**1. Introduction**

Tsunami deposit studies involve the application and use of a wide range of interdisciplinary techniques including, but not limited to, geology, stratigraphy, geochronology, geochemistry, geophysics, numerical modeling, sedimentology, micro- and macro-paleontology, geography, geomorphology, historical and ethnohistorical studies, archeology, statistical and contextual studies [1]. Such studies can support an understanding of the long-term hazard frequency as well as potential magnitude, sources, and risk of tsunamis in coastal areas. These in turn can help to underpin long-term coastal resilience planning in vulnerable locations.

Identifying past tsunami deposits in sedimentary records typically requires multiple characteristic criteria to be met [2]. For example, tsunamis can deposit distinct sedimentary units which can extend up to several kilometers and fine inland and upwards within the deposit. Similarly, distinct elevations in elemental concentrations of sodium, sulfur and chlorine (which are salinity indicators) can be observed in tsunami deposits relative to underlying and overlying sediment units. Deposits are typically preserved in coastal settings such as wetlands, which absorb wave energy and enable sediment to settle out of suspension as the tsunami wanes to normal sea level. The use of trace element, principal component, radiometric and isotope analysis to help distinguish marine from terrestrially sourced sediment and

determine the ages of specific sediment units can help to constrain a tsunami layer. A more detailed description of these examples, including a comprehensive list of tsunami deposit characteristics can be found in [1–8].

The use of geochemical proxies to characterize modern tsunami deposits and identify paleo-events has gained widespread use within the last few decades [3]. These include the characterization of elemental and magnetic susceptibility transitions in sedimentary sequences, isotope signatures, electrical conductivity and salinity [3]. In addition, such proxies have also been used to characterize the extent of tsunami inundation in areas which lack sedimentary transitions visible to the naked eye. For example, in [4–8].

A tool which has become more frequently used in tsunami deposit studies is the Itrax core scanner [9, 10]. The Itrax is a non-destructive, multifunction core scanning instrument which produces high-resolution X-ray fluorescence (XRF) elemental and magnetic susceptibility data, as well as optical and radiographic imagery. This enables the detection of very fine-scale elemental transitions in core sequences [9].

Raw Itrax elemental data of sediment cores from highly organic environments are subject to inherent uncertainties associated with organic dilution, matrix and instrumental effects [11–14]. However, organic dilution affects the raw datasets due to the closed-sum effect which in turn, can affect analytical interpretations if it is not adequately accounted for [11]. Normalization against Al can provide an organicfiltered dataset representative of the lithogenic faction of the sediment, which is more reliable for tsunami interpretation [8, 11]. Itrax studies of tsunami deposits prior to the results presented in [8] had not accounted for organic dilution in the datasets, often resulting in ambiguous Itrax interpretations of tsunami episodes.

In this study, available Itrax data for sediment cores collected from the Samoan Islands following the 2009 tsunami in this region [15, 16] (**Figure 1**), are normalized against Al and compared with tsunami Itrax observations at Little Pigeon Bay following the 2016 Kaikōura tsunami in New Zealand [8]. A distinct elevated elemental signature associated with the 2016 event was observed at Little Pigeon Bay, with comparable signatures representing characteristic types of events deeper in the sedimentary record. This provides a basis for reviewing the Samoan datasets to assess whether comparable trends are observed for the 2009 event. Distinct elevations in bromine (Br), chlorine (Cl), sulfur (S), calcium (Ca), iron (Fe), silicon (Si), titanium (Ti), rubidium (Rb) and potassium (K), are particularly targeted as these elements are known to represent marine influences [17, 18]. The findings are discussed with conclusions provided in the context of tsunami deposits and longer-term hazards in this region.

#### **1.1 The 2009 tsunami in the Samoan Islands**

The 2009 tsunami in the Samoan Islands deposited a distinct sedimentary unit in much of the coastal areas it inundated [2, 16]. The southern and eastern coastlines of Upolu Island were particularly affected [15]. In most cases, sediment deposited by the event comprised of distinct marine-derived calcareous sand units overlying dark brown and/or loamy pre-event soils. Most of these generally displayed an upwards fining within the unit from coarse sand to mud, and were commonly preserved in inundated coastal wetlands where water stagnation up to several days after the event was observed in some areas. These deposits also displayed distinct elemental elevations compared with underlying pre-event soil units as determined through coarse portable XRF (pXRF) analysis, in addition to marked changes in grain size [19].

This provides a reference for corroborating distinct elevated elemental signatures for this event in this study, and for interpreting potentially older tsunami

**29**

*Tsunami Elemental Signatures in the Samoan Islands: A Case Study*

episodes in the core profiles. For example, an interpreted older episode in a core profile might comprise of a distinct calcareous sand unit bound by an upper and a lower soil unit, and which displays distinct elevated elemental signals relative to the

*(a) Location of New Zealand and Samoa in the Southwest Pacific showing the 2016 M7.8 Kaikōura earthquake and 2009 M8.1 Samoa-Tonga earthquake locations. (b) Little Pigeon Bay site, New Zealand. (c) Core site locations in the Samoan Islands. Detailed descriptions of the Little Pigeon Bay site and core profiles are* 

The data used in this study were obtained from three cores which were sampled

The cores were sampled from coastal wetlands inland of the shore, and for the case of Manono and Ma'asina, these wetlands were exposed to inundation during the 2009 tsunami. At Ma'asina, a discernible calcareous sand deposit was observed between 0.4–0.8 m depth in the core. The embayment which this area is located has been impacted in the past by far-field tsunamis such as the 1960 Valdivia

in November 2010 using a hand-held D-Corer from three separate locations; (1) 0.7 m core extracted ~20 m inland of the shore at Ma'asina (S1); (2) 2 m core extracted ~75 m inland of the shore at Manono-uta (S2); and (3) 1.5 m core

adjacent soils that are comparable to the 2009 tsunami signature.

