1972 SSE may be about of 500 km, probably the largest subduction SSE recorded so far. The propagation of this event is difficult to assess based only on three TG records.
Since 1997 there were five large Mw~7.5, long-term slow slip events recorded by GPS in the the subduction zone of Mexico. Older SSEs are not detectable using campaign GPS measurements. Research quality tide gauge data in the Pacific coast of Mexico exists back to the year 1953 that gives a chance to recover the SSE in the PGE. Before a search for the PGE SSEs the monthly averaged tide gauge (TG) records had been processed to get rid of the long-term tide harmonics, partially reduced for the el Ninio effect, long-period solar harmonics were removed and the residual TG records smoothed.
There are two unconformities observed in the final reduced TG record at the Acapulco station in 1962 and 1972. The first step-like signal corresponds to the coseismic coastal uplift of ~22 cm produced by a doublet of earthquakes with Mw 7.1 and 7.0 in May 1962 below Acapulco. The second distinct coastal rise of 10-12 cm lasted almost one year in 1972 without any noticeable seismic activity in Guerrero. The amplitude and time span of this event closely reminds recent long-term SSEs observed at the same area. Analysis of differential residuals (with respect to the Manzanillo) of TG records reveals similar concurrent but smaller amplitude coastal rise at Puerto Angel and Salina Cruz TG stations in the Oaxaca state. The records at TG stations located NW from the Acapulco do not show any anomalous signals corresponding to the apparent 1972 slow slip in Guerrero-Oaxaca.
Our result suggest that the along coast extent of the 1972 SSE may be about of 500 km, probably the largest subduction SSE recorded so far. The propagation of this event is difficult to assess based only on three TG records.