Hard and soft measures for earthquake and tsunami disaster mitigation

A destructive earthquake struck the Kobe region on January 17, 1995, and a massive earthquake and tsunami struck eastern Japan on March 11, 2011. We present an overview of the casualty aspects of the 2011 Tohoku earthquake compared with those of the 1995 Kobe earthquake. In the Tohoku disaster, some water gates and seawalls saved some villages from the tsunami effects, though some did not. Based on these examples, we discuss the efficiency of soft and hard measures and consider their respective merits and demerits. The main causes of death in the Kobe and Tohoku EQs were, respectively, collapsing buildings and drowning in the tsunami. Although the time to death was very short in both cases, people often have more time to evacuate in the case of an interplate earthquake leading to a tsunami. Basic countermeasures against tsunamis include such hard measures as water gates, seawalls, and embankments. Soft measures need to be implemented in areas where hard measures are insufficient

Ancient and Medieval Events and Recurrence Interval of Great Kanto Earthquakes along the Sagami Trough, Central Japan, as Inferred from Historiographical Seismology

To know the Ancient and Medieval events and the recurrence interval of the great Kanto earthquakes such as the 1923 and 1703 ones generated by the subduction of the Philippine Sea plate along the Sagami trough, central Japan, I examined the latest dataset of historical records. I used only rank-A (contemporary) materials in the Online Database of Historical Documents on Japanese Earthquakes and Eruptions in the Ancient and Medieval Ages. Among destructive earthquakes in the Kanto district before the sixteenth century, the 878 Gangyo earthquake, which has been suspected an inland event generated from the Isehara fault, is considered the oldest candidate of the Kanto earthquake based on the large-scale disaster and intense aftershock activity, though tsunami is not mentioned. The 1293 Showo (Einin) Kamakura earthquake can be regarded as a great Kanto earthquake, because of severe damage in the Kanto district and remarkable aftershock activity. During the fifteenth century, both of the 1433 Eikyo and the 1495 Meio earthquakes are regarded as candidates of the Kanto earthquake; the former caused severe damage around Kamakura on the northern coast of Sagami Bay with high-aftershock activity and presumably caused tsunami, and the latter seems to have caused large tsunamis at Kamakura and the west coast of Sagami Bay. Although further investigation is necessary to clarify which one is the Kanto earthquake, we can say that an interplate earthquake probably occurred in the fifteenth century. Intervals between successive events, in 878, 1293, 1433 or 1495, 1703, and 1923, range from 140 to 270 yr except for the first interval of 415 yr. It is very difficult to confirm whether a great interplate earthquake occurred or not during this interval by means of historiographical seismology, because the Kanto district was in the worst situation of poor historical records in those days.

Los sismos intraplaca de septiembre de 2017: un breve análisis de su impacto en el territorio veracruzano

El Servicio Sismológico Nacional (SSN) reportó un sismo el día 7 de septiembre de 2017 con Mw 8.2 a una profundidad de 59 km y otro con Mw 7.1 y profundidad de 57 km el 19 de septiembre, curiosamente a 32 años del sismo interplaca de 1985 (M 8.1). Ambos eventos presentaron similitudes en cuanto al tipo de fallamiento (normal), profundidad y la placa tectónica donde se originó la ruptura. Para estudiar lo que concierne al impacto de estos sismos en el estado de Veracruz, se obtuvieron los valores máximos observados de aceleración y velocidad (PGA y PGV) durante ambos sismos en cada una de las estaciones de la Red Sísmica de Banda Ancha de Veracruz (RSBAV) y dos estaciones de banda ancha del Servicio Sismológico Nacional (SSN). Estos fueron relacionados con algunos daños reportados en esta entidad (para Xalapa con apoyo de una encuesta vía web), permitieron observar la atenuación con la distancia y analizar el contenido de frecuencias dominantes a partir de los registros de aceleración.Palabras clave: Placas tectónicas; sismo intraplaca; magnitud; intensidad; aceleración AbstractThe National Seismological Ser­vice (SSN) reported one earthquake on 7 Sep­tember 2017 with Mw 8.2 at a depth of 59 km. Another earthquake, with Mw 7.1 and depth of 57 km, occurred on September 19, curiously 32 years after the interplate earthquake of 1985 (Mw 8.1). Both recent events presented similarities as to the fault type (normal), depth and the tectonic plate where the rupture origina­ted. We study the impact of these earthquakes in the state of Veracruz, by obtaining the peak ground acceleration and velocity (PGA and PGV) at stations of the Veracruz Broadband Seismic Network (RSBAV) and two broadband stations of the National Seismological Servi­ce (SSN) during both earthquakes. We relate them to some damages reported in the Vera­cruz state (for Xalapa city with the support of a web survey), observe the attenuation with distance, and analyze the content of dominant frequencies from the records of acceleratio.Keywords: Tectonic Plates; intraplate earth­quake; magnitude; intensity; acceleration

Faulting process and coseismic stress change during the 30 January, 1973, Colima, Mexico interplate earthquake (Mw=7.6)

El 30 de enero de 1973 ocurrió un evento mayor de subducción (Mw=7.6) en la interfase de las placas de Cocos yNorteamérica, cerca del punto triple entre las placas de Rivera, Cocos y Norteamérica. Este evento podría estar relacionado condos secuencias de grandes sismos subsecuentes que ocurrieron alrededor de esta región. Aunque varios autores han analizado elmecanismo focal y la profundidad de este sismo, nosotros analizamos las características de la fuente y realizamos una inversióncinemática lineal de la distribución de deslizamientos sobre el plano de falla a través del modelado de forma de onda. Encontramosun mecanismo inverso (St=285°, Dip=16°, Ra=85°) consistente con la tectónica regional, con una profundidad de 16 km y unaliberación total de momento de 2.98x1027 dyn-cm. Los resultados muestran una distribución de deslizamiento con dos manchasprincipales, con una dislocación máxima de 199 cm y 173 cm respectivamente. Este sismo rompió dos asperezas principales enel plano de falla extendido: una en la parte inferior y al suroeste y la otra en la parte superior y al noroeste del hipocentro, conun cambio de esfuerzos de -31 y -40 bars respectivamente. El área circundante de incremento de esfuerzo podría haber influenciadola sismicidad subsecuente a una distancia de hasta 120 km del hipocentro.