*provided in [8], with the Samoan sites and profiles provided in [16, 19].*

extracted ~150 m inland of the shore at Lano (S3) (**Table 1**).

**2. Methods and materials**

**Figure 1.**

**2.1 Study area and sampling**

*DOI: http://dx.doi.org/10.5772/intechopen.85639*

*Tsunami Elemental Signatures in the Samoan Islands: A Case Study DOI: http://dx.doi.org/10.5772/intechopen.85639*

#### **Figure 1.**

*Applied Geochemistry with Case Studies on Geological Formations, Exploration Techniques…*

tsunami deposit characteristics can be found in [1–8].

naked eye. For example, in [4–8].

and longer-term hazards in this region.

**1.1 The 2009 tsunami in the Samoan Islands**

sequences [9].

determine the ages of specific sediment units can help to constrain a tsunami layer. A more detailed description of these examples, including a comprehensive list of

The use of geochemical proxies to characterize modern tsunami deposits and identify paleo-events has gained widespread use within the last few decades [3]. These include the characterization of elemental and magnetic susceptibility transitions in sedimentary sequences, isotope signatures, electrical conductivity and salinity [3]. In addition, such proxies have also been used to characterize the extent of tsunami inundation in areas which lack sedimentary transitions visible to the

A tool which has become more frequently used in tsunami deposit studies is the Itrax core scanner [9, 10]. The Itrax is a non-destructive, multifunction core scanning instrument which produces high-resolution X-ray fluorescence (XRF) elemental and magnetic susceptibility data, as well as optical and radiographic imagery. This enables the detection of very fine-scale elemental transitions in core

Raw Itrax elemental data of sediment cores from highly organic environments are subject to inherent uncertainties associated with organic dilution, matrix and instrumental effects [11–14]. However, organic dilution affects the raw datasets due to the closed-sum effect which in turn, can affect analytical interpretations if it is not adequately accounted for [11]. Normalization against Al can provide an organicfiltered dataset representative of the lithogenic faction of the sediment, which is more reliable for tsunami interpretation [8, 11]. Itrax studies of tsunami deposits prior to the results presented in [8] had not accounted for organic dilution in the datasets, often resulting in ambiguous Itrax interpretations of tsunami episodes. In this study, available Itrax data for sediment cores collected from the Samoan Islands following the 2009 tsunami in this region [15, 16] (**Figure 1**), are normalized against Al and compared with tsunami Itrax observations at Little Pigeon Bay following the 2016 Kaikōura tsunami in New Zealand [8]. A distinct elevated elemental signature associated with the 2016 event was observed at Little Pigeon Bay, with comparable signatures representing characteristic types of events deeper in the sedimentary record. This provides a basis for reviewing the Samoan datasets to assess whether comparable trends are observed for the 2009 event. Distinct elevations in bromine (Br), chlorine (Cl), sulfur (S), calcium (Ca), iron (Fe), silicon (Si), titanium (Ti), rubidium (Rb) and potassium (K), are particularly targeted as these elements are known to represent marine influences [17, 18]. The findings are discussed with conclusions provided in the context of tsunami deposits

The 2009 tsunami in the Samoan Islands deposited a distinct sedimentary unit in much of the coastal areas it inundated [2, 16]. The southern and eastern coastlines of Upolu Island were particularly affected [15]. In most cases, sediment deposited by the event comprised of distinct marine-derived calcareous sand units overlying dark brown and/or loamy pre-event soils. Most of these generally displayed an upwards fining within the unit from coarse sand to mud, and were commonly preserved in inundated coastal wetlands where water stagnation up to several days after the event was observed in some areas. These deposits also displayed distinct elemental elevations compared with underlying pre-event soil units as determined through coarse portable XRF (pXRF) analysis, in addition to marked changes in grain size [19]. This provides a reference for corroborating distinct elevated elemental signatures for this event in this study, and for interpreting potentially older tsunami

**28**

*(a) Location of New Zealand and Samoa in the Southwest Pacific showing the 2016 M7.8 Kaikōura earthquake and 2009 M8.1 Samoa-Tonga earthquake locations. (b) Little Pigeon Bay site, New Zealand. (c) Core site locations in the Samoan Islands. Detailed descriptions of the Little Pigeon Bay site and core profiles are provided in [8], with the Samoan sites and profiles provided in [16, 19].*

episodes in the core profiles. For example, an interpreted older episode in a core profile might comprise of a distinct calcareous sand unit bound by an upper and a lower soil unit, and which displays distinct elevated elemental signals relative to the adjacent soils that are comparable to the 2009 tsunami signature.
